Transition from Sequential to Concerted Proton-Coupled Electron Transfer of Water Oxidation on Semiconductor Photoanodes

Accelerating proton transfer has been demonstrated as key to boosting water oxidation on semiconductor photoanodes. Herein, we study proton-coupled electron transfer (PCET) of water oxidation on five typical photoanodes [i.e., α-Fe₂O₃, BiVO₄, TiO₂, plasmonic Au/TiO₂, and nickel-iron oxyhydroxide (Ni_1-xFe_xOOH)-modified silicon (Si)] by combining the rate law analysis of H₂O molecules with the H/D kinetic isotope effect (KIE) and operando spectroscopic studies. An unexpected and universal half-order kinetics is observed for the rate law analysis of H₂O, referring to a sequential proton-electron transfer pathway, which is the rate-limiting factor that causes the sluggish water oxidation performance. Surface modification of the Ni_1-xFe_xOOH electrocatalyst is observed to break this limitation and exhibits a normal first-order kinetics accompanied by much enhanced H/D KIE values, facilitating the turnover frequency of water oxidation by 1 order of magnitude. It is the first time that Ni_1-xFe_xOOH is found to be a PCET modulator. The rate law analysis illustrates an effective strategy for modulating PCET kinetics of water oxidation on semiconductor surfaces.