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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Review

Scalable Conformal Coating Strategies for Surface Engineering of BiVO4 Photoanodes

  • Weilong Qin 1, 2 ,
  • Ruiyuan Sun 2 ,
  • Muhammad Bilal Akbar 2 ,
  • Yang Zhou 2 ,
  • Yongbo Kuang , 2, 3
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  • 1. Materials Science and Engineering College,Jiangxi University of Science and Technology,Ganzhou 341000,China
  • 2. Key Laboratory of Advanced Fuel Cells and Electrolyzers Technology of Zhejiang Province,Ningbo Institute of Materials Technology and Engineering,Chinese Academy of Sciences,Ningbo 315201,China
  • 3. Center of Materials Science and Optoelectronics Engineering,Chinese Academy of Sciences,Beijing 100049,China

Received date: 2024-04-18

  Revised date: 2024-08-27

  Online published: 2025-02-07

Supported by

National Natural Science Foundation of China(22379153)

Ningbo Key R&D Program(2023Z147)

Ningbo 3315 Program(2023Z147)

Abstract

Solar photoelectrochemical (PEC) water splitting holds significant importance for the development of sustainable green energy. With ongoing research, the BiVO4 photoanode, a core component of PEC systems, faces challenges in scaling up and maintaining long-term stability. The superiority of fully conformal coating strategies lies in their lack of substrate size constraints, ability to suppress photo-corrosion of the BiVO4 semiconductor, creation of multifunctional interfaces, and potential synergistic action with heterojunctions and promoter catalysts, which may facilitate the stable operation of large-scale PEC water splitting devices for over 1000 hours. This review briefly introduces the basic principles of PEC water splitting and the development status of representative devices, elaborates on the important concept and main design principles of fully conformal coatings aimed at large-scale photoanodes, summarizes recent advances in materials capable of achieving fully conformal deposition coatings, including molecular catalysts, metal oxides/hydroxides, carbonized/sulfurized/phosphorized materials, and metal-organic frameworks (MOFs), and discusses key characteristics of fully conformal coatings with greater development potential. Finally, it presents a prospective view on future trends in fully conformal coatings for BiVO4 photoanodes.

Contents

1 Introduction

2 Fundamentals of PEC water splitting and develop- ment status of PEC device

3 Basic principles of fully conformal coating strategy

3.1 Fully conformal coating and its importance

3.2 Primary design principles of fully conformal coating

4 Recent progress of fully conformal coating strategy

4.1 Molecular catalyst

4.2 Metal oxides/hydroxides

4.3 Carbide/Sulfide/Phosphide

4.4 Metal-organic framework

5 Conclusion and outlook

Cite this article

Weilong Qin , Ruiyuan Sun , Muhammad Bilal Akbar , Yang Zhou , Yongbo Kuang . Scalable Conformal Coating Strategies for Surface Engineering of BiVO4 Photoanodes[J]. Progress in Chemistry, 2025 , 37(3) : 425 -438 . DOI: 10.7536/PC240414

1 Introduction

The photoelectrochemical (PEC) hydrogen production through solar-driven water splitting is a promising method for providing clean energy carriers in the future1-3. The photoanode plays a crucial role in the water splitting process by efficiently driving the four-electron reaction (2H2O → O2 + 4H+ + 4e-), achieving the conversion of hydroxide ions in the electrolyte to oxygen. Among numerous anode material candidates, bismuth vanadate (BiVO4) has attracted significant attention due to its excellent theoretical catalytic efficiency, abundance, and low cost4. However, BiVO4 is limited by bulk/surface charge recombination and electrophotochemical corrosion, making it difficult to achieve high PEC performance. Surface engineering can address most performance issues of the BiVO4 photoanode by eliminating surface states and suppressing photocorrosion5.
Surface engineering generally includes constructing heterojunctions, loading co-catalysts, forming passivation layers, and loading protective layers6. Due to the requirements of future industrialized PEC devices for core components, BiVO4 photoanodes must possess both size scalability and high oxygen evolution reaction (OER) efficiency, as well as water splitting stability of at least 1000 hours7. On the surface of a reaction device, heterojunctions and co-catalysts often dissolve, peel off, or lose activity due to poor ohmic contact, electrolyte movement, and changes in the chemical state of the catalyst. Although in reported studies, the above two strategies can achieve stability of around 100 hours8, due to the self-limiting nature of these strategies, longer-term performance is difficult to achieve. The passivation layer strategy accelerates carrier transfer by eliminating surface potential wells and reducing redox energy barriers9. Cobalt phosphate (Co-Pi) is a classic passivation layer on the BiVO4 surface and is considered to almost completely eliminate the surface states of BiVO410; however, in commonly used alkaline electrolytes, stability rapidly decays with the dissolution of Co, and under neutral conditions, it is also difficult to exceed 10 hours11. Therefore, to solve the above problems, a fundamental change in strategy is to prioritize the loading of fully conformal protective layers, such as organic polymer films, isolating the semiconductor from the electrolyte, suppressing photochemical corrosion, and ensuring continuous photoelectric conversion output from every particle converter on the photoanode semiconductor. Heterojunction and co-catalyst strategies serve as auxiliary methods to further optimize PEC performance, ultimately leading to highly efficient and long-term stable photoanodes.
The protective layer is the most common surface modification method for BiVO4 photoanodes. At present, as research progresses, small-sized photoanodes are gradually transitioning to larger sizes (Figure 1). The ohmic loss caused by the size amplification effect leads to a sharp decrease in the performance of unassisted bias-free PEC devices, greatly delaying the arrival of large-area industrial solar-to-hydrogen (STH) expected efficiency (at least 5%) [12]. Under the shift in research background, surface engineering strategies based on protective layers also change with the demands of research development, including: (1) A full conformal coating strategy with a high aspect ratio load derived from the protective layer strategy; (2) Multiple functions such as electron transport and co-catalytic properties based on further protective requirements; (3) Low-cost, scalable synthesis processes and materials.
图1 Histogram of the Number of Reported PEC Reactors Over the Past 25 Years, Classified by Small-Area Devices (Gray) and Large-Area Devices (Green)7, Where the Irradiation Area of Large-Area Devices Exceeds 50 cm2

Fig. 1 Histogram of reported PEC reactors over the last 25 years concerning the number of small-area devices (grey) and large-area devices with an illuminated area higher than 50 cm2 (green)7

In this review, starting from the current realistic dilemmas of large-scale BiVO4 photoanodes, the focus is on the full-conformal coating strategy in surface engineering. The principle of PEC water splitting and the development status of PEC devices are briefly introduced, and the importance of full-conformal coating in surface engineering and its design principles for size scaling are elaborated. The latest research progress of full-conformal coatings is highlighted, discussing different types of full-conformal coatings and their impacts on photoanodes. Finally, the future trends and potential advancements of the full-conformal coating strategy are explored, and the prospects of further improving the efficiency and stability of BiVO4 photoanodes using full-conformal coatings are discussed.

2 PEC Water Splitting Principle and Its Device Development Status

The basic principle of PEC water splitting is based on the solid-state band theory13. Under the illumination of incident light with energy higher than the semiconductor bandgap energy (>1.23 VRHE, Reversible Hydrogen Electrode, RHE), electrons are excited from the valence band (Valence Band, VB) to the conduction band (Conduction Band, CB), leading to the formation of holes and quasi-free electrons. At a standard Gibbs free energy of ΔG0 = 237 kJ·mol-1, photogenerated holes (h+) migrate to the semiconductor surface to undergo water oxidation reactions, while electrons transfer through the conductive substrate to the photocathode for water reduction reactions14. In the half-electrode photoanode reaction system (Figure 2), various near-transparent metal oxide semiconductor thin films have been developed as solar photon collectors, including: Fe2O315, WO316, TiO217, BiVO48, Ta3N518, etc. Wide bandgap (generally considered >2.2 eV) semiconductors, such as ZnO19 (direct bandgap Eg = 3.37 eV), have gradually been phased out during development due to their narrow light absorption range. The narrow bandgap semiconductor BiVO4 has a more positive VB position and strong oxidation capability, making it one of the most promising photoanode candidates (Figure 3 illustrates the main issues of BiVO4 photoanode water splitting).
图2 (a) Schematic Diagram of Photoanode PEC Water Splitting Cell; (b) Chemical Reaction Formula of PEC Water Splitting7

Fig. 2 (a) Schematic diagram of a photoanode PEC water decomposition cell;(b) PEC water decomposition chemical reaction equation7

图3 Main Issues at the BiVO4 Photoanode Water Splitting Interface21

Fig. 3 Main issues with the BiVO4 photoanode water splitting interface21

To reduce costs and maximize efficiency, unbiased full water splitting is the most ideal theoretical model20. A complete full water splitting device mainly consists of four parts, including a photoelectrolysis device, electrolyte, reactor, and membrane or separator. When assembling the equipment, four main considerations are involved, including: (1) Material selection principles: ① Preparation method of composite electrodes; ② Earth abundance; ③ Raw material cost; ④ Environmental impact; (2) Device design principles: ① Low manufacturing cost; ② High STH efficiency; ③ Long-term stability; (3) Efficiency influencing factors: ① Light absorption efficiency; ② Bulk charge transport efficiency; ③ Interfacial charge transfer efficiency; (4) Device configuration: ① Photocatalytic water splitting device; ② PEC water splitting device; ③ Photovoltaic-electrolysis water device. Among these, the PEC water splitting device has better application prospects due to its higher efficiency and lower cost advantages.
In 2021, a large-sized Cu3BiS3-BiVO4 tandem PEC water-splitting device with dimensions of 5 × 5 cm222 was reported, showing long-term stability of up to 60 hours and an unbiased STH efficiency of 2.04%. After improving the photocathode to Pt-TiO2/In0.6Cd0.4S/Cu3BiS323, the tandem assembly could achieve water splitting for over 100 hours, refreshing the STH efficiency to 2.57%. A small-sized particulate NiFeCoOx/CPF-TCB/Mo:BiVO4 photoanode assembled with perovskite/silicon solar cells set a record STH efficiency of 9.1% for photoelectrochemical tandem devices24, but its stability only lasted 10 hours. Domen et al.25 reported the currently largest photocatalytic device with an effective area of 100 m2, capable of continuous operation for 1600 hours. Based on the principle of solar-to-hydrogen conversion, this has important reference significance for the development of large-scale PEC devices. In addition, some novel water-splitting devices also show great application potential, providing new ideas for the diversified development of future PEC devices, such as: decoupled hydrogen and oxygen cells in separate battery PEC systems26, lightweight unassisted artificial leaves27, magnifying glass concentrated sunlight-indium gallium nitride photocatalytic systems28, and shuttle ion loop Fe3+/Fe2+ integrated hydrogen farm systems29. Recently, Vilanova et al.7 classified PEC devices into four main types based on electrode structure and gas separation strategies: wired back-to-back, wireless back-to-back, wired side-by-side, and membrane-free wired separated electrodes. They pointed out that investment in PEC device research should move forward, especially considering the pursuit of decarbonization. Therefore, future efforts still need continuous improvement in reactors and process flows to achieve significant cost reductions. Additionally, improving STH efficiency, enhancing the durability of catalysts, and increasing the efficiency of gas separation are key challenges that need to be addressed.

3 Basic Principles of Full Conformal Coating Strategy

3.1 Full Conformal Coating and Its Importance

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.
图4 Conformal ALD Process for Depositing Trimethylaluminum/Water32

Fig. 4 Conformal ALD process for depositing trimethylaluminum/H2O32

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.

3.2 Main Design Principles of Fully 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 hours8, 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.
图5 Summary of the Performance of BiVO4 Photoanodes Modified with Different Strategies8

Fig. 5 Summary of BiVO4 photoanodes modified with different strategies8

3.2.1 Materials and Synthetic Method Compatibility

Although the spin-coating method has achieved the surprising preparation of large-scale BiVO4 substrates up to 25 cm237, traditional deposition methods represented by this are difficult to achieve the synthesis of photoanode unit devices as large as 625 cm2 reported by Domen et al.25. Classic techniques such as electrodeposition, photoelectrodeposition, hydrothermal method, microwave synthesis, magnetron sputtering, vacuum evaporation, and atomic layer deposition are also limited to smaller equipment synthesis chambers and high manufacturing costs.
More universally applicable large-scale preparation methods generally include spraying methods and chemical solution synthesis methods. The spraying method has the advantages of adjustable size and highly uniform film formation, and is mainly divided into spray pyrolysis and direct spraying[38]. Reported studies have utilized citrate complexes with different metal cations, and spray pyrolysis can provide uniform nanoporous multi-oxide films with controllable porosity[39]. Further application of this method allows for the deposition of dense catalytic films with high coverage and optimal performance on semiconductor surfaces, such as NiFe layered hydroxide oxides and amorphous oxides[40]. Using a one-pot spray pyrolysis method, a good conductive carbon layer can be constructed on the anode substrate[41], but it remains relatively difficult to use this method to construct good conformal structures of other materials.
Direct spraying method has the advantages of simple operation and easy regulation compared with drop coating and spin coating methods, which can solve the problems of liquid distribution uniformity and batch operation, and has lower cost in large-scale coating preparation42. However, due to the significant differences in viscosity among various precursor solutions, it is difficult for the spraying method to be compatible with most coating materials. Coating materials using chemical solution synthesis are easier to achieve full conformity. For instance, polydopamine (PDA) materials with self-assembly properties can rapidly penetrate into the interior through the precursor solution and polymerize into a highly adhesive film under the action of an initiator. This allows for comprehensive encapsulation on various materials such as nano-semiconductors, noble metals, glass, and ceramics43. Such organic polymer coatings, especially those prepared by synthetic methods incorporating dimensional amplification characteristics, have shown great potential as efficient photoanode full-conformal coating materials. These coating materials, including conductive polymers such as polyaniline, polydioxythiophene, polypyrrole, polyheptazine, and metal porphyrins, have been widely used in fields like solar cells and electrochemical energy storage due to their excellent conductivity and electrochemical activity44-48. The preparation methods of these materials not only ensure their feasibility in large-scale production but also maintain high compatibility with dimensional amplification technology, giving them significant advantages in achieving efficient and scalable photoanode manufacturing.

3.2.2 Balance Between Protective and Other Functions

In the field of photoelectrochemistry, the oxygen evolution reaction (OER) of BiVO4 photoanodes has become a bottleneck limiting the efficiency of water splitting due to its sluggish kinetics49. In addition to the reported surface modification methods such as anchoring borate ions50, the full-conformal coating strategy has the same potential to improve PEC performance and offers the possibility for long-term stability. However, it is necessary to regulate and balance the relationship between the protective nature of the coating and other functionalities. Among many organic polymers that easily form full-conformal coatings, PDA has attracted widespread attention in recent years due to its unique chemical properties and biocompatibility as a natural adhesive and stabilizer. Therefore, PDA is particularly illustrative of the importance of this design principle.
The application of PDA coating in photoelectrochemistry is first reflected in its excellent protective performance. For instance, Ran et al. reported the research results on the degradation of methylene blue by BiVO4@PDA photocatalyst on cotton fabric51. After multiple cycles of use, the material still maintained 98% of its photocatalytic activity, and X-ray diffraction (XRD) results showed that the crystal structure of BiVO4 remained unchanged before and after the reaction. This indicates that the PDA coating not only provided good protection for BiVO4 (Figure 6), preventing its degradation during the catalytic process, but also enhanced the catalytic performance through a synergistic effect. In addition to its protective properties, the PDA coating also had a positive impact on the material’s light absorption, electron transport, ohmic loss, and cocatalytic capacity. For example, the BiVO4@PDA/TiO2 core-shell structure developed by Liu et al. demonstrated efficient visible-light response and stability52 (Figure 7). After five cycles of experiments, the removal rate of rhodamine B was still as high as 91.58%. The advantage of this structure is attributed to the extended π-conjugated electron system of PDA. It not only improved carrier mobility but also suppressed the aggregation of PbS quantum dots due to its natural adhesive and stabilizing properties, significantly enhancing the photocatalytic activity in the PbS@PDA/BiVO4 system53. Additionally, Celebi et al.54 constructed ZnO@C-CeO2 photoanodes through a carbonization process, where the PDA layer was converted into a graphitic carbon layer after calcination, significantly increasing the photocurrent density by 371.5 times. This achievement highlights the potential of PDA in improving the conductivity of materials. Despite PDA playing a key role in enhancing the performance of photoelectrodes, in some cases, such as reported by Peng et al.55, the carbonization of PDA may cause transient current responses at low bias voltages, affecting stability. This suggests that while pursuing high performance, it is necessary to ensure a balance between PDA's protective function and other functionalities.
图6 (a) Transmission Electron Microscopy Image of BiVO4 Particles; (b-d) BiVO4@PDA Particles[51]

Fig. 6 (a)TEM images of BiVO4 particles;(b-d) BiVO4@PDA particles

图7 Photocatalytic Mechanism of BiVO4@PDA/TiO2 Photoanode52

Fig. 7 The photocatalytic mechanism of the BiVO4@PDA/TiO2 photoanode

In summary, the application of fully conformal coatings in large-scale photoelectrochemical systems lies not only in their protective performance but also in their comprehensive enhancement of material properties. Designers should focus on improving the light absorption, electron transport, and catalytic efficiency of photoelectrodes while not compromising stability. However, how to further optimize the coating for more efficient and stable photoelectrocatalytic performance remains a subject worthy of in-depth research.

4 Recent Advances in the Fully Conformal Coating Strategy

4.1 Molecular Catalysts

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 cocatalysts56. 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 al57 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 al58 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 VRHE, 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.
图8 Investigating the Carrier Dynamics of BiVO4 and PTh/BiVO4 Thin Film Materials Using Time-Resolved Photoluminescence (TR-PL) Experiments58: (a-b) Revealing the Impact of PTh Doping on the Carrier Recombination Process in BiVO4 Through Measurements of Transient Charge Transfer Rate (ktrans) and Recombination Rate Constant (krec); (c-d) Employing 355 nm Laser Pulse Excitation to Measure Transient-State Surface Photovoltage (Transient-State SPV) and Explore the Charge Separation and Recombination Behavior of BiVO4 and PTh/BiVO4 at Different Timescales

Fig. 8 In time-resolved photoluminescence (TR-PL) experiments, the carrier dynamics characteristics of BiVO4 and PTh/ BiVO4 films were investigated. (a-b)Through the measurement of charge transfer rate (k trans) and recombination rate constant (k rec), the impact of PTh doping on the carrier recombination process in BiVO4 was revealed; (c-d)Transient surface optical voltage(transient-state SPV) measurements under 355 nm laser irradiation on a logarithmic timescale were conducted to explore the charge separation and recombination behavior of BiVO4 and PTh/ BiVO4 over different time scales

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 VRHE, 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 films60 (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 VRHE 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.
图9 Preparation Process of BiVO4/Chu/CoSi Photoanode59

Fig. 9 Preparation process of the BiVO4/Chu/CoSi photoanode

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 characteristics61-62. Currently, the longest stable duration of the PDA matrix coating can reach 5 hours @1.23 VRHE, however, after carbonization, PDA shows a dotted distribution on the BiVO4 surface, and the color of the PDA film becomes darker after N2 annealing reduction63, 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 surface64-65, 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 al66 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 VRHE.
图10 Charge Density Difference of FePc(P) Molecule, Showing Differential Charge Distribution via 2D Data Plot66: Yellow Clouds Represent Electron Accumulation and Cyan Clouds Indicate Electron Depletion (a) Top View; (b) Side View

Fig. 10 Charge density difference of FePc(P) with two-dimensional data diagram of differential charge:yellow cloud represents electron accumulation and cyan cloud denotes electron depletion;(a) top view;(b) side view

In addition, Zhang et al67 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.
图11 Schematic Diagram of CTF-BTh Forming a Full Conformal Coating on the Surface of Cu2O Photocathode and Mo:BiVO4 Photoanode67

Fig. 11 Schematic illustration depicting the formation of fully conformal coatings of CTF-BTh on both Cu2O photocathode and Mo:BiVO4 photoanode surfaces

4.2 Metal Oxides/Hydroxides

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 strategy68-71. 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 VRHE 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 NiOOH73. Among OER electrocatalysts, β-FeOOH effectively reduces the coordination number of catalytic sites and adjusts the surface electronic structure by introducing oxygen vacancies74-75. 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 surface76. 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 VRHE, and its onset potential is also reduced by 448 mV and 255 mV respectively. Additionally, by encapsulating FeOOH/BiVO4 in ZnFe layered double hydroxides77, 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 BiVO478 and promote solid/solid charge transfer; inserting Ni‑N4‑O sites can lower the reaction energy barrier80 and accelerate reaction kinetics, and the photocurrent density of the photoanode reaches 6.0 mA/cm2 at 1.23 VRHE. 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.
图12 Schematic Diagram of Charge Transfer on β-FeOOH/BiVO4 Photoanode72

Fig. 12 Illustration of the charge transfer on β-FeOOH/BiVO4 photoanode72

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 (krec) 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-catalyst83. In the Ni:FeOOH/Au/BiVO4 photoanode, Au nanoparticles generate hot holes through their unique interband transition mechanism84, 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 VRHE.
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 performance90, 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.
图13 (a) Top-view SEM images of ITO/WO3/BiVO4 1 μm and (b) ITO/WO3/BiVO4 200 nm; (c) Schematic illustration of NT and TF electrode nanostructures and the associated PEC process, highlighting the operational advantages of ITO/WO3/BiVO4 nanotube structures[79]

Fig. 13 Top view SEM micrographs of (a)ITO/WO3/BiVO4 1 μm scale; (b)ITO/WO3/BiVO4 200 nm scale;(c)Scheme of the NT and TF electrode nanostructures and the involved PEC process, highlighting the operative advantages of the ITO/WO3/BiVO4 nanotube structure

4.3 Carbides/Sulfides/Phosphides

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 oxidation92, 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 VRHESep = 95.2%)93, and its applied bias photon-to-current efficiency (ABPE) is 10.31 times higher than that of pure BiVO4.
图14 TEM, HRTEM, and SEM Images of BiVO4/g-C3N4-NS92; (a) TEM Image; (b) HRTEM Image; (c) SEM Top View; (d) SEM Side View

Fig.14 TEM, HRTEM and SEM micrographs of BiVO4/g-C3N4-NS;(a)TEM image; (b)HRTEM image; (c)SEM top view; (d)SEM side view

Moreover, compared with reduced graphene (Reduced-Graphene; r-GO)94, oxide graphene (Oxide-Graphene; o-GO) exhibits superior hole extraction capability and good stability95. After 30 hours of stability testing, NiOOH/o-GO/BiVO4 still maintains its original morphology and structure.
The amorphous FeSnOS96 obtained by annealing metal sulfide coordination compounds containing Fe/Sn, CuSCN97 obtained by spin-coating, black phosphene (BP)98, and Mxene quantum dots loaded by wet chemical processes99 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 NiOOH98, the internal thermodynamically unstable hole transport layer can be protected from redox reactions, thereby achieving good stability.

4.4 MOFs

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 VRHE, 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 photoanodes102 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-agZIF-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 VRHE. Theoretical calculations show that the introduced Co-agZIF-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.
图15 Schematic Diagram of the Structure of BiVO4@NiFe-MOFs102

Fig.15 Schematic of the structure of BiVO4@NiFe-MOFs

Moreover, N and S co-doped FeCo-MOFs also show excellent performance in PEC water splitting104. 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 catalyst105, 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).
表1 近年来采用共形涂层策略修饰BiVO4光阳极电极制备方法与性能汇总表

Table 1 Summary of reported fabrication methods and PEC performance of BiVO4 photoanodes modified with conformal coating

Photoanode Fabrication methods Coating

Current density (at 1.23 VRHE);Stability;

area

ref
CoO x /BiVO4 Drop-casting Co4O4 Cubane 5 mA/cm2 (pH 9.3 KBi);140 s;1 cm2 57
PTh-Fe/ BiVO4 In-situ polymerized PTh 4.72 mA/cm2 (pH 9 KBi);40 h;2 cm2 58
NiOOH/PANI/BiVO4 Photoelectrodeposition PANI 5.1 mA/cm2 (pH 7 KPi);3 h;2 cm2 59
BiVO4/Chu/CoSi Solution synthesis + dripping Chlorophyll-Cu

5.1 mA/cm2 (0.5 M Na2SO4);12 000 s;

\

60
BiVO4-C/N-Ag Solution synthesis + annealing PDA 2.42 mA/cm2 (0.5 M Na2SO4);5 h;1 cm2 63
BiVO4@ZCF(P)-O Anhydride-Urea process

ZnCoFe-

phthalocyanine

5.7 mA/cm2 (pH 7 KPi);40 h;\ 66

NiFeOx/CTF-BTh/Mo:

BiVO4

Solution synthesis + electrodeposition CTF-BTh 5.7 mA/cm2 (pH 9 KBi);150 h;2 cm2 67
β-FeOOH/BiVO4 Solution synthesis β-FeOOH 4.3 mA/cm2 (0.2 M Na2SO4);2 h;\ 72
BiVO4/Ni@NiOOH Dip-coating and annealing Ni@NiOOH 4.41 mA/cm2 (pH 9.5 KBi);10 h;\ 73
CQDs/FeOOH/BiVO4 Solution synthesis + spin coating β-FeOOH 2.53 mA/cm2 (0.2 M Na2SO4);2 h;\ 76
BiVO4/FeOOH/ZnFe-LDH Electrodeposition FeOOH

4.91 mA/cm2 (1 M Na2SO4);\

6000 s

77
FeOOH@1T-MoS2@BiVO4 Annealing + photodeposition 1T-MoS2 4.02 mA/cm2 ( 0.1 M KPi);8 h;2×2 cm2 78
BiVO4@Ni:FeOOH Electrodeposition Ni:FeOOH 2.86 mA/cm2 ( 0.5 M Na2SO4);2 h;1 cm2 77
BiVO4/NiFeOOH/Co-Pi Electrodeposition NiFeOOH 4.02 mA/cm2 ( 0.1 M KPi);7000 s;\ 78
FeOOH/Ni‑N4‑O/BiVO4 Spin + annealing Ni‑N4‑O 6.0 mA/cm2 (0.5 M KBi);200 h;2 cm2 80
BiVO4/Au/NiFeOOH Electrodeposition + immersion NiFeOOH 5.3 mA/cm2 ( pH 9 KBi );\ 84
WO3/BiVO4/TiO2/NiOOH Spray + sputter TiO2 5.3 mA/cm2 ( 0.5 M Na2SO4);4000 s;1 cm2 91
BiVO4/g-C3N4-NS Annealing + immersion + Annealing g-C3N4 3.12 mA/cm2 (0.2 M Na2SO4);200 s;1×5 cm2 92
CoOOH/g-C3N4/BiVO4 Ultrasonic process + Annealing + immersion g-C3N4 4.2 mA/cm2 (pH 7 KPi);6 h;\ 93
NiOOH/GO/BiVO4 Ultrasonic process o-GO 3.8 mA/cm2 (pH 7.1 KPi);34 h;2 cm2 95
FeSnOS-BiVO4 Spin coating + post-annealing FeSnOS 3.1 mA/cm2 (0.5 M Na2SO4);\ 96
BiVO4/CuSCN/NiFeO x Spin-coating + electrodeposition CuSCN 5.6 mA/cm2 (pH 9.3 KBi);15 h;2 cm2 97
NiOOH/BP/BiVO4 Ultrasonic process + Photoelectrodeposition BP 4.48 mA/cm2 (pH 7.1 KPi);60 h;2 cm2 98
OEC/MoO x /MQD/BiVO4 Solution processed MQD 5.58 mA/cm2 (pH 9.3 KBi);100 h;2 cm2 99

5 Conclusions and Prospects

In summary, this paper elaborates on the dilemma of size amplification for BiVO4 photoanodes in the development of large-scale PEC water-splitting devices, proposes a strategy for constructing scalable common coatings, and explains the importance and fundamental design principles of this strategy. It systematically summarizes recent research progress on novel full-conformal coatings for PEC water splitting, including the rising interest in full-conformal coatings of molecular catalysts, where the two-step synthesis method can provide replicable preparation templates for such catalysts, and preparation methods like electropolymerization and photoelectrochemical deposition can also be adapted into initiator-triggered solution polymerization compatible with large-area fabrication. Meanwhile, the use of such well-coated organic films demonstrates catalytic performance comparable to that of classic metal oxides and offers excellent protection, which is crucial for developing efficient, stable, and large-scale water-splitting systems.
The latest research on conformal co-catalysts of metal oxides/hydroxides has also been summarized, which can be used as the outermost layer in future interface designs to provide more and more efficient water oxidation active sites and offer a stable working environment for the intermediate layer - hole extraction layer. Besides, carbides/sulfides/phosphides and other conformal coatings that can serve as both hole transport layers and electron transport layers are promising candidates to enhance the overall hydrogen/oxygen production efficiency. To overcome the adverse effects of interlayer spacing on electron transport, such materials need to be developed towards ultra-thin structures. Conformal coating materials like MOFs have tunable electronic properties, and the current research direction is multi-metal node doping. Due to the crystalline structure of such materials that tends to cause poor interface contact, high-temperature annealing and other processes may still be required in the future to enhance interface bonding.
In summary, the fully conformal coating strategy that can achieve large-scale preparation can promote the development of PEC water splitting devices from different aspects. On the premise that the BiVO4 photoanode has good kinetics, such as bulk doping with Mo and W, constructing heterojunctions, etc., adopting a fully conformal coating with excellent interfacial compatibility to further solve problems such as photocorrosion, ohmic loss, and water splitting efficiency is expected to realize the industrialization of large-scale PEC devices. We hope that this review will bring some new inspiration to the vast number of researchers in the aspect of PEC water splitting interface design, stimulate more researchers to improve the PEC water splitting performance of BiVO4 photoanodes, and carry out more practical application explorations.
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