Motorists expect cars to go at least 300 miles between fill-ups. That's not a concern for autos that burn gasoline or diesel, but for a future in which vehicles run on nonpolluting hydrogen, adequate driving range remains a real roadblock [see "On the Road to Fuel-Cell Cars," by Steven Ashley; Scientific American, March]. Despite considerable effort, engineers have so far failed to find a way to cram enough hydrogen--the lowest-density substance in the universe--onboard cars.
Conventional approaches to compact hydrogen storage--compressing the gas to up to 10,000 pounds per square inch (psi) or cooling it down to cryogenic temperatures so that it liquefies (around ¿252 degrees Celsius)--can attain only about half the energy density needed to fit enough fuel inside something the size of a gas tank. A few years back researchers thought that hydrogen could be extracted chemically onboard from liquid hydrocarbons such as methanol, but those schemes did not pan out. Since then, solutions to this packing problem have been lacking, notwithstanding long-term research programs at General Motors, Toyota, BMW and others. But recently hints of progress have emerged. Scientists at GM and its partner HRL Laboratories in Malibu, Calif., have reported advances in two hydrogen storage technologies--cryoadsorption and destabilized complex metal hydrides.
Cryoadsorption falls somewhere between the compressed and low-temperature storage strategies. It relies on getting the gas to adhere to materials with sizable surface areas, explains James Spearot, who manages GM's hydrogen storage research efforts. First, engineers reduce the volume of the gas by cooling it to the temperature of liquid nitrogen (¿196 degrees C), which is easier to attain than liquid-hydrogen temperatures. "Then they compress it to only about 1,000 psi, which forces the hydrogen to adsorb physically into the material's many nooks and crannies," he says. The classic high-surface-area material is powdered activated carbon, but other synthetic substances may offer even more promise, including high-porosity polymers and materials made of organo-metallic molecular "cages"--hydrocarbon frameworks enclosing metal atoms.
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GM's other development concerned improvements to metal hydrides, in which lightweight metallic elements hold hydrogen. When heated, these metal hydride powders decompose, liberating the gas. High temperatures are required, however, because the metal atoms grip the hydrogen with strong covalent bonds. In recent years researchers have achieved better performance with compounds such as lithium borohydride, in which the metal atoms form weaker, ionic bonds with groups containing several hydrogen atoms.
Last year a team led by HRL's John Vajo reduced decomposition temperatures substantially by adding substances such as silicon to complex metal hydride systems. Such additives act as destabilization agents. "Essentially the metal and the destabilizer join preferentially and displace the hydrogen," explains Leslie Momoda, director of HRL's sensors and materials laboratory. Using magnesium hydride as a destabilizer for lithium borohydride, for instance, lowers the release temperature from 400 to 275 degrees C. Moreover, the hydride can store 9 percent of its weight as hydrogen, beating the oft-cited target of 6.5 percent. She hopes that HRL staffers can eventually identify a "Goldilocks" compound with sufficient adsorption capacity that will release hydrogen at 150 degrees C or even lower. Momoda admits, however, that the hydrogen pickup rate is still too slow; current materials might take 30 minutes to refuel.
Practical onboard storage would, of course, constitute only half the formula for a successful hydrogen economy; the other half would be a large-scale hydrogen distribution and refueling network. Thankfully, solving the latter issue will not likely require major technical breakthroughs--only boatloads of cash.