Decomposition mechanisms in metal borohydrides and their ammoniates

Ammoniation in metal borohydrides (MBs) with the form M(BH4)(x) has been shown to lower their decomposition temperatures with M of low electronegativity (chi(p) less than or similar to 1.6), but raise it for high-cp MBs (chi(p) less than or similar to 1.6). Although this behavior is just as desired, an understanding of the mechanisms that cause it is still lacking. Using ab initio methods, we elucidate those mechanisms and find that ammoniation always causes thermodynamic destabilization, explaining the observed lower decomposition temperatures for low-chi(p) MBs. For high-chi(p) MBs, we find that ammoniation blocks B2H6 formation-the preferred decomposition mechanism in these MBs- and thus kinetically stabilizes those phases. The shift in decomposition pathway that causes the distinct change from destabilization to stabilization around chi(p) = 1.6 thus coincides with the onset of B2H6 formation in MBs. Furthermore, with our analysis we are also able to explain why these materials release either H-2 or NH3 gas upon decomposition. We find that NH3 is much more strongly coordinated with higher-chi(p) metals and direct H-2 formation/release becomes more favorable in these materials. Our findings are of importance for unraveling the hydrogen release mechanisms in an important new and promising class of hydrogen storage materials, allowing for a guided tuning of their chemistry to further improve their properties.