Transition from Sequential to Concerted Proton-Coupled Electron Transfer of Water Oxidation on Semiconductor Photoanodes
Siqin Liu, Lei Wu, Daojian Tang, Jing Xue, Kun Dang, Hanbin He, Shuming Bai, Hongwei Ji, Chuncheng Chen, Yuchao Zhang, Jincai Zhao
Abstract
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-Fe 2 O 3, BiVO 4, TiO 2, plasmonic Au/TiO 2, and nickel–iron oxyhydroxide (Ni 1– x Fe x OOH)-modified silicon (Si)] by combining the rate law analysis of H 2 O 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 H 2 O, 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– x Fe x OOH 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– x Fe x OOH 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.