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

Abstract

Ammonia borane (AB, NH3BH3), 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 H2 release are revealed. (I) Heteropolar dihydrogen interaction (N-Hδ+···δ−H–B) and (II) homopolar dihydrogen interaction (B–Hδ−···δ−H–B) are still the most popular reaction mechanisms. However, a direct hydrogen adsorption and H2 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 H2 molecules. What is more, we discover the unexpected hydrogenation behaviors between new-formed H2 and BNH compounds. In addition, the similar H exchange reactions assisted by H2 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.

Enhanced Effect of an External Electric Field on NH3BH3 Dehydrogenation: an AIMD Study for Thermolysis

“…Accordingly, two B–H bonds are elongated and further break to release the H 2 molecule. Although a similar reaction pathway is also observed in the previous theoretical study under shock loading, 4 the H radical mainly derives from N ends through a heteropolar dihydrogen bond interaction in this work. It further confirms the importance of the heteropolar dihydrogen bond under E ext .…”

“…The MD data are analyzed stepwise using our postprocessing procedure compiled with fortran90. 4 The stable chemical bonds and molecular components are identified by the bond length and lifetime criteria. If the interaction distance of two atoms is smaller than the critical value R c , and they keep this condition for more than 10 fs, these atoms are considered to be bonded.…”

“…The target accuracy for SCF convergence is 10 –6 au. The MD data are analyzed stepwise using our postprocessing procedure compiled with fortran90 . The stable chemical bonds and molecular components are identified by the bond length and lifetime criteria.…”

“…Ammonia borane (NH 3 BH 3 , AB), as a typical hydrogen-rich material, possesses an extremely high gravimetric and volumetric hydrogen content (19.6 wt % and 0.145 kg·L –1 ), making it one of the most promising hydrogen storage materials. − Its unique physical and chemical properties, deriving from the heteropolar dihydrogen bond interaction (N–H δ+ ··· δ− H–B), have attracted extensive attention in relevant fields. − The classic three-step reaction mechanism (formula 1–3) of AB thermolysis is proposed based on theoretical and experimental studies. − …”

Structural stability, dihydrogen bonding, and pressure-induced polymorphic transformations in hydrazine borane

“…3 One of the most important findings from the extensive studies of NH 3 BH 3 is the dihydrogen bonding, N-HÁ Á ÁH-B, which contributes to the structural stability and the dehydrogenation performance of NH 3 BH 3 . [4][5][6] Moreover, high-pressure studies on NH 3 BH 3 revealed that pressure can tune the structure and thus the dehydrogenation performance of NH 3 BH 3 , and even lead to the formation of novel hydrogen storage materials upon compression. [7][8][9][10][11][12][13][14] However, NH 3 BH 3 still faces multiple challenges in its practical application as a hydrogen storage material, such as slow decomposition kinetics, relatively high reaction starting temperature, and harmful by-products that damage fuel cells.…”