Influence of heat transfer on gas and water transport in fuel cells

An analysis of transport phenomena in a proton exchange membrane fuel cell (PEMFC) is presented, with a focus on the modelling and assessment of non-isothermal and non-isobaric effects that have been neglected in previous studies. A model is formulated for a complete fuel cell taking into account diffusion through the porous electrodes of the humidified fuel (H2, CO2 and H2O(v)) and oxidant gases (O2, N2 and H2O(v)); the convective and electro-osmotic transport of liquid water in the electrodes and the membrane; and heat generation and transfer in the fuel cell. The thermodynamic equilibrium potential is calculated using the Nernst equation, and reaction kinetics is determined using the Butler–Volmer equation. Non-uniform distribution of gas pressure in the porous gas-diffusing electrodes and micro-hydrodynamics in very small pores (Knudsen diffusion) are also taken into account. The model is solved numerically to analyze fuel cell performance and water transport over a range of operating current densities. Non-uniform temperature and pressure distributions are found to have a large impact on the predicted liquid water and vapour fluxes in the anode and cathode diffusion layers. In particular, the results indicate that water management requirements (i.e., humidification or water removal) to prevent potential membrane dehydration or electrode flooding are much more conservative than predicted assuming isothermal conditions. Finally, it is found that, in the range of permeabilities of the porous electrodes used in PEMFCs (10−16–10−17 m2), Knudsen diffusion has to be taken into account in modelling gas transport.