On the Origin of High Frequency Impedance Feature in a PEM Fuel Cell

Abstract

Measured impedance spectra of a low–Pt PEM fuel cell exhibit high slope in the high–frequency range. To explain this feature, we introduce an “external” capacitance Cext connected in parallel to our recent model for the cathode catalyst layer impedance. This capacitance greatly improves the quality of spectra fitting in the high–frequency domain. We argue that the external capacitance forms due to electric double layer at the carbon/Nafion and carbon/water interface. This interface does not participate in the oxygen reduction and hence it shows up as a capacitance connected in parallel to the catalyst layer impedance. Incorporation of Cext into the model leads to lower values of CCL proton conductivity, than those reported in our previous works.