Electrochemical Impedance Spectroscopy of Unitized Reversible Fuel Cells (URFC)

Kurzfassung

Mobile and portable applications normally use secondary batteries for electrical energy supply. Electricity supply from stationary power plants required grid connection which is used for charging the batteries. Unfortunately the specific energy and the energy density of battery storage systems are rather low. Therefore fuel cells are an interesting option to overcome the limitation of energy density. However fuel cells need a continuous supply with fuel and oxidant. At some special applications, like space crafts, autonomous long distance aircrafts or stand-alone power supplies, an external refuelling is not suitable or feasible. In that case, the reactants must be regenerated by water electrolysis which can be realized with a reversible fuel cell system. An important topic for reversible fuel cell systems is the function of so-called bifunctional oxygen electrodes which generally require the combination of favourable properties for both operating modes. In particular, different catalysts for oxygen reduction and for oxygen evolution are needed. U(i) performance curves show the differences in performance and long term stability of the membrane-electrode-assembly but can not give insight into the loss distribution of the cell. Also the information about catalyst activity under the different cell conditions is limited. Therefore impedance spectroscopy is essential for gaining further insight. The impedance spectra of both electrolysis and fuel cell mode have similar frequency features. Therefore the spectra can be fitted with the same equivalent circuit and are directly comparable to each other. The changes of the properties of anode, cathode and membrane associated with the operating mode can be determined.