Sorption and Diffusion Selectivity of Methanol/Water Mixtures in NAFION

摘要

During the past 40 years membranes have gained large importance, parti cularly in separations, actuators, and fuel cells. Their advantage being the ability to control solute flux through the membrane, particularly rapid transport of one component with the exclusion of other components. In multicomponent transport through a polymer membrane, the ratio of fluxes is termed selectivity and can be caused by differences in sorption and diffusion. The former is attributed to thermodynamic interactions, while the latter is considered transport effects. This solution-diffusion mechanism was developed in the 1940s. Around 1950, ion-exchange resins were developed, allowing far greater selectivity than is possible solely from chemical interactions. Ion-exchange resins are the precursors of polymer electrolyte membranes (PEMs) used in fuel cells.Nafion is a PEM made by DuPont, consisting of a perfluorinated backbone with perfluoroether side chains that terminate in a sulfonic acid group. Nafion is widely used as a PEM in fuel cells. This work will focus on its use in the direct methanol fuel cell (DMFC), which has several benefits, such as the ability to produce energy with a renewable fuel. Also, because the DMFC is simple, converting chemical energy directly into electricity, it has the potential for high efficiency and high power density, which is ideal for portable power applications. However, an important problem called fuel crossover hinders current DMFCs running with Nafion as the PEM. Fuel crossover occurs because methanol is able to swell and pass through Nafion. In swelling the membrane, the mechanical properties are decreased and the crossover rate (methanol flux) is increased. Methanol not only reacts at the anode, where it contributes to power production, but it also passes through the membrane and reacts at the cathode producing a counter current or mixed potential, actually decreasing the power available from the fuel cell. In addition, some methanol may leave the cathode outlet as lost fuel. These effects all contribute to decreased efficiency and performance of DMFCs and require operation at low methanol feed concentrations. Unfortunately, low methanol feed concentrations limit the anode reaction rate and therefore maximum achievable power.