Understanding the Effects of Composition and Structure on the Oxygen Evolution Reaction (OER) Occurring on NiFeO<i>x</i>Catalysts

Hydrogen produced by wind- or solar energy-driven electrochemical splitting of water could be used to store renewable electrical energy or to reduce biomass or CO2 to carbon-containing fuels. The potential required for the splitting of water is larger than the thermodynamic potential due to the insufficient activity of the catalysts required for the two half reactions involved in water splitting—the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). The OER and HER occur at the anode and cathode, respectively, of the electrochemical cell. Since the overpotential for the OER can be nearly an order of magnitude larger than that for the HER, considerable attention has been devoted to finding and developing highly active OER catalysts, and in particular those based on earth-abundant elements. To date this goal has been best met with catalysts based on oxides and oxyhydroxides of Ni and Fe for alkaline electrolysis. This chapter reviews the current understanding of such catalysts and examines the role of catalyst synthesis method and percentage of Fe content on catalyst performance. Particular attention is given to the role of Fe3+ cations exchanged into the lattice of NiOOH in enhancing the OER activity of the host material. This issue is discussed from both experimental and theoretical perspectives with the aim of identifying how and why the additions of Fe3+ cations enhance catalyst performance. The chapter ends with a brief overview of recent efforts aimed at identifying elements other than Fe that can be added to Ni oxide to enhance its OER activity and elements that can be added to NiFe oxyhydroxides to further enhance their OER activity.