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

Abstract

Commercialization of automotive fuel cells requires current densities of 1.5 A/cm2 above 650 mV with Pt loadings of 0.125 mgPt/cm2 or less. Loss of high current density with cathode loadings below 0.2 mgPt/cm2 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(ds) in the Butler-Volmer equation related to a distance metric ds 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(ds). The preferred form is able to predict the correct heat of enthalpy for O2 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 mgPt/cm2 range from published data for eleven different catalyst types and cathode loadings below 0.2 mg/cm2.