The Effect of Proton Conductivity of Fe-N-C-Based Cathode on PEM Fuel cell Performance

A model-based impedance spectroscopy is used to determine proton conductivity, oxygen transport parameter, double layer capacitance and oxygen reduction reaction (ORR) Tafel slope in the Fe-N-C cathode catalyst layer (CCL) of a PEM fuel cell. Experimental spectra of two cells differing by the membrane thickness only are processed using a physics-based model for PEMFC impedance. The spectra have been measured in the range of current densities from 25 to 800 mA cm^-2. The ORR Tafel slope of both the cells shows almost linear growth with the current density. In one of the cells, the CCL proton conductivity σ _p strongly decays at the current density of 100 mA cm^-2; this decay is accompanied by the step growth of the double layer capacitance. Other minor variations of proton conductivity and double layer capacitance with the cell current occur also in a counterphase; presumed origin of this effect is discussed. The oxygen diffusion coefficient in the cathode exhibits explosive growth with the cell current. We attribute this effect to formation of temperature and pressure gradients in the CCL due to strongly non-uniform distribution of ORR rate in the electrode.