Polarization behavior of Ni-YSZ cermet anodes in YSZ fuel cells running on methane under internal reforming conditions

The polarization characteristics of the Ni-YSZ cermet anode/YSZ interface were studied as a function of gas phase composition, temperature and anodic overpotential in methane fuelled internal reforming YSZ fuel cells. AC impedance measurements carried out under open circuit conditions in the temperature range 800 to 900°C showed the presence of at least two rate-limiting processes that were influenced by temperature and gas phase composition. The apparent polarization resistance, R_app, of the Ni-YSZ cermet anode/YSZ interface was found to increase with increasing methane partial pressure and decreasing H₂O partial pressure. The activation energy related to the apparent polarization conductance R _app ⁻¹ varied between 0.7 and 1.1 eV, mainly due to the variation of P_{H 2 O}. The order of magnitude of the apparent capacitance, C_app, values calculated from AC impedance data under open circuit conditions indicate that the observed capacitance corresponds to the double layer capacitance of the Ni/YSZ interface and/or a pseudo-capacitance related to adsorption of species on the Ni-YSZ cermet electrode accompanied by charge transfer. C_app was found to decrease with increasing methane partial pressure. AC impedance measurements carried out under closed circuit conditions at 900 °C and four different P_{H 2 O} showed the appearance of a low-frequency inductive loop at high anodic over-potentials, in addition to the features observed under open circuit conditions. The effect of gas phase composition on R_app was similar to the one observed under open circuit conditions. In addition an almost exponential decrease of R_app with increasing anodic overpotential was observed. The present results are compared with results presented in literature and discussed in the frame of the mechanism of the electrochemical and catalytic reactions taking place over the anode. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.