Confinement of MgH_2 Nanoclusters within nanoporous aerogel scaffold materials

摘要

Nanoparticles of magnesium hydride were embedded in nanoporous carbon aerogel scaffold materials in order to explore the kinetic properties of hydrogen uptake and release. A new modified procedure for the synthesis of magnesium hydride nanoparticles is presented. The procedure makes use of monoliths (～0.4 cm~3) of two distinct types of nanoporous resorcinol-formaldehyde carbon aerogels loaded with dibutylmagnesium, MgBu _2. Excess MgBu_2 was removed mechanically, and the increase in mass was used as a measure of the amount of embedded MgH_2. Energy-dispersive spectrometry revealed that MgH_2 was uniformly distributed within the aerogel material. In situ synchrotron radiation powder X-ray diffraction showed that MgBu_2transformed directly to MgH _2 at T ～ 137 °C and p(H_2) = 50 bar. Two distinct aerogel samples, denoted X1 and X2, with pore volumes of 1.27 and 0.65 mL/g and average pore sizes of 22 and 7 nm, respectively, were selected. In these samples, the uptake of magnesium hydride was found to be proportional to the pore volume, and aerogels X1 and X2 incorporated 18.2 and 10.0 wt % of MgH _2, respectively. For the two samples, the volumetric MgH _2uptake was similar, ～12 vol %. The hydrogen storage properties of nanoconfined MgH_2 were studied by Sieverts' measurements and thermal desorption spectroscopy, which clearly demonstrated that the dehydrogenation kinetics of the confined hydride depends on the pore size distribution of the scaffold material; that is, smaller pores mediated faster desorption rates possibly due to a size reduction of the confined magnesium hydride.