Oxygen-deficient BaTiO_3-x perovskite as an efficient bifunctional oxygen electrocatalyst

Perovskite oxide catalysts have emerged as the most promising bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (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 100nm 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 (<1.6V) evidenced by a much reduced onset potential (1.32V) 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.