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

Abstract

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.