Hydrogen storage: Letting it go

Angew. Chem. Int. Ed. 54, 15556–15559 (2015)

Materials with small volume and low weight that can store a large amount of hydrogen are required in the development of hydrogen-powered vehicles and other mobile applications. Ammonia borane (NH₃BH₃) is a promising candidate; however, it is rather slow and inefficient at releasing the stored hydrogen. Hermann Wegner and colleagues at Justus Liebig University in Giessen have now reported a catalyst (9,10-dichlorodiboraanthracene) that enables the release of about 2.5 equivalents of molecular hydrogen per NH₃BH₃ molecule, which is the highest amount reported for metal-free catalysts.

The researchers showed that the catalyst can be used multiple times, as long as new batches of NH₃BH₃ are supplied, without compromising its catalytic activity. They also showed that the release reaction ceases upon cooling to room temperature, and quickly resumes after heating to 60 °C. Such thermal activation at a moderate temperature may be useful in applications that require intermittent release. The researchers rationalized the reaction mechanism from nuclear magnetic resonance data and first-principles calculations; the dehydrogenation is initiated by the attack of the hydride of NH₃BH₃ at the electron-deficient boron centre in the catalyst, and the rate-determining step then involves the breaking of both B–H and N–H bonds.