If the hoped-for hydrogen economy is ever to become a reality, researchers must devise efficient ways to produce and store the gas. That will require a series of breakthroughs that have been slow in coming. But researchers in the United States have hit upon a material for storing hydrogen that could be far better than the competition--just the sort of break hydrogen researchers are looking for.
Hydrogen has long been seen as a potentially green alternative to gasoline, which is produced from fossil fuels and gives off the greenhouse gas carbon dioxide when burned. When piped through a fuel cell, hydrogen molecules (H2) combine with oxygen, producing only electricity and water. At room temperature, however, hydrogen is a gas, which makes it difficult to store enough of it on board a car to drive long distances. The gas can be compressed in high-pressure tanks or cooled to a liquid at ultracold temperatures. But both of those strategies require large amounts of energy themselves.
As an alternative, researchers have been searching for materials that can hold large amounts of H2 and release it on demand. But so far the best performers, which are known as metal hydrides, hold only about 2% of their weight in hydrogen at room temperature, well below what is needed for a practical gas tank. Other materials can get up to 7% but require either high or low temperatures, and thus added energy and cost.
Last year, however, researchers led by Taner Yildirim at the National Institute of Standards and Technology in Gaithersburg, Maryland, calculated that a material made from certain metals, such as titanium, and a small hydrocarbon called ethylene should form a stable complex that could bind up to 14% of its weight in hydrogen. Adam Phillips, a physicist and postdoc in the lab of Bellave Shivaram at the University of Virginia, Charlottesville, decided to give the proposal a try.
Phillips used a laser to vaporize titanium in a gas of ethylene. The combined material settled out of the gas and on to a substrate to form a film. When Phillips added hydrogen at room temperature and weighed the result, he found the 14% added weight, just as predicted. After running a series of successful control studies, Phillips and Shivaram reported their new material on Monday at the International Symposium on Materials Issues in a Hydrogen Economy in Richmond, Virginia.
The new result is "extremely interesting," says Gholam-Abbas Nazri, a hydrogen storage expert at the General Motors Research and Development Center in Warren, Michigan. However, Nazri adds, "we have to be very cautious." There have been numerous false starts in the field before, he says. And researchers still must make the material in bulk, demonstrate that it works in that form, and show that it will release hydrogen as easily as it sops it up.
Even with those caveats, George Crabtree, a physicist at Argonne National Laboratory in Illinois, says the result "is one of the most promising developments of the last few years."