Suman Basu

Phase Change in a Polymer Electrolyte Fuel Cell

Stable high performance in a polymer electrolyte fuel cell (PEFC) requires efficient removal of product water and heat from the reaction sites. The most important coupling between water and heat transport in PEFC, through the liquid-vapor phase change, remains unexplored. This paper sheds light on physical characteristics of liquid-vapor phase change and its role in PEFC operation. A two-phase, nonisothermal numerical model is used to elucidate the phase-change effects inside the cathode gas diffusion layer (GDL) of a PEFC. Locations of condensation and evaporation are quantified. Operating conditions such as the relative humidity (RH) of inlet gases and materials properties such as the thermal conductivity of GDL are found to have major influence on phase change. Condensation under the cooler land surface is substantially reduced by decreasing the inlet RH or increasing the GDL thermal conductivity. The RH effect is more pronounced near the cell inlet, whereas the GDL thermal conductivity affects the phase-change rate more uniformly throughout the flow length.

Two-Phase Flow Maldistribution and Mitigation in Polymer Electrolyte Fuel Cells

Flow maldistribution among polymer electrolyte fuel-cell (PEFC) channels is of concern because this leads to nonuniform distributions of fuel and oxidizer, which in turn result in nonuniform reaction rates in the catalyst layers and thus detrimentally affect PEFC performance and durability. Channels with low flow rates risk flooding by liquid water. This can cause catalyst support corrosion and hence the undesirably accelerated aging of PEFCs. Multiphase flow computations are performed to examine the effects of gas diffusion layer (GDL) intrusion and manifold design on reducing flow maldistribution. Velocity field, hydrodynamic pressure, and liquid saturations are computed in the parallel gas channels using the multiphase-mixture formulation in order to quantify the flow nonuniformity or maldistribution among PEFC channels. It is shown that, when channel flow is in single phase, employing two splitter plates in the header manifold can bring down the flow maldistribution to less than half of that for the case with 20% area maldistribution due to the GDL intrusion. When channel flow occurs in the two-phase regime, the liquid-water front can be pushed downstream and the effect of GDL intrusion on the maximum liquid saturation can be decreased by more than one-third by using flow splitters.