Isostructural Oxides Sr3Ti2−xMxO7−δ (M = Mn, Fe, Co; x = 0, 1) as Electrocatalysts for Water Splitting

Abstract

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.