Fe (Oxy)hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution

Fe cations dramatically enhance oxygen evolution reaction (OER) activity when incorporated substitutionally into Ni or Co (oxy)hydroxides, serving as possible OER active sites. Pure Fe (oxy)hydroxides, however, are typically thought to be poor OER catalysts and are not well-understood. Here, we report a systematic investigation of Fe (oxy)hydroxide OER catalysis in alkaline media. At low overpotentials of ∼350 mV, the catalyst dissolution rate is low, the activity is dramatically enhanced by an AuOx/Au substrate, and the geometric OER current density is largely independent of mass loading. At higher overpotentials of ∼450 mV, the dissolution rate is high, the activity is largely independent of substrate choice, and the geometric current density depends linearly on loading. These observations, along with previously reported in situ conductivity measurements, suggest a new model for OER catalysis on Fe (oxy)hydroxide. At low overpotentials, only the first monolayer of the electrolyte-permeable Fe (oxy)hydroxide, which is in direct contact with the conductive support, is OER-active due to electrical conductivity limitations. On Au substrates, Fe cations interact with AuOx after redox cycling, leading to enhanced intrinsic activity over FeOOH on Pt substrates. At higher overpotentials, the conductivity of Fe (oxy)hydroxide increases, leading to a larger fraction of the electrolyte-permeable catalyst film participating in catalysis. Comparing the apparent activity of the putative Fe active sites in/on different hosts/surfaces supports a possible connection between OER activity and local structure.