Methanol, water and heat transport in direct methanol fuel cells for portable power

Small-scale direct methanol fuel cells (DMFCs) are expected to be next generation power sources for portable applications. High performance of a portable DMFC is determined by the methanol, water and heat transport processes, as well as their complex interactions under a wide variety of operating conditions and design regimes. The present thesis aims to developing a theoretical understanding of these transport phenomena and hence their effects on electrochemical performance of the DMFC. Based on the latest experimental observations, two-phase mathematical models have been developed.; A one-dimensional (1D) model for the membrane-electrode-backing layer assembly in a DMFC is developed for the first time to predict not only polarization curves and methanol crossover but also water crossover and transient discharge behavior coupled with the temperature evolution in a portable DMFC. The model results show good agreement with experimental data of overall cell performance, methanol and water crossover rates through the membrane, as well as reveal a positive interactive feedback mechanism between the transient temperature and methanol crossover profiles, coupled by transient cell voltage and cell energy efficiency profiles, under varied operating conditions.; Next, a multi-dimensional (multi-D) model, making use of a multi-phase mixture (M2) formulation, is developed to encompass all components in the DMFC in a single computational domain. A commercial computational fluid dynamics (CFD) software package, FluentRTM, is employed to solve species transport and electrochemical equations simultaneously. The model results discover an interesting interplay between the local current density and methanol crossover rate distributions for the first time, indicating that the anode flow field design and methanol feeding concentration are two key parameters for the optimal cell performance. When considering electron transport and interfacial liquid coverage on the cathode backing surface, the predicted results provide further insight into the geometrical effect on current density distribution and water transport through the membrane. With an accurate interfacial coverage correlation, water balance between the anode and cathode can be potentially tailored to accommodate the use of high concentration of methanol as fuel, without sacrificing cell performance.; The future work of numerical modeling should involve more complete solutions including both steady state and transient state, and more efficient solutions using improved numerical algorithms. This future work is expected to have important impact on the further development of DMFC technology as portable power.