Enhanced Charge Separation in Nanoporous BiVO4 by External Electron Transport Layer Boosts Solar Water Splitting

Abstract The optimization of charge transport with electron‐hole separation directed toward specific redox reactions is a crucial mission for artificial photosynthesis. Bismuth vanadate (BiVO4, BVO) is a popular photoanode material for solar water splitting, but it faces tricky challenges in poor charge separation due to its modest charge transport properties. Here, a concept of the external electron transport layer (ETL) is first proposed and demonstrated its effectiveness in suppressing the charge recombination both in bulk and at surface. Specifically, a conformal carbon capsulation applied on BVO enables a remarkable increase in the charge separation efficiency, thanks to its critical roles in passivating surface charge‐trapping sites and building external conductance channels. Through decorated with an oxygen evolution catalyst to accelerate surface charge transfer, the carbon‐encased BVO (BVO@C) photoanode manifests durable water splitting over 120 h with a high current density of 5.9 mA cm−2 at 1.23 V versus the reversible hydrogen electrode (RHE) under 1 sun irradiation (100 mW cm−2, AM 1.5 G), which is an activity‐stability trade‐off record for single BVO light absorber. This work opens up a new avenue to steer charge separation via external ETL for solar fuel conversion.

Synthesis of BVO@C sample: BVO@C was fabricated by using loofah as a template followed by impregnation and calcination processes.Prior to the synthesis, the loofah was pretreated by sequentially soaking it in 2% glutaraldehyde phosphate buffer for 8 h and in 5% hydrochloric acid for 3 h, and then rinsed with deionized water (DIW).Adding 40 mmol Bi (NO 3 ) 3 •5H 2 O into a mixture of 80 mL ethanol and 60 mL glycerol obtains a solution of Bi source.Adding 40 mmol NH 4 VO 3 into 40 mL of TMAH obtains a solution of V source.The two solutions were mixed with stirring at 70 ℃, appearing large amounts of yellow precipitate, and then nitric acid (65%) was added dropwise timely until the solution became transparent, forming the impregnation solution.The treated loofah was immersed in above solution for 10 h for impregnating with Bi and V sources, and then rinsed fully with ethanol/DIW.Finally, the impregnated loofah was calcined in a tubular furnace for 6 h at 600 ℃ in air, obtaining the nanoporous BVO@C sample.In order to demonstrate the function of the porous loofah template, we also directly calcined the impregnation without introducing the loofah, and the obtained sample no longer had porous structure and carbon coating.The BVO@C electrodes were fabricated by spin-coating BVO@C stock on the size-normalized fluorine-doped tin oxide (FTO) glasses (FTO: 1 cm × 2 cm, sample area: 1 cm × 1 cm), with BVO@C layer thickness regulation.For comparison, the nanoporous bare BVO electrode was also prepared through the previously reported process of electrodeposition followed by thermalization.
Modification with NiFeO x cocatalyst: NiFeO x deposition was carried out using a three-electrode PEC cell in 0.5 M potassium borate electrolyte containing 20 mM Ni(NO3)2•6H2O and 20 mM Fe(NO3)2•6H2O.Ag/AgCl electrode and Pt sheet were employed as reference and counter electrodes, respectively.Through the cyclic voltammetry (CV) scanning at a rate of 50 mV s -1 within -0.4-0.4V vs. Ag/AgCl under AM 1.5 G illumination (from front side) for two cycles, NiFeO x cocatalyst was deposited onto BVO and BVO@C electrodes.
Characterization: SEM was taken on a S-4800 microscope (Hitachi, Japan).TEM and EDS were performed on a G2 F30 S-TWIN microscope (Tecnai, America) coupled with an EDS module at 300 kV.XRD patterns were recorded on a D8 Advance diffractometer (Bruker, Germany) equipped with a Cu-Kα radiation source.
XPS spectra were obtained on an ESCALAB250Xi instrument (ThermoFisher, America) equipped with an Al-Kα source.EPR spectroscopy was carried out on an A300-10/12 spectrometer (Bruker, Germany).UV-Vis spectra were measured with a Cary 5000 spectrophotometer (Varian, America).The TAS spectra were recorded with a self-made system (TIME-TECH SPECTRA) under the pump excitation of 343 nm.
PEC Measurements: PEC performance was measured with a typical three-electrode system (CHI660d, Shanghai CH) using an Ag/AgCl reference electrode and Pt foil counter electrode under a Xe 300 W lamp (Microsolar 300, Beijing Perfectlight) as the light source coupled with an AM 1.5 G filter.The light intensity was uniformly calibrated to 100 mW cm -2 (1 sun) by an optical power meter (FZ-A, Beijing S4 Perfectlight).0.5 M potassium borate buffer solutions (pH 9.3) with and without 0.2 M Na 2 SO 3 as a sacrificial agent were used as the electrolytes for SOR and WOR measurements, respectively.LSV curves were recorded at a scan rate of 50 mV s -1 within a potential range of 0-1.3 V RHE to evaluate photocurrent densities.The stability tests in chronoamperometry were conducted at 1.23 V RHE for 145 h with several light-off phases of 5 h.IPCE curves were measured at 0.83 V RHE with specific single-wavelength filters.The gas-evolving amount was measured with a gas-tight system coupled with PEC cell (Labsolar-IIIAG, Beijing Perfectlight) and gas chromatograph (GC, Fuli, Zhejiang).The LHE can be calculated by the following equation where () is the absorbance at specific light wavelength ().And the  ABS can be calculated by integrating the LHE spectra with standard spectrum of AM 1.5 G based on an assumption of 100% photon-to-current conversion.
The  bulk and  surf can be calculated by the following equations where  SOR and  WOR are the photocurrent densities of PEC sulfate oxidation and water oxidation, respectively.
The  rec and  tra can be calculated according to the photocurrent transient response spectra by the following equations where  0 and  ∞ are the instantaneous photocurrent density and the steady-state photocurrent density, and τ is the time constant, respectively.
The ABPE can be estimated by the following equation where  is the photocurrent density at the applied potential ( RHE ) versus RHE, and  sun is 1 sun irradiation intensity (100 mW cm -2 , AM 1.5 G).
The theoretical amounts of H 2 and O 2 evolution ( H ,  O ), and the corresponding faradaic efficiency (FE) were calculated by the following equations

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Table S1.Performance comparison of BVO@C/NiFeO x photoanode in this work and other BVO-based photoanodes reported previously in literatures.
Theoretical formulas: All the potentials were converted versus RHE according to the following Nernst equation  RHE =  Ag/AgCl + 0.059 × pH +  Ag/AgCl 0 (S1) where  RHE and  Ag/AgCl are the potentials versus RHE and Ag/AgCl, respectively, and  Ag/AgCl 0 is 0.1976 V at room temperature (25 °C).