Variation of the electrocatalytic activity of isostructural oxides Sr2LaFeMnO7 and Sr2LaCoMnO7 for hydrogen and oxygen-evolution reactions

Abstract

The effect of the electronic configurations of transition metals on electrocatalytic activity, charge transport, and magnetic properties is demonstrated through the investigation of Sr₂LaFeMnO₇ and Sr₂LaCoMnO₇. The two compounds are isostructural and contain bilayer stacks of octahedrally coordinated transition metals. Despite their structural similarity, the magnetic transition temperature of Sr₂LaCoMnO₇ is significantly lower than that of Sr₂LaFeMnO₇. The electrical charge-transport properties are also different, where Sr₂LaCoMnO₇ shows considerably improved electrical conductivity. Importantly, the electrocatalytic activities for the two half-reactions of water-splitting, i.e., the hydrogen-evolution reaction (HER) and the oxygen-evolution reaction (OER), are improved in Sr₂LaCoMnO₇ compared to Sr₂LaFeMnO₇. In addition, better kinetics for the HER and OER are observed for Sr₂LaCoMnO₇, as evaluated by the Tafel method. Furthermore, the electrochemically active surface area (ECSA) shows an enhancement for Sr₂LaCoMnO₇. Therefore, the trends in electrical charge transport, the HER and OER activities, kinetics and ECSA are all similar, indicating the improved properties of Sr₂LaCoMnO₇. These changes are explained in the context of a greater bond covalency in this material due to the higher electronegativity of Co, which results in a better overlap between the transition metal d orbital and oxygen p orbital. The relation between the electrocatalytic performance and the optimum e_g orbital occupancy in Sr₂LaCoMnO₇ is also discussed.