|Different ammonia borane (AB) materials (red diamonds) show promise for meeting the DOE systems requirements for an on-board hydrogen storage material (the area inside the dotted lines). Click to enlarge. Source: DOE|
Researchers at US Department of Energy’s Pacific Northwest National Laboratory (PNNL) have made progress on developing a simple “one-pot” reaction to make ammonia borane (NH3BH3, abbreviated as AB), which at formula 19 wt% H2, is a chemical hydrogen storage material of ongoing interest for use in on-board storage systems. They report on their process in the inaugural issue of the Royal Society of Chemistry journal Energy & Environmental Science.
Ammonia borane is a stable white powder which begins to release gas upon heating to more than 70°C. With a gravimetric density of around 194 g H2 kg-1 and a volumetric density of around 146 g H2 liter-1, AB is a promising chemical hydrogen storage material.
Other work at PNNL has focused on stabilizing the material (holding ammonia borane within a scaffold of mesoporous silica templates demonstrated hydrogen storage capacities of >12 wt.%) and on manipulating the release of hydrogen from AB at predictable rates. (Earlier post.)
One issue with the material, however, has been producing it in sufficient yield.
The newly reported PNNL work showed the “surprising” result that ammonium borohydride, NH4BH4, formed in situ by the metathesis of NH4X and MBH4 salts (M = Na, Li; X = Cl, F) in liquid NH3, can be induced to decompose in an organic ether to yield AB in near quantitative yield.
The high yields of isolated AB were surprising for the following two reasons: (1) we found that it was not necessary to remove all traces of ammonia prior to addition of the organic solvent, and (2) we found that it was not necessary to add trace quantities of diborane to get quantitative yields of AB. A synthetic approach to prepare AB in quantitative yields in a single pot will provide researchers with a simple procedure to prepare AB. Furthermore, we envision that this simpler procedure can be scaled up and the solvents can be recycled. Efficient routes to the synthesis of material are an important aspect for R&D focused on discovering materials that could be used to store high densities of hydrogen for fuel cell powered applications.—“Synthesis of ammonia borane for hydrogen storage applications”
The purity of the AB prepared by this one-pot strategy is sufficient to meet the thermal stability requirements for on-board hydrogen storage, according to the researchers.
The group is currently looking at scaling up the reaction to an industrial level. This work was supported by the Office of Basic Energy Sciences of the Department of Energy, Chemical Sciences program.
David J. Heldebrant, Abhi Karkamkar, John C. Linehan and Tom Autrey (2008) Synthesis of ammonia borane for hydrogen storage applications, Energy Environ. Sci., doi:10.1039/b808865a