Decomposition mechanisms in metal borohydrides and their ammoniates

Abstract

Ammoniation in metal borohydrides (MBs) with the form M (BH 4 ) x has been shown to lower their decomposition temperatures with M of low electronegativity (χ p 1.6), but raise it for high-χ p MBs (χ p 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-χ p MBs. For high-χ p MBs, we find that ammoniation blocks B 2 H 6 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 χ 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 materials release either H 2 or NH 3 gas upon decomposition. We find that NH 3 is much more strongly coordinated with higher-χ 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.

Decomposition Mechanism of Zinc Ammine Borohydride: A First-Principles Calculation

“…Thus, the Zn cation assists the formation of with NH 3 . E b represents the calculated energy barrier.A previous study suggests that B 2 H 7 would combine with NH 3 to form NH 3 BH 3 and BH 4 in gas phase 37. It would be interesting to examine whether or not this process can occur…”

A comparison study of decomposition mechanisms of single-cation and double-cations (Li, Al) ammine borohydrides

“…In contrast, NH3 is strongly coordinated with high electronegativity metal cations (>1.6), resulting in direct H2 release in these materials. [27] Although this study has provided valuable insight of the decomposition processes of single metal cation AMBs, the results may not directly apply to double cations AMBs. For instance, the incorporation of LiBH4 (with low electronegativity of 0.98 for Li cation) into…”

“…[16,[23][24][25] The electronic structure and decomposition mechanisms of single metal cation AMBs have been studied by Density functional theory (DFT) calculation. [21,[25][26][27] Recent theoretic study indicates that the ammonia is weakly bound to the metal cations with low electronegativity (<1. 2) in AMBs, therefore tend to release ammonia at low temperature.…”

Synthesis, Crystal Structures and Thermal Properties of Ammine Barium Borohydrides

“…Alternatively, the ammine-stabilized metal borohydrides can be formed by ball milling of the ammine metal chloride and LiBH 4 , as shown for M(BH 4 ) 3 •5NH 3 (M = Ti 3+ , V 3+ ) and Ti(BH 4 ) 3 •3NH 3 [35,36]. Theoretical calculations have revealed that the metal borohydrides are always thermodynamically destabilized by ammonia, but in the case of the unstable metal borohydrides with a high Pauling electronegativity of the metal (χ P ≥ 1.6), the compounds are kinetically stabilized, as ammonia protects the metal from reduction and thereby alters the decomposition pathway and suppresses the release of B 2 H 6 [37].…”