Impedance modelling of porous electrode structures in polymer electrolyte membrane fuel cells

Electrochemical Impedance Spectroscopy (EIS) is a suitable tool for identifying the performance-related polarization processes in a polymer electrolyte membrane fuel cell. A physically meaningful impedance model is needed when drawing conclusions about further cell improvement. This study focuses on, the characterization of the porous electrode structure by applying a transmission line model (TLM) to the measured spectra. The fitting procedure is supported by the distribution of relaxation times (DRT) method enabling a separation of loss processes by their individual time constants. We are able to separate and quantify (i) the gas diffusion in the porous media (2-10 Hz), (ii) the charge transfer resistance at the Pt catalyst (2-200 Hz), and (iii) the ionic transport resistance in the catalyst layer (300-30,000 Hz), across a broad range of operating conditions (current density, relative humidity, gas compositions). The TLM approach directly reveals the electrodes' transport and reaction properties, e.g. ionic conductivity and the Tafel slope. Under high electrical load the ionic transport losses in the catalyst layer contribute more to polarization than expected. Interestingly, the oxygen reduction reaction is found to be describable with a single, current-independent Tafel slope.