Reversible perovskite electrocatalysts for oxygen reduction/oxygen evolution

Abstract

The identification of electrocatalysts mediating both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are prerequisite for the development of reversible fuel cells and rechargeable metal–air batteries. The question remains as to whether a bifunctional catalyst, or a single catalyst site, will exhibit potentials converging to +1.23 V_RHE. Transition metal-based perovskites provide tunable catalysts where site substitution can influence both ORR and OER, however substitution in the pseudo-binary phases results in an anti-correlation in ORR and OER activities. We reveal that La_xMn_yNi_1−yO_3−δ, compositions with lanthanum A-site sub-stoichiometry exhibit reversible activity correlating with the appearance of the Mn³⁺/Mn⁴⁺ redox couple. The Mn³⁺/Mn⁴⁺ couple is associated with Mn⁴⁺ co-existing with Mn³⁺ in the bulk, as La³⁺ is substituted by Ni²⁺ at the A-site to create a mixed valent system. We also show that a direct A-site substitution by the Ca²⁺ cation in La_xCa_1−xMn_yO_3−δ perovskites also results in the creation of Mn⁴⁺, the appearance of the Mn³⁺/Mn⁴⁺ redox couple, and a concomitant reversible activity. These results highlight a general strategy of optimizing oxide electrocatalysts with reversible activity.