A nonisothermal, two-phase model for polymer electrolyte fuel cells

A model fully coupling the two-phase flow, species transport, heat transfer, and electrochemical processes is developed to investigate liquid water distribution and flooding in polymer electrolyte fuel cells (PEFCs) under nonisothermal conditions. The thermal model accounts for irreversible heat and entropic heat generated due to electrochemical reactions, Joule heating arising from protonic/electronic resistance, and latent heat of water condensation and/or evaporation. A theoretical analysis is presented to show that in the two-phase zone, water transport via vapor-phase diffusion under the temperature gradient is not negligible, with a magnitude comparable to the water production rate in PEFCs. Detailed numerical results further reveal that the vapor-phase diffusion enhances water removal from the gas diffusion layer (GDL) under the channel and exacerbates GDL flooding under the land. Simultaneously, this vapor-phase diffusion provides a new mechanism for heat removal through a phase change process in which water evaporates at the hotter catalyst layer, diffuses through the interstitial spaces of the GDL, and condenses on the cooler land surface. This new heat removal mechanism resembles the heat pipe effect. Three-dimensional simulations for a full PEFC using this nonisothermal, two-phase model are presented for the first time. Separate velocity fields of gas and liquid phases are given, clearly illustrating that the vapor-phase diffusion and capillary-driven liquid water transport in a GDL aid each other in water removal along the through-plane direction under the channel area, but oppose each other along the in-plane direction between the channel area and land. (C) 2006 The Electrochemical Society.