Metal
stores more hydrogen
By
Eric Smalley,
Technology Research NewsHydrogen is the most abundant element in the universe, and when it is burned its only byproduct is water. One reason the world isn't running on hydrogen fuel is that it's hard to store.
Researchers from the National University of Singapore have made an accidental discovery that brings the promise of clean hydrogen energy a big step forward.
The challenges to using hydrogen as a fuel include finding a hydrogen storage system that is reasonably small and light, and finding a way to release the stored fuel quickly enough when it is needed.
The researchers have found a material that can store and quickly release large amounts of hydrogen. Lithium nitride can store 11.4 percent of its own weight in hydrogen, which is 50 percent more than magnesium hydride, the previous best hydrogen storage material. Other metal hydrides generally store only 2 to 4 percent of their weight.
The new material is not ready for practical applications because the temperature required to release the hydrogen is too high, but it points the way to a practical hydrogen storage material, according to Ping Chen, a senior research fellow at the National University of Singapore. "We think the main application might be... on-board hydrogen storage," he said.
The researchers discovered the material accidentally, according to Chen. "In the last three years we continuously pursued the task of storing hydrogen in lithium-carbon," he said. In the course of that work, the researchers treated lithium-carbon with nitrogen, and found that hydrogen absorption increased. When they studied the material, they found that lithium nitride was the substance responsible for the increased uptake.
Lithium nitride absorbs hydrogen when it is exposed to hydrogen that is under pressure. The chemistry involves one lithium nitride molecule combining with four hydrogen atoms to form lithium amide and lithium hydride. Reducing the pressure reverses the process. "Under low hydrogen pressure, lithium amide and [lithium] hydride react with each other and give out hydrogen gas," said Chen.
The current drawback to the researchers' material is that in order for the hydrogen to be released at one atmosphere of pressure, the storage material has to be heated to 270 degrees Celsius. It will release hydrogen at lower temperatures, but only at pressures below one atmosphere, which is the pressure of Earth's atmosphere at sea level. The researchers' next step is to find a way to release the hydrogen at a usable pressure and practical temperature.
The work is "the first to establish that alkali metal nitrides are worthy of serious consideration as viable hydrogen storage materials," said Craig Jensen, a professor of chemistry at the University of Hawaii at Manoa.
Although the thermodynamic parameters of the system render it impractical, "the paydirt reported by the authors is rich enough that it should attract a rush of prospectors into this area to search for the elusive hydrogen storage material that will open the door to the hydrogen era," Jensen said.
The researchers are working on improving the hydrogen storage performance of their system through both mechanical and chemical modifications, said Chen.
They are investigating the hydrogen storage potential of related nitrides and imides in order to find one with better temperature attributes, said Chen. New species, or forms of nitrides, imides and metal-nitride-hydrogen composites are emerging continuously, he said. "With abundant chemical information and well-developed synthetic and characteristic techniques, we believe there is huge scope for development of materials [that will work at a] practical temperature."
Another technical challenge to making a practical onboard hydrogen storage system is making sure the storage material is chemically stable, said Chen. The researchers' current prototype is sensitive to moisture, oxygen, carbon dioxide and other common substances, which makes strict conditions necessary for practical operation, he said.
It will take five to ten years to develop a practical metal nitride or imide hydrogen storage material, he said.
Chen's research colleagues were Zhitao Xiong, Jizhong Luo, Jianyi Lin and Kuang Lee Tan. They published the research in the November 21, 2002 issue of the journal Nature. The research was funded by the Singapore Agency for Science, Technology and Research.
Timeline: 5-10 years
Funding: Government
TRN Categories: Energy; Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "Interaction of Hydrogen with Metal Nitrides Imides," Nature, November 21, 2002.
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January 15/22, 2003
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