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The correlation of the electrocatalytic activity with electrical conductivity, oxygen-vacancies, and electronegativity have been studied in a series of isostructural oxides, having the so-called Ruddlesden-Popper structure. The structures of these materials comprise transition metals that are octahedrally coordinated to form a network of bilayer stacks. These materials are catalytically active for both half-reactions of water-splitting, namely oxygen-evolution reaction (OER) and hydrogen-evolution reaction (HER). They show a systematic increase in electrocatalytic activity in progression from Sr3Ti2O7 to Sr3TiMnO7, Sr3TiFeO7−δ, and Sr3TiCoO7−δ. The kinetic studies using the Tafel method indicate the same trend across the series, where the best catalyst also has the fastest kinetics for both HER and OER. In addition, the same progression is observed in the concentration of oxygen-vacancies, as well as the electrical conductivity in a wide range of temperatures, 25 °C–800 °C. The material that shows the best electrocatalytic activity, i.e., Sr3TiCoO7−δ, also has the highest electrical conductivity and the greatest concentration of oxygen vacancies in the series. The correlations observed in this work indicate that trends in electrocatalytic performance may be related to the systematic increase in electrical conductivity, electronegativity, and oxygen-vacancies, as well as the electron occupancy of eg orbitals, which can affect the strength of sigma interactions between the catalyst and reaction intermediates. [ABSTRACT FROM AUTHOR]

Herein, the effect of structure on pseudocapacitive properties in alkaline conditions is demonstrated through the investigation of isoelectronic oxides Ca2LaMn2O7 and Sr2LaMn2O7, where the difference in ionic radii of Ca2+ and Sr2+ leads to a change in structure and lattice symmetry, resulting in an orthorhombic Cmcm structure for the former and a tetragonal I4/mmm structure for the latter. While calcium and strontium do not make a direct contribution to the near‐surface faradaic processes that are essential to the pseudocapacitive properties, their effect on the structure leads to a change in the oxygen intercalation process and the associated pseudocapacitive energy storage. It is shown that Sr2LaMn2O7 has a significantly greater specific capacitance than Ca2LaMn2O7. In addition, the former shows a considerably higher‐energy density compared to the latter. Furthermore, these materials show highly stable energy‐storage properties, and retain their specific capacitance over 10 000 cycles of charge–discharge in a symmetric pseudocapacitive cell. Importantly, these findings show the structure–property relationships, where a change in the structure and lattice symmetry can result in a significant change in pseudocapacitive charge–discharge properties in isoelectronic systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]