Low temperature and rapid photothermal oxidation of liquid gallium for circular hydrogen production

Domain: physics
Published: 2026-01-20
Editorial outcome: major_revision

Abstract
Hydrogen is a clean energy carrier with significant promises for a sustainable future, yet many established production routes operate under undesirable conditions or constraints, motivating the search for alternative production pathways. Here, our approach uses photothermal oxidation of liquid gallium to generate hydrogen from both freshwater and seawater. The exposure to light thermally heats up liquid gallium droplets to a temperature suitable for fast gallium-water interaction, producing gallium oxyhydroxide and hydrogen. The light exposure also boosts the reaction by breaking the oxide layer on the surface of liquid gallium immersed in water, allowing continuous interactions between the water molecules and the surface of liquid gallium droplets. The gallium oxyhydroxide that is produced can be electrochemically reduced, allowing metal regeneration, thereby enabling circular hydrogen production. In this work, we show that photothermal activation of liquid gallium provides a rapid and circular route for generating hydrogen from diverse water sources.

Introduction
As an Earth-abundant element, hydrogen can be sourced from a variety of compounds, with water being the most significant1. Utilizing molecular hydrogen (H2) is expected to contribute to the reduction of greenhouse gas emissions2,3. Its high energy content and clean combustion make H2 an ideal candidate for a sustainable energy future, supporting efforts to mitigate climate change and promote environmental health4.

The methods for producing green H2 are primarily focused on water splitting via electrolysis, photocatalysis, plasma-based techniques, hydrothermal processes, and biological approaches5. While each method presents specific advantages, they also face inherent challenges related to efficiency, yield, complexity, scale up, and cost. Consequently, exploring alternative technological pathways is crucial to enhancing the feasibility and accessibility of green H2 in the future5.

In this work, we present an energetically favorable circular method for H2 generation utilizing only liquid metal gallium (Ga). Low temperature photothermal oxidation produces H2 under artificial or natural sunlight, and an aqueous electrochemical reduction step regenerates Ga, completing the cycle.

It is important to mention that the reactions of alloys such as aluminum (Al) with Ga have been shown to produce H2 when in contact with water, however, such systems were never energetically favorable6–11. For instance, in the case of Al-Ga, the reaction with water produces aluminum oxide, which is a very stable compound that requires high energy input to reduce6,9. In contrast, the reduction of Ga(III) species can be performed in aqueous media with standard electrochemical equipment and using relatively low-energy conditions.

Results
Liquid metals are notably absorptive of energetic electromagnetic waves, covering a broad range of wavelengths from GHz to ultraviolet (UV)13–15. The impinge electromagnetic waves imposed in the visible light range, in the presence of an oxygen source, promote photo-corrosion phenomena in metals. Historically, corrosion has been considered a detrimental process, shortening the lifespan of solid metal structures.

Here, we use light-induced corrosion to drive liquid Ga-water interaction for on-demand hydrogen generation. Liquid Ga offers advantages that facilitate this process. Notably, due to its low melting point of 29.8 °C, Ga becomes liquid just above this temperature15,16. If the surface of Ga is roughened, for instance by sonication, to make low-dimensional droplets with higher surface area, Ga can absorb light and produce heat at a much higher efficiency than that of bulk Ga17,18. Compared to mercury-based processes, such as the chlor-alkali method, the Ga-based process presented in this study is more environmentally benign due to the low toxicity of Ga19–21.

Finally, the non-polar surface of liquid Ga resists adhesion, preventing most materials from sticking under normal conditions15,22,23. This property is important in our circular process, as it enables the naturally formed oxyhydroxide to exfoliate from the surface of the liquid Ga. This natural exfoliation prevents the buildup of continuous oxidation and passivation of the surface, a problem commonly encountered with solid metals6,20,24. Instead, the formed GaOOH easily flakes off from the surface of liquid Ga, continuously exposing the bare liquid Ga surface to further oxyhydroxidation reactions by water, leading to the generation of H2. The presence of light also enhances thermal oxidation by increasing the temperature14,25, resulting in increased production of GaOOH until all the Ga is consumed (Fig. 1).

The process of photothermal corrosion of liquid metal Ga demonstrates significant thermodynamic advantages for effectively generating H2 through its reaction with water. Notably, this reaction progresses even in saline environments, where it continues to produce H2 without any degradation in performance. As a result, no additional desalination treatments are required, unlike with traditional electrolysers26. Additionally, the reaction is energetically favorable, further underscoring its viability for sustainable H2 generation. For our circular process, the theoretical circular efficiency is obtained using the following Eq. (1) in which 2 moles of Ga produce 3 moles of H2:

2Ga + 4H2O ! 2GaOOH + 3H2 (1)

Considering the molar masses of Ga as 69.723 g/mol and H2 as 2.016 g/mol, for 1 kg of Ga, an amount of H2 equal to 43.36 g can be produced with the potential energy of 1.44 kWh27. The recovery energy for the electrowinning of Ga is approximately 2.50 kW/kg28. Thus, the round-trip efficiency is 1.44 kWh/2.50 kWh × 100 = 57.6% for our circular process, considering natural solar irradiation as a free source of energy.

To increase the rate of photothermal efficiency, a sonication process for Ga is used to enhance the surface area, promoting more efficient absorption of energy from the light source18,25. The solvent used for sonication was ethanol to avoid excessive preliminary oxidation of the liquid Ga (see Methods). The Ga droplets produced after 30 min of sonication are shown in Supplementary Fig. 1a. A photoreactor shown in Supplementary Fig. 2 was set up with Ga droplets (0.2 g) placed in water (10 mL) and exposed to a halogen lamp (50 W, emitting visible light in the 400 to 800 nm range, Fig. 2). The reactor was positioned at varying distances from the lamp to investigate different energy absorption levels, as the energy absorbed by the Ga droplets decreases as the distance from the light source increases (Supplementary Fig. 2). The photothermal conversion properties of Ga droplets were demonstrated by measuring the temperature inside the reactors with and without Ga droplets under identical light irradiation conditions and volume of DI water (Supplementary Fig. 2). The maximum temperature difference, relative to the DI water-only control, peaked at 16 °C at a light intensity of 540 mW/cm2 and gradually decreased with decreasing the light intensity. While at 600 mW/cm2, both conditions exhibited closer temperatures, with a difference of 3 °C, due to the high irradiance and large thermal buffer provided by the DI water. The use of a large water volume, relative to the mass of Ga droplets, was intentional to prevent overheating or boiling while maximizing the light-to-temperature conversion efficiency for accurate evaluation of the photothermal performance for H2 production.

The analysis of the collected gas using gas chromatography-mass spectrometry (GC-MS) shows that only pure H2 is produced during the photothermal oxidation (Fig. 2b). After photothermal exposure, GaOOH crystals are produced as shown in Fig. 2c and Supplementary Fig. 1b. At 0.2 cm from the light source, the energy density is ~600 mW/cm2, equivalent to 6 suns of concentrated sunlight. The energy densities at other distances were approximately 450, 270, and 230 mW/cm2 at 2, 4 and 5 cm, respectively. The system applies visible light that heats the Ga droplets (30 min sonicated) that result in temperatures ranging from 48 °C to 93 °C, depending on the distance from the light source (Supplementary Fig. 2b). As the distance from the light source increases, the temperature and light input decrease, diminishing H2 production yield (Fig. 2d). Maximum H2 output was achieved at the light intensity of ~600 mW/cm2, when the entire volume of Ga droplets was transformed into GaOOH after just 90 min, producing ~96 mL of H2. Additionally, the evolution of H2 bubbles provided mechanical agitation to the system (Supplementary Movie 1). This experimental result aligns with the stoichiometric molarity balance described in Eq. (1), calculated based on an input of 0.2 g of Ga. As the light intensity decreased with distance according to the graph, the H2 production rate diminished. At ~230 mW/cm2, production was minimal, while at ~270 and ~450 mW/cm2, H2 production reached 38 mL and 56 mL after 5 h, respectively. This experiment illustrates the effective light absorption of Ga droplets, consistent with previous studies, showing a comparable photothermal conversion efficiency to gold nanoparticles of 20% to 30%18,25,29. We have used a rather conservative value of 22.5% in our calculations. Considering the requirement of 57.6% electrochemical reduction efficiency, this amounts to the overall circular efficiency of 12.9% for H2 production.

Furthermore, Fig. 1e contrasts H2 production using saline water and freshwater using Ga droplets (30 min sonicated), showing similar rates for both, indicating the system’s effectiveness in H2 production in saline water. To demonstrate that our process does not require a preliminary desalination step, H2 production was further evaluated using nearshore sea collected seawater (See Methods and Supplementary Fig. 3), yielding 98.4% of the theoretical maximum after 180 min using the same conditions that resulted in the same by-products as the DI water process. As shown in Supplementary Fig. 4, the collected seawater exhibits characteristic ionic composition, and the total organic carbon is measured at 3.2 ± 0.3 ppm. Although the reaction proceeded at a reduced rate, due to organic contaminants that absorb and scatter incident light and thereby reduce the effective irradiation reaching the Ga surface, the high yield indicated that the collected seawater did not impede overall completion. Elemental mapping using SEM/EDX was performed on the GaOOH product after reaction in seawater to examine possible salt incorporation. Deposits of NaCl, calcium, and potassium are initially detected (Supplementary Fig. 5). However, after washing with DI water, the salt deposits are no longer present (Supplementary Fig. 6).

In addition, the Ga concentration in the water source after the reaction was analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES), revealing minimal loss of Ga as ionic species, and confirming that Ga transitions to GaOOH in both purified and saline water (Supplementary Fig. 4). In order to highlight the effectiveness of photothermal influence on the process, a control experiment was conducted without light irradiation or heating, where the reactor was enclosed providing isolation from incident light and heat. Under otherwise identical conditions, including the same mass of Ga droplets, H2 production reached 15 mL after 48 h, corresponding to 15.5% of the maximum theoretical yield.

It is important to consider the usage of nanoparticles for efficiently absorbing light in the process. In an earlier study, Zhao et al. previously reported that bulk Ga-based liquid metals (Ga-In alloys with varying In content) can spontaneously evolve H2 gas at 23 °C without external input; however, only in strongly acidic or alkaline environments. The proposed mechanism was attributed to galvanic interactions between the oxide skin and the underlying bulk liquid Ga-In alloy, and reported that higher In content increased gas evolution in alkaline solution30. Further interesting work by Duan et al. reported the inherent reactivity of liquid metals in an aqueous environment, characterizing the behavior as “molecular scissors” activity, which included the ability to generate H2 31. In this study, a bulk eutectic alloy of Ga and indium (EGaIn) was immersed in DI water, and H2 bubbles were observed. The reported H2 production rate was ~0.13 mL g−1·h−1 of H2 at 40 °C. Under our photothermal condition, liquid Ga achieved ~27.5 mL g−1·h−1 of H2 at 48 °C (Fig. 3c). For further comparison, the EGaIn droplets system was also tested to achieve ~76.5 mL g−1·h−1 at 600 mW/cm2. Although the reaction rate was lower for the EGaIn systems, the yield, which was calculated based on the theoretical amount of Ga in the alloy, reached 21% after 36 h and 90% after 4.25 h for the bulk and particulate EGaIn, respectively. These results are consistent with both the mechanism proposed by Duan et al. and the reaction pathway described in our study. A comparison of H2 production rates is presented in Supplementary Table 1. Importantly, in our study, photothermal input enables the evolution of H2 from pure liquid Ga and also allows control of the reaction rate through light irradiation intensity.

The results imply that photothermal corrosion is the main mechanism for oxyhydroxidation, and additional NaCl saline environment does not significantly participate in the corrosion process, as it does in the corrosion of solid metals32,33. X-ray diffraction (XRD) and Raman analysis, shown in Supplementary Fig. 8 confirm the transformation of liquid Ga droplets, surrounded by their native Ga oxide layer (Ga2O3), into GaOOH following the oxyhydroxidation process34–37. Additionally, X-ray photoelectron spectroscopy (XPS) analyses of the Ga droplets before and after the reaction revealed no residual liquid Ga in the Ga 3d high-resolution spectra of the Ga droplets after the reaction (Supplementary Fig. 9)36,37. The relative intensity of the deconvoluted peak associated with -OH groups, as in GaOOH, also increased in the O 1s high-resolution spectra, supporting the formation of the oxyhydroxide material34,36,37. The valence band edges, as determined by XPS (Supplementary Fig. 9e, f), show contributions from the metallic Ga state and its thin Ga2O3 native oxide layer before the reaction. In contrast, after the reaction, the droplets exhibit contributions solely from fully oxidized materials, as in GaOOH36,38,39. To evaluate the effect of Ga droplet dimensions on H2 production, gas production was measured at different Ga sonication durations (Fig. 3a). The sonication duration was kept at a maximum of 30 min, as above this value, some direct oxidation of Ga droplets during sonication could be observed, which was counterproductive. Additionally, there is a minor difference between the Ga sizes produced at 30 min and after 30 min40. The highest H2 yield was observed with liquid Ga sonicated for 30 min (Fig. 3b). This finding suggests that droplet size is a critical factor in optimizing the efficiency of H2 production. For comparison, bulk Ga (0.2 g) was exposed to 600 W/cm2 light irradiation. While the production of GaOOH was also observed from bulk Ga (Fig. 3d), H2 gas production reached only ~32 mL even after 70 h (Fig. 3e), highlighting the critical role of forming Ga droplets in the process.

A key factor in H2 production through Ga photocorrosion is the formation of the GaOOH layer. This layer, if not removed, blocks further oxidation by limiting water access to the Ga surface. However, liquid Ga presents a distinct advantage: the oxyhydroxide layer naturally exfoliates due to its lack of polarization on the liquid metal interface, allowing water to access the surface of Ga, and the photocorrosion process can continue (Fig. 2c middle image). Light produces two effects that result in the photocorrosion of Ga. One effect is the heating of Ga droplets that enhances the photocorrosion in water, and the second is by producing extra electron-hole pairs at the interface of Ga and GaOOH41,42. Below, we present measurements and discuss the contribution of each factor to Ga photocorrosion.

To understand the effect of direct heat on corrosion, we conducted thermal oxidation experiments without light. We investigated the H2 yield at different temperatures associated with those generated by light exposure (Supplementary Fig. 2, ranging from 90 °C to 60 °C). As shown in Fig. 3d, even at 90 °C, H2 production without light reached only ~36 mL after 90 min, about a third of what was achieved with light irradiation. These results highlight the role of absorbed light in heating up the Ga droplets for driving the photocorrosion of liquid Ga.

An experiment was also conducted using a solar simulator equipped with an AM 1.5G filter (Supplementary Fig. 10). The light source was set to deliver an irradiation intensity of 600 mW/cm2 (6 suns) for comparison purposes. Under these conditions, the reactor temperature reached an average of 59 ± 3 °C, and the hydrogen yield reached 99.4% after 360 min. This corresponds to a reaction time that is 4.5 times longer than that required under halogen light irradiation at the same intensity (Fig. 2d), yet it still achieved a yield close to the theoretical maximum. In comparison, pure thermal heating at 60 °C resulted in only a 21% yield after 300 min (Fig. 3f). The experiments show the effectiveness of light absorption in the visible region than in the deep infrared, as the halogen lamp has less irradiation intensity in the infrared than the Sun. For showing the compatibility of our system for direct implementation into practical solar thermal apparatus, an additional experiment was conducted using concentrated natural sunlight (Supplementary Fig. 11), during which the reactor temperature reached 79.5 °C and a H2 yield of 98.4% was achieved after 120 min with peak irradiation of 750 mW/cm2.

Noticeably, however, direct photothermal heating does not account for the entire photocorrosion contribution. We also propose that light interacts with GaOOH, a semiconductor with multiple absorbing band states (Supplementary Fig. 9), generating electron-hole pairs at the Ga/GaOOH interface41,42. Afterwards, the quasi-depletion region at this interface facilitates electron mobility across the surface for enhancing its further oxidation. To confirm the role of GaOOH in the reaction mechanism, we conducted an experiment under acidic conditions (1.0 M HCl) using Ga droplets with an average size of 640 nm irradiated with a 600 mW/cm2 halogen light (Supplementary Fig. 12). Under these conditions, GaOOH cannot be formed. Here, H2 was initially produced. However, the Ga droplets rapidly coalesced, and no formation of GaOOH was observed, consistent with the unfavorable conditions for GaOOH formation in acidic media. After 24 h, 32 mL of H2 was produced, and approximately 83% of the initial Ga mass was recovered from the reactor. The initial H2 generation was attributed to the classic acidic dissolution models of Ga in HCl (Supplementary Fig. 12).

To confirm the role of light, a set of experiments was conducted. The schematic representation of the experimental set-up of the photooxidation process is presented in Fig. 4a. Here, photocurrent was recorded with and without light irradiation in subsequent intervals (Fig. 4b). The highest intensity light was used in these measurements. As shown in Fig. 4b, the current measurements revealed a notable increase in magnitude under illuminated conditions compared to dark conditions. This enhancement demonstrates the effectiveness of light irradiation in activating photoinduced electrochemical processes for photocorrosion. Additionally, the open-circuit voltage measurements under both dark and illuminated conditions (Fig. 4c) highlight the impact of light irradiation on the polarization potentials of Ga, suggesting the generation of additional photoexcited carriers under light illumination42. These findings demonstrate the effectiveness of light irradiation in driving the photocorrosion efficiency and the oxyhydroxidation process.

The final crucial factor in the circular process proposed in this work is the reduction of GaOOH back into Ga metal. Following H2 generation, Ga was successfully recovered from the photothermal reactor via electrochemical reduction, with a recovery efficiency of approximately 95% by weight relative to the initial Ga used (Supplementary Fig. 13). The reported 95% recovery yield is likely a conservative estimate, partly due to potential substrate degradation. Additionally, detached Ga droplets may have partially redissolved in the acidic electrolyte during recovery, further contributing to mass loss. XRD analysis of both the Ga recovered on the carbon substrate and the detached Ga after a reaction cycle revealed the characteristic broad peak of amorphous Ga, with no detectable gallium oxide crystalline by-products or alloying phases (Supplementary Fig. 14), confirming that the Ga remained chemically unaltered during the reaction.

A standard electrochemical reduction setup was used in an acidic electrolyte at a low concentration (1.0 M HCl), as schematically presented in Fig. 4d. The Ga electrode reaction (GER) was observed simultaneously with the hydrogen evolution reaction (HER) as seen in Fig. 4e on carbon paper (CP) electrode substrates43–45. In the potential region of −0.8 to −1.5 V, reduction currents begin at −0.92 V ± 0.02 V for Ga(III) on CP. The potential at 10 mA/cm2 is −1.012 ± 0.005 V vs RHE for the electroreduction of Ga(III) on CP. The open circuit potentials of the electrochemical systems are also presented (Supplementary Fig. 15). The initial cathodic current reflects the initiation of GER and HER, while the subsequent steady-state current highlights the sustained HER activity, highlighting the reducibility of Ga(III). The presence of metallic Ga post-reduction is confirmed by the observed morphology and EDS analysis verifying the elemental composition, demonstrating a successful reduction process into metallic Ga (Fig. 4f).

While GaOOH has been rarely studied as a precursor for reduction to metallic Ga, we propose that the ability of GaOOH to dissociate into Ga(III) species in an ionic form such as Ga3+ ions in acidic environments is a key factor enabling this efficient reduction reaction43. Prior to electro-recovery, GaOOH was dissociated in 2 M HCl at 90 °C for 6 h to convert it into a soluble Ga3+ species via acid-mediated leaching, allowing electroreduction to proceed in the same reaction medium. While alkaline pathways are often employed for Ga electro-recovery, the dissolution of GaOOH under such conditions is less favorable, as GaOOH remains stable over several days (see Supplementary Fig. 16). This dissociation lowers the required applied voltage, making the process less energy intensive. Consequently, this characteristic supports the viability of regenerating metallic Ga from GaOOH in a sustainable and energy-efficient manner. After electroreduction of soluble Ga(III), the recovered Ga was processed into droplets following the same sonication procedure and reintroduced into the photothermal oxidation reactor. The H2 yield using the recovered Ga was 99.4% (Supplementary Fig. 17). Carbon electrodes were selected due to their wide availability and demonstrated reliable performance for both GER and HER in our experiments. Additionally, carbon electrodes mitigate the risk of Ga alloying with metal electrodes, preventing contamination and ensuring reaction circularity. Other possibilities for circular recovery involve the dissolution of GaOOH into Ga(III) ions in sodium hydroxide, followed by electrodeposition onto other metal electrodes46. In addition, due to the liquid state of Ga under operating conditions inside the photothermal reactor, the regenerated metal avoids crystallographic trapping, ensuring sustained oxyhydroxidation reactivity upon recycling.

To evaluate the economic viability of our proof‑of‑concept circular process for H2 production, at this preliminary stage, we performed a techno‑economic analysis (see Supplementary Information). It presents levelized cost estimates for H2 per kilogram. It’s important to note that these costs are likely to improve further as the technology matures. Although Ga remains a costly material for H2 production, the presented circular process and Ga’s ability to generate H2 on demand offer considerable cost savings and practicalities. Gallium can be safely transported and stored without losing its energetic potential. Additionally, using sunlight and seawater for H2 generation helps offset the initial material cost. The reduction to Ga can be supplied by renewable energy sources, enhancing the sustainability of the process even further28. As a result, the entire H2 production cycle becomes circular and environmentally friendly.

Methods
Materials
Gallium and indium (both 99.999% purity) were purchased from Indium Corporation (USA), and ethanol absolute (99.5% purity) purchased from Merck Australia was used for the sonication process. Deionized water (18.2 MΩ·cm, total organic carbon (TOC)≤5 ppb, 25°C, Milli-Q system) was used for the reactions. Simulated seawater solutions were prepared with a sodium chloride concentration of 0.18 M (NaCl, ≥99%, ChemSupply, Australia). A seawater sample was collected from nearshore waters at the beach face in the eastern suburbs of Sydney, Australia, and filtered through a 0.45 µm syringe filter prior to use.

Synthesis of gallium droplets
The Ga droplets were synthesized through a two-step process involving thermal and ultrasonic treatment. Initially, 0.2 g of Ga was put into a borosilicate glass vial (21 mL) and placed onto a hot plate set at 70 °C for 15 min to ensure complete melting of the Ga. Following the melting process, 10 mL of ethanol was added to the borosilicate glass vial, removed from the hot plate, and immediately subjected to ultrasonication, which was carried out at a room temperature of 23 °C using a 1/8 inches sonication probe (SONICS VCX750, 750 W set at 40% amplitude and 20 kHz) inside a closed isolation cabinet. The sonication duration varied from 5, 10, 20, and 30 min which resulted in the formation of the distinct-sized Ga droplets. After sonication, the ethanol is left to evaporate at room temperature of 23 °C, leaving dry Ga droplets inside the vial. To re-disperse the Ga droplets effectively, 10 mL of DI water was added to the dried droplets. The solution was then subjected to bath sonication for 2 min.

Eutectic gallium-indium alloy (EGaIn, 75.5 wt% Ga and 24.5 wt% In) was prepared by melting the two metals on a hot plate for 2 h, and EGaIn droplets were then produced by sonication, following a procedure similar to that used for synthesizing Ga droplets.

Hydrogen production experiments and gallium photothermal oxidation reactions
Thermal process. Vials were positioned on hot plates to control the reaction temperature from 40 to 90 °C. The experiments were conducted for up to 24 h or until gas production had ceased.

Photothermal process. Vials were positioned at varying distances (0, 2, 4, and 5 cm) relative to a commercial 50 W halogen lamp (Philips MR16, GU5.3 base). A sunlight simulator (SunLite 11002-2, Abet Technologies, USA) equipped with a 100 W Xe arc lamp and an AM 1.5G filter was also used as a light source. Light intensity was measured using a digital optical power meter (Newport Optical Power Meter, model 1918-C), with the detector positioned at the same distance and orientation as the reaction vial. For concentrated natural sunlight experiments, a 30×30 cm Fresnel lens was positioned indoors to focus natural sunlight onto the reactor, producing a concentrated illumination area of 1×1 cm. As the lens and reactor were indoors, the incident natural sunlight experienced a fourfold reduction in irradiance. The experiment was conducted on an intermittently cloudy winter day. Additionally, controlled experiments were conducted in the absence of light and heat sources, with the photothermal reactor enclosed in a box to effectively shield the reactor from light and heat interference.

Experiments were conducted until gas production ceased.

Water displacement for the collection of generated hydrogen gas
The H2 produced during the reaction was collected using a water-displacement setup, in which the gas was captured in a 250 mL graduated glass column filled with water and inverted in a collection vessel. As hydrogen accumulated, it displaced the water in the column, and the gas volume was determined directly from the change in the water level. Replicate experiments (n=3) showed less than 1% deviation for the measurements at 600 mW/cm2 in Fig. 2d, e. To maintain visual clarity, individual error bars were omitted. A Nexis GC-2030 gas chromatograph (Shimadzu, Japan) was used to analyze the gas composition and quantify evolved products during the experiments. The instrument is equipped with a thermal conductivity detector (TCD) and a high-precision column suitable for detecting hydrogen (H2). No other gaseous products were detected.

Characterization
SEM, SEM/EDX, size distribution. Thermo Scientific Phenon XL G2 scanning electron microscope (SEM) was utilized for imaging and elemental mapping at an accelerating voltage of 15.0 kV. Particle size distributions were estimated from SEM images.

Raman. Raman spectroscopy was performed on a Renishaw Raman inVia Qontor using a 532 nm laser source (50 mW) with a 10 s exposure time and at a 10% laser power.

XPS. The XPS elemental analysis and valence band analysis were conducted with a Thermo Scientific ESCALAB 250 Xi, monochromated Al Kα.

XRD. The XRD patterns were obtained using a Rigaku-Smart lab-SE instrument with a scanning range of 10° to 60° at a scan rate of 1.0 θ min−1 (λ=1.5418 Å, Cu-Kα radiation).

ICP-OES. The Ga concentrations were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkinElmer, USA).

Total organic carbon (TOC) analysis. The TOC analysis was performed on the collected seawater sample using a Shimadzu TOC-LCPH instrument. The sample was filtered through a 45 µm syringe filter and diluted three times prior to analysis.

Electrochemical reduction of gallium oxyhydroxide
The electrochemical characterization and electro-reduction experiments were performed using an electrochemical workstation (CHI650E, CH Instruments Inc., USA) processed using automatic iR correction, and exported without additional smoothing or filtering, a three-electrode cell using carbon paper (AvCarb® MGL280, 1.5×2.0 cm, FuelCell Store, USA) as working electrodes, a saturated calomel electrode as the reference electrode, and a graphite rod as the counter-electrode. The data obtained from the CHI650E were saved as .txt files and subsequently plotted using Origin (OriginLab). Linear sweep voltammetry curves and chronoamperometry were carried out in 30 mL of N2-saturated 1.0 M HCl electrolyte (pH=0) prepared by diluting concentrated hydrochloric acid (37%, analytical grade, Sigma-Aldrich) with deionized water (18.2 MΩ·cm) and used straight away without storage. The linear sweep voltammetry was performed at a scan rate of 10 mV s−1. All polarization curves were automatically adjusted to account for the iR contribution from the cell. The uncompensated resistances Ru were measured using electrochemical impedance spectroscopy (EIS) on a CHI650E workstation (100 kHz–1 Hz, 5 mV amplitude). The Ru values were 1.18 ± 0.01 ohm in the electrochemical recovery cell containing the Ga(III) electrolyte and 0.39 ± 0.02 ohm in 1.0 M HCl.

The recorded potential was converted to a reversible hydrogen electrode (RHE) according to the Nernst equation: E(RHE) = E + E(SCE) + 0.059 × pH. Prior to electro-recovery, GaOOH was treated with 2.0 M HCl (pH=0) prepared by diluting 5 mL of concentrated HCl (37%, analytical grade, Sigma-Aldrich) with 25.0 mL of deionized water to obtain a 30 mL solution, and placed on a hot plate at 90 °C for 6 h to convert it into a soluble Ga3+ species via acid-mediated leaching. To quantify Ga recovery, the carbon paper electrode was weighed in its dried state both before and after the electro-recovery process. In addition to the Ga deposited on the carbon paper, any Ga droplets that detached during electroreduction and accumulated in the electrolyte bath were also collected, dried, and weighed. The total recovered mass was calculated as the sum of Ga retained on carbon paper and Ga collected from the bath. Photocurrents and polarization potentials were recorded using a liquid Ga drop as the working electrode connected to a gold wire within a glass capillary.

Electrochemical measurements were conducted in a controlled laboratory environment of 23 °C on average.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The oxidation of gallium to its oxyhydroxide is described, with release of hydrogen. Ultrasonication and 70 C were required before a photothermal reaction using a halogen lamp. The efficiency is low and requires at least an hour, depending on wavelength. Generating hydrogen in sunlight can detonate in air. The reaction relies on 5N gallium and the oxyhydroxide product is not a commodity compound. It must all be reduced back to gallium metal, more than negating the energy benefit of the hydrogen release. It would be very difficult and expensive to cost effectively scale up to even 1 kg of H2. I do not recommend publication in Nature Communications. Some additional comments ordered by line number/appearance are included below.\nLine 63: The authors say that alloys containing aluminum produce aluminum oxide compound that require high energy input to reduce to produce primary aluminum.\nYet, this is an important technology to the world’s economy and is the source of gallium needed for their reaction, produced as a minor byproduct.\nLine 64: According to the cited reference 14, sonication produces nanorods of gallium that are more efficient in producing GaO(OH) than are photothermal generated Ga nanospheres. For their current work, the authors produce polydisperse Ga nanospheres by sonication and the nanospheres are inefficient, producing much heat. The authors report a 12.9% maximum efficiency for their hydrogen production but don’t show how this was calculated and whether they included the energy input needed before and during the reaction.\nLine 74: Authors claim that Ga is environmentally friendly as it is primarily recovered as a byproduct of existing mining operation.\nThey cite Ref. 44, which describes various methods used in recovering Ga from the mining waste called “Bayer liquor”, also known as “red mud”, one of the world’s most polluting industrial wastes.\nLine 92: Authors report that the reaction of Ga with water remains highly efficient even in saline environment, such as seawater.\nOnly aqueous NaCl was tested. Seawater should be tested since it contains other metallic impurities such as iron and vanadium that could make recovery of pure Ga difficult (see Ref. 44).\nHow much of the 0.2 g of 99.999% Ga is recovered in their process? Given the 5N expense, the recovery would have to be near 100% to back up their “circular” claim.\nWhat’s the yield of hydrogen when no light is used for the entire reaction rather than cycling on/off? In Figure 3, the % hydrogen released would be much more informative than mL of hydrogen, which depends on the masses used. Also in Figure 3, specify the mW/m^2. The amount of light is said to be “6 suns of concentrated sunlight”. Was the light concentrated/focused? The flux needs to be compared to the standard AM 1.5G. It would be helpful to include the oxygen EDX map in Figure 4c.\nHow continuous is their hydrogen generation? One gets the impression it’s a labor intensive, expensive and inefficient batch process. The supplementary video is clearly sped up and the factor is not given. How much gallium would be needed for continuous supply to a 100W fuel cell, e.g. for an hour?\nLine 227: They claim that they could reduce GaO(OH) electrochemically using a carbon paper (CP) cathode at a very small scale but do not comment about greenhouse gases (GHGs) produced during this electrolysis process. They do not give the percentage of recovered Ga from the very acidic Ga electrolyte containing aqueous sulfuric acid. It therefore cannot be labeled as a green chemistry process.", "reviewer_2": "The authors present a study on the circular hydrogen production process using liquid Ga, integrating photothermal corrosion and electrochemical reduction. The concept of regenerating Ga from GaOOH via an energy-efficient pathway is interesting and addresses challenges in sustainable hydrogen generation. However, several mechanistic ambiguities, insufficient experimental evidence, and incomplete data interpretation weaken the current manuscript’s impact. I recommend major revision to address the following concerns, which are essential for validating the proposed mechanisms and strengthening the practical implications of this work.\n1. The authors utilize sonication and photothermal effects to activate Ga surfaces for corrosion, achieving rapid H₂ production (96 mL/90 min). However, the manuscript lacks a critical comparison with the \"molecular scissors\" mechanism reported by Duan’s team (ACS Appl. Mater. Interfaces 2023, DOI: 10.1021/acsami.3c15841). In their work, liquid Ga-In eutectic (eGaIn) dynamically captures oxygen atoms from molecules (e.g., H₂O, CH₃OH) via spontaneous surface oxidation, enabling molecular clipping and H₂ production without external energy input (e.g., light or heat). Key distinctions to address: Add a comparative table/schematic highlighting these differences.\n2. Quantify H₂ production rates (mL/g·h) under comparable conditions (e.g., Ga mass, reaction time) to demonstrate the trade-offs between energy input (light vs. none) and efficiency.\n3. Discuss whether combining photothermal activation with Duan’s spontaneous exfoliation (e.g., using eGaIn instead of pure Ga) could enhance durability.\n4. The single-cycle reduction analysis (EDS/morphology) does not demonstrate long-term recyclability. Multi-cycle experiments (≥10 oxidation-reduction loops) with XPS/EIS are required to assess Ga’s structural stability, surface passivation, and activity decay. Without cyclic data, claims of sustainability may lack credibility.\n5. The alternative alkaline dissolution-electrodeposition route is mentioned while not quantitatively compared to the acidic electrochemical reduction method. A life cycle assessment (LCA) or energy density analysis (kWh/kg Ga) is needed to justify the superiority of the acidic pathway in terms of energy efficiency and cost.\n6. The cost-saving argument relies on vague assertions rather than quantifiable metrics. A levelized cost of hydrogen (LCOH) model incorporating Ga recycling efficiency, energy inputs, and material loss rates must be included to substantiate economic feasibility.\nOther small suggestions：\nIn Situ Characterization: Use in situ synchrotron XRD or AFM to track GaOOH→Ga phase transformation and interfacial dynamics during reduction.\nByproduct Identification: Analyze electrolytes via HPLC/ion chromatography to evaluate reaction selectivity and side products.\nCyclic Performance Testing: Quantify Ga mass loss and H₂ yield decay over ≥5 cycles to assess practical scalability.", "reviewer_3": "This study presents a novel circular H₂ production process based on the photothermal oxidation of liquid Ga and its electrochemical regeneration, contributing to the advancement of sustainable hydrogen energy technologies. However, further validation of the regenerated Ga’s H₂ production ability, the circular efficiency’s practical significance, and the appropriateness of the light source is required. Addressing these concerns through additional experimental evidence would significantly enhance the credibility of the study and strengthen its applicability for real-world implementation.\n1.The manuscript emphasizes the role of Ga particles in photothermal conversion leading to temperature rise, but a control experiment with halogen lamp illumination without Ga particles (equivalent to Fig. S2) is missing. To quantitatively assess the contribution of photothermal heating by Ga, additional data should be provided.\n2. The study employs a halogen lamp as a solar light simulator, but its wavelength range (350–3000 nm) includes strong near-infrared (NIR) components, making it unsuitable for accurately mimicking visible solar radiation. To improve reliability, experiments using a xenon lamp with a 400–800 nm cut filter should be conducted, as this setup is commonly used in photocatalytic research. The inclusion of xenon lamp-based data is essential for validating the claims made in the manuscript.\n3. The manuscript suggests that the GaOOH/Ga heterostructure plays a key role in the photothermal oxidation process, but no control experiment directly verifies this claim. To confirm its significance, an additional experiment should be conducted where H₂ production from bulk Ga is tested in 1M HCl (a condition where GaOOH cannot form). This comparison would strengthen the argument for GaOOH’s role in the reaction mechanism.\n4. While the manuscript demonstrates the electrochemical reduction of GaOOH to metallic Ga, the H₂ production capability of the regenerated Ga has not been evaluated. To confirm the feasibility of the proposed circular process, the following additional assessments are necessary:\n4-1.H₂ evolution tests using the regenerated Ga, including electrochemical HER activity measurements and gas chromatography analysis of H₂ yield.\n4-2.Structural and compositional analysis of the regenerated Ga using XRD, XPS evaluate its purity and surface oxidation state.\nThese evaluations would provide essential evidence that the regenerated Ga maintains its reactivity and can be reused effectively."}, "author_response": "Response to referees\nR1. Reviewer #1 (Remarks to the Author):\nThe oxidation of gallium to its oxyhydroxide is described, with release of hydrogen. Ultrasonication\nand 70 C were required before a photothermal reaction using a halogen lamp. The efficiency is low and\nrequires at least an hour, depending on wavelength. Generating hydrogen in sunlight can detonate in\nair. The reaction relies on 5N gallium and the oxyhydroxide product is not a commodity compound. It\nmust all be reduced back to gallium metal, more than negating the energy benefit of the hydrogen\nrelease. It would be very difficult and expensive to cost effectively scale up to even 1 kg of H2. I do not\nrecommend publication in Nature Communications. Some additional comments ordered by line\nnumber/appearance are included below.\nAns: We thank the reviewer for the time spent on our manuscript. We addressed your comments below.\nIn addition, we would like to clarify that our process exclusively generates hydrogen at ambient pressure\nand a low temperature of 90 °C, without oxygen evolution, thereby eliminating the risk of detonation.\nWe also demonstrated that the generated GaOOH can be reduced back into metallic Ga at above 95%\n(please see R1-5), enabling a circular process with a round-trip efficiency of 57.6%. Therefore, GaOOH\nshould not be regarded as a commodity product, but rather as an intermediate state of Ga within the\nprocess.\nRegarding the scalability and economic of the process please see the techno economic analysis Table\nS1 and Figures S16 and S17 and reply to reviewer R2-6 for the levelized cost of hydrogen (LCOH) for\ntwo different scenarios (1 cycle and 24 cycles per day). The 24 cycles per day include the use of both\nsunlight and halogen lamps.\nWe amended the following sentence in the Introduction section lines 51-54, page 2 for better clarity:\n“These phenomena sustain the absorption of energy from the light source, continuously producing H\n2\nfrom water.” To “These phenomena enable sustained energy absorption from the light source,\ncontinuously generating H₂ at temperatures below the boiling point of water at atmospheric pressure,\nwithout oxygen evolution, thereby eliminating the risk of detonation.”\nR1-1 Line 63: The authors say that alloys containing aluminum produce aluminum oxide compound\nthat require high energy input to reduce to produce primary aluminum. Yet, this is an important\ntechnology to the world’s economy and is the source of gallium needed for their reaction, produced as\na minor byproduct.\nAns: Thank you for your comment. The production of aluminum metal is an important technology for\nthe world’s economy and is the main source of gallium. However, the novelty of our process presents\nconsiderable advantages, as compared to traditional reactions involving Al to Al(III) for H production.\nlower consumed energy. Aluminium oxide is much more stable material to reduce in comparison to\nGa(OOH). Our process allows the electroreduction of the formed Ga oxyhydroxide back to Ga metal in\nan aqueous solution under standard room temperature conditions and using standard equipment.\nWe added the following to clarify this point in the Introduction lines 61-63, page 2 “In contrast, the\nreduction of Ga(III) species can be performed in aqueous media with standard electrochemical\nequipment and using relatively low energy conditions.”\nmore efficient in producing GaO(OH) than are photothermal generated Ga nanospheres. For their\ncurrent work, the authors produce polydisperse Ga nanospheres by sonication and the nanospheres are\ninefficient, producing much heat.\nThe authors report a 12.9% maximum efficiency for their hydrogen production but don’t show how this\nwas calculated and whether they included the energy input needed before and during the reaction.\nAns: Thank you for your comment.\nOur process involves the synthesis of gallium nanospheres (Ga⁰) in ethanol (see Methods), followed by\nthe photothermal oxidation reaction in water. We therefore cannot consider Ga nanorods, as they\nalready represent an oxidized form of Ga.\nThe electrochemical reduction efficiency is now detailed lines 104 to 108 (page 3): “Considering the\nmolar masses of Ga as 69.723 g/mol and H₂ as 2.016 g/mol, for 1 kg of Ga an amount of H₂ equal to\n43.36 g can be produced with the potential energy of 1.44 kWh28. The recovery energy for the\nelectrowinning of Ga is approximately 2.50 kW/kg29. Thus, the round-trip efficiency is 1.44 kWh/2.50\nkWh × 100 = 57.6% for our circular process, considering natural solar irradiation as a free source of\nenergy.”\nThe 12.9% efficiency value refers to the overall circular efficiency for H₂ production, including the\nelectrochemical reduction process. The photothermal conversion efficiency of Ga nanospheres was\nreported as 25.33 ± 7.42% in reference 14 (DOI: 10.1039/c8nr08296k). This was presented in lines 147-\n148 (page 4) as follows: “Considering the requirement of 57.6% electrochemical reduction efficiency,\nthis amounts to the overall circular efficiency of 12.9% for H2 production.”\nWe considered a light conversion to heat efficiency of 22.4% for the Ga particles following reference\n18, 26 and 30, and added the following for clarity (lines 144-147, page 4): “This experiment highlights\nthe efficient light absorption of Ga particles, consistent with previous studies showing a comparable\nphotothermal conversion efficiency comparable to gold nanoparticles of 20% to 30%18,26,30. We use a\nrather conservative value of 22.5% in our calculations.”\nThus, the overall efficiency is 22.4% x 57.6% ≈ 12.9%\nThe overall circular efficiency of 12.9% for our system compares well with other established renewable\nhydrogen technologies:\n Photovoltaic + Electrolysis: Single-junction solar modules (~20%) coupled with commercial\nelectrolysers (60–70% including overpotentials) result in an overall efficiency of 12-15%.\n Direct Photochemical Water Splitting: Well-studied systems such as TiO₂-based photocatalysts\ntypically operate at <5% efficiency.\n Multi-junction “Artificial Leaf” Systems: Advanced laboratory designs, such as Nocera’s\nartificial leaf, report efficiencies of approximately 10%.\nTherefore, despite being at a proof-of-concept stage, our technology is already comparable to the best\ncommercially relevant approach (PV + electrolysis) and surpasses direct photocatalytic water-splitting\nmethods, while uniquely offering seawater compatibility and circular gallium regeneration.\nReferences for review only:\nH. Song, S. Luo, H. Huang, et al., ACS Energy Letters 2022, 7, 1043.\nD. Hansora, R. Mehrotra, E. Noh, et al., Nat. Commun. 2025, 16, 4186.\nGaNS are then transformed into nanorods upon oxidation the gallium nanorods are therefore an oxidized\nproduct of these nanospheres.\nR1-3 Line 74: Authors claim that Ga is environmentally friendly as it is primarily recovered as a\nbyproduct of existing mining operation.\nThey cite Ref. 44, which describes various methods used in recovering Ga from the mining waste called\n“Bayer liquor”, also known as “red mud”, one of the world’s most polluting industrial wastes.\nAns: Thank you for your comment. In the manuscript, we compared the environmental friendliness of\nGa-based processes to mercury-based processes such as the chlor-alkali process, which historically was\na major source of environmental contamination. In this regard, Ga can be considered environmentally\nfriendly. Furthermore, gallium has very few dedicated mining operations, thereby limiting its\nenvironmental impact and providing opportunities for the valorisation of industrial waste and e-waste.\nTo clarify this, we amended the following in lines 74-78, page 2. We amended the following: “Gallium\nis also environmentally friendly, distinct from mercury-based processes, and is primarily recovered as\na byproduct of existing mining operations” to “Compared to mercury-based processes such as the chlor-\nalkali process, the Ga-based process presented in this study is more environmentally benign due to the\nlow toxicity of Ga.”\nR1-4 Line 92: Authors report that the reaction of Ga with water remains highly efficient even in saline\nenvironment, such as seawater.\nOnly aqueous NaCl was tested. Seawater should be tested since it contains other metallic impurities\nsuch as iron and vanadium that could make recovery of pure Ga difficult (see Ref. 44).\nAns: Thank you for your comment. As suggested, we performed an experiment with seawater. Seawater\nproperties vary greatly depending on the collection location and conditions. We used surface seawater\ncollected from Eastern Suburbs of Sydney, Australia, from a relatively high pollutant area. We\nperformed a reaction using the collected seawater, which was filtered using a 0.45 µm syringe filter\nbefore the reaction. The results are presented below and reflect a slightly reduced evolution rate of H\n2\nas compared to simulated seawater, likely due to the formation of foam-like surface materials, possibly\ndue to organics contamination present in the seawater, which may reduce the rate of reaction.\nRegarding the recovery of Ga after reaction with collected seawater, nearly all the Ga has been\nconverted to GaOOH, with a yield of 98.4% of the produced H . The recovery of Ga would therefore\n2\nbe performed in a different electrolyte than seawater. As the product GaOOH is a solid, rinse techniques\ncould be used to remove soluble contaminants prior to electro-recovery. For the detail on recovery\nplease look at our response to your following comment R1-5.\nSupplementary Figure 3. Hydrogen yield (% of theoretical) from collected seawater using Ga\nparticles (640 nm average size).\nWe added the following in the main text in lines 151-155, page 4 ”To demonstrate that our process does\nnot require a preliminary desalination step, H production was further evaluated using nearshore sea\n2\ncollected seawater (See Methods and Supplementary Fig. 3), yielding 98.4% of the theoretical\nmaximum after 180 min using the same conditions that resulted in the same by-products as the DI water\nprocess. Although the reaction proceeded at a reduced rate, possibly due to organic contaminants, the\nhigh yield indicated that collected seawater did not impede the overall completion of the reaction.”\nWe added the following in the Methods: “A seawater sample was collected from nearshore waters at\nthe beach face in the eastern suburbs of Sydney, Australia, and filtered through a 0.45 µm syringe filter\nprior to use.”\nR1-5 How much of the 0.2 g of 99.999% Ga is recovered in their process? Given the 5N expense, the\nrecovery would have to be near 100% to back up their “circular” claim.\nAns: Thank you for your comment. We have performed the electrochemical reduction of the GaOOH\nback to Ga.\nAfter the electro-recovery, we weighted the cathode substrate, collected the detached Ga in the electro-\nrecovery bath, and based on the weight difference and measurement uncertainty, we calculated the\nrecovery of Ga metal above 95%.\nIn addition, since we use a closed system, very limited loss of gallium is expected, as the non-reduced\nGa(III) species remain in the electrolyte solution and can therefore participate in the next recovery cycle,\nfurther limiting any loss.\nWe added the following in the manuscript lines 283-291, page 9: “Following H generation, Ga was\n2\nsuccessfully recovered from the photothermal reactor via electrochemical reduction, with a recovery\nefficiency of approximately 95% by weight relative to the initial Ga used (Supplementary Fig. 11). The\nreported 95% recovery yield is likely a conservative estimate, partly due to potential substrate\n2\ndegradation. Additionally, detached Ga droplets may have partially redissolved in the acidic electrolyte\nduring recovery, further contributing to mass loss.”\nWe added the following in the Methods section: “To quantify Ga recovery, the carbon paper electrode\nwas weighed in its dried state both before and after the electro-recovery process. In addition to the Ga\ndeposited on the carbon paper, any Ga droplets that detached during electroreduction and accumulated\nin the electrolyte bath were also collected, dried, and weighed. The total recovered mass was calculated\nas the sum of Ga retained on the carbon paper and Ga collected from the bath.”\nWe also added Figure S11 that shows photographs of the carbon paper substrate before and after\nelectroreduction, the detached Ga in the electroreduction bath as well as SEM images of the recovery\nGa, as below:\nSupplementary Figure 11. Electro-recovery of Ga. (a) and (b) Photographs of the carbon paper\nbefore and after Ga recovery, respectively. (c) SEM image showing Ga deposited on the carbon paper.\n(e) Top-view photograph showing reduced Ga that detached from the carbon paper and accumulated in\nthe electroreduction bath. (f) SEM image of the Ga recovered from the bath after detachment from the\ncarbon paper.\nR1-6 What’s the yield of hydrogen when no light is used for the entire reaction rather than cycling\non/off? In Figure 3, the % hydrogen released would be much more informative than mL of hydrogen,\nwhich depends on the masses used. Also in Figure 3, specify the mW/m^2. The amount of light is said\nto be “6 suns of concentrated sunlight”. Was the light concentrated/focused? The flux needs to be\ncompared to the standard AM 1.5G.\nalso added labels directly in the respective graphs for clarity as below:\nFigure 3. c, Hydrogen yield as a function of the Figure 3. c, H 2 yield (% of theoretical) as a\nGa particles size distributions under light function of the Ga particles size distributions\nirradiation with a power density of 600 under light irradiation with a power density of\nmW/cm². 600 mW/cm².\nPrevious Figure 3c Updated Figure 3c\nIn response to your question about light concentration: the light was not concentrated, as a halogen lamp\nwas used as the light source for the photothermal process. The light intensity was controlled by adjusting\nthe distance between the reactor and the light source, as presented in Figure S2.\nTo compare with the standard AM 1.5G, we also performed experiments using a sun simulator (SunLite\n11002-2, Abet Technologies, USA) equipped with a 100 W Xe arc lamp and an AM 1.5G filter. Please\nsee the results below:\nWe added the following in the main text, lines 236-244, page 8: “An experiment was also conducted\nusing a solar simulator equipped with an AM 1.5G filter (Supplementary Fig. 8). The light source was\nset to deliver an irradiation intensity of 600 mW cm-² (6 suns) for comparison purposes. Under these\nconditions, the reactor temperature reached an average of 59 ± 3 °C, and the hydrogen yield reached\n99.4% after 360 min. This corresponds to a reaction time that is 4.5 times longer than that required\nunder halogen light irradiation at the same intensity (Fig. 2d), yet it still achieved a yield close to the\ntheoretical maximum. In comparison, pure thermal heating at 60 °C resulted in only a 21% yield after\n300 min (Fig. 3f). The experiments show the effectiveness of light absorption in the visible region than\nin the deep infrared as the halogen lamp has less irradiation intensity in the infrared than the Sun. For\nshowing the compatibility of our system for direct implementation into practical solar thermal\napparatus, an additional experiment was conducted using concentrated natural sunlight (Supplementary\nFig. 9), during which the reactor temperature reached 79.5 °C and a H yield of 98.4% was achieved\n2\nafter 120 min.”\nSupplementary Figure 8. Hydrogen production from DI water and Ga particles (640 nm average\nsize), and using a sunlight simulator light source. (a) Hydrogen yield (% of theoretical). (b)\nTemperature profile inside the reactor.\nThe following was added in the Methods Section: “A sunlight simulator (SunLite 11002-2, Abet\nTechnologies, USA) equipped with a 100 W Xe arc lamp and an AM 1.5G filter was also used as a light\nsource.”\nRegarding the yield of H when no light is used, we performed and additional experiment, enclosing\n2\nthe reactor in an enclosed box without using light or heat, and using the same quantity of Ga particles\nand experimental conditions, for direct comparison with previous experiments.\nThe results are as follows: After 48 hours, we produced 15 mL of H which correspond to 15.54% of\n2\nmaximum theoretical yield.\nWe added the following in the Main text lines 159-162, page 5: “In order to highlight the effectiveness\nof photothermal influence on the process, a control experiment was conducted without light irradiation\nor heating, where the reactor was enclosed providing isolation from incident light and heat. Under\notherwise identical conditions, including the same mass of Ga particles, H production reached 15 mL\n2\nafter 48 hours, corresponding to 15.5% of the maximum theoretical yield.”\nWe added the following in the Methods section: “Additionally, controlled experiments were conducted\nin the absence of light and heat sources, with the photothermal reactor enclosed in an enclosed box to\neffectively shield the reactor from light and heat interference.”\nR1-7 It would be helpful to include the oxygen EDX map in Figure 4c.\nAns: Thanks, we have updated Figure 4f to address your comment and added the oxygen mapping as\nbelow. As can be seen there is no oxygen on the surface of liquid metal droplets. It is only seen at a\nvery low intensity on carbon paper.\nPrevious Figure 4f\nR1-8 How continuous is their hydrogen generation? One gets the impression it’s a labor intensive,\nexpensive and inefficient batch process. The supplementary video is clearly sped up and the factor is\nnot given. How much gallium would be needed for continuous supply to a 100W fuel cell, e.g. for an\nhour?\nAns: Thank you for your comment. The reviewer is correct that we are demonstrating a batch process,\nhowever, it could be automated by following mature established solar-thermal process methodologies.\nThis is outside the scope of this paper but will be further investigated in the future.\nThe video was not speedup on our software or after upload on the Nature’s portal. Nevertheless, we\nhave recorded a new video of the reaction showing the whole apparatus and a close-up on the reactor\nwith a visible timer in the background. We have replaced Video S1.\nTo answer your question “How much gallium would be needed for continuous supply to a 100W fuel\ncell, e.g. for an hour”, we conducted the following calculations:\n A 100 W fuel cell (low temperature fuel cell) running for one hour requires 100 watt-hours\n(Wh) of energy.\n Using the lower heating value (HHV) of hydrogen (39.4 kWh per kilogram), this corresponds\nto approximately 6.0 grams of hydrogen if we consider an efficiency of the fuel cell of 50-55%.\n Our reaction is as follows: 2 Ga + 4 H₂O → 2 GaOOH + 3 H₂\n We would need 2 moles of Ga for 3 moles of H (corresponding to approximately 6 g of H )\n2 2\n 2 moles of Ga correspond to 2 x 69.72 = 139.44 g.\n We measured and calculated a maximum yield of reaction of 99.44% (Figure 3c)\n We would need approximately 139.44 g of gallium (138.66 g calculated) which is only ~23.5\nmL of liquid Ga in total.\nR1-9 Line 227: They claim that they could reduce GaO(OH) electrochemically using a carbon paper\n(CP) cathode at a very small scale but do not comment about greenhouse gases (GHGs) produced during\nthis electrolysis process. They do not give the percentage of recovered Ga from the very acidic Ga\nelectrolyte containing aqueous sulfuric acid. It therefore cannot be labeled as a green chemistry process.\nAns: Thanks. We did not make any claim that our process qualifies as “green chemistry”. We described\nthe novel process as a pathway for “green hydrogen” (which is according to the definition of green\nhydrogen which is production of hydrogen from renewable sources like sunlight), not green chemistry.\nor other greenhouse gases at the cell itself. The only climate-relevant emissions are therefore the CO₂\nembedded in the electricity that drives the rectifier, which depends on the carbon intensity of the power\nsupply.\nIn addition, as the reviewer pointed in R1-3, Ga is mined as a by-product of bauxite or zinc refining\nintroducing additional life-cycle burdens (ore beneficiation, acid manufacture, spent electrolyte\nneutralisation) that we have not quantified. If, however, the electrolyser is driven by renewable\nelectricity and gallium is recycled at high efficiency (above 95% after the first cycle as we demonstrated\nin your previous comment R1-5), the hydrogen product itself can meet the “green hydrogen” definition\neven though the overall process is not fully benign from a green-chemistry standpoint, a claim that we\nhave not made.\nThe authors present a study on the circular hydrogen production process using liquid Ga, integrating\nphotothermal corrosion and electrochemical reduction. The concept of regenerating Ga from GaOOH\nvia an energy-efficient pathway is interesting and addresses challenges in sustainable hydrogen\ngeneration. However, several mechanistic ambiguities, insufficient experimental evidence, and\nincomplete data interpretation weaken the current manuscript’s impact. I recommend major revision to\naddress the following concerns, which are essential for validating the proposed mechanisms and\nstrengthening the practical implications of this work.\nAns: We thank the reviewer for the valuable time spent on our manuscript.\nR2-1. The authors utilize sonication and photothermal effects to activate Ga surfaces for corrosion,\nachieving rapid H₂ production (96 mL/90 min). However, the manuscript lacks a critical comparison\nwith the \"molecular scissors\" mechanism reported by Duan’s team (ACS Appl. Mater. Interfaces 2023,\nDOI: 10.1021/acsami.3c15841). In their work, liquid Ga-In eutectic (EGaIn) dynamically captures\noxygen atoms from molecules (e.g., H₂O, CH₃OH) via spontaneous surface oxidation, enabling\nmolecular clipping and H₂ production without external energy input (e.g., light or heat).\nKey distinctions to address: Add a comparative table/schematic highlighting these differences.\nAns: Thank you for your comment. The work by Duan et al. (ACS Appl. Mater. Interfaces 2024, 16, 3,\n4212–4221) describe important mechanisms of liquid metal reactivity and “molecular scissors” effect\n(https://pubs.acs.org/doi/10.1021/acsami.3c15841).\nDuan’s system emphasizes passive, interface-driven reaction of eutectic gallium-indium liquid alloy\n(EGaIn). Their process operates without external energy input (light, heat, or sonication), relaying on\nthe inherent surface activity of the Ga-In interface.\nIn contrast, our work employs pure gallium and enhances its surface reactivity through a combination\nof sonication, which increases the material’s surface area, and photothermal activation, enabling rapid\nhydrogen production.\nWe have included the work of Duan et al. and provided a comparison, as follows (lines 163-169, p.5):\n“It is important to consider the usage of nanoparticles for efficiently absorbing light in the process.\nPrevious interesting work by Duan et al. reported the inherent reactivity of liquid metals in aqueous\nenvironment, characterizing the behavior as “molecular scissors” activity, which included the ability to\ngenerate H 30. In this study, a bulk eutectic alloy of Ga and indium (EGaIn) was immersed in DI water,\n2\nand H bubbles were observed. The reported H production rate was ~0.13 mL g-¹·h-¹ of H at 40 °C.\n2 2 2\nIn comparison, our photothermal process with Ga achieved ~27.5 mL g-¹·h-¹ of H₂ at 48 °C, which is >200\ntimes greater H production (Fig. 3c).”\ntime) to demonstrate the trade-offs between energy input (light vs. none) and efficiency.\nAns: Thank you for your comment, we have performed an additional experiment using 0.2 g of gallium\nparticles (similar quantity and size that for our previous experiments), and recorded the H generated\n2\nwhen the reactor that enclosed in a box, shielded from electromagnetic interference including light.\nWe added the following in the Main text lines 159-162, page 5: “In order to highlight the effectiveness\nof photothermal influence on the process, a control experiment was conducted without light irradiation\nor heating, where the reactor was enclosed providing isolation from incident light and heat. Under\n2\notherwise identical conditions, including the same mass of Ga particles, H production reached 15 mL\n2\nafter 48 hours, corresponding to 15.5% of the maximum theoretical yield.”\nWe added the following in the Methods section: “Additionally, controlled experiments were conducted\nin the absence of light and heat sources, with the photothermal reactor enclosed in an enclosed box to\neffectively shield the reactor from light and heat interference.”\nWe also conducted comparative experiments using EGaIn bulk and particles to benchmark against the\nwork of Duan et al. The comparison graph of the H production rate is provided in the following\n2\ncomment (R2-3).\nR2-3. Discuss whether combining photothermal activation with Duan’s spontaneous exfoliation (e.g.,\nusing eGaIn instead of pure Ga) could enhance durability.\nAns: Thank you for your comment, we performed additional experiments using 0.2 g of bulk EGaIn or\nEGaIn particles under similar light irradiation conditions and the results are as below:\nWe have added the following figure comparing the H₂ production rates in mL g-1·h-1 of liquid metals\n(Ga and EGaIn) under light irradiation, thermal activation, and in an isolated system (Faraday box).\nWe added the following supplementary Figure S5 as below:\nSupplementary Figure 5. Comparison of H₂ production rates under various experimental\nconditions using bulk and particles of Ga and EGaIn. The control condition was conducted in a\nFaraday cage without light irradiation or external heating. All other conditions involved either thermal\nor photothermal activation using light irradiation at 600 mW/cm².\nThe average size of the EGaIn particles was 400 nm.\nWe added the following text in the Main text, lines 170-177, p. 5: “For further comparison, the EGaIn\nparticle system was also tested to achieve ~76.5 mL g-¹·h-¹ at 600 mW/cm2. Although the reaction rate\nwas lower for the EGaIn systems, as compared to pure Ga, the yield, which was calculated based on the\ntheoretical amount of Ga in the alloy, reached 21% after 36 h and 90% after 4.25 h for the bulk and\nparticulate EGaIn, respectively. These results are consistent with both the mechanism proposed by Duan\net al. and the reaction pathway described in our study. A comparison of H production rates is presented\n2\nin Supplementary Fig. 5.”\n24.5 wt% In) was prepared by melting the two metals on a hot plate for 2 hours. Indium (99.999%\npurity) was sourced from Indium Corporation (USA). EGaIn particles were then produced by\nsonication, following a procedure similar to that used for synthesizing Ga particles.”\nR2-4. The single-cycle reduction analysis (EDS/morphology) does not demonstrate long-term\nrecyclability. Multi-cycle experiments (≥10 oxidation-reduction loops) with XPS/EIS are required to\nassess Ga’s structural stability, surface passivation, and activity decay. Without cyclic data, claims of\nsustainability may lack credibility.\nAns: Thank you for your comment. We are able to consistently reduce Ga(III) back to metallic Ga using\nwell-established and widely applied electro-recovery methods for metal ions. As our system contains\nonly 5N purity Ga, deionised water, and an acidic electrolyte, the recovered gallium does not exhibit\nany signs of degradation.\nDue to the liquid nature of Ga in our photothermal reactor, there is no risk of it becoming trapped in an\nunfavourable crystal structure that would hinder the oxyhydroxidation reaction, or limit the activity of\nthe Ga particles.\nIn response to your suggestion and to improve clarity, we have added the following sentence to the\nmain text (lines 322-324, p.10): “In addition, due the liquid state of Ga under operating conditions inside\nthe photothermal reactor, the regenerated metal avoids crystallographic trapping, ensuring sustained\noxyhydroxidation reactivity upon recycling.”\nWe have performed an additional experiment using the recovered Ga after one cycle of oxidation. The\nresults are presented in a new Supplementary Figure S15 below:\nSupplementary Figure 15. Hydrogen production yield (% of theoretical) from DI water and\nrecovered Ga particles using a halogen light source.\nWe have added the following in the main text (lines 313-316, p.10): “After electroreduction of the\nGa(III), the recovered Ga was processed into particles following the same sonication procedure and\nreintroduced into the photothermal oxidation reactor. The H yield using the recovered Ga was 99.4%\n2\n(Supplementary Figure 15).”\na new Supplementary Figure S12:\nSupplementary Figure 12. XRD patterns of the recovered Ga after electroreduction. (a) Recovered\nGa on carbon paper substrate. (b) Recovered Ga that detached from the carbon paper during\nelectroreduction.\nWe have added the following in the Main text (lines 288-291, p.9): “XRD analysis of both the Ga\nrecovered on the carbon substrate and the detached Ga after a reaction cycle revealed the characteristic\nbroad peak of amorphous Ga, with no detectable Ga oxide crystalline by-products or alloying phases\n(Supplementary Fig. 12), confirming that the Ga remained chemically unaltered during the reaction.”\nR2-5. The alternative alkaline dissolution-electrodeposition route is mentioned while not quantitatively\ncompared to the acidic electrochemical reduction method. A life cycle assessment (LCA) or energy\ndensity analysis (kWh/kg Ga) is needed to justify the superiority of the acidic pathway in terms of\nenergy efficiency and cost.\nAns: Thank you for your important comment. As the product of the reaction is GaOOH, the acidic\ndissolution route is preferred for using low concentration of acid. In our experiments we demonstrated\nthe high stability of GaOOH in NaOH and KOH. In contrast the use of HCl 2 M was able to dissolve\nGaOOH into Ga3+ ions effectively. Please see your R2-6’ s comment for our LCA and LCOH models.\nWe added the following in the main text (lines 306-310, p.10): “Prior to electro-recovery, GaOOH was\ndissociated in 2 M HCl at 90 °C for 6 hours to convert it into a soluble Ga³⁺ species via acid-mediated\nleaching, allowing electroreduction to proceed in the same reaction medium. While alkaline pathways\nare often employed for Ga electro-recovery, the dissolution of GaOOH under such conditions is less\nfavourable, as GaOOH remains stable over several days (see Supplementary Fig. 14).”\nSupplementary Figure 14. Photograph of GaOOH dispersed in various electrolytes: 2 M NaOH,\nThe 2 M HCl solution became visibly clear after 6 hours, indicating complete dissolution of GaOOH,\nwhereas the alkaline solutions retained suspended solid phases.\nWe added the following in the Methods for clarity “Prior to electro-recovery, GaOOH was treated with\nR2-6. The cost-saving argument relies on vague assertions rather than quantifiable metrics. A\nlevelized cost of hydrogen (LCOH) model incorporating Ga recycling efficiency, energy inputs, and\nmaterial loss rates must be included to substantiate economic feasibility.\nAns: Thank you for your important comment, we performed the energy density analysis (kWh/kg Ga)\nand calculated levelized cost of hydrogen (LCOH) as describe below (added in the Supporting\nInformation as a discussion). We considered the total gallium inventory as constant from cycle to cycle,\nas gallium can be recovered at a high rate, and the unrecovered fraction remains retained within the\nreactor between cycles.\nWe added the following in the Supporting Information:\nTechno-Economic Analysis\nLevelized Cost of Hydrogen (LCOH)\nThe calculation of the Levelized Cost of Hydrogen (LCOH) is equivalent to spreading every discounted\ncost the plant incurs over its lifetime across the total discounted value of the mass of H₂ it produces.\nThe LCOH expresses the minimum H sale price for which the present value of revenues exactly offsets\n2\nthe present value of all costs over the plant’s economic life, commonly referred to as the break-even\nprice of H .\nmass of H each year. Every cost stream (CAPEX, OPEX, end-of-life) is converted to the same\n2\nannualised base, then divided by the corresponding annual H output. All values are handled in real\nof July 2025 (1 AUD = 0.66 USD) and discounted at a project-specific real rate (r). Solar electricity\nproduction is assumed during the day.\nCapital is annualised with the familiar capital-recovery factor (CRF):\n2\n𝑟(1+𝑟)𝑛\n𝐶𝑅𝐹 = (S1)\n(1+𝑟)𝑛−1\nso that the annualised capital charge is equal to the total capital expenditure (CAPEX) multiplied by the\nCRF. Fixed and variable operating expenses are summed directly into a single annual term\n𝑂𝑃𝐸𝑋 = 𝐶 +𝐶 +∑ 𝑐 𝐹 (S2)\n𝑙𝑎𝑏𝑜𝑢𝑟 𝑚𝑎𝑖𝑛𝑡𝑒𝑛𝑎𝑛𝑐𝑒 𝑖 𝑖 𝑖\nwhere c and q are the unit cost and annual consumption of each variable input (electricity, water, etc.).\ni i\nPeriodic replacements, carbon electrodes, desiccant beds, lamp bulbs, are treated as present-value lump\nsums, discounted to year 0 and redistributed as an equivalent annual cost C via the CRF.\nrepl\nCombining all annualised costs gives the governing expression\n𝐿𝐶𝑂𝐻 =\n𝐶𝐴𝑃𝐸𝑋∙𝐶𝑅𝐹+𝑂𝑃𝐸𝑋+𝐶𝑟𝑒𝑝𝑙\n(S3)\n365 𝑀𝐻2,𝑑 𝐶𝐹\nwhere CF is the capacity factor.\nEnergy density analysis\nTo calculate the energy density analysis, we considered the electric-energy demand for the GaOOH →\nGa electrowinning step (2.50 kWh kg-Ga-1), and included compression to 200 bar and night-time lamp\noperation if required.\nResults of this calculation are summarised in the following table for three scenarios: (1) a single Ga\nproduction/regeneration cycle per day, (2) 12 production/regeneration cycles per day, and (3) 24\nproduction/regeneration cycles per day including nighttime operation.\nTable S1. Techno-economic assumptions and results for three hydrogen production scenarios.\nScenario 1 represents a low-throughput system with 1 kg/day H₂ production and 1 cycle per day.\nScenario 2 represents a medium-throughput system with 12 kg/day H₂ production and 12 cycles per\nday. Scenario 3 represents a high-throughput system with 24 kg/day H₂ production and 24 cycles per\nday, including nighttime operation.\nScenario 1 3 2\n3Main Assumptions\nH production rate (kg/d) 1 12 24\nNumber of cycles per day 1 12 24\nGallium price ($) 1,060 1,060 1,060\nPlant life (yrs) 20 20 20\nCapacity factor 95% 95% 95%\nReal discount rate 7% 7% 7%\nLabour cost - - -\nCost of electricity ($/kWh) 0.05 0.05 0.14\nResults\nElectricity – Ga Recovery (kWh/yr) 19,938 239,258 478,515\nElectricity – Compression (kWh/yr) 1,387 16,644 33,288\nElectricity – Night lamps (kWh/yr) - - 23,117\nAnnual H production (kg H /yr) 347 4,161 8,322\n2 2\nH production energy intensity (kWh/kg H ) 61.50 61.50 64.28\n2 2\nGa reduction energy intensity (kWh/kg Ga reduced) 2.50 2.50 2.50\nCAPEX ($) 181,307 220,341 257,342\nOPEX ($/yr) 3,745 16,162 46,582\nLCOH ($/kg) 39.59 5.77 7.97\nand regeneration cycle per day.\nSupplementary Figure 17. Breakdown of the levelized cost of hydrogen (LCOH) for 12 production\nand regeneration cycles per day.\nSupplementary Figure 18. Breakdown of the levelized cost of hydrogen (LCOH) for 24 production\nand regeneration cycles per day.\nof our proof‑of‑concept circular process for H production, at this preliminary stage, we performed a\n2\ntechno‑economic analysis (see Supplementary Information). It presents levelized cost estimates for H\n2\nper kilogram. It’s important to note that these costs are likely to improve further as the technology\nmatures”\nOther small suggestions：\nR2-7. In Situ Characterization: Use in situ synchrotron XRD or AFM to track GaOOH→Ga phase\ntransformation and interfacial dynamics during reduction.\nAns: Thank you for your comment. We were unable to perform in situ XRD using either a fine capillary\nor a flat sample holder due to the strong X-ray absorption by water in the electrolyte. The reduction of\nGa(III) to metallic Ga is well documented, and since our system contains only Ga species and aqueous\nelectrolyte, the process corresponds to a classical electroreduction of metal ions in aqueous media.\nParticularly, the electro-recovery of Ga has been investigated in great details in the following references\nin our manuscript:\n29 “Illés, I. B. & Kékesi, T. The production of high-purity gallium from waste LEDs by combining\nsulfuric acid digestion, cation-exchange and electrowinning. J. Environ. Chem. Eng. 11, 110391\n(2023).”\n42 “Hong, B. et al. EQCM study on the electrochemical redox behavior of gallium in alkaline solution.\nHydrometallurgy 194, 105344 (2020).”\n43 “Liu, Z., Guo, X., Tian, Q. & Xu, Z. Electrochemical behavior of gallium electrodeposition and\ninhibition of hydrogen evolution reaction in alkaline electrolyte. J. Appl. Electrochem. 53, 847-860\n(2023).”\n44 “Zhao, Z., Yang, Y., Xiao, Y. & Fan, Y. Recovery of gallium from bayer liquor: a review.\nHydrometallurgy 125-126, 115-124 (2012).”\nR2-7. Byproduct Identification: Analyze electrolytes via HPLC/ion chromatography to evaluate\nreaction selectivity and side products.\nAns: Thank you for your comment. We previously analysed the concentration of free Ga species in the\nphotothermal reactor using ICP-OES, as shown in Supplementary Fig. 4, and detected less than 10 ppm\nof elemental Ga in solution after the reaction. As 5N-purity Ga was used, no other species were detected,\nand the chemical reaction proceeded selectively toward the formation of GaOOH.\noxidation as analyzed by ICP-OES. (1): Bulk Ga under light irradiation, (2): Ga particles at 90°C in\nDI water, (3): Ga particles under light irradiation at 600 W/cm2 in DI water, and (4): Ga particles under\nlight irradiation at 600 W/cm2 in saline water. The Na to Cl ratio remained at 0.67, corresponding to the\nstoichiometric ratio of NaCl in the saline solution, indicating that no Cl participated in the reaction. The\nGa concentration in the solution slightly increased when NaCl was used, but it remained at a similar\nlevel to that observed in experiments without saline water, and the amount of released Ga was\nsignificantly lower than the concentrations of both Na and Cl.\nR2-7. Cyclic Performance Testing: Quantify Ga mass loss and H₂ yield decay over ≥5 cycles to assess\npractical scalability.\nAns: Thank you for your comment. We have answered your comment in R2.4, and in R1-5.\nWe are recovering above 95% by weight of the oxidized Ga. We have then performed an oxidation\ncycle on the recovered Ga for the production of H . Due to the liquid nature of the Ga and Ga particles,\n2\nno activity loss can be considered and the production of H2 is directly related to the Ga mass and\nphotothermal reaction parameters.\nWe added the following in R1-5 and in the manuscript lines 283-291, p.9: “Following H generation,\nrecovery efficiency of approximately 95% by weight relative to the initial Ga used (Supplementary\nFig. 11). The reported 95% recovery yield is likely a conservative estimate, partly due to potential\nsubstrate degradation. Additionally, detached Ga droplets may have partially redissolved in the acidic\nelectrolyte during recovery, further contributing to mass loss.”\nbefore and after Ga recovery, respectively. (c) SEM image showing Ga deposited on the carbon paper.\n(e) Top-view photograph showing reduced Ga that detached from the carbon paper and accumulated in\nthe electroreduction bath. (f) SEM image of the Ga recovered from the bath after detachment from the\ncarbon paper.\nWe have performed an additional experiment using the recovered Ga after one cycle of oxidation. The\nresults are presented in a new Supplementary Figure S15 below:\nSupplementary Figure 15. Hydrogen production yield (% of theoretical) from DI water and\nrecovered Ga particles using a halogen light source.\nGa(III), the recovered Ga was processed into particles following the same sonication procedure and\nreintroduced into the photothermal oxidation reactor. The H yield using the recovered Ga was 99.4%\n2\n(Supplementary Figure 15).”\nR3 This study presents a novel circular H₂ production process based on the photothermal oxidation of\nliquid Ga and its electrochemical regeneration, contributing to the advancement of sustainable hydrogen\nenergy technologies. However, further validation of the regenerated Ga’s H₂ production ability, the\ncircular efficiency’s practical significance, and the appropriateness of the light source is required.\nAddressing these concerns through additional experimental evidence would significantly enhance the\ncredibility of the study and strengthen its applicability for real-world implementation.\nAns: Thank you for your valuable comments and time spent on our manuscript.\nR3.1The manuscript emphasizes the role of Ga particles in photothermal conversion leading to\ntemperature rise, but a control experiment with halogen lamp illumination without Ga particles\n(equivalent to Fig. S2) is missing. To quantitatively assess the contribution of photothermal heating by\nGa, additional data should be provided.\nAns: Thank you for your comment, we measured the temperature inside the reactor containing DI water\nwithout Ga particles for comparison.\nTo clarify the photothermal contribution of Ga particles, we performed the suggested control\nexperiments using the same halogen lamp setup without Ga particles as shown in the updated\nSupplementary Figure S2b.\nAt the shortest distance to the light source, the contribution of Ga particles to photothermal heating was\nless distinguishable and exhibited the lowest temperature difference due to the high irradiance and the\nlarge thermal buffer provided by the DI water volume. This large water volume, relative to the mass of\nGa particles, was intentionally selected to prevent overheating or boiling, while maximising the light\nirradiation-to-temperature ratio for optimal photothermal performance.\nHowever, as the light intensity decreases with distance, the photothermal conversion properties of the\nGa particles becomes more evident, leading to a measurable and consistent temperature advantage over\nthe particle-free control.\nTo clarify this point, we added the following in the Main text of the manuscript, lines117-128, p.4: “The\nphotothermal conversion properties of Ga particles were demonstrated by measuring the temperature\ninside the reactors with and without Ga particles under identical light irradiation conditions and volume\nof DI water (Supplementary Fig. 2). The maximum temperature difference, relative to the DI water-\nonly control, peaked at 16 °C at a light intensity of 540 mW/cm2 and gradually decreased with\ndecreasing the light intensity. While at 600 mW/cm2, both conditions exhibited closer temperatures,\nwith the difference of 3 °C, due to the high irradiance and large thermal buffer provided by the DI water.\nThe use of a large water volume, relative to the mass of Ga particles was intentional to prevent\noverheating or boiling while maximizing the light-to-temperature conversion efficiency for accurate\nevaluation of the photothermal performance for H production.”\n2\nPrevious Fig S2\nUpdated Figure S2\nR3.2 The study employs a halogen lamp as a solar light simulator, but its wavelength range (350–3000\nnm) includes strong near-infrared (NIR) components, making it unsuitable for accurately mimicking\nvisible solar radiation. To improve reliability, experiments using a xenon lamp with a 400–800 nm cut\nfilter should be conducted, as this setup is commonly used in photocatalytic research. The inclusion of\nxenon lamp-based data is essential for validating the claims made in the manuscript.\nAns: Thank you for your comment. Please see R1-6 for the details of using a sunlight simulator\nequipped with a 100 W Xe arc lamp and an AM 1.5G filter.\nIn R1-6, we have performed an additional experiment and added the following supplementary figure\nS8:\nWe added the following in the main text, lines 236-247, p.8: “An experiment was also conducted using\na solar simulator equipped with an AM 1.5G filter (Supplementary Fig. 8). The light source was set to\ndeliver an irradiation intensity of 600 mW cm-² (6 suns) for comparison purposes. Under these\nconditions, the reactor temperature reached an average of 59 ± 3 °C, and the hydrogen yield reached\n99.4% after 360 min. This corresponds to a reaction time that is 4.5 times longer than that required\nunder halogen light irradiation at the same intensity (Fig. 2d), yet it still achieved a yield close to the\ntheoretical maximum. In comparison, pure thermal heating at 60 °C resulted in only a 21% yield after\n2\n300 min (Fig. 3f). The experiments show the effectiveness of light absorption in the visible region than\nin the deep infrared as the halogen lamp has less irradiation intensity in the infrared than the Sun. For\nshowing the compatibility of our system for direct implementation into practical solar thermal\napparatus, an additional experiment was conducted using concentrated natural sunlight (Supplementary\nFig. 9), during which the reactor temperature reached 79.5 °C and a H yield of 98.4% was achieved\n2\nafter 120 min.”\nWe added the following Figure as Supplementary Figure 7:\nSupplementary Figure 8. Hydrogen production from DI water and Ga particles (640 nm average\nsize), and using a sunlight simulator light source. (a) Hydrogen yield (% of theoretical). (b)\nTemperature profile inside the reactor.\nWe also conducted an additional experiment using concentrated natural sunlight, focused onto the\nphotoreactor using a Fresnel lens to form a beam of 1 cm², and included the results as Supplementary\nFigure S9.\nSupplementary Figure S9. Hydrogen production from DI water and Ga particles (640 nm average\nsize), and using concentrated natural sunlight. (a) Hydrogen yield (% of theoretical maximum). (b)\nPhotographs of the reactor showing the illuminated area (left) and temperature profile, with an average\ntemperature of 79.5°C (right).\nusing concentrated natural sunlight (Supplementary Fig. 9), during which the reactor temperature\nreached 79.5 °C and the H yield of 98.4% was achieved after 120 minutes”\nTechnologies, USA) equipped with a 100 W Xe arc lamp and an AM 1.5G filter was also used as a light\nsource.” And\n“For concentrated natural sunlight experiments, a 30 × 30 cm Fresnel lens was positioned indoors to\nfocus natural sunlight onto the reactor, producing a concentrated illumination area of 1 × 1 cm. As the\nlens and reactor were indoors, the incident natural sunlight experienced a fourfold reduction in\nirradiance. The experiment was conducted on an intermittently cloudy winter day.”\nR3.3 The manuscript suggests that the GaOOH/Ga heterostructure plays a key role in the photothermal\noxidation process, but no control experiment directly verifies this claim. To confirm its significance,\nan additional experiment should be conducted where H₂ production from bulk Ga is tested in 1M HCl (a\ncondition where GaOOH cannot form). This comparison would strengthen the argument for GaOOH’s\nrole in the reaction mechanism.\nAns: Thank you for your comment, we performed an additional experiment in 1 M HCl using the same\ntotal solution volume as in our previous experiments.\nIn the 1 M HCl solution, the nanodroplets quickly merged back together into a large drop of shiny liquid\nmetal without the formation of oxyhydroxides and oxides. The characteristic shine of the liquid metal\nrepresents the presence of a clean, oxide-free metallic surface. After 24h, we recorded 32 mL of H\n2\nproduced.\nThis was attributed to the reaction of Ga metal in HCl at a temperature of 90°C following this equation:\n2Ga + 6HCl → 2GaCl + 3H\n3 2\nThese results supported our reaction mechanism that Ga spontaneous oxyhydroxide in water of neutral\npH via photothermal corrosion, continuously producing H . See below for the picture of the particles\n2\nmerging as a large liquid Ga drop in 1 M HCl solution.\nWe added the following in the manuscript (lines 252-261, p.8): “To confirm the role of GaOOH in the\nreaction mechanism, we conducted an experiment under acidic conditions (1.0 M HCl) using Ga\nparticles with an average size of 640 nm irradiated with a 600 mW cm-² halogen light (Supplementary\nFig. 10). Under this condition GaOOH cannot be formed. Here, H was initially produced. However,\n2\nthe Ga particles rapidly coalesced, and no formation of GaOOH was observed, consistent with the\nunfavorable conditions for GaOOH formation in acidic media. After 24 h, 32 mL of H₂ was produced,\nand approximately 83% of the initial Ga mass was recovered from the reactor. The initial H generation\n2\nwas attributed to the classic acidic dissolution models of Ga in HCl (Supplementary Fig. 10).”\nlarger liquid Ga droplet, and the proposed reaction of H production in these conditions:\nsize) under halogen light irradiation (600 mW/cm2). (a) Picture showing the coalescence of Ga\nparticles in 1 M HCl. (b) Hydrogen yield (% of theoretical) in 1 M HCl, compared to equivalent\nconditions in DI water, and proposed primary respective reactions.\nR3.4 While the manuscript demonstrates the electrochemical reduction of GaOOH to metallic Ga, the\nH₂ production capability of the regenerated Ga has not been evaluated. To confirm the feasibility of the\nproposed circular process, the following additional assessments are necessary: H₂ evolution tests using\nthe regenerated Ga, including electrochemical HER activity measurements and gas chromatography\nanalysis of H₂ yield.\nAns: Thank you for your comment, this was answered in R2-4 and R2-7. We are able to consistently\nreduce Ga(III) back to metallic Ga using well-established and widely applied electro-recovery methods\nfor metal ions. As our system contains only 5N purity Ga, deionised water, and an acidic electrolyte,\nthe recovered gallium does not exhibit any signs of degradation.\nDue to the liquid nature of Ga in our photothermal reactor, there is no risk of it becoming trapped in an\nunfavourable crystal structure that would hinder the oxyhydroxidation reaction, or limit the activity of\nthe Ga particles.\nIn response to your suggestion and to improve clarity, we have added the following sentence to the\nmain text (lines 321-324, p.10): “In addition, due the liquid state of Ga under operating conditions inside\nthe photothermal reactor, the regenerated metal avoids crystallographic trapping, ensuring sustained\noxyhydroxidation reactivity upon recycling.”\nrecovered Ga particles using a halogen light source.\nWe have added the following in the main text (lines 313-316, p.10): “After electroreduction of the\nGa(III), the recovered Ga was processed into particles following the same sonication procedure and\nreintroduced into the photothermal oxidation reactor. The H yield using the recovered Ga was 99.4%\n2\n(Supplementary Figure 15).”\nWe have also performed additional XRD characterisation on the recovered gallium as below, and added\na new Supplementary Figure S11:\nSupplementary Figure 11. XRD patterns of the recovered Ga after electroreduction. (a) Recovered\nGa on carbon paper substrate. (b) Recovered Ga that detached from the carbon paper during\nelectroreduction.\nWe have added the following in the Main text (lines 288-291, p.9): “XRD analysis of both the Ga\nrecovered on the carbon substrate and the detached Ga after a reaction cycle revealed the characteristic\nbroad peak of amorphous Ga, with no Ga oxide detectable crystalline by-products or alloying phases\n(Supplementary Fig. 12), confirming that the Ga remained chemically unaltered during the reaction.”\nas the reaction exclusively produces H₂ according to reaction (1): 2Ga + 4H₂O → 2GaOOH + 3H₂.\nFigure 1 b, GC-MS chromatogram of the collected gas produced during photothermal oxidation.\nThe HER activity during Ga electro-recovery is presented in Figure 4 “e, Linear sweep voltammetry of\nGa(III) in 1.0 M HCl on carbon paper (CP) electrode substrates.” As below:\nFigure 4 e, Linear sweep voltammetry of Ga(III) in 1.0 M HCl on carbon paper (CP) electrode\nsubstrates.\nTh simultaneous occurrence the gallium electrode reaction (GER) and HER was discussed in the main\ntext: “A standard electrochemical reduction setup was used in an acidic electrolyte at a low\nconcentration (1.0 M HCl), as schematically presented in Fig. 4d. The Ga electrode reaction (GER) was\nobserved simultaneously with the hydrogen evolution reaction (HER) as seen in Fig. 4e on carbon paper\n(CP) electrode substrates41-43.”\nR3.5. Structural and compositional analysis of the regenerated Ga using XRD, XPS evaluate its purity\nand surface oxidation state.\nThese evaluations would provide essential evidence that the regenerated Ga maintains its reactivity and\ncan be reused effectively\nAns: Thank you for your comment. We performed XRD analysis on the gallium recovered on the\ncarbon paper substrate, as well as on the gallium that detached from the substrate during recovery\n(Figure S12). As the regenerated material is pure liquid gallium and the reaction does not depend on a\nspecific crystalline phase or fixed oxidation state, it can be readily reused in subsequent cycles, as\ndemonstrated in our response to your previous comment R3-4.\ntechniques. The use of 5N-purity Ga, deionised water, and an acidic electrolyte ensured reproducibility,\nwith no observable degradation. Furthermore, the liquid state of Ga in the photothermal reactor prevents\nentrapment in crystalline phases that could otherwise hinder the oxyhydroxidation process. We\ntherefore confidently concluded that the regenerated Ga does not exhibit any loss of performance, as it\nremains a pure liquid metal.\nWe have added the following discussion in the main text (please also see R2-4):\nWe have added the following sentence to the main text (lines 321-324, p.10)): “In addition, due the\nliquid state of Ga under operating conditions inside the photothermal reactor, the regenerated metal\navoids crystallographic trapping, ensuring sustained oxyhydroxidation reactivity upon recycling.”\nWe have also performed additional XRD characterisation on the recovered gallium as below, and added\na new Supplementary Figure S11:\nSupplementary Figure 12. XRD patterns of the recovered Ga after electroreduction. (a) Recovered\nGa on carbon paper substrate. (b) Recovered Ga that detached from the carbon paper during\nelectroreduction.\nWe have added the following in the Main text (lines 288-291, p.9): “XRD analysis of both the Ga\nrecovered on the carbon substrate and the detached Ga after a reaction cycle revealed the characteristic\nbroad peak of amorphous Ga, with no detectable Ga oxide crystalline by-products or alloying phases\n(Supplementary Fig. 12), confirming that the Ga remained chemically unaltered during the reaction.”\nThis study presents a novel strategy for cyclic hydrogen production by leveraging the\nphotothermal oxidation reaction of liquid gallium. It has demonstrated innovation in hydrogen\nproduction through low temperature photothermal drive using fresh water and seawater\n(without desalination), thereby offering valuable insights for the advancement of sustainable\nhydrogen energy systems. Overall, the work exhibits potential impact. It is recommended for\nacceptance after minor revisions. The authors are suggested to further elaborate on the\nfollowing aspects\nR4.1. The author should make a more systematic comparison of the advantages of this method\nwith existing green hydrogen production technologies (such as water electrolysis,\nphotocatalysis, etc.) in terms of efficiency, energy consumption, operating conditions, etc., to\nhighlight its technical advantages and application potential.\nANS: Thank you for your comment. In response to your suggestion, we have added a\ncomparison between established green hydrogen technologies in the Supplementary\nInformation, in the Techno-Economic Analysis section as follows:\n“The presented low temperature, rapid photothermal oxidation of liquid gallium enables\ncircular H production with an overall efficiency of 12.9%. For comparison, water electrolysis,\n2\nthe most established approach to green H production, achieves 60-70% efficiency but remains\n2\nelectricity-intensive, requiring between 50 to 55 kWh per kg H and producing H at $5-10 per\n2 2\nkg (at current rates) when powered by renewables 1. Thermochemical cycles convert heat into\nH but operate at high temperatures, often above 550 °C and involve multi-stage reactions with\n2\noverall efficiencies approaching of 20-45% 2. Photocatalytic water-splitting routes directly\nutilize sunlight but typically exhibit solar-to-hydrogen efficiencies below 10 %, constrained by\nlow photon-to-charge conversion, material instability, and limited visible-light absorption 3.”\nWe also amended the following in the main manuscript lines 47-50 to better highlight the\nadvantage of this method:\n“In this work, we present an energetically favorable circular method for H generation utilizing\n2\nonly liquid metal gallium (Ga) and its photothermal oxidation as the core technology.”\nto\n“In this work, we present an energetically favorable circular method for H generation utilizing\n2\nonly liquid metal gallium (Ga). Low temperature photothermal oxidation produces H₂ under\nartificial or natural sunlight, and an aqueous electrochemical reduction step regenerates Ga\ncompleting the cycle.”\nAdditional Supplementary References:\nProduction Via Water Electrolysis: A Techno-Economic and Environmental\nAssessment. Energy Environ. Sci. 15, 3583-3602 (2022).\nRecent Advances, Challenges, and Future Perspectives. ACS Energy Lett. 7, 1043-1065\n(2022).\nHydrogen Efficiency over 10%. Nat. Commun. 16, 4186 (2025).\nR4.2. The surface products of liquid metals do not fall off spontaneously in a static water\nenvironment. Is ultrasonic treatment necessary in use? If additional energy input is required, it\nshould be taken into account in the technical and economic analysis, and its impact on overall\nenergy efficiency and economy should be evaluated.\nANS: Thank you for your comment. The sonication equipment and energy consumption costs\nwere included in the LCOH calculations.\nFor better clarity we added the following in Supplementary Information, in the Techno-\nEconomic Analysis section:\n“Sonication equipment and energy costs to produce Ga particles are included in the LCOH\ncalculation. The energy consumption of the sonication step accounts for ~1% of OPEX.”\nDuring the photothermal reaction, no agitation was applied as the GaOOH layer naturally\nexfoliates from the liquid Ga due to the lack of polarisation interface on the liquid metal\ninterface. This phenomenon was described in the manuscript lines 80-87:\n“Finally, the non-polar surface of liquid Ga resists adhesion, preventing most materials from\nsticking under normal conditions15,22,23. This property is especially important in our circular\nprocess, as it enables the naturally formed oxyhydroxide to exfoliate from the surface of the\nliquid Ga. This natural exfoliation prevents the buildup of continuous oxidation and passivation\nof the surface, a problem commonly encountered with solid metals7,20,24. Instead, the formed\nGaOOH easily flakes off from the surface of liquid Ga, continuously exposing the bare liquid\nGa surface to further oxyhydroxidation reactions by water, leading to the generation of H .”\n2\nparticipated to the exfoliation of the GaOOH maintaining the liquid metal surface to stay in\ncontact to water molecules and light. See video added to SI.\nWe added the following in the manuscript to clarify the agitation mechanisms provided by the\nbubble generation, lines 137-138 “Additionally, the evolution of H₂ bubbles provided\nmechanical agitation to the system (Supplementary Video 1).”\nR4.3. The calculation basis for the hydrogen energy (1.44 kWh) mentioned in line 104 on page\n3 should be clearly listed. It is recommended to supplement the detailed calculation process\nand the calorific value data referred to.\nANS: The statement that “43.36 g of H₂ can be produced with a potential energy of 1.44 kWh”\nrefers to the chemical energy content of the hydrogen generated from 1 kg of Ga, based on\nstandard thermochemical data. From the reaction stoichiometry, 1 kg of Ga yields 43.36 g of\nH .\ncontained in 43.36 g of H is calculated as 0.04336 × 33.3 = 1.44 kWh.\n“The lower heating value of H (33.3 kWh kg-1) was used for calculating the potential energy\n2\nof the stoichiometric H produced from 1 kg of Ga, yielding 1.44 kWh (0.04336 × 33.3).”\ninfluence the reaction kinetics and the physicochemical properties of the resulting products. It\nis recommended to perform XPS or EDS mapping analysis on the GaOOH product post-\nreaction to determine the potential incorporation of elements such as Cl⁻ and Na⁺. Additionally,\nICP-MS or TOC analysis should be conducted on the seawater sample to identify specific\nconstituents and assess their possible effects on the reaction system.\nANS: Thank you very much for your comment. We performed additional analysis of the\nseawater using ICP OES and TOC analysis, and on the GaOOH product using SEM/EDX\nmappings.\nWe added the following to the manuscript lines 153-155: “As shown in Supplementary Fig. 4,\nthe collected seawater exhibits characteristic ionic composition, and the total organic carbon\nmeasured at 3.2 ± 0.3 ppm.”\nWe amended the following discussion in lines 155-158 for better clarity:\n“Although the reaction proceeded at a reduced rate, possibly due to organic contaminants, the\nhigh yield indicated that collected seawater did not impede the overall completion of the\nreaction.”\nTo\n“Although the reaction proceeded at a reduced rate, due to organic contaminants that absorb\nand scatter incident light and thereby reduce the effective irradiation reaching the gallium\nsurface, the high yield indicated that collected seawater did not impede overall completion.”\nWe added the following in the Methods, lines 425-427.\n“Total Organic Carbon (TOC) analysis: The TOC analysis was performed on the collected\nseawater sample using a Shimadzu TOC-L CPH instrument. The sample was filtered through\na 45 µm syringe filter and diluted three times prior to analysis.”\nSupplementary Figure 4. ICP-OES analysis of the collected seawater.\nAs suggested by the reviewer, we also performed additional SEM/EDX mappings of the\nGaOOH products after photothermal oxidation using the collected seawater. We added the\nfollowing in the Manuscript, lines 158-161:\n“Elemental mapping using SEM/EDX was performed on the GaOOH product after reaction in\nseawater to examine possible salt incorporation. Deposits of NaCl, calcium, and potassium are\ninitially detected (Supplementary Fig. 5). However, after washing with DI water, the salt\ndeposits are no longer present (Supplementary Fig. 6).”\nSupplementary Figure S5. SEM/EDX images of GaOOH after reaction using collected\nseawater.\nSupplementary Figure S6. SEM/EDX images of GaOOH after reaction using collected\nseawater after washing with DI water.\nR4.5. In the reply manuscript, it was mentioned that \"concentrated sunlight\" was used for the\nexperiment. Please clearly state its power density (W/cm²) to facilitate the interpretation of the\nresults and practical promotion and application.\nANS.: We appreciate the reviewer’s request for clarification. Natural sunlight was concentrated\nusing a Fresnel lens (effective collection area ≈ 470 cm2) onto a 1 cm2 focal spot (≈470×). With\nan incident irradiance of 1.6 mW/cm2, this corresponds to a focused irradiance of ~750\nmW/cm2 at the time of measurement. The experiments were performed on an intermittently\ncloudy winter day, which extended the photothermal reaction time relative to clear-sky\nconditions. As the lens and reactor were indoors, the incident natural sunlight experienced a\nsignificant reduction in irradiance. The experiment was conducted on an intermittently cloudy\nwinter day.\nWe added the following to the manuscript, lines 258-262: “For showing the compatibility of\nour system for direct implementation into practical solar thermal apparatus, an additional\nexperiment was conducted using concentrated natural sunlight (Supplementary Fig. 9), during\nwhich the reactor temperature reached 79.5 °C and a H yield of 98.4% was achieved after\n2\n120 min.”\nTo:\n“For showing the compatibility of our system for direct implementation into practical solar\nthermal apparatus, an additional experiment was conducted using concentrated natural sunlight\n(Supplementary Fig. 9), during which the reactor temperature reached 79.5 °C and a H yield\n2\nof 98.4% was achieved after 120 min with peak irradiation of 750 mW/cm2.”"}, {"round": 2, "reviewer_comments": {"reviewer_3": "The authors have carefully addressed all of my previous comments.\nAdditional experimental evidence has been provided where requested, and these revisions have significantly improved the\nclarity and robustness of the manuscript.\nI now consider the manuscript suitable for publication in this journal and recommend acceptance.", "reviewer_4": "This study presents a novel strategy for cyclic hydrogen production by leveraging the photothermal oxidation reaction of\nliquid gallium. It has demonstrated innovation in hydrogen production through low-temperature photothermal drive using\nfresh water and seawater (without desalination), thereby offering valuable insights for the advancement of sustainable\nhydrogen energy systems. Overall, the work exhibits potential impact. It is recommended for acceptance after minor\nrevisions. The authors are suggested to further elaborate on the following aspects：\n1.The author should make a more systematic comparison of the advantages of this method with existing green hydrogen\nproduction technologies (such as water electrolysis, photocatalysis, etc.) in terms of efficiency, energy consumption,\noperating conditions, etc., in order to highlight its technical advantages and application potential.\n2. The surface products of liquid metals do not fall off spontaneously in a static water environment. Is ultrasonic treatment\nnecessary in use? If additional energy input is required, it should be taken into account in the technical and economic\nanalysis, and its impact on overall energy efficiency and economy should be evaluated.\n3. The calculation basis for the hydrogen energy (1.44 kWh) mentioned in line 104 on page 3 should be clearly listed. It is\nrecommended to supplement the detailed calculation process and the calorific value data referred to.\n4. Contaminants present in seawater, such as organic matter or inorganic ions, may influence the reaction kinetics and the\nphysicochemical properties of the resulting products. It is recommended to perform XPS or EDS mapping analysis on the\nGaOOH product post-reaction to determine the potential incorporation of elements such as Cl⁻ and Na⁺. Additionally, ICP-\nMS or TOC analysis should be conducted on the seawater sample to identify specific constituents and assess their possible\neffects on the reaction system.\n5. In the reply manuscript, it was mentioned that \"concentrated sunlight\" was used for the experiment. Please clearly state its\npower density (W/cm²) to facilitate the interpretation of the results and practical promotion and application."}, "author_response": ""}, {"round": 3, "reviewer_comments": {"reviewer_4": "The authors have exemplified professionalism and rigor throughout the revision process. They have not only provided\ncomprehensive, point-by-point responses to all review comments but have also substantially enhanced the quality of the\nmanuscript through their revisions. In summary, I am satisfied with the authors' revision work. Since review have been\nadequately addressed, I recommend acceptance of this manuscript."}, "author_response": ""}]

Decision letter