Design And Performances Study Of Evaporation Systems In Vapor-feed Direct Methanol Fuel Cells

Direct methanol fuel cell（DMFC） has great potential to replace the traditional battery in the field of portable power sources. This study primarily investigates the operational and structural aspects of an electrically heated semi-passive vapor-feed DMFC（V-DMFC） and experimentally validates the effectiveness of using vaporous methanol to improve the cell performance. Results indicate that there exits an optimal value of methanol concentration and heating temperature that favors both methanol supply and control of methanol crossover. Based on the traditional structural design, a sintered porous metal- fiber pate（SPMFP） is further used to depress the impact of methanol crossover so that the cell performance can be significantly improved at a relatively higher methanol concentration. The measured cell efficiency is about 78.4% regardless of the parasitic power loss of the electric heaters. Otherwise, the whole cell efficiency will be reduced by 10% conditioned that the parasitic power loss was taken into consideration.The next part reports a novel system design coupling a catalytic combustor with a V-DMFC. As for the operation of the catalytic combustor, it is necessary to optimize oxygen feed rate, the number of capillary wicks and also catalyst loading in order to fast trigger the combustion reaction. The values of methanol concentration and methanol vapor chamber temperature both have direct effects on the cell performance. Besides, this study also applied a pervaporation membrane as the passive methanol vaporizer for a catalytic combustor. The methanol feed rate and the membrane diffusion coefficient related to the permeation flux are experimentally evaluated. Results indicate that the air feed rate（AFR） has positive effects on enhancing methanol production and also increasing the catalyst temperature. The infrared thermal analysis verifies the positive effect of increasing the AFR on the status of heat production and distribution. To validate the effectiveness of this method, a traditional bubbling-based system is prepared for comparison. It was found that the pervaporation operation outperforms the bubbling mode and facilitates self- ignition at a lower AFR. Results also validate the feasibility of changing the operating orientation of the combustor, which illuminates its potential use for portable applications. The dynamic responses of catalyst temperature to the variation of AFR are also investigated in this study. The online fuel cell tests validate that the catalytic combustor-based V-DMFC is even superior to the cell using an electric evaporator since it behaves no temperature overshot which is inevitable on the latter.Furthermore, a fully-passive V-DMFC（FPV-DMFC） based on pervaporation technology is developed. For the cathode, a SPMFP with great hydrophobicity is used to enhance water back diffusion from the cathode to the anode. Results indicate that the use of a SPMFP promotes a higher cell performance especially when a higher methanol concentration is used. Neat-methanol operation is also viable under this condition. For the anode, the use of a hydrophilic sintered porous metal-powder plate embedded in the anode current collector reduces the cell performance. In order to moisturize anode in a FPV-DMFC operated with neat methanol, three methods are experimentally compared, including water storage in a fuel reservoir, active water vapor supply and water recovery from the cathode to the anode. A water management layer for water recovery is introduced to the cathode, which is made of a quasi-superhydrophobic SPMFP to enhance water back diffusion（WBD）. Results prove that each of these methods can improve the cell performance. WBD enhancement based on the use of a SPMFP is proven to be the most effective way. It is also found that combination of different methods may more promote the cell performance. For fully-passive operation, a higher catalyst loading at the cathode helps retain stable performance when a WBD enhancement layer is used. Based on this design, the FPV-DMFC fed with neat methanol can achieve a maximum power density of 21.5m W cm^-2.