Graphite to store Hydrogen Gas
HYDROGEN-POWERED cars could travel up to 8000 kilometres on a single tank of gas thanks to a graphite storage material developed by researchers at [1]Northeastern University in Boston, Massachusetts. Nelly Rodriguez and her team claim that their graphite nanofibres can store up to three times their own weight in hydrogen under pressure at room temperature--more than ten times as much as current storage media. Rodriguez envisages the nanofibres packed into a cartridge containing enough hydrogen to power an electric car for up to 8000 kilometres. Spent cartridges could be exchanged for new ones and refilled. The vehicles would be driven by fuel cells, in which the hydrogen combines with oxygen to produce an electric current. A prototype vehicle based on fuel-cell technology, the [2]Necar II, was unveiled last May [INLINE] by Daimler-Benz. It stores the hydrogen in pressurised gas cylinders ("A tank of the cold stuff", New Scientist, 23 November 1996, p 40). Several states in the US are demanding that 2 per cent of cars on the market in 1998 have zero emissions, and hydrogen cars, which only produce water vapour, would fit the bill. Quite how the graphite nanofibres store so much hydrogen is not totally clear. Even carbon nanotubes, which are being developed by a team led by Michael Heben at the [3]National Renewable Energy Laboratory in Denver, cannot store anywhere near the levels being claimed by Rodriguez's team. Heben says: "The best figure we have been able to achieve using nanotubes for hydrogen storage is 4 per cent by weight. 300 per cent by weight of hydrogen would indeed, if true, be very interesting." He remains doubtful about the new figures, however. "The highest ratio of hydrogen to carbon in nature is found in methane, which would correspond to 25 per cent by weight," he says. If Rodriguez's figures are correct, hydrogen could account for as much as 75 per cent of the weight of a graphite nanofibre cartridge. The key to this impressive storage capacity is the regular, closely packed structure of the graphite nanofibres. The fibres are made from stacks of graphite platelets and vary from 5 to 100 millimetres in length and from 5 to 100 nanometres in diameter. Theoretical calculations of the hydrogen absorption capacity of single-crystal graphite show that 6=B72 litres of hydrogen per gram of graphite could be achieved by covering the surface of the crystal in a single layer of hydrogen molecules. The team at Northeastern University claims to have upped this figure to 30 litres. Rodriguez reckons the high capacity is due to several layers of hydrogen molecules condensing inside the "slit pores" between the platelets by capillary action. The spacing between the graphite layers is 0=B734 nanometres, while hydrogen molecules normally have an effective diameter of 0=B726 nanometres. But multiple layers of hydrogen could squeeze into the gap if the molecules were interacting strongly with electrons in the graphite. Terry Baker, a member of the team, says that when the hydrogen molecules are absorbed, they lose a lot of their vibrational and rotational energy and "shrink" to an effective radius of 0=B7064 nanometres. This leaves plenty of room for more hydrogen molecules. "We probably produce about five layers," says Rodriguez. The narrow slits stop oxygen and other larger molecules from squeezing in, and this minimises the chance of an explosive reaction. Safety will still be a major consideration, however. "I imagine some protection would be required for the cartridge," says Rodriguez. Baker discovered graphite nanofibres as long ago as 1972 when he was working for Britain's Atomic Energy Authority at Harwell, but it is only recently that Rodriguez's team has developed a process for making large amounts of them. They will not give too much away, but Baker will say that the process involves reacting hydrocarbons with carbon monoxide on bi- or tri-metallic nickel or iron-based catalytic particles. "The material itself will not be all that expensive," he says. "When the process is scaled up, it will cost less than $1 per kilogram." To pump the nanofibres full of hydrogen, they must first be washed with acid to remove metal impurities from the cata-lyst particles, and then heated to over 900 =B0C and placed under a vacuum to remove any gases already clogging up the slits. Hydrogen is then pumped in at an initial pressure of around 120 atmospheres. Rodriguez says it can take between 4 and 24 hours to fully charge them up. The pressure must then be maintained at 40 atmospheres to keep the hydrogen in place, and the gas can be released by gradually reducing the pressure. According to Rodriguez, the nanofibres can be refilled to the same capacity at least 4 or 5 times. Rodriguez presented the group's findings at the annual Materials Research Society meeting in Boston, Massachusetts, earlier this month. From New Scientist, 21 Dec 96 Copyright IPC Magazines 1996 P L A N E T S C I E N C E
Green cars go farther with graphite
Stephen Hill