Acid-Resistant BiVO₄ Photoanodes: Insolubility Control by Solvents and Weak W Diffusion in the Lattice

We have revealed for the first time that BiVO₄ photoanodes can be used even in strong acid media by mixing organic solvents into the electrolyte and depositing multilayers with a WO₃ bottom layer. In general, the BiVO₄ photoanodes are photocorrosive, especially in acid solutions. However, this shortcoming has been overcome using a combination of the two aforementioned modifications. We deduced that the contribution of each mixing organic solvent for the anti-photocorrosion of BiVO₄ in sulfuric acid solutions can be evaluated on the basis of a new empirical indicator that incorporates molecular density, the Hansen solubility parameter, and molecular polarizability. Acetone and tert-butyl alcohol were especially promising solvents for stabilizing BiVO₄ in acid media. We confirmed that the mixed organic solvents stabilized surface-emergent Bi oxide species as a passivation layer, which was generated via multilayering with a WO₃ bottom layer. During heat treatment in the fabrication process, W weakly diffused into the BiVO₄ layer and a Bi oxide layer was formed on the outermost surface because of the Bi segregation that arose from the charge compensation between W⁶⁺ and V⁵⁺ in the BiVO₄ lattice. The surface Bi oxide layer, which was protected by the mixed organic solvents, steadily served as a passivation layer for anti-photocorrosion of the underlying BiVO₄ layer. We have confirmed that the BiVO₄/WO₃ photoanodes in acetone-mixed aqueous sulfuric acid solution reliably functioned for a photoelectrochemical reaction under simulated sunlight illumination, and photoelectrochemical production of S₂O₈^2- ions was confirmed under light irradiation at λ > 480 nm. These results suggest that the BiVO₄-based photoanodes have significant potential for use in acid media in conjunction with very straightforward modifications.