Studies on effect of Ca-doping on structure and electrochemical properties of garnet-type Y_3-xCa_xFe₅O_12-δ

In this study, effect of Ca-doping on electrical and electrochemical properties of garnet structure Y_3-xCa_xFe₅O_12-δ (x ​= ​0, 0.05, 0.1, 0.3, 0.5 and 0.7) was investigated. Cubic-phase (Ia-3d) garnet was prepared through sol-gel method and sintered at 1100 ​°C for 2 ​h in air. Highest oxygen non-stoichiometry (δ ​= ​0.19) was found in x ​= ​0.3 member. X-ray absorption spectroscopy (XAS) confirmed the formation of hole carriers (Fe^._Fe) due to Ca-substitution for Y in Y₃Fe₅O₁₂. 4-Probe DC measurements showed that electrical conductivity increased with an increase in Ca amount until x ​= ​0.1 (1.58 ​S/cm at 750 ​°C) and then decreased due to decrease in the concentration of the charge carriers. Lowest area specific resistance (ASR) of 1 Ωcm² was seen at 750 ​°C in air for the symmetrical cell with x ​= ​0.3 garnet-LSGM composite electrode. Studies on pO₂ dependent ASR and impedance spectroscopy genetic programming (ISGP) analysis revealed that oxygen dissociation and partial reduction of adsorbed oxygen molecule co-limit the electrochemical performance for oxygen reduction reaction (ORR).