1. Field of the Invention
The present invention relates generally to the fields of proton conductors. More particularly, the present invention relates to solid-state proton exchange membranes.
2. Description of the Related Art
Fuel cells are electrochemical devices that convert chemical energy directly into electrical energy. Generally, the fuel cell comprises an anode, at which electrons are catalytically removed from the fuel and fed to an external circuit, and protons are catalytically removed from the fuel and fed across a proton exchange membrane to a cathode, where the electrons, protons, and an oxidant are recombined to close the circuit.
Fuel cells require hydrogen, but as is known, free hydrogen is highly reactive. Therefore, there has been interest in fuel cells that extract hydrogen from another fuel at the anode. One fuel of interest is methanol (CH3OH), which can be readily transported as a liquid and has a relatively high energy capacity (e.g., assuming 50% efficiency, 250 mL methanol can deliver about 600 watt-hours of electricity). Ethanol (CH3CH2OH) can also be used; its energy capacity is somewhat lower than that of methanol, but ethanol is far less toxic if imbibed or otherwise absorbed. Methanol or ethanol can be oxidized at the anode by exposure to an oxidant, typically oxygen from air, in the presence of a catalyst, producing CO2, H+, and e−. The skilled artisan will understand this description is not stoichiometric.
Among the challenges faced by methanol- or ethanol-fueled fuel cells are the optimization of the properties of the proton exchange membrane. The membrane should efficiently conduct protons from the anode to the cathode to complete the circuit. The membrane also should have a low methanol permeation rate, both to enhance the efficiency of methanol oxidation at the anode and to minimize the reduction of cathode potential by oxidation of methanol at the cathode.
One area of particular interest are medium temperature fuel cells, by which is meant those suitable for use at temperatures from about 100° C. to about 250° C. To operate at such a fuel cell temperature, a proton exchange membrane should have a high thermal stability. However, thermal stability is typically inversely correlated with proton conductivity, e.g., a material with a high thermal stability generally has a low proton conductivity.