Rate Law Analysis of Water Oxidation and Hole Scavenging on a BiVO4 Photoanode

Spectroelectrochemical studies employing pulsed LED irradiation are used to investigate the kinetics of water oxidation on undoped dense bismuth vanadate (BiVO4) photoanodes under conditions of photoelectrochemical water oxidation and compare to those obtained for oxidation of a simple redox couple. These measurements are employed to determine the quasi-steady-state densities of surface accumulated holes, p(s), and correlate these with photocurrent density as a function of light intensity, allowing a rate law analysis of the water oxidation mechanism. The reaction order in surface hole density is found to be first order for p(s) < 1 nm(-2) and third order for p(s) > 1 nm(-2). The effective turnover frequency of each surface hole is estimated to be 14 s(-1) at AM 1.5 conditions. Using a single-electron redox couple, potassium ferrocyanide, as the hole scavenger, only the first-order reaction is observed, with a higher rate constant than that for water oxidation. These results are discussed in terms implications for material design strategies for efficient water oxidation.