TiCl3-enhanced NaAlH4:: Impact of excess Al and development of the Al1-γTiγ phase during cycling

NaAlH4 with TiCl3 and Al were mixed by ball-milling and cycled three times. The hydrogen storage properties were monitored during cycling, and the products were characterized by synchrotron X-ray diffraction. Because of the previously described formation of Al1-yTiy with y approximate to 0.15 during cycling that traps Al beyond the amount associated with the formation of NaCl, some Na3AlH6 has no free Al to react with to form NaAlH4. This was counteracted in the present work by adding a stoichiometric amount of Al that increases the theoretical storage capacity. Due to limitations in metal diffusion small amounts of Na3AlH6 were still detected. When similar to 7 mol % more Al than the stoichiometric amount was added, the observed storage capacity increased significantly, and the Na3AlH6 content was negligible after prolonged rehydrogenation. Cycled NaAlH4 + 10 mol % TiCl3 were desorbed to two different levels, and the diffraction patterns were compared. There is no change in unit-cell dimensions during desorption, and there is no sign of changes in the bulk composition of the Al1-yTiy phase during a cycle. Adding pure Ti to a NaH + Al mixture by ball-milling in argon or hydrogen results in formation of TiH2 that is stable during at least one cycle.