Operando insights into stability of perovskite-based solar water splitting devices

Domain: chemistry
Published: 2026-01-26
Editorial outcome: accept

Abstract
Perovskite-based solar water splitting systems are promising candidates for addressing environmental challenges, exceeding the commercialization efficiency of solar-to-hydrogen (STH) at >10%. However, the operational stability remains suboptimal due to insufficient in situ/operando insights into charge carrier dynamics. Here, we investigate the role of charge accumulation on operational stability through operando catalytic modulation via near-infrared (NIR) toggling on a photothermal catalyst. Electrochemical analyses under operando NIR toggling demonstrate enhanced hydrogen evolution reaction kinetics and reduced charge recombination. In situ analyses confirm that reduced charge accumulation suppresses ion migration in the perovskite layer, thereby enhancing operational stability. The NIR-irradiated cathode delivers a photocurrent density of 25.48 mA cm–2, maintaining 90% of its initial photocurrent density at 0 V RHE for 310 h. A parallelly-illuminated coplanar system with NIR-irradiated perovskite-based water splitting cathode operates without bias, achieving a STH efficiency of 15.18%, retaining 70% of their initial performance for 115 h.

Introduction
Solar water splitting systems have been extensively studied as a promising solution to the escalating energy crisis and environmental degradation caused by fossil fuel dependency1,2. Recently, devices utilizing lead halide perovskite photo-absorbers have been at the forefront of research due to their exceptional optoelectronic properties, including low exciton binding energy, high charge transport efficiency, long carrier diffusion length, and tunable band gaps3–8. Despite their low moisture tolerance, various conductive encapsulation strategies have been developed to prevent water permeation and suppress performance loss9–16. Due to the optimal features of perovskites and advancements in encapsulation approaches, immersed perovskite-based water splitting systems have achieved an unbiased solar-to-hydrogen (STH) efficiency of 13.4% in aqueous environments, surpassing the commercial efficiency threshold of 10%17,18. However, even with complete prevention of water permeation through fully encapsulated structures, the operational stability of perovskite water splitting systems remains significantly suboptimal and is much lower compared to their photovoltaic (PV) counterparts9,19–21.

Prolonged exposure to factors such as heat, UV light, and electric fields is known to deteriorate the operational stability of perovskite devices22,23. In addition, a notable difference between perovskite water splitting systems and PV cells arises with the integration of an electrochemical compartment into the device. This introduces a timescale mismatch between photocarrier generation at the picosecond scale and electrochemical reactions at the microsecond scale24,25. This substantial discrepancy in timescales can induce charge accumulation within the multilayer device, compromising stability during continuous illumination26,27. Previous studies have focused on interface engineering of the carrier transport layer and the incorporation of efficient catalysts to mitigate these issues in perovskite-based water splitting systems9,15–17,19,21,28–36. Despite these efforts, it remains challenging to directly monitor charge accumulation dynamics within perovskite layers due to the lack of operando evaluations on the same device under operational conditions. Also, they have predominantly been limited to ex situ assessments, thereby hindering a comprehensive understanding of charge accumulation impacts during actual device operation.

Temperature modulation of electrochemical reactions offers a viable approach for mitigating and monitoring charge accumulation under operando conditions, provided it does not damage the temperature-sensitive internal layers of the device37–41. This strategy can also enhance the performance and kinetics of the state-of-the-art electrochemical catalysts. However, conventional heating methods require substantial energy because of the high heat capacity of water and pose a risk of degrading the photo-absorber along with surrounding layers. The photothermal effect, which induces localized temperature elevation in targeted areas upon simple irradiation with near-infrared (NIR) light, presents an ideal solution to this challenge42. To fully exploit the benefits of localized temperature modulation through the photothermal effect without causing thermal conduction to the perovskite layer, a recently developed catalyst-decoupled structure with thermally insulating epoxy resin can be employed14,15 (Supplementary Fig. 1). This lateral architecture allows independent thermal manipulation of the catalyst, in contrast to traditional vertical configurations where encapsulating materials with high thermal conductivity could adversely affect the perovskite layers. By integrating photothermal effects and advanced catalysts into the above-mentioned lateral design, electrochemical reaction kinetics can be modulated through toggling of NIR irradiation, enabling direct operando observation of carrier dynamics within the perovskite layer of the same device (Supplementary Note 1).

Inspired by such applicability of the photothermal effect, we investigate the electrochemical behavior of an immersed perovskite-based water splitting electrode paired with a commercial Pt-carbon-coated carbon cloth (Pt-C/CC) catalyst. NIR laser irradiation of the catalyst elevates its temperature via the photothermal effect, thereby improving the overall performance of the device, in operando. Electrochemical analyses under operando manipulation demonstrate not only enhanced hydrogen evolution reaction (HER) kinetics but also reduced charge accumulation within the multilayer structure. In a hybrid device combining a water splitting cell and a solar cell, in situ monitoring during operational stability tests reveal that reduced charge accumulation suppresses ion migration within the perovskite layer via accelerated electrochemical kinetics, thus improving long-term stability. Our results identify catalyst degradation as the dominant factor in the initial stages of stability decay, while perovskite degradation become more significant in later stages, thereby elucidating the degradation mechanism of the immersed perovskite-based solar water splitting system. Through this understanding, we demonstrate that charge accumulation is a primary cause of performance deterioration, significantly impacting overall stability. The NIR-irradiated perovskite cathode delivers a photocurrent density of 25.48 mA cm–2, retaining over 90% of its initial photocurrent density (t90) under continuous 1-sun illumination at 0 V RHE for 310 h. Coupling a perovskite-based water splitting anode with the improved cathode, a parallelly illuminated coplanar system operates without applied bias, achieving a STH efficiency of 15.18% while maintaining 70% of its initial performance (t70) for 115 h.

Results
Design and performance of immersed perovskite-based water splitting cathode

The device configuration and performance of perovskite solar cells (PSCs) are detailed in Supplementary Note 2. Based on the PSCs, a perovskite-based water splitting cathode was fabricated by positioning a Cu tape and Cu wire on the top Au electrode (Fig. 1a) (see Supplementary Fig. 2, Supplementary Note 3 and 4, and Methods for additional fabrication details about the device). The Pt-C/CC photothermal catalyst was placed at the bottom electrode, physically separated from the perovskite. All experiments were conducted under ambient conditions (RH 15–20%, 23–25 °C), unless specified. The temperature elevation of the catalyst on the cathode, immersed in 0.5 M KPi (potassium phosphate) (pH 7) electrolyte under NIR irradiation, was monitored via thermal imaging using an infrared camera, as shown in Supplementary Fig. 3. The catalyst’s temperature increased significantly within a short period despite being submerged in the electrolyte, while epoxy encapsulation effectively prevented heat transfer to the photo-absorber layer. The HER performance of the cathode was evaluated using linear sweep voltammetry (LSV) (Fig. 1b) (see Supplementary Fig. 4 for the detailed experimental setup of performance measurements with NIR irradiation). Despite being immersed in the electrolyte, the perovskite-based cathode stably operated without any degradation, indicating that our encapsulation strategy effectively prevented water permeation (Supplementary Fig. 5). The NIR laser intensity used was 2.5 A, with the optimization process described in Supplementary Fig. 6. The immersed perovskite-based cathode initially demonstrated a saturated photocurrent density (Jph) of 24.08 mA cm–2 at 0 V RHE (V vs. reversible hydrogen electrode) and an onset voltage (Von) of 0.96 V RHE. Notably, following NIR laser irradiation of the catalyst, the cathode’s performance improved, exhibiting a Jph of 25.48 mA cm–2 at 0 V RHE and Von of 1.04 V RHE. The increased onset voltage under NIR irradiation originates from enhanced catalyst performance (detailed explanation in later sections, see Supplementary Fig. 22), rather than change in band gap or photovoltage provided by the perovskite layer. Note that the thermodynamic potential of the HER does not vary due to the catalyst surface temperature (Supplementary Note 5). Operando performance observations were conducted with the NIR laser alternately turned on and off (Supplementary Fig. 7). The dark current of the water splitting cell was measured respective to the NIR irradiation (Supplementary Fig. 8). The presented dark current data via various measurements that the photocurrent density under light illumination originates from photo-induced processes rather than unfavorable attributions such as current leakages or side reactions apart from HER. Enhanced performance was observed under NIR irradiation, which reverted to the original control (without NIR) levels when the NIR laser was turned off. This indicates that the increased temperature enhanced the catalytic activity of Pt-C/CC, leading to improved overall performance of the cathode.

The incident photon-to-current conversion efficiency (IPCE) spectra demonstrated enhanced photon harvesting across all wavelengths, with integrated IPCE values under NIR irradiation closely matching the measured Jph values from LSV curves (Fig. 1c). The improvement in IPCE across the entire wavelength range suggests that the photothermal effect induced by NIR irradiation increased the catalyst’s temperature, thereby accelerating HER kinetics and enhancing both Jph and Von. Additionally, the unchanged absorption wavelength range of the IPCE spectra under both control and NIR-irradiated conditions indicates that the temperature increase of catalyst due to NIR irradiation is local and does not alter the band gap of the perovskite layer due to the thermal insulation of epoxy resin. By comparing the in situ photoluminescence (PL) spectra before and after NIR irradiation (Supplementary Fig. 9), it was observed that the PL peak position remained unchanged, and the intensity also maintained the same, which is consistent with IPCE results.

The contribution of NIR irradiation to the power of immersed perovskite-based cathode was quantitatively evaluated by measuring the improvement in the fill factor (FF) (Supplementary Fig. 10). The maximum power point (MPP) shifted to a more anodic potential (from 0.77 to 0.86 V RHE), indicating that NIR irradiation on the catalyst significantly elevated both photocurrent and photovoltage. The FF was calculated using the equation:

FF(%) = (JMPP × VMPP) / (Jph × Von) × 100% (1)

where JMPP and VMPP are the current density and potential at the MPP, respectively. The FF of control device increased from 70.07% to 77.41% upon NIR laser irradiation. This improvement in FF indicates that the photothermal effect enhances catalytic activity, leading to reduced power losses in the saturation region and increased HER reactivity even at lower voltage inputs. Notably, the cathode under NIR irradiation achieved an applied bias photon-to-current conversion efficiency (ABPE) of 20.69%. Performance parameters of 16 different devices are summarized in Supplementary Figs. 11–13 and Supplementary Table 1, demonstrating consistent performance enhancement with NIR irradiation.

The long-term stability of the cathode was evaluated chronoamperometrically under continuous 1-sun illumination at 0 V RHE (Fig. 1d). Stability tests were conducted without additional stirring systems to better assess the effects of the photothermal catalyst. The Jph of the control device declined rapidly, falling below 90% of its initial value within just 6 h of operation. In contrast, the NIR-irradiated device demonstrated a stable performance over 310 h, maintaining 90% of its initial Jph. The photocurrent drop observed in the final stage of the stability measurement for the NIR sample resulted from intentionally switching the light source off and on to confirm that the device was still responsive to illumination. For the control device, a rapid current decrease was observed during the initial stage of stability measurement, with a degradation rate of 0.52 mA cm–2 h–1 at 8 h, leading to a reduction to ~80% of the initial Jph. In comparison, the NIR-irradiated device exhibited a lower degradation rate of 0.04 mA cm–2 h–1 at 8 h, consistently preserving the initial Jph over an extended period. It is notable to mention that the effect of Pt dissolution from the counter electrode and redeposition on working electrode is neglectable, since the system operates in a potentiostatic condition within a neutral pH medium (Supplementary Fig. 14). Also, the electrolyte temperature has remained nearly constant throughout the experiment, indicating that the enhancement in stability was not influenced by temperature (Supplementary Fig. 15). Additionally, the control device exhibited a longer average Jph fluctuation period of 180.13 s, whereas the NIR-irradiated device showed a shorter average period of 30.6 s (Supplementary Fig. 16). The initial decay of Jph and current fluctuations are closely related to catalyst stability, influenced by bubble dynamics (nucleation, growth, and departure), which is discussed in later sections. The operational stability statistics of the control and NIR-irradiated cathodes are illustrated in Supplementary Fig. 17, highlighting the enhanced stability of the cathode under NIR irradiation and suggesting a correlation between improved electrochemical reaction kinetics and overall device stability. To evaluate the effect of stirring on cathode stability, performance and stability tests were conducted with and without stirring. When stirring was applied, the overpotential decreased compared to the unstirred condition, which can be attributed to improved mass transport near the catalyst (Supplementary Fig. 18a). The stability test also revealed slightly enhanced durability under stirring, indicating that increased mass transport at the catalyst–electrolyte interface contributes to improved device stability (Supplementary Fig. 18b). Notably, however, the stability achieved by stirring alone did not reach the level observed under NIR irradiation. This suggests that NIR illumination not only promotes mass transport and bubble departure but also enhances the HER kinetics, thereby suppressing charge accumulation. Such kinetic acceleration under NIR irradiation appears to play a significant role in further improving long-term stability. Furthermore, stability test under 0.5-sun illumination was conducted (Supplementary Fig. 19). The device exhibited enhanced stability compared to the original control sample, maintaining ~90% of the initial photocurrent for about 56 h. This improvement can be attributed to the reduced number of photogenerated carriers under the low-sun illumination, which reduces charge accumulation and consequently enhances the overall device stability. These results further highlight the critical role of kinetics of electrochemical reaction in determining stability of the perovskite-based water splitting device. Moreover, an encapsulated PV structure was fabricated in which the catalyst attached to the FTO side of the cathode was replaced with a Cu wire. The resulting device exhibited an open-circuit voltage (Voc) of 1.1 V and a short-circuit current density (Jsc) of about 25.1 mA cm–2 (Supplementary Fig. 20a). The device stability under both 1.05 V (near OCV) and 0 V (short-circuit, SC) conditions (Supplementary Fig. 20b). Notably, these stability tests were carried out while the device was immersed in electrolyte, closely replicating the cathode operating environment. Under near OCV conditions the device showed limited stability, with a t90 (time to reach 90% of the initial photocurrent) of 3 h. In contrast, under SC conditions the photocurrent retained nearly 90% of its initial value for more than 25 h. Considering that the control cathode exhibits a t90 of about 6 h, the stability trend can be summarized as encapsulated PV (near OCV) < cathode (control) < encapsulated PV (SC). These results indicate that charge consumption significantly influences the intrinsic stability of the perovskite layer. The H2 gas production rate of the NIR-irradiated device was measured over time using gas chromatography (GC) analysis (Fig. 1e), with the Faradaic efficiency of H2 evolution determined to be 98.29%. A comparative analysis of performance and stability values of previously reported immersed perovskite-based electrodes is summarized in Supplementary Fig. 21 and Supplementary Table 2.

Photothermal effect inducing HER kinetic improvement on electrocatalyst (EC)

To validate the enhancement of HER catalytic activity via the photothermal effect on the catalyst, the electrocatalytic properties of Pt-C/CC/FTO electrodes were evaluated in 0.5 M KPi (Supplementary Fig. 22a, 51a). The LSV curves indicated that the potentials required to achieve a current density of 10 mA cm–2 were –0.15 V RHE for the control and –0.11 V RHE under NIR irradiation. The Tafel slopes were determined to be 40.8 mV dec–1 for the control and 27.6 mV dec–1 under NIR irradiation, demonstrating accelerated catalytic activity under NIR irradiation (Supplementary Fig. 22b). Note that the Tafel slope is strongly dependent not only on temperature but also on the coverage of reaction intermediates and the rate-determining step43,44. Under NIR irradiation, the local heating and improved charge-transfer kinetics likely suppressed the excessive hydrogen coverage on Pt, leading to a more favorable surface condition for the Heyrovsky step. Furthermore, the enhancement could have resulted from the increased mass transport due to a forced convection via local heating. These effects are described in detail in later sections; however, it is necessary to confirm that electrochemical kinetics enhance under NIR irradiation, since HER overpotential originates from both mass transport and kinetic limitations. Control experiments of Pt-C/CC/FTO performance under (i) strong stirring condition (Video S1) and (ii) a thin electrolyte environment was conducted (Supplementary Fig. 23a–c). As shown in the video, vigorous stirring was imposed to establish a condition in which mass transport around the catalyst is strongly enhanced, and then compared the performance of the Pt-C/CC/FTO electrode with and without NIR irradiation (Supplementary Fig. 23d). Under stirring, the performance increased, and it improved further upon NIR irradiation. These results originate from the enhancement of further mass transport but also reaction kinetics since the limitation in mass transport has been alleviated to an extent before the NIR irradiation. In the second control experiment, the catalyst was immersed to a minimal depth such that only an ultra thin electrolyte layer was formed at the surface (Supplementary Fig. 23e). This configuration suppresses mass transport at the catalyst surface, but more importantly it prevents the formation of a sufficient temperature gradient to drive forced convection under NIR irradiation, thus separating mass transport and convection from reaction kinetics under NIR. Even under these conditions, NIR irradiation still increased the performance, which we attribute to an intrinsic kinetic enhancement. This results in a lower apparent Tafel slope despite the elevated temperature. To further elucidate the factors contributing to the increased catalytic activity, the electrochemically active surface area (ECSA) was measured by calculating the electrochemical double-layer capacitance (Cdl) (Supplementary Fig. 24). The ECSA values were 34.5 cm2 for the control and 42.9 cm2 under NIR irradiation, confirming an increase in the active area due to thermal activation (Supplementary Table 3). Also, to confirm that the increase in ECSA did not originate from an irreversible morphological change of the electrode due to NIR irradiation, the ECSA variation of electrode was observed after being cooled for 3 min and 10 min, respectively. After 3 min of cooling, ECSA was reduced to 37.5 cm2 and further decreased after 10 min, approaching the pre-irradiation value of 34.5 cm2. Based on the LSV curves of the catalyst electrode, the local heating was evaluated (Supplementary Note 6). The increased ECSA under thermal activation is likely attributed to changes in the quantity of adsorbates on the catalyst during operation. To investigate this effect, in situ Raman spectroscopy was performed on the Pt-C/CC/FTO electrode under conditions identical to the LSV measurements to monitor adsorbate changes during operation45 (Fig. 2a; detailed electrode sampling and experimental setup are shown in Supplementary Fig. 25a–c). Without an applied bias, the characteristic D- and G-band peaks of carbon cloth and carbon powder from Pt-C were observed at 1340 cm–1 and 1580 cm–1, respectively (Supplementary Fig. 25d). Upon increasing the applied bias to –0.2 V RHE, the peaks exhibited minimal shifts, suggesting insufficient chemical bonding with adsorbates to cause observable changes. When the applied bias was increased to –0.3 V RHE, a decrease in peak intensities was noted, indicating interactions with adsorbates in the electrolyte. A significant reduction in the original peaks was observed only when the bias reached –0.4 V RHE, as the electrode began interacting with a substantial quantity of adsorbates, likely including water molecules and alkali metal ions45,46 (i.e., potassium ions from KPi). Concurrent with the depletion of the D- and G-band peaks, a new peak emerged around 1642 cm–1, corresponding to interactions between water molecules and Pt particles, consistent with literature45. Under NIR irradiation, the same electrode showed similar peak dissipation and the emergence of the Pt-H2O interaction at a reduced bias of –0.2 V RHE, demonstrating decreased overpotential required to induce similar adsorbate interactions, further supporting the photothermal effect’s role in enhancing catalytic activity. Furthermore, the catalytic activity of Pt-C/CC was normalized to the respective ECSA values to compare electrochemical kinetics (Supplementary Fig. 26). The specific current density per ECSA (Js) was calculated using the equation:

Js = Jg × A / ECSA (2)

where A is the geometric area of the electrode, and Jg is the current density per geometric area. The NIR-irradiated catalyst exhibited a distinctly steeper Js slope compared to the control, even after ECSA normalization, indicating enhanced electrochemical kinetics. These combined results elucidate the effects of thermal activation on the catalyst in two aspects: increased electrochemical active area density and improved HER kinetics.

Operando electrochemical analyses of perovskite-based cathode

Electrochemical impedance spectroscopy (EIS) was performed on the immersed perovskite-based cathode under both control and NIR irradiation conditions to assess the impact of thermal activation on catalyst performance. Measurements were conducted at potentials ranging from 1.0 to 0.8 V RHE over a frequency range of 0.1 Hz to 300 kHz under 1-sun illumination. Below 0.8 V RHE, both conditions reached saturation in photocurrent density, hindering accurate measurement of charge transfer resistance. Therefore, EIS measurements were limited to the potential range of 1.0 to 0.8 V RHE. The Nyquist plots (Fig. 2b, c) were fitted using a Voigt-type equivalent circuit model (Supplementary Fig. 27 and Supplementary Note 7), with the fitting calculations summarized in Supplementary Table 4. Regardless of applied potential and NIR irradiation, there were no significant differences in Rs and RHF. However, the Bode plot revealed noticeable differences around the low-frequency region (100–0.1 Hz)47,48 (Supplementary Fig. 28). The low-frequency arc and Rct were significantly reduced under NIR irradiation compared to the control, indicating that the photothermal effect enhances the charge transfer reaction (Fig. 2b, c). A detailed interpretation of the low-frequency arc is provided in Supplementary Note 8. Transient photocurrent analysis (TPC) was performed to observe charge accumulation during operation under both conditions (Supplementary Fig. 29a). Under control conditions, a reverse current, indicative of reverse charge flow due to charge accumulation within the perovskite-based cathode, appeared after the initial decay. The observed reverse current originates from the discharge of trapped charges when the light is turned off. During illumination, some photogenerated carriers are trapped at the perovskite/charge transport layer (CTL) interface. Upon light termination, these trapped charges are released and flow in the opposite direction relative to the photocurrent, which manifests as the reverse current in the TPC signal9,49,50. For the control sample, severe charge accumulation at the catalyst/electrolyte interface interfered the charge separation and transport inside the device, leading to increased charge trapping at the perovskite/CTL interface. As a result, a pronounced reverse current was observed when the light was turned off. In contrast, for the NIR-sample, the improved catalyst kinetics reduced charge accumulation at the catalyst/electrolyte interface, which in turn mitigated charge trapping at the perovskite/CTL boundary. Moreover, intensity-modulated photovoltage spectroscopy (IMVS) was conducted under open circuit conditions with white light illumination bias, using a perturbation of 10% of the light intensity and a frequency range from 100 kHz to 0.1 Hz (Supplementary Fig. 29b, c). At ~100 kHz both conditions show high-frequency responses, indicative of fast photocarrier generation. It can be seen in both conditions that these charges are not completely consumed, where it is more severe for the control condition in that the induced photovoltage remains higher than that of the NIR condition throughout the frequency sweep. This can originate from the temporary trapping of these charges rather than forming chemical adsorbates on the electrode surface26,51, which is plausible considering the in situ Raman analysis (Fig. 2a). Although the steady-state net photocurrent is zero at open circuit, the sinusoidal light perturbation gives rise to transient interfacial charge transfer at low frequencies51. Thus, this transient interfacial electron transfer at low frequencies effectively impacts the measured photovoltage. As the intensity modulated illumination reaches low frequency near ~1 Hz, sufficiently slow for the device to undergo charge transfer, a decrease in photovoltage is observed for NIR condition as opposed to further increase in photovoltage in control condition. This is consistent with slow charge transfer at open circuit observed via EIS and thus stronger charge accumulation under control compared to when NIR is applied26,52.

To verify the impact of reduced charge accumulation on charge transport behavior in the cathode, intensity-modulated photocurrent spectroscopy (IMPS) was conducted to assess the rate of interface charge recombination and charge-transfer kinetics in the presence of the photothermal effect29,42. In Fig. 2d, e, the Nyquist plots of the complex photocurrents under control and NIR conditions were obtained under light modulation within the same potential range as EIS measurements. The charge transfer efficiency is determined by the ratio of the real photocurrents at the high- and low-frequency intercepts using the equation:

ktrans = (ktrans + krec) (3)

where ktrans and krec represent the interface recombination and charge-transfer rate constants, respectively53. Normalizing the real photocurrent to the high-frequency intercept allows the low-frequency intercept to directly reflect the charge transfer efficiency. The combined rate, ktrans + krec, is given by 2πfmin, where fmin is the frequency at the apex of the semicircle in the IMPS plots. By combining the charge-transfer efficiency and fmin, both ktrans and krec were obtained (Fig. 2f). The fmin are provided in Supplementary Table 5. As the cathodic potential increased, ktrans rose for both conditions, with the NIR-irradiated condition showing a higher ktrans, indicating that the photothermal effect accelerates HER kinetics, consistent with EIS results. Conversely, krec decreased for both control and NIR conditions with increasing cathodic bias, with krec and its intersection potential with ktrans being lower in the NIR condition than in the control. This indicates that the faster HER kinetics under NIR irradiation reduce interface recombination and facilitate carrier extraction within the cathode, enhancing performance. Moreover, the faster kinetics of the electrochemical reaction reduce charge accumulation and recombination within the perovskite cathode, contributing to improved long-term stability.

Monitoring charge accumulation during long-term operation

Stability measurements in control condition showed that the photocurrent density of the device declined rapidly, followed by a slower reduction rate, eventually reaching a stable level. To investigate the changes in bulk characteristics during each representative stage, we fabricated a device with a structure serving as a hybrid of both a solar water splitting cathode and an encapsulated solar cell (see Supplementary Note 9 and Supplementary Fig. 30). This configuration allowed us to monitor changes in carrier dynamics and band bending within the bulk perovskite using a two-electrode setup, avoiding interference from the electrolyte, catalyst degradation, or electrochemical reactions54. Figure 3a, b present the performance of cathodes under each respective conditions at 0 h, 8 h, and 45 h of operation, characterized by LSV under 1-sun illumination in 0.5 M KPi electrolyte. Performance parameters are summarized in Supplementary Fig. 31 and Supplementary Table 6. Specific time points were selected to represent the two distinct stages observed in the operational stability curve: the initial rapid decline at 8 h and the subsequent stabilization at 45 h. The corresponding long-term stability of the hybrid cathode is shown in Supplementary Fig. 32. After 8 h of operation, both the control and NIR-irradiated samples exhibited only slight decreases in Von and FF, largely retaining their initial performance. As the operation extended to 45 h, the control device showed severe degradation across all performance parameters, retaining only ~53% of its initial photocurrent. In contrast, the NIR-irradiated device maintained a stable LSV profile and demonstrated high stability, preserving ~87% of its initial photocurrent value. Additionally, the enhanced HER kinetics facilitated by the photothermal effect reduced charge accumulation during operation, resulting in a smaller decrease in onset potential after 45 h compared to the control device, aligning well with the LSV results.

We employed drive-level capacitance profiling (DLCP) and capacitance-voltage (C-V) methods to evaluate the carrier density of control and NIR-irradiated samples at 0 h (fresh) and after 8 h and 45 h of operation55,56 (Supplementary Fig. 33 and Supplementary Note 10). The corresponding interfacial trap densities were profiled at various depths for both control and NIR-irradiated devices, as shown in Fig. 3c, d. Both devices exhibited nearly identical trap site densities between the fresh state and 8 h of operation, indicating that the initial stage of operation has a negligible effect on the degradation of the perovskite layer in both control and NIR-irradiated devices. However, beyond 8 h of operation, trap densities increased more significantly on the hole transport layer (HTL) side (spiro-OMeTAD) compared to the SnO2 layer, with this increase being more pronounced in the control device than in the NIR-irradiated device. This result confirms that enhanced charge transfer reactions suppress charge accumulation at the interface of multilayer devices, leading to fewer trap states under NIR irradiation. We infer that the increased trap densities observed in the HTL side during operation are attributed to iodine migration and the organic nature of the HTL layer, which is more susceptible to hole accumulation. Furthermore, KPFM was performed to observe the change of surface potential depending on the hole accumulation. For the KPFM analysis, three different samples were prepared: fresh, control (after 45 h stability test), and NIR (after 45 h stability test) (Supplementary Fig. 34). All the three samples reveal almost identical surface topographies and roughness. The perovskite grains are not clearly visible in the topography from KPFM, as we removed the epoxy after encapsulation. In the SEM data, the surface image of the perovskite device is similar to the topography observed in KPFM; however, it was confirmed that the spiro layer remains preserved as the outermost layer (Supplementary Fig. 35). When compared to the surface work function of fresh sample, both control and NIR samples show an increase of the surface work function value. Especially, the surface potential in the control sample showed a larger increase, rising by 0.7 eV. This substantial increase is attributed to the extensive hole accumulation in HTL during operation. This finding is well matched with XPS analysis, where the C 1s spectra indicates that the HTL is more heavily oxidized due to greater hole accumulation (Supplementary Fig. 36). Therefore, the faster charge transfer reaction of photogenerated electrons reduces the accumulation of holes in the HTL layer. Iodine-related defects have lower activation energy for migration compared to other ions, making them more likely to migrate to charge-accumulated interfaces57–60. This migration leads to phase segregation within the perovskite, which is considered the primary cause of degradation in perovskite devices9,58.

We further performed XRD, cross-sectional SEM, and PL measurements to support the electrochemical characterization. In the XRD spectra of the control device, we observed a reduced peak intensity of the (100) plane and the presence of PbI2 phase with increasing operating time (Supplementary Fig. 37a). Under NIR conditions, the peak intensity of the (100) plane and PbI2 peak ratio were less pronounced than in the control device after 45 h of operation (Supplementary Fig. 37b). The corresponding SEM images of both samples are shown in Supplementary Fig. 37c, d, where pinholes were observed at the spiro-OMeTAD/perovskite interface of the control device after 45 h of operation, consistent with the trap density profiling results.

In steady-state PL analysis, the NIR condition exhibited a negligible PL peak shift during operation, whereas the control device showed a PL peak shift to higher wavelengths after 45 h (Supplementary Fig. 38). This shift is commonly associated with the migration of iodine ions, which often results in phase separation and material degradation, as supported by previous studies61,62 (Supplementary Note 11). To further investigate ion distribution as a function of operating time, we employed time-of-flight secondary ion mass spectrometry (ToF-SIMS) (Fig. 3e, f and Supplementary Fig. 39). After 8 h of operation, changes in ion distribution were insignificant; however, a pronounced distribution of I ions towards the HTL was observed after 45 h of operation in both devices. Notably, iodide migration was significantly reduced in the NIR-irradiated device compared to the control, likely due to the enhanced kinetics of the HER, which reduces charge accumulation within the device. Based on the comprehensive analysis of the experimental results, the kinetics of the electrochemical reaction are identified as the principal determinant of stability in perovskite-based water splitting devices.

Effect of NIR irradiation on bubble dynamics

DLCP and ex situ analyses indicate that the performance degradation observed during the initial operation stage of the cathode is not associated with perovskite degradation, suggesting that catalyst stability plays a critical role during this period. To verify this, stability tests were conducted on control and NIR-irradiated Pt-C/CC/FTO electrodes under galvanostatic conditions at 20 mA cm–2 to eliminate the effects of gas production differences between the two conditions (Fig. 4a). The control electrode showed an initial voltage of –0.23 V RHE which rapidly decreased to –0.33 V RHE after 10 h of operation. In contrast, the NIR-irradiated electrode exhibited a much slower voltage decrease from –0.15 V RHE to –0.20 V RHE. Additional stability evaluation through chronoamperometry measurements at –0.25 V RHE (Supplementary Fig. 40) consistently demonstrated enhanced performance stability under NIR irradiation.

To further investigate these results, the structural features of the catalysts were analyzed via transmission electron microscopy (TEM) after 10 h of galvanostatic operation (Fig. 4b–d). Unlike the fresh and NIR-irradiated electrodes, the control electrode exhibited a significant reduction in the quantity of Pt particles. Inductively coupled plasma mass spectrometry (ICP-MS) confirmed these observations, showing a marked reduction of Pt particles in the control electrode compared to the NIR-irradiated electrode (Supplementary Fig. 41). This finding substantiates that Pt particle detachment during operation is less pronounced under NIR irradiation, confirming the enhanced stability observed during the tests.

The performance degradation of Pt-based catalysts, attributed to agglomeration and detachment due to bubble evolution during operation, was further examined by comparing bubble dynamics between control and NIR-irradiated catalysts63. During potentiostatic tests of the cathode, periodic fluctuations in applied potential and current density were observed (Supplementary Fig. 16), indicative of cycles of nucleation, growth, and departure of gas bubbles on the catalyst surface64. To confirm this, real-time imaging of bubbles on the electrode was conducted (Supplementary Fig. 42). The digital images revealed that bubbles of varying sizes formed across the catalyst, but only a single large bubble significantly influenced photocurrent density fluctuations. This likely occurs because bubble size directly correlates with the deactivation of the active area, leading to a gradual decrease in photocurrent density as the bubble expands65. We opine that bubble size variability arises from the drop-casting method used for electrode fabrication, creating regions that are more favorable for nucleation and bubble coalescence65. To observe the bubble dynamics cycle in greater detail, side-profiling of the Pt-C/CC/FTO electrode under each condition was performed using a high-speed imaging camera (Fig. 4e, f). Notably, NIR irradiation accelerated the successive bubble nucleation, growth, and departure dynamics, shortening the cycle timescale by 21.5% (Fig. 4e, f and Video S2). Despite the faster growth rate, bubbles under NIR irradiation departed at smaller sizes compared to those under the control condition (Fig. 4g, h), likely due to a reduction in the local electrolyte surface tension caused by the photothermally activated catalyst. Reduced surface tension requires less buoyancy force, enabling earlier bubble release66. This was confirmed by finite-element method (FEM) modeling of gas bubbles on a wetted wall, as thermal elevation of electrode rapidly reduces the electrode coverage before departure (Supplementary Fig. 43) (detailed modeling process is described in Methods). This phenomenon also explains the less severe amplitude of current fluctuations observed with NIR irradiation, as the coverage determines the degree of fluctuation (Supplementary Fig. 44, 45). Larger bubbles exhibit greater adhesion, leading to prolonged upward movement on the electrode before decoupling, which imposes shear stress during gas evolution and adversely affects Pt particle adhesion63. Thus, the accelerated bubble dynamics cycle and reduced departure size under NIR irradiation correlate well with the observed reduction in Pt detachment, which is consistent with TEM observations. Moreover, the thermal Marangoni effect, characterized by the formation of temperature gradients, may also contribute to faster bubble departure under NIR conditions and should be considered as a contributing factor67.

The Marangoni convection generated by the thermal gradient in the electrolyte was quantitatively calculated by FEM modeling. Mapping a non-isothermal heat transfer module without the temperature difference between the electrode and electrolyte showed only an infinitesimal velocity change near the bubble due to its oscillation (Fig. 4i). However, with a temperature gradient of 30 K, the electrolyte initiated a circular convection pattern, and the velocity of the electrolyte flow increased nearly four-folds. This micro-stirring effect facilitates the easy departure of bubbles, enabling device operation without the need for external stirring systems that typically require additional input power68. This is relevant to the enhanced operational stability shown in Fig. 1d, which was achieved without the use of an additional stirring system, solely relying on NIR laser irradiation to maintain bubble dynamics and enhanced kinetics.

Degradation mechanism of perovskite-based water splitting system

Figure 5 illustrates the overall degradation mechanism of the perovskite-based water splitting cathode and the direct impact of the photothermal effect on its operational stability. When NIR irradiation is applied to the Pt-C/CC, the local temperature rapidly increases, enhancing the HER kinetics. Also, temperature gradient is formed, inducing thermal Marangoni convection, which accelerates the bubble departure. The rapid gas evolution, characterized by smaller bubble departure sizes, prevents the detachment of Pt particles, thereby improving operational stability. This catalyst degradation mechanism dominates the initial stage of stability decline in the perovskite-based cathode. In addition, the photothermal effect facilitates rapid electron extraction for HER, significantly reducing charge accumulation and iodide migration over extended period of operation, thereby suppressing perovskite degradation. In contrast, without NIR irradiation, sluggish HER kinetics result in accumulation of electrons on the FTO surface and accumulation of holes in the HTL, resulting in increased iodide migration into the HTL. This migration introduces trap states within the HTL, disrupting overall charge transport and accelerating perovskite degradation. These charge transport interruptions worsen with prolonged operation, leading to exacerbated iodide migration and further degradation of the perovskite. Based on these findings, we propose that the degradation of perovskite-based water splitting devices during operation is determined by the stability of the catalyst and the efficiency of charge extraction through electrochemical reactions, both of which are critical factors influencing the overall stability of perovskite-based water splitting systems.

Unassisted water splitting via perovskite-based electrodes

The perovskite-based anode was fabricated by attaching Ni foil to the PSC to prevent electrolyte permeation. To enhance the oxygen evolution reaction (OER) catalytic activity, a NiFe-layered double hydroxide (NiFeLDH) cocatalyst, hydrothermally synthesized on Ni foam, was attached to the Ni foil (detailed fabrication process is described in Methods and CV values in Supplementary Fig. 51b). The OER performance statistics of the anodes at 1-sun illumination in 0.5 M KPi electrolyte are provided in Supplementary Fig. 46 and Supplementary Table 7. Furthermore, photothermal heating using NIR laser was applied to OER, which improved the performance of anode device (Supplementary Fig. 47). However, stability tests revealed that the NIR irradiated anode exhibited faster degradation and poorer stability compared to the control. Post-measurement characterization further confirmed that prolonged NIR irradiation caused partial oxidation and irreversible damage to the catalyst surface. It should be noted that applying the photothermal effect to OER is not a viable strategy for our system, as it can induce continuous and irreversible damage to the NiFeLDH catalyst.

For unassisted operation without external bias, the cathode and anode were integrated into a parallelly illuminated coplanar system, as illustrated in Fig. 6a. The champion LSV curves of the cathode and anode obtained in 0.5 M KPi were superimposed to determine the operation point of the unbiased device (Fig. 6b). The intersection point of the control cathode and anode was 23.87 mA cm–2 at 0.66 V RHE, while the intersection point of the NIR-irradiated cathode and anode was 24.34 mA cm–2 at 0.76 V RHE. The actual performance of the coplanar device was evaluated using a two-electrode configuration under 1-sun illumination by scanning from –1.2 to 0.2 V (Fig. 6c). Under unbiased conditions (i.e., at 0 V), the control and NIR-irradiated coplanar devices delivered photocurrent densities of 12.08 mA cm–2 and 12.34 mA cm–2, respectively, closely matching the expected operation photocurrent densities which was obtained by halving the photocurrent density intersection point to match the integrated active area. The solar-to-hydrogen (STH) efficiency was calculated using the following equation:

STH(%) = (1.23 V × J mA cm–2) / P0 mW cm–2 × 100% (4)

where J is the photocurrent density at 0 V, and P0 is the power of the incident light. The resulting STH efficiency was 14.89% for the control device and 15.18% for the NIR-irradiated device. Supplementary Fig. 48 and Supplementary Table 8 present the performance of separately prepared 9 coplanar devices, demonstrating reproducible STH efficiency improvements with NIR irradiation. The long-term operational stability of the coplanar device was evaluated using potentiostatic chronoamperometry analysis (Fig. 6d). The control coplanar device barely retained 70% of its initial STH efficiency for 3 h of operation, whereas the NIR-irradiated device maintained 70% of its initial STH efficiency for an extended 115 h. The operational stability statistics for both control and NIR-irradiated coplanar devices are depicted in Supplementary Fig. 49. The H2 production rate of the coplanar device was measured using GC analysis (Fig. 6e). The measured amount of H2 gas was consistent with the theoretical values, confirming high Faradaic efficiencies. Comparative STH efficiency and stability data for previously reported unbiased water-splitting devices are summarized in Supplementary Fig. 50 and Supplementary Table 9. Furthermore, the potential for large-scale applicability of the NIR system is further discussed in Supplementary Note 12.

Discussion

In summary, we present direct operando and in situ observations of charge accumulation in perovskite-based water splitting cathodes and its impact on operational stability utilizing the photothermal effect. This was achieved by integrating a Pt-C/CC catalyst with the cathodes, significantly enhancing the HER kinetics through photothermal activation upon NIR irradiation. The enhanced performance resulted in a ABPE of 20.69%. Moreover, the operational stability of the perovskite-based cathodes was extended from 6 h to 310 h, maintaining ~90% (t90) of its initial photocurrent. Electrochemical analysis under operando conditions effectively monitored suppressed charge accumulation within the multilayer device using identical specimen sets. In situ analyses of the device during operation revealed that reduced charge accumulation led to suppressed iodine migration at the perovskite/HTL interface, with ion migration predominantly occurring during the later stabilized stage of operation. Observations of bubble dynamics showed that NIR irradiation accelerated gas evolution with smaller bubble sizes, effectively preventing detachment of Pt particles and thus enhancing operational stability during the initial stage. Based on these findings, a degradation mechanism for perovskite-based water splitting devices during operation is proposed, providing operando insights into charge accumulation within the perovskite layer, highlighting its role in achieving enhanced operational stability. With improved performance and stability, bias-free solar water splitting achieved a STH efficiency of 15.18%, maintaining 70% of the initial performance for 115 h in a parallelly illuminated coplanar system.

Methods
Materials

Formamidinium iodide (FAI, > 99.99%), methylammonium bromide (MABr, > 99.99%), and cyclohexylammonium iodide (CHAI, > 99.99%) were sourced from Greatcell Solar (Queanbeyan, Australia). PbI2 (99.99%, trace metals basis) was obtained from TCI Chemicals. Lead bromide (PbBr2 99.999%, trace metals basis), cesium iodide (CsI, 99.999%, trace metals basis), lithium bis(fluorosulfonyl)imide (Li-FSI, 98%), dimethylformamide (DMF, anhydrous, 99.8%), dimethyl sulfoxide (DMSO, anhydrous, 99.9%), acetonitrile (anhydrous, 99.8%), chlorobenzene (CB, anhydrous, 99.8%), 2-propanol (IPA, anhydrous, 99.5%), SnCl2·2H2O (99.99%, trace metal basis), 4-tert-butylpyridine (TBP, 96%), tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] (KF209 Co TFSI), 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamin-9,9′-spirobifluorene (spiro-OMeTAD), 4-methoxyphenethylamine (98.0%), aquivon, urea (99.0%), thioglycolic acid (TGA, > 99%), methylenediamine dihydrochloride (MDACl2, > 98.0%), potassium iodide (KI, > 99.0%), and Ni foil (100 × 100 mm, thickness 0.1 mm, annealed, 99%), potassium phosphate dibasic (K2HPO4, > 98%), potassium phosphate monobasic (KH2PO4, > 99%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Pt powders with 20% carbon black were obtained from Alfa Aesar (Ward Hill, MA, USA). Carbon cloth was purchased from Wizmac (Korea), and silver paste was obtained from Elcoat (Japan). All chemicals were used as received.

Fabrication of perovskite solar cells

Perovskite solar cells were prepared following previously reported papers69,70. More specifically, an area of 0.5 cm × 2 cm of a 2 cm × 2 cm fluorine-doped tin oxide (FTO) substrate was etched by thermal scribing method utilizing a laser etching instrument from Korthem Science (Korea). The etched substrates were cleaned with, in the order of, filtered deionized (DI) water, acetone (Duksan, Korea), and ethyl alcohol (Duksan, Korea) for 20 min, respectively. After drying the substrates by a N2 blow gun, the substrates were treated with O2 plasma 10 min. SnO2 was deposited via a chemical bath deposition method. 275 mg of SnCl2·2H2O, 1.25 g of urea, 1.25 mL of HCl (Duksan, Korea), and 25 mL of TGA were dissolved in 100 ml of DI water then was mixed by a magnetic stirring system for 20 minutes. The SnO2 thin film was grown by immersing the FTO substrates in the solution for 2 h at 90 °C. The substrates were washed with IPA and DI water for 10 min, respectively. After drying the substrates, they were annealed at 170 °C for 1 h. The perovskite composition used in this study is (FAPbI3)0.95(MAPbBr3)0.05. 1.4 M FAI, 1.4 M PbI2, 0.023 M MABr, 0.023 M PbBr2, 0.023 M CsI, and 0.5 M MACl with 3.8 mol% MDACl2 were dissolved in a 4:1 mixed solvent of DMF and DMSO for the perovskite ink. After stirring for 15 min, KI was doped (10 μmol) by using a stock solution (166 mg KI in 10 mL DMF). The precursor ink was deposited onto the prepared SnO2/FTO/Glass substrates utilizing a two-step spin-coating process; 12 s at 1000 rpm and then 20 s at 5800 rpm. Ethyl acetate (Duksan, Korea) was used as an antisolvent solution, which 800 µL of the solution was dripped onto the film during the spin-coating process, ~10 s before finishing the second step. The films were annealed using a hot plate for 60 min at 100 °C. The films were cooled to room temperature then a 25 µL of CHAI solution (6 mg CHAI in 1 mL IPA) was spin-coated onto the perovskite surface at 6000 rpm for 30 s, then was annealed at 70 °C for 10 min. The substrate was again cooled to room temperature, then 25 µL of the HTL precursor prepared by 72 mg spiro-OMeTAD dissolved in 1.2 mL chlorobenzene, followed by sequential doping via 17.5 µL Li-TFSI solution (260 mg Li-TFSI in 0.5 mL acetonitrile), 6.85 µL KF209 Co TFSI solution (375 mg KF209 Co TFSI in 1 mL acetonitrile), and 28.8 µL TBP, was spin-coated at 3000 rpm for 30 s. The HTL/perovskite/SnO2/FTO/Glass was aged overnight and then Au electrode of 80-nm was deposited via a thermal evaporator system.

Fabrication of cathode and anode

Cathode and anode were prepared following previously reported papers69,70. More specifically, the cathode was fabricated using a Cu tape with conductive adhesive which was connected to the Au layer and then gently pressed down to form contact. Then, a Cu wire was attached on the Cu tape via silver paste, which was dried in ambient condition for 3 h. Pt-C/CC was prepared by drop casting a Pt-C ink (20 mg of Pt carbon, 0.75 mL of DI water, 0.25 mL of IPA, and 75 µL of Aquivion) onto a carbon cloth and then was dried in room temperature for 24 h. The Pt-C/CC catalyst was applied to the bottom contact (FTO) via silver paste (see Supplementary Fig. 4). The catalyst area for Pt-C/CC/FTO electrode was 0.5 cm2. For the anode, Ni foil was attached to the Au layer with silver paste, and NiFeLDH/Ni foam was fabricated by dissolving Ni(NO3)2·6H2O (0.3 g), Fe(NO3)3·9H2O (0.38 g), and urea (0.6 g) in 70 mL of DI water under sonication for 15 min. The precursor solution and Ni foam were then subjected to hydrothermal synthesis in a Teflon-lined stainless-steel autoclave, followed by annealing of the resulting Ni foam for 12 h at 120 °C. The Ni foams were washed with DI water and acetone for 5 min each and dried for 24 h at 80 °C. The mass loading was ~1.5 mg cm–2 and was calculated by weighing the Ni foam before and after hydrothermal synthesis. The NiFeLDH/Ni foam catalyst was attached to Ni foil using silver paste and sealed with epoxy resin, and a Cu wire was attached to the FTO side using silver paste. The catalyst area of both Pt-C/CC/FTO and NiFeLDH/Ni foam ranged from 0.06 to 0.1 cm2.

Film characterizations

The phase evolution of the perovskite film was characterized using X-ray diffraction (XRD, Rigaku Miniflex 600 instrument, Woodlands, TX, USA). The surface morphology of the annealed samples was examined via field emission SEM (JSM-7800F, JEOL Ltd., Japan) and contact-mode atomic force microscopy (AFM, NX-10 AFM, Park Systems, Korea). ToF-SIMS measurements (TOF-SIMS V, ION-TOF GmbH, Germany) were performed using pulsed primary ions from a Cs+ (1 keV, 56.8 nA) liquid-metal ion gun for sputtering, and Bi+ and Bi3+ pulsed primary ion beams for analysis (30 keV). Raman spectra of the catalyst were obtained with a confocal Raman microscope (WITec, Germany) with an excitation wavelength of 532 nm. TEM (JEM-ARM200F, JEOL, Japan) analysis was performed at 200 kV to investigate the detachment of Pt particles. The profiling distance (X) from the junction barrier is given by: X = εε0A / C.

PV measurements

Similar setup in a previous report was utilized19. Specifically, a solar simulator (Sol3A Class AAA, Oriel Instruments, Stratford, CT, USA) was set to 1.5 AM and 1-sun conditions (100 mW cm–2) to test the performance of PSCs. The setup was calibrated using a Si reference cell (Newport Corporation, Irvine, USA). The active area of the PSC set to 0.06 cm2 using a photomask. Voltage was swept from –0.1 to 1.5 V with a rate of 0.50 V s–1 with a dwell time of 50 ms at each point to obtain J-V curves. A quantum efficiency measurement system (QEX10, PV Measurements, Inc.) was utilized to extract external quantum efficiency (EQE) spectra. J-V measurements and EQE spectra were obtained under ambient conditions (RH 15–20%, 23–25 °C).

Voltammetry analyses

A three-electrode system with a Pt wire counter electrode and an Ag/AgCl/KCl (4M) reference electrode in 0.5 M KPi (pH 7) was employed under simulated solar illumination (AM 1.5G; Newport Corporation, USA). 0.5 M KPi (pH 7±0.2) was prepared by dissolving K2HPO4 (9.343 g) and KH2PO4 (6.309 g) in 200 ml of DI water and was stored at room temperature (23–25 °C) for short-term (<1 week) use. The ohmic resistance of the 0.5 M KPi is 14.5±1.4 Ω and was calculated by EIS results with the catalytic area of 0.1 cm2. Pt wire of 1 mm in diameter with a length of 20 cm was used as the counter electrode. The reference electrode was prepared and calibrated before use by measuring the potential difference against a new Ag/AgCl/KCl (4M). LSV measurements were conducted with a reverse sweep to ensure reliability and have not been iR corrected. All measurements were controlled using a potentiostat (SI1287, Solartron, UK), and the applied potentials were adjusted to the RHE scale for comparison with previously reported values: ERHE = EAg/AgCl + 0.059 pH + 0.197. The ABPE was calculated using the LSV curve according to the following equations:

ABPE(%) = (JHER × V) / P0 × 100% (5)

ABPE(%) = (JOER × (1.23 – V)) / P0 × 100% (6)

where JHER and JOER are the measured photocurrent densities, V is the applied bias, and P0 is the incident illumination power density (i.e., AM 1.5G, 100 mW cm–2). Voltammetry analyses were conducted using a NIR laser (MDL-H-808 3W 808 nm infrared diode laser, CNI laser, China). The exposed area of the perovskite-based electrodes ranged from 0.06 to 0.1 cm2, matching the catalytic active area. ECSA measurements were conducted by calculating the average current density from the anodic (ja) and cathodic (jc) current density from corresponding CV curves, Δj = (ja – jc) / 2. Each set of CV curves was parted by a 3 min OCV to avoid any unnecessary charge buildup. Electrode area was 0.6 cm2 and specific capacitance 0.02 mF cm–2. EIS was performed using a potentiostat with a frequency analyzer (1260, Solartron, UK). Polarization and series resistances of the cathodes were measured at potentials ranging from 1.0 to 0.8 V RHE across a frequency range of 0.1 Hz to 300 kHz under 1-sun illumination with a 10-mV alternating-current amplitude. Mott-Schottky, DLCP, and C-V analyses were conducted at a fixed frequency of 10 kHz using the same frequency analyzer. The C-V analysis provided the carrier densities at the edge of the depletion region:

Nc(X) = 2 / (qεε0A2 d(1/C2) / dV) (7)

The profiling distance (X) from the junction barrier is given by: X = εε0A / C.

For the DLCP measurement, the voltage (V) was scanned from 0 V to the open circuit voltage (i.e. 1.1 V) for the hybrid device. The DLCP method employs a series of variable δV (20–200 mV) to measure the junction capacitance and extract capacitance contributions from trap states by analyzing higher-order terms. The capacitance at each δV was recorded and fitted with a polynomial function: C = C0 + C1δV + C2(δV)2 + … to determine C0 and C1. Using C0 and C1, the total carrier density (N), including both free carriers and trap density at a profiling distance X from the junction barrier, is calculated by:

Nc(X) = C03 / (2qεε0A2C1) (8)

The profiling distance (X) from the junction barrier is given by: X = εε0A / C0.

IPCE, IMVS, IMPS, and TPC measurements were conducted using an electrochemical workstation (Zennium, Zahner, Germany) and a potentiostat (PP211, Zahner, Germany). Data fitting was performed using ZView software. Gas chromatography (6500 GC system, YL Instrument, Anyang, Korea) with a pulsed discharge detector and a molecular sieve column was used to confirm H2 evolution. Rubber bulkheads were employed to seal all connections of the device, preventing gas leakage from the quartz.

High-speed imaging

High-speed imaging and time lapse of bubble formation, growth, and departure on catalyst were conducted using a high-speed camera (C320, Vision Research, New Jersey, USA) and a digital single-lens reflex (DSLR) camera (D5300, Nikon, Tokyo, Japan), respectively. Both the high-speed camera and DSLR camera were equipped with a 2X tele-converter (TELEPLUS MC7 AF 2x DGX, Kenko, Tokyo, Japan) and a 105-mm F2.8 macro lens (Nikon, Tokyo, Japan).

COMSOL Multiphysics modeling

Bubble departure in an electrolyte under varying electrode temperatures (0–30 K) was modeled using COMSOL Multiphysics (COMSOL, Stockholm, Sweden), with temperature information from experimental data. The geometry consisted of a water domain (15 mm × 15 mm) and a bubble (radius 1–2 mm) positioned at the center of the wetted wall (electrode). The laminar flow module incorporated gravity to simulate natural convection. A level set module defined two fluids: air (bubble) and water (surrounding medium), with no-flow boundaries at the top and bottom. Heat transfer was modeled using a temperature gradient defined as T = Tamb + ΔT × (x – xlim)/xlim, where Tamb = 293.15 K, ΔT = 0–30 K, and xlim = 1 mm. A wetted wall condition was applied with a slip length proportional to the minimum element size and a contact angle of π/3. Non-isothermal flow accounted for the effects of temperature on fluid behavior and bubble departure.

Data availability

All data generated in this study are provided in the Source Data file. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this paper, the authors investigate the role of charge accumulation on operational stability through operando catalytic modulation via near-infrared (NIR) toggling on a photothermal catalyst. The NIR irradiation enhances HER kinetics and inhibits charge accumulation and ion migration in the perovskite layer. As a result, a record-high STH efficiency and enhanced operational stability were obtained. The work is interesting and presents advances in the field. I recommend it for publication after dealing with the concerns listed below.\n1. For the stability test in Figure.1d, a sudden photocurrent drop from ~20 mA/cm2 to 0 was observed after ~310 h tracking. Similar results can be seen in Figure. 6d. The authors need to explain the reasons beneath.\n2. On page 8 line 180, how do the authors define the Jph fluctuation period? How are the Jph fluctuation period results obtained from Sup. Fig. 16?\n3. On page 9 line 203, the authors showed an increased ECSA value under NIR irradiation. Were the results obtained from the same device? I suggest the author monitor ECSA change on the same device during the continuous process from NIR irradiation off to on and then to off.\n4. On page 11 line 252, the authors perform TPC analysis to observe charge accumulation. Please give more test method details (e.g., bias conditions) in the Method section and give a specific explanation of how the charge flows to generate the reverse current.\n5. On page 15 line 368, stability tests under galvanostatic conditions were performed. Since charge carrier and ion distribution are related to bias conditions, will the different voltages influence the stability?\n6. Mott-Schottky analysis: The device capacity is quite different between Figure. S24 and S29 for the control and target devices, is it because the slight differences in device architecture?\n7. If the solar simulator light intensity is decreased to reduce photogenerated charge carrier density, or the catalyst and the electrolyte are changed to enhance the HER catalytic activity, will the charge carrier accumulation and ion migration be inhibited?\n8. On page 19 line 454, the authors built up an unassisted water-splitting system. Will NIR irradiation have similar effects on the OER side?", "reviewer_2": "This paper looks at the effect of NIR irradiation on a catalyst modified cathode attached to a perovskite solar cell. I cannot recommend the paper for publication at the moment as I have many questions about the reliability and accuracy of the techniques used as well as questions about whether the results have been interpreted correctly.\n(1) The authors discuss that this is an encapsulated solar cell which is connected in series with a catalyst coated electrode. There is some discussion of the configuration and why it was chosen, but I think there needs to be clearer explanation for why it is better to submerge the solar cell, rather than keeping it outside the solution and connecting the photoelectrode with a slightly longer wire?\n(2) Does the NIR heating of the carbon electrode create a temperature gradient and thereby cause a large increase in convective stirring? Is there an increase in performance due to increased mass transfer of e.g. the reactants and products to and from the electrode? Is it possible to differentiate whether the improvements are due to enhanced kinetics, or rather enhanced mass transport? There are papers that suggest that the HER on Pt nanoparticles is mass transport limited, increased mass transport would therefore legitimately increase current (e.g. https://pubs.acs.org/doi/10.1021/acsenergylett.1c00246). As far as I can see this has not been investigated? The bubble evidence supports this?\n(3) It is stated that NIR irradiation significantly enhanced both the photocurrent and the photovoltage. I can understand how the photocurrent is affected, but can the authors better explain why the photovoltage (by which I mean the open circuit photovoltage) is different? In this case the cell is at open circuit and no charge is being extracted through the contacts, how can heating of the contact influence the open circuit photovoltage inside the device?\n(4) The increased catalytic activity is partly attributed to an increased surface area which is taken from measurements of the double layer capacitance. A larger surface area is measured under NIR. I would like to see this measured several times (e.g. control, NIR, control, NIR, control) to rule out other mechanisms such as irreversible roughening of the electrode?\n(5) Impedance spectroscopy shows that the low frequency arc of the control device has a larger diameter (attributed to higher Rct) at the same voltage versus RHE. Given the complexity of the impedance spectroscopy of perovskites, can the authors give evidence that the low frequency arc is measuring RCT and not some kind of ion modulated current? Would the diameter of the arc not be expected to be quite different as at the same applied voltage versus RHE the two systems are in different states? E.g. at 0.9 V versus RHE the control cell is effectively at open circuit whereas the NIR cell still has a photocurrent of over 20 mA and may not have a linear response.\n(6) Mott-Schottky analysis is used to determine that there is no change in band-bending in the bulk perovskite. Mott-Schottky very rarely measured band bending in perovskite cells (there are many papers showing just how problematic it is as a technique for perovskites and proving that it rarely measures the space charge layer) and a great deal of work is needed here for the authors to convince us that they are actually measuring band bending. I suspect that none of the Mott-Schottky analysis here is valid?\n(7) IMVS is measured – but the interpretation is far from clear. It should mirror the EIS results at open circuit.\n(8) IMPS is measured and the data is fitted to extract ktrans and krec. All the EIS and IMPS plots need to be plotted isotropically. IMPS of perovskite solar cells measured at high frequencies measures the RC time constant of the devices (RseriesCgeo). Are the authors able to show that this is not what is being measured here? The data for both cells looks very similar, especially when it is claimed that the NIR strongly changes the kinetics. It is straightforward to calculate the RC time constant and to compare it with 1/RC from the semi-circle maximum? This needs to be ruled out.\n(9) Can the authors show that DLCP and CV methods actually measure the carrier trap density in their devices? Typically, these techniques do not accurately measure traps in perovskite devices as the ion migration dominates the response. All the same issues as for Mott Schottky analysis apply, there are many papers on this which are not discussed.", "reviewer_3": "The present manuscript discusses degradation of perovskite-based photocathode caused by charge accumulation. Comparison with and without NIR irradiation on HER catalyst and various characterization during the degradation found that sluggish HER kinetics led to the charge accumulations in the semiconductor and its degradation. The present manuscript would provide design guidelines for durable photocathodes which can be potentially considered in various semiconductors. However, my concern is what would be the original cause of the degradation. Is it due to either the catalysts degradation caused by bubbles or poor HER kinetics in the absence of NIR? Additional simple control measurements would further clarify their claims before its publication in Nat. Commun. Please find the following questions.\nFig. 1c\nThe authors claim that “the photothermal effect induced by NIR irradiation increased the catalyst’s temperature, thereby accelerating HER kinetics and enhancing both Jph and Von.” Why Von changes by HER kinetics? Von should be at 0 mA/cm2 where HER kinetics does not involve. Does the Von shift come from the thermodynamics of HER? The authors may be able to discuss the shift from Nernst equations. It may also help to evaluate the local temperature on the catalyst not just by IR camera.\nFig. S19\nTafel slope on Pt-C/CC/FTO was found to be reduced by NIR. If we simply follow Tafel equation or Butler-Volmer equation, the slope should increase at the elevated temperature. The authors can discuss the observed change in detail. I may consider that the improved kinetics helps to suppress the coverage of H leading to the smaller Tafel slope. See Sci. Rep. 5 (2015) 13801.\nAdditionally, can they show LSV of Pt-C/CC/FTO after the stability tests? It helps readers to observe how the overpotential increased after Pt detachment.\nLocal heating\nLocal heating by NIR seems to play an important role. It is also known that kinetic loss on electrocatalysts causes a local heating. Can they compare NIR power density and local heating from HER catalysis? The authors can evaluate the local heating (W/cm2) due to HER from the operating current density (A/cm2) and overpotential (V). Overpotential can be roughly evaluated from LSV at their operating current density. This analysis helps the readers to get an idea on the power density levels needed to heat up.\nThe huge bubbles and their detachments were claimed to be the cause of Pt detachment and degradation of HER. The authors limited systems without additional stirring. Can they perform control measurements with stirring and without NIR. For example, stability test of Pt-C/CC/FTO (like Fig4a), stability of photocathode (like Fig. 1d), and bubble observation (like Fig4e). This comparison will help to clarify the major cause on the degradation of photocathode either from bubbles or HER kinetics. If the bubbles are removed efficiently without NIR irradiation and HER activity is maintained at room temperature, does the photocathode degrade? Or, is the improvement of HER kinetics (like Fig. S19a) necessary?\nIf the Pt-C degraded by the bubbles in the absence of NIR is used for the fresh perovskite, can it still maintain the stability with NIR irradiation?\nKinetics of charge consumptions should be an important factor determining the degradation of perovskite. Simple measurements using PV setup (without the electrolytes and electrocatalysts) can be performed with solar irradiation at OCV and short circuit conditions while measuring IV response at certain periods. Keeping at OCV (0 mA cm2) under solar irradiation leads to charge accumulation which would cause degradation of semiconductor while keeping at the short circuit is the case of extremely fast kinetics case minimizing the charge accumulations. I believe the degradation observed in the photocathode would be positioned between them.\nImplementation potential of NIR local heating technique\nBecause the authors also emphasize its record high efficiency and large-scale applications, it would be nice to discuss how the natural solar irradiation would be managed to implement this technique in a real device. Do we need to separate NIR from the natural solar using some mirror? In their study, NIR was obtained from laser with various power. When we irradiate a unit area of perovskite, can we obtain sufficient NIR power after the split of natural sunlight? The authors can discuss its potential based on the standard solar spectrum.\nSupplementary Note 9\nThe onset potential of 1:1_control in Supplementary Note Fig 7b is quite high compared with those on other large scale samples and small devices in Main text. What is the reason? Does the cathodic photocurrent come from HER or reduction of large area catalysts region which may be reduced more easily?"}, "author_response": "In this paper, the authors investigate the role of charge accumulation on operational stability through operando catalytic modulation via near-infrared (NIR) toggling on a photothermal catalyst. The NIR irradiation enhances HER kinetics and inhibits charge accumulation and ion migration in the perovskite layer. As a result, a record-high STH efficiency and enhanced operational stability were obtained. The work is interesting and presents advances in the field. I recommend it for publication after dealing with the concerns listed below.\nRemark:\nWe sincerely appreciate the reviewer’s careful evaluation and thoughtful comments on our manuscript. In particular, we are grateful for the recognition of our work as both interesting and advanced. We believe that the revisions made in response to the reviewer’s suggestions have further strengthened the manuscript. Our detailed responses to the reviewer’s comments are provided below.\nComment 1:\nFor the stability test in Figure.1d, a sudden photocurrent drop from ~20 mA/cm2 to 0 was observed after ~310 h tracking. Similar results can be seen in Figure. 6d. The authors need to explain the reasons beneath.\nResponse:\nWe appreciate the reviewer for careful comment on our stability data. As noted by the reviewer, the photocurrent drop to 0 mA cm-2 observed in the final stage of the stability measurement for the NIR sample in Fig. 1d resulted from intentionally switching the light source off and on to confirm whether the device was still responsive to illumination. In contrast, the photocurrent drop observed in the control coplanar device in Fig. 6d was not caused by light switching but rather by device failure. We have clarified these points in the revised manuscript to avoid any potential misunderstanding.\nRevision made (colored in blue):\n(Fig. 1 revised)\nFig. 1 | Structure and performance of immersed perovskite-based cathode. a, Schematic representation of the perovskite cathode structure under NIR irradiation. b, LSV curves of control and NIR-irradiated devices under 1-sun illumination. c, Incident photon-to-current efficiency (IPCE) spectra at 0 V for both devices. d, Long-term stability of the cathodes at RHE 0 V under continuous 1-sun illumination in 0.5 M KPi (pH 7). All experiments were RHE conducted in a 0.5 M KPi (pH 7) electrolyte. e, H production rate over time for control and 2 NIR-irradiated perovskite-based cathodes, with the solid line representing the theoretical rate and circles indicating experimental values obtained via gas chromatography at 2 h intervals.\n2\n(in page 8, line 4)\nThe long-term stability of the cathode was evaluated chronoamperometrically under continuous 1-sun illumination at 0 V (Fig. 1d). Stability tests were conducted without RHE additional stirring systems to better assess the effects of the photothermal catalyst. The J of ph the control device declined rapidly, falling below 90% of its initial value within just 6 h of operation. In contrast, the NIR-irradiated device demonstrated highly stable performance over 310 h, maintaining 90% of its initial J . The photocurrent drop observed in the final stage of ph the stability measurement for the NIR sample resulted from intentionally switching the light source off and on to confirm whether the device was still responsive to illumination.\nComment 2:\nOn page 8 line 180, how do the authors define the Jph fluctuation period? How are the Jph fluctuation period results obtained from Sup. Fig. 16?\nResponse:\nThe fluctuation period of J was defined as the time interval between the onset of current ph decrease and the recovery point when the bubble detached and the current value returned. The definition of the J fluctuation is more clearly provided in Supplementary Fig. 41 (for revised ph manuscript), together with the corresponding bubble images. To clarify the fluctuation period, we have enlarged the graph in Supplementary Fig. 16 and explicitly indicated the period (Fig. R1). Note that in the NIR sample used in the stability measurements of Fig. 1d, random J ph fluctuations were observed; therefore, the average of 30 periods was calculated, yielding a value of 30.6 s. We sincerely appreciate the reviewer’s attention to such detailed aspects.\nFig. R1 J fluctuations observed during the initial operation of control and NIR-irradiated ph cathodes. Fluctuation periods are shortened in the NIR-irradiated devices.\n(Supplementary Information revised)\nSupplementary Fig. 16 | Current fluctuations in control and NIR-irradiated cathodes during stability measurements. J fluctuations observed during the initial operation of ph control and NIR-irradiated cathodes. Fluctuation periods are shortened in the NIR-irradiated devices.\nSupplementary Fig. 41 | Potentiostatic observation of perovskite-based cathode coupled with real-time imaging of bubble dynamics. Close-up imaging of bubble nucleation, growth, and departure, coupled with corresponding current density fluctuations of the cathode under potentiostatic conditions at 0 V . The fluctuation period is defined by the time between the RHE two sharp drops in stability tests, where bubble detachment and nucleation occur in immediate sequence.\nOn page 9 line 203, the authors showed an increased ECSA value under NIR irradiation. Were the results obtained from the same device? I suggest the author monitor ECSA change on the same device during the continuous process from NIR irradiation off to on and then to off.\nResponse:\nWe thank the reviewer for insightful comment on our ECSA data. We note that the proposed ECSA measurements were conducted on the same sample to observe the changes under NIR on/off conditions. However, ECSA was only recorded after NIR irradiation following the control measurement, without further measurement after switching off the NIR and allowing the sample to cool. As suggested by the reviewer, we subsequently performed ECSA measurements in the sequence of NIR off → on → off to examine whether NIR irradiation induces irreversible changes in the catalyst (Fig. R2). As expected, under NIR irradiation, the ECSA increased compared to the control. After turning off the NIR, we measured the ECSA following 3 min and 10 min of cooling, respectively, and found that the ECSA gradually returned to the control value over time. These results clearly demonstrate that the ECSA increase originates from thermal effects induced by NIR irradiation, and that no irreversible changes occur in the catalyst, confirming the significance of the operando observations. We have included the newly performed ECSA measurements in our revised manuscript. We thank the reviewer for highlighting this important aspect.\nC /cm2 (mF cm–2) C (mF) C (mF cm–2) ECSA = C /C (cm2)\ndl dl s dl s\nControl 1.15 0.69 34.5\nw/ NIR 1.43 0.858 42.9\nCooled 0.02\n1.25 0.75 37.5\n3 min\nCooled\n1.15 0.69 34.5\n10 min\nRevision made (colored in blue):\n(in page 10, line 7)\nPhotothermal effect inducing HER kinetic improvement on electrocatalyst (EC)\nTo validate the enhancement of HER kinetics via the photothermal effect on the catalyst, the electrocatalytic activities of Pt-C/CC/FTO electrodes were evaluated in 0.5 M KPi (Supplementary Fig. 22a). The LSV curves indicated that the potentials required to achieve a current density of 10 mA cm–2 were –0.15 V for the control and –0.11 V under NIR RHE RHE irradiation. The Tafel slopes were determined to be 40.8 mV dec–1 for the control and 27.6 mV dec–1 under NIR irradiation, demonstrating accelerated catalytic activity under NIR irradiation (Supplementary Fig. 22b). Note that the Tafel slope is strongly dependent not only on temperature but also on the coverage of reaction intermediates and the rate-determining step68, 69. Under NIR irradiation, the local heating and improved charge-transfer kinetics likely suppressed the excessive hydrogen coverage on Pt, leading to a more favorable surface condition for the Heyrovsky step. This results in a lower apparent Tafel slope despite the elevated temperature. To further elucidate the factors contributing to the increased catalytic activity, the electrochemically active surface area (ECSA) was measured by calculating the electrochemical double-layer capacitance (C ) (Supplementary Fig. 23) The dl ECSA values were 34.5 cm2 for the control and 42.9 cm2 under NIR irradiation, confirming an increase in the active area due to thermal activation (Supplementary Table 3). Also, to confirm that the increase in ECSA did not originate from an irreversible morphological change of the electrode 7 due to NIR irradiation, the ECSA variation of electrode was observed after being cooled for 3 min and 10 min, respectively. After 3 min of cooling, ECSA was reduced to 37.5 cm2 and further decreased after 10 min, approaching the pre-irradiation value of 34.5cm2.\n(Supplementary Information revised)\nSupplementary Fig. 23 | ECSA change of Pt-C/CC catalyst. Cyclic voltammetry of the Pt- C/CC electrode at varying scan rates (10–30 mV/s) with 5 mV/s increments in 0.5 M KPi electrolyte, a, under control, b, NIR irradiation, after being cooled for c, 3 min, and d, 10 min.\nC /cm2 (mF cm–2) C (mF) C (mF cm–2) ECSA = C /C (cm2)\ndl dl s dl s\nControl 1.15 0.69 34.5\nw/ NIR 1.43 0.858 42.9\nCooled 0.02\n1.25 0.75 37.5\n3 min\nCooled\n1.15 0.69 34.5\n10 min\nComment 4:\nOn page 11 line 252, the authors perform TPC analysis to observe charge accumulation. Please give more test method details (e.g., bias conditions) in the Method section and give a specific explanation of how the charge flows to generate the reverse current.\nResponse:\nWe appreciate the reviewer’s valuable comment in respect to TPC analysis. In response, we have revised the Methods section to include more details of the TPC measurement conditions. Specifically, TPC was measured under 0.8 V in 1-sun illumination at 0.5 M KPi (pH 7). RHE Regarding the mechanism, the observed reverse current originates from the discharge of trapped charges when the light is turned off. During illumination, some photogenerated carriers are trapped at the perovskite/charge transport layer (CTL) interface. Upon light termination, these trapped charges are released and flow in the opposite direction relative to the photocurrent, which manifests as the reverse current in the TPC signal. For control sample, severe charge accumulation at the catalyst/electrolyte interface interfered the charge separation and transport inside the device, leading to enhanced charge trapping at the perovskite/CTL interface. As a result, a pronounced reverse current was observed when the light was turned off. In contrast, for the NIR-sample, the improved catalyst kinetics reduced charge accumulation at the catalyst/electrolyte interface, which in turn mitigated charge trapping at the perovskite/CTL boundary. Consequently, the magnitude of the reverse current was 9 markedly diminished or nearly absent. We added this demonstration to our revised manuscript to help the understanding of TPC results.\nRevision made (colored in blue):\n(in page 13, line 5)\nTransient photocurrent analysis (TPC) was performed to observe charge accumulation during operation under both conditions (Supplementary Fig. 28a). Under control conditions, a reverse current, indicative of reverse charge flow due to charge accumulation within the perovskite- based cathode, appeared after the initial decay. The observed reverse current originates from the discharge of trapped charges when the light is turned off. During illumination, some photogenerated carriers are trapped at the perovskite/CTL interface. Upon light termination, these trapped charges are released and flow in the opposite direction relative to the photocurrent, which manifests as the reverse current in the TPC signal9,47,48. For control sample, severe charge accumulation at the catalyst/electrolyte interface interfered the charge separation and transport inside the device, leading to enhanced charge trapping at the perovskite/CTL interface. As a result, a pronounced reverse current was observed when the light was turned off. In contrast, for the NIR-sample, the improved catalyst kinetics reduced charge accumulation at the catalyst/electrolyte interface, which in turn mitigated charge trapping at the perovskite/CTL boundary.\nComment 5:\nOn page 15 line 368, stability tests under galvanostatic conditions were performed. Since charge carrier and ion distribution are related to bias conditions, will the different voltages influence the stability?\nResponse:\nIn this study, all galvanostatic stability tests were conducted exclusively on FTO/CC/Pt catalyst electrodes. This approach was adopted to avoid reliability issues in bubble dynamics observation, which could arise if potentiostatic conditions were applied, since varying current densities between the control and NIR samples would result in differences in bubble number and size. For typical perovskite-based water splitting cathodes, stability was assessed only under potentiostatic conditions. Nevertheless, even under galvanostatic operation, the overall stability trend is expected to remain consistent. Specifically, in order to maintain the same 10 current density, the control sample would require a more severe bias due to its lower kinetics, which would in turn accelerate ion migration. This inference can be supported by the J–V curve evolution shown in Fig. 3a, b, where the control sample exhibits pronounced decreases in both V and J over time. Consequently, even if a constant current density were applied, the on ph corresponding potential variation in the control sample would be expected to be substantially larger compared to NIR sample.\nComment 6:\nMott-Schottky analysis: The device capacity is quite different between Figure. S24 and S29 for the control and target devices, is it because the slight differences in device architecture?\nResponse:\nWe sincerely thank the reviewer for the insightful comment regarding the Mott–Schottky (M– S) analysis. As presented in Supplementary Fig. 24 and 29, the M–S plots were measured for the perovskite-based cathode and the hybrid cathode, respectively. As the reviewer pointed out, the slight differences can be attributed to the variation in device structures. Nevertheless, the capacitance values obtained from the two devices are insignificantly different. While the Y- axis in the M–S plots shows an apparent difference of about ×1.9, it should be noted that this arises from the inverse squared capacitance. Accordingly, the actual capacitance difference is only about ×1.37, which we consider to be within the range of experimental errors that may occur during device fabrication. However, we have decided to eliminate results of the M–S analysis due to its difficulty of probing C in perovskite-based devices. Please refer to dl Comment 6 from Reviewer 2. We truly appreciate the reviewer’s attention to such detailed aspects of our work.\nIf the solar simulator light intensity is decreased to reduce photogenerated charge carrier density, or the catalyst and the electrolyte are changed to enhance the HER catalytic activity, will the charge carrier accumulation and ion migration be inhibited?\nResponse:\nWe appreciate the reviewer for their insightful comment on the stability mechanism of the device. As suggested by the reviewer, we performed additional stability tests under reduced solar-light intensity. Specifically, the measurement was conducted at 0.5-sun illumination, and a photocurrent density of 10.2 mA cm-2 was observed (Fig. R3a). In addition, we conducted a potentioistatic stability test at 0 V in 0.5 M KPi electrolyte (Fig. R3b). The device exhibited RHE significantly enhanced stability compared to the original control sample, maintaining ~90% of the initial photocurrent for about 56 h. This improvement can be attributed to the reduced number of photogenerated carriers under the low-sun illumination, which reduces charge accumulation and consequently enhances the overall device stability. These results further highlight the critical role of charge accumulation at the catalyst/electrolyte interface in determining stability of the perovskite-based water splitting device. We have included this analysis in the revised manuscript to better emphasize the impact of charge accumulation. We are grateful to the reviewer for providing such a valuable and thoughtful comment on this key point.\nFig. R3 (a) Linear sweep voltammetry (LSV) curves of control device under 0.5-sun illumination. (b) Long-term stability of the cathode at 0 V under continuous 0.5-sun RHE illumination in 0.5 M KPi (pH 7).\nRevision made (colored in blue):\n(in page 8, line 18)\nThe operational stability statistics of the control and NIR-irradiated cathodes are illustrated in Supplementary Fig. 17, highlighting the enhanced stability of the cathode under NIR irradiation and suggesting a correlation between improved electrochemical reaction kinetics and overall device stability. To evaluate the effect of stirring on cathode stability, performance and stability tests were conducted with and without stirring. When stirring was applied, the overpotential 12 decreased compared to the unstirred condition, which can be attributed to improved mass transport near the catalyst (Supplementary Fig. 18a). The stability test also revealed slightly enhanced durability under stirring, indicating that increased mass transport at the catalyst– electrolyte interface contributes to improved device stability (Supplementary Fig. 18b).\nNotably, however, the stability achieved by stirring alone did not reach the level observed under NIR irradiation. This suggests that NIR illumination not only promotes mass transport and bubble detachment but also enhances the HER kinetics, thereby suppressing charge accumulation. Such kinetic acceleration under NIR irradiation appears to play a significant role in further improving long-term stability. Furthermore, stability test under 0.5-sun illumination was conducted (Supplementary Fig. 19). The device exhibited enhanced stability compared to the original control sample, maintaining ~90% of the initial photocurrent for about 56 h. This improvement can be attributed to the reduced number of photogenerated carriers under the low-sun illumination, which reduces charge accumulation and consequently enhances the overall device stability. These results further highlight the critical role of kinetics of electrochemical reaction in determining stability of the perovskite-based water splitting device.\n(Supplementary Information revised)\nSupplementary Fig. 19 | Stability test under low-sun illumination. a, Linear sweep voltammetry (LSV) curves of control device under 0.5-sun illumination. b, Long-term stability of the cathode at 0 V under continuous 0.5-sun illumination in 0.5 M KPi (pH 7). RHE\nComment 8:\nOn page 19 line 454, the authors built up an unassisted water-splitting system. Will NIR irradiation have similar effects on the OER side?\nResponse:\nFig. R4 (a) Linear sweep voltammetry (LSV) curves of anode device under 1-sun illumination at 0.5 M KPi (pH 7). (b) Long-term stability of the NIR-irradiated anode at 1.23 V under RHE continuous 1-sun illumination in 0.5 M KPi (pH 7). (c) Digital photograph of the NiFe LDH catalyst after stability test.\n(in page 21, line 18)\nThe perovskite-based anode was fabricated by attaching Ni foil to the PSC to prevent electrolyte permeation. To enhance the oxygen evolution reaction (OER) catalytic activity, a NiFe-layered double hydroxide (NiFe LDH) cocatalyst, hydrothermally synthesized on Ni foam, was attached to the Ni foil (detailed fabrication process is described in Methods). The OER performance statistics of the anodes at 1-sun illumination in 0.5 M KPi electrolyte are provided in Supplementary Fig. 45 and Supplementary Table 7. Furthermore, photothermal heating using NIR laser was applied to OER, which improved the performance of anode device (Supplementary Fig. 46). However, stability tests revealed that the NIR irradiated anode exhibited faster degradation and poorer stability compared to the control. Post-measurement characterization further confirmed that prolonged NIR irradiation caused partial burning and irreversible damage to the catalyst surface. It should be noted that applying the photothermal effect to OER is not a viable strategy for our system, as it can induce continuous and irreversible damage to the NiFe LDH catalyst.\n(Supplementary Information revised)\nSupplementary Fig. 46 | NIR irradiation in anode. a, Linear sweep voltammetry (LSV) curves of anode device under 1-sun illumination at 0.5 M KPi (pH 7). b, Long-term stability of the NIR-irradiated anode at 1.23 V under continuous 1-sun illumination in 0.5 M KPi RHE (pH 7). c, Digital photograph of the NiFe LDH catalyst after stability test.\n15\n<Reviewer 2>\nThis paper looks at the effect of NIR irradiation on a catalyst modified cathode attached to a perovskite solar cell. I cannot recommend the paper for publication at the moment as I have many questions about the reliability and accuracy of the techniques used as well as questions about whether the results have been interpreted correctly.\nRemarks:\nWe sincerely appreciate the reviewer’s thorough and critical evaluation of our manuscript. We have expanded the discussion mentioned, elaborated conceptual aspects, and conducted further experiments to clarify the reliability and accuracy of the techniques utilized in the manuscript. Detailed responses to the reviewer’s comments are provided below.\nComment 1:\nThe authors discuss that this is an encapsulated solar cell which is connected in series with a catalyst coated electrode. There is some discussion of the configuration and why it was chosen, but I think there needs to be clearer explanation for why it is better to submerge the solar cell, rather than keeping it outside the solution and connecting the photoelectrode with a slightly longer wire?\nResponse:\nWe thank the reviewer for the constructive comment on our rationale for the selected device configuration. As noted, our discussion focuses on enabling NIR-laser operation without damaging the perovskite layer and the benefits compared to a vertically stacked architecture, not on configurations with extended wires and solar cells unsubmerged. However, the laterally decoupled catalyst structure also offers advantages over such configurations for the following three reasons.\nFirstly, although encapsulation effectively suppresses moisture ingress, continuous illumination can induce local heating and accelerate thermally activated degradation, particularly in cells employing organic charge-transport layers (e.g., spiro-OMeTAD), where dopant/ion migration and redox processes are exacerbated under light/heat stress. When immersed, high specific heat capacity of the electrolyte can actively cool the cells, allowing longer operation. Secondly, eliminating external wiring simplifies the balance-of-system and shortens fabrication flow, thereby reducing process complexity and cost. Lastly, practical connections typically rely on Ag-paste contacts, and the added junctions with increased carrier path length to the catalyst can elevate series resistance, necessitating a higher applied bias at a given operating current. These considerations collectively motivated our choice to minimize 16 thermal-induced degradation, lower cost of assembly, and avoid unnecessary resistive losses. We have included these discussions in the revised supplementary information manuscript. We thank the reviewer for encouraging us to discuss this important aspect, which deepens the rationale of selecting our device configuration.\nRevision made (colored in blue):\n(Supplementary Information revised)\nSupplementary Note 1: Advantages of catalyst-decoupled lateral electrode structure\nThe cathode discussed in this study features a design where the perovskite photo-absorber is electrically connected but physically decoupled from the catalysts1,s2. This structure is cost- effective as it eliminates the need for metal foils or Field’s metal, which have traditionally been used as encapsulation materialss3-s17. Additionally, the decoupled configuration allows localized temperature elevation NIR laser irradiation without causing thermal degradation of the perovskite or adjacent carrier transport layers (Supplementary Fig. 3). This design facilitates independent manipulation of the catalyst without compromising the semiconducting properties of the perovskite. In contrast, conventional catalyst-stacked vertical configurations have interdependent layers, making operando studies challenging due to the inevitable influence of one layer on another.\nIn comparison to designs where the catalyst is connected via extended external wiring and the solar cell is kept outside the electrolyte, the laterally decoupled catalyst structure still provides clear advantages. Firstly, continuous light illumination can induce heating of the solar cell and accelerate thermal degradation. Particularly in cells with organic charge-transport layers (e.g., spiro-OMeTAD), thermal degradation is more susceptible, whereas immersion of the solar cell (with encapsulation) leverages the high specific heat capacity of the electrolyte to actively cool the cell. Secondly, eliminating external wiring simplifies the balance-of-system and shortens the fabrication flow, thereby reducing process complexity and cost. Lastly, practical connections typically rely on Ag-paste contacts, and the added junctions with increased carrier path length to the catalyst can elevate series resistance, necessitating a higher applied bias at a given operating current. Collectively, these considerations explain why our laterally decoupled configuration minimizes thermal-induced degradation, lowers cost of assembly, and avoids unnecessary resistive losses while enabling NIR-laser operation without damaging the perovskite layer.\nComment 2:\nDoes the NIR heating of the carbon electrode create a temperature gradient and thereby cause a large increase in convective stirring? Is there an increase in performance due to increased mass transfer of e.g. the reactants and products to and from the electrode? Is it possible to differentiate whether the improvements are due to enhanced kinetics, or rather enhanced mass transport? There are papers that suggest that the HER on Pt nanoparticles is mass transport limited, increased mass transport would therefore legitimately increase current (e.g. https://pubs.acs.org/doi/10.1021/acsenergylett.1c00246). As far as I can see this has not been investigated? The bubble evidence supports this?\nResponse:\nWe thank the reviewer for pointing out this factor. As the reviewer notes, we indeed observed a thermal gradient of both the catalyst and the surrounding electrolyte under NIR irradiation. An increase in performance from enhanced mass transfer is certainly possible. To assess this, we conducted LSV measurements on a perovskite cathode without NIR irradiation but with electrolyte stirring via an external magnetic stirring system (Fig. R5). In this case, a small increase in onset potential and fill factor with enhanced operation stability was observed, which we attribute to improved catalytic activity induced by enhanced mass transport. In addition, our catalyst is deposited on a carbon cloth substrate, which is intrinsically aerophilic and therefore limits exposure of active sites during the growth and detachment of H₂ bubbles. It should be noted that the stability achieved by stirring alone did not reach the level observed under NIR irradiation. This suggests that not only mass transport and bubble detachment but also HER kinetics play a significant role in improving long-term stability.\nAs the reviewer highlighted in the referenced work, Pt catalysts are typically mass- transport limited due to their inherently fast kinetics. However, it is important to emphasize that such studies have generally been conducted under strongly acidic media. In contrast, our work employs a neutral electrolyte, where kinetic barriers also play a significant role and must be overcome (J. Phys. Chem. C 2019, 123, 21554–21563). Thus, we believe that the enhanced performance observed under NIR irradiation arises from a combination of photothermal effects that improve both mass transport and reaction kinetics. We agree that a quantitative 18 deconvolution of these two contributions would require a systematic study, such as varying stirring speed and directly comparing to NIR-induced convection, which is beyond the scope of the present work. We thank the reviewer again for this valuable discussion, and we have now included a discussion of these points in the revised manuscript to clarify the mass transport and kinetic contributions in our system.\nFig. R5 (a) LSV curves of cathode devices with stirring and without stirring. (b) Stability measurements of the cathode devices with stirring and without stirring. Measurements are conducted at 0 V in a 0.5 M KPi (pH 7) electrolyte under 1-sun illumination. RHE\nRevision made (colored in blue):\n(in page 8, line 18)\nThe operational stability statistics of the control and NIR-irradiated cathodes are illustrated in Supplementary Fig. 17, highlighting the enhanced stability of the cathode under NIR irradiation and suggesting a correlation between improved electrochemical reaction kinetics and overall device stability. To evaluate the effect of stirring on cathode stability, performance and stability tests were conducted with and without stirring. When stirring was applied, the overpotential decreased compared to the unstirred condition, which can be attributed to improved mass transport near the catalyst (Supplementary Fig. 18a). The stability test also revealed slightly enhanced durability under stirring, indicating that increased mass transport at the catalyst– electrolyte interface contributes to improved device stability (Supplementary Fig. 18b).\nNotably, however, the stability achieved by stirring alone did not reach the level observed under NIR irradiation. This suggests that NIR illumination not only promotes mass transport and bubble detachment but also enhances the HER kinetics, thereby suppressing charge accumulation. Such kinetic acceleration under NIR irradiation appears to play a significant role in further improving long-term stability.\n19\n(Supplementary Information revised)\nSupplementary Fig. 18 | Effect of stirring on cathode performance and stability. a, LSV curves of cathode devices with stirring and without stirring. b, Stability measurements of the cathode devices with stirring and without stirring. Measurements are conducted at 0 V in a RHE 0.5 M KPi (pH 7) electrolyte under 1-sun illumination.\nComment 3:\nIt is stated that NIR irradiation significantly enhanced both the photocurrent and the photovoltage. I can understand how the photocurrent is affected, but can the authors better explain why the photovoltage (by which I mean the open circuit photovoltage) is different? In this case the cell is at open circuit and no charge is being extracted through the contacts, how can heating of the contact influence the open circuit photovoltage inside the device?\nResponse:\nWe sincerely thank the reviewer for raising this important point. As the reviewer points out, the external heating of the catalyst does not influence the photovoltage of the perovskite solar cell, as observed by the constant band gap by IPCE measurements. Rather the change in the onset potential of the catalyst upon NIR irradiation, –0.15 V to –0.12 V (Supplementary RHE RHE Fig. 22, in revised manuscript) via acceleration of HER kinetics, contributes to the perovskite cathode’s onset shift. We also note that the more pronounced improvement in onset potential observed for the cathode can be attributed to the difference in catalyst area: while the catalyst-alone measurement used a catalyst area of about 0.5 cm2, the cathode employed a much smaller area of about 0.1 cm2. As a result, the localized heating and other effects induced by NIR irradiation were concentrated over a smaller region, leading to a more significant enhancement. We thank the reviewer for pointing out this detail, since it can be confusing to the readers. We have included additional interpretation regarding this matter.\n(in page 6, line 5)\nThe NIR laser intensity used was 2.5 A, with the optimization process described in Supplementary Fig. 6. The immersed perovskite-based cathode initially demonstrated a saturated photocurrent density (J ) of 24.08 mA cm–2 at 0 V (V vs. reversible hydrogen ph RHE electrode) and an onset voltage (V ) of 0.96 V . Notably, following NIR laser irradiation of on RHE the catalyst, the cathode’s performance improved, exhibiting a J of 25.48 mA cm–2 at 0 V ph RHE and V of 1.04 V . The increased onset voltage under NIR irradiation originates from on RHE enhanced catalyst performance (Supplementary Fig. 22), rather than change in band gap or photovoltage provided by the perovskite layer.\nComment 4:\nThe increased catalytic activity is partly attributed to an increased surface area which is taken from measurements of the double layer capacitance. A larger surface area is measured under NIR. I would like to see this measured several times (e.g. control, NIR, control, NIR, control) to rule out other mechanisms such as irreversible roughening of the electrode?\nResponse:\nWe appreciate the reviewer’s detailed comments on the origin of the NIR-induced increase in catalytic activity and its reversibility. Following the suggestion, we re-measured the ECSA from CV scans (10–50 mV s⁻¹) under four sequential conditions: (i) control, (ii) during NIR irradiation (same scan sequence), (iii) after 3 min of cooling, and (iv) after 10 min of cooling; before each step, a 3 min at open-circuit voltage (OCV) was proceeded to minimize any residual charge build-up (Fig. R6). We observed an immediate increase in ECSA upon NIR irradiation from 34.5 cm2 to 42.9 cm2, followed by a near-linear decrease to 37.5 cm2 after 3 min of cooling and a further decrease after 10 min, approaching the pre-irradiation value. This behavior demonstrates that the enhancement is reversible and predominantly thermally driven: it diminishes as heat dissipates through forced convection and the electrolyte actively cools the electrode. Importantly, we found no evidence of irreversible electrode roughening, consistent with the modest temperature rise (20–25 °C) experienced by the Pt-C/CC electrode, which is insufficient to induce permanent morphological change under our conditions. We have included 21 such discussions with results along with experimental details in the revised manuscript and supplementary information.\nFig. R6 Cyclic voltammetry of the Pt-C/CC electrode at varying scan rates (10–30 mV/s) with 5 mV/s increments in 0.5 M KPi electrolyte, a, under control, b, NIR irradiation, after being cooled for c, 3 min, and d, 10 min.\nC /cm2 (mF cm–2) C (mF) C (mF cm–2) ECSA = C /C (cm2)\ndl dl s dl s\nControl 1.15 0.69 34.5\nw/ NIR 1.43 0.858 42.9\nCooled 0.02\n1.25 0.75 37.5\n3 min\nCooled\n1.15 0.69 34.5\n10 min\nRevision made (colored in blue):\n(in page 10, line 7)\nPhotothermal effect inducing HER kinetic improvement on electrocatalyst (EC)\nTo validate the enhancement of HER kinetics via the photothermal effect on the catalyst, the electrocatalytic activities of Pt-C/CC/FTO electrodes were evaluated in 0.5 M KPi (Supplementary Fig. 19a). The LSV curves indicated that the potentials required to achieve a current density of 10 mA cm–2 were –0.15 V for the control and –0.11 V under NIR RHE RHE irradiation. The Tafel slopes were determined to be 40.8 mV dec–1 for the control and 27.6 mV dec–1 under NIR irradiation, demonstrating accelerated catalytic activity under NIR irradiation (Supplementary Fig. 19b). To further elucidate the factors contributing to the increased catalytic activity, the electrochemically active surface area (ECSA) was measured by calculating the electrochemical double-layer capacitance (C ) (Supplementary Fig. 23) The dl ECSA values were 34.5 cm2 for the control and 42.9 cm2 under NIR irradiation, confirming an increase in the active area due to thermal activation (Supplementary Table 3). Also, to confirm that the increase in ECSA did not originate from an irreversible morphological change of the electrode due to NIR irradiation, the ECSA variation of electrode was observed after being cooled for 3 min and 10 min, respectively. After 3 min of cooling, ECSA was reduced to 37.5 cm2 and further decreased after 10 min, approaching the pre-irradiation value of 34.5cm2.\n23\n(Supplementary Information revised)\nSupplementary Fig. 23 | ECSA change of Pt-C/CC catalyst. Cyclic voltammetry of the Pt- C/CC electrode at varying scan rates (10–30 mV/s) with 5 mV/s increments in 0.5 M KPi electrolyte, a, under control, b, NIR irradiation, after being cooled for c, 3 min, and d, 10 min.\nC /cm2 (mF cm–2) C (mF) C (mF cm–2) ECSA = C /C (cm2)\ndl dl s dl s\nControl 1.15 0.69 34.5\nw/ NIR 1.43 0.858 42.9\nCooled 0.02\n1.25 0.75 37.5\n3 min\nCooled\n1.15 0.69 34.5\n10 min\n(Methods revised)\nVoltammetry analyses:\nVoltammetry analyses were conducted using a NIR laser (MDL-H-808 3W 808 nm infrared diode laser, CNI laser, China). The exposed area of the perovskite-based electrodes ranged from 0.06 to 0.1 cm2, matching the catalytic active area. A three-electrode system with a Pt wire counter electrode and an Ag/AgCl/KCl (4 M) reference electrode in 0.5 M KPi (pH 7) was employed under simulated solar illumination (AM 1.5G; Newport Corporation, USA). LSV measurements were conducted with a reverse sweep to ensure reliability. All measurements were controlled using a potentiostat (SI 1287, Solartron, UK), and the applied potentials were adjusted to the RHE scale for comparison with previously reported values: E RHE = E + 0.059 pH + 0.197. The ABPE was calculated using the LSV curve according to the Ag∕AgCl following equations:\nJHER × V\nABPE (%) = ( ) × 100%, (5)\nP0\nJOER × (1.23 – V)\nABPE (%) = ( ) × 100% (6)\nP0\nwhere JHER and JOER are the measured photocurrent densities, V is the applied bias, and P is 0 the incident illumination power density (i.e., AM 1.5G, 100 mW cm–2). ECSA measurements were conducted by calculating the average current density from the anodic (j ) and cathodic a (j ) current density from corresponding CV curves, ∆j = (ja – jc ) . Each set of CV curves was c 2 parted by a 3 min OCV to avoid any unnecessary charge build up. Electrode area was 0.6 cm2 25 and specific capacitance 0.02 mF cm–2.\nComment 5:\nImpedance spectroscopy shows that the low frequency arc of the control device has a larger diameter (attributed to higher Rct) at the same voltage versus RHE. Given the complexity of the impedance spectroscopy of perovskites, can the authors give evidence that the low frequency arc is measuring RCT and not some kind of ion modulated current? Would the diameter of the arc not be expected to be quite different as at the same applied voltage versus RHE the two systems are in different states? E.g. at 0.9 V versus RHE the control cell is effectively at open circuit whereas the NIR cell still has a photocurrent of over 20 mA and may not have a linear response.\nResponse:\nWe thank the reviewer for raising this important point. We agree that the impedance of perovskite-based electrochemical devices can involve overlapping electronic and ionic contributions, particularly at low frequency. In our case, the assignment of the large low- frequency arc to charge-transfer resistance (Rct) is supported by its voltage dependence: as the control approaches Voc, the arc diameter increases, consistent with fewer photocarriers available for interfacial charge transfer. However, if it were related with ion or defect migration within the perovskite layer, a decrease in arc diameter is expected since ion migration is most severe near open-circuit conditions. This is due to suppressed compensation of built-in field potential via illumination (J. Am. Chem. Soc. 2020, 142, 20071-20079, Nat. Energy 2018, 3, 61 - 67). Also, this voltage dependent arc at low frequency peaks around 10–1 Hz for the control and 50–10 Hz under NIR irradiation (Supplementary Fig. 26), which is relatively faster than ionic movements which occur in 1–0.1 Hz.\nWe appreciate the reviewer’s observation that, at 0.9 V vs RHE, the control and NIR- irradiated devices are in different operational states. However, this distinction is precisely what we intend to highlight. The key purpose of the impedance comparison is to show that, under the same applied potential, the two systems exhibit markedly different charge-transfer behavior, with the NIR-irradiated device sustaining significant photocurrent and thus demonstrating reduced interfacial resistance. Moreover, even if one were to compare the two devices at the same photocurrent density rather than the same applied bias, the resistance values would not be expected to match. This is because the internal distribution of carriers and degree 26 of interfacial charge accumulation depend strongly on the applied potential. We have made additional clarifications regarding the EIS measurements to explicitly address how ionic contributions are excluded. We thank the reviewer for pointing out this important consideration, which has helped us strengthen the interpretation.\nRevision made (colored in blue):\n(in page 12, line 7)\nOperando electrochemical analyses of perovskite-based cathode\nElectrochemical impedance spectroscopy (EIS) was performed on the immersed perovskite- based cathode under both control and NIR irradiation conditions to assess the impact of thermal activation on catalyst performance. Measurements were conducted at potentials ranging from 1.0 to 0.8 V over a frequency range of 0.1 Hz to 300 kHz under 1-sun illumination. Below RHE 0.8 V , both conditions reached saturation in photocurrent density, hindering accurate RHE measurement of charge transfer resistance. Therefore, EIS measurements were limited to the potential range of 1.0 to 0.8 V . The Nyquist plots (Fig. 2b, c) were fitted using a Voigt-type RHE equivalent circuit model (Supplementary Fig. 26a and Supplementary Note 7), with the fitting calculations summarized in Supplementary Table 4. Regardless of applied potential and NIR irradiation, there were no significant differences in R and R . However, the Bode plot s HF revealed noticeable differences around the low-frequency region (100–0.1 Hz)45,46 (Supplementary Fig. 27). The low-frequency arc and R were significantly reduced under NIR ct irradiation compared to the control, indicating that the photothermal effect markedly enhances the charge transfer reaction (Fig. 2b, c). Ionic contributions, are excluded from our interpretation because the observed low-frequency arc shows a clear voltage dependence, increasing near V where ionic movement is expected to be most severe due to suppressed oc compensation of built-in field66, 67. Also the peaks in Bode plot appears at 10–1 to 50–10 Hz, faster than the <1 Hz regime typically associated with ionic migration. Thus, the reduced arc under NIR irradiation is attributed to enhanced interfacial charge transfer rather than ionic effects.\n(reference 66, 67 was added)\n27\nComment 6:\nMott-Schottky analysis is used to determine that there is no change in band-bending in the bulk perovskite. Mott-Schottky very rarely measured band bending in perovskite cells (there are many papers showing just how problematic it is as a technique for perovskites and proving that it rarely measures the space charge layer) and a great deal of work is needed here for the authors to convince us that they are actually measuring band bending. I suspect that none of the Mott-Schottky analysis here is valid?\nResponse:\nWe acknowledge that the applicability of Mott–Schottky (M–S) analysis to halide perovskites has been questioned in the literature because ionic migration and interfacial effects can obscure the true depletion capacitance. Following the procedures recommended by Almora et al. (Appl. Phys. Lett., 2016, 109, 173903), we performed measurements under dark conditions and at a high-frequency window where ionic and chemical capacitances are negligible, and used a stabilized bias preconditioning to minimize slow ionic transients. However, our perovskite shows a relatively low defect density (≈1015 cm-3), a regime where chemical/ionic capacitance (C ) is expected to dominate even in high-frequency. As a result, the M–S measurements cannot s reliably determine the true depletion capacitance (C ), making any discussion of built-in dl voltage or band bending potentially unreliable. Therefore, fully accommodating reviewer’s feedback, we have removed all M–S analysis and related discussion in the revised manuscript. We appreciate the reviewer for insightful comments regarding M-S analysis, which made our manuscript more scientifically solid.\nRevision made (colored in blue):\n(in page 11, line 1)\n… the photothermal effect markedly enhances the charge transfer reaction (Fig. 2b, c). To confirm that the reduction in R was not due to changes in perovskite band bending, Mott- ct Schottky analysis was conducted under dark conditions (Supplementary Fig. 24). The flat-band potentials (E ) of the control and NIR-irradiated conditions were nearly identical, implying FB 28 that the photothermal effect did not affect band bending within the bulk perovskite, but only enhanced catalytic activity.\n(in page 13, line 1)\nSupplementary Fig. 31. After 8 h of operation, both the control and NIR-irradiated samples exhibited only slight decreases in V and FF, largely retaining their initial performance. As the on operation extended to 45 h, the control device showed severe degradation across all performance parameters, retaining only ~53% of its initial photocurrent. In contrast, the NIR- irradiated device maintained a stable LSV profile and demonstrated high stability, preserving ~87% of its initial photocurrent value. To further assess changes in band bending over time, the E of the control and NIR-irradiated devices was determined using Mott-Schottky analysis FB (Supplementary Fig. 29). Similar to the LSV curves in Fig. 3a, b, the E values of both devices FB showed negligible differences after 8 h of operation. However, after 45 h, the control device exhibited a more pronounced decrease in E compared to the NIR-irradiated device. This FB suggests that the initial decline in photocurrent density was ascribed to the degradation of Pt- C/CC rather than the bulk perovskite or the interface, as is further discussed in Fig. 4.\nAdditionally, the enhanced HER kinetics facilitated by the photothermal effect reduced charge accumulation during operation, resulting in a smaller decrease in onset potential after 45 h compared to the control device, aligning well with the LSV results.\nWe employed drive-level capacitance profiling (DLCP) and capacitance-voltage (C- V) methods to evaluate the carrier density of control and NIR-irradiated samples at 0 h (fresh) and after 8 h and 45 h of operation52,53 (Supplementary Fig. 32 and Supplementary Note 9). The corresponding interfacial trap densities were profiled at various depths for both control and NIR-irradiated devices, as shown in Fig. 3c, d. Both devices exhibited nearly identical trap site densities between the fresh state and 8 h of operation, indicating that the initial stage of operation has a negligible effect on the degradation of the perovskite layer in both control and NIR-irradiated devices, as confirmed by Mott-Schottky analysis.\nHowever, beyond 8 h of operation, trap densities increased more significantly on the hole transport layer (HTL) side (spiro-OMeTAD) compared to the SnO layer, with this increase being more pronounced in 2 the control device than in the NIR-irradiated device. This result confirms that enhanced charge transfer reactions suppress charge accumulation at the interface of multilayer devices, leading to fewer trap states under NIR irradiation.\n29\n(Supplementary Information revised)\nSupplementary Fig. 24 | Mott-Schottky analysis. a, Mott-Schottky plots of cathodes under control and NIR irradiation in dark conditions, and b, the corresponding capacitance variation with applied potential. The analysis was conducted in a 0.5 M KPi (pH 7) electrolyte.\nSupplementary Note 8: Hybrid device for in situ and operando analysis\nFor DLCP and C-V analyses, the use of a three-electrode measurement configuration is unsuitable due to potential interferences from the electrolyte, catalyst degradation, or electrochemical reactions18,19. Therefore, a two-electrode measurement setup is required. To facilitate this, we developed a hybrid-structured cathode by attaching Cu wire to FTO along with the catalyst, enabling a two-electrode configuration distinct from the traditional three- electrode system20,21 (Supplementary Fig. 29). This setup allows us to monitor changes in carrier dynamics and band bending within the same bulk perovskite immediately after long- term stability tests, free from the limitations of the three-electrode system. J-V curves for both the three-electrode and two-electrode configurations are presented in Supplementary Note Fig. 5. The two-electrode configuration demonstrated performance nearly identical to that of the PV cell, with only minor losses attributed to contact resistance. Unlike the three-electrode configuration, the NIR irradiation did not affect the performance of the two-electrode structure, indicating that photothermal-induced temperature increases exclusively influence the electrochemical reaction at the catalytic area and do not impact the bulk perovskite or charge transport layer. Additionally, Mott-Schottky analysis conducted at a fixed frequency (10 kHz) under dark conditions revealed capacitance and flat-band potential values consistent with those obtained using the PV cell (Supplementary Note Fig. 6). Consequently, the hybrid cathode configuration allows direct DLCP and C-V analyses of the same device during long-term 30 stability measurements, enabling in situ monitoring of trap density and carrier dynamics during operation. For consistency, measurements in both configurations were conducted while submerged in the electrolyte.\nTwo-electrode configuration\n8\n)\n1 / 6\n4\n1\n0\n1\n( 4\nc\ns\n2\n-\nC\n2\n0\n0.0 0.2 0.4 0.6 0.8 1.0\nVoltage (V)\nSupplementary Note Fig. 6 | Mott-Schottky analysis. Mott-Schottky analysis of the PV cell and the two-electrode configuration of the hybrid cathode.\nSupplementary Fig. 29 | Mott-Schottky analysis of hybrid cathodes. Mott-Schottky plots of a, control, and b, NIR-irradiated cathodes at 0 h (fresh) and after 8 h and 45 h of operation.\nIMVS is measured – but the interpretation is far from clear. It should mirror the EIS results at open circuit.\nResponse:\nWe thank the reviewer for highlighting the insufficient interpretation of the IMVS results. The IMVS measurements were reconducted under open circuit conditions with white light illumination bias, using a perturbation of 10% of the light intensity and a frequency range from 100 kHz to 0.1 Hz. As shown in Bode plot of Fig. R7, high frequencies around 50 kHz, both conditions exhibited a response, which reflects the prompt recombination of photogenerated electron–hole pairs, lower under NIR irradiation. For the control condition, a pronounced peak appeared in the low-frequency (~1 Hz) region, which was absent under NIR irradiation. This behavior is consistent with slow charge transfer at open circuit via EIS and serves as evidence for reduced charge accumulation under NIR irradiation, indicating stronger charge build-up in the absence of NIR. We have included a more through interpretation regarding the IMVS measurements to address how they correlate to charge accumulation dynamics. We thank the reviewer for pointing out this shortcoming.\nFig. R7 a, Transient photocurrent (TPC) decay curves under control and NIR-irradiated conditions, and b, Phase variation via intensity-modulated photovoltage spectroscopy (IMVS) analysis under control and NIR-irradiated conditions. Both measurements were performed under 1-sun illumination in a 0.5 M KPi electrolyte.\nRevision made (colored in blue):\n32\n(in page 13, line 16)\nMoreover, intensity-modulated photovoltage spectroscopy (IMVS) was conducted under open circuit conditions with white light illumination bias, using a perturbation of 10% of the light intensity and a frequency range from 100 kHz to 0.1 Hz. At ~10 kHz. Both conditions show high-frequency responses, indicative of fast photocarrier recombination dynamics, lower under NIR irradiation. In contrast, only the control exhibits a pronounced low-frequency rise near ~1 Hz, absent under NIR irradiation. This is consistent with slow charge transfer at open circuit observed via EIS and thus stronger charge accumulation under control compared to when NIR is applied (Supplementary Fig. 28b)26,49.\n(Supplementary Information revised)\nSupplementary Fig. 28 | Charge accumulation observation. a, Transient photocurrent (TPC) decay curves under control and NIR-irradiated conditions, and b, Phase variation via intensity-modulated photovoltage spectroscopy (IMVS) analysis under control and NIR-irradiated conditions. Both measurements were performed under 1-sun illumination in a 0.5 M KPi electrolyte.\nComment 8:\nIMPS is measured and the data is fitted to extract ktrans and krec. All the EIS and IMPS plots need to be plotted isotropically. IMPS of perovskite solar cells measured at high frequencies measures the RC time constant of the devices (RseriesCgeo). Are the authors able to show that this is not what is being measured here? The data for both cells looks very similar, especially when it is claimed that the NIR strongly changes the kinetics. It is straightforward to calculate 33 the RC time constant and to compare it with 1/RC from the semi-circle maximum? This needs to be ruled out.\nResponse:\nWe thank the reviewer for mentioning the interpretation of IMPS measurements. Firstly, we acknowledge that the reviewer’s suggestion to plot EIS and IMPS isotropically is a valid way to check whether the assignment of spectral features to specific processes is consistent. However, the purposes of the two techniques in this study are distinct. For EIS, our goal was to probe differences in the charge-transfer resistance close to OCV; these results are corroborated by IMVS at OCV as well as band-bending inferred from Mott–Schottky measurements. By contrast, IMPS was employed to simultaneously resolve charge-transfer and recombination kinetics across a broader operational window (0.6–0.9 V vs RHE), fully covering the saturation current region, the maximum power point, and the approach to OCV. Secondly, we agree that the high-frequency tail of IMPS can reflect the RC time constant of the device (R C ). However, the rate constants reported in this work are derived from the series geo low- to mid-frequency extrema of the IMPS spectra (10–300 Hz), whereas the geometric RC cutoff is expected at much higher frequencies: 103–105 Hz (J. Phys. Chem. C 2015, 119, 7, 3456–3465). This frequency separation of several orders of magnitude sufficiently rules out the possibility that the extracted kinetics originate from a simple RC artifact.\nAs the reviewer points out, the difference between the calculated K and K is rec trans relatively small overall. However, the divergence in K becomes more pronounced near the trans saturation-current region, while the difference in K shows an overall increase near the onset rec potential where recombination dynamics are expected to be most severe, which is reflected in the data. Furthermore, we discovered a mistake in inserting the correct f value and have min corrected it. The f are provided explicitly in Table. R1, and all calculations have been min updated. Importantly, these corrections do not change the qualitative trends or the conclusions of the observations made. Also, the figure has been re-fitted to show a more apparent difference for the respective reactions (Fig. R8).\nRHE\nTable. R1 f values extracted from IMPS measurements.\nmin\n0.6 V 0.7 V 0. 8 V 0.9 V\nRHE RHE RHE RHE\nControl 212.35 212.35 212.35 134.6\nw/ NIR 266.72 266.72 212.35 134.6\nRevision made (colored in blue):\n(in page 14, line 14)\nNormalizing the real photocurrent to the high-frequency intercept allows the low-frequency 35 intercept to directly reflect the charge transfer efficiency. The combined rate, k + k , is trans rec given by 2πf , where f is the frequency at the apex of the semicircle in the IMPS plots. By min min combining the charge-transfer efficiency and f , both k and k were obtained (Fig. 2f). min rec trans The f are provided explicitly in Supplementary Table 5.\nmin\n(Fig. 2 revised)\nFig. 2 | Electrochemical characterization under operando and in situ conditions. a, In situ Raman spectra recorded during the HER reaction on a Pt-C/CC/FTO electrode in 0.5 M KPi (pH 7). Nyquist plots of the b, control and c, NIR-irradiated cathodes. IMPS spectra of the d, control and e, NIR-irradiated conditions acquired in 0.5 M KPi (pH 7) electrolyte under 1-sun illumination at the specified applied potential. f, Charge transport and recombination rate constants derived from IMPS data within the potential range of 0.6–0.9 V .\nRHE\nSupplementary Table 5 | f values extracted from IMPS measurements.\nmin\n0.6 V 0.7 V 0. 8 V 0.9 V\nRHE RHE RHE RHE\nControl 212.35 Hz 212.35 Hz 212.35 Hz 134.6 Hz\n36\nw/ NIR 266.72 Hz 266.72 Hz 212.35 Hz 134.6 Hz\nComment 9:\nCan the authors show that DLCP and CV methods actually measure the carrier trap density in their devices? Typically, these techniques do not accurately measure traps in perovskite devices as the ion migration dominates the response. All the same issues as for Mott Schottky analysis apply, there are many papers on this which are not discussed.\nResponse:\nWe appreciate the reviewer’s concern that ion migration can compromise capacitance-based trap measurements in perovskite devices. As mentioned in our response to Comment 6, Reviewer 2, C–V and conventional M–S analyses alone cannot provide reliable information on the internal band bending or carrier dynamics of halide perovskites, because ionic migration, interfacial polarization, and chemical capacitance often mask the true depletion capacitance. However, when combined with DLCP, the reliability of these measurements is greatly enhanced. DLCP employs AC amplitude dependent fitting to separate the genuine depletion layer response from slow ionic or chemical contributions, and to map the spatial and energetic distribution of traps and carriers. Integrating DLCP with C–V or impedance spectroscopy therefore enables a more rigorous and quantitative assessment of defect-related charge dynamics within the perovskite absorber. In fact, numerous studies on various perovskite systems have employed DLCP to qualitatively map the internal trap-density distribution (Nature Energy, 2022, 7, 65–73; Science, 2020, 367, 1352–1358; ACS Appl. Mater. Interfaces, 2023, 15, 56909−56917). We sincerely appreciate the reviewer’s detailed comments on the reliability of our analysis.\n37\n<Reviewer 3>\nThe present manuscript discusses degradation of perovskite-based photocathode caused by charge accumulation. Comparison with and without NIR irradiation on HER catalyst and various characterization during the degradation found that sluggish HER kinetics led to the charge accumulations in the semiconductor and its degradation. The present manuscript would provide design guidelines for durable photocathodes which can be potentially considered in various semiconductors. However, my concern is what would be the original cause of the degradation. Is it due to either the catalysts degradation caused by bubbles or poor HER kinetics in the absence of NIR? Additional simple control measurements would further clarify their claims before its publication in Nat. Commun. Please find the following questions.\nRemark:\nWe sincerely appreciate the reviewer’s insightful comments on the original cause of the degradation. We fully agree that clarifying the origin of the observed degradation is essential to strengthen our conclusions. Based on reviewer’s suggestion, we performed additional control experiments to decouple these effects. Our detailed results and responses to the reviewer’s comments are provided below.\nComment 1:\nFig. 1c\nThe authors claim that “the photothermal effect induced by NIR irradiation increased the catalyst’s temperature, thereby accelerating HER kinetics and enhancing both Jph and Von.” Why Von changes by HER kinetics? Von should be at 0 mA/cm2 where HER kinetics does not involve. Does the Von shift come from the thermodynamics of HER? The authors may be able to discuss the shift from Nernst equations. It may also help to evaluate the local temperature on the catalyst not just by IR camera.\nResponse:\nWe sincerely thank the reviewer for raising this important point. Onset potential (V ) refers to on the potential at which a measurable photocurrent (or catalytic current) first appears, marking the initiation of the targeted electrochemical reaction. For the catalyst alone, current onset is observed at approximately –0.15 V (Supplementary Fig. 22). When integrated with the RHE perovskite PV, the built-in photovoltage (~1.05 V) shifts the onset to ~0.9 V at the cathode RHE 38 (Fig. 1b). Upon NIR irradiation, the HER kinetics are accelerated, leading to a reduction in the overpotential. Consequently, the catalyst onset shifts positively from –0.15 V to –0.12 V . RHE RHE Since the PV contribution remains unchanged, this reduction in overpotential translates into an apparent increase of V at the cathode under NIR illumination. We also note that the more on pronounced improvement in onset potential observed for the cathode can be attributed to the difference in catalyst area: while the catalyst-alone measurement used a catalyst area of about 0.5 cm2, the cathode employed a much smaller area of about 0.1 cm2. As a result, the localized heating and other effects induced by NIR irradiation were concentrated over a smaller region, leading to a more significant enhancement.\nAs the reviewer pointed out, the temperature increase induced by NIR irradiation could potentially alter the thermodynamic onset potential of the HER. According to the Nernst equation:\n2.303RT\nE(T, pH) = Eº – ( )pH (1)\nF\nwhere E(T, pH) is the equilibrium electrochemical potential, Eº is the standard electrode potential, R is universal gas constant, T is temperature, and F is Faraday constant. Under 1 atm and at pH 7, an increase in temperature of approximately 25 °C shifts the thermodynamic potential for the HER by about –34.7 mV toward more negative values. However, we note that the thermodynamic potential of the HER does not vary simply in proportion to the catalyst surface temperature. At the catalyst/electrolyte interface, HER proceeds as individual electrolyte molecules successively adsorb, undergo proton–electron transfer to generate H gas, 2 and desorb. Although the catalyst surface may be locally hot due to photothermal heating, each newly arriving electrolyte molecule retains a temperature closer to the bulk electrolyte. Consequently, the effective temperature relevant to the electrochemical reaction is not necessarily identical to the catalyst surface temperature, and the thermodynamic potential cannot be determined solely on that basis. In addition, convective electrolyte flow near the catalyst surface is expected to influence mass transport and the local energy landscape, further decoupling the thermodynamic potential from the surface temperature itself. This consideration helps explain why previous photothermal-catalysis studies, despite reporting significant surface temperature rises, have not attributed HER potential shifts due to temperature effects (Nat. Commun. 2025, 16, 3098, ACS Sustainable Chem. Eng. 2019, 7, 3710-3714). Instead, they emphasized kinetic enhancements in terms of activation energy overpotentials. While it is obvious that local heating occurs and could, in principle, alter the thermodynamic energy level, we conclude that this effect is likely much smaller than one might expect and remains difficult 39 to quantify precisely. We have incorporated this discussion into the revised manuscript to highlight the thermodynamic aspects of electrochemical reactions under photothermal conditions. We are grateful to the reviewer for prompting this deeper examination, which strengthens the scientific rigor and scope of our work.\nRevision made (colored in blue):\n(in page 6, line 5)\nThe NIR laser intensity used was 2.5 A, with the optimization process described in Supplementary Fig. 6. The immersed perovskite-based cathode initially demonstrated a saturated photocurrent density (J ) of 24.08 mA cm–2 at 0 V (V vs. reversible hydrogen ph RHE electrode) and an onset voltage (V ) of 0.96 V . Notably, following NIR laser irradiation of on RHE the catalyst, the cathode’s performance improved, exhibiting a J of 25.48 mA cm–2 at 0 V ph RHE and V of 1.04 V . The increased onset voltage under NIR irradiation originates from on RHE enhanced catalyst performance (Supplementary Fig. 22), rather than change in band gap or photovoltage provided by the perovskite layer. Note that the thermodynamic potential of the HER does not vary due to the catalyst surface temperature (Supplementary Note 5).\n(Methods revised)\nFor the cathode, Cu tape was attached to the Au layer, and a Cu wire was connected using silver paste. A Pt carbon paste was prepared by mixing 20 mg of Pt carbon, 0.75 mL of DI water, 0.25 mL of IPA, and 75 µL of Aquivon. This paste was deposited onto carbon cloth via blade coating and dried in ambient air for 24 h. After drying, the Pt-C/CC catalyst was applied to the FTO side using silver paste (see Supplementary Fig. 4). The catalyst area for FTO/CC/Pt-C electrode was 0.5 cm2. For the anode, Ni foil was attached to the Au layer with silver paste, and NiFe LDH/Ni foam was fabricated by dissolving Ni(NO ) 6H O (0.3 g), Fe(NO ) 9H O (0.38 g), 3 2 2 3 3 2 and urea (0.6 g) in 70 mL of DI water under sonication for 15 min. The precursor solution and Ni foam were then subjected to hydrothermal synthesis in a Teflon-lined stainless-steel autoclave, followed by annealing of the resulting Ni foam for 12 h at 120 °C. The Ni foams were washed with DI water and acetone for 5 min each and dried for 24 h at 80 °C. The NiFe LDH/Ni foam catalyst was attached to Ni foil using silver paste and sealed with epoxy resin, and a Cu wire was attached to the FTO side using silver paste.\n(Supplementary Information revised)\nThe temperature increase induced by NIR irradiation could potentially alter the thermodynamic onset potential of the HER. According to the Nernst equation:\n2.303RT\nE(T, pH) = Eº – ( )pH (1)\nF\nwhere E(T, pH) is the equilibrium electrochemical potential, Eº is the standard electrode potential, R is universal gas constant, T is temperature, and F is Faraday constant. Under 1 atm and at pH 7, an increase in temperature of approximately 25 °C shifts the thermodynamic potential for the HER by about –34.7 mV toward more negative values. However, we note that the thermodynamic potential of the HER does not vary simply in proportion to the catalyst surface temperature. At the catalyst/electrolyte interface, HER proceeds as individual electrolyte molecules successively adsorb, undergo proton–electron transfer to generate H gas, 2 and desorb. Although the catalyst surface may be locally hot due to photothermal heating, each newly arriving electrolyte molecule retains a temperature closer to the bulk electrolyte. Consequently, the effective temperature relevant to the electrochemical reaction is not necessarily identical to the catalyst surface temperature, and the thermodynamic potential cannot be determined solely on that basis. In addition, convective electrolyte flow near the catalyst surface is expected to influence mass transport and the local energy landscape, further decoupling the thermodynamic potential from the surface temperature itself. This consideration helps explain why previous photothermal-catalysis studies, despite reporting significant surface temperature rises, have not attributed HER potential shifts due to temperature effects48, 49. Instead, they emphasized kinetic enhancements in terms of activation energy overpotentials. While it is obvious that local heating occurs and could, in principle, alter the thermodynamic energy level, we conclude that this effect is likely much smaller than one might expect and remains difficult to quantify precisely.\n(references S48 and S49 were added)\nS48 Libo. W. et al. Photothermal-promoted anion exchange membrane seawater electrolysis on a nickel-molybdenum-based catalyst. Nat. Commun., 16, 3098, (2025)\nS49 Liu. G. et al. Enhancing Electrocatalytic Water Splitting Activities via Photothermal Effect over Bifunctional Nickel/Reduced Graphene Oxide Nanosheets. ACS Sustainable Chem. Eng., 7, 3710-3714, (2019)\nComment 2:\nTafel slope on Pt-C/CC/FTO was found to be reduced by NIR. If we simply follow Tafel equation or Butler-Volmer equation, the slope should increase at the elevated temperature. The authors can discuss the observed change in detail. I may consider that the improved kinetics helps to suppress the coverage of H leading to the smaller Tafel slope. See Sci. Rep. 5 (2015) 13801.\nResponse:\nWe appreciate the reviewer’s insightful comment on Tafel slope. Indeed, according to the Butler–Volmer relation, the Tafel slope is expected to increase with temperature. However, as highlighted in recent micro-kinetic analyses, the experimentally observed Tafel slope is strongly dependent not only on temperature but also on the coverage of reaction intermediates and the rate-determining step (Shinagawa et al., Sci. Rep., 2015, 5, 13801; ACS Energy Lett. 2025, 10, 3040−3049). Under NIR irradiation, the local heating and improved charge-transfer kinetics likely suppressed the excessive hydrogen coverage on Pt, leading to a more favorable surface condition for the Heyrovsky step. Moreover, this results in a lower apparent Tafel slope despite the elevated temperature. We have added this discussion in the revised manuscript to clarify the observed behavior.\nRevision made (colored in blue):\n(in page 10, line 10)\nThe LSV curves indicated that the potentials required to achieve a current density of 10 mA cm–2 were –0.15 V for the control and –0.11 V under NIR irradiation. The Tafel slopes RHE RHE were determined to be 40.8 mV dec–1 for the control and 27.6 mV dec–1 under NIR irradiation, demonstrating accelerated catalytic activity under NIR irradiation (Supplementary Fig. 22b). Note that the Tafel slope is strongly dependent not only on temperature but also on the coverage of reaction intermediates and the rate-determining step68, 69. Under NIR irradiation, the local heating and improved charge-transfer kinetics likely suppressed the excessive hydrogen coverage on Pt, leading to a more favorable surface condition for the Heyrovsky step. This results in a lower apparent Tafel slope despite the elevated temperature.\n(references 68 and 69 were added)\n69 van der Heijden, O. et al. Temperature-Dependent Kinetic Parameters for the Alkaline Oxygen Evolution Reaction on NiFeOOH. ACS Energy Lett. 10, 3040−3049, (2025)\nComment 3:\nAdditionally, can they show LSV of Pt-C/CC/FTO after the stability tests? It helps readers to observe how the overpotential increased after Pt detachment.\nResponse:\nWe thank the reviewer for their comments regarding the performance of catalyst after stability test. Following the reviewer’s suggestion, we evaluated the stability of the Pt-C/CC/FTO catalyst electrode by measuring its LSV performance after extended operation (Fig. R9). After 48 h of continuous operation, the onset potential shifted by ~0.23 V , resulting in a –0.36 RHE V . This degradation can be attributed to Pt detachment, which reduces the available catalytic RHE active sites and consequently requires a higher overpotential to drive the reaction. We sincerely appreciate the reviewer’s detailed and thoughtful recommendation, which helped us strengthen the manuscript.\n43\n0\n)\n2\n-\nm\n-30\nc\nA\nm\n(\n-60\ny\nt\ni\ns\n-90\nn\ne\nD\nt\nn\ne\nControl\nr\nr\nu Control_48 h\nC\n-120\n-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0\nPotential (V vs RHE)\nFig. R9 LSV curves of Pt-C/CC/FTO electrode after 48 h of operation.\nRevision made (colored in blue):\n(Supplementary Information revised)\nSupplementary Fig. 39 | Operational stability of Pt-C/CC/FTO catalyst. a, Long-term stability of control and NIR-irradiated catalysts measured under potentiostatic conditions at an applied voltage of –0.25 V in 0.5 M KPi electrolyte. b, LSV curves of Pt-C/CC/FTO RHE electrode after 48 h of operation.\nLocal heating\nLocal heating by NIR seems to play an important role. It is also known that kinetic loss on electrocatalysts causes a local heating. Can they compare NIR power density and local heating from HER catalysis? The authors can evaluate the local heating (W/cm2) due to HER from the operating current density (A/cm2) and overpotential (V). Overpotential can be roughly evaluated from LSV at their operating current density. This analysis helps the readers to get an idea on the power density levels needed to heat up.\nResponse:\nAs suggested by the reviewer, we examined the extent of local heating applied to the catalyst by evaluating the overpotential difference under the operating current density (25 mA cm-2). The calculation was performed using the following equation:\nQ = (η - η ) × J (2)\nNIR ctrl NIR op\nwhere Q is the local heating applied to the catalyst, η and η are the measured NIR ctrl NIR\noverpotential at the operation current density under two conditions, and Jop is the operation current density. At a current density of 25 mA cm-2, the control sample exhibits a potential of –0.227 V , whereas the NIR-irradiated sample shows –0.164 V , corresponding to a RHE RHE potential difference of about 63 mV. Based on this difference, we estimate that local photothermal heating contributes roughly 1.58 mW cm-2 to the system. Furthermore, using the actual incident NIR laser intensity, we calculated the extent of NIR energy absorbed by the catalyst, as described by the following equation:\nQ = I × A (3)\nNIR 0,NIR\nwhere Q is the local heating applied from NIR source, I is incident laser intensity, and A NIR 0,NIR\nis the effective optical absorption fraction of the catalyst. An NIR laser with an electrical input of 2.5 mA was applied to an area of 0.1 cm2. Considering the device operating voltage of 1.6 V and a photoelectric efficiency of 0.25, the incident NIR intensity I is calculated to be 0,NIR\napproximately 10 mW cm-2. From this, A is estimated to be about 15.8 %, indicating that the remaining energy is not absorbed because of losses in the catalyst, electrolyte, reaction cell, and beam path length. For practical commercialization of this system, it will be necessary to develop cell designs or device structures that can further enhance the NIR absorption efficiency.\n(in page 10, line 17)\nTo further elucidate the factors contributing to the increased catalytic activity, the electrochemically active surface area (ECSA) was measured by calculating the electrochemical double-layer capacitance (C ) (Supplementary Fig. 23). The ECSA values were 34.5 cm2 for dl the control and 42.9 cm2 under NIR irradiation, confirming an increase in the active area due to thermal activation (Supplementary Table 3). Also, to confirm that the increase in ECSA did not originate from an irreversible morphological change of the electrode due to NIR irradiation, the ECSA variation of electrode was observed after being cooled for 3 min and 10 min, respectively. After 3 min of cooling, ECSA was reduced to 37.5 cm2 and further decreased after 10 min, approaching the pre-irradiation value of 34.5cm2. Based on the LSV curves of the catalyst electrode, the local heating was evaluated (Supplementary Note 6).\n(Supplementary Information revised)\nSupplementary Note 6: Local heating\nWe examined the extent of local heating applied to the catalyst by evaluating the overpotential difference under the operating current density (25 mA cm-2). The calculation was performed using the following equation:\nQ = (η - η ) × J (2)\nNIR ctrl NIR op\nwhere Q is the local heating applied to the catalyst, η and η are the measured NIR ctrl NIR\noverpotential at the operation current density, and Jop is the operation current density. At a current density of 25 mA cm-2, the control sample exhibits a potential of –0.227 V , whereas RHE the NIR-irradiated sample shows –0.164 V , corresponding to a potential difference of about RHE 63 mV. Based on this difference, we estimate that local photothermal heating contributes roughly 1.58 mW cm-2 to the system. Furthermore, using the actual incident NIR laser intensity, we calculated the extent of NIR energy absorbed by the catalyst, as described by the following equation:\nQ = I × A (3)\nNIR 0,NIR\nwhere, Q is the local heating applied from NIR source, I is incident laser intensity, and NIR 0,NIR\nA is the effective optical absorption fraction of the catalyst. An NIR laser with an electrical input of 2.5 mA was applied to an area of 0.1 cm2. Considering the device operating voltage of 1.6 V and a photoelectric efficiency of 0.25, the incident NIR intensity I is calculated to be 0,NIR\napproximately 10 mW cm-2. From this, A is estimated to be about 15.8 %, indicating that the 46 remaining energy is not absorbed because of losses in the catalyst, electrolyte, reaction cell, and beam path length. For practical commercialization of this system, it will be necessary to develop cell designs or device structures that can further enhance the NIR absorption efficiency.\nComment 5:\nThe huge bubbles and their detachments were claimed to be the cause of Pt detachment and degradation of HER. The authors limited systems without additional stirring. Can they perform control measurements with stirring and without NIR. For example, stability test of Pt-C/CC/FTO (like Fig4a), stability of photocathode (like Fig. 1d), and bubble observation (like Fig4e). This comparison will help to clarify the major cause on the degradation of photocathode either from bubbles or HER kinetics. If the bubbles are removed efficiently without NIR irradiation and HER activity is maintained at room temperature, does the photocathode degrade? Or, is the improvement of HER kinetics (like Fig. S19a) necessary?\nIf the Pt-C degraded by the bubbles in the absence of NIR is used for the fresh perovskite, can it still maintain the stability with NIR irradiation?\nResponse:\nAs suggested by the reviewer, we evaluated the performance and stability of the control cathode with and without stirring. When stirring was applied, the overpotential decreased compared to the unstirred condition, which can be attributed to improved mass transport near the catalyst (Fig. R10a). The stability test also revealed slightly enhanced durability under stirring, indicating that increased mass transport at the catalyst–electrolyte interface contributes to improved device stability (Fig. R10b). Notably, however, the stability achieved by stirring alone did not reach the level observed under NIR irradiation. This suggests that NIR illumination not only promotes mass transport and bubble detachment but also enhances the HER kinetics, thereby suppressing charge accumulation. Such kinetic acceleration under NIR irradiation appears to play a significant role in further improving long-term stability.\nFurthermore, although we did not perform a dedicated experiment, we considered the expected stability behavior of fresh perovskite-based cathode with degraded Pt-C/CC catalyst. Based on our results, the control device experiences early catalyst degradation primarily due to bubble, which exacerbates charge accumulation and triggers instability. If NIR irradiation 47 were applied to such an initially degraded catalyst, we would anticipate improved stability compared to the control condition because NIR-induced enhancement of catalytic activity, bubble detachment, and mass transport would help mitigate these effects. Nevertheless, the intrinsic catalytic activity of a degraded catalyst would remain lower than that of the pristine one. As a result, even under NIR irradiation, charge consumption would be less efficient, and accumulated charge would gradually increase over time. We therefore expect the stability of this device to surpass that of the control device but still fall short of that achieved with an undegraded catalyst under NIR irradiation. We have added the stability data of cathode with and without stirring into the revised manuscript to emphasize the effect of photothermal effect. We appreciate the reviewer for thoughtful and detailed suggestion regarding the origin of stability improvement.\nFig. R10 (a) LSV curves of cathode devices with stirring and without stirring. (b) Stability measurements of the cathode devices with stirring and without stirring. Measurements are conducted at 0 V in a 0.5 M KPi (pH 7) electrolyte under 1-sun illumination. RHE\nRevision made (colored in blue):\n(in page 8, line 18)\nThe operational stability statistics of the control and NIR-irradiated cathodes are illustrated in Supplementary Fig. 17, highlighting the enhanced stability of the cathode under NIR irradiation and suggesting a correlation between improved electrochemical reaction kinetics and overall device stability. To evaluate the effect of stirring on cathode stability, performance and stability tests were conducted with and without stirring. When stirring was applied, the overpotential decreased compared to the unstirred condition, which can be attributed to improved mass transport near the catalyst (Supplementary Fig. 18a). The stability test also revealed slightly enhanced durability under stirring, indicating that increased mass transport at the catalyst– 48 electrolyte interface contributes to improved device stability (Supplementary Fig. 18b).\nNotably, however, the stability achieved by stirring alone did not reach the level observed under NIR irradiation. This suggests that NIR illumination not only promotes mass transport and bubble detachment but also enhances the HER kinetics, thereby suppressing charge accumulation. Such kinetic acceleration under NIR irradiation appears to play a significant role in further improving long-term stability.\n(Supplementary Information revised)\nSupplementary Fig. 18 | Effect of stirring. a, LSV curves of cathode devices with stirring and without stirring. b, Stability measurements of the cathode devices with stirring and without stirring. Measurements are conducted at 0 V in a 0.5 M KPi (pH 7) electrolyte under 1-sun RHE illumination.\nComment 6:\nKinetics of charge consumptions should be an important factor determining the degradation of perovskite. Simple measurements using PV setup (without the electrolytes and electrocatalysts) can be performed with solar irradiation at OCV and short circuit conditions while measuring IV response at certain periods. Keeping at OCV (0 mA cm2) under solar irradiation leads to charge accumulation which would cause degradation of semiconductor while keeping at the short circuit is the case of extremely fast kinetics case minimizing the charge accumulations. I believe the degradation observed in the photocathode would be positioned between them.\nResponse:\nWe thank the reviewer for the insightful comment regarding perovskite degradation in different device configurations. Following the suggestion, we fabricated an encapsulated PV structure in which the catalyst attached to the FTO side of the cathode was replaced with a Cu wire. The resulting device exhibited an open-circuit voltage (V ) of 1.1 V and a short-circuit current oc density (J ) of about 25.1 mA cm-2 (Fig. R11a). We then evaluated the device stability under sc both 1.05 V (near OCV) and 0 V (short-circuit, SC) conditions (Fig. R11b). Notably, these stability tests were carried out while the device was immersed in electrolyte, closely replicating the cathode operating environment. As anticipated, under near OCV conditions the device 49 showed limited stability, with a t (time to reach 90 % of the initial photocurrent) of 3 h. In 90 contrast, under SC conditions the photocurrent retained nearly 90 % of its initial value for more than 25 h. Considering that the control cathode exhibits a t of about 6 h, the stability trend 90 can be summarized as Encapsulated PV (near OCV) < Control cathode < Encapsulated PV (SC). These results indicate that charge consumption significantly influences the intrinsic stability of the perovskite layer. We have incorporated these new stability data into the revised manuscript to further substantiate our conclusion.\nFig. R11 (a) LSV curves of encapsulated PV device. (b) Stability measurements of the encapsulated PV devices at open-circuit and short-circuit conditions. Measurements are conducted in a 0.5 M KPi (pH 7) electrolyte to replicate the cathode operating environment.\nRevision made (colored in blue):\n(in page 9, line 6)\nFurthermore, stability test under 0.5 sun illumination was conducted (Supplementary Fig. 19). The device exhibited enhanced stability compared to the original control sample, maintaining ~90% of the initial photocurrent for about 56 h. This improvement can be attributed to the reduced number of photogenerated carriers under the low-sun illumination, which reduces charge accumulation and consequently enhances the overall device stability. These results further highlight the critical role of kinetics of electrochemical reaction in determining stability of the perovskite-based water splitting device. Moreover, an encapsulated PV structure was 50 fabricated in which the catalyst attached to the FTO side of the cathode was replaced with a Cu wire. The resulting device exhibited an open-circuit voltage (V ) of 1.1 V and a short-circuit oc current density (J ) of about 25.1 mA cm-2 (Supplementary Fig. 20a). The device stability sc under both 1.05 V (near OCV) and 0 V (short-circuit, SC) conditions (Supplementary Fig. 20b). Notably, these stability tests were carried out while the device was immersed in electrolyte, closely replicating the cathode operating environment. Under near OCV conditions the device showed limited stability, with a t (time to reach 90 % of the initial photocurrent) 90 of 3 h. In contrast, under SC conditions the photocurrent retained nearly 90 % of its initial value for more than 25 h. Considering that the control cathode exhibits a t of about 6 h, the stability trend 90 can be summarized as Encapsulated PV (near OCV) < Control cathode < Encapsulated PV (SC). These results indicate that charge consumption significantly influences the intrinsic stability of the perovskite layer.\n(Supplementary Information revised)\nSupplementary Fig. 20 | Encapsulated PV device. a, LSV curves of encapsulated PV device. b, Stability measurements of the encapsulated PV devices at open-circuit and short-circuit conditions. Measurements are conducted in a 0.5 M KPi (pH 7) electrolyte to replicate the cathode operating environment.\nComment 7:\nImplementation potential of NIR local heating technique\nBecause the authors also emphasize its record high efficiency and large-scale applications, it would be nice to discuss how the natural solar irradiation would be managed to implement this technique in a real device. Do we need to separate NIR from the natural solar using some mirror? In their study, NIR was obtained from laser with various power. When we irradiate a unit area of perovskite, can we obtain sufficient NIR power after the split of natural sunlight? The authors can discuss its potential based on the standard solar spectrum.\nResponse:\nWe appreciate the reviewer’s insightful suggestion to discuss how natural sunlight could be managed to realize our photothermal concept in practical devices. We envision two possible approaches for implementing the proposed photothermal perovskite-based electrochemical system in a commercialization context. The first is to employ NIR sheet irradiation to induce photothermal effects across a large-area catalyst. However, achieving a significant photothermal contribution in this manner would require the use of NIR lasers with relatively high intensity, which may lead to considerable energy loss and thus limit the practicality of this approach. Therefore, to practically implement this technology, it is essential to develop a sheet- type irradiation system with minimized NIR power loss.\nThe second approach, as suggested by the reviewer, is to utilize natural sunlight irradiation to induce the photothermal effect. One possible strategy is to spectrally split natural sunlight so that the NIR component is directed toward the catalyst while the remaining visible range is absorbed by the perovskite layer. In fact, previous studies have already demonstrated the use of spectral-split optics to simultaneously drive BiVO and perovskite photoelectrodes 4 (Adv. Funct. Mater. 2022, 32, 2110284). Therefore, by adopting a similar architecture, it should be feasible to selectively extract NIR from solar light and deliver it to the catalyst to induce a pronounced photothermal effect. However, the temperature rise achievable under direct sunlight alone is insufficient to generate a meaningful photothermal response. To overcome this limitation, a Fresnel lens can be employed. Fresnel lenses have already been applied to Si solar panels, where they concentrate sunlight onto the absorber layer. By combining spectral- splitting and Fresnel lens concentration, a sufficient NIR flux could be harvested to induce the desired photothermal effect on catalyst, thereby enabling a photothermal perovskite-based electrochemical system driven solely by natural sunlight. We have incorporated this discussion into our revised manuscript to highlight the practical applicability of this concept. We sincerely thank the reviewer for this insightful suggestion, which allowed us to refine and expand the discussion with greater technical depth.\nRevision made (colored in blue):\nSupplementary Note 9: Large-scale applicability of NIR (photothermal) system\nIn conventional PEC or PV-EC systems, the illumination area and the catalytic active area are 52 typically designed to have a 1:1 ratio. When scaling up to large-area systems, maintaining such proportions can present significant limitations. In response, employing catalysts with exceptionally high catalytic activity or by optimizing the system to enhance catalytic performance may enable comparable efficiency and stability even with a reduced catalytic area ratio. Specifically, we fabricated perovskite-based water splitting cells with dimensions of 5 × 5 cm², setting the illumination area to approximately 8 cm² while configuring the catalyst area to 2 cm² (denoted as 4:1) (Supplementary Note Fig. 6a). The performance comparison between the 1:1 sample and the 4:1 sample reveals that when the catalytic active area is limited and the catalytic activity is insufficient, the carriers generated in the illumination area are not effectively managed, resulting in a significant reduction in the fill factor (Supplementary Note Fig. 6b). Remarkably, the performance of the 4:1 sample exhibited a significant improvement depending on the NIR irradiation, comparable to performance of the 1:1 sample. This indicates that the photothermal effect demonstrates the potential to achieve superior performance with a reduced catalyst area relative to the same illumination area, offering a practical pathway for extending the applicability to large-area water splitting devices.\nFurthermore, we outline three possible routes to implement photothermal assistance.\nFirstly, a sheet-type NIR laser can be implemented, however, this has less practicality since multiple lasers with high intensity would be required for large-area irradiation. Secondly, a beam splitter can be utilized to harvest NIR or IR from natural sunlight50. Lastly, a concentration lens such as a Fresnel len can be implemented, perhaps coupled with an optical beam splitter, to create sufficient NIR flux onto the photothermal substrate.\n(reference S50 was added)\nS50 Tang. S. et al. Harvesting of Infrared Part of Sunlight to Enhance Polaron Transport and Solar Water Splitting. Adv. Funct. Mater. 32, 2110284 (2022).\nComment 8:\nSupplementary Note 9\nThe onset potential of 1:1_control in Supplementary Note Fig 7b is quite high compared with those on other large scale samples and small devices in Main text. What is the reason? Does the cathodic photocurrent come from HER or reduction of large area catalysts region which may be reduced more easily?\nResponse:\nWe sincerely thank the reviewer for the detailed comments on the onset potential of the large- scale samples. Notably, the onset potential of the 1:1_control sample is approximately 0.97 V , which corresponds to an intermediate value relative to that of our small control samples RHE (Supplementary Fig. 13). In the case of the 4:1_control sample, the photogenerated carriers from the illumination area could not be efficiently consumed, resulting in kinetic limitations compared to the 1:1_control sample, which resulted in V of 0.8 V with high overpotential. on RHE When NIR irradiation was applied, the V has increased to 0.92 V , confirming that the high on RHE overpotential in the 4:1_control sample originated from the catalytic kinetic limitations. Recent study have also reported that the performance of electrochemical reactions is strongly influenced by the ratio between the illumination and catalyst active areas, which further supports our observations (Nature Catalysis, 2025, 8, 137–146).\nJournal: Nature Communications\nManuscript number: NCOMMS-25-40476-T\nTitle: “Operando insights into stability of perovskite-based solar water splitting devices”\nAuthor(s): Chang-Seop Jeong†, Wooyong Jeong†, Juwon Yun†, Donyoung Kang, Byungjun\nKang, Hyungsoo Lee, Jun Hwan Kim, Hyungsuk Lee, and Jooho Moon*\n<Reviewer 1>\nThe authors have addressed my comments, and the manuscript can be considered for\npublication.\nRemarks:\nWe are grateful to the reviewer for the thoughtful and constructive comments. We implemented\nthe suggested control experiments and textual revisions, which have considerably improved\nthe clarity and rigor of the manuscript. Thank you for your time and valuable guidance.\n<Reviewer 3>\nThe authors have addressed my concerns. The additional control measurements are effectively\nadded in the revised documents. I believe the revised manuscript is ready for the publication.\nRemarks:\nWe are grateful for the reviewer’s insightful remarks, which helped us refine the work. The\nsuggested control experiments and edits have been fully addressed, bringing the manuscript to\na publishable form. Thank you again for your guidance.\n<Reviewer 2>\nThe authors have improved the paper and have answered some of my initial questions. I am\nstill not convinced that it is possible to conclude that the NIR heating increases the reaction\nkinetics - the authors themselves state that the picture is very complex.\nWe sincerely appreciate the reviewer’s thorough and critical evaluation of our manuscript. We\nhave expanded the discussion, elaborated conceptual aspects, and conducted further\nexperiments to clarify the reliability and accuracy of the techniques utilized in the manuscript.\nDetailed responses to the reviewer’s comments are provided below.\nComment 1:\nThe stirred experiments are a nice control, but they will differ from increased convection very\nclose to the surface with NIR heating, as well as local mixing caused by the release of bubbles.\nIt may be that the kinetics do increase, but I don't think there is enough evidence to state it\nunequivocally.\nResponse:\nWe thank the reviewer for the constructive comment regarding whether NIR heating truly\naccelerates the reaction kinetics. To decouple mass transport and kinetic effects, we designed\nadditional control experiments conducted under conditions that differentially modulate bubble\ndynamics and mass-transport contributions; (i) a strong stirring environment (Video R1), and\n(ii) a thin-layer electrolyte environment (Fig. R1a-c). As shown in the video, we imposed\nvigorous stirring (strong convection) to establish a condition in which mass transport around\nthe catalyst is strongly enhanced, and then compared the performance of the FTO/CC/Pt-C\nelectrode with and without NIR irradiation (Fig. R1d). Stirring alone improved the\nperformance, and NIR irradiation led to a further increase. Because mass-transport limitations\nwere already alleviated by stirring, the additional performance gain under NIR indicates not\nonly residual mass-transport benefits but also a kinetic contribution. In a second control\nexperiment, the catalyst was immersed to a minimal depth to form an ultrathin electrolyte layer\nat the surface (Fig. R1a-c). This suppresses mass transport at the catalyst/electrolyte surface,\nbut more importantly it prevents the formation of a sufficient temperature gradient to drive\nMarangoni convection under NIR irradiation, thus separating mass transport and convection\nfrom reaction kinetics. Even under these conditions, NIR irradiation still enhanced the\nperformance, which we attribute to an intrinsic kinetic acceleration (Fig. R1e). Across multiple\ncontrol experiments designed to decouple kinetics from mass transport, we consistently\n2\nobserve a kinetic gain under NIR. Although Pt’s intrinsic catalytic activity can be highly mass\ntransport limited, our neutral-pH operating conditions impose a kinetic bottleneck that is\nrelieved by thermal modulation. We appreciate the reviewer’s careful attention to these details.\nFig. R1 (a) Digital photograph of a thin-layer electrolyte experiment setup. (b) Front profile of\ncatalyst before electrolyte immersion. (c) Side profile of catalyst after electrolyte immersion.\n(d) Linear sweep voltammetry (LSV) curves of FTO/CC/Pt-C catalyst electrode in strong\nstirring environment. (e) LSV curves of control and NIR-irradiated FTO/CC/Pt-C catalyst\nelectrode under thin-layer shallow electrolyte environment. All experiments were conducted in\na 0.5 M KPi (pH 7) electrolyte.\nRevision made (colored in blue):\n(in Page 10, line 8)\nPhotothermal effect inducing HER improvement on electrocatalyst (EC)\nTo validate the enhancement of HER catalytic activity via the photothermal effect on the\n3\ncatalyst, the electrocatalytic properties of Pt-C/CC/FTO electrodes were evaluated in 0.5 M\nKPi (Supplementary Fig. 22a). The LSV curves indicated that the potentials required to achieve\na current density of 10 mA cm–2 were –0.15 V for the control and –0.11 V under NIR\nRHE RHE\nirradiation. The Tafel slopes were determined to be 40.8 mV dec–1 for the control and 27.6 mV\ndec–1 under NIR irradiation, demonstrating accelerated catalytic activity under NIR irradiation\n(Supplementary Fig. 22b). Note that the Tafel slope is strongly dependent not only on\ntemperature but also on the coverage of reaction intermediates and the rate-determining step43,\n44. Under NIR irradiation, the local heating and improved charge-transfer kinetics likely\nsuppressed the excessive hydrogen coverage on Pt, leading to a more favorable surface\ncondition for the Heyrovsky step. Furthermore, the enhancement could have resulted from the\nincreased mass transport due to a forced convection via local heating. These effects are\ndescribed in detail in later sections; however, it is necessary to confirm that electrochemical\nkinetics enhance under NIR irradiation, since HER overpotential originates from both mass\ntransport and kinetic limitations. Control experiments of Pt-C/CC/FTO performance under (i)\nstrong stirring condition (Video S1) and (ii) a thin electrolyte environment was conducted\n(Supplementary Fig. 23a–c). As shown in the video, vigorous stirring was imposed to establish\na condition in which mass transport around the catalyst is strongly enhanced, and then\ncompared the performance of the FTO/CC/Pt-C electrode with and without NIR irradiation\n(Supplementary Fig. 23d). Under stirring, the performance increased, and it improved further\nupon NIR irradiation. These results originate from the enhancement of further mass transport\nbut also reaction kinetics since the limitation in mass transport has been alleviated to an extent\nbefore the NIR irradiation. In a second control experiment, the catalyst was immersed to a\nminimal depth such that only an ultrathin electrolyte layer was formed at the surface\n(Supplementary Fig. 23e). This configuration suppresses mass transport at the catalyst surface,\nbut more importantly it prevents the formation of a sufficient temperature gradient to drive\nforced convection under NIR irradiation, thus separating mass transport and convection from\nreaction kinetics under NIR. Even under these conditions, NIR irradiation still increased the\nperformance, which we attribute to an intrinsic kinetic enhancement.\n4\n(Supplementary Information revised)\nSupplementary Fig. 23 | Confirming kinetic enhancement under NIR irradiation a, Digital\nphotograph of a thin-layer electrolyte experiment setup. b, Front profile of catalyst before\nelectrolyte immersion. c, Side profile of catalyst after electrolyte immersion. d, Linear sweep\nvoltammetry (LSV) curves of FTO/CC/Pt-C catalyst electrode in strong stirring environment.\ne, LSV curves of control and NIR-irradiated FTO/CC/Pt-C catalyst electrode under thin-layer\nshallow electrolyte environment. All experiments were conducted in a 0.5 M KPi (pH 7) electrolyte.\nComment 2:\nThe EIS, IMVS and IMPS are still not plotted isotropically (it is immediately obvious by the fact that the semi-circles look squashed and is a result of the x-and y- axes having the same number of units but being different lengths). I am also still concerned that the EIS (and by implication also the IMVS) may have been misinterpreted. The author response that 'However, if it (the low frequency arc) were related with ion or defect migration within the perovskite layer, a decrease in arc diameter is expected since ion migration is most severe near open-circuit conditions' is not correct for several reasons. Firstly ion migration is not most severe near open circuit (which is close to the flat ion/flat band potential). The impact of ion migration on recombination may indeed be largest close to open circuit, but this is not what the authors argue, they talk about field screening, which is largest close to short circuit not open circuit as stated in the response. Further to this the impact of ions may be to either increase or to decrease the recombination resistance close to open circuit – it depends on the specific cell and there is no clear link between the size of the arc and the interpretation of whether it is caused by ions or charge transfer as argued by the authors. In my own experience, for many cells ions reduce recombination (and hence increase recombination resistance) at open circuit. You would also expect a voltage dependence of ion modulated recombination as changing the voltage changes the Fermi-level position and the amount of recombination. I think the updated response further highlights the lack of clarity in the interpretation of the EIS, IMPS and IMVS results. I also think that there is also some confusion in the response between steady state and dynamic measurements and the resulting difference in ion behaviour. It is finally not true that a response between 1Hz and 50Hz is too fast to be ionic. In my opinion the data interpretation for these techniques has not been improved as there are many papers in the literature that show ionic responses of perovskite cells at these frequencies. The response on the IMVS and IMPS is also not strong and I still doubt some of the interpretation.\nResponse:\nWe thank the reviewer for the detailed comments on the interpretation of our electrochemical analyses. As suggested, we replotted the EIS (electrochemical impedance spectroscopy) and IMPS (intensity modulated photocurrent spectroscopy), data using identical axis scaling (isotropic plotting) (Fig. R2). As for the IMPS (intensity modulated photovoltage spectroscopy) plots, only the photovoltage and phase variation with respect to the light modulation frequency were obtained, and therefore no isotropic fitting was required. However, additional experiments were conducted to acquire data sets (Fig. R4) to improve the reliability of the analysis and to provide a more complete explanation of the observed behavior, as described after the EIS section.\nNIR-irradiated samples (b) EIS, and (d) IMPS results.\nRegarding EIS, the key question is what resistance the low-frequency (LF) arc\nrepresents. Because our device uses a perovskite/spiro/Au stack, a positive LF arc attributable to recombination/ion migration in the bulk or at perovskite interfaces can indeed appear, as the reviewer notes. To disentangle contributions observed at the cathode, we fabricated two devices separating the components: (i) an encapsulated photovoltaic (PV) stack electrically contacted with a Cu wire at the catalyst position but without the catalyst itself, and (ii) a catalyst electrode (FTO/CC/Pt-C). For the encapsulated PV stack, the LF arc emerges and increases as the bias moves from open circuit toward short-circuit (Fig. R3b). Bode analysis shows this resistance appears around ~1–10 Hz and its characteristic frequency shifts to lower values as the bias approaches short-circuit (Fig. R3c). These features are consistent with the Shockley–Read–Hall (SRH) recombination resistance originating from the bulk and/or interfaces, as reported in prior literature (Adv. Energy Mater. 2024, 14, 2400955). For the isolated catalyst electrode, an LF arc is also observed; however, its magnitude decreases as the potential is swept negative from the onset voltage, and it is further reduced under NIR illumination (Fig. R3d, e). We attribute this behavior to charge-transfer resistance at the catalyst/electrolyte interface: both improved kinetics and mass transport under NIR illumination decrease this resistance. In the Bode plots, this LF contribution appears near ~1–10 Hz and shifts to higher characteristic frequencies as the potential approaches onset and the peak frequency increases further to ~10– 50 Hz with NIR illumination (Fig. R3f). These control experiments demonstrate that (i) SRH- related and charge-transfer resistances can occupy similar frequency windows, and (ii) the LF- arc trend at the working cathode mirrors the charge-transfer behavior of the catalyst electrode but is opposite to the SRH recombination-related trend of the encapsulated PV. Consequently, interpreting the cathode LF arc as charge-transfer resistance is more consistent with the observed bias and illumination dependences. However, we need to explain why the SRH- related LF resistance observed in the encapsulated PV structure does not appear at the cathode.\nThis interpretation aligns with prior PEC-EIS analyses (Adv. Energy Mater. 2021, 11, 2003569), which recommend bias-dependent assignments of the LF arc. Near the onset voltage, charge transfer is typically the dominant bottleneck and thus the most likely origin of the LF arc. But at higher overpotentials where the photocurrent is large or the catalyst’s onset potential is exceeded, recombination-related impedances can become dominant in the LF domain. Our voltage sweeps were intentionally confined to the vicinity of the onset, so assigning the LF arc at the cathode to charge-transfer resistance is appropriate. We note that extending the sweep deeper into the high-current regime would be informative, but our instrument exhibits signal instability (“spikes”) at very high currents, preventing reliable LF-arc acquisition in that range. We hope the reviewer will understand this practical limitation.\nMoreover, examining the coherence between our EIS analysis and the IMPS results\nshows that the IMPS-derived k /k trends are consistent with the LF-arc behavior when\ntrans rec\ninterpreting the LF arc as charge-transfer resistance. The IMPS-derived rate constants exhibit\nthe evolution that as the operating bias departs from the onset voltage, k increases whereas\ntrans\nk decreases. This trend mirrors the bias dependence of the LF arc, reinforcing that the LF arc\nrec\narises predominantly from charge-transfer resistance rather than recombination.\nFinally, we have elaborated our interpretation of the IMVS measurement after further\nexperimentation. Since IMVS is conducted under open circuit potentials, the net photocurrent\nis zero. Thus, the generated photovoltage during frequency sweep is related to temporal\n8\npotential difference via photogenerated electron-hole pairs. In Fig. R4a, as the frequency is\nswept from high to low region, photoexcited electron-hole pairs are generated, inducing\nincrease in photovoltage at ~100 kHz. It can be seen in both conditions that these charges are\nnot completely consumed, where it is more severe for the control condition in that the induced\nphotovoltage remains higher than that of the NIR condition throughout the frequency sweep.\nThis can originate from the temporary trapping of these charges rather than forming chemical\nadsorbates on the electrode surface (Chem. Sci., 2016, 7, 3347; Adv. Energy Mater. 2019, 9,\n1900179), which is plausible considering our in situ Raman analysis (Fig. 2a). Although the\nsteady-state net photocurrent is zero at open circuit, the sinusoidal light perturbation gives rise\nto transient interfacial charge transfer at low frequencies (Chem. Sci., 2016, 7, 3347). Thus,\nthis transient interfacial electron transfer at low frequencies effectively impacts the measured\nphotovoltage. As the intensity modulated illumination reaches low frequency near ~1 Hz,\nsufficiently slow for the device to undergo charge transfer, a decrease in photovoltage is\nobserved for NIR condition as opposed to further increase in photovoltage in control condition.\nThis correlates well with our interpretation made for EIS and IMPS, especially acknowledging\nthat charge transfer serves as the dominant origin of LF arc near open circuit potentials.\nWe believe the additional control experiments, the PEC-EIS literature guidance, and\nthe internal consistency across EIS, IMVS, and IMPS collectively support assigning the LF arc\nto charge-transfer resistance. Under NIR illumination, a clear kinetic enhancement is observed;\nour electrochemical analyses show that this enhancement modulates the charge-transfer\nbehavior, which in turn reduces charge accumulation and improves overall device performance\nand stability. The reviewer’s comments prompted us to examine alternative interpretations we\nhad not fully considered, and this review process has strengthened our mechanistic rationale.\nWe are grateful for the reviewer’s insightful feedback.\ndata of the encapsulated PV device. Nyquist plots of FTO/CC/Pt-C catalyst electrode under (d)\ncontrol and (e) NIR-irradiated conditions. (f) Bode plots of EIS data under control, and NIR-\nirradiated conditions at the specified applied potential.\nFig. R4 (a) Photovoltage variation and (b) phase variation as a function of light-intensity\nmodulation frequency under open circuit conditions obtained via IMVS analysis of control and\nNIR conditions.\n(Fig. 2 revised)\nFig. 2 | Electrochemical characterization under operando and in situ conditions. a, In situ\nRaman spectra recorded during the HER reaction on a Pt-C/CC/FTO electrode in 0.5 M KPi\n(pH 7). Nyquist plots of the b, control and c, NIR-irradiated cathodes. IMPS spectra of the d,\ncontrol and e, NIR-irradiated conditions acquired in 0.5 M KPi (pH 7) electrolyte under 1-sun\nillumination at the specified applied potential. f, Charge transport and recombination rate\nconstants derived from IMPS data within the potential range of 0.6–0.9 V .\nRHE\n(in Page 13, line 1)\nOperando electrochemical analyses of perovskite-based cathode\nElectrochemical impedance spectroscopy (EIS) was performed on the immersed perovskite-\nbased cathode under both control and NIR irradiation conditions to assess the impact of thermal\nactivation on catalyst performance. Measurements were conducted at potentials ranging from\n1.0 to 0.8 V over a frequency range of 0.1 Hz to 300 kHz under 1-sun illumination. Below\nRHE\n0.8 V , both conditions reached saturation in photocurrent density, hindering accurate\nRHE\n11\nmeasurement of charge transfer resistance. Therefore, EIS measurements were limited to the\npotential range of 1.0 to 0.8 V . The Nyquist plots (Fig. 2b, c) were fitted using a Voigt-type\nRHE\nequivalent circuit model (Supplementary Fig. 27a and Supplementary Note 7), with the fitting\ncalculations summarized in Supplementary Table 4. Regardless of applied potential and NIR\nirradiation, there were no significant differences in R and R . However, the Bode plot\ns HF\nrevealed noticeable differences around the low-frequency region (100–0.1 Hz)47,48\n(Supplementary Fig. 28). The low-frequency arc and R were significantly reduced under NIR\nct\nirradiation compared to the control, indicating that the photothermal effect markedly enhances\nthe charge transfer reaction (Fig. 2b, c). A detailed interpretation of the low-frequency arc is\nprovided in Supplementary Note 8.\n(in Page 14, line 3)\nMoreover, intensity-modulated photovoltage spectroscopy (IMVS) was conducted under open\ncircuit conditions with white light illumination bias, using a perturbation of 10% of the light\nintensity and a frequency range from 100 kHz to 0.1 Hz (Supplementary Fig. 29b, c). At ~100\nkHz both conditions show high-frequency responses, indicative of fast photocarrier generation.\nIt can be seen in both conditions that these charges are not completely consumed, where it is\nmore severe for the control condition in that the induced photovoltage remains higher than that\nof the NIR condition throughout the frequency sweep. This can originate from the temporary\ntrapping of these charges rather than forming chemical adsorbates on the electrode surface26,\n51, which is plausible considering our in situ Raman analysis (Fig. 2a). Although the steady-\nstate net photocurrent is zero at open circuit, the sinusoidal light perturbation gives rise to\ntransient interfacial charge transfer at low frequencies51. Thus, this transient interfacial electron\ntransfer at low frequencies effectively impacts the measured photovoltage. As the intensity\nmodulated illumination reaches low frequency near ~1 Hz, sufficiently slow for the device to\nundergo charge transfer, a decrease in photovoltage is observed for NIR condition as opposed\nto further increase in photovoltage in control condition. This is consistent with slow charge\ntransfer at open circuit observed via EIS and thus stronger charge accumulation under control\ncompared to when NIR is applied26, 52.\n(reference 49 was added)\nenhancement by probing surface kinetics. Chem. Sci. 7, 3347 (2016).\n12\n(Supplementary Information revised)\nSupplementary Note 8: Low frequency arc of perovskite-based cathode\nThe key question is what resistance the low-frequency (LF) arc represents in the perovskite-\nbased electrochemical devices. Because the device uses a perovskite/spiro/Au stack, a positive\nLF arc attributable to recombination/ion migration in the bulk or at perovskite interfaces can\nindeed appear. To disentangle contributions observed at the cathode, we fabricated two devices\nseparating the components: (i) an encapsulated PV stack electrically contacted with a Cu wire\nat the catalyst position but without the catalyst itself, and (ii) a catalyst electrode (FTO/CC/Pt-\nC). For the encapsulated PV stack, the LF arc emerges and increases as the bias moves from\nopen circuit toward short-circuit (Supplementary Note Fig. 5b). Bode analysis shows this\nresistance appears around ~1–10 Hz and its characteristic frequency shifts to lower values as\nthe bias approaches short-circuit (Supplementary Note Fig. 5c). These features are consistent\nwith the Shockley–Read–Hall (SRH) recombination resistance originating from the bulk\nand/or interfaces, as reported in prior literatureS51. For the isolated catalyst electrode, an LF arc\nis also observed; however, its magnitude decreases as the potential is swept negative from the\nonset voltage, and it is further reduced under NIR illumination (Supplementary Note Fig. 5d,\ne). We attribute this behavior to charge-transfer resistance at the catalyst/electrolyte interface:\nboth improved kinetics and mass transport under NIR illumination decrease this resistance. In\nthe Bode plots, this LF contribution appears near ~1–10 Hz and shifts to higher characteristic\nfrequencies as the potential approaches onset and the peak frequency increases further to ~10–\ndemonstrate that (i) SRH-related and charge-transfer resistances can occupy similar frequency\nwindows, and (ii) the LF-arc trend at the working cathode mirrors the charge-transfer behavior\nof the catalyst electrode but is opposite to the SRH recombination-related trend of the\nencapsulated PV. Consequently, interpreting the cathode LF arc as charge-transfer resistance is\nmore consistent with the observed bias and illumination dependences. However, we need to\nexplain why the SRH-related LF resistance observed in the encapsulated PV structure does not\nappear at the cathode.\nThis interpretation aligns with prior PEC-EIS analyses, which recommend bias-\ndependent assignments of the LF arcS52. Near the onset voltage, charge transfer is typically the\ndominant bottleneck and thus the most likely origin of the LF arc. But at higher overpotentials\nwhere the photocurrent is large or the catalyst’s onset potential is exceeded, recombination-\nrelated impedances can become dominant in the LF domain. Our voltage sweeps were\n13\nintentionally confined to the vicinity of the onset, so assigning the LF arc at the cathode to\ncharge-transfer resistance is appropriate. We note that extending the sweep deeper into the\nhigh-current regime would be informative, but our instrument exhibits signal instability\n(“spikes”) at very high currents, preventing reliable LF-arc acquisition in that range.\nSupplementary Note Fig. 5 | EIS measurements of encapsulated PV and catalyst electrode\na, LSV curves of encapsulated PV device. b, Nyquist plots and c, Bode plots of EIS data of\nthe encapsulated PV device. Nyquist plots of FTO/CC/Pt-C catalyst electrode under d, control\nand e, NIR-irradiated conditions. f, Bode plots of EIS data under control, and NIR-irradiated\nconditions at the specified applied potential.\n(references S51, S52 were added)\nS51 W. Clarke. et al. Understanding the Full Zoo of Perovskite Solar Cell Impedance\nSpectra with the Standard Drift-Diffusion Model. Adv. Energy Mater. 14, 2400955\n(2024).\nS52 W. Yang. et al. Operando Analysis of Semiconductor Junctions in Multi-Layered\nPhotocathodes for Solar Water Splitting by Impedance Spectroscopy. Adv. Energy\nMater. 11, 2003569 (2021).\nJournal: Nature Communications\nManuscript number: NCOMMS-25-40476-T\nTitle: “Operando insights into stability of perovskite-based solar water splitting devices”\nAuthor(s): Chang-Seop Jeong†, Wooyong Jeong†, Juwon Yun†, Donyoung Kang, Byungjun\nKang, Hyungsoo Lee, Jun Hwan Kim, Hyungsuk Lee, and Jooho Moon*\n<Reviewer 1>\nThe authors have addressed my comments, and the manuscript can be considered for\npublication.\nRemarks:\nWe are grateful to the reviewer for the thoughtful and constructive comments. We implemented\nthe suggested control experiments and textual revisions, which have considerably improved\nthe clarity and rigor of the manuscript. Thank you for your time and valuable guidance.\n<Reviewer 3>\nThe authors have addressed my concerns. The additional control measurements are effectively\nadded in the revised documents. I believe the revised manuscript is ready for the publication.\nRemarks:\nWe are grateful for the reviewer’s insightful remarks, which helped us refine the work. The\nsuggested control experiments and edits have been fully addressed, bringing the manuscript to\na publishable form. Thank you again for your guidance.\n<Reviewer 2>\nThe authors have made a sustained effort to answer the questions and to carry out additional\ncontrol experiments. I am now happy for this to be published.\nWe sincerely thank the reviewer’s positive evaluation of our manuscript. We appreciate your\nrecognition of our additional control experiments and revisions, and we are pleased that you\nnow support publication. Detailed responses to the reviewer’s comment are provided below.\nComment 1:\nOne minor comment remains which is that the EIS and IMVS is still not plotted isotropically\n(it is immediately obvious by the fact that the semi-circles look squashed and is a result of the\nx-and y- axes having the same number of units but being different lengths).\nResponse:\nWe thank the reviewer for the helpful comment regarding the plotting of EIS and IMVS. For\nFigure 2, all four plots have an isotropic 1:1 axis aspect ratio (equal scaling of x and y) with\nidentical number of units and lengths, making a perfect square. Supplementary Figure 27 has\nbeen re-plotted isotropically (Fig. R1). This makes all impedance Nyquist plots — both in main\nmanuscript and supplementary information document — isotropic, eliminating any distortion\nin the figures. We again appreciate the reviewer’s careful consideration to these details.\nFig. R1 Isotopically plotted EIS. Fitted Nyquist plots of (a) control samples and (b) NIR-\nirradiated samples.\n(Supplementary Information revised)\nSupplementary Fig. 27 | Interpretation of electrochemical impedance spectroscopy (EIS).\na, Voigt equivalent circuit model used for interpreting EIS results. Fitted Nyquist plots of b,\ncontrol and c, NIR-irradiated conditions using the equivalent circuit. Fitted parameters are\nsummarized in Supplementary Table 4.\n3"}]

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