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

Abstract

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.

Solar-to-Pharmaceutical Raw Material Production: Photoelectrochemical Naphthoquinone Formation Using Stabilized BiVO4 Photoanodes in Acid Media

“…The kinetics study of surface reactions of naphthalene adsorption and naphthoquinone desorption is necessary to elucidate the origin of this phenomenon. In the BiVO 4 /WO 3 photoanodes used in the acetone-mixed sulfuric acid electrolytes, the onset potential at around 0.6 V Ag/AgCl was not sufficiently low compared to the other PEC reactions of BiVO 4 -based photoanodes in neutral electrolytes such as Na 2 SO 4 and KHCO 3 . ,, This could be due to the surface property of BiVO 4 in the environment in which the outermost amorphous BiO x passivation layer was protected by the surrounding acetone molecules against the sulfuric acid attack . The surface property of BiVO 4 /WO 3 photoanodes would have another key for the enhancement of the PEC activity.…”

“…The variation of the photoanode appearance also confirmed this deterioration. The photoilluminated part at 60 min exhibited allochroism, which originates from the residue of decomposed Bi–V–O species, and it was completely dissolved after 180 min (Figure c) due to the photocorrosion of BiVO 4 …”

“…In contrast, in the present study, the BiVO 4 layer remained stable during the PEC reaction in a 0.5 M H 2 SO 4 H 2 O–acetone (60 vol %) mixed solution containing 5 mM naphthalene shown in Figure c,f. We previously reported that mixing aqueous sulfuric acid electrolytes with organic solvents such as acetone and tert -butyl alcohol for BiVO 4 /WO 3 photoanodes is remarkably effective in preventing the photocorrosion of BiVO 4 . We have revealed that Bi oxide (BiO x ) species were generated at the outermost surface via multilayering of BiVO 4 with a WO 3 bottom layer, and the surface amorphous BiO x layer, which was protected by the chemisorbed acetone with low affinity for water molecules, steadily served as a passivation layer for the anti-photocorrosion of the underlying BiVO 4 layer .…”

“…In this context, some advances have enhanced the stability of BiVO 4 to expand its applicability in prismatic reaction systems. − For instance, in neutral solutions, overlayer coatings of CoO x on BiVO 4 photoanodes were reported to prevent dissolution, and Yao et al discussed the self-passivation mechanism at the surface . In the case of a basic solution (pH = 12), Lee et al demonstrated the stabilization of BiVO 4 using an atomic layer deposited TiO 2 overlayer .…”

“…Very recently, we revealed a new approach to operate BiVO 4 /WO 3 photoanodes stably even in acid electrolytes mixed with organic solvents such as acetone. A Bi oxide-based self-passivation layer formed during the fabrication process at the outermost surface due to W’s weak diffusion into the BiVO 4 lattice from the WO 3 bottom layer . These protection conditions enabled the realization of PEC reactions even in 0.5 M sulfuric acid solutions under strong ultraviolet (UV) light and simulated sunlight illumination.…”