Enhancement of Electrocatalytic and Pseudocapacitive Properties as a Function of Structural Order in A₂Fe₂O₅ (A = Sr,...

Significant enhancements of electrocatalytic activities for both half-reactions of water-electrolysis, i.e., oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), as well as pseudocapacitive charge-storage properties are demonstrated upon changing the structural order in a perovskite-type system. The structural change is prompted by the increase in the ionic radius of the A-site ion in A₂Fe₂O₅. The structure of Sr₂Fe₂O₅ consists of alternating layers of FeO₆ octahedra and FeO₄ tetrahedra, whereas Ba₂Fe₂O₅ comprises seven different coordination geometries for Fe. We note that the catalytically active metal, i.e., iron, and the oxygen stoichiometry are the same for both materials. Nevertheless, the change in the structural order results in significantly greater electrocatalytic activity of Ba₂Fe₂O₅, manifested in smaller overpotentials, smaller charge-transfer resistance, greater electrocatalytic current, and faster reaction kinetics. In addition, this material shows significantly enhanced pseudocapacitive properties, with greater specific capacitance and energy density compared to Sr₂Fe₂O₅. These findings indicate the important role of structural order in directing the electrochemical properties.