Improved polymer electrolyte membranes for direct methanol fuel cells can be made by any of a variety of processes in which cross-linked polystyrene sulfonic acid (PSSA) is mobilized within electrochemically inert matrices of poly(vinylidene fluoride) (PVDF). Alternatively, other matrix materials can be substituted for, or blended or copolymerized with, PVDF. The principal advantage of these membranes over polymer electrolyte membranes made of other materials is that they are less permeable to methanol; this translates to less crossover of methanol in molecular form (denoted "methanol crossover" for short). Methanol crossover is undesired because it wastes fuel and degrades fuel-cell performance, as explained below.
Figure 1 schematically illustrates a typical direct liquid-feed methanol fuel cell in operation. The polymer electrolyte membrane is part of a membrane/electrode composite-material laminate known in the industry as a membrane/electrode assembly. The anode is preferably made from a carbon-supported Pt/Ru catalyst; the cathode is preferably made from a carbon-supported Pt catalyst. An aqueous solution of methanol is circulated past the anode, while air or oxygen is circulated past the cathode.
Oxidation of methanol at the anode generates carbon dioxide, electrons, and protons. The electrons travel through an external electrical load, to which they deliver the electrical energy generated by the chemical reactions in the fuel cell. The polymer electrolyte membrane serves as a medium for transport of the protons to the cathode, where the protons combine with electrons and oxygen, producing water. To the degree to which the polymer electrolyte membrane allows conduction of electrons, electrical energy is diverted from the external load; and to the degree to which the membrane allows methanol crossover, the methanol fuel is consumed unproductively at the cathode.
It would not be possible, within the limits of this article, to present a comprehensive description of the many alternative materials, techniques, and processes that could be used in fabricating PSSA/PVDF membranes and membrane/electrode assemblies that contain them. The best that can be done here is to present an example of a preferred approach in which (1) a PVDF membrane matrix is prepared; (2) the membrane matrix is impregnated with a solution of styrene, divinylbenzene, and a small amount of a polymerization initiator; (3) the styrene and divinylbenzene are copolymerized within the membrane matrix; (4) the membrane is sulfonated; and (5) the membrane is sandwiched between electrode films.
The PVDF membrane matrix can be prepared by hot pressing of PVDF powder or of a PVDF membrane cast from solution. Alternatively, one can start with a commercial film made of PVDF or a copolymer of PVDF and hexafluoropropylene. The membrane matrix is impregnated by immersion in a bath of styrene, divinylbenzene, and azobis(isobutyronitrile) (AIBN) or another suitable polymerization initiator. The proportion of AIBN is typically between 0.3 and 0.4 weight percent. The proportions of styrene and divinylbenzene govern the amount of cross-linking.
After removal from the bath, the membrane is heated to a temperature between 150 and 170 °C and pressed at 500 to 2,000 psi (3.4 to 14 kPa) for as long as it takes to increase the density of the membrane by 15 to 25 percent. The membrane is then sulfonated by immersion in a solution comprising 15 percent of chlorosulfonic acid in chloroform for 24 hours. The sulfonated membrane is washed in distilled water, then hydrolyzed in distilled water at a temperature of 65 °C. At the end of this process, there is a sulfonic acid group attached to almost every aromatic ring in the membrane. A membrane/electrode assembly is then fabricated by hot-pressing the membrane, while it is still in its hydrated state, between catalyzed, polytetrafluoroethylene-impregnated porous carbon electrode layers.
Figure 2 presents an example of experimental data that show that the methanol-crossover rate of a membrane/electrode assembly made with PSSA/PVDF was less than that of one made with an expensive commercial perfluorocarbon proton-exchange membrane that has been the membrane of choice for methanol fuel cells in recent years. Another advantage of a PSSA/PVDF membrane over the commercial membrane arises in connection with oxygen-flow rates. The necessary circulation of oxygen past the cathode undesirably tends to dry the membrane, thereby increasing its electrical resistivity. Therefore, to minimize the drying effect, it is preferable to operate a fuel cell at the smallest feasible oxygen flow. A fuel cell containing a PSSA/PVDF can function well at a small oxygen flow, whereas one containing a membrane of the commercial material performs poorly at a small oxygen flow.
This work was done by G. K. Surya Prakash, George A. Olah, Marshall C. Smart, and Qungie J. Wang of the University of Southern California and Sekharipuram Narayanan of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category.
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Refer to NPO-20378, volume and number of this NASA Tech Briefs issue,and the page number.