Enhancement of Electrocatalytic and Pseudocapacitive Properties as a Function of Structural Order in A[sub.2]Fe[sub.2]O[sub.5]

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 A[sub.2]Fe[sub.2]O[sub.5]. The structure of Sr[sub.2]Fe[sub.2]O[sub.5] consists of alternating layers of FeO[sub.6] octahedra and FeO[sub.4] tetrahedra, whereas Ba[sub.2]Fe[sub.2]O[sub.5] 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 Ba[sub.2]Fe[sub.2]O[sub.5], 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 Sr[sub.2]Fe[sub.2]O[sub.5]. These findings indicate the important role of structural order in directing the electrochemical properties.