Interpretation of Low Temperature Solid Oxide Fuel Cell Electrochemical Impedance Spectra

Electrochemical impedance spectroscopy was performed on low temperature solid oxide fuel cells with yttria-stabilized zirconia electrolytes and different electrode materials and morphologies. Three loops are seen in a Nyquist plot; the high frequency loop is attributed to the electrolyte and series resistance. The intermediate and low frequency loops are influenced by the material and morphology of both electrodes. To clarify which elementary processes contribute to each loop, kinetic Monte Carlo simulations of a solid oxide fuel cell were performed to calculate the reaction rates for each elementary process. The rates fall into three groupings, allowing the identification of processes with corresponding features in the impedance spectra. Vacancy diffusion processes occur at the highest frequency, agreeing with the usual assignment of the high frequency loop with series resistance. Chemical reactions at the anode have an intermediate frequency, suggesting that the intermediate frequency loop is dominated by anode reactions. Low frequency reactions include electrochemical reactions, chemical reactions at the cathode, and water formation and desorption at the anode. This agrees with the experimental findings of the strong dependence of the low frequency loop on the bias voltage and the dominance of the cathode reactions in the low frequency regime.