Mass Activity Elucidation of Cobalt‐Based Oxygen Evolution Catalysts Utilizing Depth‐Resolved Spectroscopy in the Presence of Various Cations with Chloride

The oxygen evolution reaction (OER) in water electrolysis is a kinetically sluggish reaction that requires catalysts with high electrocatalytic activity. Cobalt oxide (CoO_x) is among the best performing OER catalysts and has been reported to have "bulk activity" with active sites distributed within the material. Herein, we examined the distribution of OER activity over μg cm^-2 loaded CoO_x deposited on anti-corrosive Ti substrate in 1 M KOH with and without various cations with chloride (X-Cl, X=Li, Na, K). Depth-resolved X-ray absorption spectroscopy and depth-profiling X-ray photoelectron spectroscopy were employed. In a thin sample with ~20 nm thickness, uniform oxidation was observed, but a thicker sample with ~80 nm thickness had active oxidized phases close to the surface and an unexpected inactive Co⁰ phase concurrently occupying about three-quarters of the CoO_x layer in the depth direction, which accounted for the mass activity loss. The formation of Co⁰ in the bulk is attributed to the galvanic replacement reaction between CoO_x and the metallic Ti substrate, which is not observed on the carbon substrate. Operando Raman spectroscopy demonstrated that the type of cations rather than Cl^- impacts the OER performance likely due to their interaction with superoxo species.