Preparation of LiBH4@carbon micro–macrocellular foams: tuning hydrogen release through varying microporosity

Abstract

Microporous–macroporous carbononaceous monolith-type materials, prepared through a hard template method using silica as exo-templating matrices, have been impregnated with an etheric solution of LiBH₄ to prepare LiBH₄@carbon samples. It has been shown that the amorphous character of LiBH₄ is largely favoured when developing the carbon microporosity (pores smaller than 2 nm) and that, as a consequence, the LiBH₄ dehydrogenation is strongly enhanced at low temperatures. The onset temperature of dehydrogenation can be decreased to 200 °C and hydrogen capacity reaching 4.0 wt% is obtained at 300 °C with the carbon having the largest microporous volume, whereas the hydrogen release for bulk LiBH₄ is negligible at the same temperature. It is suggested both from DSC and from pressure values reached upon hydrogen release that the enthalpy of the dehydrogenation reaction is strongly modified. In addition to some irreversible reactions with carbon surface groups (and even with the carbon matrix itself according to ¹¹B MAS NMR spectroscopy, which reveals the formation of B–C bonds), the explanation for such modification could lie in the LiBH₄ destabilization through confinement to the nanoscale range and associated amorphization. The above feature, where LiBH₄ crystalline character is tuned through the imposed microporosity, can be considered as a highly promising approach to control the hydrogen release temperature of complex hydrides.