On the electrochemistry of metal–YSZ single contact electrodes

Two different theoretical impedance models were fitted to experimental spectra measured on Ni–YSZ and Pt–YSZ single contact electrodes in H 2–H 2O atmospheres at 1273 K. The models assume that only the activities of the adsorbed species and the electrode overpotential govern the rate limiting reaction steps, thus ignoring any diffusion limitations. The two models consider either one or two active species, respectively. Equivalent circuits were constructed for both models, incorporating also the impedance response due to the current constriction near the three-phase boundary and the electrolyte resistance. The equivalent circuits were fitted to spectra recorded at open circuit potential as well as polarized electrodes. In both cases, only the model assuming two adsorbed species was able to adequately reproduce the experimental data. In the analysis of the impedance spectra, the charge transfer resistance, R ct, was found to contribute only a few percent to the polarization resistance, R p. Chemical processes such as adsorption, desorption and possibly reactions between adsorbed species seem to dominate R p. However, the present models could not resolve the identity of these processes.