Reversible hydrogen sorption in the composite made of magnesium borohydride and silica aerogel

Abstract

Magnesium borohydride Mg(BH4)2 is a promising hydrogen storage material as it releases high hydrogen storage capacity at mild desorption temperatures, but it is still limited by slow hydrogen release kinetics and by the harsh conditions required to re-hydrogenate this compound. In this work, composites made of commercial Mg(BH4)2 and synthesized silica aerogel microparticles were prepared by thermal treatment in hydrogen under 120 bar and 200ºC. As a result, the sorption properties of the hydride are improved: calorimetric measurements show that decomposition temperature is reduced by 60ºC, and the typical 3-step decomposition mechanism of Mg(BH4)2 changes to a single-step mechanism in range of 220-400°C. The kinetics of the first dehydrogenation at 300ºC was two times faster in Mg(BH4)2-SiO2 composites than in the case of bulk γ-Mg(BH4)2. Additionally, the re-hydrogenation of this material at comparatively moderate conditions of 390ºC and 110 bar is presented for the first time, achieving cyclability with a reversible release of hydrogen up to 6wt%. Different amounts of hydrogen were exchanged depending on the temperature of desorption (300ºC or 400ºC) and the presence or absence of silica aerogel. This result indicates that silica aerogel chemically interacts with Mg(BH4)2, acting as an additive, which can result in different hydrogenation-dehydrogenation routes in which different amounts and types of intermediates are formed, influencing the kinetics and the cyclability.