Charge-Transfer Effects in Ni-Fe and Ni-Fe-Co Mixed-Metal Oxides for the Alkaline Oxygen Evolution Reaction

Ni-Fe and Ni-Fe-Co mixed-metal oxide (MMO) films were investigated as electrocatalysts for the oxygen evolution reaction (OER) in 0.1 M KOH. In an effort to optimize MMO morphology, aniline was used as a capping agent to produce high-surface-area Ni-Fe-Co films on Raney nickel supports. This catalyst exhibits enhanced mass activity in comparison to the Ni-Fe OER electrocatalysts reported to date. Cyclic voltammetry shows changes in the potential of the Ni2+/(3+) transitions in Fe- or Co-containing MMO films. In situ X-ray absorption spectroscopy (XAS) analysis confirms that Fe acts to stabilize Ni in the 2+ oxidation state, while Co facilitates oxidation to the 3+ state. The results of this study support the recent claims that Fe (not Ni) is the OER active site. The OER enhancement of the ternary Ni-Fe-Co catalyst results from two effects: (1) the charge-transfer effects of Co result in the formation of the conductive (NiOOH)-O-II phase at lower overpotential, thus activating the Fe sites which are otherwise inaccessible to electron transfer in the nonconductive Ni-II(OH)(2) host lattice, and (2) XAS analysis shows that the presence of Co effectively "shrinks" the Ni and Fe local geometry, likely resulting in an optimized Fe-OH/OOH bond strength. In addition, analysis of heat-treatment effects indicates that calcination at 400 degrees C improves the OER activity of Ni-Fe-Co but deactivates Ni-Fe. Annealing studies under argon show that MMO surfaces with a hydrated Ni(OH)(2) phase and a crystalline NiO phase exhibit nearly identical OER activities. Finally, the morphology of the MMO catalyst film on Raney Ni support provides excellent catalyst dispersion and should result in high active-site utilization for use in technologically relevant gas-diffusion electrodes.