Variation and influence of the local structure around Ti in NaAlH4 doped with a Ti-based precursor

The kinetics and hydrogen storage capacity of Ti-doped sodium alanate prepared by ball milling of an AlxTi((1-x)) nanocomposite as Ti-containing starting material with NaH remain comparably stable upon cycling under hydrogen. The reaction rate of this material is comparable to that of TiCl3-doped samples. Extended X-ray absorption fine structure (EXAFS) analysis revealed that forming an AlxTi((1-x)) nanocomposite by the reduction of TiCl4 by Al and subsequent annealing stabilizes the short-range order around Ti. The presence of stable Al-Ti phases at the beginning of the reaction prevents the subsequent formation of a Ti-Al cluster upon cycling under hydrogen, which is found in the case of doping with TiCl3.When the atomic-scale behavior of this new material is compared with that of the TiCl3- or Ti colloid-doped sodium alanates, it is found that the chemical state of Ti as well as its local structure seem to be relevant for the stability of the storage capacity and desorption/absorption reaction rate upon cycling under hydrogen.