Dynamics of Fe Adsorption and Desorption from CoOxHy During Oxygen Evolution Reaction Electrocatalysis

Iron plays a central and critical role in the water oxidation mechanism and the activity of transition-metal oxides and (oxy)hydroxides. Tracking Fe dynamics (deposition/dissolution/electrolyte transport) and unraveling the chemistries of various Fe active sites under oxygen-evolution reaction (OER) conditions are important for catalyst design, particularly for applications in alkaline electrolysis. Here, we use CoOxHy thin films as a platform to investigate Fe transport and reactivity at the catalyst-electrolyte interface and its impact on OER activity. We find that the deposition/dissolution of the surface-absorbed Fe species is governed by the transport of soluble Fe species and applied potential. Soluble Fe species in the electrolyte adsorb on CoOxHy under stirred electrolyte conditions. Accelerated Fe desorption is observed with a more-positive OER potential. The surface-localized Fe sites generated by absorption from soluble Fe species have a higher OER turnover frequency (TOFFe) compared to Fe in codeposited CoFeOxHy films. Operando X-ray absorption spectroscopy shows structural similarity between reference Fe oxyhydroxides and surface Fe sites on CoOxHy, contrasting with Fe sites within the CoOxHy structure made by codeposition, where Fe shows a different apparent X-ray absorption edge energy. The OER activity of the surface-absorbed Fe decreased by Fe desorption but was recoverable by redepositing Fe species under non-OER conditions.