A first-principles study of Sc-doped NaAlH4 for reversible hydrogen storage

Density function theory calculations have been carried out to investigate energetics, structure, and NMR chemical shielding of Sc-doped NaAlH4 (0 0 1) surface. Sc atom was found to occupy the interstitial site and interact directly with three immediate neighboring AlH4− groups and indirectly with two additional AlH4− groups, forming an extended complex structure of ScAl3H12–2AlH4. In the ScAl3H12–2AlH4 structure, the Al–Al distances between the Al atom in ScAl3H12 and the Al in the neighboring AlH4− are 3.154 Å. Hydrogen desorption energies from many positions of the complex were reduced significantly from that of the clean surface. This extended structure is more efficient for dehydrogenating NaAlH4 than the similar TiAl3H12 complex that was identified in the Ti-doped NaAlH4 surfaces. Furthermore, our calculated NMR shielding of Sc-doped NaAlH4 shows that the Al atoms in direct contact with Sc have very different shielding values from the Al atoms in the center of H tetrahedra. These result may provide an explanation for the 27Al MAS NMR chemical shifts observed experimentally.