Pem Fuel Cells

The proton exchange membrane cell, which was developed by Gen­eral Electric for NASA's Gemini space program nearly four decades ago, is the favored technology for auto applications because it is compact, runs at a low operating temperature, permits an adjustable power put-put, and can be started relatively rapidly. Innovations made in the 1980s at Los Alamos National Laboratories made the PEM cell more practi­cal by substantially cutting the amount of precious metal catalyst needed to coat the cell's ultra-thin polymer membrane.

Ballard's prototype fuel-cell units, which are used by DaimlerChrys­ler, Ford, Honda, and Nissan (and others yet unacknowledged), com­prise a series of carbon plate/PEM electrode assemblies. "Each assembly includes five main components," explained Paul Lancaster, Vice Presi­dent, Finance at Ballard. "At either end is an electrode made of a carbon material with a coating of platinum-family catalyst that ionizes hydro­gen on one side of the unit and oxygen on the other. In the middle is a thin proton exchange membrane, which is a rubbery hydrophilic poly­mer electrolyte with solid sulfonic acid sites bonded onto it. These sites allow protons to be transported selectively through the membrane."

Although the latest enhancements to fuel cells are rather recent, re­searchers at Ballard and presumably GM have worked out most of the technical problems, boosting the stack's power density by determining how to keep the membranes moist but not flooded, and by optimizing the flow lines that transport hydrogen, oxygen, and water through the stacks. Ballard, which has obtained nearly 400 patents in PEM technol­ogy, intends to have a car-sized power unit ready to go within four years at prices comparable to 1C engines/according to Lancaster.

Clearly, fuel cells provide some advantages over 1C engines: they are more efficient in extracting energy from fuel; they are quieter; and they could form the basis of a zero- or very-low-emissions engine that runs on a renewable fuel. It should be noted that some engineers expect practical fuel-cell vehicles to be hybridized with batteries, ultracapacitors, or oth­er energy storage devices to allow them to be run at lower power output levels and, thus, at higher efficiencies. "Fuel-cell stacks operate at 50 to 70% efficiency in the current power load of interest - about 60% in around-town driving," explained Byron McCormick, Go-director of GM's Glo­bal Alternative Propulsion Center.

Getting the Cost Out

Until recently, building a fuel cell sufficiently powerful to run a car was costly - even more than a vehicle powered by electrochemical bat­teries or a hybrid drive. To attain the power levels of a standardr-issue 1C engine in a midsize sedan, a fuel cell needs to produce from 60 to 90 kW. When NASA first started using fuel-cell technology in space, a PEM fuel cell cost about $500,000 per kW. Today that price has dropped to around $500 per kW - but that means that a fuel-cell engine still costs about $25,000, which is around seven times the price of a typical 1C engine (about $3500).

Working for several years with specialists from Ford and Daimler-Chrysler, Ballard researchers studied the automotive industry's needs for low-cost, high-volume fuel-cell stack manufacturing and specifically designed the Mark 900 unit to accommodate them. "The key to develop­ing an efficient supply chain," explained Lancaster, "is to choose low-cost, readily available materials and cheap, scaleable, automated manu­facturing processes. We did an actual commercial plant study for the annual production of 300,000 vehicle equivalents, considering the build­ing^ logistics and other crucial details.

Facing the challenge of making economical fuel-cell units, Ballard worked with Ford and DaimlerChrysler to optimize its latest stack de­sign for production. Said Lancaster, "Whereas the Mark 700 systems were basically hand-crafted units that needed carbon plates that were individually machined for two hours from blanks costing $ 100, the Mark 900 unit is made using a carbon sheet material called Grafoil which is supplied by U.S. Carbon Graftek. This soft, natural graphite material comes in rolls. The sheet is first roller-embossed, die-cut, then impreg­nated, and heat-treated. Now each plate costs a few dollars." The man­ufacturing process also reportedly employs other high-volume produc­tion processes such as injection molding. Each PEM fuel-cell stack com­prises hundreds of these identical plates sandwiched between polymeric membranes.