Single-walled carbon nanotubes/lithium borohydride composites for hydrogen storage: role of in situ formed LiB(OH)4, Li2CO3 and LiBO2 by oxidation and nitrogen annealing

Lithium Borohydride (LiBH4), from the family of complex hydrides has received much attention as a potential hydrogen storage material due to its high hydrogen energy densities in terms of weight (18.5 wt%) and volume (121 kg H-2 per mol). However, utilization of LiBH4 as a hydrogen carrier in off- or on-board applications is hindered by its unfavorable thermodynamics and low stability in air. In this study, we have synthesized an air stable SWCNT@LiBH4 composite using a facile ultrasonication assisted impregnation method followed by oxidation at 300 degrees C under ambient conditions (SWLiB-A). Further, part of the oxidized sample is treated at 500 degrees C under nitrogen atmosphere (SWLiB-N). Upon oxidation in air, the in situ formation of lithium borate hydroxide (LiB(OH)(4)) and lithium carbonate (Li2CO3) on the surface of the composite (SWLiB@LiBH4) is observed. But in the case of SWLiB-N, the surface hydroxyl groups [OH4](-) completely vanished leaving porous LiBH4 with SWCNT, LiBO2 and Li2CO3 phases. Hydrogen adsorption/desorption experiments carried out at 100 degrees C under 5 bar H-2 pressure showed the highest hydrogen adsorption capacity of 4.0 wt% for SWLiB-A and 4.3 wt% for SWLiB-N composites in the desorption temperature range of 153-368 degrees C and 108-433 degrees C respectively. The observed storage capacity of SWLiB-A is due to the H+ and H- coupling between in situ formed Li+[B(OH)(4)](-), Li2+[CO3](-) and Li+[BH4](-). Whereas in SWLiB-N, the presence of positively charged Li and B atoms and LiBO2 acts as a catalyst which resulted in reduced de-hydrogenation temperature (108 degrees C) as compared to bulk LiBH4. Moreover, it is inferred that the formation of intermediate phases such as Li+[B(OH)(4)](-), Li2+[CO3](-) (SWLiB-A) and Li+[BO2](-) (SWLiB-N) on the surface of the composites not only stabilizes the composite under ambient conditions but also resulted in enhanced de- and re-hydrogenation kinetics through catalytic effects. Further, these intermediates also act as a barrier for the loss of boron and lithium through diborane release from the composites upon dehydrogenation. Furthermore, the role of in situ formed intermediates such as LiB(OH)(4), Li2CO3 and LiBO2 on the stability of the composite under ambient conditions and the hydrogen storage properties of the SWCNT@LiBH4 composite are reported for the first time.