Oxygen Sponges for Electrocatalysis: Oxygen Reduction/Evolution on Nonstoichiometric, Mixed Metal Oxides

Electrocatalysis of oxygen reduction and evolution (ORR and OER) have become of significant importance due to their critical role in the performance of electrochemical energy conversion and storage devices, such as fuel cells, electrolyzers, and metal air batteries. While efficient ORR and OER have been reported using noble-metal based catalysts, their commercialization is cost prohibitive. In this Perspective, we discuss the potential of nonprecious metal based, mixed electronic-ionic conducting oxides (i.e., perovskites, double perovskites, and Ruddlesden-Popper (R-P) oxides) for efficient oxygen electro-catalysis at high and low temperatures. The nonstoichiometry of oxygen in these materials provides key catalytic properties that facilitate efficient ORR/OER electrocatalysis. We discuss the importance of surface structure and composition as critical parameters to understand and tune the ORR/OER activity of these oxides. We argue that techniques facilitating controlled synthesis and characterization of the surface structures are key at achieving a correlation between structure and activity of these materials. We make the case for combinatorial approaches involving quantum chemical calculations combined with detailed characterization, controlled synthesis, and testing as effective ways for developing the fundamental knowledge at the molecular level required to guide the design of efficient nonstoichiometric, mixed metal oxides for oxygen electrocatalysis. We conclude by summarizing current advances and devising future directions in this area.