Numerical Simulation Of Transport Phenomena In Proton Exchange Membrane Fuel Cells

Two mathematical models are developed in this dissertation. The first one is a two-dimensional, steady-state model for liquid-feed direct memanol fuel cell (LFDMFC), another one is a three-dimensional, comprehensive, steady-state mathematical model for H2/O2 proton exchange membrane fuel cell (PEMFC). The transport phenomena, electrochemical characteristics and the current-potential profile of the fuel cells are studied. The fuel cells performance of H2/O2 PEMFC and DMFC predicted by models are compared with experimental results respectively, and reasonable agreements are achieved.At first, a two-dimensional across-the-channel mathematical model for simulation of a direct methanol fuel cell is described. The model accounts simultaneously for electrochemical kinetics, hydrodynamics, and multicomponent transport, and fully accounts for the mixed potential effects of methanol oxidation at the cathode as a result of methanol crossover caused by convection, diffusion and electro-osmosis. Transport in the diffusion layers and catalyst layers are described by a superposition of Knudsen diffusion and Stefan-Maxwell diffusion, and electrochemical kinetics for anodic methanol oxidation and cathodic oxygen reduction are described by Tafel equation. The two-dimensional distributions of concentrations of reactants, the two-dimensional distributions of current densities, the crossover flux of methanol from the anode to the cathode, and current-voltage curve for the fuel cell have been calculated. The obtained results indicate that the concentrations of reactants in the catalyst layers in front of the current collectors are very low, that reduce the utility of catalyst; the current density at the edges of the channels is many times greater than the mean current density. Effects of variable methanol inlet concentration and cathode pressure on fuel cell performance and methanol crossover are analyzed.Then, a three-dimensional, comprehensive, steady-state mathematical model is described to investigate the fluid flow, heat transfer, species transport and electrochemical reaction in the PEM fuel cells. The studied domain consists of fluid channels, diffusion layers and catalyst layers of anode and cathode, and membrane. The transport phenomena occurred in the whole fuel cell is described by the generalized equation, and different physical parameters and source terms are employed for different layers. The flow characteristics, distributions of temperature, potentials, and chemical components in the 3-D space are obtained by resolving thetransport equation set, and coupling the electrochemical kinetics equations. The general differential equations are solved by method based on volume-control finite-discrete computation fluid dynamics (CFD) technique. The effect of thermal conductivity of membrane on the distribution of temperature in the fuel cell is discussed, and the effect of inlet velocity and porosity of porous diffusion layers on the fuel cell performance are analyzed. The results show that high inlet velocity and porosity is favorable for fuel cell performance.Based on above-mentioned 3-D mathematical model, a comparison study of PEMFC with conventional and interdigitated flow fields has been conducted at last. The flow characteristics, distributions of current density and chemical components, and the performance of these two different designs are calculated and compared. The flow and mass transport characteristics are analyzed in detail, which indicate that strong forced convection is produced in the interdigitated flow field, which consist of dead-end gas channel that force the gases through the porous electrodes. Results of comparison show that forced convection induced by the interdigitated flow field in the diffusion layer effectively enhances mass transport of reactants and products, thus leading to a higher cell performance and the limiting current density.