(471e) Perovskite Bifunctional Oxygen Cathode Catalysts

Development of cost-effective and highly active catalysts for electrochemical energy storage and conversion applications is a critical element in currently studied sustainable energy technologies. In particular, metal–air batteries represent the most promising energy storage systems for portable electronics, electrical vehicles, and stationary applications for storing the clean energy obtained from wind, solar, and power plants. Unlike the traditional intercalation electrodes in Li-ion batteries, the porous air cathode in the metal–air cell is capable of taking reactant O₂ from the atmosphere, instead of storing it in the electrodes. The unique configuration results in a vastly improved theoretical specific energy density of 1086 W h kg⁻¹ for Zn–air and 5028 W h kg⁻¹ for Li–air cells, two of the most promising metal–air batteries. These advanced electrochemical energy technologies; however, rely greatly on the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are a pair of the most important and technologically pertinent electrochemical reactions. However, the ORR and OER typically have very high overpotentials with slow kinetics, requiring catalysts containing large amount of precious metals such as Pt and Ir. These precious metals are prohibitively expensive and their supply is limited, which make these catalysts unsustainable for modern society.

To avoid the use of precious metal catalysts, some promising nonprecious metal catalysts (NPMCs) have been studied in the past decade. As a matter of fact, compared to the ORR, electrocatalysis of OER is more challenging due to the extremely oxidative conditions at high potentials (>1.4 V vs RHE), where most of currently studied carbon-based catalysts degrade due to the rapid oxidation of carbon. Perovskite oxide catalysts have emerged as the most promising bifunctional ORR and OER catalysts for electrochemical energy conversion and storage. In this work, a new type of oxygen-deficient BaTiO_3−x has been synthesized using a sol–gel method followed by a reductive heat treatment at 1300 °C in vacuum. The prepared perovskite nanoparticles have an average particle size on the order of 100 nm with uniform size distribution. X-ray diffraction shows that this perovskite catalyst consists of a significant amount of hexagonal BaTiO_3−x. State-of-the-art IrO₂ nanoparticles were also prepared in this work, which were used for reference and has excellent OER activity. Importantly, the oxygen-deficient perovskite catalysts exhibited high catalytic activity simultaneously for the ORR and the OER in alkaline electrolyte. The more challenged OER activity measured with the perovskite exceeds the IrO₂ catalyst at relatively low potentials (<1.6 V) evidenced by a much reduced onset potential (1.32 V) and increased current density. In order to clearly elucidate the structure of the oxygen-deficient BaTiO_3−x catalysts, X-ray and neutron diffraction experiments were further carried out, indicating that the hexagonal phase in the best performing BaTiO_3−x catalyst is oxygen-deficient with a stoichiometry of BaTiO_2.76. The oxygen vacancies in the perovskite crystal structure may lead to vastly enhanced electrocatalytic activity toward the ORR and OER. This work demonstrates a new type of highly efficient perovskite bifunctional catalyst for electrochemical energy technologies relying on oxygen electrocatalysis.