Stability and Reversibility of Lithium Borohydrides Doped by Metal Halides and Hydrides

In an effort to develop reversible metal borohydrides with high hydrogen storage capacities and low dehydriding temperature, doping LiBH 4 with various metal halides and hydrides has been conducted. Several metal halides such as TiCl 3, TiF 3, and ZnF 2 effectively reduced the dehydriding temperature through a cation exchange interaction. Some of the halide doped LiBH 4 are partially reversible. The LiBH 4 + 0.1TiF 3 desorbed 3.5 wt % and 8.5 wt % hydrogen at 150 and 450 °C, respectively, with subsequent reabsorption of 6 wt % hydrogen at 500 °C and 70 bar observed. XRD and NMR analysis of the rehydrided samples confirmed the reformation of LiBH 4 . The existence of the (B 12 H 12 ) −2 species in dehydrided and rehydrided samples gives insight into the resultant partial reversibility. A number of other halides, MgF 2, MgCl 2, CaCl 2, SrCl 2, and FeCl 3, did not reduce the dehydriding temperature of LiBH 4 significantly. XRD and TGA-RGA analyses indicated that an increasing proportion of halides such as TiCl 3, TiF 3, and ZnCl 2 from 0.1 to 0.5 mol makes lithium borohydrides less stable and volatile. Although the less stable borohydrides such as LiBH 4 + 0.5TiCl 3, LiBH 4 + 0.5TiF 3, and LiBH 4 + 0.5ZnCl 2 release hydrogen at room temperature, they are not reversible due to unrecoverable boron loss caused by diborane emission. In most cases, doping that produced less stable borohydrides also reduced the reversible hydrogen uptake. It was also observed that halide doping changed the melting points and reduced air sensitivity of lithium borohydrides.