Elucidating the mechanism of MgB<sub>2</sub>initial hydrogenation via a combined experimental–theoretical study

Ray, Keith G.; Klebanoff, Leonard E.; Lee, Jonathan R. I.; Stavila, Vitalie; Heo, Tae Wook; Shea, Patrick; Baker, Alexander A.; Kang, ShinYoung; Bagge‐Hansen, Michael; Liu, Yi‐Sheng; White, J. L.; Wood, Brandon C.

doi:10.1039/c7cp03709k

Cited by 24 publications

(62 citation statements)

References 42 publications

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“…The product obtained from the hydrogenation of MgB 2 ‐THF at 700 bar and 300 °C, was seen to undergo a slow, steady mass loss between 100–600 °C. This finding echoes a recent report by Ray et al . of the surface‐only uptake of <1 wt.% H by MgB 2 at 364–391 °C under 145 bar which showed only trace amounts of Mg(BH 4 ) 2 .…”

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confidence: 90%

Kinetic Enhancement of Direct Hydrogenation of MgB₂ to Mg(BH₄)₂ upon Mechanical Milling with THF, MgH₂, and/or Mg

et al. 2019

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Modification of magnesium diboride, MgB 2 , by mechanical milling with THF, MgH 2 , and/or Mg results in a lowering of the conditions required for its direct, bulk hydrogenation to magnesium borohydride, Mg(BH 4 ) 2 , by 300 bar and 100°C. Following mechanical milling with MgH 2 or THF and Mg, MgB 2 can be hydrogenated to Mg(BH 4 ) 2 at 300°C under 700 bar of H 2 while achieving~54-71 % conversion to the borohydride. The discovery of a means of dramatically lowering the conditions required for the hydrogenation of MgB 2 is an important step towards the development of a practical onboard hydrogen storage system based on hydrogen cycling between Mg(BH 4 ) 2 and MgB 2 . We suggest that mechano-milling with THF, Mg, and/or MgH 2 may possibly introduce defects in the MgB 2 structure which enhance hydrogenation. The ability to activate the MgB 2 through the introduction of structural defects transcends its relevance to hydrogen storage, as a method of overcoming its chemical inertness provides the key to harnessing other interesting properties of this material.Magnesium borohydride, Mg(BH 4 ) 2 , is one of the few materials with a gravimetric H 2 density (14.9 wt % H) that is sufficient to meet the requirements of practical on-board PEM fuel cell applications [1] and also possesses thermodynamics (ΔH°= 39 kJ mol À 1 H 2 , ΔS = 112 J K À 1 mol À 1 H 2 ) that permit reversible H 2 release under moderate pressure and temperature. [1][2] However,

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Section: Figuresupporting

confidence: 90%

Kinetic Enhancement of Direct Hydrogenation of MgB₂ to Mg(BH₄)₂ upon Mechanical Milling with THF, MgH₂, and/or Mg

et al. 2019

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“…[ 26 ] However, owing to the high energy required to break stable BB bonds and the large diffusion barrier of H in MgB 2 , the reversible ratio is still lower than 20% even under a pressure as high as 15 MPa. [ 27 ] Another method for improving the reversibility of Mg(BH 4 ) 2 is to mechanically mill MgB 2 with Mg/MgH 2 in solvents. [ 28 ] Although milling process creates defects that improve the reversibility of MgB 2 to some extent, the physical contact between Mg/MgH 2 and MgB 2 was largely limited owing to the uncontrollable distribution with large particle size; more importantly, only the first cycle of hydrogen storage could be achieved owing to the difficult formation of MgB 2 via the direct dehydrogenation of bulk Mg(BH 4 ) 2 .…”

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confidence: 99%

Heterostructures Built in Metal Hydrides for Advanced Hydrogen Storage Reversibility

et al. 2020

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Hydrogen storage is a vital technology for developing on‐board hydrogen fuel cells. While Mg(BH4)2 is widely regarded as a promising hydrogen storage material owing to its extremely high gravimetric and volumetric capacity, its poor reversibility poses a major bottleneck inhibiting its practical applications. Herein, a facile strategy to effectively improve the reversible hydrogen storage performance of Mg(BH4)2 via building heterostructures uniformly inside MgH2 nanoparticles is reported. The in situ reaction between MgH2 nanoparticles and B2H6 not only forms homogeneous heterostructures with controllable particle size but also simultaneously decreases the particle size of the MgH2 nanoparticles inside, which effectively reduces the kinetic barrier that inhibits the reversible hydrogen storage in both Mg(BH4)2 and MgH2. More importantly, density functional theory coupled with ab initio molecular dynamics calculations clearly demonstrates that MgH2 in this heterostructure can act as a hydrogen pump, which drastically changes the enthalpy for the initial formation of BH bonds by breaking stable BB bonds from endothermic to exothermic and hence thermodynamically improves the reversibility of Mg(BH4)2. It is believed that building heterostructures provides a window of opportunity for discovering high‐performance hydrogen storage materials for on‐board applications.

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“…This shift is attributed to the existence of MgB 2 (not shown here but can be found in ref. 18) and is expected to be absent in kinetic byproducts such as MgB 12 H 12 spectrum . Hence, these results imply that the sample without rGO dehydrogenates to MgB 2 without forming significant amount of byproducts such as MgB 12 H 12 .…”

Section: Examplesmentioning

confidence: 99%

In‐Situ/Operando X‐ray Characterization of Metal Hydrides

et al. 2019

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In this article, the capabilities of soft and hard X-ray techniques, including X-ray absorption (XAS), soft X-ray emission spectroscopy (XES), resonant inelastic soft X-ray scattering (RIXS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), and their application to solid-state hydrogen storage materials are presented. These characterization tools are indispensable for interrogating hydrogen storage materials at the relevant length scales of fundamental interest, which range from the micron scale to nanometer dimensions. Since nanostructuring is now well established as an avenue to improve the thermodynamics and kinetics of hydrogen release and uptake, due to properties such as reduced mean free paths of transport and increased surface-to-volume ratio, it becomes of critical importance to explicitly identify structure-property relationships on the nanometer scale. X-ray diffraction and spectroscopy are effective tools for probing size-, shape-, and structure-dependent material properties at the nanoscale. This article also discusses the recent development of in-situ soft X-ray spectroscopy cells, which enable investigation of critical solid/liquid or solid/gas interfaces under more practical conditions. These unique tools are providing a window into the thermodynamics and kinetics of hydrogenation and dehydrogenation reactions and informing a quantitative understanding of the fundamental energetics of hydrogen storage processes at the microscopic level. In particular, in-situ soft X-ray spectroscopies can be utilized to probe the formation of intermediate species, byproducts, as well as the changes in morphology and effect of additives, which all can greatly affect the hydrogen storage capacity, kinetics, thermodynamics, and reversibility. A few examples using soft X-ray spectroscopies to study these materials are discussed to demonstrate how these powerful characterization tools could be helpful to further understand the hydrogen storage systems.[a] Dr.

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Elucidating the mechanism of MgB₂initial hydrogenation via a combined experimental–theoretical study

Cited by 24 publications

References 42 publications

Kinetic Enhancement of Direct Hydrogenation of MgB₂ to Mg(BH₄)₂ upon Mechanical Milling with THF, MgH₂, and/or Mg

Kinetic Enhancement of Direct Hydrogenation of MgB₂ to Mg(BH₄)₂ upon Mechanical Milling with THF, MgH₂, and/or Mg

Heterostructures Built in Metal Hydrides for Advanced Hydrogen Storage Reversibility

In‐Situ/Operando X‐ray Characterization of Metal Hydrides

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Elucidating the mechanism of MgB2initial hydrogenation via a combined experimental–theoretical study

Cited by 24 publications

References 42 publications

Kinetic Enhancement of Direct Hydrogenation of MgB2 to Mg(BH4)2 upon Mechanical Milling with THF, MgH2, and/or Mg

Kinetic Enhancement of Direct Hydrogenation of MgB2 to Mg(BH4)2 upon Mechanical Milling with THF, MgH2, and/or Mg

Heterostructures Built in Metal Hydrides for Advanced Hydrogen Storage Reversibility

In‐Situ/Operando X‐ray Characterization of Metal Hydrides

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Elucidating the mechanism of MgB₂initial hydrogenation via a combined experimental–theoretical study

Kinetic Enhancement of Direct Hydrogenation of MgB₂ to Mg(BH₄)₂ upon Mechanical Milling with THF, MgH₂, and/or Mg

Kinetic Enhancement of Direct Hydrogenation of MgB₂ to Mg(BH₄)₂ upon Mechanical Milling with THF, MgH₂, and/or Mg