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

Abstract

© 2017 American Chemical Society. The hydrogen storage properties of eutectic melting 0.68LiBH 4 -0.32Ca(BH 4 ) 2 (LiCa) as bulk and nanoconfined into a high surface area, S BET = 2421 ± 189 m 2 /g, carbon aerogel scaffold, with an average pore size of 13 nm and pore volume of V 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 (?/? = 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 H 2 release rate temperature, T max , between bulk and nanoconfined LiCa during the second cycle is ? max ˜ 40 °C, which over five cycles becomes smaller, ? max ˜ 10 °C. The high temperature, T 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 LiBH 4 has a strong effect in only the first cycle of H 2 release. The hydrogen storage capacity is stable for bulk and nanoconfined LiCa in the second to the fifth cycle, which contrasts to nanoconfined LiBH 4 where the H 2 storage capacity continuously decreases. Bulk and nanoconfined LiCa have hydrogen storage capacities of 5.4 and 3.7 wt % H 2 in the fifth H 2 release, which compare well with the calculated hydrogen contents of LiBH 4 only and in LiCa, which are 5.43 and 3.69 wt % H 2 , respectively. Thus, decomposition products of Ca(BH 4 ) 2 appear to facilitate the full reversibility of the LiBH 4 , and this approach may lead to new hydrogen storage systems with stable energy storage capacity over multiple cycles of hydrogen release and uptake. (Graph Presented).