In a PEFC stack, end cells are subjected to severe conditions compared to the center cell, sometimes resulting in poor end cell performance and early freeze out. In this work, the concept of using controlled temperature gradients to non-parasitically remove excess water from end cell during PEFC stack shutdown has been numerically investigated. To investigate the end cell water transport, an integrated modeling approach focusing both at stack and single cell level is presented. The stack thermal model is developed to obtain detailed temperature distribution across the PEFC stack. Extending the results of the stack thermal model into a single cell level, a two-phase unit fuel cell model is developed to investigate the water and thermal transport in the PEFC components after shutdown, which for the first time includes thermo-osmotic flow in the membrane. The model accounts for capillary and phase-change induced flow in the porous media, and thermo-osmotic and diffusive flow in the polymer membrane. The single cell model is used to estimate the local water distribution with land/channel boundary condition, and the experimentally validated stack thermal model provided the transient temperature boundary condition to simulate the end cells. Model results indicate that a favorable temperature gradient can be formed in the stack to enhance the water drainage rate, thus enhancing the anode end cell cold start performance.
Visualization of Fuel Cell Water Transport and Performance Characterization under Freezing Conditions
