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 C-ext 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 C-ext into the model leads to lower values of CCL proton conductivity, than those reported in our previous works. (C) The Author(s) 2019. Published by ECS.