Dynamics of Fe\nAdsorption and Desorption from CoO<i><sub>x</sub></i>H<i><sub>y</sub></i> During Oxygen\nEvolution Reaction Electrocatalysis

Iron\nplays a central and critical role in the water oxidation\nmechanism\nand the activity of transition-metal oxides and (oxy)hydroxides. Tracking\nFe dynamics (deposition/dissolution/electrolyte transport) and unraveling\nthe chemistries of various Fe active sites under oxygen-evolution\nreaction (OER) conditions are important for catalyst design, particularly\nfor applications in alkaline electrolysis. Here, we use CoO<i><sub>x</sub></i>H<i><sub>y</sub></i> thin films as\na platform to investigate Fe transport and reactivity at the catalyst-electrolyte\ninterface and its impact on OER activity. We find that the deposition/dissolution\nof the surface-absorbed Fe species is governed by the transport of\nsoluble Fe species and applied potential. Soluble Fe species in the\nelectrolyte adsorb on CoO<i><sub>x</sub></i>H<i><sub>y</sub></i> under stirred electrolyte conditions. Accelerated\nFe desorption is observed with a more-positive OER potential. The\nsurface-localized Fe sites generated by absorption from soluble Fe\nspecies have a higher OER turnover frequency (TOF<sub>Fe</sub>) compared\nto Fe in codeposited CoFeO<i><sub>x</sub></i>H<i><sub>y</sub></i> films. <i>Operando</i> X-ray absorption\nspectroscopy shows structural similarity between reference Fe oxyhydroxides\nand surface Fe sites on CoO<i><sub>x</sub></i>H<i><sub>y</sub></i>, contrasting with Fe sites within the CoO<i><sub>x</sub></i>H<i><sub>y</sub></i> structure made\nby codeposition, where Fe shows a different apparent X-ray absorption\nedge energy. The OER activity of the surface-absorbed Fe decreased\nby Fe desorption but was recoverable by redepositing Fe species under\nnon-OER conditions.