Dehydrogenation and Rehydrogenation of Ammonia Borane under Shock Loading: Ab Initio Molecular Dynamics Simulations

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Ammonia borane (AB, NH 3 BH 3 ), as a hydrogen-rich material, has been attracting great interest in the field of hydrogen storage. However, the mechanism of its dehydrogenation and rehydrogenation is not sufficiently understood yet. In this work, the initial decomposition process of AB under shock loading is investigated using the ab initio molecular dynamics method. The results show that the B–H bond breaking plays a more important role in the reaction initiation. Three main reaction pathways for H 2 release are revealed. (I) Heteropolar dihydrogen interaction (N-H δ+ ··· δ− H–B) and (II) homopolar dihydrogen interaction (B–H δ− ··· δ− H–B) are still the most popular reaction mechanisms. However, a direct hydrogen adsorption and H 2 liberation mechanism (III) is uncovered. The H radical readily adsorbs a B atom to form the pentacoordinate boron-containing species and efficiently activates the adjacent B–H bond, which further ruptures to form H 2 molecules. What is more, we discover the unexpected hydrogenation behaviors between new-formed H 2 and BNH compounds. In addition, the similar H exchange reactions assisted by H 2 are also explored. The probable rehydrogenation mechanisms are proposed based on our results. These theoretical calculations not only give a comprehensive understanding about AB decomposition under shock loading but also shed light on its rehydrogenation for hydrogen storage.