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

We have revealed for the first time that BiVO photoanodes can be used even in strong acid media by mixing organic solvents into the electrolyte and depositing multilayers with a WO bottom layer. In general, the BiVO 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 BiVO 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 -butyl alcohol were especially promising solvents for stabilizing BiVO 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 WO bottom layer. During heat treatment in the fabrication process, W weakly diffused into the BiVO layer and a Bi oxide layer was formed on the outermost surface because of the Bi segregation that arose from the charge compensation between W and V in the BiVO 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 BiVO layer. We have confirmed that the BiVO/WO photoanodes in acetone-mixed aqueous sulfuric acid solution reliably functioned for a photoelectrochemical reaction under simulated sunlight illumination, and photoelectrochemical production of SO ions was confirmed under light irradiation at λ > 480 nm. These results suggest that the BiVO-based photoanodes have significant potential for use in acid media in conjunction with very straightforward modifications.