Complex Ammine Titanium(III) Borohydrides as Advanced Solid Hydrogen-Storage Materials with Favorable Dehydrogenation Properties

Ammine metal borohydrides (AMBs), with high hydrogen contents and favorable dehydrogenation properties, are receiving intensive research efforts for their potential as hydrogen storage materials. In this work, we report the successful synthesis of three ammine titanium borohydrides (denoted as ATBs), Ti(BH4)(3)center dot 5NH(3), Li2Ti(BH4)(5)center dot 5NH(3), and Ti(BH4)(3)center dot 3NH(3) via metathesis reaction of metal chloride ammoniates (TiCl3 center dot 5NH(3) and TiCl3 center dot 3NH(3)) and lithium borohydride. These ATBs present favorable stability, owing to the coordination with NH3 groups, compared to the unstable Ti(BH4)(3) at room temperature. Dehydrogenation results revealed that Ti(BH4)(3)center dot 5NH(3), which theoretically contains 15.1 wt % hydrogen, is able to release similar to 13.4 wt % H-2 plus a small amount of ammonia. This occurred via a single-stage decomposition process with a dehydrogenation peak at 130 degrees C upon heating to 200 degrees C. For Li2Ti(BH4)(5)center dot 5NH(3), a three-step decomposition process with a total of 15.8 wt % pure hydrogen evolution peaked at 105, 120, and 215 degrees C was observed until 300 degrees C. In the case of Ti(BH4)(3)center dot 3NH(3), a release of 14 wt % pure hydrogen via a two-step decomposition process with peaks at 109 and 152 degrees C can be achieved in the temperature range of 60-300 degrees C. Isothermal TPD results showed that over 9 wt % pure hydrogen was liberated from Ti(BH4)(3)center dot 3NH(3) and Li2Ti(BH4)(5)center dot 5NH(3) within 400 mm at 100 degrees C. Preliminary research on the reversibility of this process showed that dehydrogenated ATBs could be partly recharged by reacting with N2H4 in liquid ammonia. These aforementioned preeminent dehydrogenation performances make ATBs very promising candidates as solid hydrogen storage materials. Finally, analysis of the decomposition mechanism demonstrated that the hydrogen emission from ATBs is based on the combination reaction of B H and N-H groups as in other reported AMBs.