Optimizing the Copper Content in GdBa0.5Sr0.5Co2-XCuxO5+δ Double Perovskites as the High Performance Oxygen Electrodes for Reversible Solid Oxide Cells

Abstract

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. H2 + 0.5O2 → H2O 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, REBaCo2O5+δ (RE: rare earth elements and Y), feature the layered structure with alternating configuration of REOδ 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 GdBa0.5Sr0.5Co2-xCuxO5+δ (0 ≤ x ≤ 2). Intermediate-size Gd3+ facilitates layered structure formation, while introducing Sr at Ba sites is beneficial to electrical conductivity, due to alleviated lattice distortion. Comprehensive studies have been directed to optimize the copper content in the selected oxides. After synthesis by a self-combustion sol-gel method, the samples were systematically characterized, including identification of the phase composition and crystal structure, thermal stability and chemical reactivity with solid electrolytes, thermal expansion behavior, oxygen content, total electrical conductivity and electrochemical properties. The adopted characterization methods include the X-ray diffraction at room and high temperatures with Rietveld refinements, iodometric titration and thermogravimetric tests, dilatometric measurements, total electrical conductivity studies. Also, laboratory-scale symmetrical and full cells have been manufactured and characterized concerning morphology of the electrodes, electrode-electrolyte interface, electrode polarization resistance with distribution of relaxation times method, as well as power output. High copper content (x > 1.15) was found as hindering factor for synthesis of pure samples. Bearing in mind a need to reduce cobalt content, GdBa0.5Sr0.5Co2-xCuxO5+δ compositions with x = 1, 1.05, 1.1, and 1.15 were selected for further evaluation. The oxides could be synthesized as practically phase-pure, revealing the desired P/4mmm tetragonal symmetry. All of them maintain stable lattice structure up to 1000 °C in air. They are chemically compatible with La0.8Sr0.2Ga0.8Mg0.2O3-δ and Ce0.9Gd0.1O2-δ electrolytes up to at least 900 °C. A moderate thermal expansion coefficients were measured, which decrease with the increasing Cu-doping content, from 15.8·10-6 K-1 (x = 1) to 14.5·10-6 K-1 (x = 1.15) in the temperature range of 300-900 °C. The equilibrated oxygen content in the considered samples ranges from 5.69 (x = 1) to 5.49 (x = 1.15). All oxides exhibit a sufficiently high total conductivity, above 10 S cm-1 in the working temperature range of 600-900 °C. The recorded electrochemical impedance spectra indicate that the lowest polarization resistance (0.017 Ω cm2 at 850 °C) can be obtained for the electrode with GdBa0.5Sr0.5Co0.9Cu1.1O5+δ composition (on La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolyte), which is lower than for the higher cobalt-content GdBa0.5Sr0.5CoCuO5+δ, 0.025 Ω cm2 at the same temperature. The developed GdBa0.5Sr0.5Co0.9Cu1.1O5+δ material used as the oxygen electrode in the La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolyte-supported full cell enabled to achieve power density output exceeding 1.2 W cm-2 at 850 °C, and 0.57 A cm-2 current density at 1.5 V in electrolysis mode at 700 °C. All the evidence prove that Cu-doping is an effective approach, since it allows for limiting the Co content and obtaining the desirable performance at the same time.