Improvement of the kinetics of hydrogen release from ammonia borane confined in silica aerogel

Abstract

Ammonia borane is a promising hydrogen storage material due to its high gravimetric capacity (19.6 % wt), but it also presents limitations such as a slow hydrogen release with a long induction time, a difficult regeneration, or the formation of foams and gaseous by-products during thermolysis. Previous studies have shown that by nanoconfinement of ammonia borane within a porous support some of these limitations can be overcome due to the reduction and stabilization of ammonia borane particle size. However, this effect was only observed with moderate ammonia borane loadings, as with higher loadings the pores of the support became obstructed. In this work, silica aerogels produced by CO2 drying, with pore volumes up to 2 cm3/g, have been used to confine ammonia borane. The influence of the amount of ammonia borane loaded on the aerogel support on the thermal and structural properties of the material has been analyzed. It has been found that more than 60 wt% of ammonia borane can be effectively stored in the pores of the aerogel support. The resulting material shows faster hydrogen release kinetics by thermolysis at 80ºC, due to a significant reduction in the mea size of ammonia borane after confinement and the participation of SiOH and SiOSi groups of silica aerogel in the decomposition mechanism.