(Invited) Operando Optical Spectroscopy Analyses of Photoelectrochemical Water Oxidation Kinetics

Abstract

Water oxidation is a key kinetic bottleneck in many photoelectrochemical systems for solar to fuels and chemicals. We have recently been developing an operando optical spectroscopic analysis to assay these reaction kinetics in metal oxide photoanodes. This technique, which we refer to as photoinduced absorption spectroscopy, is based around measuring the optical absorption of oxidising species (‘surface holes’) generated in photoanodes under anodic bias and pulsed (typically 5 s) light irradiation. These analyses have allowed us to observe a non-linear dependence of water oxidation kinetics on surface hole density, most typically a 3rd order rate law dependence under circa 1 sun irradiation conditions. In my talk I will talk about three different applications of this PIAS approach to investigate the function of BiVO4, Fe2O3 and TiO2 photoanodes. Firstly I will discuss how this non-linear dependence can be exploited to determine the photoelectrochemically active surface area of photoanodes as function of metal oxide nanostructuring. I will then go on to demonstrate that, when normalised for matched surface hole densities, water oxidation kinetics measured in under light irradiation or in the dark at strong anodic bias are identical, and discuss how this conclusion can be exploited to quantify the photovoltage under irradiation. Finally I will discuss deviation from rate law behaviour observed under > 1 sun irradiation conditions, and its mechanistic origin.