Optimizing the Copper Content in GdBa 0.5 Sr 0.5 Co 2-X Cu x O 5+δ Double Perovskites as the High Performance Oxygen Electrodes for Reversible Solid Oxide Cells

Sustainable development and better protection of environment are among the top priority issues for human society. Seeking for cleaner and less greenhouse-emissive energy resources or solutions has been a dominating trend around the world, since daily functioning of the current society is still dependent on fossil energy sources, such as oil, coal, and also natural gas. Tremendous efforts have been made to develop alternative energy production technologies, among them, fuel cells have been found as very promising ones. Especially, reversible Solid Oxide Cells (rSOCs) have become famous for their all-solid structure, which consist of the porous fuel electrode, dense solid oxide electrolyte, porous oxygen electrode, and interconnects. Reversible SOCs can effectively work either in the fuel cell (e.g. H 2 + 0.5O 2 → H 2 O and heat) or electrolyzer modes, making them useful for grid balancing and utilization of surplus electrical energy. It is worth noting that the oxygen electrode has a decisive role regarding performance and long-term operation of the whole cell. Until now most of the widely-studied and also practically-implemented oxygen electrode materials are cobalt-based perovskite-type oxides or related compounds having the relatively high concentration of Co. This can be regarded as one of drawbacks, due to the toxicity, carcinogenicity, high costs, and poorly available resources of cobalt, despite impressive electrocatalytic activity and excellent electrochemical properties of such materials. Accordingly, numerous attempts have been carried out, which aim at replacing Co with other transition metal elements, such as Ni, Fe, Mn, and Cu, at least to some extent. For example, copper-doping into Co-based perovskites enabled to limit the excessive thermal expansion, as well as positively influenced redox stability. The A-site ordered double perovskites, REBaCo 2 O 5+δ (RE: rare earth elements and Y), feature the layered structure with alternating configuration of REO δ and BaO layers along the c -axis, which generates fast diffusion channels for migration of oxygen anions, as well as the materials exhibit excellent electrical conductivity. In this work, an attempt has been made to further optimize the compounds, with the target compositions proposed as GdBa 0.5 Sr 0.5 Co 2-x Cu x O 5+δ (0 ≤ x ≤ 2). Intermediate-size Gd 3+ facilitates layered structure formation, while introducing Sr at Ba sites is beneficial to electrical conductivity, due to alleviated lat