ElectrocatalyticProperties of Oxygen-Deficient PerovskitesCa₃Fe_3–xMn_xO₈ (x = 1–2) forthe Hydrogen Evolution Reaction

Abstract

We have demonstrated a systematic trend in the electrocatalytic activity for the hydrogen evolution reaction (HER) and its correlations with transition-metal type, structural order, and electrical conductivity. The materials studied in this work, Ca₃FeMn₂O₈ (CaFe_1/3Mn_2/3O_3–1/3), Ca₃Fe_1.5Mn_1.5O₈, and Ca₃Fe₂MnO₈, belong to the family of oxygen-deficient perovskites and show a gradual increase in the ordering of oxygen vacancies. Ca₃FeMn₂O₈ (CaFe_1/3Mn_2/3O_3–1/3) contains randomly distributed oxygen vacancies, which begin to order in Ca₃Fe_1.5Mn_1.5O₈, and are fully ordered in Ca₃Fe₂MnO₈. The gradual increase in the structural order is associated with a systematic enhancement of the electrocatalytic activity for HER in acidic conditions, Ca₃FeMn₂O₈ < Ca₃Fe_1.5Mn_1.5O₈ < Ca₃Fe₂MnO₈. While the improvement of the HER activity is also associated with an increase in the Fe content, we have shown that the type of structural order plays a more important role. We demonstrated this effect by control experiments on an analogous material where all Mn was substituted by Fe, leading to a different type of structural order and showing an inferior HER activity compared to the above three materials. Furthermore, electrical conductivity studies in a wide range of temperatures, 25–800 °C, indicate that the trend in the electrical conductivity is the same as that of the HER activity. These findings reveal several important structure–property relationships and highlight the importance of synergistic effects in enhancing the electrocatalytic properties.