Reversibility of LiBH₄ Facilitated by the LiBH₄–Ca(BH₄)₂ Eutectic

The hydrogen storage properties of eutectic melting 0.68LiBH₄–0.32Ca­(BH₄)₂ (LiCa) as bulk and nanoconfined into a high surface area, <i>S</i>_BET = 2421 ± 189 m²/g, carbon aerogel scaffold, with an average pore size of 13 nm and pore volume of <i>V</i>_tot = 2.46 ± 0.46 mL/g, is investigated. Hydrogen desorption and absorption data were collected in the temperature range of RT to 500 °C (Δ<i>T</i>/Δ<i>t</i> = 5 °C/min) with the temperature then kept constant at 500 °C for 10 h at hydrogen pressures in the range of 1–8 and 134–144 bar, respectively. The difference in the maximum H₂ release rate temperature, <i>T</i>_max, between bulk and nanoconfined LiCa during the second cycle is Δ<i>T</i>_max ≈ 40 °C, which over five cycles becomes smaller, Δ<i>T</i>_max ≈ 10 °C. The high temperature, <i>T</i>_max ≈ 455 °C, explains the need for high temperatures for rehydrogenation in order to obtain sufficiently fast reaction kinetics. This work also reveals that nanoconfinement has little effect on the later cycles and that nanoconfinement of pure LiBH₄ has a strong effect in only the first cycle of H₂ release. The hydrogen storage capacity is stable for bulk and nanoconfined LiCa in the second to the fifth cycle, which contrasts to nanoconfined LiBH₄ where the H₂ storage capacity continuously decreases. Bulk and nanoconfined LiCa have hydrogen storage capacities of 5.4 and 3.7 wt % H₂ in the fifth H₂ release, which compare well with the calculated hydrogen contents of LiBH₄ only and in LiCa, which are 5.43 and 3.69 wt % H₂, respectively. Thus, decomposition products of Ca­(BH₄)₂ appear to facilitate the full reversibility of the LiBH₄, and this approach may lead to new hydrogen storage systems with stable energy storage capacity over multiple cycles of hydrogen release and uptake

Li₅(BH₄)₃NH: Lithium-Rich Mixed Anion Complex Hydride

The Li₅(BH₄)₃NH complex hydride, obtained by ball milling LiBH₄ and Li₂NH in various molar ratios, has been investigated. Using X-ray powder diffraction analysis the crystalline phase has been indexed with an orthorhombic unit cell with lattice parameters <i>a</i> = 10.2031(3), <i>b</i> = 11.5005(2), and <i>c</i> = 7.0474(2) Å at 77 °C. The crystal structure of Li₅(BH₄)₃NH has been solved in space group <i>Pnma</i>, and refined coupling density functional theory (DFT) and synchrotron radiation X-ray powder diffraction data have been obtained for a 3LiBH₄:2Li₂NH ball-milled and annealed sample. Solid-state nuclear magnetic resonance measurements confirmed the chemical shifts calculated by DFT from the solved structure. The DFT calculations confirmed the ionic character of this lithium-rich compound. Each Li⁺ cation is coordinated by three BH₄^– and one NH^2– anion in a tetrahedral configuration. The room-temperature ionic conductivity of the new orthorhombic compound is close to10^–6 S/cm at room temperature, with activation energy of 0.73 eV

Design of a Nanometric AlTi Additive for MgB₂‑Based Reactive Hydride Composites with Superior Kinetic Properties

Solid-state hydride compounds are a promising option for efficient and safe hydrogen-storage systems. Lithium reactive hydride composite system 2LiBH₄ + MgH₂/2LiH + MgB₂ (Li-RHC) has been widely investigated owing to its high theoretical hydrogen-storage capacity and low calculated reaction enthalpy (11.5 wt % H₂ and 45.9 kJ/mol H₂). In this paper, a thorough investigation into the effect of the formation of nano-TiAl alloys on the hydrogen-storage properties of Li-RHC is presented. The additive 3TiCl₃·AlCl₃ is used as the nanoparticle precursor. For the investigated temperatures and hydrogen pressures, the addition of ∼5 wt % 3TiCl₃·AlCl₃ leads to hydrogenation/dehydrogenation times of only 30 min and a reversible hydrogen-storage capacity of 9.5 wt %. The material containing 3TiCl₃·AlCl₃ possesses superior hydrogen-storage properties in terms of rates and a stable hydrogen capacity during several hydrogenation/dehydrogenation cycles. These enhancements are attributed to an in situ nanostructure and a hexagonal AlTi₃ phase observed by high-resolution transmission electron microscopy. This phase acts in a 2-fold manner, first promoting the nucleation of MgB₂ upon dehydrogenation and second suppressing the formation of Li₂B₁₂H₁₂ upon hydrogenation/dehydrogenation cycling