Reversible perovskite electrocatalysts for oxygen reduction/oxygen evolution

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 Mn Ni_1- O_3- , 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 Ca_1- Mn O_3- 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.