Hydrogen sorption and reaction mechanisms of nanoconfined 2LiBH4-NaAlH4

Nanoconfinement of 2LiBH4-NaAlH4 in carbon aerogel scaffold (CAS) via direct melt infiltration is proposed for reversible hydrogen storage. Weight ratios of CAS:2LiBH4-NaAlH4 (2:1, 2:1.5, and 1:1) and melt infiltration temperatures at 185 and 310 °C are varied to obtain the most suitable condition for nanoconfinement. It is found that the sample melt infiltrated at 310 °C under CAS:2LiBH4-NaAlH4 weight ratio of 2:1, denoted as s1@310 offers the best results based on effective nanoconfinement, high hydrogen content released, and fast dehydrogenation kinetics. Further studies on reversibility shows that up to 89% and 74% H2 with respect to theoretical storage capacity release from s1@310 during the 1st and 2nd dehydrogenations (T = 400 °C under vacuum), respectively, while those of milled 2LiBH4-NaAlH4 are only 57% and 32%, respectively. The reaction between LiBH4 and NaAlH4 during nanoconfinement produces LiAlH4 and NaBH4. During dehydrogenation, an intermediate phase (LiNa2AlH6) is formed and it decomposes in the temperature range of 203-350 °C. Moreover, significant reduction in decomposition temperature of NaBH4 of s1@310 with respect to those of milled 2LiBH4-NaAlH4 and pristine NaBH4 (ΔT = 137 and 197 °C, respectively) is accomplished. Reversibility of NaBH4, LiAlH4, and LiBH4 are achieved together with unreacted LiH and metallic Al (T = 400 °C, p(H2) = 80 bar for 12 h). Although irreversibility of metallic Al leads to deficient content of hydrogen desorbed in the 2nd cycle, greater amount of hydrogen reproduced of s1@310 with respect to milled sample is obtained. This is due to the recovery of NaBH4 after rehydrogenation of s1@310, whereas that of milled 2LiBH4-NaAlH4 was not possible due to the evaporation of metallic Na during dehydrogenation.