<p><span>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. Despite this, 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 is pH dependent. Operando hard X-ray absorption spectroscopy characterizations show a pH-induced change 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 OER conditions. </span><a name="_Hlk195171735"></a><span>Combining the kinetic isotope effect and the apparent activation energy with theoretical verification, we provide the mechanistic insight on the possible OER pathway for CoOOH.<span> </span></span><span>In addition, CoOOH demonstrate a stable cell potential at 100 mA cm<sup>-2</sup> for 400 hours in a proton exchange membrane water electrolyzer. These results provide insight into both the fundamental electrochemical properties of CoOOH and its potential for practical device performance. </span></p>
