Impact of oxygen-vacancies on electrical conductivity and electrocatalytic activity of La3-xCaxFe2GaO9-δ (x = 0, 2; δ = 0, 1)

Abstract

The effect of oxygen-vacancies on functional properties is demonstrated through the investigation of LaCa2Fe2GaO8, which has a perovskite-related structure, where oxygen-vacancies appear in every third layer, converting the octahedral coordination into tetrahedral. This effect is highlighted when the properties of this material are compared to that of La3Fe2GaO9 (LaFe0.67Ga0.33O3), which has the same Fe/Ga stoichiometry but lacks oxygen-vacancies. Since the electrocatalytically active metals in ABO3-δ catalysts are the transition metals, the role of the A-site metals (La and Ca) is to modify the structural features of the material. While the electrocatalytic performance of these systems is lower than that of the state of the art catalysts, they show an excellent example of structure-activity relationships. The vacancy-ordered LaCa2Fe2GaO8 shows better HER electrocatalytic performance compared to La3Fe2GaO9. The former also shows a faster HER kinetics. We have also studied the electrical conductivity in a wide range of temperatures, indicating the enhanced electrical charge-transport properties of LaCa2Fe2GaO8. Therefore, both electrical conductivity and HER electrocatalysis are improved due to the ordered arrangement of oxygen-vacancies in LaCa2Fe2GaO8 as compared to La3Fe2GaO9.