Oxygen transport in the low-Pt catalyst layer of a PEM fuel cell: Impedance spectroscopy study

A model for PEM fuel cell impedance taking into account the pore size distribution (PSD) in the cathode catalyst layer is developed. Experimental PSD is approximated by pores of three sizes (small, medium and large) and in each kind of pores, the oxygen diffusion coefficient is allowed to have a separate value. The model is fitted to experimental impedance spectra of a low-Pt PEM fuel cell. The oxygen diffusivities of small and medium pores exhibit rapid growth with the cell current density, while in large pores, this parameter remains nearly constant. We show that oxygen reduction occurs mainly in the small and medium pores, leaving the large pores for mass transport only. This effect explains the discrepancy between small effective oxygen diffusivity of PEMFC catalyst layer measured in situ in operating cells by limiting current method, and much larger value of this parameter determined from ex situ experiments using Loschmidt cell.