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

The hydrogen storage properties of eutectic melting 0.68LiBH(4)-0.32Ca(BH4)(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 degrees C (Delta T/Delta t = 5 degrees C/min) with the temperature then kept constant at 500 degrees 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 Delta T-max approximate to 40 degrees C, which over five cycles becomes smaller, Delta T-max approximate to 10 degrees C. The high temperature, T-max approximate to 455 degrees 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 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 LiBH4 where the H-2 storage capacity continuously decreases. Bulk and nanoconfined LiCa have hydrogen storage capadties of 5.4 and 3.7 wt % H-2 in the fifth H-2 release, which compare well with the calculated hydrogen contents of LiBH4 only and in LiCa, which are 5.43 and 3.69 wt % H-2, 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.