Nd_(1.8)Ce_(0.2)CuO_(4+δ):Ce_(0.9)Gd_(0.1)O_(2-δ) as a composite cathode for intermediate-temperature solid oxide fuel cells

摘要

The (100 - x)Nd_(1.8)Ce_(0.2)CuO_(4+δ):(x)Ce_(0.9)Gd_(0.1)O_(2-δ) (x = 00, 10, 20 and 30 vo.%) composite systems are obtained by impregnating a stoichiometric solution of cerium and gadolinium nitrates followed by sintering at 900 ℃ for 4 h. Impregnating the Ce_(0.9)Gd_(0.1)O_(2-δ) not only inhibits the growth of the host Nd_(1.8)Ce_(0.2)CuO_(4+δ) grains during sintering but also enlarges the oxygen reduction reaction zone by introducing a nanosized phase that is ionically conductive, which significantly decreases the electrode polarization resistance of the composite cathode. A minimum polarization resistance value of 0.23 ± 0.02 Ω cm~2 is obtained at 700 ℃ for a (80)Nd_(1.8)Ce_(0.2)CuO_(4+δ):(20)Ce_(0.9)Gd_(0.1)O_(2-δ) composite cathode, and this value is attributed to the optimal dispersion into the porous Nd_(1.8)Ce_(0.2)CuO_(4+δ) matrix. The impedance spectra are modeled using an electrical equivalent model that consists of a mid-frequency Z_(R1-cpe) circuit (parallel combination of R_1 and constant phase element (CPE)) and a low-frequency Gerischer impedance. The Gerischer impedance decreases significantly when Ce_(0.9)Gd_(0.1)O_(2-δ) infiltrates the Nd_(1.8)Ce_(0.2)CuO_(4+δ) matrix. The oxygen partial pressure-dependent polarization study suggests a medium-frequency response, which is due to charge transfer step; however, the low-frequency response corresponds to the non-charge transfer oxygen adsorption-desorption and the diffusion process during the overall oxygen reduction reaction process.