NEED TO KNOW: STAYING POWER Although micro fuel cells are likely to cost more than batteries, they will last much longer. Fuel cells can be replenished hundreds of thousands of times without degradation, whereas batteries typically can be charged only a few hundred times. CIRCUITOUS JOURNEY In fuel cells, electrons are freed from hydrogen fuel atoms at the anode, leaving positively charged ions. As the electrons travel through an outside circuit to power a load, the positive hydrogen ions head through the electrolyte toward the cathode, where the charged particles combine with oxygen drawn from the air to form water--the only waste product of fuel cells.

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Micro fuel cells are being touted as the hot portable energy source of the future. They pack a lot more punch than batteries and yield only water as a by-product. Yet the revolution in small power sources is not likely to occur until the second half of this decade, when developers expect to unveil miniaturized fuel cells for third-generation cellular phones, laptop computers, personal digital assistants and other portable electronics. "Potential military and consumer users," reports Christopher Dyer, a fuel cell researcher and editor of the International Journal of Power Sources, "say they expect micro fuel cells to make inroads into markets now dominated by batteries within the next five years"--three years if key breakthroughs are made. As it stands today, prototype micro fuel cells still fall short of the mark. MICRO POWER: Tiny prototype fuel cell uses hydrogen gas to generate electricity. Fuel cells are relatively simple devices that are similar to batteries. Both generate electricity chemically. And both depend on electrodes (an anode and a cathode) connected by an electrolyte. Fuel cells, however, convert hydrogen or hydrocarbon molecules rather than solid electrodes into electricity. Fuel cells feature a specialized polymer or conductive liquid electrolyte that allows positive ions to pass but blocks electrons. Most micro fuel cell designs rely on a solid electrolyte called a proton exchange membrane (PEM) to create the charge separation. The hard part in realizing the portable fuel cell future has been finding the best way to extract the energy. Larger fuel cells cannot just be scaled down. "As fuel cells shrink in size," Dyer says, "the engineering challenges multiply, requiring a difficult balance of providing sufficient power and convenience while minimizing the size and the cost." Energy content is not the problem. In practice, a kilogram of hydrogen fuel can deliver from 1,000 to 23,000 watt-hours of energy, whereas the best lithium batteries now range from 175 to 300. But today's prototype micro fuel cells barely reach 100. Although some developers are using hydrogen fuel stored chemically in canisters, most designers have opted for methanol, a cheap and widely available fuel. Breaking down methanol into hydrogen ions is chemically slow and thus limits power output. Platinum and ruthenium are typically employed to catalyze the reaction, but those elements are costly, so their use must be minimized, says Chao-Yang Wang, director of the Electrochemical Engine Center at Pennsylvania State University. Other problems include fuel leakage through the membrane, excessive heat buildup, moisture retention, and corrosion of the PEM by methanol. To avoid PEM degradation, most designers dilute methanol in water (to less than 5 percent), thereby yielding less energy. Many are working to make PEMs more robust. Robert Hockaday of New York City–based Manhattan Scientifics, for example, reports that his group has proprietary techniques that enable its cells to use 50 percent methanol fuel concentrations. The final major design hurdle is to ensure that micro fuel cells can be manufactured at low cost. Manhattan Scientifics, Mechanical Technology in Albany, N.Y., and researchers from Motorola and Los Alamos National Laboratory are applying microchip fabrication techniques to their designs, an approach suited to low-cost, high-volume production. These integrated-circuit-like cells tend to produce small amounts of power, though. Taking an entirely different design approach is Medis Technologies in Yehud, Israel. The Medis fuel cell, says the company's general manager, Zvi Rehavi, employs a liquid electrolyte, which avoids the PEM's drawbacks. It also relies on catalysts that incorporate extremely fine grained powders of electrically conductive polymers, thereby reducing the amount of expensive platinum-family metals needed. Medis has a deal with the Sagem Group (a French cell phone maker) and is building a pilot plant that can produce 50 million micro fuel cell units a year. The Medis cell can also use ethanol for fuel--a useful feature for travelers. Says Rehavi: "I could pull a bottle of good vodka out of a hotel minibar, pour some into a fuel cartridge and place it in the fuel cell." Cheap vodka would presumably work, too. image: Fraunhofer Institute for Solar Energy Systems

--BY STEVEN ASHLEY