Two methods for improving the release or storage of hydrogen in ammonia borane enhance its potential for future use
Hydrogen has a much greater energy density than gasoline by mass, but a much lower one by volume, hindering its wider use as a fuel. The US Department of Energy has a target of 9 wt% for on-board hydrogen storage by 2015. Ammonia borane (NH3BH3) has a hydrogen content of 19 wt% and has been investigated extensively as the material to meet the challenge. Releasing hydrogen from ammonia borane, however, is problematic: kinetic barriers require temperatures above 100 °C and side products such as borazine hamper the process.
Ping Chen and colleagues from the Dalian Institute of Chemical Physics in China have now incorporated1 cobalt and nickel catalysts into ammonia borane. At a loading of only 2 mol%, these do not reduce the hydrogen capacity significantly, and allow 5.8 wt% hydrogen to be released at temperatures around 60 °C. It also reduced the amount of borazine formed—none could be detected. The material itself also suffered less from deformation.
Ho-kwang Mao of the Carnegie Institution of Washington and co-workers have used high pressure to incorporate2 more hydrogen into ammonia borane. At around 6 GPa under a hydrogen atmosphere, between one and two molecules of extra hydrogen were absorbed per molecule of ammonia borane. Crushing the solid improved its contact with hydrogen and thus the rate at which it was taken up. This extra capacity raised the level of hydrogen in the material to around 30 wt%, giving increased hope that this material could feature in the hydrogen-storage devices of the future.
References
He, T. et al. Nanosized Co- and Ni-catalyzed ammonia borane for hydrogen storage. Chem. Mater. 10.1021/cm900672h (2009).
Lin, Y., Mao, W. L. & Mao, H.-k. Storage of molecular hydrogen in an ammonia borane compound at high pressure. Proc. Natl Acad. Sci. USA 10.1073/pnas.0903511106 (2009)
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Withers, N. Improving capacity and release. Nature Chem (2009). https://doi.org/10.1038/nchem.251
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DOI: https://doi.org/10.1038/nchem.251