A novel aided-cation strategy to advance the dehydrogenation of calcium borohydride monoammoniate

Abstract

The crystal structure of a promising hydrogen storage material, calcium borohydride monoammoniate (Ca(BH 4 ) 2 $NH 3 ), is reported. Structural analysis revealed that this compound crystallizes in an orthorhombic structure (space group Pna2 1 ) with unit-cell parameters of a ¼ 8.4270A, b ¼ 12.0103 A, c ¼ 5.6922A and V ¼ 576.1121 A 3 , in which the Ca atom centrally resides in a slightly distorted octahedral environment furnished by five B atoms from BH 4 units and one N atom from the NH 3 unit. As Ca(BH 4 ) 2 $NH 3 tends to release ammonia rather than hydrogen when heated in argon, a novel aided-cation strategy via combining this compound with LiBH 4 was employed to advance its dehydrogenation. It shows that the interaction of the two potential hydrogen storage substances upon heating, based on a promoted recombination reaction of BH and NH groups, enables a significant mutual dehydrogenation improvement beyond them alone, resulting in more than 12 wt% high-pure H 2 (>99%) released below 250 C. The synergetic effect of associating the dihydrogen reaction with mutually aided-metal cations on optimizing the dehydrogenation of this kind of composites may serve as an alternative strategy for developing and expanding the future B-N-H systems with superior and tuneable dehydrogenation properties.

Ammine Calcium and Strontium Borohydrides: Syntheses, Structures, and Properties

“…[15] The structure is composed of hcp octahedra layersi nt he bc plane (Figure 3a)s tacked along the a axis in the order AAA, which are connected by dihydrogen bonds between H dÀ from BH 4 À and H d+ + from NH 3 ( Figure 3b). The CaÀNb ond lengths are 2.52 , which is similar to 2.50 i nC a(NH 3 )(BH 4 ) 2 , [21] and the CaÀBd istances of 2.83 a re within the range of 2.82-2.97 in aCa(BH 4 ) 2 .…”

“…Pbcn [20] Pna2 1 [21] Mg(NH 3 ) n Cl 2 -Fm3 m [17] Cmmm [47] unknown Mg(NH 3 ) n (BH 4 ) 2 -Fm3 m [17] Pcab [17] unknown…”

Tailoring the Properties of Ammine Metal Borohydrides for Solid‐State Hydrogen Storage

“…For the ammine compounds based on the more stable metal borohydrides such as LiBH 4 or Ca(BH 4 ) 2 , ammonia is released at~120 and 140 8C, respectively, from open systems, that is, p(NH 3 )~0, even though the n/m ratio is 'optimal' (i.e., n/m 1) and the crystal structures contain strong dihydrogen bonds. [20,55] On the other hand, hydrogen is released in closed systems, p(NH 3 ) > 0, during thermolysis of Li(NH 3 ) 4/3 BH 4 and Ca(BH 4 ) 2 ·2NH 3 , which in fact suggests that hydrogen is released in a solid-gas dihydrogen elimination reaction between the metal borohydride and ammonia gas. Metal borohydrides with low electronegativity of the metals are destabilized by ammonia, while metal borohydrides with high electronegativity of the metals are stabilized by ammonia.…”

“…[15] Since then, a range of novel ammine metal borohydrides have been examined, [17][18][19][20] and one of the most successful is Zn(BH 4 ) 2 ·2NH 3 , which releases 8.9 wt % of pure H 2 below 115 8C within 15 min. [21] Remarkably, Zn(BH 4 ) 2 is not stable on its own and can be stabilized by less electronegative atoms in bimetallic borohydrides.…”

NH3BH3/LiBH4·NH3 modified by metal hydrides for advanced dehydrogenation

“…(b) LiHeAB/LiBH 4 $NH 3 , 2LiHe2AB/LiBH 4 $NH 3 and 3LiHe3AB/ LiBH 4 $NH 3 $dehydrogenated to 250 C. FTIR spectra of LiBH 4 after heating to 250 C were also measured for comparison. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 8 1 0 1 e1 8 1 0 7 attribute to the combination of H dþ and H dÀ , leading to the formation of BeN bonds [36]. Upon full dehydrogenation, the BeH stretching modes in the dehydrogenation products appear at the frequencies of 2218, 2286, and 2380 cm À1 , and the BH 2 deformation bands are assigned to the absorptions at 1120 cm À1 , which is well consistent with the BH 4 À anion [37].…”