Advances in PEM fuel cell membranes

Touting versatility, adaptability and portability the proton exchange membrane (PEM) fuel cell is emerging as the winner in many of the primary applications that fuel cells can satisfy and service, according to a recent report (No. C-235) by Business Communications Co. (BCC; Norwalk, CT; www.bccresearch.com). The research firm predicts that U.S. sales of the PEM and other components making up the fuel cell stack, now $149 million/yr, will increase at an annual average growth rate of 26% to $475 million by 2008. "Enhancements in the PEM's ion-selective, polymer membranes, including durability, performance and lower costs, are driving PEM fuel cell market growth," says Anna Crull, BCC consultant in membrane science who coauthored the report with Col. Dick Hooker (U.S., retired). "An improvement in any one of these parameters makes the membrane more suitable for the specific application, whether it be automotive, portable or stationary/distributive power applications," she adds.

Overcoming temperature limitations

The initial challenge in developing electrolyte membrane materials for PEMs is the ability to endure prolonged exposure to the designated temperature range and the feed composition). "Many polymer membranes operate most effectively within a limited temperature range," says Crull. "But a polymer electrolyte membrane that operates around 150[degrees]C instead of less than 100[degrees]C is much less sensitive to carbon monoxide poisoning, which in turn gives more flexibility to the type of feed used; this can lower costs by allowing a reduction in the purity of the feed," she continues.

Celanese AG's (Kronberg, Germany; www.celanese.com) Celtec membrane, made of the heat-resistant polymer, polybenzimidazole (PBI) (CEP, Nov. 2002, p. 15), enables the fuel cell to operate at temperatures up to 200[degrees]C. Celanese Ventures, the research arm of Celanese, has been manufacturing high-temperature membrane electrode assemblies (MEAs) featuring Celtec membranes since Sept of 2002, at its pilot plant in Frankfurt-Hochst Industrial Park (Germany).

Performance polymers

Perfluorinated polymers are used to manufacture a number of today's commercially available high-performance fuel cell membranes, including Asahi Glass Co.'s (Tokyo, Japan; www.agc.co.jp/english) Flemion, Asahi Kasei's (Tokyo; www.asahi-kasei.co.jp) Aciplex, and the Nafion family of membranes from DuPont Corp. (Wilmington, DE' www.dupont.com). Other high-performance membranes include W. L. Gore & Associates' (Elkton, MD; www.gore.com) Gore Select, an expanded' polytetrafluoroethylene, and Ballard Power Systems' (Burnaby, B.C., Canada; www.ballard.com) BAM family of membranes, which were developed with low cost in mind. BAM, a sulfonated copolymer of trifluorostyrene and substituted-trifluorostyrene monomers, is made by a condensation polymerization reaction that does not involve direct fluoridation. In contrast, DuPont's Nafion 117 is a directly perfluorinated polymer membrane composed of a tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymer. "Since BAM uses a monomer that is already fluorinated, Ballard does not need fluorine-handling facilities, which are expensive to operate and maintain," Crull points out.

Market breakthrough

Moving the PEM into the commercial marketplace will require more than membrane optimization, notes Crull. High-volume automated manufacturing of MEAs, greater carbon monoxide tolerances at the anode, lower-cost carbon media, more-compatible MEA interfaces and lower loadings of a higher-activity catalyst are crucial criteria. The use of platinum in catalyst development for PEM fuel cells is expected to prevail for the foreseeable future because of its ability to catalyze chemical reactions and to be recycled, predicts Crull. However, the use of platinum poses unique challenges. Its availability is limited, and it's not easy to mine.

Today's PEM fuel cells range from 5 kW to 75 kW. Thus, it is likely that a variety of power sources will coexist with the fuel cell for a considerable period after its initial acceptance in the marketplace.