Cooperative Fe-Sites in Nickel and Cobalt Oxyhydroxides

With incorporated iron, nickel and cobalt oxyhydroxides display the highest known oxygen evolution activities of any catalyst in alkaline media. To leverage this high activity in low temperature electrolysis systems, a comprehensive understanding of where Fe-based active sites form, their local chemical structure, and how this structure evolves under electrochemical conditioning is critical. Here we show that incorporation of Fe into NiO x H y and CoO x H y films during constant potential chronoamperometry yields NiFeO x H y and CoFeO x H y catalysts with extremely high TOFs of up to ~40 s -1 and 0.7 s -1 , respectively, at 350 mV overpotential. Further, we discover that the TOF of these materials is linearly related to the amount of Fe incorporated with this method. We propose a mechanism involving cooperative catalysis involving multiple Fe sites to explain this linear dependence. Density functional theory calculations are used to show this cooperativity may arise from increased ability to delocalize oxidative charge in active sites comprised of neighboring FeO x H y monomers. We then show that cycling these films lowers the per-Fe TOF substantially, to a level similar to that of co-deposited NiFe and CoFe films. This loss of activity during cycling and similarity to co-deposited films in which Fe occupies intraplanar lattice sites is consistent with migration of high activity Fe clusters initially adsorbed at surface edges and defects during chronoamperometry to less active sites within the oxyhydroxide lattice via cycling-driven dynamic substrate behavior. Further evidence for this Fe relocation into bulk lattice sites is provided using the redox features of adsorbed Ni 2+ and Co 2+ as probes for monitoring the extent to which ions are incorporated into the bulk of NiO x H y and CoO x H y .