HYDROGEN seems the ideal fuel of the future 鈥� 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 (see Graphic).
To create the clathrate, the team subjected a mixture of hydrogen and water to a pressure of about 2000 atmospheres, while keeping the temperature steady at 27 掳C. At first the two separated, and the hydrogen formed a bubble surrounded by ice. But when cooled to 鈭�24 掳C, the water and hydrogen merged to form the clathrate (Science, vol 297, p 2247).
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Ho-kwang Mao, a member of the Carnegie team, told 快猫短视频 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鈥檚 prototype hydrogen-powered vehicles have to use hydrogen in liquid form. But this must be kept at a temperature of 鈭�253 掳C, which requires impractical and expensive liquid helium as a coolant. By contrast, liquid nitrogen is 鈥渃heap and inexhaustible鈥�, says Wendy Mao. It is also environmentally friendly. 鈥淭he only by-products are water and nitrogen.鈥�
快猫短视频s may also be getting closer to tapping an inexhaustible supply of hydrogen. In a separate paper in the same issue of Science this week (vol 297, p 2243), 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 (TiO2), which acts as a catalyst. The researchers found that by first evaporating the TiO2 in a natural gas flame, some carbon atoms become incorporated into it. That boosted the efficiency of the photochemical splitting to 11 per cent 鈥� 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.
Both pieces of research bring us one step closer to what many policy analysts see as the ultimate goal: a pollution-free, fully renewable hydrogen-based energy system. 鈥淥ne of the biggest issues in hydrogen power is how to store the stuff in cars,鈥� said Peter Hoffman, editor of Hydrogen and Fuel Cell Letters. 鈥淭here鈥檚 no single best solution so far.鈥�
Cold storage of hydrogen in clathrate may be something that nature has been doing all along. Astronomers once believed that small bodies such as asteroids and comets, or Jupiter鈥檚 icy moons, must have lost virtually all the hydrogen they might once have contained. Now it seems these bodies could be harbouring large amounts of hydrogen in ice after all 鈥� and we could even use them as interplanetary sources of rocket fuel some day.