Effect of Sm³⁺ content on the properties and electrochemical performance of Sm_xSr_{1 - X}CoO_{3 - δ} (0.2 ≤ x ≤ 0.8) as an oxygen reduction electrodes on doped ceria electrolytes

Sm_xSr_{1 - x}CoO_{3 - δ} (SSCx) materials are promising cathodes for IT-SOFCs. The influence of Sm content in SSCx (0.2 ≤ x ≤ 0.8) oxides on their oxygen nonstoichiometry, oxygen desorption, thermal expansion behavior, electrical conductivity and electrochemical activity for oxygen reduction is systematically studied by iodometric titration, oxygen-temperature programmed desorption (O₂-TPD), dilatometer, four-probe DC conductivity, electrochemical impedance spectroscopy (EIS) and three-electrode polarization test, respectively. Iodometric titration experiments demonstrate that the electrical charge neutrality compensation in SSCx proceeds preferably through the oxidation of cobalt ion for high Sm ³⁺ contents (x ≥ 0.6). However, it proceeds mainly through the creation of oxygen vacancies at x ≤ 0.5. O₂-TPD shows SSC5 possesses the highest oxygen desorption ability among the range of SSCx materials tested. The thermal expansion coefficients (TECs) are high between the transition temperature and 900 °C, showing values typically larger than 20 × 10^-6 K^-1. All dense materials show high electrical conductivity with a maximum value of ∼1885 S cm^-1 for SSC6 in air, while SSC5 has the highest electrical conductivity in nitrogen. EIS analysis of porous electrodes demonstrates that SSC5 has the lowest area specific resistance (ASR) value (0.42 Ω cm²) at 600 °C. Cathodic overpotential testing demonstrates that SSC5 also has the largest exchange current density of 60 mA cm^-2 at 600 °C in air.