Oxygen diffusion impedance in proton exchange membrane fuel cells - insights into electrochemical impedance spectra and equivalent electrical circuit modeling

In Randles circuit, the oxygen diffusion impedance connects in series with the Oxygen Reduction Reaction charge-transfer resistance, implying that the Cathode Catalyst Layer (CCL) governs oxygen diffusion. The oxygen diffusion impedance can be connected in series with the ORR circuit, implying that Gas Diffusion Layer (GDL) rules oxygen diffusion. The common finite Warburg element and an alternative expression for diffusion through GDL derived by Kulikovsky were tested and compared. For this, experimental data obtained from measurements on various cells geometries, MEA types and operating conditions were used. All cases yield the same trend: the low frequency diffusion impedance in PEMFC mainly relates to the GDL, or both the GDL and the CCL, the latter to a significantly lower extent. EI spectra measured for different cathode Pt-loadings suggest that the ionomer film inside the CCL contributes rather to the high-frequency resistance, increasingly at lower catalyst loading. Hence, the physically most appropriate equivalent electrical circuit is a Kulikovsky diffusion impedance connected in series with the ORR kinetics circuit. Comparing global and local impedance spectra confirmed the significant contribution of perturbation-induced oxygen concentration oscillations to the low frequency loop in Nyquist plots: analyzing local impedance data measured near the oxygen inlet allows to minimize their impact.