Modeling and optimization of the DMFC system: Relating materials properties to system size and performance

Abstract

In designing a direct methanol fuel cell and evaluating the appropriateness of new materials for inclusion, 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 can be mitigated by proper system design. Simple engineering models can be useful tools in facilitating this type of system design. In this paper, an analytical model is developed to determine the oxygen concentration profile in the cathode backing layer and flow channel along a one-dimensional cross-section of the fuel cell. An existing analytical model is then used to determine the methanol concentration profile in the anode backing layer and membrane and the methanol crossover current along the same cross-section. Applying a fixed cell potential and using Tafel kinetics to describe the charge-transfer reactions at the anode and the cathode, the local current density and the rates of methanol and oxygen consumption are determined. The process repeats for a fixed number of cross-sections down the entire length of the flow channels, and the average current density and stoichiometric ratios are calculated at the end. The model is applied to examine the effects of new low-crossover membranes and to suggest new design parameters for those membranes. Also, an analysis is presented in which the tradeoff between stack power and the size of system components is examined for a range of methanol and oxygen flow rates.