An Exploration of the Promises and Limitations of Passive Direct Methanol Fuel Cells

Rosenthal, Neal Stephen

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An Exploration of the Promises and Limitations of Passive Direct Methanol Fuel Cells

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While Direct Methanol Fuel Cells (DMFC) have a promising future as a long-lasting and environmentally friendly energy source, the use of balance of plant (BOP) equipment, such as pumps, fans, and compressors, create a complex system that can significantly reduce plant efficiency and increase cost. As an alternative, passive DMFCs have been designed and studied due to their ability to run under ambient conditions without any BOP equipment. However, before they become a feasible energy source, more must be understood about their promise and limitations. In this thesis, performance of a self-designed and constructed passive DMFC was investigated. In addition, an analytical mathematical model was developed in order to gain a better understanding of the limitations of the passive DMFC. The model was compared with literature's data to ensure reliability. Passive DMFCs, consisting of one to twelve Membrane Electrode Assemblies (MEAs) were designed, constructed and tested. The smaller scale fuel cell was optimized using different setups and elaborately tested using a variety of fuels, most notably methanol chafing gel, to determine an optimal performance curve. The larger fuel cells were further used to test for long-term performance and practical feasibility. The compact four-cell units could run for at least 24 hours and can provide performance akin to an AA battery. A larger 12-cell fuel cell was also designed and built to test feasibility as a convenient power supply for camping equipment and other portable electronics, and was tested with neat methanol and methanol gel. In all fuel cell prototypes, polarization plots were obtained, along with open circuit voltage (OCV) plots and long-term performance plots. While it is currently not possible to differentiate which methanol fuel source is the best option without a more thorough investigation, methanol gel has shown great potential as a readily available commercial fuel. The three largest restrictions in passive DMFC performance are 1) slow mass transfer of fuel to the anode, 2) slow kinetics of methanol and oxygen electrodes, and 3) methanol crossover. The developed model correctly predicts the effect of methanol crossover and the resulting crossover current on OCV as well as on performance of the fuel cell over the entire voltage-current range. Further, the model correctly predicts the effect of increasing methanol feed concentration on reduced OCV but increased limiting current density. The effect of the proton exchange membrane thickness is also well explained. Finally, the model describes the significant power losses from larger overpotentials, as well as crossover current, and the resulting significant heat generated and low efficiency. Overall, PDMFCs show great promise for potential application provided the cost can be reduced significantly.

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