Simulation of HT-PEMFC AC Impedance Spectra: Relaxation Impedance and Identification of Oxygen Reduction Reaction Mechanism

The scope of this work is to shed some light to the oxygen reduction reaction mechanism for HTPEMFC utilizing polarization and EIS data, in order to extract the elusive ORR kinetics. To do so first we propose a simple three step reaction scheme, then based on the proposed mechanism we derive an analytic electrode impedance equation at the activation regime of operation and develop a Monte Carlo type algorithm to estimate the values of the respective kinetic constants, by simultaneously fitting polarization-dc bias and the corresponding galvanostatic AC Spectra. All data in this study were produced by a 1-D finite element macro-homogeneous physical model of the fuel cell. Our results clearly demonstrate that 1) when the fuel cell is operated at the activation regime two arcs at most, (charge transfer and relaxation), constitute ORR impedance with the latter being dominant and 2) the effectiveness of the proposed algorithm.