Effects of Nano-confinement on Hydrogen Dynamics in TiCl3-Doped NaAlH4 Studied using Quasi-Elastic Neutron Scattering (QENS).

Dynamics of hydrogen in 4 mol {\%} TiCl3-doped NaAlH4 nanoconfined within the mesoporous SBA15 was studied using QENS. Results show a higher fraction of hydrogen is mobile within these nanoconfined systems relative to bulk TiCl3-doped NaAlH4 but with overall localized motion. Nanostructured Ti-doped NaAlH4 exhibited poorer long-range diffusion kinetics than bulk materials of the same composition. Around 15{\%} of hydrogen participated in long-range motion at 400K (relative to 5{\%} in bulk). Around 65{\%} to 75{\%} of all available hydrogen atoms are mobile at 400K in localized dynamics described by rotation of AlH$_{\mathrm{x}}$ units. Less than 10{\%} hydrogen was mobile at higher temperatures of 420K and 450K, possibly because of the formation of Si-Al phases. Dynamic models indicate that nanostructured samples demonstrated slightly shorter jump lengths (2.09 {\AA} to 2.16 {\AA}) relative to bulk Ti-doped NaAlH4 (2.27 {\AA}).