The application of hydrogen as an energy vector requires the development of new hydrogen storage systems. Magnesium borohydride Mg(BH4)2 is a promising hydrogen storage material because of its high hydrogen storage capacity (14,8wt% H2, 0.112 Kg/L). However, it is still limited by slow hydrogen release kinetics and by the harsh conditions required for reversible hydrogen sorption due to the formation of stable intermediates.
Promising composites were made of commercial Mg(BH4)2 and silica aerogel microparticles by thermal treatment in hydrogen under 120 bar and 200ºC. Silica aerogel particles with pore volume up to 2cm3/g were prepared in High pressure lab at University of Valladolid, and the composites were made at H2 lab in Pavia, Italy. http://h2lab.unipv.it/Home
As a result, the sorption properties of the hydride were significantly improved due to the destabilization of the hydride by silica:
- Calorimetric measurements showed that decomposition temperature of the hydride was reduced by 60ºC in the composite
- The typical 3-step decomposition mechanism of Mg(BH4)2 changed 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 H2 was presented for the first time, achieving cyclability with a reversible release of hydrogen of 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 indicated that silica aerogel chemically interacted with Mg(BH4)2, acting as an additive, which can result in different hydrogenation-dehydrogenation routes, enhancing the kinetics and the cyclability.
Miriam Rueda- Project LIGHT HYDROGEN STORAGE MATERIALS FOR MOBILE APPLICATIONS, 18ZSEW99