Mechanism of complete dehydrogenation of ammonia borane in electrochemical alkaline environment

Abstract

Ammonia borane (NH3BH3, AB) has been considered to be a promising chemical hydrogen storage material. The dehydrogenation reaction of AB on the surface of MgSiP2 is investigated by the first-principles method. We have verified the two conventional intramolecular dehydrogenation pathways in the AB hydrolysis reaction, B-distal dehydrogenation (the rate-determining step (RDS) was 0.71 eV) and B-N bond dissociation dehydrogenation (the RDS was 1.00 eV). A new reaction route for dehydrogenation, the BN alternate dehydrogenation, was also explored. The reaction heat of the RDS was 0.71 eV. Note that the entire reaction process in the path of the dissociation of the BN bond can completely release the 6H protons in AB. With the help of the dissociation of H2O, the H2 output ratio of 1:6 can be achieved. In addition, we also considered the influence of *OH– on the RDS of each dehydrogenation reaction in alkaline environment. The calculations have found that the presence of *OH– is beneficial to reduce the reaction heat of dehydrogenation in the AB hydrolysis reaction, making the reaction heat of the RDS of B-distal dehydrogenation is changed from 0.71 eV to 0.15 eV, the B-N bond dissociation dehydrogenation is changed from 1.00 eV to 0.09 eV and the BN alternate dehydrogenation is changed from 0.71 eV to 0.15 eV. This shows that *OH– greatly promotes the production of H2 from the dehydrogenation of AB.