Electrochemical behaviour of Ni-cermet anodes containing a proton-conducting ceramic phase on YSZ substrate

Solid oxide fuel cell cermet anodes with proton-conducting ceramic phases, Ni-SrZr0.95Y0.05O2.975 (Ni-SZY), Ni-CaZr0.95Y0.05O2.975 (Ni-CZY) and Ni-SrCe0.475Zr0.475Y0.05O2.975 (Ni-SCZY), have been analysed by electrochemical impedance spectroscopy. The anodes were sintered on opposing faces of yttria-stabilised zirconia (YSZ) electrolyte and the polarisation behaviour studied in the temperature range 600–900°C in various regimes of H2 and H2O partial pressures. The ceramic component of the Ni-CZY and Ni-SCZY cermets form an insulating phase at the interface with YSZ. Impedance spectra are composed of two dominant rate-limiting contributions attributable to electrode processes with relaxation frequencies ca. 103 and 1Hz at 800°C. Both high- and low-frequency responses are sensitive to H2O partial pressure and temperature, with activation energies in the range 1.02–1.25 and 1.19–1.35eV, respectively. Factors influencing the origin of the rate-limiting processes are discussed, including transport limitations (oxide-ion and electronic) in the solid phases and microstructure. Proton conductivity may assist in accelerating the kinetics of the anodic reaction by widening the effective reaction area in electrodes optimised in terms of Ni content, oxide-ion conductivity and microstructure.