Electrolytes for Long-Life, Ultra Low-Power Direct Methanol Fuel Cells

The fuel cells must have very low energy losses, including low methanol permeability, and must allow the use of highly concentrated fuels. This chapter examines the existing fuel cell technologies in light of these new requirements. Portable direct methanol fuel cells are potentially excellent power sources for small electronic devices because of the high energy density of pure methanol. In the direct methanol fuel cell (DMFC), liquid methanol is fed directly to the anode compartment of the proton exchange membrane (PEM) fuel cell. This provides several advantages over its hydrogen counterpart, namely that liquid methanol has a higher volumetric energy density than hydrogen and the generation, storage, and transportation of methanol is facile. Several fluorinated membranes have been developed for use as proton exchange membranes in both the PEM and DMFC systems. The fluorinated polymers are typically synthesized by copolymerization of tetrafluoroethylene and perfluorinated vinyl ether with sulfonyl acid fluoride. Due to its high ionic conductivity under fully hydrated conditions, several groups have explored modified Nafion membranes for DMFCs. In this alternative approach, many research studies have proposed the modification of the perflurosulfonic acid matrix of Nafion by impregnation with acids that have low solubility, solution casting with solubalized oxygenated acids, and composites of nonconducting polymers with Nafion. Some attention has recently been given to the design of novel inorganic proton exchange membranes. Successful design of such membranes could potentially overcome the drawbacks of the polymer-based electrolytes by exhibiting comparable proton conductivities and ultra-low methanol crossover at reasonable cost.