Solar-Assisted PEM Water Electrolysis with Symmetric IrO₂ Electrodes for Hydrogen-Rich Water Production

Abstract

Hydrogen-rich water (HRW) has attracted significant attention for its physiological and therapeutic potential, driving efforts to develop a green and direct production approach. In particular, if solar energy could be utilized to power the process and the power-generation and water-production modules could be integrated into a single device, it would greatly enhance portability and user convenience, making it an ideal solution for personalized healthcare and outdoor applications. We demonstrate solar-assisted proton exchange membrane (PEM) electrolysis using symmetric IrO₂ electrodes at both cathode and anode to directly generate HRW. The symmetric design simplifies manufacturing, mitigates lifetime mismatch and metal-ion cross-contamination. IrO₂ films were electrodeposited on stainless steel substrates and annealed at 400–700 °C. When coupled with a 100 cm² Si solar cell, the electrode annealed at 550 °C—featuring ~6 nm IrO₂ nanocrystals embedded in an amorphous matrix—exhibited the highest hydrogen production rate. At an applied voltage of 4 V, this 550 °C-annealed IrO₂ electrode produced approximately 1800 μmol h⁻¹ of H₂, corresponding to about 44 mL h⁻¹ of H₂ at 25 °C and 1 atm. Corrosion tests show the HRW is less aggressive to iron than DI, RO, and tap water, suggesting better compatibility with metallic components. During water splitting, the oxidation–reduction potential (ORP) rapidly decreases to <−300 mV within 0–10 min and then stabilizes, with the 550 °C–annealed electrode exhibiting the lowest ORP. Upon air exposure, the ORP increases by ~200 mV over 45–70 min yet remains reductive for >120 min, indicating persistent dissolved H₂ and sustained performance. Overall, the symmetric IrO₂ architecture provides a green, stable, and direct route to HRW production.

1. Introduction

Hydrogen is concurrently recognized as a carbon-free, clean energy carrier with high gravimetric energy density. When H₂ dissolves in water, it forms hydrogen-rich water (HRW), which has attracted increasing attention for its potential antioxidative and health-promoting effects [1,2,3,4,5]. Various water electrolyzer technologies have been studied and comprehensively reviewed in the literature; however, most of these investigations have focused on hydrogen as a fuel or energy-storage medium rather than as a dissolved therapeutic agent [6,7].

Alkaline electrolysis, anion exchange membrane (AEM) electrolysis, proton exchange membrane (PEM) electrolysis, and solid oxide electrolysis (SOE) are all widely studied for H₂ production. These four technologies differ mainly in their electrolytes, operating temperature regimes, catalyst requirements, and overall system behavior. Alkaline electrolysis is commonly used to produce hydrogen in large-scale (commercial) applications. It uses liquid alkaline electrolytes such as KOH or NaOH and does not require noble-metal electrocatalysts. However, some degree of gas crossover between half-cells is inevitable and may reduce H₂ purity. Hybrid heterojunction electrocatalysts, nano-heterostructured electrocatalysts, nanointerface electrocatalysts, and porous structures have been shown to enhance the performance of the oxygen evolution reaction (OER) and/or hydrogen evolution reaction (HER) [8,9,10,11]. AEM electrolysis is similar to conventional alkaline electrolysis and uses a solid AEM as the separator and ion-conducting medium, typically paired with a dilute alkaline or near-neutral electrolyte. Noble-metal-free catalysts are feasible for AEM; nevertheless, membrane and catholyte stability and durability are still under development. In both alkaline and AEM electrolysis, H₂ and hydroxide ions (OH-) are produced at the cathode. The OH- traverses the diaphragm/membrane to the anode, where it participates in forming O₂ and regenerating H₂O. Performance improvements, including increased current density and enhanced stability, have been demonstrated through electrode-material optimization [12,13]. PEM electrolysis uses a proton-conducting membrane with pure-water feed at low temperatures. It delivers high current density, fast response, low gas crossover, and high-purity H₂, but depends on scarce noble metals (Pt, Ir) and costly membranes. At the anode, water is oxidized to generate O₂, protons (H⁺), and electrons (e⁻). The protons traverse the proton-conducting membrane to the cathode, while the electrons travel through the external circuit. At the cathode, protons and electrons recombine to produce H₂. Improvements achieved by altering materials and structures have been widely discussed [14,15,16]. SOE operates at high temperatures using ceramic oxygen-ion (or protonic) conductors. At the cathode, steam is reduced to H₂ and oxide ions (O²⁻). The O²⁻ ions traverse the solid electrolyte to the anode, where they are oxidized to form O₂ and release electrons. The oxygen is discharged at the anode surface, while the electrons flow through the external circuit to the cathode. The use of novel electrode materials for performance enhancement has been explored by different groups [17,18].

Among these electrolysis technologies, PEM is typically faster due to the high activity and large electrochemically active area of noble-metal electrodes (usually Pt), and it is safer because it avoids liquid caustic electrolytes. The PEM water electrolyzer (PEMWE) is therefore an important research focus. In studies of PEMWE, IrO₂ has been demonstrated to be a highly stable OER catalyst [19,20]. The influences of porosity, crystallinity, and nanostructure on the performance of IrO₂ oxygen evolution catalysts have been extensively analyzed [21,22]. Cherevko et al. investigated the electrocatalytic performance of thermally oxidized Ir (IrO₂) toward the OER and the HER, and compared its performance with that of Ir, Ru, and RuO₂ [23]. After a potential sweep to the OER potential for 5 mA cm⁻², Ir dissolution was <0.1 ng cm⁻², indicating exceptional stability. However, despite its high stability, IrO₂ shows relatively low activity, requiring an overpotential >400 mV to reach a current density of 5 mA cm⁻², a level that may only be acceptable for PEMWEs when powered by low-cost renewable electricity. They concluded that IrO₂ remains the catalyst of choice for the OER and may be considered for the HER when Pt contamination is a concern. IrO₂ is stable and thus suitable for medical and healthcare applications. For example, activated iridium oxide (AIROF) microelectrodes have been widely studied to maximize charge-injection capacity and have been investigated both in vitro and in living organisms [24,25].

Solar-assisted PEMWE offers several advantages over a grid- or fossil-powered system without solar input. By coupling photovoltaics either directly or indirectly, it lowers operating electricity costs and carbon intensity, enabling genuinely green hydrogen with near-zero upstream emissions [26,27]. Direct-coupled systems are simple, reliable, and efficient, but face system-level constraints, especially in PV array configuration and tight voltage/current matching under variable irradiance. Indirect coupling, often integrated with an inverter and optionally batteries, stabilizes output, smooths irradiance swings, and reduces grid reliance, but increases system cost, complexity, and maintenance needs. Theoretical and experimental enhancements in solar-assisted PEMWE have been reported, including optimizing the number of PV cells in series and parallel, energy management strategies, and relative sizing [26,27,28]. These studies demonstrate green hydrogen production via solar-assisted PEMWE and indicate that further investigation is warranted.

A potential by-product of water electrolysis is HRW, which has recently been widely studied for its possible human health benefits [1,2,3,4,5]. In PEMWE, protons (H⁺) travel to the cathode of the membrane electrode assembly (MEA) water electrolyzer and combine with electrons to generate high-purity hydrogen gas (H₂), which is typically collected by the water-displacement method. A portion of this hydrogen can dissolve into residual water along its escape path, yielding HRW. Vaishnavi et al. reported that HRW exhibits superior antimicrobial activity to chlorhexidine mouthwash and may be of potential relevance to preventing dental disease [4]. Kuzmanovic et al. found that a six-week HRW intervention improved muscle performance and elevated serum free testosterone and cortisol [5]. Their findings support the potential health benefits of HRW and highlight the need for further research, especially on environmentally friendly, straightforward, and cost-effective HRW production. Systematic studies on HRW produced by water electrolyzers are still lacking. The simultaneous generation of HRW during water electrolysis represents an emerging direction that offers both added value and cross-disciplinary potential. Using IrO₂ on both electrodes in PEM water electrolysis is unconventional, since Pt-based cathodes normally minimize precious-metal demand. Nevertheless, a symmetric IrO₂ MEA can unify catalyst fabrication, avoid Pt crossover, reduce heterogeneous metal contamination, and promote a more uniform corrosion pattern.

In this work, we investigate a solar-assisted PEMWE system that employs IrO₂ at both electrodes, emphasizing its potential for portable and off-grid applications without reliance on conventional electricity. We further examine how growth conditions influence electrochemical performance and characterize the HRW generated by this approach.

2. Materials and Methods

2.1. Electrodeposition

IrO₂ thin film was deposited onto stainless steel to fabricate electrodes for water electrolysis (Figure 1a). The deposition solution was prepared following a modified Yamanaka’s method [29]. In brief, 3 mM hydrogen hexachloroiridate hydrate (H₂IrCl₆·6H₂O) was dissolved in deionized water under magnetic stirring, and 100 mM oxalic acid (H₂C₂O₄·2H₂O) was added to form a stable iridium–oxalate complex. The pH of the solution was adjusted to 10.5 by the gradual addition of K₂CO₃. The resulting solution was aged in the dark at room temperature for one week until it turned dark blue, indicating stabilization of the precursor. Electrochemical deposition was subsequently performed in the stabilized solution at room temperature using a Model 600E Series Electrochemical Analyzer (CH Instruments, Inc., Austin, TX, USA). The working electrode was polarized at a constant potential of 0.52 V (vs. Ag/AgCl) for 60 min, which enabled the continuous formation and growth of nano-porous iridium oxide (IrO₂) on the stainless-steel electrode substrate.

2.2. Fabrication of MEA and Water Electrolysis Cell

After electrodeposition, the nano-porous IrO₂-coated stainless steel electrodes were annealed in a furnace at 400–700 °C for 0.5 h. As shown in Figure 1b,c, IrO₂ electrodes were used as the anode and cathode, positioned on both sides of a polymer electrolyte membrane (PEM; Nafion 117, DuPont, Wilmington, DE, USA), forming an anode/PEM/cathode module that served as the MEA for water electrolysis. The electrolysis cell was subsequently operated in a solar cell-assisted configuration for hydrogen generation (Figure 1c). The additional bias required for water splitting was supplied by a 100 cm² Si-based solar cell (0.48–1.26 W) illuminated by a tungsten lamp. Commercial Pt-Nb electrodes were also used as a Pt-Nb/PEM/Pt-Nb module in the MEA for comparison. Deionized (DI), reverse osmosis (RO), and tap water were used, and each had a pH near 7 during electrolysis. The I-V curve of the solar-assisted water electrolyzer was recorded at ambient conditions. Cyclic voltammetry (CV) was performed in 1.0 M H₂SO₄ using a glassy carbon working electrode, a platinum counter electrode, and an Ag/AgCl reference electrode. The oxidation-reduction potential (ORP) of the HRW was measured using an ORP meter. All experiments were conducted at room temperature under atmospheric pressure. A transmission electron microscope (FEI Tecnai G2 F20 S-TWIN; FEI, Hillsboro, OR, USA) was used to study the structure of the IrO₂ coatings.

3. Results and Discussion

TEM images and electron diffraction patterns of IrO₂ coatings are presented in Figure 2. The structure of the as-grown IrO₂ coating remains amorphous, as no lattice or diffraction pattern is observed. After annealing at 400 °C, the presence of a diffraction pattern indicates that the IrO₂ coating exhibits a mixed crystalline and amorphous structure. When the annealing temperature is increased to 550 °C, IrO₂ nanocrystals and a diffraction pattern are both observed. The lattice spacing of these nanocrystals, approximately 6 nm in size, is about 0.315 nm, corresponding to the IrO₂(110) phase. As the annealing temperature is further increased to 700 °C, large IrO₂ nanocrystals, ~20 nm in size, are present across almost the entire surface of the TEM image, suggesting a highly crystallized structure of IrO₂ coating. Notably, electrodeposition has yielded stable IrO₂ films when cyclic voltammetry (0.05–0.55 V vs. SCE, 50 mV s⁻¹, 50 cycles) is immediately followed by potential pulsing over the same potential window for 400–1000 pulses [30]. However, for PEMWE electrodes, a porous, high-surface-area IrO₂ is desirable to increase the electrochemically active surface area and maintain high OER performance while reducing the overall Ir loading compared with dense films. Therefore, the working electrode was polarized at a constant potential of 0.52 V (vs. Ag/AgCl) for 60 min.

Figure 3 shows the I-V curve of the MEA using Pt-Nb, as-grown IrO₂ and annealed IrO₂ electrodes for solar-assisted water electrolysis. The initial potential follows the order: 550 °C annealed IrO₂ < 400 °C annealed IrO₂ < Pt-Nb < as-grown IrO₂ < 700 °C annealed IrO₂. The electrode containing small IrO₂ nanocrystals (~6 nm) shows the lowest initial potential, indicating the best performance for water electrolysis among our samples. At a fixed potential of 2.5 V, the order of current density is as follows: 700 °C annealed IrO₂ < as-grown IrO₂ < 400 °C annealed IrO₂ < Pt-Nb < 550 °C annealed IrO₂. A higher current density indicates a greater flow of current per unit area, generally reflecting a higher rate of electrochemical reactions. The 550 °C annealed IrO₂ shows the highest current density (~103 mA/cm²), surpassing even that of Pt-Nb (~80 mA/cm²). Siracusano et al. studied nanosized IrO₂ anode electrocatalysts and suggested that crystalline IrO₂ particles of appropriate size embedded within an amorphous matrix likely reduce sintering, thereby potentially improving current density [31]. In our case, the 550 °C annealed IrO₂, composed of small IrO₂ nanocrystals (~6 nm), may exhibit comparable behavior, potentially resulting in the highest current density among our samples. When the potential exceeds 3 V, the Pt-Nb sample exhibits the highest current density, while the 550 °C annealed IrO₂ still shows a higher current density compared to the other IrO₂ samples. The current density of the sample containing ~6 nm IrO₂ nanocrystals embedded within an amorphous matrix is 52 mA/cm² at 2 V and 285 mA/cm² at 4 V, respectively.

Ngo et al. studied the influence of amorphous/crystalline IrO₂ interfaces and reported that grain boundaries affect the current density; accordingly, the reduced current density observed for the sample containing larger IrO₂ crystals likely stems from a combined effect of diminished surface area and a lower fraction of the more active amorphous phase in the catalyst [32]. In our study, the IrO₂ electrode annealed at 550 °C exhibited superior electrocatalytic performance, primarily attributed to its nanocrystalline structure and grain-boundary effects. First, the nanocrystalline domains introduce numerous grain boundaries, which are regions of lower activation energy. In IrO₂, both electrons and ions can migrate along these boundaries via vacancy-assisted hopping with lower energy barriers, facilitating charge transport and improving overall electrolysis efficiency. In contrast, the sample annealed at 700 °C, characterized by enlarged grains and fewer boundaries as confirmed by TEM selected-area diffraction (SAD; Figure 2), showed reduced catalytic activity due to diminished grain-boundary conduction. Moreover, grain-boundary defects and structural irregularities serve as electrochemically active sites, whose kinetics critically influence reaction rates. The 550 °C IrO₂ electrode, maintaining a mixed amorphous–nanocrystalline architecture, provides both abundant active sites and efficient charge-transfer pathways, resulting in higher current density and enhanced electrocatalytic activity. Compared with fully amorphous or excessively crystallized samples, this nanocrystalline configuration represents an optimal balance between reaction kinetics and structural stability.

Figure 4a,b show the H₂ and O₂ production rates as a function of the applied voltage. For all samples, the H₂ production rate is approximately twice that of O₂, indicating the complete decomposition of H₂O into hydrogen and oxygen. As shown in Figure 3, the gas production rate depends almost linearly on the current density. When the applied voltage is below 2.5 V, the gas production rate of the Pt-Nb sample is comparable to that of the 550 °C annealed IrO₂, indicating similar performance between these two samples. However, when the voltage exceeds 3 V, the Pt-Nb sample exhibits the highest H₂ and O₂ production rates, while the 550 °C annealed IrO₂ sample remains the best performer among the IrO₂ samples. At 4 V, the 550 °C-annealed IrO₂ sample produced approximately ~1800 μmol H₂ and ~1000 μmol O₂ per hour. This corresponds to about 44 mL h⁻¹ of H₂ and 22.5 mL h⁻¹ of O₂ at 25 °C and 1 atm. The low H₂ production rate is likely attributable to suboptimal electrode pairing, wherein IrO₂ is employed at both electrodes, thereby increasing the HER overpotential relative to Pt-based cathodes.

In the context of PV-coupled outdoor operation with low and fluctuating irradiance, the rationale for adopting a symmetric IrO₂|IrO₂ MEA is to optimize robustness at moderate, dynamic potentials rather than to prioritize peak performance at sustained high voltages (>3 V) characteristic of an asymmetric Pt–Nb configuration. A symmetric IrO₂ MEA provides three system-level advantages: (i) IrO₂ has been evaluated both in vitro and in vivo and is used in healthcare-related applications [24,25], so employing IrO₂ for hydrogen-rich water does not introduce new human-safety concerns, unlike the uncertainty surrounding Pt–Nb; (ii) our IrO₂ coating on stainless steel enables very low Ir loading, yielding a more cost-effective electrode than Pt–Nb and insulating the system from the high cost and price volatility of Pt-based materials; and (iii) process symmetry standardizes catalyst-layer fabrication and assembly on both electrodes, eliminating Pt crossover, reducing heterogeneous-metal contamination, and mitigating asymmetric, metal-specific degradation for more uniform and predictable corrosion behavior. Consistent with this rationale, Cherevko et al. identified IrO₂ as the catalyst of choice for the OER and as a viable option for the HER when Pt contamination is a concern [23]. Importantly, because the device is powered by coupled photovoltaic modules under outdoor conditions where sustained high voltages cannot be guaranteed, symmetric IrO₂ operates robustly at moderate potentials with dynamic voltage swings, enabling effective hydrogen-rich water production while lowering requirements on the PV source—smaller-area modules and even lower-voltage amorphous-silicon panels become feasible. In contrast, although Pt–Nb can exhibit higher instantaneous performance under ideal, sustained high-voltage conditions, the heterogeneous-bipolar stack is more prone to cross-electrode metal migration and differential degradation, adding long-term reliability and maintenance uncertainty. Collectively, these advantages simplify process control, mitigate durability risks inherent to dissimilar electrodes, and enhance long-term stability—benefits that are more consequential for PV-coupled, fluctuating-power operation than maximizing performance only at >3 V.

Figure 5 shows the average cyclic voltammetry over 20 cycles at room temperature for the 550 °C annealed IrO₂ sample. The C1, C2, and C3 peaks have been reported to be related to the hydrogen adsorption reaction, redox couples Ir(IV)/Ir(VI) and Ir(III)/Ir(IV), respectively [33,34]. The authors have suggested that these peaks observed for IrO₂/C depend on the IrO₂ structure, with amorphous iridium oxide typically contributing to featureless peaks. In our sample, the C2 and C3 peaks are clearly observed, whereas the C1 peak is not well-defined. This observation is most likely a result of the crystalline IrO₂ particles (~6 nm) embedded within an amorphous matrix, as shown in Figure 2. The coexistence of crystalline and amorphous structures contributes to the simultaneous presence of both featureless and well-defined characteristics.

To characterize the HRW, we performed ORP measurements. The ORP values of HRW, deionized (DI) water, reverse osmosis (RO) water, and tap water are listed in

Table 1. ORP value of different waters.

Water Type	ORP (mV)
Tap water	251
RO water	170
DI water	157
HRW (550 °C annealed)	−332

. The substantially negative ORP measured for the HRW (−332 mV at the start of data collection, after drifting from an initial value of about −400 mV) reflects residual dissolved molecular hydrogen generated using the 500 °C annealed IrO₂ electrode. Control waters lacking measurable H₂ all displayed positive ORP values (>+150 mV). These findings establish that the preparation successfully generated HRW with a strong reducing redox environment. To further assess the influence of redox potential differences, we immersed a 304 stainless-steel nail (model Y-03; Yun-Chang Hardware Tools Co., Ltd., Changhua, Taiwan) in each water sample for three months under ambient conditions. As shown in Figure 6, visible surface corrosion was observed on the nails in all water samples, with the extent following the order: tap > RO ≈ DI > HRW, consistent with the initial ORP values. The results suggest that dissolved molecular hydrogen may mitigate oxidative corrosion processes. This observation can be rationalized by established mechanisms through which dissolved hydrogen suppresses oxidation. Molecular hydrogen is a reductant and can consume dissolved oxygen. This lowers the concentration of oxidizing species. Recent work (e.g., Badley et al. [35]) shows that H₂ can scavenge reactive oxygen species such as hydroxyl radicals, which may slow surface oxidation. Their findings suggest that H₂ may slow metal corrosion in HRW. Dissolved hydrogen also lowers the ORP, shifting the solution toward more reducing conditions and decreasing the thermodynamic driving force for metal oxidation. The synergistic action of these effects makes HRW a less oxidizing environment and is consistent with the reduced corrosion observed in this study.

Figure 7a shows the ORP evolution during electrolysis using different IrO₂ electrodes. The PEMWEs were integrated into a commercial water dispenser. In addition to the ORP meter, a dissolved hydrogen meter (ENH-1000, Trustlex Inc., Osaka, Japan) was used to monitor the H₂ concentration in HRW, as shown in the inset of Figure 7a. Because the ORP signal often exhibited an initial transient with a highly variable peak during the first ≈ 30 s, the ORP reading was recorded only after approximately 30 s, once the value had stabilized. The most negative ORP observed was −515 mV using the IrO₂ electrode annealed at 500 °C, at a dissolved H₂ concentration of ~965 ppb; however, the value subsequently decreased substantially. According to the Nernst equation, a dissolved H₂ level of ~965 ppb corresponds to an ORP in the negative 500 mV range, consistent with the measured value. For all samples, during the first 10 min of electrolysis, the ORP decreases sharply, followed by only a slight additional decline over the subsequent 50 min. After 1 h of electrolysis, the ORP values for the as-grown and 400, 550, and 700 °C-annealed IrO₂ samples were −310, −366, −388, and −315 mV. The 550 °C-annealed IrO₂ sample shows the highest current density (Figure 3), producing more H₂; consequently, the dissolved H₂ concentration increases and the ORP becomes more negative. Because dissolution of H₂ in water is mildly exothermic, its equilibrium solubility decreases as temperature rises; warmer conditions therefore retain less dissolved hydrogen than cooler ones. After preparation, the dissolved H₂ concentration decays as the liquid re-equilibrates with air’s extremely low hydrogen partial pressure, and the loss rate is accelerated by higher temperature, larger gas–liquid interfacial area, agitation, and permeable packaging. These temperature- and time-dependent losses of dissolved hydrogen shorten the practical shelf life of HRW.

Figure 7b presents the ORP in H₂-enriched DI, RO, and tap water as a function of open-air exposure time. Upon ambient, open-air exposure, all three HRW samples display a time-dependent loss of reducing potential (increase in ORP toward more positive values). During the first 70 min of open-air exposure, the ORP of the DI water sample increased almost linearly from below −400 mV to approximately −190 mV, after which it remained essentially stable near −190 mV with further exposure. For the RO water sample, a near-linear climb from −320 mV to ~−140 mV occurred in the first 45 min, with the ORP then remaining near −140 mV. The tap water sample, initially the least negative (~−250 mV), also exhibited an almost linear increase to ~−50 mV over 45 min and thereafter remained essentially stable at that value. ORP increased by approximately 200 mV in all samples; only the DI water sample showed a delayed plateau at about 70 min, whereas the others stabilized by roughly 45 min. Our data show that the HRW prepared in this work is still effective after 120 min of open-air exposure. The more negative value for DI water is likely due to the near absence of residual oxidants (free chlorine, chloramine, higher-valent metal ions) that would otherwise consume H₂ or keep the mixed potential more positive.

4. Conclusions

In this study, electrochemically deposited IrO₂ coatings on stainless steel were thermally tuned (400–700 °C) to optimize a solar-assisted water electrolyzer, in which identical IrO₂ electrodes functioned as both anode and cathode for simultaneous hydrogen production and generation of HRW. Microstructural evolution from amorphous (as-grown) to mixed amorphous/crystalline (400 °C) and finally to highly crystalline IrO₂ with enlarged grains (700 °C) was confirmed by TEM. Annealing at 550 °C produced ~6 nm IrO₂ (110) nanocrystals embedded in an amorphous matrix, delivering the lowest initial cell potential and the highest current density (~103 mA/cm² at 2.5 V), outperforming the commercial Pt-Nb benchmark in that voltage window. Stoichiometric H₂:O₂ (≈2:1) production verified efficient water splitting. HRW produced herein exhibited reduced corrosion of iron surfaces relative to deionized (DI), reverse-osmosis (RO), and tap water. The 550 °C electrode yielded the most negative ORP (−388 mV after 1 h of electrolysis), consistent with increased dissolved H₂ levels. ORP time-course monitoring revealed a sharp initial drop (first 10 min), followed by a slower approach to a plateau. The superior performance is most likely attributable to the synergy between nanocrystalline and amorphous domains. These findings highlight a pathway toward green, stable, solar-assisted PEMWE with reduced noble-metal reliance for distributed hydrogen and HRW production.