Improved reversible dehydrogenation of 2LiBH4–MgH2 composite by the controlled formation of transition metal boride

Abstract

In this paper, we report a careful study of the properties and mechanisms of a 2LiBH₄–MgH₂ composite with a niobium oxide (Nb₂O₅) additive. It was found that directly milling with a small amount of the Nb₂O₅ additive exerted a negligible effect on the dehydrogenation kinetics of the 2LiBH₄–MgH₂ composite in the first cycle. However the recycled sample showed notable property advantages over the neat 2LiBH₄–MgH₂ composite. Phase/chemical state analyses and control experiments were conducted to gain insight into the chemical transformation of the Nb-containing species. It was found that the Nb₂O₅ additive reacted with LiBH₄ to form a niobium boride (NbB₂) heterogeneous nucleation agent, via a niobium hydride (NbH₂) intermediate, in the dehydrogenation process of the composite sample. The retarded formation of the NbB₂ is a major mechanistic reason for the observed drastic property change of the composite upon cycling. As a solution, we employed a novel three-step method to controllably promote the formation of the targeted NbB₂ species and improve its dispersion state. As expected, the 2LiBH₄–MgH₂–0.08Nb₂O₅ composite sample, prepared using the new method, exhibits rapid dehydrogenation kinetics and high cyclic stability, which compare favorably with the literature results of the 2LiBH₄–MgH₂ composites with transition metal additives.