A 3D, Multiphase, Multicomponent Model of the Cathode and Anode of a PEM Fuel Cell

TLDR: In this paper, a computational fluid dynamics multiphase model of a proton exchange membrane (PEM! fuel cell) was presented, which accounts for three-dimensional transport processes including phase change and heat transfer, and includes the gas-diffusion layers and gas flow channels for both anode and cathode, as well as a cooling channel.

Abstract: A computational fluid dynamics multiphase model of a proton-exchange membrane ~PEM! fuel cell is presented. The model accounts for three-dimensional transport processes including phase change and heat transfer, and includes the gas-diffusion layers ~GDL! and gas flow channels for both anode and cathode, as well as a cooling channel. Transport of liquid water inside the gas-diffusion layers is modeled using viscous forces and capillary pressure terms. The physics of phase change is accounted for by prescribing local evaporation as a function of the undersaturation and liquid water concentration. Simulations have been performed for fully humidified gases entering the cell. The results show that different competing mechanisms lead to phase change at both anode and cathode sides of the fuel cell. The predicted amount of liquid water depends strongly on the prescribed material properties, particularly the hydraulic permeability of the GDL. Analysis of the simulations at a current density of 1.2 A/cm 2 show that both condensation and evaporation take place within the cathode GDL, whereas condensation prevails throughout the anode, except near the inlet. The three-dimensional distribution of the reactants and products is evident, particularly under the land areas. For the conditions investigated in this paper, the liquid water saturation does not exceed 10% at either anode or cathode side, and increases nonlinearly with current density. The operation of proton-exchange membrane ~PEM! fuel cells depends not only on the effective distribution of air and hydrogen, but also on the maintenance of an adequate cell operating temperature and fully humidified conditions in the membrane. The fully humidified state of the membrane is crucial to ensuring good ionic conductivity and is achieved by judicious water management. Water content is determined by the balance between various water transport mechanisms and water production. The water transport mechanisms are electro-osmotic drag of water ~i.e., motion of water molecules attaching to protons migrating through the membrane from anode to cathode!; back diffusion from the cathode ~due to nonuniform concentration!; and diffusion and convection to/from the air and hydrogen gas streams. Water production depends on the electric current density and phase change. Without control, an imbalance between production and removal rates of water can occur. This can result in either dehydration of the membrane, or flooding of the electrodes, which are both detrimental to performance. A common water management technique relies on the humidification of the air and hydrogen gas streams. At higher current densities, the excess product water is removed by convection via the air stream, and the rate of removal is controlled by adjusting moisture content in concert with pressure drop and temperature in the flow channels. Thermal management is also required to remove the heat produced by the electrochemical reaction in order to prevent drying out of the membrane, which in turn can result not only in reduced performance but also in eventual rupture of the membrane. Thermal management, which is performed via forced convection cooling in larger stacks, is also essential for the control of the water evaporation or condensation rates. The operation of a fuel cell and the resulting water and heat distributions depend on numerous transport phenomena including charge-transport and multicomponent, multiphase flow, and heat transfer in porous media. The complexity and interaction of these processes and the difficulty in making detailed in situ measurements have prompted the development of a number of numerical models. The theoretical framework was laid out in early one-dimensional numerical models of the membrane-electrode. 1-3 A quasi-twodimensional model based on concentrated solution theory was also proposed by Newman and Fuller, 4 and a full two-dimensional model including flow channels but no electrodes was also presented by Nguyen and White. 5 This model was refined in a number of subsequent studies to account for the porous electrodes and interdigitated