Strategies for Enhancing BiVO₄ Photoanodes for PEC Water Splitting: A State-of-the-Art Review

Abstract

Bismuth vanadate (BiVO₄) has attracted significant attention as a photoanode material for photoelectrochemical (PEC) water splitting due to its suitable bandgap (~2.4 eV), strong visible light absorption, chemical stability, and cost-effectiveness. Despite these advantages, its practical application remains constrained by intrinsic limitations, including poor charge carrier mobility, short diffusion length, and sluggish oxygen evolution reaction (OER) kinetics. This review critically summarizes recent advancements aimed at enhancing BiVO₄ PEC performance, encompassing synthesis strategies, defect engineering, heterojunction formation, cocatalyst integration, light-harvesting optimization, and stability improvements. Key fabrication methods—such as solution-based, vapor-phase, and electrochemical approaches—along with targeted modifications, including metal/nonmetal doping, surface passivation, and incorporation of electron transport layers, are discussed. Emphasis is placed on strategies to improve light absorption, charge separation efficiency (η_sep), and charge transfer efficiency (η_trans) through bandgap engineering, optical structure design, and catalytic interface optimization. Approaches to enhance stability via protective overlayers and electrolyte tuning are also reviewed, alongside emerging applications of BiVO₄ in tandem PEC systems and selective solar-driven production of value-added chemicals, such as H₂O₂. Finally, critical challenges, including the scale-up of electrode fabrication and the elucidation of fundamental reaction mechanisms, are highlighted, providing perspectives for bridging the gap between laboratory performance and practical implementation.

1. Introduction

The escalating concerns over global warming and the depletion of fossil fuels have intensified the demand for clean and renewable energy sources [1,2,3,4]. In this context, hydrogen has emerged as a promising energy carrier due to its high energy density and environmentally benign characteristics [5,6,7,8]. In particular, photoelectrochemical (PEC) water splitting utilizing solar energy has attracted considerable interest as a sustainable approach for hydrogen production. PEC water splitting provides an efficient and cost-effective route to directly convert solar energy into chemical energy, exhibiting higher energy conversion efficiency compared to conventional electrolysis methods [9,10,11].

In PEC systems, the selection of photoelectrode materials is critical for achieving high efficiency and stability. An ideal photoelectrode should possess strong visible light absorption, efficient charge separation and transport, appropriate band alignment, and chemical stability in aqueous environments [4,12,13,14]. Among various semiconductors, bismuth vanadate (BiVO₄) has drawn substantial attention as a material that meets these requirements [15,16,17].

BiVO₄ exhibits several advantageous properties for PEC hydrogen production, including a suitable bandgap (~2.4 eV), strong visible light absorption, cost-effectiveness, and environmental benignity [18,19,20,21]. Its band structure is particularly favorable for water oxidation, making BiVO₄ a widely studied n-type photoelectrode [22,23,24]. Moreover, BiVO₄ demonstrates high chemical stability, resulting in minimal performance degradation under long-term operation [25,26,27].

Despite these promising characteristics, BiVO₄ faces intrinsic limitations that hinder its practical application. Low charge carrier mobility, short minority carrier diffusion length, and sluggish oxygen evolution reaction (OER) kinetics are major challenges [28,29,30], leading to a notable gap between theoretical and actual PEC performance. To overcome these limitations, various strategies have been explored, including nanostructure engineering, heteroatom doping, heterojunction formation, cocatalyst integration, and surface passivation [31,32,33].

Nanostructure engineering increases the surface area and shortens charge transfer distances, thereby enhancing the photocurrent [34,35]. Heteroatom doping improves electrical conductivity and tunes the band structure [36,37]. Heterojunction formation promotes charge separation and extends light absorption [38,39]. Cocatalyst integration lowers the overpotential for OER and accelerates reaction rates [40,41], while surface passivation mitigates surface defects and suppresses charge recombination [42,43]. These strategies are often combined to achieve significant improvements in BiVO₄ PEC performance.

This review aims to provide a comprehensive overview of recent research trends in BiVO₄-based PEC hydrogen production. We discussed the fundamental properties of BiVO₄, synthesis methods, performance enhancement strategies, and PEC system configurations.

2. Principle of Operation for Photoanode in PEC Water Splitting

Electrochemical (EC) water splitting is initiated by photon absorption in the semiconductor material of the photoelectrode. For this process to occur, incident photons must possess sufficient energy to excite electrons from the valence band (VB) to the conduction band (CB), i.e., when the photon energy (hν) is equal to or greater than the semiconductor’s bandgap energy (E_g), generating electron–hole pairs that drive subsequent chemical reactions:

$$SC+hv\left(\ge E_g\right)\to e_{CB}^{-}+h_{VB}^{+}:Light absorption$$

(1)

The photogenerated charge carriers participate in spatially separated redox reactions. Electrons in the CB (e_CB⁻) with potentials more negative than 0.0 V_RHE migrate to the photoelectrode surface and reduce protons to hydrogen, typically facilitated by a hydrogen evolution cocatalyst (HEC):

$$4H^{+}+4e_{CB}^{-}\to 2H_2 :HER$$

(2)

Simultaneously, holes in the VB (h_VB⁺) with potentials more positive than 1.23 V_RHE can oxidize water to oxygen, typically facilitated by an oxygen evolution cocatalyst (OEC):

$$2H_{2}O+4h_{VB}^{+}\to O_2+4H^{+}:OER$$

(3)

Overall, water splitting proceeds as

$$2H_{2}O\to 2H_2+O_2, ∆G^{°}=238 \mathrm{k}\mathrm{J} {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$$

(4)

To theoretically split water, a semiconductor must satisfy two criteria: (i) a bandgap energy (E_g) exceeding 1.23 eV, and (ii) a CB edge more negative than 0.0 V_RHE and a VB edge more positive than 1.23 V_RHE [44]. In practice, overpotentials associated with the hydrogen and oxygen evolution reactions—approximately 0.05 V for HER and 0.25 V for OER—require a larger effective bandgap (>1.5 eV) to drive the reactions efficiently.

Hydrogen production efficiency is often limited by electron–hole recombination, which reduces the availability of charge carriers for water splitting reactions. PEC cells employ a dual-electrode configuration, producing O₂ at the anode and H₂ at the cathode [45]. This spatial separation of oxidation and reduction reactions enhances energy conversion efficiency, improves operational stability, and reduces recombination losses compared to conventional photocatalytic systems using dispersed semiconductor powders. By maintaining distinct reaction sites, PEC cells optimize light absorption and charge carrier dynamics, enabling more reliable hydrogen production.

Figure 1 illustrates that when the semiconductor absorbs light, electron–hole pairs are generated in the CB and VB, respectively. Due to the band bending at the semiconductor–electrolyte interface, holes migrate toward the surface, where they initiate the OER, oxidizing water. Concurrently, the photogenerated electrons are transported through a transparent conducting substrate, such as fluorine-doped tin oxide (FTO), and collected into an external circuit. An applied potential (E_app) is often used to provide the necessary energetic boost for the electrons, enabling them to travel to the metallic cathode. At the cathode, these electrons drive the hydrogen evolution reaction (HER), leading to hydrogen production. This synchronized process of water oxidation and reduction effectively converts light energy into chemical energy.

Figure 1. (a) Schematic of PEC cell in two-electrode configuration. Graphic visualization of PEC energy diagrams for PEC water splitting using (b) a photoanode; (c) a photocathode. Reprinted with permission from Ref. [46], Copyright © 2022, ECS Advances.

Figure 1. (a) Schematic of PEC cell in two-electrode configuration. Graphic visualization of PEC energy diagrams for PEC water splitting using (b) a photoanode; (c) a photocathode. Reprinted with permission from Ref. [46], Copyright © 2022, ECS Advances.

This bias voltage is generally supplied by a photovoltaic (PV) cell positioned behind the photoanode, which captures any transmitted photons and converts them into electrical power. In this tandem configuration, the PV device enhances the overall efficiency of the PEC system. The primary goal of this design is to achieve effective and sustainable hydrogen generation via water splitting.

To better evaluate the efficiency and intrinsic limitations of charge transfer in photoelectrodes, a well-established experimental methodology allows for the quantification of key performance parameters, including the photocurrent density (J_ph), absorbed photon flux (J_abs), and the rates of surface recombination (J_sr) and bulk recombination (J_br) [17]. Photogenerated charge carriers on a photoanode face two competing fates: they can either contribute to the desired photocurrent or undergo recombination, which constitutes a major energy loss pathway. Surface recombination occurs when charges recombine at the interface before reacting with the electrolyte, whereas bulk recombination refers to the loss of carriers within the semiconductor material. The balance between these two pathways is a critical factor determining the overall performance and efficiency of the photoanode in solar-driven water splitting.

When highly reactive species, such as hydrogen peroxide (H₂O₂) or sodium sulfite (Na₂SO₃), are introduced as sacrificial reagents, the surface recombination rate is significantly mitigated. Due to their strong tendency to undergo oxidation, these species provide kinetically favorable pathways for hole transfer to the electrolyte. Under such conditions, photogenerated holes are efficiently consumed in oxidation reactions, preventing their recombination with electrons at the photoanode surface.

By analyzing these recombination pathways, researchers can gain insights into the key factors limiting charge carrier dynamics, which in turn determine the overall efficiency of a photoelectrode. During the water oxidation reaction, the net photocurrent—denoted as J_H2O—reflects the portion of absorbed photon flux (J_abs) that successfully contributes to oxygen evolution after accounting for losses due to surface and bulk recombination:

$$J_{H_{2}O}=J_{abs}-(J_{sr}+J_{br})$$

(5)

The establishment of a semiconductor–electrolyte junction, commonly referred to as a semiconductor–liquid (SCL) junction, is a crucial process in PEC cells featuring photoelectrodes without a buried junction. The SCL junction facilitates the separation of photogenerated electron–hole pairs at the interface, directing electrons toward the external circuit and holes toward the electrolyte. Unlike buried junctions formed by stacking two semiconductors, the SCL junction operates directly at the semiconductor–electrolyte boundary, presenting unique challenges and opportunities for optimizing PEC efficiency and stability.

3. Synthesis Methods of BiVO₄ Photoelectrodes

The synthesis method plays a critical role in achieving high-efficiency photoelectrodes [16], as it directly determines the nature and concentration of defects within the material. These defects, in turn, influence the overall conductivity, its type, and the effectiveness of any applied modifications.

Table 1. Summary of materials, fabrication method, electrolyte condition, and PEC performance of BiVO₄-based photoanodes.

Materials	Fabrication Method	Annealing Condition (Temp., Time, Atmosphere)	Electrolyte Condition	Photocurrent Density @ 1.23 VRHE (mA/cm2)
Bare BiVO4 [47]	Electrodeposition BiOI + VO(acac)2 drop	450 °C, 2 h, air	0.1 M Na2SO4 (pH 6)	0.65
NiOOH/FeOOH/BiVO4 [48]	Electrodeposition BiOI + VO(acac)2 drop + cocatalysts deposition	450 °C, 2 h, air	0.1 M Na2SO4	~1.3
CdS/NiFe-LDH/BiVO4 [49]	BiOI + VO(acac)2 + CdS hydrothermal + NiFe-LDH deposition	450 °C, 2 h, air	0.5 M Na2SO4	3.10
BiVO4/NiO composite [50]	BiOI + VO(acac)2 conversion + NiO HT	450 °C, 2 h, air	0.5 M Na2SO4	1.20
CoV-LDH/Ag/BiVO4 [51]	BiOI + VO(acac)2→CoV-LDH/Ag coating	450 °C, 2 h, air	0.5 M Na2SO4 + 0.1 M glycerol	7.15
Bare BiVO4 [52]	BiOI electrodeposition + V2O5 electrodeposition + calcination	475 °C, 2 h, air	0.5 M potassium borate (pH 9.5)	2.2
Mo-BiVO4/TiO2/FeOOH [53]	BiOI + VO/(Mo dopant) + TiO2 + FeOOH	450 °C, 2 h, air	0.1 M Na2SO4	0.81
BiVO4/PbS QDs/ZnS [54]	BiOI + VO(acac)2→BiVO4 + PbS/ZnS (SILAR)	450 °C, 2 h, air	0.5 M KH2PO4 + 1.0 M Na2SO3	5.19

provides a summary of common synthetic techniques and the resulting morphologies of BiVO₄. The fabrication of BiVO₄ on substrates—primarily fluorine-doped tin oxide (FTO)—typically begins with the deposition of bismuth (Bi) and vanadium (V) precursors, which are subsequently converted in situ into a BiVO₄ film at temperatures ranging from 400 to 500 °C, either in air or under a controlled atmosphere.

Most BiVO₄ films produced by this method exhibit a monoclinic phase with relatively poor crystallinity, though some show characteristics of a tetragonal phase, as indicated by the weak splitting of the XRD peak around 2θ ≈ 19° [55]. Despite the low crystallinity, BiVO₄ possesses “defect tolerance” [2], which allows it to maintain efficient charge separation. As a result, crystallinity is not the primary factor determining the performance of BiVO₄ in PEC applications.

Figure 2 presents representative exemplars for selected synthesis routes discussed in Section 3.1,Section 3.2,Section 3.3,Section 3.4,Section 3.5; methods that are not depicted in Figure 2 are referenced textually

Mapping of panels to sections: Figure 2a→Section 3.1 (Hydrothermal), Figure 2b→Section 3.2 (Electrodeposition), Figure 2c→Section 3.3 (BiOI conversion), Figure 2d→Section 3.4 (Sol–gel). Methods in Section 3.5 (e.g., aerosol/ALD) are illustrated in Figure 2e,f.

3.1. Preparation of Nanoporous BiVO₄ Films

A representative hydrothermal morphology (nanoporous BiVO₄) is shown in Figure 2a, a variety of solution-based approaches—such as soaking, dip coating, and impregnation—have been extensively employed for the preparation of BiVO₄ photoanodes [56,57,58,59]. In the soaking method, the substrate is immersed in a precursor solution for an extended duration, allowing surface chemical reactions to occur; although straightforward, this technique often requires long processing times [56]. Dip-coating provides greater control by immersing the substrate in the solution followed by withdrawal at a regulated speed, enabling tunable film thickness through adjustment of the withdrawal rate [57]. Impregnation involves introducing the precursor solution into porous supports, which are subsequently dried and calcined, making this approach particularly effective for producing catalyst-supported BiVO₄ materials with high specific surface area [58,59].

3.2. Spin Coating, Drop-Casting, Spray Pyrolysis, and Electrospray Deposition

An electrodeposited BiOI precursor prior to conversion is illustrated in Figure 2; and thin-film deposition techniques—including spin coating, drop-casting, spray pyrolysis, and electrospray deposition (ESD)—have also been applied in BiVO₄ fabrication [20,60,61,62]. In spin coating, a small quantity of precursor solution is dispensed onto a rotating substrate, where centrifugal forces ensure the formation of a uniform thin film [60]. Drop-casting involves placing droplets of the precursor solution onto the substrate and allowing them to dry under ambient conditions; however, this method can result in film thickness variations [20]. Spray pyrolysis directs an aerosolized precursor onto a heated substrate, initiating pyrolysis and forming a continuous thin film [61]. Electrospray deposition (ESD) employs a high-voltage electric field to generate a fine mist of the precursor solution, enabling precise deposition of nanoparticles or uniform films onto the substrate surface [62].

Figure 2. Representative morphologies for selected BiVO₄ synthesis routes. (a) Automatic dip coating machine. Reprinted with permission from ref. [48], Copyright © MDPI, 2018. Schematic representation for the synthesis by (b) hydrothermal, reprinted with permission from ref. [63], Copyright © Royal Society of Chemistry, 2023; (c) spin coating, reprinted with permission from ref. [64], Copyright © MDPI, 2021; (d) mechanism of the in situ generation of NiFeO_x catalyst on BiVO₄, reprinted with permission from ref. [65], Copyright © 2020, Wiley-VCH GmbH; (e) ion exchange technique, reprinted with permission from ref. [66], Copyright © 2016, American Chemical Society; (f) electrophoretic deposition of BiVO₄, reprinted with permission from ref. [67], Copyright © 2021, Research square.

Figure 2. Representative morphologies for selected BiVO₄ synthesis routes. (a) Automatic dip coating machine. Reprinted with permission from ref. [48], Copyright © MDPI, 2018. Schematic representation for the synthesis by (b) hydrothermal, reprinted with permission from ref. [63], Copyright © Royal Society of Chemistry, 2023; (c) spin coating, reprinted with permission from ref. [64], Copyright © MDPI, 2021; (d) mechanism of the in situ generation of NiFeO_x catalyst on BiVO₄, reprinted with permission from ref. [65], Copyright © 2020, Wiley-VCH GmbH; (e) ion exchange technique, reprinted with permission from ref. [66], Copyright © 2016, American Chemical Society; (f) electrophoretic deposition of BiVO₄, reprinted with permission from ref. [67], Copyright © 2021, Research square.

3.3. Sol–Gel Method, Wet-Chemical Process, Hydrothermal, Solvothermal, SILAR

Cross-sectional features after BiOI→BiVO₄ conversion are shown in Figure 2c; enlarged views and phase verification are compiled; solution-based synthesis routes—including hydrothermal, solvothermal, wet-chemical, successive ionic layer adsorption and reaction (SILAR), and sol–gel processes—are widely utilized for fabricating nanostructured BiVO₄ [68,69,70,71,72]. Hydrothermal synthesis, conducted in an aqueous medium under elevated temperature and pressure, enables the growth of highly crystalline BiVO₄ [68]. Solvothermal synthesis, analogous to hydrothermal synthesis but carried out in organic solvents, provides additional versatility in tailoring particle size and morphology [69]. Wet-chemical methods encompass various liquid-phase synthesis techniques, offering flexibility in adjusting material composition and properties [70]. The SILAR technique, a layer-by-layer deposition process, is effective for thin-film growth under ambient conditions, making it a cost-efficient route for large-scale fabrication [71]. The sol–gel method involves generating a sol from metal precursors in solution, which subsequently transforms into a gel and undergoes heat treatment to yield solid BiVO₄, facilitating uniform nanostructure formation at low temperatures and scalability [72].

3.4. Chemical Vapor Deposition (CVD), Reactive Co-Sputtering, and Atomic Layer Deposition (ALD)

A dense, sol–gel-derived BiVO₄ layer is depicted in Figure 2d; roughness tuning vs. annealing temperature is summarized; post-synthesis modification techniques—such as annealing, plasma treatment, and surface grafting—are commonly employed to enhance the structural, electronic, and interfacial properties of BiVO₄ thin films [73,74,75,76]. Annealing at elevated temperatures improves crystallinity, reduces structural defects, and enhances charge transport [73]. Plasma treatment introduces reactive species to the film surface, enabling controlled modification of surface chemistry and improving catalytic activity, wettability, or interfacial charge transfer [74,75]. Surface grafting involves covalently attaching functional molecules to the BiVO₄ surface, allowing tailored chemical functionalities that enhance PEC performance, stability, or compatibility with cocatalysts [76].

3.5. In Situ Coordination, Ion Exchange Technique, and Grafting

As shown in Figure 2e,f, Chemical modification strategies—including in situ coordination, ion exchange, and surface grafting—allow precise tailoring of BiVO₄ composition and structure during or after synthesis [77,78,79,80,81,82]. In situ coordination facilitates controlled formation of coordination complexes during synthesis, influencing crystal structure, morphology, and optoelectronic properties [77]. Ion exchange techniques substitute selected ions within the BiVO₄ lattice, enabling precise doping or compositional tuning to enhance optical absorption and catalytic activity [78]. Grafting, as a post-synthesis surface functionalization, allows covalent attachment of molecular species or functional groups to the BiVO₄ surface, improving surface reactivity, stability, or compatibility with cocatalysts [79].

Electrochemical fabrication methods—including electrodeposition and photoelectrophoretic deposition (PEPD)—have also been successfully applied for BiVO₄ photoanodes [80,81,82]. Electrodeposition uses an externally applied current to deposit BiVO₄ from a precursor solution onto a conductive substrate, providing precise control over film thickness, morphology, and stoichiometry by tuning deposition parameters [80,81]. PEPD employs a photoinduced electric field to drive nanoparticle deposition onto the substrate, forming uniform, size-controlled nanoparticle-based BiVO₄ films with homogeneous distribution [82].

4. Key Factors Influencing the PEC Performance of the BIVO₄ Photoanode

The photocurrent density (J_PEC) is a fundamental parameter for assessing the PEC performance of a photoelectrode. For a BiVO₄ photoanode, J_PEC can be calculated according to the following equation [31]:

$$J_{PEC}=J_{max}\times LHE\times {\eta }_{trans}\times {\eta }_{sep}$$

(6)

$$A\mathrm{r}\mathrm{r}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{i}\mathrm{u}\mathrm{s} \mathrm{e}\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:\mathrm{k}=\mathrm{A}\mathrm{e}\mathrm{x}\mathrm{p}(-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{A}\mathrm{O}\mathrm{P}}}{\mathrm{R}\mathrm{T}}})$$

(7)

In Equation (6), J_max represents the theoretical maximum photocurrent density attainable by BiVO₄, which has been reported to be approximately 7.5 mA/cm². According to this relationship, the PEC efficiency of BiVO₄-based photoanodes can be significantly improved by enhancing the light-harvesting efficiency (LHE), charge separation efficiency η_sep, and charge transport efficiency η_trans). Optimization of one or more of these factors—either individually or synergistically—is crucial for maximizing the overall performance of the photoanode in solar water splitting applications [83].

The rapid advancement of BiVO₄-based photoanodes has been driven by various modification strategies, including elemental doping, post-treatment methods, incorporation of oxygen evolution catalysts (OECs), and integration of electron transport layers (ETLs) or hole transport layers (HTLs), in addition to nanostructuring techniques, as summarized in

Table 2. Summary of modification strategies for enhancing the PEC performance of BiVO₄-based photoanodes.

Techniques	Materials/Structures/Methods	Functionality
1. Synthesis and Film Morphology Engineering	Electrodeposition [84] Hydrothermal synthesis [85] Hollow nanospheres [86] Metal–organic decomposition [86] RF sputtering [87]	(1) Controls morphology and film thickness. (2) Increases surface area. (3) Improves light absorption and bulk charge transport (ηbulk). (4) Precisely controls composition and film density. (5) Enables conformal coating on complex nanostructures.
2. Doping	Mo, W [88]. Mo and W [89]. Ti, Zr [90]	(1) Increases bulk conductivity. (2) Tunes the bandgap. (3) Reduces bulk recombination.
3. Surface and Cocatalyst Modification	FeOOH, NiOOH [20] NiOOH thin layer [91] RuO2, IrO2 [92] CoPi [93]	(1) Lowers OER overpotential. (2) Improves surface reaction kinetics (ηsurf). (3) Increases chemical stability. (4) Accelerates oxygen evolution reaction. (5) Enhances charge transfer efficiency at the surface.
4. Heterojunction Engineering	WO3 [94] Cu2O [95] BiVO4/MoS2 [96] BiVO4/CdS/TiO2 [97] BiVO4/Ag NPs [98] PbS Quantum Dots [54] NiOx [99]	(1) Increases electron–hole separation efficiency. (2) Improves quantum efficiency. (3) Enhances light absorption. (4) Boosts performance via surface resonance effect. (5) Provides a direct pathway for hole extraction. (6) Expands absorption into the near-infrared and enhances energy conversion efficiency.
5. Post-treatment and Passivation	Plasma (Ar, O2) [100] Controlled annealing [101] Strain engineering [102] Small organic compounds [101,102] ALD Al2O3/TiO2 [103]	(1) Passivates surface defects. (2) Improves crystallinity. (3) Tunes electronic band structure. (4) Increases stability and reduces recombination. (5) Creates a protective layer against photocorrosion.
6. Other Treatment Methods	Antireflection coatings [98] Controlled defect creation [104] Laser-sintering [105]	(1) Enhances photon absorption. (2) Improves electrical conductivity. (3) Promotes desired defect states for enhanced activity.

. The effectiveness of these strategies is multifaceted; for instance, ETLs not only reduce charge recombination at the interface between BiVO₄ and ETL, as well as within the bulk of the photoelectrode, but they also significantly enhance surface charge transfer. This improvement arises from the influence of the underlying ETL on the deposition and structural characteristics of the overlying BiVO₄ layer.

As shown in Figure 3, Park et al. developed a heterostructured photoanode by decorating anodic WO₃ “nanocoral” arrays with ultrafine BiVO₄ nanoparticles (~10 nm) via spin coating. This linked configuration established an efficient electron transfer pathway while preserving the nanocoral morphology. Under simulated solar illumination, the BiVO₄/WO₃ nanocoral electrode exhibited a photocurrent density of approximately 2.4 times that of bare WO₃. More notably, incident photon-to-current conversion efficiency (IPCE) at 410 nm increased by a factor of 8.3, indicating substantially enhanced light absorption and charge carrier utilization. This study demonstrates that nanoscale heterojunction engineering combined with optimized morphology control can synergistically improve both optical harvesting and interfacial charge dynamics in PEC systems [106].

Figure 3. PEC performances of the WO₃ nanocorals and BiVO₄/WO₃ (BW) photoanodes of 5-BW, 10-BW, and 30-BW: (a) photocurrent densities by linear sweep voltammetry (LSV), (b) Nyquist plots and equivalent circuits, (c) PEC H₂ production diagram as a function of time, and (d) IPCE spectra. Reprinted with permission from Ref. [106], Copyright © 2024, Royal Society of Chemistry.

Figure 3. PEC performances of the WO₃ nanocorals and BiVO₄/WO₃ (BW) photoanodes of 5-BW, 10-BW, and 30-BW: (a) photocurrent densities by linear sweep voltammetry (LSV), (b) Nyquist plots and equivalent circuits, (c) PEC H₂ production diagram as a function of time, and (d) IPCE spectra. Reprinted with permission from Ref. [106], Copyright © 2024, Royal Society of Chemistry.

4.1. Introducing Extrinsic/Intrinsic Defects Through Doping

Doping plays a critical role in enhancing the performance of BiVO₄ photoelectrodes by modulating their electronic and chemical properties. A key strategy for improving PEC efficiency involves either the introduction of intrinsic defects, such as oxygen vacancies, or the incorporation of foreign metal ions into the crystal lattice. Recent studies have provided valuable insights into different approaches for defect engineering, highlighting how such modifications can influence charge separation and transport processes.

Kong et al. demonstrated that Ni doping in BiVO₄, achieved via electrodeposition, significantly enhances PEC performance through defect engineering. Ni-doped BiVO₄ samples were categorized according to Ni content. The nanoparticle size decreased with increasing Ni content: 500 nm for pristine BiVO₄, 440 nm for 3-Ni-BiVO₄, 310 nm for 5-Ni-BiVO₄, and 200 nm for 10-Ni-BiVO₄. Controlled Ni incorporation promotes the formation of oxygen vacancies and surface V⁴⁺ species, which simultaneously improve charge separation and catalytic activity at the electrode–electrolyte interface. These synergistic effects effectively suppress recombination and accelerate charge transport. The morphology of Ni-doped BiVO₄ are shown in Figure 4a [107]. As a result, the optimized 5-Ni-BiVO₄ exhibited a photocurrent density of 2.39 mA/cm² at 1.23 V_RHE, approximately 2.5 times higher than pristine BiVO₄ (0.94 mA/cm²), along with an IPCE of 45% (400–450 nm) and an ABPE of 0.55%, as shown is Figure 4b. In contrast, excessive Ni in 10-Ni-BiVO₄ introduced additional recombination centers, highlighting the importance of finely tuning oxygen vacancy concentrations to achieve optimal PEC efficiency [107].

Figure 4. (a) SEM images of Ni-doped BiVO₄. (b) J–V curves of pure BiVO₄ and Ni-doped BiVO₄. Reprinted with permission from Ref. [107], Copyright © 2024, Springer Nature. (c) Schematic illustration of photoanode fabrication process (d) HR-TEM images of Ba:BiVO₄/HfO₂/NiPt. (e) J–V curves of pure BiVO₄ and Ni-doped BiVO₄. Reprinted with permission from Ref. [108], Copyright © 2024, Wiley-VCH GmbH.

Figure 4. (a) SEM images of Ni-doped BiVO₄. (b) J–V curves of pure BiVO₄ and Ni-doped BiVO₄. Reprinted with permission from Ref. [107], Copyright © 2024, Springer Nature. (c) Schematic illustration of photoanode fabrication process (d) HR-TEM images of Ba:BiVO₄/HfO₂/NiPt. (e) J–V curves of pure BiVO₄ and Ni-doped BiVO₄. Reprinted with permission from Ref. [108], Copyright © 2024, Wiley-VCH GmbH.

Shin et al. employed a synergistic combination of Ba doping, HfO₂ passivation, and NiPt alloy cocatalyst decoration to enhance charge transport and interfacial kinetics. Ba doping into the BiVO₄ lattice introduced beneficial defect states, improving carrier separation. A thin HfO₂ layer, deposited via atomic layer deposition (ALD), served as a surface passivation layer, suppressing surface recombination and stabilizing the electrode, as shown in Figure 4c. High-resolution transmission electron microscopy (HR-TEM) confirmed successful dopant incorporation into the BiVO₄ lattice and the formation of well-defined interfaces. Lattice fringes with an interplanar spacing of 0.31 nm corresponded to the (121) plane of highly crystallized BiVO₄, whereas the top-side fringes with a spacing of 0.19 nm were attributed to the Ni (111) plane, indicating intimate contact between the cocatalyst and the photoanode surface. This multi-step surface modification strategy led to a substantial improvement in PEC performance, achieving a photocurrent density of 3.1 mA/cm² at 1.23 V_RHE under AM 1.5G illumination. These results highlight the effectiveness of combining electronic structure tuning with interfacial engineering to overcome key limitations of BiVO₄, in particular, poor surface kinetics and charge recombination. The HR-TEM images and photocurrent curves validating the synergistic effect of Ba-doping, HfO₂ passivation, and NiPt decoration are presented in Figure 4d,e [108].

In conclusion, these studies collectively demonstrate that defect engineering through doping, when coupled with meticulous interfacial engineering, constitutes a powerful approach to significantly enhance the PEC performance of BiVO₄ photoanodes.

4.2. Heterojunction with ETL (Electron Transport Layer)

Despite its favorable optical bandgap and stability under neutral pH conditions, BiVO₄ photoanodes suffer from poor charge separation and inefficient electron transport, which limit their practical PEC performance. Traditional strategies, such as embedding WO₃ as a buried electron transport layer, can enhance electron extraction but necessitate thick absorbers, conflicting with the inherently low charge mobility in BiVO₄.

Yu et al. addressed these limitations by applying an ultrathin Co-phthalocyanine (CoPc) surface layer to the BiVO₄ photoanode, effectively tuning its surface hydrophilicity. XRD confirmed the formation of monoclinic BiVO₄ on FTO without observable CoPc peaks, likely due to its low loading and high dispersion, while Raman spectra revealed characteristic C–C and C–N vibrational bands of CoPc, verifying its successful incorporation and stronger interfacial coupling in solvothermally treated samples. The successful incorporation of CoPc and the improved interfacial coupling are supported by the XRD and Raman results shown in Figure 5a,b, where characteristic peaks and morphological changes are clearly observed. This interface modification improved interaction with the electrolyte and suppressed surface recombination, resulting in a photocurrent density of 4.0 mA/cm² at 1.23 V_RHE—approximately 3.1 times higher than that of pristine BiVO₄—along with enhanced IPCE and operational stability. The ultrathin interface layer overcomes the drawbacks of thick buried ETLs, providing an elegant solution for performance improvement without compromising charge mobility [109].

Figure 5. (a) XRD patterns; (b) Raman spectra and SEM images of the BVO@CoPc-S photoanodes. Reprinted with permission from Ref. [109], Copyright © 2025, Royal Society of Chemistry; (c) SEM image of Mo:BiVO₄, (d) XRD of Mo:BiVO₄ and CPF-TCB/Mo:BiVO₄; (e) schematic illustration of the electropolymerization of CPF-TCB on Mo:BiVO₄. Reprinted with permission from Ref. [110], Copyright © 2024, Royal Society of Chemistry.

Figure 5. (a) XRD patterns; (b) Raman spectra and SEM images of the BVO@CoPc-S photoanodes. Reprinted with permission from Ref. [109], Copyright © 2025, Royal Society of Chemistry; (c) SEM image of Mo:BiVO₄, (d) XRD of Mo:BiVO₄ and CPF-TCB/Mo:BiVO₄; (e) schematic illustration of the electropolymerization of CPF-TCB on Mo:BiVO₄. Reprinted with permission from Ref. [110], Copyright © 2024, Royal Society of Chemistry.

Furthermore, Yang et al. introduced an organic–inorganic heterojunction by conformally coating a conjugated polycarbazole framework (CPF-TCB) onto nanoporous Mo-doped BiVO₄. The morphology, XRD patterns, and schematic representation of CPF-TCB deposition on Mo:BiVO₄ are presented in Figure 5c–e, highlighting the conformal coverage and structural features of the hybrid interface. This hole transport layer (HTL) forms a type-II band alignment with BiVO₄, facilitating efficient hole extraction and suppressing recombination. After loading a NiFeCoO_x cocatalyst, the photoanode achieved a record-high water oxidation photocurrent density of 6.66 mA/cm² at 1.23 V_RHE. When integrated into unassisted tandem PEC devices, it delivered solar-to-hydrogen conversion efficiencies of up to 9.02%, setting benchmarks for organic–inorganic hybrid photoelectrodes under operational conditions [110].

Overall, the integration of an external ETL via the carbon shell provides a powerful and scalable strategy to overcome the intrinsic charge transport limitations of oxide semiconductors, offering promising avenues for the development of high efficiency and durable PEC energy conversion systems.

4.3. Hole-Transport Layers (HTLs) for BiVO₄ Photoanodes

Beyond electron transport layers, hole transport layers (HTLs) play a critical role in enhancing the surface charge extraction of BiVO₄. By creating favorable band offsets for hole transfer, passivating surface trap states that promote interfacial recombination, and offering abundant well-dispersed oxygen evolution sites when coupled with cocatalysts, HTLs directly improve the charge transfer efficiency (η_trans) under water oxidation conditions [111,112,113,114].

Cui et al. demonstrated that inserting a partially oxidized 2D bismuthene layer between BiVO₄ and NiFeOOH enhanced interfacial band bending, passivated V_O-related traps, and extended hole lifetime. The interfacial hole transport mechanism and performance enhancement enabled by such interlayers are schematically illustrated in Figure 6a,b. This strategy yielded a 5.8-fold increase in photocurrent compared to bare BiVO₄, achieving 3.4 ± 0.2 mA/cm² at +0.8 V_RHE and 4.7 ± 0.2 mA/cm² at +1.23 V_RHE, with stable operation under illumination [111].

Figure 6. (a) Schematic illustration of interfacial hole transport in bare BiVO₄ photoanode and (b) interfacial hole transport pathway in NiFeV/B-BiVO₄ photoanode. Reprinted with permission from Ref. [113], Copyright © 2021, Royal Society of Chemistry. (c) Photocurrent density stability of photoanodes at 0.8 V_RHE. Reprinted with permission from Ref. [111], Copyright © 2022, Wiley-VCH GmbH. (d) LSV curves of BiVO₄, B-BiVO₄, NiFeV/BiVO₄, and NiFeV/B-BiVO₄ photoanodes under AM 1.5G illumination in 1.0 M potassium borate buffer (pH 9.3, scan rate: 10 mV/s). Reprinted with permission from Ref. [113], Copyright © 2021, Royal Society of Chemistry.

Figure 6. (a) Schematic illustration of interfacial hole transport in bare BiVO₄ photoanode and (b) interfacial hole transport pathway in NiFeV/B-BiVO₄ photoanode. Reprinted with permission from Ref. [113], Copyright © 2021, Royal Society of Chemistry. (c) Photocurrent density stability of photoanodes at 0.8 V_RHE. Reprinted with permission from Ref. [111], Copyright © 2022, Wiley-VCH GmbH. (d) LSV curves of BiVO₄, B-BiVO₄, NiFeV/BiVO₄, and NiFeV/B-BiVO₄ photoanodes under AM 1.5G illumination in 1.0 M potassium borate buffer (pH 9.3, scan rate: 10 mV/s). Reprinted with permission from Ref. [113], Copyright © 2021, Royal Society of Chemistry.

In another approach, Li et al. employed a serial HTL architecture by sequentially depositing Fe₂O₃ (facilitating bulk-to-surface hole transfer) and NiOOH/FeOOH (promoting surface-to-electrolyte transfer) on BiVO₄. The photocurrent stability and LSV curves of these stepwise HTLs are shown in Figure 6c,d, confirming the substantial improvements in charge transfer efficiency. The resulting photoanode delivered 2.24 mA/cm² at 1.23 V_RHE (≈2.95× over pristine BiVO₄). At the same potential, η_bulk and η_surface were enhanced by 1.63× and 2.62×, respectively, demonstrating stepwise improvements in charge transfer. When coupled with a commercial Si photovoltaic for self-biasing, the device sustained 2.60 mA/cm², corresponding to a solar-to-hydrogen (STH) efficiency of 3.2%, underscoring the practicality of sequential HTLs [112].

Expanding this concept, Meng et al. reported that NiFeV-LDH deposited on borate-treated BiVO₄ (B-BiVO₄) served as an interfacial hole transport/storage layer. The optimized NiFeV/B-BiVO₄ photoanode achieved 4.6 mA/cm² at 1.23 V_RHE with an early onset potential of 0.2 V_RHE, while retaining ≥80% of the initial photocurrent after 24 h and maintaining ~95% faradaic efficiency. Mechanistic studies revealed a substantial increase in Z_surface (~90% vs. 31% for bare BiVO₄) and reduced charge transfer resistance (R_ct), confirming accelerated interfacial hole transfer upon LDH integration [113].

Collectively, these studies highlight that rational HTL design—from 2D bismuthene insertion to serial inorganic HTLs and LDH-based transport/storage layers—offers complementary pathways to lower interfacial barriers, enhance band bending, and provide robust scaffolds for cocatalyst integration, thereby substantially boosting both activity and stability of BiVO₄ photoanodes.

4.4. Cocatalyst and Surface Modification Strategies for BiVO₄ Photoanodes

A wide range of cocatalyst deposition and surface modification strategies have been developed to improve the PEC performance of BiVO₄ photoanodes. These approaches primarily aim to accelerate OER kinetics, passivate surface trap states, and suppress interfacial charge recombination, thereby enhancing both photocurrent density and operational stability.

Table 3. Summary of representative cocatalyst and surface overlayer strategies for enhancing the PEC performance of BiVO₄ photoanodes.

Structure/Strategy	Cocatalyst or Overlayer	Performance Highlight	Key Effect	Ref.
One-step PEC deposition	FeOOH (inner) + Co–Sil (outer)	6.10 mA/cm2 @1.23 V	Dual-layer cocatalyst boosts charge separation and reduces recombination	[115]
Fluoride-assisted in situ passivation	F− ions	Long-term stability >100 h @0.6 V	Surface passivation and cocatalyst reactivation	[116]
Porphyrin-based surface ligand	Co–He (Co–O–V linkage)	5.3 mA/cm2 @1.23 V, Von = 0.07 V	Low overpotential and efficient hole transfer	[117]
Organic ligand modification	Co2+ + BTC ligand	4.82 mA/cm2, onset 0.22 V	Surface passivation + cocatalyst anchoring	[118]
Magnetic overlayer	Co-doped Fe3O4	1.9× higher OER activity, Faradaic efficiency >85%	Improves surface kinetics and protects BiVO4	[119]
Dual cocatalyst immersion	FeOOH + Co(OH)2	2.56 mA/cm2, 71.6% retention (10 h)	Synergistic catalytic enhancement + stability	[120]
Bilayer MOF cocatalyst	Fe-MOF/Ni-MOF	1.80 mA/cm2, V_on dropped from 0.9 V to 0.69 V	Facilitated interfacial charge transfer	[121]
Room-temp photodeposition	Co-Pi, Ni-Bi, Mn-Pi/BiVO4	Hole transfer efficiency up to 94.5 % @1.23 VRHE	Conformal, uniform cocatalyst deposition	[122]
Facet-selective cocatalyst loading	Selective facet-modified MnOx	0.74 mA/cm2 @1.23 VRHE	Maximize catalytic activity by crystal facet control	[123]
In situ solvothermal growth	COF–Azo	1.38 mA/cm2 @1.23 VRHE	Improves carrier separation, lowers impedance, and accelerates OER	[124]
Bulk Mo doping + surface molecular catalyst deposition	CoPOM	4.32 mA/cm2 @1.23 VRHE	Conductivity enhancement + catalytic activation	[125]

summarizes representative strategies such as dual-layer deposition, ligand-assisted passivation, and facet-selective cocatalyst loading, highlighting their impact on photocurrent density, onset potential, Faradaic efficiency, and durability.

Among these strategies, Dong et al. reported a dual modification approach of coupling CdS nanoparticles with NiFe-LDH nanosheets on BiVO₄. XRD confirmed the monoclinic scheelite phase of BiVO₄, while the absence of distinct CdS and NiFe-LDH peaks was attributed to the low CdS loading and amorphous nature of NiFe-LDH. XPS verified the coexistence of CdS and NiFe-LDH through Ni²⁺ and Fe³⁺ peaks, and SEM/HRTEM revealed a defect-rich amorphous nanosheet structure providing abundant active sites. The CdS nanoparticles broadened the light absorption range and facilitated charge transfer, while the NiFe-LDH overlayer functioned as a cocatalyst to accelerate OER kinetics and passivate surface traps. This synergistic interface engineering significantly improved charge separation and transport properties, resulting in a photocurrent density of 3.1 mA cm⁻² at 1.23 V_RHE—about 5.8 times higher than bare BiVO₄—and excellent stability under continuous illumination. The enhanced photocurrent density, crystallographic features, and XPS confirmation of the cocatalyst loading are shown in Figure 7a–d [49].

Figure 7. (a) LSV curves and (b) XRD patterns of NiFe-LDH/CdS/BiVO₄, NiFe-LDH/BiVO₄, CdS/BiVO₄, and pristine BiVO₄ photoanodes measured in 0.5 M Na₂SO₄ electrolyte (pH 6.1) at 1.23 V_RHE. (c,d) XPS spectra of Ni 2p and Fe 2p, respectively. Reprinted with permission from Ref. [49], Copyright © 2024, MDPI. (e) LSV and OER polarization curves of the photoanodes measured without illumination Reprinted with permission from Ref. [124], Copyright © 2024, Royal Society of Chemistry. (f) LSV curves recorded under intermittent AM 1.5 G illumination in 0.5 M borate buffer (pH 9.0) with a cathodic scan rate of 10 mV/s. Reprinted with permission from Ref. [125], Copyright © 2024, Royal Society of Chemistry.

Figure 7. (a) LSV curves and (b) XRD patterns of NiFe-LDH/CdS/BiVO₄, NiFe-LDH/BiVO₄, CdS/BiVO₄, and pristine BiVO₄ photoanodes measured in 0.5 M Na₂SO₄ electrolyte (pH 6.1) at 1.23 V_RHE. (c,d) XPS spectra of Ni 2p and Fe 2p, respectively. Reprinted with permission from Ref. [49], Copyright © 2024, MDPI. (e) LSV and OER polarization curves of the photoanodes measured without illumination Reprinted with permission from Ref. [124], Copyright © 2024, Royal Society of Chemistry. (f) LSV curves recorded under intermittent AM 1.5 G illumination in 0.5 M borate buffer (pH 9.0) with a cathodic scan rate of 10 mV/s. Reprinted with permission from Ref. [125], Copyright © 2024, Royal Society of Chemistry.

Beyond inorganic systems, organic framework materials such as MOFs and Covalent organic frameworks (COFs) offer a molecularly tunable platform with intrinsically high surface area and porosity, enabling precise control over catalytic site density and efficient mass transport. Guo et al. reported the in situ growth of COF–Azo and COF–Ben on BiVO₄, yielding hybrids with reduced charge transfer resistance, lower onset potential, and higher photocurrent densities compared to pristine BiVO₄. The improved OER activity of the COF–Azo modified electrode is evidenced by the polarization and LSV curves in Figure 7e. EIS confirmed enhanced interfacial kinetics, and the intimate COF–BiVO₄ heterojunction facilitated efficient hole extraction and suppressed recombination, leading to markedly improved OER activity [124]. Similarly, Feng et al. demonstrated that combining Mo doping with cobalt polyoxometalate (CoPOM) deposition effectively enhanced bulk charge transport, lowered OER overpotential, and delivered significantly higher photocurrent densities with improved operational stability. The photocurrent enhancement and stability of the CoPOM-modified BiVO₄ are illustrated in Figure 7f [125]. Collectively, these results demonstrate that both inorganic cocatalysts/overlayers and MOF/COF-based organic frameworks provide complementary strategies to overcome sluggish surface kinetics and interfacial recombination in BiVO₄ photoanodes, ultimately enabling high efficiency and durable PEC water splitting devices.

4.5. Light Absorption Efficiency

BiVO₄ possesses a bandgap of approximately 2.4 eV, enabling the absorption of photons with wavelengths ranging from 300 to 520 nm [126]. Under AM 1.5G solar illumination, the theoretical maximum photocurrent density (J_max) is 7.5 mA/cm² [20]. In practice, however, optical losses due to light refraction and reflection reduce effective light absorption, yielding an actual theoretical photocurrent density of only 4.45 mA/cm² [127]. This discrepancy highlights that light absorption efficiency is a critical factor limiting the PEC performance of BiVO₄-based photoanodes.

Enhancing light absorption is, therefore, essential for improving PEC activity. Effective strategies include constructing heterostructures and integrating optical elements into the BiVO₄ architecture, both of which can strengthen photon–matter interactions [16,128]. These approaches increase photon utilization and, consequently, improve the overall performance of the photoanode.

4.5.1. Band Edge Engineering

Reducing the bandgap of semiconductor photocatalysts is an established strategy to broaden their light absorption spectrum, potentially extending it into the near-infrared region. Among various approaches, defect engineering via doping is particularly effective for BiVO₄, as it introduces defect levels within the band structure, modifies the electronic configuration, shifts the band edges, and enhances light absorption. Careful control of dopants and defect states allows the bandgap to be narrowed without compromising charge transport, resulting in improved PEC performance.

Fan Feng et al. systematically investigated the synergistic effects of Mo doping and surface modification with a cobalt polyoxometalate (CoPOM) on BiVO₄ photoanodes. UV–Vis absorption spectra showed similar absorption onsets of around 515 nm for all samples, and Tauc plot analysis indicated bandgap energies of ~2.56 eV, confirming that Mo doping and CoPOM deposition do not alter the fundamental absorption edge or introduce significant parasitic light absorption. This suggests that the observed performance enhancement originates primarily from improved charge transport and interfacial catalytic activity rather than changes in light-harvesting properties. Photocurrent–potential curves showed that Mo doping alone increased the photocurrent from ~0.65 to ~2.53 mA/cm² at 1.23 V_RHE, while the combination of Mo–BiVO₄/CoPOM further enhanced it to ~4.32 mA/cm². The absorption spectra, Tauc plots, and density of states analysis supporting these results are shown in Figure 8a–c. As a result, this represents a ~6.6-fold improvement over pristine BiVO₄ and ~1.7-fold compared to Mo-doped BiVO₄ alone. The enhancement is attributed to improved conductivity through Mo doping and accelerated water oxidation kinetics via CoPOM catalysis. Electrochemical impedance spectroscopy (EIS) confirmed a significant reduction in charge transfer resistance, while the optical appearance of the electrodes validated enhanced visible light response [125].

Similarly, Elnagar et al. demonstrated that Mo-doped BiVO₄ modified with a CoPOM surface layer exhibited a slight bandgap narrowing and extended light absorption into the longer-wavelength visible region [129]. The UV–Vis absorption spectra and Tauc plots confirming this bandgap narrowing are presented in Figure 8d,e. This, combined with surface modification, resulted in synergistic improvement in PEC performance.

Furthermore, Zhou et al. reported that co-doping BiVO₄ with Mo and Ni effectively narrows the bandgap and enhances light-harvesting capability. UV–Vis spectra showed a red-shift in the absorption edge, corresponding to a bandgap reduction of approximately 0.15–0.20 eV compared to pristine BiVO₄. The co-doped photoanode exhibited a photocurrent density of ~3.6 mA/cm² at 1.23 V_RHE, over 40% higher than undoped BiVO₄. EIS analysis confirmed improved charge transfer resistance, and the color change in the electrodes reflected the enhanced visible light response. These results indicate that band-edge tuning via Mo–Ni co-doping effectively extends light absorption and boosts PEC activity [130].

These findings collectively confirm that band-edge engineering through doping and defect creation remains a promising and actively evolving strategy to enhance the PEC performance of BiVO₄ photoanodes in solar water splitting.

4.5.2. Constructing Heterojunction Structure

Constructing heterojunctions is an effective strategy to enhance the light-harvesting efficiency (LHE) of BiVO₄, suppress charge recombination, and accelerate OER kinetics. By coupling BiVO₄ with suitable semiconductors or cocatalysts, researchers have developed advanced heterostructures that significantly improve PEC water splitting performance.

Seo et al. demonstrated the effectiveness of a p–n heterojunction between BiVO₄ and PbS quantum dots (QDs), combined with a ZnS overlayer, for enhanced PEC hydrogen production [54]. Nanoporous BiVO₄ films were prepared via electrodeposition, followed by PbS QD sensitization using the successive ionic layer adsorption and reaction (SILAR) method. The PbS QDs extended light absorption into the near-infrared region and improved charge separation through the p–n junction interface. Subsequently, a conformal ZnS overlayer was deposited to passivate surface defects on the QDs, reducing non-radiative recombination.

As a result, the photocurrent density at 1.23 V_RHE increased from 2.92 mA/cm² for bare BiVO₄ to 4.88 mA/cm² after PbS QD sensitization, and further to 5.19 mA/cm² with the ZnS overlayer. The J–V and stability curves of these heterostructured electrodes are shown in Figure 9a,b, confirming the improved photocurrent and durability. EIS confirmed a substantial reduction in charge transfer resistance (R_ct), indicating enhanced surface kinetics. Stability tests over 2 h revealed retention rates of 91.24% for BiVO₄/PbS and 96.20% for BiVO₄/PbS/ZnS, emphasizing the role of passivation layers in maintaining operational durability. This study highlights that combining heterojunction engineering with surface passivation can simultaneously maximize efficiency and stability in BiVO₄-based PEC systems.

Similarly, Zhang et al. reported highly efficient BiVO₄ single-crystal nanosheets modified via phosphorus doping and selective Ag decoration. The synthetic route and structural modification strategy are schematically illustrated in Figure 9c. This dual modification generated oxygen vacancies and improved charge migration pathways, leading to a ~2.8-fold increase in methylene blue degradation compared to pristine BiVO₄ under visible light irradiation [131].

Li et al. fabricated nanostructured BiVO₄/Co-Pi heterostructures using a vanadium-infused interaction method. The incorporation of the Co-Pi cocatalyst facilitated efficient hole extraction and prolonged carrier lifetime, enabling a photocurrent density of 2.9 mA/cm² at 1.23 V_RHE under simulated solar illumination [47]. These results underscore the effectiveness of heterojunction construction and cocatalyst integration for enhancing the PEC performance of BiVO₄ photoanodes.

In another example, Gil-Rostra et al. developed ITO/WO₃/BiVO₄/CoPi multishell nanotube arrays using a soft-templating technique [132]. The intimate WO₃/BiVO₄ interface generated an internal electric field that enhanced charge separation, while the CoPi overlayer improved surface OER kinetics. In Figure 9d, LSV curves confirmed a substantial increase in photocurrent upon CoPi deposition, and the band diagram illustrates the overall charge transport pathway: photoholes generated in BiVO₄ migrate to the surface to drive OER, whereas photoexcited electrons are efficiently funneled through the WO₃ shell and ITO layer to the external circuit. This heterostructured architecture enables efficient light absorption within BiVO₄, while the nanoscale tubular design shortens the electron diffusion path, thereby minimizing ohmic losses and maximizing charge collection. The corresponding band diagram describing the fate of photogenerated carriers is presented in Figure 9e. The resulting hierarchical electrode exhibited outstanding stability and PEC performance under one-sun illumination.

These studies collectively demonstrate that combining heterojunction formation with surface modification strategies can simultaneously maximize optical absorption, charge separation, and operational stability of BiVO₄-based photoanodes, paving the way toward efficient solar fuel generation.

4.5.3. Optics-Based Elements

Tuning the morphological features of photocatalysts provides an effective strategy to modulate light scattering and reflection, thereby enhancing overall light absorption and improving catalytic performance [133]. Three-dimensional (3D) inverse opal architectures, in particular, have emerged as a powerful design due to their highly ordered macro–mesoporous networks and substantially increased specific surface areas [134].

A recent study reported the fabrication of a 3D core–shell inverse opal photoanode composed of FTO/TiO₂/BiVO₄, using a combination of atomic layer deposition and electrodeposition techniques [135]. This configuration creates a uniform opal framework that facilitates rapid electron transport and improved light management, enabling the photoanode to achieve a photocurrent density of 4.11 mA/cm² at 1.23 V_RHE, significantly outperforming comparable planar structures.

For example, Marinković et al. synthesized zircon-type BiVO₄ nanoparticles (2–8 nm) via a facile colloidal route. The nanosized BiVO₄ exhibited a tetragonal zircon structure with enhanced optical absorption and a significantly enlarged surface-to-volume ratio. These features led to superior photocatalytic degradation of methyl orange compared with commercial TiO₂, underscoring the potential of ultrafine BiVO₄ nanoparticles for efficient light harvesting and environmental remediation The photoluminescence spectra and photodegradation performance confirming these results are presented in Figure 10a,b [136].

Similarly, Zhao et al. demonstrated that controlling the crystal orientation of BiVO₄ thin films has a critical impact on PEC behavior. BiVO₄ films preferentially oriented along the [010] direction exhibited markedly improved carrier separation and mobility compared to [119]-oriented or randomly oriented samples. The synthesis process and characterization of these catalytic films are schematically illustrated in Figure 10c. As a result, the [010]-oriented BiVO₄ delivered a photocurrent density of 0.2 mA/cm² at 1.21 V_RHE, outperforming [119]-oriented (0.056 mA/cm²) and random films (0.11 mA/cm²) [137]. Despite these significant improvements in light absorption, optimizing light utilization remains a key challenge in advancing the performance of BiVO₄ photoanodes.

4.6. Photogenerated Charge Separation Efficiency (η_sep)

The primary bottleneck in the PEC efficiency of BiVO₄ photoanodes is the rapid recombination of photogenerated electron–hole pairs. This limitation is further exacerbated by the low carrier mobility of BiVO₄, which is approximately 10⁻² cm²/V·s, resulting in substantial charge recombination before the charges can reach the electrode surface [138]. Enhancing charge migration is, therefore, essential to unlock the full PEC potential of BiVO₄.

To mitigate recombination, extensive strategies have been developed to improve carrier transport and separation. These include elemental doping to modulate carrier density and defect states, construction of heterojunctions to establish favorable band alignments, and interface engineering with cocatalysts to accelerate surface charge transfer. Additionally, defect engineering, such as introducing oxygen vacancies, can further promote charge separation by creating internal electric fields or shallow trap states that reduce recombination.

Representative examples are summarized in

Table 4. Summary of η_sep and PEC activity in modified BiVO₄ photoanodes. η_sep and η_inj extracted under AM 1.5G; all values measured at 1.23 V_RHE unless noted.

Photoanodes	JPEC @1.23 VRHE (mA/cm2)	ηsep (%)	ηinj (%)	Modification Method	Ref.
WO3/S:Bi2O3/(Ga,W):BiVO4/Co-Pi	5.10	N/R	N/R	Interface design	[140]
Co3O4/BiVO4	~2.3	N/R	N/R	Cocatalyst interface	[141]
Plasma-treated N-doped BiVO4	1.39	~4.6× higher vs. pristine	N/R	N doping + oxygen vacancies	[142]
Co:BiVO4/Mo:BiVO4	2.09	77.8	86.5	Homojunction (doped layers)	[139]
Zn:BiVO4/Mo:BiVO4	2.70	65.0	89.0	Homojunction	[143]
Ni-BiVO4/FeOOH	3.02	N/R	73.3	Homojunction (+OER overlayer)	[144]
BiVO4/SnO2 (heterostructure)	5.61	97	N/R	Heterojunction + cocatalyst	[145]
Ov-BiVO4 (VOx engineered)	6.29	94	96	VOx (oxygen vacancy-engineered)	[146]

, which highlights the PEC performance and η_sep of selected BiVO₄-based photoanodes. For instance, the introduction of a WO₃/S:Bi₂O₃/(Ga,W):BiVO₄/Co–Pi interface structure significantly improved photocurrent density through optimized charge transfer pathways [135], while oxygen vacancy-engineered BiVO₄ (Ov-BiVO₄) achieved an outstanding η_sep of 94% with a photocurrent density of 6.29 mA/cm² [139]. These results collectively demonstrate that tailored modifications in BiVO₄ can directly translate to higher charge separation efficiency and overall PEC activity.

Taken together, these findings underscore that η_sep serves as a crucial performance metric in evaluating BiVO₄-based photoanodes. Continued progress in tuning bulk properties, interface chemistry, and defect states has already shown remarkable improvements, but further breakthroughs will likely depend on synergistically integrating multiple strategies.

Therefore, improving the charge separation efficiency of BiVO₄ requires a comprehensive consideration of multiple strategies, including the utilization of internal electric fields and the modulation of conductivity and carrier density. The following section will examine these representative approaches in detail, with a focus on their underlying mechanisms and demonstrated performance.

4.6.1. Utilizing in Internal Electric Field

A practical approach to overcome the intrinsically low charge separation efficiency (η_sep) of BiVO₄ is the construction of heterojunctions that induce a built-in internal electric field (IEF). This IEF bends the energy bands at the interface, directing electrons and holes in opposite directions and thereby extending carrier lifetimes. As a result, the overall PEC water splitting performance is significantly enhanced.

For example, Bhat et al. constructed a triple planar SnO₂/WO₃/BiVO₄ heterojunction. The staggered (type-II) band alignment, along with the SnO₂ hole-blocking layer, facilitated interfacial charge separation and electron transport, yielding a photocurrent density of ~2.01 mA/cm² at 1.23 V_RHE under front illumination, with remarkably high IPCE values of ~90% (front) and ~80% (back) at 400 nm. The photocurrent response and impedance characteristics of this triple planar heterojunction are presented in Figure 11a,b. The authors attributed this enhancement to efficient interfacial separation and reduced recombination under front illumination, consistent with the effect of a built-in field across the multilayer junction [147].

Similarly, Liang et al. fabricated surface-dispersed WO₃/BiVO₄ heterojunction arrays (BiVO₄-nanoparticle@WO₃-nanoflake). Decorating WO₃ nanoflakes uniformly with ~20–50 nm BiVO₄ nanoparticles increased the effective junction area and mitigated interfacial hole accumulation, thereby enhancing charge separation. The schematic formation process and corresponding LSV curves of these heterojunction arrays are shown in Figure 11c,d. The resulting composite achieved a photocurrent density of 3.53 mA/cm² for PEC water oxidation—approximately twice that of WO₃ alone—while maintaining good stability, indicating a robust IEF over the nanojunction network [148].

In another optics-plus interface example, Cao et al. engineered an FTO/WO₃/BiVO₄ heterojunction and applied a mild NaOH post-treatment to modify the BiVO₄ surface chemistry. The heterojunction provided an interfacial driving force for carrier separation, while the surface treatment accelerated water oxidation kinetics, together yielding ~1.75 mA/cm² at 1.23 V_RHE—more than double that of WO₃,as shown in Figure 11e [149].

Collectively, these studies demonstrate that establishing heterointerfaces (SnO₂/WO₃/BiVO₄; WO₃/BiVO₄) and tuning surface chemistry can create or strengthen internal electric fields, which (i) promote directional carrier migration, (ii) suppress interfacial recombination, and (iii) enhance photocurrent and IPCE without the need for precious metal cocatalysts.

4.6.2. Enhancing Electrical Conductivity and Carrier Concentration

Enhancing the electrical conductivity of BiVO₄ has emerged as an important strategy to improve its intrinsic material properties [150]. Doping, in particular, is widely recognized as an effective means of tuning the electronic structure and thereby increasing conductivity [151]. Both metal and nonmetal dopants have been successfully incorporated into BiVO₄ for this purpose [152]. Among them, tungsten (W) and molybdenum (Mo) are especially effective: substituting V⁵⁺ ions with W⁶⁺ or Mo⁶⁺ ions significantly enhance the electronic conductivity of BiVO₄ [153].

Enhancing the electrical conductivity of BiVO₄ is a critical strategy to facilitate efficient charge transport and suppress recombination losses during PEC water splitting. Improved conductivity not only enables faster carrier movement but also contributes directly to higher photocurrent generation. Wu et al. demonstrated that Mo doping in BiVO₄ significantly lowers the interfacial charge transfer resistance by 2–3 orders of magnitude under illumination. This finding indicates that interfacial kinetics, rather than bulk resistance, dominate the PEC performance of pristine BiVO₄. As a result, the enhanced conductivity and accelerated charge transfer led to a remarkable increase in photocurrent density, reaching values ~2.7 times higher than those of undoped BiVO₄ at 1.23 V_RHE. Figure 12a–c present the LSV curves and Mott–Schottky plots, confirming the enhanced donor density and reduced resistance [154].

Similarly, Chaudhari et al. reported that introducing a conductive carbon network into BiVO₄ through MOF-derived Bi–V–O/carbon composites (BVC) effectively reduced charge transport resistance and enlarged the electrochemically active surface area, leading to enhanced PEC water oxidation activity and additional applicability in energy storage systems. The corresponding LSV, stability, and EIS results supporting these improvements are shown in Figure 12d–f [155].

Taken together, these results underscore that strategies improving the electrical conductivity of BiVO₄—whether through cation doping or conductive composite engineering—consistently enhance PEC performance by lowering resistance, increasing carrier density, and enabling more efficient charge extraction.

4.7. Charge Transfer Efficiency (η_trans)

Enhancing the charge transfer efficiency η_trans at the photoanode/electrolyte interface is crucial for realizing efficient PEC performance in BiVO₄. Although photogenerated holes in BiVO₄ can reach the electrode surface to drive the OER, a substantial fraction is often lost through recombination with electrons during transport or at the interface. To assess this limitation, η_trans is generally determined by comparing the photocurrent densities measured in the presence and absence of a hole scavenger such as Na₂SO₃, which suppresses surface recombination and reflects the ideal case of complete hole utilization. The efficiency can be expressed as

$${\eta }_{transfer}\left(\%\right)={\displaystyle \frac{J_{H_{2}O}}{J_{{Na}_2{SO}_3}}}\times 100$$

(8)

where J_H2O represents the photocurrent density obtained in water oxidation, and J_Na2SO3 corresponds to the photocurrent density measured with Na₂SO₃ present.

This evaluation emphasizes that the sluggish surface reaction kinetics of BiVO₄ remain a major bottleneck for water oxidation. Accordingly, considerable effort has been devoted to loading suitable cocatalysts onto BiVO₄, which not only suppress surface recombination but also lower the reaction overpotential. These strategies have been consistently reported to enhance η_trans, resulting in improved photocurrent densities and overall PEC water splitting efficiency.

Co-Catalysts Based on Metal (Oxy) Hydroxides

Incorporating metal (oxy)hydroxide (M-OOH) cocatalysts, such as FeOOH and NiFeOOH, onto BiVO₄-based photoanodes has emerged as one of the most effective approaches to enhance charge transfer efficiency (η_trans). These cocatalysts supply abundant active sites, suppress surface recombination, and accelerate water oxidation kinetics [156].

For instance, Li et al. fabricated a micro–nanostructured FeOOH/BiVO₄/WO₃ photoanode via a combination of hydrothermal, electrodeposition, and impregnation methods. The resulting heterostructure exhibited a photocurrent density of ~2.04 mA/cm² at 1.23 V_RHE, nearly doubling that of the FeOOH-free counterpart (~1.09 mA/cm²), along with a positive shift in onset potential from 0.80 to 0.60 V_RHE. The SEM morphology, surface photovoltage, and dark current characteristics supporting these improvements are shown in Figure 13a–c [157].

Similarly, Creasey et al. constructed a WO₃/BiVO₄/NiFeOOH photoanode using a scalable aerosol-assisted CVD process. The incorporation of NiFeOOH not only stabilized the heterojunction but also suppressed BiVO₄ dissolution, enabling a stable photocurrent of 1.75 mA/cm² at 1.23 V_RHE that was sustained for 24 h under one-sun illumination, as shown in Figure 12d,e [158].

Another study by Li et al. reported BiVO₄/CoPi electrodes prepared through in situ electrodeposition. The CoPi layer increased photocurrent by ~1.8× relative to bare BiVO₄, achieving 1.39 mA/cm² at 1.23 V_RHE, while simultaneously enhancing applied-bias photon-to-current efficiency (ABPE) and reducing onset potential—highlighting its role in effective passivation and charge separation. The J–V curves and ABPE values of the BiVO₄/CoPi photoanodes are illustrated in Figure 13f–h [159].

Collectively, these findings confirm that M-OOH cocatalyst coatings on BiVO₄ or BiVO₄-based heterostructures significantly promote charge transfer, reduce onset potential, enhance stability, and thereby improve overall PEC performance.

4.8. Stability

The operational stability of BiVO₄ photoanodes is a key factor for their practical application, as maintaining a consistent photocurrent over extended periods is essential. Photocorrosion, caused by undesirable redox reactions between photogenerated electrons and holes within the BiVO₄ lattice, can significantly reduce charge extraction efficiency. This degradation is further exacerbated by light-induced leaching of V⁵⁺ ions, which distorts the lattice and causes long-term structural damage. Recent studies have highlighted two primary strategies to mitigate these effects: the incorporation of protective surface layers and the optimization of electrolyte composition, both of which are discussed in the following sections.

4.8.1. Protection Layer Incorporation

Photocorrosion in BiVO₄ photoanodes primarily arises from photogenerated holes reacting with surface atoms or via light-assisted dissolution processes, resulting in structural degradation and reduced stability. A practical approach to mitigate these effects is the application of a protective layer, which physically blocks corrosive interactions while still allowing hole transfer to support OER.

For example, Zhang et al. systematically examined the operando photostability of BiVO₄ in near-neutral electrolytes using a scanning flow cell, directly tracking dissolution during light-driven OER. They demonstrated that the dissolution rate is strongly dependent on the electrolyte composition (borate > phosphate > citrate), and that citrate provides kinetic protection via hole scavenging. Distinct dissolution potentials for Bi and V were identified, and time-resolved measurements revealed that photocurrent and dissolution can evolve independently, highlighting that surface chemistry control is essential for achieving long-term stability. These findings underscore the need for barrier or passivation layers to decouple corrosion from charge extraction on BiVO₄ surfaces. Figure 14a–f provides direct visual confirmation of these dynamics, coupling photocurrent/thickness profiles with dissolution rate measurements and post-mortem morphology analyses under borate and phosphate conditions [160].

Similarly, Wang et al. reviewed interface regulation strategies for BiVO₄, emphasizing artificial protection layers—such as ultrathin ALD oxides (TiO₂, Al₂O₃) and organic/organic–inorganic interlayers—that suppress photocorrosion while maintaining hole transfer. The review highlighted the design principles for these coatings, which include (i) blocking electrolyte attack, (ii) passivating surface traps, and (iii) forming favorable band alignment for OER catalysis, thereby lowering interfacial resistance, shifting onset potentials, and improving operational durability under AM 1.5G illumination. Representative protection strategies and schematic illustrations of stabilized BiVO₄ electrodes are shown in Figure 14g–i. [161].

Collectively, these examples demonstrate that protective layers—whether polymeric or ALD-oxide—are central to stabilizing BiVO₄ photoanodes, mitigating operando dissolution, and sustaining interfacial hole transport necessary for efficient oxygen evolution, ultimately enabling durable and high-performance PEC devices.

4.8.2. Dual-Protection Strategies for Durable BiVO₄ Photoanodes

Photocorrosion and dissolution of BiVO₄—especially the leaching of V⁵⁺ ions—represent critical challenges to achieving long-term PEC stability. A practical approach to mitigate these effects involves tuning the electrolyte composition to suppress dissolution and stabilize the surface chemistry.

Lei et al. demonstrated that conformal NiO_x overlayers deposited via ALD, combined with the deliberate addition of NaVO₃ to the electrolyte, provided a synergistic stabilization effect. The NiO_x coating suppressed surface charge recombination and facilitated hole transfer by stabilizing Bi–O bonds, while dissolved vanadate species helped re-establish chemical equilibrium and compensate for V loss. This dual-protection strategy enabled NiO_x/BiVO₄ photoanodes to achieve an applied-bias photon-to-current efficiency (ABPE) of 2.05% with a fill factor of 47.1%, and, remarkably, operational durability exceeding 2100 h under continuous PEC operation. The long-term stability and proposed anticorrosion mechanism of the NiO_x-protected BiVO₄ electrodes are illustrated in Figure 15a,b [162].

More recently, Lee et al. introduced a complementary approach by selectively engineering surface oxygen vacancies (VO) via a surface chemical reduction (SCR) process, followed by deposition of an ultrathin TiO₂ layer (≈5 nm) via ALD. The VO-rich surface promoted strong oxygen-end networking with TiO₂, yielding a nearly pinhole-free protective layer that minimized interfacial recombination while maintaining efficient charge tunneling. When further coupled with a cobalt phosphate (CoPi) oxygen evolution catalyst, the optimized SCR-BiVO₄/TiO₂/CoPi photoanode exhibited a stable photocurrent density of 3.9 mA/cm² at 1.23 V_RHE, with charge transfer efficiency of up to 97% and sustained photostability for over 50 h while generating stoichiometric H₂ and O₂. The PEC performances in different electrolytes and the schematic of charge kinetics in this optimized multilayer system are shown in Figure 15c–e. [163].

These findings confirm that electrolyte composition is a critical factor in BiVO₄ stability. By adding protective species (e.g., phosphate buffers, vanadium ions) or coupling with passivation layers (e.g., NiO_x), the dissolution rate can be significantly suppressed while preserving efficient hole transfer, making electrolyte engineering a powerful tool for enabling durable PEC water splitting devices.

4.8.3. Synergistic Surface–Electrolyte Protection Strategies

The identity of the electrolyte—including buffer anions/cations and pH—has been reported to govern not only interfacial oxygen evolution kinetics but also the chemical pathways of photocorrosion in BiVO₄. In phosphate electrolytes, competitive reactions between water oxidation and lattice dissolution proceed via phosphate–bismuth surface interactions, whose rates accelerate under illumination and elevated potentials, as illustrated in Figure 16a. These findings emphasize that stability should be assessed alongside kinetics rather than inferred from photocurrent alone, as demonstrated by the contrasting stability profiles in Figure 16b [164].

In borate buffers, a frequently observed “activation” effect has been attributed to trace Fe impurities, which deposit as an ultrathin oxyhydroxide layer that passivates surface traps and lowers interfacial resistance. Under optimized conditions, bare BiVO₄ photoanodes have been reported to reach ≈4.5 mA/cm² at 1.23 V_RHE and surface Fe activation layers further boost photocurrent and enhance photostability by promoting Fe-layer formation on the surface. Comparable enhancements across Li⁺ Na⁺, and K⁺ borates support the view that Fe-mediated passivation, rather than alkali identity, governs this activation behavior. The photocurrent response before and after Fe impurity removal, as well as the effect of Fe²⁺ addition, are presented in Figure 16c [165]. Nevertheless, quantitative correlations between Fe concentration and performance gain remain limited and merit systematic study.

Beyond conventional phosphate and borate systems, bicarbonate electrolytes—including mixed or non-aqueous variants—have been shown to mitigate vanadium leaching, especially when supplemented with saturated V⁵⁺. This approach reduces morphological degradation and preserves optical density over extended operation, underscoring electrolyte engineering as a viable lever for durability improvement. The long-term stability of BiVO₄ photoanodes in aqueous and mixed MeCN/NaHCO₃ electrolytes is shown in Figure 16d [166].

Electrochemical conditioning protocols, such as controlled pre-bias or photocharging sequences, have also been demonstrated to enhance operational stability by tuning surface states and optimizing interfacial charge transfer pathways. These effects are synergistic with electrolyte composition, suggesting that combined strategies may yield more durable BiVO₄ photoanodes [167].

In summary, electrolyte design directly shapes both surface kinetics and corrosion chemistry in BiVO₄ photoanodes. Phosphate media can accelerate degradation without stabilization measures [164]; borate buffers “activate” BiVO₄ via Fe-assisted passivation [165]; bicarbonates, particularly with V⁵⁺ supplementation, suppress vanadium loss and slow morphological decay [166]; and electrochemical conditioning provides an orthogonal route to stabilize interfacial dynamics [167]. Future studies are encouraged to report kinetic and stability metrics in parallel, clearly specify buffer composition and pH ranges, and combine electrolyte engineering (e.g., Fe-controlled borate or V⁵⁺-supplemented bicarbonate) with conditioning protocols to enable reproducible and robust long-term PEC operation.

5. Emerging Applications of BiVO₄ Photoelectrodes in Solar Water Splitting and Beyond

While materials strategies (doping, heterojunctions, HTLs/overlayers, and electrolyte engineering) have substantially improved BiVO₄ at the electrode level, practical relevance is established only in overall water splitting devices, where a BiVO₄ photoanode is paired with a photocathode or photovoltaic element to operate unassisted (0 V external bias) or at low bias. This section surveys recent device architectures (PEC–PEC tandems, PEC–PV hybrids, monolithic vs. wired configurations) and applies a consistent set of performance benchmarks under AM 1.5G: (i) J_PEC at 1.23 V_RHE to normalize anodic OER kinetics, (ii) STH (%) as a device-level energy conversion metric, (iii) electrolyte/pH and applied bias for fair comparison, and (iv) durability (h) with supporting evidence (e.g., gas-tight collection and faradaic efficiency) when available. Building on these data, we discuss such as energy-level matching in tandems, reduction in ohmic/contact losses, membrane and electrode spacing design with hydrodynamic considerations, as well as pre-conditioning protocols and corrosion-mitigating electrolytes (e.g., impurity-controlled borate or vanadate-buffered media). The accompanying comparison table includes only reports with complete entries to provide a quantitative basis for assessing genuine progress and guiding the integration of BiVO₄ into scalable modules.

Table 5. Comparison of BiVO₄ photoanodes for overall water splitting with complete reporting.

Photoanodes	Bias	Electrolyte	JPEC @1.23 VRHE (mA/cm2)	STH (%)	Ref
NiOOH/FeOOH/BiVO4/SnO2//TTO//TOPCon-Si	Unbiased	1.0 M potassium borate, pH 9	1.40	1.72	[168]
Nanocone/Mo:BiVO4/Fe(Ni)OOH	Unassisted	Phosphate buffer, pH 7	5.82 ± 0.36	6.2	[33]
BiVO4/NiOOH/FeOOH (top)//Cu2O/CuO/TiO2 (bottom)	Unassisted	0.1 M Na2SO4, pH 6	2.05	0.27	[169]
BiVO4/FeOOH (oxygen vacancy gradient; FeOOH OEC)	Unassisted	1 M borate buffer (pH ≈ 9)	7.0	8.4	[170]
BiVO4/Cu2O/NiFe-LDH	Unassisted	0.1 M Na2SO4, pH 6	5.01	1.18	[171]
Mo:BiVO4 + polycarbazole HTL (CPF-TCB) + NiFeCoOx OEC	Unassisted	K–borate buffer (pH ≈ 9–10)	≈6.6	~9	[110]
CoPi/W:BiVO4/Ni	Unassisted	K-phosphate buffer (pH 7)	1.5	2.1–6.3	[172]

summarizes BiVO₄-based photoanodes for overall water splitting under unbiased conditions, highlighting that the integration of oxygen evolution catalysts, Mo doping, surface nano-structuring, and organic hole transport layers can substantially enhance both J_PEC and STH efficiencies. Among the reported systems, Mo:BiVO₄ combined with a polycarbazole HTL and NiFeCoO_x OEC exhibits one of the highest performances (J_PEC ≈ 6.6 mA/cm², STH ≈ 9%), emphasizing the importance of optimizing charge separation and catalytic activity in achieving high-efficiency photoanodes. [110].

5.1. BiVO₄-Based Tandem System for Comprehensive Water Splitting

Achieving efficient overall water splitting without external bias requires that the top photoanode generates sufficient photovoltage while maintaining strong light absorption and operational stability. BiVO₄, with a bandgap of ~2.4 eV and robust visible light absorption, remains a promising candidate. To overcome its intrinsic limitation in photovoltage, researchers have explored tandem PEC configurations that combine BiVO₄ with additional light-harvesting materials.

A notable example is a BiVO₄ nanocone/Fe(Ni)OOH photoanode coupled with a perovskite solar cell. Under AM 1.5G illumination, this system achieved unassisted overall water splitting with a STH efficiency of up to 6.2%, leveraging complementary light absorption and efficient interfacial charge transfer. The standalone PEC BiVO₄-based component exhibited a photocurrent density of 5.82 ± 0.36 mA/cm² at 1.23 V_RHE The optical absorption mechanism of the nanocone substrate and the corresponding J–V curves confirming this photocurrent enhancement are shown in Figure 17a,b [33].

Another recent advancement involves a monolithically integrated tandem system combining a BiVO₄ photoanode with a Cu₂O photocathode, both protected by conformal layers. Sitaaraman et al. fabricated a Mo-BiVO₄/TiO₂/FeOOH photoanode paired with a Cu₂O/TiO₂/MoS₂ photocathode, forming an unassisted tandem PEC cell. The TiO₂ layers serve as protective barriers and facilitate charge extraction, whereas FeOOH and MoS₂ act as cocatalysts to accelerate the OER and HER. Under AM 1.5G illumination, the individual electrodes delivered ~0.81 mA/cm² (photoanode) and –1.88 mA/cm² (photocathode), and the assembled tandem achieved an unassisted current density of approximately 65.3 µA/cm², with enhanced stability compared to unprotected devices. The LSV response, unassisted stability test, and energy band diagram of these tandem PEC devices are illustrated in Figure 17c–e [53].

The solar-to-hydrogen efficiency for tandem devices is calculated as

$${\eta }_{STH}\left(\%\right)={\left[{\displaystyle \frac{J_{sc}\left(\frac{\mathrm{m}\mathrm{A}}{{\mathrm{c}\mathrm{m}}^2}\right)\times \left(1.23V\right)\times {\eta }_F}{P_{Total}\left(\frac{\mathrm{m}\mathrm{W}}{{\mathrm{c}\mathrm{m}}^2}\right)}}\right]}_{AM1.5G}$$

(9)

Here, J_sc represents the short-circuit photocurrent density corresponding to the rate of hydrogen generation expressed as current density, η_F is the Faradaic efficiency for hydrogen production (typically near 100%), and P_Total denotes the total solar irradiance intensity, set at 100 mW/cm² under AM 1.5G conditions. By overlaying the current–voltage (J–V) curve of the photoanode with those of the photocathode or photovoltaic cells, the operating point of the tandem device can be determined, defined by the short-circuit photocurrent density at 0 V applied bias. However, the actual device performance often falls short of the predicted operating point due to overpotentials between the two electrodes. This discrepancy arises because the J–V curves of individual half-cells (including photovoltaics), referenced to the reversible hydrogen electrode (RHE) scale, do not account for resistive losses such as ohmic resistance and pH gradients present between the components.

5.2. PEC Cells for the Generation of Value-Added Chemicals

While PEC systems are primarily designed for overall water splitting, there is increasing interest in their application for the selective synthesis of value-added chemicals, such as hydrogen peroxide (H₂O₂), via partial oxidation or reduction under solar illumination. H₂O₂ can be selectively produced when the PEC system is appropriately engineered, exploiting its redox potentials of 1.77 V_RHE (via 2e⁻ water oxidation) and 0.68 V_RHE (via oxygen reduction).

The fundamental half-reactions for PEC-based H₂O₂ generation are summarized as follows:

$$\mathrm{Oxygen} \mathrm{evolution} \mathrm{reaction}:2H_{2}O+4h^{+}\to 4H^{+}+O_2, E^{°}=1.23{ V}_{\mathit{RHE}}$$

(10)

$$H_2{O}_2\mathrm{production} \mathrm{by} \mathrm{water} \mathrm{oxidation}:2H_{2}O+2h^{+}\to 2H^{+}+H_2{O}_2, { E}^{°}=1.77 V_{\mathit{RHE}}$$

(11)

$$\mathrm{Hydrogen} \mathrm{evolution} \mathrm{reaction}:2H^{+}+2e^{-}\to H_2, E^{°}=0.0{ V}_{\mathit{RHE}}$$

(12)

$$H_2{O}_2\mathrm{production} \mathrm{by} \mathrm{oxygen} \mathrm{reduction}:O_2+2H^{+}+2e^{-}\to H_2{O}_2, E^{°}=0.68{ V}_{\mathit{RHE}}$$

(13)

These reactions highlight that by controlling the PEC environment and electrode materials, the pathways for partial oxidation or reduction can be preferentially promoted, enabling efficient and selective H₂O₂ production.

Recent advances have highlighted BiVO₄-based systems as promising platforms for PEC hydrogen peroxide production. Wan et al. demonstrated that phosphate-modified BiVO₄ (PBVO) photoanodes undergo dynamic anion exchange with bicarbonate (HCO₃⁻) at the semiconductor–electrolyte interface, which accelerates charge transfer and enhances the selectivity of the two-electron water oxidation pathway. This modification achieved an average Faradaic efficiency of 82.6% (maximum 92.1%) with a H₂O₂ production rate of 66.5 µmol/h, while the formation of H₂PO₄⁻ intermediates suppressed H₂O₂ overoxidation. The LSV curves and charge balance plots confirming the selective H₂O₂ production on PBVO-2 photoanodes are shown in Figure 18a,b [173]. Under AM 1.5G illumination in a H-type PEC cell, PBVO-2 photoanodes exhibited ~1.6-fold higher photocurrent density than pristine BiVO₄, maintained a solar-to-hydrogen efficiency of 1.27%, and enabled continuous H₂O₂ accumulation up to 2.34 × 10⁻³ M.

Complementarily, Shi et al. reported a heterojunction photocathode based on p-BiVO₄/SnO₂/NiNC, which delivered a H₂O₂ generation rate of 65.46 µmol/h with a Faradaic efficiency of 76.1%. LSV revealed significantly enhanced photocurrent density at 0.4 V_RHE compared to pristine p-BVO and other oxide-modified photocathodes (TiO_x, NiO_x, ZnO), confirming the charge transport benefit of the SnO₂ interlayer. The IPCE reached a maximum of 8.5%, and I–t measurements demonstrated excellent stability with photocurrent maintained above 0.15 mA/cm² and only 8.9% loss after 20 h of operation under O₂-saturated conditions. Integration with a Mo-doped BiVO₄ photoanode in a tandem cell achieved nearly quantitative H₂O₂ selectivity. The chopped J–V curves, transient current responses, and Faradaic efficiency data supporting this high selectivity are presented in Figure 18c–e [174].

Collectively, these studies demonstrate that surface anion modulation and rational heterojunction design enable efficient and selective PEC conversion of water to H₂O₂, establishing BiVO₄-based systems as a viable platform for solar-driven production of value-added oxidants.

6. Challenges and Perspectives

Despite significant advances in BiVO₄ electrodes over the past four decades, these materials still fall short of the requirements for practical applications, highlighting several challenges and opportunities.

Photocurrent Density Limitations: While recent reports have achieved photocurrent densities up to 6.0 mA/cm² at 1.23 V_RHE, this performance remains below the theoretical limit of 7.5 mA/cm². Improving PEC water splitting efficiency, therefore, requires enhancements in light absorption, charge transfer, and carrier separation. A deeper understanding of factors influencing these processes could guide the design of more efficient BiVO₄ electrodes. In particular, the role of crystal orientation on PEC performance remains underexplored, representing a promising avenue for future research. Additionally, synthesizing BiVO₄ electrodes with multicomponent and multifunctional catalysts could provide additional active sites for hydrogen and oxygen evolution while promoting more effective charge separation.

Challenges in Understanding PEC Mechanisms: Understanding the fundamental PEC reaction mechanisms remains challenging. Most studies have prioritized performance improvements, often at the expense of mechanistic insight. Limited access to advance in situ techniques has hindered in-depth exploration of water oxidation on BiVO₄. Approaches such as infrared spectroscopy, X-ray spectroscopy, and transient absorption spectroscopy could enable observation of reaction dynamics and intermediates at the molecular or atomic scale. Developing these methodologies will be essential for advancing the mechanistic understanding of PEC water oxidation.

Scaling Up BiVO₄-Based Photoanodes: Current research largely involves small-area BiVO₄ photoanodes (~1.0 cm²), whereas practical deployment demands large-area materials with high uniformity and crystallinity. Fabrication via electroplated BiOI conversion faces challenges in maintaining uniformity across large substrates due to increased resistance, nonuniform films, and nonlinear diffusion of reactants and products. Developing scalable and cost-effective synthesis strategies for large-area, high-quality photoelectrodes will be critical for commercial applications.

Design of Tandem PEC Systems: Tandem configurations using dual- or three-electrode setups are common in practical PEC systems. A feasible approach for real-world devices involves compartmentalization with membranes, along with careful optimization of electrode spacing, membrane design, and pressure balance. Tandem PEC modules often integrate multijunction or perovskite photovoltaic cells, requiring precise matching of energy levels between the photoanode and photocathode. Further engineering efforts, including fluid dynamics optimization, are needed to maximize device performance under operational conditions.

Advanced/Operando Spectroscopy and Diagnostics: To convert performance gains into durable operation, buried-interface processes must be observed under realistic bias, illumination, and electrolyte conditions. High-energy-resolution X-ray absorption (HERFD-XANES/XAS) at the Bi L and V K edges can track oxidation-state shifts, ligand fields, and local coordination during OER, revealing whether interlayers/overlayers stabilize Bi–O and V–O polyhedra or merely delay dissolution. Ambient-pressure/operando XPS resolves surface terminations, adsorbates (e.g., phosphate/borate/vanadate), and HTL-derived functional groups that modulate band bending and recombination velocities. Operando GIWAXS/XRD identifies phase transformations (e.g., BiPO₄ signatures, amorphization), while Raman/IR spectroelectrochemistry reports reaction intermediates and lattice strain. Charge-dynamics probes—IMPS/EIS to deconvolute η_sep vs. η_trans, transient absorption (fs–µs) and time-resolved photoluminescence to quantify interfacial transfer constants, and UV–Vis spectroelectrochemistry to monitor polaron/charge accumulation—link kinetic rate constants to optical populations. Finally, pairing photocurrent with operando dissolution (e.g., ICP-MS in a scanning flow cell or EQCM) closes the gap between apparent activity and true stability by quantifying Bi/V loss in real time.

Standardization and Reporting of Stability: To ensure comparability across studies, we recommend (i) co-reporting J–V (or ABPE) with operando dissolution traces on the same electrode; (ii) specifying light intensity (AM 1.5G), temperature, pH, buffer composition and impurities (e.g., trace Fe), bias protocols (hold vs. scan; scan rate), and operation time; (iii) adopting common endurance metrics (e.g., current retention and dissolution yield after 10–24 h at a defined potential); and (iv) providing raw spectra and analysis scripts for XAS/XPS/Raman together with IMPS/EIS fitting models. Such practices reduce ambiguity between kinetic- and corrosion-driven losses and accelerate translation to device design.

Diagnostics for Scale-Up: As devices move beyond ~1 cm², spatial non-uniformities dominate. Integrating spatially resolved tools—scanning photoelectrochemical microscopy (SPECM), light-beam-induced current mapping, micro-XRD/µ-XRF, or X-ray/optical tomography—can identify local recombination hot spots, catalyst delamination, and electrolyte transport limitations, informing fluid dynamics optimization and contact/grid design for large-area modules.

Given these multifaceted challenges, interdisciplinary collaboration is increasingly important. Combining expertise from materials science, physical chemistry, surface science, and advanced characterization techniques will be essential to develop more stable and efficient BiVO₄-based photoelectrodes for PEC water splitting.