Tuning Bifunctional Oxygen Electrocatalysts by Changing the A‐Site Rare‐Earth Element in Perovskite Nickelates

Abstract Perovskite‐structured (ABO 3 ) transition metal oxides are promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In this paper, a set of epitaxial rare‐earth nickelates (RNiO 3 ) thin films is investigated with controlled A‐site isovalent substitution to correlate their structure and physical properties with ORR/OER activities, examined by using a three‐electrode system in O 2 ‐saturated 0.1 m KOH electrolyte. The ORR activity decreases monotonically with decreasing the A‐site element ionic radius which lowers the conductivity of RNiO 3 (R = La, La 0.5 Nd 0.5 , La 0.2 Nd 0.8 , Nd, Nd 0.5 Sm 0.5 , Sm, and Gd) films, with LaNiO 3 being the most conductive and active. On the other hand, the OER activity initially increases upon substituting La with Nd and is maximal at La 0.2 Nd 0.8 NiO 3 . Moreover, the OER activity remains comparable within error through Sm‐doped NdNiO 3 . Beyond that, the activity cannot be measured due to the potential voltage drop across the film. The improved OER activity is ascribed to the partial reduction of Ni 3+ to Ni 2+ as a result of oxygen vacancies, which increases the average occupancy of the e g antibonding orbital to more than one. The work highlights the importance of tuning A‐site elements as an effective strategy for balancing ORR and OER activities of bifunctional electrocatalysts.