115 - A 3D, Multiphase, Multicomponent Model of the Cathode...

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Unformatted text preview: A 3D, Multiphase, Multicomponent Model of the Cathode and Anode of a PEM Fuel Cell T. Berning and N. Djilali z Institute for Integrated Energy Systems, University of Victoria, Victoria, British Columbia V8W 3P6, Canada 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. © 2003 The Electrochemical Society. @ DOI: 10.1149/1.1621412 # All rights reserved. Manuscript submitted January 13, 2003; revised manuscript received June 9, 2003. Available electronically November 12, 2003. 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 tempera- ture 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 trans- port mechanisms and water production. The water transport mecha- nisms are electro-osmotic drag of water ~ i.e. , motion of water mol- ecules attaching to protons migrating through the membrane from anode to cathode ! ; back diffusion from the cathode ~ due to nonuni- form 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....
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115 - A 3D, Multiphase, Multicomponent Model of the Cathode...

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