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 electrolysers; 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 oxidised nickel states were present before and after the Fe incorporation, with negligible change in their redox potentials. However, the discharge kinetics of the Ni states shows a significant 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 determine the intrinsic reaction time constant per surface Fe site is <0.1 s, which is two 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 OER occurs via charge accumulation on primarily Ni centres in these catalysts, followed by hole transport to the surface Fe species where oxygen evolution occurs.