A Tandem Water Splitting Cell Based on Nanoporous BiVO4

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A Tandem Water Splitting Cell Based on Nanoporous BiVO4 Photoanode Co-catalyzed by Ultra-Small Cobalt Borate Sandwiched with Conformal TiO2 Layers Dongqi Xue, Miao Kan, Xufang Qian, and Yixin Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03078 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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ACS Sustainable Chemistry & Engineering

A Tandem Water Splitting Cell Based on Nanoporous BiVO4 Photoanode Co-catalyzed by Ultra-Small Cobalt Borate Sandwiched with Conformal TiO2 Layers Dongqi Xue1, Miao Kan1, Xufang Qian1, and Yixin Zhao1,2* 1. School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China 2. Shanghai Institute of Pollution Control and Ecological Security, Shanghai200092, P.R. China E-mail: [email protected] School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China E-mail: [email protected]

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ABSTRACT: The BiVO4 photoanode with 2.4 eV band gap and a-Si photocathode with 1.7 eV band gap are ideal to form a self-driven tandem photoelectrochemical (PEC) water splitting cell. The main obstacles for such tandem cells are the relatively low activity and stability of BiVO4 photoanode. Herein, we demonstrate a highly active undoped BiVO4 co-catalyzed with ultrasmall cobalt borate (CoBi) via photo-assisted electrodeposition. The conformal TiO2 layer deposited on BiVO4 surface helps the formation of ultra-small CoBi co-catalysts with excellent charge transfer properties. The resulting BiVO4 photoanode exhibited a remarkable stable photocurrent of ~2.5 mA/cm2 at 1.23 V vs. RHE. By combining the highly active and stable BiVO4 photoanode with a-Si photocathode, a solar-driven water splitting tandem water splitting cell is successfully fabricated with up to 3% solar-to-hydrogen (STH) conversion efficiency, a fairly high efficiency yet reported for undoped BiVO4 photoanode.

KEYWORDS: Water splitting, Bismuth vanadate, Amorphous titanium dioxide, Co-catalysts INTRODUCTION An overall photoelectrochemical (PEC) water splitting is one of most promising approaches to convert solar energy into solar fuels.1-4 Among PEC water splitting, photoanode and photocathode are used for oxygen and hydrogen evolution reactions, respectively. However, it is so difficult to realize an overall water splitting by an ideal photocathode or photocathode. It is a promising strategy to construct a tandem photoelectrochemical cell for water splitting. The a-Si based photocathode with different tunable band gap and output potential are an ideal candidate for tandem photoelectrochemical cell. Generally, the overall water splitting or solar-to-hydrogen (STH) conversion efficiency is mainly limited by the performance of photoanode. A high performance photoanode requires suitable band gap, valence band position and chemical stability


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for oxygen evolution reaction. Tremendous research efforts have been invested into exploring candidate materials for photoanode.5-12 Among these photoanodes materials, BiVO4 have been one of most promising candidates because of suitable band gap, earth abundance, facile and low cost preparation etc.1, 3, 12-19 However, the PEC performance of BiVO4 is far below its theoretical maximum value due to its poor electron-hole separation, short diffusion length and thermodynamically slow interface reaction.20-22 Furthermore, its practical application is restricted by its electrode stability.21-24 Therefore, various techniques are employed to deal with these above mentioned problem of BiVO4 (some techniques listed in Table S1). For example, metal elemental doping has been developed to improve electrical conductivity and diffusion length; morphology and structure regulation has been adopted to enhance charge separation and electron transportation properties, various oxygen evolution co-catalysts help reduce the kinetic barrier for water oxidation reaction.17, 21, 23-27 Among above methods to promote BiVO4 performance, employing oxygen evolution reaction (OER) co-catalysts is one of the simplest but effective one. Co-based co-catalysts especially CoPi has been widely investigated because of their low cost and confirmed excellent catalytic activity in PEC water splitting system.22, 28-30 The CoPi co-catalysts usually require the using of phosphate buffer to stabilize the CoPi, however the phosphate buffer solution is some corrosive to BiVO4 photoanode.31 Recently, another proton-accepting borate buffer with less corrosive to BiVO4 has become another alternative electrolyte for water splitting.20, 32-33 Coincidently, a novel Co based OER catalyst of cobalt borate (CoBi) demonstrate high activities and can be electrodeposited onto BiVO4 photoanodes as co-catalyts in borate electrolyte. Until now, the CoBi cocatalyzed BiVO4 photoanode did not exhibit comparable high PEC performance and stability as previously reported BiVO4 photoanodes with other co-catalysts, which could be due to low


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activities of CoBi co-catalysts or the inefficient charge transfer between CoBi co-catalysts and BiVO4 photoanode.25, 34 Here, we demonstrated a CoBi co-catalyzed undoped BiVO4 photoanode with superior PEC performance and enhanced stability by utilizing ultra-small photo-assisted electrodeposited CoBi co-catalysts sandwiched with double conformal TiO2 layers. The conformal TiO2 interface layer not only help form the ultra-small CoBi co-catalysts with enhanced charge transfer but also significantly enhance stability. This highly active and stable BiVO4 photoanode was then successfully coupled with a-Si photocathode to constitute a stable overall solar-driven tandem water splitting cell with excellent STH efficiency. EXPERIMENTAL SECTION Fabrication of BiVO4 Electrodes: BiVO4 thin films were fabricated on fluorine-doped tin oxide (FTO)-coated glass (15 ohm⋅cm) via a previously reported electrochemical deposition method.17 Briefly, 3.32 g KI was dissolved in 50 mL deionized water and adjusted to pH~1.7 using 1M HNO3. Later, 0.97 g Bi(NO3)3⋅5H2O was added to KI solution, resulting in a transparent red-orange solution after 20 min stirring. The red-orange solution was mixed with 20 mL anhydrous ethanol containing 0.4968 g benzoquinone (PBQ) followed by stirring thoroughly for a few minutes. A typical three-electrode system was used for electrochemical-deposition. A fluorine-doped tin oxide (FTO) working electrode (WE) cleaned in water and ethanol under sonication, a Ag/AgCl reference electrode (RE), and a platinum counter electrode (CE) were used. Cathodic deposition was performed potentiostatically at -0.1 V vs. Ag/AgCl at RT for 180 s. A red BiOI film was deposited on FTO substrate. Appropriate dosage of DMSO solution containing 0.2 M vanadyl-acetylacetonate was dropped on BiOI films, then heated at 450 ℃ (ramping rate, 2 ℃/min) in a furnace to form BiVO4. After cooled to ambient temperature,


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BiVO4 electrodes with excess V2O5 were soaked in 1 M NaOH solution for 30 min with stirring. Then the clean BiVO4 electrodes were rinsed with deionized water and dried at RT. Atomic layer deposition of TiO2: A conformal TiO2 layer was deposited by atomic layer deposition system (ALD f-100-4, MNT). Briefly, each ALD cycle consisted of a 15 ms pulse of deionized H2O, followed by 200 ms pulse of Tetrakis (dimethylamino) titanium (TDMAT) with waiting time of 5 s. Between each pulse, a 15 s purge was performed under a 20 mL/s flow of N2(g). During the deposition, the reaction chamber was 160 ℃, the TDMAT precursor was 85 ℃, and the H2O was 40 ℃. And the thickness of TiO2 films was controlled by applied cycles. Photo-assisted electrodeposition of CoBi co-catalysts: CoBi co-catalysts were photo-assisted electrodeposited onto BiVO4 electrodes via a typical photoelectrochemical deposition method carried out in 1 M potassium borate buffer solution (pH=9.5) containing 10 µM Co2+. The potassium borate buffer solution was prepared by dissolving 1 M H3BO3 in deionized water, then adjusting pH to 9.5 with potassium hydroxide. Then, Co(NO3)2⋅6H2O was added to above solution with stirring, resulting in a clear solution. The deposition of CoBi was carried out in a three-electrode system with BiVO4 as working electrode (WE), Ag/AgCl as reference electrode (RE), and platinum as counter electrode (CE). The CoBi was deposited on BiVO4 at 0.477 V vs. Ag/AgCl with the assistance of AM 1.5 G for 500 s. Photoelectrochemical measurements: Photoelectrochemical measurements were performed in a cubic quartz container (5 cm × 5 cm × 7 cm) using a typical three-electrode configuration with BiVO4 as working electrode (WE), Ag/AgCl as reference electrode (RE), and platinum as counter electrode (CE). The gap between electrodes was 1.0 cm. In most experiments, the electrolyte was 1 M potassium borate buffer solution (pH=9.5) unless otherwise stated. For light illumination, a 500 W Xe arc lamp with ~100 mW cm-2 light intensity (CEL-S500, Ceaulighttech


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Co. Ltd., China) coupled with an AM 1.5 global filter was used.. Photocurrent-potential (I-V), cyclic-voltammetry (C-V), amperometric (I-T) curves were obtained by an electrochemical workstation (CHI 630e, Chenhua Co. Ltd., China). All electrochemical characterizations were performed with Zahner Instrument (Zahner-PP211, Zahner Co. Ltd., Germany). Specifically, Electrochemical Impedance Spectroscopy (EIS) was measured from 100 mHz to 10 kHz at 0.6 V vs. RHE under ~100 mW cm-2 with white light source. Intensity Modulated Photocurrent/Photovoltage Spectroscopy (IMPS/IMVS) were measured to compare the electron transport and recombination dynamics under variable light intensity. The electron transit time (τd) and electron lifetime (τn) in cell devices can be obtained by τd = 1/2πfIMPS, and τn = 1/2πfIMVS, where fIMPS/fIMVS is the frequency of the minimum imaginary component in IMPS/IMVS measurements. These measurements were performed with white light source from 100K to 1 Hz. Incident photon to current efficiency (IPCE) was obtained by combining a monochromator (GLORIA-X500A, Zolix Co. Ltd., China) with an electrochemical workstation (CHI 630e, Chenhua Co. Ltd., China). Briefly, photocurrent was measured at 1.23 V vs. RHE under variable wavelength with corresponding light intensity recorded by an optical power meter (CEL-NP2000, Ceaulighttech Co. Ltd., China). Then, IPCE values were calculated by formula. Tandem water splitting cell: The improved BiVO4/TiO2/CoBi/TiO2 (1 cm × 1 cm) photoanode and a-Si photocathode (1 cm × 1 cm) are coupled to construct water splitting cell. A-SiC:H films were fabricated in a PECVD cluster tool system. The intrinsic a-SiC:H films was deposited using CH4, SiH4 and H2 gas mixtures at 200℃ substrate temperature. Deposition pressure was kept at 550 mTorr and RF power density was in the range of 8-20 mW/cm2. The flow-rates were varied from 0 to 12 sccm for CH4, keeping the SiH4 flow constant (20sccm). The a-Si (2-jn)


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photocathode was fabricated on textured Asahi U-type SnO2:F, where a-SiC:H as the active layer, a-SiC:B:H was the p-layer and deposited from SiH4, CH4, and B2H6 gas mixtures, while the nlayer was prepared using SiH4 and PH3 gas mixtures.35 The performance of water splitting cell was measured as similar as other photoelectrochemical measurements in this paper. H2 and O2 measurements: Oxygen and hydrogen evolution were measured at 1.23 V vs. RHE under AM 1.5 in an airtight reactor. Before measurements, the reactor with electrolyte was purged with ultrahigh purity Ar for 20 min. A certain volume of gas extracted from reactor was analyzed by a gas chromatograph with thermal conductivity detector (GC-7900, Ceaulighttech Co. Ltd., China). Materials characterization: Scanning-electron microscopy (SEM) was performed on MIRA3 (SEM) & INCA X-Act (EDS) with a 5 kV accelerating voltage. Transmission-electron microscopy (TEM) was performed on TALOS F200X (FEI) with a 200 kV accelerating voltage. X-ray diffraction (XRD) was performed on a Lab XRD-6100 X-ray diffract meter. The UV-Vis spectra was measured on an Agilent Cary 60. X-ray photoelectron spectroscopy (XPS) was performed on AXIS UltraDLD. RESULTS AND DISCUSSION Herein, we fabricated an undoped nanoporous BiVO4 photoanode with ~100-200 nm sized phase pure BiVO4 nanocrystals using an electro-deposition methods shown in Figure S1. This undoped nanoporous BiVO4 exhibited a low photoelectrochemical performance and low stability for water splitting. To enhance the performance and stabilize the photoelectrode, a conformal TiO2 layer and CoBi co-catalysts were deposited onto the BiVO4. Although these different BiVO4 samples have been deposited with conformal TiO2 or CoBi co-catalysts, their XRD patterns show the same phase pure BiVO4 XRD peaks without any new XRD peaks as shown in


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Figure 1a, which is consistent with amorphous properties of conformal TiO2 layer and CoBi.25, 34 The deposition of conformal TiO2 layer and CoBi co-catalysts has little impact on the morphology of BiVO4 photoelectrode. Figure 1b shows that pristine nanoporous BiVO4 films consist of~100-200 nm sized nanocrystals with smooth surface.17 After the deposition of a conformal TiO2 layer, BiVO4/TiO2 showed no distinct difference from BiVO4 as shown in Figure 1c. In contrast, the BiVO4/CoBi sample deposited with CoBi co-catalysts shows some additional particles on the surface of BiVO4 nanocrystallines as illustrated in Figure 1d. These particles could be assigned as the deposited CoBi co-catalysts. Interestingly, there is no such obvious particles found on the surface of BiVO4/TiO2/CoBi shown in Figure 1e, in which the BiVO4 was first deposited with conformal TiO2 layer before the deposition of CoBi co-catalysts. The morphological difference between BiVO4/CoBi and BiVO4/TiO2/CoBi suggests that the conformal TiO2 layer deposited on BiVO4 surface could obviously affect the morphology of the deposited CoBi co-catalysts. Once the BiVO4/TiO2/CoBi was further deposited with another conformal TiO2 layer to form a conformal TiO2 layer sandwiched BiVO4/TiO2/CoBi/TiO2 sample, its morphology is also similar with BiVO4/TiO2/CoBi without apparent CoBi co-catalysts as depicted in Figure 1f.


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Figure 1. a) XRD patterns of these different BiVO4 samples; Top-view SEM images of b) bare BiVO4; c) BiVO4/TiO2; d) BiVO4/CoBi; e) BiVO4/TiO2/CoBi; f) BiVO4/TiO2/CoBi/TiO2.

All above the XRD and SEM characterizations show the deposition of conformal TiO2 and CoBi co-catalysts have little effect on crystal structure and morphology although there exist a few differences among these modified BiVO4 samples. XPS of all BiVO4 samples deposited with CoBi co-catalysts or TiO2 layer all showed similar Bi (+3), V (+5), O (-2) signals as shown in Figure S5a-c, which confirm the successful deposition of TiO2 and CoBi. Figure 2a shows all BiVO4 electrodes modified with single or dual TiO2 layer sandwiched samples display Ti 2p3/2 signals. BiVO4/CoBi sample exhibits strong Co peaks of Co 2p1/2 and Co 2p3/2, demonstrating successful CoBi deposition onto BiVO4.25 Figure 2b reveals XPS of BiVO4/TiO2/CoBi/TiO2 and BiVO4/TiO2/CoBi samples show relatively weak Co peaks compared to BiVO4/CoBi, which suggests less CoBi loading with the conformal TiO2 interlayer. The HRTEM characterizations were adapted to further explore the conformal TiO2 layer and CoBi co-catalysts deposited on BiVO4 nanocrystals. As shown in Figure 2c, the surface of BiVO4/TiO2 nanocrystals is covered by a ~2-5 nm thickness conformal amorphous layer, which should be the amorphous TiO2 layer. Figure 2d shows the BiVO4/CoBi sample is covered with ~10-20 nm uneven amorphous layer. And this amorphous layer should be the CoBi co-catalysts. These HRTEM results reveal that the size of regular photo-assisted electro-deposited CoBi cocatalysts are relatively large up to hundred nm, which is consisted with the SEM images in Figure 1. With the deposition of conformal TiO2 interlayer, the ultra-small CoBi nanoparticle cocatalysts were successfully deposited on BiVO4 as shown Figure 2e. In the dual conformal TiO2 layer sandwiched BiVO4/TiO2/CoBi/TiO2 sample, another conformal TiO2 layer (noted by


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yellow line) covers the deposited CoBi nanoparticle as shown in Figure 2f. These XPS and HRTEM results indicate the conformal TiO2 interlayer could help the deposition of ultra-small CoBi nanoparticles. In all, we successfully obtained a conformal TiO2 outer layers protected ultra-small CoBi co-catalysts deposited on BiVO4 with a conformal TiO2 interlayer.

Figure 2. XPS spectra of a) Ti element; b) Co element among various photoanodes; HRTEM images of c) BiVO4/TiO2; d) BiVO4/CoBi; e) BiVO4/TiO2/CoBi; f) BiVO4/TiO2/CoBi/TiO2.

Figure 3a shows the J-V curves of pristine BiVO4, BiVO4/TiO2, BiVO4/CoBi, BiVO4/TiO2/CoBi and BiVO4/TiO2/CoBi/TiO2 measured in 1 M potassium borate buffer solution (pH=9.5) under AM 1.5 illumination. The BiVO4 showed a relatively low performance with a high onset potential and low photocurrent density of 1.7 mA/cm2 at 1.23 V vs. RHE, which could be caused by quick electron-hole recombination, low charge separation and slow electron


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transfer in BiVO4 photoelectrode.13, 17-18 Furthermore, the BiVO4 photoelectrode exhibited a low stability for water splitting and its photocurrent quickly decrease to ~0.5 mA/cm2 (Figure 3b). With the help of deposited CoBi co-catalysts, the BiVO4/CoBi exhibited a significantly enhanced performance. The J-V curve of BiVO4/CoBi photoelectrode shows a ~200 mV lower onset potential and the photocurrent density increases to 2.5 mA/cm2 at 1.23 V vs. RHE, suggesting the enhanced charge separation and charge transfer by the co-catalyst.25-26 However, the BiVO4/CoBi photoelectrode also shows low stability and its photocurrent slowly decrease to only ~1.0 mA/cm2 (Figure 3b, blue line). The observed decrease of photocurrent could be ascribed to either surface recombination, the inefficient charge separation/transfer or the instability of CoBi co-catalysts. The BiVO4/TiO2 photoelectrode with a conformal layer of amorphous TiO2 did not indicate remarkable difference compared to bare BiVO4 in J-V curves because a conformal amorphous TiO2 layer usually does not affect the charge transfer.36-37 Interestingly, the BiVO4/TiO2 photoelectrode then exhibited a higher and more stable J-T curve than the BiVO4, suggesting the passivation effect of conformal TiO2 layer.38-41 With the conformal TiO2 interlayer, the BiVO4/TiO2/CoBi photoelectrode exhibits both a higher photocurrent of 3.3 mA/cm2 and ~50 mV lower onset potential for photocurrent than the BiVO4/CoBi. We inferred such improvement could be ascribed to the synergetic effect of the conformal TiO2 layer passivation and the enhanced charge separation and transportation by the ultra-small CoBi cocatalysts. Although BiVO4/TiO2/CoBi photoeletrode show superior PEC performance at 1.23 V vs. RHE under AM 1.5, it still suffered undesirable stability issue and the photocurrent slowly decrease with similar decay trend as BiVO4/CoBi as shown in Figure 3b. These results suggest the instability of CoBi co-catalysts might affect the stability of both BiVO4/CoBi and BiVO4/TiO2/CoBi. The BiVO4/TiO2/CoBi/TiO2 photoelectrode with another conformal TiO2


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layer protection has a slightly lower photocurrent of 3.0 mA/cm2 at 1.23 V and ~50 mV higher onset potential than BiVO4/TiO2/CoBi. This could be mainly caused by the outmost conformal TiO2 layer induced electron transfer and reaction kinetic barrier. Although the outermost TiO2 layer hindered electron transfer, the J-T curves of BiVO4/TiO2/CoBi/TiO2 shows a stable photocurrent of ~2.5 mA/cm2 without obvious decay in 2 h test (a longer stability test of improved BiVO4 samples could be obtained in supporting information Figure S4) .

Figure 3. a) J-V curves; b) J-T curves at 1.23 V vs. RHE of BiVO4/TiO2/CoBi/TiO2, BiVO4/TiO2/CoBi, BiVO4/CoBi, BiVO4/TiO2 and pristine under AM 1.5; c) Intensity modulated photocurrent spectroscopy (IMPS); d) Intensity modulated photovoltage spectroscopy (IMVS). All measurements were carried out in 1 M potassium borate buffer solution (pH=9.5).


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The electron transfer properties of these different BiVO4 photoanodes were also explored by Intensity modulated photocurrent spectroscopy (IMPS) and Intensity modulated photovoltage spectroscopy (IMVS). Nyquist plots of typical IMPS and IMVS spectra measured on different BiVO4 photoanodes were shown in Figure S7, Figure S8 respectively. And Figure S6 shows the EIS of various electrodes at 0.6 V vs. RHE irradiation with white light source. Both BiVO4 and BiVO4/TiO2 exhibited approximately highest resistance due to the high kinetic barrier for water oxidation, BiVO4/TiO2 has a slightly slower charge transportation time (τd) and longer charge lifetime (τn) than BiVO4 as shown in Figure 3c and 3d. The results confirm passivation effect of conformal TiO2 layer on BiVO4, which has also been observed in other photoelectrodes38-41. The BiVO4/CoBi showed much less Rct than BiVO4 and BiVO4/TiO2, implying CoBi can significantly reduce the water oxidation kinetic barrier and promote charge transportation. Interestingly, BiVO4/TiO2/CoBi showed the lowest resistance among these different BiVO4 photoelectrodes, which is consistent with its best J-V performance. BiVO4/TiO2/CoBi/TiO2 exhibited similar Rct and a slightly slower charge transportation time (τd) compared to BiVO4/TiO2/CoBi in Figure 3c, which reveal that outermost conformal TiO2 layer could efficiently protect co-catalysts with little hinder impact on the electron transfer. Furthermore, BiVO4/TiO2/CoBi/TiO2 has the longest charge lifetime (τn) than others shown in Figure 3d, suggesting the passivation effect. Figure S9 shows the incident photo-to-electron conversion efficiency (IPCE) curves of various BiVO4 photoanodes. BiVO4/TiO2/CoBi reached the highest quantum efficiency of 50% corresponding to remarkable PEC behavior, which was only 10% lower than the theoretical limitation of BiVO4 in Na2SO3. The maximum IPCE values of BiVO4/TiO2/CoBi/TiO2, BiVO4/TiO2 and BiVO4 are 45%, 20% and 15% respectively. All above


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results indicate that the BiVO4/TiO2/CoBi/TiO2 exhibites the best performance with balanced high activities and enhanced stability for PEC water splitting.

Figure 4. a) J-V curves of BiVO4/TiO2/CoBi/TiO2, bare BiVO4, Si under AM 1.5 in a threeelectrode system; b) J-V curves of BiVO4/TiO2/CoBi/TiO2-Si compared to BiVO4-Si and BiVO4Si in Na2SO3 under AM 1.5 in a two-electrode system (BiVO4/TiO2/CoBi/TiO2 or BiVO4 as anode, Si as cathode); c) J-T curves of BiVO4/TiO2/CoBi/TiO2-Si, BiVO4-Si, and Pt-Si under AM 1.5 (no additional bias).

This BiVO4/TiO2/CoBi/TiO2 photoanode was then coupled with a-Si photocathode with 1.6 V Voc to construct a tandem overall water splitting cell. Figure 4a listed the J-V curves of BiVO4, BiVO4/TiO2/CoBi/TiO2 and a-Si in a three-electrode system under AM 1.5. Based on the intersection of a-Si with bare BiVO4 and BiVO4/TiO2/CoBi/TiO2 photoanode, the short-circuit photocurrents of ~1.5 and ~2.5 mA/cm2 were achieved, respectively. The J-V curves of BiVO4Si and BiVO4/TiO2/CoBi/TiO2-Si tandem photoelectrochemical water splitting cells under AM 1.5 irradiation were shown in Fig. 4b, BiVO4/TiO2/CoBi/TiO2-Si exhibited higher photocurrent and more favorable fill factor than BiVO4-Si and realized an efficient unbiased solar-driven water splitting. Furthermore, its performance is comparable to theoretical limitation photocurrent


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for BiVO4-Si using Na2SO3 as sacrificial agent as shown in Figure 4b. Then the J-T curves of BiVO4/TiO2/CoBi/TiO2-Si tandem PEC water splitting cell in Figure 4c shows a stable output. BiVO4/TiO2/CoBi/TiO2-Si showed superior cell stability, displaying no any photocurrent decays. While, BiVO4-Si implied much worse stability with photocurrent decline from ~1.5 mA/cm2 to ~1.0 mA/cm2 after only 0.5 h. The oxygen and hydrogen evolution at BiVO4/TiO2/CoBi/TiO2 and a-Si photoelectrodes are measured by gas chromatograph in Figure S10. The molar ratio of oxygen and hydrogen is approximately 1:2 and their molar amounts are also closed to theoretical value calculated by J-T curves, suggesting high faraday efficiency. To confirm the contribution of BiVO4 photoanode, we set control experiments of replacing BiVO4 photoanode with a Pt counter electrode. As shown in Figure 4c, control tandem configuration show negligible photocurrent. In all, BiVO4/TiO2/CoBi/TiO2 showed excellent solar-driven unbiased PEC behavior for a tandem photoelectrochemical water cell once it is coupled with a-Si photocathode with 1.6 V Voc with efficient solar spectrum utilization. CONCLUSIONS A stable and high performance nanoporous BiVO4 photoanode can be achieved by using double conformal TiO2 layer sandwiched ultra-small CoBi nanoparticle co-catalysts. The conformal TiO2 interlayer helps the deposition of ultra-small CoBi co-catalysts via photo-assisted electrodeposition, and the outmost conformal TiO2 stabilizes both the BiVO4 photoanode and CoBi co-catalysts. The ultra-small CoBi co-catalyzed BiVO4 photoanode exhibits significantly enhanced the charge transfer and reduced water oxidation kinetic barrier. And solar-driven tandem water splitting cells were successfully constituted by couple this stable BiVO4 photoanode with a 1.6 V a-Si photocathode to efficient utilizing solar spectrum for high performance and stable overall water splitting under AM 1.5 without bias. In the all, the ultra-


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small CoBi and conformal TiO2 layer stabilization could be a promising strategy to co-catalyze and stabilize BiVO4 photoanode with promoted charge transfer and repressed electron-hole recombination. Such BiVO4 with the conformal TiO2 sandwiched co-catalysts configuration would be a promising photoanode candidate for tandem solar-driven water splitting cells and strategy for enhancing and stabilizing photoanodes. ASSOCIATED CONTENT Supporting Information The fundamental characterization results of pristine BiVO4 photoanode were given in Figure S1. Optimization of concentration of Co2+ was investigated on photo-assisted electrodeposition of CoBi co-catalysts in Figure S2. ABPE curves of different modified BiVO4 photoanodes were calculated and listed in Figure S3. A longer J-T measurement of improved BiVO4 photoanodes were shown in Figure S4. XPS spectra of all containing elements and a larger scale were shown in Figure S5. Electrochemical impendence spectrum (EIS) of these different BiVO4 photoanodes were performed in Figure S6. Nyquist plots of typical IMPS and IMVS spectra measured on different BiVO4 photoanodes were shown in Figure S7, Figure S8 respectively. Incident photonto-electron conversion efficiency (IPCE) of BiVO4 photoanodes were shown in Figure S9. H2 and O2 evolution based on constituted water splitting cell were measured by GC and presented in Figure S10. Characterizations of a-Si photocathode were given in Figure S11. And a table of recent research on water splitting using BiVO4 photoanodes is listed in Table S1. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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ACKNOWLEDGMENTS YZ acknowledges the support of the NSFC (Grant 21777096 and 21303103) and Huoyingdong Grant (151046) REFERENCES (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S., Solar Water Splitting Cells. Chem. Rev. 2010, 110 (11), 6446-6473, 10.1021/cr1002326. (2) Grätzel, M., Photoelectrochemical cells. Nature 2001, 414, 338, 10.1038/35104607. (3) Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37, 10.1038/238037a0. (4) Hisatomi, T.; Kubota, J.; Domen, K., Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem. Soc. Rev. 2014, 43 (22), 7520-7535, 10.1039/c3cs60378d. (5) Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J. H.; Nocera, D. G., Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts. Science 2011, 334 (6056), 645. (6) Hikita, Y.; Nishio, K.; Seitz, L. C.; Chakthranont, P.; Tachikawa, T.; Jaramillo, T. F.; Hwang, H. Y., Band Edge Engineering of Oxide Photoanodes for Photoelectrochemical Water Splitting: Integration of Subsurface Dipoles with Atomic-Scale Control. Adv. Energy Mater. 2016, 6 (7), 1502154, 10.1002/aenm.201502154.


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Cells

to

over

9%.

Chem.

Mater.

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8398-8405,

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For Table of Contents Use Only

Brief synopsis: A spontaneous, solar-driven tandem water splitting cell by facile connection between excellent BiVO4 photoanode and a-Si photocathode


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A Tandem Water Splitting Cell Based on Nanoporous BiVO4

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