In this thesis, the development of a three-dimensional, single-phase, non-isothermal model for the cathode of a Proton Exchange Membrane fuel cell is presented. The model is implemented in a commercial Computational Fluid Dynamics code, developed by Fluent Inc., and, within the single-phase assumption, accounts for most of the transport phenomena present in a PEMFC. Investigations were carried out on the implementation of a agglomerate catalyst model within a three-dimensional geometry, different approaches to catalyst modelling, effects of mixing and convection in serpentine flow fields, and the effects of channel length and permeability on roles of mixing and convection within serpentine configurations. The three-dimensional implementation of the agglomerate model shows that two-dimensional model geometries were incapable of accurately predicting current density distributions along the length of the flow channel. Shifts in the current distribution are seen as the dominating loss transitions from electron transport to mass transport. Thin-interface, discrete, and agglomerate approaches to catalyst modelling were assessed. It was found that there were significant differences in the predicted polarization curves for each model. The agglomerate model exhibited the mass-transport drop-off at high current densities, despite the absence of liquid water. The resulting polarization curves also showed differing slopes between the approaches due to different dominating losses. Application of the model was carried out for serpentine flow fields where it was observed that the effect of mixing due to the bends resulted in increases of approximately 1% in the average current density at high load, while the role of convection was more significant with an increase of about 25%. The effect of varying channel length was assessed for serpentine domains. Lengths of 19.25mm, 39.5mm, and 80mm were considered with constant active areas. A length of 19.25mm showed the highest increase in current density for both mixing and convection with increases of 2% and 28%, respectively. Longer channel lengths were seen to exhibit flow bypass at permeabilities in excess of 10_u m 2 and showed increases in the current density on the order of 0.42% and 7.82% for mixing and convection respectively.