Nafion® blend membranes for the direct methanol fuel cell

The direct methanol fuel cell (DMFC) will replace lithium ion batteries in portable electronic devices, where current DMFCs operate at double the power density of current lithium ion batteries with instant recharge (refuel) capability. However, the DMFC could provide much higher power densities if the heart of the fuel cell - the polymer electrolyte membrane (PEM) - were more resistant to the fuel (methanol). Therefore, the development of Nafion® (the most commonly used PEM in fuel cells) alternatives that exhibit high proton conductivity and low methanol permeability (high selectivity) is an active area of research. Polymer blends are a simple and cost-effective method to develop membranes that conjoin both of the desired transport properties. To date, there is limited research in the area of PEM blends, and furthermore, a number of unresolved questions. In this study, the effects of blend composition, chemistry, and processing on polymer morphology and subsequently the transport properties and selectivity were investigated. A key result in this study was comparing solution cast membranes to heat pressed membranes. Both heat pressed membranes and unannealed solution cast membranes resulted in immiscible blends with low selectivity, while annealed solution cast membranes improved miscibility and subsequently selectivity. These results were confirmed, where the DMFC performance of an annealed solution cast blend membrane of Nafion®/poly(vinyl alcohol) (PVA) (with only 5 wt% PVA) was 33% higher than Nafion® (the commercial standard) at a high methanol fuel concentration (8 M).