Simulation and experimental characterization of the water management for an open-cathode direct methanol fuel cell that utilizes a liquid barrier layer

Water management is a critical issue in a practical direct methanol fuel cell (DMFC) system. In this research, the interaction of water management and performance of a DMFC stack with an open-cathode design utilizing a liquid barrier layer (LBL) is studied. As compared to the traditional design, the novel DMFC stack has a passive water-recovery mechanism and eliminates the water collection and replenishment devices on the cathode side, reducing the complexity and size of the system. However, water management of the new DMFC stack can impose significant operating constraints if the water balance is not well-controlled. The purpose of this research is to analytically and experimentally study the effects of the change of the key variables on the water balance of the novel DMFC stack.;A model was developed to simulate the cell performance, rate of methanol crossover, and multi-component mass transport of the novel DMFC stack. A dimensionless water balance parameter, chi, based on the conservation of mass of the water inside the stack, was also created to facilitate the study of the water balance of the stack.;A water management map of the novel DMFC stack was created based on the developed model. The modeling results were validated with the data from our experiments on this novel stack design. The results showed that the stack temperature dominates the control of water management of this DMFC stack design. Increases in the operating current density and the rate of methanol crossover favor the water recovery of the stack. However, the most effective way to change the stack from water-loss mode to water-recovery mode is to reduce the stack temperature. The results also showed that the novel DMFC stack (under the same material properties) could operate in water-recovery or water-neutrality mode only for stack temperatures of 50 °C or lower, when the current density was under the nominal design value of 150 mA/cm2. The developed model can simulate the trend of the cell performance and water management of the stack by varying the key variables, such as stack temperature, solution molarity, and the porosity of the LBL. The modeling results also showed that the LBL has a more significant effect on the water balance and cell performance than the CGDL does. By increasing the porosity of the LBL 30%, the cell performance is increased significantly but the vent rate of the water vapor is also increased, resulting in a water-loss mode. The results showed that a decrease of the porosity of the LBL enhances the water recovery of the stack, but that the cell performance is degraded. (Full text of this dissertation may be available via the University of Florida Libraries web site. Please check http://www.uflib.ufl.edu/etd.html).