Electrocatalytic Properties of Oxygen-Deficient Perovskites Ca 3 Fe 3- x Mn x O 8 ( x = 1-2) for the Hydrogen Evolution Reaction.

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, Ca 3 FeMn 2 O 8 (CaFe 1/3 Mn 2/3 O 3-1/3 ), Ca 3 Fe 1.5 Mn 1.5 O 8 , and Ca 3 Fe 2 MnO 8 , belong to the family of oxygen-deficient perovskites and show a gradual increase in the ordering of oxygen vacancies. Ca 3 FeMn 2 O 8 (CaFe 1/3 Mn 2/3 O 3-1/3 ) contains randomly distributed oxygen vacancies, which begin to order in Ca 3 Fe 1.5 Mn 1.5 O 8 , and are fully ordered in Ca 3 Fe 2 MnO 8 . The gradual increase in the structural order is associated with a systematic enhancement of the electrocatalytic activity for HER in acidic conditions, Ca 3 FeMn 2 O 8 < Ca 3 Fe 1.5 Mn 1.5 O 8 < Ca 3 Fe 2 MnO 8 . 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.