Advanced impedance study of polymer electrolyte membrane single cells by means of distribution of relaxation times

Electrochemical impedance spectroscopy data of a polymer electrolyte membrane fuel cell are compiled across a wide range of operating conditions. In all cases, homogeneous operating conditions are applied over the entire cell area to set a clearly defined operating point and to derive unambiguous parameter dependencies. As a novelty, this diverse set of impedance spectra is deconvoluted by the distribution of relaxation times (DRT) method in the frequency range from 0.5 Hz to 60 kHz. In H2/air-operation five polarization contributions with different time constants are registered. The assignment to gas diffusion in the gas diffusion layer and catalyst layer, the oxygen reduction reaction at the Pt-catalyst and proton transport processes in the cathode catalyst layer is supported by a systematic analysis of parameter dependencies. Furthermore, DRT deconvolution in H2/H2-operation reveals three “new” anode contributions, usually hidden by the dominating cathode contributions. They are assigned to gas diffusion and hydrogen oxidation, including charge-transfer and proton transport in the anode catalyst layer. It is confirmed that the polarization contributions at the cathode side sum up to 96% for typical operating conditions, but the share changes to 80% cathode and 20% anode at low hydrogen partial pressure.