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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Nanoconfinement of Molecular Magnesium Borohydride Captured in a Bipyridine-Functionalized Metal-Organic Framework

The lower limit of metal hydride nanoconfinement is demonstrated through the coordination of a molecular hydride species to binding sites inside the pores of a metal-organic framework (MOF). Magnesium borohydride, which has a high hydrogen capacity, is incorporated into the pores of UiO-67bpy (Zr6O4(OH)4(bpydc)6 with bpydc2- = 2,2'-bipyridine-5,5'- dicarboxylate) by solvent impregnation. The MOF retained its long-range order, and transmission electron microscopy and elemental mapping confirmed the retention of the crystal morphology and revealed a homogeneous distribution of the hydride within the MOF host. Notably, the B-, N-, and Mg-edge XAS data confirm the coordination of Mg(II) to the N atoms of the chelating bipyridine groups. In situ 11B MAS NMR studies helped elucidate the reaction mechanism and revealed that complete hydrogen release from Mg(BH4)2 occurs as low as 200 °C. Sieverts and thermogravimetric measurements indicate an increase in the rate of hydrogen release, with the onset of hydrogen desorption as low as 120 °C, which is approximately 150 °C lower than that of the bulk material. Furthermore, density functional theory calculations support the improved dehydrogenation properties and confirm the drastically lower activation energy for B-H bond dissociation.

Nanoconfinement, Metal Hydrides, Metal-Organic Fra↗
DOE OSTI2023

Electrolytes in Multiple-Phase Hydrogen Storage Reactions

Multiple-phase hydrogen storage materials such as metal alanates and borohydrides, and destabilized systems offer the possibility of high hydrogen storage capacity with favorable thermodynamics. However, the multiphase nature of these materials intrinsically limits the kinetics due to the required transport of species between phases, which are typically in dry powder form. To address this limitation, the influence of added electrolytes is explored. This approach is motivated by analogy with similar multiphase battery reactions that show reduced kinetic limitations while necessarily containing electrolytes. Previous experimental results showing improved kinetics for MgH2/Sn (using a LiBH4/KBH4 eutectic electrolyte) and NaAlH4 (using a diglyme electrolyte) are further analyzed in terms of this analogy. The results show that the analogy is useful and rate constants are increased. Importantly, the inclusion of an electrolyte also appears to alleviate the continuously decreasing rates with the extent of reaction, which is characteristic of many multiphase hydrides. Instead, reaction rates are approximately constant until near completion. Together, these effects can lead to >10× shorter overall reaction times. In addition, new results are presented for the hydrogenation of MgB2 using Li/K/CsI and Li/K/CsCl eutectic electrolytes, where >60% conversion to Mg(BH4)2 was demonstrated at 350 bar.

08 HYDROGEN↗
DOE OSTI2024

Optimization and purification of magnesium borides fabricated by combustion synthesis and by high-temperature sintering

Hydrogen as a fuel provides several benefits over the use of fossil fuels; however, one challenge in utilizing hydrogen as an energy carrier revolves around its storage. Achieving sufficient volumetric hydrogen density in a storage solution will facilitate hydrogen’s adoption for use in both stationary and mobile applications. Solid-state hydrogen storage provides a promising pathway to solving this problem. However, the hydrogenation of these materials is characterized by slow kinetics and extreme thermodynamic conditions. Magnesium borohydride (Mg(BH4)2) is a promising material in hydrogen storage due to its reversible properties and a theoretical hydrogen capacity of 14.9 wt.%. To synthesize this material, combustion synthesis of magnesium borides (MgBx) has been implemented with the aim to further lower thermodynamic requirements for direct hydrogenation. A drawback of this process is the potential formation of oxide contaminants, which decrease hydrogenation/dehydrogenation and recyclability performance. The present work focuses on identifying parameters useful for reducing contamination and evaluating potential pathways to the purification of magnesium borides with the goal of improving their quality.

Camarena, Miguel Joaquin↗
DOE OSTI2025

Fabrication and characterization of nanoscale magnesium diboride and tetraboride for propulsion and hydrogen storage applications

Abstract: Boron-loaded propellants have the potential to dramatically increase the performance of solid fuel ramjets, ducted rockets, and hybrid rocket engines. However, difficult ignition of boron decreases the combustion efficiency of these propellants. One approach to solving this problem involves the use of magnesium diboride, MgB2, which ignites easier than boron. Magnesium tetraboride, MgB4, potentially offers greater energetic performance as B has a higher energy density than Mg. However, the effect of the higher boron/metal ratio on the ignition and combustion is unclear. Nanoscale MgB2 particles and quasi 2D structures are promising propellant ingredients, but the oxidation and combustion properties of nanoscale MgB4 remain unknown. Nanoscale magnesium borides are also of interest as precursors for the synthesis of magnesium borohydride, Mg(BH4)2, a promising hydrogen storage material, but hydrogenation of MgB4 has not been studied yet. The objectives of the present work included synthesis, purification, and high-energy ball milling of MgB2 and MgB4 powders as well as investigation of their hydrogen uptake, thermal decomposition, oxidation, and combustion. The powders were fabricated by combustion synthesis and by heating in a tube furnace. The latter method was superior in the synthesis of MgB4. Oxide impurities in the synthesized powders were removed by acid leaching. Nanoscale powders were obtained by ball-mill exfoliation. The hydrogen intake of the obtained magnesium borides was examined at 700 bar and 300 ℃ and did not reveal any advantage of MgB4 over MgB2. Their thermal decomposition and oxidation were investigated with thermogravimetric analysis (TGA), while their combustion was studied using laser ignition and high-speed video recording. TGA has confirmed prior observations of multistep decomposition of magnesium borides, where each step involves formation of a boride with a higher B/Mg ratio and evaporation of formed magnesium. The oxidation rates of the borides are much higher than that of boron at temperatures over 1200 °C for MgB2 and over 900 °C for MgB4. The burning rates of non-milled MgB₂ and MgB₄ powders were much higher than for the used submicron boron. Milling the MgB₂ and MgB₄ powders further increased their burning rates. The milled MgB4 burned 7.5 times faster than submicron boron.

Combustion of metals, Solid fuels, Propellants, Hy↗
DOE OSTI2025

Orientational Disorder of NH3 in Hexammine Magnesium Borohydride

Hexammine magnesium borohydride, Mg(NH3)6(BH4)2, consists of adducted NH3 molecules locked in a matrix of Mg cations and borohydride anions. It is a candidate material for hydrogen storage, with 16.8 wt % hydrogen stored in both the NH3 and borohydride anions. It may also be of interest as an Mg2+-conducting electrolyte in solid-state batteries. Its crystal structure has, until now, eluded a proper structural solution due to ambiguity regarding the NH3 position and behavior. In this work, we show using synchrotron X-ray diffraction that the room-temperature structure can be solved only with a model assuming the orientational disorder of ammonia molecules within the crystal structure. Cooling the sample to 120 K yields additional Bragg peaks, which can be solved only with a unit cell expansion consistent with the freezing of the orientational freedom of ammonia molecules. Using this insight from the structure solution, we performed a full assignment of the vibrational modes in the room-temperature infrared spectrum.

08 HYDROGEN↗
DOE OSTI2024

Ca-dimers, solvent layering, and dominant electrochemically active species in Ca(BH4)2 in THF

Abstract Divalent ions (Mg, Ca, and Zn) are being considered as competitive, safe, and earth-abundant alternatives to Li-ion electrochemistry, but present challenges for stable cycling due to undesirable interfacial phenomena. We explore the formation of electroactive species in the electrolyte Ca(BH 4 ) 2 ∣THF using molecular dynamics coupled with a continuum model of bulk and interfacial speciation. Free-energy analysis and unsupervised learning indicate a majority population of neutral Ca dimers and monomers with diverse molecular conformations and an order of magnitude lower concentration of the primary electroactive charged species – the monocation, $${\rm{CaBH}}_{4}^{+}$$ CaBH 4 + – produced via disproportionation of neutral complexes. Dense layering of THF molecules within ~1 nm of the electrode surface strongly modulates local electrolyte species populations. A dramatic increase in monocation population in this interfacial zone is induced at negative bias. We see no evidence for electrochemical activity of fully-solvated Ca 2+ . The consequences for performance are discussed in light of this molecular-scale insight.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Materials Data on Li2MgB4(H17N3)2 by Materials Project

(Li(BH4)2)2Mg(NH3)6 crystallizes in the tetragonal P4_32_12 space group. The structure is one-dimensional and consists of four Mg(NH3)6 clusters and two Li(BH4)2 ribbons oriented in the (0, 0, 1) direction. In each Mg(NH3)6 cluster, Mg2+ is bonded in an octahedral geometry to six N3- atoms. There are a spread of Mg–N bond distances ranging from 2.22–2.28 Å. There are three inequivalent N3- sites. In the first N3- site, N3- is bonded in a distorted trigonal non-coplanar geometry to one Mg2+ and three H+0.76+ atoms. There is one shorter (1.02 Å) and two longer (1.03 Å) N–H bond length. In the second N3- site, N3- is bonded in a distorted trigonal non-coplanar geometry to one Mg2+ and three H+0.76+ atoms. There is one shorter (1.02 Å) and two longer (1.03 Å) N–H bond length. In the third N3- site, N3- is bonded in a distorted trigonal non-coplanar geometry to one Mg2+ and three H+0.76+ atoms. There is two shorter (1.02 Å) and one longer (1.03 Å) N–H bond length. There are nine inequivalent H+0.76+ sites. In the first H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one N3- atom. In the second H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one N3- atom. In the third H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one N3- atom. In the fourth H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one N3- atom. In the fifth H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one N3- atom. In the sixth H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one N3- atom. In the seventh H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one N3- atom. In the eighth H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one N3- atom. In the ninth H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one N3- atom. In each Li(BH4)2 ribbon, Li1+ is bonded in a 8-coordinate geometry to eight H+0.76+ atoms. There are a spread of Li–H bond distances ranging from 1.94–2.28 Å. There are two inequivalent B3- sites. In the first B3- site, B3- is bonded in a tetrahedral geometry to four H+0.76+ atoms. All B–H bond lengths are 1.23 Å. In the second B3- site, B3- is bonded in a tetrahedral geometry to four H+0.76+ atoms. All B–H bond lengths are 1.23 Å. There are eight inequivalent H+0.76+ sites. In the first H+0.76+ site, H+0.76+ is bonded in a water-like geometry to one Li1+ and one B3- atom. In the second H+0.76+ site, H+0.76+ is bonded in a water-like geometry to one Li1+ and one B3- atom. In the third H+0.76+ site, H+0.76+ is bonded in a distorted single-bond geometry to two equivalent Li1+ and one B3- atom. In the fourth H+0.76+ site, H+0.76+ is bonded in a water-like geometry to one Li1+ and one B3- atom. In the fifth H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one B3- atom. In the sixth H+0.76+ site, H+0.76+ is bonded in a water-like geometry to one Li1+ and one B3- atom. In the seventh H+0.76+ site, H+0.76+ is bonded in a distorted single-bond geometry to two equivalent Li1+ and one B3- atom. In the eighth H+0.76+ site, H+0.76+ is bonded in a single-bond geometry to one B3- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgB4(H10N)2 by Materials Project

MgB4(NH10)2 crystallizes in the orthorhombic P2_12_12_1 space group. The structure is zero-dimensional and consists of four MgB4(NH10)2 clusters. Mg2+ is bonded to eight H+0.80+ atoms to form distorted MgH8 hexagonal bipyramids that share edges with four BH4 tetrahedra. There are a spread of Mg–H bond distances ranging from 1.97–2.11 Å. There are four inequivalent B3- sites. In the first B3- site, B3- is bonded to four H+0.80+ atoms to form BH4 tetrahedra that share an edgeedge with one MgH8 hexagonal bipyramid. There is two shorter (1.22 Å) and two longer (1.24 Å) B–H bond length. In the second B3- site, B3- is bonded to four H+0.80+ atoms to form BH4 tetrahedra that share an edgeedge with one MgH8 hexagonal bipyramid. There are a spread of B–H bond distances ranging from 1.21–1.25 Å. In the third B3- site, B3- is bonded to one N3- and three H+0.80+ atoms to form BH3N tetrahedra that share an edgeedge with one MgH8 hexagonal bipyramid. The B–N bond length is 1.58 Å. There is one shorter (1.21 Å) and two longer (1.23 Å) B–H bond length. In the fourth B3- site, B3- is bonded to one N3- and three H+0.80+ atoms to form BH3N tetrahedra that share an edgeedge with one MgH8 hexagonal bipyramid. The B–N bond length is 1.59 Å. There is one shorter (1.21 Å) and two longer (1.23 Å) B–H bond length. There are two inequivalent N3- sites. In the first N3- site, N3- is bonded in a tetrahedral geometry to one B3- and three H+0.80+ atoms. All N–H bond lengths are 1.03 Å. In the second N3- site, N3- is bonded in a tetrahedral geometry to one B3- and three H+0.80+ atoms. All N–H bond lengths are 1.03 Å. There are twenty inequivalent H+0.80+ sites. In the first H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the second H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the third H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the fourth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the fifth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the sixth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the seventh H+0.80+ site, H+0.80+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the eighth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the ninth H+0.80+ site, H+0.80+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the tenth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the eleventh H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the twelfth H+0.80+ site, H+0.80+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the thirteenth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the fourteenth H+0.80+ site, H+0.80+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the fifteenth H+0.80+ site, H+0.80+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the sixteenth H+0.80+ site, H+0.80+ is bonded in a distorted L-shaped geometry to one Mg2+ and one B3- atom. In the seventeenth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the eighteenth H+0.80+ site, H+0.80+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the nineteenth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the twentieth H+0.80+ site, H+0.80+ is bonded in a distorted L-shaped geometry to one Mg2+ and one B3- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgB4(H10N)2 by Materials Project

MgB4(NH10)2 crystallizes in the orthorhombic P2_12_12_1 space group. The structure is zero-dimensional and consists of four MgB4(NH10)2 clusters. Mg2+ is bonded to seven H+0.80+ atoms to form distorted MgH7 pentagonal bipyramids that share a cornercorner with one BH3N tetrahedra and edges with three BH4 tetrahedra. There are a spread of Mg–H bond distances ranging from 1.96–2.15 Å. There are four inequivalent B3- sites. In the first B3- site, B3- is bonded to four H+0.80+ atoms to form BH4 tetrahedra that share an edgeedge with one MgH7 pentagonal bipyramid. There are a spread of B–H bond distances ranging from 1.21–1.24 Å. In the second B3- site, B3- is bonded to four H+0.80+ atoms to form BH4 tetrahedra that share an edgeedge with one MgH7 pentagonal bipyramid. There are a spread of B–H bond distances ranging from 1.21–1.24 Å. In the third B3- site, B3- is bonded to one N3- and three H+0.80+ atoms to form BH3N tetrahedra that share a cornercorner with one MgH7 pentagonal bipyramid. The B–N bond length is 1.59 Å. There is two shorter (1.21 Å) and one longer (1.24 Å) B–H bond length. In the fourth B3- site, B3- is bonded to one N3- and three H+0.80+ atoms to form BH3N tetrahedra that share an edgeedge with one MgH7 pentagonal bipyramid. The B–N bond length is 1.59 Å. There are a spread of B–H bond distances ranging from 1.21–1.24 Å. There are two inequivalent N3- sites. In the first N3- site, N3- is bonded in a tetrahedral geometry to one B3- and three H+0.80+ atoms. All N–H bond lengths are 1.03 Å. In the second N3- site, N3- is bonded in a tetrahedral geometry to one B3- and three H+0.80+ atoms. There is one shorter (1.02 Å) and two longer (1.03 Å) N–H bond length. There are twenty inequivalent H+0.80+ sites. In the first H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the second H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the third H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the fourth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the fifth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the sixth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one N3- atom. In the seventh H+0.80+ site, H+0.80+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the eighth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the ninth H+0.80+ site, H+0.80+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the tenth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the eleventh H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the twelfth H+0.80+ site, H+0.80+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the thirteenth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the fourteenth H+0.80+ site, H+0.80+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the fifteenth H+0.80+ site, H+0.80+ is bonded in a bent 120 degrees geometry to one Mg2+ and one B3- atom. In the sixteenth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the seventeenth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the eighteenth H+0.80+ site, H+0.80+ is bonded in a distorted L-shaped geometry to one Mg2+ and one B3- atom. In the nineteenth H+0.80+ site, H+0.80+ is bonded in a single-bond geometry to one B3- atom. In the twentieth H+0.80+ site, H+0.80+ is bonded in a water-like geometry to one Mg2+ and one B3- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on LiMgB3(H9N)2 by Materials Project

LiMgB3(NH9)2 crystallizes in the hexagonal P6_3 space group. The structure is three-dimensional. Li1+ is bonded to six H+0.67+ atoms to form LiH6 octahedra that share corners with six equivalent BH4 tetrahedra. There is three shorter (1.97 Å) and three longer (2.01 Å) Li–H bond length. Mg2+ is bonded to two N3- and six H+0.67+ atoms to form MgH6N2 hexagonal bipyramids that share edges with three equivalent BH4 tetrahedra. There are one shorter (2.12 Å) and one longer (2.13 Å) Mg–N bond lengths. There are three shorter (2.10 Å) and three longer (2.13 Å) Mg–H bond lengths. B3- is bonded to four H+0.67+ atoms to form BH4 tetrahedra that share corners with two equivalent LiH6 octahedra and an edgeedge with one MgH6N2 hexagonal bipyramid. The corner-sharing octahedra tilt angles range from 45–54°. There are a spread of B–H bond distances ranging from 1.22–1.24 Å. There are two inequivalent N3- sites. In the first N3- site, N3- is bonded in a distorted trigonal non-coplanar geometry to one Mg2+ and three equivalent H+0.67+ atoms. All N–H bond lengths are 1.03 Å. In the second N3- site, N3- is bonded in a distorted tetrahedral geometry to one Mg2+ and three equivalent H+0.67+ atoms. All N–H bond lengths are 1.03 Å. There are five inequivalent H+0.67+ sites. In the first H+0.67+ site, H+0.67+ is bonded in a bent 120 degrees geometry to one Li1+ and one B3- atom. In the second H+0.67+ site, H+0.67+ is bonded in a single-bond geometry to one N3- atom. In the third H+0.67+ site, H+0.67+ is bonded in a distorted bent 150 degrees geometry to one Li1+ and one B3- atom. In the fourth H+0.67+ site, H+0.67+ is bonded in a distorted L-shaped geometry to one Mg2+ and one B3- atom. In the fifth H+0.67+ site, H+0.67+ is bonded in a distorted L-shaped geometry to one Mg2+ and one B3- atom.

36 MATERIALS SCIENCE↗