Confinement of NaAIH4 in Nanoporous Carbon: Impact on H2 Release, Reversibility, and Thermodynamics

Metal hydrides are likely candidates for the solid state storage of hydrogen. NaAlH4 is the only complex metal hydride identified so far that combines favorable thermodynamics with a reasonable hydrogen storage capacity (5.5 wt %) when decomposing in two steps to NaH, Al, and H-2. The slow kinetics and pool-reversibility of the hydrogen desorption can be combatted by the addition of a Ti-based catalyst. the an alternative approach we studied the influence of a reduced NaAlH4 particle size and the presence of a carbon SUpport. We focused oil NaAlH4/porous carbon nanocomposites prepared by melt infiltration. The NaAlH4 was confined in the mainly 2-3 nm pores of the carbon, resulting in a lack of long-range order in the NaAlH4 structure. The hydrogen release profile was modified by contact with the carbon; even for similar to 10 nm NaAlH4 on a nonporous carbon material the decomposition of NaAlH4 to NaH, Al, and H, now led to hydrogen release in a single step. This was a kinetic effect, with the ternperature at which the hydrogen was released depending oil the NaAlH4 feature size. However, confinement a nanoporous carbon material was essential to not Only achieve low H, release temperatures, but also rehydrogenation at mild conditions (e.g., 24 bar H, at 150 degrees C). Not only had the kinetics of hydrogen sorption improved, but the thermodynamics had also changed. When hydrogenating at conditions at which Na3AlH6 would be expected to be the stable phase (e.g., 40 bar H, at 160 degrees C), instead nanoconfined NaAlH4,as formed, indicating a shift of the NaAlH4 <-> Na(3)AH(6)(, thermodynamic equilibrium ill these nanocomposites compared to bulk materials.