The Self-Passivation Mechanism in Degradation of BiVO4 Photoanode

摘要

class="head no_bottom_margin" id="sec1title">IntroductionArtificial photosynthesis, which captures and stores solar energy in the chemical bonds of a fuel, is a potential solution to the energy storage problem (). Solar-assisted water splitting to produce hydrogen fuel has received significant research interest in this regard (, , , ). The photoelectrochemical cell (PEC) is a versatile tool for photolysis of water where a semiconductor is used to harvest solar energy and an external bias is applied to facilitate the water splitting reactions (, , , ). Such electrochemical water photolysis was first reported by Fujima and Honda using a TiO2 (band gap = 3.0–3.2 eV) photoanode (). For more efficient photolysis, a narrower bandgap and a valence band edge above 2.0 eV versus reversible hydrogen electrode (RHE) is mandatory to provide sufficient overpotential for holes to oxidize water at the anode. Simultaneously, a negative conduction band edge is required at the cathode for electrons to reduce water. The efficiency of light absorption is principally determined by the bandgap of the semiconductor, which is the basis of theoretical calculations of solar to hydrogen conversion efficiency. The bandgap is also a measure of the stability of a compound, and thus semiconductors with narrow bandgaps are usually vulnerable to degradation in photoelectrodes ().BiVO4 photoanode has a narrower bandgap of 2.4 eV versus RHE, which contributes to a high, theoretical solar-to-hydrogen conversion efficiency of 9.2% (, ). This remarkable efficiency has drawn tremendous research interest, yet its vulnerability to photocorrosion has been its weakness. BiVO4 suffers from chemical instability in both acidic and alkaline conditions. Besides, its degradation is notably promoted by applied bias and light illumination when used as a photoanode in PEC (, ). Researchers have examined mainly three strategies to protect BiVO4 electrodes against photocorrosion: class="simple" style="list-style-type:none" id="olist0010">

(a)Adopt an external passivation layer on the photoanode to avoid direct contact with the electrolyte (href="#bib6" rid="bib6" class=" bibr popnode">Fan et al., 1983, href="#bib12" rid="bib12" class=" bibr popnode">Hu et al., 2014, href="#bib23" rid="bib23" class=" bibr popnode">McDowell et al., 2014),

(b)Use co-catalysts to alter the thermodynamic reduction/oxidation potential of photo-generated holes/electrons, to facilitate oxygen/hydrogen evolution reactions instead of detrimental side reactions (href="#bib14" rid="bib14" class=" bibr popnode">Kim and Choi, 2014, href="#bib29" rid="bib29" class=" bibr popnode">Seabold and Choi, 2012, href="#bib40" rid="bib40" class=" bibr popnode">Zhong and Gamelin, 2009),

(c)Manipulate the electrolyte composition to retard dissolution. For example, when the electrolyte is saturated with V ions, dissolution of BiVO4 photoanode is suppressed (href="#bib15" rid="bib15" class=" bibr popnode">Lee and Choi, 2017).Considerable experimental effort has been devoted to the enhancement of stability via the three strategies, but the effort expended in understanding the degradation mechanism is limited. Several studies have fragmentally indicated the anodic photo-degradation propensity of the V ion in BiVO4 (href="#bib5" rid="bib5" class=" bibr popnode">Ding et al., 2013, href="#bib15" rid="bib15" class=" bibr popnode">Lee and Choi, 2017, href="#bib23" rid="bib23" class=" bibr popnode">McDowell et al., 2014), which probably prompted Choi et al. to saturate the electrolyte with V ions to retard its degradation (href="#bib15" rid="bib15" class=" bibr popnode">Lee and Choi, 2017). Although, this approach reduced photodegradation, it did not throw any light on the degradation mechanism. A recent study by Toma et al. did confirm the dissolution of V from BiVO4, but it concluded that the rates of dissolution of both Bi and V would eventually become stoichiometric upon extensive corrosion (href="#bib32" rid="bib32" class=" bibr popnode">Toma et al., 2016). In this paper we propose a variation to this mechanism of degradation by in-depth mechanistic and theoretical studies that will contribute to the fundamental understanding. BiVO4 has emerged as a versatile platform for understanding the photoelectrochemical behavior of transition metal oxide semiconductors (href="#bib30" rid="bib30" class=" bibr popnode">Sharp et al., 2017), where the observed material behavior could potentially be applied to similar types of materials.