Effect of Electrode Surface Area Distribution on High Current Density Performance of PEM Fuel Cells

Commercialization of automotive fuel cells requires current densities of 1.5 A/cm 2 above 650 mV with Pt loadings of 0.125 mg Pt /cm 2 or less. Loss of high current density with cathode loadings below 0.2 mg Pt /cm 2 in Pt/C electrodes is an issue that current kinetic/transport models are reported inadequate to explain. We show that this effect is much less at a given loading with the NSTF catalyst type electrodes and explain these differences using a model based on elementary kinetic gas theory and known molecule/surface interaction mechanisms that take place in the Knudsen regime. The result is an additional pre-exponential scaling factor f ( d s ) in the Butler-Volmer equation related to a distance metric d s describing the catalyst surface area distribution. We approximate this distance metric by the inverse of the surface area per unit volume of the electrode and define and test two possible functional forms for f ( d s ). The preferred form is able to predict the correct heat of enthalpy for O 2 physisorption and the observed ratio of current densities at V(iR-free) = 0.7 V for NSTF compared to Pt/C dispersed electrodes in the 0.05 to 0.15 mg Pt /cm 2 range from published data for eleven different catalyst types and cathode loadings below 0.2 mg/cm 2 .