Gilbert M. Brown,
(865) 576-2756,
E-mail

Postdoc

Douglas Knight
Phone: (865) 576-1305
E-mail

Complex Metal Hydrides for Hydrogen Storage

The objective of this program is to develop the chemistry for a reversible hydrogen storage system based on complex hydrides, chosen mostly from the borohydrides, amines, imide/amides, alane or the alanates of the light elements in the periodic table. The work conducted at ORNL takes advantage of our expertise in the synthesis and characterization of oxygen and moisture sensitive organic, metal organic, and inorganic materials.

Metal borohydride compounds have high hydrogen content, and Al(BH₄)₃ (structure shown above) is a liquid at room temperature with the highest volumetric hydrogen content of any metal borohydride. We have conducted temperature dependent pyrolysis studies, and our conclusion is that the first step is conversion of an Al-BH₄ bond to Al-H and BH₃, ultimately forming AlH₃. Borane dimerizes to form diborane B₂H₆. The yield of H₂ is consistent with that from dehydration of AlH₃ and conversion of diborane to a (BH)_n polymeric product and H₂. Our temperature programmed decomposition studies with gas analysis by mass spectroscopy (TPD-MS) of other metal borohydrides, including low melting temperature eutectic mixtures of Li, Na, K, and Mg borohydrides, supports the general conclusion that the fundamental reaction of these materials is

M-BH₄ -> M-H + BH₃
A rapid, solvent-free reaction occurs between Al(BH₄)₃ and NH₃ as shown:
Al(BH₄)₃ + 2NH₃ <-> Al(BH₄)₃-2NH₃

to produce a solid product. The decomposition chemistry of this material changes so that the hydrogen in the ammonia is also evolved. Borazine is observed as a trace product during TPD-MS suggesting this compound decomposes through an ammonia borane (NH₃BH₃) intermediate. The structure of the ammonia adduct is under investigation by X-ray diffraction (synchrotron radiation), NMR, and vibrational spectroscopy (FTIR, Raman).

Mg(BH₄)₂ also reacts with ammonia in a solvent-less reaction to form the complex Mg(BH₄)₂(NH₃)₂. The hydrogen in the ammonia is also evolved as this compound is decomposed. The hydrogen pressure dependence of the decomposition reaction suggests the reaction proceeds by formation of MgH₂ and ammonia borane. Work is in progress to find a way to make the decomposition of metal borohydrides reversible.

We have investigated the reaction of Al(BH₄)₃ with diborane to form the AlB₄H₁₁ material which is also a promising hydrogen storage material.

Specialized equipment available for this project include several vacuum lines with teflon stopcocks, two inert atmosphere glove boxes with oxygen and water meters, a computer controlled high pressure (200 bar) pressure-temperature-composition device (Sievert’s apparatus), and a thermogravimetric apparatus in one of the glove boxes.