Bifunctional oxygen electrocatalysis in alkaline media: addressing the challenge of high activity, durability and cost effectiveness

Abstract

Oxygen electrochemistry plays a major role in energy conversion and storage devices, particularly in the field of fuel cells, metal-air batteries and water electrolyzers. Especially metal-air batteries have drawn much attention over the last years due to high theoretical energy densities. The major challenge in metal-air batteries is to design and develop economically feasible, earth-abundant, inexpensive, efficient electrocatalysts for the oxygen evolution reaction (OER) as well as the oxygen reduction reaction (ORR). High overpotentials for ORR and OER are reported especially for lithium-air batteries and precious metal electrocatalysts are commonly used. The choice of an alkaline electrolyte widens significantly the material base and the possibility for cost reduction. This contribution discusses the options available and presents a promising bifunctional catalyst that is presently favored at DLR. A wide-range of catalysts was screened and many suitable electrocatalysts and combination of catalyst were identified. In particular, IrO2, Co3O4, Mn3O4 and La0.6Ca0.4CoO3 are most promising bi-functional catalysts. A promising combination is Co3O4 - as the most suitable catalyst for OER - and Ag or Ni which are efficient OER catalysts. Ag/Co3O4 electrodes show high current densities and good long-term stability at reasonable cost and the optimum ratio between the active materials was determined [1-3]. It was shown that the catalyst, electrode thickness, temperature and electrolyte concentration have the highest impact on the current densities, while changing porosity always had a negative impact on the current densities of the investigated electrodes. This contribution discusses the characterization efforts and the investigation of the apparent synergistic effect present in this electrocatalyst mixture.