Decomposition mechanisms in metal borohydrides and their ammoniates

摘要

Ammoniation in metal borohydrides (MBs) with the form$\mathcal{M}$(BH$_4$)$_x$ has been shown to lower their decompositiontemperatures with $\mathcal{M}$ of low electronegativity ($\chi_p \lesssim1.6$), but raise it for high-$\chi_p$ MBs ($\chi_p \gtrsim 1.6$). Although thisbehavior is just as desired, an understanding of the mechanisms that cause itis still lacking. Using \emph{ab initio} methods, we elucidate those mechanismsand 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 B$_2$H$_6$formation---the preferred decomposition mechanism in these MBs---and thuskinetically stabilizes those phases. The shift in decomposition pathway thatcauses the distinct change from destabilization to stabilization around$\chi_p=1.6$ thus coincides with the onset of B$_2$H$_6$ formation in MBs.Furthermore, with our analysis we are also able to explain why these materialsrelease either H$_2$ or NH$_3$ gas upon decomposition. We find that NH$_3$ ismuch more strongly coordinated with higher-$\chi_p$ metals and direct H$_2$formation/release becomes more favorable in these materials. Our findings areof importance for unraveling the hydrogen release mechanisms in an importantnew and promising class of hydrogen storage materials, allowing for a guidedtuning of their chemistry to further improve their properties.