Potential-Sensing Electrochemical AFM Shows CoPi as a Hole Collector and Oxygen Evolution Catalyst on BiVO₄ Water-Splitting Photoanodes

Oxide/(oxy)hydroxide overlayers such as cobalt (oxy)hydroxide phosphate (CoPi) enhance the performance of BiVO4 water-spitting photoanodes, but the mechanism of this enhancement remains unclear. We show that if the BiVO4 layer is thin and incompletely covers an underlying conductive glass, the performance dramatically decreases as CoPi loading is increased. This is consistent with direct contact between the CoPi and conducting glass that leads to "shunt" recombination of photogenerated holes accumulated in the CoPi. For thicker BiVO4 layers that completely cover the conducting glass, these shunt pathways are blocked. We then use a nanoelectrode atomic force microscopy probe to measure, in operando, the electrochemical potential of CoPi on thick BiVO4 films under illumination. We find that CoPi is charged to a potential necessary to drive water oxidation at a rate consistent with the measured photocurrent. CoPi acts as a hole collector and is the principal driver of water oxidation on BiVO4.