In this dissertation, we present two major lines of numerical investigation based on a control-volume approach to solve coupled, nonlinear differential equations. The first model is developed to provide better understanding of the water management in PEMFC operating at less than 100ºC, under transient conditions. The model provides explanations for the observed differences between hydration and dehydration time constants during load change. When there is liquid water at the cathode catalyst layer, the time constant of the water content in the membrane is closely tied to that of liquid water saturation in the cathode catalyst layer, as the vapor is already saturated. The water content in the membrane will not reach steady state as long as the liquid water flow in the cathode catalyst layer is not at steady state. The second model is to optimize the morphological properties of HT-PEMFCs components so as to keep water generated as close as possible to the membrane to help reduce ionic resistance and thereby increase cell performance. Humidification of the feed gas at room temperature is shown to have minimal effects on the ionic resistance of the membrane used in the HT-PEMFC. Feed gases must be humidified at higher temperature to have effects on the ionic resistance. However, humidification at such higher temperatures will require complex system design and additional power consumption. It is, therefore, important to keep the water generated by the electrochemical reaction as close as possible to the membrane to hydration the membrane so as to reduce the ionic resistance and thereby increase cell performance. The use of cathode MPL helps keep the water generated close to the membrane and decreasing the MPL porosity and pore size will increase the effectiveness of the MPL in keep the water generated close to the membrane. The optimum value of the MPL porosity depends on the operating conditions of the cell. Similarly, decreasing the GDL porosity helps keep water close to the membrane and the optimum value of the GDL porosity depends on the operating conditions of the cell. / text