A fuel cell operates like a refuelable battery; it will generate electricity as long as it is supplied with hydrogen and oxygen. A proton-exchange membrane (PEM) fuel cell (right) is composed of two thin, porous electrodes, an anode and a cathode, separated by a solid polymer membrane electrolyte. Platinum-based catalysts coat one side of each electrode. After hydrogen atoms enter the cell (1), the anode catalyst splits them into electrons and protons (2). The electrons move along an external circuit to power a drive motor (3), while the protons migrate through the membrane (4) to the cathode. The catalyst on that side combines the protons with returning electrons and with oxygen from the air to generate water and heat (5). Many cells are piled in stacks to produce higher voltages (6).
Hydrocarbon structural block
Hydrocarbon structural block
Hydrocarbon membranes last longer, generate more energy and cost less than current fluorocarbon types, maker PolyFuel claims. The company's concept incorporates blocks of highly conductive polymer species to promote the passage of protons, increasing energy production. These conductive materials are tied to blocks of high-strength polymers that reinforce the membrane's structure, improving durability. Because the two types of polymers have a low chemical affinity for each other, they segregate themselves during processing into the different functional blocks, easing manufacture.
Resistance to subzero temperatures has long been a key goal for developers of fuel-cell stacks. When stacks freeze, the water inside turns to ice, which can puncture membranes and block pipes. Last year Honda engineers demonstrated that the fuel-cell power plant in its latest FCX hatchback (right) will start up repeatedly at -20 degrees Celsius. Researchers at DaimlerChrysler and General Motors have shown similar results with frozen stacks in the laboratory (below). The trick seems to lie in keeping all water inside the system in the vapor state.
but suffer from significant drawbacks: about one third of the energy available from the fuel is needed to keep the temperature low enough to preserve the element in a liquid state. And despite bulky insulation, evaporation through seals robs these systems every day of about 5 percent of the total stored hydrogen.
Several alternative storage technologies are under development, but no surefire advances have occurred. "There's quite a good distance between what can be demonstrated in the lab and a fully engineered storage system that's affordable, long-lasting and compact," says Lawrence Burns, vice president for research and development and planning at GM.
Probably the foremost candidates for a storage technology are metal hydride systems in which various metals and alloys hold hydrogen on their surfaces until heat releases it for use. "Think of a sponge for hydrogen," explains Robert Stempel, chairman of ECD Ovonic, a part of Texaco Ovonic Hydrogen Systems, the leader in this area. The hydrogen gas is fed into the storage tank under pressure and chemically bonds to the crystal lattice of the metal in question through a reaction that absorbs heat. The resulting compounds are called metal hydrides. Waste heat from the stack is used to reverse the reaction and release the fuel. In January, GM and Sandia National Laboratories launched a four-year, $10-million program to develop metal hydride storage systems based on sodium aluminum hydride.
Because metal hydride storage systems tend to be heavy (about 300 kilograms), researchers at Delft University of Technology in the Netherlands have developed a way to store hydrogen in water ice—as a hydrogen hydrate, in which hydrogen is trapped in molecule-size cavities in ice. Water, of course, is significantly lighter than metal alloys. This approach is unexpected because hydrogen hydrates are notoriously difficult to make, typically requiring low temperatures and extremely high pressures, on the order of 36,000 psi. Working with sci entists at the Colorado School of Mines, the Delft team came up with a "promoter" chemical—tetrahydrofuran—that stabilizes gas hydrates under much less extreme pressure conditions, only 1,450 psi. Theoretically, it should be possible to get about 120 liters (120 kilograms) of water to store about six kilograms of hydrogen.
Freezing Stacks several hundred people gathered behind the state capitol building in Albany, N.Y., to hear Governor George E. Pataki welcome the lease by New York State of a pair of Honda FCX hydrogen fuel-cell cars one cold, blustery late November morning in 2004. What made the event notable was the temperature of the air. All previous fuel-cell vehicle demonstration programs had been situated in warmer climes to ensure that the fuel-cell stacks would not freeze up. In previous designs, subzero temperatures could convert any liquid water into expanding ice crystals that can puncture membranes or rupture water lines. Early in the year Honda engineers demonstrated that their fuel-cell units could withstand winter conditions, an important engineering achievement for the fuel-cell research community.
After the speech, Ben Knight, vice president of R&D for American Honda, explained that the new freeze-resistant 2005 FCX models will start up repeatedly at -20 degrees C. Other car companies, including DaimlerChrysler and GM,
Nobody really knows how to store enough hydrogen fuel in a reasonable volume.
have also claimed success with cold-starting test stacks in the lab [see box on preceding page].
Besides its ability to start up in midwinter temperatures, the 2005 version of Honda's FCX fuel-cell car—a four-seat compact hatchback—showcases other technical advances over the model released two years earlier. The new FCX is unusual, for example, because it employs an ultracapacitor—a device that stores energy in the electric fields between charged electrode plates—to provide short bursts of supplementary power for passing and hill climbing. Most other automakers use batteries for this purpose.
Infrastructure Issues later on that November day an even more enthusiastic crowd assembled for the second half of the planned ceremonies at the nearby headquarters of Plug Power, the Latham, N.Y.-based maker of stationary hydrogen fuel-cell energy units for backup power applications. The cheering group of mostly Plug Power workers were there to celebrate the opening of a hydrogen fueling station that they had co-developed with Honda engineers. The Home Energy Station II contains a miniature chemical plant—a steam reformer—that extracts hydrogen fuel from piped-in natural gas using a steam-based pro-
Hydrogen Gas Stations
Filling stations that dispense hydrogen fuel are still rare. Currently about 70 hydrogen refueling units are operating worldwide: two dozen each in the U.S. and Europe, a dozen in Japan and 10 elsewhere. Filling up a car with pressurized hydrogen, demonstrated above by a Ford Focus FCV fuel-cell car, typically takes about five minutes. An electrical ground wire must be attached to the car beforehand to avoid sparks. At its Torrance, Calif., headquarters, American Honda has built a service station (below) that splits water into hydrogen fuel and oxygen using power generated by a solar photovoltaic array. This would be the ultimate in green hydrogen production.
cess. "it's half the size of the The hydrogen previous version," said Rog-
pr Saillant, CEO of Plug infrastructure is Power. "Besides refueling the classic vehicles, the system feeds some of the hydrogen into a chicken-and-egg fuel-cell stack to produce dilemma electricity for our headquarters building, which is also warmed in part by waste heat generated by the unit."
With great fanfare, one of the FCXs wheeled up to the fuel-dispensing pump—a metal box the size of a luxury kitchen stove that had been installed in the company parking lot. A state official first grounded the car by attaching a wire to the vehicle. He then dragged the fuel hose from the pump to the FCX's refueling port, inserted the nozzle and locked it into place. The unit finished filling the car's tank after about five or six minutes. Knight explained that the pump produces enough purified hydrogen to refill a single fuel-cell vehicle a day.
Afterward, Knight discussed the problems facing the development of a hydrogen infrastructure: "It's the classic chicken-and-egg dilemma," he said. "There's no demand for cars and trucks with limited fueling options, but no one wants to make the huge investment to create a fueling infrastructure unless there are fleets of vehicles on the road. So the question is: How do we create demand?" [see "Questions about a Hydrogen Economy," by Matthew L. Wald; Scientific American, May 2004].
A study by GM has estimated that $10 billion to $ 15 billion would pay to build 11,700 new fueling stations—enough so a driver would always be within two miles of a hydrogen station in major urban areas and so there would be a station every 25 miles along main highways. That concentration of mostly urban hydrogen stations would support an estimated one million fuel-cell vehicles, it says. "Twelve billion dollars, that's chump change when cable operators are plunking down $85 billion for cable system installations," exclaims Ballard's Campbell.
The Latham filling station—along with several dozen others scattered from Europe to California to Japan—embodies the first halting steps toward the construction of an infrastructure. Soon, Campbell says, about 70 hydrogen refueling stations will be operating worldwide, and California's Hydrogen Highway program has set a goal of 200 stations.
A National Academy of Sciences committee recently estimated that the transition to a "hydrogen economy" will probably take decades, because tough challenges remain. These include how to produce, store and distribute hydrogen in sufficient quantities and at reasonable cost without releasing greenhouse gases that contribute to atmospheric warming. Unfortunately, the extraction of hydrogen from methane generates carbon dioxide, a major greenhouse gas. If the energy sources for electrolysis (the splitting of water into hydrogen and oxygen using electricity) burn fossil fuels, they, too, would generate carbon dioxide. And hydrogen is a highly leak-prone gas that could escape from cars and production plants into the atmosphere,
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