Role of Potassium in Electrocatalytic Water Oxidation Investigated in a Volume‐Active Cobalt Material at Neutral pH

Abstract The oxygen evolution reaction (OER) is crucial in systems for sustainable production of hydrogen and other fuels. Catalytic OER materials often undergo potential‐induced redox transitions localized at metal sites. For volume‐active catalyst‐materials, these are necessarily coupled to charge‐compensating relocation of ions entering or leaving the material, which is insufficiently understood. The binding mode and mechanistic role of redox‐inert ions for a cobalt‐based oxyhydroxide material (CoCat) when operated at neutral pH in potassium‐phosphate (KPi) electrolyte are investigated by i) determination of K:Co and P:Co stoichiometries for various KPi‐concentrations and electrode potentials, ii) operando X‐ray spectroscopy at the potassium and cobalt K‐edges, and iii) novel time‐resolved X‐ray experiments facilitating comparison of K‐release and Co‐oxidation kinetics. Potassium likely binds non‐specifically within water layers interfacing Co‐oxyhydoxide fragments involving potassium–phosphate ion pairs. The potassium‐release kinetics are potential‐independent with a fast‐phase time‐constant of about 5 s and thus clearly slower than the potential‐induced Co oxidation of about 300 ms. It is concluded that the charge‐compensating ion flow is realized neither by potassium nor by phosphate ions, but by protons. The results reported here are likely relevant also for a broader class of volume‐active OER catalyst materials and for the amorphized near‐surface regions of microcrystalline materials.