Effect of Co doping on the electrochemical properties of Sr ₂Fe_1.5Mo_0.5O₆ electrode for solid oxide fuel cell

Co is doped to Sr₂Fe_1.5Mo_0.5O₆ to enhance its electrochemical activity as the cathode for intermediate- temperature solid oxide fuel cells. Pure cubic perovskites of Sr ₂Fe_1.5-xCo_xMo_0.5O₆ (SF_1.5-xC_xM, x = 0, 0.5, 1) are synthesized using a glycine-nitrate combustion progress. The average thermal expansion coefficient varies from 15.8 to 19.8 × 10^-6 K^-1. The electrical conductivity increases while its activation energy decreases with increasing Co content. X-ray photoelectron spectroscopy analysis demonstrates mixed valences of Fe, Co and Mo, suggesting small polaron hopping mechanism. Electrical conductivity relaxation (ECR) measurement shows that the surface exchange coefficient increases about two orders of magnitude when the content increases from x = 0 to x = 1.0, i.e. from 2.55 × 10^-5 to 2.20 × 10^-3 cm s^-1 at 750 °C. ECR also exhibits that chemical diffusion coefficient increases with Co content. Density Functional Theory calculation demonstrates that oxygen vacancy formation energy decreases with Co content, suggesting high oxygen vacancy concentration at high Co content. Impedance spectroscopy on symmetric cells consisting of SF_1.5-xC _xM electrodes and La_0.8Sr_0.2Ga _0.8Mg_0.2O_3-δ electrolytes shows that Co doping is very effective in reducing the interfacial polarization resistance, from 0.105 Ω cm² to 0.056 Ω cm² at 750 °C. These results suggest that Co doping into Sr₂Fe_1.5Mo _0.5O₆ can substantially improve its electrochemical performance. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.