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Publication Date: 09/1/2009
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New Hydrogen Storage Brings Fuel Cell Autos Closer
Los Alamos, NM — With every announcement of another automobile manufacturer's fuel cell initiative come wonderful pronouncements and forecasts that are still far from being fulfilled for a number of reasons. One of these over-arching stumbling blocks has been the lack of a suitable fuel storage system for pure hydrogen — sometimes referred to as the "ideal" fuel. The problem is that there has been no really effective way of compressing large amounts of hydrogen into a fuel tank that would work on a fuel cell-powered auto. Test vehicles on the roads now require frequent fillups because hydrogen tank capacity is so small. But that may be changing.

A new method for "recycling" hydrogen-containing fuel materials could open the door to economically viable hydrogen-based vehicles. As reported in Angewandte Chemie, a German Chemical Society Journal, Los Alamos National Laboratory and University of Alabama researchers working within the U.S. Department of Energy's Chemical Hydrogen Storage Center of Excellence appear to have made an important advance in hydrogen storage science.

Hydrogen is in many ways an ideal fuel for transportation. It is abundant and can be used to run a fuel cell, which is much more efficient than internal combustion engines. When a pure hydrogen/oxygen fuel cell is used, there are zero harmful emissions; the exhaust consists of water vapor. In fact the water is so pure, drinking-water needs for NASA astronauts have been met with fuel-cell created water. The water is the result of combining hydrogen and oxygen atoms to produce electricity in the fuel cell, along with lots of waste heat and water vapor.

Making it Practical
For use in transportation, a fuel ideally should be lightweight to maintain overall fuel efficiency and pack a high energy content into a small volume. This hasn't happened until recently because under normal conditions, pure hydrogen has a low energy density per unit volume, presenting technical challenges for its use in vehicles capable of traveling 300 miles or more on a single fuel tank — a benchmark target set by the Department of Energy.

Consequently, until now, the universe's lightest element has been considered by some as a lightweight in terms of being a viable transportation fuel.

In order to overcome some of the energy density issues associated with pure hydrogen, work within the Chemical Hydrogen Storage Center of Excellence has focused on using a class of materials known as chemical hydrides. Hydrogen can be released from these materials and potentially used to run a fuel cell. These compounds can be thought of as "chemical fuel tanks" because of their hydrogen storage capacity.

Ammonia borane is an attractive example of a chemical hydride because its hydrogen storage capacity approaches a whopping 20percent by weight. The chief drawback of ammonia borane, however,has been the lack of energy-efficient methods to reintroduce hydrogen back into the spent fuel once it has been released. In other words, until recently, after hydrogen release, ammonia borane couldn't be adequately recycled.

Joint Reserch Effort
Los Alamos researchers have been working with University of Alabama colleagues on developing methods for the efficient recycling of ammonia borane. The research team made a breakthrough when it discovered that a specific form of dehydrogenated fuel, called polyborazylene, could be recycled with relative ease using modest energy input. This development is a significant step toward using ammonia borane as a possible energy carrier for transportation purposes.

"This research represents a breakthrough in the field of hydrogen storage and has significant practical applications," said Dr. GenePeterson, leader of the Chemistry Division at Los Alamos. "The chemistry is new and innovative, and the research team is to be commended on this excellent achievement."

The Chemical Hydrogen Storage Center of Excellence is one of three Center efforts funded by DOE. The other two focus on hydrogen sorption technologies and storage in metal hydrides. The Center of Excellence is a collaboration between Los Alamos, Pacific Northwest National Laboratory, and academic and industrial partners. Referring to the work described in the Angewandte Chemie article, Los Alamos researcher John Gordon, corresponding author for the paper, said, "Collaboration encouraged by our Center model was responsible for this breakthrough. At the outset there were myriad potential reagents with which to attempt this chemistry."

"The predictive calculations carried out by University of Alabama professor Dave Dixon's group were crucial in guiding the experimental work of Los Alamos postdoctoral researcher Ben Davis,"Gordon added. "The excellent synergy between these two groups clearly enabled this advance."

The research team currently is working with colleagues at The Dow Chemical Company, another Center partner, to improve overall chemical efficiencies and move toward large-scale implementation of hydrogen-based fuels within the transportation sector.


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