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.
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Zhang, C. Hydrogen storage: Letting it go. Nat Energy 1, 15022 (2016). https://doi.org/10.1038/nenergy.2015.22
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