Hydrogen storage in ammonia borane: {\em Ab initio} study of the de- and rehydrogenation mechanisms*

Using {\em ab initio} density functional calculations, we study the microscopic mechanism of hydrogen release from ammonia borane (NH$_3$BH$_3$) and the reverse process leading to its subsequent recharging with hydrogen. Our total energy surfaces indicate the most favorable pathways to thermally convert the NH$_3$BH$_3$ molecular solid to the energetically preferred polymer NH$_2$BH$_2$ and molecular hydrogen. To prevent formation of undesirable side-products such as the cyclic compound borazine (N$_3$B$_3$H$_6$) or other complexes that would prevent subsequent rehydrogenation, we propose to enclose AB in narrow carbon nanotubes. In this constrained space, we investigate possible rehydrogenation pathways using atomic and molecular hydrogen as well as selected protonation agents.