Confinement of NaAlH4in Nanoporous Carbon: Impact on H2Release, Reversibility, and Thermodynamics

Metal hydrides are likely candidates for the solid state storage of hydrogen. NaAlH 4 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 poor reversibility of the hydrogen desorption can be combatted by the addition of a Ti-based catalyst. In an alternative approach we studied the influence of a reduced NaAlH 4 particle size and the presence of a carbon support. We focused on NaAlH 4 /porous carbon nanocomposites prepared by melt infiltration. The NaAlH 4 was confined in the mainly 2−3 nm pores of the carbon, resulting in a lack of long-range order in the NaAlH 4 structure. The hydrogen release profile was modified by contact with the carbon; even for ∼10 nm NaAlH 4 on a nonporous carbon material the decomposition of NaAlH 4 to NaH, Al, and H 2 now led to hydrogen release in a single step. This was a kinetic effect, with the temperature at which the hydrogen was released depending on the NaAlH 4 feature size. However, confinement in a nanoporous carbon material was essential to not only achieve low H 2 release temperatures, but also rehydrogenation at mild conditions (e.g., 24 bar H 2 at 150 °C). Not only had the kinetics of hydrogen sorption improved, but the thermodynamics had also changed. When hydrogenating at conditions at which Na 3 AlH 6 would be expected to be the stable phase (e.g., 40 bar H 2 at 160 °C), instead nanoconfined NaAlH 4 was formed, indicating a shift of the NaAlH 4 ↔Na 3 AlH 6 thermodynamic equilibrium in these nanocomposites compared to bulk materials.