Charge Transport Enhancement in BiVO4 Photoanode for Efficient Solar Water Oxidation

Photoelectrochemical (PEC) water splitting in a pH-neutral electrolyte has attracted more and more attention in the field of sustainable energy. Bismuth vanadate (BiVO4) is a highly promising photoanode material for PEC water splitting. Additionally, cobaltous phosphate (CoPi) is a material that can be synthesized from Earth’s rich materials and operates stably in pH-neutral conditions. Herein, we propose a strategy to enhance the charge transport ability and improve PEC performance by electrodepositing the in situ synthesis of a CoPi layer on the BiVO4. With the CoPi co-catalyst, the water oxidation reaction can be accelerated and charge recombination centers are effectively passivated on BiVO4. The BiVO4/CoPi photoanode shows a significantly enhanced photocurrent density (Jph) and applied bias photon-to-current efficiency (ABPE), which are 1.8 and 3.2 times higher than those of a single BiVO4 layer, respectively. Finally, the FTO/BiVO4/CoPi photoanode displays a photocurrent density of 1.39 mA cm−2 at 1.23 VRHE, an onset potential (Von) of 0.30 VRHE, and an ABPE of 0.45%, paving a potential path for future hydrogen evolution by solar-driven water splitting.

The surface modification of oxygen evolution catalysts is a more promising method for enhancing the PEC performance of BiVO 4 photoanodes than its other counterparts, due to the effective passivation of the surface charge recombination centers and improving the interfacial OER kinetics for PEC water oxidation. For instance, J. Hu et al. successfully synthesized iron oxhydroxide (FeOOH) with different crystalline phases (α-, β-, and δ-) through a regulated solvothermal pathway, where the electrocatalytic OER activity of β-FeOOH was highest [26]. Carbon quantum dots (CQDs) are also able to remarkably improve electrocatalytic OER activity owing to the increased O v density; at the same time, they can form heterojunctions with semiconductors, thereby effectively promoting charge separation and transport [27]. Moreover, L. Wu et al. synthesized the nickel boride (NiB) layer by adjusting the composition of the neutral electrolyte. The borates with B-O bonds become promoters of catalyst activity by accelerating proton-coupled electron transfer and interact with Ni 2+ ions to inhibit charge recombination on BiVO 4 surface, thereby reporting a J ph of 6.0 mA cm −2 at 1.23 V RHE [14]. Cobaltous phosphate (CoPi) is an effective electrocatalyst for water oxidation and was first reported by G. Nocera and W. Kanan, who also demonstrated that hydrogen phosphate ions are proton acceptors in oxygen production reactions under neutral pH conditions [28]. Moreover, J. Durrant et al. discussed the oxidation degree of the CoPi catalyst on the BiVO 4 photoanode under simulated sunlight irradiation and determined the appropriate degree of catalyst oxidation to drive substantial water oxidation. Additionally, the relative kinetics of water oxidation on the surfaces of electrocatalyst and semiconductor and the kinetics of holes transfer to electrocatalysts were discussed for the first time [18].
In this work, a BiVO 4 thin film was initially fabricated, which is inherently beneficial for charge transport through the adjustment the electrodeposition time of the BiOI precursor film and the dropping of excessive vanadium source solution for annealing in a muffle furnace. Then, the cobaltous phosphate (CoPi) layer was electrodeposited on optimal BiVO 4 electrodes while the composition of neutral electrolyte was regulated by adding cobaltous nitrate and phosphate species. Finally, the FTO/BiVO 4 /CoPi photoanode was successfully prepared. Combining the electrocatalytic and photoelectric technologies, the CoPi for OER catalytic activity was investigated in detail and notable results were obtained, e.g., a J ph of 1.39 mA cm −2 (at 1.23 V RHE ) and an undoped ABPE of 0.45% under AM 1.5 G illumination.

Preparation of BiOI Precursor Film
The BiOI precursor film was electrodeposited on an FTO glass substrate with an effective area of 1.5 cm × 2 cm in a three-electrode system, where a Pt-foil was used as the counter electrode and an Ag/AgCl with saturated KCl solution was used as the reference electrode. The BiOI precursor deposition solution was fabricated by dissolving lactic acid (0.03 M), KI (0.4 M), and Bi(NO 3 ) 3 ·5H 2 O (0.015 M) in deionized water (100 mL), with 1,4-Benzoquinone (0.046 M) in ethanol (40 mL) solution. The pH of the mixed solution was adjusted to 3.7 by adding 0.1 M nitric acid aqueous solution after stirring for 20 min. Initially, a 60 s deposition was conducted at −0.40 V Ag/AgCl to prevent the falling off of BiOI films from the surface of the FTO substrate. After the initial deposition, the BiOI film was obtained at a constant voltage of −0.25 V Ag/AgCl with different deposition durations, then rinsed thoroughly with deionized water and dried in a drying oven.

Preparation of BiVO 4 Electrode
The vanadium source solution was prepared by dissolving VO(acac) 2 (0.5 M) in dimethylsulfoxide (10 mL). The as-prepared BiOI precursor film was dropped into the superfluous vanadium source solution. The electrode was then shifted to a muffle furnace and annealed for about 12 h. The heating rate was 3 • C/min to 120 • C, 0.67 • C/min to 280 • C, and 1.41 • C/min to 450 • C, and then held at 450 • C for 1 h. All the annealing processes ended with furnace cooling. After annealing, the electrodes were immersed in a 1.0 M NaOH solution for 15 min with gentle stirring to wash off the V 2 O 5 on the BiVO 4 surface. The prepared BiVO 4 electrodes were rinsed thoroughly with deionized water and dried in a drying oven.

Photo-Assisted Electrodeposition of CoPi Cocatalyst
The CoPi cocatalyst was electrodeposited on the BiVO 4 electrode under AM 1.5 G simulated sunlight by using the three-electrode system containing the solutions of NaH 2 PO 4 (0. larly, an Ag/AgCl with a saturated KCl solution was used as the reference electrode and a Pt-foil as the counter electrode. The deposition voltage and time were −0.40 V Ag/AgCl and 90 s, respectively.

Characterizations
The crystallinity and structure of BiVO 4 films were examined by X-ray diffraction (XRD, Ultima-iv with Cu/K α radiation). Surface morphologies were observed via scanning electron microscope (SEM, Germany Zeiss SUPRA 55). The transmittance and absorption of BiVO 4 films were measured via a Shimadzu UV-3600 spectrophotometer. Ultraviolet photoelectron spectroscopy (UPS) was performed using a PHI 5000 VersaProbe with an energy value of a He I source with 21.22 eV. The PEC performance was obtained by using the CHI 660E electrochemical workstation under a three-electrode system, where an Ag/AgCl with saturated KCl solution was used as the reference electrode, with Pt-foil as the counter electrode and the BiVO 4 photoanode as the working electrode. Photoelectrochemical impedance spectroscopy (PEIS) was assessed under light conditions, and its frequency ranged from 10 −1 Hz to 10 5 Hz. Mott-Schottky (M-S) measurements were used to calculate the flat band potential (E fb ) of the BiVO 4 films and the analysis of defects in junctions formed at the interface of semiconductor electrolytes.

Results and Discussion
BiVO 4 film with different thicknesses can be prepared by adjusting the electrodeposition time of the BiOI precursor film, i.e., the BiOI-1 precursor film deposited at 300 s, BiOI-2 at 330 s, and BiOI-3 at 360 s, respectively, and then dropping excessive vanadium source solution to anneal in muffle furnace. The resulting BiVO 4 films were labeled as BiVO 4 -1, BiVO 4 -2, and BiVO 4 -3, and the corresponding thicknesses were 358.5 nm, 424.3 nm, and 485.6 nm, respectively ( Figure S1). The X-ray diffraction (XRD) patterns of BiVO 4 film with three different thicknesses are shown in Figure 1a. The existence of three major diffraction peaks (011), (-121), and (040) and the standard monoclinic BiVO 4 (JCPDS Card No.14-0688) peaks without any extra peaks, confirm the high crystallinity and pureness of the as-fabricated BiVO 4 thin films [1]. Figure 1b shows that the smaller FWHM (full width at half maxima) values of different diffraction peaks demonstrate larger crystal grains in the BiVO 4 -2 film. The grain size can also be calculated according to the Scherrer formula [29]: where K is the Scherrer constant, λ is the wavelength of the X-ray sources (0.15406 nm), D is crystallite size (nm), β is the FWHM value, and θ is the Bragg angle at peak position. Figure 1c displays the proportion diagram of grain size distribution, and the BiVO 4 -2 obviously accounts for a large proportion in areas with a large grain size distribution. In addition, the average grain sizes of BiVO 4 -1, BiVO 4 -2, and BiVO 4 -3 are 15.54, 16.28, and 15.51 nm, respectively. The surface scanning electron microscopy (SEM) images of the BiVO 4 -1, BiVO 4 -2, and BiVO 4 -3 films are shown in Figure 1d-f. It can be seen that Figure 1e depicts larger crystalline grains compared to those of BiVO 4 -1 ( Figure 1d). The BiVO 4 -1 film consists of many small grains with obvious voids on their surfaces, which are not conducive to charge transport, possibly due to the increase in charge recombination centers on the BiVO 4 -1 film surface. On the other hand, a quasi-uniform BiVO 4 -2 film with large grains can be obtained by increasing the deposition time of the BiOI precursor film, which is directly related to its effective thermodynamic/kinetic growth under a sufficient annealing temperature. However, the BiVO 4 -3 film shows a stacked structure and more voids than the BiVO 4 -2 film as the deposition time of the precursor film increases ( Figure 1f). Therefore, highly compact and quasi-uniform BiVO 4 -2 films are better for photo-assisted electrodeposition with a CoPi catalyst. ters on the BiVO4-1 film surface. On the other hand, a quasi-uniform BiVO4-2 film with large grains can be obtained by increasing the deposition time of the BiOI precursor film, which is directly related to its effective thermodynamic/kinetic growth under a sufficient annealing temperature. However, the BiVO4-3 film shows a stacked structure and more voids than the BiVO4-2 film as the deposition time of the precursor film increases ( Figure  1f). Therefore, highly compact and quasi-uniform BiVO4-2 films are better for photo-assisted electrodeposition with a CoPi catalyst. The energy band gap (Eg) of the BiVO4 semiconductor was gained from the transmission spectra in the wavelength range of 300 nm to 1500 nm, as shown in Figure 2a. The transmittance value of the BiVO4-2 film is lower than that of the BiVO4-1 and BiVO4-3 films, indicating that it has a higher absorbance. The FTO was used as a substrate, and the Eg value was calculated using the equations below [30]: where n is an index equal to 0.5 in a direct band-gap semiconductor, d is the thickness of the BiVO4 film, T is the transmission, hν is the energy of a photon, α is the absorption coefficient, and C is a constant. The BiVO4 films with three different thicknesses display a similar Eg value of 2.41 eV (Figure 2b). The energy band information of the BiVO4 film was calculated via ultraviolet photoelectron spectroscopy (UPS), as we can see in Figure 2c. According to the secondary electron cut-off (SEC) edge and valence band (VB) position, the work function (Ф) of BiVO4 was calculated as 5.57 eV [31]; meanwhile, the EF and EV are determined as −5.57 eV and −7.23 eV, respectively (Supplementary Materials, Note S1).
In combination with its optical band gap (Eg), at a value of 2.41 eV, the conduction band (EC, vs. vacuum) of the BiVO4 semiconductor can be obtained, and its value is -4.82 eV. The energy band gap (E g ) of the BiVO 4 semiconductor was gained from the transmission spectra in the wavelength range of 300 nm to 1500 nm, as shown in Figure 2a. The transmittance value of the BiVO 4 -2 film is lower than that of the BiVO 4 -1 and BiVO 4 -3 films, indicating that it has a higher absorbance. The FTO was used as a substrate, and the E g value was calculated using the equations below [30]: where n is an index equal to 0.5 in a direct band-gap semiconductor, d is the thickness of the BiVO 4 film, T is the transmission, hν is the energy of a photon, α is the absorption coefficient, and C is a constant. The BiVO 4 films with three different thicknesses display a similar E g value of 2.41 eV (Figure 2b). The energy band information of the BiVO 4 film was calculated via ultraviolet photoelectron spectroscopy (UPS), as we can see in Figure 2c. According to the secondary electron cut-off (SEC) edge and valence band (V B ) position, the work function (Φ) of BiVO 4 was calculated as 5.57 eV [31]; meanwhile, the E F and E V are determined as −5.57 eV and −7.23 eV, respectively (Supplementary Materials, Note S1). In combination with its optical band gap (E g ), at a value of 2.41 eV, the conduction band (E C , vs. vacuum) of the BiVO 4 semiconductor can be obtained, and its value is -4.82 eV. The photocurrent density potential (J-V) curves of pure BiVO 4 photoanodes with different thicknesses corresponding to the differing BiVO 4 /CoPi photoanodes are shown in Figure 2d. It can be seen that the BiVO 4 -2 photoanodes are significantly superior to other BiVO 4 photoanodes, indicating that the BiVO 4 film with the fewest charge recombination centers was obtained by adjusting the electrodeposition time of the BiOI precursor film and the annealing temperature. In addition, we can also see that the J ph and the fill factor of all BiVO 4 photoanodes increased after CoPi catalyst surface modification. Of these, the BiVO 4 -2/CoPi photoanode has a maximum J ph value of 1.39 mA cm −2 . Therefore, BiVO 4 -2 with optimal thickness was selected in this work in order to study the effect of CoPi catalyst on OER occurrence and PEC performance. The potential relative to the Ag/AgCl reference electrode (V Ag/AgCl ) can be converted into V RHE using the Nernst equation [32]: while ABPE was obtained from the J-V response of the photoanodes according to the following equation [33]: where V RHE is the potential relative of a relative hydrogen electrode (RHE), V H 2 O/O 2 is the oxidation potential for oxygen (1.23 V RHE ), and P sun is the simulated sunlight intensity (100 mW/cm 2 ). The electrocatalytic OER activities of BiVO 4 and BiVO 4 /CoPi electrodes were measured by linear sweep voltammetry (LSV) tests in 0.2 M Na 2 HPO 4 /NaH 2 PO 4 solution (pH = 6.5) under dark conditions. Figure 3a depicts the LSV curves with a 100% iR drop compensation and a 0.1 mV s −1 scanning rate of the OER catalysts. In general, a catalyst's OER activity is typically evaluated based on its overpotential at a current density of 10 mA cm −2 [27]. Therefore, the BiVO 4 /CoPi displays the overpotential value of 0.99 V RHE , which is smaller than that of BiVO 4 (1.87 V RHE ) at 10 mA cm −2 and the Tafel slope of BiVO 4 /CoPi (106.0 mV dec −1 ) is also obviously lower than that of BiVO 4 (265.7 mV dec −1 ), indicating that the CoPi catalyst is able to drive reactions at lower overpotentials and play an excellent role in BiVO 4 surface modification (Figure 3b The photocurrent density potential (J-V) curves of pure BiVO4 photoanodes with different thicknesses corresponding to the differing BiVO4/CoPi photoanodes are shown in Figure 2d. It can be seen that the BiVO4-2 photoanodes are significantly superior to other BiVO4 photoanodes, indicating that the BiVO4 film with the fewest charge recombination photocurrent density of BiVO4 and BiVO4/CoPi photoanodes increases sharply, confirming the rapid photo-generated carrier generation, separation, and transport, without the need for excess drive potentials. The J-V curves based on BiVO4 and BiVO4/CoPi photoanodes are displayed in Figure 3e. Specifically, the Jph values at 1.23 VRHE are determined to be 0.75 mA cm −2 and 1.39 mA cm −2 in sequence. Furthermore, Figure 3f exhibits the ascalculated ABPE and the BiVO4/CoPi photoanode (0.45%) is about ~3 times higher than the BiVO4 photoanode (0.14%), indicating a simultaneous upgrade in PEC performance. The charge transport kinetics were explored according to the electrochemical impedance test. Figure 4a,b exhibits the photoelectrochemical impedance spectroscopy (PEIS) characterization of the BiVO4 and BiVO4/CoPi photoanodes under illumination. The equivalent circuit is shown in Figure 4a; RW can be attributed to the resistance of the electrolyte solution, and Rct and Csc represent the charge transport resistance and capacitance in the space charge region of the electrode/electrolyte interface. The fitted results are shown in Table 1, the chi-square values of the BiVO4 and BiVO4/CoPi photoanodes are both less than 0.02, and we can also see that the raw data and the fitted results match well (Figure 4a). Similar RW values (18.2-18.8 Ω) indicate the stability of the test environment, and the remarkably smaller Rct value of the BiVO4/CoPi photoanode (137.9 Ω) implies a more effective charge transport at the electrode/electrolyte interface. The larger Csc value of the BiVO4/CoPi photoanode (5.20 × 10 −4 F) indicates that the ability of the charge collection is strengthened. Generally, the PEIS-derived low-frequency region (10 -1~1 0 1 Hz) normally represents the mass transfer reactions in the electrode/electrolyte interface [34]. As shown in Figure 4b, the reduction of interface impedance (|Z|) also proves that the CoPi catalyst accelerates the mass transfer process. Moreover, the peaks in the Bode diagrams for the BiVO4 photoanode are located at low frequencies (10 -1~1 0 1 Hz), while the BiVO4/CoPi photoanode is located between 10 1 and 10 2 Hz (insert in Figure 4b). The rapid response of the BiVO4/CoPi photoanode to frequency indicates that charge transport and The charge transport kinetics were explored according to the electrochemical impedance test. Figure 4a,b exhibits the photoelectrochemical impedance spectroscopy (PEIS) characterization of the BiVO 4 and BiVO 4 /CoPi photoanodes under illumination. The equivalent circuit is shown in Figure 4a; R W can be attributed to the resistance of the electrolyte solution, and R ct and C sc represent the charge transport resistance and capacitance in the space charge region of the electrode/electrolyte interface. The fitted results are shown in Table 1, the chi-square values of the BiVO 4 and BiVO 4 /CoPi photoanodes are both less than 0.02, and we can also see that the raw data and the fitted results match well (Figure 4a). Similar R W values (18.2-18.8 Ω) indicate the stability of the test environment, and the remarkably smaller R ct value of the BiVO 4 /CoPi photoanode (137.9 Ω) implies a more effective charge transport at the electrode/electrolyte interface. The larger C sc value of the BiVO 4 /CoPi photoanode (5.20 × 10 −4 F) indicates that the ability of the charge collection is strengthened. Generally, the PEIS-derived low-frequency region (10 −1~1 0 1 Hz) normally represents the mass transfer reactions in the electrode/electrolyte interface [34]. As shown in Figure 4b, the reduction of interface impedance (|Z|) also proves that the CoPi catalyst accelerates the mass transfer process. Moreover, the peaks in the Bode diagrams for the BiVO 4 photoanode are located at low frequencies (10 −1~1 0 1 Hz), while the BiVO 4 /CoPi photoanode is located between 10 1 and 10 2 Hz (insert in Figure 4b). The rapid response of the BiVO 4 /CoPi photoanode to frequency indicates that charge transport and mass transfer simultaneously accelerated. The Mott-Schottky (M-S) measurement was also introduced in order to study the junction formed at the semiconductor-electrolyte interface in reaction to the applied potential (V Ag/AgCl , i.e., the aforementioned E appl ). Figure 4c reveals that the 1/C 2 increases with the potential V Ag/AgCl in the presence of the space charge region (SCR), indicating n-type properties for the BiVO 4 semiconductor. Moreover, the conduction band position of BiVO 4 semiconductor relative to normal hydrogen electrode (NHE) can be obtained from the flat band potential (E fb ), donor density (N D ), the effective density of states functions in the conduction band (N c ), and the effective mass of the electron (m * n ), according to the following equations [35,36]: where C sc is the SCR capacitance in the semiconductor-electrolyte interface, A is the active device area, ε 0 is the permittivity of vacuum, ε is the relative dielectric coefficient, k B is Boltzmann constant, e is the unit charge, and T represents the temperature. The  The surface charge transfer efficiency (ηtran) and bulk charge separation efficiency (ηsep) of the BiVO4 photoanode were further investigated in order to determine the reasons for the significantly improved photocurrent density after surface modification with CoPi catalyst. The integrated photocurrent density (Jabs) of BiVO4 and BiVO4/CoPi photoanodes can be obtained according to wavelength-dependent light harvesting efficiency (LHE) and the standard AM 1.5G solar spectrum, utilizing the following formulas [1]:  The calculated E C value range is -4.82 ± 0.02 eV, using the above equations, which is consistent with the E C of -4.82 eV obtained from the UPS measurement. According to the E g value (2.41 eV) calculated by transmission, the valance band (E V , vs. NHE) is 2.79 V NHE , which thermodynamically supports the occurrence of water oxidation with oxygen production by solar-driven water splitting. In contrast, the E fb value and N D value of the BiVO 4 /CoPi quasi-semiconductor are 0.40 V NHE and 8.87 × 10 20 cm −3 , respectively (Figure 4d). The lower N D value demonstrates that the CoPi catalyst passivates defects (e.g., charge recombination centers) on the BiVO 4 film surface, reducing electron-hole recombination during charge transport to the electrode/electrolyte interface, thereby improving photocurrent density. We can also clearly observe that the CoPi catalyst effectively reduces the defect pinholes on the BiVO 4 surface through a top-view SEM image of the BiVO 4 /CoPi photoanode ( Figure S2). The increase in E fb of 0.02 V NHE also indicates that the BiVO 4 /CoPi photoanode is more beneficial for water oxidation in thermodynamics ( Figure S3).
The surface charge transfer efficiency (η tran ) and bulk charge separation efficiency (η sep ) of the BiVO 4 photoanode were further investigated in order to determine the reasons for the significantly improved photocurrent density after surface modification with CoPi catalyst. The integrated photocurrent density (J abs ) of BiVO 4 and BiVO 4 /CoPi photoanodes can be obtained according to wavelength-dependent light harvesting efficiency (LHE) and the standard AM 1.5 G solar spectrum, utilizing the following formulas [1]: where J abs is the integrated photocurrent density, λ e is the absorption cut-off wavelength that is linked to the band gap, N ph (λ) is the photo flux, and A (λ) is the wavelength-dependent absorption, covering wavelengths from 350 to 800 nm (Figure 5a). The λ e values for BiVO 4 and BiVO 4 /CoPi were determined to be at 514 nm and 520 nm, suggesting CoPi can also effectively broaden and heighten the LHE range of the BiVO 4 photoanode (Figure 5b), and giving the J abs values of 6.29 mA cm −2 and 6.68 mA cm −2 , respectively ( Figure 5c). Moreover, the transient photocurrent response spectra of the BiVO 4 and BiVO 4 /CoPi photoanodes are shown in Figure 5d, while the η tran can be obtained through the measured photocurrents associated with the "light off" state and the "light on" state, according to the following formulas [32]: where J ss is the photocurrent density in steady state and J inst signifies the instantaneous photocurrent density. Accordingly, the η tran value of the BiVO 4 /CoPi photoanode (82.1%) has increased in comparison to the BiVO 4 photoanode (76.0%), demonstrating that the CoPi catalyst accelerates holes transfer to the electrode/electrolyte interface and then oxidizes water. Moreover, we can observe in Figure 5d that the BiVO 4 /CoPi photoanode has a spike peak in the "light off" state compared with the BiVO 4 photoanode. The negative current transient suggests that there is significant back electron/hole recombination after "light off" [39], reiterating that the BiVO 4 /CoPi photoanode has a larger capacitance value (Table 1), which demonstrates that the CoPi catalyst can delay charge recombination and promote the OER's continuous progress. Additionally, the η sep can be calculated using following equation [32]: The BiVO 4 and BiVO 4 /CoPi photoanodes' calculated η sep values are 15.7%, and 25.4%, respectively. The obvious improvement of the η sep value indicates that the CoPi catalyst is also conducive to promoting the rapid separation of electron-hole pairs in the BiVO 4 body.
lowing equation [32]: The BiVO4 and BiVO4/CoPi photoanodes' calculated ηsep values are 15.7%, and 25.4%, respectively. The obvious improvement of the ηsep value indicates that the CoPi catalyst is also conducive to promoting the rapid separation of electron-hole pairs in the BiVO4 body.

Conclusions
In summary, a highly compact and quasi-uniform BiVO4 film was obtained at a suitable electrodeposition time and annealing temperature. After the successful electrodeposition of the CoPi catalyst on the optimized BiVO4 electrode, the FTO/BiVO4/CoPi photoanodes were fabricated, and their PEC performances were systematically investigated. Due to the surface modification of the CoPi catalyst, i.e., passivating charge recombination centers on the BiVO4 surface and promoting the separation of electron-hole pairs in the BiVO4 body, the interface impedance (|Z|) of the mass transfer process was decreased, while significantly enhancing the ηtran value of 82.1% and ηsep value of 25.4%. The Jph and ABPE of the BiVO4 photoanode were increased by about ~2 times and ~3 times, respectively, demonstrating that the CoPi catalyst can accelerate holes transfer from the BiVO4 semiconductor to the catalyst in order to be competitive with water oxidation by holes in the semiconductor and to improve PEC performance. The results of this study may open

Conclusions
In summary, a highly compact and quasi-uniform BiVO 4 film was obtained at a suitable electrodeposition time and annealing temperature. After the successful electrodeposition of the CoPi catalyst on the optimized BiVO 4 electrode, the FTO/BiVO 4 /CoPi photoanodes were fabricated, and their PEC performances were systematically investigated. Due to the surface modification of the CoPi catalyst, i.e., passivating charge recombination centers on the BiVO 4 surface and promoting the separation of electron-hole pairs in the BiVO 4 body, the interface impedance (|Z|) of the mass transfer process was decreased, while significantly enhancing the η tran value of 82.1% and η sep value of 25.4%. The J ph and ABPE of the BiVO 4 photoanode were increased by about~2 times and~3 times, respectively, demonstrating that the CoPi catalyst can accelerate holes transfer from the BiVO 4 semiconductor to the catalyst in order to be competitive with water oxidation by holes in the semiconductor and to improve PEC performance. The results of this study may open up the possibility for the future design and construction of extremely effective photoanodes for solar-driven water splitting.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest:
The authors declare no conflict of interest.