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 S2O82- 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.
Keywords: BiVO4; acid resistance; anti-photocorrosion; high stability; photoelectrodes; solar energy conversion.