Hydrogen-powered cars that dont pollute the environment are a step closer thanks to a new discovery that promises to solve the main problem holding back the technology: The lack of a safe, efficient system for onboard storage.

Explains Andrew Weller from the Dept. of Chemistry at the University of Bath in the U.K., "Hydrogen has a low density and it only condenses into a liquid at −252°C. This makes it difficult to use conventional storage systems. High-pressure gas containers would need 3-in.-thick steel walls, making them too heavy and large for cars.

Scientists have experimented with ways of storing hydrogen by locking the gas into metal lattices. "The U.S. Dept. of Energy has said that it wants 6% of the weight of hydrogen storage systems to be hydrogen in order to give hydrogen powered cars the same mileage/tank of fuel as petrol-based systems, says Weller.

"While metal hydrides and metal organic framework materials can give this kind of ratio, they only work at temperature extremes which are difficult to engineer into ordinary vehicles, says Weller. Metal hydrides only work above 300°C and metal organic framework materials only work at liquid nitrogen temperatures −198°C).

The University of Bath researchers, however, discovered a new material that works at room temperature and at atmospheric pressure at the flick of a switch. The material, made from a heavy metal (rhodium), only has a weight ratio of 0.1%. This isnt enough for a whole hydrogen tank, says Weller. But the material could be used in combination with metal hydride sources to store and release energy instantaneously until the main tank hits 300°C, its normal operating temperature.

The researchers made the discovery while investigating how hydrogen affects metals. They found that an organo-metallic compound containing six rhodium atoms and 12 hydrogen atoms would absorb two molecules of hydrogen at room temperature and atmospheric pressure and would release the molecules when a small electric current was applied. This kind of take-up and release at the atomic scale is what makes the material a candidate for solving the hydrogen storage problem.

The researchers are now looking at ways of printing the material onto sheets that could be stacked together and encased to form a storage tank. Potentially this tank could sit beside a metal hydride tank and kick into action when the driver hits the accelerator, giving the metal hydride tank time to heat up. Copies of articles in scientific journals describing the work are available from the University of Bath press office.

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University of Bath

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