# Design of Heterostructured Mo:BiVO4/Tio2 and Mo:BiVO4/Co-Pi Photoanodes for Efficiency and Durability Comparison In Solar-Driven Water Oxidation Under Acidic Conditions

Adeline Blot, Cédric Boissière, Olivier Durupthy, Christel Laberty-Robert

► To cite this version:

Adeline Blot, Cédric Boissière, Olivier Durupthy, Christel Laberty-Robert. Design of Heterostructured Mo:BiVO4/Tio2 and Mo:BiVO4/Co-Pi Photoanodes for Efficiency and Durability Comparison In Solar-Driven Water Oxidation Under Acidic Conditions. ChemPhotoChem, 2025, 9 (11), 10.1002/cptc.202500222. hal-05318548

HAL Id: hal-05318548

https://hal.sorbonne-universite.fr/hal-05318548v1

Submitted on 1 Dec 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Design of Heterostructured Mo:BiVO4/Tio2 and Mo:BiVO4/
Co-Pi Photoanodes for Efficiency and Durability Comparison
In Solar-Driven Water Oxidation Under Acidic Conditions

Adeline Blot, Cédric Boissière, Olivier Durupthy,* and Christel Laberty-Robert*

# 1. Introduction

Photoelectrochemical (PEC) water-splitting is a highly promising approach for greener dihydrogen production thanks to the elimination of the overpotential needed in conventional electrolysis.[1] The design of tandem cells constituted of a photocathode and photoanode is a solution for dihydrogen production without external bias input.[1–5] Nevertheless, the bottleneck resides in the photoanode material. Indeed, the oxidation of water on this electrode, required for the production of one O₂ molecule (OER), necessitates double amount of photocharges compared to the production of one H₂ molecule (HER).[1,4] Among the number of semiconductors having favorable valence band edge for OER, bismuth vanadate (BiVO₄) is the most promising due to its low bandgap 2.4 eV, and a conduction band edge (0.02 V<sub>RHE</sub>) the closest above HER potential (0.0 V<sub>RHE</sub>).[4,6–12] Theoretically, 11% of the standard AM 1.5 G solar spectrum can be absorbed, which corresponds to 9.2% of solar-to-hydrogen (STH) conversion efficiency with a maximum photocurrent of 7.5 mA cm⁻².[13,14]

However, the main constraints for achieving high STH conversion with BiVO₄-based photoanodes are related to their poor electron-hole charge transport and their slow water oxidation kinetics. Additionally, in order to create a dual-bandgap PEC device to produce dihydrogen without the need for an external bias, stability issues in the electrolyte buffer must be taken into account.[4,8,13,15] Indeed, Cu₂O-based photocathodes have shown best results for HER under acidic conditions and its lower bandgap of 1.4 eV could allow an efficient coupling with photoanode.[16] To overcome these limitations many efforts have been made to optimize the photoanode, including i) surface engineering, ii) bulk modification, and iii) interface engineering.[8,17–22] BiVO₄ photoanode stability in acidic conditions is a significant challenge when developing PEC devices in this pH range (see Table S1).[4,16] In acidic conditions the kinetics of OER are slower compared to neutral and alkaline electrolyte where the amount of hydroxides ions is higher.[23] There is only a few research conducted in acidic media, leading to a lack of understanding on the predominant factors that affects the overall photoanode efficiency.[24–28] Competing with OER, the photogenerated electron or hole may react with their close environment (material/electrolyte) producing active species that degrade the electrode. It has been shown that photocorrosion takes place at the surface of the photoanode immersed in the electrolyte under illumination.[3] This phenomena is eventually enhanced by the set bias potential.[3,15] To improve BiVO₄-based photoanode efficiency, a co-catalyst is frequently used to improve the OER reaction kinetics and limit the undesired reaction that conduces to electrode photocorrosion.[7] This contributes to enhanced electrode durability with the reduction of recombination centers linked to the band bending effect and modification the surface chemistry. The co-catalyst is beneficial for hole capture, thereby promoting for example, OH–h⁺
interaction.[29] P.Plate et al. and Irani et al. have highlighted the importance of complete film coverage on the electron-collector before adding a co-catalyst.[30,31] They have shown that direct contact of the co-catalyst with the electron collector induces recombination.

For long-time sustainability, passivation layers deposition at the electrode-electrolyte interface seems to be a promising approach[12,25,32,33] To achieve this effect, the additional layers must be chemically stable in the electrolyte and under the operating conditions (light exposure, wide range of potential).[34] Atomic layer deposition (ALD) of TiO₂ is the most common approach that has been developed to protect the electrode surface. Generally, the thickness of the layer used vary from 1 nm to few nanometers.[33,35] In most cases, an ultrathin film of TiO₂ is associated with co-catalyst deposition. However, with an extremely thin TiO₂ layer, its passivation property remains questionable due to the lack of post-mortem characterization (imaging) for interface stability and analysis of the TiO₂ ultrathin layer alone.

S. Hilliard et al. have demonstrated that sol-gel chemistry coupled with dip-coating was an interesting and easily scalable method to generate BiVO₄ photoanodes.[36] They achieved a photocurrent density of 2.1 mA cm⁻² at 1.23 V<sub>RHE</sub> in 1 M potassium phosphate buffer (pH 7) under 350 mW cm⁻² (the simulated light was 3.5 times higher than standardize conditions).

In this study, we addressed the improvement of both the efficiency and durability of the BiVO₄ photoanode for the oxygen evolution reaction (OER) under acidic conditions, with the specific objective of integrating it into a full photoelectrochemical device alongside a copper-based photocathode. To ensure compatibility with the copper electrode, we selected a 1 M potassium phosphate buffer as the electrolyte, providing a chemically stable and operationally relevant environment for both components. We first evaluated the photoelectrochemical performance of Mo:BiVO₄ photoanodes modified with a protective TiO₂ layer, deposited via sol-gel chemistry and dip-coating. This combination of bulk doping (Mo) and surface passivation (TiO₂) significantly improved electrode stability but led to a noticeable decrease in photocurrent. To overcome this limitation and achieve both durability and high performance, we subsequently introduced a cobalt-phosphate (Co-Pi) co-catalyst onto the Mo:BiVO₄ surface via electrodeposition. Interestingly, a key strength of this study lies in the design of well-defined heterostructures that synergistically enhance both stability and performance, while intentionally limiting the number of processing steps. Our strategy relies on cost-effective, scalable techniques—primarily sol-gel chemistry coupled with dip-coating and electrodeposition—to develop a practical and industrially relevant approach to photo-electrode fabrication. Notably, the sol-gel route enables the formation of crystalline TiO₂ layers even at thicknesses below 10 nm, which is rarely achievable using conventional ALD techniques that typically yield amorphous films. This crystallinity is expected to contribute to the improved chemical and mechanical stability of the electrode under the operating conditions tested.

# 2. Results and Discussion

## 2.1. Pristine BiVO₄ Photoanode

Pure BiVO₄ and Mo:BiVO₄ films were synthesized from an adapted synthesis method developed by Hilliard et al.[36] Mo:BiVO₄ photo-anode was synthesized to modify the charge carrier dynamic and then limit the electron-hole recombination.[37–40] Figure 1 depicts the characterization of BiVO₄ and Mo:BiVO₄. In Figure 1a, the XRD pattern of pristine BiVO₄ and Mo:BiVO₄ electrodes reveal the presence of a shoulder on the peaks at 35° and 46° in 2<sup>θ</sup>. The characteristic doublets observed for (200), (020) and (204), (024) peaks correspond respectively to the monoclinic scheelite (m-s) and the tetragonal scheelite (t-s).[10] This agrees with Pamar et al.[38] FEG-SEM imaging shows dense films with well sintered nanoparticles of size ranging from 30 to 430 nm, with the majority of particles between 50 and 150 nm (Figure 1c,e). In Figure 1d,f, a film thickness of 110 nm is estimated from the cross-section images, consistent with that determined by ellipsometry analysis. (Table S2, Supporting Information) Complete coverage of the FTO substrate is observed in electrochemistry, with the absence of oxidation-reduction peaks during cyclic voltammetry measurement in 50 mM ferro-ferri solution (Figure S1, Supporting Information). Bandgap value (Eg) of 2.65 and 2.63 eV are determined from ellipsometry for BiVO₄ and Mo:BiVO₄, respectively. (Figure 1b and S2, Supporting Information). To quantify the percentage of Mo⁶⁺ incorporate into the BiVO₄ photoanode, X-Ray fluorescence was conducted with detection limits (LOD) and quantifications limits (LOQ) of 0.013 and 0.44 µg for Bi, and 0.008, and 0.026 µg for Mo and V. The quantity of Mo was estimated to be 0.56 mol% giving to the following chemical formula of BiV₀.₉₄Mo₀.₀₀₆O₄ (Table S3, Supporting Information).

The oxidation state of Bi, V, Mo, and O atoms are investigated by X-ray photoelectron spectroscopy (XPS). The fitted spectra are presented in Figure 2 (survey spectra Figure S3, Supporting Information). Pristine BiVO₄ and Mo:BiVO₄ exhibit the characteristic spin-orbit splitting for Bi and V atoms with Bi 4f<sub>5/2</sub>, Bi 4f<sub>7/2</sub> and V 2p<sub>1/2</sub>, V 2p<sub>3/2</sub> signals.[11,41] Their binding energies are summarized in Table 1. XPS spectrum in the Mo 3d binding energy domain confirms its presence in the doped sample. The O 1s, corresponding to the binding energy for the lattice oxide, is detected at 529.82 eV for BiVO₄ and 529.43 eV for Mo:BiVO₄. The presence of V(V) oxide is also observed at 530.59 and 530.41 eV for BiVO₄ and Mo:BiVO₄, respectively. This corresponds to the oxidation state of V in VO₄³⁻ units.

## 2.2. PEC Performances Evaluation

The PEC performances of the prepared pure BiVO₄ and Mo:BiVO₄ electrodes are summarized in Figure 3 in order to compare the OER efficiency and durability when bulk and surface chemistry are modified.

Pristine BiVO₄ photoanode analysis reveals a maximum photocurrent density of 1.2 mA cm⁻² at 1.23 V<sub>RHE</sub> for front illumination with an onset potential of 0.6 V<sub>RHE</sub> (Figure 3a). The
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Pristine BiVO₄ and Mo:BiVO₄ characterization: a) XRD pattern of pristine BiVO₄ (black curve), Mo:BiVO₄ (red curve) gray curve annealed FTO substrate (gray curve), PDF-04-010-5710 BiVO₄ tetragonal scheelite (blue straight-line) and PDF-04-010-5713 BiVO₄ monoclinic scheelite (black straight-line); b) BiVO₄ (black curve) and Mo:BiVO₄ (red curve) banggap determination by ellipsometry; FEG-SEM surface imaging of c) BiVO₄ e) Mo:BiVO₄; FEG-SEM cross-section imaging of d) BiVO₄ f) Mo:BiVO₄.</figcaption></figure>

dependence of the photocurrent density on the electrode's side of irradiation indicates electron or hole transport limitations within the material's bulk. Pure BiVO₄ sample exhibits comparable values for back and front illumination, indicating that our photo-electrode does not suffer from photocarriers transport limitation. This is due to the thickness of our electrode layer that does not exceed ≈100 nm, which corresponds to the diffusion length of photocharges in BiVO₄. This limitation becomes negligible.[42] In comparison, the photocurrent of the Mo:BiVO₄ at 1.23 V~RHE~ increases by a factor of 1.4 compared to pristine BiVO₄, reaching a value of 1.7 mA cm⁻² in front illumination. However, under back illumination, the photocurrent of the Mo:BiVO₄ photoanode at 1.23 V~RHE~ decreased by a factor of 2.6, accompanied by an anodic shift of the onset potential by 150 mV. The doped photoanode shows a considerably higher photocurrent than pure BiVO₄ under front illumination. This increase in photocurrent density is not attributed to changes in specific areas since the electrochemically active surface area of BiVO₄ and Mo:BiVO₄ is comparable (Figure

S4, Supporting Information). Instead, the better performance under front-side rather than back-side illumination provides proofs of an enhanced electron transport due to Mo⁶⁺ doping. The poor electron transport in the bulk of the material is no longer an obstacle to photocurrent production. Nevertheless, due to doping, the OER efficiency is reduced for back-lighting, due to poor holes diffusion.

The stability of the electrodes was then assessed through chronoamperometric measurements conducted over a 5-h period. In Figure 3b, pure BiVO₄ produced a photocurrent density of ≈0.8 mA cm⁻² at 1.23 V~RHE~ after 45 min of analysis, which decreased to 0.3 mA cm⁻² after 2 h and further dropped to 0.03 mA cm⁻² after 5 h. The presence of sharp recombination peaks indicates that improving the separation and transport of electrons is not fast enough to enhance the efficiency of OER for photocharges. Additionally, the gradual decrease in photocurrent value over time reveals competitive reactions occurring in parallel with OER, contributing to the degradation of the
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Room-temperature XPS spectra of air-exposed samples. Color code: black spectra corresponding to pristine BiVO₄ and red spectra corresponding to Mo:BiVO₄.</figcaption></figure>

<table><caption>**Table 1.** Binding energy in eV of bi 4f 7/2, bi 4f 5/2, V 2p 3/2, and V 2p 1/2 in the electrodes of BiVO₄ and Mo:BiVO₄; Δ (Bi) = E(Bi 4f 5/2) - E(Bi 4f 7/2), Δ (V 2p) = E(V 2p 1/2) - E(V 2p 3/2).</caption><thead><tr><th>Samples</th><th>Bi 4f 7/2</th><th>Bi 4f 5/2</th><th>Δ (Bi)</th><th>V 2p 3/2</th><th>V 2p 1/2</th><th>Δ (V)</th></tr></thead><tbody><tr><td>BiVO<sub>4</sub></td><td>158.80</td><td>164.11</td><td>5.30</td><td>516.35</td><td>523.83</td><td>7.48</td></tr><tr><td>Mo:BiVO<sub>4</sub></td><td>159.14</td><td>164.44</td><td>5.30</td><td>516.70</td><td>524.10</td><td>7.40</td></tr></tbody></table>

photoanode itself. By doping with Mo, the Mo:BiVO₄ photoanode showed higher photocurrent density over time, reaching 1.2 mA cm⁻² at 1.23 V<sub>RHE</sub> after 45 min, which corresponds to the maximum photocurrent of pristine BiVO₄. After 2 h of operation, the photocurrent density of the undoped-BiVO₄ electrode declined significantly to 0.6 mA cm⁻², and continued to drop to just 0.04 mA cm⁻² after 5 h, indicating pronounced instability under the tested conditions. In contrast, the Mo-doped BiVO₄ film exhibited markedly enhanced stability, maintaining a substantially higher photocurrent over the same period. This clear improvement can be attributed to the beneficial effect of molybdenum doping, which is known to increase electrical conductivity and promote more efficient charge carrier separation. By facilitating better extraction and utilization of photogenerated holes and/or electrons, Mo doping likely reduces charge recombination processes, thereby extending the operational lifetime and performance of the photoanode.

FEG-SEM post-mortem analysis after 5 h of chronoamperometric measurements reveals a significant electrode degradation for both photoelectrodes BiVO₄ and Mo:BiVO₄ (Figure 4). Interestingly, the thickness of the photoelectrode seems to be

thinner under front irradiation compared to back irradiation. We notice the presence of pores between (10 and 30) nm in both photoelectrodes, confirming their degradation and corrosion (Figure S5, Supporting Information). Interestingly, the degradation seems to be reduced with backlighting, which comfort the hypothesis that the rate of holes consumption accumulated at the electrode-electrolyte interface plays a crucial role in determining the type of reaction occurring at the interface.

## 2.3. Photocorrosion Study

We investigated further photoelectrode degradation by studying its chemical stability in the electrolyte under both dark and light conditions (Figure 5). After immersing the electrode in potassium phosphate electrolyte (pH 6) without exposure to light for 5 h, the electrode's morphology remains unchanged (Figure 5a,b). However, when subjected to simulated sunlight at 100 mW cm⁻² for an additional 5 h, we observed partial dissolution of the electrode (Figure 5c). This dissolution is attributed to photocorrosion phenomena,[43,44] involving the preferential dissolution of V and the formation of BiPO₄ under illumination.[15,45,46]
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Photoelectrochemical measurements in 1 M potassium phosphate buffer (pH 6) under simulated illumination at 100 mW cm⁻² a) linear voltage sweeps measured at BiVO₄ thin films with/without illumination in front-side (black) and in back-side (gray dots) and Mo:BiVO₄ thin films in front-side (red) and in back-side (pink dots); b) chronoamperometric measurement BiVO₄ (black thicker line) and Mo:BiVO₄ (red line) thin films at 1.23 V<sub>RHE</sub> with chopped on/off front-side illumination of 90s for 2 h.</figcaption></figure>

Post-mortem FEG-SEM analyses confirm the degradation of the photoelectrode, with the appearance of pores. We hypothesize that this degradation is linked to accumulation of holes at the

electrode-electrolyte interface. Moreover, at pH = 6 the holes react more efficiently with the solution to produce radicals that will enhance BiVO₄ dissolution.

The addition of Na₂SO₃ was then employed as a diagnostic tool to isolate the role of photogenerated holes in electrode degradation, allowing us to differentiate between intrinsic material instability and surface oxidation processes. 1 M sodium sulfite (Na₂SO₃) is introduced to the phosphate electrolyte buffer as a sacrificial agent, slightly raising the pH to 6.8 (Figure 6). In Figure 6a, it can be seen an increase of photocurrent to 3.6 mA cm⁻² for Mo:BiVO₄ compared to 1.7 mA cm⁻² at 1.23 V<sub>RHE</sub> 3.6 mA cm⁻² with no Na₂SO₃. Additionally, we observe a cathodic shifts of ≈0.4 mV for the onset potential upon the addition of the sacrificial agent. These results confirm the dependence of surface chemistry on the onset potential. The significant increase of photocurrent density in presence of sulfites confirms that recombination phenomena are a major limiting factor for our photoelectrodes. Interestingly, the chronoamperometry curve revealed better stability and fewer electron-hole recombination, as demonstrated by the little variation of the steady-state current.

The addition of SO₃²⁻ ions allows twice-lower electron-hole pair to be generated compared to the OER reaction. The stability obtained for an easier reaction confirms that photogenerated holes drives the anodic corrosion (Figure 6b). Also, we hypothesized that the presence of Mo⁶⁺ doping can influence the stability of vanadium's oxidation state within the material or passivate the surface state.

Doping BiVO₄ with Mo⁶⁺ modifies the properties of the material's bulk by increasing the donor density, which enhances electron conductivity and simultaneously moderates the accumulation of holes, diminishing the photodegradation phenomena.

After 5 h of chronoamperometric measurement, the photocurrent value for this photoelectrode remains the same (Figure S6, Supporting Information). FEG-SEM post-mortem analysis showed

<figure><img src="image_5.png" /><figcaption>**Figure 4.** FEG-SEM post-mortem imaging of BiVO₄ (110 nm) a) surface; b) cross-section and Mo:BiVO₄ c) surface; d) cross-section.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 5.** FEM-SEM imaging of Mo:BiVO$_4$ with a scale of 200 nm a) after synthesis without any external perturbation, b) immersed in 1 M potassium phosphate electrolyte (pH 6) in dark conditions during 5 h, c) immersed in 1 M potassium phosphate electrolyte (pH 6) in light conditions during 5 h with 100 mW cm$^{-2}$ of simulated sunlight.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Photoelectrochemical measurements in 1 M potassium phosphate buffer with 1 M additional Na$_2$SO$_3$ (pH 6.8) under front simulated illumination at 100 mW cm$^{-2}$. a) Linear voltage sweeps measured at Mo:BiVO$_4$ thin films front irradiation; b) chronoamperometric measurement on Mo:BiVO$_4$ thin films at 1.23 V$_{RHE}$ with chopped on/off illumination of 90 s for 2 h.</figcaption></figure>

that the particles are still well sintered and dense (Figure S7, Supporting Information).

Therefore, passivating layers including OER catalysts are indispensable to protect the BiVO$_4$ and Mo:BiVO$_4$ surface from photo-corrosion. This have already shown stable operation for 1100 and 500 h.[46,47]

## 2.4. Surface Engineering

To protect the photoelectrode, we have first modified the electrode-electrolyte interface through the deposition of thin TiO$_2$ passivation layer.

The TiO$_2$ passivation layer was deposited by sol-gel chemistry coupled with the dip-coating, as developed by Sum et al.[48]

A 3-layers sample with an average thickness of 5 nm was made (Table S4, Supporting Information). After calcination, the presence of anatase TiO$_2$ crystallized was confirmed by Raman spectroscopy due the appearance of a peak at 144 cm$^{-1}$ (Figure 7a). For Mo:BiVO$_4$/TiO$_2$ with 3 nm of TiO$_2$ passivation layer, the linear voltage sweep exhibits a photocurrent density at 1.23 V$_{RHE}$ of 0.43 mA cm$^{-2}$ (Figure 7b). Compared to the 1.19 mA cm$^{-2}$ for Mo:BiVO$_4$ (110 nm) electrode, the decrease of photocurrent density is only 2.7-times smaller. According to chronoamperometric measurements, the sample (Figure 7c) shows great stability, with only one residual recombination peak. Its average photocurrent density value at the plateau corresponds to ≈0.3 mA cm$^{-2}$.

Imaging of the Mo:BiVO$_4$/TiO$_2$—3 nm surface after PEC measurement for ≈3 h reveals a sharper surface with a darker area in the particle/particle contact region than before PEC testing (see Figure 8a,b). The cross-section analysis shows a slight reduction in thickness and the presence of a few larger particles on the surface, which could be attributed to the generation of bismuth phosphate nanoparticles (Figure 8c,d). However, sustainability is greatly improved, as the film remains dense. Nevertheless, the overall photocurrent decreased using this approach.

## 2.5. Cobalt-Phosphate (Co-Pi) Co-Catalyst

To enhance both OER kinetics and durability, we selected Co-Pi as co-catalyst and proceeded to its electrodeposition onto the surface of the Mo:BiVO$_4$ photoanode. First, various deposition times were applied to determine the best conditions for photoanode efficiency. Figure 9 shows the surface of the photoanode after 5 min of electrodeposition, revealing the formation of small particles on the surface. However, the film does not uniformly cover the entire surface of the photoanode (Figure 9b).

Figure 10c shows that the best result is obtained for 5 min of deposition with a photocurrent density of 1.9 mA cm$^{-2}$ at 1.23 V$_{RHE}$ measured in linear sweep mode. In comparison with Mo:BiVO$_4$ current-voltage curve, a cathodic shift of 0.1 mV is observed (Figure 9d). Additionally, a change in the shape of the photocurrent density appeared with the co-catalyst, resulting in a small bump directly after the onset potential. Interestingly, the photocurrent of the co-catalyst photoelectrode for the first 2 h is higher as attended and similar to that of Mo:BiVO$_4$ (Figure 9e). However, after 2 h, the photocurrent reaches a value of 0.6 mA cm$^{-2}$.

FEG-SEM post-mortem analysis revealed porosity on the photoanode with pore size between 10 and 35 nm (Figure 10a). But,
<figure><img src="image_8.png" /><figcaption>**Figure 7.** a) Raman spectroscopy of TiO₂ ≈ 50 nm deposited on Si (black curve) as reference for Mo:BiVO₄/TiO₂-5 nm (in pink). Photoelectrochemical measurements. b) Linear voltage sweeps measured at Mo:BiVO₄ (110 nm) reference photoanode in gray and Mo:BiVO₄/TiO₂-3 nm in green. c) Chronoamperometric measurements with chopped on/off irradiation of 90s during 1 h at 1.23 V<sub>RHE</sub> on Mo:BiVO₄/TiO₂-3 nm.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 8.** FEG-SEM surface imaging of Mo:BiVO₄/TiO₂-3 nm a) before PEC measurements b) after PEC measurements; FEG-SEM cross-section imaging of Mo:BiVO₄/TiO₂-3 nm c) before PEC measurements d) after PEC measurement.</figcaption></figure>

cross-section imaging showed that most of the thickness is maintained after the PEC measurements (Figure 10b). We believe that the improvement in photoanode stability is linked to a faster holes collection and use through OER thanks to the co-catalyst. This modification limits their accumulation on the surface of the electrode, thus avoiding the presence of reactive species that contribute to photocorrosion on the electrode surface.

The change of shape in linear sweep voltammetry (LSV) on the sample with Co-Pi co-catalyst suggests that other electrochemical reactions occur. To further understand this behavior, CV-curves are measured in 1 M potassium phosphate buffer (pH 6) under front illumination (see Figure 11). The first cycle distinguished itself from the following ones, with the largest oxidation bump observed at 0.56 V. Interestingly, the maximum of photocurrent density was the smallest for the first scan from 0.2 to 1.3 V. From the 2nd cycle, all
<figure><img src="image_10.png" /><figcaption>**Figure 9.** FEG-SEM surface imaging of a) bare Mo:BiVO₄ thin film, b) Mo:BiVO₄ after 5 min co-pi electrodeposition. Photoelectrochemical measurements c) linear voltage sweeps measured at Mo:BiVO₄/Co-Pi thin films with/without illumination on sample with 60 s, 110 s, 5 min, 10 min Co-Pi electrodeposition time; d) Linear voltage sweeps comparison of Mo:BiVO₄ (in red) and Mo:BiVO₄/Co-Pi with 5 min electrodeposition time e) chronoamperometric measure-
ment on Mo:BiVO₄/Co-Pi with 5 min electrodeposition at 1.23 V<sub>RHE</sub> with chopped on/off illumination of 90 s for 2 h.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 10.** FEG-SEM Post-mortem imaging of Mo:BiVO₄/Co-Pi photoanode of a) surface b) cross-section.</figcaption></figure>

the CV curves presented an identical shape. Interestingly, adding a
scan in reduction significantly increases the photocurrent density
at 1.23 V<sub>RHE</sub>, reaching 2.4 mA cm<sup>-2</sup>. This enhancement improved
the PEC efficiency by 30%. These results confirm that a modifica-
tion of the electrode occurs during the first electron–hole pairs
generation. In their work, M.W. Kanan et al., have evidenced ‘a
self-repair cycle’ using cyclic voltammetry.[49] In Figure 11b,c,
XPS analysis before any experiment and immediately after the first

linear scan under illumination confirm the evolution of the surface
layer of Mo:BiVO₄ photoanode. Following electrodeposition, the
presence of different oxidation states of Co in the Co-Pi layer is
identified. The fit on the XPS spectra in Figure 11b corresponds
to Co 2p in Co₃O₄, showing the presence of both Co(II) and
Co(III). After the initial linear voltage sweeps under illumination,
the Co binding energy peak seems to partially disappear, with a
new contribution observed at 807 eV. Conversely, the binding
<figure><img src="image_12.png" /><figcaption>**Figure 11.** a) Cyclic voltammetry of $BiV_{0.994}Mo_{0.06}O_4/Co-Pi$ in 1 M potassium phosphate buffer (pH 6) under 100 mW cm$^{-2}$ illumination. XPS study of $BiV_{0.994}Mo_{0.06}O_4/Co-Pi$ after electrodeposition (black curve) and after first illumination of LSV measurements (green) with the binding energy of b) Co atom and c) P atom.</figcaption></figure>

energy of phosphorous, P 2p, remains unchanged. These results confirm the dynamic behavior of Co-Pi as demonstrated by the group of M.W. Kanan, where Co (II) is oxidized and the phosphorous remains in the form of $HPO_4$ throughout the process.

Finally, the electrolyte buffer after several CV measurements was subjected to ICP-AES to determine whether the $Co^{2+}$ recovery was total or partial. The analysis revealed the presence of 429 ng of cobalt ions in the electrolyte solution with a surface exposition of 0.281 cm$^2$ for the $Mo:BiVO_4/Co-Pi$. This analysis revealed that the 'self-repair' property of Co-Pi is not complete despite the significant improvement in photocurrent.

To identify whether the amount of cobalt detected in ionic form could be linked to a partial recovery of the catalyst in Co (II) after several cyclic voltammetry, we calculated the mass of cobalt oxidized based on the difference in photocurrent density observed during the oxidation/reduction scans at 0.6 $V_{RHE}$ in the bump potential range. We approximate the time for $Co^{2+}$ to be oxidized to $Co^{3+}$ during the scanning from 0.44 V to 0.62 V. Given a scan rate of 10 mV s$^{-1}$, this time interval (t) is defined as 18 s. For the first cycle, the difference in photocurrent is approximately ≈0.25 mA cm$^{-2}$. With an illuminated surface area of 0.281 cm$^2$, we calculated a current i of 0.07025 mA. Using the definition of the current = $Q/t$, with Q the electric charge (C) and t the time (s), the number of electrons ($N_e$) can be calculated

with: $N_e = Q/e$ giving a value of $7.89 \times 10^{15}$ electrons. If we consider that each electron is used in the oxidation of Co (II) into Co (III), we deposited 772 ng of Co. This mass is superior to the mass measured by ICP in the electrolyte after the CVs. If we calculate with the exact same process the Co regenerate from the following cycles, the difference in photocurrent is ≈0.13 mA cm$^{-2}$ that corresponds to a current i of 0.037 mA. It gives a deposition of 405 ng of Co. This result shows that an incomplete recovery of Co(II) during the electrocatalysis is operating. However, the quantity of Co(II) deposited during the following cycle does not have an impact on the OER.

### 3. Conclusion

Dense $BiVO_4$ and Mo-doped $BiVO_4$ photoanodes were successfully synthesized using sol-gel dip-coating techniques, resulting in a scheelite structure composed of monoclinic and tetragonal polymorphs with an average film thickness of 110 nm. Our study reveals that under acidic conditions, the performance of pure $BiVO_4$ photoanode is constrained by three main factors: i) limited electron transfer within the material, ii) rather slow kinetics for OER, and iii) accumulation of holes at the electrode-electrolyte interface. These issues lead to the recombination of photogenerated electron-hole
pairs and photocorrosion, significantly impacting the overall photo-
anode efficiency and sustainability for OER. Bulk modification
through doping enhances electron mobility within the material,
resulting in an increase of the photocurrent density from 1.2 to
1.7 mA cm⁻². We have developed an efficient and cost-effective
process for depositing TiO₂ protection using sol–gel chemistry
and dip-coating. A 3 nm thin TiO₂ layer is easily obtained with good
crystallinity in the anatase structure. This layer exhibits a stable pho-
tocurrent of ≈0.3 mA cm⁻² for 1 h at 1.23 V<sub>RHE</sub>. Post-mortem analy-
sis confirms the effective passivation properties of the thin TiO₂
layer deposited by dip-coating, with the absence of porosity and
better thickness retention.

With the addition of Na₂SO₃ as sacrificial agent, Mo:BiVO₄ photoanodes exhibit a high photocurrent density reaching up to 3.6 mA cm⁻² with high stability during 5 h. It demonstrates the potential of enhancing photocurrent density and durability by quickly consuming photogenerated holes for OER. Surface engineering involving the addition a Co-Pi co-catalyst overlayer on Mo:BiVO₄ photoanode improves the OER kinetics by a cathodic shift in the onset potential and enhances the photocurrent density to 1.9 mA cm⁻² at 1.23 V<sub>RHE</sub> in conditions close to the use in a final device. Furthermore, it notably enhances the long-term stability of the photoanode, preserving its thickness for over 5 h. Nevertheless, a detailed examination during different stages of cyclic voltammetry experiments revealed an incomplete recovery of Co(II) within the cyclic voltammetry process. Additionally, scanning in reduction showed better photocurrent density in the presence of Co-Pi co-catalyst compared to linear sweeps, emphasizing the incomplete self-repair property. When compared to other BiVO₄-based photoanodes designed for OER under acidic conditions, our photoanode currently exhibits one of the highest OER efficiencies to date (see Table S1, Supporting Information). Future possibilities may include surface modification with more durable catalysts, such as those based on iron (Fe) or nickel (Ni), to further enhance photocurrent density.

4. Experimental Section

Synthesis of the BiVO<sub>4</sub>-Based Photoanode: BiVO<sub>4</sub> and
Mo:BiVO<sub>4</sub> films

To synthesize multilayer BiVO₄ and doped-Mo:BiVO₄ photoanodes, sol–gel chemistry has been coupled with the dip-coating deposition technique. The syntheses are based on a previous work developed by S. Hilliard.[36] For the undoped-BiVO₄ precursor solution, 1.08 mmol of bismuth (III) nitrate pentahydrate (ACS reagent ≥98.0%) and 1.08 mmol of vanadium (IV) oxide acetylacetonate (ACS reagent ≥95.0%) are dissolved in a mixture of 130.8 mmol acetylacetone (ACS reagent ≥99.0%) and 28.1 mmol acetic acid. For the doped Mo:BiVO₄ precursor solution, 1%mol of the vanadium precursor is replaced by molybdenum (VI) dioxide bis(acetylacetone) (ACS reagent ≥95.0%) given 1.069 mmol of vanadium precursor and 0.0108 mmol of Mo⁶⁺ precursor. They are first put in an ultrasonic bath for 10 min at 35 °C before being magnetically stirred overnight. For the deposition, a commercial glass of F-doped SnO₂ (FTO) 80 nm/ glass substrate from (SOLEMS: YSUB/ASAH120/1) with effective resis- tivity measured in the 200—250 Ω range is used as an electrode

substrate. The films are produced by dip coating (Ossila Dip-Coater) and controlling the relative humidity (RH) (<5%). A first ultra-thin layer is obtained at a deposition rate of 1 mm s⁻¹ to help anchor the subsequent layers. After each deposited layer, the non-conductive side of the substrate is wiped with ethanol and acetone and the electrode is annealed at 400 °C for 1—2 min. Six layers of the precursor solution are deposited at a withdrawing speed of 8 mm s⁻¹ resulting in an average thickness of 18 nm/layer. Finally, the films are crystallized by a calcination step at 450 °C for 1 hr in air.

Synthesis of the BiVO₄-Based Photoanode: TiO₂ Passivation Layer:

For the deposition of thin TiO₂ layers by sol–gel synthesis coupled with dip-coating, a first solution Sₐ of TiCl₄: EtOH 1:5 volumetric ratio (v:v) is prepared. Then, 1.6 g from Sₐ is poured into another flask where 5.0 g of absolute ethanol and 0.4 g of Milli-Q water are added giving solution Sₐ ( [Ti⁴⁺] = 0.2 M). Finally, Sₐ was then diluted with EtOH:H₂O 1:2 (v:v) to obtain a [Ti⁴⁺] concentration of 0.125 M. The dip-coating was done in a deposition chamber with a relative humidity below 10%. A withdrawal speed of 0.8 mm s⁻¹ was used to obtain two or three layers of TiO₂. An intermediate heat treatment of 1–2 min at 350 °C was performed between each layer of TiO₂, followed by a final calcination for 1 hr at 450 °C under O₂. The films were deposited onto silicon wafers and Mo:BiVO₄ photoanodes

Synthesis of the BiVO₄-Based Photoanode: Cobalt-Phosphate
(Co-Pi) Co-catalyst Layer

Co-Pi is electrodeposited onto the surface of the Mo:BiVO₄ photoanode using an electrolyte containing Co precursors. A solution Sₐ of a 0.1 M phosphate buffer electrolyte is made from potassium phosphate monobasic, KH₂PO₄ (ACS reagent ≥99.0%) and potassium hydroxide, KOH (ACS reagent ≥90.0%). A Solution Sₐ is prepared by dissolving 29.5 mmol of Cobalt (II) nitrate hexahydrate, Co(NO₃)₂·6H₂O (ACS reagent ≥98.0%), in 35 mL of Milli-Q water. Sₐ is then poured into Sₐ. Electrodeposition is carried out at 1.713 V<sub>RHE</sub> constant voltage for deposition times ranging from 1 min to 10 min, without stirring, in a 3-electrode cell. The photoanode is the working electrode, a coiled platinum wire is the counter electrode, and an Ag/AgCl KCl saturated electrode is used as the reference.

**Characterization: X-ray Diffraction (XRD)**

X-ray diffractograms were acquired in Bragg-Brentano geometry using a Bruker Discover D8 apparatus, equipped with a Cu anode (1.54 Å) and a LynxEye XE – T detector, over a 2θ range from 5° to 70° with a step size of 0.02°.

**Characterization: UV–Visible Spectroscopy**

UV/visible light absorption measurements were performed with an
Agilent Technologies Cary Series 5000 UV/visible/near-infrared spec-
trometer using a double beam with a background of FTO defined.

**Characterization: Fluorescence**

X-ray fluorescence measurements were performed on an Epsilon
3XL energy dispersive spectrometer (@Malvern-Panalytical).
Bismuth (Lα) and molybdenum (Kα) were measured with a voltage
of 50 kV and an intensity of 6 µA for 300 s. For vanadium ($K\alpha$) during 60 s, a voltage of 12 kV and an intensity of 25 µA were applied. For all three elements, a calibration was established using standard solutions of 1 g L⁻¹ deposited in different volumes on polycarbonate membranes. The powdered samples were then prepared in the same way. For samples on substrates, a prolene membrane (highly transparent to X-rays) was used to deposit the sample. When the sample is thin, there are no matrix effects, and the signal intensity is directly proportional to the mass of the element present in the analyzed area (0.5 cm²).

## Characterization: FEG-SEM EDX Imaging

Field emission gun-scanning electron microscopy (FEG-SEM) and energy-dispersive X-ray spectroscopy (EDX) were carried out using a Hitachi SU-70 equipped with Oxford X-Max beam with 50 mm² X-Max silicon drift EDS detector with a 50 mm² tungsten filament operated at 10–15 kV. High-resolution transmission electron microscopy (HR-TEM) was done using a JEOL JEM-2010.

Raman spectroscopy was conducted on a LabRam HR 800 Horiba-Jobin Yvon spectrometer with green wavelength at 514.5 nm: grid with 600 lines or 1800 lines per mm, objective × 50 uis2 (NA 0,75), ×100 uis2 (NA 0,9) of Olympus, spot size of [1,2] µm analyzed.

## Characterization: Photoelectrochemical Performances:

The PEC performance of BiVO₄ photoanodes was evaluated using an AMETEK Solartron Analytical Modulab potentiostat coupled with a Newport Xenon lamp under AM 1.5 G illumination (100 mW cm⁻²) with both front side (through the film-electrolyte interface) and back side (through the FTP back-contact) exposure. LSV was measured from -0.2 V to 0.74 V versus Ag/AgCl. KCl sat. reference electrode (E°Ag/AgCl = 0.197 V vs NHE) using a scan rate of 10 mV s⁻¹. All current density curves were referenced to the reversible hydrogen electrode (RHE), using the Nernstian relation of:

$$V_{RHE} = \text{Evs.Ag/AgCl} + 0.197 + 0.059 \times \text{pH} \quad (1)$$

The photocurrent was stable for at least five scans and reproducible. The third scans acquired with illumination for all investigated electrodes are collected. Chronoamperometry measurement under alternative irradiation was performed using the potentiostatic mode at a fixed potential of 1.23 V$_{RHE}$. The geometric active area of the BiVO₄ working electrode was defined by Hi-Bond polyamide scotch tape (HB830-19) and measured 0.28 cm². Electrical contacts were made by conductive copper scotch (3M Scotch 1245), which was adhered directly to the FTO exposed portion of the photoelectrode. The electrolyte used during electrochemical and photoelectrochemical tests was a 1 M potassium phosphate buffer of pH 6.07 and a 1 M potassium phosphate buffer enriched with 1 M Na₂SO₃ of pH 6.77.

## Characterization: XPS Fitting Procedure

XPS analyses were performed using an Omicron Argus X-ray photoelectron spectrometer, equipped with a monochromated Al Kα radiation source (hv = 1486.6 eV) and a 280 W electron beam power. The emission of photoelectrons from the sample was analyzed at a take-off angle of 45° under ultrahigh vacuum conditions

($\le$10⁻⁹ mbar). Spectra were carried out with 100 eV pass energy for the survey scan and 20 eV pass energy for the C 1s, O 1s, N 1s, Bi 4f, V 2p, Mo 3d, P 2p, and Co 2p regions. Binding energies were calibrated against the C 1s (C–C) binding energy at 284.8 eV and element peak intensities were corrected by Scofield factors. The peak areas were determined after subtraction of a (LINEAR—SHIRLEY—U 2 TOUGAARD) background (Figure S3, Supporting Information). The spectra were fitted using Casa XPS v.2.3.15 software (Casa Software Ltd, U.K.) and applying a Gaussian/Lorentzian ratio g/l equal to 70/30.

Ellipsometry were performed on a J.A Woollam Ellipsometer to determine the thickness and the bandgap (Eg) of each sample. The method is described in supporting information.

## Characterization: Electrochemically Active Surface Area:

The comparison of the double layer capacitance ($C_{DL}$) of the samples brings information on the electrochemically active surface area (EASA). $C_{DL}$ was determined following the methodology described in the supporting information.

## Characterization: ICP-AES

Inductive coupled plasma-atomic emission spectrometry was conducted on a Spectrogreen spectrometer. The samples were diluted by a factor of 10 in a solution HNO₃ 1% prepared with ultrapure milliQ water and pure HNO₃ 68%. For each wavelength, three measurements were performed.

## Acknowledgements

Adeline Blot thanks the Institut des Matériaux (IMat) for Ph.D. Grant. Antoine Miche is gratefully acknowledged for the help in conducting XPS experiments. The spectrometer was funded by Sorbonne Université, CNRS, and Région Ile de France, and is part of FCMat. The authors acknowledge the X-rays facility (ITODYS-LISA, Université Paris Cité, Paris, France) and Sophie Nowak assisted by Ivonne Cocca, who helped in conducting FX analyses. Prof. Rémi Losno is greatly acknowledged for the ICP-AES analysis conducted at IPGP, Institut de Physique du Globe de Paris, Université Paris Cité, Paris, France. They acknowledge David Montero from FCMat of Sorbonne Université for SEM-FEG-EDX analysis. Dr. Gwenael Gouadec is also greatly acknowledged for the help conducting Raman spectroscopy experiments at the Laboratoire Monaris, Sorbonne Université, France.
[1] F. Amano, in *Solar-to-Chemical Convers. Photocatalytic Photoelectrochem Process*, Wiley-VCH GmbH, Weinheim, Germany **2021**.
[2] M. Kumar, B. Meena, P. Subramanyam, D. Suryakala, C. Subrahmanyam, *NPG Asia Mater.* **2022**, *14*, 1.
[3] M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, *Chem. Rev.* **2010**, *110*, 6446.
[4] C. Jiang, S. J. A. Moniz, A. Wang, T. Zhang, J. Tang, *Chem. Soc. Rev.* **2017**, *46*, 4645.
[5] M. Tayebi, B. K. Lee, *Renewable Sustainable Energy Rev.* **2019**, *111*, 332.
[6] S. Wang, K. Wan, J. Feng, Y. Yang, S. Wang, *J. Mater. Sci. Technol.* **2025**, *217*, 182.
[7] Y. Bai, Z. Fang, Y. Fang, C. Lin, H. Bai, W. Fan, *Chem. Commun.* **2025**, *61*, 5264.
[8] K. R. Tolod, S. Hernández, N. Russo, *Catalysts* **2017**, *7*, 13.
[9] J. K. Cooper, S. Gul, F. M. Toma, L. Chen, P. A. Glans, J. Guo, J. W. Ager, J. Yano, I. D. Sharp, *Chem. Mater.* **2014**, *26*, 5365.
[10] S. Tokunaga, H. Kato, A. Kudo, *Chem. Mater.* **2001**, *13*, 4624.
[11] G. Li, Y. Bai, W. F. Zhang, *Mater. Chem. Phys.* **2012**, *136*, 930.
[12] J. H. Kim, J. S. Lee, *Adv. Mater.* **2019**, *31*, 1806938 (30).
[13] H. L. Tan, R. Amal, Y. H. Ng, *J. Mater. Chem. A.* **2017**, *5*, 16498.
[14] R. P. Antony, M. Zhang, K. Zhou, S. C. J. Loo, J. Barber, L. H. Wong, ACS *Omega*. **2018**, *3*, 2724.
[15] S. Zhang, I. Ahmet, S. H. Kim, O. Kasian, A. M. Mingers, P. Schnell, M. Kölbach, J. Lim, A. Fischer, J. K.J., et al Mayrhofer, ACS Appl. Energy Mater. **2020**, *3*, 9523.
[16] R. S. Moakhar, S. M. Hosseini-Hosseinabad, S. Masudy-Panah, A. Seza, M. Jalali, H. Fallah-Arani, F. Dabir, S. Gholipour, Y. Abdi, M. Bagheri-Hariri, et al., *Adv. Mater.* **2021**, *33*, 2007285.
[17] M. A. Gaikwad, U. P. Suryawanshi, U. V. Ghorpade, J. S. Jang, M. P. Suryawanshi, J. H. Kim, *Small* **2022**, *18*, 1.
[18] D. Chen, Z. Xie, Y. Tong, Y. Huang, *Energy Fuels.* **2022**, *36*, 9932.
[19] M. Rohloff, B. Anke, D. Wiedemann, A. C. Ulpe, O. Kasian, S. Zhang, C. Scheu, T. Bredow, M. Lerch, A. Fischer, Zeitschrift für Phys. Chemie **2020**, *234*, 655.
[20] J. Huang, T. Lin, L. Lin, G. Ma, Z. Zhang, S. Handschuh-Wang, A. Meng, P. Han, B. He, ACS Appl. Energy Mater. **2024**, *7*, 10710.
[21] B. Liu, X. Wang, Y. Zhang, K. Wan, L. Xu, S. Ma, R. Zhao, S. Wang, W. Huang, *Adv. Energy Mater.* **2025**, *15*, 2403835.
[22] C. M. Suarez, S. Hernández, N. Russo, *Appl. Catal. A Gen.* **2015**, *504*, 158.
[23] J. C. Fornaciari, L. C. Weng, S. M. Alia, C. Zhan, T. A. Pham, A. T. Bell, T. Ogitsu, N. Danilovic, A. Z. Weber, *Electrochim. Acta.* **2022**, *405*, 139810.
[24] L. Gao, X. Long, S. Wei, C. Wang, T. Wang, F. Li, Y. Hu, J. Ma, J. Jin, *Chem. Eng. J.* **2019**, *378*, 122193.

[25] X. Wan, Y. Xu, X. Wang, X. Guan, Y. Fu, C. Hu, H. Hu, N. Rong, *Appl. Surf. Sci.* **2022**, *573*, 151492.
[26] C. H. Ahn, N. G. Deshpande, H. S. Lee, H. K. Cho, *Small Methods.* **2021**, *5*, 2000753.
[27] M. Zhou, J. Bao, Y. Xu, J. Zhang, J. Xie, M. Guan, C. Wang, L. Wen, Y. Lei, Y. Xie, *ACS Nano.* **2014**, *8*, 7088.
[28] X. Zhong, H. He, M. Yang, G. Ke, Z. Zhao, F. Dong, B. Wang, Y. Chen, X. Shi, Y. Zhou, *J. Mater. Chem. A.* **2018**, *6*, 10456.
[29] L. Fu, Z. Li, X. Shang, Y. Zheng, *J. Mater. Chem. A.* **2025**, *13*, 13951.
[30] P. Plate, C. Höhn, U. Bloeck, P. Bogdanoff, S. Fiechter, F. F. Abdi, R. Van De Krol, A. C. Bronneberg, ACS Appl. Mater. Interfaces. **2021**, *13*, 2428.
[31] R. Irani, P. Plate, C. Höhn, P. Bogdanoff, M. Wollgarten, K. Höflich, R. Van De Krol, F. F. Abdi, *J. Mater. Chem. A.* **2020**, *8*, 5508.
[32] J. Xie, C. Guo, P. Yang, X. Wang, D. Liu, C. M. Li, *Nano Energy.* **2017**, *31*, 28.
[33] E. Usman, M. B. Vishlaghi, A. Kahraman, N. Solati, S. Kaya, ACS Appl. Mater. Interfaces. **2021**, *13*, 60602.
[34] R. Liu, Z. Zheng, J. Spurgeon, X. Yang, *Energy Environ. Sci.* **2014**, *7*, 2504.
[35] M. T. McDowell, M. F. Lichterman, J. M. Spurgeon, S. Hu, I. D. Sharp, B. S. Brunschwig, N. S. Lewis, *J. Phys. Chem. C.* **2014**, *118*, 19618.
[36] S. Hilliard, D. Friedrich, S. Kressman, H. Strub, V. Artero, C. Laberty-Robert, ChemPhotoChem **2017**, *1*, 273.
[37] G. Talasila, S. Sachdev, U. Srivastva, D. Saxena, S. S. V. Ramakumar, *Energy Rep.* **2020**, *6*, 1963.
[38] K. P. S. Parmar, H. J. Kang, A. Bist, P. Dua, J. S. Jang, J. S. Lee, *ChemSusChem* **2012**, *5*, 1926.
[39] Y. Liang, T. Tsubota, L. P. A. Mooij, R. Van De Krol, *J. Phys. Chem. C* **2011**, *115*, 17594.
[40] J. A. Seabold, K. Zhu, N. R. Neale, *Phys. Chem. Chem. Phys.* **2014**, *16*, 1121.
[41] M. Huang, J. Bian, W. Xiong, C. Huang, R. Zhang, *J. Mater. Chem. A.* **2018**, *6*, 3602.
[42] A. J. E. Rettie, H. C. Lee, L. G. Marshall, J. F. Lin, C. Capan, J. Lindemuth, J. S. McCloy, J. Zhou, A. J. Bard, C. B. Mullins, *J. Am. Chem. Soc.* **2013**, *135*, 11389.
[43] S. Zhang, M. Rohloff, O. Kasian, A. M. Mingers, K. J. J. Mayrhofer, A. Fischer, C. Scheu, S. Cherevko, *J. Phys. Chem. C.* **2019**, *123*, 23410.
[44] F. M. Toma, J. K. Cooper, V. Kunzelmann, M. T. McDowell, J. Yu, D. M. Larson, N. J. Borys, C. Abelyan, J. W. Beeman, M. K., et al Yu, *Nat. Commun.* **2016**, *7*, 1.
[45] M. Favaro, F. F. Abdi, M. Lamers, E. J. Crumlin, Z. Liu, R. Van De Krol, D. E. Starr, *J. Phys. Chem. B* **2018**, *122*, 801.
[46] D. K. Lee, K. S. Choi, *Nat. Energy* **2018**, *3*, 53.
[47] Y. Kuang, Q. Jia, G. Ma, T. Hisatomi, T. Minegishi, H. Nishiyama, M. Nakabayashi, N. Shibata, T. Yamada, A. Kudo, et al, *Nat. Energy* **2017**, *2*, 1.
[48] J. Sum, O. Durupthy, N. Krins, C. Laberty-Robert, *J. Electrochem. Soc.* **2022**, *169*, 030522.
[49] M. W. Kanan, Y. Surendranath, D. G. Nocera, *Chem. Soc. Rev.* **2009**, *38*, 109.