Thermal Stability of Li₂B₁₂H₁₂and its Role in the Decomposition of LiBH₄

Abstract

The purpose of this study is to compare the thermal and structural stability of single phase Li2B12H12 with the decomposition process of LiBH4. We have utilized differential thermal analysis/thermogravimetry (DTA/TGA) and temperature programmed desorption-mass spectroscopy (TPD-MS) in combination with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy to study the decomposition products of both LiBH4 and Li2B12H12 up to 600 °C, under both vacuum and hydrogen (H2) backpressure. We have synthesized highly pure single phase crystalline anhydrous Li2B12H12 (Pa-3 structure type) and studied its sensitivity to water and the process of deliquescence. Under either vacuum or H2 backpressure, after 250 °C, anhydrous Li2B12H12 begins to decompose to a substoichiometric Li2B12H12-x composition, which displays a very broad diffraction halo in the d-spacing range 5.85-7.00 Å, dependent on the amount of H released. Aging Pa-3 Li2B12H12 under 450 °C/125 bar H2 pressure for 24 h produces a previously unobserved well-crystallized β-Li2B12H12 polymorph, and a nanocrystalline γ-Li2B12H12 polymorph. The isothermal release of hydrogen pressure from LiBH4 along the plateau and above the melting point (Tm = 280 °C) initially results in the formation of LiH and γ-Li2B12H12. The γ-Li2B12H12 polymorph then decomposes to a substoichiometric Li2B12H(12-x) composition. The Pa-3 Li2B12H12 phase is not observed during LiBH4 decomposition. Decomposition of LiBH4 under vacuum to 600 °C produces LiH and amorphous B with some Li dissolved within it. The lack of an obvious B-Li-B or B-H-B bridging band in the FTIR data for Li2B12H(12-x) suggests the H poor B12H(12-x) pseudo-icosahedra remain isolated and are not polymerized. Li2B12H(12-x) is persistent to at least 600 °C under vacuum, with no LiH formation observable and only a ca. d = 7.00 Å halo remaining. By 650 °C, Li2B12H(12-x) is finally decomposed, and amorphous B can be observed, with no LiH reflections. Further studies are required to clarify the structural symmetry of the β- and γ-Li2B12H12 polymorphs and substoichiometric Li2B12H(12-x).

Hydrogen Sorption in Erbium Borohydride Composite Mixtures with LiBH4 and/or LiH

“…One of the most promising candidates for solid state hydrogen storage applications is LiBH 4 , which has a theoretical capacity of 18.5 wt % H 2 . The practical hydrogen amount that can be desorbed is 13.8 wt %, due to the formation of very stable LiH during decomposition, but LiBH 4 is still among the materials with the highest hydrogen capacity considered for solid state storage [1,2].…”

“…The practical hydrogen amount that can be desorbed is 13.8 wt %, due to the formation of very stable LiH during decomposition, but LiBH 4 is still among the materials with the highest hydrogen capacity considered for solid state storage [1,2]. However, it requires very tough conditions for rehydrogenation [3] and suffers from capacity loss on cycling due to the formation of higher boranes [4]. Another class of materials to be considered is rare earth (RE) borohydrides, with hydrogen capacities varying between 9.0 wt % for Y(BH 4 ) 3 and 5.5 wt % for Yb(BH 4 ) 3 [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19].…”

“…Gennari et al investigated the composite mixtures of 6LiBH 4 and RECl 3 with RE = Ce, Gd and found a decrease in decomposition temperature of LiBH 4 due to an in situ formed REH 2 phase [17]. Additionally, the decomposed composite showed the ability to reabsorb 20% of the initial hydrogen content.…”

“…Additionally, the decomposed composite showed the ability to reabsorb 20% of the initial hydrogen content. Frommen et al investigated the composite mixtures of LiBH 4 , LiH and RE(BH 4 ) 3 with RE = La, Er [27]. It was reported that the desorption temperature of LiBH 4 in the mixture with RE = Er decreased by about 100 • C compared to pure LiBH 4 .…”

“…Frommen et al investigated the composite mixtures of LiBH 4 , LiH and RE(BH 4 ) 3 with RE = La, Er [27]. It was reported that the desorption temperature of LiBH 4 in the mixture with RE = Er decreased by about 100 • C compared to pure LiBH 4 . Reabsorption at 340 • C under 100 bar H 2 yielded a hydrogen uptake of 66% after desorption against vacuum, and 80% after desorption against 5 bar H 2 , respectively, of the initially release of 3 wt % H 2 .…”