Electrochemical Impedance Spectroscopy of Unitized Reversible Fuel Cells (URFC)

Kurzfassung

In mobile and portable applications electrical energy is normally supplied by secondary batteries since the use of electricity from stationary power plants required grid connection. Unfortunately the specific energy and the energy density of those energy 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 possible or feasible. In that case the reactants must be regenerated by water electrolysis and a reversible fuel cell system is needed. An important topic 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. Polarisation curves showing the differences in performance and long term stability of the membrane-electrode-assembly but can not give an overview about the activity of the catalysts. Also there are limited information’s about the behaviour of the electrodes under the different cell conditions. Therefore the impedance spectroscopy is necessary. While operating the same cell in electrolysis and fuel cell mode the impedance spectra of both shows the same elements. So for simulation the same equivalent circuit have to be used and the data are directly comparable to each other. The changing of the properties of anode, cathode and membrane through the operating mode can be shown.