| This dissertation describes fundamental properties of gas diffusion media (GDM) and their relationship to the mass transport in proton exchange membrane fuel cells (PEMFCs). First, the accuracy of solving the multi-component equations for PEMFC by using a computational fluid dynamics (CFD) technique is examined. This technique uses an approximated multi-component (AMC) model with a correction term that guarantees the overall mass balance. Accuracy is assessed by comparing the species concentrations computed with the Maxwell-Stefan and the AMC model. This comparison is important because the structure of some CFD programs does not permit the direct use of the Maxwell-Stefan equations. Here, it is shown that the maximum error between the two models is less than 5%.;Second, the ratio of tortuosity to porosity, known as the MacMullin number, is reported for different carbon cloth and carbon paper GDM. This analysis show that only carbon cloths GDM follow the commonly accepted Bruggeman equation and that carbon paper GDM have a different relationship between the tortuosity and the porosity. These differences are discussed in terms of path length created by the orientation of fibers of each GDM.;Third, data for the hydrophilic and hydrophobic pore size distributions (PSD) are presented for two types of GDM used in PEMFCs. The data were obtained by using two common measurement methods, intrusion porosimetry (IP) and the method of standard porosimetry (MSP). The use of multiple working fluids to access hydrophilic and hydrophobic pores is discussed as well as the limitations associated with structural changes of the GDM during the tests. The differences in interpretations of the data between the two methods for both GDM have significant implications relative to the distribution of hydrophilic and hydrophobic pores that control liquid water transport.;Finally, a two-phase mass-transport-only model (MTOM) that incorporates the tortuosity and the PSD data described above is presented. The model provides an understanding of the effect of PSD in the water transport by decoupling it from other factors. The MTOM shows that differences in GDM structure produce significant differences in the liquid saturation. |
