Electrochemical impedance study of PEM fuel cells. Experimental diagnostics and modeling of air cathodes

The impedance response of a H-2/air PEM FC was investigated in conditions relevant for the operation of low-power fuel cells, used for portable applications (cell temperature close to ambient and moderate humidification). The dependence of EIS pattern on cell voltage, humidification temperature, and air flow rate was examined systematically. The spectrum of the air cathode at room temperature was found to contain two arcs, the potential dependence of which was analyzed in terms of the flooded-agglomerate model for gas diffusion electrodes. The high-frequency (HF) loop is responsible for processes occurring in the;cathode catalyst layer: interfacial charge transfer and mass transport of air in the pores of the catalyst layer (agglomerate diffusion) and in the Nafion layer surrounding the catalyst particles (thin film diffusion). On the basis of its flow rate dependence, the low-frequency loop (LF) was assigned to the mass transport Limitation which appears in the backing due to liquid water accumulation. The model equations lead to excellent fits to experimental data and enable evaluating data of practical significance-exchange current and Tafel plot slope, as well as the parameters characterizing agglomerate and thin-film diffusion. The individual overpotential losses due to kinetic, ohmic, and the three types of mass transport limitations were estimated in a typical case. The results were used to interpret the dependence of spectra on operating parameters: air flow rate, type and temperature of humidification. Also, the electrochemically active surface area of the electrocatalyst was evaluated from double layer capacitances.