First-Principles Study of Intrinsic Defects in Ammonia Borane

Solid ammonia borane is a promising hydrogen storage material but suffers from a slow and ill-controlled dehydrogenation process. We studied intrinsic point defects that might play a role for hydrogen release by means of first principles calculations augmented with van der Waals (vdW) corrections. The vdW corrections proved to be crucial for structural properties and also for energies in some cases. For vacancy and interstitial defects of single H as well as of molecular (NH3, BH3) type, we determined formation energies and local lattice structures of the defects in various charge states. Atomic H-related vacancies and interstitials exist predominantly in charged states in agreement with their chemical, i.e., protonic or hydridic, character. For molecular defects, some NH3- and BH3-related neutral defects have rather low formation energies, suggesting that the dehydrogenation of undoped ammonia borane is initiated by the cleavage of the B-N bond. The relaxation pictures associated with H-related defects can explain the observation of a variety of oligomeric products in experiment. Besides, some low-energy defects are found to form H-2 molecules spontaneously and thus might catalyze the dehydrogenation reaction of doped ammonia borane.