Conformal coatings first appeared in the 1960s in the Soviet Union and in Finland in 1974 with the independent development of gas-phase thin-film deposition technology: Atomic Layer Deposition (ALD)^［30-31］ (Figure 4). Conformal, which means forming a uniform coating layer on the surface of an object, has been widely used in capacitors, microelectronics, fuel cells, solar cells, catalytic interfaces, textiles, and pharmaceuticals, etc.^［32］. In photoelectrocatalytic water splitting, greater emphasis is placed on the full encapsulation of conformal coatings. To meet the developmental needs of large-sized photoanode semiconductors, conformal coatings need to meet other basic requirements, including: (1) materials and preparation methods simultaneously meeting mass production, enabling the coating to fully encapsulate single particles; (2) having good protective properties: effectively suppressing the photochemical corrosion of BiVO₄ without affecting the light absorption and charge transport performance of the electrode; (3) integrating multiple functions by selecting different coating materials and optimizing structural design.

In terms of deposition technology, it is challenging to construct a well-coated conformal layer on the surface of semiconductors with 3D nanoporous complex structures such as BiVO₄. Traditional crystalline oxide coatings, such as TiO₂ thin films deposited using techniques like magnetron sputtering and vacuum thermal evaporation^[21], are limited by the deposition method and crystal size, allowing the coating to cover only the semiconductor surface. Additionally, electro-deposition technology, influenced by pulse current and the diffusion current gradient on the substrate surface, often fails to produce uniform conformal coatings; photoelectrochemical deposition, induced by light, tends to deposit preferentially in areas with high catalytic activity sites, resulting in non-uniform deposition density and making it difficult to achieve good coatings. Therefore, a good deposition technique with adjustable deposition size is crucial.

In terms of coating morphology, the built-in electric field of the semi-conformal coating is non-uniform, and the diffusion concentration gradient of majority carriers (electrons) in the space charge region is large. Even when constructing p-n junctions or loading hole transport layers for strong hole extraction or reducing transmission path impedance, some carriers still require a longer migration distance. The full-conformal coating can independently encapsulate one or several adjacent nanoparticles on the photoanode semiconductor: on the one hand, it ensures that the electron transport distance remains as unchanged as possible; on the other hand, even as the electrode operating time increases, electrolyte erosion will not cause short-circuiting of large areas of semiconductor nanoparticles. In addition, the uniform full-conformal coating maximizes the incident light path. During the water oxidation process of BiVO₄ photoanodes, V⁵⁺ within the lattice easily leaches out in the photochemical solution, leading to an irrepressible rapid decay in stability ^[33-34]. In PEC devices, defects such as ohmic losses caused by size effects further increase the difficulty of maintaining long-term stability and exacerbate the situation on larger electrodes. Therefore, the surface of BiVO₄ photoanodes requires carefully designed full-conformal coatings.

The industrial stability standard condition for PEC devices is 1000 h, and currently, only the photoanode reported by Kuang et al.^［35］ has achieved 1100 h of stable water splitting. A small number of works have broken through the hundred-hour level, while more works still remain below a hundred hours^{［8, 36］} (Fig. 5). Good interface coating design coupled with efficient crystal engineering may bring new hope. Among these, the most fundamental and important design principles that need to be followed are for the more core size-scalable full-conformal coating design.

Recent studies have shown that molecular catalysts exhibit high activity and structural diversity in photoelectrochemical water splitting and can achieve turnover frequencies (TOFs) for water oxidation in the thousands, far surpassing inorganic cocatalysts^［56］. However, they are rarely integrated into modular systems for overall water splitting. To achieve solar energy conversion without sacrificial agents, researchers have started combining molecular catalysts with visible-light-responsive semiconductors. One approach is to immobilize molecular catalysts on porous BiVO₄ surfaces to form hybrid photoelectrodes. This method not only enhances photoelectrochemical performance but also allows modulation of the interfacial structure of the final molecular catalyst/BiVO₄ composite by altering the bridging ligands, leading to its widespread application.

In 2017, Wang et al^［57］ constructed molecular cobalt catalysts with specific geometric configurations and electronic properties. Compared to previously reported molecular catalysts, these exhibit higher photocurrent density and Incident Monochromatic Photon-Electron Conversion Efficiency (IPCE). More notably, the two-step preparation approach in this study provides a template for the synthesis of molecular catalysts. The first step involves the synthesis and design of molecular cobalt catalysts, while the second step introduces bridging ligands. Through a self-assembly process, the amino functional groups of the ligand coordinate with Co ions on cobalt-based Metal Organic Frameworks (MOFs) materials, and the other end of the ligand interacts with oxygen atoms on the surface of the BiVO₄ electrode, thereby connecting the molecular cobalt catalyst to the BiVO₄ electrode. This process occurs at room temperature without requiring additional catalysts or high-temperature treatment. Miao et al^［58］ also synthesized PTh/BiVO₄ photoanodes using a two-step solution method. The [FeCl₄] active unit within the polythiophene (PTh) matrix is an efficient OER catalyst that optimizes the surface reaction kinetics of the photoanode (Figure 8). Even in seawater, it exhibits excellent stability and corrosion resistance, increasing the photocurrent density of the BiVO₄ photoanode from 1.61 mA/cm² to 4.72 mA/cm² @1.23 V_RHE, with a long-term stability of 40 hours in PEC seawater splitting. The advantage of the solution method lies in controlling the soaking time to achieve a coating of appropriate thickness, which easily balances catalytic efficiency and stability.

In addition, well-coated polyaniline (PANI) can be used as a hole transport layer at the interface. Wang et al.^［59］ prepared NiOOH/PANI/BiVO₄ (NPB) photoanodes using a photoelectrochemical deposition method. The water oxidation photocurrent of the NPB photoanode reached 3.31 mA/cm² @1.23 V_RHE, showing good water oxidation stability after 3 hours of PEC water splitting, with excellent Faradaic efficiency of approximately 97.22%. PANI demonstrated significant potential for PEC applications. However, electro/photoelectrochemical deposition methods can easily lead to excessive loading of organic thin films. Compared with the gain from the early onset potential, this method still significantly hinders PEC performance, especially when redundant coatings fall off, resulting in stability degradation. Gradient-distributed Co ions and Si ions, along with modified chlorophyll organic compounds, have also been used as fully conformal multifunctional thin films^［60］ (Figure 9). Among these, Co ions promote the transport of photogenerated holes, while Si ions enhance the hydrophilicity of the device. The presence of the film effectively reduces the barrier of the oxidation reaction, facilitates photogenerated carrier transfer, suppresses vanadium loss, and improves the stability of the BiVO₄ photoanode. After modification, the BiVO₄ photoanode exhibited a photocurrent density of 5.1 mA/cm² @1.23 V_RHE during testing, with an onset potential negatively shifted by 350 mV, maintaining stable water splitting for up to 12,000 seconds with only an 8.2% decrease. In this study, replacing Mg²⁺ in the porphyrin ring with Cu²⁺ altered the electronic properties and photophysical performance of the material. Cu²⁺ has different electronic structures and oxidation states, which may lead to changes in light absorption spectra, photogenerated carrier generation, and transport efficiency. This substitution can improve the material's response to specific wavelengths of light, thereby enhancing its application potential in optoelectronic devices such as solar cells, photocatalysts, and photodetectors. This research provides important insights for designing other similar inorganic-organic hybrid thin-film materials.

PDA can be synthesized onto the substrate in one step due to its high adhesion without the need for bridging ligands and has been confirmed to possess good electron transfer characteristics^［61-62］. Currently, the longest stable duration of the PDA matrix coating can reach 5 hours @1.23 V_RHE, however, after carbonization, PDA shows a dotted distribution on the BiVO₄ surface, and the color of the PDA film becomes darker after N₂ annealing reduction^［63］, which is not conducive to the light absorption of semiconductors. Since the PDA film structure uses —OH and —NH₂ as redox bonds, its catalytic ability is relatively weak and requires coordination with highly active metals. Co-Pi as a passivator is not suitable as a ligand, and even under the confinement effect of PDA, it will not change the accumulation of holes on the surface^［64-65］, eventually leading to rapid performance degradation. A more promising strategy is to use transition metal ions such as Ni, Fe, and Co for coordination, which is beneficial for the efficient utilization of holes. Overall, PDA, as an inexpensive full-conformal material, still has great room for development.

In addition to the activity and function of key sites on the fully conformal coating being worth studying, the organic matrix, which accounts for the largest proportion as a whole, should also receive more attention. Pan et al^［66］ proposed a BiVO₄ photoanode coated with ZnCoFe polyphthalocyanine (ZCF) using pyrazine (Pz) as a ligand. The metal phthalocyanine-based shell can improve the water oxidation efficiency of the BiVO₄@Metal core-shell structured photoelectrode through the bidirectional axial coordination of pyrazole because pyrazole molecules, as an effective bridging ligand, enable a close connection between metal phthalocyanine and the BiVO₄ core. This connection not only promotes the efficient transport of photogenerated carriers but also enhances the activity of the water oxidation reaction by modulating the electronic structure, lowering the Fe d-band center and orbital spin (Figure 10). Moreover, the bidirectional axial coordination of pyrazole can also enhance the stability of the photoanode, allowing it to maintain efficient water oxidation performance during a prolonged 40-hour operation at 1.23 V_RHE.

In addition, Zhang et al^［67］ reported a covalent triazine organic framework (Covalent Triazine Frameworks, CTF-BTh) polymer film completely without coordinated metal ions (Figure 11). The water splitting efficiency of the NiFeOx/CTF-BTh/Mo:BiVO₄ photoanode reaches 3.7%. Combining Cu₂O and Mo:BiVO₄, two different types of photoelectrodes, with CTF-BTh forms effective p-n junctions and type II heterostructures. This hybrid material exhibits excellent electron transport properties, which can significantly enhance the efficiency of water splitting reactions. Moreover, this material can also serve as a surface protective layer to effectively prevent the photoelectrode from oxidative corrosion during long-term use. Among them, the NiFeO _x/CTF-BTh/Mo:BiVO₄ photoanode shows only a 10% decay after 150 hours of water splitting, and throughout the entire water splitting process, the Faradaic efficiency for hydrogen and oxygen production is close to 100%. This represents the highest performance record for fully conformal coatings currently in use.

Depositing p-type Group Ⅷ metal (such as Fe, Co, Ni) oxides or hydroxides on BiVO₄ photoanodes as water oxidation electrocatalysts to improve photoelectrocatalytic efficiency has been proven to be an effective strategy^［68-71］. These p-type co-catalysts can effectively extract photogenerated holes from the BiVO₄ bulk and store them, thereby suppressing electron-hole recombination. Zhang et al.^［72］ used a simple soaking method to deposit a 2 nm thick ultrathin β-FeOOH layer on worm-like BiVO₄, and due to the abundant oxygen vacancies in this crystalline nanolayer, it exhibited a photocurrent density of 4.3 mA/cm² @1.23 V_RHE under AM 1.5 G, which is twice that of amorphous FeOOH prepared by electrodeposition and close to the performance of the Ni/BiVO₄ photoanode with in-situ generated NiOOH^［73］. Among OER electrocatalysts, β-FeOOH effectively reduces the coordination number of catalytic sites and adjusts the surface electronic structure by introducing oxygen vacancies^［74-75］. Such changes in structure and electronic properties significantly enhance the activity of the OER reaction. Especially in ultrathin β-FeOOH, the abundant oxygen vacancies not only promote the rapid migration of holes (Figure 12) but also enhance the ability of holes to capture electrons, thus effectively driving highly oxidative holes to migrate outward. This migration mechanism helps reduce energy loss at the photoanode/electrolyte interface, thereby improving the overall OER catalytic efficiency. Although FeOOH theoretically has catalytic activity, its conductivity is poor, and it is difficult to achieve efficient catalysis at low bias. To address this issue, researchers have improved its conductivity and light capture capability by loading carbon quantum dots (CQDs) on the FeOOH surface^［76］. These CQDs can efficiently capture ultraviolet and visible light, significantly improving the catalytic kinetics of FeOOH and suppressing carrier recombination. Experimental data show that the photocurrent of FeOOH loaded with CQDs is 10.7 times and 2.98 times higher than that of pure BiVO₄ and FeOOH/BiVO₄ respectively @0.8 V_RHE, and its onset potential is also reduced by 448 mV and 255 mV respectively. Additionally, by encapsulating FeOOH/BiVO₄ in ZnFe layered double hydroxides^［77］, a hierarchical structure was formed, which not only optimizes the separation and transfer of photogenerated electron-hole pairs but also enhances the material's response to visible light, thereby significantly improving PEC efficiency. Furthermore, inserting a 1T-MoS₂ layer at the BiVO₄/FeOOH interface can significantly improve the surface charge accumulation of BiVO₄^［78］ and promote solid/solid charge transfer; inserting Ni‑N₄‑O sites can lower the reaction energy barrier^［80］ and accelerate reaction kinetics, and the photocurrent density of the photoanode reaches 6.0 mA/cm² at 1.23 V_RHE. The introduction of single-atom Ni-N4-O forms stable chemical bond bridges, establishing a strong interface between the OEC and semiconductor photoanode, suppressing the degradation of co-catalytic materials during the photoelectrochemical process, ultimately setting a record of 200 h stable water splitting.

Based on the understanding that Ni-doped FeOOH can achieve a highly active catalytic phase, Zhang et al.^［81］ developed a novel photoanode material: ultrathin Ni:FeOOH (~8 nm) decorated nanoporous BiVO₄. Through intensity-modulated photocurrent spectroscopy hole scavenger measurements, it was found that Ni:FeOOH mainly improves the recombination situation of photogenerated carriers by reducing the surface recombination rate constant (k_rec) rather than directly participating in the catalytic reaction. Subsequently, Fang et al.^［82］ achieved accelerated separation of photogenerated carriers by loading Co-Pi onto Ni:FeOOH@BiVO₄, allowing holes to be quickly captured by the Co-Pi layer and participate in the catalytic cycle, thereby significantly improving the efficiency of hydrogen and oxygen evolution. The valence band edge of Co-Pi is more positive than the oxidation potential of other transition metal ions in the co-catalyst^［83］. In the Ni:FeOOH/Au/BiVO₄ photoanode, Au nanoparticles generate hot holes through their unique interband transition mechanism^［84］, which not only improves the separation efficiency of photocarriers but also enhances the water oxidation activity of NiFeOOH, ultimately achieving a high photocurrent density of up to 5.3 mA/cm² at 1.23 V_RHE.

In addition to Ni\Fe oxyhydroxides and hydroxides, the large interlayer spacing and controllable structure of Co(OH) _x may endow it with excellent ionic conductivity and a large active area for OER in alkaline electrolytes^[13]. However, due to the change in valence state during the catalytic cycle, Co²⁺ tends to dissolve after formation, thus loading stable Co-based co-catalysts on the BiVO₄ surface remains a challenge. Using Co₃O₄ as a buried heterojunction with a surface conformal coating of NiOOH has been reported as a feasible solution; the synthesized NiOOH/Co₃O₄/BiVO₄ photoanode achieved stable operation for 90 hours^[85]. In fact, even if the intermediate layer Co₃O₄ cocatalyst can chemically passivate the surface states, its water oxidation performance is still inferior to that of CoOOH formed by contacting the electrolyte medium, since CoOOH can accumulate more photogenerated holes in the bulk phase, providing more active sites^[86-87], but the formation of CoOOH will cause the electrode to decay almost uncontrollably^[88]. Therefore, novel composite Co-based catalysts need further development in the future.

The photoelectrochemical performance of BiVO₄ photoanodes is limited by their slower charge transport speed. To address this, researchers have adopted the approach of combining BiVO₄ with other ultra-thin metal oxides to form heterostructures, such as BiVO₄/Fe₂O₃, BiVO₄/ZnO, and BiVO₄/MoO₃, etc.^［89］. This structural design aims to optimize surface charge transport characteristics to promote charge separation and transport while maintaining the other excellent properties of BiVO₄ as much as possible. Among numerous BiVO₄-based photoanode materials, the WO₃/W:BiVO₄ NW photoanode structure doped with W nanowires (NW) has demonstrated better photoelectrochemical performance^［90］, where BiVO₄ serves as the primary light absorption layer and WO₃ acts as the electron-conducting support layer. However, insufficient charge separation efficiency and issues like photocorrosion limit the solar-to-hydrogen conversion efficiency. To solve these problems, researchers have proposed new photoelectrode architectures. Gil-Rostra et al. fabricated substrate-supported nanotubes (NTs)^［79］ (Figure 13) using a soft template method, which consist of concentric layers of indium tin oxide (ITO), WO₃, and BiVO₄, along with a thin CoPi layer. This structure not only provides a high electrochemical surface but also minimizes resistive losses, thereby enhancing the catalytic efficiency for OER. The fabrication method of these multilayered nanotube electrodes holds potential for large-scale production and exhibits good stability, opening new avenues for the development of PEC water splitting. Additionally, the addition of CoPi facilitates surface reactions between photogenerated holes and water, further improving catalytic performance. Despite significant progress achieved with this method, the onset potential remains relatively high, which is one of the issues future research needs to address. In another study, Thirumalaisamy et al.^［91］ inserted a TiO₂ passivation layer between the WO₃/BiVO₄ semiconductor and the NiOOH co-catalyst layer. This structural improvement significantly enhanced photocurrent density and stability. By optimizing the combination strategy of spraying and sputtering TiO₂ coating layers, it can play a key role in thin-film growth, blocking surface defects, and improving surface charge carrier separation efficiency during the PEC water splitting process.

Simple carbides, phosphides, sulfides, etc. are usually precursors of substances with oxygen evolution activity and are thermodynamically unstable. Ultrathin (~2 nm) graphitic phase C₃N₄ nanosheets (g-C₃N₄) can effectively transmit and store holes for water oxidation^［92］, breaking through the charge transport limitations of layered structures with weak van der Waals forces (Figure 14). After g-C₃N₄ modification of BiVO₄, the photocurrent density increased to seven times that of the original BiVO₄, and it has high hydrogen production capability. Further studies show that ultrathin g-C₃N₄ can achieve ultrafast hole extraction at a lower bias voltage under solar illumination. Due to the staggered band structure and Fermi level pinning effect, an optimized electric field is formed between g-C₃N₄ and BiVO₄, which can drive rapid transfer of interface holes to the photoanode/electrolyte surface. Further loading of the cocatalyst CoOOH results in the CoOOH/g-C₃N₄/BiVO₄ photoanode achieving a significant water oxidation photocurrent of 4.2 mA/cm² at 1.23 V_RHE (η_Sep = 95.2%)^［93］, and its applied bias photon-to-current efficiency (ABPE) is 10.31 times higher than that of pure BiVO₄.

Moreover, compared with reduced graphene (Reduced-Graphene; r-GO)^［94］, oxide graphene (Oxide-Graphene; o-GO) exhibits superior hole extraction capability and good stability^［95］. After 30 hours of stability testing, NiOOH/o-GO/BiVO₄ still maintains its original morphology and structure.

The amorphous FeSnOS^［96］ obtained by annealing metal sulfide coordination compounds containing Fe/Sn, CuSCN^［97］ obtained by spin-coating, black phosphene (BP)^［98］, and Mxene quantum dots loaded by wet chemical processes^［99］ are all considered to be good photo-generated hole storage layers. The p-n heterojunctions constructed with BiVO₄ can efficiently extract bulk carriers and prevent the accumulation of holes in the Helmholtz layer. Further, by conformally depositing well-coated co-catalysts such as NiOOH^［98］, the internal thermodynamically unstable hole transport layer can be protected from redox reactions, thereby achieving good stability.

Metal-organic frameworks (MOFs) have gained widespread attention in the field of PEC water splitting as a new type of catalytic material. Among them, mixed-metal MOFs exhibit significant potential due to their unique structural and performance advantages. The advantages of mixed-metal MOFs in PEC water splitting are mainly reflected in their ability to provide more active sites and more effectively separate photogenerated charge carriers. Li et al. prepared NiFe bimetallic MOFs through in-situ etching and coated them onto BiVO₄ photoanodes (Figure 15), achieving a photocurrent density of 4.61 mA/cm² @1.23 V_RHE, demonstrating excellent PEC performance. This indicates that the introduction of bimetallic MOFs can significantly enhance PEC performance. However, the interaction between BiVO₄ particles and micrometer-sized MOF crystals may negatively affect PEC performance. To address this issue, researchers have explored the application of two-dimensional MOFs. The ultra-thin two-dimensional MOF structure can provide a large number of active sites and shorten the charge carrier diffusion path, thereby enhancing PEC performance. Pan et al. developed Ov-BiVO₄@NiFe-MOFs photoanodes^［102］ with excellent stability over 10 hours, where the thickness of NiFe-MOFs is approximately 15 nm. This composite material exhibited superior performance in PEC water splitting. Constructing amorphous MOF glass catalysts can also significantly increase the contact rate of key solid-solid interfaces and improve catalyst utilization. Due to the excellent conductivity, stability, and open active sites of MOF glass, compared to pristine BiVO₄ photoanodes, the Co-a_gZIF-62/NiO/BiVO₄ photoanode reported by Song et al.^［103］ demonstrated significantly enhanced photoelectrochemical water oxidation activity and stability, achieving a photocurrent density of 5.34 mA/cm² @1.23 V_RHE. Theoretical calculations show that the introduced Co-a_gZIF-62 as a co-catalyst enhances interfacial reaction kinetics. Due to the rapid extraction and utilization of holes, it avoids the accumulation of holes in the bulk and at the interface, allowing the photoanode to maintain excellent continuous water oxidation for 20 hours.

Moreover, N and S co-doped FeCo-MOFs also show excellent performance in PEC water splitting^［104］. Introducing N and S elements into FeCo-MOFs can further optimize their structure and performance, improve the efficiency of electron and hole transfer, and expose more catalytic active sites. For instance, the optimized BiVO₄/NS-FeCo-MOFs photoanode exhibits a photocurrent density of 5.23 mA/cm², which is 4.84 times higher than that of the pristine BiVO₄ photoanode. Additionally, this composite material also demonstrates excellent long-term stability, maintaining 96% of the initial current after 5 hours. ZnCo-MOFs nanosheets, as a new type of catalyst^［105］, also show potential in PEC water splitting. Their large specific surface area helps improve charge transfer and suppresses the photochemical corrosion of BiVO₄ during the water oxidation reaction. In practical applications, ZnCo-MOFs nanosheets loaded onto nano-worm-like BiVO₄ enable the storage and efficient transfer of photogenerated holes, retaining 95% of the initial photocurrent density after stable operation for 24 hours.

This article summarizes the research progress of conformal coatings used to modify BiVO₄ photoanodes and their photoelectrochemical performance (see Table 1 for details).