Improved polymer electrolyte membranes for direct methanol fuel cells can be made by impregnating the baseline membrane material with cross-linked polystyrene (a copolymer of styrene and divinylbenzene). The baseline membrane material is a perfluorosulfonic acid-based hydrophilic, proton-conducting ion-exchange polymer sold under the trade name "Nafion". The principal benefit afforded by the impregnation is a reduction in permeability by 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 thereby degrades fuel-cell performance.
To demonstrate this concept, membranes were prepared as follows:
- Baseline membranes were cut slightly larger than the final size needed for fuel-cell membrane/electrode assemblies (MEAs). The membranes were dried, weighed, and marked for identification.
- Solutions of 1, 3, 5, and 8 weight percent styrene/divinylbenzene in methyl chloride with 1 weight percent benzoyl peroxide as an activator were prepared for use in impregnation and polymerization.
- The membranes were placed in the solutions in test tubes, which were then capped to prevent exposure to air.
- Each capped test tube was heated to a temperature between 60 and 65 °C for 16 or more hours, until polymerization was complete.
- After cooling to room temperature, the test tubes were cracked to remove the styrenated membranes.
- Excess polystyrene (the portion not cross-linked with the baseline membranes) was removed by immersing the samples in methyl chloride at room temperature for various times ranging from 1 to 24 hours.
- The styrenated membranes were sulfonated by immersing them in a solution of ClSO3H/CH3Cl for 16 hours.
- The sulfonated membranes were placed in distilled water at room temperature, the beaker was covered, and the water was brought to a rapid boil. Samples of the water were cooled to room temperature and tested for Cl– ions by use of one or two drops of AgNO3 solution. In each case, if a positive result was found, the membranes were removed and placed into another, previously heated beaker of distilled water. This was repeated until the test for Cl– ions yielded a negative result.
Both baseline membranes and membranes prepared as described above were tested to characterize them with respect to ion-exchange capacity, water content, cell resistance, permeability by methanol, and proton conductivity. The best one of the prepared membranes was fabricated into a complete MEA, using Pt/Ru anode and Pt black cathode catalysts. An MEA was also made from a baseline membrane. Both MEAs were then tested in a fuel cell. The results of the test (see table) show decreased methanol crossover in the case of the styrenated membrane. The results also show decreased fuel-cell performance in this case. The decrease in performance has been tentatively attributed to incompatibility of the styrenated membranes with the electrode structures and bonding conditions, which were optimized for the unstyrenated membranes. The implication is that it should be possible to recover lost fuel-cell performance by optimizing the electrode structures and bonding conditions for styrenated membranes.
These Fuel-Cell Performance Figures were obtained using MEAs containing two different membranes.
This work was done by J. Kosek, M. Hamdan, and A. B. LaConti of Giner, Inc., for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tspunder the Materials category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center,
Commercial Technology Office,
Attn: Steve Fedor,
Mail Stop 4 —8,
21000 Brookpark Road,
Cleveland, Ohio 44135.
Refer to LEW-16669.