Abstract
Our first-principles study of Ca(NH2BH3)2 reveals that the gas phase energy barrier for the first H2 release is 1.90 eV via a Ca⋯H transition state and 1.71 eV via an N–H⋯B transition state for the second H2 release. In the dimer, the barrier for H2 release from the bridging [NH2BH3]− species is 1.60 eV via an N–H⋯B transition state, and 0.94 eV via an N–H⋯B transition state for the non-bridging [NH2BH3]− species. Analysis of the atomic charge distribution shows that the mechanism of dehydrogenation is determined by the charge transfer between the transition state and the initial state: the less the charge transfer, the lower the barrier to dehydrogenation.
Highlights
One of the most important problems in hydrogen fuel cell technology is the lack of safe and highly efficient hydrogen storage materials [1]
To understand the dehydrogenation mechanism of Ca(NH2BH3)2, we first studied the basic properties of the compound in the gas phase
The shortest Hþ/HÀ distance that appears in either [NH2BH3]À group was about 2.56 A, which is longer than the maximum dihydrogen bond length (2.4 A)
Summary
One of the most important problems in hydrogen fuel cell technology is the lack of safe and highly efficient hydrogen storage materials [1]. Because of its high storage capacity (19.6 wt%) and moderate dehydrogenation temperature, ammonia borane is considered to be a promising on-board hydrogen storage material [2]. The thermal decomposition of NH3BH3 involves three steps evolving one equivalent H2 per step, at temperatures of w110, 150, and >500 C, yielding a final BN product [3e6]. The final step is not considered practical for hydrogen storage because of the very high reaction temperature. Direct use of NH3BH3 is unsuccessful because of borazine formation and the low dehydrogenation kinetics at typical proton exchange membrane fuel cell operating temperatures [2,7,8]. The overall hydrogen storage capacity was reduced by addition of these species, which do not release hydrogen at the operation temperature
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