Phase Boundaries and Reversibility of LiBH₄/MgH₂ Hydrogen Storage Material

Abstract

High Resolution Image Download MS PowerPoint Slide The coupled system LiBH 4 + 1 / 2 MgH 2 ↔ LiH + 1 / 2 MgB 2 + 2H 2 demonstrates improved hydrogen cycling thermodynamics compared to either LiBH 4 or MgH 2 alone; in effect, formation of MgB 2 “destabilizes” the decomposition of LiBH 4 . Here we establish the thermodynamically and kinetically stable region of the H 2 pressure−temperature phase diagram for reversible hydrogen storage in TiCl 3 -catalyzed LiBH 4 + 1 / 2 MgH 2 . Although MgB 2 formation was thermodynamically favored at elevated temperature, it was kinetically more favorable for MgH 2 and LiBH 4 to decompose independently in a two-step dehydrogenation starting with MgH 2 ↔ Mg + H 2 . At high temperature and low H 2 pressure, direct LiBH 4 decomposition is both thermodynamically allowed and kinetically favored; thus, the second dehydrogenation step from LiBH 4 produced LiH and amorphous boron along with the Mg metal from the first step. From this state, recombination of LiH with amorphous boron had very poor kinetics and the system did not fully rehydrogenate. Applying an H 2 gas overpressure of at least 3 bar during dehydrogenation, however, suppressed direct decomposition of LiBH 4 and reaction of Mg with LiBH 4 produced LiH and MgB 2, which was fully reversible.