Steady Flow and Pulsed Performance Trends of High Concentration DMFCs
McCarthy, Larry K.
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Direct Methanol Fuel Cells (DMFCs) are a promising source of energy due to their potentially high energy density, facilitated fuel delivery and storage, and precluded fuel processing. However, DMFCs have several challenges which need to be resolved before they can replace existing energy sources. Some of the challenges include lower power density, relatively high cost, and uncertain reliability. These issues are all promoted, at least in part, by the methanol crossover phenomenon, wherein membrane permeability allows the undesirable species transport of methanol from anode to cathode. This phenomenon also causes the requirement of dilute fuel mixtures, which is undesirable from an energy density viewpoint. Steady flow polarization curves were first analyzed at various concentrations. An optimal concentration range was found wherein both methanol crossover and concentration losses were effectively minimized. During the study of transient phenomena, the fuel was first temporarily discontinued. It was found that a significant cell potential enhancement occurred due to anodic fuel concentration reduction and thus depleting the reactant crossover. The percentage voltage increase was considerably greater at higher concentrations. Based on the fuel discontinuation, a hydraulic pulsing operation was developed and tested. During some of these continuous pulsing schemes, fuel discontinuation did not result in an instantaneous cell potential enhancement mainly due to the internal inertia of the membrane. Nonetheless, a significant cell potential and fuel efficiency enhancement was observed. In addition, the pulse of both fuel and current density resulted in a significant power density increase.