Impedance and Resistivity of Low-Pt Cathode in a PEM Fuel Cell

Analysis of impedance model for the low-Pt cathode catalyst layer (CCL) in a PEM fuel cell is reported. The CCL is modeled as a cylindrical pore with the Nafion film separating the open pore volume from the Pt/C surface. In the limit of fast oxygen transport through the open pore, analytical expressions for the CCL impedance, Nafion film impedance and for the ohmic CCL resistivity R-ccl (Ohm cm(2)) are derived. The characteristic frequency of film impedance is independent of film oxygen transport parameters and it is only 1.73 times less than the frequency of faradaic process in the CCL, which impedes separation of these processes by impedance spectroscopy. A fast version of algorithm for distrubution of relaxation times calculation is developed and used to illustrate the problem. R-ccl exhibits rapid growth in the vicinity of limiting current density in the Nafion film, manifesting "overlinear" oxygen transport loss reported in experiments. For typical low-Pt cell parameters, this growth occurs at the cell current around 1 A cm(-2). The model leads to a simple relation for the Nafion film transport resistivity (s cm(-1)); this relation is compared to semi-empirical and model relations available in literature.