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Energy, Environmental, and Catalysis Applications
Enhanced Photoelectrochemical Water Oxidation Performance on BiVO4 by Coupling of CoMoO4 as Hole-transfer and conversion Co-catalyst Jinyan Du, Xiaohui Zhong, Huichao He, Ji Huang, Minji Yang, Gaili Ke, Jun Wang, Yong Zhou, Faqin Dong, Qin Ren, and Liang Bian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13130 • Publication Date (Web): 13 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018
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ACS Applied Materials & Interfaces
Enhanced Photoelectrochemical Water Oxidation Performance on BiVO4 by Coupling of CoMoO4 as Hole-transfer and conversion Cocatalyst Jinyan Du,† Xiaohui Zhong,† Huichao He,†* Ji Huang, † Minji Yang,† Gaili Ke,† Jun Wang, † Yong Zhou,‡ Faqin Dong, †
Qin Ren,† and Liang Bian†
† State
Key Laboratory of Environmental-Friendly Energy Materials, Key Laboratory of Solid Waste Treatment and
Resource Recycle of Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China. ‡
Ecomaterials and Renewable Energy Research Center, School of Physics, Nanjing University, Nanjing 211102,
China. KEYWORDS ： BiVO4/CoMoO4; Water oxidation; Hole-to-oxygen kinetics; Charge separation and transfer; Synergistic catalytic effect
ABSTRACT: The manipulation of interfacial charge separation and transfer is one of primary breakthroughs to improve the water oxidation activity and stability of BiVO4 photoanode. In the present work, CoMoO4-coupled BiVO4 (BiVO4/CoMoO4) film was designed and prepared as photoanode for photoelectrochemical water oxidation. Compared with the bare BiVO4 film, obviously improved photoelectrochemical water oxidation performance was observed on the BiVO4/CoMoO4 film. Specifically, a higher water oxidation photocurrent density of 3.04 mA/cm2 at 1.23 V vs. RHE was achieved on the BiVO4/CoMoO4 photoanode, which is about 220% improvement over bare BiVO4 photoanode (1.34 mA/cm2 at 1.23 V vs. RHE). In addition, the BiVO4/CoMoO4 film photoanode was of better stability and faster hole-to-oxygen kinetics for water oxidation, without significant activity attenuation for 6 hours reaction at 0.65 V vs. RHE. The enhanced water oxidation performance on BiVO4/CoMoO4 film photoanode can be ascribed to the synergistic effect of following factors. (i) In thermodynamics, the photogenerated holes of BiVO4 are directionally transferred to CoMoO4 through their physical coupling interface and valance band potentials matching. (ii) In kinetics, the transferred holes induce the formation of Co3+ active sites on CoMoO4 that could synergistically oxidize H2O to molecular O2 with stable activity. 1 ACS Paragon Plus Environment
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INTRODUCTION Photoelectrochemical (PEC) water splitting has been widely studied as a clean pathway to produce hydrogen fuel from renewable solar energy.1 Since the oxygen evolution reaction on semiconductor photoanode involves a four-hole process with kinetical difficulty,2 the study of high performance photoanodes is a research focus of PEC water splitting field. In recent years, BiVO4 has been intensively investigated as a photoelectrode material for solar water oxidation.3 In general, BiVO4 is of several advantages for PEC water oxidation. First, BiVO4 has an indirect band gap of ~2.4 eV and suitably positioned valence band edge (~2.40 V vs. RHE) relative to water oxidation potential (1.23 V vs. RHE),3, 4 allowing it absorbs about 11% solar energy to drive water oxidation. Second, BiVO4 exhibits good thermodynamic stability in neutral and weak alkaline solution, showing potential application prospect for seawater splitting.5 Third, the raw materials and approaches for the production of BiVO4 are easily-obtained and low-cost.6 In theory, a solar-to-hydrogen conversion efficiency (ηSTH) as high as 9.3% can be achieved on BiVO4 under standard solar light irradiation,7 very close to the ηSTH of commercial requirement for water splitting (10%). That indicates a maximum photocurrent of 7.6 mA/cm2 that originated from water oxidation reaction can be measured on BiVO4 photoanode.8 Unfortunately, poor charge transfer and separation property as well as sluggish PEC kinetics impede the actual water oxidation performance of BiVO4 photoanode.3, 9 Specifically, the charge transfer and separation property in bulk and on surface mainly affect the water oxidation activity of BiVO4 photoanode. And the PEC kinetics related to interfacial charge transfer and separation has direct influence on the stability and activity of BiVO4 photoanode. Recent studies have demonstrated that the limitations of charge transfer and separation in BiVO4 bulk and on BiVO4 surface can be greatly overcome through composition and morphology tuning.3,
9
Typically,
doping of partial sites of V with W and Mo, or introducing oxygen vacancies into BiVO4 both can improve the bulk and surface charge migration property of BiVO4 evidently.10, 11 As for the kinetics, BiVO4 coupled with proper oxygen-evolution catalysts have shown accelerated PEC reaction kinetics. For example, BiVO4 coupled with FeOOH or NiOOH is of better activity and stability for PEC water oxidation.7 It is noted that the PEC water oxidation reaction only occurs at the photoanode-electrolyte interface that are related to the inter-reaction of H2O molecule and photogenerated holes. For the BiVO4 photoanode, once the sluggish PEC kinetics governs its water oxidation reaction, the photogenerated 2 ACS Paragon Plus Environment
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ACS Applied Materials & Interfaces
holes on BiVO4 surface will not be consumed quickly. And then, the accumulated holes will result in the photooxidation of BiVO4 and trigger the dissolution of V5+, which be called BiVO4 anodic photocorrosion.12 In theory, when the photogenerated holes on BiVO4 photoanode devote to water oxidation reaction quickly enough, the BiVO4 anodic photo-corrosion will be kinetically suppressed and thus its PEC reaction stability and activity can be enhanced simultaneously. In summary, the manipulation of interfacial charge transfer and separation is one of primary breakthroughs to enhance the solar water oxidation performance of BiVO4 photoanode. In the works of Li et al., sandwich-structured photoanodes consisting of semiconductor, hole-storage layer and molecular co-catalyst have shown high water oxidation performance.13, 14 For such sandwichstructured photoanodes, the hole-storage layer plays the role to mediate interfacial charge transfer from semiconductor to co-catalyst for water oxidation. Additionally, enhanced water oxidation performance was achieved BiVO4 photoanode by modified with a hybrid structure of layered double hydroxides and charge-transfer medium.
[15-17]
Inspired by these BiVO4-based photoanodes, BiVO4 coupled with
oxygen-evolution catalyst with good hole-transfer capability is expected to highly efficient PEC water oxidation performance. Recently, CoMoO4 has attracted much research interest as excellent electrochemical catalyst and supercapacitor material due to its good conductivity, high electrochemical activity and stability.
18, 19
Compared to Co3O4, the larger cell parameters and higher conductivity of
CoMoO4 determine its high rate capability and electrocatalytic activity.20 Accordingly, nanostructured CoMoO4 exhibits substantially higher oxygen-evolution performance and stability even than the IrO2 and Co3O4 benchmark.19 In addition, CoMoO4 is a stable solid with potential photocatalytic activity,21 suggesting the possibility of interfacial charge transfer between CoMoO4 and BiVO4 through their band potentials matching. Therefore, we accurately anticipate that BiVO4 coupled with CoMoO4 may be of high PEC water oxidation activity and stability through their synergistic effect of charge-transfer and catalytic activity. Based on above analyses, CoMoO4-coupled BiVO4 film (BiVO4/CoMoO4) was investigated as photoanode for PEC water oxidation in present work. A higher water oxidation photocurrent density of 3.04 mA/cm2 at 1.23 V vs. RHE was achieved on the BiVO4/CoMoO4 film photoanode, which is about 220% improvement over bare BiVO4 film photoanode (1.34 mA/cm2 at 1.23 V vs. RHE). Additionally, the BiVO4/CoMoO4 film photoanode has exhibited satisfactory stability, without significant loss of 3 ACS Paragon Plus Environment
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photoactivity for 6 hours water oxidation reaction. The enhanced PEC water oxidation performance on BiVO4/CoMoO4 film is mainly attributed to the synergistic effect of following factors: (i) Thermodynamically, the photogenerated holes on BiVO4 surface are directionally transferred to CoMoO4 through their physical coupling interface and valance band potentials matching. (ii) Kinetically, the transferred holes induce the formation of Co3+ active sites on CoMoO4 that could synergistically oxidize H2O to molecular O2 with stable activity. We believe this work provides new insights of designing and evaluating metal-oxide co-catalysts to boost the water oxidation performance of semiconductor-photoanodes both in thermodynamics and kinetics.
2. EXPERIMENTAL SECTION 2.1 Preparation of BiVO4, BiVO4/CoMoO4 and BiVO4/Co3O4 film The BiVO4 film was prepared on FTO glass substrate (Sheet Resistance
