Molecular dynamics simulations of hydrogen production from ammonia borane: Dual promotion by CO2 and alternating electric field

Abstract

Ammonia borane (AB) is known for its high hydrogen storage density. This study aims to investigate the effects of CO₂ atmosphere and electric field (EF) on the mechanism of hydrogen production from AB pyrolysis. The variations of the main products, chemical bonds and the detailed decomposition pathways of AB were obtained from the reactive force field molecular dynamics (ReaxFF-MD) simulations. First, under no EF, the H₂ yield in S3 system (AB/CO₂ molar ratio of 2.89) is higher than that of other systems. Comparing different EF conditions, it is found that S3 system has the highest yield of H₂ and H₂O when the EF frequency (ν_EF) is 0.005 fs⁻¹. The high-frequency EF increases the reaction rate while reducing the formation of the by-product NH₃. The initial decomposition of AB is dominated by the cleavage of BH and NH bonds, as well as more intermolecular H transfer. The high-frequency EF significantly enhanced the activation of AB and promoted the pyrolysis dehydrogenation of AB·NH₃BH₃ → H₂ + NH₂BH₂ is the dominant pathway. When the value of ν_EF exceeds 0.001 fs⁻¹, the proportion of this pathway gradually decreases with increasing ν_EF. The main reaction pathway of CO₂ is hydrogenation to generate CO₂H fragments. The apparent activation energy of S3 system in the presence of optimal CO₂ ratio and EF is 53.9 kJ/mol, which is lower than 80.0 kJ/mol of S1 (without CO₂ and EF) and 68.6 kJ/mol of S3 (with CO₂ but without EF). The coupling effect of CO₂ and high-frequency alternating EF significantly reduces the reaction energy barrier of AB pyrolysis dehydrogenation. By leveraging the combined effects of CO₂ and EF, both the yield and quality of H₂ are improved. This approach not only achieves efficient hydrogen conversion but also contributes to carbon neutrality.