Abstract: A composition consisting essentially of a perovskite crystalline structure includes ions of a first metal M1 which occupies an A-site of the perovskite crystalline structure and ions of a second metal M2 which occupies a B-site of the perovskite crystalline structure. M2 has two oxidation states capable of forming a redox couple suitable for reversibly catalyzing an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER). The composition also includes ions of a third metal M3 at least a portion of which substitutes for M1 in the A-site of the perovskite crystalline structure, and at least a portion of which optionally also substitutes for M2 in the B-site of the perovskite crystalline structure. At least some of the ions of M3 have a different oxidation state to the ions of M1. The composition also includes atoms of an element X, which is a chalcogen.

Abstract: A composition consisting essentially of a perovskite crystalline structure, the composition comprising: ions of a first metal M1 which occupies an A-site of the perovskite crystalline structure; ions of a second metal M2 which occupies a B-site of the perovskite crystalline structure, M2 having two oxidation states capable of forming a redox couple suitable for reversibly catalyzing an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER); ions of a third metal M3 at least a portion of which substitutes for M1 in the A-site of the perovskite crystalline structure, and at least a portion of which optionally also substitutes for M2 in the B-site of the perovskite crystalline structure, at least some of the atoms M3 having a different oxidation state to the atoms M1; and atoms of an element X, which is a chalcogen; wherein the metal ions M1, M2 and M3 are present in the atomic ratios (a) or (b): (a) 25 to 49.