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Issue 3, 2013
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Methods to stabilize and destabilize ammonium borohydride

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Abstract

Ammonium borohydride, NH4BH4, has a high hydrogen content of ρm = 24.5 wt% H2 and releases 18 wt% H2 below T = 160 °C. However, the half-life of bulk NH4BH4 at ambient temperatures and pressures, ∼6 h, is insufficient for practical applications. The decomposition of NH4BH4 (ABH2) was studied at variable hydrogen and argon back pressures to investigate possible pressure mediated stabilization effects. The hydrogen release rate from solid ABH2 at ambient temperatures is reduced by ∼16% upon increasing the hydrogen back pressure from 5 to 54 bar. Similar results were obtained using argon pressure and the observed stabilization may be explained by a positive volume of activation, ca. 73 ± 17 cc mol−1, in the transition state leading to hydrogen release. Nanoconfinement in mesoporous silica, MCM-41, was investigated as alternative means to stabilize NH4BH4. However, other factors appear to significantly destabilize NH4BH4 and it rapidly decomposes at ambient temperatures into [(NH3)2BH2][BH4] (DADB) in accordance with the bulk reaction scheme. The hydrogen desorption kinetics from nanoconfined [(NH3)2BH2][BH4] is moderately enhanced as evidenced by a reduction in the DSC decomposition peak temperature of ΔT = −13 °C as compared to the bulk material. Finally, we note a surprising result, storage of DADB at temperature <−30 °C transformed, reversibly, the [(NH3)2BH2][BH4] into a new low temperature polymorph as revealed by both XRD and solid state MAS 11B MAS NMR.

Graphical abstract: Methods to stabilize and destabilize ammonium borohydride

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Publication details

The article was received on 17 Jul 2012, accepted on 29 Aug 2012 and first published on 30 Aug 2012


Article type: Paper
DOI: 10.1039/C2DT31591B
Citation: Dalton Trans., 2013,42, 680-687
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    Methods to stabilize and destabilize ammonium borohydride

    T. K. Nielsen, A. Karkamkar, M. Bowden, F. Besenbacher, T. R. Jensen and T. Autrey, Dalton Trans., 2013, 42, 680
    DOI: 10.1039/C2DT31591B

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