Hydrogen-based fuel cells are becoming increasingly popular as an alternative to crude oil-based internal combustion engines. Specifically, hydrogen can be converted to electricity through the use of a hydrogen-oxygen fuel cell. The by-product of this type of a fuel cell is water, making this a “green” or environmentally-friendly technology. At the heart of the fuel cell is the proton exchange membrane (PEM), which transports protons from the anode to the cathode while providing electronic insulation between them. There are many types of electrolyte materials, each with specific limitations. Generally, such materials either have too low a proton mobility or don't operate in the intermediate temperature range (100 to 300° C.) to be useful for intermediate-temperature fuel cells.
Some of the most popular electrolyte materials are polymer exchange membranes, phosphoric acid membranes, and solid oxide membranes. Polymer exchange membranes, or more specifically solid organic polymer poly-perfluorosulfonic acids such as Nafion®, require hydration to maintain high proton conductivity. However, this limits their operation to temperatures below 100° C., thus requiring the use of expensive noble metal catalysts such as platinum. These electrolytes also suffer from fuel cross-over due to their porous hydrated nature. Phosphoric acid membranes are typically operated from 150° C. to 200° C. Since these membranes are liquid electrolytes, they suffer from membrane leakage and fuel cross-over problems. They also require the use of expensive platinum catalysts. Solid oxide membranes are typically operated between 700° C. to 1000° C., a temperature range in which the use of platinum as an electrode material can be reduced. Additionally, this temperature range is used to achieve the desired oxide anion conductivity. Since, these membranes are solid in nature, they do not suffer from fuel cross-over problems. However, there remains a temperature region between about 100° C. and 300° C. for which no one membrane currently available can provide optimum performance.
Thus, what is needed are materials and compounds for use in proton exchange membranes which are able to operate in a wide variety of temperature ranges, including in the intermediate temperature range of between about 100° C. to 300° C.