Cooperative Fe Sites on Transition Metal (Oxy)hydroxides for High Oxygen Evolution Activity

Fe-containing transition-metal (oxy)hydroxides are record-activity oxygen-evolution reaction (OER) electrocatalysts in alkaline media and ubiquitously form as the active surface phase across many materials systems. The complexity, heterogeneity, and dynamics of the Fe sites within the (oxy)hydroxide has slowed mechanistic understanding of how and where the Fe-based active sites form—information critical for designing catalysts and electrolytes with higher activity and stability. We show that where Fe species in the electrolyte incorporate into host Ni or Co (oxy)hydroxides depends on the electrochemical history and the structural properties of host material. Substantially less Fe is incorporated from Fe-spiked electrolyte into Ni (oxy)hydroxide at anodic potentials, past the nominally Ni2+/3+ redox wave, compared to during potential cycling. The Fe adsorbed under constant anodic potentials leads to high OER activity which we attribute to under-coordinated “surface” Fe. By systematically controlling the concentration of surface Fe, we discover that the per-Fe OER turn-over frequency (TOF-Fe) increases linearly with the Fe concentration, suggesting a changing OER mechanism with increased Fe concentration. We propose a mechanism involving multiple, cooperative Fe sites in FeOx clusters. The magnitude of TOF-Fe depends on the host material, with new Fe:NiOxHy showing record-high TOF-Fe of ~ 40 s-1 at 350 mV overpotential.