Spectroelectrochemical Studies of Oxygen Evolution Reaction Kinetics for Surface-Incorporated Iron in Nickel Oxyhydroxide Electrocatalysts

NixFe1–xOyHz is the state-of-the-art catalyst for the oxygen evolution reaction (OER) in alkaline water electrolyzers; however, understanding the impact of Fe on the active sites, reaction mechanism, and consequently intrinsic activity has been under intense debate. In this work, operando UV–vis spectroscopy was used to investigate Fe-free NiOxHy and NiOxHy with Fe selectively incorporated onto the surface. At oxygen-evolution potentials, similar oxidized nickel states were present before and after the Fe incorporation, with negligible changes in their redox potentials. However, the discharge kinetics of the Ni states show a substantial acceleration after the introduction of Fe, consistent with an increase in OER kinetics upon Fe incorporation and formation of active Ni–Fe species. Using optical spectroscopy, we determined the intrinsic reaction time constant per surface Fe site is <0.1 s, which is 2 orders of magnitude faster than Ni sites not in proximity to surface Fe sites (∼10 s), and also an order of magnitude faster than Ni sites in pure NiOxHy (∼1 s). Consequently, we propose that the OER occurs via charge accumulation primarily on Ni centers in these catalysts, followed by hole transport to the surface Fe species where oxygen evolution occurs.