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