Gas storage: X-ray structure shows improved hydrogen storage material

Solid storage solution

Mark Bowden, David Heldebrant, Gregory Schenter and Tom Autrey and Abhi Karkamkar of the Pacific Northwest National Laboratory, in Richland, Washington state and Thomas Proffen of the Lujan Neutron Scattering Center, at Los Alamos National Laboratory, in New Mexico, explain how there has been a resurgence of interest in hydrogen-rich boron compounds because of the potential for using them as a safe chemical energy supply for hydrogen fuel cells. Ammonia borane (NH₃BH₃) for instance contains 196 grams of dihydrogen per kilogram (147 g per litre) and is relatively stable under atmospheric conditions. As such, researchers have studied details of this compound almost exhaustively.

A much less well-known compound, the so-called "diammoniate of diborane" (DADB, which has the formula [(NH₃)₂BH₂]⁺[BH₄]^-) was actually first synthesised more than two decades before ammonia borane and has the potential to be an even more useful storage material. Bowden and colleagues point out that the stoichiometry of DADB is known but its actual chemical structure was the subject of some discussion for many years until chemists settled on the borohydride formula after a definitive series of experiments. However, although powder diffraction and nuclear magnetic resonance spectroscopy, together with Raman solution spectroscopy have hinted at details of the precise atomic arrangement, no defining crystal structure has been obtained, until now.

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Previously, they had suggested that ammonium borohydride decomposed to DADB through the intermediate and transient formation of ammonia borane. The team has now provided evidence of this through an in situ XRD experiment, which they say shows a clear acceleration of DADB formation when ammonia borane and ammonium borohydride are mixed in the solid state. "The crystal structure of DADB [itself] was solved from a combination of X-ray and neutron powder diffraction patterns using the Rietveld refinement programme TOPAS," the team says. "For neutron diffraction, an isotopically enriched sample containing boron-11 and D was prepared."

The compound can be stored for several months under an inert atmosphere, as can ammonia borane, the team emphasises. However, heating of DADB produced some intriguing results. The team found that heating to 200 Celsius led to two distinct decompositions that released hydrogen gas at each stage. This is similar to the way in which ammonia borane decomposes and so is perhaps not entirely surprising, except that the first stage decomposition DADB occurs at 85 Celsius rather than the 110 Celsius required for initial ammonia borane break down and the overall kinetics are much faster. This bodes well for the use of DADB in investigations of hydrogen storage as a potentially viable alternative to ammonia borane.

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