Directly synthesized cobalt oxyhydroxide as an oxygen evolution catalyst in proton exchange membrane water electrolyzers.

The limited choice of oxygen evolution reaction catalysts for proton exchange membrane water electrolyzers hinders their large-scale commercialization. Cobalt-based catalysts are promising candidates and usually undergo surface reconstruction into CoOOH-like structures. However, the directly synthesized CoOOH has not yet been investigated in acidic environments. Here, we show that the CoOOH is active across the whole pH range, while its redox features are pH dependent. Operando hard X-ray absorption spectroscopy characterizations show a pH-induced change in Co oxidation onset, but no change in the coverage of redox-active Co species before the oxygen evolution reaction. The pH-dependent catalytic performance is connected to the interfacial Co oxidative transformations under electrocatalytic conditions. By combining the kinetic isotope effect and the apparent activation energy with theoretical verification, we offer the mechanistic discussion of the possible reaction pathway for CoOOH. In addition, CoOOH demonstrates a stable cell potential of 100 mA cm for 400 h in a proton exchange membrane water electrolyzer. These results shed light on both the fundamental electrochemical properties of CoOOH and its potential for practical device applications.