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Researchers have revealed a new single-stage method for recharging the hydrogen storage compound ammonia borane. The breakthrough makes hydrogen a more attractive fuel for vehicles and other transportation modes.
In an article appearing in the March 18 edition of Science magazine, Los Alamos National Laboratory (LANL) and University of Alabama researchers working within the U.S. Department of Energy’s Chemical Hydrogen Storage Center of Excellence describe a significant advance in hydrogen storage science.
Hydrogen is in many ways an ideal fuel. It possesses a high energy content per unit mass when compared to petroleum, and it can be used to run a fuel cell, which in turn can be used to power a very clean engine. On the down side, H2 has a low energy content per unit volume versus petroleum (it is very light and bulky). The crux of the hydrogen issue has been how to get enough of the element on board a vehicle to power it a reasonable distance.
And what is the significance of the breakthrough?
“It’s probably to be determined,” said LANL’s Andrew Sutton, who headed the project and has worked at the lab for four years. “It raises the potential ability of recharging the hydrogen storage compound ammonia borane.”
“This is a critical step if we want to use hydrogen as a fuel for the transportation industry,” said David Dixon, Ph.D., the Robert Ramsay Chair of Chemistry at The University of Alabama and one of the article’s co-authors.
In this approach, ammonia borane in a fuel tank produces hydrogen which, when combined with oxygen in the vehicle’s fuel cell, releases energy. That energy is then converted to electricity that powers an electric motor. Water is the only emission.
After hydrogen is released from the ammonia borane, a residue, which the researchers refer to as “spent fuel,” remains.
“The spent fuel stays in the car, and we need to add hydrogen back to it in order to use it again,” Dixon said. “What this paper describes is an efficient way to add the hydrogen back to make the ammonia borane again. And it can be done in a single reactor.”
Work at LANL and elsewhere has focused on chemical hydrides for storing hydrogen, with one material in particular, ammonia borane, taking center stage. Ammonia borane is attractive because its hydrogen storage capacity approaches 20 percent by weight — enough that it should, with appropriate engineering, permit hydrogen-fueled vehicles to go farther than 300 miles on a single “tank,” a benchmark set by the U.S. Department of Energy.
This “one pot” method represents a significant step toward the practical use of hydrogen in vehicles by potentially reducing the expense and complexity of the recycle stage. Regeneration takes place in a sealed pressure vessel using hydrazine and liquid ammonia at 40 degrees Celsius and necessarily takes place off-board a vehicle. The researchers envision vehicles with interchangeable hydrogen storage “tanks” containing ammonia borane that are used, and sent back to a factory for recharge.
The Chemical Hydrogen Storage Center of Excellence was one of three Center efforts funded by DOE. The other two focused on hydrogen sorption technologies and storage in metal hydrides. The Center of Excellence was a collaboration between Los Alamos, Pacific Northwest National Laboratory, and academic and industrial partners.
LANL researcher Dr. John Gordon, a corresponding author for the paper, credits collaboration encouraged by the Center model with the breakthrough.
“Crucial predictive calculations carried out by University of Alabama Professor Dave Dixon’s group guided the experimental work of the Los Alamos team, which included researchers from both the Chemistry Division and the Materials Physics and Applications Division at LANL,” Gordon said.
The success of this particular advance built on earlier work by this team (see: Angew. Chem. Int. Ed. 2009, 37, 6812).
Input from colleagues at Dow Chemical (also a Center Partner), indicated that an alternative approach to the work in the Angew. Chem. paper would be required if ammonia borane recycle were to be feasible on a large scale. Armed with this information, it was “the insight, creativity and hard work of Sutton of the Chemistry Division at LANL that provided the key to unlocking the ‘one-pot’ chemistry,” Gordon said.