Stabilization of Nanosized Borohydrides for Hydrogen Storage: Suppressing the Melting with TiCl 3 Doping

Lightweight complex hydrides, M(BH4)n (M = Li, Na, Mg, and Ca; n = 1 for Li and Na, n = 2 for Mg and Ca), are believed to be promising hydrogen storage materials with extreme high hydrogen density up to 18.5 mass %. However, these materials suffer high dehydrogenation temperature, melting, and reversibility problems, which exclude them from the list of practical hydrogen storage systems. Herein, borohydrides (M(BH4)n-Ti, with M = M1 or M2 and n = 1 or 2), were modified with TiCl3 via a wet chemistry approach, and in some cases this led to the formation of solvent-stabilized nanoparticles. As a result of TiCl3 modification, the melting before hydrogen release was suppressed as evidenced by DSC and thermal microscopy observations. Furthermore, the hydrogen release temperature of M(BH4)n-Ti was significantly reduced. For example, the dehydrogenation temperature of NaBH4-Ti was reduced from 570 to 120 °C. Ti modification was also found to improve to some extent the reversibility of the doped materials. In particular, up to 2 mass% H2 was reversibly cycled for Ca(BH4)2-Ti at 300 °C and 9 MPa H2 pressure, in comparison to 400 °C and 70 MPa for pristine Ca(BH4)2. This study demonstrates a simple method to synthesize surfactant-free Ti-doped nanosized borohydrides, and by removing the melting of these materials, it provides a new path toward the stabilization of borohydride particles at the nanoscale.