Electricity-free hydrogen production from the air

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

Abstract
The ever-growing demand for electricity and clean water restricts widespread application of hydrogen production via water electrolysis or photocatalytic water splitting. Here, we present a self-sufficient electricity-free air-to-hydrogen system that integrates radiative cooling-enhanced water adsorption with synergistic photocatalysis and photothermal conversion by fabricating spectral selective absorbing/emitting hygroscopic hydrogen evolution nanofiber membranes to harvest atmospheric moisture and produce hydrogen. Leveraging the nocturnal radiative cooling effect, we expand the operational relative humidity range of nanofiber membranes and enhance both water collection capacity and kinetics. The collected water undergoes efficient gas-phase water splitting for H2 production during the day through photothermal catalytic processes without electrical and liquid water assistance. The hydrogen production rate of the scale-up air-to-H2 system under outdoor natural light reaches 6467.55 µmol·m-2·h-1. Extrapolating this experimentally validated rate to land-based deployment demonstrates the potential for large-scale hydrogen generation, with practical feasibility dependent on regional humidity and solar conditions. Thus, our approach cost-effectively addresses green-H2 scarcity without demanding natural freshwater and electricity, thereby providing an archetype for global sustainable development.

Introduction
Producing hydrogen (H2) fuel from sunlight and water through photocatalytic (PC) water splitting offers a promising pathway to carbon neutrality1–4. However, this process necessitates a continuous supply of clean fresh water, as impurities and contaminants can compromise the stability and longevity of photocatalytic systems5,6. With over 4 billion people facing freshwater scarcity7,8, relying solely on photocatalytic H2 production fails to adequately address the intertwined issues of energy and water9. While numerous studies have explored the use of seawater as a water source for PC H2 production10–12, the ions present in seawater can be absorbed onto the catalyst surface, potentially affecting its stability or generating undesirable byproducts13–15. Additionally, this approach is challenging to implement in inland areas that are far from coastlines. Therefore, there is an urgent need to identify alternative water sources for H2 production that do not compromise clean water supplies and have fewer geographical restrictions.

Atmospheric humidity, which contains approximately 13 trillion tons of water16, represents one of the most promising sources for clean water17–19. Adsorption-based atmospheric water harvesting (AWH) technologies utilize hygroscopic materials such as metal-organic frameworks (MOFs)19–22 to effectively capture moisture from the air. By supplying heat energy, the adsorbed water can subsequently be desorbed to obtain clean water. Therefore, the alternating adsorption-desorption process of MOFs can provide a reliable source of clean water for PC H2 production.

Furthermore, most PC systems exhibit limited solar spectrum utilization for H2 production3. Only photons with energy exceeding the semiconductor bandgap can be harnessed. This limitation means that only ultraviolet (UV) light and a small portion of visible (Vis) light are used, while the majority of solar energy is wasted as heat9,23,24. Additionally, many PC systems rely on liquid water as the reaction medium, which presents several challenges. For instance, catalyst components may leach into the water, and liquid water can hinder the absorption of solar radiation12,25,26. To address these issues, the heat generated during the photocatalytic process can be harnessed to drive the desorption of AWH, releasing water vapor. Subsequently, gas-phase water vapor can serve as the feedstock for PC reactions. This approach not only enhances solar spectrum utilization but also alleviates the limitations associated with clean liquid-phase water, improving reaction rates and long-term stability.

This study introduces an air-to-H2 system that employs multilayer hygroscopic hydrogen evolution (HHE) nanofiber membranes (NFMs) for efficient green H2 production, with clean water generated as a byproduct (Fig. 1a). Fabricated via electrospinning, the HHE NFMs consist of a photothermal catalytic (PTC) top layer of Pt/TiO2@PVDF and a porous hygroscopic bottom layer of MIL-101(Cr)@PAN (Fig. 1b). Capitalizing on the high solar absorbance and high infrared emissivity of the PTC layer (Fig. 1c), the system achieves radiative cooling-enhanced atmospheric water harvesting (AWH) at night, and photocatalytic-driven H2 production during the day by decomposing the water vapor released from the hygroscopic layer. Specifically, the distribution of MIL-101(Cr) within the highly porous hygroscopic layer facilitates rapid, spontaneous moisture adsorption from ambient air at night while simultaneously mitigating particle aggregation. Concurrently, the radiative cooling effect of the PTC layer elevates the local humidity, thereby accelerating moisture capture and extending the effective hygroscopic range (Fig. 1d). Under daytime solar irradiation, visible and near-infrared (NIR) wavelengths are converted into thermal energy to desorb water vapor from the hygroscopic layer, while ultraviolet (UV) light is absorbed to activate Pt/TiO2 for the photocatalytic (PC) production of H2. These advantages are substantiated through experimental and theoretical validation. A large-scale prototype system was demonstrated in outdoor operations over 14 days, yielding a maximum H2 production rate of 764.2 mL·m⁻² under 6 h of natural sunlight. This approach enables the direct, passive production of green H2 from air, entirely independent of liquid water input, thereby offering a promising pathway toward a sustainable future.

Results
The adsorption capacity of the hygroscopic layer and the range of humidity it can adsorb are crucial for the performance of HHE system. Generally, the moisture adsorption kinetics of the hygroscopic layer depend on the diffusion of water vapor between and within the MIL-101(Cr) particles embedded in the nanofiber. A porous, flexible green hygroscopic layer is prepared by using a co-spinning method with MIL-101(Cr) and PAN (Figs. 2a and S1). The X-ray diffraction (XRD) patterns clearly indicate that the diffraction peaks of the synthesized MIL@PAN NFMs align well with those of the MIL-101(Cr) crystals, confirming the presence of the MOF in the hygroscopic layer (Fig. S2). Scanning electron microscopy (SEM) images show that the porous hygroscopic layer exhibits a honeycomb-like structure and the merged nanofibers form the walls of the honeycomb (Figs. 2b and S3), which results in a high porosity of 0.93 (Fig. S4) and therefore facilitates moisture diffusion27,28. The uniformly distributed MIL-101(Cr) particles (Fig. S5) along the nanofibers impart a hierarchical roughness and nanoporous structure to the MIL@PAN NFMs (Fig. 2b). N2 adsorption-desorption measurements (Fig. S6) reveal that the adsorption isotherms exhibit typical Type I characteristics with a notable hysteresis loop, confirming the presence of a significant number of micropores in the MIL@PAN NFMs. The measured pore size distribution indicates that the micropore sizes are centered around 1.479 nm, which is larger than the kinetic diameter of water molecules (0.27–0.32 nm) (Fig. S7), allowing for the adsorption of a large amount of water molecules29. Material hydrophilicity (Fig. 2e, left) and capillary effect of porous NFMs promotes rapid moisture contact and penetration. As shown in Fig. 2c, the droplet is quickly absorbed and penetrates both horizontally and vertically, confirming the great water adsorption capability of the hygroscopic layer.

To evaluate the moisture adsorption performance of the hygroscopic layer, the moisture adsorption process was assessed gravimetrically at 25°C under various humidity levels (Fig. S8). Experimental results show that the dry NFMs exhibit water adsorption capacities of 0.698, 0.765, 0.821, and 0.872 g water layer g-1 at 60, 70, 80, and 90% relative humidity (RH), respectively. Furthermore, this hygroscopic layer can rapidly adsorb moisture from the air within the first hour (Fig. S8B). Additionally, this layer demonstrates great cycling stability, retaining its adsorption performance even after multiple cycling tests (Fig. S9). However, as the air humidity decreases, the moisture absorption of the hygroscopic layer drops rapidly. At 50% RH, the absorption capacity is about 0.179 g water layer g-1, limiting the applicability of this hygroscopic film under low humidity conditions.

Therefore, to broaden the humidity range for practical applications of the hygroscopic layer and accelerate the moisture absorption process, we further positioned the high infrared emissivity PTC layer on the top layer of the hygroscopic layer (Figs. 2d and S10). The thickness of the porous PTC layer is about 130 µm, which allows the flow of moisture. Besides, as shown in Fig. 2e, the hydrophobic PTC layer and the hydrophilic hygroscopic layer form a Janus structure, which enhances the transfer of moisture. Meanwhile, due to the inherent high emissivity of PVDF, the PTC layer exhibit great emissivity of 0.95 in the atmospheric transparency window (8–13 µm) (Fig. 1c). Essentially, the adsorption-based AWH converts atmospheric moisture into liquid water for storage through phase change, releasing heat to the surroundings. By utilizing the radiative cooling effect of PTC layer, the long-wavelength (infrared, IR) thermal radiation can be emitted from the surface of HHE NFMs to the cold sky to cool it below the ambient temperature during the night (Fig. 2f). Given the constant absolute humidity of the environment, this cooling effect increases the effective RH near the hygroscopic layer30–32.

The practical radiative cooling effect was tested by recording the surface temperature of Pt/TiO2@PVDF NFMs and ambient temperature on the outdoor roof in real time via thermocouples (Fig. S11). As shown in Fig. 2g, we initiated the test at around 19:00 h local time, with an average nighttime ambient temperature of around 18.6°C and an average humidity of approximately 50% RH. The PTC layer was positioned to face the clear sky to enable passive radiative cooling, reducing the surface temperature below its ambient. Temperature drops of ~6°C were consistently observed, with a maximum temperature drop of 8.19°C, confirming the good practical radiative cooling property of the PTC layer. Experiments were also conducted to characterize the moisture adsorption performance of the radiative cooling enhanced system in real-world scenarios (Fig. S12). Figure 2h shows a comparison of the water adsorption capacities between the samples with radiative cooling enhancement and those without. Due to the increase in effective RH from the radiative cooling effect, the sample with radiative cooling enhancement exhibits faster adsorption rates and greater total adsorption capacity. At an average humidity of 80% RH, the radiative cooling enhanced sample achieved a water adsorption capacity of 0.90 g water layer g-1 (Fig. 2i), demonstrating nearly 10% improvement in performance compared to their non-radiative cooling enhanced HHE NFMs counterparts (0.82 g water layer g-1).

To further quantify the impact of radiative cooling on effective RH under lower humidity conditions, we calculated the effective RH increased by the radiative cooling effect near the surface of the PTC layer under environmental humidity of 50% RH at various temperature (see the “Methods” section and Fig. S13). The results indicate that with a temperature drop of 6.01°C, the average effective humidity increased by approximately 40%, rising from 50% RH to about 70% RH (Fig. S13). Controlled experiments were also conducted in an environmental chamber. The effective relative humidity after cooling was set as the adsorption environment, and we tested the theoretical water absorption capacity of the radiative cooling-enhanced HHE NFMs under low humidity conditions (30–60% RH). These specific indoor conditions can indirectly simulate and reflect the water collection performance of radiative cooling in outdoor environments. As shown in Fig. 2j, the adsorption range of the hygroscopic layer is significantly broadened. Initially, it is nearly impossible to adsorb moisture at 30% RH, but after radiative cooling enhancement, it can achieve an adsorption capacity of 0.29 g water layer g-1. At 40, 50, and 60% RH under 25°C, the theoretical adsorption capacities increased by 0.64, 0.61, and 0.18 g water layer g-1, respectively. This indicates that the radiative cooling-enhanced HHE NFMs possess a broader range of moisture adsorption and greater moisture adsorption capacity.

To further achieve photothermal conversion for water vapor release and PC H2 production, Platinum-loaded titanium dioxide (Pt/TiO2) has been chosen as a PTC material due to its simple synthesis method, good light absorption, superior charge separation, effective photothermal conversion, great catalytic activity, and good chemical stability33,34. Pt/TiO2 powder was synthesized by depositing Pt nanoparticles (1 wt%) onto commercial TiO2 using a photodeposition method33 (see the “Methods” section, and the optimal pt load has been verified as shown in Fig. S14). Figs. S15 and S16 display the SEM and TEM images, along with the XRD pattern of Pt/TiO2 (Fig. S17), confirming the successful synthesis of Pt/TiO2. These catalyst nanoparticles were then co-spun with PVDF to produce Pt/TiO2@PVDF NFMs, which also serves as the PTC layer continuously deposited above the hygroscopic layer. SEM and TEM images of the PTC layer (Figs. S18 and S19) reveal that the interlaced Pt/TiO2@PVDF nanofibers are arranged in layers, creating a porous structure that facilitates vapor escape and provides abundant active sites for vapor decomposition. Comparisons between Pt/TiO2@PVDF nanofibers and pure PVDF nanofibers (Fig. S20) indicate that the surface of the Pt/TiO2@PVDF nanofibers is significantly rougher, due to the uniform distribution of catalyst particles on the surface of PVDF nanofibers. Thus, the PTC layer exhibits high solar absorption with an average absorption rate of 76.6% (Fig. 1c). This enhancement is attributed to the use of high solar absorption Pt/TiO2 nanoparticles and the rough surface of the Pt/TiO2@PVDF nanofibers, as the rough surface significantly increases the scattering of incident light within the PTC layer, thereby improving solar radiation absorption.

The indoor H2 production tests were first studied in a glass reactor under AM 1.5G (1000 W·m-2) irradiation for 6 h with H2 quantification by gas chromatography (GC) (Fig. S21). The vapor generation rate was measured by placing the 2×2 cm HHE NFMs on an electronic balance, while the surface temperature of the HHE NFMs was recorded using an infrared radiation meter. Prior to the experiment, the HHE NFMs had already undergone saturated adsorption in a 90% RH environment. The optimal catalyst loading mass of PTC layer and the optimal thickness of hygroscopic layer were found to be 4.0 mg·cm−2 and 11 mm, respectively (Fig. S22). With this loading, the temperature on the upper surface of the HHE NFMs increased rapidly from room temperature to 58.0°C within 10 minutes, subsequently maintaining above 68°C (Fig. 3a). Heat is then conducted downward, raising the temperature of the hygroscopic layer (Fig. 3b). The water adsorbed in the hygroscopic layer was released by nearly 80% (approximately 120 mg) within 1 h, with a vapor generation rate reaching 0.298 kg·m−2·h−1 (Fig. 3c). Additionally, 3D models of the HHE NFMs were established based on finite element analysis to simulate the vapor generation process under natural light, and the results align well with the experimental findings (Fig. 3c).

As the Pt/TiO2 nanoparticles fixed on the PVDF NFMs are in intimate contact with the vapor, photogenerated charge carriers participate in the H2 evolution reaction at the active sites of the catalyst under sunlight (Fig. 3d). Since only the solid-phase catalyst and the gas-phase vapor and H2 participate in the reactions within the HHE system, this process can be classified as a biphasic reaction. Under the optimal catalyst loading mass and optimal thickness of HHE NFMs, the maximum H2 production rate of the HHE NFMs during the first hour reaches 2035 µmol·m⁻²·h⁻¹ in indoor test (Fig. 3f), which is about four times the H2 production rate (565.7 µmol·m-2·h-1) under triphasic conditions (liquid-phase water, solid-phase Pt/TiO2 nanoparticles, and gas-phase H2) (Fig. 3f). To further verify that the H2 production performance in a biphasic system surpasses that in a triphasic system, a vapor feeding reaction condition was established (refer to the Method section and Fig. S23A). This setup reliably maintained the relative humidity inside the reactor above 80% RH (as confirmed in Fig. S23B). The results show that the H2 production rate of biphasic system with vapor feeding is about 2543.2 µmol·m-2·h−1 during the first hour of reaction, which is higher than HHE NFMs biphasic system (2035.2 µmol·m−2·h−1), as shown in Fig. S24A. However, both of these two biphasic systems show four 4-4.5 times higher than that of the triphasic system. However, the H2 production rate of HHE NFMs biphasic system decreases because of the gradual release of water vapor from hygroscopic layer. And the H2 production rate of biphasic system with vapor feeding keeps nearly constant H2 production rate (Fig. S24B). Therefore, after 4 h of sunlight illumination, the average H2 production rate of biphasic system with vapor feeding is about twice that of HHE NFMs biphasic system under indoor condition. This enhancement can be attributed to the continuous vapor supply in the biphasic system with vapor feeding, which ensures stable H2 production performance, further validating the advantages of biphasic systems over triphasic systems.

H2 production tests of HHE NFMs under different humidity conditions were also simultaneously conducted in indoor environments. As humidity increased, the enhanced moisture adsorption by the HHE NFMs led to a corresponding improvement in H2 production (as shown in Fig. S25). When the HHE NFMs contained no adsorbed water, no H2 was detected in the reactor, confirming that the H2 originated from the decomposition of adsorbed water. To further verify the source of H2, the source of hydrogen protons in the reactants was qualitatively investigated by using D2O as an isotope tracer instead of H2O. The experimental results show that D2 (m/z=4) signal is observed after illumination in HHE NFMs which adsorbed D2O (Fig. S26A). However, only H2 (m/z=2) signal is found after illumination in HHE NFMs which adsorbed H2O (Fig. S26B). In addition, O2 was observed after the reaction (Fig. S27), and the isotope detection of H218O also found that the reactor produced 18O2 (Fig. S28), verifying that both H2 and O2 came from adsorbed water from the air.

To confirm the synergistic effect between photocatalytic and photothermal processes in the HHE system, we measured H2 production under different light conditions (Fig. S29). The results show that although no H2 was detected under visible and infrared light, significant liquid water condensed on the reactor walls (Fig. S30). Meanwhile, under UV light, the H2 production rate was only about 457.7 µmol·m⁻²·h⁻¹ in the first hour, which is much lower than under AM 1.5G illumination. This supports our design concept that the utilization of full spectrum significantly enhances the overall H2 production in the HHE system. Subsequent repeatability tests on the HHE NFMs show a stable H2 production performance, with no noticeable decline observed (Fig. S31). To validate the versatility of the HHE system, we also used Au/TiO2 catalysts co-spun with PVDF as the PTC layer. While the average H2 production rate over 6 h is lower than using Pt/TiO2@PVDF NFMs, it still surpasses the H2 production rate of the Au/TiO2 triphasic reaction system (Fig. S32).

The great H2 production performance of our HHE biphasic system compared to the triphasic reaction system can be attributed to the effects of water state and temperature. These effects can be analyzed from both thermodynamic and kinetic perspectives, involving three key steps: the adsorption of water molecules, the adsorption of hydrogen atoms, and the PC H2 evolution reaction. In order to investigate the thermodynamic mechanism of effects of the state of water and temperature, the Gibbs adsorption free energies of two main reaction steps, including the adsorption of water molecule and a hydrogen atom in different states of water and temperatures were calculated by using density functional theory (DFT) method. The calculated adsorption energies of water molecule and hydrogen atom of biphasic system both are obviously lower compared to those of triphasic system under the same temperature (298 K) (Figs. 3g and S33, S34). The potential-determining step (PDS) for the biphasic system is the formation of H2 gas with an energy change of 0.35 eV, where the PDS turns into the adsorption of H2O with an uphill barrier of 0.54 eV in the case of triphasic system. This suggests that gas-phase water can lower the thermodynamic barrier in PC reaction process, which agrees well with experimental results (Figs. 3f and S35). On the other hand, the temperature can also affect the Gibbs adsorption free energies. Consistent with experimental results, the higher reaction temperature in the biphasic system leads to a lower barrier for the H2 formation process, thus accelerating the PC H2 evolution. Therefore, gas-phase water and elevated temperature can modulate the adsorption of reaction intermediate with an optimal bonding strength, ultimately boosting the intrinsic catalytic performance. Besides, the temperature increase induced by photothermal effects further accelerates electron transport between Pt and TiO2 and within Pt itself, thereby reducing the recombination of photogenerated carriers and improving the overall photocatalytic H2 evolution efficiency35–37.

Additionally, from a kinetic perspective, the diffusion coefficient of H2 in the gas phase is significantly greater than in the liquid phase11,12. Consequently, during the release process, H2 encounters considerable resistance when crossing liquid water, making it difficult to detach from the photocatalyst surface. In contrast, in the biphasic system, H2 encounters less resistance while diffusing through the gas phase, facilitating its release and collection. As temperature increases, H2 transport resistance decreases, allowing it to desorb more easily from catalytic active sites. This difference further enhances the H2 production efficiency of the biphasic system.

Scaling up the HHE system is essential for practical applications. The size of the HHE NFMs was sequentially scaled up by 5 times and then by 25 times, from 0.0004 m2 to 0.002 m2, and then to 0.01 m2. Then, HHE NFMs of different sizes were arranged in an array within the reactors (Figs. 4a and S36, S37). Under three typical weather conditions (namely sunny, partially cloudy, and overcast), the scaled-up HHE reactor was exposed to natural light to produce H2, with a weather station (Fig. S38) recording temperature, light intensity, and humidity in real-time (Specific definition of different weather condition can be founded in Note S1). In other large-scale photocatalytic reactions, increasing the size typically results in a decrease in reaction rates when conducted under outdoor natural light5,9,10,12. However, it is noteworthy that our HHE system demonstrated a substantial increase in H2 production rates under large-scale outdoor natural light conditions. As shown in Fig. 4b, the 0.04 m2 reactor, composed of an array of 0.01 m2 HHE NFMs, yielded H2 production totals of 38,805.3 µmol·m−2, 32,103 µmol·m−2, and 22,789.7 µmol·m−2 under sunny (786 W·m−2), partially cloudy (716.5 W·m−2), and overcast (397.6 W·m−2) conditions, respectively. During the first hour of outdoor test, the maximum H2 production rates can reach 12148.5 µmol·m−2·h−1, 9286.4 µmol·m−2·h−1, and 7617.5 µmol·m−2·h−1, with a maximum solar to H2 efficiency (STH) of 0.10%, 0.07%, and 0.085%, respectively. However, with the decrease of light intensity and the gradual release of moisture in the hygroscopic layer, the H2 production rate gradually decreased. Therefore, the average H2 production rates were 6467.55 µmol·m−2·h−1, 5350.5 µmol·m−2·h−1, and 3798.28 µmol·m−2·h−1 during the 6 h outdoor test, which is 5.71 times, 4.73 times and 3.36 times higher than that under indoor conditions with 1000 W·m−2 AM 1.5G simulated sunlight (Fig. 4c). Due to environmental variability and dimensional scaling of the HHE NFMs, the distinct H2 production performance observed between indoor and outdoor conditions (contrary to trends reported in prior studies) demands a rigorous investigation into the underlying mechanisms.

The enhancement of H2 production performance under outdoor larger-scale system can be attributed to the combined synergistic thermodynamic and kinetic effects. From the thermodynamic perspective, in the outdoor air-to-H2 system, the HHE NFMs exhibit both higher energy input and higher surface temperature of PTC layer (Note S2 and S3). As shown in Fig. 4b, the temperature of the PTC layer under outdoor open area conditions can rise from 45.9°C to 71.8°C within 30 s (Fig. S39), reaching a maximum surface temperature of 82.2°C (average ~76°C) (Fig. 4d), which is about 14°C higher than that of indoor conditions. As demonstrated by DFT calculations (Fig. 3h), the increased reaction temperature within the biphasic system lowers the energy barrier for H2 formation, thereby accelerating the H2 evolution rate. Thus, the enhanced surface temperature under outdoor conditions thermodynamically boosts H2 production rates compared to indoor systems. From the kinetic perspective, the enhancement of H2 production rates in HHE NFMs originates from scaling effects. In the indoor condition, a 0.0004 m2 HHE NFMs was enclosed within a glass reactor, whereas the outdoor system employed a 0.01 m2 HHE NFMs array integrated into a 0.25 m2 reactor. COMSOL simulations of the photothermal water desorption process for HHE NFMs of varying sizes revealed that moisture in the hygroscopic layer escapes not only from the top PTC layer but also from the sides of the hygroscopic layer (Figs. 4e and S40). However, as the HHE NFMs size increases, a greater proportion of vapor escapes through the PTC layer (Figs. 4f and S41). This indicates that larger HHE NFMs dimensions enhance reactant availability by increasing vapor contact with catalysts on the PTC layer, thereby elevating vapor concentration in the PTC layer and kinetically enhancing H2 production rates.

Next, we prepared an array of HHE NFMs with a total area of 0.25 m2 (Fig. S42) for large-scale H2 production of the HHE system (Fig. 5a, b). After 14 days of moisture adsorption and H2 production experiments, we verified the outdoor long-term feasibility and stability of the HHE system (Figs. 5d and S43, Supplementary Movie 1), with each square meter of HHE NFMs gathering up to approximately 0.31 kg of water from the air and 764 mL of H2 under 6 h effective sunlight. During the two-week outdoor H2 production tests, no significant degradation in H2 production performance was observed (Figs. 5d and S44). The observed fluctuations correlated with daily weather variations, confirming that the efficiency remained stable under operational conditions. No detectable changes were observed in HHE NFMs before and after testing, confirming material stability (Figs. S45, S46). Subsequently, after two days of operation, the HHE system was connected to a fuel cell with an output power rate of 2 W, successfully lighting nearly 1000 commercial LED bulbs continuously (Fig. 5c and Supplementary Movie 2), demonstrating the practical applicability of the HHE system. Given its excellent performance, we conducted a comprehensive assessment of the H2 production of the HHE system across terrestrial regions from 60°N to 60°S latitude (Fig. S53). Considering local solar intensity and average nighttime humidity conditions, application of HHE system could produce over 4.5×104 GL of H2 per year (approximately 4.1×109 kg) across even just 0.1% of the global land area. Future research could focus on reactor optimization and material synergy to accelerate the moisture capture-to-conversion process, ultimately boosting H2 production and solar utilization efficiency. For example, simplified reactor structures with solar-tracking capabilities will enhance solar energy utilization. Meanwhile, strengthened integration of hygroscopic materials and photocatalysts through precisely engineered pore structures, tailored surface properties, and enhanced catalytic activity will optimize airflow organization, light utilization. Overall, this study introduces a passive air-to-H2 system without requiring any liquid water input, offering potential solutions to energy and water scarcity in developing regions.

Methods
Materials
Titanium dioxide (P25 TiO2, purity ≥99.5%), chloroplatinic acid hexahydrate (H2PtCl6·6H2O, purity ≥99.9%), gold (III) chloride trihydrate (HAuCl4·3H2O purity ≥99.9%), chromium (III) nitrate nonahydrate (Cr(NO3)3·9H2O, 99.99%), 1,4-benzenedicarboxylic acid (HBDC, 99%), and absolute ethanol (EtOH, purity ≥99.7%) were obtained from Aladdin Biochemical Technology Co., Ltd., China. Polyacrylonitrile (PAN, Mw=150,000, Ispin, China) was purchased from Kaneka Co., Ltd., Japan. N,N-Dimethylformamide (DMF, ≥99.5%) was supplied by Macklin Biochemical Co., Ltd., China. All chemicals were used without any further treatment. Ultrapure water (18.2 megaohm·cm) was obtained by using Mili-Q Synthesis System. Deuterium oxide (D2O, 99.9%).

Synthesis of Pt/TiO2 and Au/TiO2 nanoparticles
Photodeposition was conducted within a Pyrex-topped reaction cell. In a typical process, 200 mg of TiO2 powder was suspended in 20 ml water at 25°C, and the mixture was then ultrasonicated for 30 min. After ultrasonication, a stoichiometric amount of H2PtCl6·6H2O solution was added slowly under vigorous stirring (450 rpm). The amount of Pt metal added was 1 wt%, relative to the TiO2. Then, the reaction vessel purged with argon and irradiation under a 300 W Xe lamp for 1 h. The photodeposited powders were washed and filtered (300 ml water) before drying at 80°C for 12 h to obtain dry Pt/TiO2 nanoparticles. The optimal pt load has been verified. Accordingly, Pt/TiO2 catalysts with 0.5, 1, and 2 wt% Pt loadings (denoted as 0.5Pt/TiO2, 1Pt/TiO2, and 2Pt/TiO2) by adjusting the amount of H2PtCl6 solution during synthesis were prepared and tested. Similarly, the Au/TiO2 nanoparticles were prepared using the same process.

Synthesis of MIL-101(Cr) nanoparticles
The reactant mixture with a molar composition of 1 Cr(NO3)3·9H2O: 1 HBDC: 2.87 HNO3: 267 H2O was poured into a Teflon-lined stainless steel autoclave and then followed by heating at 220°C for 8 h. After being cooled to room temperature naturally, the green precipitates were obtained by centrifugation and washed several times with DI water, DMF, and hot ethanol. Finally, the product was dried in a vacuum oven at 150°C for 12 h.

Synthesis of MIL@PAN NFMs
A precursor solution for fabricating the MIL@PAN NFMs was prepared by ultrasonically dispersing 20 wt% MIL-101(Cr) in DMF for 2 h, followed by the addition of 5 wt% PAN and vigorous magnetic stirring of the blend for 12 h. This solution was electrospun into nanofibers using a commercial electrospinning apparatus (Huizhi Electrospinning Co., Ltd., China). Key electrospinning parameters, including a 15 cm working distance, a 1 mL·h⁻¹ flow rate, and a 20 kV applied voltage, were meticulously controlled. The electrospinning time was adjusted across multiple hours to achieve varying membrane thicknesses under constant ambient conditions (25±1°C, 50±3% RH). The obtained light green NFMs27, exhibiting a multilayer honeycomb morphology, were then vacuum-dried at 100°C for 12 h.

Preparation of Pt/TiO2@PVDF NFMs and Au/TiO2@PVDF NFMs
10 wt% Pt/TiO2 nanoparticles and 8 wt% of PVDF was dissolved in DMF under vigorous stirring for 12 h at 65°C after 2 h of ultrasonic treatment. Resultant solution was electrospun under 1 mL·h−1 feeding rate, 20 kV voltage, and the tip-roller distance of 15 cm was maintained. The as-prepared Pt/TiO2@PVDF NFMs was deposited on a glossy paper-covered roller. In addition, the ambient temperature and RH were 25±1°C and 50±3%, respectively. Finally, the gray Pt/TiO2@PVDF NFMs was dried at 100°C under vacuum for 12 h. Similarly, the Au/TiO2@PVDF NFMs were prepared using the same process.

Preparation of HHE NFMs
Preparation of Pt/TiO2@PVDF NFMs as described above. Differently, multilayer MIL@PAN NFMs were covered on the spinning receiving disc for the electrospinning of Pt/TiO2@PVDF. After several hours, the obtained bilayer NFMs were dried at 100°C for 12 h under vacuum. Finally, the prepared bilayer NFMs were denoted as HHE NFMs.

Characterization
A comprehensive suite of characterization techniques was employed to analyze the materials. The sample morphology was examined using scanning electron microscopy (SEM, Zeiss EVO MA10), with elemental mapping conducted on a TESCAN MIRA LMS system equipped for energy-dispersive X-ray spectroscopy (EDX). High-resolution transmission electron microscopy (HR-TEM) and corresponding EDX analysis were performed on a JEOL JEM-2100F. Crystal structure was determined by X-ray diffraction (XRD) on a PANalytical X’Pert3 powder diffractometer using Cu Kα radiation (λ=1.54184 Å). Optical properties were assessed via ultraviolet-visible (UV-Vis) spectroscopy on a Shimadzu UV-3600 spectrophotometer. Surface temperature profiles and thermal images were captured with an FLIR EX-8 infrared camera. Chemical functional groups were identified by Fourier transform infrared (FTIR) spectroscopy using a Bruker Tensor 27 spectrometer. Textural properties, including specific surface area and pore volume, were measured via N2 physisorption at 77 K on a Micromeritics ASAP 2020 system. Gas composition was monitored in real-time using a ThermoStar mass spectrometer.

Indoor moisture adsorption measurement
Indoor moisture adsorption measurements were conducted following a standardized procedure. First, square samples (2 cm in length) were thoroughly dried in an oven at 100°C until their mass stabilized. Subsequently, the samples were transferred to a constant temperature and humidity testing machine (QHP-150BE, LICHEN Technology) set to 25°C and varying RH levels. The mass change during testing was recorded by an electronic balance (YOUSHENG, 0.1 mg) integrated with the system. To evaluate cyclic stability, the samples underwent repeated 6-h adsorption periods at 90% RH and 25°C, interspersed with 1-h desorption periods at 100°C. The adsorption capacity for each cycle was calculated according to Eq. (1).

Cads = Δm / m0 (1)

where Cads is the moisture adsorption capacity based on unit weight of hygroscopic layer g water layer g-1, Δm is the moisture adsorption quantity (g), and m0 is the weight of dried hygroscopic layer (g).

The relationship of absolute humidity, relative humidity and effective relative humidity
The ambient relative humidity (RH) is the amount of water vapor in the air as a percentage of the total amount that could be held at its current temperature, and can be expressed as:

RH = Pv / P*Ta (2)

where Pv is the partial pressure of water vapor in the air, called absolute humidity, independent of temperature, and P*Ta is the saturated vapor pressure of water vapor at the current temperature. Correspondingly, the effective relative humidity near the sorbent (RHe) is expressed as:

RHe = Pv / P*Tsurface (3)

where P*Tsurface is the saturated vapor pressure of water vapor after the surface temperature drops by 6.01°C.

Outdoor radiative cooling enhanced temperature and moisture adsorption measurement
The temperature tests for the outdoor radiative cooling enhanced AWH are presented in Fig. S10. The experiments were conducted on the rooftop of AC2 at City University of Hong Kong in Kowloon Tong on March 12, 2024. HHE NFMs samples were placed in an insulated cavity while exposed to the clear night sky. A polyethylene (PE) film served as a wind break to minimize convective heat loss. Due to the excellent infrared transmittance of this PE film, it does not hinder the HHE NEFs from radiating heat into space38. Ambient temperature was measured using two thermocouples, while the sample temperature was monitored with a K-type thermocouple positioned at the yellow marker in Fig. S11.

The moisture adsorption tests for the outdoor radiative cooling enhanced AWH are presented in Fig. S12. The experiment took place on August 8, 2024, at the AC2 rooftop of City University of Hong Kong, located in Kowloon Tong, Hong Kong. HHE NFMs with an approximate area of 100 cm2 were positioned on electronic balances with a precision of ±0.1 mg. For the radiative cooling-enhanced group (left panel of Fig. S12), the electronic balance surface was covered with a PE film to minimize wind interference and leave the window for radiative cooling. In contrast, the control group without radiative cooling (right panel of Fig. S12) was enclosed within an acrylic chamber wrapped with aluminum foil to block thermal radiation to outer space. Both of them was left enough gaps for air to get in. Prior to the commencement of the experiment, the test samples were subjected to a vacuum oven at 100°C to determine their dry mass. Throughout the experiment, the mass of the sample, ambient temperature, and relative humidity were recorded every minute. The water absorption quantity was calculated using Eq. (1).

Indoor theoretical radiative cooling enhanced water adsorption test
Simulation tests were conducted in a constant temperature and humidity chamber with controlled temperature and relative humidity. First, calculate the corresponding effective relative humidity after the average temperature drops by 6.01°C by using Eq. (3), and use the result as the environment in the constant temperature and humidity box. Subsequently, a square sample with a side length of 2 cm was dried in a vacuum oven at 100°C for 2 h, and then placed in a box to absorb moisture. At the same time, an electronic balance with an accuracy of 0.1 mg was used to record the mass change of the sample.

Photocatalytic hydrogen production measurement in triphasic system
For H2 production measurement of Pt/TiO2 particles in DI water, 20 ml of deionized water was added to the transparent reactor chamber. To prepare the catalyst suspension, 20 mg of Pt/TiO2 nanoparticles were immersed in water and subsequently added to a stirring rotor. Prior to initiating the photocatalytic reaction, the solution in the reactor underwent ultrasound treatment to achieve a homogeneous catalyst suspension. The light source used in the experiment is a 300 W xenon lamp, which is coupled with an AM 1.5G filter (Fig. S21). The height of the light source is adjusted to ensure that the light intensity reaching the solution surface is maintained at 1000 W·m-2. An injection of argon gas was carried out for a duration of 30 minutes to purge and remove as much air as possible from the reactor. During the photocatalytic reaction, gas samples were collected from the reactor at hourly intervals. The gas composition of the samples was subsequently quantitatively measured using meteorological chromatography (GC) to determine the H2 yield within the reactor. Similarly, the H2 production of Au/TiO2 were measured using the same process.

Indoor hydrogen production measurement of 0.0004 m2 HHE NFMs
The indoor H2 production measurement of HHE NFMs was carried out similarly to the triphasic system measurement. 0.0004 m2 of the HHE NFMs, previously saturated with water at 25°C and 90% RH, was put on the bottom of the transparent reactor chamber without adding any liquid water. The remaining testing process in the HHE system was similar to that of the triphasic system H2 production.

Indoor hydrogen production measurement of a biphasic system with vapor feeding
The biphasic system with vapor feeding is designed based on methods reported in the literature39,40 (schematic and actual setup are shown in Fig. S23A). Specifically, 20 mg of Pt/TiO2 was dispersed in 1 mL of ethanol solution and sonicated for 30 minutes. Then, the mixture was drop-cast onto a 4 cm² frosted glass plate placed on a hotplate at 60°C. After solvent evaporation, the Pt/TiO2 catalyst remained deposited on the glass plate, which was subsequently dried overnight in an oven at 80°C. The catalyst-coated glass plate was then placed in a quartz reactor, with 20 mL of DI water positioned beneath the sample holder to serve as a vapor source. This setup maintained a relative humidity above 80% RH inside the reactor (as confirmed in Fig. S23B). The reactor was purged with argon for 30 minutes to remove air prior to irradiation under AM 1.5G simulated sunlight (100 mW/cm2) for H2 production test.

Indoor solar evaporation experiment
Indoor solar evaporation experiments were performed under stable ambient conditions (~25°C, ~45% RH). Prior to illumination, the HHE NFM sample (4 cm2) was first brought to moisture saturation by exposure to 90% RH at 25°C. Desorption and evaporation were then driven by simulated AM 1.5G solar irradiation at an intensity of 1000 W·m−2. The mass change of the sample was recorded at 5 min intervals throughout the one-sun illumination period. These time-resolved mass data were subsequently used to calculate the water evaporation rate (kg·m−2·h−1).

Isotope tracer experiment
Firstly, 2 cm × 2 cm HHE NFMs was placed in a vacuum oven and dried at 100°C for 12 h. Then the HHE NFMs was quickly removed and placed in a glass reactor for sealing. At the same time, dry Ar is introduced into liquid phase pure D2O. Then the wetted Ar is used to purge for 6 h to ensure that the hygroscopic layer of HHE NFMs can completely adsorb D2O. The reactor was then exposed to AM 1.5G light for 6 h, and then the gas products after reaction were introduced into the mass spectrometer to detect the gases composition. Similarly, the gas composition test of using H2O was measured by using the same process. For the isotope tracer experiment of oxygen, the liquid phase pure H218O is replace D2O and N2 is used to take the H218O vapor into the reactor.

Outdoor hydrogen production measurement of 0.002 m2 HHE NFMs
The HHE NFMs, with an area of 0.002 m2, were placed in a stainless steel reactor (Fig. 4a, middle). After sealing the reactor, it was purged with argon gas for 30 min and subsequently exposed to natural light. A weather station recorded solar radiation intensity, outdoor temperature, and humidity in real time. The amount of H2 produced was measured using gas chromatography.

Outdoor hydrogen production measurement of 0.01 m2 HHE NFMs
Four HHE NFMs, each with an area of 0.01 m2, were placed in an aluminum alloy reactor (Fig. 4a, right). After sealing, the reactor was purged with argon gas for 1 h. The reaction system was then positioned on the rooftop of AC2 at City University of Hong Kong in Kowloon Tong and exposed to natural light. A weather station recorded solar radiation intensity, outdoor temperature, and humidity in real time. The amount of H2 produced was measured using gas chromatography.

Outdoor daytime surface temperature measurement of 0.01 m2 HHE NFMs
Two pieces of HHE NFMs (one has PTC layer and the other does not) were positioned inside a plastic box without the quartz glass covering under outdoor condition. The high walls around the plastic box are used to block wind and prevent it from affecting the experiment. Both of the two HHE NFMs were wrapped with insulating foam on all sides and underneath. An infrared thermal imager was used to record the upper surface temperature distribution of HHE NFMs exposed to outdoor sunlight.

Outdoor hydrogen production measurement of a large-scale HHE system
Twenty-five 0.01 m2 HHE NFMs were arranged in an array and filled into a large aluminum alloy reactor (Fig. S32). After complete sealing, the closed reaction system was purged with argon gas for 4 h. The reaction system was then positioned on the rooftop of AC2 at City University of Hong Kong in Kowloon Tong and exposed to natural light. A weather station was used to record solar radiation intensity, outdoor temperature, and humidity in real time. The amount of H2 produced was measured using a GC system. After the outdoor H2 production test, the reactor have to be opened to make sure that the HHE NFMs were exposed to the night sky for radiative cooling and to adsorb the moisture from the air for the next daytime H2 production test. The quartz glass cover plate is part of a modular design with a reusable rubber ring system and quick-install clamp mechanism, ensuring consistent sealing after repeated reactor disassembly and reassembly (as shown in Fig. S42). The process of screwing the large-scale air-to-H2 reactor together serves a dual purpose: securing the quick-install clamp assembly while applying compression to the rubber ring and quartz glass to create a tight fit and good sealing. Rigorous gas monitoring protocols were implemented: initial gas composition measurements were taken before each H2 production experiment, followed by bihourly sampling to detect potential leaks through nitrogen (N2) level monitoring by using gas chromatography (GC). Throughout extensive outdoor testing of the large-scale system, no gas leakage incidents were observed.

STH efficiency
The STH efficiency of HHE system was calculated using Eq. (4):

STH = Energy output as H2 / Incident light energy = rH2 × ΔG0 / Plight × A (4)

where rH2 is the experimental H2 production rate, ΔG0 is the standard Gibbs free energy, taken as 237 kJ·mol⁻¹ for liquid water and 228.6 kJ·mol⁻¹ for gas-phase water splitting at 298 K, and Plight is the incident light intensity, and A is the irradiated area.

3D Numerical simulation
The simulations based on the finite element method were performed using COMSOL Multiphysics version 6.0. Three modules, namely laminar flow, humid air heat transfer, and moisture transport in the air, are used to investigate the concentration, temperature, and velocity distribution of evaporation process. Here, the simulation of water vapor photocatalytic H2 production is ignored because its energy consumption only accounts for 0.1% of the incident solar energy, which is too small compared to other energies. Given the complexity of coupling reaction kinetics and phase-transport phenomena in the H2 production process, fully integrating these effects would significantly increase computational demands. In this COMSOL study, our primary objective was to validate that increasing the size of HHE NFMs (while maintaining a constant height) enhances the water vapor transport rate from the PTC layer, thereby supplying more reactants for the photothermal catalytic reaction. To maintain computational tractability, we simplified the model by focusing on the evaporation dynamics under illumination. 3D components were constructed for simulation, and the geometric parameters of the model are shown in Fig. S47. Free triangular elements are used in meshing the model. The relative tolerance in the steady-state solver was set to 0.0141.

Our numerical simulations were conducted using COMSOL Multiphysics 6.0, employing a finite element approach to model the system. The primary goal of this study was to computationally verify that upscaling the HHE NFMs at a constant thickness augments the water vapor transport flux from the PTC layer, thereby enhancing reactant supply for the subsequent photothermal catalysis. To this end, we utilized three core modules, including laminar flow, humid air heat transfer, and moisture transport in air, to resolve the temperature, concentration, and velocity fields during water evaporation under one-sun illumination. Given the considerable computational cost associated with fully coupling complex reaction kinetics with phase-transport phenomena, and considering that the energy consumption of the photocatalytic step is negligible, we simplified the model by focusing exclusively on the evaporation dynamics. The 3D geometry, detailed in Fig. S47, was discretized using free triangular elements, and a relative tolerance of 0.01 was set in the steady-state solver to ensure computational efficiency.

The water vapor flow in the open area is simulated by solving Eq. (5) and Eq. (6):

∂cv / ∂t + D∇2cv + u · ∇cv = Rv (5)

cv = φcsat (6)

where cv represents the concentration of water vapor, D is the diffusion coefficient, u denotes the velocity field induced by vapor diffusion. The saturation concentration is given by csat, φ is the relative humidity, and Rv is steam generation rate. Furthermore, the diffusion of water vapor, gv, within the gas phase is defined by the following equation:

gv = -ρg Deff ∇w (7)

Here, the effective diffusion coefficient is given by Deff = Dva ε4/3 s10/3, and Dva is the vapor-air diffusion coefficient with a value of 2.6×10−5 m2/s. The moisture content within the HHE NFMs was determined experimentally. A correlation fitted from the measured water adsorption capacity at various humidity levels (Fig. S48) was incorporated into the model to define the internal moisture distribution. Furthermore, ambient RH variations influence the evaporation energy demand of the HHE NFMs by altering the environmental vapor pressure (and thus vapor density). The accompanying heat transfer was simulated by solving the following energy conservation equations:

ρCp ∂T / ∂t + ρCp ∇T + k∇2T = Qv (8)

where T is temperature, ρ, Cp, and k represent the density, specific heat capacity, and thermal conductivity, respectively. To account for the endothermic process of evaporation, the associated latent heat absorption is incorporated as a volumetric heat source term Qv, given by:

Qv = -Hv Mw Rv (9)

where Hv and Mw are the latent heat of evaporation and molecular weight of water, respectively. Incident illumination was modeled as a heat flux boundary condition on the top surface of HHE NFMs. The assembly was treated as a single domain, justified by the PTC layer’s minimal thickness (130 µm) relative to the 1.1 cm hygroscopic layer. The model incorporates surface heat losses from convection and radiation to a 298.15 K environment and includes the temperature dependence of csat. The detailed simulated material properties and boundary conditions are cataloged in Tables S3 and S4.

DFT simulation
All spin-polarized DFT computations were performed by using the Vienna Ab initio Simulation Package (VASP 6.4)42. The exchange-correlation interaction was described by the Generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional43. Projector augmented-wave (PAW) potential with a kinetic energy cutoff of 400 eV was employed to treat the ion-electron interaction. To account for van der Waals (VdW) interactions, Grimme’s DFT-D3 method with the Becke-Johnson damping function was used44. The GGA+U approach was applied with a U–J value of 4.0 eV to account for the Ti 3d electrons45–47.

Convergence criteria for the structural optimization were set to ensure that the total energies per atom and forces on each atom reached values less than 10-5 eV and 0.02 eV Å−1, respectively. The anatase TiO2 model (tetragonal, space group I41/amd) with lattice parameters a=b=3.78 Å, c=9.62 Å, was sourced from an open-access database (The Materials Project). Next, a four-layered TiO2 (101) slab with 5×3 supercell was constructed based on X-ray diffraction (XRD) results, where the bottom layer was fixed during structural optimization. A 24-atom Pt cluster (Pt24) was initially placed on TiO2 (101) surface. All slab structures included a vacuum region of 20 Å. For Brillouin zone sampling, gamma k-point was utilized. The optimized structures of Pt24/TiO2 (101) and the adsorbed H2O and H atom on Pt24 are shown in Fig. S49. The Gibbs free energy of adsorption was computed based of the following equation:

ΔG = ΔE + ΔEZPE - TΔS + ΔGpH (10)

where ΔE is the electronic energy difference between the free-standing and adsorption states of the intermediates; ΔEZPE and ΔS are the changes in zero-point energies (ZPE) and entropy, respectively, which are obtained from the vibrational frequency calculations; T is the temperature; ΔGpH is the free energy correction to account for the effect of solution pH value, which can be calculated from the following equation:

ΔGpH = KBT × pH × In10 (11)

In this study, free-standing H2 and H2O served as the reference states, respectively. The solvent effect was taken into account by using the implicit solvation model based on the VASP sol48,49. The relative permittivity of liquid water and gaseous water were set to be 80 and 1, respectively. The Gibbs free energies in the bi- and tri-phasic systems are dependent on the environment (bi- and tri-phasic system), temperature (from 298 K to 373 K) and the entropy change ΔS. The thermodynamic properties of water and H2 were referenced from the Handbook of Fundamentals of Classical Thermodynamics50.

Calculation of the potential of H2 production from HHE systems across terrestrial regions from 60°N to 60°S latitude throughout the year
Firstly, based on the light intensity and hydrogen production data collected from long-term outdoor testing of the air-to-H2 system, we calculated the average hydrogen production rate RH2 (mL·m−2·h−1) under different average solar radiation (Plight). And based on these data, a relationship curve between average solar radiation and average hydrogen production rate was fitted (Fig. S54A), and the fitting formula is:

RH2 = 3.73 Plight 0.53 / 12 (12)

Secondly, based on the data obtained from the radiative cooling enhanced atmospheric water harvesting, we also established the relationship between RH and the water adsorption saturation (α) of HHE NFMs (Fig. S54B), and fitted a curve with the following formula:

α = 1 / (1 + (RH / 0.336912)-6.25821) + 0.9996 (13)

Therefore, based on the above two fitting curves, the annual average hydrogen production per square meter of land (H2,year (L·m−2·year−1)) can be obtained:

H2,year = α tday RH2 / 1000 (14)

Here, the direct solar irradiation all over the world can be obtained from the Global Solar Atlas (https://globalsolaratlas.info) (Fig. S55). And the Global nighttime average humidity distribution can be obtained from the Asia-Pacific Data-Research Center, Reanalysis Data, NCEP Reanalysis (http://apdrc.soest.hawaii.edu/las/v6/dataset?catitem=16801) (Fig. S56). Finally, the calculated annual average H2 production data will be visualized on a map for reference, and further calculations will be made to determine the amount of H2 available for the air-to-H2 system from 0.1% of the land.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Journal : Nature Communications\nManuscript ID : NCOMMS-25-00566-T\nTitle : Electricity-Free Hydrogen Production from the Air\nThe study proposes to capture moisture from the air and produce hydrogen without the need for electricity or liquid water.\nThe demonstration unit is based on a radiative cooling-enhanced atmospheric water harvesting (AWH) with a photothermal\ncatalytic (PTC) process to extract and split water. A multilayer nanofiber membrane composed of a hygroscopic bottom layer\n(MIL@PAN NFMs) that captures water from the air and a photothermal catalytic top layer (Pt/TiO₂@PVDF NFMs) that\nutilizes sunlight to drive the hydrogen evolution reaction has been used. The following are the specific comments that need\nto be addressed,\n1. Page 5, line 15, the claim that “demonstrating nearly 10% improvement in performance compared to the sample without\nthe radiative cooling enhanced one (0.82 gwater-1 ·glayer-1 )”. It is not clear whether the same setup has been used to\nperform the experiments without radiative cooling or it has been taken from literature.\n2. The manuscript claims that a hydrogen production rate of 6467.55 µmol·m⁻²·h⁻¹ under outdoor conditions, which is nearly\nsix times higher than indoor experiment. The plausible factors enhancing the hydrogen production rate for outdoor\nexperiments compared to indoor experiments must be mentioned.\n3. Page 18, line 10, mentions “The GGA + U method was used with U-J value of 4.0 for Ti 3d electrons”. The .xyz file of the\ncatalyst slab used for the computation may be included in the Electronic Supplement.\n4. Is numerical vibrational frequency computation was done on optimized catalyst slab, for estimating delta G.\n5. If it is so which space group and arrangement of original unit cell was used, must be mentioned.\n6. The claim that “The system can theoretically produce ~4.5 × 10⁴ GL (gigaliters) of hydrogen annually across 0.1% of\nglobal land areas.” is a very tall claim considering the amount of catalytic material and their production requirement.\n7. Hydrogen production happens only when sunlight is available. Cloudy, rainy days and nighttime will not produce\nhydrogen. Though moisture collection happens at night, there will be no hydrogen production.\n8. The solar-to-hydrogen (STH) efficiency of 0.1%–0.085% is reported in manuscript is very much lower than electrolysis-\nbased hydrogen production, which is around 15–20%. It is not clear how the system may compete against electrolysis.\n9. The hygroscopic and photocatalytic materials (Pt/TiO₂@PVDF and MIL@PAN NFMs) need to maintain their performance\nover long-term outdoor exposure. Potential degradation due to UV exposure, environmental pollutants, or material fatigue\nover time could reduce efficiency. The cycling stability of the moisture adsorption-desorption process must be further tested\nfor real-world applications.\n10. Is there any other material than platinum Pt and Au which can be used which is cheaper and abundant. A selection of\nthese two elements must be justified.\n11. Page 17, the results from 3D numerical simulations must be presented.\n12. The system is a breakthrough in sustainable hydrogen production, but low efficiency, weather dependence, and\nscalability challenges must be addressed before commercial deployment.\n13. Despite these limitations, the study presents a promising off-grid hydrogen production technology for arid and water-\nscarce regions, contributing to sustainable energy transitions.\nBased on the above comments, the manuscript is “recommended with minor revisions” for publication after incorporation of\ncomments.", "reviewer_2": "This manuscript presents a hydrogen production device that captures moisture from ambient air using MIL@PAN nanofiber\nmembranes (NFMs) with a radiative cooler (Pt/TiO2@PVDF) during nighttime, and the collected water is subsequently\nconverted into hydrogen via a photothermal catalytic process using hygroscopic H2 evolution nanofiber membranes (HHE\nNFMs) during daylight hours. While the research topic is interesting, the manuscript requires significant revisions based on\nthe following comments.\n1) Please provide the energy dispersive spectroscopy data and the specific surface area (m2/g) of the honeycomb-like\nstructure composed of MIL@PAN.\n2) The authors should evaluate the energy balance of the hydrogen production device for both indoor and outdoor\nexperiments.\nOptical characteristics: The photothermal catalytic (PTC) top layer, consisting of Pt/TiO2@PVDF NFMs, absorbs supplied\nenergy as heat, with an absorptance of 0.95 in the ultraviolet range and 0.7 in the visible-near infrared spectrum. It\nsimultaneously achieves radiative cooling, with an emissivity of 0.95 in the mid-infrared range.\nIn the indoor experiment under AM 1.5G conditions (1,000 W/m2), the surface temperature of the hydrogen production\ndevice was maintained at 68 oC, and the evaporation rate was 0.298 kg/m2 h. The net radiative power absorbed by the\ndevice was calculated as 713 W/m2 by integrating the product of absorptance and supplied power over the wavelength.\nThermal radiation within the indoor environment is negligible. Latent heat absorption from water desorption was measured at\nendothermic heat of the hydrogen conversion process.\nHowever, in the outdoor experiment, the surface temperature of the device increased to 82.2 oC. This raises questions about\nthe energy balance, when the net absorbed power under 1,000 W/m2 solar flux is 713 W/m2, while the thermal emission of\nthe device is calculated as 858.9 W/m2 at 82.2 oC. Additionally, water desorption from the porous MIL@PAN material\nabsorbs latent heat at 2,260 kJ/kg-water, alongside the endothermic reaction heat of hydrogen production. This discrepancy\nsuggests that the energy balance cannot be fully explained, even though the system is described as entirely insulated.\nIt is also noted that the outdoor solar flux is measured as 786 W/m2, significantly lower than the indoor value of 1,000 W/m2.\nThis further complicates the energy balance, as the heat transfer coefficient of the system with outdoor air should logically be\nhigher than that with indoor air. The authors should clarify these discrepancies and provide a detailed analysis of the energy\nbalance under outdoor conditions.\n3) The numerical simulation conducted using COMSOL Multiphysics lacks sufficient details. Specifically, when desorbed\nwater molecules undergo gas-phase water splitting for hydrogen production via a photothermal catalytic process in the\nindoor experiment, this endothermic reaction likely impacts the overall energy balance, which was not explained in the\nmanuscript. Please discuss the effect of this phenomenon. Additionally, more comprehensive information regarding the\ninitial and boundary conditions, as well as the analytical scheme used, should be included.\n4) Why was MIL-101(Cr) used for water harvesting from ambient air? The moisture sorption performance (both kinetics and\ncapacity) of MIL-101(Cr) is significantly lower than that of other materials under low humidity conditions. While the radiative\ncooler was utilized to enhance the moisture sorption performance of MIL-101(Cr) under low-humidity conditions, it also\nreduces net energy absorption during daylight hours due to thermal radiation. Alternatively, materials with higher sorption\ncapacity at low humidity could be considered. Please discuss the advantages of combining the MIL@PAN layer with the\nradiative cooler in this context.\n5) It is recommended to describe the performance of the hydrogen production device under outdoor conditions from an\nenergy perspective. For example, the energy consumption required to produce a unit mass of hydrogen (J/g or J/mol) could\nbe calculated and compared with other studies.\n6) The hydrogen production device used for outdoor experiments appears to be insulated. The authors should discuss the\ninsulation materials utilized and calculate the overall thermal resistances of the device in each direction.\n7) Although the net energy absorbed by the hydrogen production device under outdoor conditions is lower than that under\nindoor conditions, how can the hydrogen production rate increase? Please consider the following factors:\nOutdoor experiments: Solar flux of 786 W/m2, thermal emission from the surface (absorptance/emissivity of 0.95), and a\nhigher heat transfer coefficient due to windy conditions.\nIndoor experiments: Power flux of 1,000 W/m2 from the solar simulator, regulated thermal emission due to the indoor\nenvironment, and a lower heat transfer coefficient.\nWhen the energy flux is high and a high surface temperature is achieved, improved efficiency can be explained\nthermodynamically using the Gibbs energy relation. However, this does not align with the lower energy environment\ndescribed in outdoor experiments.\n8) One potential reason for higher efficiency under outdoor conditions could relate to the wavelength distribution and\nintensity of the solar simulator used in the indoor tests. Please provide specifications for the solar simulator, including its\nwavelength range and intensity distribution.", "reviewer_3": "The author discusses a method that enhances atmospheric water harvesting using radiative cooling and achieves hydrogen\nproduction via photothermal and photocatalytic water splitting in the vapor phase. I have three main concerns regarding this\npaper:\nRegarding Radiative Cooling\nThe author states that the PTC layer has a high emissivity, enabling radiative cooling at night. Figure 2g shows that the\nsurface temperature of the HHE NMFs is approximately 6°C lower than the ambient air temperature. However, the author\ndoes not explicitly mention that this result was obtained with the HHE NMFs directly exposed to air—this detail can only be\nfound in Supplementary Figure S10. In subsequent system experiments, the PTC layer and the water-absorbing layer were\nplaced under a transparent window, as can be seen in the video materials. Under these conditions, the radiative cooling\neffect should be significantly weaker than what is presented in Figure 2g. Similarly, I suspect that the infrared temperature\nmeasurements shown in Figure 4d were also taken without the transparent window, although the author does not clarify this.\nThese results, therefore, do not reflect the conditions with a transparent window. Given these concerns, the data presented\nin the earlier sections of the paper do not fully support the analysis and discussion of the system operation results in the later\nsections.\nRegarding Hydrogen Production Rate under Outdoor Natural Conditions\nThe author exaggerates the superiority of the outdoor experimental results over those obtained under indoor illumination in\nboth the abstract and main text. This comparison is unnecessary because the two conditions differ significantly in terms of\noperating temperatures and geometric characteristics, making them fundamentally incomparable.\nRegarding Scalability\nThe author's estimation of hydrogen production based on 0.1% of the global land area lacks credible data support and has\nno scientific value.\nGiven the inconsistencies between fundamental data and system results, as well as the arbitrary nature of comparisons and\nestimations, I recommend rejecting the paper."}, "author_response": "Point-to point response to reviewers’ comments on the manuscript\n‘Electricity-Free Hydrogen Production from the Air’\nWe wish to thank the three Reviewers for their comments. First, we have carefully\n5 studied all these comments. Then, we made a major revision of our initial manuscript\nby accounting for the comments of the three Reviewers as much as possible and in a\nconstructive way. We sincerely apologize for any repetition in our responses, as some\nof the reviewers’ comments addressed overlapping aspects of the work. Please rest\nassured that each response has been carefully tailored to address the specific concerns\n10 raised. We greatly appreciate your kind understanding and patience in reviewing this\nmaterial. We believe that this revision leads to a substantial improvement of our initial\nmanuscript.\nBelow, we explain how all the comments of the three Reviewers have been taken into\naccount in our revision. In this response letter, the reviewers’ comments to our original\n15 manuscript are provided in black, and our point-by-point responses as well as the\ncorresponding changes are shown in blue.\n[General comment]: The study proposes to capture moisture from the air and produce\nhydrogen without the need for electricity or liquid water. The demonstration unit is\nbased on a radiative cooling-enhanced atmospheric water harvesting (AWH) with a\n5 photothermal catalytic (PTC) process to extract and split water. A multilayer nanofiber\nmembrane composed of a hygroscopic bottom layer (MIL@PAN NFMs) that captures\nwater from the air and a photothermal catalytic top layer (Pt/TiO₂@PVDF NFMs) that\nutilizes sunlight to drive the hydrogen evolution reaction has been used. The following\nare the specific comments that need to be addressed.\n10 [Response]: We sincerely thank the reviewer for reviewing our work and giving the\nvaluable comments! We have addressed all the comments through further\nexperimentation and have made revisions on the manuscript, as specified below. We\nhope our response satisfies you.\n15 [Comment 1]: Page 5, line 15, the claim that “demonstrating nearly 10% improvement\nin performance compared to the sample without the radiative cooling enhanced one\n(0.82 -1 ·g -1)”. It is not clear whether the same setup has been used to perform\ngwater layer\nthe experiments without radiative cooling or it has been taken from literature.\n[Response]: Thank you very much for your careful reading and useful comment. We\n20 apologize for the lack of clarity in our original manuscript. Here, we aim to demonstrate\nthat the radiative cooling enhanced HHE NFMs exhibit a 10% improvement in water\nuptake capacity compared to their non-radiative cooling enhanced counterparts which\ncomes from our own HHE NFMs. The same setup has been used to perform the\nexperiments with and without radiative cooling HHE NFMs. As the reviewer rightly\n25 pointed out, we have supplemented relevant information in the revised manuscript and\nmethod part to prevent potential ambiguity (revised manuscript, page 5, line 13-17).\n[Comment 2]: The manuscript claims that a hydrogen production rate of 6467.55\nµmol·m⁻²·h⁻¹ under outdoor conditions, which is nearly six times higher than indoor\n2\nexperiment. The plausible factors enhancing the hydrogen production rate for outdoor\nexperiments compared to indoor experiments must be mentioned.\n[Response]: We sincerely thank the reviewer for these insightful comments. While the\nreasons for the enhanced H production rate were mentioned on page 9, line 17 of the\n2\n5 original manuscript, we acknowledge that our explanation may not have been\nsufficiently clear. Here, we provide a more detailed analysis to clarify why the hydrogen\nproduction rate of the air-to-H₂ system under outdoor conditions is 6 times higher than\nthat observed in indoor conditions. We analyze that this enhancement arises from the\ncombined synergistic thermodynamic and kinetic effects.\n10 1. Thermodynamic perspective:\nA comprehensive energy analysis of the photothermal catalytic (PTC) layer surface\nreveals that under outdoor conditions, the upper surface absorbs significantly more\nsolar energy, leading to elevated surface temperatures. This thermal effect\nthermodynamically favors the hydrogen evolution reaction. The specific mechanisms\n15 are elaborated as follows:\n(1) Discrepancies in spectral intensity distribution between indoor simulated sunlight\nand outdoor natural light\nFirst, we provide detailed specifications of the simulated solar light source used in\nindoor experiments. The xenon lamp system (Newport Model 67005 with Power\nmanufacturer's spectral irradiance profile and our measured spectral distribution when\nusing the AM1.5G filter. Notably, comparative analysis reveals that, in the ultraviolet\nrange and visible range (625-800 nm), the filtered xenon lamp exhibits higher irradiance\nthan natural sunlight. However, in the 450-625 nm of visible light, filtered xenon lamp\n25 exhibits lower irradiance. While these spectral mismatches were shown in the figure,\nthese differences alone cannot fully explain the six times increase in her performance\nobserved under outdoor conditions.\nOZONE FREE is what we used). B) The spectral irradiance of natural sunlight and\n300W Xe light with AM1.5g filter.\n5 (2) Light scattering by condensed liquid films on glass reactor surfaces\nThe solar energy absorbed by the surface (𝑃 ) of HHE NFMs under outdoor\n𝑠𝑢𝑛\nconditions and indoor conditions can be calculated by1:\n∞\n𝑃 =∫ [𝜀 (𝜆,𝜃 )𝐼 (𝜆)]𝑑𝜆\n𝑠𝑢𝑛 𝑠 𝑠𝑢𝑛 𝐴𝑀1.5\n0\nwhere 𝜀 (𝜆,𝜃 ) represents the absorptance/emittance of the surface of HHE NFMs,\n𝑠 𝑠𝑢𝑛\n10 which is PTC layer, and I (𝜆) represents the AM1.5 solar irradiance. In the indoor\nAM1.5\nH production tests, the HHE NFMs were encapsulated within a quartz glass reactor\n2\nto maintain a sealed environment and prevent hydrogen leakage. Theoretically, the P\nsun\ncalculation should be based on the standard simulated solar radiation of 1000 W/m².\nHowever, under indoor conditions, the temperature of the reactor's top glass window\n15 and sidewalls remained close to ambient temperature (~25°C), and water vapor released\nduring evaporation condensed on these surfaces, forming liquid droplets beneath the\nglass window. Optical characterization reveals that these condensate droplets induce\nnotable light scattering and absorption. Consequently, the actual photon flux reaching\nthe PTC layer was reduced. To validate this conclusion, we set the simulated solar\n20 irradiance to 1000 W/m², as shown in Fig. R2A. Under illumination, Fig. R2B shows\nthat liquid film rapidly formed and persisted on the underside of the quartz glass cover\nplate. To evaluate the impact of this liquid film on light intensity, a quartz plate with an\nanalogous liquid film was positioned above a radiometer at the same height as the\n4\nreactor's quartz cover plate. The results from Fig. R2C demonstrated a significant\nirradiance reduction due to light scattering by the liquid film, decreasing from 1000\nW/m² to 745 W/m². Consequently, the effective solar irradiance absorbed by the HHE\nNFMs surface within the reactor under indoor conditions (P ) was calculated as\nsun, indoor\n5 534.1 W/m². The presence of the liquid film in the enclosed reactor thus reduced the\nsolar irradiance absorbed by the PTC layer, leading to both (i) lower surface\ntemperature compared to the open-environment measurement under indoor conditions\nand (ii) lower photoexcitation of photocatalyst to drive water splitting.\nfilter. B) Photo of H production test under indoor Xe lamp and the condensed water\n2\nvaper under the quartz glass. C) The test photo of solar irradiance when the quartz glass\nis covered by condensed water vapor.\nHowever, in the outdoor air-to-H₂ system, the Video S1 reveals that although a thin\n15 liquid film initially forms on the quartz glass surface, it rapidly dissipates under natural\nsolar irradiation, resulting in negligible light-scattering effects. Under normal outdoor\nmidday sunlight (1000 W/m2), the power received by the PTC layer surface is about\naverage outdoor solar irradiance of 786 W/m², the effective irradiance absorbed by\n20 the PTC layer (P ) reaches 569.1 W/m². This demonstrates that P\nsun, outdoor sun, outdoor\n(569.1 W/m²) > P (534.1 W/m²). The superior solar energy absorption capacity\nsun, indoor\nof HHE NFMs under outdoor conditions is therefore confirmed.\nBesides, most researchers intuitively assume that higher wind speeds in outdoor\nenvironments enhance convective heat transfer. However, in our outdoor air-to-H₂\n25 system, the reactor body is constructed using aluminum (Al). This Al reactor undergoes\nslight heating under sunlight, resulting a temperature rise to slightly above ambient\ntemperature. Therefore, it can be inferred that the solar radiation absorbed by the reactor\nbody is offset by external natural convective cooling, with negligible impact on the\nsurface temperature of PTC layer.\nenergy absorption thus higher surface temperature of PTC layer (revised manuscript\nFig. 4d). As demonstrated by our DFT calculations (revised manuscript Fig. 3h), the\nincreased reaction temperature within the biphasic system lowers the energy barrier for\nH formation, thereby accelerating the hydrogen evolution rate. Thus, from a\n2\n10 thermodynamic perspective, the enhanced surface temperature of HHE NFMs under\noutdoor conditions contributes significantly to the improvement in hydrogen production\nrates compared to indoor conditions.\n2. Kinetic perspective\nThe kinetic enhancement of H production rates in HHE NFMs primarily originates\n2\nfrom scaling effects. In indoor condition, a 4×10-4 m2 HHE NFMs was enclosed within\na glass reactor, whereas the outdoor system employed a 0.01 m2 HHE NFMs array\nintegrated into a 0.25 m2 reactor. COMSOL simulations of the photothermal water\nrelease process for HHE NFMs of varying sizes revealed that moisture in the\nhygroscopic layer escapes not only from the top PTC layer but also from the sides of\n20 the hygroscopic layer (revised manuscript Fig. 4e). However, as the HHE NFMs size\nincreases, a greater proportion of vapor escapes through the PTC layer (revised\nmanuscript Fig. 4f). This indicates that larger HHE NFMs dimensions enhance reactant\n(i.e. water molecules) availability by increasing vapor contact with catalysts on the PTC\nlayer, thereby elevating vapor concentration in the PTC layer and kinetically enhancing\n25 hydrogen production rates.\nIn summary, under natural outdoor sunlight irradiation, both thermodynamic\nand kinetic factors synergistically promote H production. Consequently, the large-\n2\nscale outdoor air-to-H₂ system achieves significantly higher hydrogen production rates.\nAdditionally, we have refined the descriptions of H production rate enhancements in\n2\n6\nthe main text and supplementary materials (revised manuscript page 9, line 16 to page\n10, line 7, and revised Note S1 and Note S2) to better elucidate why the large-scale\noutdoor air-to-H system exhibits superior performance.\n5 improve the clarity and rigor of our mechanistic explanations in the revised manuscript.\n[Comment 3]: Page 18, line 10, mentions “The GGA + U method was used with U-J\nvalue of 4.0 for Ti 3d electrons”. The .xyz file of the catalyst slab used for the\ncomputation may be included in the Electronic Supplement.\n10 [Response]: Thank you for your kind suggestion! We have put the structural files of\nthe catalyst into the Electronic Supplementary Materials for reference by both reviewers\nand readers.\n[Comment 4]: Is numerical vibrational frequency computation was done on optimized\n15 catalyst slab, for estimating delta G.\n[Response]: Thank you very much for your kind comments. We have done the\nnumerical vibration frequency computation on optimized catalyst slab. The process was\ndescribed in the calculation details (revised manuscript page 19, line 21-22).\n20 [Comment 5]: If it is so which space group and arrangement of original unit cell was\nused, must be mentioned.\n[Response]: We sincerely appreciate the reviewer's careful reading and constructive\nfeedback. In response to these valuable comments, we have incorporated additional\ncomputational details in the revised manuscript (page 19, line 11-14) for enhanced\n25 clarity and reproducibility. The supplementary content is presented as follows:\n‘The model of anatase TiO (tetragonal, space group I41/amd), with lattice parameters\n2\na = b = 3.78 Å, c = 9.62 Å, was obtained from open-access database The Materials\nProject. Then, a 4 layered (101) surface with 5×3 supercell was built based on XRD\nresult, where the bottom layer was fixed during optimization.’\n7\n[Comment 6]: The claim that “The system can theoretically produce ~4.5 × 10⁴ GL\n(gigaliters) of hydrogen annually across 0.1% of global land areas.” is a very tall claim\nconsidering the amount of catalytic material and their production requirement.\n5 [Response]: We appreciate the reviewer's kind suggestions and insights. The reviewer\nhas a profound understanding of the annual hydrogen production, required materials,\nand preparation processes. In the current stage, this paper utilizes Pt/TiO as the\n2\nphotocatalyst (Pt content is 1%wt), with a catalyst usage of 40g per square meter of\nHHE NFMs. Considering a land usage of 0.1%, which is approximately 1.49×1011 m2,\n10 the required amount of catalyst is estimated to be around 5.9×106 tons of Pt/TiO .\n2\ntechnology to prepare HHE NFMs. As electrospinning is a relatively cost-effective and\nrapid technique, it enables us to swiftly fabricate HHE NFMs within a limited\ntimeframe. Therefore, we concur with the reviewer's viewpoint that while\n15 implementing our proposed air-to-H system would demand lots of catalyst material\n2\nwith 0.1% of global land usage, the manufacturing process might be relatively\nstraightforward.\nAdditionally, the statement in our original text, \"The system can theoretically produce\n~4.5 × 10⁴ GL (gigaliters) of hydrogen annually across 0.1% of global land areas,\" is\n20 based on ideal conditions. Our quantitative projections and system-level analyses of\nglobal air-to-H system deployment not only highlight its scalability but also provide\n2\nintuitive feeling of the potential of H production by using atmospheric moisture, which\n2\nis a practical prospect for the future use of the system. At the same time, it can guide us\nto select a reasonable location (such as location with operatable relative humidity (RH)\n25 and humidity) and let us clearly understand where to build the air-to-H system we\n2\nproposed to obtain better benefits. Furthermore, in the supporting materials, we have\nelaborated on and illustrated our calculation process for the reviewer and readers to\nreference (revised supplementary materials Note S3).\n8\n[Comment 7]: Hydrogen production happens only when sunlight is available. Cloudy,\nrainy days and nighttime will not produce hydrogen. Though moisture collection\nhappens at night, there will be no hydrogen production.\n[Response]: We sincerely appreciate the reviewer’s thoughtful comments. Based on\n5 our experimental results, the system demonstrates optimal H production efficiency\n2\nunder clear-sky conditions. While the HHE NFMs can still generate hydrogen under\novercast conditions (with UV radiation and relatively weak visible light), the H\n2\nproduction rate decreases proportionally to the available photon flux (revised\nmanuscript Fig. 4b). We concur with the reviewer’s observation that H production\n2\n10 ceases during nighttime or rainy periods, a limitation shared by most solar-driven\nsystems (photovoltaic2,3, photoelectrochemical4,5, solar driven seawater desalination6,7).\nHowever, this work specifically focuses on enabling photothermal catalytic H\n2\nproduction under liquid water-free conditions by utilizing atmospheric moisture and\nnatural sunlight. During nighttime or precipitation events, the elevated RH facilitates\n15 passive water harvesting from air, which subsequently enhances daytime photocatalytic\nactivity. This dual-functional mechanism allows simultaneous energy and water\nproduction in arid regions.\nWhile hydrogen production pauses at night, the system actively prepares for daytime\noperation through water storage. Furthermore, the radiative cooling capability during\n20 nighttime can enhance atmospheric water harvesting by dissipating heat to outer space,\nhighlighting the unique advantage of our system compared to conventional\nphotocatalytic8,9 or photo-electrocatalytic4,5 approaches.\nWe thank the reviewer for raising this important point, which has allowed us to clarify\nthe system’s operational strategy.\n25\n[Comment 8]: The solar-to-hydrogen (STH) efficiency of 0.1%–0.085% is reported in\nmanuscript is very much lower than electrolysis-based hydrogen production, which is\naround 15–20%. It is not clear how the system may compete against electrolysis.\n[Response]: We sincerely thank the reviewer for the detailed review. Here, we have\n9\ncreated a table to highlight the differences between our system and electrocatalytic\napproaches, along with a comparative analysis.\nFirst of all, the proposed air-to-H system is not intended to compete\n2\nwith electrocatalysis but rather to complement it. Large-scale centralized H\n2\n5 production near water sources and electricity supply may prioritize electrocatalysis,\nwhile our system provides decentralized solutions for water-scarce regions,\ncontributing to sustainable hydrogen economies.\nSecondly, as we emphasized throughout our work, the key strength of this system lies\nin its ability to produce H without requiring liquid water and an electrical grid. By\n2\n10 directly utilizing atmospheric moisture and solar energy, it overcomes the reliance of\ntraditional electrolysis H production on purified water/alkaline electrolytes and stable\n2\npower supplies.10,11 Additionally, air-to-H system is particularly suitable for off-grid\n2\nscenarios (e.g., arid/semi-arid regions like the Sahara Desert, where nighttime humidity\naverages ~35% RH), where electrocatalytic systems struggle to operate.\n2\noperation, which is totally a passive way as no electricity is needed in the intermediate\nstage. In contrast, grid-dependent electrocatalytic hydrogen production may generate\nindirect emissions unless powered entirely by renewable energy.\nWe thank the reviewer for raising these critical points, which have allowed us to better\n20 articulate the unique value and applicability of our work.\n[Comment 9]: The hygroscopic and photocatalytic materials (Pt/TiO₂@PVDF and\nMIL@PAN NFMs) need to maintain their performance over long-term outdoor\nexposure. Potential degradation due to UV exposure, environmental pollutants, or\n25 material fatigue over time could reduce efficiency. The cycling stability of the moisture\nadsorption-desorption process must be further tested for real-world applications.\n[Response]: We sincerely thank the reviewer for this constructive comment. The\ncycling stability of HHE NFMs is indeed critical for the long-term operation of the\nsystem. To address this, we conducted additional extensive cyclic tests involving\n10\nprolonged illumination and repeated water adsorption/desorption processes.\n1. Indoor cycling tests:\nAfter each indoor H production experiment, the HHE NFMs were exposed to ambient\n2\nair for natural moisture adsorption for subsequent cycles of H production. In fact, we\n2\n5 performed multiple water adsorption and desorption processes (at least 10 times) using\nthe same HHE NFMs, as shown in Fig. R3A. Meanwhile, repeated indoor cycling tests\n(Fig. R3B) demonstrated consistent H production without noticeable performance\n2\ndecay (at least 8 times).\n2. Outdoor cycling tests:\n2 2\nproduction performance was observed (revised manuscript Fig. 5d and revised\nsupplementary materials Fig. S33). The observed fluctuations correlated with daily\nweather variations (e.g., cloud cover, humidity), confirming that the efficiency\nremained stable under operational conditions. After a long time of outdoor experiments,\n15 as shown in Fig. R3C, we took out the HHE NFMs after outdoor cycling reaction for\nindependent testing, and found that its water adsorption and release capacity and H\n2\nproduction capacity did not significantly decline, which verifies that the outdoor\nenvironment has negligible influence on our HHE NFMs. We also added this data to\nthe supporting materials for reference (revised supplementary materials Fig. S34).\n20 3. Material Characterization\nSEM analysis revealed intact honeycomb-like porous structures in the hygroscopic\nlayer after prolonged outdoor exposure, with MIL-101(Cr) uniformly distributed on\nPAN nanofibers as shown in Fig. R3D, which verifies the structure stability of\nhygroscopic layer.\nevenly dispersed on PVDF nanofibers after long time outdoor test as shown in Fig. R3E\n(added to revised supplementary materials Fig. S35). No detectable changes were\nobserved in XRD patterns before and after testing, confirming material stability (Fig.\nR3F).\nworld operational conditions. We thank the reviewers for highlighting this important\naspect, which has allowed us to strengthen the stability discussion in the revised\nmanuscript page 10, line 13-17.\nPAN NFMs at 25 °C and 90% RH (desorption at 100 °C). B) Time course of H\n2\nduring\nthe indoor cyclic testing of HHE NFMs. C) The water adsorption and desorption test\nand hydrogen production test after long time outdoor experiment. D) The SEM and\nphoto of PTC layer after long time outdoor experiment. F) The XRD test result of PTC\nlayer before and after long time outdoor experiment.\n[Comment 10]: Is there any other material than platinum Pt and Au which can be used\n15 which is cheaper and abundant. A selection of these two elements must be justified.\n[Response]: We sincerely appreciate the reviewer’s insightful comments. In the\n12\nproposed air-to-H system, TiO was selected as the photocatalyst for water vapor\n2 2\nsplitting due to its chemical stability, low cost, and environmental friendliness.\nHowever, the rapid electron-hole recombination in TiO limits its photocatalytic\n2\nactivity and quantum efficiency.12,13 To address this, strategies such as metal loading\n5 have been widely adopted to suppress recombination rates and enhance charge carrier\nmobility, thereby improving the overall H evolution performance14,15. Potential\n2\ncocatalysts include noble metals (e.g., Pt, Au, Ag, Pd) and non-noble metals (e.g., Fe,\nCu, Ni, Zn). 15 While non-noble metals are more cost-effective, they exhibit certain\nlimitations compared to noble metals.\nresonance (SPR) effect, which localize incident light and generate high-energy hot\nelectrons.16 This significantly broadens the light-responsive range of TiO and enhances\n2\nphotothermal conversion efficiency, while maintaining high stability against\noxidation17,18. These properties are critical for daytime water desorption and hydrogen\n15 production in the air-to-H system. Some non-noble metals (e.g., Cu, Co) may exhibit\nenvironments. 19\nBesides, noble metals (Pt, Au) possess high work functions, forming Schottky barriers\nat the TiO interface.20,21 This effectively traps photogenerated electrons, suppresses\n2\n20 electron-hole recombination, and prolongs carrier lifetimes. Concurrently, noble metals\nprovide high-density active sites to enhance hydrogen evolution rates.\nBased on these considerations, Pt and Au were selected as cocatalysts to demonstrate\nthe feasibility of the concept of this work. Their synergistic advantages, efficient\nelectron trapping and SPR-driven photothermal effects, outperform the single-function\n25 optimization achievable with non-noble metals. We acknowledge that further research\non non-noble metals could improve material stability and balance performance with\ncost. Future work will explore such approaches to develop lower-cost hydrogen\nproduction solutions.\n13\n[Comment 11]: Page 17, the results from 3D numerical simulations must be presented.\n[Response]: We sincerely appreciate the reviewer's thoughtful reminder. The purpose\nof this analysis is to illustrate the water vapor transport dynamics across HHE NFMs\nsurfaces at varying scales (revised manuscript Fig. 4e). To clearly visualize these\n5 phenomena, we selected cross-sectional computational results for presentation in the\noriginal manuscript. As requested, the full 3D simulation results have been included in\nthe revised supplementary materials (revised supplementary materials Fig. S30) for\nreference. Additionally, the 3D computational results are provided below for the\nreviewer's convenience.\nmaps of HHE NFMs (3D simulation results)\n[Comment 12]: The system is a breakthrough in sustainable hydrogen production, but\n15 low efficiency, weather dependence, and scalability challenges must be addressed\nbefore commercial deployment.\n[Response]: We sincerely thank the reviewer for this recognition of our work and the\nprofound insights into the critical challenges. The issues of efficiency improvement,\nweather dependency, and scalability raised by the reviewers are indeed core obstacles\n20 that must be addressed for the commercialization of this technology. We fully\nacknowledge these limitations and will conduct further in-depth research on the\nchallenges highlighted by the reviewer. For example, regarding H production\n2\n14\nefficiency, enhancement of the water adsorption capacity of the hygroscopic layer by\nincorporating hygroscopic salts into the MOF materials to form a hierarchical water-\nabsorbing structure can be addressed. Additionally, photothermal catalysts with broader\nlight absorption ranges can be developed to maximize solar energy utilization. To\n5 address weather dependency, more works can be done by adding heat energy storage\nsystems to harvest excess heat during sunny periods, which could supply clean water to\narid regions during cloudy or rainy conditions, even if H production is paused.\nbalancing performance and cost by utilizing more affordable and readily available\n10 catalysts to reduce overall system costs. We fully agree that achieving commercial\ndeployment requires a better balance among efficiency, stability, and cost. The primary\ngoal of this work is to provide a proof-of-concept and preliminary optimization\npathways for this emerging field, while elucidating the effects of surface temperature\nand size effect on H production rates. The reviewer’s valuable feedback will\n2\n15 significantly guide our future research, and we look forward to collaborating with the\nbroader scientific community to advance this technology toward maturity.\n[Comment 13]: Despite these limitations, the study presents a promising off-grid\nhydrogen production technology for arid and water-scarce regions, contributing to\n20 sustainable energy transitions.\n[Response]: We sincerely thank the reviewer for the strong affirmation of our work's\nsignificance, particularly the recognition of the potential contributions to off-grid\nhydrogen production in arid, water-scarce regions and the sustainable energy transition.\nThe highlighted limitations (e.g., efficiency and scalability challenges) align closely\n25 with our own priorities, and they serve as a strong impetus for further exploration of\nviable pathways toward practical implementation. Once again, we deeply appreciate\nyour constructive feedback and encouragement.\n[General comment]: This manuscript presents a hydrogen production device that\ncaptures moisture from ambient air using MIL@PAN nanofiber membranes (NFMs)\nwith a radiative cooler (Pt/TiO2@PVDF) during nighttime, and the collected water is\n5 subsequently converted into hydrogen via a photothermal catalytic process using\nhygroscopic H2 evolution nanofiber membranes (HHE NFMs) during daylight hours.\nWhile the research topic is interesting, the manuscript requires significant revisions\nbased on the following comments.\n[Response]: We sincerely thank the reviewer for reviewing our work and giving the\n10 valuable comments! We have addressed all the comments through further\nexperimentation and have made revisions on the manuscript, as specified below. We\nhope our response will satisfy you.\n[Comment 1]: Please provide the energy dispersive spectroscopy data and the specific\n15 surface area (m2/g) of the honeycomb-like structure composed of MIL@PAN.\n[Response]: We sincerely thank the reviewer for these valuable suggestions. As\nrecommended, the energy-dispersive spectroscopy (EDS) data and specific surface area\nanalysis of the honeycomb-like MIL@PAN structure have been incorporated into the\nrevised manuscript and supplementary materials. The EDS elemental mapping of the\n20 honeycomb structure is presented in Fig. R5.\nThe C and N elements from the PAN nanofibers are primarily distributed along the\nwalls of the honeycomb structure, with minor amounts observed in the central regions,\nconfirming the presence of nanofibers throughout the structure, consistent with the\nmorphology shown in revised manuscript Fig. 2b. Similarly, the Cr, O, and F elements\n25 from MIL-101(Cr) are localized along the walls, with partial distribution in the central\nregions, further verifying the uniform dispersion of MIL-101(Cr) along the nanofibers\nand the formation of a highly porous architecture.\nBrunauer–Emmett–Teller (BET) surface area measurements reveal that MIL@PAN\nexhibits a high specific surface area of 1790.8 m²/g, suggesting the abundant accessible\n16\nsurface area.\nFig. R5 SEM and elements mapping images of honeycomb-like structure\nTo address the reviewer’s comments, we have made necessary changes and included\n5 the updated results in the revised supplementary materials Fig. S3.\n[Comment 2]: The authors should evaluate the energy balance of the hydrogen\nproduction device for both indoor and outdoor experiments.\nOptical characteristics: The photothermal catalytic (PTC) top layer, consisting of\n2\nin the ultraviolet range and 0.7 in the visible-near infrared spectrum. It simultaneously\nachieves radiative cooling, with an emissivity of 0.95 in the mid-infrared range.\nIn the indoor experiment under AM 1.5G conditions (1,000 W/m2), the surface\ntemperature of the hydrogen production device was maintained at 68 oC, and the\n15 evaporation rate was 0.298 kg/m2 h. The net radiative power absorbed by the device\nwas calculated as 713 W/m2 by integrating the product of absorptance and supplied\npower over the wavelength. Thermal radiation within the indoor environment is\nnegligible. Latent heat absorption from water desorption was measured at 187 W/m2.\nThe remaining power, 526 W/m2, corresponds to the sum of heat transferred to the\n20 surrounding air and the endothermic heat of the hydrogen conversion process.\nHowever, in the outdoor experiment, the surface temperature of the device increased to\n82.2 oC. This raises questions about the energy balance, when the net absorbed power\n17\nunder 1,000 W/m2 solar flux is 713 W/m2, while the thermal emission of the device is\ncalculated as 858.9 W/m2 at 82.2 oC. Additionally, water desorption from the porous\nMIL@PAN material absorbs latent heat at 2,260 kJ/kg-water, alongside the\nendothermic reaction heat of hydrogen production. This discrepancy suggests that the\n5 energy balance cannot be fully explained, even though the system is described as\nentirely insulated.\nIt is also noted that the outdoor solar flux is measured as 786 W/m2, significantly lower\nthan the indoor value of 1,000 W/m2. This further complicates the energy balance, as\nthe heat transfer coefficient of the system with outdoor air should logically be higher\n10 than that with indoor air. The authors should clarify these discrepancies and provide a\ndetailed analysis of the energy balance under outdoor conditions.\n[Response]: We sincerely appreciate the reviewer’s careful reading and constructive\nfeedback. A rigorous energy conservation analysis of the HHE NFMs under outdoor\nsolar conditions is indeed critical. Below, we provide a detailed analysis of the heat\n15 transfer processes involved. Here, we further divide the experiments under indoor and\noutdoor conditions (tests in open environments and tests in closed reactor) for separate\ncalculations to verify energy conservation.\n1) Indoor energy balance on the surface of HHE NFMs\nIn the indoor surface temperature test, the HHE NFMs was placed on an electronic\n20 balance for surface temperature test and water evaporation test, where the electronic\nbalance has the glass wall on the surrounding to protect the HHE NFMs from being\naffected by the wind. Therefore, as we shown in revised manuscript Fig. 3C, the surface\ntemperature of PTC layer increased from ~25 ℃ to ~58 ℃ within 10 min and finally\narrived around 68 ℃. Therefore, in energy balance calculation, we choose to use the\n25 surface temperature of 58 ℃ in the calculation, as under this temperature the\nevaporation rate is the highest. Therefore, the P can be calculated by:\nsun\n∞\n𝑃 =∫ [𝜀 (𝜆,𝜃 )𝐼 (𝜆)]𝑑𝜆\n𝑠𝑢𝑛 𝑠 𝑠𝑢𝑛 𝐴𝑀1.5\nsun\nUnder indoor condition, the passive cooling to the universe can be ignored. Therefore,\n18\nunder the indoor condition,\n𝑃 (𝑇 ) = 𝜀 ×𝜎×(𝑇4 −𝑇4 ) = 221.8 W/m2\n𝑟𝑎𝑑 𝑠 𝑠 𝑎𝑚𝑏\nThe calculation of non-radiative cooling follows:\n𝑃 = ℎ (𝑇 −𝑇 )\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒,𝑖𝑛𝑑𝑜𝑜𝑟 𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒,𝑖𝑛𝑑𝑜𝑜𝑟 𝑠,𝑖𝑛𝑑𝑜𝑜𝑟 𝑎𝑚𝑏,𝑖𝑛𝑑𝑜𝑜𝑟\nthe\nℎ = 10.2 W/(m2·K). Therefore, 𝑃 = 336.6\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒,𝑖𝑛𝑑𝑜𝑜𝑟 𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒,𝑖𝑛𝑑𝑜𝑜𝑟\nW/m2.\nAccording to the original manuscript, the evaporation rate under 1 sun illumination in\nthe open indoor environment is approximately equivalent to that in the outdoor\n10 environment.\n𝑃 = ℎ 𝑚̇ = 187 W/m2.\n𝑒𝑣𝑎𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑒𝑣𝑎𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛\nTherefore, under indoor open area,\n𝑃 +𝑃 +𝑃 +𝑃 = 746.4 W/m2.\n𝑒𝑣𝑎𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒,𝑖𝑛𝑑𝑜𝑜𝑟 𝑟𝑎𝑑,𝑖𝑛𝑑𝑜𝑜𝑟 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛,𝑖𝑛𝑑𝑜𝑜𝑟\nThis value of 746.4 W/m2 closely aligns with P =718.9 W/m2. The extra energy loss\nsun\n15 may come from slight inaccuracy of the measured indoor temperature, because under\nthe simulated sunlight, the space inside the electronic balance also heats up, so\n𝑃 may be overestimated here. Therefore, the calculation results\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒,𝑖𝑛𝑑𝑜𝑜𝑟\nvalidate the energy balance at the PTC layer surface under indoor open area\ncondition.\n20 2) Outdoor energy balance on the surface of HHE NFMs under open area\nIn the outdoor experiments, the HHE NFMs were placed in an open plastic enclosure\n(without quartz glass shielding) for surface temperature test, effectively operating\nunder windless conditions due to the enclosure’s walls. As shown in the temperature\ndistribution data in the original manuscript, the maximum surface temperature reached\n25 82°C. While this peak temperature was used in the initial radiative cooling power\ncalculations in the comment, we acknowledge that using the average surface\ntemperature of 76°C would yield a more representative energy balance. Furthermore,\nthe solar irradiance during peak temperature conditions (noon) was measured as ~970\nW/m² (close to 1 sun intensity), whereas the value of 760 W/m² cited in the original\n19\nmanuscript represents the daily average irradiance between 10:00 AM and 4:00 PM.\nRegarding the radiative cooling power calculation, after thorough literature review and\nvalidation, we have refined the energy balance framework. Under outdoor conditions,\nas shown in Fig. R6A, the energy conservation equation for the HHE NFMs surface\n5 should be expressed as:\n𝑃 =𝑃 (𝑇 )−𝑃 (𝑇 )+𝑃 +𝑃 +𝑃\n𝑠𝑢𝑛 𝑟𝑎𝑑 𝑠 𝑎𝑡𝑚 𝑎𝑚𝑏 𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑒𝑣𝑎𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛\nwhere P , P (T ), P (T ), P , P and P is the absorbed solar\nsun rad s atm amb non-radiative evaperation reaction\npower, radiated power by the PTC layer, absorbed radiation power from the atmosphere,\nnon-radiative cooling power, power absorbed by water evaporation process, and power\n10 for hydrogen production. T and T is the temperature of PTC layer surface exposed\ns amb\nto the sky and ambient temperature, which is 76℃ and 35℃, respectively.\nWhen the PTC layer is exposed to the sunlight, the P can be calculated by1:\nsun\n∞\n𝑃 =∫ [𝜀 (𝜆,𝜃 )𝐼 (𝜆)]𝑑𝜆\n𝑠𝑢𝑛 𝑠 𝑠𝑢𝑛 𝐴𝑀1.5\n0\nwhere I (𝜆) represents the AM1.5 solar irradiance. When the clouds move to not\nAM1.5\n15 block the sun, the light intensity is about 1sun, and the actual value is about 970 W/m2.\nTherefore, the P =697.33 W/m2.\nsun\nDue to the high infrared emissivity of PTC layer, it can spontaneously radiate heat\nthrough the atmospheric window. Under the condition of zero humidity, no clouds and\nclean air atmosphere (no aerosol, etc.), the theoretical value of surface radiation\n20 (𝑃 (𝑇 )) to the sky should be calculated according to the following equation:22\n𝑟𝑎𝑑,𝑖𝑑𝑒𝑎𝑙 𝑠\n∞\n𝑃 (𝑇 )=∫ ∫ [𝜀 (𝜆,𝜃)𝐼 (𝜆,𝑇 )]𝑑𝜆𝑐𝑜𝑠𝜃𝑑Ω\n𝑟𝑎𝑑,𝑖𝑑𝑒𝑎𝑙 𝑠 𝑠 𝐵𝐵 𝑠\n∩ 0\n𝜋/2 2𝜋\nwhere ∫ 𝑑Ω = ∫ 𝑠𝑖𝑛𝜃𝑑𝜃∫ 𝑑𝜙 is the hemisphere angular integral, and 𝜀 (𝜆,𝜃)\n∩ 0 0 𝑠\n2ℎ𝑐2 1\nis the spectral and angular emissivity of the PTC layer. 𝐼 (𝜆,𝑇 ) =\n𝐵𝐵 𝑠 𝜆5 ℎ𝑐\n−1\n𝑒𝜆𝑘𝐵𝑇𝑠\nindicates the spectral radiance of a blackbody, where h is Planck’s constant, c is the\n25 speed of light in vacuum, and k is the Boltzmann constant.\nB\nIn this theoretical condition, the theoretical maximum radiation power of our PTC layer\nis:\n20\n𝑃 (𝑇 ) = 499.6 W/m2.\n𝑟𝑎𝑑,𝑖𝑑𝑒𝑎𝑙 𝑠\nHowever, in practice, the atmospheric humidity, aerosol content, and cloud coverage\nsignificantly influence radiative cooling power. First, water vapor exhibits strong\nabsorption of infrared radiation beyond wavelength of 16 μm.23 As humidity increases,\n5 the 16–22 μm radiative window becomes substantially attenuated or even closed.\nSpecifically, when environmental RH rises from 20% to 100%, the peak atmospheric\ntransmittance in the primary radiative window (8–13 μm) decreases from 90% to 60%.\nCompared to dry conditions, most secondary radiative windows either disappear (e.g.,\n16–22 μm) or narrow (e.g., 2.5–5 μm) under high humidity.22,24 Additionally,\n10 atmospheric aerosols (suspended solid/liquid particles) induce Rayleigh and Mie\nscattering across both solar and thermal infrared spectra.25,26 Higher humidity\nexacerbates aerosol scattering, leading to increased infrared attenuation. In Hong Kong,\na coastal humid city where our outdoor experiments were conducted, summer aerosols\nare particularly pronounced. At typical experimental conditions (average RH =\n15 60~80%), aerosol scattering suppresses the atmospheric transmittance in the primary\nwindow by up to 20%.23\nTo conservatively account for the combined effects of humidity and aerosols on\nradiative cooling under cloudless skies, we introduced a correction factor R in our\ncalculation.23 Generally, the smaller the R is, the greater the impact of the environment\n20 on the radiation cooling effect. In order to simplify the calculation and comprehensively\nconsider the influence of RH (60%-80% RH) and aerosol effect in the air, we adopted\na conservative value of R = 0.8 in our calculations. Therefore, the actual radiative\ncooling power can thus be expressed as:\nP = R×P = 0.8×499.6 = 399.68 W/m2.\nrad,Ts rad,ideal\ncloud occlusion follows:22\n∞\n𝑃 (𝑇 )=∫ 𝜀 (𝜆,𝜃)∫ [𝜀 (𝜆,𝜃)𝐼 (𝜆,𝑇 )]𝑑𝜆𝑐𝑜𝑠𝜃𝑑Ω\n𝑎𝑡𝑚,𝑛𝑜𝑛𝑐𝑙𝑜𝑢𝑑 𝑎𝑚𝑏 𝑠 𝑎𝑡𝑚 𝐵𝐵 𝑎𝑏𝑚\n∩ 0\nwhere 𝜀 (𝜆,𝜃) = 1−𝑡(𝜆)1/𝑐𝑜𝑠𝜃 , is the emissivity of atmosphere, 𝑡(𝜆) is the\n𝑎𝑡𝑚\n21\ntransmittance of the atmosphere.\nFurthermore, the presence of clouds significantly impacts radiative cooling\nperformance.27 Clouds often behave as near-blackbody emitters in the mid-infrared\nspectrum, becoming opaque to outgoing thermal radiation. This enhances atmospheric\n5 re-radiation and blocks heat dissipation from the Earth's surface, thereby reducing\nradiative cooling power. As demonstrated by Zhao et al.28, the decline in radiative\ncooling power is proportional to the increased cloud cover duration as compared to\nclear-sky conditions. Besides, the presence of cloud layers leads to increased\natmospheric re-radiation, resulting in a higher P (T ). Taking June 2024 as an\natm amb\n10 example (when outdoor experiments were conducted), Hong Kong experienced\npredominantly cloudy conditions with an average cloud cover of 86% (f ) (The cloud\nc\ncover data comes from Hong Kong Observatory). Even on clear days, abundant floating\ncloud layers persisted, which reduced the radiative cooling power of the PTC layer and\nincreased the environmental radiation incident on the PTC. Here, we do not account for\n15 the cooling power reduction caused by cloud cover but focus solely on its effect on\nenhancing P .\natm\nIn the case of cloud cover, the received ambient radiation P is:23\natm, undercloud\n∞\n𝑃 =∫ 𝜀 (𝜆,𝜃)∫ [𝜀 (𝜆,𝜃)𝐼 (𝜆,𝑇 )𝜏 (𝜆,𝑅𝐻,𝑇 ,𝜃)]𝑑𝜆𝑐𝑜𝑠𝜃𝑑Ω\n𝑎𝑡𝑚,𝑢𝑛𝑑𝑒𝑟𝑐𝑙𝑜𝑢𝑑 𝑠 𝑐𝑙𝑜𝑢𝑑 𝐵𝐵 𝑎𝑚𝑏 𝑎𝑡𝑚 𝑎𝑚𝑏\n∩ 0\nwhere 𝜀 (𝜆,𝜃) is the is the emissivity of cloud, here we take 𝜀 (𝜆,𝜃) = 1.\n𝑐𝑙𝑜𝑢𝑑 𝑐𝑙𝑜𝑢𝑑\n20 𝜏 (𝜆,𝑅𝐻,𝑇 ,𝜃) is the emissivity of atmosphere under cloud. Therefore,\natm 𝑎𝑚𝑏\n𝑃 (𝑇 ) = 𝑓𝑃 +(1−𝑓)𝑃 = 257.3 W/m2.\natm 𝑎𝑚𝑏 𝑐 𝑎𝑡𝑚,𝑢𝑛𝑑𝑒𝑟𝑐𝑙𝑜𝑢𝑑 𝑐 𝑎𝑡𝑚,𝑛𝑜𝑛𝑐𝑙𝑜𝑢𝑑\nThe calculation of non-radiative cooling follows:\n𝑃 = ℎ (𝑇 −𝑇 )\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑠 𝑎𝑚𝑏\nWhere ℎ represents non-radiative heat transfer coefficient. In this setup,\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒\n25 thermal insulation materials were applied to the sidewalls, and a windbreak layer was\nimplemented to minimize additional heat loss caused by wind. Consequently, heat\ntransfer in this configuration is dominated by natural convection. Following established\nnatural convection heat transfer correlations, the calculated non-radiative heat transfer\n22\ncoefficient is ℎ = 8.0 𝑊/(𝑚2∙𝐾). Therefore,\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒\n𝑃 = ℎ (𝑇 −𝑇 ) = 328 W/m2\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑠 𝑎𝑚𝑏\nAccording to the original manuscript, the evaporation rate under 1 sun illumination in\nthe open indoor environment is approximately equivalent to that in the outdoor\n5 environment.\n𝑃 = ℎ 𝑚̇ = 187 W/m2.\n𝑒𝑣𝑎𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑒𝑣𝑎𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛\nIt is mentioned in the draft that under outdoor conditions, the STH in sunny days is\n0.1%. Therefore,\n𝑃 = 𝑆𝑇𝐻∙𝑃 = 1 W/m2.\n𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝐴𝑀1.5\n𝑃 (𝑇 )−𝑃 (𝑇 )+𝑃 +𝑃 +𝑃 = 658.38 W/m2\n𝑟𝑎𝑑 𝑠 𝑎𝑡𝑚 𝑎𝑚𝑏 𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑒𝑣𝑎𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛\nThis value of 658.38 W/m2 closely aligns with P =697.33 W/m2. The residual\nsun\nenergy discrepancy likely arises from unaccounted energy losses. This calculation\nresults validates the energy balance at the PTC layer surface under outdoor open\n15 area condition.\n3) Outdoor energy balance on the surface of HHE NFMs in the large scale reactor\nWhen the HHE NFMs were placed in the outdoor large scale reactor, the quartz glass\nwindow can block most of the radiative cooling into the universe under sunny day,\ndue to its low transmittance in atmospheric radiation window, as shown in Fig. R6C.\nlight intensity here is still calculated based on natural light:\n∞\n𝑃 =∫ [𝜀 (𝜆,𝜃 )𝐼 (𝜆)]𝑑𝜆\n𝑠𝑢𝑛 𝑠 𝑠𝑢𝑛 𝐴𝑀1.5\ntemperature was tested under midday. So, 𝑃 =697.33 W/m2.\n𝑠𝑢𝑛\ndissipation should follow:\n𝑃 (𝑇 ) = 𝜀 ×𝜎×(𝑇4 −𝑇4 )\n𝑟𝑎𝑑 𝑠 𝑠,𝑜𝑢𝑡𝑑𝑜𝑜𝑟 𝑎𝑚𝑏,𝑜𝑢𝑡𝑑𝑜𝑜𝑟\nHere, as it is difficult to get the temperature inside the reactor, the surface temperature\nof PTC layer and ambient temperature also taken as 76℃ and 35 ℃. Therefore,\n23\n𝑃 (𝑇 ) = 314.3 W/m2.\n𝑟𝑎𝑑 𝑠\nMost researchers intuitively assume that higher wind speeds in outdoor environments\nenhance convective heat transfer. However, in our outdoor air-to-H₂ system, the reactor\nbody is constructed of aluminum (Al). This Al reactor undergoes slight heating under\n5 sunlight (moderate temperature rises, slightly above ambient temperature, here taken as\n35°C). Therefore, it can be inferred that the solar radiation absorbed by the reactor body\nis offset by external natural convective cooling, with negligible impact on the surface\ntemperature of PTC layer. For energy analysis focusing solely on the PTC layer, since\nthe HHE NFMs are enclosed within reactors in both indoor and outdoor configurations,\n10 natural convective heat transfer occurs at the PTC layer surface in all cases.\n𝑃 = ℎ (𝑇 −𝑇 ) = 328 W/m2.\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑠 𝑎𝑚𝑏\nFurthermore, when the HHE NFM was placed in a reactor (either under outdoor\nconditions or indoor conditions), the evaporation rate of the HHE NFM was greatly\nreduced in a closed environment as shown in Fig. R6B (revised supplementary\n15 materials Fig. S34). The obviously lower evaporation rate in the closed system is mainly\nbecause (i) the significantly higher relative humidity in the closed system inhibits\nevaporation,29,30 and (ii) the convection in the closed system is much less than that in\nthe open system.31,32\nTherefore, the evaporation power was calculated as:\n20 𝑃 = ℎ 𝑚̇ = 32.5 W/m2.\n𝑒𝑣𝑎𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑒𝑣𝑎𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛\nIt is mentioned in the manuscript that under outdoor conditions, the STH in sunny days is\n0.1%. Therefore,\n𝑃 = 𝑆𝑇𝐻∙𝑃 = 1 W/m2.\n𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝐴𝑀1.5\nTherefore, in outdoor large scale reactor,\n25 𝑃 +𝑃 +𝑃 +𝑃 = 675.8 W/m2\n𝑒𝑣𝑎𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒,𝑖𝑛𝑑𝑜𝑜𝑟 𝑟𝑎𝑑,𝑖𝑛𝑑𝑜𝑜𝑟 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛,𝑖𝑛𝑑𝑜𝑜𝑟\nThis value of 675.8 W/m2 closely aligns with P =697.3 W/m2 within the outdoor\nsun\nlarge scale reactor. The residual energy discrepancy likely arises from unaccounted\nenergy losses.\nTherefore, based on our calculations, HHE NEMs verify energy conservation in\n24\nboth outdoor and indoor conditions.\nFig. R6 A) Schematic diagram and heat transfer processes of PTC layer under outdoor\nopen area. B) The water adsorption and desorption test and hydrogen production test\n5 with the humidity, temperature, and solar radiation intensity under outdoor fixed\nreactor condition. C) The transmittance of quartz glass window.\n[Comment 3]: The numerical simulation conducted using COMSOL Multiphysics\nlacks sufficient details. Specifically, when desorbed water molecules undergo gas-phase\n10 water splitting for hydrogen production via a photothermal catalytic process in the\nindoor experiment, this endothermic reaction likely impacts the overall energy balance,\nwhich was not explained in the manuscript. Please discuss the effect of this\nphenomenon. Additionally, more comprehensive information regarding the initial and\nboundary conditions, as well as the analytical scheme used, should be included.\n15 [Response]: We sincerely thank the reviewer for these insightful comments. As noted,\nthe photothermal catalytic water vapor splitting process is endothermic, and the\nabsorbed energy would indeed influence the overall energy flow in simulations.\nHowever, as calculated in our previous response, the energy consumed by the H\n2\n25\nevolution reaction accounts for only 0.10% of the total solar input (approximately 1\nW/m2), which is negligible compared to other energy pathways. Given the complexity\nof coupling reaction kinetics and phase-transport phenomena in the hydrogen\nproduction process, fully integrating these effects would significantly increase\n5 computational demands. In this COMSOL study, our primary objective was to validate\nthat increasing the size of HHE NFMs (while maintaining a constant height) enhances\nthe water vapor transport rate from the PTC layer, thereby supplying more reactants (i.e.\nwater molecules) for the photothermal catalytic reaction. To maintain computational\ntractability, we simplified the model by focusing on the evaporation dynamics under\n10 illumination.\nAs suggested, we have clarified this rationale in the revised manuscript (page 17,\nline 19-28) and expanded the description of initial/boundary conditions in the revised\nsupplementary materials Table S3. In addition, we have added a more detailed\nsimulation scheme in the revised manuscript for the reference of reviewers (page 17,\n15 line 16-19 and page 18, line10-15).\n[Comment 4]: Why was MIL-101(Cr) used for water harvesting from ambient air? The\nmoisture sorption performance (both kinetics and capacity) of MIL-101(Cr) is\nsignificantly lower than that of other materials under low humidity conditions. While\n20 the radiative cooler was utilized to enhance the moisture sorption performance of MIL-\n101(Cr) under low-humidity conditions, it also reduces net energy absorption during\ndaylight hours due to thermal radiation. Alternatively, materials with higher sorption\ncapacity at low humidity could be considered. Please discuss the advantages of\ncombining the MIL@PAN layer with the radiative cooler in this context.\n25 [Response]: We sincerely thank the reviewer for the constructive feedback. In\natmospheric water harvesting systems, MIL-101(Cr) has been widely adopted due to its\nhigh porosity, exceptional adsorption capacity, and broad operational humidity\nrange.33,34 The rationale for selecting MIL-101(Cr) as the hygroscopic material in this\nwork is as follows:\n26\n(1) MIL-101(Cr) exhibits ultrahigh water adsorption capacity (1.2–1.4 g/g) under\nmoderate to high humidity conditions (RH > 60%),34,35 significantly outperforming\nsilica gel (0.3–0.4 g/g) and zeolites (0.2–0.3 g/g).36 Its rapid adsorption kinetics further\nensure efficient water vapor capture.37\n5 (2) The material demonstrates outstanding regeneration performance, requiring a low\ndesorption temperature of 60~80°C,34 which aligns well with solar thermal energy\navailability.\n(3) MIL-101(Cr) maintains structural stability under operational conditions and long-\nterm stable water adsorption capacity.38\nreduced efficacy under low-humidity conditions (RH < 30%), where its adsorption\ncapacity drops to 0.1–0.2 g/g due to its larger pore size.37 To address this, we have\nincluded a comparative Table R1 to analyze the advantages and limitations of various\nhygroscopic materials under low-humidity conditions.\nmaterials.\nWater uptake\nMaterials capacity, RH=20% Advantages Disadvantages\n（g/g）\nWide moisture absorption\nrange, large capacity under Water uptake capacity is\nMIL-101(Cr) 0-0.1534,35,39 high humidity,39 high cycle relatively low under low\nstability and low regeneration humidity conditions\nenergy consumption3839\nUltra microporous structure The synthesis cost is\n(pore size 0.6 nm), designed high, and the adsorption\nMOF-80140-42 0.25-0.340-42 for low humidity conditions, capacity is limited under\nfast adsorption kinetics under high humidity\nlow humidity conditions42 conditions.\nModified with amino group,\ncapacity under high\nthe material has strong\nhumidity conditions and\nCAU-10-H43-45 0.2-0.2543-45 hygroscopic capacity (0.1g/g)\nhigh water desorption\nunder extreme low humidity\ntemperature (80-\nconditions (RH=10%)44,45\n100 ℃)45,46\nEasy deliquescence,\npoor circulation\nHygroscopic salt stability, and the\nHigh water absorption\n(taking LiCl as 0.3-0.536,47 deliquescent solution is\ncapacity at low humidity47\nan example)36 corrosive to the\nequipment and difficult\nto control.36,47\nPAN fiber enhances\nhydrophilicity, grading holes\naccelerate mass transfer, and\nMIL@PAN\n0.1-0.2 radiation cooling can improve Composite materials\n(This work)\nlocal humidity to enhance\nwater adsorption under low\nhumidity conditions\nAs demonstrated in the table, most hygroscopic adsorbents or absorbents with strong\nwater uptake capabilities under low-humidity conditions struggle to maintain high\nadsorption capacities at elevated humidity levels. Notably, hygroscopic salts exhibit\nstrong water adsorption across both low- and high-humidity ranges51. However, they\n5 are prone to deliquescence after moisture absorption, leading to severe cycling\ninstability. The resulting liquid salt solutions are difficult to control and may cause\nequipment corrosion47. Furthermore, our statistical analysis reveals that nighttime\naverage humidity exceeds 40% across most global land areas. This ensures that the\noperational environment for our air-to-H system predominantly falls within the\n2\n28\nmoderate-to-high humidity regime (RH≥40%), where the HHE NFMs can achieve\natmospheric water harvesting even in relatively low-humidity environments through\nradiative cooling.\nTherefore, in this work, we propose integrating MIL-101(Cr) with radiative cooling to\n5 develop the MIL@PAN layer, which offers the following advantages:\n(1) Multi-layer honeycomb-like structure\nThe electrospinning technique ensures uniform dispersion of MIL-101(Cr)\nnanoparticles on PAN nanofibers, preventing particle agglomeration and preserving\ntheir high specific surface area. Additionally, the introduction of PAN enhances\n10 hydrophilicity and promoting water molecule adsorption and diffusion under low-\nhumidity conditions.\n(2) Radiative cooling enhanced atmospheric water harvesting\nBy coupling the MIL@PAN layer with the PTC layer, radiative cooling is achieved.\nDuring nighttime, the HHE NFMs surface temperature is reduced through radiative\n15 cooling to outer space, thereby increasing the effective surface relative humidity (RH).\nThis effect is particularly critical in low-humidity environments, elevating ambient RH\nfrom suboptimal levels (e.g., 30% RH) to values favorable for adsorption (e.g., 45%\nRH), significantly enhancing water uptake capacity of MIL@PAN.\n(3) Optimized energy utilization\nthe remaining energy is sufficient to drive the desorption process in the hygroscopic\nlayer. However, due to the quartz glass covering above the HHE NFMs, the daytime\nradiative cooling will be decreased, which will minimize the energy loss by radiative\ncooling during the daytime. This mechanism also establishes a stable, continuous vapor\n25 flux under outdoor conditions to sustain the photothermal H evolution reaction.\n2\n(4) Improved system stability and scalability\nThe PAN matrix acts as a flexible substrate, enhancing mechanical stability.\nFurthermore, the MIL@PAN composite can be mass-produced via electrospinning,\nfacilitating large-scale manufacturing and practical deployment of the system.\n29\n[Comment 5]: It is recommended to describe the performance of the hydrogen\nproduction device under outdoor conditions from an energy perspective. For example,\nthe energy consumption required to produce a unit mass of hydrogen (J/g or J/mol)\ncould be calculated and compared with other studies.\n[Response]: We sincerely appreciate the reviewer’s insightful suggestion. In this study,\nwe employed the solar-to-H (STH) efficiency to quantify the conversion efficiency\n2\nfrom solar energy to H energy in the photocatalytic process. And the STH efficiency\n2\nis inversely proportional to the ‘energy consumption required to produce a unit mass of\n10 hydrogen’. Following the recommendation of reviewer, we have included a\ncomprehensive comparison between our air-to-H system with other photocatalytic H\n2 2\nproduction technologies as shown in Table R2 (revised supplementary materials Table\nS1).\nTable R2 The comparison of air-to-H system with other photocatalytic H production\n2 2\n15 technologies.\nSolar\nenergy\nrequired to\nSTH\nPhotocatalyst produce Feedstock Light source Ref\nEfficiency\none mol of\nhydrogen\n(J/mol)\nVaper\nharvested for\nthe air\nThis\nPt/TiO 0.10% 2.3×108 (without Natural sunlight\n2\nwork\nusing\nsacrificial\nagent)\nevaporated\nRhCrOx-Al:SrTiO 0.09% 2.63×108 Natural sunlight 48\n3\nfrom pure\nliquid water\nCoOOH/Rh loaded\nPure vapor 300 W Xe lamp\nSrTiO :Al coated with 0.4% 5.93×107 49\n3\nfeeding AM 1.5G\nTiO\nx\nLiquid\nPt/TiO 0.002% 1.19×1010 500 W Hg lamp 50\n2\nseawater\nRu-modified SrTiO :\nLa, Rh/Au colloid 0.1% 2.37×108 300 W Xe lamp 51\nwater\n(40 wt%)/BiVO :Mo\ng-C N /ITO/Co–BiVO 0.028% 8.46×108 52\n3 4 4\nwater AM 1.5 G\nLiquid water\nRh/CrO Au/CoO 300 W Xe lamp\nx x\n0.053% 4.47×108 containing 53\nmodified PbTiO /BiVO λ > 420 nm\n3 4\nFe2+/Fe3+\nBulk pure 300W Xe lamp\nBiVO /Au/CdS 0.054% 4.39×108 54\n4\nwater AM 1.5 G\nBulk pure 300 W Xe lamp\nCoO /CdTe/V-In S 1.31% 1.87×107 55\nx 2 3\nwater λ≥300 nm\nBulk pure 300 W Xe lamp\nPt/CoOx/ZnIn S /WO 1.52% 1.56×107 56\n2 4 3\nwater AM 1.5 G\nMethanol\nCu/TiO 0.002% 1.19×1010 aqueous Natural sunlight 57\n2\nsolution\nMethanol\n300W Xe\nCu/TiO 0.074% 3.2×108 aqueous 57\nsolution\n365 mW LED\nPdPSA-CdS 0.1% 2.3×108 aqueous 58\nλ=420 nm\nsolution\nBulk pure 300 W Xe lamp\nPt-IrO /3D-g-C N 0.06% 3.95×108 59\n2 3 4\nwater λ≥420 nm\nEthanol\nNi–Ag/CN 0.02% 1.19×109 aqueous 60\nx\nAM 1.5 G\nsolution\n[Comment 6]: The hydrogen production device used for outdoor experiments appears\nto be insulated. The authors should discuss the insulation materials utilized and\ncalculate the overall thermal resistances of the device in each direction.\n5 [Response]: We sincerely thank the reviewer for raising this important consideration.\nIn our outdoor large-scale reactor setup, a thin foam layer (0.5 cm thickness) was\napplied to the periphery of the reactor. However, this foam serves as a protective\ncushioning layer to prevent quartz glass breakage during transportation and handling,\nrather than functioning as a thermal insulation barrier.\nsupport frame. The reactor body is composed of aluminum alloy reaction baseplate,\nquartz glass window, stainless steel spacers, and rubber gaskets. And the reactor frame\nis composed of extruded aluminum alloy profiles. Crucially, the aluminum reactor body\nremains uninsulated and is directly exposed to ambient air. Therefore, thermal\n15 resistance beneath the HHE NFMs is around 8.77×10-5 m2·K/W (attributed to the 2 cm-\nthick aluminum plate) and the thermal resistance of the reactor periphery is around\n0.143 m2·K/W (resulting from the combined 3 cm aluminum casing and 0.5 cm rubber\ninsulation layer).\n20 [Comment 7]: Although the net energy absorbed by the hydrogen production device\nunder outdoor conditions is lower than that under indoor conditions, how can the\n32\nhydrogen production rate increase? Please consider the following factors:\nOutdoor experiments: Solar flux of 786 W/m2, thermal emission from the surface\n(absorptance/emissivity of 0.95), and a higher heat transfer coefficient due to windy\nconditions.\nthermal emission due to the indoor environment, and a lower heat transfer coefficient.\nWhen the energy flux is high and a high surface temperature is achieved, improved\nefficiency can be explained thermodynamically using the Gibbs energy relation.\nHowever, this does not align with the lower energy environment described in outdoor\n10 experiments.\n[Response]: We sincerely thank the reviewers for these insightful comments. While the\nreasons for the enhanced hydrogen production rate were mentioned on the original\nmanuscript, we acknowledge that our explanation may not have been sufficiently clear.\nHere, we provide a more detailed analysis to clarify why the hydrogen production rate\n15 of the air-to-H₂ system under outdoor conditions is 6 times higher than that observed in\nindoor conditions. We hypothesize that this enhancement arises from the combined\nsynergistic thermodynamic and kinetic effects.\n1) Thermodynamic perspective:\nA comprehensive energy analysis of the PTC layer surface reveals that under outdoor\n20 conditions, the upper surface absorbs significantly more solar energy, leading to\nelevated surface temperatures. This thermal effect thermodynamically favors the\nhydrogen evolution reaction. The specific mechanisms are elaborated as follows:\n(1) Light scattering by condensed liquid films on glass reactor surfaces\nThe solar energy absorbed by the surface (𝑃 ) of HHE NFMs under outdoor\n𝑠𝑢𝑛\n25 conditions and indoor conditions can be calculated by:22\n∞\n𝑃 =∫ [𝜀 (𝜆,𝜃 )𝐼 (𝜆)]𝑑𝜆\n𝑠𝑢𝑛 𝑠 𝑠𝑢𝑛 𝐴𝑀1.5\n0\nwhere 𝜀 (𝜆,𝜃 ) represents the absorptance/emittance of the surface of HHE NFMs,\n𝑠 𝑠𝑢𝑛\nwhich is PTC layer, and I (𝜆) represents the AM1.5 solar irradiance. In the indoor\nAM1.5\nhydrogen production tests, the HHE NFMs were encapsulated within a quartz glass\n33\nreactor to maintain a sealed environment and prevent hydrogen leakage. Theoretically,\nthe P calculation should be based on the standard simulated solar radiation of 1000\nsun\nW/m². However, under indoor conditions, the temperature of the reactor's top glass\nwindow and sidewalls remained close to ambient temperature (~25°C), and water vapor\n5 released during evaporation condensed on these surfaces, forming liquid droplets\nbeneath the glass window. Optical characterization reveals that these condensate\ndroplets induce notable light scattering and absorption. Consequently, the actual photon\nflux reaching the PTC layer was reduced. To validate this conclusion, we set the\nsimulated solar irradiance to 1000 W/m², as shown in the Fig. R7A. Fig. R7B shows\n10 that, under illumination, a liquid film rapidly formed and persisted on the underside of\nthe quartz glass cover plate. To evaluate the impact of this liquid film on light intensity,\na quartz plate with an analogous liquid film was positioned above a radiometer at the\nsame height as the reactor's quartz cover plate. The results from Fig. R7C demonstrated\na significant irradiance reduction due to light scattering by the liquid film, decreasing\n15 from 1000 W/m² to 745 W/m². Consequently, the effective solar irradiance absorbed\nby the HHE NFMs surface within the reactor under indoor conditions (P ) was\nsun, indoor\ncalculated as 534.1 W/m². The presence of the liquid film in the enclosed reactor thus\nreduced the solar irradiance absorbed by the PTC layer, leading to a lower surface\ntemperature compared to the open-environment measurement under indoor conditions.\nfilter. B) Photo of H production test under indoor Xe lamp and the condensed water\n2\nvaper under the quartz glass. C) The test photo of solar irradiance when the quartz glass\nis covered by condensed water vapor.\n34\nalthough a thin liquid film initially forms on the quartz glass surface, it rapidly\ndissipates under natural solar irradiation, resulting in negligible light-scattering effects.\nUnder normal outdoor midday sunlight (1000 W/m2), the power received by the\nPTC layer surface is about 713 W/m2, which is larger than indoor condition of\n5 534.1 W/m2. Even under an average outdoor solar irradiance of 786 W/m², the effective\nirradiance absorbed by the PTC layer (P ) reaches 569.1 W/m². This\nsun, outdoor\ndemonstrates that P (569.1 W/m²) > P (534.1 W/m²). The superior\nsun, outdoor sun, indoor\nsolar energy absorption of HHE NFMs under outdoor conditions is therefore confirmed.\nBesides, most researchers intuitively assume that higher wind speeds in outdoor\n10 environments enhance convective heat transfer. However, in our outdoor air-to-H₂\nsystem, the reactor body is constructed using aluminum (Al). This Al reactor undergoes\nslight heating under sunlight, resulting a temperature rise to slightly above ambient\ntemperature. Therefore, it can be inferred that the solar radiation absorbed by the reactor\nbody is offset by external natural convective cooling, with negligible impact on the\n15 surface temperature of PTC layer.\nConsequently, in the outdoor air-to-H₂ system, the HHE NFMs exhibit both higher solar\nenergy absorption and higher surface temperature of PTC layer (revised manuscript Fig.\n4d). As demonstrated by our DFT calculations (revised manuscript Fig. 3h), the\nincreased reaction temperature within the biphasic system lowers the energy barrier for\nH formation, thereby accelerating the hydrogen evolution rate. Thus, from a\n2\n20 thermodynamic perspective, the enhanced surface temperature of HHE NFMs under\noutdoor conditions contributes significantly to the improvement in hydrogen production\nrates compare to the indoor conditions.\n2) Kinetic perspective\noriginates from scaling effects. In indoor condition, a 4×10-4 m2 HHE NFMs was\nenclosed within a glass reactor, whereas the outdoor system employed a 0.01 m2 HHE\nNFMs array integrated into a 0.25 m2 reactor. COMSOL simulations of the\nphotothermal water release process for HHE NFMs of varying sizes revealed that\n35\nmoisture in the hygroscopic layer escapes not only from the top PTC layer but also from\nthe sides of the hygroscopic layer (revised manuscript Fig. 4e). However, as the HHE\nNFMs size increases, a greater proportion of vapor escapes through the PTC layer\n(revised manuscript Fig. 4f). This indicates that larger HHE NFMs dimensions enhance\n5 reactant (i.e. water molecules) availability by increasing vapor contact with catalysts on\nthe PTC layer, thereby elevating vapor concentration in the PTC layer and kinetically\nenhancing hydrogen production rates.\nIn summary, under natural outdoor sunlight irradiation, both thermodynamic and\nkinetic factors synergistically promote H production. Consequently, the large-scale\n2\n10 outdoor air-to-H₂ system achieves significantly higher H production rates. Additionally,\n2\nwe have refined the descriptions of H production rate enhancements in the revised\n2\nmanuscript and revised supplementary materials (revised manuscript page 9, line 16 to\npage 10, line 7, and supplementary materials Note S1) to better elucidate why the large-\nscale outdoor air-to-H system exhibits superior performance.\n2\n15\n[Comment 8]: One potential reason for higher efficiency under outdoor conditions\ncould relate to the wavelength distribution and intensity of the solar simulator used in\nthe indoor tests. Please provide specifications for the solar simulator, including its\nwavelength range and intensity distribution.\n20 [Response]: We sincerely thank the reviewer for the kind comments and suggestion.\nThe Xenon lamp system (Newport Model 67005 with Power Supply 69911) equipped\nwith an AM1.5G filter was employed. Fig.R8 presents the spectral irradiance of Xenon\nlamp we used and our measured spectral distribution when using the AM1.5G filter.\nNotably, comparative analysis reveals that, in the ultraviolet and visible range (625-800\n25 nm), the filtered xenon lamp exhibits higher irradiance than natural sunlight. However,\nin 450-625 nm of visible light, filtered xenon lamp exhibits lower irradiance. While\nthese spectral mismatches were shown in the figure, these differences alone cannot fully\nexplain the increase in the performance observed under outdoor conditions.\nOZONE FREE is what we used). B) The spectral irradiance of natural sunlight and\n300W Xe light with AM1.5g filter.\n[General comment]: The author discusses a method that enhances atmospheric water\nharvesting using radiative cooling and achieves hydrogen production via photothermal\nand photocatalytic water splitting in the vapor phase. I have three main concerns\n5 regarding this paper:\n[Response]: We sincerely appreciate the reviewer for the time and valuable feedback\non our work. We explained our experimental process and results in detail, and made\ncareful revisions based on the reviewer's comments, as specified below. We hope our\nresponse will sufficiently address the concerns.\n10\n[Comment 1]: Regarding Radiative Cooling\nThe author states that the PTC layer has a high emissivity, enabling radiative cooling at\nnight. Figure 2g shows that the surface temperature of the HHE NMFs is approximately\n6°C lower than the ambient air temperature. However, the author does not explicitly\n15 mention that this result was obtained with the HHE NMFs directly exposed to air—this\ndetail can only be found in Supplementary Figure S10. In subsequent system\nexperiments, the PTC layer and the water-absorbing layer were placed under a\ntransparent window, as can be seen in the video materials. Under these conditions, the\nradiative cooling effect should be significantly weaker than what is presented in Figure\n20 2g. Similarly, I suspect that the infrared temperature measurements shown in Figure 4d\nwere also taken without the transparent window, although the author does not clarify\nthis. These results, therefore, do not reflect the conditions with a transparent window.\nGiven these concerns, the data presented in the earlier sections of the paper do not fully\nsupport the analysis and discussion of the system operation results in the later sections.\n25 [Response]: We sincerely thank the reviewer for the careful reading and constructive\ncomments, and we apologize for any misunderstandings caused by insufficient clarity\nin the original manuscript.\nWhile we mentioned the radiative cooling-enhanced atmospheric water harvesting\nprocess during nighttime in the text, we did not explicitly emphasize the experimental\n38\nprotocol. The requirement for exposing the HHE NFMs to the night sky for radiative\ncooling was detailed in the Methods section under ‘Outdoor radiative cooling-enhanced\ntemperature and moisture adsorption measurement’ (page 14, lines 3–10). In these\nexperiments, the HHE NFMs were covered with a polyethylene (PE) film and equipped\n5 with K-type thermocouples to record temperature changes, as shown in revised\nsupplementary materials Fig. S11. The PE film was selected for its high infrared\ntransmittance (ensuring unimpeded radiative cooling at night) and its ability to\nminimize wind-induced convective heat transfer, which is a standard practice in\nradiative cooling studies.1 Experimental results demonstrated that the high emissivity\n10 of the PTC layer reduced the surface temperature by an average of 6°C compared to\nsamples without the PTC layer under identical conditions.\nWe further clarify that subsequent experiments investigating radiative cooling enhanced\natmospheric water harvesting are detailed in the Methods section (Page 14, lines 16–\n20). In these experiments, two identical HHE NFMs were placed on electronic balances\n15 to monitor nighttime mass changes. For the radiative cooling-enhanced group (left\npanel of Fig. S11), the electronic balance surface was covered with a PE film to\nminimize wind interference. In contrast, the control group without radiative cooling\n(right panel of Fig. S11) was enclosed within an acrylic chamber wrapped with\naluminum foil to block thermal radiation to outer space. Both of them were left enough\n20 gaps for air to get in. Results demonstrated that the radiative cooling-enhanced HHE\nNFMs exhibited a 10% enhancement in total water uptake compared to the non-\nradiative cooling control group.\nIn outdoor experiments, the HHE NFMs were indeed enclosed within the large-scale\nair-to-H reactor. This design choice was necessitated by the requirement to maintain a\n2\n25 sealed environment during daytime H\n2\nproduction tests to prevent H\n2\nleakage.\nRegarding the reviewer’s concern about the quartz glass impacting radiative cooling\nperformance, we clarify that the quartz glass cover plate was intentionally opened\nduring nighttime operations to allow direct exposure of the HHE NFMs to ambient\nair for moisture adsorption. Under these conditions, the quartz glass does not interfere\n39\nwith radiative cooling. We apologize for not explicitly detailing this operational\nprotocol in the original manuscript. To address this omission, we have emphasized the\nnighttime radiative cooling exposure process in the revised manuscript, specifically\nstating that the HHE NFMs are exposed to the night sky to enable radiative cooling.\nsystem have been added to the ‘Methods: Outdoor hydrogen production measurement\nof large-scale HHE system’ subsection to provide the reviewer with full experimental\ntransparency (page 17, line 4-6).\nFurthermore, regarding the reviewer's query about the temperature recordings of the\nmethods in the revised manuscript (page 16, line 22-28) for the reviewers' reference.\nActually, when the HHE NFMs were placed within the reactor and isolated with a quartz\nglass, we were unable to capture their surface temperature using an infrared camera.\nTherefore, during the temperature measurement process, we positioned the HHE NFMs\n15 in an open environment (without quartz glass covering) inside a plastic box. Due to the\nenclosure by the box's walls, the setup was essentially under conditions without external\nairflow. In this setup, our HHE NFMs film could radiate heat to the space. In contrast,\nwhen the HHE NFMs were placed inside a sealed reactor, the quartz glass would hinder\nheat radiation towards space due to its low transmittance in infrared spectrum.\nunderestimated the surface temperature of the PTC layer inside the sealed reactor. To\naddress this disparity, we conducted an analysis of the radiative cooling process on the\nsurface of the PTC layer under these two distinct conditions.\n(1) Daytime radiative cooling power of PTC layer under outdoor open area condition\n25 (without quartz glass covering)\nDue to the high infrared emissivity of PTC layer, it can spontaneously radiate heat\nthrough the atmospheric window. Under the condition of zero humidity, no clouds and\nclean air atmosphere (no aerosol, etc.), the theoretical value of surface radiation\n(𝑃 (𝑇 )) to the sky should be calculated according to the following equation:22\n𝑟𝑎𝑑,𝑖𝑑𝑒𝑎𝑙 𝑠\n40\n∞\n𝑃 (𝑇 )=∫ ∫ [𝜀 (𝜆,𝜃)𝐼 (𝜆,𝑇 )]𝑑𝜆𝑐𝑜𝑠𝜃𝑑Ω\n𝑟𝑎𝑑,𝑖𝑑𝑒𝑎𝑙 𝑠 𝑠 𝐵𝐵 𝑠\n∩ 0\n𝜋/2 2𝜋\nwhere ∫ 𝑑Ω = ∫ 𝑠𝑖𝑛𝜃𝑑𝜃∫ 𝑑𝜙 is the hemisphere angular integral, and 𝜀 (𝜆,𝜃)\n∩ 0 0 𝑠\n2ℎ𝑐2 1\nis the spectral and angular emissivity of the PTC layer. 𝐼 (𝜆,𝑇 ) =\n𝐵𝐵 𝑠 𝜆5 ℎ𝑐\n−1\n𝑒𝜆𝑘𝐵𝑇𝑠\nindicates the spectral radiance of a blackbody, where h is Planck’s constant, c is the\n5 speed of light in vacuum, and k is the Boltzmann constant. T and T is the\nB s amb\ntemperature of PTC layer surface exposed to the sky and ambient temperature, which\nis 76℃ (here we take the average temperature in the open area rather than the maximum\ntemperature because the average temperature is more representative of the surface\ntemperature of PTC layer) and 35℃, respectively.\nis:\n𝑃 (𝑇 ) = 499.6 W/m2.\n𝑟𝑎𝑑,𝑖𝑑𝑒𝑎𝑙 𝑠\nAt the same time, the PTC layer also receives radiation from the environment, and its\ncalculation method is:\n∞\n15 𝑃 (𝑇 )=∫ 𝜀 (𝜆,𝜃)∫ [𝜀 (𝜆,𝜃)𝐼 (𝜆,𝑇 )]𝑑𝜆𝑐𝑜𝑠𝜃𝑑Ω\n𝑎𝑡𝑚 𝑎𝑚𝑏 𝑠 𝑎𝑡𝑚 𝐵𝐵 𝑎𝑏𝑚\n∩ 0\nwhere 𝜀 (𝜆,𝜃) = 1−𝑡(𝜆)1/𝑐𝑜𝑠𝜃， where 𝜀 (𝜆,𝜃) = 1−𝑡(𝜆)1/𝑐𝑜𝑠𝜃 , is the\n𝑎𝑡𝑚 𝑎𝑡𝑚\nemissivity of atmosphere, 𝑡(𝜆) is the transmittance of the atmosphere. Therefore, the\ntheoretical radiation received from the environment is 𝑃 (𝑇 )=136 W/m2.\n𝑎𝑡𝑚 𝑎𝑚𝑏\nTherefore, the heat power taken away by radiation is:\n20 𝑃 = 𝑃 (𝑇 )−𝑃 (𝑇 ) = 363.6 W/m2.\n𝑟𝑎𝑑,𝑤/𝑜 𝑤𝑖𝑛𝑑𝑜𝑤 𝑟𝑎𝑑,𝑖𝑑𝑒𝑎𝑙 𝑠 𝑎𝑡𝑚 𝑎𝑚𝑏\n(2) Radiative cooling power of PTC layer in large outdoor reactor (covered by quartz\nglass)\nDue to the quartz glass blocking radiative heat dissipation to outer space, the PTC layer\ncan only radiate heat to the surrounding environment within the enclosed reactor. Under\n25 these conditions,\n𝑃 = 𝜀 ×𝜎×(𝑇4−𝑇4 ) = 314.4 W/m2.\n𝑟𝑎𝑑,𝑤 𝑤𝑖𝑛𝑑𝑜𝑤 𝑠 𝑎𝑚𝑏\n𝑟𝑎𝑑,𝑤/𝑜 𝑤𝑖𝑛𝑑𝑜𝑤 𝑟𝑎𝑑,𝑤 𝑤𝑖𝑛𝑑𝑜𝑤\ntemperature of the HHE NFMs inside the enclosed reactor compared to those operating\nin open environments without quartz glass covering. However, the temperature\ndifference between the two configurations is minimal, leading to negligible practical\n5 impact on the overall thermal behavior.\nIn our subsequent analysis, the primary focus was on demonstrating that temperature\nelevation enhances H production rates. A slight underestimation of the PTC layer’s\n2\nsurface temperature under enclosed conditions does not affect the validity of this\nconclusion, as the relative temperature trends remain consistent. Consequently, the use\n10 of open-environment test results (without glass covering) for system performance\nanalysis does not compromise the conclusion of our study.\nWe appreciate the reviewer’s diligence in ensuring methodological rigor and have\nincorporated the relevant changes into the revised manuscript.\n15 [Comment 2]: Regarding Hydrogen Production Rate under Outdoor Natural\nConditions.\nThe author exaggerates the superiority of the outdoor experimental results over those\nobtained under indoor illumination in both the abstract and main text. This comparison\nis unnecessary because the two conditions differ significantly in terms of operating\n20 temperatures and geometric characteristics, making them fundamentally incomparable.\n[Response]: We sincerely appreciate the reviewer’s meticulous review and constructive\nfeedback. Through careful consideration of the comments, we strongly believe it is\nessential to enhance the original manuscript based on the reviewer's insights,\nparticularly emphasizing the different environments of indoor and outdoor experiments,\nas well as the impact of varying sizes of HHE NFMs on the entire H production process\n2\nto avoid reader confusion. Besides, we have deleted the comparison of H production\n2\nrate under indoor and outdoor conditions in the abstract (revised manuscript page 1,\nline 29-30). In addition, we have emphasized the reasons for the increase in H\n2\nproduction rate under outdoor conditions and the comparison with indoor environments\n42\nin the revised manuscript.\nAdditionally, we intend to provide further explanations to elucidate the significance and\ncriteria behind contrasting outdoor experiments with indoor experiments.\nFirst of all, the comparison between indoor and outdoor experiments holds profound\n5 engineering implications. Historically, photocatalysis research has predominantly\nfocused on catalyst modification52,61, with limited attention to reactor design and\nsystem-level integration8,62,63. However, transitioning laboratory-scale photocatalysis to\nreal-world applications necessitates rigorous outdoor validation under natural sunlight.\nSuch comparisons reveal the catalyst’s true performance beyond the constrained\n10 wavelength ranges of simulated solar light, thereby guiding catalyst development\ntoward practical outdoor compatibility.62 This principle extends beyond solar H\n2\nproduction. Systems for solar energy conversion or atmospheric water harvesting\nsimilarly require outdoor testing to bridge the gap between laboratory efficiency metrics\nand field performance, ultimately informing scalable engineering solutions.\n2\nsystem exhibits superior -H\n2\nproduction rates\nunder outdoor conditions compared to controlled indoor environments represents a\nsignificant departure from prior studies. To our knowledge, existing research on\nphotocatalytic systems consistently reports performance degradation when\ntransitioning from indoor to outdoor conditions8,48,57,63. Besides, in other large-scale\n20 photocatalytic reactions, increasing the size typically results in a decrease in reaction\nrates when conducted under outdoor natural light.8,48,57 To elucidate the underlying\nmechanisms driving the observed performance enhancement in our air-to-H system,\n2\nwe conducted a rigorous comparative analysis and mechanistic study. This investigation\naims to advance fundamental understanding of gas-phase photothermal catalytic\n25 systems and propose new performance enhancement pathways beyond traditional\ncatalyst modification strategies. Based on our analyses, the dimension and environment\nof air-to-H system are crucial. Conventional photocatalysis studies primarily\n2\ninvestigate liquid-phase water splitting using particulate catalysts, where reactor scale\nhas minimal impact on performance. In contrast, our work focuses on gas-phase\n43\nphotothermal catalytic hydrogen production via HHE NFMs, which adsorb atmospheric\nmoisture at night and generate steam for daytime photocatalytic splitting. Here, both\nthe dimensions of HHE NFMs and their operational environment critically influence\nhydrogen evolution:\ntemperature rise on HHE NFMs thermodynamically accelerates H production.\nelevating reactant concentrations at catalytic interfaces.\nThese findings highlight the importance of optimizing dimensions and environmental\n10 adaptability in practical applications, offering key insights for scaling air-to-H systems.\nof area-normalize hydrogen production rate (μmol·m-2·h-1), enabling direct comparison\nwith other photocatalytic H production systems. A comprehensive benchmarking\n2\nanalysis against other systems has been added to the supplementary materials (Table\nWe have to emphasize that we highly respect the patient reading and well intentioned\nreminders of the reviewers. Indeed, we should not only focus on the differences in\nindoor and outdoor performance, but neglect to emphasize the reasons behind this\nprocess. Therefore, we have deleted the strong contrast in the abstract. And in the\n20 description of the revised manuscript, clearer explanations have been added to clarify\nthe reasons for the differences in indoor and outdoor performance, so that readers can\nbetter understand the focus of our work (page 9, line 16 to page 10, line 7). We also put\nthem here for your reference.\n‘Due to environmental variability and dimensional scaling of the HHE NFMs, the\n25 distinct H production performance observed between indoor and outdoor conditions\n2\n(contrary to trends reported in prior studies) demands a rigorous investigation into the\nunderlying mechanisms.\nThe enhancement of H production performance under outdoor larger scale system can\n2\nbe attributed to synergistic thermodynamic and kinetic effects. From the\n44\nthermodynamic perspective, in the outdoor air-to-H₂ system, the HHE NFMs exhibit\nboth higher energy input and higher surface temperature of PTC layer (Note S1 and\nNote S2). As shown in Fig. 4b, the temperature of the PTC layer under outdoor open\narea conditions can rise from 45.9°C to 71.8°C within 30 seconds (Fig. S29), reaching\n5 a maximum surface temperature of 82.2°C (average ~76 °C) (Fig. 4d), which is about\n14°C higher than that of indoor conditions. As demonstrated by DFT calculations (Fig.\n3h), the increased reaction temperature within the biphasic system lowers the energy\nbarrier for H formation, thereby accelerating the H evolution rate. Thus, the enhanced\n2 2\nsurface temperature under outdoor conditions thermodynamically boosts hydrogen\n10 production rates compared to indoor systems. From the kinetic perspective, the\nenhancement of H production rates in HHE NFMs originates from scaling effects. In\n2\nthe indoor condition, a 0.0004 m2 HHE NFMs was enclosed within a glass reactor,\nwhereas the outdoor system employed a 0.01 m2 HHE NFMs array integrated into a\n0.25 m2 reactor. COMSOL simulations of the photothermal water desorption process\n15 for HHE NFMs of varying sizes revealed that moisture in the hygroscopic layer escapes\nnot only from the top PTC layer but also from the sides of the hygroscopic layer (Fig.\n4e and S31). However, as the HHE NFMs size increases, a greater proportion of vapor\nescapes through the PTC layer (Fig. 4f). This indicates that larger HHE NFMs\ndimensions enhance reactant availability by increasing vapor contact with catalysts on\n20 the PTC layer, thereby elevating vapor concentration in the PTC layer and kinetically\nenhancing hydrogen production rates.’\n[Comment 3]: The author’s estimation of hydrogen production based on 0.1% of the\nglobal land area lacks credible data support and has no scientific value.\n25 [Response]: We sincerely thank the reviewer for this highly valuable suggestion and\ntheir meticulous review. We apologize for omitting the detailed estimation methodology\nfor the global-scale application potential of the proposed air-to-H system in the original\n2\nmanuscript. To address this, we have added a comprehensive description of the\ncalculation process to the supplementary materials Note S3. For the reviewer’s\n45\nconvenience, we summarize the estimation process below:\nFirstly, based on the light intensity and hydrogen production data collected from long-\nterm outdoor testing of the air-to-H system, we calculated the average H production\n2 2\nrate𝑅 (mL∙m-2∙h-1) under different average solar radiation (P ). And based on these\n𝐻2 light\n5 data, as shown in Fig. R9A, a relationship curve between average solar radiation and\naverage hydrogen production rate was fitted, and the fitting formula is:\n𝑃 0.53\n𝑙𝑖𝑔ℎ𝑡\n𝑅 = 3.73( )\n𝐻2 12\nSecondly, based on the data obtained from the radiative cooling enhanced atmospheric\nwater harvesting, we also established the relationship between RH and the water\n10 adsorption saturation (α) of HHE NFMs, as shown in Fig. R9B, and fitted a curve with\nthe following formula:\n1.00189\nα = − +0.99965\n1+(𝑅𝐻/0.33612)6.25821\nTherefore, based on the above two fitting curves, the annual average hydrogen\nproduction per square meter of land (H /(L·m-2·year-1)) can be obtained:\n2,year\n𝑡𝑅\n15 𝐻 = α\n𝐻2\n∙day\n2,𝑦𝑒𝑎𝑟 1000\nHere, the direct solar irradiation all over the world can be obtained from Global Solar\nAtlas, as shown in Fig. R10 (revised supplementary materials Fig. S44). And Global\nnighttime average humidity distribution can be obtained from Asia-Pacific Data-\nresearch Center, Reanalysis Data, NCEP Reanalysis, as shown in Fig. R11 (revised\n20 supplementary materials Fig. S45).\nFinally, the calculated annual average H production data will be visualized on a map\n2\nfor reference, and further calculations will be made to determine the amount of H\n2\navailable for the air-to-H system from 0.1% of the land. Therefore, our calculations\n2\nhave reliable and sufficient information sources.\n2\ndeployment not only highlight its scalability but also provide intuitive feeling of the\npotential of H production potential by using atmospheric moisture, which is a practical\n2\nprospect for the future use of the system. At the same time, it can guide us to select a\n46\nreasonable location and let us clearly understand where to build the system to obtain\nbetter benefits. 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Nature Reviews Methods Primers 3, 42 (2023). https://doi.org:10.1038/s43586-023-\n00226-x\n1331-1347 (2022). https://doi.org:10.1007/978-3-030-63713-2_43\n40 63 Gao, M., Peh, C. K., Zhu, L., Yilmaz, G. & Ho, G. W. Photothermal Catalytic Gel Featuring\nSpectral and Thermal Management for Parallel Freshwater and Hydrogen Production. Advanced\nEnergy Materials 10, 2000925 (2020). https://doi.org:10.1002/aenm.202000925\n52"}, {"round": 2, "reviewer_comments": {"reviewer_1": "As my comments have been successfully incorporated, the manuscript is recommended for publication.", "reviewer_2": "The authors answered all the comments correctly, so I recommend this manuscript for publication in Nature\nCommunications.", "reviewer_3": "Although the author has addressed my questions, the responses are clearly not very persuasive. I still have reservations\nabout this paper.\n1. Although the author added a description that the quartz glass cover plate was intentionally removed during nighttime\noperations to allow direct exposure of the HHE NFMs to ambient air for moisture adsorption, I would like to inquire how the\nfrequent removal and reinstallation of the quartz glass cover in this integrated hydrogen production and water collection\nsystem maintains its seal. The revised version in Fig. 4b shows the hydrogen production rate under typical weather\nconditions during the day, while Fig. 5d only presents hydrogen production results. Fig. 2 is a standalone analysis of\nradiation cooling water collection, but I cannot find any experimental results in the entire paper that demonstrate the\ncontinuous operation of the large-scale air-to-hydrogen system (as shown in Fig. 5a) from daytime hydrogen production to\nnighttime water collection. Therefore, I believe that the author's statement about the removal of the quartz glass cover plate\nat night lacks credibility.\n2. Regarding the explanation of the HHE NFM surface temperature records in Fig. 4g, the author conducted a theoretical\nanalysis of the radiation cooling power with and without the quartz glass in order to account for the temperature differences\nof the PTC surface. In the response on page 42, the first five lines confirm that there is indeed a difference between the two\nconfigurations. However, the next sentence immediately concludes that the temperature difference between the two is\nminimal, and its overall impact on the thermal behavior is negligible. This conclusion seems overly forced and illogical.\nAdditionally, the revised version of the figure does not contain Fig. 4g, which further heightens the reviewer’s doubts."}, "author_response": "NCOMMS-25-00566-A\nPoint-to point response to reviewers’ comments on the manuscript\n‘Electricity-Free Hydrogen Production from the Air’\nReviewer #1 (Remarks to the Author):\nAs my comments have been successfully incorporated, the manuscript is recommended\nfor publication.\n[Response]: We sincerely thank the reviewer for the positive feedback. We are glad\nthat our revisions have addressed the reviewer’s concerns, and we are grateful for\nhis/her endorsement to publish in Nature Communications.\nReviewer #2 (Remarks to the Author):\nThe authors answered all the comments correctly, so I recommend this manuscript for\npublication in Nature Communications.\n[Response]: We thank the reviewer for the positive feedback. We are glad that our\nrevisions have correctly answered the reviewer’s comments, and we are grateful for\nhis/her endorsement to publish in Nature Communications.\nReviewer #3 (Remarks to the Author):\nAlthough the author has addressed my questions, the responses are clearly not very\npersuasive. I still have reservations about this paper.\n[Response]: We sincerely appreciate the reviewer for the time and valuable feedback\non our work. We explained our experimental process and results in detail, and made\ncareful revisions based on the reviewer's comments, as specified below. We hope our\nresponse will sufficiently address the concerns.\nComment 1: Although the author added a description that the quartz glass cover plate\nwas intentionally removed during nighttime operations to allow direct exposure of the\nHHE NFMs to ambient air for moisture adsorption, I would like to inquire how the\nfrequent removal and reinstallation of the quartz glass cover in this integrated hydrogen\nproduction and water collection system maintains its seal.\nThe revised version in Fig. 4b shows the hydrogen production rate under typical\nweather conditions during the day, while Fig. 5d only presents hydrogen production\nresults. Fig. 2 is a standalone analysis of radiation cooling water collection, but I cannot\nfind any experimental results in the entire paper that demonstrate the continuous\noperation of the large-scale air-to-hydrogen system (as shown in Fig. 5a) from daytime\nhydrogen production to nighttime water collection. Therefore, I believe that the author's\nstatement about the removal of the quartz glass cover plate at night lacks credibility.\n[Response]: We sincerely thank the reviewer for the careful reading and constructive\ncomments. To clarify concerns regarding potential gas leakage during repeated reactor\ndisassembly and reassembly, we provide a detailed description of the large-scale\nreactor's structural design and assembly process. The analysis results show that our\nmodular air-to-hydrogen reactor design enables frequent disassembly and\nreassembly to achieve continuous cyclic operation between nighttime radiative\ncooling enhanced atmospheric water harvesting and daytime photothermal vapor\nsplitting for H production, while maintaining excellent sealing integrity\n2\nthroughout the process.\nFirst of all, as shown in Fig. R1A, the large-scale air-to-H system features a modular\n2\ndesign with a reusable rubber ring system and quick-install clamp mechanism to\nensure airtightness. The reactor consists of the following components from bottom to\ntop: reactor base, rubber ring, transparent window, gasket, and quick-install clamp\nassembly. During installation, the rubber ring (with diameter approximately twice the\ngroove depth) is first secured in the precisely machined groove of the reactor base. The\nquartz glass is then positioned above the gasket, followed by placement of the gasket\nalong the glass perimeter. Finally, the clamp assembly is installed and fastened using\nthreaded screws. The screw tightening process serves dual purposes: securing the clamp\nassembly while simultaneously applying compressive force to the gasket and quartz\nglass. As shown in Fig. R1B, the rubber ring undergoes significant compression,\nforming a tight seal against the quartz glass to achieve sealing. The entire assembly\nprocess requires approximately 3 minutes (excluding screw tightening time), as\ndemonstrated in the supplementary installation video R1. Furthermore, rigorous gas\nmonitoring protocols were implemented: initial gas composition measurements were\ntaken before each experiment, followed by bihourly sampling to detect potential leaks\nthrough nitrogen (N ) level monitoring by using gas chromatography (GC). Throughout\n2\nextensive outdoor testing of the large-scale system, no gas leakage incidents were\nobserved.\nBesides, the nighttime atmospheric water harvesting process with the glass removed is\nillustrated in Fig. R1C. Under this condition, the reactor base is mounted on the support\nframe and covered with a polyethylene (PE) film, which effectively prevents\ninterference from wind and potential precipitation while maintaining optimal radiative\ncooling performance. The reactor design incorporates quick-connect ports on the side\npanels that remain open during nighttime operation to facilitate sufficient airflow for\nmoisture adsorption. These ports are connected to valved tubing during daytime\noperation to ensure reactor sealing (Fig. R1D).\nFinally, in addition to presenting the nighttime radiative cooling-enhanced atmospheric\nwater harvesting and daytime hydrogen production results in the revised manuscript,\nwe have conducted comprehensive 24-hour continuous water\nadsorption/desorption and H production tests, as shown in the Fig. R1E (also\n2\nincluded in Fig. S34 in the revised supplementary materials). Due to technical\nlimitations in directly monitoring real-time mass changes of HHE NFMs within the\nlarge-scale reactor during operation, we implemented an alternative experimental\napproach. During nighttime water harvesting with the reactor cover open, we\nsimultaneously conducted separate tests on individual HHE NFMs samples to\ncharacterize the complete daily water adsorption/desorption cycle, while the remaining\nHHE NFMs in the reactor were used for subsequent daytime H production experiments.\nalso added to the revised manuscript page 17, line 4-15.\nFig. R1 A) 3D model of large-scale air-to-H system. B) The rubber ring before and\n2\nafter tightening. C) Radiative cooling enhanced atmospheric water harvesting during\nthe night. D) The reactor connected to pipes with valves. E) 24h-continuous moisture\nadsorption and desorption test and hydrogen production test.\nComment 2: Regarding the explanation of the HHE NFM surface temperature records\nin Fig. 4g, the author conducted a theoretical analysis of the radiation cooling power\nwith and without the quartz glass in order to account for the temperature differences\nof the PTC surface. In the response on page 42, the first five lines confirm that there is\nindeed a difference between the two configurations. However, the next sentence\nimmediately concludes that the temperature difference between the two is minimal, and\nits overall impact on the thermal behavior is negligible. This conclusion seems overly\nforced and illogical. Additionally, the revised version of the figure does not contain Fig.\n4g, which further heightens the reviewer’s doubts.\n[Response]: We sincerely appreciate the reviewer’s thoughtful comments. The Fig. 4g\nin the original response should be Fig.4d. In our previous analysis, we calculated the\nradiative heat transfer at the HHE NFMs surface under outdoor clear-sky conditions,\ncomparing scenarios with and without the quartz glass cover plate. Due to the inability\nto measure the internal HHE NFMs surface temperature without compromising reactor\nairtightness, we assumed identical surface temperatures between the covered and\nuncovered configurations for radiative heat transfer calculations. The calculations\nyielded radiative heat transfer values of 363.6 W/m² (without glass cover) and 314.4\nW/m² (with glass cover), showing a relatively small difference of 49.2 W/m².\nHowever, when considering other heat dissipation mechanisms, particularly natural\nconvection:\n𝑃 = ℎ (𝑇 −𝑇 ) = 328 W/m2,\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒 𝑠 𝑎𝑚𝑏\nwhere ℎ represents non-radiative heat transfer coefficient. In this setup,\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒\nthermal insulation materials were applied to the sidewalls, and a windbreak layer was\nimplemented to minimize additional heat loss caused by wind. Consequently, heat\ntransfer in this configuration is dominated by natural convection. Therefore, calculated\nnon-radiative heat transfer coefficient is ℎ = 8.0 𝑊/(𝑚2 ∙𝐾). Under this\n𝑛𝑜𝑛−𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑣𝑒\ncondition, the radiative heat transfer difference represents only 7% of the total\nheat dissipation. This relatively small contribution supports our initial assessment that\nthe effect of quartz glass on the overall thermal performance of HHE NFMs under\ndaytime sunlight conditions is relatively small and therefore proposed to be negligible.\nFurthermore, even if we take this effect into account, the quartz glass slightly reduces\nradiative cooling effect during the day, which theoretically leading to marginally higher\nsurface temperatures in enclosed configurations. However, this observation actually\nstrengthens our primary finding, as the reactor-enclosed HHE NFMs would still\nmaintain temperatures slightly above both the measured 76 °C in open outdoor\nconditions, and more certainly above the 58-68 °C range observed in indoor\nsimulated sunlight tests. Consequently, our fundamental conclusion that elevated\nsurface temperatures under outdoor conditions enhance H production rates remains\n2\nfully supported."}, {"round": 3, "reviewer_comments": {"reviewer_4": "Any science can only benefit the world if it is transformed into engineering. I think engineering-related works should also be\neligible for publication in Nature Communicationse and I will no longer able to review this manuscript."}, "author_response": "NCOMMS-25-00566B-Z\nPoint-to point response to reviewers’ comments on the manuscript\n‘Electricity-Free Hydrogen Production from the Air’\nGeneral response:\nWe wish to sincerely thank the two additional reviewers (Reviewer #4 and Reviewer\n#5) for their comments. Both reviewers explicitly mentioned that they agreed that we\nhave satisfactorily addressed the comments from Reviewer #3 during the previous\nrevision. Both reviewers provide additional new comments. First, we have carefully\nstudied all these comments. We have made a major revision of our initial manuscript\nby accounting for the comments of the two Reviewers as much as possible and in a\nconstructive way. We sincerely apologize for any repetition in our responses, as some\nof the reviewers’ comments addressed overlapping aspects of the work. Please rest\nassured that each response has been carefully tailored to address the specific concerns\nraised. We greatly appreciate your kind understanding and patience in reviewing this\nmaterial. We believe that this revision leads to a substantial improvement of our initial\nmanuscript.\nBelow, we explain how all the comments of the two additional reviewers (Reviewer#4\nand Reviewer #5) have been taken into account in our revision. In this response letter,\nthe reviewers’ comments to our original manuscript are provided in black, and our\npoint-by-point responses as well as the corresponding changes are shown in blue.\n[General comment]:\nI have carefully read the Rev #3's comments and the authors' responses, and I personally\nbelieve that the authors have well addressed the concerns raised by the Rev #3.\nThis is an engineering system work that contains many engineering details. The\ndiscussion between the Rev #3 and the authors on these details is conducive to the\nimprovement of engineering experiments and the enhancement of reproducibility.\nThis work proposes a dual-functional composite nanofiber membrane (HHE NFMs)\nsystem that does not require electrical energy input and relies on air water vapor to\ndirectly produce hydrogen, integrating the hygroscopic and photothermal-\nphotocatalytic synergistic hydrogen production mechanisms driven by radiation\ncooling. The paper is ingeniously conceived and novelly designed, with obvious\noriginality and application potential, especially suitable for green hydrogen energy\nsolutions in arid or water-scarce areas. The authors conducted systematic material\nconstruction, structural characterization, climate adaptation testing, and theoretical\nsimulations, and the research content is rich.\n[Response]: We sincerely thank the reviewer for providing the valuable comments! We\nhave addressed all the comments through further\nexperimentation and have made revisions on the manuscript, as specified below. We\nhope our response will satisfy you.\n[Comment 1]: The experimental conditions for the Pt/TiO2@PVDF nanofiber\nmembranes (NFMs) and MIL-101(Cr)@PAN nanofiber membranes used in the article\nneed further explanation. For example: Why was the Pt loading of 1 wt% chosen? Have\nyou done any experiments to optimize the loading? What is the basis for selecting the\nloading and membrane thickness of MIL-101(Cr)? Is there any optimization\nexperimental data to support this?\n[Response]: We sincerely appreciate the reviewer's thorough review and valuable\ncomments regarding the optimization of Pt loading and loading and membrane\nthickness of MIL-101(Cr).\nprior to experimental design. While Pt deposition on TiO surfaces significantly\n2\nenhances photocatalytic H production, this improvement does not scale infinitely with\n2\nincreasing Pt content1-3. The literature consistently indicates an optimal Pt loading range\nof 0.5-2 wt%4-6. Accordingly, we systematically prepared and tested Pt/TiO₂ catalysts\nwith 0.5, 1, and 2 wt% Pt loadings (denoted as 0.5Pt/TiO₂, 1Pt/TiO₂, and 2Pt/TiO₂) by\nadjusting the amount of H PtCl solution during synthesis.\n2 6\nThe H production performance was evaluated using 20 mg of catalyst dispersed in 20\n2\nmL deionized water within a transparent reactor. The suspension was homogenized by\nultrasonication before illumination with a 300 W xenon lamp equipped with an AM\n1.5G filter, maintaining an irradiance of 1000 W·m-2 at the solution surface. The reactor\nwas purged with argon for 30 minutes, and gas samples were analyzed hourly by gas\nchromatography (GC). As shown in Fig. R1a, the H production rate initially increased\n2\nand then decreased with higher Pt loading, reaching an optimum at 1 wt% Pt. This trend\naligns perfectly with established literature findings5,7, confirming our selection of 1 wt%\nPt as the optimal loading.\nConcerning the MIL-101(Cr) loading in the hygroscopic layer, our design aimed to\nmaximize water uptake capacity while maintaining structural integrity. However,\nexcessive MIL-101(Cr) incorporation led to particle agglomeration (Fig. R1b) and non-\nuniform fiber formation, which would compromise both water uptake kinetics and total\ncapacity. We therefore optimized the loading to achieve maximum MIL-101(Cr)\ncontent without sacrificing distribution homogeneity, as demonstrated in Fig. R1c.\nFurthermore, we optimized the hygroscopic layer thickness by fabricating HHE NFMs\nwith varying thicknesses while maintaining identical photothermal catalytic layers.\nSince H production rates decrease over time, we compared the initial hourly production\n2\nrates across different thicknesses. As shown in Fig. R1d, the H production rate first\n2\nincreased and then decreased with increasing hygroscopic layer thickness, peaking at\napproximately 11 mm. This optimal thickness was subsequently adopted for all further\nexperiments. The above results have been included into the revised supplementary\n3\nmaterials (Fig. S14 and Fig. S22) and revised manuscript page 6, line 11.\nFig. R1 a) Optimization of Pt loading mass. b) SEM image of xcessive MIL-101(Cr)\nincorporation led to particle agglomeration. c) SEM image of optimized the loading\nmass of MIL-101(Cr) content without sacrificing distribution homogeneity. d)\nOptimization of the thickness of HHE NFMs for H evolution (H production rate\n2 2\nduring the first hour).\n[Comment 2]: It is recommended that the authors further improve the description of\nthe experimental conditions, such as the specific definition of different weather\nconditions (sunny, partly cloudy, and overcast) and the frequency of recording\nenvironmental parameters, to facilitate other researchers to repeat the experiment.\n[Response]: We sincerely thank the reviewer for their careful reading and constructive\nsuggestions. We have now provided a more detailed description of the outdoor\nexperimental conditions in the Supporting Information. During outdoor testing, we\nemployed a portable weather station to systematically monitor ambient conditions,\nrecording solar irradiance, temperature, and humidity at one-minute intervals.\nA) Sunny conditions (with average solar intensity is around 786.0 W·m-2): With\nminimal cloud coverage throughout the testing period.\nB) Partially cloudy conditions (with average solar intensity around 716.5 W·m-2):\nFeaturing intermittent cloud passage causing temporary shading of the reactor.\nC) Overcast conditions (with average solar intensity around 397.6 W·m-2): With\npersistent cloud coverage frequently obstructing solar irradiation.\nThe specific definition of different weather conditions has been included in\nsupplementary materials Nots S1.\nBesides, the comprehensive time-resolved data for solar irradiance and temperature\nduring each weather condition have been provided in revised manuscript Fig. 4b.\nAdditionally, we have included comprehensive time-resolved data for solar irradiance,\ntemperature, and humidity during outdoor testing in the revised supplementary\nmaterials (Fig. R2 and revised Fig. S43). These datasets provide valuable reference for\nreaders to understand the environmental parameters affecting system performance\nunder real-world conditions.\nFig. R2 Operational status of the large-scale HHE system over a 14-day period. H\n2\n5\nproduction, solar radiation intensity, H production rate and STH efficiency at\n2\ncorresponding humidity and temperature during the 14-day period.\n[Comment 3]: The data presented in the chart is relatively complete, but it is\nrecommended to add error bars or standard deviations of repeated experiments to the\ngraph to reflect the stability and reliability of the experimental results.\n[Response]: We sincerely appreciate the reviewer's meticulous review. We would like\nto clarify that error bars were already included in the relevant figures of the original\nmanuscript (as indicated in the revised manuscript Fig. 2f, and revised supplementary\nmaterials Fig. S22A and Fig. S29). Furthermore, to comprehensively demonstrate the\nreproducibility and stability of our HHE NFMs, we have conducted additional repeated\ntests and incorporated updated error bars in revised manuscript Fig. 4c and revised\nsupplementary materials Fig. S22B, Fig. S32.\nBesides, Fig. R3 also shows the new added figures in the manuscript. These\nenhancements provide stronger statistical validation of our experimental results.\nFig. R3 a) Comparison of average H production rates between HHE NFMs of different\n2\nsizes and varying reaction conditions. b) Optimization of the thickness of HHE NFMs\nfor H evolution (H production rate during the first hour). c) H production rate over\n2 2 2\n6h of Au/TiO and Pt/TiO in triphasic and biphasic system.\n2 2\n[Comment 4]: The author mentioned that the humidity adsorption capacity of MIL-\n101(Cr) varies greatly at different humidity levels (for example, it decreases\nsignificantly at 50% humidity). Can you further provide adsorption data at lower\nhumidity conditions (such as 20%, 30%, 40%) to clarify the applicable humidity lower\nlimit of this material?\n[Response]: We sincerely thank the reviewer for their insightful comments. In revised\nmanuscript Fig. 2j (also provided as Fig. R4a for reference), we have presented the\nmoisture adsorption performance of the HHE NFMs. The dark-colored bars in the\nfigure represent the water uptake capacity under conventional conditions (without\nradiative cooling enhancement). As correctly observed by the reviewer, the HHE NFMs\nexhibit limited performance at low humidity levels without radiative cooling\nenhancement: the adsorption capacity is 0.179 g-1 ·g-1 at 50% RH, merely 0.05\nwater layer\ng-1 ·g-1 at 40% RH, and virtually negligible at 30% RH.\nwater layer\nThis limitation originates from the unique water adsorption mechanism of MIL-101(Cr).\nAlthough featuring exceptionally large pore sizes (~2.9 nm and ~3.4 nm) and\noutstanding moisture capture capacity at high humidity levels, its adsorption\nmechanism relies not on simple monolayer-multilayer adsorption but on cooperative\nadsorption. Individual water molecules exhibit weak interactions with the pore walls.\nThe process requires initial water molecule adsorption at hydrophilic Cr sites to form\nnucleation centers8,9. These nucleated water molecules then facilitate H bonding with\n2\nadditional water molecules, ultimately triggering instantaneous capillary condensation\nthat fills the entire pore cavity through a chain reaction-like process. Under low\nhumidity conditions, insufficient water molecules are available to form stable\nnucleation centers, preventing the chain reaction from initiating and resulting in low\nadsorption capacity.\nThrough radiative cooling-enhanced atmospheric water harvesting, we effectively\nlower the surface temperature of the material during nighttime operation10. This\nlocalized cooling significantly increases the relative humidity at the HHE NFMs surface,\nthereby promoting the formation of nucleation centers at hydrophilic sites.\nConsequently, radiative cooling not only accelerates the moisture adsorption kinetics\nbut also enhances the maximum water uptake capacity across various humidity levels,\nextending the effective operating range to below 30% RH. As shown in the yellow bars\nof Fig. R4a, the radiative cooling-enhanced HHE NFMs achieve a water adsorption\ncapacity of 0.29 gwater·glayer⁻¹ even at 30% RH, with notable improvements observed\n7\nacross all tested humidity levels. Unfortunately, due to equipment limitations in our\nlaboratory, we were unable to test performance below 20% RH as we cannot reliably\nmaintain such low humidity conditions. Additionally, we have included further\nexperimental data on moisture adsorption under low humidity conditions without\nradiative cooling in the revised supplementary materials Fig. S8 (Fig. R4b and R4c) to\nprovide comprehensive reference for the reviewers.\nFig. R4 a) The theoretical enhancement of water adsorption capacity due to the effects\nof radiation cooling. b) and c) Moisture adsorption kinetics of hygroscopic layer at\n25 °C under various humidities.\n[Comment 5]: Some of the terms used in the article are too subjective, such as\n\"ingeniously combine\" and \"we are surprised to find\". We suggest using more neutral\nterms, such as \"systematically integrate\" and \"it is noteworthy that\".\n[Response]: We sincerely appreciate the reviewer's careful review and thoughtful\nfeedback. We have revised the original manuscript accordingly, adopting more neutral\nlanguage in the relevant descriptions as suggested. The modifications can be found on\nPage 1, line 23-24 and Page 10, line 4 of the revised manuscript.\n[Comment 6]: To determine whether there are microstructural changes (such as\nagglomeration or detachment of MOF particles) in MIL-101(Cr)@PAN nanofibers after\nmultiple adsorption-desorption cycles, it is recommended to supplement with TEM or\nSEM high-magnification characterization images after long-term cycling.\n[Response]: We sincerely thank the reviewer for their thorough review and valuable\nsuggestions. In the original manuscript, we have included SEM and elemental mapping\n8\nimages of MIL-101(Cr)@PAN after long-term cycling tests in the supplementary\nmaterials Fig. S3A (as shown in Fig. R5a). After long-term outdoor testing, the\nhoneycomb structure remained intact and did not undergo significant deformation.\nAdditionally, as kindly suggested by the reviewer, we have provided further\nsupplementary figures for reference. As shown in Fig. R5b (added in revised\nsupplementary materials Fig. S3B), the TEM of MIL101(Cr)@PAN results demonstrate\nthat MIL-101(Cr) remains firmly anchored on the nanofibers without any observed\ndetachment or agglomeration, indicating excellent structural stability and cycling\ndurability of the composite material. These findings further support the reliability of\nour proposed material system for long-term operational applications.\nFig. R5 a) The SEM and element mapping images of honeycomb-like structure after\nlong time outdoor experiment. b) The TEM image of MIL101(Cr)@PAN.\n[General comment]: I have carefully reviewed the revisions and acknowledge that the\nauthors have addressed Reviewer #3’s concerns by supplementing structural evidence\nto demonstrate sealing integrity during cover removal and by providing quantitative\nthermal analysis showing the negligible impact of the quartz glass on the overall reactor\nthermal performance.\nIn this manuscript, the authors report on spectral selective absorbing/emitting HHE\nnanofiber membranes (NFMs) employed as a self-sufficient, electricity-free air-to-H\n2\nsystem. This system combines radiative cooling with advanced photothermal catalytic\nprocesses. The H production rate of scale-up air-to-H system under outdoor natural\n2 2\nlight reaches 6467.55 µmol·m-2·h-1. However, some analytical results are not\nsufficiently convincing, and the study suffers from insufficient in-depth mechanistic\ninvestigations, which hinders a comprehensive assessment of its practical application\npotential. Consequently, I cannot recommend this paper for publication in Nature\nCommunications in its current form (see below the details).\n[Response]: We sincerely thank the reviewer for reviewing our work and giving the\nvaluable comments! We have addressed all the comments through further\nexperimentation and have made revisions on the manuscript, as specified below. We\nhope our response will satisfy you.\n[Comment 1]: As shown in Fig. 3d, the schematic diagram of PTC vapor\ndecomposition reactions indicates H and O as the primary products. However, the\n2 2\nauthors have not provided experimental evidence of O . Additionally, the author should\n2\npresent evidence regarding the formation of other potential oxidation products. The\nambiguity surrounding the anodic reaction pathway introduces risks of unaccounted\nside reactions.\n[Response]: We sincerely thank the reviewer for their constructive comments. During\nindoor experimental testing, we monitored the gas generation throughout the HHE\nNFMs reaction process, with results shown in Fig. R6a. The ratio of generated H to\n2\n10\noxygen (O ) was consistently close to 2:1, with minor deviations attributed to minimal\n2\ngas leakage during transfer and measurement procedures.\nTo further verify O generation, we conducted experiments using H₂¹⁸O and analyzed\n2\nthe O composition. Specifically, a 2 cm × 2 cm HHE NFMs sample was first dried in\n2\na vacuum oven at 100°C for 12 hours. The sample was then rapidly transferred and\nsealed in a glass reactor. Simultaneously, dry nitrogen gas was bubbled through pure\nliquid H₂¹⁸O, and this moisture-saturated nitrogen stream was introduced into the\nreactor for 6 hours to ensure complete adsorption of H₂¹⁸O by the hygroscopic layer of\nthe HHE NFMs. Subsequently, the reactor was exposed to AM 1.5G simulated sunlight\nfor 4 hours. The gaseous products were then introduced into a mass spectrometer for\ncomposition analysis. Parallel control experiments were performed using H₂¹⁶O under\nidentical conditions. The results clearly demonstrated the detection of ¹⁸O₂ in the\ngaseous products from the H₂¹⁸O experiment, while no ¹⁸O₂ was detected in the H₂¹⁶O\ncontrol experiment. Additional experiments using D₂O confirmed that the generated H\n2\noriginated exclusively from the adsorbed water vapor. These findings provide\nconclusive evidence that the H and O produced by the HHE NFMs system derive from\n2 2\nphotocatalytic water splitting. Besides, the above results have been added into\nsupplementary materials (Fig. S27 and S28) for the references of reviewers and readers.\nsolar light. Mass spectrometer signals of gas generated from HHE NFMs after\nillumination by using b) H 18O, c) H 16O, and d) D O.\n2 2 2\n[Comment 2]: Can the authors provide quantitative evidence (e.g. interfacial\nadsorption energies, charge-transfer measurements) to enhance readers’ understanding\nof why the biphasic system is better than the triphasic system?\n[Response]: We sincerely appreciate the reviewer’s valuable suggestion. We have\ncalculated the electronic properties including charge density difference (CDD) and\nprojected density of states (PDOS) to distinguish biphasic system and triphasic system.\nAs shown in Fig. R7a, b and c, Pt cluster of biphasic system accept less electron from\nTiO (101) substrate, leading to upshift of p orbital and d orbital center compared to\n2\nthat of triphasic system. As a results, the adsorption strength, and electron transfer of\nwater molecule and hydrogen atom of biphasic system both are larger than those of\ntriphasic system under the same temperature (298 K) (See Fig. R7d, e, f and g). The\nabove results have been provided in the revised Supplementary Materials (Figure S33\nand S34).\nby Bader charge for a) biphasic system and b) triphasic system (The silver, blue and red\nballs indicate Pt, Ti and O, respectively. The yellow and blue colors represent charge\naccumulation and depletion. The isosurface values are set at ±5.0×10-3 e/Å3). c) PDOS\nfor Pt/TiO of biphasic system and triphasic. The charge density difference and the\n2\ncorresponding electron transfer analyzed by Bader charge for d) adsorbed H O* of\n2\nbiphasic system and e) adsorbed H O* of triphasic system, f) adsorbed H* of triphasic\n2\nsystem and g) adsorbed H* of triphasic system (The silver, blue, red and white balls\nindicate Pt, Ti, O and H, respectively. The yellow and blue colors represent charge\naccumulation and depletion. The isosurface values are set at ±5.0×10-3 e/Å3).\n[Comment 3]: It is better to compare this system with other recently reported\nphotocatalytic air-to-H systems in the literature, including parameters such as STH and\n2\nquantum efficiency.\n[Response]: We sincerely appreciate the reviewer’s valuable suggestion. We have\nincorporated a comparative analysis between the proposed Air-to-H₂ system and other\nphotocatalytic Air-to-H₂ systems in the revised manuscript (as shown in Table R1).\nGiven the limited number of studies focusing specifically on photocatalytic air-to- H\n2\nsystems, we have further expanded the comparison to include representative vapor-fed\nand moisture evaporated from pure water photocatalytic H production systems. The\n2\ncomprehensive comparison, including key performance metrics and operational\nparameters, has been provided in the revised Supplementary Materials (Table S2).\nTable R1 The comparisons of photocatalytic air-to-H system.\nPhotocatalyst Feedstock System scale Ref.\nefficiency efficiency source\nMoisture\nNatural This\nPt/TiO harvested 0.10% NA. 2.5×10-1 m2\n2\nsunlight work\nfrom the air\nTiO -coated Moisture 0.08% 0.11% 1.075×10-3 m2 Natural 11\nx\n3 3\n:Al LED (380\nnm)\n300W Xe\nlamp（100\nTiO -coated\nx\nMoisture mW\nCoOOH/Cr\n2\nharvested 0.12% 0.11% 1.075×10-3 m2 cm−2） 11\nO /Rh/SrTiO\n3 3\nfrom the air\nand UV\n:Al\nLED (380\nnm)\nMoisture\nharvested\nfrom the air\n(with Natural\nPt-TiO 0.0859 % NA. 2.83×10-3 m2 12\n2\npolyethylene sunlight\nglycol as\nsacrificial\nagent)\nMoisture\nharvested\nSimulated\nfrom the air\nsunlight\n(with\nPt-TiO 0.14% NA. 5×10-2 m2 13\n2 （100 mW\nethylene\nglycol as cm−2）\nsacrificial\nagent)\nSrTiO :Al- Moisture UV LED\nRhCrO - from the air (365 nm,\nx\ny\ncm2.)\nMoisture\nRhCrOx- evaporated Natural\n0.09% NA. 2.5×10-3 m2 15\nAl:SrTiO from pure sunlight\n3\nliquid water\nXenon\nlamp with\nMoisture filter\nPy- evaporated (λ > 420 n\n0.064% 7.9% 3.14×10-4 m2 16\nHMPA@Pt from pure m,\nwater 100 mW c\nm2), UV\nLED (420\nnm)\nCoOOH/Rh AM 1.5G\nloaded simulated\nPure vapor\nSrTiO :Al 0.4% NA. 4×10-4 m2 sunlight 17\n3\nfeeding\ncoated with (~100 mW\nTiO cm2)\nx\nPure vapor Simulated\nfeeding sunlight\nPt-CN 0.26% NA 4×10-4 m2 18\n(flow rate (100 mW\n2 mL min−1) cm-2)\n[Comment 4]: Specially, regarding nighttime operation: does the system store\nsufficient water to sustain daytime hydrogen production?\n[Response]: We sincerely appreciate the reviewer’s thorough review and insightful\n15\ncomments. Due to the good moisture harvesting capability of the HHE NFMs under\nnighttime conditions, the material can absorb sufficient amounts of water vapor even in\nlow-humidity environments. The radiative cooling effect at the surface of the HHE\nNFMs further enhances this process by effectively reducing the local temperature and\nincreasing the relative humidity nearby, thereby significantly improving the water\nuptake performance of the hygroscopic layer.\nBased on our daytime tests, the amount of water adsorbed during the night is adequate\nto support H production throughout the following day. Under cloudy or partially\n2\ncloudy conditions, the system maintains continuous H production over the entire\n2\ndaytime period. Even under clear-sky conditions, the stored moisture enables at least 6\nhours of efficient H production under effective illumination. To validate the stability\n2\nof the material after long-term outdoor exposure as well as the reliability of the\nadsorption–desorption process, we have provided detailed data on both H production\n2\nand moisture changes of the HHE NFMs during extended outdoor testing in the\nSupporting Information. The results confirm that the stored water is gradually released\nover 6 hours of illumination, ensuring consistent H production during the daytime (As\n2\nshown in Fig. R8 and revised supplementary materials Fig. S44).\nFig. R8 24h-continuous moisture adsorption and desorption test and H production test\n2\nafter long time outdoor experiment, with the humidity, temperature, and solar radiation\n16\nintensity.\n[Comment 5]: Please supplement data on the hygroscopic and hydrogen production\nperformance under varying humidity levels (30%–90% RH).\n[Response]: We sincerely thank the reviewers for their valuable comments. We have\nsupplemented the manuscript with comprehensive data on both hygroscopic\nperformance and H production efficiency across varying humidity levels (30%–90%\nperformance under different relative humidity conditions, have also been included in\nthe Supporting Information to ensure full transparency and reproducibility. The above\nresults have been provided in the revised Supplementary Materials (Fig. S8 and S25).\nFig. R9 a) and b) Moisture adsorption kinetics of hygroscopic layer at 25 °C under\nvarious humidities. c) H production rate of HHE NFMs (0.0004 m2) under indoor\n2\nsimulated sunlight. d) Average H production rate of 4h sunlight illumination.\n2\n17\n[Comment 6]: Please conduct supplementary cyclic tests to evaluate the hygroscopic\nand hydrogen production stability of the composite system.\n[Response]: We sincerely appreciate the reviewer's valuable suggestions. We have\nconducted long-term outdoor testing to evaluate the cycling stability of the large-scale\nair-to-H₂ system, as shown in revised supplementary materials Fig. S33. We also put\nthe result in Fig. R10a for your references. Besides, the cycling stability of the\nhygroscopic layer has also been thoroughly examined, and the results are provided in\nFig. R10b (included in the revised Supplementary materials Fig. S9). Additionally, we\nperformed multiple cycling experiments under indoor conditions using 2 cm × 2 cm\nHHE NFMs (Fig. R10c and revised supplementary materials Fig. S31), which further\nconfirm the stability of the integrated system.\nFig. R10 a) Operational status of the large-scale HHE system over a 14-day period. H\n2\nproduction, solar radiation intensity, H production rate and STH efficiency at\n2\ncorresponding humidity and temperature during the 14-day period. b) Cycling stability\nof moisture adsorption−desorption of MIL@PAN and pure PAN NFMs at 25 °C and\n90% RH (desorption at 100 °C). c) H production cycling stability of HHE NFMs.\n2\n[Comment 7]: While radiative cooling is claimed to enhance hygroscopicity, the\nmoisture saturation time is only 1 hour. How is this purported advantage substantiated?\n18\n[Response]: We sincerely thank the reviewer for raising this important question. The\nradiative cooling effect on the surface of the HHE NFMs effectively reduces the local\ntemperature of the hygroscopic layer. This decrease in temperature significantly\nincreases the local relative humidity around the HHE NFMs, thereby enhancing their\nmoisture capture performance. As a result, radiative cooling contributes to two key\nimprovements: a) It accelerates the moisture adsorption rate of the HHE NFMs,\nand b) It increases the maximum water uptake capacity of the material. In outdoor\ntests comparing HHE NFMs with and without the PTC layer, we observed that the\nsamples with the PTC layer exhibited faster moisture adsorption kinetics (Fig. R11a)\nand achieved nearly 10% higher maximum water uptake capacity (Fig. R11a and\nR11b).\nBesides, following established methodologies reported in the literature10,19, we\nreplicated radiative cooling-like conditions under controlled laboratory settings to\nsystematically evaluate the hygroscopic performance of HHE NFMs under idealized\nradiative cooling-enhanced scenarios. The results demonstrate that radiative cooling\nextends the effective operating range of the HHE NFMs and increases the maximum\nwater uptake capacity under all RH (Fig. R11c). The minimum required RH is reduced\nfrom 40% RH to 30 % RH. This improvement significantly expands the potential\napplication range of the air-to- H system and enhances the maximum water supply\n2\navailable for H production.\n2\ncooling effect as a function of time during the outdoor test. b) The maximum water\nuptake comparison of HHE NFMs with/without radiative cooling during the outdoor\ntest. c) The theoretical enhancement of water adsorption capacity due to the effects of\nradiation cooling at an ambient temperature of 25°C during the indoor test.\n[Comment 8]: Please explicitly state the functional contribution of each component\nwithin the main text.\n[Response]: We sincerely appreciate the reviewer’s valuable suggestion. In the original\nmanuscript (page 2, line 30 – page 3, line 13), we provided a detailed introduction to\neach component of the HHE NFMs and their respective functions. To present this\ninformation more clearly and avoid potential misunderstandings, we have carefully\nrefined and elaborated the description in this section. The revised content is provided\nbelow and has been explicitly marked in the revised manuscript for easy reference by\nboth the reviewers and readers.\n‘This study introduces an innovative air-to-hydrogen system that employs multilayer\nhygroscopic hydrogen evolution (HHE) nanofiber membranes (NFMs) for efficient\ngreen H production, with clean water generated as a byproduct (Fig. 1a).\n20\n(PTC) top layer of Pt/TiO₂@PVDF and a porous hygroscopic bottom layer of MIL-\n101(Cr)@PAN (Fig. 1b). Capitalizing on the high solar absorptivity and high infrared\nemissivity of the PTC layer (Fig. 1c), the system achieves radiative cooling-enhanced\natmospheric water harvesting (AWH) at night, and photocatalytic-driven H production\n2\nduring the day by decomposing the water vapor released from the hygroscopic layer.\nSpecifically, the distribution of MIL-101(Cr) within the highly porous hygroscopic layer\nfacilitates rapid, spontaneous moisture adsorption from ambient air at night while\nsimultaneously mitigating particle aggregation. Concurrently, the radiative cooling\neffect of the PTC layer elevates the local humidity, thereby accelerating moisture\ncapture and extending the effective hygroscopic range (Fig. 1d). Under daytime solar\nirradiation, visible and near-infrared (NIR) wavelengths are converted into thermal\nenergy to desorb water vapor from the hygroscopic layer, while ultraviolet (UV) light\nis absorbed to activate Pt/TiO₂ for the photocatalytic (PC) production of H .’\n2\n[Comment 9]: The original DFT calculations used energy barrier data at 100°C.\nHowever, since the actual maximum operational temperature did not reach 100°C,\nthis data is unreliable. Please provide supplementary DFT calculations at the actual\noperational temperatures to demonstrate the trend of decreasing reaction energy barriers\nwith increasing temperature.\n[Response]: We sincerely thank the reviewer for their highly valuable suggestion. We\ncalculated the Gibbs energy of a photocatalytic reaction in the biphasic system over the\nPt/TiO surface at various temperature (including our reaction temperature around 60 ℃\n2\nunder indoor conditions and around 80 ℃ under outdoor conditions) and reaction\nenergy barriers of potential determining step (PDS) for the biphasic system at various\ntemperature. As shown in Fig. R12 (b), the reaction energy barriers of PDS generally\nexhibit a linear relationship with temperature. These results have been included in the\nrevised supplementary materials (Fig. S35) for the reference of reviewers and readers.\nover the Pt/TiO surface at various temperature. b) Reaction energy barriers of potential\n2\ndetermining step (PDS) for the biphasic system at various temperature.\n[Comment 10]: Please assess the solar energy utilization efficiency of the system.\n[Response]: We sincerely thank the reviewer for their highly valuable suggestion. The\nsolar-to-hydrogen (STH) conversion efficiency of the large-scale outdoor air-to-H\n2\nsystem has been calculated. Additionally, we have now specifically highlighted and\nannotated the solar energy utilization efficiency in the relevant figures as indicated in\nFig. R13 (also provided in supplementary materials Fig. S43). For the solar efficiency\ncalculations, we assumed that after each 6-hour outdoor test, the moisture within the\nHHE NFMs was completely released. This assumption formed the basis for determining\nthe solar energy utilized during the water evaporation process. Therefore, the\ncalculation of the power utilized by water evaporation (P ) can be calculated as:\nevaperation\nP =(h + 𝐶 ∆𝑇)𝑚̇\nevaporation evaporation p,w\nwhere, h is water vapor desorption isosteric heats of MIL-101(Cr), which is\nevaporation\ntaken from the experimental results of Yan et.al (53 kJ/mol), C is heat capacity of\np,w\nwater (4.182×103 J·kg−1·K−1), ∆𝑇 is the temperature increased of HHE NFMs (here,\nwe use the average temperature rise on the surface of HHE NFMs as the standard),\nand\n𝑚̇ is the desorption rate of water vapor. h is taken from the experimental results\nevaporation\nof Yan et.al (53 kJ/mol). Therefore, the solar to water vapor efficiency (STW) could be\n22\ncalculated by:\nEnergy output as H O 𝑃\n2 evaporation\nSTW = =\nIncident light energy P\nlight\nTherefore, the solar utilization efficiency (EFF ) is calculated by:\nsun\nEFF =STH+STW\nsun\nFig. R13 Operational status of the large-scale HHE system over a 14-day period. H\n2\nproduction, solar radiation intensity, Average H production rate, STH efficiency and\nday period.\n[Comment 11]: Please compare the hydrogen production performance between the\ntwo-phase system and the system after in-situ moisture absorption, rather than the three-\nphase system under conditions of abundant water supply.\n[Response]: We sincerely thank the reviewer for their constructive suggestion. In the\noriginal manuscript, we compared the H production performance of catalysts in liquid-\n2\n23\nphase environments primarily to highlight the advantages of our air-to- H system over\n2\ntraditional liquid-phase catalytic systems. For instance, our system eliminates the need\nfor purified liquid water input, as it autonomously harvests moisture from air to provide\na virtually unlimited clean water source. Additionally, our system demonstrates\nsignificantly higher H production rates compared to conventional triphasic catalytic\n2\nsystems.\nThe reviewer’s suggestion to include a biphasic system for comparison is highly\nvaluable, as it would provide clearer insights into the catalyst’s performance under\nbiphasic conditions. Although it is challenging to maintain a continuous vapor feeding\nin our experimental setup, we designed an experimental condition simulating biphasic\nreactions based on methods reported in the literature11,16 (schematic and actual setup\nare shown in Fig. R14a). Specifically, 20 mg of Pt/TiO was dispersed in 1 mL of\n2\nethanol solution and sonicated for 30 minutes. The mixture was then drop-cast onto a 4\ncm² frosted glass plate placed on a hotplate at 60°C. After solvent evaporation, the\nPt/TiO catalyst remained deposited on the glass plate, which was subsequently dried\n2\novernight in an oven at 80°C. The catalyst-coated glass plate was then placed in a quartz\nreactor, with 20 mL of DI water positioned beneath the sample holder to serve as a\nvapor source. This setup maintained a relative humidity above 80% RH inside the\nreactor (as confirmed in Fig. R14b). The reactor was purged with argon for 30 minutes\nto remove air prior to irradiation under AM 1.5G simulated sunlight (100 mW/cm2) for\nH production test. The results show that the H production rate of biphasic system with\n2 2\nvapor feeding is about 2543.2 µmol·m-2·h-1 during the first hour of reaction, which is\nhigher than HHE NFMs biphasic system (2035.2 µmol·m-2·h-1), as shown in Fig. R14c.\nHowever, both of these two biphasic systems show four 4-4.5 times higher than that of\nthe triphasic system. However, the H production rate of HHE NFMs biphasic system\n2\ndecreases because of the gradual release of water vapor from hygroscopic layer. And\nthe H production rate of biphasic system with vapor feeding keeps nearly constant H\n2 2\nproduction rate (Fig. R14d). Therefore, after 4h of sunlight illumination, the average H\n2\nproduction rate of biphasic system with vapor feeding is about twice that of HHE NFMs\n24\nbiphasic system under indoor condition. This enhancement can be attributed to the\ncontinuous vapor supply in the biphasic system with vapor feeding, which ensures\nstable H production performance, further validating the advantages of two-phase\n2\nsystems over three-phase systems. These findings have been included in the revised\nsupporting materials (Fig. S23 and 24) and revised manuscript page 7, line 15-29 for\nthe reference of reviewers and readers.\nFig. R14 a) Schematic and actual setup of biphasic system with vapor feeding. b) The\nrecord of RH in the biphasic photoreactor. c) H production rate comparison of triphasic\n2\nand biphasic reaction system in the first hour. d) H production comparison of HHE\n[Comment 12]: The mechanism of thermal-photocatalytic synergy remains ambiguous.\nIt is not elucidated how infrared photothermal effects facilitate the separation/transfer\nof photogenerated charges.\n[Response]: We sincerely thank the reviewer for their careful reading and constructive\n25\ncomments. We would like to clarify and emphasize that the photothermal catalytic\nprocess described in this work refers to a synergistic process that combines\nphotothermal and photocatalytic mechanisms for enhanced solar energy utilization. The\nultraviolet (UV) portion of the spectrum is primarily responsible for exciting the\nTiO catalyst to generate electron-hole pairs that drive the redox reaction, with Pt\n2\ncocatalyst to facilitate charge separation6. Meanwhile, visible and near-\ninfrared (NIR) light, along with energy from the relaxation of electron-hole pairs\ngenerated above the bandgap energy, is converted into heat. This thermal energy\ndrives the release of water vapor from the hygroscopic layer, thereby supplying the\nreactant for the photocatalytic H evolution reaction.\nNFMs were irradiated with only visible and NIR light, the release of steam was\nobserved as condensation on the reactor walls, but no H was detected (Fig. R15a and\n2\nb). This confirms that photons in this energy range are insufficient to excite charge\ncarriers in TiO for redox reactions. Conversely, under UV irradiation alone, a\n2\nmeasurable amount of H was produced, though the yield was significantly lower than\n2\nthat under full-spectrum illumination. This demonstrates that UV light is essential for\ngenerating electron-hole pairs in TiO , while visible-NIR light contributes mainly\n2\nthrough photothermal effects. Notably, full-spectrum irradiation substantially enhances\nthe H production performance of the air-to- H system. In the Pt/TiO catalytic system,\n2 2 2\nPt serves as an effective electron acceptor, promoting charge separation and reducing\nelectron-hole recombination. The temperature increase induced by photothermal effects\nfurther accelerates electron transport between Pt and TiO and within Pt itself,\n2\nthereby reducing the recombination of photogenerated carriers and improving the\noverall photocatalytic H evolution efficiency20-22. Additionally, DFT calculations\n2\nindicate that in the biphasic air-to-H system, higher reaction temperatures lower the\n2\nenergy barrier for H generation, thermodynamically favoring the reaction. Elevated\n2\ntemperatures also reduce H transport resistance, facilitating desorption from active\n2\nsites. These combined effects synergistically enhance the H production efficiency of\n2\nthe photocatalytic system.\nIn response to the reviewer’s suggestion, we have not only elaborated on the\npromotional role of the photothermal process in the photocatalytic reaction in the\nrevised manuscript but also added further explanation regarding its positive impact on\ncharge carrier migration (page10, line 5 to line 31).\nFig. R15 a) H production rate under different light conditions during the first hour. b)\n2\nwall of reactor.\nTiO2 (Pt/TiO2) catalysts prepared by the polygonal barrel-sputtering method. Applied Catalysis\nB: Environmental 254, 7-14 (2019).\nhttps://doi.org:https://doi.org/10.1016/j.apcatb.2019.04.075\nproduction performance of TiO2 hollow spheres. Separation and Purification Technology 361,\n131586 (2025). https://doi.org:https://doi.org/10.1016/j.seppur.2025.131586\nfrom Naphthalene by Various TiO2 Photocatalysts: Impact of Pt Loading and Formation of\nIntermediates. Catalysts 11 (2021).\nAu/TiO2 Photocatalysts: A Comparative Study. Topics in Catalysis 56, 1139-1151 (2013).\nhttps://doi.org:10.1007/s11244-013-0080-8\nfrom a water-ethanol solution: a comparison of activities of Au TiO2 and Pt TiO2. Journal of\nPhotochemistry and Photobiology A: Chemistry 89, 177-189 (1995).\nhttps://doi.org:https://doi.org/10.1016/1010-6030(95)04039-I\nand photonic efficiency of Pt, Au and Pd deposited on TiO2 catalyst. Water Research 38, 3001-\n3008 (2004). https://doi.org:https://doi.org/10.1016/j.watres.2004.04.046\nProduction Over Pt/TiO2. ENERGY & ENVIRONMENTAL MATERIALS 3, 202-208 (2020).\nhttps://doi.org:https://doi.org/10.1002/eem2.12068\nApplications. Advanced Functional Materials 34, 2304788 (2024).\nhttps://doi.org:https://doi.org/10.1002/adfm.202304788\nInterplay between Topology and Surface Charge in Graphene Oxide Membranes During\nHumidity Induced Swelling. ACS nano 17, 21923-21934 (2023).\nhttps://doi.org:10.1021/acsnano.3c08260\nCommunications Engineering 2, 35 (2023). https://doi.org:10.1038/s44172-023-00082-3\nHydrogen Production via Multifunctional Hygroscopic‐Photocatalytic Hydrogel\nNanocomposite. Advanced Functional Materials (2025).\nhttps://doi.org:10.1002/adfm.202512738\ncycle. Nano Energy 128 (2024). https://doi.org:10.1016/j.nanoen.2024.109879\nsunlight. Chemical Engineering Journal 510 (2025). https://doi.org:10.1016/j.cej.2025.161832\nof moisture in ambient air: Toward reagent-free hydrogen production. Journal of\nPhotochemistry and Photobiology A: Chemistry 401, 112757 (2020).\n28\nhttps://doi.org:10.1016/j.jphotochem.2020.112757\nsplitting coupled to water purification. Nature Water 1, 952-960 (2023).\nhttps://doi.org:10.1038/s44221-023-00139-9\nfor photocatalytic hydrogen evolution from water vapor. Applied Catalysis B: Environmental\n338 (2023). https://doi.org:10.1016/j.apcatb.2023.123074\nphotocatalytic water splitting. Nature communications 13, 5698 (2022).\nhttps://doi.org:10.1038/s41467-022-33439-x\nAdvanced Functional Materials 34 (2024). https://doi.org:10.1002/adfm.202313058\nsorbents-based atmospheric water harvesting. Cell Reports Physical Science 3 (2022).\nhttps://doi.org:10.1016/j.xcrp.2022.100879\nphotothermal effect of Cu/TiO2 nanoparticles. International Journal of Hydrogen Energy 48,\n6336-6345 (2023). https://doi.org:https://doi.org/10.1016/j.ijhydene.2022.05.027\nPhotocatalytic Hydrogen Production from Saturated Vapor of VOC Aqueous Solutions.\nAdvanced Functional Materials n/a, e18394 (2025).\nhttps://doi.org:https://doi.org/10.1002/adfm.202518394\nMonolayer MoS2. ACS nano 16, 2921-2927 (2022). https://doi.org:10.1021/acsnano.1c10030\nPoint-to point response to reviewers’ comments on the manuscript\n‘Electricity-Free Hydrogen Production from the Air’\nReviewer #4 (Remarks to the Author):\nAny science can only benefit the world if it is transformed into engineering. I think engineering-related works should also be\neligible for publication in Nature Communicationse and I will no longer able to review this manuscript.\nReviewer #5 (Remarks to the Author):"}]

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