Characterization of (La_1-xSr_x)(s)MnO₃ and Doped Ceria Composite Electrodes in NO_x-Containing Atmosphere with Impedance Spectroscopy

This study used electrochemical impedance spectroscopy (EIS) to characterize composite metal oxide electrodes in atmospheres containing NO, NO2, and O2. Symmetrical cells with electrodes of (La1−xSrx)sMnO3 [(x, s)=(0.15, 0.9) and (0.5, 0.99)] and doped ceria [Ce0.9Gd0.1O2 and Ce1−xGdxO2 (x=0.1, 0.2)] were subjected to EIS while varying the temperature (from 300 to 600°C), the composition of the atmosphere, and the gas flow. The impedance spectra were fitted to equivalent circuits, and common arcs were identified and sought related to physical and chemical processes. The electrodes had a much lower polarization resistance (Rp) when NO or NO2 was present in the atmosphere at low temperatures (300–400°C) than in air. The impedance spectra for electrodes in 1% NO in Ar were dominated by a low frequency arc at high temperatures (500–600°C). This arc seemed to be a type of conversion arc, which is related to a gaseous intermediate (possibly NO2), formed from NO, through which the electrode reaction occurs. Indications were found that the electrodes are not electrochemically active toward NO around open-circuit voltages. ©2010 The Electrochemical Society