Photoelectrochemical water splitting cells at elevated pressure using BiVO_4 and platinized III-V semiconductor photoelectrodes

Domain: physics
Published: 2025-12-13
Editorial outcome: major_revision

Abstract
Direct production of pressurized green hydrogen via photoelectrochemical water splitting reduces the need for mechanical compression and mitigates bubble-related losses. However, existing demonstrations have been limited to atmospheric pressure. Here, we bridge this gap by designing, constructing, and testing a high-pressure flow cell for photoelectrochemical water splitting using two configurations. In a back-illuminated BiVO4-based photoelectrochemical cell, increased pressure suppresses bubble evolution and alleviates photocurrent saturation under concentrated sunlight: at 10 suns, the photocurrent rises from ~3× at 1 bar to ~7× at 5 bar. Direct operando imaging of the electrode surfaces confirms that this improvement comes primarily from suppressed bubble evolution. Conversely, a front-illuminated platinized triple-junction III-V-based photoelectrochemical cell shows limited pressure dependence up to 8 bar due to its dispersed catalyst and long carrier diffusion length. These findings highlight the differing response of photoelectrochemical devices to elevated pressure and demonstrate a viable pathway toward scalable, high-pressure solar-driven hydrogen production.

Introduction
As the transition from fossil fuels to sustainable energy sources accelerates, solar water splitting emerges as a promising strategy to store intermittent solar energy in the form of hydrogen and other energy-dense molecules that are compatible with existing infrastructures1. Three main types of solar water splitting systems have been demonstrated: (1) photovoltaic-electrolyzer (PV–EC) configurations, where a PV cell is coupled with an electrolyzer, (2) photoelectrochemical (PEC) cells, which integrate light absorption and electrochemical reactions in a single device2, and (3) photocatalytic (PC) systems, where the reactions occur at the surfaces of suspended or immobilized particles. For PC water splitting systems, near-unity conversion yields have been achieved under UV irradiation3. Moreover, the feasibility of scale-up to larger areas using photocatalyst sheets was demonstrated with a 100 m2 outdoor prototype panel reactor system3–5, although the maximum solar-to-hydrogen (STH) efficiency was only ~0.76%5. A slightly higher STH efficiency of 1.21% was recently demonstrated with a PC water splitting system that utilizes an I⁻/I⁻ redox mediator. Employing MoSe-loaded halide perovskites (CH(NH)PbBr I) for H evolution and NiFe-layered double hydroxide-modified BiVO for O evolution, this 700 cm2 system demonstrated stable operation over one week under natural sunlight6. PV-EC systems are more technologically mature and show high STH efficiencies, but suffer from thermal losses when PV panels operate at higher temperature7,8. These losses can be mitigated by thermally coupling the PV and electrolyzer cells, where water acts as both a coolant for the photoabsorber and as an electrolyte9,10. This has resulted in STH efficiencies greater than 20% at the device level when using solar concentration11, and up to 8.5% without solar concentration12. However, more than 30% of the generated heat is still lost11. PEC systems offer an alternative by directly integrating semiconductors and electrocatalysts in one single device, allowing for synergistic effects, such as improved reaction kinetics and mass transport, through system heating from thermalized and sub-band gap photons13–17. Recent laboratory-scale PEC devices (~1 cm2) have already demonstrated STH efficiencies of ~20%18,19, highlighting significant potential for efficient solar-to-fuel conversion.

With record-high STH efficiencies demonstrated, efforts are now focused on scaling up PEC water splitting systems. In general, PEC water splitting can be scaled by increasing the photoactive area per device20, by deploying a greater number of devices2,20,21, and/or by utilizing higher solar concentrations5,9,22. Among these strategies, the use of higher solar concentrations has been particularly promising for achieving economically viable devices with high power densities9,11,23. However, the photocurrent increases sub-linearly with solar concentration, causing the performance to plateau23–25. This sub-linear increase has also been reported in studies using monochromatic light under low irradiance (<0.4 kW m−2) to investigate charge transfer mechanisms26–28. While the exact cause is not always clear, several factors may be responsible, including ohmic losses (from the substrate, the electrolyte, or from gas bubbles)23,25, surface recombination28, bubble-induced light scattering29–31, and blocking of the PEC active area by bubbles31–33. Understanding and disentangling these effects is essential for optimizing system performance and enabling the efficient scaling of solar water splitting systems.

One approach to addressing performance saturation in PEC water splitting under concentrated solar irradiance is to operate the system at elevated pressure. This is especially relevant as gas bubbles can limit the performance when their volume fraction in the electrolyte becomes too high. The volume fraction of the gas bubbles can be effectively controlled by adjusting the operating pressure34. We recently reported multiphysics simulations showing that bubble-induced performance losses can be minimized by increasing the operating pressure of PEC devices to ~6–8 bar35. Furthermore, producing hydrogen directly at high(er) pressure aligns well with the requirements of most downstream processes (e.g., fuel cells, ammonia production, methanol synthesis), minimizing the need for energy-intensive mechanical compression36,37. Despite these advantages, all PEC water splitting demonstrations to date have been conducted at atmospheric pressure. As illustrated in Fig. 1a, this represents a clear research gap that needs to be addressed.

In this study, we experimentally demonstrate PEC water splitting at elevated pressure using a custom-designed high-pressure laminar flow cell. Two different PEC configurations were investigated. In the first configuration, a back-illuminated BiVO photoanode was employed in a PEC cell operating at pressures up to 8 bar under concentrated solar irradiance (up to 10 suns). At 10 suns, a significant photocurrent increase of ~40% was achieved when operating above 5 bar instead of at atmospheric pressure. This improvement is primarily attributed to the suppression of gas bubble formation on the photoelectrode surface. In the second configuration, a platinized triple-junction (3J) III-V photoelectrode was used in an integrated PEC cell. Interestingly, even under front illumination, we observed only a ~10% photocurrent reduction due to gas bubbles, and increasing the operating pressure had minimal impact on performance. We attribute this to the long carrier diffusion length in the III-V semiconductor and the high hydrophilicity of the surface, which facilitates bubble detachment. Furthermore, stability measurements reveal that an increase in operating pressure has no significant impact on the degradation of photoelectrode materials. Our quantitative findings reveal that the effects of increasing operating pressure on PEC cell performance are configuration-dependent, thereby offering valuable insights for scaling PEC water splitting devices.

Results
High-pressure flow cell for photoelectrochemical water splitting
A schematic of the custom-designed high-pressure PEC cell is shown in Fig. 1b, an exploded view of the cell design is shown in Fig. S1, and the digital photographs of the setup can be found in Figure S2. Further details on the cell design and construction are available in the Methods section. High pressure operation was accomplished by supplying compressed gas into the cell while regulating the outflow rate of gases with a back-pressure controller (Bronkhorst High-Tech, uncertainty: 0.2%). Note that this approach to regulate the pressure was chosen because it offers easy control and experimental convenience in a lab-based setting; in practical applications, PEC water splitting cells can self-pressurize through gas production during operation, requiring only a pressure relieve valve as a control mechanism. Prior to PEC experiments, the setup was tested for safe operation up to 8 bar (a); higher pressures have not been tested to comply with current safety guidelines. For illumination, AM1.5G (100 mW cm−2) simulated sunlight was used with an optional Fresnel lens to achieve solar concentrations up to 10 suns (see Supplementary Note S1 and Fig. S3 and Fig. S4).

Our high-pressure PEC cell features two key innovations. First, it enables laminar electrolyte flow between two parallel electrodes at elevated pressure, facilitated by an optimized flow distributor (as shown in Fig. S2c). Second, the cell includes two observation windows (see Fig. S1), allowing real-time in situ visualization of the evolving gas bubbles from multiple angles (see Fig. S6) at elevated pressure. To confirm that the electrolyte flow profile is laminar, we conducted particle imaging velocimetry (PIV) measurements. A detailed description of the measurements is provided in Methods and Fig. S5. Colormaps of electrolyte velocity at different operating pressures are shown in Fig. 2a. At a constant flow rate of ~4.6 mL s−1 (used for most of our experiments), the electrolyte flow field remains uniformly distributed between the two electrodes, with minimal variation across different operating pressures. Characteristic parabolic velocity (u) profiles are observed at 1, 3 and 5 bar (see Fig. 2b), with an average velocity of ~1 cm s−1. Beyond liquid flow visualization, our versatile cell design also enables monitoring of gas bubble dynamics from different viewpoints using multiple cameras (as shown in Fig. S6) at elevated pressure.

Back-illuminated single photoanode cell configuration
We first utilized the high-pressure flow cell (HPFC) described above to investigate BiVO photoanodes at elevated pressure and concentrated back-side illumination. The experimental setup consisted of a BiVO working electrode, a platinum mesh counter electrode, and an Ag/AgCl reference electrode. Undoped BiVO photoelectrodes (without co-catalysts) were fabricated via a previously reported electrodeposition method38, as detailed in the Methods section. The structural and optical properties of the synthesized BiVO are consistent with those of typical electrodeposited BiVO 38, as confirmed by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-vis spectroscopy and film profilometry (Figs. S7–S8).

Figure 3a shows the linear sweep voltammetry (LSV) curves measured at atmospheric pressure (1 bar) under various solar concentrations, with a final measurement at 1 sun (dashed curve) to confirm reproducibility. As expected, the photocurrent increases with higher solar irradiance. However, the increase is less pronounced than anticipated; for instance, under an irradiance of ~10 suns, the photocurrent is only ~3 times higher than that observed at 1 sun. To better illustrate the correlation between photocurrent and solar concentration, we extracted the photocurrent at 1.23 V versus the reversible hydrogen electrode (RHE) and normalized it to the value at 1 sun (Jc=x =Jc=1 ). The normalized photocurrents, plotted as red data points in Fig. 3b, reveal a saturation at higher solar concentrations. This behavior is consistent with previous reports in literature. For comparison, we include the data from Vilanova et al.23, who observe a similar trend (orange data points in Fig. 3b) despite significant differences in experimental conditions. Their study utilized α-Fe O photoelectrodes (active area ~6.25 cm2) in 1 M KOH (pH = 13.6) electrolyte. A comparable photocurrent saturation was also reported by Holmes-Gentle et al. for α-FeO and BiVO photoelectrodes under much higher solar concentrations (40 to 360 suns)25.

A possible explanation for the observed photocurrent saturation is the increased formation of gas bubbles at higher concentrations. Gas bubbles are known to block active sites on the photoelectrode39–42, reducing its effective catalytic area, and to scatter incident light29,33, diminishing the number of absorbed photons. While bubble-induced convection may enhance local mass transfer33,43, its effect is negligible at the relatively low current densities in this study (e.g., ~5 mA cm⁻² at 10 suns for unmodified (bare) BiVO photoanodes without hole scavenger). Since our experiments employed back-side illumination—where light does not pass through the gas bubbles before reaching the BiVO photoanode—the contribution of light scattering should also be minimal. We therefore hypothesize that photocurrent saturation is primarily due to electrode deactivation associated with the increased gas bubble coverage on the surface of BiVO at higher solar concentrations.

To test the hypothesis above, we performed the LSV measurements using an electrolyte containing 0.5 M sodium sulfite (Na SO ) as a hole scavenger. Due to its more favorable thermodynamics and kinetics44, sulfite oxidation replaces water oxidation, preventing gas bubble formation (see Supplementary Video 1). The normalized photocurrents (Jc=x =Jc=1 ) in the presence of hole scavenger are plotted as blue data points in Fig. 3b. The photocurrent increases with higher concentrations, and the increase is more pronounced compared to that in the absence of hole scavenger. Note that the normalization excludes the effect of reaction kinetics, as all values are expressed as multiples of their respective photocurrents at 1 sun. The absence of bubbles during Na SO oxidation accounts for the increased normalized photocurrent compared to water oxidation. The blue-shaded area in Fig. 3b highlights the disparity, which is primarily attributed to the surface coverage of gas bubbles.

Interestingly, even in the presence of a hole scavenger, the normalized photocurrent still does not increase proportionally with solar concentration. The purple-shaded area in Fig. 3b highlights the difference between the expected one-to-one photocurrent increase with solar concentration and the observed increase in the presence of hole scavenger. This observation suggests that factors other than gas bubble coverage limit the photocurrent of BiVO at higher concentrations. One possible limiting factor is mass transfer; however, this is unlikely to occur in our setup, as the electrolyte is continuously circulated at a relatively high flow rate of ~4.6 mL s−1, and varying the scan rate has an negligible effect on the LSV curves (see Fig. S9). Vilanova et al. suggested that substrate sheet resistance was a primary limiting factor in their study23, but this is unlikely to be the case for the current densities observed in our study. Using a previously established relationship22, we estimate that the potential drop due to substrate sheet resistance only increases from ~5 to ~20 mV as the solar concentration increases from 1 to 10 suns. In view of the relatively shallow profile of the J-V curve (Fig. S9), this will indeed have a negligible effect on the current density. We tentatively attribute this loss (represented by the purple-shaded area in Fig. 3b) to increased bulk recombination, which has indeed been reported for BiVO under higher light intensities28,45.

We briefly note that temperature variations during the LSVs under different solar concentrations can be ignored in our study as several measures were implemented to minimize its effect. First, the electrolyte was circulated using a rotary pump with a flow rate of ~4.6 mL s−1, resulting in an average linear flow velocity of ~1 cm s−1 between the two electrodes (see Fig. 2 for a visualization of the electrolyte flow velocity). Second, a ~5 min resting period was included between experiments to prevent continuous heating of the electrolyte. Indeed, the electrolyte temperature variation, measured using a K-type thermocouple (see Fig. S10a), was found to be less than 2.5 °C between the highest and lowest solar concentrations (see Fig. S10b).

We then investigated how pressure elevation affects the PEC performance of the back-illuminated BiVO photoanode. Higher pressures are expected to suppress gas bubble formation35, potentially enhancing the photocurrent, especially at higher solar concentrations. Figure 4a shows the normalized photocurrent of BiVO at 1.23 V vs. RHE at different pressures in a 0.1 M KP electrolyte (without hole scavenger). The corresponding LSVs under different concentrations are shown in Fig. S11a–c. While the photocurrent at 1 sun remains largely unaffected by pressure, the photocurrent increases significantly with pressure at higher solar concentrations. At ~3 suns, pressure elevation improves the normalized photocurrent by ~30%, and at ~10 suns, the enhancement even reaches ~50%. This pressure induced improvement in photocurrent is more clearly illustrated in Fig. 4b. At 1 sun, the photocurrent remains nearly constant at ~0.5 mA cm−2 across the pressure range of 1–8 bar. At ~3 suns, however, the photocurrent increases from ~0.9 mA cm−2 at 1 bar to ~1.2 mA cm−2 at 5 bar. The effect is more pronounced at ~10 suns, where the photocurrent increases from ~1.7 mA cm−2 at 1 bar to ~2.5 mA cm−2 at 5 bar. Furthermore, as shown in Fig. S11, the onset potential of the BiVO photoelectrode remains unchanged with varying operating pressure. These findings demonstrate a key advantage of operating PEC water splitting devices under elevated pressure, particularly at higher solar concentrations.

To determine if the pressure-induced enhancement in photocurrent stems from the suppression of bubble evolution (or a decrease in bubble size, see below), we performed the same measurements in the presence of a Na SO hole scavenger (so that no bubbles are generated), and the LSVs are presented in Fig. S11d. The normalized photocurrent remained unchanged with increasing pressure for all solar concentration factors (see Fig. 4c). This indeed confirms that the increase in photocurrent with pressure, observed in Fig. 4a, b, is due to the suppression of bubble evolution. This result is consistent with direct visual observations from our PEC cell, the design of which allows in situ monitoring of gas bubble evolution on the photoelectrode from multiple angles (front- and side-view, see the schematic illustration and photographs in Fig. S6). Representative images of gas bubbles on BiVO photoelectrode at 1 bar and 5 bar are shown in Fig. 4e, f, respectively, with corresponding videos available in Supplementary Videos 2–4. At 5 bar, noticeably fewer bubbles were observed compared to 1 bar, as evidenced by the images and videos. In addition, we quantified the average bubble size (D ) and number of bubbles (N ) under different pressures and solar concentrations, as shown in Fig. 4d. Both D and N decrease with increasing pressure, especially at higher solar concentrations. These findings confirm that the suppression of gas bubble evolution is the primary mechanism behind the improved photocurrent observed at elevated pressures.

It should be noted that while an increase in operational pressure effectively suppresses gas bubble evolution on the BiVO photoelectrode (see Fig. S12), the normalized photocurrent at 8 bar in the absence of a hole scavenger (~5.3) remains lower than that in the presence of scavenger (~6.8). We attribute this discrepancy to the resolution limits of our imaging setup. Although the images suggest “bubble free” conditions at 8 bar, nano- or micro-bubbles below our detection threshold may still be present and block a significant fraction of the electrode surface. To support this hypothesis, we estimated the Laplace pressure46,47 for bubbles of varying sizes (see Supplementary Note S2 and Fig. S13). For bubbles with a radius ≤ 0.2 µm, the Laplace pressure at bubble/electrolyte interface exceeds 8 bar, meaning such small-sized bubbles are unlikely to be suppressed under our experimental conditions. The detection threshold of the bubble radius with our microscopic camera lens is about 10 µm.

Although bubble nucleation events could not be directly resolved, recent work on bubble evolution on TiO micro-photoelectrodes showed that elevated pressure facilitates bubble detachment, primarily due to higher dissolved gas saturation48. Consistent with these findings, our measurements (Fig. 4e, f) revealed smaller and fewer bubbles under elevated pressure. This behavior can be tentatively explained by the formation of a supersaturated boundary layer (SSBL) of dissolved gas near the (photo)electrode at higher pressure. Our recent simulation showed that the SSBL facilitates bubble detachment from the surface of the (photo)electrode via Marangoni convection49. Additional contributing factors may include an increased Laplace pressure and a larger critical radius for nucleation at higher pressures, both of which inhibit initial bubble formation. Interfacial properties, such as the gas–liquid contact angle, may also play a role in pressure-dependent detachment dynamics, but direct experimental measurement of contact angles and nano/micro bubbles on semiconductor surfaces (e.g., BiVO photoelectrode) under elevated pressure remains technically challenging. High-resolution tools such as atomic force microscopy and spectroscopy techniques50–60 and molecular dynamics simulation61,62 will be essential, which are beyond the scope of this study. Furthermore, the porous morphology of our BiVO samples (see Fig. S7) introduces additional complexity, as it may lead to random spatial distribution of nanobubbles, making their detection even more complicated.

To further elucidate the effects of pressure on gas bubble evolution throughout the whole surface of the 2 cm × 2 cm BiVO photoelectrode, we conducted macroscopic imaging of oxygen bubbles. The experimental setup was modified accordingly (Fig. S14a) and introduced in the Methods section. At 1 bar, bubbles appeared randomly across the porous BiVO surface (Fig. S14b), with larger bubble sizes observed near the top of the electrode; we tentatively attribute this observation to the presence of hydrostatic pressure gradients. This insight is useful for the scale-up of PEC devices, as non-uniform bubble detachment may result in spatial variations in performance. However, at pressures above 2 bar, bubble presence was already negligible (Fig. S14b). This observation is consistent with our localized observations shown in Fig. 4e, f.

To investigate whether pressure elevation introduces any effect on the stability of the back-illuminated BiVO photoanode, potentiostatic experiments were performed with the same PEC cell at 1.23 V vs. RHE. Illumination intensities of 1 sun and 10 suns were applied. As shown in Fig. S15, the photocurrent decreases with time, indicating degradation; this is expected since our BiVO is uncatalyzed and has no protection layer25,63. However, the photocurrent decrease remains similar at different pressures, suggesting that pressure elevation has minimal impact on the degradation behavior of the BiVO photoelectrode. Moreover, no gas or liquid leakage was observed during the extended high-pressure operation, indicating reliable structural mechanical integrity and operational stability of the system.

A heat transfer model was developed to evaluate the thermal effect associated with the prolonged operation under higher solar concentration, as detailed in Supplementary Note S3 (see Fig. S16). The simulated results were validated against experimentally measured bulk electrolyte temperature, as shown in Fig. S17a. Despite its simplicity, this heat transfer model effectively captures the thermal trends observed during stability testing under ~10 suns illumination. Specifically, the measured bulk electrolyte temperature increased by approximately 5 °C at 1 bar and 3 °C at 5 bar under continuous illumination, while the model predicts a comparable rise of ~5 °C for both pressures. In addition, the simulation reveals that the maximum surface temperature rise of the photoelectrode can reach ~13 °C above ambient (Fig. S17b–d). While this value may be somewhat overestimated due to simplified boundary assumptions—namely, adiabatic treatment of the electrode sealing interfaces and the electrolyte boundary adjacent to the counter electrode—it nonetheless highlights the potential for localized heating at the photoelectrode surface. Such a temperature rise can indeed facilitate interfacial kinetics64 and could potentially accelerate material degradation under extended operation65. These findings emphasize the importance of effective thermal management in high-pressure PEC systems under concentrated sunlight. Advanced cooling strategies, such as those demonstrations performed by Haussener and co-authors9,11,25, can be implemented to prevent photoelectrode overheating under concentrated sunlight. Importantly, the consistent trends observed across under different pressures, both experimentally and in simulation, support the validity of our comparative performance analysis under elevated pressures.

Front-illuminated integrated PEC water splitting cell configuration
We now turn to the effect of pressure on the front-illuminated integrated PEC water splitting cell. In this configuration, front illumination is necessary due to the non-transparent contact layer on the backside of the PV cell. The photoelectrode is a platinized triple-junction (3J) III-V (GaInP/GaAs/Ge) PV cell, prepared as described in the Methods section. The digital photographs of the photoelectrode are shown in Fig. S18. The photoelectrode functions as a photocathode, where the nanoparticulate Pt catalyst facilitates the HER (2H+ + 2e− → H ) and an IrOx/TaOx/Ti mesh (mmo electrode, China) serves as the counter electrode catalyzing the oxygen evolution reaction (OER, 2H O → O + 4H+ + 4e−). An Ag/AgCl electrode was used as the reference. To optimize gas bubble imaging on the electrode surface, the working electrode was tilted at ~45° from the horizontal. This adjustment was necessary for the integrated PEC cell but not for the BiVO photoelectrodes due to three reasons. First, the area of the III-V photocathode is a lot smaller (~1/6) than that of the BiVO photoelectrodes. Second, the edges of the photocathode need to be protected with silicone resin to prevent direct contact with the aqueous solution (see Fig. S18). Third, keeping the photocathode tilted (e.g., at 45° from horizontal) reduces the density of bubble plume. The counter electrode (CE) was carefully placed to avoid shadowing effects from both the CE itself and from the oxygen bubbles that are generated at its surface. The 3J III-V PV cell is made of GaInP/GaAs/Ge junctions with varying bandgaps (1.8/1.4/0.7 eV) to absorb photons from the entire spectrum, including the infrared region66. To ensure effective illumination of the bottom Ge junction, the experiments were conducted with a dual light source solar simulator, with a halogen lamp providing the infrared part of the spectrum, as shown in Figure S19. Unless otherwise specified, a Fresnel lens was not used for these measurements. The schematic illustration of the experimental setup is shown in Fig. 5a. The PEC measurements were conducted following the protocols proposed by Ben-Naim et al. for III-V photoelectrodes67. LSV curves were scanned from a reverse bias (−0.24 V vs. RHE) to more positive bias with a scan rate of 20 mV s−1. The initial ~160 mV of each LSV scan was conducted without illumination to obtain the dark current. Measurements were halted when the current density approached 0 mA cm−2 to prevent surface oxidation associated with passing anodic current67.

Figure 5b presents the LSV curves of the platinized 3J III-V photoelectrode at various operating pressures. Measurements were initially conducted at 1 bar, followed by 3 bar, 5 bar, and 8 bar, with a final measurement at 1 bar to confirm reproducibility. Notably, the LSV curves are nearly identical for all pressures, indicating that an increase in pressure has no significant impact on photocurrent in this configuration. This finding is unexpected as images taken during the measurements (Fig. 5d–g) clearly indicate variations in bubble formation on the photoelectrode at different pressures, with corresponding videos available in Supplementary Videos 5–8. In other words, the lack of photocurrent limitation suggests that the presence of bubbles does not hinder performance. To confirm this, we conducted measurements in the presence of 0.5 M sodium persulfate (Na S O ) as an electron scavenger68,69, which eliminates gas evolution (Fig. 5h and Supplementary Video 9). The resulting LSV curves (Fig. 5c) also show no pressure dependence, and the photocurrents are similar to those measured in the absence of electron scavenger (Fig. 5b), further supporting the conclusion that gas bubbles do not significantly impact photocurrent in this system.

We also examined the behavior of bubbles over an extended operation period. We performed chronoamperometry at 1 bar and 8 bar, and the results are shown in Fig. 5i. To better describe the effect of bubble detachment to the photocurrent, we plot the ΔJ and Δt in the J-t curves in Fig. 5j; ΔJ represents the increase in photocurrent due to bubble detachment and Δt is the bubble growth time prior to detachment. While bubbles form and detach cyclically (more frequently at 1 bar vs. 8 bar, i.e., Δt < Δt , as expected), their impact on photocurrent (ΔJ) remains similar (~10-15%), suggesting that bubbles do not persistently block catalytic sites.

The lack of pressure-dependence on the photocurrent means that bubbles do not reduce light absorption or block catalytically active surface sites on the photoelectrode. We speculate that three factors contribute to this observation. First, the platinized 3J III-V photoelectrode exhibits high hydrophilicity (liquid contact angle ~35°, see Fig. S20). This means that bubbles detach more easily from the surface, minimizing coverage losses. Second, while bubbles can reflect and scatter light, previous studies suggest that the smooth interface of gas bubbles leads to minimal diffuse scattering29,33, with most reflected light being redirected away rather than lost. Additionally, due to refraction at the curved bubble surface, some of the redirected light can still be absorbed by other parts of the photoelectrode, rather than escaping entirely. Third, the Pt catalyst layer on the 3J III-V cell is very thin (~1 nm) and likely forms dispersed nanoclusters rather than a continuous film, as illustrated in Fig. 6. Even when some clusters are temporarily blocked by bubbles, the long carrier diffusion lengths in III-V semiconductors70,71 ensure that the photogenerated carriers can still reach Pt sites not covered by a bubble and thus contribute to the HER. Note that this is markedly different from metal oxide semiconductors like BiVO , which have diffusion lengths of 10–100 nm. Since this value is much shorter than the radius of a typical adhered gas bubble, nearly all the carriers generated directly below the gas bubble will recombine. Together, these factors explain why gas bubble formation in the platinized 3J III-V photoelectrode does not significantly reduce the photocurrent, even at elevated pressure, enabling efficient hydrogen production at 8 bar without performance loss.

We briefly note that the impact of solar concentration on the front-illuminated integrated PEC water splitting cell configuration was also investigated. The LSVs and the normalized photocurrent (Jc=x/Jc=1 ) are shown in Fig. S21. Unlike the case of BiVO , the photocurrent increases proportionally with illumination intensity, i.e., the photocurrent at 3 suns is ~3-times that at 1 sun. This observation further confirms that light absorption and bubble-induced losses are not limiting the photocurrent in our platinized 3J III-V photoelectrode.

Our findings suggest that the impact of gas bubble evolution on photocurrent is linked to the diffusion length of photoexcited charge carriers. Although light scattering by adhering bubbles does not significantly impact the photocurrent for photoelectrodes with long diffusion lengths, this is markedly different for short diffusion length materials, where most of the charge carriers generated directly below a surface-adsorbed bubble will recombine. In addition, for PEC cell designs where light must pass through bubble plume before reaching the photoelectrode, light scattering may still induce considerable optical losses30,35,72. Other important bubble-related effects, such as bubble-induced corrosion and localized photocorrosion due to bubble-induced light concentration33, are beyond the scope of this study. Further investigation using advanced techniques like scanning photocurrent microscopy (SPCM)31 is needed to fully assess these phenomena.

We will now discuss some general limitations of our study and provide an outlook for operating PEC devices at elevated pressure. First, in this initial demonstration of high-pressure PEC water splitting, only two representative and contrasting systems are selected: BiVO as a metal oxide photoanode under back-illumination and a platinized triple-junction III-V photocathode under front-illumination. These photoelectrodes were intentionally chosen as benchmark photoelectrodes since they have been extensively studied in PEC water splitting research and beyond16,17,66,73–77. They are relatively stable and have demonstrated adequately high photocurrent; this ensures that performance variations in our study can be attributed primarily to pressure-related effects rather than material-related factors, thus providing a reliable basis to assess their impact on device operation. At the same time, expanding this comparison to additional semiconductors, such as α-FeO and WO photoanodes or Sb S and Cu O photocathodes, would offer further insights into any material-specific pressure effects. These studies are part of our ongoing efforts and will be reported in due course.

On the system level, the benefits from operating PEC water splitting cells at elevated pressure should be ensured to outweigh the associated penalties. In addition to the suppression of bubble-induced losses, reducing or eliminating the need for downstream hydrogen compression represents a significant advantage. In practice, the elevated pressure PEC cell can be connected directly to high-pressure H tanks, solid-state H storage materials, or any applications that require elevated pressure operation (e.g., Haber-Bosch or hydrogenation reactors), as illustrated in Fig. S22. For instance, when compressing H to 875 bar, doing so by starting from 1 bar consumes ~80% more energy than starting from 20 bar (~4.8 vs. ~2.7 kWh kg−1)78–80. In principle, the system can be self-pressurized via gas evolution during operation, potentially avoiding additional energy input for pressurization other than the thermodynamic penalty of higher-pressure operation. In the pressure range of up to 20 bar, our recent analysis has shown that the benefits largely outweigh the thermodynamic penalty35. Nevertheless, we acknowledge that achieving and maintaining elevated pressure introduces technical and economic challenges, including the need for reinforced cell housings, thicker optical windows, enhanced sealing, pressure-rated tubing, and precise back-pressure control—all of which would increase capital and maintenance costs. A comprehensive techno-economic analysis is needed to evaluate the economic trade-offs and inform possible scale-up configurations.

Another factor not yet fully considered here is gas separation. We utilized precise fluid dynamic control of the electrolyte flow to ensure laminar condition and keep the evolved O and H gas bubbles closer to their respective electrodes, effectively minimizing gas crossover (see flow velocity colormaps in Fig. 2a). However, the purity of the separated gases has not been quantified. Such a membraneless separation strategy via hydrodynamically guided flow has shown promise in simplifying system architecture81,82, but we suspect that the incorporation of ion exchange membranes (e.g., Nafion® 117) might be required to achieve the highest purity and to guarantee safe operation. Moreover, the use of ion exchange membranes would enable the PEC cell to operate under differential pressure between the anodic and cathodic compartments. Prior studies have shown that hydrogen permeability through Nafion membranes remains largely independent of the applied pressure differential (up to 5 bar), indicating that pressure differences have a negligible effect on gas crossover83.

In summary, we operated photoelectrochemical water splitting devices at elevated pressure up to 8 bar using two cell configurations. For the back-illuminated BiVO -based PEC cell, a saturation in photocurrent is observed under AM1.5G illumination and 1 bar. An increase in operating pressure alleviates this saturation under high solar irradiance, improving the normalized photocurrent by ~40% at 5 bar under ~10× simulated sunlight, with marginal additional benefit at 8 bar. Direct operando imaging of the electrode surfaces reveals that this enhancement is primarily due to suppressed gas bubble evolution. In contrast, for the front-illuminated, integrated 3J III-V PEC water splitting device, the photocurrent remains largely unaffected by gas bubble evolution and increased operating pressure. This can be attributed to the high hydrophilicity of the platinized top surface, but more importantly, to the long charge carrier diffusion length in III-V photoelectrodes. This causes the photocurrent to increase proportionally with light intensity up to a solar concentration factor of at least 3× for a platinized GaInP/GaAs/Ge PEC cell. Additionally, stability measurements revealed that operation at increased pressure does not significantly impact the degradation of these photoelectrode materials. These results demonstrate the feasibility of operating PEC water splitting cells at elevated pressure, highlighting distinct effects of gas bubble evolution, pressure, cell configuration, and photoelectrode material properties. These findings offer fundamental insights that are directly relevant to PEC device design, bringing PEC water splitting a step closer to a practical application.

Methods
Design and fabrication of the high-pressure flow cell
The high-pressure flow cell (HPFC) was custom-designed and constructed in our workshop according to the schematics in Fig. 1b, with an exploded view of the cell design shown in Figure S1, and the digital photographs in Figure S2. The electrolyte was circulated using a high-pressure gear pump (LAB-9, GATHER Industrie). The bottom electrolyte reservoir has an approximate volume of ~400 mL, and a 3D printed structure made from transparent, high-temperature resistant plastic (VisiJet M2S-HT90) was installed to serve as a flow homogenizer / distributor. The flow homogenizer ensured that the electrolyte flow between the two electrodes in the reactor is laminar, as shown by the flow field obtained from particle image velocimetry (PIV) measurements (see Methods: Fluid flow field visualization and Fig. 2). The reactor part was 3D printed using stainless steel. A pair of sample holders can be installed, which allows a face-to-face arrangement of two 2 cm × 2 cm (maximum) samples with a distance of ~4 mm. The maximum effective area of the photoelectrodes is therefore ~4 cm2, which is constrained by the size of the illumination window (see Figure S1). Note that the sample holders are exchangeable; in fact, we customized another pair of sample holders which enabled the front-illuminated photocathode to be tilted at 45° in the reactor. A pair of glass windows on the side walls were installed for better operando monitoring of the gas bubble evolution, see Figure S1. The cell was pressurized by supplying compressed N (or O for certain cases). The cell pressure was controlled using a back-pressure controller (Bronkhorst High-Tech, uncertainty: 0.2%), visible in Fig. 1b and Fig. S2. The highest pressure achieved is 8 bar(a), and no leakage or explosion happened during our experiments. A protective shield was constructed to isolate personnel from any potential safety risks.

We chose a membrane-free flow cell design to facilitate operando (or in situ) measurements at higher pressure, e.g., through particle image velocimetry (PIV) and multi-angle bubble imaging; without flow, the gas bubbles would accumulate and prohibit these experiments. However, during our experiments, we noticed that it is challenging to avoid mixing of the high-pressure gas with the liquid flow at the outlets. As a result, gas bubbles will form when the liquid re-enters the reactor due to cavitation and disturb the measurement (or lead to product crossover). Undesired cavitation happened more often at higher flow rates (e.g., >10 mL s−1). We solved this problem by inserting capillary tubes (inner diameter of ~1.5 mm) underneath the electrolyte level and maintaining the electrolyte flow rate at a moderate level, i.e., ~4.6 mL s−1. A schematic illustration of the liquid outlets as well as the gas inlets and outlets is shown in Fig. S23a, while a digital photograph of the capillary tube is presented in Fig. S23b. This solution was effective for most of our experiments, but we note that they might need to be evaluated on a case-to-case basis. For long-term operation of the high-pressure PEC device, a membrane or separator between the electrodes may be mandatory to assure effective product separation.

Electrodeposition of the BiVO photoelectrode
The BiVO electrodeposition recipe used here follows that previously reported74. 3.32 g of potassium iodide (≥99%, Santa Cruz Biotechnology) was dissolved in 50 mL of deionized water (18.2 MΩ) to yield a 0.4 M potassium iodide solution. To this solution, 0.1 mL of nitric acid (>69.0%, Honeywell) and 0.04 M bismuth nitrate pentahydrate (≥98%, Acros Organics) were added. The mixture was magnetically stirred until all salts were completely dissolved. A 20 mL ethanolic solution was prepared, which contains 0.225 M p-benzoquinone (≥98%, Alfa Aesar). The aqueous Bi-I precursor solution was then slowly added to the ethanolic solution under stirring, yielding a clear, dark red mixture.

BiOI nanosheet arrays were electrodeposited onto fluorine-doped tin oxide (FTO) substrates (3 × 3 cm2, 3 mm thick, sheet resistance: 10 Ω sq−1, Sigma Aldrich). The electrodeposition was carried out using a three-electrode configuration with a platinum coil (0.5 mm diameter) as the counter electrode and an Ag/AgCl (saturated KCl; XR300, Radiometer Analytical) electrode as the reference electrode. A constant potential of −0.1 V vs. Ag/AgCl was applied until a charge of 200 mC cm−2 was reached, forming red-orange BiOI films.

The as-deposited BiOI films were subsequently coated with 50 µL cm−2 of 0.2 M vanadyl acetylacetonate (≥99%, Acros Organics) solution in dimethyl sulfoxide (≥99.9%, VWR Life Science). The coated samples were annealed on a hot plate at 450 °C for 2 h with a ramping rate of 2 K min−1 to induce the conversion to monoclinic BiVO . Residual V O was removed by immersing the films in a 1 M NaOH (≥98%, Sigma Aldrich) solution for 30 min.

The morphology of the BiVO films was characterized by scanning electron microscopy (SEM, GeminiSEM 360 instrument, Zeiss), see Fig. S7. The X-ray diffraction (XRD) pattern of the BiVO /FTO sample and the Tauc plot for indirect bandgap estimation are shown in Fig. S8a and S8b, respectively. The film thickness was determined to be ~10 µm using a DEKTAK 8 profilometer (Fig. S8c).

Design and fabrication of the 3J III-V photoelectrode
Platinized triple-junction (3J) III-V photoelectrodes were fabricated following our previously reported procedure84, The device architecture was based on a GaInP/GaAs/Ge (PV) cell (AZUR SPACE Solar Power GmbH) with a photoactive area of 1 cm2. Each cell was supplied with a protective GaAs cap layer to prevent surface degradation during shipping and storage, and the back contact consisted of an Ag/Au metallization. Prior to the TiO deposition, the GaAs cap layer was etched by wet chemical etching.

A 50 nm-thick TiO protection layer was then deposited by atomic layer deposition (ALD) using a Picosun R200 Advanced system, with tetrakis(dimethylamido)titanium (TDMAT, 99%, Strem Chemicals, Inc.) and Milli-Q H O serving as the precursor and oxidizer, respectively. The deposition temperature was maintained at 130 °C, resulting in an amorphous TiO layer. To promote the hydrogen evolution reaction (HER), a 1 nm Pt catalyst layer was subsequently deposited by electron beam (e-beam) evaporation (base pressure of 2 × 10−6 mbar) at a rate of 1 Å s−1. For improved adhesion, a 2 nm Ti interlayer was deposited between the Pt and TiO layers.

For photoelectrochemical (PEC) testing, the samples were mounted on the glass substrates using Ag conductive tape (3M) applied along the substrate’s midline. To ensure good ohmic contact, conductive Ag epoxy (Circuitworks CW2400, Chemtronics) was applied between the Ag tape and the back-side metallization of the III-V cell, followed by curing on a hot plate at 110 °C for 10 min. The electrode edges were sealed with a black two-part insulating silicone resin (101RF, Microset Products Ltd, see Figure S18), leaving only the active surface exposed. The electrochemically active area (~0.6–0.7 cm2) was determined from digital images using ImageJ software (Fig. S18d). A representative photograph of the completed photoelectrode is shown in Fig. S18.

PEC measurements
The instrumentation diagram of the test bench for the PEC measurement is shown in Fig. 1b. A dual light-source solar simulator (Wacom-WXS-100S-L2H AM 1.5G MM) was used as the light source. A set of Fresnel lenses (Sankuai, purchased from Amazon.de) with different focal lengths was used to increase the solar concentration. The solar irradiance was calibrated using a USB spectrometer (USB-2000+, Ocean Optics), see the Supplementary Note S1 and the measured irradiance shown in Fig. S3. The highest solar concentration obtained was ~10 suns for a Fresnel lens with a focal length of ~300 mm. We managed to illuminate the full sample area (~4 cm2) under ~10 suns, as shown by the photograph taken during the PEC tests in Fig. S4. However, it is worth noting that the irradiance was not homogenous throughout the sample (Fig. S3b and S3c). This inhomogeneity in solar irradiance is acceptable in our study, as our primary focus is pressure-dependence of the PEC water splitting cells performance. Nonetheless, this issue has to be dealt carefully for the real-world operation of such a PEC device under higher solar concentrations (e.g., c > 10). A photon homogenizer would be a viable solution, as reported in ref. 11.

For the PEC test with a back-illuminated BiVO photoelectrode cell configuration, a small portion of the prepared BiVO sample was etched using 1 M HCl (Fluka), on which electrical contact was made using a copper wire and conductive tape. The contact was then sealed properly with a black insulating two-part silicone resin (101RF, Microset). The BiVO sample was then fixed into the sample holder (visible in Fig. S1) using a rubber O-ring and the silicon resin. The resulting photoactive area of the BiVO samples was ~4 cm2. A Pt mesh (0.198 mm diameter wire, 99.9%, Alfa Aesar) was glued onto a 30 mm × 30 mm × 3 mm (length × width × thickness) glass substrate and used as the counter electrode. After assembly, the working and counter electrode were arranged face-to-face, with a gap of ~4 mm. A miniature Ag/AgCl (saturated KCl) electrode (PalmSens BV) was used as the reference electrode. Note that the reference electrode was calibrated against a master RE (saturated KCl) prior to measurement, and potential shift (ΔV) was ensured to be less than ±5 mV for each experiment. Electrochemical measurements were performed in a three-electrode configuration using a VersaSTAT 3 potentiostat/galvanostat (AMETEK). The measured potentials (E ) were converted to the reversible hydrogen electrode (RHE) scale using the Nernst equation:

E RHE (V) = E Ag=AgCl (V) + 0.0591 × pH + E0 Ag=AgCl (1)

here, E RHE is the applied potential vs. RHE and E0 Ag=AgCl is the potential of the reference electrode (0.198 V at 25 °C). No iR corrections was conducted for all reported voltammograms.

For the front-illuminated PEC cell configuration, most of the setup (e.g., reference electrode, potentiostat) remained the same, except for the sample holders (see Fig. S1) and the counter electrode. The sample holders were changed to a self-made glass sample holder to enable irradiating the platinized 3J III-V photoelectrode at a tilted angle (~45°, referring to the horizontal plane). An IrOx/TaOx/Ti mesh was used as the counter electrode. To avoid the shadowing effect from the counter electrode and gas bubble evolution during the reaction, the counter electrode was placed above the light path, as shown by the schematic in Fig. 5a.

A 0.1 M potassium phosphate (KP, pH = 7 ± 0.1) solution was used as the electrolyte for most of the experiments and was prepared from KH PO (Sigma-Aldrich, ≥99.0%) and K HPO ·3H O (Sigma-Aldrich, ≥99.0%) to obtain the desired pH. Either 0.5 M Na SO (Sigma-Aldrich, ≥99.0%) or 0.5 M Na S O (Sigma-Aldrich, ≥99.0%) was added to the electrolyte as the hole or electron scavenger, respectively. The water used in all experiments was obtained from a Milli-Q integral system with a resistivity of 18.2 MΩ cm. Fresh electrolyte solution was prepared and used for each individual measurement, and the electrolyte was stored in a glass beaker.

The testing protocol for the back-illuminated BiVO PEC cell was as follows: the electrolyte (0.1 M KP, pH = 7 ± 0.1) was purged with O gas for at least 1 h, then transferred into the HPFC. The high-pressure O gas supply was connected to the cell and maintained at the desired pressure. The rotary pump was started to circulate the electrolyte for about 15 min prior to the measurements. A 15 min gap was scheduled to ensure the electrolyte flow was in steady state after adjusting the operating pressure. The solar concentration was varied by adding Fresnel lens with various focal lengths. A 5 min gap was scheduled before conducting the next experiment to minimize the effect of heating at different solar concentrations. A K-type thermocouple was used to monitor the electrolyte temperature at the outlets of the cell. All relevant parameters (e.g., pressure, electric potential, current, flow rate, etc.) were fed through the data acquisitions connected to a computer. The PEC tests of the front-illuminated 3J III-V PEC cell followed a similar protocol, except that the compressed gas was changed to N . All the measurements were conducted at room temperature (~25 °C).

Gas bubble visualization
Two complementary imaging setups were used to observe gas bubble evolution on the electrodes from different angles during operation, as shown by the schematic in Fig. S6. For the back-illuminated PEC cell, a high-speed camera (HS5-Q Quad HD+ Resolution (2560 × 2048 pixels) with a Navitar Zoom 6000 Lens System, FasTec) was used to capture the side-view of gas bubble evolution, and the shadowgraphy of bubbles on the electrodes was captured using a LaVision camera system (Imager SX 6M CCD camera, 2752 × 2200 pixels). For the front-illuminated PEC cell, only the high-speed camera was used to observe the side-view of gas bubbles. The camera was fixed higher than the 3J III-V photoelectrode so that the surface of the sample was visible. Neutral density (ND) filters with various ND factors, purchased from Thorlabs, were used depending on the light condition to enhance the imaging quality. The localized telescopic imaging configuration achieved a spatial resolution of approximately 10 µm over a limited field of view (~5 × 4 mm), suitable for capturing detailed bubble dynamics in selected regions of interest (see Fig. 4e, f). The images were processed using ImageJ, with additional validation using DaVis 10 (LaVision). To enhance image clarity, we applied post-processing techniques in ImageJ, including background subtraction and contrast normalization. These helped improve visual distinction of gas bubbles, although lighting non-uniformities and optical limitations still affected the overall image quality. Nevertheless, since the experimental conditions are consistent across all tests, relative comparisons between our experimental results remain valid.

To enable full-field visualization of the bubble evolution on the BiVO photoanode (2 cm × 2 cm), the setup was further modified as shown in the schematic illustration in Figure S14a. The same BiVO and Ag/AgCl electrodes were kept as the working and reference electrodes, but the counter electrode was a platinum wire instead of the platinum mesh to reduce visual obstruction. The PEC cell was operated at 0.5 mA cm−2 in static 0.1 M KP (pH = 7 ± 0.1), and bubble distribution was recorded using a Zeiss Makro-Planar T* 2/100 lens coupled to a high-speed camera (HS5-Q, 2560 × 2048 pixels), with a resolution limit of ~45–50 μm in bubble diameter. The representative images of bubbles on the BiVO surface under different pressures are shown in Fig. S14b.

Fluid flow field visualization
The electrolyte flow field between the two parallel electrodes was visualized using particle imaging velocimetry (PIV, LaVision). The working principle of the PIV measurement is schematically illustrated in Fig. S5a, and is briefly described as follows: polyamid spheres (20 µm diameter, LaVision) were seeded in liquid at a seeding density of approximately 1 million particles per mL, these microparticles have similar density as Milli-Q water. The mixture was stirred overnight to obtain a stable suspension. Theseeded liquid was then fed into the flow cell and circulated using a rotary pump (LAB-9, GATHER Industrie). A double-pulse laser (532 nm) and light sheet optics were used to generate two consecutive laser sheets, and the illuminated particles were captured by a camera (LaVision Imager SX 6M). The laser pulse duration in our measurements ranged from 10 to 30 ms (Δt in Fig. S5a), optimized based on the seeding density and the camera’s spatial resolution. The digital photographs of the experimental setup are shown in Fig. S5b.

It is important to note that the fluid flow visualization results presented in Fig. 2 were obtained using the ‘default’ sample holders (as shown in Fig. S1). Changing the sample holders—such as when using the customized version for the front-illuminated PEC experiments—will alter the fluid flow field between the electrodes, even if the flow rate remains the same.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors designed a high-pressure flow cell for PEC water splitting. A back-illuminated BiVO4-based PEC cell and a front-illuminated platinized triple-junction (3J) III-V-based PEC cell were tested. It is found that at a solar concentration of 10 , increasing the operating pressure from 1 to 5 bar causes the photocurrent to increase from 3  to almost 7  the photocurrent under 1 sun for the BiVO4-based PEC cell. However, the photocurrent scales proportionally to the illumination intensity and does not saturate in the 3J III-V-based PEC cell. The findings demonstrate useful information for scaling up PEC water splitting cells. However, several significant comments and suggestions must be addressed before the manuscript can be recommended for publication:\n1. Considering that the authors only tested a back-illuminated BiVO4-based PEC cell and a front-illuminated platinized triple-junction (3J) III-V-based PEC cell in the manuscript, it is not clear whether “elevated pressure” is applicable to all photoelectrochemical water splitting cells. The title seems too big, and it is suggested the authors to modify the title to make it more focus.\n2. The Abstract for Nature Communications is too long. According to the requirement of the journal, the abstract should be less than 150 words.\n3. The authors demonstrated that elevated pressure can alleviate the effect of bubbles on the PEC performance of the BiVO4-based PEC cell. Whether more energy should be input to maintain the elevated pressure? The energy conversion efficiency is important for possible scale-up applications. It is suggested the authors to discuss and comment on the total energy conversion efficiency in this elevated pressure system.\n4. For possible scale-up applications, how to collect the produced hydrogen while maintain the elevated pressure in the system is important. It is suggested the authors to discuss and comments on this point.\n5. This elevated pressure system is expected to produce a mixture of hydrogen and oxygen gases. How to effectively separate the hydrogen and oxygen gases is important for practical applications. It is suggested the authors to detail the possible methods ore technologies for efficient separation of the produced hydrogen and oxygen gases.\n6. It is important to confirm whether the PEC system can be stably operated at elevated pressure for a long time. Longer stability measurement to hundreds of hours is recommended to evaluate the reliability of the system.\n7. This work aims to provide a new pathway for scaling PEC water splitting devices. It is suggested to discuss some recent works related to scale-up PEC devices to broaden the readership (Nat. Commun. 2025, 16, 990; Nat. Commun. 2025, 16, 2792; Nat. Energy 2024, 9, 272-284).", "reviewer_2": "This manuscript presents a novel and comprehensive study of the operation of photoelectrochemical (PEC) water splitting cells under high-pressure conditions. Using a custom-designed high-pressure laminar flow cell, the authors successfully demonstrate—for the first time—PEC hydrogen production at pressures up to 8 bar. The investigation of photoelectrodes such as BiVO₄ reveals how pressure-induced suppression of bubble formation influences photocurrent behavior. The experimental methodology is rigorous, and the findings are significant, offering practical guidance for the future scale-up of PEC systems. However, several experimental aspects require clarification or further elaboration to make the manuscript more complete, particularly regarding system stability under prolonged operation.\n1. It is recommended that the authors expand the scope of comparison, for instance by benchmarking the pressure-induced enhancement in photocurrent density against other established photoelectrode systems (e.g., α-Fe₂O₃, WO₃, Sb₂S₃).\n2. Additional comparative data on bubble evolution under different pressure gradients should be included to systematically elucidate the effects of pressure on bubble nucleation, size, and distribution.\n3. Although the manuscript focuses on short-term PEC performance enhancement, it is recommended to include stability data from long-term operation under pressurised conditions (e.g., 10 hours).\n4. While the authors state that the bulk electrolyte temperature change is less than 2.5°C, , local heating of the photoelectrode surface under 10 times solar irradiance over extended periods may affect interfacial dynamics, which should be carefully considered.\n5. It would be valuable for the authors to briefly discuss how the high-pressure PEC module might be integrated into a practical solar water splitting system.\n6. For better clarify the author’s point of view and attract more readers, it is recommended to add citations.(Angew. Chem. Int. Ed. 2020, 59, 23094-23099; Nat. Sustain.2025,doi.org/10.1038/s41893-025-01530-y; Matter 2024,7,2278-2293; Angew. Chem. Int. Ed. 2024,136,e202317414.).", "reviewer_3": "This manuscript demonstrates the operating photoelectrochemical (PEC) water splitting cells at elevated pressures (up to 8 bar), addressing a bottleneck in scalable solar hydrogen production. The authors developed a custom high-pressure flow cell and investigated two configurations: (1) a back-illuminated BiVO₄-based PEC cell, and (2) a front-illuminated integrated platinumized triple-junction (3J) III-V PEC device. For the BiVO₄ system, increased pressure suppressed gas bubble formation, resulting in up to a 50% improvement in photocurrent under 10 suns illumination. Operando imaging confirmed that the improved performance stemmed from reduced bubble coverage and size. In contrast, the 3J III-V PEC device exhibited pressure-independent photocurrent due to its long carrier diffusion length, thin dispersed Pt catalyst layer, and highly hydrophilic surface, which facilitated bubble detachment and minimized recombination losses. The manuscript underscores the configuration-dependent effects of pressure on PEC performance and highlights the benefit of high-pressure operation: directly generating pressurized hydrogen to reduce downstream compression energy costs. Overall, I think this is a great paper and the manuscript could meet the standard of Nature Communications after addressing the following issues.\n1. The figure clarity should be improved. The authors need to make sure that the readers can gain the key breakthrough of this work by reading the introduction and figures. However, figures only contain the recorded data without highlighting the novelty of this work. For example, in Figure 1, they need to add Figure 1c to highlight the novelty as compared with the previous approach. Besides, there are too many labels in Figure 1b about the whole system, and I do not know how significance of this work as compared with other works. The same things happen in other figures, the authors need to improve the figure quality and highlight the key novelty.\n2. To add pressure for hydrogen production, the long-term stability and safety issues require a second consideration. During large-scale applications, will the materials degrade under cyclic pressure/continuous pressure? Is there any auxiliary equipment required for pressure adding, and will this increase the total cost of the hydrogen production?\n3. The authors focused more on the breakthrough at the systematic level, and the materials used are simple. However, I still want the authors to clarify why BiVO4 is applied in this system. What is the key advantage as compared with other catalysts? Besides, instead of only simple materials characterization in supplementary information, is there any difference/breakthrough on the materials level?\n4. Please expand the discussion on pressure-dependent bubble nucleation dynamics. The manuscript attributes performance gains to suppressed bubble evolution at higher pressure, but the underlying physical mechanism (e.g., Laplace pressure, critical bubble size, contact angle effects) should be further elaborated.\n5. The observation of bubbles plays a significant role in understanding the reason for high water splitting. Therefore, I recommend improving the bubble imaging resolution. Can you specify the optical resolution limits of your cameras, and whether any contrast-enhancing techniques (e.g., DIC, phase contrast) were attempted? Suggest future tools (e.g., AFM, total internal reflection microscopy) to visualize nanobubbles.\n6. Since this technique requires 10 sun for water splitting, the authors need to consider possible heat accumulation at high solar flux. Could longer operation at 10 suns lead to cumulative heating or catalyst degradation?", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to reviewers’ comments\nManuscript ID: NCOMMS-25-32194-T\nTitle: Operating photoelectrochemical water splitting cells at elevated pressure\nIn this response letter, the reviewers’ comments to our original manuscript are provided\nin black, and our point-by-point responses as well as the corresponding changes to the\nmanuscript are shown in blue.\nReviewers' Comments:\nReviewer #1:\nThe authors designed a high-pressure flow cell for PEC water splitting. A back-illuminated\nBiVO -based PEC cell and a front-illuminated platinized triple-junction (3J) III-V-based\noperating pressure from 1 to 5 bar causes the photocurrent to increase from 3× to almost\n7× the photocurrent under 1 sun for the BiVO -based PEC cell. However, the photocurrent\n4\nscales proportionally to the illumination intensity and does not saturate in the 3J III-V-\nbased PEC cell. The findings demonstrate useful information for scaling up PEC water\nsplitting cells. However, several significant comments and suggestions must be\naddressed before the manuscript can be recommended for publication:\n1. Considering that the authors only tested a back-illuminated BiVO -based PEC cell\n4\nand a front-illuminated platinized triple-junction (3J) III-V-based PEC cell in the\nmanuscript, it is not clear whether “elevated pressure” is applicable to all\nphotoelectrochemical water splitting cells. The title seems too big, and it is suggested\nthe authors to modify the title to make it more focus.\nResponse: We thank the reviewer for the suggestion. Although it is likely that the\n“elevated pressure” condition and effects will apply to more than the\nphotoelectrodes tested, we agree that it is more appropriate for our title to be more\nspecific. The title is now changed from “Operating photoelectrochemical water\nsplitting cells at elevated pressure” to “Operating photoelectrochemical water\nsplitting cells at elevated pressure: a demonstration using BiVO and platinized III-V\n4\nsemiconductor photoelectrodes”.\nAssociated changes to the Manuscript and Supplementary Information:\n• Title (Manuscript), page 1, line 1 – 2: “Operating photoelectrochemical water\nsplitting cells at elevated pressure: a demonstration using BiVO and platinized III-\n• Title (Supplementary Information), page 1, line 1 – 3: “Operating\nphotoelectrochemical water splitting cells at elevated pressure: a demonstration\nusing BiVO and platinized III-V semiconductor photoelectrodes”\n4\n2. The Abstract for Nature Communications is too long. According to the requirement of\nthe journal, the abstract should be less than 150 words.\nResponse: We thank the reviewer for reminding us about this requirement. We have\nnow shortened the Abstract to be 150 words.\nAssociated changes to the Manuscript:\n• Abstract (Manuscript), page 1 – 2, line 16 – 28 “Direct production of pressurized\ngreen hydrogen via photoelectrochemical (PEC) water splitting reduces the need\nfor mechanical compression and mitigates bubble-related losses. However,\nexisting demonstrations have been limited to atmospheric pressure. To bridge this\ngap, we designed, constructed, and tested a high-pressure flow cell for PEC water\nsplitting using two configurations. In a back-illuminated BiVO -based PEC cell,\n4\nincreased pressure suppresses bubble evolution and alleviates photocurrent\nsaturation under concentrated sunlight: at 10 suns, the photocurrent rises from\n3× at 1 bar to ~7× at 5 bar. Direct operando imaging of the electrode surfaces\nconfirms that this improvement comes primarily from suppressed bubble\nevolution. Conversely, a front-illuminated platinized triple-junction III-V-based\nPEC cell shows limited pressure dependence up to 8 bar due to its dispersed\ncatalyst and long carrier diffusion length. These findings highlight the differing\nresponse of PEC devices to pressure and demonstrate a viable pathway toward\nscalable, high-pressure PEC hydrogen production.”\n3. The authors demonstrated that elevated pressure can alleviate the effect of bubbles\non the PEC performance of the BiVO -based PEC cell. Whether more energy should\n4\nbe input to maintain the elevated pressure? The energy conversion efficiency is\nimportant for possible scale-up applications. It is suggested the authors to discuss\nand comment on the total energy conversion efficiency in this elevated pressure\nsystem.\nResponse: We thank the reviewer for raising up this important point. Reducing the\nneed for downstream hydrogen compression and suppression of bubble-induced\nlosses represent significant benefits of operating PEC water splitting cells at elevated\npressure. In our experiments, we supplied compressed gas to the cell for controlled\nconditions; however, in practical applications, the system can be self-pressurized\nthrough gas production during operation. At such, no additional energy input is\nrequired for system pressurization other than the thermodynamic penalty of higher-\npressure operation. Our recent analysis shows that the benefits still largely outweigh\nthis thermodynamic penalty in the pressure range of up to 20 bar.1 Nevertheless, we\nfully acknowledge that increasing the operating pressure may also introduce\nnontrivial technical and economic challenges. These include the need for thicker\nglass components, more robust sealing strategies to prevent electrolyte and gas\nleakage, and the use of high-pressure-rated tubing and control systems—all of which\ncan increase the capital and maintenance costs. Detailed techno-economic\nassessment beyond the scope of our current study is thus necessary to\ncomprehensively evaluate the economic feasibility of operating PEC systems at\nelevated pressures. We have now added a brief discussion regarding the points above\nin the revised Manuscript.\nAssociated changes to the Manuscript:\n• Results (Manuscript), page 5, line 111 – 117: “High pressure operation was\naccomplished by supplying compressed gas into the cell while regulating the\noutflow rate of gases with a back-pressure controller. Note that this approach to\nregulate the pressure was chosen because it offers easy control and experimental\nconvenience in a lab-based setting; in practical applications, PEC water splitting\ncells can self-pressurize through gas production during operation, requiring only\na pressure relieve valve as a control mechanism.”\n• Results (Manuscript), page 29, line 537 – 555: “On the system level, the benefits\nfrom operating PEC water splitting cells at elevated pressure should be ensured\nto outweigh the associated penalties. In addition to the suppression of bubble-\ninduced losses, reducing or eliminating the need for downstream hydrogen\ncompression represents a significant advantage. In practice, the elevated\npressure PEC cell can be connected directly to high-pressure H tanks, solid-state\n2\n(e.g., Haber-Bosch or hydrogenation reactors), as illustrated in Fig. S22. For\ninstance, when compressing H to 875 bar, doing so by starting from 1 bar\n2\nconsumes ~80% more energy than starting from 20 bar (~4.8 vs. ~2.7 kWh kg-1).78-\noperation, potentially avoiding additional energy input for pressurization other\nthan the thermodynamic penalty of higher-pressure operation. In the pressure\nrange of up to 20 bar, our recent analysis has shown that the benefits largely\noutweigh the thermodynamic penalty.35 Nevertheless, we acknowledge that\nachieving and maintaining elevated pressure introduces technical and economic\nchallenges, including the need for reinforced cell housings, thicker optical\nwindows, enhanced sealing, pressure-rated tubing, and precise back-pressure\ncontrol—all of which would increase capital and maintenance costs. A\ncomprehensive techno-economic analysis is needed to evaluate the economic\ntrade-offs and inform possible scale-up configurations.”\n4. For possible scale-up applications, how to collect the produced hydrogen while\nmaintain the elevated pressure in the system is important. It is suggested the authors\nto discuss and comments on this point.\nResponse: We appreciate this valuable suggestion from the reviewer. We fully agree\nthat gas separation and collection will be important in practical applications, but we\nemphasize that the primary focus of our current study is to explore the effect of\npressure elevation on the performance of PEC water splitting cells. Nevertheless, we\nhave now considered the possible solutions for collecting or utilizing the produced H\n2\nat high pressure in scale-up applications. Three possible strategies are proposed for\nstoring or utilizing the high-pressure H gas generated by the PEC water splitting cells,\n2\nas illustrated schematically in Fig. R1. First, the produced H can be stored in a high-\n2\npressure gas cylinders for transportation and later use. Second, it can be absorbed by\nthe solid-state H storage materials, such as MgNi, ZrCr, and LaNi -based metal\n2 5\nhydrides. Third, the produced high-pressure H can be directly fed into high-pressure\n2\nhydrogen applications, such as Haber–Bosch reactors. This way, the elevated\npressure can be maintained while collecting or directly utilizing the pressurized H .\nInformation.\nFigure R1. Schematic illustrations of three possible strategies for high-pressure H\n2\nstorage/utilization. Option 1: the high-pressure H is stored in a gas cylinder; Option\n2\n2: the high-pressure H is absorbed by the solid-state H storage materials, such as\n2 2\nMgNi, ZrCr, and LaNi -based metal hydrides; Option 3: the high-pressure H is directly\n5 2\nfed into high-pressure hydrogen applications such as a Haber–Bosch reactor. Note\nthat the oxygen is produced and released to the environment at nearly atmospheric\npressure. Such a differential- pressure operation can be enabled by using a proton-\nexchange-membrane (e.g., Nafion@ 117). Note that the modules such as gas-liquid\nseparator, H gas purification are not included in this schematic illustration.\n• Results (Manuscript), page 29, line 540 – 555: “In practice, the elevated pressure\nPEC cell can be connected directly to high-pressure H tanks, solid-state H\n2 2\nstorage materials, or any applications that require elevated pressure operation\n(e.g., Haber-Bosch or hydrogenation reactors), as illustrated in Fig. S22. For\ninstance, when compressing H₂ to 875 bar, doing so by starting from 1 bar\nconsumes ~80% more energy than starting from 20 bar (~4.8 vs. ~2.7 kWh kg-1).78-\noperation, potentially avoiding additional energy input for pressurization other\nthan the thermodynamic penalty of higher-pressure operation. In the pressure\nrange of up to 20 bar, our recent analysis has shown that the benefits largely\noutweigh the thermodynamic penalty.35 Nevertheless, we acknowledge that\nachieving and maintaining elevated pressure introduces technical and economic\nchallenges, including the need for reinforced cell housings, thicker optical\nwindows, enhanced sealing, pressure-rated tubing, and precise back-pressure\ncontrol—all of which would increase capital and maintenance costs. A\ncomprehensive techno-economic analysis is needed to evaluate the economic\ntrade-offs and inform possible scale-up configurations.”\n• Supplementary Information, page 38: Figure R1 has now been added as Fig. S22\nin the revised version.\n5. This elevated pressure system is expected to produce a mixture of hydrogen and\noxygen gases. How to effectively separate the hydrogen and oxygen gases is\nimportant for practical applications. It is suggested the authors to detail the possible\nmethods ore technologies for efficient separation of the produced hydrogen and\noxygen gases.\nResponse: We appreciate this insightful and practical suggestion from the reviewer.\nGas separation in the present work is achieved by carefully controlling the electrolyte\nflow dynamic, i.e., the produced H and O gas bubbles are kept close to the gas\n2 2\nevolving electrodes by the laminar electrolyte flow (see the flow colormaps in Fig. 2a,\nManuscript). Although the purity of the produced gases has not been quantified, this\nmembraneless strategy has been shown to be promising.2,3 Nevertheless, it is\nsuspected that incorporating ion-exchange membranes (e.g., Nafion® 117) would be\nrequired for extremely high purity and safe operation, especially under elevated\npressure conditions. The use of ion-exchange membranes also allows operating the\nanodic and cathodic compartments of the PEC cells under differential pressure. Prior\nstudies have shown that hydrogen permeability through Nafion membranes remains\nlargely independent of the applied pressure differential (up to 5 bar), indicating that\npressure differences have a negligible effect on gas crossover.4 We have now included\nthis brief discussion in the revised Manuscript.\nAssociated changes to the Manuscript:\n• Results (Manuscript), page 30, line 556 – 568: “Another factor not yet fully\nconsidered here is gas separation. We utilized precise fluid dynamic control of the\nelectrolyte flow to ensure laminar condition and keep the evolved O and H gas\n2 2\nbubbles closer to their respective electrodes, effectively minimizing gas crossover\n(see flow velocity colormaps in Fig. 2a). However, the purity of the separated gases\nhas not been quantified. Such membraneless separation strategy via\nhydrodynamically guided flow has shown promise in simplifying system\narchitecture,81,82 but we suspect that the incorporation of ion exchange\nmembranes (e.g., Nafion® 117) might be required to achieve the highest purity and\nto guarantee safe operation. Moreover, the use of ion exchange membranes would\nenable the PEC cell to operate under differential pressure between the anodic and\ncathodic compartments. Prior studies have shown that hydrogen permeability\nthrough Nafion membranes remains largely independent of the applied pressure\ndifferential (up to 5 bar), indicating that pressure differences have a negligible\neffect on gas crossover.83”\n6. It is important to confirm whether the PEC system can be stably operated at elevated\npressure for a long time. Longer stability measurement to hundreds of hours is\nrecommended to evaluate the reliability of the system.\nResponse: We agree with the reviewer that it is important to evaluate whether the\nelevated pressure has any impact on device stability. For the front-illuminated\nplatinized III-V PEC cell, the photocurrent degradation remained comparable\nbetween 1 and 8 bar, as illustrated in Fig. 5i of the Manuscript. In the revised\nManuscript, we have now included additional stability measurements for the back-\nilluminated, BiVO -based PEC water splitting cell under varying pressures.\n1.23 V vs. RHE, in a 0.1 M potassium phosphate buffer (KP, pH = 7) with an electrolyte\ni\nflow of 4.6 mL s-1. Two illumination intensities were applied: 1 sun (AM1.5G) and 10\nsuns. Since uncatalyzed and undoped BiVO samples were used in our study,\n4\nphotocurrent decrease is expected;5,6 however, we emphasize that the focus here is\nto understand if pressure elevation introduces any effect on degradation. The CA test\nat 1 sun was terminated when the photocurrent dropped by ~50%; while under 10\nsuns, the test was stopped at ~100% photocurrent loss. Interestingly, as shown in Fig.\nR2, no significant difference is observed in the normalized photocurrent plots at\ndifferent pressure, both at 1 and 10 suns. This suggests that the BiVO degradation\n4\nmechanism is the same at different pressures. Overall, our study does not suggest\nany presence of pressure-induced degradation, but further detailed investigations are\nrequired to reveal the mechanisms. Finally, within the pressure range investigated, no\ngas or liquid leakage was detected during prolonged high-pressure operation,\nconfirming the mechanical integrity and operational robustness of the system. This\ndiscussion has now been included in the Manuscript and the Supplementary\nInformation.\nFigure R2. Stability measurements of the back-illuminated, BiVO -based PEC water\n4\nsplitting cell under different pressures. (a) Chronoamperometry (CA) measurements\nunder AM1.5G 1 sun illumination; (b) the same CA data divided by the respective J\nmax\nfor clearer comparison. (c) and (d) present the corresponding CA results under\nconcentrated illumination (10 suns). In all tests, undoped and uncatalyzed BiVO was\n4\nused as the photoanode, with a platinum mesh as the counter electrode (CE), and\nAg/AgCl as the reference electrode. The PEC cells were operated potentiostatically at\n1.23 V vs. RHE in a 0.1 M potassium phosphate (KP) buffer solution (pH = 7), an\ni\nelectrolyte flow rate of 4.6 mL s-1 was maintained during the stability measurements.\nThe CA test was terminated upon ~50% photocurrent loss at 1 sun and near-complete\ndegradation at 10 suns.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 21, line 362 – 371: “To investigate whether pressure\nelevation introduces any effect on the stability of the back-illuminated BiVO\n4\nphotoanode, potentiostatic experiments were performed with the same PEC cell\nat 1.23 V vs. RHE. Illumination intensities of 1 sun and 10 suns were applied. As\nshown in Fig. S15, the photocurrent decreases with time, indicating degradation;\nthis is expected since our BiVO is uncatalyzed and has no protection layer.25,63\nsuggesting that pressure elevation has minimal impact on the degradation\nbehavior of the BiVO photoelectrode. Moreover, no gas or liquid leakage was\n4\nobserved during the extended high-pressure operation, indicating reliable\nstructural mechanical integrity and operational stability of the system.”\n• Supplementary Information, page 25: Figure R2 has now been added as Fig. S15\nin the revised version.\n7. This work aims to provide a new pathway for scaling PEC water splitting devices. It is\nsuggested to discuss some recent works related to scale-up PEC devices to broaden\nthe readership (Nat. Commun. 2025, 16, 990; Nat. Commun. 2025, 16, 2792; Nat.\nEnergy 2024, 9, 272-284).\nResponse: We thank the reviewer for this valuable suggestion and for pointing out\nthese important relevant works. We have now expanded the discussion in the revised\nManuscript to cover more recent advances in the field, and the suggested references\nhave been included as Refs. 6, 20, and 21.\nAssociated changes to the Manuscript:\n• Introduction (Manuscript), page 2, line 38 – 46: “For PC water splitting systems,\nnear-unity conversion yields have been achieved under UV irradiation.3 Moreover,\nthe feasibility of scale-up to larger areas using photocatalyst sheets was\ndemonstrated with a 100 m2 outdoor prototype panel reactor system,3-5 although\nthe maximum solar-to-hydrogen (STH) efficiency was only ~0.76%.5 A slightly\nhigher STH efficiency of 1.21% was recently demonstrated with a PC water\nsplitting system that utilizes an I ⁻/I⁻ redox mediator. Employing MoSe -loaded\n3 2\nhalide perovskites (CH(NH ) PbBr I ) for H evolution and NiFe-layered double\n2 2 3-x x 2\nhydroxide-modified BiVO for O evolution, this 700 cm2 system demonstrated\n4 2\nstable operation over one week under natural sunlight.6”\n• Introduction (Manuscript), page 3, line 59 – 62: “With record-high STH efficiencies\ndemonstrated, efforts are now focused on scaling up PEC water splitting systems.\nIn general, PEC water splitting can be scaled by increasing the photoactive area\nper device,20 by deploying a greater number of devices,2,20,21 and/or by utilizing\nhigher solar concentrations.5,9,22”\nReviewer #2:\nThis manuscript presents a novel and comprehensive study of the operation of\nphotoelectrochemical (PEC) water splitting cells under high-pressure conditions. Using\na custom-designed high-pressure laminar flow cell, the authors successfully\ndemonstrate—for the first time—PEC hydrogen production at pressures up to 8 bar. The\ninvestigation of photoelectrodes such as BiVO₄ reveals how pressure-induced\nsuppression of bubble formation influences photocurrent behavior. The experimental\nmethodology is rigorous, and the findings are significant, offering practical guidance for\nthe future scale-up of PEC systems. However, several experimental aspects require\nclarification or further elaboration to make the manuscript more complete, particularly\nregarding system stability under prolonged operation.\nResponse: We sincerely appreciate the reviewer’s positive feedback. We have carefully\naddressed all the listed comments/suggestions and believe that the revision has\nsignificantly strengthened our study.\nThe point-to-point responses are as follow:\n1. It is recommended that the authors expand the scope of comparison, for instance by\nbenchmarking the pressure-induced enhancement in photocurrent density against\nother established photoelectrode systems (e.g., α-Fe₂O₃, WO₃, Sb₂S₃).\nResponse: We appreciate the reviewer’s suggestion. While we agree that extending\nthe scope of comparison to other semiconductors such as α-Fe O , WO , Sb S , and\n2 3 3 2 3\nCu O would further enrich the understanding of pressure effects across different\n2\nmaterial platforms, we emphasize that the focus of our study is to demonstrate the\nfirst high-pressure PEC water splitting cells. Therefore, we intentionally selected two\nrepresentative and contrasting systems: a metal oxide (BiVO ) and a platinized triple-\n4\njunction III-V semiconductor. This selection encompasses both photoanode and\nphotocathode configurations as well as front- and back-illumination cell geometries.\nWe believe this selection provides sufficient breadth to establish the general\nfeasibility and performance trends under elevated pressure. We therefore kindly ask\nthe reviewer’s understanding that further expansion of scope to include other\npromising semiconductors is part of our planned future work and will be reported in\ndue course. We have added a brief discussion to acknowledge this point in the revised\nManuscript. Moreover, as also noted in our response to Comment 1 of Reviewer #1,\nwe have modified the Manuscript title to more accurately reflect the scope of the\ncurrent study.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 28 – 29, line 521 – 536: “We will now discuss some\ngeneral limitations of our study and provide an outlook for operating PEC devices\nat elevated pressure. First, in this initial demonstration of high-pressure PEC water\nsplitting, only two representative and contrasting systems are selected: BiVO as\n4\na metal oxide photoanode under back-illumination and a platinized triple-junction\nIII-V photocathode under front-illumination. These photoelectrodes were\nintentionally chosen as benchmark photoelectrodes since they have been\nextensively studied in PEC water splitting research and beyond.16,17,66,73-77 They are\nrelatively stable and have demonstrated adequately high photocurrent; this\nensures that performance variations in our study can be attributed primarily to\npressure-related effects rather than material-related factors, thus providing a\nreliable basis to assess their impact on device operation. At the same time,\nexpanding this comparison to additional semiconductors, such as α-Fe O and\n2 3\nWO photoanodes or Sb S and Cu O photocathodes, would offer further insights\n3 2 3 2\ninto any material-specific pressure effects. These studies are part of our ongoing\nefforts and will be reported in due course.”\n• Title (Manuscript), page 1, line 1 – 2: “Operating photoelectrochemical water\nsplitting cells at elevated pressure: a demonstration using BiVO and platinized III-\n• Title (Supplementary Information), page 1, line 1 – 3: “Operating\nphotoelectrochemical water splitting cells at elevated pressure: a demonstration\nusing BiVO and platinized III-V semiconductor photoelectrodes”\n4\n2. Additional comparative data on bubble evolution under different pressure gradients\nshould be included to systematically elucidate the effects of pressure on bubble\nnucleation, size, and distribution.\nResponse: We appreciate the reviewer’s insightful suggestion. To further elucidate\nthe pressure effects on gas bubble evolution, we have performed additional\nexperiments focusing on the oxygen bubble dynamics from the surface of the BiVO\n4\nphotoanode. To enhance visualization over the full 2 cm × 2 cm electrode area, the\nexperimental setup was modified, as shown in Fig. R3a. In short, the same BiVO and\ncounter electrode was a platinum wire instead of the platinum mesh to reduce visual\nobstruction. The PEC cell was operated galvanostatically at 0.5 mA cm-2 in 0.1 M KP\ni\n(pH = 7). A macroscopic Zeiss Makro-Planar T* 2/100 lens coupled with a high-speed\ncamera (HS5-Q, 2560 × 2048 pixels) was used to capture bubble distribution,\nachieving a detection threshold of ~45–50 µm in bubble diameter.\nRepresentative images (Fig. R3b) clearly show that increasing pressure significantly\nsuppresses gas bubble formation on the BiVO surface, consistent with localized\n4\nobservations presented in Fig. 4e–f of the Manuscript. At 1 bar, bubbles were\nrandomly distributed across the porous BiVO layer, with larger bubble sizes observed\n4\nnear the top of the electrode—which can be attributed to hydrostatic pressure\ngradients. However, bubble presence was already negligible at pressures above 2 bar.\nThis insight should be considered for the scale-up design of PEC systems.\nWe also appreciate the reviewer’s suggestion to investigate bubble nucleation events.\nUnfortunately, direct observation of nucleation events was not feasible due to the\nlimited resolution of our current imaging setup. However, related studies, such as a\nrecent work on bubble evolution on TiO micro-photoelectrodes,7 confirm that\n2\nelevated pressure facilitates bubble detachment due to increased dissolved gas\nsaturation—aligned with previous simulation results.8 Finally, we emphasize that the\ntopic of gas bubble dynamics attracts increasing interests from researchers in the\nfield, and a full understanding of this complex dynamics under pressure remains an\nopen question. Advanced experimental and theoretical techniques such as atomic\nforce microscopy 9-11 and molecular dynamics simulations 12,13 will be essential for\nfuture investigation.\nAdditional discussion has now been added to the revised Manuscript and\nSupplementary Information.\nFigure R3. Macroscopic shadowgraphy of oxygen bubble evolution from the BiVO\n4\nphotoanode. (a) Schematic of the measurement setup. (b) Representative images of\nbubble formation at 1, 2, and 3 bar, respectively. The working electrode (BiVO , 2 cm\n4\n× 2 cm active area) is identical to that used in the main study. A platinum wire was\nemployed as the counter electrode (instead of the platinum mesh) to minimize visual\nobstruction, and a saturated Ag/AgCl electrode served as the reference. The PEC cell\nwas operated galvanostatically at 0.5 mA cm-2 in 0.1 M potassium phosphate buffer\n(KP, pH = 7), with the electrolyte held static. AM1.5G 1 sun illumination was used.\ni\nImaging was conducted using a Zeiss Makro-Planar T* 2/100 lens coupled to a high-\nspeed camera (HS5-Q, 2560 × 2048 pixels), yielding a bubble detection threshold of\napproximately 45–50 µm. Note that gas bubbles observed at the top of the frames\ncorrespond to the region of electrical contact, where a small amount of silicone epoxy\nwas applied for insulation.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 20, line 352 – 361: “To further elucidate the effects of\npressure on gas bubble evolution throughout the whole surface of the 2 cm × 2 cm\nBiVO photoelectrode, we conducted macroscopic imaging of oxygen bubbles.\nthe Methods section. At 1 bar, bubbles appeared randomly across the porous\nBiVO surface (Fig. S14b), with larger bubble sizes observed near the top of the\n4\nelectrode; we tentatively attribute this observation to the presence of hydrostatic\npressure gradients. This insight is useful for the scale-up of PEC devices, as non-\nuniform bubble detachment may result in spatial variations in performance.\nHowever, at pressures above 2 bar, bubble presence was already negligible (Fig.\nS14b). This observation is consistent with our localized observations shown in Fig.\n4 e-f.”\n• Results (Manuscript), page 17, line 315 – 333: “Although bubble nucleation events\ncould not be directly resolved, recent work on bubble evolution on TiO micro-\n2\nphotoelectrodes showed that elevated pressure facilitates bubble detachment,\nprimarily due to higher dissolved gas saturation.48 Consistent with these findings,\nour measurements (Fig. 4e–f) revealed smaller and fewer bubbles under elevated\npressure. This behavior can be tentatively explained by the formation of a\nsupersaturated boundary layer (SSBL) of dissolved gas near the (photo)electrode\nat higher pressure. Our recent simulation showed that the SSBL facilitates bubble\ndetachment from the surface of the (photo)electrode via Marangoni convection.49\nAdditional contributing factors may include an increased Laplace pressure and a\nlarger critical radius for nucleation at higher pressures, both of which inhibit initial\nbubble formation. Interfacial properties, such as the gas–liquid contact angle,\nmay also play a role in pressure-dependent detachment dynamics, but direct\nexperimental measurement of contact angles and nano/micro bubbles on\nsemiconductor surfaces (e.g., BiVO photoelectrode) under elevated pressure\n4\nremains technically challenging. High-resolution tools such as atomic force\nmicroscopy and spectroscopy techniques 50-60 and molecular dynamics\nsimulation 61,62 will be essential, which are beyond the scope of this study.\nFurthermore, the porous morphology of our BiVO samples (see Fig. S6)\n4\nintroduces additional complexity, as it may lead to random spatial distribution of\nnanobubbles, making their detection even more complicated.”\n• Methods (Manuscript), page 38 – 39, line 757 – 766: “To enable full-field\nvisualization of the bubble evolution on the BiVO photoanode (2 cm × 2 cm), the\n4\nsetup was further modified as shown in the schematic illustration in Fig. S14a. The\nsame BiVO and Ag/AgCl electrodes were kept as the working and reference\n4\nelectrodes, but the counter electrode was a platinum wire instead of the platinum\nmesh to reduce visual obstruction. The PEC cell was operated at 0.5 mA cm-2 in\nstatic 0.1 M KP (pH = 7), and bubble distribution was recorded using a Zeiss\ni\nMakro-Planar T* 2/100 lens coupled to a high-speed camera (HS5-Q, 2560 × 2048\npixels), with a resolution limit of ~45–50 μm in bubble diameter. The\nrepresentative images of bubbles on the BiVO surface under different pressures\n4\nare shown in Fig. S14b.”\n• Supplementary Information, page 23 – 24: Figure R3 has now been added as Fig.\nS14 in the revised version.\n3. Although the manuscript focuses on short-term PEC performance enhancement, it is\nrecommended to include stability data from long-term operation under pressurised\nconditions (e.g., 10 hours).\nResponse: We thank the reviewer for this constructive suggestion. As noted in our\nresponse to Comment 6 of Reviewer #1, additional long-term stability measurements\nhave now been conducted on the back-illuminated BiVO -based PEC water splitting\n4\ncell under different pressures. Chronoamperometry (CA) tests were conducted under\npotentiostatic conditions at 1.23 V vs. RHE, in a 0.1 M potassium phosphate buffer\n(KP, pH = 7) with an electrolyte flow of 4.6 mL s-1. Two illumination intensities were\ni\napplied: 1 sun (AM1.5G) and 10 suns. Since uncatalyzed and undoped BiVO samples\n4\nwere used in our study, photocurrent decrease is expected;5,6 however, we emphasize\nthat the focus here is to understand if pressure elevation introduces any effect on\ndegradation. The CA test at 1 sun was terminated when the photocurrent dropped by\n~50%; while under 10 suns, the test was stopped at ~100% photocurrent loss.\nInterestingly, as shown in Fig. R2, no significant difference is observed in the\nnormalized photocurrent plots at different pressure, both at 1 and 10 suns. This\nsuggests that the BiVO degradation mechanism is the same at different pressures.\nbut further detailed investigations are required to reveal the mechanisms. Finally,\nwithin the pressure range investigated, no gas or liquid leakage was detected during\nprolonged high-pressure operation, confirming the mechanical integrity and\noperational robustness of the system. This discussion has now been included in the\nManuscript and the Supplementary Information.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 21, line 362 – 371: “To investigate whether pressure\nelevation introduces any effect on the stability of the back-illuminated BiVO\n4\nphotoanode, potentiostatic experiments were performed with the same PEC cell\nat 1.23 V vs. RHE. Illumination intensities of 1 sun and 10 suns were applied. As\nshown in Fig. S15, the photocurrent decreases with time, indicating degradation;\nthis is expected since our BiVO is uncatalyzed and has no protection layer.25,63\nsuggesting that pressure elevation has minimal impact on the degradation\nbehavior of the BiVO photoelectrode. Moreover, no gas or liquid leakage was\n4\nobserved during the extended high-pressure operation, indicating reliable\nstructural mechanical integrity and operational stability of the system.”\n• Supplementary Information, page 25: Figure R2 has now been added as Fig. S15\nin the revised version.\n4. While the authors state that the bulk electrolyte temperature change is less than\n2.5°C, local heating of the photoelectrode surface under 10 times solar irradiance\nover extended periods may affect interfacial dynamics, which should be carefully\nconsidered.\nResponse: We are grateful to the reviewer for highlighting this important issue. To\nbetter assess the thermal effects, we have now measured the bulk electrolyte\ntemperature using a K-type thermocouple during stability tests under 10 suns\nillumination (see setup in Fig. S10a, Supplementary Information). The results, shown\nas symbols in Fig. R5a, indicate that the electrolyte temperature increased by\napproximately 5 °C at 1 bar and 3 °C at 5 bar under continuous illumination. Notably,\nthe similar temperature trends observed at different pressures suggest that our\nperformance comparisons under elevated pressures remain valid.\nNevertheless, we fully agree with the reviewer that bulk electrolyte temperature\nmeasurements do not necessarily reflect localized heating at the photoelectrode\nsurface, which may influence interfacial reaction kinetics and material stability under\nconcentrated light.14-16 Direct measurement of the photoelectrode surface\ntemperature is challenging with our high-pressure PEC cell geometry, so we\nsimulated the temperature distribution at the photoanode surface using a heat\ntransfer model, detailed below.\nModel descriptions\nProblem definition and assumptions\nWe simplified the PEC cell geometry (see Fig. 1b, Manuscript and Fig. S1,\nSupplementary Information) for simulation of coupled heat transfer in solids and\nfluids. The BiVO /FTO photoanode was modelled as a quartz substrate with\n4\nequivalent thickness. The fluid domain (electrolyte) was assumed to be liquid water\nunder fully developed laminar flow, with inlet velocity taken from PIV measurements\n(Fig. 2, Manuscript). An inward heat flux of 5600 W m⁻² (corresponding to 56%\nabsorption under 10 suns, per Ref. 17) was applied to the illuminated quartz surface.\nThe edges of the electrode, which are in contact with the rubber gasket and stainless-\nsteel cell body in our setup (see Fig. S1, Supplementary Information), as well as the\nright-side boundary of the fluid domain (in contact with the counter electrode), were\ntreated as adiabatic boundaries, as shown in Fig. R4a and R4c. As a result, the\nsimulated surface temperature of the photoelectrode represents a worst-case\nscenario for thermal accumulation.\nFluid dynamics\nThe following continuity equation for incompressible Newtonian fluids that conserves\nmass was considered,\n𝜌𝛻 ∙𝐯 = 0 (R1)\nwhere 𝐯 is the velocity field. The density of the fluid phase (𝜌) is constant. This means\nthat the only way to change the mass of the computational domain (Ω ) is by\nconvection, which is expressed in the following equation.\n𝜌(𝐯∙𝛻)𝐯 = 𝛻∙[−𝑝𝐈+𝐊]+𝐅 (R2)\n𝐊 = 𝜇(𝛻𝐯+(𝛻𝐯)T) (R3)\nMomentum conservation was solved using the Navier-Stokes equation in (eqn. R2).\n𝑝\nis pressure, 𝐈 is the identify tensor, while 𝐅 is the external volume forces, e.g., gravity.\n𝐊 is the viscous stress tensor, and expressed in (eqn. R3), while 𝜇 is dynamics\nviscosity.\nBoundary conditions\nThe liquid inlet was considered as fully-developed laminar flow,\n2\n2𝑥\n𝐯 = (0, 1.5𝑈{1−( ) })\n𝐿𝑥\nwhere 𝐿 is the width of the flow channel and equals 4 mm, and 𝑈 denotes the\n𝑥\naverage inlet flow velocity. The 𝑈 values were obtained from our PIV measurement\npresented in Fig. 2a (Manuscript) and set as 7.33 mm s-1. The outlet pressure was set\nas 0 bar and 4 bar, respectively, to mimic the pressure settings of 1 bar and 5 bar in\nour experiments. The side walls were treated as no slip boundaries, as shown in Fig.\nR4b.\nHeat transfer in solids and fluids\nThe heat transfer in the solid part was solved using the following equation.\n𝑑 𝜌𝐶 𝐯∙𝛻𝑇 +𝛻∙𝐪 = 𝑑 𝑄+𝑞 (R5)\n𝑧 𝑝 𝑧 0\n𝐪 = −𝑑 𝑘𝛻𝑇 (R6)\n𝑧\nHere, 𝑑 is the thickness of the domain and equals 30 mm. 𝐶 is the specific heat\n𝑧 𝑝\ncapacity of the fluid, 𝐯∙𝛻𝑇 represents the convection term, and 𝛻∙𝐪 solves heat\nconduction (divergence of heat flux). The heat flux 𝐪 is defined in (eqn. R6), with 𝑘\nrepresenting the thermal conductivity of the material (𝑘 = 1.4 W (m K)−1 for quartz\nglass). On the right-hand side of the equation, 𝑑 𝑄 denotes the volumetric heat\n𝑧\nsource, and 𝑞 is the surface heat source.\nassumed. The viscous dissipation term was included as\n𝑄 = 𝜏:𝛻𝐯 (R7)\nvd\nwhere 𝑄 is the dissipative heat generation term, 𝜏 is the viscous stress tensor.\nvd\nBoundary conditions\nAn inward heat flux of 5600 W m⁻² (corresponding to 56% absorption under 10 suns,\nper Ref. 17) was applied at the illuminated section of the BiVO photoanode. The edges\n4\nof the electrode, which are in contact with the rubber gasket and stainless-steel cell\nbody in our setup (see Fig. S1, Supplementary Information), as well as the right-side\nboundary of the fluid domain (in contact with the counter electrode), were treated as\nadiabatic boundaries, as shown in Fig. R4c. The numerical settings for other\nboundaries are shown in Fig. R4c.\nNumerical treatment\nThe model was solved in fully coupled mode using COMSOL Multiphysics® 6.2 with\nthe PARDISO solver. A P1+P1 scheme was used for pressure-velocity coupling, and\ntime integration was performed using the generalized alpha method with adaptive\ntime stepping. The computational domain was discretised into 79,927 elements,\nyielding an average mesh quality of 0.86 (with 1.0 indicating an ideal mesh).\nSimulations were run on a high-performance workstation (Intel Xeon E5-2650 v2, 64\ncores, 256 GB RAM) with a relative tolerance of 0.005.\nFigure R4. Two-dimensional heat transfer model used for our simulations. The\nsimplified geometry of our setup is shown in (a). (b) Governing equations and\nboundary conditions for the fluid flow model. (c) Heat transfer model in the solid and\nfluid domains. The BiVO /FTO sample was modelled using a quartz glass piece, with\n4\nthe electrolyte was treated as liquid water. An inward heat flux of 5600 W m⁻²\n(corresponding to 56% absorption under 10 suns, per Ref. 17) was applied at the\nilluminated section of the BiVO photoanode. The edges of the electrode, which are in\n4\ncontact with the rubber gasket and stainless-steel cell body in our setup (see Fig. S1,\nSupplementary Information), as well as the right-side boundary of the fluid domain (in\ncontact with the counter electrode), were treated as adiabatic boundaries in panels\n(a) and (c). As a result, the simulated surface temperature of the photoelectrode\nrepresents a worst-case scenario for thermal accumulation. The liquid flow is\nconsidered as the fully-developed laminar flow, with the average velocity obtained\nfrom our PIV measurements (shown in Fig. 2a, Manuscript).\nFigure R5a compares the simulated surface temperature with experimental bulk\nmeasurements. Both show ~5 °C rise under steady-state illumination, validating the\nmodel’s ability to capture thermal behavior. In addition, Figure R5b-R5d shows a\nmaximum surface temperature rise of ~13 °C at the photoelectrode under both\npressure conditions. While this estimate may be somewhat exaggerated due to\nsimplified boundary assumptions—specifically, the adiabatic treatment of the\nelectrode sealing interfaces and the electrolyte boundary adjacent to the counter\nelectrode—it nonetheless underscores the potential for localized heating at the\nphotoelectrode surface. Such a temperature increase can indeed facilitate reaction\nkinetic18 and accelerate photoelectrode material degradation14. These findings\nemphasize the importance of thermal management in high-pressure PEC systems\nunder concentrated sunlight. Advanced cooling strategies, such as those\ndemonstrated in Ref. 19 and Ref. 6,17, can be implemented to prevent photoelectrode\noverheating under concentrated sunlight.\nImportantly, the similar thermal trends observed in both our measurements and\nsimulations at different pressures support the validity of our comparative\nperformance analysis, as illustrated in Fig. R5. We have added a new Supplementary\nNote detailing the model and results, and included additional discussion in the\nManuscript.\nFigure R5. Thermal measurement and simulation results during the stability test\nunder ~10 suns illumination. (a) Comparison between simulated and measured\ntemperature rise in the bulk electrolyte at different pressures. (b) Simulated surface\ntemperature of the photoelectrode after ~3.5 hours stability tests. Temperature\ndistribution colormaps at the end of the test at (c) 1 bar and (d) 5 bar. During the\nstability measurement, undoped and uncatalyzed BiVO was used as the photoanode,\n4\nwith a platinum mesh as the counter electrode (CE), and Ag/AgCl as the reference\nelectrode. The PEC cells were operated potentiostatically at 1.23 V vs. RHE in a 0.1 M\npotassium phosphate (KP) buffer solution (pH = 7), and an electrolyte flow rate of 4.6\ni\nmL s-1 was maintained during the stability measurements. A K-type thermocouple\nwas used to measure the electrolyte temperature, according to the digital\nphotographs of the measurement setup in Fig. S10a, Supplementary Information.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 21 – 22, line 372 – 393: “A heat transfer model was\ndeveloped to evaluate the thermal effect associated with the prolonged operation\nunder higher solar concentration, as detailed in Supplementary Note S4 (see Fig.\nS16). The simulated results were validated against experimentally measured bulk\nelectrolyte temperature, as shown in Fig. S17a. Despite its simplicity, this heat\ntransfer model effectively captures the thermal trends observed during stability\ntesting under ~10 suns illumination. Specifically, the measured bulk electrolyte\ntemperature increased by approximately 5 °C at 1 bar and 3 °C at 5 bar under\ncontinuous illumination, while the model predicts a comparable rise of ~5 °C for\nboth pressures. In addition, the simulation reveals that the maximum surface\ntemperature rise of the photoelectrode can reach ~13 °C above ambient (Fig.\nS17b-S17d). While this value may be somewhat overestimated due to simplified\nboundary assumptions—namely, adiabatic treatment of the electrode sealing\ninterfaces and the electrolyte boundary adjacent to the counter electrode—it\nnonetheless highlights the potential for localized heating at the photoelectrode\nsurface. Such a temperature rise can indeed facilitate interfacial kinetics64 and\ncould potentially accelerate material degradation under extended operation.65\nThese findings emphasize the importance of effective thermal management in\nhigh-pressure PEC systems under concentrated sunlight. Advanced cooling\nstrategies, such as those demonstrations performed by Haussener and co-\nauthors,9,11,25 can be implemented to prevent photoelectrode overheating under\nconcentrated sunlight. Importantly, the consistent trends observed across under\ndifferent pressures, both experimentally and in simulation, support the validity of\nour comparative performance analysis under elevated pressures.”\n• Supplementary Information, page 26 – 32: the multiphysics model introduced\nabove has been included as Supplementary Note S4 in the revised version.\n• Supplementary Information, page 29 – 32: Figure R4 and Figure R5 have now been\nadded as Fig. S16 and Fig. S17 in the revised version, respectively.\n5. It would be valuable for the authors to briefly discuss how the high-pressure PEC\nmodule might be integrated into a practical solar water splitting system.\nResponse: We sincerely thank the reviewer for this important suggestion. As noted in\nour response to Comment 4 of Reviewer #1, three possible strategies are proposed\nfor storing or utilizing the high-pressure H gas generated by the PEC water splitting\n2\ncells, as illustrated schematically in Fig. R1. First, the produced H can be stored in a\n2\nhigh-pressure gas cylinders for transportation and later use. Second, it can be\nabsorbed by the solid-state H storage materials, such as MgNi, ZrCr, and LaNi -\n2 5\nbased metal hydrides. Third, the produced high-pressure H can be directly fed into\n2\nhigh-pressure hydrogen applications, such as hydrogenation reactors. This way, the\nelevated pressure can be maintained while collecting or directly utilizing the\npressurized H . We have now added a brief discussion in the revised Manuscript and\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 29, line 545 – 555: “In principle, the system can be self-\npressurized via gas evolution during operation, potentially avoiding additional\nenergy input for pressurization other than the thermodynamic penalty of higher-\npressure operation. In the pressure range of up to 20 bar, our recent analysis has\nshown that the benefits largely outweigh the thermodynamic penalty.35\nNevertheless, we acknowledge that achieving and maintaining elevated pressure\nintroduces technical and economic challenges, including the need for reinforced\ncell housings, thicker optical windows, enhanced sealing, pressure-rated tubing,\nand precise back-pressure control—all of which would increase capital and\nmaintenance costs. A comprehensive techno-economic analysis is needed to\nevaluate the economic trade-offs and inform possible scale-up configurations.”\n• Supplementary Information, page 38: Figure R1 has now been added as Fig. S22\nin the revised version.\n6. For better clarify the author’s point of view and attract more readers, it is\nrecommended to add citations.(Angew. Chem. Int. Ed. 2020, 59, 23094-23099; Nat.\nSustain.2025,doi.org/10.1038/s41893-025-01530-y; Matter 2024,7,2278-2293;\nAngew. Chem. Int. Ed. 2024,136,e202317414.).\nResponse: We appreciate the reviewer’s suggestion. The key conclusions of the\nrecommended papers have now been integrated to enhance the Discussion section,\nand cited as Refs. 16, 17, 60, 62 in the revised Manuscript.\nAssociated changes to the Manuscript:\n• Results, page 28 – 29, line 522 – 533: “First, in this initial demonstration of high-\npressure PEC water splitting, only two representative and contrasting systems are\nselected: BiVO as a metal oxide photoanode under back-illumination and a\n4\nplatinized triple-junction III-V photocathode under front-illumination. These\nphotoelectrodes were intentionally chosen as benchmark photoelectrodes since\nthey have been extensively studied in PEC water splitting research and\nbeyond.16,17,66,73-77 They are relatively stable and have demonstrated adequately\nhigh photocurrent; this ensures that performance variations in our study can be\nattributed primarily to pressure-related effects rather than material-related\nfactors, thus providing a reliable basis to assess their impact on device operation.”\nReviewer #3:\nThis manuscript demonstrates the operating photoelectrochemical (PEC) water splitting\ncells at elevated pressures (up to 8 bar), addressing a bottleneck in scalable solar\nhydrogen production. The authors developed a custom high-pressure flow cell and\ninvestigated two configurations: (1) a back-illuminated BiVO₄-based PEC cell, and (2) a\nfront-illuminated integrated platinumized triple-junction (3J) III-V PEC device. For the\nBiVO₄ system, increased pressure suppressed gas bubble formation, resulting in up to a\n50% improvement in photocurrent under 10 suns illumination. Operando imaging\nconfirmed that the improved performance stemmed from reduced bubble coverage and\nsize. In contrast, the 3J III-V PEC device exhibited pressure-independent photocurrent\ndue to its long carrier diffusion length, thin dispersed Pt catalyst layer, and highly\nhydrophilic surface, which facilitated bubble detachment and minimized recombination\nlosses. The manuscript underscores the configuration-dependent effects of pressure on\nPEC performance and highlights the benefit of high-pressure operation: directly\ngenerating pressurized hydrogen to reduce downstream compression energy costs.\nOverall, I think this is a great paper and the manuscript could meet the standard of Nature\nCommunications after addressing the following issues.\nResponse: We sincerely appreciate the reviewer’s positive feedback. We have carefully\naddressed all the listed comments/suggestions. The point-to-point responses are as\nfollow:\n1. The figure clarity should be improved. The authors need to make sure that the readers\ncan gain the key breakthrough of this work by reading the introduction and figures.\nHowever, figures only contain the recorded data without highlighting the novelty of\nthis work. For example, in Figure 1, they need to add Figure 1c to highlight the novelty\nas compared with the previous approach. Besides, there are too many labels in Figure\n1b about the whole system, and I do not know how significance of this work as\ncompared with other works. The same things happen in other figures, the authors\nneed to improve the figure quality and highlight the key novelty.\nResponse: We sincerely thank the reviewer for this constructive suggestion. In the\nrevised Manuscript, we have significantly improved figure clarity to better\ncommunicate the core innovation of our work, while assuring the formatting is\nconsistent throughout the Manuscript. As an example, Figure 1 has been redesigned\nin the revised version and is now presented as Fig. R6. A new schematic (Fig. R6a)\nhas been added to clearly illustrate the research gap and motivation for operating PEC\nwater splitting devices at elevated pressure. The original system overview (previously\nFig. 1b) has been moved to the Supplementary Information (now Fig. S1) to reduce\nvisual complexity and enhance readability in the main text.\nAdditionally, we have carefully revised the figure annotations across the Manuscript\nto better highlight the novelty and significance of our findings, as suggested. We hope\nthese changes will improve the overall clarity and impact of the figures.\nFigure R6. (a) Motivation for operating PEC water splitting devices at elevated\npressure, and (b) schematic illustration of the high-pressure flow cell (HPFC)\ndeveloped in this work. In (a), a typical PEC water splitting setup is shown on the left,\nhighlighting that all demonstrations to date operate under atmospheric pressure. In\ncontrast, most downstream hydrogen applications—such as fuel cells,\nhydrogenation processes, and ammonia synthesis—require high-pressure hydrogen,\nas indicated on the right. This reveals a critical research gap between current\nacademic PEC studies and industrial-scale requirements. Panel (b) presents the\nschematic of the HPFC designed, constructed, and tested in this study. An exploded\ntechnical view is provided in Fig. S1, and the digital photographs of the assembled\nHPFC are shown in Fig. S2. The electrolyte is circulated by a rotary pump, achieving\nlaminar flow between the electrodes via an optimized flow distributor (see Fig. S2c).\nThe system is pressurized using external gas (N₂ or O₂, depending on the reaction),\nand the operating pressure is regulated by a back-pressure controller. Simulated\nAM1.5G illumination—with optional Fresnel lenses—enables solar concentrations\nup to ~10 suns. Electrolyte temperature is monitored at the outlet via a K-type\nthermocouple. The downstream pressure requirements for hydrogen infrastructure\nare adapted from Referecne20. For clarity, standard fittings (e.g., nuts, connectors,\nPTFE tubing) are omitted in the schematic.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 7 – 8: Figure R6 has now been added as Fig. 1 in the\nrevised version.\n• Supplementary Information, page 2 – 3: Figure 1b in the previous version\n(Manuscript) has now been added as Fig. S1 in the revised version.\n2. To add pressure for hydrogen production, the long-term stability and safety issues\nrequire a second consideration. During large-scale applications, will the materials\ndegrade under cyclic pressure/continuous pressure? Is there any auxiliary equipment\nrequired for pressure adding, and will this increase the total cost of the hydrogen\nproduction?\nResponse: We thank the reviewer for this valuable comment. We have addressed the\nconcern regarding long-term stability in our responses to Comment 6 of Reviewer #1\nand Comment 3 of Reviewer #2 by conducting additional long-term stability\nmeasurements on the back-illuminated BiVO -based PEC cell under varying\n4\npressures. Chronoamperometry (CA) tests were conducted under potentiostatic\nconditions at 1.23 V vs. RHE, in a 0.1 M potassium phosphate buffer (KP, pH = 7) with\ni\nan electrolyte flow of 4.6 mL s-1. Two illumination intensities were applied: 1 sun\n(AM1.5G) and 10 suns. The CA test at 1 sun was terminated when the photocurrent\ndropped by ~50%; while under 10 suns, the test was stopped at ~100% photocurrent\nloss. Since uncatalyzed and undoped BiVO samples were used in our study,\n4\nphotocurrent decrease is expected;5,6 however, we emphasize that the focus here is\nto understand if pressure elevation introduces any effect on degradation. Interestingly,\nas shown in Fig. R2, no significant difference is observed in the normalized\nphotocurrent plots at different pressure, both at 1 and 10 suns. This suggests that the\nBiVO degradation mechanism is the same at different pressures. Overall, our study\n4\ndoes not suggest any presence of pressure-induced degradation, but further detailed\ninvestigations are required to reveal the mechanisms. Finally, within the pressure\nrange investigated, no gas or liquid leakage was detected during prolonged high-\npressure operation, confirming the mechanical integrity and operational robustness\nof the system. This discussion has now been included in the Manuscript and the\nSupplementary Information.\nRegarding the reviewer’s comment on the need of further auxiliary equipment, we\nhave also addressed this in our response to Comment 3 of Reviewer #1. Reducing the\nneed for downstream hydrogen compression and suppression of bubble-induced\nlosses represent significant benefits of operating PEC water splitting cells at elevated\npressure. In our experiments, we supplied compressed gas to the cell for controlled\nconditions; however, in practical applications, the system can be self-pressurized\nthrough gas production during operation. At such, no additional energy input is\nrequired for system pressurization other than the thermodynamic penalty of higher-\npressure operation. Our recent analysis shows that the benefits still largely outweigh\nthis thermodynamic penalty in the pressure range of up to 20 bar.1 Nevertheless, we\nfully acknowledge that increasing the operating pressure may also introduce\nnontrivial technical and economic challenges. These include the need for thicker\nglass components, more robust sealing strategies to prevent electrolyte and gas\nleakage, and the use of high-pressure-rated tubing and control systems—all of which\ncan increase the capital and maintenance costs. Detailed techno-economic\nassessment beyond the scope of our current study is thus necessary to\ncomprehensively evaluate the economic feasibility of operating PEC systems at\nelevated pressures. We have now added a brief discussion regarding the points above\nin the revised Manuscript.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 21, line 362 – 371: “To investigate whether pressure\nelevation introduces any effect on the stability of the back-illuminated BiVO\n4\nphotoanode, potentiostatic experiments were performed with the same PEC cell\nat 1.23 V vs. RHE. Illumination intensities of 1 sun and 10 suns were applied. As\nshown in Fig. S15, the photocurrent decreases with time, indicating degradation;\nthis is expected since our BiVO is uncatalyzed and has no protection layer.25,63\nsuggesting that pressure elevation has minimal impact on the degradation\nbehavior of the BiVO photoelectrode. Moreover, no gas or liquid leakage was\n4\nobserved during the extended high-pressure operation, indicating reliable\nstructural mechanical integrity and operational stability of the system.”\n• Results (Manuscript), page 29, line 537 – 555: “On the system level, the benefits\nfrom operating PEC water splitting cells at elevated pressure should be ensured\nto outweigh the associated penalties. In addition to the suppression of bubble-\ninduced losses, reducing or eliminating the need for downstream hydrogen\ncompression represents a significant advantage. In practice, the elevated\npressure PEC cell can be connected directly to high-pressure H tanks, solid-state\n2\n(e.g., Haber-Bosch or hydrogenation reactors), as illustrated in Fig. S22. For\ninstance, when compressing H₂ to 875 bar, doing so by starting from 1 bar\nconsumes ~80% more energy than starting from 20 bar (~4.8 vs. ~2.7 kWh kg-1).78-\noperation, potentially avoiding additional energy input for pressurization other\nthan the thermodynamic penalty of higher-pressure operation. In the pressure\nrange of up to 20 bar, our recent analysis has shown that the benefits largely\noutweigh the thermodynamic penalty.35 Nevertheless, we acknowledge that\nachieving and maintaining elevated pressure introduces technical and economic\nchallenges, including the need for reinforced cell housings, thicker optical\nwindows, enhanced sealing, pressure-rated tubing, and precise back-pressure\ncontrol—all of which would increase capital and maintenance costs. A\ncomprehensive techno-economic analysis is needed to evaluate the economic\ntrade-offs and inform possible scale-up configurations.”\n• Supplementary Information, page 25: Figure R2 has now been added as Fig. S15\nin the revised version.\n3. The authors focused more on the breakthrough at the systematic level, and the\nmaterials used are simple. However, I still want the authors to clarify why BiVO is\n4\napplied in this system. What is the key advantage as compared with other catalysts?\nBesides, instead of only simple materials characterization in supplementary\ninformation, is there any difference/breakthrough on the materials level?\nResponse: We thank the reviewer for the thoughtful comment. Our primary focus in\nthe present study is to investigate the impact of operating pressure and gas bubble\ndynamics on the overall performance of PEC water splitting devices, rather than\nintroducing breakthroughs at the materials level. Accordingly, BiVO was intentionally\n4\nselected as a model photoanode due to its well-established performance, relative\nstability, and extensive characterization in the literature. BiVO is widely regarded as\n4\none of the highest-performing and most studied complex oxide photoelectrodes for\nPEC water oxidation. The use of benchmark material like BiVO allows for a clearer\n4\nattribution of performance changes to pressure-related effects, rather than to\nchanges in material properties. We have clarified this point in the revised Manuscript\nto prevent potential confusion.\nAssociated changes to the Manuscript:\n• Results (Manuscript), page 28 – 29, line 521 – 536: “We will now discuss some\ngeneral limitations of our study and provide an outlook for operating PEC devices\nat elevated pressure. First, in this initial demonstration of high-pressure PEC water\nsplitting, only two representative and contrasting systems are selected: BiVO as\n4\na metal oxide photoanode under back-illumination and a platinized triple-junction\nIII-V photocathode under front-illumination. These photoelectrodes were\nintentionally chosen as benchmark photoelectrodes since they have been\nextensively studied in PEC water splitting research and beyond.16,17,66,73-77 They are\nrelatively stable and have demonstrated adequately high photocurrent; this\nensures that performance variations in our study can be attributed primarily to\npressure-related effects rather than material-related factors, thus providing a\nreliable basis to assess their impact on device operation. At the same time,\nexpanding this comparison to additional semiconductors, such as α-Fe O and\n2 3\nWO photoanodes or Sb S and Cu O photocathodes, would offer further insights\n3 2 3 2\ninto any material-specific pressure effects. These studies are part of our ongoing\nefforts and will be reported in due course.”\n4. Please expand the discussion on pressure-dependent bubble nucleation dynamics.\nThe manuscript attributes performance gains to suppressed bubble evolution at\nhigher pressure, but the underlying physical mechanism (e.g., Laplace pressure,\ncritical bubble size, contact angle effects) should be further elaborated.\nResponse: We appreciate the reviewer’s insightful comment on pressure-dependent\nbubble nucleation dynamics. As noted in our response to Comment 2 of Reviewer #2,\nwe have conducted additional imaging experiments using a macroscopic\nshadowgraphy setup to observe oxygen bubble evolution on the BiVO photoanode\n4\nsurface (Fig. R3). Representative images (Fig. R3b) clearly show that increasing\npressure significantly suppresses gas bubble formation on the BiVO surface,\n4\nconsistent with localized observations presented in Fig. 4e–f of the Manuscript. At\n1 bar, bubbles were randomly distributed across the porous BiVO layer, with larger\n4\nbubble sizes observed near the top of the electrode—which can be attributed to\nhydrostatic pressure gradients. However, bubble presence was already negligible at\npressures above 2 bar. This insight should be considered for the scale-up design of\nPEC systems.\nWe also appreciate the reviewer’s suggestion to investigate pressure-dependent\nbubble nucleation dynamics. Unfortunately, direct observation of nucleation events\nwas not feasible due to the limited resolution of our current imaging setup. However,\nrelated studies, such as a recent work on bubble evolution on TiO micro-\n2\nphotoelectrodes,7 confirm that elevated pressure facilitates bubble detachment due\nto increased dissolved gas saturation—aligned with previous simulation results.8\nFinally, we emphasize that the topic of gas bubble dynamics attracts increasing\ninterests from researchers in the field, and a full understanding of this complex\ndynamics under pressure remains an open question. Advanced experimental and\ntheoretical techniques such as atomic force microscopy 9-11 and molecular dynamics\nsimulations 12,13 will be essential for future investigation.\nAdditional discussion has now been added to the revised Manuscript and\nSupplementary Information.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 20, line 352 – 361: “To further elucidate the effects of\npressure on gas bubble evolution throughout the whole surface of the 2 cm × 2 cm\nBiVO photoelectrode, we conducted macroscopic imaging of oxygen bubbles.\nthe Methods section. At 1 bar, bubbles appeared randomly across the porous\nBiVO surface (Fig. S14b), with larger bubble sizes observed near the top of the\n4\nelectrode; we tentatively attribute this observation to the presence of hydrostatic\npressure gradients. This insight is useful for the scale-up of PEC devices, as non-\nuniform bubble detachment may result in spatial variations in performance.\nHowever, at pressures above 2 bar, bubble presence was already negligible (Fig.\nS14b). This observation is consistent with our localized observations shown in Fig.\n4 e-f.”\n• Results (Manuscript), page 17, line 315 – 333: “Although bubble nucleation events\ncould not be directly resolved, recent work on bubble evolution on TiO₂ micro-\nphotoelectrodes showed that elevated pressure facilitates bubble detachment,\nprimarily due to higher dissolved gas saturation.48 Consistent with these findings,\nour measurements (Fig. 4e–f) revealed smaller and fewer bubbles under elevated\npressure. This behavior can be tentatively explained by the formation of a\nsupersaturated boundary layer (SSBL) of dissolved gas near the (photo)electrode\nat higher pressure. Our recent simulation showed that the SSBL facilitates bubble\ndetachment from the surface of the (photo)electrode via Marangoni convection.49\nAdditional contributing factors may include an increased Laplace pressure and a\nlarger critical radius for nucleation at higher pressures, both of which inhibit initial\nbubble formation. Interfacial properties, such as the gas–liquid contact angle,\nmay also play a role in pressure-dependent detachment dynamics, but direct\nexperimental measurement of contact angles and nano/micro bubbles on\nsemiconductor surfaces (e.g., BiVO photoelectrode) under elevated pressure\n4\nremains technically challenging. High-resolution tools such as atomic force\nmicroscopy and spectroscopy techniques 50-60 and molecular dynamics\nsimulation 61,62 will be essential, which are beyond the scope of this study.\nFurthermore, the porous morphology of our BiVO samples (see Fig. S6)\n4\nintroduces additional complexity, as it may lead to random spatial distribution of\nnanobubbles, making their detection even more complicated.”\n• Methods (Manuscript), page 38 – 39, line 757 – 766: “To enable full-field\nvisualization of the bubble evolution on the BiVO photoanode (2 cm × 2 cm), the\n4\nsetup was further modified as shown in the schematic illustration in Fig. S14a. The\nsame BiVO and Ag/AgCl electrodes were kept as the working and reference\n4\nelectrodes, but the counter electrode was a platinum wire instead of the platinum\nmesh to reduce visual obstruction. The PEC cell was operated at 0.5 mA cm-2 in\nstatic 0.1 M KP (pH = 7), and bubble distribution was recorded using a Zeiss\ni\nMakro-Planar T* 2/100 lens coupled to a high-speed camera (HS5-Q, 2560 × 2048\npixels), with a resolution limit of ~45–50 μm in bubble diameter. The\nrepresentative images of bubbles on the BiVO surface under different pressures\n4\nare shown in Fig. S14b.”\n• Supplementary Information, page 23 – 24: Figure R3 has now been added as Fig.\nS14 in the revised version.\n5. The observation of bubbles plays a significant role in understanding the reason for\nhigh water splitting. Therefore, I recommend improving the bubble imaging resolution.\nCan you specify the optical resolution limits of your cameras, and whether any\ncontrast-enhancing techniques (e.g., DIC, phase contrast) were attempted? Suggest\nfuture tools (e.g., AFM, total internal reflection microscopy) to visualize nanobubbles.\nResponse: We appreciate the reviewer’s valuable comment and constructive\nsuggestion. In our current study, two imaging setups were employed: a high-speed\nmicroscopic imaging system using a telescopic lens for localized observation (as in\nFig. 4e–f, Manuscript), and a macroscopic imaging system using a Zeiss Makro-\nPlanar T* 2/100 lens to capture bubble evolution over the entire 2 cm × 2 cm BiVO\n4\nelectrode surface (as in Fig. S14b). The macroscopic system, coupled with a 2560 ×\n2048 pixel high-speed camera (HS5-Q), provides an estimated spatial resolution of\n~45–50 µm. The localized telescopic setup achieves higher resolution (~10 µm),\nalbeit over a smaller field of view (~5 mm × 4 mm). These values define the current\noptical detection thresholds in our experiments.\nDue to hardware constraints and the use of thick optical windows in the pressurized\ncell, contrast-enhancing techniques such as Differential Interference Contrast (DIC)\nor phase contrast microscopy were not feasible. These methods require specialized\nobjectives, precise optical alignment, and short working distances, all of which are\nincompatible with our sealed high-pressure PEC system.\nTo enhance image clarity, we applied post-processing techniques in ImageJ, including\nbackground subtraction and contrast normalization. These helped improve visual\ndistinction of gas bubbles, although lighting non-uniformities and optical limitations\nstill affected the overall image quality. Nevertheless, since the experimental\nconditions are consistent across all tests, relative comparisons between our\nexperimental results remain valid.\nWe fully agree that future investigations into nanobubble dynamics will benefit from\nhigher-resolution tools. Techniques such as atomic force microscopy (AFM), total\ninternal reflection microscopy (TIRM), and high-speed confocal imaging are under\nactive consideration in our ongoing work to explore nucleation, growth, and\ndetachment processes under pressure.\nWe have now clarified these limitations and future directions in the revised\nManuscript.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 20, line 352 – 361: “To further elucidate the effects of\npressure on gas bubble evolution throughout the whole surface of the 2 cm × 2 cm\nBiVO photoelectrode, we conducted macroscopic imaging of oxygen bubbles.\nthe Methods section. At 1 bar, bubbles appeared randomly across the porous\nBiVO surface (Fig. S14b), with larger bubble sizes observed near the top of the\n4\nelectrode; we tentatively attribute this observation to the presence of hydrostatic\npressure gradients. This insight is useful for the scale-up of PEC devices, as non-\nuniform bubble detachment may result in spatial variations in performance.\nHowever, at pressures above 2 bar, bubble presence was already negligible (Fig.\nS14b). This observation is consistent with our localized observations shown in Fig.\n4 e-f.”\n• Methods (Manuscript), page 38 – 39, line 737 – 766: “Two complementary imaging\nsetups were used to observe gas bubble evolution on the electrodes from different\nangles during operation, as shown by the schematic in Fig. S6. For the back-\nilluminated PEC cell, a high-speed camera (HS5-Q Quad HD+ Resolution (2560 ×\n2048 pixels) with a Navitar Zoom 6000 Lens System, FasTec) was used to capture\nthe side-view of gas bubble evolution, and the shadowgraphy of bubbles on the\nelectrodes was captured using a LaVision camera system (Imager SX 6M CCD\ncamera, 2752 × 2200 pixels). For the front-illuminated PEC cell, only the high-\nspeed camera was used to observe the side-view of gas bubbles. The camera was\nfixed higher than the 3J III-V photoelectrode so that the surface of the sample was\nvisible. Neutral density (ND) filters with various ND factors were used depending\non the light condition to enhance the imaging quality. The localized telescopic\nimaging configuration achieved a spatial resolution of approximately 10 µm over a\nlimited field of view (~5 mm × 4 mm), suitable for capturing detailed bubble\ndynamics in selected regions of interest (see Fig. 4e-f). The images were\nprocessed using ImageJ, with additional validation using DaVis 10 (LaVision). To\nenhance image clarity, we applied post-processing techniques in ImageJ,\nincluding background subtraction and contrast normalization. These helped\nimprove visual distinction of gas bubbles, although lighting non-uniformities and\noptical limitations still affected the overall image quality. Nevertheless, since the\nexperimental conditions are consistent across all tests, relative comparisons\nbetween our experimental results remain valid.\nTo enable full-field visualization of the bubble evolution on the BiVO photoanode\n4\n(2 cm × 2 cm), the setup was further modified as shown in the schematic\nillustration in Fig. S14a. The same BiVO and Ag/AgCl electrodes were kept as the\n4\nworking and reference electrodes, but the counter electrode was a platinum wire\ninstead of the platinum mesh to reduce visual obstruction. The PEC cell was\noperated at 0.5 mA cm-2 in static 0.1 M KP (pH = 7), and bubble distribution was\ni\nrecorded using a Zeiss Makro-Planar T* 2/100 lens coupled to a high-speed\ncamera (HS5-Q, 2560 × 2048 pixels), with a resolution limit of ~45–50 μm in\nbubble diameter. The representative images of bubbles on the BiVO surface\n4\nunder different pressures are shown in Fig. S14b.”\n6. Since this technique requires 10 sun for water splitting, the authors need to consider\npossible heat accumulation at high solar flux. Could longer operation at 10 suns lead\nto cumulative heating or catalyst degradation?\nResponse: We thank the reviewer for rising this important issue. As noted in our\nresponse to Comment 4 of Reviewer #2, we conducted additional simulations and\nexperiments to monitor the thermal effect of continuous illumination of ~10 suns on\nthe BiVO₄ photoelectrode. The heat transfer model is described in Fig. R4 and the\nresults are shown in Fig. R5.\nIn brief, the simulated results were validated against experimentally measured bulk\nelectrolyte temperature, as shown in Fig. R5a. Despite its simplicity, this heat transfer\nmodel effectively captures the thermal trends observed during stability testing under\n~10 suns illumination. Specifically, the measured bulk electrolyte temperature\nincreased by approximately 5 °C at 1 bar and 3 °C at 5 bar under continuous\nillumination, while the model predicts a comparable rise of ~5 °C for both pressures.\nIn addition, the simulation reveals that the maximum surface temperature rise of the\nphotoelectrode can reach ~13 °C above ambient (Fig. R5b-R5d). While this estimate\nmay be somewhat overestimated due to simplified boundary assumptions—namely,\nadiabatic treatment of the electrode sealing interfaces and the electrolyte boundary\nadjacent to the counter electrode—it nonetheless highlights the potential for\nlocalized heating at the photoelectrode surface. Such a temperature rise can\nenhance interfacial kinetics 18 and could potentially accelerate material degradation\nunder extended operation 14. These findings emphasize the importance of effective\nthermal management in high-pressure PEC systems under concentrated sunlight.\nAdvanced cooling strategies, such as those demonstrated in Ref. 19 and Ref. 17, can be\nimplemented to prevent photoelectrode overheating under concentrated sunlight.\nImportantly, the consistent trends observed across under different pressures, both\nexperimentally and in simulation, support the validity of our comparative\nperformance analysis under elevated pressures.\nWe have now addressed and discussed this point in the revised Manuscript and\nSupplementary Information.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 21 – 22, line 372 – 393: “A heat transfer model was\ndeveloped to evaluate the thermal effect associated with the prolonged operation\nunder higher solar concentration, as detailed in Supplementary Note S4 (see Fig.\nS16). The simulated results were validated against experimentally measured bulk\nelectrolyte temperature, as shown in Fig. S17a. Despite its simplicity, this heat\ntransfer model effectively captures the thermal trends observed during stability\ntesting under ~10 suns illumination. Specifically, the measured bulk electrolyte\ntemperature increased by approximately 5 °C at 1 bar and 3 °C at 5 bar under\ncontinuous illumination, while the model predicts a comparable rise of ~5 °C for\nboth pressures. In addition, the simulation reveals that the maximum surface\ntemperature rise of the photoelectrode can reach ~13 °C above ambient (Fig.\nS17b-S17d). While this value may be somewhat overestimated due to simplified\nboundary assumptions—namely, adiabatic treatment of the electrode sealing\ninterfaces and the electrolyte boundary adjacent to the counter electrode—it\nnonetheless highlights the potential for localized heating at the photoelectrode\nsurface. Such a temperature rise can indeed facilitate interfacial kinetics64 and\ncould potentially accelerate material degradation under extended operation.65\nThese findings emphasize the importance of effective thermal management in\nhigh-pressure PEC systems under concentrated sunlight. Advanced cooling\nstrategies, such as those demonstrations performed by Haussener and co-\nauthors,9,11,25 can be implemented to prevent photoelectrode overheating under\nconcentrated sunlight. Importantly, the consistent trends observed across under\ndifferent pressures, both experimentally and in simulation, support the validity of\nour comparative performance analysis under elevated pressures.”\n• Supplementary Information, page 26 – 32: the multiphysics model introduced\nabove has been included as Supplementary Note S4 in the revised version.\n• Supplementary Information, page 29 – 32: Figure R4 and Figure R5 have now been\nadded as Fig. S16 and Fig. S17 in the revised version, respectively.\nReviewer #4:\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nResponse: We sincerely appreciate the reviewer’s effort and time for reviewing our work.\nWe have carefully addressed all the listed comments/suggestions. We hope that this\nrevised version of our Manuscript and Supplementary Information will be considered\nfavourably.\nReferences\nphotoelectrochemical water splitting via multiphysics modeling. Nature\nCommunications 15, 4944 (2024).\nproduct crossover in solar-driven, membrane-less water splitting. Cell Reports\nPhysical Science 2, 100358 (2021).\nCurrent Opinion in Electrochemistry 32, 100881,\ndoi:https://doi.org/10.1016/j.coelec.2021.100881 (2022).\nThe Journal of Physical Chemistry C 119, 25145-25155 (2015).\ntransformations of bismuth vanadate photoanodes. Nature Communications 7,\n12012, doi:10.1038/ncomms12012 (2016).\nPhotoelectrochemical behaviour of photoanodes under high photon fluxes.\nJournal of Materials Chemistry A 11, 23895-23908 (2023).\ndynamics of photoelectrochemical water splitting. Physics of Fluids 36 (2024).\non bubble detachment from planar (photo) electrodes. Cell Reports Physical\nScience 5 (2024).\nin operando analysis of water-splitting catalysts and interfaces. Nature Energy 3,\n46-52 (2018).\nand temporal resolution. Energy & Environmental Science 8, 2863-2885 (2015).\nhydrogen evolution. Energy & Environmental Science 16, 2068-2079 (2023).\nheterogeneous wettability surface by using molecular dynamics simulation.\nInternational Journal of Hydrogen Energy 112, 160-171 (2025).\nnanometre bubbles: an in situ molecular dynamics study. Nanoscale 13, 11242-\n11249 (2021).\nOperating Conditions on the Degradation of BiVO4 Photoanodes. Advanced\nFunctional Materials, 2505102.\nphotocatalytic formic acid dehydrogenation. Joule 2, 549-557 (2018).\nof photo (electro) catalysts towards solar water splitting. Chemical Science 14,\n3415-3427 (2023).\nhydrogen production system using a concentrated integrated\nphotoelectrochemical device. Nature Energy, 1-11 (2023).\nmonolithic Mo: BiVO 4/SnO 2/Si by thermally increasing the minority carrier\ndiffusion length. Energy & Environmental Science 9, 2044-2052 (2016).\nelectrochemical hydrogen generator operating under concentrated solar\nirradiation. Nature Energy 4, 399-407 (2019).\nPure Hydrogen from Solar Power. Angewandte Chemie International Edition,\ne202218850 (2023)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all comments from reviewers and the manuscript can be accepted for publication in the current version.", "reviewer_2": "The author has already provided excellent solutions to the previously raised questions, but there are a few minor points below that require clarification.\n1. Could the author elaborate on how the maximum pressure value measured in the experimental setup was determined, and whether this value was constrained by safety considerations?\n2. What methodology was employed to determine the effective electrode area used in the photoelectrochemical (PEC) tests?\n3. How does variation in pressure influence the onset potential of the photoelectrode?", "reviewer_3": "I am happy with the revison and the manuscript could be accepted in its current form.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to reviewers’ comments\nManuscript ID: NCOMMS-25-32194A\nTitle: Operating photoelectrochemical water splitting cells at elevated pressure: a\ndemonstration using BiVO and platinized III-V semiconductor photoelectrodes\nin black, and our point-by-point responses as well as the corresponding changes to the\nmanuscript are shown in blue.\nReviewers' Comments:\nReviewer #1:\nThe authors have addressed all comments from reviewers and the manuscript can be\naccepted for publication in the current version.\nResponse: We sincerely appreciate the reviewer’s effort in reviewing our work and\nproviding the positive feedback. We are glad to know that the current version of our\nmanuscript is acceptable for publication.\nReviewer #2:\nThe author has already provided excellent solutions to the previously raised questions,\nbut there are a few minor points below that require clarification.\nResponse: We sincerely appreciate the reviewer’s positive feedback. We have carefully\naddressed all the additional minor comments/suggestions point-by-point below:\n1. Could the author elaborate on how the maximum pressure value measured in the\nexperimental setup was determined, and whether this value was constrained by\nsafety considerations?\nResponse: We appreciate the reviewer’s practical question. During our experiments,\nthe operating pressure was measured using a back-pressure controller (Bronkhorst\nHigh-Tech, uncertainty: 0.2%). The maximum pressure achieved was 8 bar(a), and\nhigher pressure has not been tested to comply with our current safety guidelines.\nTo avoid further confusion, this information has now been further clarified in our\nrevised Manuscript.\nAssociated changes to the Manuscript:\n• Results (Manuscript), page 5, line 111 – 119: “High pressure operation was\naccomplished by supplying compressed gas into the cell while regulating the\noutflow rate of gases with a back-pressure controller (Bronkhorst High-Tech,\nuncertainty: 0.2%). Note that this approach to regulate the pressure was chosen\nbecause it offers easy control and experimental convenience in a lab-based\nsetting; in practical applications, PEC water splitting cells can self-pressurize\nthrough gas production during operation, requiring only a pressure relieve valve as\na control mechanism. Prior to PEC experiments, the setup was tested for safe\noperation up to 8 bar(a); higher pressures have not been tested to comply with\ncurrent safety guidelines.”\n2. What methodology was employed to determine the effective electrode area used in\nthe photoelectrochemical (PEC) tests?\nResponse: For BiVO photoelectrodes, the effective area was determined by the size\n4\nof the illumination windows, which is 4 cm2 (visible in the inset in Fig. R1). For the\nplatinized 3J III-V photoelectrodes, the effective area was measured by analyzing\ndigital photographs of the photoelectrodes in ImageJ. The digital photographs\nincluded a portion of a ruler so that the scale could be set, as shown in Fig. R2. This\ninformation has now been clarified in the revised Manuscript and Supplementary\nInformation.\nFigure R1. Exploded view of the high-pressure flow cell (HPFC) developed for this\nstudy. The electrolyte (not shown) is circulated using a rotary pump, establishing\nlaminar flow between the electrodes via an optimized flow distributor (see Fig. S2c).\nThe system is pressurized with external gas (N or O , depending on the reaction),\n2 2\nand the operating pressure is regulated by a back-pressure controller. Simulated AM1.5G\nillumination—optionally concentrated using Fresnel lenses—enables solar\nintensities up to ~10 suns. The area of the illumination window (visible in the inset) is\n~4 cm2, which determines the maximum effective area of the photoelectrodes.\nObservation windows are fabricated from 5 mm-thick toughened glass, allowing\ndirect visualization of gas bubble evolution during operation at elevated pressure up\nto 8 bar. Standard fittings (e.g., nuts, connectors, PTFE tubing) are omitted in the\nschematic for clarity.\n[FIGURE REDACTED]\nFigure R2. Measurement of the effective area of the platinized 3J III-V photoelectrode.\nThe opensource software ImageJ was used, while the scale was calibrated with the\nruler visible in the digital photograph. For this electrode, the measured effective area\nwas 0.66 cm2.\nAssociated changes to the Manuscript and Supplementary Information:\n• Methods (Manuscript), page 29, line 598 – 602: “The reactor part was 3D printed\nusing stainless steel. A pair of sample holders can be installed, which allows a\n“face-to-face” arrangement of two 2 cm × 2 cm (maximum) samples with a\ndistance of ~4 mm. The maximum effective area of the photoelectrodes is\ntherefore ~4 cm2, which is constrained by the size of the illumination window (see\nFig. S1).”\n• Methods (Manuscript), page 35, line 666 – 679: “Samples were prepared for the\nPEC experiment in a “lollipop” format. Ag conductive tape was applied lengthwise\non the middle of glass substrate. To ensure a good ohmic contact, conductive Ag\nepoxy (Circuitworks CW2400, Chemtronics) was applied between Ag conductive\ntape and the back-side metallization of the prepared III-V cell. The conductive\nepoxy was cured on a hot plate at 110 °C for 10 min. The edges were covered using\na black insulating two-part silicone resin (101RF, Microset Products Ltd, see Fig.\nS18) and the remaining electrochemically active surface was determined from\ndigital images using the open-source software ImageJ, as shown in Fig. S18d. The\nresulting photoactive area of the sample is ~0.6 – 0.7 cm2. The fabricated\nphotoelectrode is shown in Fig. S18.”\n• Supplementary Information: Figure S1 has now been updated with Fig. R1 in the\nrevised version.\n• Supplementary Information: Figure R2 has now been added as Fig. S18d in the\nrevised version.\n3. How does variation in pressure influence the onset potential of the photoelectrode?\nResponse: We thank the reviewer for this insightful question. The effect of pressure\non the onset potential can already be observed in Fig. S11 (reproduced as Fig. R3\nbelow) and Figs. 5b-c (reproduced as Fig. R4 below). Overall, pressure change did not\nlead to a noticeable impact to the onset potential of either the BiVO or the platinized\n4\n3J III-V photoelectrodes. To further support this observation, we conducted additional\nlinear sweep voltammetry (LSV) measurements with BiVO photoelectrode under\n4\nchopped-light condition (1 sun). As shown in the LSV curves in Fig. R5, the onset\npotential of the photoelectrode remains unchanged with pressure variation. This\ninformation has now been clarified in the revised Manuscript and Supplementary\nInformation.\nFigure R3. (a) LSV scans of the back-illuminated BiVO photoelectrode at different\n4\npressure under AM1.5G (c = 1), the electrolyte was a 0.1 M KP (pH = 7) solution. (b)\ni\nand (c) show the same results as in panel (a) but under concentration of c ≈ 3 and c ≈\n10, respectively. Additional measurement at 1 bar (dashed curve) was performed\nafter that at 8 bar to check for reproducibility. (d) Same as (a) but with the addition of\nthe hole scavenger (0.5 M Na SO ). The compressed O gas was used for the\n2 3 2\nelectrolyte purging and cell pressurization. The scan rate for the LSV was 20 mV s-1\nand the electrolyte was circulated by a rotary pump at a constant flow rate of\napproximately 4.6 mL s-1.\nFigure R4. Linear sweep voltammetry (LSV) curves of the platinized 3J III-V\nphotoelectrode measured under AM1.5G 1 sun illumination at different pressures (a)\nin a 0.1 M KP solution (pH = 7) and (b) in a 0.1 M KP solution containing 0.5 M Na S O\ni i 2 2 8\nas electron scavenger.\nFigure R5. Chopped-light (1 sun) J−V characteristics of BiVO photoelectrode at\n4\ndifferent pressures, measured at a scan rate of 20 mV s-1 in 0.1 M KP (pH = 7). The\ni\nonset potentials for each curve are indicated with the red circles. Compressed O gas\n2\nwas used for electrolyte purging and cell pressurization. The electrolyte was\ncirculated by a rotary pump at a constant flow rate of approximately 4.6 mL s-1.\nAssociated changes to the Manuscript and Supplementary Information:\n• Results (Manuscript), page 15, line 275 – 282: “At 1 sun, the photocurrent remains\nnearly constant at ~0.5 mA cm-2 across the pressure range of 1 – 8 bar. At ~3 suns,\nhowever, the photocurrent increases from ~0.9 mA cm-2 at 1 bar to ~1.2 mA cm-2\nat 5 bar. The effect is more pronounced at ~10 suns, where the photocurrent\nincreases from ~1.7 mA cm-2 at 1 bar to ~2.5 mA cm-2 at 5 bar. Furthermore, as\nshown in Fig. S11, the onset potential of the BiVO photoelectrode remains\n4\nunchanged with varying operating pressure. These findings demonstrate a key\nadvantage of operating PEC water splitting devices under elevated pressure,\nparticularly at higher solar concentrations.”\n• Supplementary Information: Figure R5 has now been added as Fig. S11e in the\nrevised version.\nReviewer #3:\nI am happy with the revison and the manuscript could be accepted in its current form.\nResponse: We sincerely appreciate the reviewer’s effort in reviewing our work and\nproviding the positive feedback. We are glad to hear that the current version of our\nmanuscript is acceptable for publication.\nReviewer #4:\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nResponse: We sincerely appreciate the co-reviewer’s positive feedback and effort in\nevaluating our work."}, {"round": 3, "reviewer_comments": {"reviewer_2": "The author has answered all my questions and I don't have any more for now."}, "author_response": ""}]

Decision letter