Unveiling the Electrocatalytic Activity of the GdBa_0.5Sr_0.5Co_2-CuO_5+δ (x ≥ 1) Oxygen Electrodes for Solid Oxide Cells

The A-site cation-ordered GdBa_0.5Sr_0.5Co_2-CuO_5+δ (GBSCC) double perovskites are evaluated regarding the development of high-performance oxygen electrodes for reversible solid oxide cells (rSOCs). The aims are to maximally decrease the content of toxic and expensive cobalt by substitution with copper while at the same time improving or maintaining the required thermomechanical and electrocatalytic properties. Studies reveal that compositions with 1 ≤ x ≤ 1.15 are particularly interesting. Their thermal and chemical expansions are decreased, and sufficient transport properties are observed. Complementary density functional theory calculations give deeper insight into oxygen defect formation in the considered materials. Chemical compatibility with La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM) and Ce_0.9Gd_0.1O_2-δ (GDC) solid electrolytes is evaluated. It is documented that the GdBa_0.5Sr_0.5Co_0.9Cu_1.1O_5+δ oxygen electrode enables obtaining very low electrode polarization resistance (R_p) values of 0.017 Ω cm² at 850 °C as well as 0.111 Ω cm² at 700 °C, which is lower in comparison to that of GdBa_0.5Sr_0.5CoCuO_5+δ (respectively, 0.026 and 0.204 Ω cm²). Systematic distribution of relaxation times analyses allows studies of the electrocatalytic activity and distinguishing elementary steps of the electrochemical reaction at different temperatures. The rate-limiting process is found to be oxygen atom reduction, while the charge transfer at the electrode/electrolyte interface is significantly better with LSGM. The studies also allow elaborating on the catalytic role of the Ag current collector as compared with Pt. The electrodes manufactured using materials with x = 1 and 1.1 permit reaching high power outputs, exceeding 1240 mW cm^-2 at 850 °C and 1060 mW cm^-2 at 800 °C, for the LSGM-supported cells, which can also work in the electrolysis mode.