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

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 eg orbital occupancy in Sr2LaCoMnO7 is also discussed.