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All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Thermal conversion of unsolvated Mg(B3H8)2 to BH4- in the presence of MgH2

In the search for energy storage materials, metal octahydrotriborates, M(B3H8)n, n=1,2, are promising candidates but their synthesis suffers from residual solvents which tend to interact and greatly alter their decomposition mechanism. Therefore, we studied the thermal conversion of unsolvated Mg(B3H8)2 to BH4 -: as synthesized, and in the presence of MgH2. The conversion of our unsolvated Mg(B3H8)2 starts at ~100°C and yields ~22 wt% of BH4 - along with the formation of (closo-hydro)borates and volatile boranes. This loss of boron (B) is a sign of poor cyclability of the system. However, the addition of MgH2 to unsolvated Mg(B3H8)2 drastically increases the thermal conversion to 85-88wt% of BH4 - while simultaneously decreasing the amounts of B-losses. Our results strongly indicate that the presence of activated MgH2 substantially decreases the formation of (closohydro) borates and provides the necessary H2 for the B3H8-to-BH4 conversion. This is the first report of a metal octahydrotriborate system to selectively convert to BH4 - under moderate conditions of temperature (200°C) in less than 1h, making the MgB3H8-MgH2 system very promising for energy storage applications.

Gigante, Angelina↗
DOE OSTI2020

Materials Data on Be(B3H8)2 by Materials Project

Be(B3H7)2H2 crystallizes in the monoclinic P2_1/c space group. The structure is zero-dimensional and consists of four hydrogen molecules and four Be(B3H7)2 clusters. In each Be(B3H7)2 cluster, Be2+ is bonded in a 3-coordinate geometry to one B3- and three H1+ atoms. The Be–B bond length is 1.99 Å. There is two shorter (1.54 Å) and one longer (1.56 Å) Be–H bond length. There are six inequivalent B3- sites. In the first B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H1+ atoms. There are a spread of B–H bond distances ranging from 1.20–1.27 Å. In the second B3- site, B3- is bonded in a 4-coordinate geometry to four H1+ atoms. There are a spread of B–H bond distances ranging from 1.20–1.43 Å. In the third B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H1+ atoms. There are a spread of B–H bond distances ranging from 1.19–1.27 Å. In the fourth B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to one B3- and three H1+ atoms. The B–B bond length is 1.79 Å. There are a spread of B–H bond distances ranging from 1.20–1.33 Å. In the fifth B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to one B3- and three H1+ atoms. The B–B bond length is 1.54 Å. There are a spread of B–H bond distances ranging from 1.19–1.39 Å. In the sixth B3- site, B3- is bonded in a 2-coordinate geometry to one Be2+, two B3-, and two H1+ atoms. There is one shorter (1.19 Å) and one longer (1.34 Å) B–H bond length. There are fourteen inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the second H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the third H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the fourth H1+ site, H1+ is bonded in an L-shaped geometry to two B3- atoms. In the fifth H1+ site, H1+ is bonded in an L-shaped geometry to two B3- atoms. In the sixth H1+ site, H1+ is bonded in an L-shaped geometry to one Be2+ and one B3- atom. In the seventh H1+ site, H1+ is bonded in an L-shaped geometry to one Be2+ and one B3- atom. In the eighth H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the ninth H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the tenth H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the eleventh H1+ site, H1+ is bonded in an L-shaped geometry to two B3- atoms. In the twelfth H1+ site, H1+ is bonded in a 2-coordinate geometry to two B3- atoms. In the thirteenth H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the fourteenth H1+ site, H1+ is bonded in an L-shaped geometry to one Be2+ and one B3- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on B2H8N by Materials Project

B3H8B(NH4)2 is High Pressure (4-7GPa) Tellurium structured and crystallizes in the orthorhombic P2_12_12_1 space group. The structure is zero-dimensional and consists of four B(NH4)2 clusters and four B3H8 clusters. In each B(NH4)2 cluster, B3- is bonded in a tetrahedral geometry to two N2- and two H1+ atoms. There is one shorter (1.59 Å) and one longer (1.60 Å) B–N bond length. Both B–H bond lengths are 1.21 Å. There are two inequivalent N2- sites. In the first N2- site, N2- is bonded to one B3- and three H1+ atoms to form corner-sharing NBH3 tetrahedra. All N–H bond lengths are 1.03 Å. In the second N2- site, N2- is bonded to one B3- and three H1+ atoms to form corner-sharing NBH3 tetrahedra. All N–H bond lengths are 1.03 Å. There are eight inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a single-bond geometry to one N2- atom. In the second H1+ site, H1+ is bonded in a single-bond geometry to one N2- atom. In the third H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the fourth H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the fifth H1+ site, H1+ is bonded in a single-bond geometry to one N2- atom. In the sixth H1+ site, H1+ is bonded in a single-bond geometry to one N2- atom. In the seventh H1+ site, H1+ is bonded in a single-bond geometry to one N2- atom. In the eighth H1+ site, H1+ is bonded in a single-bond geometry to one N2- atom. In each B3H8 cluster, there are three inequivalent B3- sites. In the first B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H1+ atoms. There are a spread of B–H bond distances ranging from 1.21–1.27 Å. In the second B3- site, B3- is bonded in a 4-coordinate geometry to four H1+ atoms. There are a spread of B–H bond distances ranging from 1.21–1.48 Å. In the third B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H1+ atoms. There is two shorter (1.22 Å) and one longer (1.26 Å) B–H bond length. There are seven inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in an L-shaped geometry to two B3- atoms. In the second H1+ site, H1+ is bonded in an L-shaped geometry to two B3- atoms. In the third H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the fourth H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the fifth H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the sixth H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom. In the seventh H1+ site, H1+ is bonded in a single-bond geometry to one B3- atom.

36 MATERIALS SCIENCE↗