Combined Effects of Functional Cation and Anion on the Reversible Dehydrogenation of LiBH4

Catalyst doping has been widely employed as an effective method for improving the H-exchange kinetics of complex hydrides. However, a viable hydrogen storage material requires favorable properties on both kinetic and thermodynamic aspects to allow dehydrogenation and rehydrogenation reactions to proceed at moderate temperatures. In the present study, titanium halides (TiCl3 or TiF3) were mechanically milled with LiBH4 and examined with respect to their effects on the hydrogen storage properties of LiBH4. Experimental study showed that both halides are effective promoters. In particular, TiF3 exhibits a superior promoting effect to its analogue, TiCl3, on the reversible dehydrogenation of LiBH4. Phase characterization and chemical state analysis results showed that both halides react with the host LiBH4, resulting in the in situ formation of the catalytically active Ti hydride. However, the two halogen anions differ substantially from each other in terms of state and function. Associated with the dissolution of LiCl into LiBH4, the Cl- anion can readily replace [BH4](-) and exerts a stabilizing effect on the hexagonal phase of LiBH4. By contrast, the F- anion may partially substitute the anionic H in both LiBH4 (hydrogenated state) and LiH (dehydrogenated state) lattices, resulting in a favorable modification of the hydrogen-exchange thermodynamics of LiBH4. The observed promoting effect of TiF3 on the reversible dehydrogenation of LiBH4 should be understood from the combined effects of functional cation and anion. In this regard, TiF3 provides a prototype catalyst that can simultaneously modify the H-exchange kinetics and thermodynamics of complex hydrides.