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Progress in Synthesizing Ammonia Borane for Use in H2 Storage Systems

Ab
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.

Resources

  • 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

Comments

K

The storage and release figures are truly impressive.

The synthesis described sounds expensive to me but other readers can and will tell us about that.

Treehugger

This ammonia borane is not a panacea since you have to recycle the by-product once the H2 is release, and I don't see that to be practical unless you use some kind of 100 pounds cartridge that you recycle. Maybe but far from ideal

David Ahlport

re: Treehugger
Thats why it might make sense, not as a transportation medium, but for grid storage.

Engineer-Poet

If hydrogen gas doesn't make sense for grid storage due to losses in conversion, it makes even less sense to use ammonia borane.

Michael

OT: interesting hybrid fuel cell research...

Hybrid fuel cell

"Carbon fuel cells offer very high efficiency of conversion and can yield two to three times the amount of energy for a given amount of coal compared to conventional thermal generation"
- John Irvine, University of St Andrews, UK

besides Carbon, does this type of hybrid extend to higher efficiency for other type fuel cells?

Michael

OT: interesting hybrid fuel cell research...

Hybrid fuel cell

"Carbon fuel cells offer very high efficiency of conversion and can yield two to three times the amount of energy for a given amount of coal compared to conventional thermal generation"
- John Irvine, University of St Andrews, UK

besides Carbon, does this type of hybrid extend to higher efficiency for other type fuel cells?

Michael

apologies for double-post...

Reality Czech

The "hybrid" cell looks like the SRI International DCFC. The SRI cell is about 70% efficient compared to the Cooper DCFC's 80%, but it has greater current capacity.

Mannstein

When dehydrogenating AB at high temperature borazine is released which poisons the fuel cell. The gas is also poisonous to humans. Wonder how they plan to get around that difficulty?

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