Application of Fourier-based transforms to impedance spectra of small-diameter tubular solid oxide fuel cells

Recent demonstrations of direct utilization of hydrocarbon fuels have stimu lated an automotive interest in solid oxide fuel cells for reformerless aux iliary power units with high power density, high chemical-to-electrical eff iciency, and low exhaust emissions. Furthermore, recent designs with small- diameter oxide tubes appear to be well-suited to accommodate repeated cycli ng under rapid changes in electrical load and in cell operating temperature s. To understand the limiting transient processes in these small-tube fuel cell designs, we applied an analysis approach which requires no a priori eq uivalent circuit model assumptions. This approach was applied to the electr ochemical impedance spectroscopy (EIS) data measured from such cells in the temperature range from 585 to 888 degreesC. In this way, the complex, over lapping arc EIS details (seen in Cole-Cole plots) were transformed in a net work-model-independent way into a spectrum of relaxation times. We extended the deconvolution method to allow peak fitting and integration to calculat e the resistances of individual processes within the cathode polarization, which becomes limiting in comparison to either anode or electrolyte at temp eratures below about 700 degreesC. With the new results, the process with t he highest apparent activation energy can be targeted to improve cathode de velopment. (C) 2001 The Electrochemical Society.