Effect of Sm~(3+) 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

摘要

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 (O_2-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. O_2-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 cm~2) 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.