The present invention relates generally to the synthesis of improved proton-exchange membrane (PEM) materials for use in fuel cells. More specifically, the invention relates to the synthesis of hybrid organic-inorganic PEM materials based on polyimide block copolymers that can be used in direct methanol fuel cells (DMFC).
In 1839 Grove discovered the first fuel cell by observing the production of electricity from organic chemicals using precious metal electrodes. However, it was not until the development of proton-exchange membrane (PEM) materials that fuel cells began to show their potential as a viable power source. Fuel cells directly convert chemical energy into electrical energy. Typical fuels such as hydrogen, methanol, and ethanol can be directly oxidized and reduced utilizing a Pt or PtRu catalyst to generate power. Unlike batteries, fuel cells can theoretically operate indefinitely by simply supply more fuel. The efficiency of fuel conversion to power is 40–60% for fuel cells, while current internal combustion engines are only 20–30% efficient. Furthermore, fuel cells based on H2 as a feed source generate only water (no green house gases).
One application of the present invention is in the area of Direct Methanol Fuel Cell (DMFC) technology. A DMFC electrochemically converts methanol and oxygen into electrical energy by utilizing a Pt or PtRu catalyst. At the anode of a DMFC, methanol is catalytically oxidized into one mole of CO2, six protons, and six electrons; while at the cathode O2 is reduced in the presence of protons and electrons to yield H2O. The electrons produced at the anode are used to do work in an outside circuit, and then recombined with oxygen and protons that diffuse from the anode through a solid electrically insulating electrolyte medium to the cathode to complete the electrochemical reaction. This entire electrochemical process has a theoretical cell voltage of 1.21 V at 25° C., which is schematically illustrated in FIG. 1. In order for the DMFC to operate, the separating electrolyte medium provides electrical insulation and high proton conductivity. This role is fulfilled by the PEM by separating these half-cell reactions in the fuel cell. The entire ensemble of anode, PEM, and cathode is known as a membrane electrode assembly (MEA).
Currently, the state of the art PEM material used in fuel cells is Nafion® (DuPont, United States). Nafion® is a polytetrafluoroethylene (PTFE) polymer that is polymerized in the presence of sulfonic acid containing fluorinated α-olefin. This type of chemistry leads to a random structure of Nafion®, which is schematically illustrated in FIG. 2 revealing its PTFE backbone and pendant sulfonic acid group. Nafion® has been demonstrated as an excellent PEM material, although its high material cost is a barrier for consideration in many practical proton-exchange membrane fuel cell (PEMFC) applications. Nafion® is almost exclusively used in Direct Hydrogen and Direct Methanol Fuel Cells (DHFC and DMFC) because of its excellent proton conductivity and chemical, electrochemical, and mechanical stability.
While Nafion® possesses excellent material properties, such as chemical inertness and high proton conductivity, Nafion® also has incredibly high methanol permeability. This high methanol permeability results in the unwanted diffusion of methanol from the anode to the cathode. Currently, DMFCs suffer from fuel cross over, which limits the concentration of methanol that can be fed to the fuel cell. This prevents the fuel cell from achieving higher current densities and power outputs due to poor utilization of the PtRu catalyst. Furthermore, increasing the methanol concentration in the feed would lead to increased utilization of the catalyst, which may lead to decreased catalyst loadings. Swelling of PEM materials when hydrated leads to decreased mechanical properties, and difficulty in integrating the PEM into a fuel cell stack without creating very thick films. Thicker PEM films results in more material utilization leading to more expensive fuel cell stacks, and increases the proton resistance, which leads to decreased fuel cell power output.
In order to compensate for methanol crossover, a dilute feed mixture of water and methanol is fed to the anode. The main problem with using a dilute methanol-water mixture is that it increases the activation overpotential at the anode. This anode overpotential impacts the overall fuel cell performance by reducing power output. Methanol crossover effects exhibit a trade-off relationship between conductivity and methanol permeability, as illustrated in FIG. 3 for several different types of PEM materials. Decreasing the rate of methanol crossover would allow the use of higher concentrations of methanol-water feed mixtures, which would increase catalyst efficiency, DMFC power output, and potentially fuel utilization.
DMFCs utilizing Nafion® to form the MEA are limited to a fuel cell operating temperature below or near 80° C. in order to maintain proper hydration of the PEM. Proper hydration of a Nafion® PEM is critical for good proton conductivity and overall fuel cell performance. Low temperatures are also required to maintain the material integrity of Nafion®, which is lost at temperatures near 100° C. This loss in material integrity is due to the hydrated state of Nafion® and its glass transition temperature (Tg) near 100° C. due to water plasticization. If the DMFC were operated at higher temperatures, enhanced diffusion rates and reaction kinetics for methanol oxidation, oxygen reduction, and water and CO2 desorption could be realized, resulting in a more efficient fuel cell. Additionally, operating a DMFC near 150° C. would significantly retard CO catalyst poisoning, and improve fuel cell efficiency and performance. Therefore, the high cost of power associated with DMFC technology is partly due to the PEM material limitations.
The advancement of DMFC technology would benefit significantly by the development of novel PEM materials that enable higher DMFC power outputs by minimizing methanol diffusion across the PEM. PEM materials possessing this property could be made into thinner members, which would decrease ohmic losses and improve fuel cell performance. Currently, Nafion® is almost exclusively used as the PEM in a DMFC because of its excellent electrically insulating and high proton conductivity properties. However, Nafion® is very expensive, suffers from a high rate of methanol diffusion across the anode to cathode, and the aforementioned material limitations previously discussed. A technical challenge for DMFC technology is the engineering of cheaper PEM materials without sacrificing proton conductivity and chemical stability. Furthermore, lowering methanol crossover would increase the power output of the DMFC by increasing the kinetics by increasing the concentration of methanol in the feed, which in turn would improve the catalyst's efficiency.
Against this background, the present invention was developed.