The direct methanol fuel cell (DMFC) has the potential to replace lithium-ion rechargeable batteries in portable electronic devices. However, current DMFCs experience significant power density and efficiency losses due to high methanol crossover (flux). This is a fundamental transport property associated with polymer electrolyte membranes (PEMs), where Nafion^® (DuPont) is the most frequently used PEM in the DMFC. The fabrication of a PEM can be achieved through a variety of methods. Our chosen method for membrane fabrication is solution casting. In this method, the PEM is dissolved in a solvent and the membrane is formed through solvent evaporation.

In this study, membrane preparation techniques and post-treatments will be shown to have important roles in suppressing methanol crossover and increasing proton conductivity, thereby affecting DMFC performance. Solvent choice, casting environment, annealing temperature, and post-acidification are parameters that affect the transport properties (proton conductivity and methanol permeability) and fuel cell performances of PEMs. The same polymer cast from different solvents with varying thermodynamic properties (i.e., boiling point, viscosity) exhibits differences in proton conductivity, methanol permeability, and DMFC performance. Inert and ambient casting environments have shown dramatic differences in both proton conductivity and methanol permeability experiments. Annealing at different temperatures has been shown to enhance selectivity (ratio of proton conductivity and methanol permeability), while post-acidification increases proton conductivity.

These fundamental differences in transport properties suggest different morphologies. The recast Nafion^® membranes have been analyzed using x-ray scattering and electron microscopy and these results validate this transport-morphology relationship. Dynamic light scattering results show that dilute solutions of Nafion^® in different solvents yield varying aggregate sizes (micelles), which can be attributed to the differences in solubility parameters of the solvent and the hydrophobic (backbone) and hydrophilic (side chain) portions of Nafion^®. The structure of the micelle has a significant influence of the final morphology of the membrane. Infrared spectroscopy experiments do not suggest chemical interactions are occurring in the membrane as a result of differences in membrane preparation or treatment.

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