Hydrogen Storage in Trimetallic Borohydrides: a Crystal Structure Prediction and Ab Initio Molecular Dynamics Simulations Study

Abstract

Trimetallic borohydrides have emerged as promising candidates for hydrogen storage due to their unique structural and chemical properties. Nevertheless, the literature on trimetallic borohydrides is limited to only a few studied examples. Thermodynamic properties of trimetallic borohydrides can be tuned by changing the composition and stoichiometry of the compound. In this regard, the hydrogen storage potential of a series of new Al-based trimetallic borohydrides, including alkali, earth alkali, and transition metals with different electronegativities, with a general formula of LiAlM(BH 4 ) 5–7 (M = Na, Mg, Sc, Y, Zn, and Mo) has been investigated using various computational tools. Due to the limited knowledge about the crystal structure of trimetallic borohydrides, first, a crystal structure prediction study has been accomplished to determine the lowest-energy crystal structures of Al-based trimetallic borohydrides using the recently developed highly parallel FFCASP tool together with subsequent electronic structure calculations. Iso-energetic crystal structures with different metal-borohydride coordinations were obtained with FFCASP especially for the higher borohydride stoichiometries, indicating a potential energy landscape with shallow minima, which make easier the phase transformations under temperature. The convex hull of the ternary LiBH 4 –Al(BH 4 ) 3 -M(BH 4 ) x (M = Na, Mg, Zn, and Y) system indicated that while both LiAlNa(BH 4 ) 5 and LiAlZn(BH 4 ) 6 are stable, LiAlMg(BH 4 ) 6 and LiAlY(BH 4 ) 7 are metastable. Hydrogen release dynamics in these predicted structures was studied with the help of ab initio molecular dynamics (AIMD) simulations. Among the trimetallic borohydrides, AIMD simulations indicated that LiAlZn(BH 4 ) 6 has a favorable hydrogen release temperature, starting from 392 K.