Molecular clustering unlocks high-performance hydrovoltaics across temperatures from −35 °C to 80 °C

Domain: neuroscience
Published: 2026-02-04
Editorial outcome: major_revision

Abstract
Hydrovoltaic technologies face challenges of low conversion efficiency and narrow operational temperature ranges, limiting their practical applications in extreme environments. Here, we propose a molecular clustering strategy that leverages organic molecules to interact with organic salt anions, forming stable composite clusters. These clusters enhance water’s phase change energy barrier and thermal stability while mitigating electrostatic shielding effects, effectively overcoming ion transport bottlenecks across a wide temperature range. The hydrogel achieves an operational temperature range from −35 °C to 80 °C and increases power density by an order of magnitude compared to existing technologies. Furthermore, the hydrogel demonstrates exceptional thermal and mechanical stability, maintaining stretchability above 1000% and stable performance under harsh conditions such as freezing and high heat. These advancements enable hydrovoltaic systems to operate reliably in flexible electronics, environmental monitoring, and self-powered devices across extreme environments, providing sustainable energy solutions for diverse and demanding scenarios.

Introduction
Meeting the world’s growing demand for renewable and sustainable energy calls for innovative approaches capable of harvesting power from widely available resources1–3. Among these, water stands out due to the enormous energy potential stored in natural processes, such as evaporation, precipitation, and flow—estimated to exceed the global annual energy consumption by several orders of magnitude4. Hydrovoltaic technologies harness this energy by converting the interaction energy of water molecules (e.g., from vapor, droplets, or flowing streams) with nanomaterial interfaces into electrical output (Fig. 1a)5–8. Compared with conventional solar or wind energy systems, hydrovoltaics offer broader accessibility and lower dependence on external conditions, making them highly promising for applications in unmanned platforms, low-altitude economic products, and other lightweight, distributed devices. However, despite this potential, hydrovoltaic performance under extreme temperatures remains a critical bottleneck, severely limiting real-world implementation.

The core challenge lies in the temperature-sensitive hydrogen-bond network of water (Fig. 1b). At elevated temperatures, hydrogen bonds readily break, while in subzero conditions they become rigid, in both cases disrupting ion dissociation and transport crucial for hydrovoltaic conversion. This temperature sensitivity stems from the temperature dependence of water’s Gibbs free energy. The phase with the lowest Gibbs free energy is thermodynamically most stable, and for water the gas phase is lowest at higher temperatures while the solid phase is lowest at lower temperatures, which explains water’s tendency to vaporize when heated and to freeze when cooled. To tackle these issues, researchers have tried using high concentrations of inorganic salts or organic components rich in hydrophilic groups to lower the liquid water’s Gibbs free energy across an expanded temperature range9,10. Inorganic salts enhance stability by the strong ion–water interactions, which disrupt the water’s hydrogen-bonding network and suppress ice crystallization at subzero temperatures. Moreover, at elevated temperatures, the strong hydration shells around ions increase the energy required for water molecules to escape into the vapor phase, thereby raising the fast-vaporizing point. However, high ionic concentrations can introduce strong electrostatic shielding that inhibits charge transport. Organic molecules are typically low-freezing-point solvent species that act as co-solvents by partially replacing water. These molecules depress the freezing point by increasing the fraction of non-freezable bound water. Their vapor pressures are generally lower than that of water, thereby reducing the overall vapor pressure of the hydrogel and raising the temperature at which rapid evaporation commences. Nonetheless, organic solvents often exhibit limited dissociation and high viscosity, leading to fewer mobile ions and reduced conductivity. Previous reports in hydrovoltaic research also investigated the temperature-dependent viability of energy harvesting, typically employing organic compounds, such as ethylene glycol or glycerol, and inorganic salts like lithium chloride and calcium chloride11–16. The rationale is that such additives’ intrinsic antifreeze properties can extend device operability at sub-zero temperatures. However, these reports also note that at elevated temperatures reduced internal water availability within the generator can restrict sustained ion transport, thereby limiting steady electrical output. Consequently, existing hydrovoltaic devices still exhibit modest power densities (on the order of 100 μW cm–2) across a wide temperature window, making them unsuited for deployment in harsh or variable real-world environments.

Here, we present a molecular clustering strategy to overcome these limitations, employing an integrated system of organic salts and organic molecules that induce the formation of stable composite clusters (Fig. 1c). These clusters not only increase the effective surface charge, drawing in more water molecules, but also mitigate electrostatic shielding between ions. As a result, our molecularly clustered hydrogel maintains robust thermodynamic stability and efficient ion transport across an unprecedented temperature range from –35 °C to 80 °C, achieving power densities up to an order of magnitude greater than previous reports (Fig. 1e). Furthermore, the hydrogel retains remarkable mechanical integrity under these extreme temperatures, demonstrating over 1000% strain and stable electrochemical performance. Together, these attributes offer a reliable path toward self-powered devices in both frigid and high-temperature settings. Overall, this molecular clustering concept addresses the longstanding issue of hydrogen-bond fragility in hydrovoltaic systems, enabling high efficiency under conditions where conventional approaches fail. Beyond hydrovoltaic power generation, this design principle provides a broadly applicable framework for creating functional materials capable of withstanding extreme thermal environments. We anticipate that our findings will stimulate further innovations in flexible electronics, environmental monitoring, aerospace applications, and biomedical devices, where durable and adaptive energy-harvesting materials are becoming increasingly vital.

Results
Design and characterization of molecularly clustered hydrogels
To achieve efficient hydrovoltaic performance over a broad temperature range, we first developed an all-organic hydrogel system based on a molecular clustering concept. Acrylamide (AAm) and sodium vinyl sulfonate (SV) were chosen as the polymerizable backbone, providing both a robust gel matrix and sulfonate groups that contribute mobile charges for hydrovoltaic conversion17–21. To further increase ion availability, we incorporated sodium sulfamate (SS), whose strongly polar –SO3Na moiety dissociates to release additional Na+22,23. However, without extra intermolecular interactions, the sulfamate anion (NH2SO3−) freely migrates along with Na+ in the hydrogel, leading to net charge dissipation and diminished power output (Fig. 1d).

To address this issue while balancing ion dissociation and matrix stability, we introduced glycolic acid (GA) and sodium glycolate (SG), both bearing similar hydrophilic functionalities, to induce “molecular clustering” of anions around GA (Fig. 1c). According to the principle of like-dissolves-like and Lewis acid–base interactions, GA’s hydroxyl and carboxyl groups can coordinate with the lone-pair electrons on SS/SG anions, forming robust organic clusters. Notably, SG, having a higher dissociation tendency, generates glycolate anions and Na⁺ preferentially and thus suppresses over-dissociation of GA, retaining its molecular integrity. This clustering confines anions around GA, mitigating random migration for hydrovoltaic conversion and enhancing surface charge for stabilizing water in hydrogels.

We fabricated these molecularly clustered hydrogels via UV-initiated polymerization in an open environment (Fig. S1). During UV exposure, free-radical polymerization produces a three-dimensional crosslinked matrix (Fig. S2), while controlled evaporation of excess water further promotes close contact among GA, SS, and SG, yielding a tightly crosslinked hydrogel architecture. The evaporation step facilitates the controlled transport of the internal solution, thereby promoting interactions among the components24,25. As shown in Fig. S3, Cryo-SEM imaging reveals the hydrogel microstructure after immersion in pure water for 8 h. The evaporation step densifies the porous network and reduces the pore diameter. XRD results show enhanced diffraction peaks for hydrogels prepared via the water evaporation step, as presented in Fig. S4. This finding indicates that water evaporation promotes closer packing of the polymer network26,27, thereby facilitating close contact among GA, SS, and SG.

Fourier-transform infrared (FTIR) spectroscopy revealed pronounced shifts in GA’s –OH and –COOH peaks as SS and SG were added (Fig. S5a), consistent with intermolecular interactions that facilitate the clustering process. The thickness-normalized small angle X-ray scattering (SAXS) patterns shown in Fig. 2a, b indicate that sequential incorporation of SS, SG, and GA preserves the hydrogels’ homogeneity and amorphous character28,29. A comparison of peak intensities shows that co-introduction of SS and SG produces a marked increase in scattering intensity. This behavior is attributable to the fact that both additives are dissociable small organic molecules that generate anions bearing organic functional groups upon dissociation. Electrostatic repulsion among these charged species loosens the internal network and increases structural disorder, thereby enhancing the scattering intensity. By contrast, incorporation of GA leads to a pronounced reduction in scattering intensity, to values below those observed for hydrogels containing SS or the SS-SG combination. This decrease indicates that GA reduces disorder within the hydrogel network. The scattering intensity of the SS-SG-GA hydrogel lies between that of the anion-only and the GA-only hydrogels, indicating interactions among SS, SG, and GA.

To evaluate the stability of these clusters, we performed density functional theory (DFT) calculations on three proposed binding sequences (SS-SG-GA, SG-SS-GA, and SG-GA-SS), finding similar binding energies in the range of –5.09 eV to –5.65 eV (Fig. S6). Among them, SG-GA-SS exhibited the most negative binding energy (–5.65 eV), suggesting slightly enhanced stability. Simulated infrared spectra based on this representative arrangement (Fig. S7) corroborated the experimental peak shifts, indicating that GA effectively coordinates with SS and SG anions to form stable organic clusters.

Broadening the operating temperature range
To elucidate how molecular clustering improves hydrogel stability under extreme temperatures, we first performed DFT simulations to evaluate the surface potential and hydration enthalpy of the SG-GA-SS cluster (Fig. 2c, d). Compared with alone SS anion, the presence of molecular cluster significantly increases the negative charge on the sulfonate (from −0.641 eV to −0.658 eV) while concurrently raising the positive partial charge on amino hydrogen atoms (up to +0.331 eV). Additionally, the hydration enthalpy of the cluster drops by a factor of ~1.6 relative to SS alone, suggesting enhanced water-binding capacity that helps maintain liquid water over a broader temperature window.

Next, we conducted molecular dynamics (MD) simulations to visualize the water distribution around SG-GA-SS versus an only-SS anion. Radial distribution functions (g(r)) reveal abundant water molecules within 1–8 Å of each hydrophilic site (Fig. S8), with the amino group of clusters exhibiting particularly high local water density due to its strong polarity (Fig. 2e, f). At equilibrium, the cluster’s first and second hydration shells collectively display nearly three times more hydrogen bonds than in the SS-only scenario (Fig. 3a, b). In line with this observation, FTIR spectra show a blue shift in the O–H peaks (Fig. 3c)30, and the Raman spectrum exhibits enhanced O–H intensities at 3630 cm−1 (Fig. 3d)31. These results confirm that GA-driven clustering strengthens water–cluster hydrogen bonding while partially reducing water–water interactions.

To quantitatively assess water retention in high and low temperature regimes, we measured phase-change enthalpies, including isothermal tests that better reflect real-world applications (Fig. 3e, f). Compared with the SS-only system, the SG-GA-SS cluster increases the average vaporization enthalpy by ~23% and elevates the onset temperature for rapid vaporization to 84 °C, prolonging water retention by ~30%. Under freezing conditions, the cluster reduces the average freezing enthalpy by ~20% and shifts the onset of fast freezing down to −41 °C, enabling a ~54% longer duration of liquid water. This observation is consistent with previous DFT calculations and MD simulations, which indicate that the presence of GA molecules reduces the hydration enthalpy of the clusters. As a result, a higher proportion of water molecules become associated with the clusters. Compared to free water molecules, breaking the hydrogen bonds between these bound water molecules requires greater energy input, thereby shifting the onset temperature of rapid vaporization to a higher range. Simultaneously, the energy barrier for the bound water molecules to form ice crystals with surrounding water is increased, leading to an extension of the fast-freezing onset temperature toward lower values. These enhancements surpass those of typical glycol-based hydrogels by roughly 10 °C on both the high- and low-temperature ends (Fig. S10), underscoring how molecular clustering imparts stronger thermodynamic stability to retained water.

Finally, we evaluated the molecularly clustered hydrogel’s behavior under temperatures from −35 °C to 80 °C, focusing on optical transparency and mechanical strength (Figs. S11 S12). Throughout this range, the molecularly clustered hydrogel retains stable surface morphology and clear optical properties, with tensile strains of around 1000% and only a 3–6% deviation from room-temperature performance—significantly outperforming pure hydrogels without clusters (45–52%). In addition, the deviation in Young’s modulus for the molecularly clustered hydrogel was generally less than 15%, whereas that of the cluster-free hydrogel consistently exceeded 15%. Under multi-axial extension, it achieves large-area deformation (surface strain ~1800%) without fracturing (Fig. 3g). Rheological behavior is shown in Fig. S13, the results revealed that the hydrogel consistently exhibited a storage modulus (G’) greater than the loss modulus (G”) at –35 °C, 25 °C, and 80 °C, signifying characteristic of soft and elastic solids32. We attribute this robust resilience to the GA-induced clusters that firmly bind water, mitigating the structural damage typically caused by high-temperature vaporization or low-temperature freezing. Overall, the molecularly clustered hydrogel maintains liquid water and mechanical flexibility from −35 °C to 80 °C. Beyond the hydrovoltaic application, hydrogels employed in diverse fields, such as flexible electronics and biomedicine also demand thermal and mechanical stability of the confined liquid water under varying temperature conditions33,34, as the crystallization of water at subzero temperatures or evaporation at elevated temperatures can severely impair their structural integrity and functional performance. Compared with representative hydrogels reported for soft robotics35–37, flexible electronics38–40, bioelectronics41–43, and biomedical44–46 applications, the molecularly clustered hydrogel achieves superior thermal stability of liquid water over an extended temperature window (Fig. 3h). Furthermore, relative to state-of-the-art hydrogels used in many soft multifunctional electronic platforms, this hydrogel exhibits larger tensile strain and lower Young’s modulus over a wider temperature range, indicative of its stronger ductility (Fig. S14, Table S2 and Table S3). This broad temperature tolerance highlights the potential of our molecular clustering approach for self-powered devices and other wide-temperature applications.

Enhancing hydrovoltaic performance
Effective ion transport over a wide temperature range is crucial for hydrovoltaic conversion. Unlike most ionic conductors, hydrovoltaic effects rely on the fixation of anions and the directed migration of cations, accumulating net charges at the terminal interface47. Within the molecularly clustered hydrogel, clustering of SS and SG anions induced by GA molecules is expected to alleviate the steric hindrance experienced by Na+ during transport. We quantify the degree of steric hindrance acting on Na+ using thermodynamic entropy. The mathematical definition of entropy used in this analysis is given in the Methods section. Larger entropy values correspond to a higher degree of disorder and to greater freedom of Na+ movement. Analysis of the MD simulations indicates that Na+ ions in the molecularly clustered system exhibit higher entropy than those in the SS-only system (Fig. S15a). This finding implies that formation of molecular clusters reduces the steric hindrance acting on Na+. The resulting free Na+ transport contributes to enhanced hydrovoltaic performance. DFT calculations provide complementary evidence for this conclusion by showing that the attractive interaction between SS anions and Na+ in the molecularly cluster is markedly smaller than the corresponding interaction energy in the SS-only system (Fig. S15b). Such reduction reduces steric hindrance acting on Na+, thereby facilitating Na+ mobility.

To quantify the spontaneous ion separation within the molecularly clustered hydrogel at −35 °C, 25 °C, and 80 °C, we measured its ion conductivity (Fig. 4a) under an external electric field. Incorporation of the SG-GA-SS cluster led to conductivities of 61.17, 67.31, and 79.47 mS cm−1, respectively. Notably, the conductivity at −35 °C improved by over 50% than many previous ionogels (Fig. S16 and Table S4), indicating that molecular clustering by GA effectively promotes ion dissociation and transport. Further source-meter tests (Fig. 4b) showed that the SG-GA-SS cluster reduces the hydrogel’s electrical resistance by about 33% at low temperatures, highlighting the formation of more favorable ion channels through molecular clustering. The reduction in resistance is more significant at low temperature than at higher temperature, which aligns with the well-established temperature dependence of ionic mobility. Ionic conduction inherently diminishes at lower temperatures due to reduced thermal energy and polymer chain dynamics48. For hydrogels without clusters, the simultaneous effects of decreased ionic mobility and reduced water phase fluidity lead to a pronounced increase in electrical resistance. By contrast, SG-GA-SS cluster mitigates these adverse temperature effects.

In real-world conditions, temperature gradients often drive water evaporation and ion diffusion. To examine the molecularly clustered hydrogel’s hydrovoltaic behavior under these conditions, we conducted non-isothermal experiments (Fig. 4c), maintaining one end at ambient temperature (sealed) and heating the other with an evaporation outlet, applying a 10–25 K temperature difference. As the gradient increased, the SG-GA-SS cluster substantially boosted the open-circuit voltage. Under a 25 K temperature gradient, the response potential difference was enhanced by 183% relative to the control, revealing that this cluster effectively facilitates spontaneous anion–cation separation and charge accumulation. This phenomenon arises because increasing temperature enhances polymer chain dynamics and reduces viscosity, thereby facilitating ion transport49,50. Combined with the thermal stability of liquid water confined within the molecularly clustered hydrogel, these factors lead to a more pronounced increase in potential difference compared with the hydrogel without clusters.

Active metal electrodes, such as copper, aluminum, and zinc have been widely adopted to enhance electrical output and broaden application scenarios in recent hydrovoltaic studies12,15,19,51–54. Accordingly, we evaluated the open-circuit voltage (Voc) of molecularly clustered hydrogels using these three metals as electrodes. By comparing the average Voc over the same measurement period, with error bars indicating the standard deviation of fluctuations during this interval (Fig. S17), it was observed that zinc electrodes yielded a higher average Voc with smaller fluctuations, indicating superior stability. Furthermore, comparisons were made between hydrogels containing only SS and molecularly clustered hydrogels. Results demonstrated that, regardless of electrode type, the molecularly clustered hydrogel consistently exhibited high Voc, confirming that electrode selection does not alter the conclusion that molecular clustering enhances charge accumulation and hydrovoltaic performance.

Building on these improved properties, GA-based molecularly clustered hydrogels achieved consistently high hydrovoltaic performance from −35 °C to 80 °C (Fig. S18), with peak output power exceeding most reported devices by at least one order of magnitude (Table S1). We next assembled 28 individual hydrogel cells in parallel (Fig. 4d) on a flexible PET substrate, using zinc and carbon pastes as electrodes. Remarkably, the half-decay time of the hydrovoltaic open-circuit voltage maintained up to 120 h at both −35 °C and 80 °C (Fig. 4e)—outperforming many literature reports operating around room temperature (Fig. S19, Table S5). Such endurance underscores the promise of hydrovoltaic technology for sustained energy harvesting in extreme environments.

To prove that the voltage and current are generated from the hydrovoltaic effect, additional tests were conducted using two same graphite felt electrodes and asymmetric graphite felt–platinum inert electrodes. As presented in Fig. S20, when two identical graphite felt electrodes were employed, localized evaporation within the hydrogel induced an internal ion concentration gradient. This promoted directional ion diffusion, leading to charge separation and the generation of both open-circuit voltage and short-circuit current. When platinum–graphite felt inert electrodes were used, the inherent electrode potential difference synergized with evaporation-driven ion diffusion, thereby enhancing electrical output. In contrast, the slow surface oxidation of the zinc-active electrode generates a sustained flux of metal cations, which facilitates ionic diffusion and thereby enhances electrochemical performance. These findings are consistent with previously reported mechanisms in related literature15,51.

Furthermore, recognizing that water evaporation from the hydrogel during the hydrovoltaic conversion process is intrinsically unidirectional and therefore limits practical application, we evaluated a cyclic operating protocol consisting of one hour of evaporation followed by ten minutes of rehydration. As shown in Fig. S21, after six such cycles the device retained 87% of its initial open-circuit voltage and 86% of its initial short-circuit current. These results indicate that, owing to strong hydrogen-bonding interactions between molecular clusters and water molecules, dehydrated hydrogels readily reabsorb water and can undergo repeated evaporation cycles. This behavior is analogous to the charging process of conventional batteries, in which water uptake restores device functionality. Additionally, the combination of moisture absorption and evaporation has been employed to extend the duration11,15,55, while a recently reported innovative internal water circulation mechanism has demonstrated to enable sustained device operation56.

Universality and applications
To explore the universality of the molecular clustering strategy, we replaced SS in the SG-GA-SS cluster of the SV-based hydrogel with sodium methanesulfonate (SM) and examined the phase transition behavior. It was observed that, compared to the system with SM alone, the presence of the SG-GA-SM cluster significantly reduced the initial freezing transition temperature and increased the initial vaporization transition temperature of the hydrogel (Fig. S22a, b), indicating an effective broadening of the operational temperature range. In addition, when SV was substituted with 2-acrylamido-2-methylpropanesulfonic acid sodium salt (AMPS-Na) in the SG-GA-SS cluster system, the clustering strategy remained effective in widening the temperature window relative to the system with SS alone (Fig. S22c, d), further supporting the general applicability of the molecular clustering approach in enhancing the thermal stability of hydrogels. Additionally, the hydrovoltaic generators based on molecular clusters demonstrated higher open-circuit voltage and short-circuit current compared to those containing only SS or SM (Fig. S23), confirming the broad universality of the clustering strategy in improving hydrovoltaic generation performance.

To explore broader applications, we incorporated the molecularly clustered hydrogel to fabricate a pressure sensor by sandwiching it between 3D-printed stainless-steel electrodes (Fig. 4f). Under external pressing, the contact area between the hydrogel and electrodes increases, thereby enhancing the short-circuit current and enabling pressure detection. As shown in Fig. 4g, even at −35 °C and 80 °C, the sensor retained an effective response, indicating that molecular clustering preserves effective ion transport and mitigates both high-temperature vaporization and low-temperature freezing effects. These results illustrate the potential for self-powered sensing and other multifunctional devices in harsh conditions.

Methods
Materials
Chemicals including sodium vinyl sulfonate (SV, 25 wt% in water), GA (GA, 98%), sodium glycollate (SG, 98%), acrylamide (AAM), and 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone as UV initiator (UV-I, ≥98.0%, HPLC) were purchased from Aladdin. SS (SS, 98%) and N,N’-methylenebis(acrylamide) (MBAA, AR) were purchased from Macklin. Graphite felt with a thickness of one millimeter was customized by Xinye Electronic Material Manufacturers (Taobao, China). Zinc electrodes with a thickness of one millimeter were purchased from Zhoukou Guangjule Trading Co., Ltd., China. DI water was obtained from local sources (Dalian, China).

Fabrication of molecularly clustered hydrogels
SG-GA-SS cluster hydrogels were prepared by introducing 5.5 mmol SV solute, GA, SS, SG, 3 g AAM, 0.0037 g MBAA, and 0.0054 g UV-I in 7 ml of DI water for pre-solution, which was then stirred on a magnetic stirrer under 50 °C heating and 500 rpm for 15 min until the solution became clear. The hot solution was poured into the mold and irradiated with 365 nm ultraviolet light under 45 W of irradiation power for 40 min. The ambient temperature and relative humidity should be controlled at 25 °C and 60% to ensure a good sample yield during the polymerization process. The amount of SS in SS-based hydrogels is the sum of SG, SS and GA in SG-GA-SS cluster hydrogels. For the hydrogel without clusters in electrical measurements and hydrovoltaic experiments, the SV solute is 5.5 × 3 mmol for ensuring the same amount of Na+ ions as the SG-GA-SS cluster hydrogel.

GA, SS, and SG are key components for validating the molecular clustering strategy. Since the primary focus of this study is to investigate the effect of their clusters on hydrogel properties, we employed a fixed 1:1:1 molar ratio to ensure compositional consistency and simplify the analysis of clustering behavior. Furthermore, we evaluated the hydrovoltaic performance of the hydrogel under this ratio at varying concentrations. As shown in Fig. S24, the highest performance was achieved at 5.5 mmol. Therefore, this concentration was adopted in all experiments to ensure optimal hydrovoltaic output, thereby achieving high-efficiency hydrovoltaic performance across a broad temperature range.

Material Characterization
Characteristic groups and the O-H bond intensity of water in hydrogels were detected by Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS20, USA). The newly synthesized hydrogel was characterized to determine the strength of the O-H bonds, thereby assessing the stability of water in the samples. When characterizing the interactions between functional groups in the hydrogel, dried samples were used to eliminate the influence of water hydroxyl groups. The hydrogen-bond intensity among water molecules in hydrogels was analyzed via Raman spectroscopy with a 532 nm excitation laser (HR Evolution). All Raman spectra were fitted by overlapping peak resolution through the Gaussian function. Surface states of molecularly clustered hydrogels under different temperatures were captured by microscopy (Jiangnan N-300M, China). The above two characterizations were performed on newly synthesized hydrogels to assess the stability of water. The porous networks in molecularly clustered hydrogels were observed by a cryo-SEM (Zeiss Sigma 300) with a cryo-transfer system (Quorum PP3010T), and the samples were pre-soaked in DI water for 24 h. The hydrogel samples used were square specimens with dimensions of 1 × 1 × 0.2 cm, which were fully immersed in water. During immersion, the pores within the hydrogel expanded, and subsequent freezing preserved the swollen structure for observation. It is worth noting that the high-water content of hydrogels limits the applicability of most characterization techniques for observing their internal micro-porous structures. Consequently, the majority of hydrogel studies employ cryo-SEM to visualize pore distribution57–59. Although the observed pores may differ from those in the native state and thus cannot provide a fully accurate quantification, using the same pretreatment procedure and characterization conditions allows for reliable qualitative comparison.

Thermodynamic measurements
The thermodynamic behaviors of hydrogels during vaporization and freezing processes were assessed using Differential Scanning Calorimetry (DSC; METTLER TOLEDO DSC3). All hydrogel samples used for characterization were cut from newly synthesized hydrogels. For the freezing analysis, hydrogel samples were hermetically sealed within aluminum crucibles to prevent water loss, and subjected to a cooling protocol that reduced the temperature from 30 °C to −100 °C at a controlled rate of −5 °C min⁻¹, under a nitrogen atmosphere with a flow rate of 50 mL min⁻¹. The heat flow was continuously recorded to detect exothermic phase transitions indicative of freezing. In the vaporization assessment, hydrogel samples were placed in aluminum crucibles with perforations to allow vapor release. These samples were then heated from 30 °C to 200 °C at a rate of 5 °C min⁻¹, under identical nitrogen flow conditions. The resulting heat flow data were analyzed to identify endothermic transitions corresponding to water vaporization. The onset, peak, and completion temperatures of these vigorous phase transitions were extracted from the DSC thermograms, and the associated enthalpy changes (ΔH) were determined through integration of the area under the transition peaks. The start and end temperatures of the peak were used to define the boundaries of the liquid-phase water stability temperature range.

The water retention endurance within the hydrogels was evaluated using a quasi-isothermal heat flux measurement scheme, which enabled the characterization of isothermal phase-change behavior under both high and low-temperature conditions. This approach involves maintaining the hydrogel at its initially vigorous phase transition temperature for 60 min under constant temperature and pressure conditions. The vigorous phase transition temperature refers to the onset temperature of the phase transition, i.e., the temperature corresponding to the initial peak observed during the respective heating or cooling protocol. This temperature is reached starting from room temperature at a rate of 5 °C min−1 for heating or −5 °C min−1 for cooling. Throughout the entire measurement cycle, the nitrogen gas flow rate was maintained at 50 mL min−1. The mass of each sample tested was controlled to approximately 15 mg. Prior to all measurements, the instrument was calibrated according to the manufacturer’s reference guidelines. The changes in the heat flux curve are observed, and the time corresponding to the step change indicates the maximum duration for which the water content within the hydrogel sample is maintained.

Mechanical properties characterization
Hydrogels were cut into dumbbell-shaped specimens according to reported methods to enable tensile testing60,61. Samples were equilibrated at controlled temperatures of −35 °C, 25 °C, and 80 °C prior to testing to assess thermal stability and mechanical performance across a broad temperature spectrum. Tensile tests were conducted using a material testing machine (XK-206s) operating at a constant strain rate of 50 mm min−1. The stress-strain response was quantified by normalizing the applied force to the initial cross-sectional area and the measured elongation to the initial gauge length. Young’s modulus was calculated from the linear region of the stress-strain curve, corresponding to the 0–5% strain interval62. The sample area before and after stretching was estimated using the width of a fingertip as a reference, which was then used to calculate the areal strain.

Electrochemical measurements
All experiments were conducted under meticulously controlled conditions within a programmable environmental chamber (DHS-GDJS-100B), where temperature and humidity were precisely regulated. For the evaluation of hydrovoltaic performance, high-purity zinc and graphite felt electrodes were utilized to characterize the electrochemical properties during hydrovoltaic conversion. Hydrogels, fabricated to dimensions of 1 × 5 × 2 mm, was positioned with electrodes affixed at each end. To minimize external influences, the system was hermetically sealed with cling film, leaving a 1 cm² window exposed to ensure water evaporation. The water gradient established across the hydrogel facilitated ion migration, which in turn generated an electrical output. The electrical performance of the system was continuously monitored in real-time using a multimeter (Keithley 6510).

The electrical resistance of the hydrogels was determined using the four-wire method, facilitated by a Keithley 2450 sourcemeter. During testing, the sample assembly was carried out in strict accordance with the procedures used in the aforementioned hydrovoltaic performance evaluation. This approach ensures that the measured resistance accurately reflects the resistance under hydrovoltaic operational conditions. This method minimizes the influence of lead and contact resistances, ensuring precise measurements. Samples were prepared by attaching four equidistant probes along the surface of the material. The outer two probes supplied a constant current, while the inner two probes measured the voltage drop across a defined length of the sample. This configuration allowed for direct calculation of the sample resistance according to Ohm’s law. The measurements were conducted within a programmable environmental chamber (DHS-GDJS-100B), with the sourcemeter configured to perform multiple sweeps to ensure data reliability. The collected data were processed to extract the resistance values, which were averaged over repeated measurements to account for any variability.

Temperature gradient-response ion transport was measured by positioning identical zinc electrodes symmetrically on opposite sides of the hydrogel samples with an insulating tape applied to the surface. During the testing process, the sample assembly was consistent with that used during the assessment of the hydrovoltaic performance. This consistency ensures that the characterized ion transport pathways and distances align with those during hydrovoltaic operation, thereby accurately reflecting the ion transport performance under actual operating conditions. Uniform heating was applied using a silicone rubber heater (Taobao, China), which was applied under one side of the hydrogel. The open-circuit voltage between the two electrodes was measured in real-time using a Keithley 6510 multimeter (Tektronix, USA).

The ionic conductivity of the material was assessed using a broadband dielectric impedance spectrometer (CONCEPT-80, NOVOCONTROL). We prepared thin hydrogel samples with a uniform thickness of 2 mm for measurements under different temperature conditions to ensure consistency in the experimental setup. The contact area between the electrode and the hydrogel was controlled at 2 mm2. Impedance spectra were recorded across a frequency range from 0.8 MHz to 0.1 Hz, with a voltage amplitude of 1 V to mitigate nonlinear effects17. Measurements were conducted immediately after removing the samples from the controlled environment to minimize any environmental influence. Each impedance spectrum was collected at incremental frequencies and analyzed using an appropriate equivalent circuit model to extract the real and imaginary components of impedance. The ionic conductivity (σ) was calculated from the impedance data using the following equation:

σ = δ / (R × A) (1)

where δ represents the measured thickness of the sample, A is the cross-sectional area of the sample in contact with the electrodes, and R is the resistance obtained from impedance fitting. The resistance (R) was determined from the impedance data by analyzing the real part of the impedance (Zreal) divided by the magnitude of the impedance (|Z|).

Quantum chemistry calculations
All quantum chemical calculations, including the evaluation of thermodynamic properties, were conducted using Gaussian 16, ensuring a comprehensive analysis of the intermolecular interactions and hydration effects in the studied system. Geometric optimization of molecular structures was conducted using the B3LYP DFT method combined with the 6–31G basis set. Frequency analyses were performed to verify that all optimized geometries correspond to true minima on the potential energy surface. To evaluate binding energy and hydration enthalpy values, the M05-2X functional was employed with the 6-311++G basis set, chosen for its efficacy in accurately describing long-range van der Waals interactions63,64. Binding energy of the SG-GA-SS cluster was computed as the difference in total energy between the fully optimized SG-GA-SS cluster and the sum of the energies of the individual SG, GA, and SS molecules. Hydration enthalpy was determined by the enthalpy difference between calculations in the gas phase and those utilizing the polarizable continuum model (PCM) to simulate the solvent environment. Enthalpy values were extracted from thermodynamic calculations using the Shermo procedure65. Electrostatic potential maps were generated and analyzed by GaussView.

Molecular dynamics simulations
MD simulations were conducted using the LAMMPS software package. The simulations employed the optimized potentials for liquid simulations all-atom (OPLS-AA) force field to model systems comprising SG, GA, SS, and water molecules66. Specifically, the SG-GA-SS system contained 40 SG, GA, and SS molecules, alongside 2830 water molecules, while the SS system included 120 SS molecules with an identical number of water molecules. Water molecules were represented using the TIP3P model, as implemented in LAMMPS. Force-field parameters and partial atomic charges for the molecules were derived using the LigParGen web-based service67. The molecular systems were placed in a cubic simulation box with dimensions of 45 × 45 × 45 nm³, and the initial configurations were generated randomly using the Packmol package. The systems underwent simulated annealing, starting with an energy minimization step to relax the system to a local energy minimum, with convergence criteria set to 1.0 × 10⁻⁷ for both energy and force and a maximum of 10,000 iterations. Following minimization, the systems were equilibrated under a number-pressure-temperature (NPT) ensemble conditions for 1.5 million timesteps, with a timestep of 1.0 fs. Thermodynamic properties, including temperature, pressure, potential energy, kinetic energy, and volume, were recorded every 1000 timesteps during the equilibration phase to monitor system stability. Temperature and pressure were maintained at 298 K and 1 atm, respectively, using a Nosé-Hoover thermostat and barostat with damping parameters of 100 and 1000 timesteps.

The equilibrated state at timestep 50000 was selected for subsequent analyses. Radial distribution functions (RDF) were computed for specific atom pairs of interest, with cutoff distances set to 8.0 Å. The resulting RDFs were averaged over the entire simulation duration and recorded in output files. The final atomic configurations and velocities were saved in both data and custom dump formats for further analysis. Snapshots of the most probable hydration sheaths were sampled from the simulation trajectory using Material studio 2020. Hydrogen bond (H-bond) analysis was performed using a custom Python script68. The script identified H-bonds based on a geometric criterion: the distance between two oxygen atoms was < 3.0 Å, and the O–H···O angle exceeded 150°69. The script tracked bond lengths, determining the minimum and maximum lengths observed. Upon completion of the analysis, the average bond length was calculated and reported along with the minimum and maximum bond lengths. H-bond visualizations were generated using Material Studio 2020, where H-bonds inside the first and remaining hydration shell of hydrophilic groups were separately visualized.

Entropy calculations were performed based on MD simulation data70. A Python script, employing the NumPy and Matplotlib libraries, was used for data processing and visualization71. Entropy, S, was calculated using the following expression:

S = kB ln [ (kB T e^2 / ℏ^2) M δ + I ] (2)

where kB refers to the Boltzmann constant, ℏ is the reduced Planck constant, e is the Euler’s number and T corresponds to the temperature. M and I are the mass matrix and the unity matrix, respectively. The covariance matrix, δ, captures the coordinate fluctuations, defined as δij = 〈(xi − 〈xi〉)(xj − 〈xj〉)〉. Input data consisted of atomic coordinates obtained from an MD simulation dump file with a timestep of 1 fs and an atom count of 80. Atomic positions in the x, y, and z directions were extracted, converted from angstroms to meters, and used to construct covariance matrices reflecting atomic positional fluctuations.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The work by Prof. Li develops a hydrogel system for hydrovoltaic technologies applicable in extreme environments. By combining organic molecules to induce the aggregation of organic salt anions into the polymer network, the resulting hydrogels serve as a hydrovoltaic system that exhibits a wide range of operational temperatures and high conversion efficiency, along with significant mechanical strength across this temperature range. This study systematically investigates the structure, functionality, and applications of hydrovoltaics through multiple characterizations and simulations. The referee recommends publication after addressing the following comments:\n1.In Figure 1, the schematic is unclear regarding the interaction between the main chains and the small molecules. It is suggested that the chemical structures of the polymer and organic salts are included in the figure. Additionally, the main text lacks a description of the differences in Gibbs free energy of water in different phases and temperatures.\n2.The rationale for requiring controlled evaporation of excess water during sample preparation is unclear. Although the authors state that this process promotes close contact among GA, SS, and SG, yielding a tightly crosslinked hydrogel architecture, how is this statement substantiated? Furthermore, how does the evaporation of water influence the hydrovoltaic and mechanical behavior of the hydrogels?\n3.Since the authors assert that the advanced hydrovoltaic behavior arises from the aggregation of organic salt anions within the polymer network. How to prove this statement? for instance, SAXS or other characterizations?\n4.Given that the hydrovoltaic behavior arises from the diffusion of ions due to water evaporation through temperature gradients, this behavior appears to be unidirectional and lacks reversible features as water evaporates from the hydrogels. How do the authors propose to address this issue for practical applications?\n5.Figure S9 suggests that the mechanical performance of the hydrogel is independent of the measured temperature, indicating the elastic behavior. However, the sample appears transparent at room temperature but becomes opaque at -35 degrees, suggesting microphase separation (Figure 3h). How does the rheological behavior of the sample manifest across the wide range of measured temperatures?", "reviewer_2": "This study proposed a molecular clustering strategy to overcome long-standing challenges for hydrovoltaic technology in ion transport, thermal stability, and energy conversion efficiency. The authors expanded the functional temperature range of hydrovoltaic systems, which traditionally suffer from poor performance under extreme environmental conditions, and simultaneously enhance mechanical robustness—features that are critical for real-world deployment in environmental sensors and self-powered systems.The overall logic of the paper is clear, with well designed experiments and substantial data. However, there are still some aspects that need further improvement.\n(1) For the evaluation of hydrovoltaic performance, the authors used high-purity zinc and graphite felt electrodes to characterize the electrochemical properties during hydrovoltaic conversion. However, zinic electrode is susceptible to corrosion. The auhtors need to prove that the voltage/current are generated from hydrovoltaic effect but not faradic effect by using two same graphite electrodes, and compare the difference in their performance by using different inert electrodes, for example gold or platium electrodes.\n(2) Introduction section: in addition to comparing the shortcomings of existing technologies, a more in-depth analysis of the principles and effects of other similar strategies for improving material properties can be introduced.\n(3) Methods section: In the preparation method of hydrogels, some details that may affect the experimental results are not described in detail. For example,in the material characterization part, the preparation process of some test samples is not clearly described. The pretreatment steps of the samples used for SEM and other observation before freezing treatment may affect the accurate observation of the microstructure of hydrogels. In the electrochemical performance tests, the basis for choosing some test conditions is not fully explained. For example, when measuring ionic conductivity, the frequency range of 0.8 MHz - 0.1 Hz is selected.\n(4)Discussion section: The in-depth exploration of the underlying mechanisms of some phenomena could be further enhanced. For example, the results of molecular dynamics simulations could be combined to analyze from more microscopic perspectives such as the interaction energy and steric hindrance between ions and molecular clusters. In addition, the discussion on the universality of this molecular clustering strategy in other similar hydrogel systems or different types of hydrovoltaic devices is insufficient.", "reviewer_3": "The manuscript by Li et al. presents an interesting molecular-clustering strategy for broad-temperature hydrogels in hydrovoltaic applications. The authors introduce an innovative, fully organic system that tunes hydration free energy. The materials they used exhibit outstanding mechanical strength and ion conductivity, and the straightforward synthesis suggests broad reproducibility. Given its novelty and potential to advance hydrovoltaic conversion, I recommend this manuscript for publication in Nature Communications, pending suitable revisions to enhance clarity and completeness.\n1. The manuscript would benefit from inclusion of the hydrovoltaic current across the full intended temperature range. Please provide these data, ideally in a concise plot, to demonstrate the performance trends under varying thermal conditions.\n2. The claim of sustained mechanical integrity is important. In Fig. S9, please include quantitative metrics, such as percentage change in tensile modulus, compared to a control hydrogel. This will more clearly highlight the clustering hydrogel’s advantage.\n3. In Fig. 2a and 2b, the text attributes the increased surface charge density and reduced hydration enthalpy solely to the GA molecule. However, these effects likely arise from the complete molecular cluster. Please revise the description to accurately reflect the contribution of the entire clustering assembly.\n4. The DSC data on hydrogel endurance are intriguing. To ensure rigor, please provide full experimental parameters (e.g., heating rate, sample mass, calibration standards).\n5. The reported 1800% area strain is impressive. Please outline the calculation method in either the main text or the Supplementary Information.\n6. In Fig. 4b–c, the hydrogel’s resistance advantage is more pronounced at low temperatures, whereas its potential‐difference advantage is more pronounced at high temperatures. Adding a brief discussion to explain these trends. Perhaps in terms of the intrinsic temperature dependence of ion mobility would strengthen the interpretation.", "reviewer_4": "In the manuscript, the authors present a high-performance hydrogel designed using a molecular clustering strategy. This hydrogel shows significant promise for real-world applications in extreme environments. The manuscript is well-written and includes clear and informative figures. Therefore, I recommend it for publication in Nature Communications after revision. To further improve the paper’s quality, I have listed the following comments and suggestions for the authors’ consideration.\n1.As indicated by the title, the key innovation is the hydrogel’s ability to work across extreme temperatures ranging from −35 °C to 80 °C. However, the introduction section provides too little discussion about Temperature Comparisons with reported results. Figure S11, which presents a Comparison of temperature ranges, should be moved from the supplementary information to the main text and provide more detailed discussion.\n2. All citations in Figures 1e, S11, S12, and S14 are incomplete, lacking the issue and article number. For example, “Nat. Commun. 2024” in Figure 1e. More importantly, many citations, such as “Nat. Commun. 2024” in Figure 1e, cannot be found in the Reference List. This is a big mistake.\n3. The preparation of the designed hydrogels is not clearly described in the manuscript and lacks explicit details, such as optimal ratios or concentrations. The authors should provide more detailed preparation methods to ensure that readers can easily reproduce the reported hydrogels."}, "author_response": "Response to reviewers’ comments\nReply: We sincerely appreciate the time and effort that all reviewers dedicated to providing feedback on\nour manuscript. Point-by-point responses to the other reviewers have been listed behind, of which\nresponse to Reviewer #1 begins on Page 1, response to Reviewer #2 begins on Page 8, response to\nReviewer #3 begins on Page 15, and response to Reviewer #4 begins on Page 18.\nReviewer #1:\nThe work by Prof. Li develops a hydrogel system for hydrovoltaic technologies applicable in extreme\nenvironments. By combining organic molecules to induce the aggregation of organic salt anions into the\npolymer network, the resulting hydrogels serve as a hydrovoltaic system that exhibits a wide range of\noperational temperatures and high conversion efficiency, along with significant mechanical strength\nacross this temperature range. This study systematically investigates the structure, functionality, and\napplications of hydrovoltaics through multiple characterizations and simulations. The referee\nrecommends publication after addressing the following comments:\nReply: We sincerely appreciate you for the important comments, which help us to greatly improve the\nquality of our manuscript. We have revised the manuscript carefully and organized our point-by-point\nresponse to these comments below. All corresponding revisions have been highlighted in red in the\nrevised Manuscript and Supplementary Information.\n1. In Figure 1, the schematic is unclear regarding the interaction between the main chains and the small\nmolecules. It is suggested that the chemical structures of the polymer and organic salts are included in\nthe figure. Additionally, the main text lacks a description of the differences in Gibbs free energy of water\nin different phases and temperatures.\nReply: We sincerely appreciate your thorough review and the valuable comments you have raised. We\nhave addressed these points in detail below, organizing our responses into three specific sections.\n1) In the revised Manuscript, we have updated Figure 1 to more clearly illustrate the interactions\nbetween the main chains and the small molecules. We would like to emphasize that the primary focus of\nour original study was on the interactions among the small molecules themselves, rather than between\nthe small molecules and the main chains. Nevertheless, in response to your concern, we conducted FTIR\nspectroscopy analysis to investigate possible interactions between poly(sodium vinylsulfonate) chains\nand the small molecules. The results have been included as Fig. S3b.\nFig. S3 FTIR spectra of pure PAAM hydrogel with sequential addition of GA, SS, SG and SV.\n1 / 24\nAs shown in the Fig. S3, the introduction of SV monomer decreased the carboxyl peak of glycolic acid\nand the hydroxyl peaks of both glycolic acid and sodium glycolate. These changes suggest the presence\nof interactions between the poly(sodium vinylsulfonate) chains and the glycolic acid/sodium glycolate\nmolecules. However, considering that such interactions are not the central focus of this paper, we have\nonly schematically indicated them as dotted lines in Fig. 1c without extensive discussion in the main text.\nWe hope this approach addresses your comment appropriately.\n2) As suggested, we have included the chemical structures of the main polymer and the organic salts\nin Fig. 1. The structure of the poly(sodium vinylsulfonate) chain is presented in Fig. 1a, and that of the\norganic salts is shown in Figure 1c.\n3) We apologize for omitting a discussion of the Gibbs free energy differences of water across phases\n2 / 24\nand temperatures. In the revised Manuscript we added the following paragraph to the Introduction section:\n“The core challenge lies in the temperature-sensitive hydrogen-bond network of water (Fig. 1b). At\nelevated temperatures, hydrogen bonds readily break, while in subzero conditions they become rigid, in\nboth cases disrupting ion dissociation and transport crucial for hydrovoltaic conversion. This temperature\nsensitivity stems from the temperature dependence of water's Gibbs free energy. The phase with the\nlowest Gibbs free energy is thermodynamically most stable, and for water the gas phase is lowest at\nhigher temperatures while the solid phase is lowest at lower temperatures, which explains water’s\ntendency to vaporize when heated and to freeze when cooled. To tackle these issues, researchers have\ntried using high concentrations of inorganic salts or organic components rich in hydrophilic groups to\nlowering the liquid water’s Gibbs free energy across an expanded temperature range.1, 2”\n2. The rationale for requiring controlled evaporation of excess water during sample preparation is unclear.\nAlthough the authors state that this process promotes close contact among GA, SS, and SG, yielding a\ntightly crosslinked hydrogel architecture, how is this statement substantiated? Furthermore, how does the\nevaporation of water influence the hydrovoltaic and mechanical behavior of the hydrogels?\nReply: We sincerely appreciate you highlighting this factor. We have addressed these points in detail\nbelow, organizing our responses into three specific sections.\n1) Regarding the role of water evaporation in forming tightly crosslinked hydrogel architecture during\nsample preparation, we systematically compared the microstructures and XRD spectra of hydrogels\nprepared with and without this evaporation step. The corresponding description has been added in\n“Design and characterization of molecularly clustered hydrogels” section of the revised Manuscript,\nwhich highlighted in red as follows:\n“During UV exposure, free-radical polymerization produces a three-dimensional crosslinked matrix (Fig.\nS2), while controlled evaporation of excess water further promotes close contact among GA, SS, and SG,\nyielding a tightly crosslinked hydrogel architecture. The evaporation step facilitates the controlled\ntransport of the internal solution, thereby promoting component aggregation.3, 4 As shown in Fig. S3,\nCryo-SEM imaging reveals the hydrogel microstructure after immersion in pure water for 8 hours. The\nevaporation step densifies the porous network and reduces the pore diameter. XRD results show enhanced\ndiffraction peaks for hydrogels prepared via the water evaporation step, as presented in Fig. S4. This\nfinding indicates that water evaporation promotes closer packing of the polymer network,5, 6 thereby\nfacilitating close contact among GA, SS and SG.”\nFig. S3 Cryo-SEM images of hydrogels obtained from preparation processes without and with a water\nevaporation step.\n3 / 24\nFig. S4 XRD patterns of hydrogels obtained from preparation processes with and without a water\nevaporation step.\n2) Regarding the influence of water evaporation in hydrovoltaic performance, we compared\nhydrovoltaic open-circuit voltage (V ) and short-circuit current (I ) of hydrogels prepared with and\noc sc\nwithout the evaporation step. The corresponding description has been added in “Supplementary Text”\nsection of the revised Supplementary Information as follows:\n“As shown in Fig. S25, evaporation increased V by 10.01% and I by 9.20%. This improvement\noc sc\ncorrelates with reduced water content and increased ion concentration after evaporation. Specifically,\ncovered samples during preparation retained a water content of 0.42 g g-1, while uncovered samples had\n0.27 g g-1, confirming evaporation lowers water content and raises ion concentration.”\nFig. S25 Hydrovoltaic performance of hydrogels obtained from preparation processes with and without\na water evaporation step.\n3) Regarding the influence of water evaporation in mechanical behavior, we compared tensile\nproperties of hydrogels prepared with and without the evaporation step. The corresponding description\nhas been added in “Supplementary Text” section of the revised Supplementary Information as follows:\n“As shown in Fig. S26, evaporation increased the maximum tensile strain by 9.73% and the maximum\nstress by 31.81%, indicating greater ductility. This enhancement is closely linked to microstructural\nchanges induced by evaporation. As revealed in Fig. S3, evaporation yielded a tightly pore network with\na reduced average pore size. This promotes even stress distribution across the entire network, rather than\nconcentration at localized pores, thereby improving mechanical behavior.”\n4 / 24\nFig. S26 Mechanical behavior of hydrogels obtained from preparation processes with and without a water\nevaporation step.\n3. Since the authors assert that the advanced hydrovoltaic behavior arises from the aggregation of organic\nsalt anions within the polymer network. How to prove this statement? for instance, SAXS or other\ncharacterizations?\nReply: We sincerely thank you for the constructive suggestion. Following the recommendation, we\ncompared the SAXS results of hydrogels containing sodium sulfanilate (SS), sodium sulfanilate and\nsodium glycolate (SS-SG), as well as sodium sulfanilate, sodium glycolate, and glycolic acid (SS-SG-\nGA). The corresponding results and discussion have been incorporated into the “Design and\ncharacterization of molecularly clustered hydrogels” section in the revised Manuscript and marked in red,\nas detailed below:\n“The thickness-normalized small angle X-ray scattering (SAXS) patterns shown in Fig. 2a,b indicate that\nsequential incorporation of SS, SG and GA preserves the hydrogels’ homogeneity and amorphous\ncharacter.7, 8 A comparison of peak intensities shows that co-introduction of SS and SG produces a marked\nincrease in scattering intensity. This behavior is attributable to the fact that both additives are dissociable\nsmall organic molecules that generate anions bearing organic functional groups upon dissociation.\nElectrostatic repulsion among these charged species loosens the internal network and increases structural\ndisorder, thereby enhancing the scattering intensity. By contrast, incorporation of GA leads to a\npronounced reduction in scattering intensity, to values below those observed for hydrogels containing SS\nor the SS-SG combination. This decrease indicates that GA reduces disorder within the hydrogel network,\nconsistent with aggregation induced by GA.”\nFig. 2: Material Characterization of molecular clustering and simulation analysis of molecular\nclusters-enhanced water binding. a, b SAXS spectra and 2D mapping of pure PAAM/SV hydrogel with\n5 / 24\nsequential addition of SS, SG and GA.\n4. Given that the hydrovoltaic behavior arises from the diffusion of ions due to water evaporation through\ntemperature gradients, this behavior appears to be unidirectional and lacks reversible features as water\nevaporates from the hydrogels. How do the authors propose to address this issue for practical applications?\nReply: We sincerely thank you for this valuable question. Although water evaporation from the hydrogel\nis unidirectional, the dehydrated hydrogel can reabsorb water and be reused for evaporation cycles,\nsimilar to the charging process of conventional batteries in which the hydrovoltaic generator reabsorbs\nwater. To assess feasibility, we performed hydrovoltaic performance measurements, as shown in Fig. S21.\nIn the revised Manuscript, we have added the corresponding description in the section “Enhancing\nhydrovoltaic performance” as below.\n“Furthermore, recognizing that water evaporation from the hydrogel during the hydrovoltaic conversion\nprocess is intrinsically unidirectional and therefore limits practical application, we evaluated a cyclic\noperating protocol consisting of one hour of evaporation followed by ten minutes of rehydration. As\nshown in Fig. S21, after six such cycles the device retained 87% of its initial open-circuit voltage and\n86% of its initial short-circuit current. These results indicate that, owing to strong hydrogen-bonding\ninteractions between molecular clusters and water molecules, dehydrated hydrogels readily reabsorb\nwater and can undergo repeated evaporation cycles. This behavior is analogous to the charging process\nof conventional batteries, in which water uptake restores device functionality. Additionally, the\ncombination of moisture absorption and evaporation has been employed to extend the duration,9, 10, 11\nwhile a recently reported innovative internal water circulation mechanism has demonstrated to enable\nsustained device operation.12”\nFig. S21 Open-circuit voltage and short-circuit current generated by the molecularly clustered hydrogel\nduring cyclic testing with 1-hour evaporation and 10-minute water-absorption intervals.\n5. Figure S9 suggests that the mechanical performance of the hydrogel is independent of the measured\ntemperature, indicating the elastic behavior. However, the sample appears transparent at room\ntemperature but becomes opaque at -35 degrees, suggesting microphase separation (Figure 3h). How does\nthe rheological behavior of the sample manifest across the wide range of measured temperatures?\nReply: We sincerely thank you for pointing out this difference and apologize for not clarifying it in the\noriginal Manuscript. The apparent opacity observed at −35 ℃ was in fact due to light reflection. The\nattached video of the stretching process provides a clearer demonstration. The rheological behavior of\nthe hydrogel across the temperature range has been added in the “Broadening the operating temperature\nrange” section, as follows:\n6 / 24\n“Under multi-axial extension, it achieves large-area deformation (surface strain ~1800%) without\nfracturing (Fig. 3h). Rheological behavior is shown in Fig. S13, the results revealed that the hydrogel\nconsistently exhibited a storage modulus (G’) greater than the loss modulus (G”) at –35 ℃, 25 ℃, and\n80 ℃, signifying characteristic of soft and elastic solids.13 We attribute this robust resilience to the GA-\ninduced clusters that firmly bind water, mitigating the structural damage typically caused by high-\ntemperature vaporization or low-temperature freezing.”\nFig. S13 Rheological behavior of the molecularly clustered hydrogel under –35 ℃, 25 ℃, and 80 ℃\n7 / 24\nReviewer #2:\nThis study proposed a molecular clustering strategy to overcome long-standing challenges for\nhydrovoltaic technology in ion transport, thermal stability, and energy conversion efficiency. The authors\nexpanded the functional temperature range of hydrovoltaic systems, which traditionally suffer from poor\nperformance under extreme environmental conditions, and simultaneously enhance mechanical\nrobustness—features that are critical for real-world deployment in environmental sensors and self-\npowered systems. The overall logic of the paper is clear, with well-designed experiments and substantial\ndata. However, there are still some aspects that need further improvement.\nReply: Thank you very much for your positive evaluation of our work. We also appreciate your valuable\nsuggestions, which have helped us further improve the quality of the manuscript. Our point-by-point\nresponses are as follows. All corresponding revisions have been highlighted in red in the revised\nManuscript and Supplementary Information.\n1. For the evaluation of hydrovoltaic performance, the authors used high-purity zinc and graphite felt\nelectrodes to characterize the electrochemical properties during hydrovoltaic conversion. However, zinc\nelectrode is susceptible to corrosion. The authors need to prove that the voltage/current are generated\nfrom hydrovoltaic effect but not faradic effect by using two same graphite electrodes, and compare the\ndifference in their performance by using different inert electrodes, for example gold or platinum\nelectrodes.\nReply: We thank you for highlighting this important factor. In accordance with your suggestion, we have\nadded the performance comparison with inert electrodes to the “Enhancing hydrovoltaic performance”\nsection of the revised Manuscript. The changes are as follows.\n“To prove that the voltage and current are generated from the hydrovoltaic effect, additional tests were\nconducted using two same graphite felt electrodes and asymmetric graphite felt–platinum inert electrodes.\nAs presented in Fig. S20, when two identical graphite felt electrodes were employed, localized\nevaporation within the hydrogel induced an internal ion concentration gradient. This promoted\ndirectional ion diffusion, leading to charge separation and the generation of both open-circuit voltage\nand short-circuit current. When platinum–graphite felt inert electrodes were used, the inherent electrode\npotential difference synergized with evaporation-driven ion diffusion, thereby enhancing electrical\noutput. In contrast, the slow surface oxidation of the zinc-active electrode generates a sustained flux of\nmetal cations, which facilitates ionic diffusion and thereby enhances electrochemical performance. These\nfindings are consistent with previously reported mechanisms in related literature.9, 14”\nFig. S20 Hydrovoltaic performance of the molecularly clustered hydrogel with symmetric graphite felt\nelectrodes versus with a graphite felt–platinum inert electrode pair.\n8 / 24\nIn order to improve the completeness of our study, we have incorporated the following additional\nexperiments and analyses into the “Enhancing hydrovoltaic performance” section in the revised\nManuscript. These additions clarify our rationale for employing zinc as the electrode material and hope\nto address your concerns.\n“Active metal electrodes such as copper, aluminum, and zinc have been widely adopted to enhance\nelectrical output and broaden application scenarios in recent hydrovoltaic studies.9, 14, 15, 16, 17, 18, 19\nAccordingly, we evaluated the open-circuit voltage (V ) of molecularly clustered hydrogels using these\noc\nthree metals as electrodes. By comparing the average V over the same measurement period, with error\noc\nbars indicating the standard deviation of fluctuations during this interval (Fig. S17), it was observed that\nzinc electrodes yielded a higher average V with smaller fluctuations, indicating superior stability.\noc\nFurthermore, comparisons were made between hydrogels containing only sodium sulfamate (SS) and\nmolecularly clustered hydrogels. Results demonstrated that, regardless of electrode type, the molecularly\nclustered hydrogel consistently exhibited high V , confirming that electrode selection does not alter the\noc\nconclusion that molecular clustering enhances charge accumulation and hydrovoltaic performance.”\nFig. S17 Comparison of the electrical performance between systems incorporating molecular clustering\nand those using sodium sulfamate (SS) alone under graphite felt and different metal electrodes.\n2. Introduction section: in addition to comparing the shortcomings of existing technologies, a more in-depth\nanalysis of the principles and effects of other similar strategies for improving material properties can be\nintroduced.\nReply: We sincerely thank you for this insightful suggestion. As suggested, we have revised the\nIntroduction to provide a more detailed explanation of how inorganic salts and organic molecules broaden\nthe hydrogel’s operational temperature range and of the effects these additives produce. The\ncorresponding revisions are highlighted in red as follows.\n“Inorganic salts enhance stability by the strong ion–water interactions, which disrupt the water’s\nhydrogen-bonding network and suppress ice crystallization at subzero temperatures. Moreover, at\nelevated temperatures, the strong hydration shells around ions increase the energy required for water\nmolecules to escape into the vapor phase, thereby raising the fast-vaporizing point. However, high ionic\nconcentrations can introduce strong electrostatic shielding that inhibits charge transport. Organic\nmolecules are typically low-freezing-point solvent species that act as co-solvents by partially replacing\nwater. These molecules depress the freezing point by increasing the fraction of non-freezable bound water.\nTheir vapor pressures are generally lower than that of water, thereby reduing the overall vapor pressure\nof the hydrogel and raising the temperature at which rapid evaporation commences. Nonetheless, organic\nsolvents often exhibit limited dissociation and high viscosity, leading to fewer mobile ions and reduced\n9 / 24\nconductivity.”\n3. Methods section: In the preparation method of hydrogels, some details that may affect the experimental\nresults are not described in detail. For example, in the material characterization part, the preparation\nprocess of some test samples is not clearly described. The pretreatment steps of the samples used for\nSEM and other observation before freezing treatment may affect the accurate observation of the\nmicrostructure of hydrogels. In the electrochemical performance tests, the basis for choosing some test\nconditions is not fully explained. For example, when measuring ionic conductivity, the frequency range\nof 0.8 MHz - 0.1 Hz is selected.\nReply: Thank you very much for your valuable review. We apologize for not providing sufficient details\non-details\non material preparation and characterization. In the revised Manuscript, we have added the description\nof the sample preparation process prior to cryo-SEM observation in the “Materials characterization”\nsection. In addition, we have included the rationale for the frequency range selected in the ionic\nconductivity measurements in the “Electrochemical measurements” section. Revisions are highlighted in\nred below.\n“The porous networks in molecularly clustered hydrogels were observed by a cryo-SEM (Zeiss Sigma\n300) with a cryo-transfer system (Quorum PP3010T), and the samples were pre-soaked in DI water for\n24 hours. The hydrogel samples used were square specimens with dimensions of 1 × 1 × 0.2 cm, which\nwere fully immersed in water. During immersion, the pores within the hydrogel expanded, and\nsubsequent freezing preserved the swollen structure for observation. It is worth noting that the high-water\ncontent of hydrogels limits the applicability of most characterization techniques for observing their\ninternal micro-porous structures. Consequently, the majority of hydrogel studies employ cryo-SEM to\nvisualize pore distribution.20, 21, 22 Although the observed pores may differ from those in the native state\nand thus cannot provide a fully accurate quantification, using the same pretreatment procedure and\ncharacterization conditions allows for reliable qualitative comparison.”\n“Impedance spectra were recorded across a frequency range from 0.8 MHz to 0.1 Hz, with a voltage\namplitude of 1 V to mitigate nonlinear effects. The frequency range was determined with reference to\nprevious related studies in the literature.23”\nTo further improve the description of the hydrogel preparation in the Methods section, in addition to the\ntwo specific revisions you highlighted, we have also made the following corresponding changes to\nprovide a clearer and more detailed account of our experimental procedures. The added description is\nalso highlighted in red below.\ni) Material Characterization\nCharacteristic groups and the O-H bond intensity of water in hydrogels were detected by Fourier\ntransform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS20, USA). The newly synthesized\nhydrogel was characterized to determine the strength of the O-H bonds, thereby assessing the stability of\nwater in the samples. When characterizing the interactions between functional groups in the hydrogel,\ndried samples were used to eliminate the influence of water hydroxyl groups. The hydrogen-bond\nintensity among water molecules in hydrogels was analyzed via Raman spectroscopy with a 532 nm\nexcitation laser (HR Evolution). All Raman spectra were fitted by overlapping peak resolution through\nthe Gaussian function. Surface states of molecularly clustered hydrogels under different temperatures\nwere captured by microscopy (Jiangnan N-300M, China). The above two characterizations were\nperformed on newly synthesized hydrogels to assess the stability of water.\n10 / 24\nii) Thermodynamic measurements\nThe thermodynamic behaviors of hydrogels during vaporization and freezing processes were assessed\nusing Differential Scanning Calorimetry (DSC; METTLER TOLEDO DSC3). All hydrogel samples used\nfor characterization were cut from newly synthesized hydrogels.\niii) Mechanical properties characterization\nHydrogels were cut into dumbbell-shaped specimens according to reported methods to enable tensile\ntesting.24, 25\niv) Electrochemical measurements\nThe electrical resistance of the hydrogels was determined using the four-wire method, facilitated by a\nKeithley 2450 source meter. During testing, the sample assembly was carried out in strict accordance\nwith the procedures used in the aforementioned hydrovoltaic performance evaluation. This approach\nensures that the measured resistance accurately reflects the resistance under hydrovoltaic operational\nconditions.\nTemperature gradient-response ion transport was measured by positioning identical zinc electrodes\nsymmetrically on opposite sides of the hydrogel samples with an insulating tape applied to the surface.\nDuring the testing process, the sample assembly was consistent with that used during the assessment of\nthe hydrovoltaic performance. This consistency ensures that the characterized ion transport pathways and\ndistances align with those during hydrovoltaic operation, thereby accurately reflecting the ion transport\nperformance under actual operating conditions.\nThe ionic conductivity of the material was assessed using a broadband dielectric impedance spectrometer\n(CONCEPT-80, NOVOCONTROL). We prepared thin hydrogel samples with a uniform thickness of 2\nmm for measurements under different temperature conditions to ensure consistency in the experimental\nsetup. The contact area between the electrode and the hydrogel was controlled at 2 mm2.\n4. Discussion section: The in-depth exploration of the underlying mechanisms of some phenomena could\nbe further enhanced. For example, the results of molecular dynamics simulations could be combined to\nanalyze from more microscopic perspectives such as the interaction energy and steric hindrance between\nions and molecular clusters. In addition, the discussion on the universality of this molecular clustering\nstrategy in other similar hydrogel systems or different types of hydrovoltaic devices is insufficient.\nReply: We sincerely thank you for the constructive suggestions. We will address these comments in two\nparts, first by exploring the underlying mechanism and then by discussing the universality of our strategy.\n1) Mechanism exploration: In the original Manuscript, our MD analysis focused on interactions\nbetween water molecules and hydrophilic functional groups in SS, GA and SG. These MD results were\ncomplemented by DFT calculations that quantified the interaction energies and identified the most stable\nconfigurations among the studied anions and GA molecules. The stable configuration was then adopted\nto analyze the hydration enthalpies. The combined MD and DFT evidence provided the mechanistic\ninterpretation for the experimentally observed widening of the hydrogel temperature range. In the revised\nManuscript, according to your suggestions, we have added further mechanistic investigation on\ncomparative analysis of the steric hindrance to Na+ transport in SS-only and molecular clustering systems\nthe MD results. These transport analyses were interpreted together with DFT-calculated interaction\nenergies between Na+ and molecular clusters to elucidate how cluster formation may contribute to the\nobserved improvement in hydrovoltaic performance. The added description has been added in the\n11 / 24\n“Enhancing hydrovoltaic performance” section and highlighted in red below.\n“Unlike most ionic conductors, hydrovoltaic effects rely on the fixation of anions and the directed\nmigration of cations, accumulating net charges at the terminal interface. Within the molecularly clustered\nhydrogel, clustering of SS and SG anions induced by GA molecules is expected to alleviate the steric\nhindrance experienced by Na+ during transport. We quantify the degree of steric hindrance acting on Na+\nusing thermodynamic entropy. The mathematical definition of entropy used in this analysis is given in\nthe Methods section. Larger entropy values correspond to a higher degree of disorder and to greater\nfreedom of Na+ movement. Analysis of the MD simulations indicates that Na+ ions in the molecularly\nclustered system exhibit higher entropy than those in the SS-only system (Fig. S15a). This finding implies\nthat formation of molecular clusters reduces the steric hindrance acting on Na+. The resulting free Na+\ntransport contributes to enhanced hydrovoltaic performance. DFT calculations provide complementary\nevidence for this conclusion by showing that the attractive interaction between SS anions and Na+ in the\nmolecularly cluster is markedly smaller than the corresponding interaction energy in the SS-only system\n(Fig. S15b). Such reduction reduces steric hindrance acting on Na+, thereby facilitating Na+ mobility.\nFig. S15 a Thermodynamic entropy reflecting the degree of steric hindrance acting on Na+ in SS-only\nand molecularly clustered system based on the results from MD. b DFT-calculated interaction energy of\nNa+ with the SS anions in both the SS-only system and the cluster.\nIn addition, as you have noted, the fundamental mechanisms underlying some phenomena warrant in-\ndepth exploration. To address this, beyond the aspects mentioned above, we have also provided additional\ndiscussion on other experimental observations presented in the revised Manuscript, as detailed below.\ni) Broadening the operating temperature range\nTo quantitatively assess water retention in high and low temperature regimes, we measured phase-change\nenthalpies, including isothermal tests that better reflect real-world applications (Fig. 3e,f). … This\nobservation is consistent with previous DFT calculations and molecular dynamics simulations, which\nindicate that the presence of GA molecules reduces the hydration enthalpy of the clusters. As a result, a\nhigher proportion of water molecules become associated with the clusters. Compared to free water\nmolecules, breaking the hydrogen bonds between these bound water molecules requires greater energy\ninput, thereby shifting the onset temperature of rapid vaporization to a higher range. Simultaneously, the\nenergy barrier for the bound water molecules to form ice crystals with surrounding water is increased,\nleading to an extension of the fast-freezing onset temperature toward lower values.\n12 / 24\nii) Enhancing hydrovoltaic performance\nFurther source-meter tests (Fig. 4b) showed that the SG-GA-SS cluster reduces the hydrogel’s electrical\nresistance by about 33% at low temperatures, highlighting the formation of more favorable ion channels\nthrough molecular clustering. The reduction in resistance is more significant at low temperature than at\nhigher temperature, which aligns with the well-established temperature dependence of ionic mobility.\nIonic conduction inherently diminishes at lower temperatures due to reduced thermal energy and polymer\nchain dynamics.26 For hydrogels without clusters, the simultaneous effects of decreased ionic mobility\nand reduced water phase fluidity lead to a pronounced increase in electrical resistance. By contrast, SG-\nGA-SS cluster mitigates these adverse temperature effects.\nTo examine the molecularly clustered hydrogel’s hydrovoltaic behavior under these conditions, we\nconducted non-isothermal experiments (Fig. 4c)… Under a 25 K temperature gradient, the response\npotential difference was enhanced by 183% relative to the control, revealing that this cluster effectively\nfacilitates spontaneous anion–cation separation and charge accumulation. This phenomenon arises\nbecause increasing temperature enhances polymer chain dynamics and reduces viscosity, thereby\nfacilitating ion transport.27, 28 Combined with the thermal stability of liquid water confined within the\nmolecularly clustered hydrogel, these factors lead to a more pronounced increase in potential difference\ncompared with the hydrogel without clusters.\n2) Universality: In our original Manuscript, the hydrogel was prepared using sodium vinyl sulfonate\n(SV), sodium sulfamate (SS), glycolic acid (GA), and sodium glycolate (SG) as the primary functional\ncomponents. We note that hydrogels employed in hydrovoltaic applications typically rely on sulfonate\ngroups as the dissociative functional groups.17, 23, 29 To evaluate the universality of this strategy, we\nconducted two additional sets of experiments in which sodium 2-acrylamido-2-methylpropane sulfonate\n(AMPS-Na) and sodium methanesulfonate (SM) were used to replace SV and SS, respectively. The\ntemperature-dependent behavior and hydrovoltaic performance of these modified systems were\nsubsequently characterized. The details have been added in the “Universality and applications” section\nin the revised Manuscript, as shown below.\n“To explore the universality of the molecular clustering strategy, we replaced SS in the SG-GA-SS cluster\nof the SV-based hydrogel with sodium methanesulfonate (SM) and examined the phase transition\nbehavior. It was observed that, compared to the system with SM alone, the presence of the SG-GA-SM\ncluster significantly reduced the initial freezing transition temperature and increased the initial\nvaporization transition temperature of the hydrogel (Fig. S22a,b), indicating an effective broadening of\nthe operational temperature range. In addition, when SV was substituted with 2-acrylamido-2-\nmethylpropanesulfonic acid sodium salt (AMPS-Na) in the SG-GA-SS cluster system, the clustering\nstrategy remained effective in widening the temperature window relative to the system with SS alone\n(Fig. S22c,d), further supporting the general applicability of the molecular clustering approach in\nenhancing the thermal stability of hydrogels. Additionally, the hydrovoltaic generators based on\nmolecular clusters demonstrated higher open-circuit voltage and short-circuit current compared to those\ncontaining only SS or SM (Fig. S23), confirming the broad universality of the clustering strategy in\nimproving hydrovoltaic generation performance.”\n13 / 24\nFig. S22 Differential scanning calorimetry (DSC) heat-flow curves for four hydrogel samples: SV\nhydrogel with SM only, SV hydrogel containing SG-GA-SM clusters, AMPS-Na hydrogel with SS only,\nand AMPS-Na hydrogel containing SG-GA-SS clusters. The onset temperatures for freezing and\nvaporization are indicated.\nFig. S23 Hydrovoltaic performance for four hydrogel samples: SV hydrogel with SM only, SV hydrogel\ncontaining SG-GA-SM clusters, AMPS-Na hydrogel with SS only, and AMPS-Na hydrogel containing\nSG-GA-SS clusters.\n14 / 24\nReviewer #3:\nThe manuscript by Li et al. presents an interesting molecular-clustering strategy for broad-temperature\nhydrogels in hydrovoltaic applications. The authors introduce an innovative, fully organic system that\ntunes hydration free energy. The materials they used exhibit outstanding mechanical strength and ion\nconductivity, and the straightforward synthesis suggests broad reproducibility. Given its novelty and\npotential to advance hydrovoltaic conversion, I recommend this manuscript for publication in Nature\nCommunications, pending suitable revisions to enhance clarity and completeness.\nReply: We sincerely thank you for your positive evaluation and thoughtful, constructive suggestions.\nThe comments have materially improved the clarity and completeness of the manuscript. Below, we\nprovide point-by-point responses to each of your comments. All corresponding revisions have been\nhighlighted in red in the revised Manuscript and Supplementary Information.\n1. The manuscript would benefit from inclusion of the hydrovoltaic current across the full intended\ntemperature range. Please provide these data, ideally in a concise plot, to demonstrate the performance\ntrends under varying thermal conditions.\nReply: We thank you for this constructive suggestion. We have added a new figure (Fig. S18) showing\nthe hydrovoltaic current across the entire effective temperature range.\nFig. S18 Short-circuit current (I ) of molecularly clustered hydrogels over the temperature range from –\nsc\n35 ℃ to 80 ℃.\n2. The claim of sustained mechanical integrity is important. In Fig. S9, please include quantitative metrics,\nsuch as percentage change in tensile modulus, compared to a control hydrogel. This will more clearly\nhighlight the clustering hydrogel’s advantage.\nReply: We thank you for the valuable suggestion. Following the advice, we have added quantitative\nmetrics as Fig. S12b to highlight the advantage of the clustering hydrogel. The corresponding description\nhas been included in the “Broadening the operating temperature range” section of the revised Manuscript,\nhighlighted in red as follows:\n“Finally, we evaluated the molecularly clustered hydrogel’s behavior under temperatures from −35 to\n80 ℃, focusing on optical transparency and mechanical strength (Fig. S11 and S12). Throughout this\nrange, the molecularly clustered hydrogel retains stable surface morphology and clear optical properties,\nwith tensile strains of around 1000% and only a 3–6% deviation from room-temperature performance—\nsignificantly outperforming pure hydrogels without clusters (45–52%). In addition, the deviation in\nYoung’s modulus for the molecularly clustered hydrogel was generally less than 15%, whereas that of\nthe cluster-free hydrogel consistently exceeded 15%.”\n15 / 24\nFig. S12 b Comparison of mechanical-performance deviation of hydrogels with and without SG–GA–\nSS clusters.\n3. In Fig. 2a and 2b, the text attributes the increased surface charge density and reduced hydration enthalpy\nsolely to the GA molecule. However, these effects likely arise from the complete molecular cluster. Please\nrevise the description to accurately reflect the contribution of the entire clustering assembly.\nReply: We sincerely thank you for the careful review of our work. In the revised Manuscript, we have\nmodified the corresponding statements accordingly. The changes are highlighted in red in the\n“Broadening the operating temperature range” section of the revised Manuscript, as detailed below:\n“Compared with a lone SS anion, the presence of molecular cluster significantly increases the negative\ncharge on the sulfonate (from −0.641 eV to −0.658 eV) while concurrently raising the positive partial\ncharge on amino hydrogen atoms (up to +0.331 eV).”\n4. The DSC data on hydrogel endurance are intriguing. To ensure rigor, please provide full experimental\nparameters (e.g., heating rate, sample mass, calibration standards).\nReply: In response to your suggestion, we have provided the complete experimental parameters for the\nDSC measurements in the “Thermodynamic measurements” section of the revised Manuscript. The added\ndetails are highlighted in red for clarity, as shown below.\n“The water retention endurance within the hydrogels was evaluated using a quasi-isothermal heat flux\nmeasurement scheme, which enabled the characterization of isothermal phase-change behavior under\nboth high and low-temperature conditions. This approach involves maintaining the hydrogel at its initially\nvigorous phase transition temperature for 60 minutes under constant temperature and pressure conditions.\nThe vigorous phase transition temperature refers to the onset temperature of the phase transition, i.e., the\ntemperature corresponding to the initial peak observed during the respective heating or cooling protocol.\nThis temperature is reached starting from room temperature at a rate of 5 °C min-1 for heating or\n−5 °C min-1 for cooling. Throughout the entire measurement cycle, the nitrogen gas flow rate was\nmaintained at 50 mL min-1. The mass of each sample tested was controlled to approximately 15 mg. Prior\nto all measurements, the instrument was calibrated according to the manufacturer’s reference guidelines.\nThe changes in the heat flux curve are observed, and the time corresponding to the step change indicates\nthe maximum duration for which the water content within the hydrogel sample is maintained.”\n5. The reported 1800% area strain is impressive. Please outline the calculation method in either the main\ntext or the Supplementary Information.\nReply: According to your suggestion, we have added the method for calculating areal strain to the\n“Mechanical properties characterization” section of the main text. The added details are highlighted in\n16 / 24\nred for clarity, as shown below.\n“The sample area before and after stretching was estimated using the width of a fingertip as a reference,\nwhich was then used to calculate the areal strain.”\n6. In Fig. 4b–c, the hydrogel’s resistance advantage is more pronounced at low temperatures, whereas its\npotential‐difference advantage is more pronounced at high temperatures. Adding a brief discussion to\nexplain these trends. Perhaps in terms of the intrinsic temperature dependence of ion mobility would\nstrengthen the interpretation.\nReply: We sincerely thank you for the constructive comments. In the revised Manuscript, we have\nincorporated the corresponding discussion in the “Enhancing hydrovoltaic performance” section in the\nrevised Manuscript, as detailed below:\n“Further source-meter tests (Fig. 4b) showed that the SG-GA-SS cluster reduces the hydrogel’s electrical\nresistance by about 33% at low temperatures, highlighting the formation of more favorable ion channels\nthrough molecular clustering. The reduction in resistance is more significant at low temperature than at\nhigher temperature, which aligns with the well-established temperature dependence of ionic mobility.\nIonic conduction inherently diminishes at lower temperatures due to reduced thermal energy and polymer\nchain dynamics.26 For hydrogels without clusters, the simultaneous effects of decreased ionic mobility\nand reduced water phase fluidity lead to a pronounced increase in electrical resistance. By contrast, SG-\nGA-SS cluster mitigates these adverse temperature effects.”\n“To examine the molecularly clustered hydrogel’s hydrovoltaic behavior under these conditions, we\nconducted non-isothermal experiments (Fig. 4c) … Under a 25 K temperature gradient, the response\npotential difference was enhanced by 183% relative to the control, revealing that this cluster effectively\nfacilitates spontaneous anion–cation separation and charge accumulation. This phenomenon arises\nbecause increasing temperature enhances polymer chain dynamics and reduces viscosity, thereby\nfacilitating ion transport.27, 28 Combined with the thermal stability of liquid water confined within the\nmolecularly clustered hydrogel, these factors lead to a more pronounced increase in potential difference\ncompared with the hydrogel without clusters.”\n17 / 24\nReviewer #4:\nIn the manuscript, the authors present a high-performance hydrogel designed using a molecular clustering\nstrategy. This hydrogel shows significant promise for real-world applications in extreme environments.\nThe manuscript is well-written and includes clear and informative figures. Therefore, I recommend it for\npublication in Nature Communications after revision. To further improve the paper’s quality, I have listed\nthe following comments and suggestions for the authors’ consideration.\nReply: We sincerely appreciate your positive evaluation, and are very grateful for your professional and\ninsightful recommendations. These comments have greatly enhanced the quality of our manuscript. In\nthe following, we respectfully provide a detailed, point-by-point response to each of the raised concerns.\nAll corresponding revisions have been highlighted in red in the revised Manuscript and Supplementary\nInformation.\n1. As indicated by the title, the key innovation is the hydrogel’s ability to work across extreme temperatures\nranging from −35 °C to 80 °C. However, the introduction section provides too little discussion about\nTemperature Comparisons with reported results. Figure S11, which presents a Comparison of temperature\nranges, should be moved from the supplementary information to the main text and provide more detailed\ndiscussion.\nReply: We sincerely appreciate your insightful and thorough comments. We have addressed this question\nin the following two aspects. The corresponding changes are highlighted in red in the revised Manuscript.\n1) We added to the Introduction a comparison of the operating temperature range with previous\nhydrovoltaic studies as below. Relevant literature data have been presented in Fig. 1e.\n“Previous reports in hydrovoltaic research also investigated the temperature-dependent viability of\nenergy harvesting, typically employing organic compounds such as ethylene glycol or glycerol, and\ninorganic salts like lithium chloride and calcium chloride.9, 11, 19, 30, 31, 32 The rationale is that such additives'\nintrinsic antifreeze properties can extend device operability at sub-zero temperatures. However, these\nreports also note that at elevated temperatures reduced internal water availability within the generator\ncan restrict sustained ion transport, thereby limiting steady electrical output. Consequently, existing\nhydrovoltaic devices still exhibit modest power densities (on the order of 100 μW cm–2) across a wide\ntemperature window, making them unsuited for deployment in harsh or variable real-world environments.”\n2) Fig. S11 has been moved into the main text as Fig. 3h, and the additional discussion has been added\nto the “Broadening the operating temperature range” section as below.\n“Overall, the molecularly clustered hydrogel maintains liquid water and mechanical flexibility from −35\nto 80 °C. Beyond the hydrovoltaic application, hydrogels employed in diverse fields such as flexible\nelectronics and biomedicine also demand thermal and mechanical stability of the confined liquid water\nunder varying temperature conditions,33, 34 as the crystallization of water at subzero temperatures or\nevaporation at elevated temperatures can severely impair their structural integrity and functional\nperformance. Compared with representative hydrogels reported for soft robotics35, 36, 37, flexible\nelectronics38, 39, 40, bioelectronics41, 42, 43, and biomedical44, 45, 46 applications, the molecularly clustered\nhydrogel achieves superior thermal stability of liquid water over an extended temperature window (Fig.\n3h). Furthermore, relative to state-of-the-art hydrogels used in many soft multifunctional electronic\nplatforms, this hydrogel exhibits larger tensile strain and lower Young’s modulus over a wider\ntemperature range, indicative of its stronger ductility (Fig. S14, Table S2,3).”\n18 / 24\n2. All citations in Figures 1e, S11, S12, and S14 are incomplete, lacking the issue and article number. For\nexample, “Nat. Commun. 2024” in Figure 1e. More importantly, many citations, such as “Nat. Commun.\n2024” in Figure 1e, cannot be found in the Reference List. This is a big mistake.\nReply: Thank you for highlighting this important point. In the revised Manuscript, all citations in the\nfigure have been clearly labeled with their corresponding reference numbers from the Reference List.\nFig. 1: e Comparison of the maximum power density at different temperatures between the hydrovoltaic\ndevices from this study and existing devices (Table S1).\nFig. 3: h Comparison of temperature range of the molecularly clustered hydrogel with reported\napplications in soft robots, flexible electronics, bioelectronics, and biomedicine.\nFig. S14 Comparison with the reported Young's modulus and uniaxial tensile strain of flexible electronic\nmaterials highlights the softness and stretchability of the molecular clustering hydrogel, maintained\nacross ambient conditions as well as at extreme temperatures of -35 ℃ and 80 ℃.\n19 / 24\nFig. S16 Ion conductivity of molecularly clustered hydrogel at -35℃, 25℃, and 80℃ compared with\nreported ionogels under different temperatures.\nFig. S19 Half-decay time of the hydrovoltaic open-circuit voltage of molecular clustering hydrogels in\nthis study compared with previous works under different temperatures.\n3. The preparation of the designed hydrogels is not clearly described in the manuscript and lacks explicit\ndetails, such as optimal ratios or concentrations. The authors should provide more detailed preparation\nmethods to ensure that readers can easily reproduce the reported hydrogels.\nReply: We sincerely appreciate you for raising this point. We have revised the section \" Fabrication of\nmolecularly clustered hydrogels\" to include a detailed justification for the selected ratios of components\nwithin the molecular clusters and a discussion on concentration optimization. The corresponding\nmodifications have been highlighted in red for clarity in the revised Manuscript as below.\n“GA, SS, and SG are key components for validating the molecular clustering strategy. Since the primary\nfocus of this study is to investigate the effect of their clusters on hydrogel properties, we employed a\nfixed 1:1:1 molar ratio to ensure compositional consistency and simplify the analysis of clustering\nbehavior. Furthermore, we evaluated the hydrovoltaic performance of the hydrogel under this ratio at\nvarying concentrations. As shown in Fig. S24, the highest performance was achieved at 5.5 mmol.\nTherefore, this concentration was adopted in all experiments to ensure optimal hydrovoltaic output,\nthereby achieving high-efficiency hydrovoltaic performance across a broad temperature range.”\n20 / 24\nFig. S24 Open-circuit voltage and short-circuit current of molecularly clustered hydrogels under standard\nexperimental conditions with varying component amounts of SG, GA, and SS at a fixed ratio of 1:1:1.\n21 / 24"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have adequately addressed most of the reviewers' comments. However, regarding Comment 3 on the demonstration of organic salt anion aggregation via SAXS, the interpretation remains debatable. SAXS reflects the electron density difference between phases, if present. The incorporation of GA leads to a pronounced reduction in scattering intensity, suggesting that the system becomes more homogeneous. This result appears inconsistent with the authors’ statement.", "reviewer_2": "The authors have addressed all my concerns. I think it is now acceptable for publicaiton in Nature Communications.", "reviewer_3": "All the concerns raised by the reviewer have been thoroughly addressed, and the manuscript is now recommended for publication in Nature Communications.", "reviewer_4": "The authors have carefully addressed my concerns and have made significant revisions and improvements to the paper. I recommend that this paper be published in Nature Communications."}, "author_response": "Response to reviewers’ comments\nReply: We sincerely appreciate the time and effort that all reviewers dedicated to providing feedback on\nour manuscript. The concerns raised by Reviewer 1 have been responded as follows, and the manuscript\nhas been revised accordingly. We hope these changes will satisfactorily resolve the reviewer’s concerns.\nReviewer #1:\nThe authors have adequately addressed most of the reviewers' comments. However, regarding Comment\n3 on the demonstration of organic salt anion aggregation via SAXS, the interpretation remains debatable.\nSAXS reflects the electron density difference between phases, if present. The incorporation of GA leads\nto a pronounced reduction in scattering intensity, suggesting that the system becomes more homogeneous.\nThis result appears inconsistent with the authors’ statement.\nReply: Thank you very much for your professional and constructive review. We also apologize for the\ncontentious wording in the original manuscript. Here, “aggregation” essentially refers to the\nestablishment of interactions between glycolic acid (GA) molecules and the aminated sulfonate\nanions (SS) and the glycolate anions (SG). This phenomenon is distinct from conventional aggregation-\ninduced phase separation and therefore yields different SAXS signatures. Our focus is on whether GA\nmolecules interact with the anions. Because such interactions are responsible for the broadened\ntemperature range and the improved electrochemical performance. As you pointed out, the decreased\nSAXS scattering intensity indicates that the system becomes more homogeneous; however, so long as\nthe interactions are present, this observation is fully consistent with our conclusions. In addition, we have\nincluded new control characterization data and have carefully revised the manuscript.\nFor your convenience, the analysis procedure from the previous submission and the analysis of the newly\nadded data in this revision are summarized below. We hope this will allay your concerns.\nIn the previous revised manuscript, we observed that introduction of the anions (SS-SG)\nalone increased\nthe SAXS scattering intensity, which we attribute to electrostatic repulsion among the anions that loosens\nthe internal network and increases structural disorder. By contrast, addition of GA molecules attenuated\nthe SAXS scattering intensity, indicating that GA interacts with the anions and weakens their electrostatic\nrepulsion, which is consistent with your observation of “more homogeneous”.\nTo address your concerns, we have added SAXS measurements for a GA-only hydrogel, a control in\nwhich electrostatic repulsion is not pronounced. As shown in Fig. 2a, the GA-only hydrogel exhibits the\nlowest scattering intensity and appears the most homogeneous among the controls. The scattering\nintensity of the SS-SG-GA hydrogel lies between that of the SS-SG-only hydrogels and the GA-only\nhydrogel, indicating interactions among SS, SG and GA.\nTo avoid misunderstanding, we have replaced all occurrences of the term “aggregation” in the original\ntext with “interactions”. And we have revised the original sentence “This decrease indicates that GA\nreduces disorder within the hydrogel network, consistent with aggregation induced by GA.” to the\nfollowing: “This decrease indicates that GA reduces disorder within the hydrogel network. The scattering\nintensity of the SS-SG-GA hydrogel lies between that of the anion-only and the GA-only hydrogels,\nindicating interactions among SS, SG, and GA.”\nFig. 2: Material Characterization of molecular clustering and simulation analysis of molecular\nclusters-enhanced water binding. a SAXS spectra of pure PAAM/SV hydrogel with sequential addition\nof SS, SG, GA, and GA-only molecules. b SAXS 2D mapping of SS-SG and SS-SG-GA added hydrogels.\nReviewer #2:\nThe authors have addressed all my concerns. I think it is now acceptable for publication in Nature\nCommunications.\nReply: We are sincerely grateful for your recommendation of acceptance. Your insightful critiques have\nsignificantly improved the manuscript.\nReviewer #3:\nAll the concerns raised by the reviewer have been thoroughly addressed, and the manuscript is now\nrecommended for publication in Nature Communications.\nReply: We sincerely thank you for recommending acceptance. Your constructive suggestions\nsignificantly strengthened our work.\nReviewer #4:\nThe authors have carefully addressed my concerns and have made significant revisions and\nimprovements to the paper. I recommend that this paper be published in Nature Communications.\nReply: We gratefully acknowledge your endorsement of publication. Your careful comments\nsignificantly helped enhance the clarity and rigor of the manuscript."}, {"round": 3, "reviewer_comments": {"reviewer_1": "This version is fine."}, "author_response": ""}]

Decision letter