Finding a way to safely transport hydrogen is an important concern for researchers and engineers investigating ways to introduce fuel cell-powered vehicles. In June 2008, the ACerS Bulletin published an article on Savannah River National Lab and Toyota research on some prototype systems based an a system of chemical absorbents contained within porous-walled glass microspheres. The idea then was that these microspheres would carry encapsulated absorbent and when the hydrogen was needed, the gas would be released either through a chemical or mechanical means. The benefit of the microspheres, themselves, is that they are strong, recyclable and, at the macro scale, flow like a liquid and therefore can be pumped.
Although our article didn’t provide details about what kind of absorbents were being tested, a different group of investigators from the University of Alabama and the Los Alamos National Lab say they have one: ammonia borane. The general concept is that AB can act as a hydride and soak up the hydrogen. Then, once the hydrogen is released, the AB could then be regenerated and reused.
There are some basic requirements for such a hydrogen fuel system. It needs to be lightweight, it needs to have a relative high energy density (higher than just hydrogen gas), be recyclable, be safe, and, ultimately, meet the DOE 300 miles per fuel tank benchmark.
The LANL and UA, working together under the umbrella of the DOE’s Chemical Hydrogen Storage Center of Excellence, tackled the energy density issue by focusing on hydrides, in particular AB, because as noted in a press release, “its hydrogen storage capacity approaches a whopping 20 percent by weight.”
The easy part was identifying the hydrogen storage capacity of AB; the hard part was figuring out a way to economically recycle it. Eventually, the team realized that one form of dehydrogenated AB – polyborazylene – could be reused fairly efficiently, i.e., without a great dead of additional energy.
One of the LANL researchers, John Gordon, praised the collaborative work with UA. “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 team reports that is also working with Dow Chemical to tweak the chemical processes in the AB–hydrogen system and develop prototype applications.
A paper on this research appears in Angewandte Chemie International Edition.