Tailoring the Co4+/Co3+ active sites in a single perovskite as a bifunctional catalyst for the oxygen electrode reactions

Abstract

Developing a non-precious metal electrocatalyst for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is desirable for low-cost energy conversion devices. Herein, we designed and developed a new class of layered cation ordered single perovskite oxides (Pr_0.9Ca_0.1Co_0.8Fe_0.2O_3-δ) with an optimum ratio of the Co⁴⁺/Co³⁺ oxidation state and oxygen vacancy for oxygen electrode reactions. Catalytic activities are investigated as a function of electronic structure and surface composition. A moderate amount of Ca and Fe dopants keeps the B-site Co cations at a higher oxidation state (Co⁴⁺) and generates a vast amount of an oxygen defect rich structure. The improved performance in the ORR and OER is explained by the increase in the sites of Co⁴⁺ cations, a state responsible for enhanced catalytic activity. A hypothesis for how doped Ca fraction affects the adsorbed oxygen species and contributes to catalytic activity is discussed. This work sheds light on the influence of crystal structure on the catalytic property and reports that ORR and OER activities are affected not only by oxygen vacancy concentration but also by the oxidation state of the transition metal in the perovskite oxide.