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.
Wang et al. (Mon,) studied this question.
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