Theory and Application of Electrochemical Impedance Spectroscopy for Fuel Cell Characterization

Kurzfassung

Separation of different electrochemical and ohmic contributions to the current/voltage U(i) characteristics requires additional experimental techniques like Electrochemical Impedance Spectroscopy (EIS). The application of EIS is an approach to determine parameters which have proved to be indispensable for the characterization and development of fuel cell electrodes and electrolyte electrode assemblies [1]. By varying the operating conditions of the fuel cell and by simulation of the measured EIS with an appropriate equivalent circuit, it is possible to split the cell impedance into electrode impedances and electrolyte resistance. Integration in the current density domain of the individual impedance elements enables the calculation of the individual overpotentials in the fuel cell and the determination of the voltage loss fraction. In the presentation examples of EIS applied to different fuel cell systems and operating conditions will be shown: • Polymer Electrolyte Fuel Cells (PEFC) with symmetrical gas supply, with hydrogen and oxygen at different cell potentials • Solid Oxide Fuel Cells (SOFC) supplied with hydrogen with different water partial pressures and operated at different temperatures • Oxygen reduction at the cathode of an Alkaline Fuel Cell (AFC) Furthermore, the theory of impedance spectra measured at fuel cells with electrodes changing their state with time e.g. anode surface changing during CO poisoning of PEFC anodes and water flooding of the cathode during “dead end” operation mode of the PEFC will be discussed in the presentation. Also first experimental results of locally resolved EIS measured simultaneously on 5 cells of a SOFC stack will be presented and discussed. For the evaluation of the measured impedance spectra a porous electrode model was used. After a brief overview of different porous electrode models used in literature for fitting experimental data the porous electrode model of Göhr [2] will be discussed in detail. This porous electrode model includes different impedance contributions like impedance of the interface porous layer/pore, interface porous layer/electrolyte, interface porous layer/bulk, impedance of the porous layer and impedance of the pores filled by electrolyte. [1] N. Wagner, “Electrochemical power sources – Fuel cells” in Impedance Spectroscopy: Theory, Experiment, and Applications, 2nd Edition, Edited by Evgenij Barsoukov and J. Ross Macdonald, John Wiley&Sons, Inc., ISBN: 0-471-64749-7, pp. 497-537, 2005 [2] H. Göhr, Electrochemical Applications, 1 (1997) 2 (http://www.zahner.de)