Phase Boundaries and Reversibility of LiBH₄/MgH₂ Hydrogen Storage Material

Abstract

The coupled system LiBH4 + 1/2MgH2 ↔ LiH + 1/2MgB2 + 2H2 demonstrates improved hydrogen cycling thermodynamics compared to either LiBH4 or MgH2 alone; in effect, formation of MgB2 "destabilizes" the decomposition of LiBH4. Here we establish the thermodynamically and kinetically stable region of the H2 pressure−temperature phase diagram for reversible hydrogen storage in TiCl3-catalyzed LiBH4 + 1/2MgH2. Although MgB2 formation was thermodynamically favored at elevated temperature, it was kinetically more favorable for MgH2 and LiBH4 to decompose independently in a two-step dehydrogenation starting with MgH2 ↔ Mg + H2. At high temperature and low H2 pressure, direct LiBH4 decomposition is both thermodynamically allowed and kinetically favored; thus, the second dehydrogenation step from LiBH4 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 H2 gas overpressure of at least 3 bar during dehydrogenation, however, suppressed direct decomposition of LiBH4 and reaction of Mg with LiBH4 produced LiH and MgB2, which was fully reversible.