Hydrogen utopia comes two steps closer

HYDROGEN SEEMS the ideal fuel of the future — clean-burning, efficient and in potentially limitless supply. But to store it in liquid form requires expensive high-pressure tanks. Now researchers may have cracked the problem, with a way of storing hydrogen in blocks of ice.

Wendy Mao and her colleagues at the Carnegie Institution of Washington have found that at high enough pressure, hydrogen molecules can be trapped inside cages made of ice, known as clathrates. Unlike other gases such as methane that are known to get trapped in clathrates, hydrogen molecules were thought too small to remain imprisoned. But it turns out that under enough pressure, the molecules stuff themselves into the cages in pairs or foursomes.

To create the clathrate, the team subjected a mixture of hydrogen and water to a pressure of about 2,000 atmospheres, while keeping the temperature steady at 27{+0}C. At first the two separated, and the hydrogen formed a bubble surrounded by ice. But when cooled to -24{+o}C, the water and hydrogen merged to form the clathrate.

Ho-Kwang Mao, a member of the Carnegie team, told New Scientist that once the clathrate has formed, it should be possible to store it at low pressure using liquid nitrogen as a coolant. Most of today's prototype hydrogen-powered vehicles have to use hydrogen in liquid form. But this must be kept at a temperature of 253{+o}C, which requires impractical and expensive liquid helium as a coolant. By contrast, liquid nitrogen is "cheap and inexhaustible," says Wendy Mao. It is also eco-friendly.

Scientists may also be getting closer to tapping an inexhaustible supply of hydrogen. In a separate paper in Science, a University of Pittsburgh team reports a breakthrough in using light to split water into hydrogen and oxygen.

The team managed to do this on a semiconductor chip coated with a chemically modified layer of titanium dioxide (TiO{-2}), which acts as a catalyst. The researchers found that by first evaporating the TiO{-2} in a natural gas flame, some carbon atoms become incorporated into it. That boosted the efficiency of the photochemical splitting to 11 per cent — better than previous attempts by a factor of 10. This may eventually provide a way to use solar energy directly to make storable hydrogen fuel.