Quasi-Elastic Neutron Scattering (QENS) Studies of Hydrogen Dynamics for Nano-Confined NaAlH4

The hydrogen dynamics of nano-confined sodium alanate (NaAlH4) has been studied using quasi-elastic neutron scattering (QENS). Results indicate thermodynamic destabilization is responsible for reduced desorption temperatures of NaAlH4 upon confinement within the nanopores of a metal organic framework (MOF). Both the bulk (microscale) NaAlH4 and the nanoconfined NaAlH4 data were fitted to re-orientation models which yielded corresponding percent mobile hydrogen and jump lengths. The jump lengths calculated from the nano-NaAlH4 were $\approx $2.5 {\AA}, and in conformity with those jump lengths determined for bulk NaAlH4 of $\approx $2.3 {\AA}. As much as 18 {\%} of the hydrogen atoms were estimated to be mobile in the nano-NaAlH4 sample even at relatively low temperatures of 350 K. In contrast, bulk NaAlH4 shows less than 7 {\%} mobile H-atoms even at higher temperatures of $\approx $450 K. The activation energy for the long range is 3.1meV.