Fabrication of Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ) Catalysts with Enhanced Electrochemical Performance by Removing an Inherent Heterogeneous Surface Film Layer

摘要

Complex perovskite oxides are considered promising candidates as bifunctional catalysts for catalytic oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs). As a class of oxide-based catalyst to replace noble metal-based catalysts, the perovskite structure, ABO_3, has great structural/ chemical flexibility that may enhance electrochemical performance. Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ) (BSCF5582) is a widely known perovskite oxide that is a strong candidate for oxygen membrane and solid oxide fuel cell (SOFC) cathode materials. Shao-Horn and co-workers showed that BSCF5582 has an e_g ≈1.2 of t_(2g)~5e_g~(1.2) as a band descriptor in the electronic configuration of Co cations, and exhibits the highest OER activity among perovskite oxide candidates, with a performance nearly identical to IrO_2 in terms of catalytic activity. Any transition metal oxide perovskite that has an e_g filling (σ~* orbital occupation) of close to 1 shows maximum ORR activity, because the B-site transition metal-oxygen covalency between the metal 3d and oxygen 2p orbitals determines catalytic activity. This highlights that that the optimization of a complex BSCF-based perovskite oxide enables it to be applicable as a promising bifunctional catalyst in diverse oxygen electrochemical applications such as metal-air batteries and fuel cells. BSCF is a complex perovskite oxide with dynamic defect chemistry and a broad range of oxygen stoichiometric (3-δ) changes from δ = 2.2 to 2.8, which depends on temperature and oxygen partial pressures (pO_2), while keeping the cubic symmetry of the crystal structure.