Water Oxidation and Degradation Mechanisms of BiVO Photoanodes in Bicarbonate Electrolytes.

The photoelectrochemical hydrogen peroxide evolution reaction (HPER) has attracted increasing attention as an environmentally friendly approach to generate a commercially and industrially valuable water oxidation product. BiVO photoanodes operated in bicarbonate-containing electrolytes have been shown to offer remarkable performance characteristics for HPER, with HCO serving as a reaction mediator. However, the factors affecting the stability of both the semiconductor photoanode and the aqueous electrolyte remain poorly understood. Here, we investigated BiVO photoanodes to quantitatively assess the roles of electrolyte composition, bias potential, and illumination on competitive reaction pathways associated with HPER, oxygen evolution reaction, and photocorrosion. Our results confirm that HCO serves as a highly efficient mediator, leading to rapid hole extraction and near complete suppression of interfacial recombination on BiVO. In addition, these favorable hole transfer kinetics significantly decrease the rate of photocorrosion, leading to dramatically enhanced stability compared to bicarbonate-free electrolytes. While the elevated pH of unbuffered bicarbonate electrolyte leads to gradual chemical attack of BiVO, the stability is greatly enhanced in near-neutral buffered bicarbonate electrolytes. Finally, we confirm that HCO is regenerated during the photoanodic reaction, though pH swings during operation in an unbuffered electrolyte can lead to electrolyte instabilities. Overall, we find that BiVO photoanodes operating in buffered bicarbonate-containing solutions exhibit significantly enhanced stability and can efficiently drive water oxidation reactions, including HPER, thus providing a route to robust production of high value oxidation products.