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High-pressure Compound Could Be Key to Hydrogen-powered Vehicles

Hydrogen-powered cars sound like a great idea, but how do you stuff enough hydrogen into a small, portable volume that's practical as fuel? Help could be on the way in the form of a hydrogen-rich compound discovered by researchers in the Stanford Institute for Materials and Energy Science at the Department of Energy's SLAC National Accelerator Laboratory.

The newly discovered material is a high-pressure form of ammonia borane, a solid material which itself is already imbued with ample hydrogen. By working with the parent material at high pressure in an atmosphere artificially enriched with hydrogen, the scientists were able to ratchet up the hydrogen content of the material by roughly 50 percent.

"Including the hydrogen already stored in ammonia borane, this new material can store around 30 weight percent in total," said SIMES researcher Yu Lin, lead author of a paper describing the work that was published last week in the online edition of Proceedings of the National Academy of Sciences.

The Department of Energy has set a target for hydrogen-powered vehicles to have an on-board storage system able to hold 9 percent, by weight, of hydrogen, by 2015. The new compound, called ammonia borane-hydrogen, contains more than triple that amount.

But there's a fly in the hydrogen: the sought-after storage system must function at ambient pressure and temperature. The process Lin used to get the added hydrogen into the ammonia borane has to take place at a minimum pressure that is approximately 60,000 times the usual pressure at the surface of the Earth.

"For energy applications, we need to stabilize the material near ambient conditions," said Lin, a graduate student in geological and environmental sciences. Currently, most hydrogen-powered machines use either compressed hydrogen gas or liquid hydrogen, which needs to be maintained at high pressure or very low temperature, respectively, relative to ambient temperature and pressure. These methods have associated safety concerns in the case of compressed hydrogen and require significant energy for cooling in the case of liquid hydrogen.

There is currently no material that satisfies all of the requirements for on-board fuel storage for hydrogen-powered vehicles, according to Lin, who co-authored the PNAS paper with SIMES physicist Wendy Mao and geophysicist Ho-kwang Mao, senior staff scientist at the Geophysical Laboratory at the Carnegie Institution of Washington.

"If the material can be stabilized at or near ambient conditions with a large amount of hydrogen content, then I think it will be very promising," Lin said. There are potentially several ways to help do this. For example, there might be some "alternative chemical paths, like adding some catalyst to try to stabilize the system," Lin said.

If Lin and Mao succeed, ammonia borane could move one step closer to becoming an everyday storage material for hydrogen. Also closer to a reality would be scientists' and environmentalists' dream of powering cars with oxygen from the air and hydrogen from a fuel cell, while pumping out only water from the exhaust pipe.

—Casey Lindberg and Louis Bergeron
  SLAC Today, May 11, 2009

Casey Lindberg is a science-writing intern and Louis Bergeron is a science writer at Stanford News Service.