Humidity-induced dynamic coordination drives the oscillatory migration of ions for sustainable energy harvesting

Domain: neuroscience
Published: 2026-02-12
Editorial outcome: major_revision

Abstract
Moisture-induced ion diffusion in nanostructured materials is a promising route for high-performance power generation, yet achieving the continuous ion migration necessary for long-term operation remains challenging. Here, we report a moisture-electricity conversion mechanism capable of sustained electrical output. We demonstrate that humidity fluctuations regulate the dynamic coordination between iodide ions (I-) and iodine molecules (I2), driving the oscillatory migration of I- to generate a continuous alternating ion current (AC). Crucially, this coordination process avoids charge exchange, while electricity generation is achieved through moisture-driven ion transport. The device achieves a current output of 33.2 µA cm-² and exhibits sustainable performance recovery under natural humidity fluctuations. This mechanism remains effective even under minimal humidity gradients (13% RH), ensuring adaptability to diverse weather conditions. Our strategy, applicable to various gel materials, utilizes environmental humidity fluctuations as a power source for ion diffusion, offering a fundamental solution for long-term, autonomous energy harvesting.

Introduction
The ubiquitous presence of water in the environment provides a natural reservoir for green energy1. Generating electricity through the electric coupling between functional materials and various forms of water—such as flow, waves, droplets, moisture and evaporation—a new type of energy conversion effect has emerged, which is defined as hydrovoltaic technology2–4. Compared with conventional photovoltaic and wind power technologies, which rely on intermittent and variable sources such as sunlight and wind under specific environmental conditions, hydrovoltaic technology demonstrates significant potential due to the ubiquity of water resources in nature5. Among hydrovoltaic technologies, moisture- and evaporation-driven generators have attracted increasing attention for their virtues of ubiquity, spontaneity, and direct current output6. Evaporation-based power generation represents a passive means of energy harvesting that leverages natural processes instead of mechanical motion7. However, its long-term performance is constrained by the reliance on continuous access to liquid water and is susceptible to climatic fluctuations8. Moisture-electric generators (MEGs) are destined to exploiting omnipresent power sources due to the abundance, accessibility, and all-weather availability of moisture in the ambient environment9–12.

Nevertheless, most existing MEGs operate in a unidirectional migration process of water and ions13–15. When the water and ions migration tend to equilibrate, the MEGs will stop outputting electrical energy, reflecting a limited power generation duration16–18, particularly in terms of current duration5. Current efforts to improve device lifespan include developing numerous strategies to maintain or enhance water and ion concentration gradients, such as constructing asymmetric or heterogeneous structures19,20, optimizing electrodes21,22, and engineering functional materials23. However, unidirectional ion migration leads to ion accumulation on one side of the device, generating an internal reverse electric field that impedes the movement of subsequent ions, thereby causing the device's power generation to decay. Some studies have attempted to introduce redox reactions to consume the ions accumulated at the electrodes, thereby mitigating the reverse electric field and enabling long-term power generation; however, electrode redox reactions should ideally be avoided in pure hydrovoltaic devices24. Our previous work achieved 600 h of power generation by introducing photocatalysis to consume accumulated hydrogen ions and restore the concentration gradient25, but this method requires an additional energy input and suffers from limited performance recovery after multiple cycles. Therefore, solving the lifetime issue requires new strategies that overcome the limitations of unidirectional ion transport and promote bidirectional ion transport.

For the moisture-induced ion-diffusion phenomenon, returning the migrated ions by introducing humidity fluctuations is expected to rebuild the ion concentration gradient, making it an effective method to restore power generation performance. However, when humidity fluctuates in the opposite direction, most power generation devices do not give rise to ionic back migration23,26,27. This is because the moisture adsorption power generation driven by ion concentration difference and the moisture desorption power generation dominated by ion hydration take place in the same direction28. In very few studies, humidity fluctuations have been observed to induce ionic back migration29,30. For example, Zhao et al. noticed that the moisture adsorption/desorption process resulted in a water content difference in the transmembrane ionic liquid layer, which triggered a bidirectional transmembrane ionic flux, enabling the generation of an AC electrical signal for continuous energy harvesting30. However, the ion channel membranes described above demand a fine mesopore size and an average membrane thickness of 74 nm. The block copolymerization fabrication technology utilized has the disadvantages of high synthesis difficulty and harsh reaction conditions30,31, which will limit its promotion in practical applications. Further efforts are still required to explore new materials and mechanisms for ionic back-migration, thereby enabling sustainable power generation through the daily cycle of humidity.

In this work, we propose that the dynamic coordination between I- and I2 to synthesize I3- is regulated by external humidity fluctuations. In principle, this enables perpetual reconstruction of iodide ion concentrations and reciprocal ion migration driven by natural humidity cycles, without any external energy input. Based on this mechanism, we designed a hydrogel-based sustainable moisture-electric generator (SMEG). The upper surface of the SMEG is exposed to the air, while the lower surface and the surrounding area are isolated from the air (Fig. 1a). Under humidity fluctuations, I- ions in SMEG undergo continuous oscillatory migration, resulting in the sustained AC power generation. Detailed power generation processes are provided in Supplementary Fig. 1. The process of I- ions oscillation migration induced by moisture absorption and desorption was confirmed by a combination of electrical experimental tests, Raman spectroscopy, and theoretical calculations. Compared to traditional MEGs, our SMEG has excellent stability and sustained power generation capabilities. As shown in Fig. 1b, our device can continuously and stably generate electricity for nearly two weeks without performance degradation under the high-low humidity switching environment simulated in the laboratory (25±3 °C). Furthermore, even after being stored under laboratory conditions for seven months (30-65% RH, 25±3°C), the device was still able to operate continuously for one month (Supplementary Fig. 2), demonstrating its outstanding long-term storage stability and sustained performance exceeding one month. We tested four different hydrogels loaded with I3- ions, and all of them exhibited similar AC sustained power generation phenomena. This indicates that the power generation mechanism applies with minimal requirements for pore size and the chemical composition of the hydrogel skeleton material. Furthermore, our device exhibits excellent environmental adaptability, responding to small humidity fluctuations as low as 2% RH across a wide humidity range, demonstrating immense potential for continuous energy harvesting from ambient humidity variations. Overall, this work provides a mechanism and a material system for ion back-migration, offering an optimized platform for realizing stable, high-performance, and sustainable moisture-based electricity-generating devices.

Results
Design and characterization of SMEG
The SMEG in question consists of a porous carbon upper electrode, a hydrogel functional layer surrounded by a sealing film and a carbon lower electrode (Fig. 1a). The hydrogel functional layer is composed of acrylamide (AM), 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and sodium iodide (NaI) (Fig. 2a). AM mainly serves to provide structural support and improve water retention (Supplementary Fig. 3), while AMPS with sulphonic acid groups enables the functional layer to dissociate hydrogen ions through interaction with water molecules32–34. The use of a sealing film ensures that air contacts the functional layer from the top, facilitating the formation of ionic concentration differences. The introduction of NaI with medium hygroscopicity is the unique design of the functional layer, which allows endowing the device with three key properties:
(i) Due to the Hofmeister effect35–37, I- is a Chaotropic ion that makes the hydrogel skeleton more sparse, favoring ionic transport, and weakens the ion screening of the charged skeleton3,38,39.
(ii) NaI with appropriate hygroscopicity (Supplementary Fig. 4) provides the functional layer with excellent responsiveness to changes in humidity (absorption of moisture at relatively high humidity and desorption of moisture at relatively low humidity)40,41.
(iii) The following reactions occur in the acidic environment provided by the sulfonic acid group of AMPS, and the corresponding equations are as follows:
4I- + 4H+ + O2 = 2H2O + 2I2 (1)
I- + I2 ⇌ I3- (2)
In this case, Eq. (1) is the pre-reaction; Eq. (2) is a reversible dynamic coordination equilibrium, which is the core reaction of the power generation principle. The directionality of Eq. (2) is regulated by the water molecule content (humidity fluctuations) (for details, see the mechanism validation section). By comparing the microscopic morphologies of the NaI-treated and non-NaI-treated hydrogels, it is proven that the pore structure of the hydrogels becomes more sparse after the addition of I- (Fig. 2b,c). As seen in the inset of Fig. 2c, I3- is introduced into the hydrogel (the hydrogel changes from transparent to brown), confirming the presence of Eq. (2) and the generation of a large amount of I3- (I- itself is colorless and I3- is brown).

The entire device preparation process is described in the experimental section and supplementary information (Supplementary Fig. 5). To further illustrate the material properties, a series of characterizations were carried out on the moisture absorption/desorption, the distribution and type of iodine elements present in the hydrogel. Firstly, in addition to optimizing the hydrogel skeleton, NaI also provides good moisture absorption capacity, maintaining the hydrated state of the hydrogel. To demonstrate this, we compared the changes in hydrogels with and without NaI in an air environment (55% RH). The hydrogel without NaI becomes dried out and hardened after 48 h, whereas the one with NaI retains its original size even after being exposed to air for one month (Supplementary Fig. 6). At the same time, the moderate moisture uptake capacity of NaI enables the hydrogel to exhibit rapid moisture uptake and release behavior when switching between high humidity and low humidity (using a hydrogel thickness of 3 mm). As shown in Fig. 2d, a piece of gel was first pre-treated by placing it in a humidity chamber at 15% RH for 12 h, and then placed at 98% RH and 15% RH for 12 h, respectively, showing its good humidity responsiveness (Fig. 2d).

Secondly, the NaI distribution is analyzed by an Energy Dispersive Spectrometer (EDS) (Fig. 2e and Supplementary Fig. 7), and the iodine and sodium elements are evenly distributed throughout the hydrogel, confirming the uniform loading of NaI on its skeleton. Furthermore, iodine exists in three forms within the gel: I-, I2, and I3-. In the hydrated state, the laser Raman spectra confirms the presence of I3- and I2 (Supplementary Fig. 8)42–44. Additionally, the X-ray Photoelectron Spectroscopy (XPS) of the dried hydrogel demonstrates that the hydrogel contains I3-, I2, and I- (Fig. 2f)45,46, with I3- having the highest proportion (Supplementary Table 1). This is consistent with our hypothesis that Eq. (2) tends to synthesize I3- under low humidity conditions. In summary, the hydrogel possesses the pore structure necessary for rapid ion mobility, good moisture uptake and release capabilities, and the dynamic coordination of I- and I2 to synthesize I3- is the key to device performance and power generation mechanism.

Electrical output performance and influencing factors of SMEG
The current signal associated with a large humidity switch (15% RH-98% RH) within 24 h reaches 33.2 µA at 98% RH changes reversely to −31.4 µA at 15% RH (Fig. 3a). Since the effective working area of the device is 1 cm2, the current density value is numerically equal to the current value. When the humidity is switched, the current amplitude displays a rapid increase, followed by a gradual decay. At the inception of environment switching, the rate of moisture absorption and desorption is the highest, thus engendering a comparatively substantial I- concentration difference and a rapid increase in current. As time elapses, the water exchange between the device and the environment is progressively balanced, the rate of moisture absorption and desorption diminishes, and the ionic motion slows down, consequently leading to a gradual decline in the device’s performance.

Subsequently, the influences of a series of preparation parameters on the power generation performance of the SMEG are studied. Firstly, the effect of NaI solution concentration is investigated, where the solution soaking time is fixed at 24 h (Fig. 3b). The results show that the device has optimal performance output when the NaI concentration is 1 M. In the case where the NaI concentration is 0.01 M, the device generates almost no electricity due to an insufficient number of ions participating in dynamic coordination. Conversely, when the NaI concentration is excessively high, the performance also deteriorates. This attenuation phenomenon may be attributed to the high concentration of NaI that tend to form ion pairs and the high osmotic pressure-induced dehydration or even collapse of the gel skeleton47–50, thus hindering ion transport (Supplementary Figs. 9 and 10). In the subsequent investigation, the concentration of NaI solution is fixed at 1 M, while the soaking time in NaI is varied. The impact of soaking time on device performance is shown in Fig. 3c and Supplementary Fig. 11a. The overall trend shows that the output performance increases with soaking time and gradually reaches saturation. This is likely because the gel gradually absorbs more NaI, approaching its maximum absorption capacity over time (Supplementary Fig. 12). The sulfonic acid groups of AMPS dissociate hydrogen ions when interacting with water molecules, which promotes the formation of I2. Thus, the effect of different AMPS contents on the device performance is investigated (Fig. 3d and Supplementary Fig. 11b). The experimental results demonstrate that the device output current gradually increases with AMPS content before reaching a saturation plateau. This trend can be explained as follows: The sulfonic acid groups in AMPS provide additional H+ ions, which facilitate the conversion of I- to I3-, thereby enhancing device performance. However, when the H+ concentration reaches a certain threshold, the conversion between I- and I3- approaches equilibrium. Beyond this point, further increases in AMPS content no longer significantly promote I3- generation, resulting in the observed current stabilization. The positive correlation between I3- content and current intensity, as clearly demonstrated in Supplementary Fig. 13, provides compelling experimental evidence to support our proposed mechanism.

The effect of hydrogel thickness on the device performance is further investigated (Fig. 3e and Supplementary Fig. 11c). The device current generation gradually increases and reaches saturation with increasing thickness. At a thickness of 9 mm, the moisture absorbing current can reach up to 30 µA and the desorbing current up to −30.5 µA. There is no obvious improvement in performance with further increases in thickness, but the electrical output lasts longer. When the thickness is thin, rapid water saturation causes a low humidity gradient in the gel, and the ion flux is small, resulting in a weak electrical signal. When the thickness is greater than 9 mm, the humidity gradient and the hydrogel ion flux gradually reach saturation, so that the device performance gradually saturates. It is noted that the time for the device to reach the peak current after the humidity changes also increases with thickness (Supplementary Fig. 11d), which is due to the fact that the greater the thickness, the greater the distance over which the ions are transported. The voltage signals at different thicknesses are further examined (Supplementary Fig. 14a) and the voltage gradually augments with the increase in thickness. In particular, for devices of 9 mm thickness, voltage changes were recorded during a single humidity cycle, and the results showed a trend similar to that of current (Supplementary Fig. 14b). Subsequently, the internal resistance of the 9-mm-thick device was evaluated via its current-voltage characteristics. The results indicate that even at 15% relative humidity, SMEG maintains an internal resistance of 390 Ω (Supplementary Fig. 15a). It can be inferred that the relatively low internal resistance is unfavorable for the formation of a substantial ionic concentration gradient, which ultimately results in a smaller output voltage. On the other hand, the low voltage may also originate from the internal ionic distribution prior to humidity fluctuations. Under steady-state conditions, iodide ions remain homogeneously distributed within the gel. Although humidity fluctuations trigger dynamic coordination and regenerate a concentration gradient, the pre-existing uniform distribution may diminish the net ion separation, thereby constraining the maximum voltage that can be produced. To comprehensively assess the power-output characteristics of the device, we measured the power density under various external load resistances (Supplementary Fig. 15b). A maximum power density of 0.049 µW cm-2 was obtained.

Interestingly, as shown in Fig. 3a–e, we observed that the device performance during absorption and desorption varies: in some cases, the performance is similar, while in others, the absorption current is 1.5-2 times higher than the desorption current. This phenomenon may originate from the different I-/I3- ratios within the hydrogel, which in turn lead to differences in its absorption and desorption capacities. Consequently, the device exhibits distinct performance characteristics during the absorption and desorption processes. The detailed analysis is provided in Supplementary Notes 1-2 and Supplementary Figs. 13, 16 and 17. Furthermore, to further elucidate the role of the sealing film, we systematically varied the thickness of the gel layer to examine the influence of sealing films on device performance under 98% RH. As illustrated in Supplementary Fig. 18, devices with sealing films consistently exhibited superior performance compared to their unsealed counterparts. Notably, the performance gap between sealed and unsealed devices widened as the gel thickness increased. We attribute this phenomenon to the expanded lateral surface area of the gel exposed to ambient moisture at greater thicknesses, which reduces the ionic concentration gradient between the top and bottom electrodes, thereby degrading the electrical performance.

The humidity switching interval is another influencing factor (Fig. 3f), where the interval is the time between switching from 98% RH (15% RH) to 15% RH (98% RH). We compared three groups of switching intervals, and the reverse current amplitudes when high humidity turns to low humidity are extracted. The current of the device in the first group with an interval of 5 h is about −3 µA; the device current is up to −4.58 µA in the second group of 10 h intervals; the device current is up to −6.22 µA in the third group of 15 h intervals. The longer the humidity switching interval time, the higher the performance of the device can be recovered. This is because the longer the interval time, the more complete the Eq. (2) is. After switching the humidity, the ion concentration gradient in the reverse direction is larger, thus generating a larger current signal.

In addition, the influence of the number of holes in the top electrode on the electrical performance was also investigated, we conducted a series of experiments using top electrodes with different numbers of holes (5, 10, 15, 20, and 25 holes, each with a diameter of 1.5 mm) to investigate their influence on device performance during the humidification process. As shown in Supplementary Fig. 19, when the number of holes was small, the limited contact area with ambient moisture led to slower performance enhancement and lower peak output. In contrast, devices with a higher number of holes exhibited faster performance increase and higher peak currents, due to a larger exposed surface area facilitating more efficient moisture uptake. These findings are consistent with the proposed working mechanism.

Finally, we compared several recently reported MEGs, as summarized in Supplementary Table 2. The results indicate that the SMEG based on the dynamic-coordination-driven ion-concentration-reconstruction mechanism offers several key advantages, including a broad operational humidity range, the ability to be activated by small humidity variations, a low performance-decay rate, and robust long-term energy output. Concurrently, the maximum power density (VOC × JSC) of our device stands among the top tier when compared to other pure hydrovoltaic devices (i.e., those operating without electrode redox reactions). These features collectively underscore the strong potential of our device for sustainable and durable moisture-enabled energy generation.

Verification for electricity-generating mechanism
In order to explore the electricity-generating mechanism of the SMEG, the device electrical tests were first carried out under different moisture action modes (Fig. 4a). Four moisture action modes are considered: (i) moisture absorption/desorption on the upper surface of the device; (ii) moisture absorption/desorption on the lower surface of the device; (iii) moisture absorption/desorption on both surfaces of the device; (iv) sealing of the device on both sides. The results demonstrate that the devices operating under the first two asymmetric moisture action modes exhibit good power generation performance. In contrast, the current output is disordered when porous electrodes are employed on both the upper and lower surfaces. When the device is sealed on both sides, the current signal is negligible. The above phenomena prove that the power generation signal is closely related to the directional moisture absorption and desorption behavior of the device.

Secondly, to verify whether the observed AC generation originates from the hygroscopic nature of the salts, a range of hygroscopic salts (hygroscopicity of LiCl > CaCl2 > MgCl2 > NaI > NaF) were incorporated into the hydrogel41, and the power generation performance of the devices was examined at high and low humidity switching conditions (98% RH-15% RH) (Fig. 4b). Only the device with added NaI exhibits moisture absorption and desorption-induced AC power generation. When other types of hygroscopic salt devices switch between high humidity and low humidity, it will not reverse, although the current will fluctuate to a certain extent.

Since only the NaI-soaked hydrogel changes color, it is concluded that the AC generation phenomenon is related to the iodine element. Subsequently, various halogenated salts were added to the hydrogels for testing. The NaF, NaCl, and NaBr control devices exhibit no AC generation phenomenon, whereas the NaI-treated hydrogel demonstrates an AC electrical signal (Supplementary Fig. 20). Together with the observation that NaI-treated hydrogels turn brown, it indicates that the AC generation phenomenon is associated with the dynamic coordination between I- and I2 to form I3-, thereby underscoring the specificity of the element iodine. However, the limited contribution of Na+ to the power generation performance may be attributed to the fact that the migration of Na+ primarily relies on the movement of water molecules, whereas I- ions benefit from an additional driving force provided by the dynamic coordination process.

In order to elucidate the moisture absorption- and desorption-induced AC power generation phenomenon associated with elemental iodine, the following principle hypotheses are proposed. Firstly, two reactions occur during the immersion of P(AM-co-AMPS) hydrogels in NaI solution, creating prerequisites for device use (Refer to Eqs. (1) and (2) in section “Design and characterization of SMEG”). Secondly, the forward and reverse directions of the Eq. (2) are regulated by humidity. Precisely, in a dry environment, Eq. (2) proceeds in the forward direction, I- and I2 tend to coordinate to synthesize I3-; in a humid environment, Eq. (2) proceeds in the reverse direction, I3- tends to dissociate into I2 and I-. This humidity-modulated dynamic coordination serves as the core driving force for the device’s power generation. On this basis, when the device is switched from a low-humidity environment (15% RH) to a high-humidity environment (98% RH), the dissociation of I3- occurs on the air-contacting side of the device surface to produce I2 and I-, which contributes to a decrease in the concentration of I3- and an increase in the concentration of I-. However, considering that I- ions are smaller and move faster than I3- ions51,52, it is believed that the overall downward movement of I- that dominates the ionic current. The current will first increase and then decrease gradually to a near-zero level. This decrease is due to the gradual equilibration of the dynamic coordination. Concurrently, the voltage also shows an increase first and then a decrease next, illustrating a similar change pattern as electric current (Supplementary Fig. 14b). When the device is switched from a high humidity environment (98% RH) to a low humidity environment (15% RH), I- and I2 synthesize into I3- in the surface layer on the air-contacting side of the functional layer, prompting I- to flow back. This reciprocating oscillation of I- enables the device to generate electricity continuously.

To validate Eqs. (1) and (2), we compared the power generation differences of hydrogels with different AMPS contents in the previous section, proving the importance of the acid environment induced by the dissociation of protons from the AMPS sulfonate group. To further confirm the contribution of hydrogen ions, we immersed PAM gels in NaI solution with added HCl, and the device power generation performance was restored to a level comparable to that of the P(AM-co-AMPS)-NaI hydrogel device (Fig. 4c). In contrast, the PAM-NaI control device showed very weak power generation phenomenon without HCl addition, indicating the importance of I3- to the moisture-induced AC power generation phenomenon. The above results strongly indicate the presence of Eq. (1), and the introduction of hydrogen ions changes the hydrogel color from transparent to dark brown, confirming the presence of I3- and Eq. (2).

Further, to illustrate the generalizability of the dynamic coordination strategy, hydrogels with different backbone charges, namely P(AM-co-AMPS), PAM, poly[2-(methacryloyloxy)ethyl] trimethyl ammonium chloride (PMETAC), ploy[2-(methacryloyloxy)ethyl] dimethyl-(3-sulfonatopropyl) ammonium hydroxide (PDMAPS), were immersed in NaI and NaI (HCl), respectively. The hydrogel devices immersed in NaI (HCl) all show similar AC signals (Fig. 4d), indicating that the dynamic coordination strategy is universal and has low requirements for gel backbone materials. The small difference in the amplitude of the electrical signals under different charged skeletons maybe due to the salt-in effect of NaI, which loosens the hydrogel skeleton. A higher salt concentration shorten the Debay length and inhibits the electrical screening of the charged skeleton by the bilayer.

The ion transport dynamic originates mainly from moisture absorption- and desorption-induced I- concentration gradients rather than interfacial charge modulation. When PAM, PMETAC, and PDMAPS are immersed in acid-free NaI, the hydrogel currents show very weak oscillations (Supplementary Fig. 21). This may be due to the introduction of a small amount of I3- during the heat treatment of the prepared devices, giving the device AC power generation capability. Taking PAM hydrogel as an example, it can be seen that the hydrogel remains transparent after being immersed in NaI, and becomes slightly yellow (Supplementary Fig. 22) after heat treatment, indicating the introduction of a small amount of I3-. The yellowing may be attributed to the partial ionization of water molecules within the hydrogel, generating a small amount of H+, which then reacts with I- to form a trace amount of I3-.

To verify that the dynamic coordination equilibrium is regulated by humidity, we performed Raman characterization on the PAM-AMPS hydrogel in different humidity environments (Fig. 4e). The hydrogel was tested after being placed in a 15% RH humidity chamber for 12 h. Raman intensity maps relevant to I3- at ≈110 cm-1 are shown (Fig. 4e-I). Next, the hydrogel was placed in a 98% RH humidity chamber for 30 min, and the laser Raman test was performed again. An overall weakening of the peak intensity of I3- is found (Fig. 4e-II), indicating a decrease in the content of I3- and proving that I3- tends to dissociate under high humidity. Further, the hydrogel was transferred to a 15% RH humidity chamber for 30 min, and the Raman test was performed again. The results show that the concentration of I3- becomes high again (Fig. 4e-III). Subsequently, the continuous and dynamic evolution of the Raman spectra of the hydrogel was monitored during humidity switching cycles, as shown in Supplementary Fig. 23. The results obtained after transferring the sample from a 15% RH humidity chamber to the laboratory ambient environment for testing are shown in Supplementary Fig. 23a. These data reveal a gradual decrease in I3- concentration from its initially high level, providing strong evidence that I3- dissociates into I2 and I- during the moisture-absorption process. Conversely, after placing the sample in a 98% RH humidity chamber for 30 min and then transferring it back to the laboratory ambient environment for Raman characterization, the results shown in Supplementary Fig. 23b demonstrate that I3- is regenerated when the hydrogel undergoes moisture desorption. The experiments performed provide strong evidence that the Eq. (2) is regulated by humidity.

To further demonstrate that Eq. (2) is regulated by humidity, we carried out theoretical calculations. Two conditions were setup for simulation analysis, one for the dissociation of I3- under water-rich condition and the other under water-poor condition. The dissociation of I3- under water-rich condition was analyzed using the NEB method (Fig. 4f). In this case, I3- is wrapped with 44 water molecule explicit solvent shell and the outer layer is treated with an implicit solvent model. The results show that the dissociation energy barrier in the presence of an aqueous solvent is 18.2 kcal/mol, which can be significantly carried out at room temperature. To provide a clearer understanding of the dissociation behavior of the I3- and its interaction with surrounding water molecules, we provided an illustration of the I3- dissociation pathway (see Supplementary Fig. 24). In contrast, scanning using a flexible surface under solvent-free condition yields a dissociation energy barrier of up to 23.36 kcal/mol (Fig. 4g), which is too slow to proceed at room temperature. Combined with the scan path trends, the water explicit and implicit solvent models reduce the dissociation energy barrier between I- and I2 to a reasonable range compared to the anhydrous solvent case, so that I3- is able to dissociate smoothly in water-rich environments. It becomes kinetically more favorable to dissociate I3- in water because the I3- → I- + I2 reaction is a single molecule reaction (first-order rate constant), whereas the I- + I2 → I3- inverse reaction is a bi-molecule reaction (second-order rate constant). In a water-poor or anhydrous environment, it is difficult to cross the energy barrier at room temperature due to the high energy barrier, and thus the reaction always tends to return to I3-. Since the reverse process in the water-rich state (Fig. 4f, looking from right to left) need to overcome a small energy barrier, and this energy barrier gives I- the opportunity to escape at water enrichment. For the energy curve calculated in the anhydrous environment, the path is reversed to see that there is almost no energy barrier, and it is a direct monotonic energy drop, which makes I- escape after coming out and follow the energy curve back to the reactants. Therefore, the calculation results also support the hypothesis that Eq. (2) is regulated by humidity.

Environment-adaptive applications
The cyclic change in humidity caused by the diurnal cycle in daily life is a reliable source of energy. For example, on a sunny day, the humidity reaches a maximum on the eve of sunrise and gradually decreases to a minimum as the sun rises; then the humidity rises slowly as the sun sets and night falls, repeating this cycle (Fig. 5a). Given the above scenario, we simulated the process in a temperature and humidity chamber in order to better observe the response of the device to humidity, with the temperature set at 25 °C and the humidity varying cyclically from 40-90% RH in a cycle of 12 h (Fig. 5b). The current curve of the device in the above environment is also recorded, and the results show that the device is very sensitive to humidity changes and generates power cyclically with the humidity cycle. In addition, we conducted experiments to assess the device’s performance under real outdoor environmental conditions (Supplementary Fig. 25). The results confirm that the device can effectively respond to ambient humidity fluctuations and continuously generate electricity, thereby underscoring the practical potential of SMEG.

Further, small-amplitude humidity fluctuations are also frequently encountered in real-world environments. To assess the performance of SMEG under such practical conditions, we first examined its current response under modest humidity variations. As shown in Fig. 5c, the device maintains stable and reproducible switching behavior, demonstrating a high degree of adaptability even when the humidity change is relatively minor. To further evaluate the sensitivity limit of the device, we investigated its response to extremely small humidity perturbations. Remarkably, SMEG can reliably detect variations as low as 2% RH, underscoring its exceptional responsiveness and broad applicability across diverse humidity conditions (Supplementary Fig. 26; Supplementary Tables 3 and 4). Meanwhile, we evaluated the device's oscillatory energy-generating behavior under these small humidity fluctuations. As presented in Fig. 5d, SMEG continues to exhibit AC-type power generation even when the humidity varies by only 13-14% RH under both high-humidity and low-humidity conditions. This capability highlights the device’s strong environmental adaptability and confirms that dynamic coordination-driven ion oscillation remain effective over narrow humidity ranges.

Methods
Chemicals and materials
Unless otherwise stated, all reagents and solvents were used without further purification. Acrylamide (AM, Macklin), 2-acrylamido-2-methylpropanesulfonic acid (AMPS, Sigma-Aldrich), N,N′-methylene bisacrylamide (MBAA, Sigma-Aldrich), 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959, Aladdin), [2-(methacryloyloxy)ethyl] trimethylammonium chloride (METAC, Aladdin), [2-(methacryloyloxy)ethyl] dimethyl-(3-sulfonatopropyl) ammonium hydroxide (DMAPS, HEOWNS), and lithium chloride (LiCl, Sigma-Aldrich). Calcium chloride, magnesium chloride, sodium iodide, potassium iodide, sodium bromide, sodium chloride, sodium fluoride, and hydrochloric acid were analytically pure and purchased from Chengdu Kelong Chemical Co., Ltd. Hydrochloric acid was diluted to 1 M for use.

The preparation process of SMEG
a) Preparation of hydrogel precursor solution and gel: Firstly, weigh the gel monomer AM (6 g), AMPS (x g, where x = 0, 0.7, 1.4, 2.1, 2.8, 3.5, 4.2 or 4.9), the crosslinking agent MBAA (15 mg), and the initiator Irgacure 2959 (15 mg). Put the above materials into 30 mL of deionized water, and stir for 20 min with a magnetic stirrer at room temperature until all are dissolved. The precursor solution was added dropwise into a mold (the mold size can be customized), and the precursor solution was irradiated by a UV lamp with a power of 30 W for 20 min to prepare a hydrogel successfully. The preparation of PEMEATC and PDMAPS did not require using AM, and the other steps remained unchanged.
b) Introduction of NaI: it was sufficient to soak the prepared hydrogel in NaI solution for 48 h.
c) Water content control of the gel: the soaked gel was placed in a drying oven at 80 °C for about 3 h, and then placed in the atmosphere or a specified humidity environment for 12 h to wait for its moisture absorption and desorption equilibrium (i.e., the volume was similar to that of the original gel, or no longer changed significantly).
d) Electrode preparation and assembly: Carbon electrodes were prepared by commercial carbon paste coated on PET film, waiting for its natural drying to be used. The porous electrodes were prepared by using a laser marking machine to punch holes with a diameter of 1.5 mm and a spacing of 0.4 mm, totaling 5 × 5 holes. Conductive carbon tape was used to connect the carbon fiber leads to the carbon electrodes, and the carbon fiber leads were sealed with transparent tape to prevent liquid condensed on the sidewall of the humidity chamber from entering the carbon fiber leads. Two carbon electrodes were placed on both sides of the hydrogel, the top electrode was a porous carbon electrode and the bottom electrode was a normal carbon electrode. The perimeter of the gel was sealed by a sealing film, and the effective working area of the device is 1 cm2. The detailed process is shown in Supplementary Fig. 5.

Material characterization
The cross-sectional morphology of the gel was characterized by field emission scanning electron microscopy (SEM) (Apreo 2C, Thermo Fisher, US). Elemental analyses in the gel-dried state were made by an EDS unit attached to the SEM instrument and X-ray photoelectron spectroscopy (XPS) (K-Alpha, Thermo Scientific, US). XPS data were analyzed using Avantage 6.6.0 and plotted with Origin 2024. The triiodide ion characterization of gels in the water-containing state was performed using a laser confocal Raman spectrometer (Horiba, Japan), which was tested in ambient laboratory conditions (55% RH, 25 °C), a laser of 532 nm, and a laser intensity of 3.2%. Raman mapping data were analyzed using LabSpec 6 software.

Electrical measurements
Half a month of data was obtained using a semiconductor parameter analyzer (Keithley 4200A-SCS). Short-time currents and voltages were recorded using a digital source meter (Keithley 2450). The circuit parameter for the open circuit voltage test was a current of 0 nA, and the circuit parameter for the short circuit current test was a voltage of 0 V. Current-voltage characteristics were measured using an electrochemical analyzer (CHI760E, CH Instruments, USA) with a voltage window of −10 mV to 10 mV and a scan rate of 1 mV/s. All electrical data were analyzed and plotted using Origin 2024.

Temperature and humidity conditions for electrical measurements
Except for the experiments shown in Fig. 5b and Supplementary Fig. 25, all other tests were conducted in humidity chambers provided by saturated salt solutions40,53, with the laboratory temperature maintained at 25 ± 3 °C. Temperature and humidity variations in the simulated environment provided by the constant temperature and humidity chamber (DHTHM-27-80-P-SD) in Fig. 5b and Supplementary Fig. 26. The humidity of the real environment was recorded by a Rotronic hygrometer (Rotronic Hygrolog NT, Switzerland) with a sampling interval of 5 s.

Theoretical calculation Methods
All calculations were performed using the ORCA version 6.0 program package54 with BLYP55,56-D3BJ57,58 functions and the ma59-def2-SVP60 basis set. The ma-def2-SVP basis set was chosen because of the anionic nature of I- and its explicitly soluble droplets. The def2/J61 auxiliary basis set was also used. Pure functions combined with the resolved identity (RI)62 approximation were used to model explicitly solvated I3- systems consisting of 135 atoms (44 water molecules explicit solvent shells and three iodine atoms) and a large number of hydrogen bonds. A water density (SMD)63-based solvation model was used as an implicit solvation model outside of the explicit solvent shell. The cleavage and dissociation of I3- were simulated by relaxation surface scanning, since the reaction does not have an effective product equilibrium geometry (a smooth decrease in energy after an energy barrier). The calculations were performed without using any solvation model to simulate systems with little water content. In contrast, the explicitly solvated I3- was analyzed using the naked elastic band (NEB)64 method with climb images to determine the minimum energy reaction path for its cleavage and dissociation.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, Lu et al. studied the moisture-based energy harvesting through oscillatory migration of ions driven by humidity fluctuations-induced dynamic coordination. This research work is somewhat meaningful and interesting. However, this article has many aspects that need improvement. Many further experiments and explanation should be included in the revision. Here are the comments.\n1. It seemed that the authors controlled the humidity fluctuations in a humidity chamber and the humidity ranged from the 15% to 98%. What was the temperature for the test? Was the measurement conducted under variable temperature condition or constant temperature condition? The authors did not provide any details on this point in the Methods part. This is quite unrigorous.\n2. Influence of some optimized synthesis preparation was strange. In Figure 3b, why did the current decrease when the NaI concentration was higher than 1 M? It was hard to understand. In Figure 3c, for the different soaking time, why did the current decrease at 24 h but increase at 48 h for desorption? These results should be well explained in the manuscript.\n3. In the proposed mechanism, the reaction 4I-+4H++O2=2H2O+2I2 was not reversible. This hinted that I- ions would be continuously consumed. That was to say, when the I- ions completely converted to I2, the device would not work. How many adsorption/desorption circles could the device operate?\n4. In Figure 1b, why the current in the adsorption process was always larger than that in desorption process?\n5. What was the NaI adsorption capacity or the content in the hydrogel? This should be also measured. It seemed that the NaI content in the hydrogel would influence the current.\n6. Actually I2 will also slowly evaporate through sublimation at room temperature. This means that the I2 in the hydrogel will slowly decrease. Would the authors observe a decline in current after long-term performance measurements?\n7. Why the Na+ ion cannot be transported in hydrogel? This is essential and should be discussed in the revised manuscript.\n8. What about the open circuit potential for the device at steady state? Corresponding measurement is missing.\n9. In Figure 4e, the Raman intensity maps of P(AM-co-AMPS)-NaI gel in different humidity environments were displayed. The authors mentioned that it was placed in a “humidity bottle”. What was the “humidity bottle”? I was curious about that how the authors perform the Raman characterizations in different humidity environments. Corresponding experimental conditions need to be put in the Methods parts.\n10. Some minor errors need to be noted. There should be a space between the number and the unit. Please check.", "reviewer_2": "This manuscript, entitled \" Humidity fluctuations-induced dynamic coordination drives the oscillatory migration of ions for sustainable energy harvesting\" introduces a hydrogel-based sustainable moisture-electric generator (SMEG) based on the dynamic coordination between I- and I2 to synthesize I3-. Specifically, the authors reasonably demonstrated through Raman spectroscopy and molecular dynamics simulations that the dynamic coordination is regulated by the adsorption and desorption of moisture, endowing it with the ability to continuously operate in an environment with cyclic changes in air humidity. It has been tested to be able to work stably for nearly two weeks in a simulated environmental chamber. In general, the concept of ion oscillation migration driven by dynamic coordination is novel and impressive. It has shown remarkable potential for enabling the continuous and stable operation of moisture-based energy-harvesting devices. However, several concerns need to be addressed before it can be published in Nature communications. Please see the following detailed comments.\n[Comment 1] In line 24, the abstract states that “The coordination process of I- does not involve charge exchange, and the charge transfer behavior (electricity generation) is still achieved through moisture-driven ion transport.” However, the text appears to lack corresponding evidence for this claim. Please provide a detailed explanation of this behavior.\n[Comment 2] In line 70, the definitions of relative high humidity and low humidity are not clearly delineated in the preceding text. Please provide a specific explanation for these terms.\n[Comment 3] In line 105, it is mentioned that “The use of a sealing film ensures that air contacts the functional layer from the top, facilitating the formation of ionic concentration differences.” However, what is the performance of the device when there is no sealing film around it? This could be further elucidated through a control experiment, which would help to corroborate the working mechanism of the device.\n[Comment 4] In line 112, it is stated that the material possesses a moderate hygroscopic capacity. However, the degree of this hygroscopic capacity is not specifically described, and the current description is somewhat vague. It is recommended to supplement the text with comparative data on the hygroscopic capacities of commonly used adsorbents.\n[Comment 5] In line 258, the ranking of the hygroscopic capabilities of different salts is provided without experimental evidence or reference to relevant literature. It is necessary to supplement this by either conducting experiments to measure the hygroscopic properties of the salts or by citing pertinent studies that have previously reported such rankings.\n[Comment 6] In line 262, the power generation effects of hydrogels treated with other salt solutions are mentioned, but the underlying mechanisms for each are not explained. It is suggested to provide a brief explanation of the working principles for these cases.\n[Comment 7] During the device fabrication process, what is the impact of the drying temperature of the hydrogel on the gel properties, and why was 80°C specifically chosen?\n[Comment 8] It is suggested to enlarge a single cycle of data in Fig. 1b to more clearly highlight the characteristics of the data.\n[Comment 9] Why do Fig. 3b and 3c exhibit significant differences in forward and reverse current performance, while the forward and reverse currents in Fig. 3a are at the same level?\n[Comment 10] In Fig. 3d, it is suggested to further increase the content of AMPS and measure the corresponding current performance to see if the current continues to decrease. Adding one or two more data points would make the trend more evident.\n[Comment 11] The curves in Fig. 4a are recommended to be labeled with numbers or letters for differentiation. The current presentation is not conducive to reader comprehension.\n[Comment 12] It is suggested to provide a comparative table of similar devices (including aspects such as lifetime, humidity range, and working principles) to highlight the advantages of this device. The current presentation relies solely on textual descriptions, which lacks intuitiveness.\n[Comment 13] The performance of the device under real environmental conditions is missing. It is recommended to supplement the curves showing the device's current performance fluctuating with humidity in a real atmospheric environment to demonstrate its working state under actual conditions.\n[Comment 14] In Fig. 5d, only two sets of humidity difference data are provided. It is suggested to supplement with additional data points.\n[Comment 15] It is recommended to include images showing the relationship between the number of breathable pores in the top electrode and the performance changes, in order to illustrate the correlation between the contact area of moisture and the device performance.", "reviewer_3": "Comments:\nLu et al. proposed a novel moisture-electricity conversion mechanism based on the dynamic coordination between iodine ions and iodine molecules to keep a continuous ions migration for a long operating lifespan in MEG. While this strategy exhibits promising extensibility to diverse functional materials and broad humidity adaptability, its practical implementation faces notable limitations. The material only achieves voltage (~0.2 V) and current density (~33 μA/cm2) performance even at extremely high humidity environment (98%RH), which is an order of magnitude lower than recent related literature (Nat Commun 15, 3329 (2024), Nat Commun 15, 6100 (2024)). In addition, more evidence is needed to understand the mechanism by which this material generates electrical signals in a humidity environment. Therefore, in my opinion, this manuscript can be accepted after major revisions for publishing in Nature Communications.\n1. Currently, most MEG studies based on functional hydrogels attribute the generation of electrical signals to the dissociation of protons (H⁺) from functional groups and their migration along the concentration gradient. However, this manuscript uses NaI to neutralize the dissociated hydrogen ions and uses the migration of negatively charged ions to generate electrical signals. Therefore, the gradient within the SMEG must be inferior to the previous one, resulting in a weaker electrical signal, which is also demonstrated by the performance in the manuscript. Given this significant performance gap, the proposed mechanism may offer limited advancement for the field.\n2. Figure 1b shows that the SMEG has high cycling stability. However, the reversible reaction is difficult to maintain the ion gradient, as evidenced by the significant current decay from 10 μA to 2 μA within 24 hours. The positive and negative jumps of the electrical signal caused by the adsorption-desorption switching also limit the practical application scenarios of SMEG.\n3. In Line 52-54, the authors mentioned ‘… the moisture desorption power generation dominated by ion hydration take place in the same direction’. Iodide ions also appear to form hydrates. How did the authors exclude this factor from their explanation?\n4. While the proposed mechanism identifies PAMPS and NaI as the key functional components in the SMEG system, the rationale for using P(AM-co-AMPS) as the polymer matrix remains unclear. Although the authors note PAM's hydrophilic nature, this alone does not sufficiently justify its incorporation into the copolymer structure. Does PAM actively participate in power generation, or does it just play a role as a structural support? In what ways do P(AM-co-AMPS) copolymers have advantages over pure PAMPS?\n5. The coordinates on the right side of Figure 2D seem to be wrong, as desorption and adsorption should be a continuous process in this figure. Did the authors mean to say desorption rate (%)?\n6. In Figures 3b and c, the current during adsorption is 1.5 to 2 times that during desorption, whereas in other variables studied, the current during adsorption and desorption are similar. The authors should explain this.\n7. In Line 286-288, the authors mentioned that ‘However, since I3- moves much slower than I-, it is the overall downward movement of I- that dominates the ionic current (Fig.4a)’. Figure 4A does not seem to justify this statement. The authors should add more evidence to demonstrate the difference in diffusion rates between I3- and I- in the material.\n8. In Figure 4b, why are the positive and negative signs of the current signals of CaCl2 and LiCl opposite?\n9. In Figure 4c, the authors compared different hydrogels with NaI, and found that PAM-NaI generate almost no current. Based on the mechanistic explanation in the manuscript, it can be speculated that this is because there is no release of hydrogen ions. However, previous literatures (Energy Environ. Sci., 2024, 17, 3788-3796, Nano Energy, 2024, 110590) has demonstrated that PAM also releases and migrates hydrogen ions during moisture adsorption, thereby producing current. The authors should clarify this discrepancy.\n10. The font of ‘°C’ in Figure 5b seems to be different from other words. And I suggest that the author use ‘h’ instead of ‘s’ for the horizontal axis.\n11. The language of the manuscript should be improved. Many prepositions are used incorrectly. For example, in line 184-185, ‘… NaI concentration is of 0.01 M, …’ should be ‘…NaI concentration is 0.01 M…’ and ‘participating in the dynamic coordination’ should be ‘participating in dynamic coordination’.", "reviewer_4": "The authors design and fabricate a sustainable power generator SMEG based on the dynamic coordination of I- induced by varying humidity. The device shows great potential in small-scale green and sustainable energy access. However, there are several concerns that must be addressed before the manuscript can be considered for publication:\n1. Figure 3d shows that the performance is the best when the AMPS content is 2.8g, but it does not explain why excessive AMPS reduces the performance.\n2. The Raman intensity maps in Figure 4e do not give the color scale annotation.\n3. Explain in detail how moisture leads to the alternating changes in the I- concentration.\n4. The principal comparison between moisture power generators and other novel power generation strategies should be added to the introduction.\n5. The moisture power generator reported in this work should be compared with the literature to demonstrate its advantages.\n6. A theoretical analysis should be given on whether the dynamic mechanism based on I- can be extended to other halogen ions."}, "author_response": "Point-to-point response to the Reviewers’ comments on the manuscript\n“Humidity fluctuations-induced dynamic coordination drives the oscillatory\nmigration of ions for sustainable energy harvesting”\nDear Editor and Reviewers,\nWe sincerely appreciate the time and effort that the Editor and Reviewers have\ndevoted to evaluating our manuscript. We are grateful for your insightful and\nconstructive comments, which have helped us to significantly improve the quality and\nclarity of our work. In response, we have carefully addressed all the comments and\nsuggestions raised by the four Reviewers. A major revision of the manuscript has been\ncarried out, with changes made in a constructive and thoughtful manner. We believe\nthese revisions have substantially enhanced the overall presentation, scientific rigor,\nand impact of the manuscript.\nBelow, we explain how all the comments of the Reviewers have been taken into\naccount in our revision. Since some comments of the Reviewers turn out to be quite\nsimilar, our answers to such comments are inevitably similar. To facilitate the reading\nand understanding of our response to the comments of the Reviewers, we have\nchosen to render each response self-contained while taking the risk of making our\nresponse appear somewhat repetitive.\nAll changes made in the revised manuscript are highlighted in color.\nReviewer #1\n[General comment] In this work, Lu et al. studied the moisture-based energy\nharvesting through oscillatory migration of ions driven by humidity fluctuations-\ninduced dynamic coordination. This research work is somewhat meaningful and\ninteresting. However, this article has many aspects that need improvement. Many\nfurther experiments and explanation should be included in the revision. Here are the\ncomments.\n[Response] We sincerely thank you for your thorough evaluation of our work and for\nrecognizing its significance and originality. We fully acknowledge that the manuscript\nstill required improvements in several aspects, and we are truly grateful for your\nconstructive comments and suggestions. In response to the issues you raised, we have\nsupplemented relevant experimental data and further clarified the underlying\nmechanism of the work. Additionally, we have made comprehensive revisions to\nenhance the scientific rigor and clarity of the manuscript. We believe that these\nmodifications have substantially improved the scientific rigor and overall quality of the\nmanuscript. Our detailed responses to your comments are provided below.\n[Comment 1] It seemed that the authors controlled the humidity fluctuations in a\nhumidity chamber and the humidity ranged from the 15% to 98%. What was the\ntemperature for the test? Was the measurement conducted under variable temperature\ncondition or constant temperature condition? The authors did not provide any details\non this point in the Methods part. This is quite unrigorous.\n[Response] We thank you for your careful review and for pointing out the omission of\ntemperature conditions in the Methods section. Indeed, this was an oversight on our\npart. The experiments were conducted under laboratory conditions, with the ambient\ntemperature maintained at 25 ± 3 ℃. The relative humidity was controlled using\nsaturated salt solutions: 15% RH achieved with saturated lithium chloride (LiCl)\nsolution, and 98% RH with saturated potassium sulfate (K SO ) solution.\n2 4\nWe have now incorporated this information into both the main text and the\nMethods section to enhance the rigor and completeness of the manuscript.\n[Comment 2] Influence of some optimized synthesis preparation was strange. In Figure\n3b, why did the current decrease when the NaI concentration was higher than 1 M? It\nwas hard to understand. In Figure 3c, for the different soaking time, why did the current\ndecrease at 24 h but increase at 48 h for desorption? These results should be well\nexplained in the manuscript.\n[Response] We sincerely thank you for this critical question. Theoretically speaking,\nincreasing the concentration of NaI should provide more I- ions for migration, which\nwould be beneficial to the device’s output performance. However, we observed that the\noptimal performance was achieved at a NaI concentration of 1 M, while a further\nincrease to 2 M led to a decrease in output. This counterintuitive behavior can be\nunderstood by the Hofmeister effect.\nIndeed, I- is a typical chaotropic ion, which, at moderate concentrations, can\nsignificantly enhance hydrogen bonding between water molecules and the polymer\nnetwork. This increases the solubility of the gel, making its structure more swollen and\nwater-rich, which in turn facilitates ion migration and diffusion. As reported by Swann\net al.1, I- can markedly promote the swelling behavior of pH-responsive hydrogels\nwithin a certain concentration range.\nHowever, when the NaI concentration increases beyond the optimal range, the\nchaotropic effect diminishes or even reverses. This is primarily due to two factors2-4:\n(1) At high concentrations, Na+ and I- ions are more likely to form ion pairs,\nreducing the proportion of free iodide ions available for migration;\n(2) The high ionic strength leads to increased osmotic pressure, which can induce\ndehydration or compression of the hydrogel network, thereby hindering ion transport.\nOur experimental results suggest that at 1 M NaI, an optimal balance is achieved\namong ion concentration, hydration effect, and gel structure, leading to the best power\noutput. This observation aligns well with the current understanding of the\nconcentration-dependent Hofmeister effect.\nTo further verify this, we performed SEM characterization of the hydrogel\nnetwork at 2 M NaI, which clearly revealed that high salt concentrations compromise\nthe polymer structure (Supplementary Fig. 10), impairing ion transport.\nAdditionally, in response to your comment on Fig. 3c, we repeated the experiments.\nWe found that the performance drop after 24 h immersion was due to individual device\nissues. The overall trend shows that the output performance increases with soaking time\nand gradually reaches saturation. This is likely because the gel gradually adsorbs more\nNaI, approaching its maximum adsorption capacity over time (Supplementary Fig. 12).\nWe sincerely thank you for your comment, which helped us more clearly interpret\nthe underlying causes of this phenomenon.\nWe have refined the explanation of the relevant phenomena in the revised\nmanuscript.\nSupplementary Fig. 10 Scanning electron microscope images after soaking in 2M NaI for 4\ndays. a Cross-sectional electron microscope image. b Local magnification image. This indicates\nthat the gel structure has collapsed, hindering the effective transport of ions.\nFig. 3c The current performance pattern of devices after fixing NaI concentration of 1M and\ndifferent time of gel immersion (gel thickness of 3 mm).\nSupplementary Fig. 12 Sodium iodide adsorption in hydrogels after different soaking time in\ntime and gradually approached saturation.\n1. Swann, J. M., Bras, W., Topham, P. D., Howse, J. R. & Ryan, A. J. Effect of the Hofmeister\nAnions upon the Swelling of a Self-Assembled pH-Responsive Hydrogel. Langmuir 26,\n10191-10197 (2010).\n2. Lo Nostro, P. & Ninham, B. W. Hofmeister phenomena: an update on ion specificity in\nbiology. Chem. Rev. 112, 2286-2322 (2012).\n3. Ball, V. Specific ion effects in hydrogels. Molecules 29, 5990-6006 (2024).\n4. Gregory, K. P. et al. Understanding specific ion effects and the Hofmeister series. Phys.\nChem. Chem. Phys. 24, 12682-12718 (2022).\n[Comment 3] In the proposed mechanism, the reaction 4I-+4H++O =2H O+2I was\n2 2 2\nnot reversible. This hinted that I- ions would be continuously consumed. That was to\nsay, when the I- ions completely converted to I , the device would not work. How many\n2\nadsorption/desorption circles could the device operate?\n[Response] We sincerely appreciate your insightful comment. Indeed, the reaction 4I-\n+ 4H+ + O → 2H O + 2I is thermodynamically irreversible. However, it is important\n2 2 2\nto note that molecular iodine (I ) has very limited solubility in water, but can be\n2\nstabilized in the presence of iodide ions (I-) through the formation of triiodide ions (I -).\n3\nspecies, a phenomenon well documented in the literature5.\nIn other words, the formation of I - effectively limits the complete conversion of\n2\nsignals remain clearly detectable even in devices stored for up to seven months,\nindicating that a substantial amount of I- remains unconverted and exists in the form of\nI - (Fig. R1).\nIn response to the question regarding how many humidity adsorption/desorption\ncycles the device can undergo, we conducted long-term testing on a device that had\nbeen stored under laboratory conditions for seven months (30-65% RH, 25 ± 3 ℃). As\nshown in Supplementary Fig. 2， the device was still capable of generating a continuous\ncurrent output for over one month, demonstrating its functional durability and\noperational stability over extended periods.\nIt is worth noting that, although the initial performance of the long-stored device\nwas comparable to that of a freshly prepared one, the performance degradation appeared\nearlier during testing. This suggests that long-term exposure to ambient environmental\nconditions may lead to a gradual decline in device stability over time.\nSupplementary Fig. 2 Current performance of the device after being stored under laboratory\nconditions (30-65% RH, 25 ± 3 ℃) for 7 months, followed by continuous operation under\nalternating high- and low-humidity environments for one month.\n5. Palmer, D. A., Ramette, R. W. & Mesmer, R. E. Triiodide ion formation equilibrium and\nactivity coefficients in aqueous solution. J. Solution Chem. 13, 673-683 (1984).\n[Comment 4] In Figure 1b, why the current in the adsorption process was always\nlarger than that in desorption process?\n[Response] Thank you for raising this important question. Through systematic\nexperimentation and comprehensive characterization, we have demonstrated that this\nphenomenon arises from the different ratios of iodide ions (I-) to triiodide ions (I -)\n3\ninside the hydrogel, which in turn leads to differences in its moisture adsorption and\ndesorption capabilities. As a result, the device exhibits higher performance during the\nadsorption process than during the desorption process. A detailed analysis is presented\nas follows:\nFirstly, based on the experimental results presented in Fig. 2d, we speculate that\nthe performance difference between the adsorption and desorption and adsorption stages arises from\nthe asymmetric moisture adsorption and desorption capacities of the device.\nSecondly, as shown in Fig. 3e, a comparison of device performances using\nhydrogels with different thicknesses reveals that when the gel thickness is less than 9\nmm, the device tends to exhibit higher output during the adsorption stage and lower\noutput during desorption. In contrast, when the thickness is equal to or greater than 9\nmm, the performance becomes symmetric or even slightly higher during the desorption\nstage. Therefore, we infer that this phenomenon is thickness-dependent, and is likely\ncaused by the influence of hydrogel thickness on moisture transport and ionic dynamics.\nFrom the above analysis, it can be inferred that the thickness of the hydrogel\ninfluences its moisture adsorption and desorption behavior, which is intrinsically\nrelated to the ratio between the quantities of iodide ions (I-) and triiodide ions (I -)\n3\nwithin the gel. Specifically, I- ions are primarily responsible for moisture adsorption\n(e.g., the hygroscopicity of NaI is greater than that of NaF6-8), while I - ions, formed by\n3\nthe coordination between I- and I , possess a larger linear structure with a more diffuse\n2\ncharge distribution, making them less capable of forming stable hydration shells. At a\nfixed NaI concentration, a thicker hydrogel introduces more sulfonic acid groups,\nthereby increasing the availability of protons (H+), which promotes the conversion of I-\ninto I -. As a result, thinner gels contain a higher proportion of I-, leading to stronger\n3\nmoisture adsorption and enhanced device performance during the adsorption stage. As\nthe gel thickness increases, the ratio of I- to I - decreases, giving rise to more balanced\n3\nadsorption/desorption behavior, or even stronger performance during the desorption\nstage. In summary, thin hydrogels tend to show higher performance during moisture\nadsorption, while thicker hydrogels exhibit more balanced or even desorption-\ndominated behavior, owing to the shifting I-/I - ratio.\n1. Moisture adsorption and desorption tests of hydrogels with different\nthicknesses. To investigate the effect of thickness on moisture behavior, hydrogels of\nidentical compositions but different thicknesses (5 mm, 9 mm, and 13 mm) were tested.\nFirst, the hydrogels were placed in a 15% RH humidity chamber for 12 hours to\nsimulate the device working process (ambient conditions: 25 °C, 50% RH). Then, they\nwere transferred to a 98% RH humidity chamber to evaluate their moisture adsorption\ncapacity. The mass of each sample was measured every 10 minutes, with three repeated\nweighings to obtain the average value. After 12 hours of moisture adsorption, the\nhydrogels were again placed in the 15% RH chamber to test their desorption behavior,\nfollowing the same weighing protocol. As shown in Supplementary Fig. 15, thinner\nhydrogels exhibited stronger moisture adsorption than desorption, while thicker\nhydrogels showed the opposite trend, which is consistent with our hypothesis.\nCombined with the data in Fig. 3e, these results confirm that the asymmetry in device\nperformance during adsorption and desorption is primarily attributed to the different\nmoisture handling capacities of hydrogels with varying thicknesses.\n2. The difference in moisture adsorption/desorption capacity originates from\nthe varying I-/I - ratios. To investigate this, P(AM-co-AMPS) hydrogels with\n3\nthicknesses of 3 mm, 9 mm, and 15 mm were soaked in equal volumes of 1 M NaI\nsolution for 4 days to ensure equilibrium between I- and I -. As shown in Supplementary\nwith increasing gel thickness, indicating a higher I - content. Subsequently, these three\n3\nsolutions were used to soak identical 3 mm-thick PAM hydrogels, denoted as PAM ,\n9 15\nrevealed a progressive increase in the intensity of the I - characteristic peaks from PAM\n3 3\nto PAM , suggesting a decreasing I- to I - ratio in the soaking solutions used, and\n15 3\ntherefore in the resulting hydrogels.\n3. The I-/I - ratio affects moisture adsorption/desorption performance. To\n3\nfurther demonstrate that the ratio between I- and I - influences the adsorption and\n3\ndesorption behavior, the electrical performances of PAM , PAM , and PAM were\n3 9 15\nevaluated, as shown in Supplementary Fig. 13b. The results indicate that as the\nproportion of I - increases, the adsorption performance of the samples gradually\n3\napproaches their desorption performance. Moreover, the overall outputs of the devices\nwere found to be positively correlated with the I - content in the hydrogel. This\n3\nphenomenon likely originates from the enhanced dynamic coordination effect at higher\nI - concentrations, which induces a larger I- concentration gradient and thus leads to\n3\nstronger electrical output.\nThrough the above analysis, experiments, and characterizations, we have\ndemonstrated that the different ratios of I- to I - within the hydrogel lead to variations\n3\nin its moisture adsorption and desorption capacities. This ultimately results in the\nobserved asymmetry in device performance, with higher output during the adsorption\nphase compared to the desorption phase. We sincerely thank you for raising this\nimportant point, which has deepened our understanding of the device’s working\nmechanism.\nSupplementary Fig. 15 Comparison of moisture adsorption and desorption capacities of NaI-\nbased hydrogels with different thicknesses. a Moisture adsorption of hydrogels after being\nmaintained at 15% RH for 12 h and then transferred to 98% RH. b Moisture desorption of the same\nhydrogels after an additional 10 h at 98% RH, followed by transfer to 15% RH.\nSupplementary Fig. 16 The state of NaI solutions (1 M, equal volumes) after immersing\nhydrogels of different thicknesses for 4 days. It is evident that the thicker the hydrogel, the darker\nthe solution color, indicating a higher concentration of triiodide ions. The inset in the upper left\nshows the appearance of three PAM gels of the same thickness after immersion in the above three\nNaI solutions followed by drying.\nSupplementary Fig. 13 Raman characterization and device performance of PAM , PAM , and\n3 9\nPAM hydrogels. a The Raman spectra indicate that the triiodide content in PAM , PAM , and\n15 3 9\nPAM hydrogels increases sequentially, suggesting a corresponding decrease in the I-/I - ratio. b\n15 3\nDevice performance of the hydrogels, with the adsorption-to-desorption performance ratios of 2.44,\n1.96, and 1.17, respectively.\n6. Miñambres, L., Méndez, E., Sánchez, M. N., Castaño, F. & Basterretxea, F. J. Water uptake\nproperties of internally mixed sodium halide and succinic acid particles. Atmos. Environ.\n45, 5896-5902 (2011).\n7. Albayrak, C., Barim, G. & Dag, O. Effect of hygroscopicity of the metal salt on the\nformation and air stability of lyotropic liquid crystalline mesophases in hydrated salt-\nsurfactant systems. J. Colloid Interface Sci. 433, 26-33 (2014).\n8. Peng, C., Chen, L. & Tang, M. A database for deliquescence and efflorescence relative\nhumidities of compounds with atmospheric relevance. Fundam. Res. 2, 578-587 (2022).\n[Comment 5] What was the NaI adsorption capacity or the content in the hydrogel?\nThis should be also measured. It seemed that the NaI content in the hydrogel would\ninfluence the current.\n[Response] We sincerely thank you for this insightful comment. The amount of sodium\niodide (NaI) retained within the hydrogel is indeed a critical factor influencing the\ndevice performance. To investigate this influence, we quantified the NaI content in\nhydrogels soaked for different durations using a gravimetric method, as detailed below:\n1. The dry mass of the hydrogel was accurately measured and recorded as m₁.\n2. The hydrogel was then soaked in 1 M NaI solution for 1 h, 6 h, 12 h, 24 h, 48 h,\nand 96 h, respectively.\n3. After soaking, the hydrogel was removed, thoroughly rinsed with deionized water\nto remove surface-adsorbed NaI, and then dried completely.\n4. The dry mass after soaking was measured and recorded as m₂.\n5. The amount of adsorbed NaI (m) was calculated as m = m₂ − m₁.\nAs shown in Supplementary Fig. 12, the NaI content in the hydrogel increased\nwith soaking time and gradually approached saturation. Moreover, when correlated\nwith the electrical output data in Fig. 3c, a clear relationship was observed: a higher\nNaI content corresponds to a stronger current output.\nWe have incorporated this experimental data and analysis into the revised\nmanuscript and supplementary files to improve the completeness of the study.\nSupplementary Fig. 12 Sodium iodide adsorption in hydrogels after different soaking time in 1 M\nNaI solution.\nFig. 3c The current performance pattern of devices after fixing NaI concentration of 1M and\ndifferent time of gel immersion (gel thickness of 3 mm).\n[Comment 6] Actually I will also slowly evaporate through sublimation at room\n2\ntemperature. This means that the I in the hydrogel will slowly decrease. Would the\n2\nauthors observe a decline in current after long-term performance measurements?\n[Response] It is true that I can gradually evaporate via sublimation at room\n2\ntemperature. In fact, sublimated iodine can be visually observed on the stopper of the\nglass bottle containing the saturated salt solution, as shown in Fig. R2.\nHowever, this sublimation primarily occurs on the surface of the gel, and in our\ndevice configuration, only the porous upper electrode is exposed to the environment.\nAs a result, a significant amount of iodine is expected to remain within the interior of\nthe gel, where it continues to support device operation.\nTo verify this assumption, we tested a device that had been stored under laboratory\nconditions for seven months (30-65% RH, 25 ± 3 ℃). As shown in Supplementary Fig.\n2, the initial output performance of the aged device was comparable to that of a freshly\nprepared one, and it maintained stable operation for a considerable period. However,\nafter approximately 10 days, we observed a gradual decline in performance, which may\nbe attributed to a critical threshold of iodine loss through sublimation. This suggests\nthat as iodine sublimation reaches a critical point, the device exhibits a gradual decline\nin performance corresponding to the loss of iodine.\nFig. R2 Sublimation of iodine deposited on the glass stopper. The left shows a new glass stopper,\nwhile the right shows one after being used for a period of time.\nSupplementary Fig. 2 Current performance of the device after being stored under laboratory\nconditions (30-65% RH, 25 ± 3 °C) for 7 months, followed by continuous operation under\nalternating high- and low-humidity environments for one month.\n[Comment 7] Why the Na+ ion cannot be transported in hydrogel? This is essential\nand should be discussed in the revised manuscript.\n[Response] We thank you for this insightful question, which we acknowledge to be\ninsufficiently discussed in the original manuscript. In theory, sodium ions (Na+) are\nmobile and may migrate together with water molecules during the adsorption and\ndesorption processes, likely in the same direction as I- ions. However, since both Na+\nand I- are initially evenly distributed within the hydrogel (as shown in Supplementary\nFig. 7), the driving force generated solely by water movement may be limited.\nIn contrast, I- ions can form dynamic coordination species, which introduce an\nadditional concentration gradient that drives their directional migration. Therefore, the\nobserved signal is most likely attributed to anion (I-) concentration differences.\nFurthermore, Supplementary Fig. 19, which compares the performance of devices\nbased on different sodium salts, also suggests that the oscillating signal is mainly\ngoverned by the behavior of iodide anions. However, this does not imply that sodium\nions are immobile. On the contrary, their limited mobility may in fact contribute to the\nunexpectedly low voltage observed in the device, as voltage arises from ion separation,\nwhile current reflects ion migration.\nWe have added a discussion on the potential mobility of Na⁺ ions in the revised\nmanuscript to improve the completeness and clarity of the analysis.\nSupplementary Fig. 7 EDS pattern of P(AM-co-AMPS)-NaI gel cross-section. a Electron\nmicroscopy of cross-section morphology. b Distribution of sodium elements in the cross-\nsection.\nSupplementary Fig. 19 The current performance of P(AM-co-AMPS) gels treated with\nNaI, NaBr, NaCl, and NaF at high and low humidity. Only the gel treated with NaI has AC\npower generation, indicating that this phenomenon is related to the dynamic coordination of I-\nand I .\n2\n[Comment 8] What about the open circuit potential for the device at steady state?\nCorresponding measurement is missing.\n[Response] We thank you for the above question regarding the open-circuit voltage\n(V ) measurements. Since the voltage output of our device is relatively less\nOC\npronounced compared to the current output, it was not emphasized in the main text.\nHowever, we have indeed conducted V measurements for devices with different\nOC\nthicknesses, and the results are presented in Supplementary Fig. 14a.\nIn particular, for devices of 9 mm thickness, voltage changes were recorded during\na single humidity cycle, and the results showed a trend similar to that of current.\n(Supplementary Fig. 14b). We have refined the relevant descriptions in the revised\nmanuscript to enhance the logical flow and overall readability of the paper.\nSupplementary Fig. 14 Voltage data from SEMG. a Voltage performance patterns for devices of\ndifferent thicknesses. b Individual voltage performance of P(AM-co-AMPS)-NaI device with a\nthickness of 9 mm. It shows a similar change pattern as electric current.\n[Comment 9] In Figure 4e, the Raman intensity maps of P(AM-co-AMPS)-NaI gel in\ndifferent humidity environments were displayed. The authors mentioned that it was\nplaced in a “humidity bottle”. What was the “humidity bottle”? I was curious about\nthat how the authors perform the Raman characterizations in different humidity\nenvironments. Corresponding experimental conditions need to be put in the Methods\nparts.\n[Response] We sincerely thank you for this valuable comment, which has helped us to\nimprove the completeness of our manuscript. The “humidity bottle” refers to a sealed\ncontainer containing a saturated salt solution used to generate a specific humidity level\n(as shown in Fig. R3). We utilized the humidity generated by these saturated solutions\nto control the environmental conditions during the experiments. At the same time, in\norder to avoid misunderstandings, we have replaced “humidity bottle” in the manuscript\nby “humidity chamber.” The detailed Raman characterization process is as follows:\nThe hydrogel was first placed in a 15% RH humidity chamber, where the humidity\nwas controlled by a saturated lithium chloride (LiCl) solution. After 12 hours of\nequilibration, the hydrogel was removed and subjected to Raman characterization under\nambient laboratory conditions (55% RH, 25 ℃). Raman spectra were collected using\na 532 nm laser source, with the laser power set to 3.2% and an acquisition time of 3\nminutes per spectrum. Due to the relatively narrow wavelength range, the acquisition\ntime remained short.\nSubsequently, the hydrogel was transferred into a 98% RH humidity chamber,\ngenerated by a saturated potassium sulfate (K SO ) solution. After 30 minutes (chosen\n2 4\nbased on the observed device performance, which typically reaches a peak around this\ntime), the hydrogel was again subjected to Raman spectra under the same conditions.\nFinally, the hydrogel was placed back into the 15% RH humidity chamber for\nanother 30 minutes, followed by a third round of Raman measurements. Through this\nprocedure, we obtained the Raman intensity profiles of triiodide ions (I -) under\n3\ndifferent humidity states, as shown in Fig. 4e.\nWe have included these experimental conditions in the Methods section of the\nrevised manuscript to facilitate a clearer understanding of our procedures.\nFig. R3 Experimental setup of humidity bottles used for controlling relative humidity.\n[Comment 10] Some minor errors need to be noted. There should be a space between\nthe number and the unit. Please check.\n[Response] Accounting for this comment, we have carefully reviewed the text and have\ncorrected the missing spaces between numerical values and their corresponding units\nthroughout the manuscript.\nReviewer #2\n[General comment] This manuscript, entitled \"Humidity fluctuations-induced dynamic\ncoordination drives the oscillatory migration of ions for sustainable energy harvesting\"\nintroduces a hydrogel-based sustainable moisture-electric generator (SMEG) based on\nthe dynamic coordination between I- and I to synthesize I -. Specifically, the authors\n2 3\nreasonably demonstrated through Raman spectroscopy and molecular dynamics\nsimulations that the dynamic coordination is regulated by the adsorption and\ndesorption of moisture, endowing it with the ability to continuously operate in an\nenvironment with cyclic changes in air humidity. It has been tested to be able to work\nstably for nearly two weeks in a simulated environmental chamber. In general, the\nconcept of ion oscillation migration driven by dynamic coordination is novel and\nimpressive. It has shown remarkable potential for enabling the continuous and stable\noperation of moisture-based energy-harvesting devices. However, several concerns\nneed to be addressed before it can be published in Nature communications. Please see\nthe following detailed comments.\n[Response] We sincerely thank you for the positive evaluation and constructive\nsuggestions regarding our work. We are grateful for your recognition of the new concept\nproposed in this study—humidity fluctuation-induced dynamic coordination driving\noscillatory ion migration—as well as for your affirmation of our experimental design,\nRaman spectroscopy characterization, molecular dynamics simulations, and the\ndemonstration of the device’s long-term stability.\nIn response to your valuable suggestions and specific questions, we have carefully\nrevised the manuscript point by point. The revision includes additional experimental\ndata, theoretical explanations, and mechanistic analysis to further enhance the scientific\nrigor and completeness of the paper.\nWe believe that the revision has significantly improved the quality of the\nmanuscript. Our detailed responses to each of your comments are provided below.\n[Comment 1] In line 24, the abstract states that “The coordination process of I- does\nnot involve charge exchange, and the charge transfer behavior (electricity generation)\nis still achieved through moisture-driven ion transport.” However, the text appears to\nlack corresponding evidence for this claim. Please provide a detailed explanation of\nthis behavior.\n[Response] We thank you for the insightful question and recognize the need to further\nelaborate on the proposed mechanism. The coordination process involving iodide ions\n(I-) does not involve charge transfer. Specifically, I- acts as a Lewis base, donating a\nlone pair of electrons to molecular iodine (I ), which serves as a Lewis acid. This\n2\ninteraction forms a coordinate bond through σ–σ* orbital interaction9. Such\ncoordination is essentially an intermolecular association rather than a redox reaction,\nand thus, no electron transfer occurs during the formation of triiodide (I -).\ntransfer, it is currently understood in the field that humidity-driven ion migration leads\nto ionic accumulation at the electrode interfaces. These accumulated ions interact\nelectrostatically with the electrons in the electrode, thereby inducing electron\ndisplacement in the external circuit, which manifests as an electric signal.\nIn summary, while the coordination process itself is not redox-active, the ion\nmigration triggered by humidity fluctuations creates an electrochemical environment\nwhere electrostatic interactions at the electrode interface drive electron movement,\nresulting in power generation.\n9. Pimentel, G. C. The bonding of trihalide and bifluoride ions by the molecular orbital\nmethod. J. Chem. Phys. 19, 446-448 (1951).\n[Comment 2] In line 70, the definitions of relative high humidity and low humidity are\nnot clearly delineated in the preceding text. Please provide a specific explanation for\nthese terms.\n[Response] We appreciate your feedback and acknowledge the need to clarify the\ndefinitions of these terms. “Relatively high humidity” and “relatively low humidity”\nare used to describe humidity levels relative to the device’s previous environment. For\nexample, if a device is initially stored at 50% RH and then transferred to 70% RH, the\nlatter is considered as a relatively high-humidity environment, and the device will\nexhibit moisture-adsorption behavior. Conversely, if the device is moved from a high\nhumidity environment to a low one, the new environment is referred to as relatively\nlow humidity, and the device will demonstrate moisture-desorption behavior.\nThese definitions are inherently relative and depend on the specific conditions to\nwhich the device was previously exposed. We have refined the relevant descriptions to\nensure clearer and more coherent expressions throughout the manuscript.\n[Comment 3] In line 105, it is mentioned that “The use of a sealing film ensures that\nair contacts the functional layer from the top, facilitating the formation of ionic\nconcentration differences.” However, what is the performance of the device when there\nis no sealing film around it? This could be further elucidated through a control\nexperiment, which would help to corroborate the working mechanism of the device.\n[Response] We sincerely appreciate your insightful question. To address this concern,\nwe conducted dedicated experiments to verify the effect. Specifically, we\nsystematically varied the thickness of the gel layer to examine the influence of sealing\nfilms on device performance under 98% RH.\nAs illustrated in Supplementary Fig. 17, devices with sealing films consistently\nexhibited superior performance compared to their unsealed counterparts. Notably, the\nperformance gap between sealed and unsealed devices widened as the gel thickness\nincreased. We attribute this phenomenon to the expanded lateral surface area of the gel\nexposed to ambient moisture at greater thicknesses, which reduces the ionic\nconcentration gradient between the top and bottom electrodes, thereby degrading the\nelectrical performance.\nThese findings have been incorporated into the revised manuscript with detailed\ndiscussions to provide further clarification.\nSupplementary Fig. 17 Current output of devices with and without side sealing films.\n[Comment 4] In line 112, it is stated that the material possesses a moderate\nhygroscopic capacity. However, the degree of this hygroscopic capacity is not\nspecifically described, and the current description is somewhat vague. It is\nrecommended to supplement the text with comparative data on the hygroscopic\ncapacities of commonly used adsorbents.\n[Response] We sincerely appreciate your valuable suggestion for a more precise\ndescription of the material’s hygroscopic capacity. In this context, the term “appropriate\nhygroscopic capacity” refers to the material’s optimal hygroscopic property for stable\noperation in varying humidity environments. Specifically, sodium iodide (NaI) exhibits\nthe most suitable moisture adsorption capacity for maintaining device functionality. In\ncontrast, lithium chloride (LiCl) demonstrates excessively high hygroscopicity—even\nafter one month of storage, the gel in devices still retained a high water content, as\nshown in Supplementary Fig. 4. This excessive moisture retention may adversely affect\ndevice performance in humid environments. On the other hand, sodium fluoride (NaF)\nshows insufficient moisture absorption capacity, with the gel drying out and\ncrystallizing after just 3 days of atmospheric exposure.\nImportantly, sodium iodide effectively maintains the gel’s original state,\ndemonstrating what we term as “appropriate moisture hygroscopic capacity”. We have\nsupplemented the manuscript with comparative data and more detailed explanations to\nclarify this point.\nSupplementary Fig. 4 Evidence of the appropriate hygroscopic capacity of NaI.\n[Comment 5] In line 258, the ranking of the hygroscopic capabilities of different salts\nis provided without experimental evidence or reference to relevant literature. It is\nnecessary to supplement this by either conducting experiments to measure the\nhygroscopic properties of the salts or by citing pertinent studies that have previously\nreported such rankings.\n[Response] We sincerely appreciate your valuable suggestion regarding the need for a\nmore precise description of the material’s moisture absorption capabilities. In response,\nwe have incorporated a reference to prior studies that systematically rank the\nhygroscopic properties of different salts8, thereby supporting the classifications\npresented in our manuscript.\n8. Peng, C., Chen, L. & Tang, M. A database for deliquescence and efflorescence relative\nhumidities of compounds with atmospheric relevance. Fundam. Res. 2, 578-587 (2022).\n[Comment 6] In line 262, the power generation effects of hydrogels treated with other\nsalt solutions are mentioned, but the underlying mechanisms for each are not explained.\nIt is suggested to provide a brief explanation of the working principles for these cases.\n[Response] We appreciate your suggestion and have provided a concise explanation of\nthe power generation mechanism for hydrogels treated with other salt solutions. When\nhydrogels are treated with hygroscopic salts, they exhibit two key characteristics: (1)\nthe ability to capture moisture from air, and (2) the presence of abundant mobile ions\nwithin the gel. With these properties, when the hydrogel is exposed to humidity, a\nvertical moisture gradient forms within the device (precisely, the top surface is exposed\nto environment while the bottom remains sealed). Driven by this gradient, ions migrate\ndownward owing to the moisture gradient. Due to the different migration rates of anions\nand cations, concentration differences develop between the upper and lower electrodes,\nthereby generating electrical signals.\n[Comment 7] During the device fabrication process, what is the impact of the drying\ntemperature of the hydrogel on the gel properties, and why was 80°C specifically\nchosen?\n[Response] The drying temperature of hydrogels primarily affects the time required to\nachieve optimal water content within the gel. When the temperature is too low, the\ndrying process becomes excessively prolonged. Conversely, excessively high\ntemperatures lead to rapid evaporation, resulting in uneven heating of the gel and\nsubsequent cracking failure.\nTo balance between processing time and maintaining gel integrity, we selected\n80 °C as the optimal drying temperature. Furthermore, during the drying process, the\nhydrogels were repositioned every 30 minutes to ensure uniform heat distribution.\n[Comment 8] It is suggested to enlarge a single cycle of data in Fig. 1b to more clearly\nhighlight the characteristics of the data.\n[Response] To account for your suggestion, we have enlarged the single-cycle data in\nFig. 1b to enable clearer visualization of the data characteristics. This modification will\nfacilitate readers’ understanding of the specific features and trends within the data cycle.\nFig.1 b Current performance of SMEG in repeated cycles of high and low humidity for 14 days (gel\nthickness of 3 mm), with an enlarged view on the right to highlight the data characteristics.\n[Comment 9] Why do Fig. 3b and 3c exhibit significant differences in forward and\nreverse current performance, while the forward and reverse currents in Fig. 3a are at\nthe same level?\n[Response] The phenomenon in question is closely related to the thickness of the\nhydrogel: Figs. 3b and 3c correspond to a 3 mm thick hydrogel, while Fig. 3a uses a\n15 mm thick hydrogel. The difference in thickness leads to variations in the internal\nratio of I- to I -, which subsequently affects the moisture adsorption and desorption\n3\ncapacities of the hydrogel. This ultimately results in the observed performance\ndifferences between the adsorption and desorption processes. The detailed analysis is\nas follows:\nFirstly, based on the experimental results presented in Fig. 2d, we speculate that\nthe performance difference between the adsorption and desorption stages arises from\nthe asymmetric moisture adsorption and desorption capacity of the device.\nSecondly, as shown in Fig. 3e, a comparison of device performance using\nhydrogels with different thicknesses reveals that when the gel thickness is less than 9\nmm, the device tends to exhibit higher output during the adsorption stage and lower\noutput during desorption. In contrast, when the thickness is equal to or greater than 9\nmm, the performance becomes symmetric or even slightly higher during the desorption\nstage. Therefore, we infer that this phenomenon is thickness-dependent, and is likely\ncaused by the influence of hydrogel thickness on moisture transport and ionic dynamics.\nBased on the above analysis, it can be inferred that the thickness of the hydrogel\ninfluences its moisture adsorption and desorption behavior, which is intrinsically\nrelated to the ratio between the quantities of iodide ions (I-) and triiodide ions (I -)\n3\nwithin the gel. Specifically, I- ions are primarily responsible for moisture adsorption\n(e.g., the hygroscopicity of NaI is greater than that of NaF6-8), while I - ions, formed by\n3\nthe coordination between I- and I , possess a larger linear structure with a more diffuse\n2\ncharge distribution, making them less capable of forming stable hydration shells. At a\nfixed NaI concentration, a thicker hydrogel introduces more sulfonic acid groups,\nthereby increasing the availability of protons (H+), which promotes the conversion of I-\ninto I -. As a result, thinner gels contain a higher proportion of I-, leading to stronger\n3\nmoisture adsorption and enhanced device performance during the adsorption stage. As\nthe gel thickness increases, the ratio of I- to I - decreases, leading to more balanced\n3\nadsorption/desorption behavior, or even stronger performance during the desorption\nstage. In summary, thin hydrogels tend to show higher performance during moisture\nadsorption, while thicker hydrogels exhibit more balanced or even desorption-\ndominated behavior, owing to the shifting I-/I - ratio.\n1. Moisture adsorption and desorption tests of hydrogels with different\nthicknesses. To investigate the effect of thickness on moisture behavior, hydrogels of\nidentical compositions but different thicknesses (5 mm, 9 mm, and 13 mm) were tested.\nFirst, the hydrogels were placed in a 15% RH humidity chamber for 12 hours to\nsimulate the device working process (ambient conditions: 25 °C, 50% RH). Then, they\nwere transferred to a 98% RH humidity chamber to evaluate their moisture adsorption\ncapacity. The mass of each sample was measured every 10 minutes, with three repeated\nweighings to obtain the average value. After 12 hours of moisture adsorption, the\nhydrogels were again placed in the 15% RH chamber to test their desorption behavior,\nfollowing the same weighing protocol. As shown in Supplementary Fig. 15, thinner\nhydrogels exhibited stronger moisture adsorption than desorption, while thicker\nhydrogels showed the opposite trend, which is consistent with our hypothesis.\nCombined with the data in Fig. 3e, these results confirm that the asymmetry in device\nperformance during adsorption and desorption is primarily attributed to the different\nmoisture handling capacities of hydrogels with varying thicknesses.\n2. The difference in moisture adsorption/desorption capacity originates from\nthe varying I-/I - ratios. To investigate this, P(AM-co-AMPS) hydrogels with\n3\nthicknesses of 3 mm, 9 mm, and 15 mm were soaked in equal volumes of 1 M NaI\nsolution for 4 days to ensure equilibrium between I- and I -. As shown in Supplementary\nwith increasing gel thickness, indicating a higher I - content. Subsequently, these three\n3\nsolutions were used to soak identical 3 mm-thick PAM hydrogels, denoted as PAM ,\n9 15\nrevealed a progressive increase in the intensity of the I - characteristic peaks from PAM\n3 3\nto PAM , suggesting a decreasing I- to I - ratio in the soaking solutions used, and\n15 3\ntherefore in the resulting hydrogels.\n3. The I-/I - ratio affects moisture adsorption/desorption performance. To\n3\nfurther demonstrate that the ratio between I- and I - influences the adsorption and\n3\ndesorption behavior, the electrical performances of PAM , PAM , and PAM were\n3 9 15\nevaluated, as shown in Supplementary Fig. 13b. The results indicate that as the\nproportion of I - increases, the adsorption performance of the samples gradually\n3\napproaches their desorption performance. Moreover, the overall outputs of the devices\nwere found to be positively correlated with the I - content in the hydrogel. This\n3\nphenomenon likely originates from the enhanced dynamic coordination effect at higher\nI - concentrations, which induces a larger I- concentration gradient and thus leads to\n3\nstronger electrical output.\nThrough the above analysis, experiments, and characterizations, we have\ndemonstrated that the different ratios of I- to I - within the hydrogel lead to variations\n3\nin its moisture adsorption and desorption capacities. This ultimately results in the\nobserved asymmetry in device performance, with higher output during the adsorption\nphase compared to the desorption phase. We sincerely thank you for raising this\nimportant point, which has deepened our understanding of the device’s working\nmechanism.\nSupplementary Fig. 15 Comparison of moisture adsorption and desorption capacities of NaI-\nbased hydrogels with different thicknesses. a Moisture adsorption of hydrogels after being\nmaintained at 15% RH for 12 h and then transferred to 98% RH. b Moisture desorption of the same\nhydrogels after an additional 10 h at 98% RH, followed by transfer to 15% RH.\nSupplementary Fig. 16 The state of NaI solutions (1 M, equal volumes) after immersing\nhydrogels of different thicknesses for 4 days. It is evident that the thicker the hydrogel, the darker\nthe solution color, indicating a higher concentration of triiodide ions. The inset in the upper left\nshows the appearance of three PAM gels of the same thickness after immersion in the above three\nNaI solutions followed by drying.\nSupplementary Fig. 13 Raman characterization and device performance of PAM , PAM , and\n3 9\nPAM hydrogels. a The Raman spectra indicate that the triiodide content in PAM , PAM , and\n15 3 9\nPAM hydrogels increases sequentially, suggesting a corresponding decrease in the I-/I - ratio. b\n15 3\nDevice performance of the hydrogels, with the adsorption-to-desorption performance ratios of 2.44,\n1.96, and 1.17, respectively.\n6. Miñambres, L., Méndez, E., Sánchez, M. N., Castaño, F. & Basterretxea, F. J. Water uptake\nproperties of internally mixed sodium halide and succinic acid particles. Atmos. Environ.\n45, 5896-5902 (2011).\n7. Albayrak, C., Barim, G. & Dag, O. Effect of hygroscopicity of the metal salt on the\nformation and air stability of lyotropic liquid crystalline mesophases in hydrated saltnsurfactant systems. J. Colloid Interface Sci. 433, 26-33 (2014).\n8. Peng, C., Chen, L. & Tang, M. A database for deliquescence and efflorescence relative\nhumidities of compounds with atmospheric relevance. Fundam. Res. 2, 578-587 (2022).\n[Comment 10] In Fig. 3d, it is suggested to further increase the content of AMPS and\nmeasure the corresponding current performance to see if the current continues to\ndecrease. Adding one or two more data points would make the trend more evident.\n[Response] Following your recommendations, we have repeated the relevant\nexperiments and included two additional data points with higher AMPS concentrations\nin Fig. 3d. The experimental results demonstrate that the device output current\ngradually increases with AMPS content before reaching a saturation plateau.\nThis trend can be explained as follows. The sulfonic acid groups in AMPS provide\nadditional H⁺ ions, which facilitate the conversion of I- to I -, thereby enhancing device\n3\nperformance. However, when the H+ concentration reaches a certain threshold, the\nconversion between iodide and triiodide ions approaches equilibrium. Beyond this\npoint, further increases in AMPS content no longer significantly promote I₃⁻ generation,\nresulting in the observed current stabilization.\nThe positive correlation between I - content and current intensity, as clearly\n3\ndemonstrated in Supplementary Fig. 13, provides compelling experimental evidence to\nsupport our proposed mechanism.\nFig. 3d The current performance characteristics of SMEG devices with different AMPS contents\n(gel thickness of 3 mm).\nSupplementary Fig. 13 Raman characterization and device performance of PAM , PAM , and\n3 9\nPAM hydrogels. a The Raman spectra indicate that the triiodide content in PAM , PAM , and\n15 3 9\nPAM hydrogels increases sequentially, suggesting a corresponding decrease in the I-/I - ratio. b\n15 3\nDevice performance of the hydrogels, with the adsorption-to-desorption performance ratios of 2.44,\n1.96, and 1.17, respectively.\n[Comment 11] The curves in Fig. 4a are recommended to be labeled with numbers or\nletters for differentiation. The current presentation is not conducive to reader\ncomprehension.\n[Response] Following your recommendation, we have made the following\nimprovements to Fig. 4a:\n1. Adding detailed annotations to the curves;\n2. Modifying certain expressions for clarity.\nIn the modified figure, solid arrows indicate moisture adsorption performance\nwhile dashed arrows denote moisture desorption performance.\nFig. 4a Current profiles when different surfaces of the device are exposed to moisture, and the inset\nshows the schematic diagram of four device states.\n[Comment 12] It is suggested to provide a comparative table of similar devices\n(including aspects such as lifetime, humidity range, and working principles) to\nhighlight the advantages of this device. The current presentation relies solely on textual\ndescriptions, which lacks intuitiveness.\n[Response] Following your suggestion, we have reviewed and summarized recent\nliterature in the relevant field, as presented below:\nTable 2. The performance comparison of current moisture electric generators.\nDurability\nOpen Short and output\nOperating\nFunctional Working Area circuit circuit patterns in a Work Decay\nElectrode humidity Ref.\nmaterials principles (cm2) voltage current humidity- time rate\n(%)\n(V) (μA) fluctuations\nenvironment\nDirected ion\nμA/cm2\nRedox\nDirected ion\n>25 ~75.1\nMEL-PCL C-C migration + 6 0.28 1.8 80 No, DC 11\ndays %\nPC\nPVA-AlgNa Al-C Directed ion 1 1.3 1310 80 Yes, DC >30 ~96% 12\nbased hydrogel migration days\nLiCl@CP- Directed ion >10 ~21.7\nC-C 9 0.78 9.2 60±5 Yes, DC 13\nCB@CP migration days %\nDirected ion\n>100\nZn-C migration + 2.25 1.25 5.67 80 No, DC ~24%\nh\nRedox\naASA 14\nDirected ion\nC-C 2.25 0.21 2.81 80 No, DC None None\nmigration\nDirected ion\n>5\nC-Al migration+ None 1.8 1200 90 No, DC ~33%\nPAM- days\nRedox 15\nLiCl/CMC\nDirected ion\nC-C None ~0.35 ~15 30 No, DC None None\nmigration\nPAM-LiCl/ Directed ion >800 ~39.8\nCFP-Ag 4 1.28 5.9 60 (35℃) No, DC 16\nSiNWs migration h %\nDirected ion\nMXene aerogel ~97.5\nCu-Cu migration + 1 0.6 1200 20 Yes, DC >12 h 17\n/PAM-CaCl2 %\nRedox\nPEDOT:PSS-\nPVA-PAAs- Directed ion\n60 ( ΔT = ~44.2\nbased Cu-Cu migration + 4.9 0.215 ~29.2 No, DC >25 h 18\n3.7 K) %\nanisotropic TE\nhydrogel\nP(VDF- Directed ion\n>30\nTrFE)/PAN/CN C-Al migration + 2 0.85 256 35-45 Yes, DC 96% 19\ndays\nW-LiCl Redox\nSelf-excited\nP(MEDSAH- 15000\nAu- Au chemoelectr 4 0.4 10 100 No, AC 99.9% 20\nco-AA) s\nical reaction\nOscillatory\n>3\nILs Ag/AgCl migration of 0.785 0.15 2 35-81 Yes, AC ~51% 21\ndays\nions\nILs/Clustered Oscillatory\n0.00012 >30 Nearly\npolycation Ag/AgCl migration of 0.0009 22 73 Yes, AC 22\n5 days 0%\nmembrane ions\nNew device\n>14 Nearly\nOscillatory days 0%\nP(AM-co- This\nC-C migration of 1 0.028 33 15-98 Yes, AC After 7 months\nAMPS)-NaI work\nions device\n>30\n39.5%\ndays\nCalculation of decay rate:\n For DC-type devices, the decay rate is calculated as:\nDecay rate =\n𝐼𝑚𝑎𝑥−𝐼𝑚𝑖𝑛×100%\n𝐼𝑚𝑎𝑥\nwhere I and I represent the maximum and minimum current values during the\nmax min\nmeasured period.\n For AC-type devices, the decay rate is calculated as:\n Decay rate =\n𝐼𝑝𝑒𝑎𝑘,1−𝐼𝑝𝑒𝑎𝑘,𝑒𝑛𝑑×100%\n𝐼𝑝𝑒𝑎𝑘,1\nwhere I is the peak current of the first cycle, and I is the peak current of the last\npeak,1 peak,end\ncycle within the measured period.\nFrom the foregoing table, it can be concluded that in most current moisture-enabled power generation devices, the primary mechanism is humidity-induced directional ion migration. In some high-performance devices, redox reactions may also play a role. While the direct current (DC) generation mode based on directional ion migration offers the advantage of easy direct utilization, it relies on a limited supply of mobile ions, which imposes a theoretical limit on its operational lifetime and often results in rapid performance degradation.\nIn contrast, the alternating current (AC) generation mode can continuously re-establish ion concentration gradients through external humidity fluctuations.\nTheoretically speaking, this enables perpetual power generation with a significantly lower degradation rate. Therefore, the working mode demonstrated in this study highlights a theoretical advantage in the context of sustainable power generation.\n10. Song, M., Kim, D., Lee, H., Han, H. & Jeon, S. Synergistic effect of a Berlin green framework for highly efficient moisture-electric energy transformation. Energy\nEnviron. Sci. 17, 5421-5428 (2024).\n11. Duan, P. et al. Moisture-based green energy harvesting over 600 hours via\nphotocatalysis-enhanced hydrovoltaic effect. Nat. Commun. 16, 239 (2025).\n12. Yang, S. et al. Green moisture-electric generator based on supramolecular hydrogel\nwith tens of milliamp electricity toward practical applications. Nat. Commun. 15, 3329\n(2024).\n13. Tan, J. et al. Self-sustained electricity generator driven by the compatible integration\nof ambient moisture adsorption and evaporation. Nat. Commun. 13, 3643 (2022).\n14. Zhou, J., Ren, Z., Cui, X., Liu, X. & Lu, X. Bioinspired interfacial design of robust\naramid nanofiber composite films for high‐performance moisture‐electric generators.\nAdv. Energy Mater. 15, 2404840 (2025).\n15. Cao, F. et al. Efficient energy harvesting: High power wearable humidity generators\nwith PAM-LiCl/CMC structure. Nano Energy 134, 110590 (2025).\n16. Duan, W. et al. Silicon nanowire/ionic hydrogel-based hybrid moist-electric generators\nwith enhanced voltage output and operational stability. Energy Environ. Sci. 17, 3788-\n3796 (2024).\n17. Zhao, K. et al. Humidity-tolerant moisture-driven energy generator with mxene\naerogel-organohydrogel bilayer. ACS Nano 17, 5472-5485 (2023).\n18. Zhang, M. et al. Long-term serviceable ionic thermoelectric hydrogel with temperature\nand moisture dual-driven waste energy harvesting capability. Small 21, e2501960\n(2025).\n19. Hu, Y. et al. Phyto-inspired sustainable and high-performance fabric generators via\nmoisture absorption-evaporation cycles. Sci. Adv. 10, eadk4620 (2024).\n20. Long, Y. et al. Moisture-induced autonomous surface potential oscillations for energy\nharvesting. Nat. Commun. 12, 5287 (2021).\n21. Tang, J. et al. Circadian humidity fluctuation induced capillary flow for sustainable\nmobile energy. Nat. Commun. 13, 1291 (2022).\n22. Zhao, Y. et al. Diurnal humidity cycle driven selective ion transport across clustered\npolycation membrane. Nat. Commun. 15, 7161 (2024).\n[Comment 13] The performance of the device under real environmental conditions is\nmissing. It is recommended to supplement the curves showing the device's current\nperformance fluctuating with humidity in a real atmospheric environment to\ndemonstrate its working state under actual conditions.\n[Response] To account for your recommendation, we have conducted experiments to\nevaluate the performance of the device under real environmental conditions. The tests\nwere carried out from June 6 to June 8, 2025, with the device placed outside the window\nof the laboratory at the Center for Ultra-Precision Surface Manufacturing. As shown in\nSupplementary Fig. 22, the ambient relative humidity fluctuated between 33% and 65%,\nand the corresponding current output ranged from -15.8 μA to 8.2 μA. The results\ndemonstrate that the device can effectively respond to ambient humidity variations and\ncontinuously generate electricity. Notably, the current response during the desorption\nprocess was significantly higher than during the adsorption process, which may be\nattributed to the relatively higher temperature during desorption.\nFurthermore, the data show that the device is capable of responding to humidity\nchanges as small as 2% RH, highlighting its exceptional sensitivity to humidity\nfluctuations.\nSupplementary Fig. 22 Current data of SEMG tested continuously for two days in a real\noutdoor environment. a Current response to environmental changes. b Enlarged view of Fig. a. c\nTemperature and humidity data of the outdoor environment. d Enlarged view of Fig. c.\n[Comment 14] In Fig. 5d, only two sets of humidity difference data are provided. It is\nsuggested to supplement with additional data points.\n[Response] We appreciate your suggestion. The purpose of Fig. 5d is to demonstrate\nthat the device can still generate alternating signals under small humidity differences.\nWe deliberately selected the smallest humidity gradient achievable using saturated salt\nsolutions to evaluate the device’s performance during both adsorption (low to high RH)\nand desorption (high to low RH) processes. These data points effectively show that the\ndevice remains functional even under very small humidity fluctuations. Given the\nlimited range of humidity conditions provided by saturated salt solutions, we believe\nthe existing data are sufficient to support our conclusion. Therefore, we consider that it\nis not necessary to include additional data points under these conditions.\n[Comment 15] It is recommended to include images showing the relationship between\nthe number of breathable pores in the top electrode and the performance changes, in\norder to illustrate the correlation between the contact area of moisture and the device\nperformance.\n[Response] To account for the recommendation, we conducted a series of experiments\nusing top electrodes with different numbers of holes (5, 10, 15, 20, and 25 holes, each\nwith a diameter of 1.5 mm) to investigate their influence on device’s performance\nduring the humidification process. As shown in Supplementary Fig. 18, when the\nnumber of holes was small, the limited contact area with ambient moisture led to slower\nperformance enhancement and lower peak output. In contrast, devices with a higher\nnumber of holes exhibited faster performance increase and higher peak currents, due to\na larger exposed surface area facilitating more efficient moisture uptake. These findings\nare consistent with the proposed working mechanism. We have included the\ncorresponding figures and detailed data in the revised manuscript to illustrate this\ncorrelation more clearly.\nSupplementary Fig. 18 Current output of 3 mm-thick devices with varying numbers of holes in\nthe top electrode.\nReviewer #3\n[General comment] Lu et al. proposed a novel moisture-electricity conversion\nmechanism based on the dynamic coordination between iodine ions and iodine\nmolecules to keep a continuous ions migration for a long operating lifespan in MEG.\nWhile this strategy exhibits promising extensibility to diverse functional materials and\nbroad humidity adaptability, its practical implementation faces notable limitations. The\nmaterial only achieves voltage (~0.2 V) and current density (~33 μA/cm2) performance\neven at extremely high humidity environment (98%RH), which is an order of magnitude\nlower than recent related literature (Nat Commun 15, 3329 (2024), Nat Commun 15,\n6100 (2024)). In addition, more evidence is needed to understand the mechanism by\nwhich this material generates electrical signals in a humidity environment. Therefore,\nin my opinion, this manuscript can be accepted after major revisions for publishing in\nNature Communications.\n[Response] We sincerely thank you for recognizing our proposed moisture-electricity\nconversion strategy based on the dynamic coordination mechanism between iodide ions\n(I-) and iodine molecules (I ). We fully understand and acknowledge your concern\n2\nregarding the current device performance. Although this strategy demonstrates\npromising potential in terms of material versatility and humidity adaptability, its output\nperformance is indeed lower than those reported by some recently published papers\n(e.g., Nat. Commun. 15, 3329 (2024); Nat. Commun. 15, 6100 (2024)). We would like\nto underline that the objective of the present work is different from mainstream\nmoisture-electric generators (MEGs) that focus on producing stable direct current (DC)\nunder static high-humidity conditions. Instead, our work aims to achieve continuous\nalternating current (AC) output under naturally fluctuating ambient humidity. The\nproposed dynamic coordination mechanism, driven by the reversible reaction between\nI- and I , enables periodic oscillatory migration of ions, offering a novel approach\n2\ntoward sustainable moisture-induced energy harvesting.\nRegarding your valuable suggestion on the need for more mechanistic evidence,\nwe completely agree and have added additional experimental data in the revised\nmanuscript to support the proposed mechanism. We are also actively exploring new\nmaterial systems and device architectures to improve the output performance of\nindividual devices and to develop scalable integrated strategies that may facilitate\npractical applications.\nWe believe this work provides both theoretical and experimental foundations for\nfuture advancements and contributes meaningful insight into the development of\nsustainable power generation in the field of moisture-induced energy harvesting.\nOnce again, we sincerely thank you for your valuable comments, which have\ngreatly helped us further improve the quality of the manuscript. We genuinely hope that\nthe revised version now meets your expectations. We remain open to any further\nsuggestions you may have and are happy to make additional revisions if needed.\n[Comment 1] Currently, most MEG studies based on functional hydrogels attribute the\ngeneration of electrical signals to the dissociation of protons (H⁺) from functional\ngroups and their migration along the concentration gradient. However, this manuscript\nuses NaI to neutralize the dissociated hydrogen ions and uses the migration of\nnegatively charged ions to generate electrical signals. Therefore, the gradient within\nthe SMEG must be inferior to the previous one, resulting in a weaker electrical signal,\nwhich is also demonstrated by the performance in the manuscript. Given this significant\nperformance gap, the proposed mechanism may offer limited advancement for the field.\n[Response] We are very grateful for your in-depth comparative analysis of our work\nwith traditional proton migration-based hydrovoltaic power generation mechanisms.\nIndeed, most current hydrogel-based MEG studies attribute the generation of electrical\nsignals to the directional migration of H+ released by acidic groups driven by\nconcentration gradients. In our work, we used NaI to neutralize the dissociated H+ in\nthe material. In addition, in view the inherent humidity fluctuations in the natural\natmospheric environment, we focused on the ion migration behavior caused by the\ndynamic coordination of iodine by humidity fluctuations so as to explore a new ion\nmigration driving mechanism. It is expected to utilize the inherent day and night\nhumidity fluctuations in nature to achieve long-lasting performance output.\nWe fully recognize that the overall output performance of our device is indeed\nlower than that of the proton-dominated system due to the suppression of high-mobility\nproton participation. However, we believe that this dynamic coordination-driven ion\noscillation migration mechanism is still of great significance for the following reasons:\n1. Provide a new perspective on ion migration mechanism: By introducing\niodine anion migration instead of traditional H⁺ migration, we have broadened the\nresearch scope of humidity-responsive ion transport mechanism, which is particularly\nsuitable for some application scenarios where it is not suitable or desirable to introduce\nprotons (such as systems with high requirements for material chemical stability).\n2. Provide a driving force for theoretical sustainable power generation: Most\ntraditional devices are DC power generation modes, relying on ion concentration\ngradients to drive ion unidirectional movement. However, the concentration gradient of\nmost devices is difficult to maintain and rebuild, or additional energy is required to re-\nestablish the concentration gradient to make the device work again. The dynamic\ncoordination mechanism driven by moisture fluctuations will theoretically continuously\noutput electrical energy in the natural environment without the need for other additional\nenergy input.\n3. Provide a basis for subsequent performance optimization: Although the\ncurrent output is still at a low level, we believe that this strategy has laid the foundation\nfor the construction of a moisture sustainable power generation system, and has room\nfor further optimization of performance by building directional ion channels and\nimproving ion mobility.\nAt the same time, the significant performance difference may be caused by\ninefficient ion transport and interface charge conversion. Our subsequent work will\ndraw on some excellent strategies to further improve device performance, mainly\nincluding the following aspects:\n1. Device structure optimization: We plan to draw on technologies such as\ndirectional freezing and cryogelation to construct a hydrogel network with directional\npore structure and strong deformation resistance. This type of structure helps the\nefficient migration of ions under humidity gradients, and is expected to significantly\nimprove the current density and working stability of the device.\n2. Improvement of interface charge conversion efficiency: We will modify the\nsurface nanostructure of the inert electrode surface (such as constructing a rough\nstructure of nanoparticles or introducing a microporous structure) to increase the\neffective contact area of the electrode and reduce the interface impedance, thereby\nimproving the efficiency of ion and electron coupling; introduce an intermediate layer\nmaterial with high ion-electron coupling ability (such as PEDOT:PSS, etc.) between\nthe electrode and the gel. This type of materials has good charge transfer performance\nand can effectively promote the efficient conversion of ions to electrons at the interface,\nthereby ameliorating output stability and energy utilization efficiency.\n3. Optimization of the internal ion system: On the basis of maintaining the non-\nredox mechanism, we will explore ions or ion composite systems with higher mobility\nand stronger polarizability to enhance the ion response rate and migration efficiency.\n4. Exploitation of the temperature and humidity synergy: Considering that the\nambient temperature and humidity usually have coupled fluctuation characteristics, our\nsubsequent work attempts to design devices capable of exploiting the humidity\nresponse mechanism and the energy of the ambient temperature so as to synergistically\npromote device performance.\nWe believe that through the above multi-dimensional collaborative optimization\nstrategy, the performance of devices based on the moisture-driven dynamic\ncoordination mechanism can be significantly improved, and its practical applications\ncan be expanded.\nIn summary, although the current performance of our device cannot be compared\nwith the one of the mainstream MEGs based on the proton migration mechanism, the\nconceptual exploration and new mechanism path it provides promote the diversified\ndevelopment of theories and material systems in this field, while being extremely\nvaluable in the direction of sustainable power generation.\n[Comment 2] Figure 1b shows that the SMEG has high cycling stability. However, the\nreversible reaction is difficult to maintain the ion gradient, as evidenced by the\nsignificant current decay from 10 μA to 2 μA within 24 hours. The positive and negative\njumps of the electrical signal caused by the adsorption-desorption switching also limit\nthe practical application scenarios of SMEG.\n[Response] We thank you for pointing out that ion concentration is difficult to maintain.\nThis phenomenon is also a key research object in the field. Many papers have\nintroduced advanced strategies to enhance and maintain ion concentration gradients.\nHowever, one highlight of our work is the proposition of a dynamic coordination-driven\nion oscillation migration mechanism induced by humidity fluctuations. Another\nhighlight is that the ion concentration gradient is reestablished through this mechanism\nto achieve long-term stable operation of the device. Therefore, our strategy sacrifices\nthe maintenance of ion concentration and turns to the inherent moisture fluctuations in\nthe atmospheric environment to rebuild the concentration gradient through a reversible\ndynamic coordination mechanism, which allows achieving long-term power generation.\nConcerning the problem of positive and negative transitions in the electrical signal\noutput by the device during the adsorption-desorption switching process, note that,\nsince a moisture-based power generation device relies on environmental humidity\nfluctuations to drive ion oscillation migration, it usually exhibits alternating electrical\nsignals that change with humidity cycles. Regarding the application of AC signals, we\nexplored feasible strategies for its applications with respect to the following aspects:\n1. Signal regulation through rectification and energy storage unit: Low-\nvoltage rectification circuit (such as Schottky bridge rectification) and micro-\ncapacitor/supercapacitor can be integrated to achieve effective rectification and\nsmoothing of AC signals, thereby outputting stable DC voltage, which can be directly\nused in low-power electronic devices (such as LEDs, sensors)20,21,23.\n2. Use of humidity fluctuation signals for functional control or environmental\nperception: We notice that positive and negative transitions actually reflect the periodic\nfluctuations of ambient humidity, so the AC signal can be used as frequency encoding\nor threshold trigger signals in some scenarios such as environmental perception and\nintelligent control. For example, this periodic change can be applied in intelligent\nventilation, respiratory monitoring24, and humidity control systems.\n3. Combining it with bionic circuits or AC adaptive systems: In recent years,\nstudies have used low-frequency AC signals in the fields of neural synapse simulation\nand frequency-responsive sensors25,26. We believe that such signals can be expanded to\nfrequency-sensitive systems27 (such as soft electronics, skin electronics, etc.).\nIn summary, although the output signal of the SMEG has positive and negative\ntransitions, we believe that this feature is surmountable through optimization of\nelectronic circuit design, system integration, and functional application. In addition, it\nhas unique advantages in some specific applications.\n20. Long, Y. et al. Moisture-induced autonomous surface potential oscillations for energy\nharvesting. Nat. Commun. 12, 5287 (2021).\n21. Tang, J. et al. Circadian humidity fluctuation induced capillary flow for sustainable mobile\nenergy. Nat. Commun. 13, 1291 (2022).\n23. Komazaki, Y., Nobeshima, T., Hirama, H., Watanabe, Y., Suemori, K. & Uemura, S.\nHygroelectric energy harvesting by daily humidity cycles and its thermodynamics. Adv.\nEnergy Sustainability Res. 6, 2400342 (2025).\n24. Kano, S., Jarulertwathana, N., Mohd-Noor, S., Hyun, J. K., Asahara, R. & Mekaru, H.\nRespiratory monitoring by ultrafast humidity sensors with nanomaterials: A review.\nSensors 22, 1251 (2022).\n25. Kim, Y., Lee, C. W. & Jang, H. W. Neuromorphic hardware for artificial sensory systems:\nA review. J. Electron. Mater. 54, 3609-3650 (2025).\n26. Shim, H. et al. Stretchable elastic synaptic transistors for neurologically integrated soft\nengineering systems. Sci. Adv. 5, eaax4961 (2019).\n27. Shim, H. et al. Artificial neuromorphic cognitive skins based on distributed biaxially\nstretchable elastomeric synaptic transistors. Proc. Natl. Acad. Sci. U.S.A. 119, e2204852119\n(2022).\n[Comment 3] In Line 52-54, the authors mentioned ‘… the moisture desorption power\ngeneration dominated by ion hydration take place in the same direction’. Iodide ions\nalso appear to form hydrates. How did the authors exclude this factor from their\nexplanation?\n[Response] We sincerely thank you for raising this insightful question. Indeed, iodide\nions are known to form hydration shells, and the potential influence of ion hydration on\nions transport behavior cannot be entirely excluded. We appreciate the opportunity to\nclarify this point further.\nIn our system, although hydration may provide a certain level of driving force, we\nbelieve that the dominant driving factor for ion migration—particularly under humidity\nfluctuations—is the ion concentration gradient. This judgment is based on both\nphysicochemical considerations and experimental evidence:\nFirst, in terms of ion properties, iodide ions have a relatively large ionic radius (2.2\nÅ) and a lower charge density compared to smaller cations like sodium (1.02 Å).\nConsequently, their electrostatic interaction with water molecules is weaker. Reported\nhydration energies support this view: the hydration energy of Na+ is approximately –\n365 kJ/mol, while that of I- is around -275 kJ/mol28. This suggests that iodide ions\nexperience a less pronounced hydration effect, and thus a relatively weaker hydration-\ninduced driving force.\nSecond, from the experimental perspective, if ion hydration were the main driving\nforce, the direction of ion migration (and thus the resulting electrical signal) would\nremain the same during both moisture adsorption and desorption, as hydration and\ndehydration occur symmetrically. However, as shown in Fig. 4a, the directions of\ncurrent signals during moisture adsorption and desorption are opposite, which aligns\nwell with our proposed mechanism: humidity fluctuations induce a concentration\ngradient of mobile iodide ions, and this gradient drives their directional migration.\nBased on the above analysis, we conclude that in our device, the hydration energy\nof iodide ions is insufficient to dominate ion transport. Therefore, we focused our\ndiscussion on the role of the humidity-induced ion concentration gradient.\nWe are grateful for your thoughtful suggestion, which has helped us to improve the\nclarity and completeness of our explanation.\nFig. 4a Current profiles when different surfaces of the device are exposed to moisture, and the inset\nshows the schematic diagram of four device states.\n28. Marcus, Y. Thermodynamics of solvation of ions. Part 5.—Gibbs free energy of hydration\nat 298.15 K. Journal of the Chemical Society, Faraday Trans. 87, 2995-2999 (1991).\n[Comment 4] While the proposed mechanism identifies PAMPS and NaI as the key\nfunctional components in the SMEG system, the rationale for using P(AM-co-AMPS)\nas the polymer matrix remains unclear. Although the authors note PAM's hydrophilic\nnature, this alone does not sufficiently justify its incorporation into the copolymer\nstructure. Does PAM actively participate in power generation, or does it just play a role\nas a structural support? In what ways do P(AM-co-AMPS) copolymers have\nadvantages over pure PAMPS?\n[Response] We sincerely thank you for this important question, which prompted us to\nclarify the role of PAM in the system and to provide a more comprehensive rationale\nfor the copolymer design.\n1. Role of PAM in the power generation process:\nWe agree that PAM does not directly participate in the moisture-driven power\ngeneration mechanism. Instead, it mainly serves to provide structural support and\nimprove water retention. Experimental results from the PAM-NaI system show\nnegligible electrical output under humidity changes, confirming that PAM is not an\nactive functional component in the energy conversion process. Additionally, we\nattempted to fabricate pure PAMPS hydrogels and found them to be highly brittle and\ndifficult to handle during preparation (see Supplementary Fig. 3 and Supplementary\nVideo-R). After soaking in the same NaI solution and exposure to ambient air for one\nweek, the pure PAMPS gels exhibited significant shrinkage, indicating limited water\nretention, which may adversely affect ion mobility.\n2. Advantages of P(AM-co-AMPS) over pure PAMPS:\nThe P(AM-co-AMPS) copolymer offers superior physical properties compared to\npure PAMPS. Pure PAMPS hydrogels are mechanically rigid and brittle, making them\nsusceptible to damage during swelling and deswelling cycles induced by humidity\nfluctuations. This can compromise the structural integrity and long-term performance\nof the device. Our performance tests also showed that PAMPS–NaI devices, although\ninitially outputting higher currents, suffered from rapid signal decay and poor cycling\nstability (Fig. R4). In contrast, incorporating flexible AM segments into the copolymer\nimproves chain mobility and mechanical toughness, enabling the formation of a more\nrobust and elastic network. This enhances device durability and supports stable\noperation over time.\nWe have refined the relevant descriptions in the revised manuscript. We greatly\nappreciate your valuable suggestions, which have helped us strengthen the explanation\nof our material design rationale.\nSupplementary Fig. 3 The advantage of incorporating AM into the hydrogel.\nFig. R4 Electrical output performance of the pure PAMPS-NaI device under humidity variations.\n[Comment 5] The coordinates on the right side of Figure 2D seem to be wrong, as\ndesorption and adsorption should be a continuous process in this figure. Did the\nauthors mean to say desorption rate (%)?\n[Response] The original purpose of this figure was to illustrate the hydrogel’s moisture\nresponse behavior under different humidity conditions—namely, moisture adsorption\nat high humidity and desorption at low humidity—thereby demonstrating its excellent\nhumidity responsiveness.\nWe fully agree with your point that “adsorption and desorption should be presented\nas a continuous process.” Initially, our intention was to emphasize the desorption ratio.\nTo more clearly present the hydrogel’s adsorption and desorption capacities under\ndifferent humidity conditions, we recalculated the values based on the mass at the start\nof desorption, which allowed the data to correspond with the left axis for easier\ncomparison. However, after your valuable comments, we recognize that this approach\nwas misleading.\nTo ensure both clarity and scientific accuracy, we have now presented the complete\ncontinuous data without any special recalculations. At the same time, we optimized the\npresentation of Fig. 2d so that readers can intuitively grasp the key information.\nFig. 2d Relative mass change during adsorbing (98% RH) and desorbing (15% RH) of the gel, 3\nmm thick gel.\n[Comment 6] In Figures 3b and c, the current during adsorption is 1.5 to 2 times that\nduring desorption, whereas in other variables studied, the current during adsorption\nand desorption are similar. The authors should explain this.\n[Response] Thank you for your careful review and constructive questions. The main\ndifference between Figs. 3a and 3b-c lies in the thickness of the hydrogel. In addition,\nFig. 3d was revised in response to Comment 10 from Reviewer 2, where we re-\ninvestigated the influence of AMPS content and found that it also leads to differences\nin moisture adsorption and desorption performance. The initial data presented in Fig.\n3d (now completely replaced) did not precondition the device in a 15% RH environment.\nInstead, the test was conducted by directly transferring the device from a relatively\nstable environment (~60% RH, laboratory conditions) into a 98% RH environment,\nwhich led to results showing little difference between adsorption and desorption\nbehavior. Therefore, we re-conducted the experiments under controlled conditions and\nhave provided the updated Fig. 3d accordingly.\nWe greatly appreciate your attention to this phenomenon and have designed a series\nof experiments to investigate it. Our results indicate that the underlying cause is the\ndifference in the moisture adsorption and desorption capacities of the hydrogels,\nwhich arises from varying ratios of iodide ions (I-) to triiodide ions (I -) within the hydrogel\n3\nmatrix. This, in turn, leads to the observed differences in device performance during the\nadsorption and desorption processes. The detailed analysis is given as follows:\nFirstly, based on the experimental results presented in Fig. 2d, we speculate that\nthe performance difference between the adsorption and desorption stages arises from\nthe asymmetric moisture adsorption and desorption capacities of the device.\nSecondly, as shown in Fig. 3e, a comparison of device performance using\nhydrogels with different thicknesses reveals that when the gel thickness is less than 9\nmm, the device tends to exhibit higher output during the adsorption stage and lower\noutput during desorption. In contrast, when the thickness is equal to or greater than 9\nmm, the performance becomes symmetric or even slightly higher during the desorption\nstage. Therefore, we infer that this phenomenon is thickness-dependent, and is likely\ncaused by the influence of hydrogel thickness on moisture transport and ionic dynamics.\nFrom the above analysis, it can be inferred that the thickness of the hydrogel\ninfluences its moisture adsorption and desorption behavior, which is intrinsically\nrelated to the ratio between the quantities of iodide ions (I-) and triiodide ions (I -)\n3\nwithin the gel. Specifically, I- ions are primarily responsible for moisture adsorption\n(e.g., the hygroscopicity of NaI is greater than that of NaF6-8), while I - ions, formed by\n3\nthe coordination between I- and I , possess a larger linear structure with a more diffuse\n2\ncharge distribution, making them less capable of forming stable hydration shells. At a\nfixed NaI concentration, a thicker hydrogel introduces more sulfonic acid groups,\nthereby increasing the availability of protons (H+), which promotes the conversion of I-\ninto I -. As a result, thinner gels contain a higher proportion of I-, leading to stronger\n3\nmoisture adsorption and enhanced device performance during the adsorption stage. As\nthe gel thickness increases, the ratio of I- to I - decreases, giving rise to more balanced\n3\nadsorption/desorption behavior, or even stronger performance during the desorption\nstage. In summary, thin hydrogels tend to show higher performance during moisture\nadsorption, while thicker hydrogels exhibit more balanced or even desorption-\ndominated behavior, owing to the shifting I-/I - ratio.\n1. Moisture adsorption and desorption tests of hydrogels with different\nthicknesses. To investigate the effect of thickness on moisture behavior, hydrogels of\nidentical compositions but different thicknesses (5 mm, 9 mm, and 13 mm) were tested.\nFirst, the hydrogels were placed in a 15% RH humidity chamber for 12 hours to\nsimulate the device working process (ambient conditions: 25 °C, 50% RH). Then, they\nwere transferred to a 98% RH humidity chamber to evaluate their moisture adsorption\ncapacity. The mass of each sample was measured every 10 minutes, with three repeated\nweighings to obtain the average value. After 12 hours of moisture adsorption, the\nhydrogels were again placed in the 15% RH chamber to test their desorption behavior,\nfollowing the same weighing protocol. As shown in Supplementary Fig. 15, thinner\nhydrogels exhibited stronger moisture adsorption than desorption, while thicker\nhydrogels showed the opposite trend, which is consistent with our hypothesis.\nCombined with the data in Fig. 3e, these results confirm that the asymmetry in device\nperformance during adsorption and desorption is primarily attributed to the different\nmoisture handling capacities of hydrogels with varying thicknesses.\n2. The difference in moisture adsorption/desorption capacity originates from\nthe varying I-/I - ratios. To investigate this, P(AM-co-AMPS) hydrogels with\n3\nthicknesses of 3 mm, 9 mm, and 15 mm were soaked in equal volumes of 1 M NaI\nsolution for 4 days to ensure equilibrium between I- and I -. As shown in Supplementary\nwith increasing gel thickness, indicating a higher I - content. Subsequently, these three\n3\nsolutions were used to soak identical 3 mm-thick PAM hydrogels, denoted as PAM ,\n9 15\nrevealed a progressive increase in the intensity of the I - characteristic peaks from PAM\n3 3\nto PAM , suggesting a decreasing I- to I - ratio in the soaking solutions used, and\n15 3\ntherefore in the resulting hydrogels.\n3. The I-/I - ratio affects moisture adsorption/desorption performance. To\n3\nfurther demonstrate that the ratio between I- and I - influences the adsorption and\n3\ndesorption behavior, the electrical performances of PAM , PAM , and PAM were\n3 9 15\nevaluated, as shown in Supplementary Fig. 13b. The results indicate that as the\nproportion of I - increases, the adsorption performance of the samples gradually\n3\napproaches their desorption performance. Moreover, the overall output of the devices\nwere found to be positively correlated with the I - content in the hydrogel. This\n3\nphenomenon likely originates from the enhanced dynamic coordination effect at higher\nI - concentrations, which induces a larger I- concentration gradient and thus leads to\n3\nstronger electrical output.\nThrough the above analysis, experiments, and characterizations, we have\ndemonstrated that the different ratios of I- to I - within the hydrogel lead to variations\n3\nin its moisture adsorption and desorption capacities. This ultimately results in the\nobserved asymmetry in device performance, with higher output during the adsorption\nphase compared to the desorption phase. We sincerely thank you for raising this\nimportant point, which has deepened our understanding of the device’s working\nmechanism.\nFig. 3d The current performance characteristics of SMEG devices with different AMPS contents\n(gel thickness of 3 mm).\nSupplementary Fig. 15 Comparison of moisture adsorption and desorption capacities of NaI-\nbased hydrogels with different thicknesses. a Moisture adsorption of hydrogels after being\nmaintained at 15% RH for 12 h and then transferred to 98% RH. b Moisture desorption of the same\nhydrogels after an additional 10 h at 98% RH, followed by transfer to 15% RH.\nSupplementary Fig. 16 The state of NaI solutions (1 M, equal volumes) after immersing\nhydrogels of different thicknesses for 4 days. It is evident that the thicker the hydrogel, the darker\nthe solution color, indicating a higher concentration of triiodide ions. The inset in the upper left\nshows the appearance of three PAM gels of the same thickness after immersion in the above three\nNaI solutions followed by drying.\nSupplementary Fig. 13 Raman characterization and device performance of PAM , PAM , and\n3 9\nPAM hydrogels. a The Raman spectra indicate that the triiodide content in PAM , PAM , and\n15 3 9\nPAM hydrogels increases sequentially, suggesting a corresponding decrease in the I-/I - ratio. b\n15 3\nDevice performance of the hydrogels, with the adsorption-to-desorption performance ratios of 2.44,\n1.96, and 1.17, respectively.\n6. Miñambres, L., Méndez, E., Sánchez, M. N., Castaño, F. & Basterretxea, F. J. Water uptake\nproperties of internally mixed sodium halide and succinic acid particles. Atmos. Environ.\n45, 5896-5902 (2011).\n7. Albayrak, C., Barim, G. & Dag, O. Effect of hygroscopicity of the metal salt on the\nformation and air stability of lyotropic liquid crystalline mesophases in hydrated saltnsurfactant systems. J. Colloid Interface Sci. 433, 26-33 (2014).\n8. Peng, C., Chen, L. & Tang, M. A database for deliquescence and efflorescence relative\nhumidities of compounds with atmospheric relevance. Fundam. Res. 2, 578-587 (2022).\n[Comment 7] In Line 286-288, the authors mentioned that ‘However, since I - moves\n3\nmuch slower than I-, it is the overall downward movement of I- that dominates the ionic\ncurrent (Fig.4a)’. Figure 4A does not seem to justify this statement. The authors should\nadd more evidence to demonstrate the difference in diffusion rates between I - and I- in\n3\nthe material.\n[Response] We fully agree with your assessment that Figure 4a alone is insufficient to\nconclusively attribute the signal to I-. Consequently, we have provided additional data\nto demonstrate the difference in diffusion rates between ionic species.\nIn general, the diffusion rate of I - is lower than that of I-, primarily because I-\n3\npossesses a simpler structure, a smaller hydrated radius, and experiences less resistance\nduring migration. In contrast, I - is larger in size and its diffusion is more restricted.\napproximately twice that of I -29, further confirming that I- migrates more rapidly than\nthe corresponding description to substantiate this conclusion.\n29. Darrall, K. G. & Oldham, G. The diffusion coefficients of the tri-iodide ion in aqueous\nsolutions. J. Chem. Soc. A 0, 2584-2586 (1968).\n[Comment 8] In Figure 4b, why are the positive and negative signs of the current\nsignals of CaCl and LiCl opposite?\n2\n[Response] Thank you very much for your interest in the polarity difference of\nelectrical signals exhibited by hydrogels introduced with LiCl and CaCl . Initially, we\n2\nwere also skeptical when we observed this phenomenon, but repeated experiments\nconsistently confirmed this result. In our hydrogel system, this difference can likely be\nexplained by the ion-selective migration mechanism under humidity-driven conditions.\nSpecifically, in hydrogels containing LiCl, Li+ ions possess a smaller ionic\nhydration radius (approximately 241 pm) and a relatively weaker hydration energy\n(around -475 kJ/mol). In contrast, in hydrogels containing CaCl , the divalent Ca2+ ions\n2\nexhibit a larger hydration radius (approximately 272 pm) and much stronger hydration\nenergy (approximately -1505 kJ/mol)28. As a result, in the LiCl-based hydrogel, the\nmigration is primarily dominated by Li⁺ ions, whereas in the CaCl -based hydrogel, the\ntend to remain in regions with higher moisture content30. Consequently, the main\nmigrating species in this case becomes Cl- ions.\nTherefore, the polarity reversal of the electrical signal may originate from the\ndifference in dominant migrating ions in the two systems, which is collectively\ninfluenced by ionic hydration radius, hydration energy, and valence state. Similar\nphenomena have also been reported in studies on evaporation-induced electricity\ngeneration, where high-valence ions more readily interact electrostatically with the\nframework, leading to signal reversal31. Although the system in these studies differs\nfrom ours, the underlying mechanism offers valuable insight. It is also worth noting\nthat ion migration behavior may vary further depending on the specific hydrogel\nnetwork.\n28. Marcus, Y. Thermodynamics of solvation of ions. Part 5.—Gibbs free energy of hydration\nat 298.15 K. Journal of the Chemical Society, Faraday Trans. 87, 2995-2999 (1991).\n30. Wang, H. et al. Moisture adsorption-desorption full cycle power generation. Nat. Commun.\n13, 2524 (2022).\n31. Anwar, T. & Tagliabue, G. Salinity-dependent interfacial phenomena toward hydrovoltaic\ndevice optimization. Device 2, 100287 (2024).\n[Comment 9] In Figure 4c, the authors compared different hydrogels with NaI, and\nfound that PAM-NaI generate almost no current. Based on the mechanistic explanation\nin the manuscript, it can be speculated that this is because there is no release of\nhydrogen ions. However, previous literatures (Energy Environ. Sci., 2024, 17, 3788-3796, Nano Energy, 2024, 110590) has demonstrated that PAM also releases and\nmigrates hydrogen ions during moisture adsorption, thereby producing current. The\nauthors should clarify this discrepancy.\n[Response] We sincerely thank you for raising this valuable point and for highlighting\nthe potential discrepancy between our explanation and recent literature (Energy Environ.\nSci., 2024, 17, 3788–3796; Nano Energy, 2024, 110590). We take this issue very\nseriously and would like to further clarify our experimental results and mechanistic\ninterpretation.\nUpon carefully reviewing the two referenced studies, we found that the\nobservation of H⁺ release from PAM-based hydrogels in those works likely originates\nfrom the incorporation of highly hygroscopic salts, such as LiCl. These salts can\nstrongly promote moisture adsorption and trigger water phase transition processes,\nthereby facilitating the partial ionization of water molecules and leading to the release\nof a certain amount of protons.\nIn our experiments, we also observed a slight yellowing of the pure PAM hydrogel\nafter NaI soaking and thermal drying (Supplementary Fig. 21), which suggests the\npossible formation of a small amount of I -. This indirectly indicates that under certain\n3\nconditions, minor water ionization might occur within the PAM matrix. However, in\nterms of device output, the PAM-NaI system exhibits very weak current signals (0.36\nμA during adsorption and -0.15 μA during desorption, Fig. 20). Compared with\nsystems containing sulfonic acid groups or external proton donors, this significantly\nlower output implies a negligible contribution from H+ ions.\nFurthermore, in the study by Duan et al. (Energy Environ. Sci., 2024, 17, 3788-3796), their Figs. 4e-f present a comparison among various metal ions (e.g., Ca2+, Co3+,\nAl3+), where ions with larger hydration radii (such as Al3+) show markedly reduced\nmigration ability, and the corresponding current drops from 16 μA to 4 μA. This\nstrongly supports the view that the device performance is primarily governed by the\nexternally introduced migrating ions, rather than by trace amounts of H+ generated from\nwater ionization.\nTherefore, we acknowledge that PAM may release a small amount of H+ under\nspecific conditions. However, based on our experimental results and comparative\nanalysis, we believe that its influence on iodide ion behavior within the device is\nminimal. In our original manuscript, the expression “almost no current generation” was\nintended to emphasize the relatively weak performance of the system, rather than to\nsuggest the complete absence of current in an absolute sense.\nTo avoid misunderstanding, we have revised and clarified the relevant descriptions\nin the revised manuscript. Once again, we thank you for pointing out this important\nissue, which has helped us further refine our mechanistic explanation.\nSupplementary Fig. 21 The PAM gel remained transparent after 48 h soaking in NaI, while\nthe gel became slightly yellowish after 3h treatment in the drying chamber, indicating that a\nsmall amount of I - had been introduced.\nPDMAPS soaked with NaI. PAM, PMETAC, and PDMAPS gels showed a weak oscillation\nphenomenon, probably due to the introduction of I - into the gels by heat treatment.\nions (Energy Environ. Sci., 2024, 17, 3788–3796).\n[Comment 10] The font of ‘°C’ in Figure 5b seems to be different from other words.\nAnd I suggest that the author use ‘h’ instead of ‘s’ for the horizontal axis.\n[Response] We sincerely thank you for the careful examination of the figure details and\nthe valuable suggestions. The inconsistency in the “°C” font in Fig. 5b was indeed\ncaused by an improper character format setting during image preparation. We have\ncorrected this issue and standardized the font throughout the figure to ensure a\nconsistent and professional appearance.\nAdditionally, we fully agree with your suggestion regarding the time unit on the\nx-axis. To more accurately reflect the experimental timescale, we have replaced “s”\nwith “h”, which better corresponds to the actual cycle duration and the logical\npresentation of the data. The revised figure has been updated accordingly in the revised\nmanuscript (Fig. 5b).\nFig. 5b Current profiles of SMEG under simulated diurnal humidity changes in the test chamber as\nwell as real-time humidity and temperature profiles in the test chamber.\n[Comment 11] The language of the manuscript should be improved. Many prepositions\nare used incorrectly. For example, in line 184-185, ‘… NaI concentration is of 0.01\nM, …’ should be ‘…NaI concentration is 0.01 M…’ and ‘participating in the dynamic\ncoordination’ should be ‘participating in dynamic coordination’.\n[Response] We sincerely thank you for pointing out the grammatical and prepositional\ninaccuracies in our manuscript. Accounting for your comment, we have carefully\nproofread the entire text and improved the language of our manuscript.\nReviewer #4\n[General comment] The authors design and fabricate a sustainable power generator\nSMEG based on the dynamic coordination of I- induced by varying humidity. The device\nshows great potential in small-scale green and sustainable energy access. However,\nthere are several concerns that must be addressed before the manuscript can be\nconsidered for publication:\n[Response] We sincerely thank you for the positive evaluation of our work, particularly\nfor recognizing the potential of our humidity-fluctuation-induced dynamic coordination\nmechanism for sustainable energy generation in small-scale green energy applications.\nWe have carefully analyzed and responded to each of your comments , and the\ncorresponding changes and additions have been made and clearly marked in the revised\nmanuscript.\n[Comment 1] Figure 3d shows that the performance is the best when the AMPS content\nis 2.8g, but it does not explain why excessive AMPS reduces the performance.\n[Response] To further clarify the influence of AMPS content on device performance,\nwe conducted additional experiments and included two new data points with higher\nAMPS concentrations in the revised manuscript (Fig. 3d). The updated results show\nthat the device current does not peak at 2.8 g of AMPS, as previously suggested, but\nrather continues to increase with increasing AMPS content and eventually reaches a\nsaturation point.\nThis trend can be explained by the following mechanism: the sulfonic acid groups\n(–SO H) in AMPS act as proton (H+) donors, which promote the conversion of I- to I -,\n3 3\nthereby enhancing the current output of the device. However, once the majority of I-\nhas been converted and the system approaches an I-/I - equilibrium, further increases in\n3\nformation. As a result, the current output begins to plateau. To support this explanation,\nwe have added a correlation analysis of the relation between the I - concentration and\n3\ncurrent intensity (Supplementary Fig. 13), which clearly demonstrates a positive\ncorrelation.\nFig. 3d The current performance characteristics of SMEG devices with different AMPS contents\n(gel thickness of 3 mm).\nSupplementary Fig. 13 Raman characterization and device performance of PAM , PAM ,\n3 9\nand PAM hydrogels. a The Raman spectra indicate that the triiodide content in PAM , PAM ,\n15 3 9\nand PAM hydrogels increases sequentially, suggesting a corresponding decrease in the I-/I - ratio.\n15 3\nb Device performance of the hydrogels, with the adsorption-to-desorption performance ratios of 2.44,\n1.96, and 1.17, respectively.\n[Comment 2] The Raman intensity maps in Figure 4e do not give the color scale\nannotation.\n[Response] Following your suggestion, we have added the missing color scale label\n(“Intensity (cnts)”) to the Raman intensity map in Fig. 4e, ensuring the completeness\nand readability of the graphical data.\nFig. 4e P(AM-co-AMPS)-NaI gel in different humidity environments with Raman intensity maps\nat 110 cm-1 (I - symmetric telescopic vibration), after being placed in 15% RH humidity chamber\n3\nfor 12 h (e-I), and then put into 98% RH humidity chamber for 30 min (e-II), and then put back into\n15% RH humidity chamber for 30 min (e-III).\n[Comment 3] Explain in detail how moisture leads to the alternating changes in the I-\nconcentration.\n[Response] In view of your foregoing comment, we have provided a more detailed\nexplanation in the manuscript regarding how humidity fluctuations drive the periodic\nvariation of I- concentration. The specific mechanism is as follows:\nAssuming that the device, initially equilibrated under a moderate humidity\ncondition (e.g., 60% RH), is transferred to a higher humidity environment (e.g., 75%\nRH), the top surface of the hydrogel begins to adsorb moisture. This adsorption\npromotes the dissociation of nearby I - into I- and I . The dissociation of I - establishes\n3 2 3\na top-to-bottom I- concentration gradient within the hydrogel, driving the downward\nmigration of I- ions along with associated water molecules, thereby generating an ionic\ncurrent. After a certain period, the system reaches a new equilibrium under the higher\nhumidity.\nSubsequently, when the device is exposed to a lower humidity condition (e.g., 43%\nRH), the hydrogel begins to desorb moisture from its top surface. In this state, I- and I\n2\ntend to re-coordinate to form I -, leading to a decrease in I- concentration at the surface.\nmigration of I- ions and produces an ionic current in the opposite direction.\nWith cyclic fluctuations in environmental humidity, the reversible dissociation and\nrecombination of I - cause alternating I- concentration gradients, which continuously\n3\ndrive the oscillatory migration of I- ions. This mechanism enables the SMEG device to\nsustainably generate alternating current (AC) output, as illustrated in Supplementary\nFig. 1.\nWe have refined the description of this process in the revised manuscript and\nSupplementary Information to enhance its clarity and overall quality.\nSupplementary Fig. 1 Schematic diagram of the AC power generation of SMEG.\n[Comment 4] The principal comparison between moisture power generators and other\nnovel power generation strategies should be added to the introduction.\n[Response] We sincerely thank you for your valuable suggestion. We fully agree that\nincluding a comparison between moisture-electric power generation and other\nemerging energy-harvesting strategies in the Introduction would help to better clarify\nthe research positioning and novelty of our work. In response to your suggestion, we\nhave added a comparison of several representative emerging energy-harvesting\ntechnologies reported in recent years and elaborated on the advantages of moisture-\nelectric generators, such as environmental adaptability and long-term sustainability.\nThese revisions have been incorporated into the Introduction section of the revised\nmanuscript. For your convenience, the modified text is reproduced below:\n“The ubiquitous presence of water in the environment provides a natural reservoir\nfor green energy1. Generating electricity through the electric coupling between\nfunctional materials and various forms of water—such as flow, waves, droplets,\nmoisture and evaporation—a new type of energy conversion effect has emerged, which\nis defined as hydrovoltaic technology2,3,4. Compared with conventional photovoltaic\nand wind power technologies, which rely on intermittent and variable sources such as\nsunlight and wind under specific environmental conditions, hydrovoltaic technology\ndemonstrates significant potential due to the ubiquity of water resources in nature5.\nAmong hydrovoltaic technologies, moisture- and evaporation-driven generators have\nattracted increasing attention for their virtues of ubiquity, spontaneity, and direct current\noutput6. Evaporation-based power generation represents a novel, passive means of\nenergy harvesting that leverages natural processes instead of mechanical motion7.\nHowever, its long-term performance is constrained by the reliance on continuous access\nto liquid water and is susceptible to climatic fluctuations8. Moisture-electric generators\n(MEGs) are destined to exploiting omnipresent power sources due to the abundance,\naccessibility, and all-weather availability of moisture in the ambient environment9, 10, 11,\n12.\nNevertheless, most existing MEGs operate in a unidirectional migration process\nof water and ions13,14,15. When the water and ions migration tend to equilibrate, the\nMEGs will stop outputting electrical energy, reflecting a limited power generation\nduration (minutes to hours)16,17,18. Although numerous strategies have been developed\nto enhance or maintain water and ion concentration gradients—such as constructing\nasymmetric or heterogeneous structures19,20, optimizing electrodes21,22, and engineering\nfunctional materials23. The number of directionally migrating ions introduced into the\nsystem is inherently limited, resulting in a restricted duration of power generation.\nWithout external energy input, it is therefore challenging to achieve long-term and\nsustainable electricity generation.”\n[Comment 5] The moisture power generator reported in this work should be compared\nwith the literature to demonstrate its advantages.\n[Response] Thank you for your valuable suggestion, which has helped us better\nhighlight the advantages of this work. Accounting for this suggestion, we have reviewed\nand summarized recent literature in the relevant field, as presented below:\nSupplementary Table 2. The performance comparison of current moisture electric generators.\nDurability\nOpen Short and output\nOperating\nFunctional Working Area circuit circuit patterns in a Work Decay\nElectrode humidity Ref.\nmaterials principles (cm2) voltage current humidity- time rate\n(%)\n(V) (μA) fluctuations\nenvironment\nDirected ion\nμA/cm2\nRedox\nDirected ion\n>25 ~75.1\nMEL-PCL C-C migration + 6 0.28 1.8 80 No, DC 11\ndays %\nPC\nPVA-AlgNa Directed ion >30\nAl-C 1 1.3 1310 80 Yes, DC ~96% 12\nbased hydrogel migration days\nLiCl@CP- Directed ion >10 ~21.7\nC-C 9 0.78 9.2 60±5 Yes, DC 13\nCB@CP migration days %\nDirected ion\n>100\nZn-C migration + 2.25 1.25 5.67 80 No, DC ~24%\nh\nRedox\naASA 14\nDirected ion\nC-C 2.25 0.21 2.81 80 No, DC None None\nmigration\nDirected ion\n>5\nC-Al migration+ None 1.8 1200 90 No, DC ~33%\nPAM- days\nRedox 15\nLiCl/CMC\nDirected ion\nC-C None ~0.35 ~15 30 No, DC None None\nmigration\nPAM-LiCl/ Directed ion >800 ~39.8\nCFP-Ag 4 1.28 5.9 60 (35℃) No, DC 16\nSiNWs migration h %\nDirected ion\nMXene aerogel ~97.5\nCu-Cu migration + 1 0.6 1200 20 Yes, DC >12 h 17\n/PAM-CaCl2 %\nRedox\nPEDOT:PSS-\nPVA-PAAs- Directed ion\n60 ( ΔT = ~44.2\nbased Cu-Cu migration + 4.9 0.215 ~29.2 No, DC >25 h 18\n3.7 K) %\nanisotropic TE\nhydrogel\nP(VDF- Directed ion\n>30\nTrFE)/PAN/CN C-Al migration + 2 0.85 256 35-45 Yes, DC 96% 19\ndays\nW-LiCl Redox\nSelf-excited\nP(MEDSAH- 15000\nAu- Au chemoelectr 4 0.4 10 100 No, AC 99.9% 20\nco-AA) s\nical reaction\nOscillatory\n>3\nILs Ag/AgCl migration of 0.785 0.15 2 35-81 Yes, AC ~51% 21\ndays\nions\nILs/Clustered Oscillatory\n0.00012 >30 Nearly\npolycation Ag/AgCl migration of 0.0009 22 73 Yes, AC 22\n5 days 0%\nmembrane ions\nNew device\n>14 Nearly\nOscillatory days 0%\nP(AM-co- This\nC-C migration of 1 0.028 33 15-98 Yes, AC After 7 months\nAMPS)/NaI work\nions device\n>30\n39.5%\ndays\nCalculation of decay rate:\n For DC-type devices, the decay rate is calculated as:\nDecay rate =\n𝐼𝑚𝑎𝑥−𝐼𝑚𝑖𝑛×100%\n𝐼𝑚𝑎𝑥\nwhere I and I represent the maximum and minimum current values during the\nmax min\nmeasured period.\n For AC-type devices, the decay rate is calculated as:\n Decay rate =\n𝐼𝑝𝑒𝑎𝑘,1−𝐼𝑝𝑒𝑎𝑘,𝑒𝑛𝑑×100%\n𝐼𝑝𝑒𝑎𝑘,1\nwhere I is the peak current of the first cycle, and I is the peak current of the\npeak,1 peak,end\nlast cycle within the measured period.\nFrom this table, it can be concluded that in most current moisture-electric\ngenerators, the primary mechanism is moisture-induced directional ion migration. In\nsome high-performance devices, redox reactions may also play a role. While the DC\ngeneration mode based on directional ion migration offers the advantage of easy direct\nutilization, it relies on a limited supply of mobile ions, which imposes a limit on its\noperational lifetime and often results in rapid performance degradation.\nIn contrast, the AC generation mode can continuously re-establish ion\nconcentration gradients through external humidity fluctuations. Theoretically speaking,\nthis enables perpetual power generation with a significantly lower degradation rate.\nTherefore, the working mode demonstrated in this study highlights a theoretical\nadvantage in the context of sustainable power generation.\n10. Song, M., Kim, D., Lee, H., Han, H. & Jeon, S. Synergistic effect of a Berlin green framework for highly efficient moisture-electric energy transformation. Energy\nEnviron. Sci. 17, 5421-5428 (2024).\n11. Duan, P. et al. Moisture-based green energy harvesting over 600 hours via\nphotocatalysis-enhanced hydrovoltaic effect. Nat. Commun. 16, 239 (2025).\n12. Yang, S. et al. Green moisture-electric generator based on supramolecular hydrogel\nwith tens of milliamp electricity toward practical applications. Nat. Commun. 15, 3329\n(2024).\n13. Tan, J. et al. Self-sustained electricity generator driven by the compatible integration\nof ambient moisture adsorption and evaporation. Nat. Commun. 13, 3643 (2022).\n14. Zhou, J., Ren, Z., Cui, X., Liu, X. & Lu, X. Bioinspired interfacial design of robust\naramid nanofiber composite films for high‐performance moisture‐electric generators.\nAdv. Energy Mater. 15, 2404840 (2025).\n15. Cao, F. et al. Efficient energy harvesting: High power wearable humidity generators\nwith PAM-LiCl/CMC structure. Nano Energy 134, 110590 (2025).\n16. Duan, W. et al. Silicon nanowire/ionic hydrogel-based hybrid moist-electric generators\nwith enhanced voltage output and operational stability. Energy Environ. Sci. 17, 3788-\n3796 (2024).\n17. Zhao, K. et al. Humidity-tolerant moisture-driven energy generator with mxene\naerogel-organohydrogel bilayer. ACS Nano 17, 5472-5485 (2023).\n18. Zhang, M. et al. Long-term serviceable ionic thermoelectric hydrogel with temperature\nand moisture dual-driven waste energy harvesting capability. Small 21, e2501960\n(2025).\n19. Hu, Y. et al. Phyto-inspired sustainable and high-performance fabric generators via\nmoisture absorption-evaporation cycles. Sci. Adv. 10, eadk4620 (2024).\n20. Long, Y. et al. Moisture-induced autonomous surface potential oscillations for energy\nharvesting. Nat. Commun. 12, 5287 (2021).\n21. Tang, J. et al. Circadian humidity fluctuation induced capillary flow for sustainable\nmobile energy. Nat. Commun. 13, 1291 (2022).\n22. Zhao, Y. et al. Diurnal humidity cycle driven selective ion transport across clustered\npolycation membrane. Nat. Commun. 15, 7161 (2024).\n[Comment 6] A theoretical analysis should be given on whether the dynamic\nmechanism based on I- can be extended to other halogen ions.\n[Response] We sincerely thank you for raising this insightful question. From a\ntheoretical standpoint, the core of our proposed dynamic mechanism lies in the\nreversible coordination reaction between I- and I (I- + I ↔ I -), which is a unique\n2 2 3\nchemical behavior of iodine. Other halide ions (such as Cl-, Br-, and F-) do not exhibit\nsimilar reversible coordination under standard conditions and are generally unable to\nform stable polyhalide anions. As such, they are not capable of supporting a comparable\ndynamic ion migration mechanism. This conclusion is further supported by our\nexperimental results. As shown in Supplementary Fig. 19, only the NaI-treated hydrogel\ndevice exhibited alternating current (AC) output, whereas devices treated with other\nhalide salts did not display such behavior. These findings confirm the fact that the AC\nsignal originates from the unique reversible coordination between I- and I .\nNaBr, NaCl, and NaF at high and low humidity.\n2 “Humidity fluctuations-induced dynamic coordination drives the oscillatory\n3 migration of ions for sustainable energy harvesting”\n6 thank the reviewers for their thoughtful and constructive evaluations throughout this\n7 extended review process. We are grateful for the additional efforts made to obtain\n8 further expert assessment, and we fully understand the importance of addressing the\n9 remaining concerns in order to strengthen the rigor and clarity of our study. We have\n10 carefully considered all points raised in the new review round, including the request for\n11 additional experimental evidence regarding small humidity step responses, further\n12 clarification and discussion of the device’s power limitations, and a more rigorous\n13 examination of the migrating ionic species. In this revision, we have conducted new\n14 experiments, expanded mechanistic analyses, refined key discussions, and substantially\n15 revised both the main text and Supplementary Information to fully address these\n16 concerns. All revised contents are marked in yellow in the revised manuscript.\n18\n21 were satisfactory. However, some critical issues need to be improved.\n22 [Comment 1] Actually, the Raman spectroscopy was only performed under ambient\n23 laboratory conditions (55% RH). Before the Raman spectroscopy test, the sample was\n24 placed in two humidity chambers (98% and 15%). Could the authors provide\n25 continuous and dynamic change data for Raman spectroscopy under changed humidity?\n27 for controlling relative humidity. It is easy to understand that humidity can reach 98%\n28 through natural water evaporation. But how to make the humidity in such a setup reach\n29 15%?\n30 [Response] We sincerely thank you for the constructive comments. We fully understand\n31 the importance of monitoring the dynamic evolution of Raman spectra under\n32 continuously changing humidity conditions. However, performing in-situ Raman\n33 spectroscopy while dynamically controlling the humidity is technically challenging and\n34 may potentially affect the stability of the Raman instrument. To obtain equivalent\n35 information reflecting the spectral evolution under humidity variation, we adopted an\n36 alternative time-dependent measurement strategy.\n39 the surrounding humidity changed instantaneously. Although the ambient humidity\n40 during the test remained constant, the gel sample itself was undergoing a continuous\n41 moisture desorption (from 98% RH to 55% RH) or moisture adsorption (from 15% RH\n42 to 55% RH) process. Therefore, by extending the overall spectral acquisition time,\n43 we could effectively capture the dynamic spectral evolution associated with this humidity-\n44 driven transition.\n46 extended the acquisition time per point from 1 s to 4 s, and disabled SWIFT mode. As\n47 a result, each Raman spectrum was recorded over approximately 20 min. For\n48 comparison, the spectra shown in Fig. 4e were acquired with a shorter duration of about\n49 2 min to represent the state of triiodide ions as close as possible to their equilibrium\n50 condition inside the humidity chambers. The results obtained after transferring the\n51 sample from a 15% RH humidity chamber to the laboratory ambient environment for\n52 testing are shown in Supplementary Fig. 23a. These data reveal a gradual decrease in\n53 triiodide concentration from its initially high level, providing strong evidence that\n54 triiodide dissociates into elemental iodine and iodide ions during the moisture-\n55 absorption process. Conversely, after placing the sample in a 98% RH humidity\n56 chamber for 30 min and then transferring it back to the laboratory ambient environment\n57 for Raman characterization, the results shown in Supplementary Fig. 23b demonstrate\n58 that triiodide is regenerated when the hydrogel undergoes moisture desorption.\n59\n60\n62 at 110 cm-1 (I - symmetric telescopic vibration). a, after being placed in 15% RH humidity chamber\n3\n63 for 12 h (e-I), and then put into 98% RH humidity chamber for 30 min (e-II), and then put back into\n64 15% RH humidity chamber for 30 min (e-III).\n65\n67 intensity maps at 110 cm-¹ (I₃- symmetric stretching vibration). a Transfer from a 15% RH\n68 humidity chamber to the testing environment (~55% RH). b Transfer from a 98% RH humidity\n69 chamber to the testing environment (~55% RH).\n70\n72 humidity in the chambers was not controlled by natural water evaporation alone. Instead,\n73 we employed saturated salt solutions, which establish fixed equilibrium vapor pressures\n74 corresponding to specific relative humidities at a given temperature. This approach\n75 follows the well-established saturated salt solution method1,2. In our experiments, the\n76 low-humidity condition (15%) was achieved using a saturated LiCl solution, which\n77 strongly binds water molecules and thus significantly lowers the equilibrium vapor\n78 pressure.\n80 the relevant literature to facilitate the reader’s understanding of this method in the\n81 revised manuscript (Section Methods).\n82 1. Young, J. F. Humidity control in the laboratory using salt solutions—a review. J. Appl. Chem.\n83 17, 241-245 (1967).\n84 2. Wexler A., Hasegawa S. Relative humidity-temperature relationships of some saturated salt\n85 solutions in the temperature range 0° to 50°C. J. Res. Natl. Bur. Stand. 53, 19-26 (1954).\n86\n87 [Comment 2] For MD simulation, the authors did not give the geometric model in the\n89 solvent shell? On the other hand, the authors are encouraged to simulate the dynamic\n90 iodide ion dissociation process during the entire humidity change process.\n91 [Response] Thank you for pointing out the missing information in the Methods section.\n94 molecules, while no water molecules were included in the anhydrous model. To provide\n95 a clearer understanding of the dissociation behavior of the triiodide ion and its\n96 interaction with surrounding water molecules, we have added an illustration of the I₃⁻\n97 dissociation pathway (see Supplementary Fig. 24). Finally, we have updated the\n98 relevant details of the computational methods in the revised manuscript to present the\n99 simulation information more accurately, clearly, and comprehensively.\n101 dissociation process during the entire humidity change process,” we fully agree that\n102 such a simulation could, in principle, provide deeper mechanistic insight. We initially\n103 evaluated this possibility; however, conventional classical MD methods are inherently\n104 incapable of describing bond formation and cleavage, and therefore cannot capture the\n105 dissociation process directly. Although ab initio molecular dynamics (AIMD) could\n106 theoretically address this issue, several fundamental and practical challenges arise when\n107 attempting to simulate iodide ion dissociation and coordination under dynamically\n108 changing humidity. These include: (i) the need for a quantum-mechanical treatment to\n109 accurately describe bond breaking/forming and charge redistribution; (ii) the\n110 requirement of an explicit and sufficiently large solvent representation; and (iii) the\n111 necessity of extensive time-scale sampling to capture humidity-dependent dynamics.\n113 resource demands, making such simulations infeasible under current resources.\n115 between scientific rigor and computational feasibility. Two representative equilibrium\n116 states—an anhydrous environment and a water-rich environment—were selected to\n117 capture the essential influence of humidity on the iodide ion dissociation behavior and\n118 structural features. This approach provides reliable and reproducible quantitative\n119 insights while maintaining consistency with the experimental observations. Moreover,\n120 the time-dependent Raman spectroscopy results further complement our simulations by\n121 revealing the dynamic aspects of the dissociation process experimentally, thereby\n122 strengthening the overall mechanistic understanding.\n123\n3\n125 environment.\n126\n127 [Comment 3] In Supplementary Fig. 14, why is the voltage of the device so low (<30\n128 mV)? Except from short circuit current and open circuit voltage, the authors are also\n129 encouraged to provide other parameters, such as internal resistance and power density.\n131 [Response] We sincerely thank you for raising this important question regarding the\n132 low voltage (<30 mV) observed in Supplementary Fig. 14 and for encouraging us to\n133 provide additional device parameters. Following your suggestion, we conducted\n134 supplementary measurements of the 9-mm-thick device’s internal resistance, which\n135 helps clarify the origin of the low-voltage output. Generally, in moisture-enabled\n136 generators, the output voltage is typically governed by the degree of ionic separation\n137 between the two electrodes3. To investigate the device’s internal resistance, the current-\n138 voltage (I-V) characteristics of the device were measured. Accordingly, the\n139 measurement results are displayed in Supplementary Fig. 15a. It can be concluded that\n140 the internal resistance of our device was determined to be approximately 183 Ω at 98%\n142 resistance. In contrast, most reported moisture-electric generators possess internal\n143 resistances within the range of a few kΩ to several MΩ4-6. This significant difference\n144 suggests that the low internal resistance of our device may be the cause of its limited\n145 voltage output. This finding also offers a clear direction for future improvement:\n146 increasing the device resistance through tailored material or structural design may\n147 significantly enhance the voltage. On the other hand, the relatively low voltage may\n148 also stem from the internal ionic distribution prior to humidity fluctuations. At steady\n149 state, iodide ions are uniformly distributed. Although humidity fluctuations induce\n150 dynamic coordination and regenerate a concentration gradient, the pre-existing\n151 homogeneous distribution may reduce the effective ion separation, thereby limiting the\n152 attainable voltage.\n154 conditions using external loads ranging from 100 Ω to 10⁵ Ω. The load power density\n155 was calculated as P = I2R/A, where I is the load current, R is the load resistance, and A\n156 is the active device area. As shown in Supplementary Fig. 15b, the load power density\n157 achieved was 0.049 μW/cm² at a load of 200 Ω, consistent with the low internal\n158 resistance of the SMEG. Although many recent studies reported high power outputs,\n159 most such devices inevitably include contributions from metal–air battery reactions at\n160 the electrodes7,8. By contrast, our SMEG relies solely on ion migration induced by the\n161 ionic concentration gradient without electrode redox involvement. As shown in\n163 relatively low (we calculated the maximum power density (V × J ), as some\nOC SC\n164 literature did not provide load power density data), it still stands among the top tier in\n165 the field of pure hydrovoltaic devices utilizing inert electrodes—systems that operate\n166 without contributions from electrode redox reactions to the device performance.\n168 primary purpose is to demonstrate a new long-lifetime electricity-generation\n169 mechanism. Substantial improvements are expected in future iterations. For example,\n170 several previously validated strategies—such as material/structural optimization and\n171 electrode/interface engineering—could be incorporated to further enhance the output\n172 performance9.\n173\n175 current-voltage (I-V) characteristics of the SMEG under various relative humidity (RH) levels. b\n177 from100 Ω to 105 Ω. The power density was calculated as P = I2R/A, where I is the load current, R\n178 is the load resistance, and A is the active device area.\n179\nShort\nPower Minimum\nOpen circuit\ndensity Operating humidity\nFunctional Elec- Working circuit current Work Decay\nPM = humidity change Ref.\nmaterials trode principles voltage density time rate\nVOC×JSC (%) response\nVOC (V) JSC\n(μW/cm2) (%)\n(μA/cm2)\nPVA-AlgNa\nDirected ion\nbased Al-C 1.3 1310 1703 80 10 >720 h ~96% 10\nmigration\nhydrogel\nAu@sta Directed ion\nBGC-NC inless- migration + 1.17 2770 3240.9 90 15 10 h ~95% 11\nsteel Redox\nDirected ion\n>100 h ~24%\nZn-C migration + 1.25 2.52 3.15 80 10\n(V) (V)\nRedox\naASA 12\nDirected ion\nC-C 0.21 1.25 0.2625 80 None None None\nmigration\nDirected ion\n>120 h ~33%\nC-Al migration+ 1.8 1200μA 113 90 20\nPAM- (V) (V)\nRedox 13\nLiCl/CMC\nDirected ion\nC-C ~0.35 ~15μA None 30 None None None\nmigration\nPAM-LiCl/ Directed ion >800 h ~39.8%\nCFP-Ag 1.28 1.475 1.888 60 (35℃) 10 14\nSiNWs migration （V） (V)\nMXene Directed ion\naerogel/PAM- Cu-Cu migration + 0.6 1200 720 20 15 >12 h ~97.5% 15\nCaCl2 Redox\nPEDOT:PSS-\nPVA-PAAs- Directed ion\n60 ( ΔT = >26 h ~44.2%\nbased Cu-Cu migration + 0.215 5.96 1.2814 None 16\n3.7 K) （V） （V）\nanisotropic TE\nhydrogel\nP(VDF- Directed ion\nTrFE)/PAN/C C-Al migration + 0.85 128 108.8 35-45 20 >720 h 96% 17\nNW-LiCl Redox\nOscillatory\nNearly\nILs/Clustered migration of\nAg/ 0%\npolycation ions driven 0.00025 10615.7 ~2.65 73 9 >672 h 5\nAgCl (record 8\nmembrane by humidity\ntimes)\nfluctuations\nOscillatory\nmigration of\nAg/\nILs ions driven 0.15 2.55 0.3825 35-81 20 >72 h ~51% 18\nAgCl\nby humidity\nfluctuations\nOscillatory\nmigration of\nP (DMAPS- Nearly\nC-C ions driven 0.11 0.55 0.06 15-93 33 >33h 19\nco-AM)-LiCl 0%\nby humidity\nfluctuations\nLiCl@CP- Directed ion\nC-C 0.78 0.83 0.65 60±5 10 >240 h ~21.7% 20\nCB@CP migration\nSelf-excited\nP(MEDSAH-\nAu-Au chemoelectri 0.4 1.5 0.6 100 None 15000 s 99.9% 21\nco-AA)\ncal reaction\nDirected ion\nMEL-PCL C-C migration + 0.28 1.8 0.504 98 25 >600 h ~75.1% 3\nPC\nDirected ion\nPSSA-PDDA C-C 0.95 0.05 0.0475 25 10 >150h 96% 22\nmigration\n(PSSA-\nDirected ion\nPVA)/(HPC- C-C 1.1 0.0089 0.00979 20 20 >11.7h 60% 23\nmigration\nKGM)\nNew device\nNearly\n0%\n>336 h (Con-\nOscillatory\ntinuous\nmigration of\nP(AM-co- record) This\nC-C ions driven 0.028 33 0.924 15-98 2\nAMPS)/NaI After 7 months work\nby dynamic\ndevice\ncoordination\n39.5%\n(record\n>720 h\n30\ntimes)\n183 maximum power density (V × J ) in this table.\nOC SC\n184\n185\n186 3. Duan, P. et al. Moisture-based green energy harvesting over 600 hours via photocatalysis-\n187 enhanced hydrovoltaic effect. Nat. Commun. 16, 239 (2025).\n188 4. Wang, H. et al. Moisture adsorption-desorption full cycle power generation. Nat. Commun. 13,\n189 2524 (2022).\n190 5. Zhao, Y. et al. Diurnal humidity cycle driven selective ion transport across clustered polycation\n191 membrane. Nat. Commun. 15, 7161 (2024).\n192 6. Lu, X. et al. Hierarchically porous fiber-based nanofluidic diode as an efficient multimode\n193 hygroelectric generator. Adv. Energy Mater. 12, 2202634 (2022).\n194 7. Tan, J. et al. Harvesting energy from atmospheric water: grand challenges in continuous\n195 electricity generation. Adv. Mater. 36, e2211165 (2024).\n196 8. Xu X, Yang X, Zhang Z, et al. Identification of metal–air batteries from water energy harvesters.\n198 9. Zang, S. et al. Moisture power generation: from material selection to device structure\n199 optimization. ACS Nano 18, 19912-19930 (2024).\n200 10. Yang, S. et al. Green moisture-electric generator based on supramolecular hydrogel with tens\n201 of milliamp electricity toward practical applications. Nat. Commun. 15, 3329 (2024).\n202 11. Song, M., Kim, D., Lee, H., Han, H. & Jeon, S. Synergistic effect of a Berlin green framework\n203 for highly efficient moisture-electric energy transformation. Energy Environ. Sci. 17, 5421-\n204 5428 (2024).\n205 12. Zhou, J., Ren, Z., Cui, X., Liu, X. & Lu, X. Bioinspired interfacial design of robust aramid\n206 nanofiber composite films for high‐performance moisture‐electric generators. Adv. Energy\n208 13. Cao, F. et al. Efficient energy harvesting: High power wearable humidity generators with PAM-\n210 14. Duan, W. et al. Silicon nanowire/ionic hydrogel-based hybrid moist-electric generators with\n211 enhanced voltage output and operational stability. Energy Environ. Sci. 17, 3788-3796 (2024).\n212 15. Zhao, K. et al. Humidity-tolerant moisture-driven energy generator with mxene aerogel-\n213 organohydrogel bilayer. ACS Nano 17, 5472-5485 (2023).\n214 16. Zhang, M. et al. Long-term serviceable ionic thermoelectric hydrogel with temperature and\n215 moisture dual-driven waste energy harvesting capability. Small 21, e2501960 (2025).\n216 17. Hu, Y. et al. Phyto-inspired sustainable and high-performance fabric generators via moisture\n217 absorption-evaporation cycles. Sci. Adv. 10, eadk4620 (2024).\n218 18. Tang, J. et al. Circadian humidity fluctuation induced capillary flow for sustainable mobile\n219 energy. Nat. Commun. 13, 1291 (2022).\n220 19. Fu, C. et al. Moisture adsorption-desorption induced ion reciprocating diffusion in zwitterionic\n221 polymer for alternating-cycle power generation Adv. Funct. Mater. e11590 (2025).\n222 20. Tan, J. et al. Self-sustained electricity generator driven by the compatible integration of\n223 ambient moisture adsorption and evaporation. Nat. Commun. 13, 3643 (2022).\n224 21. Long, Y. et al. Moisture-induced autonomous surface potential oscillations for energy\n225 harvesting. Nat. Commun. 12, 5287 (2021).\n226 22. Wang, H. et al. Bilayer of polyelectrolyte films for spontaneous power generation in air up to\n227 an integrated 1,000 V output. Nat. Nanotechnol. 16, 811-819 (2021).\n228 23. Zhao, K. et al. Moisture-driven energy generation by vertically structured polymer aerogel on\n229 water-collecting gel. Nano Energy 126, 109645 (2024).\n230\n231 [Comment 4] Under natural conditions, achieving extreme humidity levels of 15% or\n232 98% is difficult. The author gives the performance of humidity between 30% and 43%,\n233 as well as 84% and 98%. This range is more representative of realistic conditions. How\n234 much does the current change for every 5% increase in humidity? Is it quantifiable?\n236 [Response] We thank you for highlighting the importance of quantifying the device\n237 response to small humidity fluctuations. To address this, we performed additional\n238 measurements using a programmable humidity chamber, since the humidity bottles\n239 used previously cannot produce the fine RH increments required. The chamber was set\n240 to cover a broad humidity range (35-95% RH) at 25 °C with a target step of 5% RH to\n241 probe the current response as humidity increased.\n243 limited precision of the humidity chamber—particularly during the humidity decrease\n244 phase—the actual RH increments could not be held exactly at 5% in every step. The\n245 quantified results are summarized in Supplementary Table 3. At 25 °C the mean\n246 measured humidity increment was 5.71 ± 0.64%, and the corresponding mean current\n247 increase was 3.9 ± 0.69 µA per step. During the moisture-desorption process the\n248 chamber did not reliably produce a comparable stepwise RH decrease under the same\n249 settings; consequently, the current changes during desorption did not show a clear\n250 staircase pattern and could not be robustly quantified.\n252 conclude that the device can respond to humidity changes of approximately ~2% RH,\n253 provided a stable humidity controller is available. Therefore, we retested the device’s\n254 response to a 2% RH incremental change, as shown in Supplementary Fig. 26b. Due to\n255 the precision limitations of the humidity chamber, the actual humidity increment\n256 approached 2% only when operating above 60% RH. Consequently, we selected 12 data\n257 points above 60% RH to calculate the mean and standard deviation of the response. The\n258 results show that for a humidity increment of 2.34 ± 0.22%, the corresponding current\n259 increment was 1.14 ± 0.14 μA.\n261 to the revised manuscript to clarify the device responsiveness to small humidity\n262 fluctuations.\n263\n264\n266 provided by a constant temperature and humidity chamber (DHTHM-27-80-P-SD). a Current\n267 performance response of the device under a 5% RH change. During the humidity decrease phase,\n268 the humidity chamber cannot maintain a stable humidity gradient, thus limiting the quantitative\n269 analysis. b Current performance response of the device under a 2% RH change. Due to limitations\n270 in equipment precision, the chamber cannot consistently provide the specified humidity change.\n271\n272\n274 change under 5% RH change settings.\nSet RH (%) 40 45 50 55 60 65 70 75 80 85 90 95\nActual\nHumidity\n6.4 5.1 6.3 6.4 6.5 6.4 5.6 5.6 5.1 4.9 5.1 5.1\nIncrement\n(%)\nCurrent\nIncrement 3.80 4.50 5.50 4.20 4.21 4.24 3.82 3.78 3.46 3.08 3.10 3.20\n(μA)\n275\n276\n277\n278\n279\n280\n281\n283 change under 2% RH change settings.\nSet RH (%) Actual Humidity Increment (%) Current Increment (μA)\n45 3.0 1.80\n47 3.6 2.90\n49 3.2 2.10\n51 3.8 2.46\n53 3.2 2.00\n55 3.4 2.01\n57 3.1 1.74\n59 3.1 1.73\n61 2.4 1.36\n63 2.1 1.39\n65 2.6 1.30\n67 2.4 1.18\n71 2.8 1.25\n73 2.3 1.02\n75 2.4 1.05\n77 2.0 0.98\n79 2.4 1.09\n81 2.2 1.01\n83 2.3 1.08\n85 2.2 0.99\n284\n285\n286\n287\n288\n289 [Comment 5] The authors did not adequately summarize recent advances in moisture-\n290 enabled electricity generation. In the Introduction part, they simply noted the challenge\n291 of achieving long-term sustainable power generation from moisture, attributing the\n292 limitation primarily to the inherently finite number of directionally migrating ions.\n294 capable of continuous operation for up to 200 days with outstanding performance (Adv.\n296 highlight the significance and value of incorporating iodine into the moisture-based\n297 device. The authors evaluated the store stability (7 months). Actually, this is not the\n298 stability of continuous operation.\n299 [Response] Thank you for your careful evaluation and valuable suggestions. We have\n300 now thoroughly summarized recent advances in moisture-enabled electricity generation\n301 and revised the relevant parts of the Introduction to better highlight the significance and\n302 value of introducing iodine into the device. The main revisions are as follows:\n303 “However, the unidirectional migration of ions typically leads to their\n304 accumulation on one side of the device, where the resulting back electric field\n305 suppresses further ion transport, causing the electrical output to decay. Some studies\n306 have attempted to introduce redox reactions to consume the accumulated ions at the\n307 electrode, thereby mitigating the back electric field and enabling long-term power\n308 generation. Nevertheless, redox reactions at the electrodes should be avoided as much\n309 as possible in pure hydrovoltaic devices. In our previous work, we introduced\n310 photocatalysis to consume the accumulated protons and restore the concentration\n311 gradient, achieving 600 hours of electricity generation. However, this approach requires\n312 external energy input and suffers from limited recovery after multiple cycles. Therefore,\n313 addressing the lifetime issue requires new strategies that can break the limitation of\n314 unidirectional ion transport and enable back-and-forth ion motion.”\n316 hydrovoltaic devices, where no electrode redox reactions occur and the electricity is\n317 generated solely through humidity-driven ion migration. Regarding the study you\n318 mentioned (Adv. Funct. Mater. 202406901), we acknowledge its excellent performance\n319 and its ability to operate continuously for 200 days. This work indeed demonstrates an\n320 ingenious device design that maintains an almost permanent humidity gradient, which\n321 is commendable. However, the study does not exclude the possible contribution of\n322 electrode redox reactions, and thus a metal–air battery effect may still contribute to both\n323 the device lifetime and its electrical output8. Since such systems may involve a\n324 combination of metal–air battery mechanisms and moisture-enabled electricity\n325 generation, we did not highlight this type of work in the Introduction.\n327 months). Actually, this is not the stability of continuous operation,” we apologize for\n328 the ambiguity in our previous description. We have revised the wording to ensure\n329 precise expression. The revised description is as follows:\n330 “Furthermore, even after being stored under laboratory conditions for seven\n331 months (30–65% RH, 25 ± 3 °C), the device was still able to operate continuously for\n332 one month (Supplementary Fig. 2), demonstrating its outstanding long-term storage\n333 stability and sustained performance exceeding one month.”\n334\n335 8. Xu X, Yang X, Zhang Z, et al. Identification of metal–air batteries from water energy\n336 harvesters. Droplet 2, e80 (2023).\n337\n338 [Comment 6] Table 2 does not demonstrate an obvious performance advantage for the\n339 proposed materials. AC output mode is not an important advantage. Is the introduction\n340 of iodine universally applicable to enhancing the performance of other types of\n341 moisture-based devices? The author should summarize the real and comparable\n342 advantages of the proposed material through comparison in Table 2.\n343 [Response] We sincerely appreciate your careful review of Supplementary Table 2 and\n344 your constructive comments.\n346 coordination of iodine ions can be regulated by humidity fluctuations. In principle, this\n347 enables perpetual reconstruction of iodide ion concentrations and reciprocal ion\n348 migration driven by natural humidity cycles, without any external energy input. During\n349 daily humidity cycles, this dynamic coordination-driven reversible ion transport\n350 induces alternating ion current (AC) output. In essence, the AC output mode is merely\n351 an external manifestation of reversible ion migration. In the original table, AC was\n352 included only for descriptive and classification convenience. We have now removed all\n354 and optimized. Thank you again for pointing out this issue.\n356 applicable to enhancing the performance of other types of moisture-based devices?” In\n357 the field of moisture-enabled power generation without electrode redox reactions, the\n358 fundamental mechanism generally relies on humidity gradients or ion concentration\n359 gradients driving directional ion transport7,24-26. In this context, “other types of\n360 moisture-based devices” should refer to devices based on different material systems or\n361 device architectures. Current materials used for moisture-enabled generators include\n362 inorganic carbon materials, metal oxides, polymers, biomaterials, MOFs/COFs, and\n363 various composites26,27. Device architectures include planar structures, sandwich\n364 structures, and heterostructures25. In our present study, we investigated hydrogel-based\n365 polymer systems and sandwich-structured devices and found that the mechanism\n366 exhibits good generality among hydrogels with different charged backbones. Based on\n367 our current understanding, to allow humidity-driven dynamic coordination to function\n368 in other devices, the device structure should ensure that the direction of humidity-\n369 induced transport aligns with the ion transport direction, and the material system must\n370 provide efficient ion-conduction pathways and compatibility with iodide salts.\n372 structured or heterostructured devices based on composite or polymer materials.\n374 work.\n376 comparable advantages of our device. As shown in the updated table, although some\n377 reported devices exhibit long lifetimes and high performance, their use of active metal\n378 electrodes suggests the possible involvement of electrode redox reactions. In addition,\n379 they typically require strictly controlled, constant humidity environments, which limits\n380 their applicability under real-world conditions, and many exhibit noticeable\n381 performance decay. Since these devices rely on water-content or ion-concentration\n382 gradients, they inevitably face the gradual reduction of driving gradients, leading to\n383 performance decay and eventual failure—an inherent limitation for long-term stability.\n385 reconstruction of ion-concentration gradients exhibits clear and meaningful advantages,\n386 including a broad operational humidity range, sensitivity to small humidity variations,\n387 a markedly reduced performance-decay rate, and stable long-term power generation. In\n388 terms of output performance, although the power density is not high, our device remains\n389 within the leading group of pure hydrovoltaic devices employing inert electrodes,\n390 where no electrode redox reactions are involved.\n391\nShort\nPower Minimum\nOpen circuit\ndensity Operating humidity\nFunctional Elec- Working circuit current Work Decay\nPM = humidity change Ref.\nmaterials trode principles voltage density time rate\nVOC×JSC (%) response\nVOC (V) JSC\n(μW/cm2) (%)\n(μA/cm2)\nPVA-AlgNa\nDirected ion\nbased Al-C 1.3 1310 1703 80 10 >720 h ~96% 10\nmigration\nhydrogel\nAu@sta Directed ion\nBGC-NC inless- migration + 1.17 2770 3240.9 90 15 10 h ~95% 11\nsteel Redox\nDirected ion\n>100 h ~24%\nZn-C migration + 1.25 2.52 3.15 80 10\n(V) (V)\nRedox\naASA 12\nDirected ion\nC-C 0.21 1.25 0.2625 80 None None None\nmigration\nDirected ion\n>120 h ~33%\nC-Al migration+ 1.8 1200μA 113 90 20\nPAM- (V) (V)\nRedox 13\nLiCl/CMC\nDirected ion\nC-C ~0.35 ~15μA None 30 None None None\nmigration\nPAM-LiCl/ Directed ion >800 h ~39.8%\nCFP-Ag 1.28 1.475 1.888 60 (35℃) 10 14\nSiNWs migration （V） (V)\nMXene Directed ion\naerogel/PAM- Cu-Cu migration + 0.6 1200 720 20 15 >12 h ~97.5% 15\nCaCl2 Redox\nPEDOT:PSS-\nPVA-PAAs- Directed ion\n60 ( ΔT = >26 h ~44.2%\nbased Cu-Cu migration + 0.215 5.96 1.2814 None 16\n3.7 K) （V） （V）\nanisotropic TE\nhydrogel\nP(VDF- Directed ion\nTrFE)/PAN/C C-Al migration + 0.85 128 108.8 35-45 20 >720 h 96% 17\nNW-LiCl Redox\nOscillatory\nNearly\nILs/Clustered migration of\nAg/ 0%\npolycation ions driven 0.00025 10615.7 ~2.65 73 9 >672 h 5\nAgCl (record 8\nmembrane by humidity\ntimes)\nfluctuations\nILs Ag/ Oscillatory 0.15 2.55 0.3825 35-81 20 >72 h ~51% 18\nAgCl migration of\nions driven\nby humidity\nfluctuations\nOscillatory\nmigration of\nP (DMAPS- Nearly\nC-C ions driven 0.11 0.55 0.06 15-93 33 >33h 19\nco-AM)-LiCl 0%\nby humidity\nfluctuations\nLiCl@CP- Directed ion\nC-C 0.78 0.83 0.65 60±5 10 >240 h ~21.7% 20\nCB@CP migration\nSelf-excited\nP(MEDSAH-\nAu-Au chemoelectri 0.4 1.5 0.6 100 None 15000 s 99.9% 21\nco-AA)\ncal reaction\nDirected ion\nMEL-PCL C-C migration + 0.28 1.8 0.504 98 25 >600 h ~75.1% 3\nPC\nDirected ion\nPSSA-PDDA C-C 0.95 0.05 0.0475 25 10 >150h 96% 22\nmigration\n(PSSA-\nDirected ion\nPVA)/(HPC- C-C 1.1 0.0089 0.00979 20 20 >11.7h 60% 23\nmigration\nKGM)\nNew device\nNearly\n0%\n>336 h (Con-\nOscillatory\ntinuous\nmigration of\nP(AM-co- record) This\nC-C ions driven 0.028 33 0.924 15-98 2\nAMPS)/NaI After 7 months work\nby dynamic\ndevice\ncoordination\n39.5%\n(record\n>720 h\n30\ntimes)\n395 maximum power density (V × J )in this table.\nOC SC\n396\n397 3. Duan, P. et al. Moisture-based green energy harvesting over 600 hours via photocatalysis-\n398 enhanced hydrovoltaic effect. Nat. Commun. 16, 239 (2025).\n399 5. Zhao, Y. et al. Diurnal humidity cycle driven selective ion transport across clustered polycation\n400 membrane. Nat. Commun. 15, 7161 (2024).\n401 7. Tan, J. et al. Harvesting energy from atmospheric water: grand challenges in continuous\n402 electricity generation. Adv. Mater. 36, e2211165 (2024).\n403 8. Xu X, Yang X, Zhang Z, et al. Identification of metal–air batteries from water energy harvesters.\n405 9. Zang, S. et al. Moisture power generation: from material selection to device structure\n406 optimization. ACS Nano, 18, 19912-19930 (2024).\n407 10. Yang, S. et al. Green moisture-electric generator based on supramolecular hydrogel with tens\n408 of milliamp electricity toward practical applications. Nat. Commun. 15, 3329 (2024).\n409 11. Song, M., Kim, D., Lee, H., Han, H. & Jeon, S. Synergistic effect of a Berlin green framework\n410 for highly efficient moisture-electric energy transformation. Energy Environ. Sci. 17, 5421-\n411 5428 (2024).\n412 12. Zhou, J., Ren, Z., Cui, X., Liu, X. & Lu, X. Bioinspired interfacial design of robust aramid\n413 nanofiber composite films for high‐performance moisture‐electric generators. Adv. Energy\n415 13. Cao, F. et al. Efficient energy harvesting: High power wearable humidity generators with PAM-\n417 14. Duan, W. et al. Silicon nanowire/ionic hydrogel-based hybrid moist-electric generators with\n418 enhanced voltage output and operational stability. Energy Environ. Sci. 17, 3788-3796 (2024).\n419 15. Zhao, K. et al. Humidity-tolerant moisture-driven energy generator with mxene aerogel-\n420 organohydrogel bilayer. ACS Nano 17, 5472-5485 (2023).\n421 16. Zhang, M. et al. Long-term serviceable ionic thermoelectric hydrogel with temperature and\n422 moisture dual-driven waste energy harvesting capability. Small 21, e2501960 (2025).\n423 17. Hu, Y. et al. Phyto-inspired sustainable and high-performance fabric generators via moisture\n424 absorption-evaporation cycles. Sci. Adv. 10, eadk4620 (2024).\n425 18. Tang, J. et al. Circadian humidity fluctuation induced capillary flow for sustainable mobile\n426 energy. Nat. Commun. 13, 1291 (2022).\n427 19. Fu, C. et al. Moisture adsorption-desorption induced ion reciprocating diffusion in zwitterionic\n428 polymer for alternating-cycle power generation Adv. Funct. Mater. e11590 (2025).\n429 20. Tan, J. et al. Self-sustained electricity generator driven by the compatible integration of\n430 ambient moisture adsorption and evaporation. Nat. Commun. 13, 3643 (2022).\n431 21. Long, Y. et al. Moisture-induced autonomous surface potential oscillations for energy\n432 harvesting. Nat. Commun. 12, 5287 (2021).\n433 22. Wang, H. et al. Bilayer of polyelectrolyte films for spontaneous power generation in air up to\n434 an integrated 1,000 V output. Nat. Nanotechnol. 16, 811-819 (2021).\n435 23. Zhao, K. et al. Moisture-driven energy generation by vertically structured polymer aerogel on\n436 water-collecting gel. Nano Energy 126, 109645 (2024).\n437 24. Fu, C. et al. Characterization methods on moisture-enabled power generator: mechanism,\n438 parameters and applications. J. Phys. D: Appl. Phys. 57, 373003 (2024).\n439 25. Wang, X. et al. Hydrovoltaic technology: from mechanism to applications. Chem. Soc. Rev. 51,\n440 4902-4927 (2022).\n441 26. Shao, B. et al. Electricity generation from phase transitions between liquid and gaseous water.\n443 27 Wang, X. et al. Advances in harvesting water and energy from ubiquitous atmospheric moisture.\n445\n446\n447\n448 [Comment 7] The overall English writing requires improvement. The conclusion, in\n449 particular, is convoluted and fails to provide a succinct summary of the key findings,\n450 their underlying principles, and the broader significance of the work.\n451 [Response] We sincerely thank you for the constructive comment regarding the clarity\n452 and organization of the manuscript, particularly the conclusion section. We agree that\n453 the previous conclusion did not sufficiently distill the key findings, underlying\n454 mechanism, and broader significance of the work. In response, we have thoroughly\n455 revised the English writing throughout the manuscript and substantially reorganized the\n456 conclusion to ensure a concise and coherent summary.\n458 “In summary, we propose a sustainable moisture-electric energy conversion strategy\n459 based on ion dynamic coordination that enables the continuous reconstruction of ionic\n460 concentration gradients. The device operates by absorbing or desorbing moisture as the\n461 ambient humidity rises or falls, which in turn triggers the dissociation or re-coordination\n462 of I- ions. This reversible process regenerates a concentration gradient in a system that\n463 would otherwise have reached equilibrium. We validate this mechanism through\n464 electrical measurements, Raman spectroscopy, and theoretical calculations, and further\n465 establish its generality across hydrogel matrices with neutral, anionic, and cationic\n466 backbones. Benefiting from this mechanism, the SMEG operates reliably over a wide\n467 humidity range of 15-98% RH, exhibits reversible responses to small humidity cycles\n468 of only 13% RH, and is even able to detect humidity variations as low as 2% RH. These\n469 capabilities enable long-term, steady energy generation under natural humidity\n470 fluctuations. This work unveils a previously unrecognized moisture-to-electricity\n471 conversion mechanism governed by reversible dynamic coordination, which\n472 reconstructs ionic gradients without external energy input. It overcomes the inherent\n473 limitation of conventional MEGs whose output rapidly decays as ion distributions\n474 equilibrate, and establishes a new design framework for green, universal, and\n475 sustainable small-scale power sources. Despite these advances, the present system\n476 remains constrained by its relatively low output power and the gradual volatilization of\n477 iodine. Future efforts integrating temperature–humidity coupling strategies, optimizing\n478 electrode and interfacial architectures, and exploiting less volatile coordination species\n479 may further enhance the power density, lifetime, and long-term stability of the device.”\n481 scientific impact of the manuscript, and we hope they address your concerns.\n482\n483\n486 addressed Reviewer #3’s concerns, I think that most technical questions are reasonably\n487 answered; however, the central limitation—weak device performance—of the SMEG\n488 remains.\n490 power real-world devices. In the response to Comment 2, the authors claim that the\n491 rectified/storage-integrated device “can be directly used in low-power electronic\n492 devices (such as LEDs, sensors)”, but the reported voltage, current, and power are\n493 orders of magnitude below the requirements even for lighting a small visible LED.\n495 required to power the LED, which is an impractical architecture. The present\n496 demonstration in the manuscript does not substantiate utility as a power source.\n499 device’s generality and long-term durability are good, which could serve as a platform\n500 for future performance optimization. Most of the responses to technical questions are\n501 convincing. Although the authors’ response to Comment 8 is not perfectly rigorous (they\n502 should also compare the hydration radius, number, and energy of Ca2+ and Cl- to\n503 conclude that the migrating species is Cl-), their idea is correct.\n504 [Response] We first sincerely appreciate you for the thorough evaluation of our work\n505 and for recognizing the mechanistic insights, material generality, and long-term\n506 durability demonstrated in this study. We would also like to clarify that the statement in\n507 our response to Reviewer #3—“Signal regulation through rectification and energy\n508 storage units … can be directly used in low-power electronic devices (such as LEDs,\n509 sensors)”—was intended solely as a conceptual illustration of potential future\n510 application scenarios, emphasizing that alternating humidity-induced signals can be\n511 rectified into direct current signals with practical utility. This was meant to address\n513 limit the practical application scenarios of SMEG,” rather than to imply that the present\n515 the remaining concerns raised by you.\n516 1. Concerning the limited power output\n518 provide sufficient voltage or power to operate conventional electronic components such\n519 as commercial LEDs. For this reason, we have intentionally avoided presenting any\n520 data suggesting such functionality in the manuscript. As the reviewer rightly pointed\n521 out, powering sensors with lower power consumption is likely to be a crucial future\n522 application scenario for this type of device.\n524 (MEGs) shows a consistent trend of dramatic performance improvement. Early MEGs\n525 delivered power densities as low as 18.4 nW cm⁻², whereas subsequent progress has\n526 increased this value by nearly four orders of magnitude (e.g., ~0.11 mW cm⁻²)7,10. Thus,\n527 although our present SMEG exhibits modest performance, it establishes a robust\n528 mechanistic and material platform for future optimization. Moreover, this version of\n529 the device represents only the first-generation demonstration, whose primary purpose\n530 is to reveal a new long-life energy-harvesting mechanism. Substantial improvements\n531 are expected in future iterations.\n533 subsequent efforts will focus on several synergistic directions"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors well addressed most questions of the reviewers and most of the responses were satisfactory. However, some critical issues need to be improved.\n1. Actually, the Raman spectroscopy was only performed under ambient laboratory conditions (55% RH). Before the Raman spectroscopy test, the sample was placed in two humidity chambers (98% and 15%). Could the authors provide continuous and dynamic change data for Raman spectroscopy under changed humidity? On the other hand, Figure R3 provided the experimental setup of humidity bottles used for controlling relative humidity. It is easy to understand that humidity can reach 98% through natural water evaporation. But how to make the humidity in such a setup reach 15%?\n2. For MD simulation, the authors did not give the geometric model in the Methods Part. In the calculation, how many water molecules are there for the explicit solvent shell? On the other hand, the authors are encouraged to simulate the dynamic iodide ion dissociation process during the entire humidity change process.\n3. In Supplementary Fig. 14, why is the voltage of the device so low (<30 mV)? Except from short circuit current and open circuit voltage, the authors are also encouraged to provide other parameters, such as internal resistance and power density. These parameters are also essential to the device.\n4. Under natural conditions, achieving extreme humidity levels of 15% or 98% is difficult. The author gives the performance of humidity between 30% and 43%, as well as 84% and 98%. This range is more representative of realistic conditions. How much does the current change for every 5% increase in humidity? Is it quantifiable? It is essential to study small-scale humidity fluctuations.\n5. The authors did not adequately summarize recent advances in moisture-enabled electricity generation. In the Introduction part, they simply noted the challenge of achieving long-term sustainable power generation from moisture, attributing the limitation primarily to the inherently finite number of directionally migrating ions. However, recent studies have already demonstrated that some moisture-based devices capable of continuous operation for up to 200 days with outstanding performance (Adv. Funct. Mater. 202406901). I recommend that the authors revise the introduction to highlight the significance and value of incorporating iodine into the moisture-based device. The authors evaluated the store stability (7 months). Actually, this is not the stability of continuous operation.\n6. Table 2 does not demonstrate an obvious performance advantage for the proposed materials. AC output mode is not an important advantage. Is the introduction of iodine universally applicable to enhancing the performance of other types of moisture-based devices? The author should summarize the real and comparable advantages of the proposed material through comparison in Table 2.\n7. The overall English writing requires improvement. The conclusion, in particular, is convoluted and fails to provide a succinct summary of the key findings, their underlying principles, and the broader significance of the work.", "reviewer_2": "I have no other comments for this revised manuscript.", "reviewer_4": "The authors have addressed all my concerns in the initial submission. I agree with the responses and the modifications in the updated submission. I suggest the manuscript can be accepted for publication in this Journal.", "reviewer_5": "As an additional reviewer assessing whether the authors have adequately addressed Reviewer #3’s concerns, I think that most technical questions are reasonably answered; however, the central limitation—weak device performance—of the SMEG remains.\nI agree on the major concern of Reviewer #3 that the SMEG’s output is too low to power real-world devices. In the response to Comment 2, the authors claim that the rectified/storage-integrated device “can be directly used in low-power electronic devices (such as LEDs, sensors)”, but the reported voltage, current, and power are orders of magnitude below the requirements even for lighting a small visible LED. Stacking over a hundred of SMEG cells, each with tens of square centimeters area, is required to power the LED, which is an impractical architecture. The present demonstration in the manuscript does not substantiate utility as a power source. Relative to energy generation, the device is more suitable for sensing applications.\nThis work does provide new perspectives and methods in the field of MEGs. The device’s generality and long-term durability are good, which could serve as a platform for future performance optimization. Most of the responses to technical questions are convincing. Although the authors’ response to Comment 8 is not perfectly rigorous (they should also compare the hydration radius, number, and energy of Ca2+ and Cl- to conclude that the migrating species is Cl-), their idea is correct."}, "author_response": "1 Point-to-point response to the Reviewers’ comments on the manuscript\n2 “Humidity fluctuations-induced dynamic coordination drives the oscillatory\n3 migration of ions for sustainable energy harvesting”\n6 thank the reviewers for their thoughtful and constructive evaluations throughout this\n7 extended review process. We are grateful for the additional efforts made to obtain\n8 further expert assessment, and we fully understand the importance of addressing the\n9 remaining concerns in order to strengthen the rigor and clarity of our study. We have\n10 carefully considered all points raised in the new review round, including the request for\n11 additional experimental evidence regarding small humidity step responses, further\n12 clarification and discussion of the device’s power limitations, and a more rigorous\n13 examination of the migrating ionic species. In this revision, we have conducted new\n14 experiments, expanded mechanistic analyses, refined key discussions, and substantially\n15 revised both the main text and Supplementary Information to fully address these\n16 concerns. All revised contents are marked in yellow in the revised manuscript.\n18\n21 were satisfactory. However, some critical issues need to be improved.\n22 [Comment 1] Actually, the Raman spectroscopy was only performed under ambient\n23 laboratory conditions (55% RH). Before the Raman spectroscopy test, the sample was\n24 placed in two humidity chambers (98% and 15%). Could the authors provide\n25 continuous and dynamic change data for Raman spectroscopy under changed humidity?\n27 for controlling relative humidity. It is easy to understand that humidity can reach 98%\n28 through natural water evaporation. But how to make the humidity in such a setup reach\n29 15%?\n30 [Response] We sincerely thank you for the constructive comments. We fully understand\n31 the importance of monitoring the dynamic evolution of Raman spectra under\n32 continuously changing humidity conditions. However, performing in-situ Raman\n33 spectroscopy while dynamically controlling the humidity is technically challenging and\n34 may potentially affect the stability of the Raman instrument. To obtain equivalent\n35 information reflecting the spectral evolution under humidity variation, we adopted an\n36 alternative time-dependent measurement strategy.\n39 the surrounding humidity changed instantaneously. Although the ambient humidity\n40 during the test remained constant, the gel sample itself was undergoing a continuous\n41 moisture desorption (from 98% RH to 55% RH) or moisture adsorption (from 15% RH\n42 to 55% RH) process. Therefore, by extending the overall spectral acquisition time,\n43 we could effectively capture the dynamic spectral evolution associated with this humidity-\n44 driven transition.\n46 extended the acquisition time per point from 1 s to 4 s, and disabled SWIFT mode. As\n47 a result, each Raman spectrum was recorded over approximately 20 min. For\n48 comparison, the spectra shown in Fig. 4e were acquired with a shorter duration of about\n49 2 min to represent the state of triiodide ions as close as possible to their equilibrium\n50 condition inside the humidity chambers. The results obtained after transferring the\n51 sample from a 15% RH humidity chamber to the laboratory ambient environment for\n52 testing are shown in Supplementary Fig. 23a. These data reveal a gradual decrease in\n53 triiodide concentration from its initially high level, providing strong evidence that\n54 triiodide dissociates into elemental iodine and iodide ions during the moisture-\n55 absorption process. Conversely, after placing the sample in a 98% RH humidity\n56 chamber for 30 min and then transferring it back to the laboratory ambient environment\n57 for Raman characterization, the results shown in Supplementary Fig. 23b demonstrate\n58 that triiodide is regenerated when the hydrogel undergoes moisture desorption.\n59\n60\n62 at 110 cm-1 (I - symmetric telescopic vibration). a, after being placed in 15% RH humidity chamber\n3\n63 for 12 h (e-I), and then put into 98% RH humidity chamber for 30 min (e-II), and then put back into\n64 15% RH humidity chamber for 30 min (e-III).\n65\n67 intensity maps at 110 cm-¹ (I₃- symmetric stretching vibration). a Transfer from a 15% RH\n68 humidity chamber to the testing environment (~55% RH). b Transfer from a 98% RH humidity\n69 chamber to the testing environment (~55% RH).\n70\n72 humidity in the chambers was not controlled by natural water evaporation alone. Instead,\n73 we employed saturated salt solutions, which establish fixed equilibrium vapor pressures\n74 corresponding to specific relative humidities at a given temperature. This approach\n75 follows the well-established saturated salt solution method1,2. In our experiments, the\n76 low-humidity condition (15%) was achieved using a saturated LiCl solution, which\n77 strongly binds water molecules and thus significantly lowers the equilibrium vapor\n78 pressure.\n80 the relevant literature to facilitate the reader’s understanding of this method in the\n81 revised manuscript (Section Methods).\n82 1. Young, J. F. Humidity control in the laboratory using salt solutions—a review. J. Appl. Chem.\n83 17, 241-245 (1967).\n84 2. Wexler A., Hasegawa S. Relative humidity-temperature relationships of some saturated salt\n85 solutions in the temperature range 0° to 50°C. J. Res. Natl. Bur. Stand. 53, 19-26 (1954).\n86\n87 [Comment 2] For MD simulation, the authors did not give the geometric model in the\n89 solvent shell? On the other hand, the authors are encouraged to simulate the dynamic\n90 iodide ion dissociation process during the entire humidity change process.\n91 [Response] Thank you for pointing out the missing information in the Methods section.\n94 molecules, while no water molecules were included in the anhydrous model. To provide\n95 a clearer understanding of the dissociation behavior of the triiodide ion and its\n96 interaction with surrounding water molecules, we have added an illustration of the I₃⁻\n97 dissociation pathway (see Supplementary Fig. 24). Finally, we have updated the\n98 relevant details of the computational methods in the revised manuscript to present the\n99 simulation information more accurately, clearly, and comprehensively.\n101 dissociation process during the entire humidity change process,” we fully agree that\n102 such a simulation could, in principle, provide deeper mechanistic insight. We initially\n103 evaluated this possibility; however, conventional classical MD methods are inherently\n104 incapable of describing bond formation and cleavage, and therefore cannot capture the\n105 dissociation process directly. Although ab initio molecular dynamics (AIMD) could\n106 theoretically address this issue, several fundamental and practical challenges arise when\n107 attempting to simulate iodide ion dissociation and coordination under dynamically\n108 changing humidity. These include: (i) the need for a quantum-mechanical treatment to\n109 accurately describe bond breaking/forming and charge redistribution; (ii) the\n110 requirement of an explicit and sufficiently large solvent representation; and (iii) the\n111 necessity of extensive time-scale sampling to capture humidity-dependent dynamics.\n113 resource demands, making such simulations infeasible under current resources.\n115 between scientific rigor and computational feasibility. Two representative equilibrium\n116 states—an anhydrous environment and a water-rich environment—were selected to\n117 capture the essential influence of humidity on the iodide ion dissociation behavior and\n118 structural features. This approach provides reliable and reproducible quantitative\n119 insights while maintaining consistency with the experimental observations. Moreover,\n120 the time-dependent Raman spectroscopy results further complement our simulations by\n121 revealing the dynamic aspects of the dissociation process experimentally, thereby\n122 strengthening the overall mechanistic understanding.\n123\n3\n125 environment.\n126\n127 [Comment 3] In Supplementary Fig. 14, why is the voltage of the device so low (<30\n128 mV)? Except from short circuit current and open circuit voltage, the authors are also\n129 encouraged to provide other parameters, such as internal resistance and power density.\n131 [Response] We sincerely thank you for raising this important question regarding the\n132 low voltage (<30 mV) observed in Supplementary Fig. 14 and for encouraging us to\n133 provide additional device parameters. Following your suggestion, we conducted\n134 supplementary measurements of the 9-mm-thick device’s internal resistance, which\n135 helps clarify the origin of the low-voltage output. Generally, in moisture-enabled\n136 generators, the output voltage is typically governed by the degree of ionic separation\n137 between the two electrodes3. To investigate the device’s internal resistance, the current-\n138 voltage (I-V) characteristics of the device were measured. Accordingly, the\n139 measurement results are displayed in Supplementary Fig. 15a. It can be concluded that\n140 the internal resistance of our device was determined to be approximately 183 Ω at 98%\n142 resistance. In contrast, most reported moisture-electric generators possess internal\n143 resistances within the range of a few kΩ to several MΩ4-6. This significant difference\n144 suggests that the low internal resistance of our device may be the cause of its limited\n145 voltage output. This finding also offers a clear direction for future improvement:\n146 increasing the device resistance through tailored material or structural design may\n147 significantly enhance the voltage. On the other hand, the relatively low voltage may\n148 also stem from the internal ionic distribution prior to humidity fluctuations. At steady\n149 state, iodide ions are uniformly distributed. Although humidity fluctuations induce\n150 dynamic coordination and regenerate a concentration gradient, the pre-existing\n151 homogeneous distribution may reduce the effective ion separation, thereby limiting the\n152 attainable voltage.\n154 conditions using external loads ranging from 100 Ω to 10⁵ Ω. The load power density\n155 was calculated as P = I2R/A, where I is the load current, R is the load resistance, and A\n156 is the active device area. As shown in Supplementary Fig. 15b, the load power density\n157 achieved was 0.049 μW/cm² at a load of 200 Ω, consistent with the low internal\n158 resistance of the SMEG. Although many recent studies reported high power outputs,\n159 most such devices inevitably include contributions from metal–air battery reactions at\n160 the electrodes7,8. By contrast, our SMEG relies solely on ion migration induced by the\n161 ionic concentration gradient without electrode redox involvement. As shown in\n163 relatively low (we calculated the maximum power density (V × J ), as some\nOC SC\n164 literature did not provide load power density data), it still stands among the top tier in\n165 the field of pure hydrovoltaic devices utilizing inert electrodes—systems that operate\n166 without contributions from electrode redox reactions to the device performance.\n168 primary purpose is to demonstrate a new long-lifetime electricity-generation\n169 mechanism. Substantial improvements are expected in future iterations. For example,\n170 several previously validated strategies—such as material/structural optimization and\n171 electrode/interface engineering—could be incorporated to further enhance the output\n172 performance9.\n173\n175 current-voltage (I-V) characteristics of the SMEG under various relative humidity (RH) levels. b\n177 from100 Ω to 105 Ω. The power density was calculated as P = I2R/A, where I is the load current, R\n178 is the load resistance, and A is the active device area.\n179\nShort\nPower Minimum\nOpen circuit\ndensity Operating humidity\nFunctional Elec- Working circuit current Work Decay\nPM = humidity change Ref.\nmaterials trode principles voltage density time rate\nVOC×JSC (%) response\nVOC (V) JSC\n(μW/cm2) (%)\n(μA/cm2)\nPVA-AlgNa\nDirected ion\nbased Al-C 1.3 1310 1703 80 10 >720 h ~96% 10\nmigration\nhydrogel\nAu@sta Directed ion\nBGC-NC inless- migration + 1.17 2770 3240.9 90 15 10 h ~95% 11\nsteel Redox\nDirected ion\n>100 h ~24%\nZn-C migration + 1.25 2.52 3.15 80 10\n(V) (V)\nRedox\naASA 12\nDirected ion\nC-C 0.21 1.25 0.2625 80 None None None\nmigration\nDirected ion\n>120 h ~33%\nC-Al migration+ 1.8 1200μA 113 90 20\nPAM- (V) (V)\nRedox 13\nLiCl/CMC\nDirected ion\nC-C ~0.35 ~15μA None 30 None None None\nmigration\nPAM-LiCl/ Directed ion >800 h ~39.8%\nCFP-Ag 1.28 1.475 1.888 60 (35℃) 10 14\nSiNWs migration （V） (V)\nMXene Directed ion\naerogel/PAM- Cu-Cu migration + 0.6 1200 720 20 15 >12 h ~97.5% 15\nCaCl2 Redox\nPEDOT:PSS-\nPVA-PAAs- Directed ion\n60 ( ΔT = >26 h ~44.2%\nbased Cu-Cu migration + 0.215 5.96 1.2814 None 16\n3.7 K) （V） （V）\nanisotropic TE\nhydrogel\nP(VDF- Directed ion\nTrFE)/PAN/C C-Al migration + 0.85 128 108.8 35-45 20 >720 h 96% 17\nNW-LiCl Redox\nOscillatory\nNearly\nILs/Clustered migration of\nAg/ 0%\npolycation ions driven 0.00025 10615.7 ~2.65 73 9 >672 h 5\nAgCl (record 8\nmembrane by humidity\ntimes)\nfluctuations\nILs Ag/ Oscillatory 0.15 2.55 0.3825 35-81 20 >72 h ~51% 18\nAgCl migration of\nions driven\nby humidity\nfluctuations\nOscillatory\nmigration of\nP (DMAPS- Nearly\nC-C ions driven 0.11 0.55 0.06 15-93 33 >33h 19\nco-AM)-LiCl 0%\nby humidity\nfluctuations\nLiCl@CP- Directed ion\nC-C 0.78 0.83 0.65 60±5 10 >240 h ~21.7% 20\nCB@CP migration\nSelf-excited\nP(MEDSAH-\nAu-Au chemoelectri 0.4 1.5 0.6 100 None 15000 s 99.9% 21\nco-AA)\ncal reaction\nDirected ion\nMEL-PCL C-C migration + 0.28 1.8 0.504 98 25 >600 h ~75.1% 3\nPC\nDirected ion\nPSSA-PDDA C-C 0.95 0.05 0.0475 25 10 >150h 96% 22\nmigration\n(PSSA-\nDirected ion\nPVA)/(HPC- C-C 1.1 0.0089 0.00979 20 20 >11.7h 60% 23\nmigration\nKGM)\nNew device\nNearly\n0%\n>336 h (Con-\nOscillatory\ntinuous\nmigration of\nP(AM-co- record) This\nC-C ions driven 0.028 33 0.924 15-98 2\nAMPS)/NaI After 7 months work\nby dynamic\ndevice\ncoordination\n39.5%\n(record\n>720 h\n30\ntimes)\n183 maximum power density (V × J ) in this table.\nOC SC\n184\n185\n186 3. Duan, P. et al. Moisture-based green energy harvesting over 600 hours via photocatalysis-\n187 enhanced hydrovoltaic effect. Nat. Commun. 16, 239 (2025).\n188 4. Wang, H. et al. Moisture adsorption-desorption full cycle power generation. Nat. Commun. 13,\n189 2524 (2022).\n190 5. Zhao, Y. et al. Diurnal humidity cycle driven selective ion transport across clustered polycation\n191 membrane. Nat. Commun. 15, 7161 (2024).\n192 6. Lu, X. et al. Hierarchically porous fiber-based nanofluidic diode as an efficient multimode\n193 hygroelectric generator. Adv. Energy Mater. 12, 2202634 (2022).\n194 7. Tan, J. et al. Harvesting energy from atmospheric water: grand challenges in continuous\n195 electricity generation. Adv. Mater. 36, e2211165 (2024).\n196 8. Xu X, Yang X, Zhang Z, et al. Identification of metal–air batteries from water energy harvesters.\n198 9. Zang, S. et al. Moisture power generation: from material selection to device structure\n199 optimization. ACS Nano 18, 19912-19930 (2024).\n200 10. Yang, S. et al. Green moisture-electric generator based on supramolecular hydrogel with tens\n201 of milliamp electricity toward practical applications. Nat. Commun. 15, 3329 (2024).\n202 11. Song, M., Kim, D., Lee, H., Han, H. & Jeon, S. Synergistic effect of a Berlin green framework\n203 for highly efficient moisture-electric energy transformation. Energy Environ. Sci. 17, 5421-\n204 5428 (2024).\n205 12. Zhou, J., Ren, Z., Cui, X., Liu, X. & Lu, X. Bioinspired interfacial design of robust aramid\n206 nanofiber composite films for high‐performance moisture‐electric generators. Adv. Energy\n208 13. Cao, F. et al. Efficient energy harvesting: High power wearable humidity generators with PAM-\n210 14. Duan, W. et al. Silicon nanowire/ionic hydrogel-based hybrid moist-electric generators with\n211 enhanced voltage output and operational stability. Energy Environ. Sci. 17, 3788-3796 (2024).\n212 15. Zhao, K. et al. Humidity-tolerant moisture-driven energy generator with mxene aerogel-\n213 organohydrogel bilayer. ACS Nano 17, 5472-5485 (2023).\n214 16. Zhang, M. et al. Long-term serviceable ionic thermoelectric hydrogel with temperature and\n215 moisture dual-driven waste energy harvesting capability. Small 21, e2501960 (2025).\n216 17. Hu, Y. et al. Phyto-inspired sustainable and high-performance fabric generators via moisture\n217 absorption-evaporation cycles. Sci. Adv. 10, eadk4620 (2024).\n218 18. Tang, J. et al. Circadian humidity fluctuation induced capillary flow for sustainable mobile\n219 energy. Nat. Commun. 13, 1291 (2022).\n220 19. Fu, C. et al. Moisture adsorption-desorption induced ion reciprocating diffusion in zwitterionic\n221 polymer for alternating-cycle power generation Adv. Funct. Mater. e11590 (2025).\n222 20. Tan, J. et al. Self-sustained electricity generator driven by the compatible integration of\n223 ambient moisture adsorption and evaporation. Nat. Commun. 13, 3643 (2022).\n224 21. Long, Y. et al. Moisture-induced autonomous surface potential oscillations for energy\n225 harvesting. Nat. Commun. 12, 5287 (2021).\n226 22. Wang, H. et al. Bilayer of polyelectrolyte films for spontaneous power generation in air up to\n227 an integrated 1,000 V output. Nat. Nanotechnol. 16, 811-819 (2021).\n228 23. Zhao, K. et al. Moisture-driven energy generation by vertically structured polymer aerogel on\n229 water-collecting gel. Nano Energy 126, 109645 (2024).\n230\n231 [Comment 4] Under natural conditions, achieving extreme humidity levels of 15% or\n232 98% is difficult. The author gives the performance of humidity between 30% and 43%,\n233 as well as 84% and 98%. This range is more representative of realistic conditions. How\n234 much does the current change for every 5% increase in humidity? Is it quantifiable?\n236 [Response] We thank you for highlighting the importance of quantifying the device\n237 response to small humidity fluctuations. To address this, we performed additional\n238 measurements using a programmable humidity chamber, since the humidity bottles\n239 used previously cannot produce the fine RH increments required. The chamber was set\n240 to cover a broad humidity range (35-95% RH) at 25 °C with a target step of 5% RH to\n241 probe the current response as humidity increased.\n243 limited precision of the humidity chamber—particularly during the humidity decrease\n244 phase—the actual RH increments could not be held exactly at 5% in every step. The\n245 quantified results are summarized in Supplementary Table 3. At 25 °C the mean\n246 measured humidity increment was 5.71 ± 0.64%, and the corresponding mean current\n247 increase was 3.9 ± 0.69 µA per step. During the moisture-desorption process the\n248 chamber did not reliably produce a comparable stepwise RH decrease under the same\n249 settings; consequently, the current changes during desorption did not show a clear\n250 staircase pattern and could not be robustly quantified.\n252 conclude that the device can respond to humidity changes of approximately ~2% RH,\n253 provided a stable humidity controller is available. Therefore, we retested the device’s\n254 response to a 2% RH incremental change, as shown in Supplementary Fig. 26b. Due to\n255 the precision limitations of the humidity chamber, the actual humidity increment\n256 approached 2% only when operating above 60% RH. Consequently, we selected 12 data\n257 points above 60% RH to calculate the mean and standard deviation of the response. The\n258 results show that for a humidity increment of 2.34 ± 0.22%, the corresponding current\n259 increment was 1.14 ± 0.14 μA.\n261 to the revised manuscript to clarify the device responsiveness to small humidity\n262 fluctuations.\n263\n264\n266 provided by a constant temperature and humidity chamber (DHTHM-27-80-P-SD). a Current\n267 performance response of the device under a 5% RH change. During the humidity decrease phase,\n268 the humidity chamber cannot maintain a stable humidity gradient, thus limiting the quantitative\n269 analysis. b Current performance response of the device under a 2% RH change. Due to limitations\n270 in equipment precision, the chamber cannot consistently provide the specified humidity change.\n271\n272\n274 change under 5% RH change settings.\nSet RH (%) 40 45 50 55 60 65 70 75 80 85 90 95\nActual\nHumidity\n6.4 5.1 6.3 6.4 6.5 6.4 5.6 5.6 5.1 4.9 5.1 5.1\nIncrement\n(%)\nCurrent\nIncrement 3.80 4.50 5.50 4.20 4.21 4.24 3.82 3.78 3.46 3.08 3.10 3.20\n(μA)\n275\n276\n277\n278\n279\n280\n281\n283 change under 2% RH change settings.\nSet RH (%) Actual Humidity Increment (%) Current Increment (μA)\n45 3.0 1.80\n47 3.6 2.90\n49 3.2 2.10\n51 3.8 2.46\n53 3.2 2.00\n55 3.4 2.01\n57 3.1 1.74\n59 3.1 1.73\n61 2.4 1.36\n63 2.1 1.39\n65 2.6 1.30\n67 2.4 1.18\n71 2.8 1.25\n73 2.3 1.02\n75 2.4 1.05\n77 2.0 0.98\n79 2.4 1.09\n81 2.2 1.01\n83 2.3 1.08\n85 2.2 0.99\n284\n285\n286\n287\n288\n289 [Comment 5] The authors did not adequately summarize recent advances in moisture-\n290 enabled electricity generation. In the Introduction part, they simply noted the challenge\n291 of achieving long-term sustainable power generation from moisture, attributing the\n292 limitation primarily to the inherently finite number of directionally migrating ions.\n294 capable of continuous operation for up to 200 days with outstanding performance (Adv.\n296 highlight the significance and value of incorporating iodine into the moisture-based\n297 device. The authors evaluated the store stability (7 months). Actually, this is not the\n298 stability of continuous operation.\n299 [Response] Thank you for your careful evaluation and valuable suggestions. We have\n300 now thoroughly summarized recent advances in moisture-enabled electricity generation\n301 and revised the relevant parts of the Introduction to better highlight the significance and\n302 value of introducing iodine into the device. The main revisions are as follows:\n303 “However, the unidirectional migration of ions typically leads to their\n304 accumulation on one side of the device, where the resulting back electric field\n305 suppresses further ion transport, causing the electrical output to decay. Some studies\n306 have attempted to introduce redox reactions to consume the accumulated ions at the\n307 electrode, thereby mitigating the back electric field and enabling long-term power\n308 generation. Nevertheless, redox reactions at the electrodes should be avoided as much\n309 as possible in pure hydrovoltaic devices. In our previous work, we introduced\n310 photocatalysis to consume the accumulated protons and restore the concentration\n311 gradient, achieving 600 hours of electricity generation. However, this approach requires\n312 external energy input and suffers from limited recovery after multiple cycles. Therefore,\n313 addressing the lifetime issue requires new strategies that can break the limitation of\n314 unidirectional ion transport and enable back-and-forth ion motion.”\n316 hydrovoltaic devices, where no electrode redox reactions occur and the electricity is\n317 generated solely through humidity-driven ion migration. Regarding the study you\n318 mentioned (Adv. Funct. Mater. 202406901), we acknowledge its excellent performance\n319 and its ability to operate continuously for 200 days. This work indeed demonstrates an\n320 ingenious device design that maintains an almost permanent humidity gradient, which\n321 is commendable. However, the study does not exclude the possible contribution of\n322 electrode redox reactions, and thus a metal–air battery effect may still contribute to both\n323 the device lifetime and its electrical output8. Since such systems may involve a\n324 combination of metal–air battery mechanisms and moisture-enabled electricity\n325 generation, we did not highlight this type of work in the Introduction.\n327 months). Actually, this is not the stability of continuous operation,” we apologize for\n328 the ambiguity in our previous description. We have revised the wording to ensure\n329 precise expression. The revised description is as follows:\n330 “Furthermore, even after being stored under laboratory conditions for seven\n331 months (30–65% RH, 25 ± 3 °C), the device was still able to operate continuously for\n332 one month (Supplementary Fig. 2), demonstrating its outstanding long-term storage\n333 stability and sustained performance exceeding one month.”\n334\n335 8. Xu X, Yang X, Zhang Z, et al. Identification of metal–air batteries from water energy\n336 harvesters. Droplet 2, e80 (2023).\n337\n338 [Comment 6] Table 2 does not demonstrate an obvious performance advantage for the\n339 proposed materials. AC output mode is not an important advantage. Is the introduction\n340 of iodine universally applicable to enhancing the performance of other types of\n341 moisture-based devices? The author should summarize the real and comparable\n342 advantages of the proposed material through comparison in Table 2.\n343 [Response"}]

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