Oxygen Transport Impedance in a Polymer Electrolyte Membrane Fuel Cell Equivalent Electrical Circuit

One common way to interpret the data of Electrochemical Impedance Spectroscopy (EIS) with Polymer Exchange Membrane Fuel Cells (PEMFC) consists in using an Equivalent Electrical Circuit (EEC). There are however various issues in EEC modeling, among which the location and expression of the oxygen transport impedance. In this work, we compare the results obtained using a Randles circuit with those of an EEC where the oxygen diffusion impedance is connected in series with the circuit of the Cathode Catalyst Layer (CCL). In the Randles circuit, the oxygen transport impedance is in series with the charge transfer resistance of the Oxygen Reduction Reaction (ORR), implying that the CCL (pores and/or ionomer) is governing oxygen diffusion. In the other case, the oxygen diffusion impedance is outside of the CCL circuit, which implicates that the Gas Diffusion Layer (GDL) governs oxygen diffusion. In addition, two expressions of the GDL oxygen diffusion impedance were tested: the usual finite Warburg impedance and an alternative expression derived by Kulikovsky that considers the impact of the double-layer capacity on oxygen concentration at the CCL/GDL interface. The parameters obtained with these EEC are used to estimate the main characteristic diffusion length, for various cells and operating conditions. The same trend was observed in all cases: the values of the characteristic diffusion length are found to be of the order of the GDL thickness.