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.