Among the different hydrogen storage alternatives studied, hydrides are some of the most promising materials, but these compounds still present thermodynamic (stability) and kinetic (hydrogen release rate) limitations. The production of hydride micro and nanoparticles can contribute to reduce the kinetic limitations, but the preservation of the structure of the nanoparticles during hydrogen cycling, with the associated physical and chemical changes, still is an issue.
We study the stabilization of hydrogen-storing hydride into mesoporous aerogels produced by supercritical CO2 drying, formed either as monoliths or as microparticles. Aerogels are mesoporous materials, with typical pore sizes of 10 – 100 nm, high surface areas (400 m2/g – 1500 m2/g), low densities (typically 0.25 g/cm3 – 0.5 g/cm3, although densities as low as 0.003 g/cm3 can be achieved) and high porosities (92 – 98%). With pore volumes up to 4 cm3/g, aerogels offer potential hydride loading capacities significantly above those of alternative porous materials. For example, activated carbon shows pore volumes in the range of 1 cm3/g. Moreover, silica and metal oxide aerogels present excellent thermal stability, and the high porosity and surface area of these materials provide favourable mass transfer conditions, with gas diffusion coefficients of the order of 0.1 cm2/s , of similar order of magnitude as bulk gas diffusion coefficients.
Hydride nanoparticles can be physically stabilized by forming and loading them into mesoporous aerogels. The size and size distribution of nanoparticles can be efficiently controlled if the particles are formed inside the pores of the aerogel, since particle growth is restrained by the pore structure of the support. Furthermore, the incorporation of nanoparticles in aerogels can contribute to solve problems of agglomeration and sinterization during hydrogen cycling, because particles are isolated inside the pores of the aerogel. Additionally, the aerogel can be used to support additional compounds, as for example catalysts.