Functional anion concept: effect of fluorine anion on hydrogen storage of sodium alanate

Doping NaAlH4 with Ti- catalyst has produced a promising hydrogen storage system that can be reversibly operated at moderate temperature conditions. Of the various dopant precursors, TiCl3 was well recognized due to its pronounced catalytic effect on the reversible dehydrogenation processes of sodium aluminium hydrides. Quite recently we experimentally found that TiF3 was even better than TiCl3 in terms of the critical hydrogen storage properties of the doped hydrides, in particular the dehydriding performance at Na3AlH6/NaH + Al step at moderate temperature. We present here the DFT calculation results of the TiF3 or TiCl3 doped Na3AlH6. Our computational studies have demonstrated that F- and Cl- anions differ substantially from each other with regard to the state and function in the doped sodium aluminium hydride. In great contrast to the case of chloride doping where Cl- anion constitutes the dead weight'' NaCl, the fluoride doping results in a substitution of H- by F- anion in the hydride lattice and accordingly, a favorable thermodynamics adjustment. These results well explain the observed dehydriding performance associated with TiF3/TiCl3-doping. More significantly, the coupled computational and experimental efforts allow us to put forward a functional anion'' concept. This renews the current mechanism understanding in the catalytically enhanced sodium alanate.