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

Abstract

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)3·5NH3, Li2Ti(BH4)5·5NH3, and Ti(BH4)3·3NH3 via metathesis reaction of metal chloride ammoniates (TiCl3·5NH3 and TiCl3·3NH3) 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)3·5NH3, which theoretically contains 15.1 wt % hydrogen, is able to release ∼13.4 wt % H2 plus a small amount of ammonia. This occurred via a single-stage decomposition process with a dehydrogenation peak at 130 °C upon heating to 200 °C. For Li2Ti(BH4)5·5NH3, a three-step decomposition process with a total of 15.8 wt % pure hydrogen evolution peaked at 105, 120, and 215 °C was observed until 300 °C. In the case of Ti(BH4)3·3NH3, a release of 14 wt % pure hydrogen via a two-step decomposition process with peaks at 109 and 152 °C can be achieved in the temperature range of 60–300 °C. Isothermal TPD results showed that over 9 wt % pure hydrogen was liberated from Ti(BH4)3·3NH3 and Li2Ti(BH4)5·5NH3 within 400 min at 100 °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.