Journal
ADVANCED SUSTAINABLE SYSTEMS
Volume 7, Issue 7, Pages -Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/adsu.202300008
Keywords
cobalt oxyhydroxide; operando spectroscopy; oxygen evolution reaction; electrocatalysis; X-ray absorption spectroscopy
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The binding mode and mechanistic role of redox-inert ions for a cobalt-based oxyhydroxide material (CoCat) under neutral pH in potassium-phosphate (KPi) electrolyte were investigated. It was determined that potassium ions bind non-specifically within water layers and do not play a role in charge-compensation during the reaction. It was concluded that the ion flow is compensated by protons instead of potassium or phosphate ions. These findings may have implications for other volume-active OER catalyst materials and amorphized near-surface regions of microcrystalline materials.
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.
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