Enabling Solar Water Oxidation by BiVO₄ Photoanodes in Basic Media

Titanium dioxide (TiO2) deposited by atomic layer deposition (ALD) has been the most commonly used protection layer to enhance chemical and photoelectrochemical stabilities of photoelectrodes. In this study, we report a new electrochemical deposition method that can place a thin, conformal TiO2 coating layer on a photoelectrode. This method takes <1 min and may serve as a practical alternative to ALD for the deposition of a TiO2 layer. The uniform quality of the TiO2 protection layer was confirmed by demonstrating the chemical stability of the BiVO4/TiO2 electrode in strongly basic media (pH 12 and 13) where BiVO4 readily dissolves. More importantly, the high-quality TiO2 protection layer made it possible to comparatively investigate photoelectrochemical properties and stabilities of the BiVO4 and BiVO4/TiO2 electrodes, which was critical to elucidate the effect that the chemical instability of BiVO4 in basic media has on the rate of photocorrosion. Systematic photoelectrochemical studies for sulfite oxidation and water oxidation provided a coherent understanding of how the interplay among the relative rates of interfacial charge transfer, surface recombination, and photocorrosion affects the photocurrent generation and photostability of BiVO4. On the basis of this understanding, stable photocurrent generation for water oxidation could be achieved at pH 12 over 20 h using a BiVO4/TiO2/FeOOH/NiOOH electrode where FeOOH/NiOOH served as oxygen evolution catalyst. The results and discussion contained in this study provide new insights into the understanding of photocurrent decay caused by photocorrosion involving dissolution, enabling the development of effective strategies to achieve stable photocurrent generation.