Theory and Application of Porous Electrodes in Fuel Cell Characterization

Kurzfassung

The most common method used to characterize the electrochemical performance of fuel cells is recording of current/voltage U(i) curves. Separation of electrochemical and ohmic contributions to the 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 all types of fuel cell electrodes and electrolyte electrode assemblies. In addition to EIS semi-empirical approaches based on simplified mathematical models can be used to fit experimental U(i) curves. 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 case of using a three electrode cell configuration with a reference electrode one can determine directly the corresponding overvoltage. For the evaluation of the measured impedance spectra the porous electrode model of Göhr was used. 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.