If hydrogen is the prince of fuels, then coal is the dirty old man. Hydrogen's promise as the clean fuel of the future has been frustrated by two problems: how to make it, and how to move it. Now, researchers report they may have a solution to both. In a serendipitous discovery, they found that coal, the dirty but abundant fuel of the past, traps hydrogen when milled with cyclohexene, a common chemical. Ground coal is cheap and easy to transport and appears to be reusable, making it a possible candidate for practical hydrogen storage.
Hydrogen is certainly abundant. Oodles of hydrogen atoms float in the ocean bound up in water molecules, and many more are trapped in hydrocarbons such as natural gas and oil. Prying the hydrogen loose from those molecules takes a lot of energy, though, and storing it once it has been extracted also poses major problems. Hydrogen gas is explosive when exposed to oxygen, and it's so lightweight that not much of it will fit in a fuel tank. For a hydrogen fuel-cell car to cover the long distances that standard gasoline and diesel vehicles drive routinely, there needs to be a significant advance in storage techniques.
In the 21 June issue of Journal of the American Chemical Society, Angela Leuking, a chemical engineer in the Department of Energy and Geoenvironmental Engineering at Pennsylvania State University in State College, and colleagues, describe a new, one-step process to both produce and store hydrogen. The group had been intrigued by research that had managed to store hydrogen in graphite and wondered if they could make a similar storage material out of coal. As a first step, they put coal in a mill along with some steel balls and a little cyclohexene as a lubricant. The mill spun the contents at 400 revolutions per minute, and the coal was gradually smashed into a powder by the steel balls. The researchers then put the powder in a thermogravimetric analyzer to let it release any contaminating gases that might have stuck to it before they added hydrogen, but they noticed something odd: The sample was losing a lot of mass. They sampled the gas it was giving off with a mass spectrometer and found, shockingly, that it was almost all hydrogen. When they examined the milled coal under a transmission electron microscope, they found little bits of diamond--a clue that hydrogen gas was present. (Hydrogen helps carbon turn to diamond at low pressures.) The team's best guess was that the pressure inside the mill had been high enough to force the cyclohexene to change chemically and give off hydrogen, which then was trapped by the coal.
"I haven't seen anything like this before, and I think it's something we need," says Anne Dillon, a physical chemist at the National Renewable Energy Lab in Boulder, Colorado. Although the milled coal traps only 2% hydrogen by weight, both Dillon and Leuking suspect it could be tweaked to hold more.