Proton Conduction and Oxygen Reduction Kinetics in PEM Fuel Cell Cathodes: Effects of Ionomer-to-Carbon Ratio and Relative Humidity

The electrode in a proton exchange membrane (PEM) fuel cell is composed of a carbon-supported Pt catalyst coated with a thin layer of ionomer. At the cathode, where the oxygen reduction reaction occurs, protons arrive at the catalyst sites through the thin ionomer layer. The resistance to this protonic conduction (R-H(+),cath) through the entire thickness of the electrode can cause significant voltage losses, especially under dry conditions. The R-H(+),cath in the cathode with various ionomer/carbon weight ratios (I/C ratios) was characterized in a H-2/N-2 cell using ac impedance under various operating conditions. AC impedance data were analyzed by fitting R-H(+),cath, cathode capacitance (C-cath), and high frequency resistance to a simplified transmission-line model with the assumption that the proton resistance and the pseudocapacitance are distributed uniformly throughout the electrode. The proton conductivity in the given types of electrode starts to drop at I/C ratios of approximately < 0.6/1 or an ionomer volume fraction of similar to 13% in the electrode. The comparison to H-2/O-2 fuel cell performance shows that the ohmic loss in the electrode can be quantified by this technique. The cell voltage corrected for ohmic losses is independent of relative humidity (RH) and the electrode's I/C ratio, which indicates that electrode proton resistivity rho(+)(H),cath (ratio of R-H(+),cath over cathode thickness) is indeed an intrinsic RH-dependent electrode property. The effect of RH on the ORR kinetics was further identified to be rather small for the range of RH studied (>= 35% RH).