Hydrogen Storage Capacity Loss in a LiBH₄–Al Composite

Abstract

A detailed investigation of the decomposition reactions and decay in the hydrogen storage capacity during repeated hydrogen release and uptake cycles for the reactive composite LiBH4–Al (2:3) is presented. Furthermore, the influence of a titanium boride, TiB2, additive is investigated. The study combines information from multiple techniques: in situ synchrotron radiation powder X-ray diffraction, Sieverts measurements, simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy, solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), and Raman spectroscopy. The decomposition of LiBH4–Al results in the formation of LiAl, AlB2, and Li2B12H12 via several reactions and intermediate compounds. The TiB2 additive appears to have a limited effect on the decomposition pathway of the samples, but seems to facilitate formation of intermediate species at lower temperatures compared to the sample without additive. Solid solutions of Li x Al1–x B2 or Al1–x B2 are observed during decomposition and from Rietveld refinement the composition of the solid solution is estimated to be Li0.22Al0.78B2. The intercalation of Li in the AlB2 structure is further investigated by 11B and 27Al MAS NMR spectra of the LiH-AlB2 and AlB2 samples (presented in Supporting Information). Hydrogen release and uptake for LiBH4–Al reveals a significant loss in the hydrogen storage capacity, that is, after four cycles a capacity of about 45% remains, and after 10 cycles, the capacity is degraded to approximately 15% of the theoretically available hydrogen content. This capacity loss may be due to the formation of Li2B12H12, as observed by 11B MAS NMR and Raman spectroscopy. Formation of Li2B12H12 has previously been observed during the decomposition of LiBH4, but it has not been reported earlier in the LiBH4–Al (2:3) system.

Dehydrogenation of Surface-Oxidized Mixtures of 2LiBH4 + Al/Additives (TiF3 or CeO2)

“…XPS suggests the formation of LiBO 2 rather than Li 2 O on the surface of our samples and the segregation of Al to sub-surface layers. As a result of the surface oxidation, a reduction of the dehydrogenation temperature was observed here and compared with similar materials carefully protected against surface oxidation [17][18][19][20][21][22]. A reduction of the dehydrogenation activation barrier was proposed by Kang et al [35] if LiBH 4 or the intermediate LiBH donate one electron (each), to a catalyst on their surfaces.…”

“…The multistep nature of the dehydrogenation reaction of the 2LiBH 4 + Al is shared with the dehydrogenation of LiBH 4 [36] and the RHC LiBH 4 + MgH 2 [22,37]. Reports of LiBH 4 -Al dehydrogenation in several molar proportions also described a multistep mechanism for the dehydrogenation reaction [17,[19][20][21]38]. In the materials presented here, the first dehydrogenation step occurred at low temperature, i.e., 100-110 • C. And the main dehydrogenation step occurred between 200-300 • C, finishing at 400 • C. The main dehydrogenation temperature interval is close to the temperature predicted by Siegel et al for the dehydrogenation reaction of 2LiBH 4 + Al, 277 • C [18].…”

“…To reduce the dehydrogenation temperature, improve reaction kinetics or reversibility, LiBH 4 has been mixed with several compounds in different proportions. The list includes but is not limited to other borohydrides such as: Ca(BH 4 ) 2 [2], NaBH 4 [3], or Mg(BH 4 ) 2 [4], other complex hydrides such as LiNH 2 [5], alanates of Li or Na [6,7], binary hydrides such as MgH 2 [8][9][10], CaH 2 [11][12][13], TiH 2 [12], halide salts such as LiCl [14], oxides [15], scaffolds [16], and metals such as Mg, Ti, V, Cr, Sc, or Al [12,17]. The main characteristics of these mixtures are collated in Table 1.…”

Complex and liquid hydrides for energy storage

“…A detailed investigation of the decomposition reactions and decay in the hydrogen storage capacity during repeated hydrogen release and uptake up to ten cycles for the reactive composite LiBH 4 -Al (2:3) is conducted with and without the additive titanium boride, TiB 2 [201,202] [14] and Raman spectroscopy [202].…”

Hydrogen Desorption Properties of Bulk and Nanoconfined LiBH4-NaAlH4

“…One way to alter kinetic properties for hydrogen release of reactive composites of LiBH 4 is by additives using transition metals such as Ti, V, Cr or Sc [8][9][10][11]. Other metals like Al and Mg have also shown significant destabilization abilities as a reactive hydride composite (RHC) with LiBH 4 , by reducing the enthalpy of reversible dehydrogenation and rehydrogenation reactions [12][13][14]. Al nanoparticles have also been considered as an Al-source but the properties are generally inhibited due to an oxide layer, which limits the reactivity [10].…”