Recent demonstrations of direct utilization of hydrocarbon fuels have stimulated an automotive interest in solid oxide fuel cells for reformerless auxiliary power units with high power density, high chemical-to-electrical efficiency, and low exhaust emissions. Furthermore, recent designs with small-diameter oxide tubes appear to be well-suited to accommodate repeated cycling under rapid changes in electrical load and in cell operating temperatures. To understand the limiting transient processes in these small-tube fuel cell designs, we applied an analysis approach which requires no a priori equivalent circuit model assumptions. This approach was applied to the electrochemical impedance spectroscopy ͑EIS͒ data measured from such cells in the temperature range from 585 to 888°C. In this way, the complex, overlapping arc EIS details ͑seen in Cole-Cole plots͒ were transformed in a network-model-independent way into a spectrum of relaxation times. We extended the deconvolution method to allow peak fitting and integration to calculate the resistances of individual processes within the cathode polarization, which becomes limiting in comparison to either anode or electrolyte at temperatures below about 700°C. With the new results, the process with the highest apparent activation energy can be targeted to improve cathode development.Solid oxide fuel cells ͑SOFCs͒ are established as electrochemical devices for stationary electrical power generation. 1 Because of their high power conversion efficiency and low exhaust emissions, 2,3 these energy conversion devices have also been suggested for use in automotive auxiliary power units ͑APUs͒ to meet the increasing demands for on-board electrical power. Futhermore, recent laboratory-based demonstrations have given evidence of direct oxidation of hydrocarbon fuels 4,5 at the SOFC anode, without carbon deposition, at sufficiently low temperatures. Consequently, the likelihood of developing a compact, reformerless APU that can operate on the same fuel as the internal combustion engine now seems to be more realistic.The development of practical SOFCs for automotive use can benefit from fundamental knowledge about ac impedance related interfacial processes and associated microstructures. Since a SOFC is a complicated electrochemical system consisting of heterogeneous phases of the electrodes 6,7 and the electrolyte, 8 improvements in cell performance and manufacturing technology can be better focused if there is a means to separate the individual impedance-related processes within a single cell. These processes include the transport of ions and electrons through ͑and reacting at͒ the interphase boundaries. Each of these processes may depend upon temperature.Therefore, the main objective of the present work is to estimate the influence of temperature ͑580-888°C͒ on the separate component impedances in fast-thermal-response cells of potential automotive interest.
ApproachWe adopt and extend an approach that does not require any a priori assumptions of an equivalent network model to rep...