Proton-conductive (solid-electrolyte) membranes made from sulfonated poly(phenylether sulfone), plus membrane/electrode assemblies containing these membranes, have been developed for use in methanol fuel cells. These membranes offer two important advantages over traditional fuel-cell membranes made of a commercial perfluorosulfonic acid-based ion-exchange polymer:
- Whereas the traditional membranes cost about $900/m2 (as of 1997), the present membranes are expected to cost between $5/m2 and $10/m2.
- The traditional membranes are somewhat permeable by methanol; crossover by methanol is a parasitic process that reduces fuel-cell efficiency. The present membranes offer greater resistance to methanol crossover.
The figure illustrates the synthesis of sulfonated poly(phenylether sulfone). Degrees of sulfonation can be controlled to obtain polymers of various equivalent molecular weights. Any of these polymers or a mixture of them can be dissolved in dimethyl formamide (DMF) to form a casting "dope." Other ingredients can be added to the mixture to modify the properties of the membranes to be formed subsequently. To form a membrane, one casts the dope on a suitable surface in air, by use of a casting apparatus with a doctor blade.
Of the membranes of this kind tested thus far, the best one was made from a 75-percent portion of sulfonated poly(phenylether sulfone) of 620 daltons equivalent molecular weight plus a 25-percent portion of the unsulfonated base polymer. The incorporation of the unsulfonated base polymer adds strength and reduces crossover, relative to the pure sulfonated polymer. The casting dope formed by dissolving this mixture of polymers in DMF is not a true solution and, instead, appears to be more like an emulsion.
Casting twice has been found to be essential to formation of a stable dope and casting of a useful membrane. On first casting, the dope separates into what are presumed to be ionomeric (sulfonated) and inert (unsulfonated) phases. The polymer as thus cast is redissolved in DMF to form a new dope. Upon casting from the new dope, phase separation does not occur; the precise physicochemical mechanism responsible for this phenomenon has not been established, though it has been conjectured to be a consequence of absorption of a small amount of water from the air immediately after the first casting.
The fabrication of a membrane/electrode assembly includes, among other things, hot-pressing a membrane between carbon papers that have been coated with electrode-catalyst/liquid-ionomer mixtures, with the coated sides in contact with the membrane. To achieve adequate adhesion between the anode and the membrane, it is necessary to modify the anode-side membrane surface, prior to hot pressing, by rubbing it with a small amount of carbon-supported anode catalyst.
The electrical characteristics of fuel cells containing membrane/electrode assemblies of this type are similar to those of similarly dimensioned fuel cells containing traditional membrane/electrode assemblies. However, the present membrane/electrode assemblies operate with less methanol crossover; for example, the membrane/electrode assembly made with the best membrane exhibited about as much methanol crossover as would be expected of a membrane/electrode assembly containing a traditional membrane of three times the thickness.
This work was done by Shiao-Ping Yen, Andrew Kindler, Andre Yavrouian, and Gerald Halpert of Caltech for NASA's Jet Propulsion Laboratory.
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