Estimation of Thermodynamic Properties of Metal Hydroborates

Abstract

Metal hydroborates from M(BH 4 ) n to M x (B 12 H 12 ) y currently attract a strong interest for potential applications in the field of hydrogen storage and more recently as solid ionic conductors. While thermodynamic data for the alkali borohydrides (MBH 4 ) are well known, experimental data for other compounds are cruelly lacking. Using a combination of theoretical (DFT) and experimental data, we calculate the lattice enthalpy for a series of borohydrides. The comparison with literature data show that these lattice enthalpies are very similar to those of the corresponding metal bromides. This similarity is at the origin of a good correlation between the formation enthalpy of bromides and borohydrides which is then used to estimate the previously unknown formation enthalpy of M II (BH 4 ) 2 (M II = divalent cation) and Ln(BH 4 ) 3 (Ln = lanthanide). The Gibbs free energy of formation of M 2 (B 12 H 12 ) (M = alkali metal) is also estimated.[a] Prof.

Thermal Conversion of Unsolvated Mg(B3H8)2 to BH4– in the Presence of MgH2

“…The presence of NaBr and MgBr 2 can potentially play a role, albeit minor, in the decomposition reaction of Mg­(B 3 H 8 ) 2 -MgH 2 . It has been shown that the ionic radii of Br – and BH 4 – are of similar size, making it the formation of mixed salts, such as Mg­(BH 4 ) 2– x Br x , possible during heating of Mg­(B 3 H 8 ) 2 in the presence of NaBr or MgBr 2 . Furthermore, the ionic substitution of halide for BH 4 was shown to modify the stability of metal borohydrides, specifically of NaBH 4 and LiBH 4 . , However, with our experimental data available, it is very difficult to appreciate the extent of this mixed salt formation.…”

“…The unsolvated Mg(B 3 H 8 ) 2 was prepared according to the procedure recently reported by Moury et al 12 The compound was analyzed by 11 B solid-state MAS NMR at 25 °C, and the chemical shifts are shown in Figure 1. The peak assignments and quantification from the − are of similar size, 45 making it the formation of mixed salts, such as Mg(BH 4 ) 2−x Br x , possible during heating of Mg(B 3 H 8 ) 2 in the presence of NaBr or MgBr 2 . Furthermore, the ionic substitution of halide for BH 4 was shown to modify the stability of metal borohydrides, specifically of NaBH 4 and LiBH 4 .…”

“…The new and pertinent result presented here is that MgH 2 , which has been added to keep a constant Mg/B ratio as in Mg­(BH 4 ) 2 , does not catalyze the further decomposition of Mg­(B 3 H 8 ) 2 but rather favors the rehydrogenation reaction and prevents the release of toxic boranes. In this case, the thermodynamic driving force is the formation of Mg­(BH 4 ) 2 , which has a formation enthalpy of Δ f H ° = −208 kJ/mol . This raises the possibility that excess MgH 2 may also favor the rehydrogenation of other compounds of the form MgB x H y .…”

Boron Hydrogen Compounds: Hydrogen Storage and Battery Applications

“…The overall enthalpy of reaction for the dehydrogenation of Mg(BH 4 ) 2 (∆ f H • = −208 kJ/mol) to form MgB 2 (∆ f H • = −91.96 kJ/mol) and hydrogen can be calculated [45][46][47] to be equal to +116 kJ/mol, i.e., less than 30 kJ/mol per hydrogen molecule released, which is, in principle, in the correct range for a hydrogen storage material [13]. The first step of a dehydrogenation reaction of BH 4…”

“…For alkali borohydrides, thermodynamical data are available [47], but only few other experimental data are available. Using the experimental values of the formation enthalpy of Mg(BH 4 ) 2 [45] and La(BH 4 ) 3 [63], the formation enthalpy of other M(BH 4 ) 2 and M(BH 4 ) 3 compounds were estimated, assuming that the lattice enthalpy of bromides and borohydrides with the same metal ion were identical within about 15 kJ/mol [46]. The experimental formation enthalpy of NH 4 B 3 H 8 (−530 ± 33 kJ/mol) [64], (NH 4 ) 2 B 10 H 10 (−359.2 ± 10 kJ/mol) [65], and of guanidinium and other nitrogen-based closo-borates was reported [66].…”