Insight Into the Impacting Mechanism of Cocatalysts on Electrochemical Potential and PEC Activity of BiVO4 Photoelectrode

Abstract

Although some cobalt compounds, especially Co-Pi, have been widely studied as cocatalysts for BiVO₄ in photoelectrochemical water splitting, there is still a lack of in-depth understanding of why these cocatalysts work and what kind of catalyst works. Here, we compared the structure changes of several cobalt compounds during the PEC process by in situ Raman and XPS spectra and found that the amorphous structure of Co-Pi allows it to contain a large number of cobalt ions with different valence states inside by changing its structure and coordination. It makes Co-Pi behave like redox couples, which can regulate the Fermi level of BiVO₄. Under illumination, with the generation of high valence cobalt ions in Co-Pi, the hole quasi-Fermi level of BiVO₄ is correspondingly pinned at a more anodic E_f-redox level. This improves the electrode’s photovoltage and carrier transport efficiency and makes it easier for the photoelectrode to meet the thermodynamic conditions of the OER reaction. We validated this conclusion using various cobalt and nickel compounds and proposed corresponding models. This provides a valuable reference in the selection and design of cocatalysts for different photoelectrodes and catalytic reactions.