Mechanistic Study of the Synergy between Iron and Transition Metals for the Catalysis of the Oxygen Evolution Reaction

Abstract

A comprehensive study of the synergy between Fe and six transition metals (M=Ti, Co, Ni, Cu, Ag, Au), and how their M-Fe oxides electrocatalyze the oxygen evolution reaction (OER) was undertaken. Measurements were made using metal disks as the working electrodes and the addition of Fe³⁺ ions to the 1 m KOH electrolyte. The surfaces of the metal disks were oxidized after the OER. Interestingly, Fe interacted synergistically with all metal oxide layers except for that of Ti, resulting in enhanced catalytic activity for the OER. At an overpotential (η) of 400 mV, the current densities of the Ni and Ag disks in the Fe³⁺ ions-spiked electrolyte increased by 253 and 132 times, respectively, whereas it was only 20-30 times for the Co and Cu disks (compared with the OER in pure KOH at η=400 mV). The Tafel slopes of the Fe, Co, Ni, Cu, and Ag disks in 1 m KOH+Fe³⁺ electrolyte were in the range of 29-42 mV dec^-1 . The surface morphology and post-OER concentration of Fe in the catalysts could not be used to account for differences in the OER activities. Cyclic voltammetry showed that improvements in the OER performance were accompanied by changes in the redox features of the metal disk electrodes, which indicated the presence of electronic interactions between them and the Fe³⁺ . Strikingly, this was not observed between Ti and the Fe³⁺ ions, which could explain the lack of synergy between Ti and Fe³⁺ towards the OER catalysis. Electrochemical impedance spectroscopy indicated that the charge-transfer resistances of all the electrodes (except Ti) decreased after the addition of Fe³⁺ ions. Fe plays an important role in all these observed phenomena and we propose that the surface-adsorbed Fe species serve as the main active sites for OER in these synergistic M-Fe combinations.

Keywords: iron active site; mixed metal oxides; oxygen evolution reaction; synergistic effect.