Modeling and optimization of the direct methanol fuel cell system : relating materials properties to system size and performance
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When designing a direct methanol fuel cell and evaluating the appropriateness of new materials, it is helpful to consider the impact of material properties on the performance of a complete system. To some degree, poor fuel utilization and performance losses from methanol crossover and low reactant concentrations can be mitigated by proper system design. In order to facilitate system design, an analytical model is developed to evaluate the methanol and oxygen concentration profiles across the membrane electrode assembly of the direct methanol fuel cell. In the first part of this work, the model is used to determine fuel utilization as a function of the feed concentration, backing layer properties, and membrane properties. A minimum stoichiometric ratio is determined based on maintaining zero-order methanol kinetics, which allows the fuel efficiency to be optimized by controlling these physical properties. The size of system components such as the methanol storage tank and the fuel pump can be estimated based on the minimum methanol flow rate that those components must produce to deliver a specified current; in this way, the system-level benefits of reduced membrane crossover can be evaluated. In the second section, the model is extended by using the Bulter-Volmer equation to describe the anodic and cathodic overpotentials along a single cross-section of the fuel cell. An iterative technique is then used to determine the methanol and oxygen concentration profiles in the flow channels. The model is applied to examine the benefits of new low-crossover membranes and to suggest new design parameters for those membranes. Also, the tradeoff between the power output of the fuel cell stack and the size of system components is examined across a range of methanol and oxygen flow rates.