Magnesium Nanocrystals Embedded in a Metal–Organic Framework: Hybrid Hydrogen Storage with Synergistic Effect on Physi‐ and Chemisorption

Abstract

Hydrogen is considered to be a promising energy carrier for the future. In order to use it as a fuel, a compact, safe, and efficient hydrogen storage system should be developed. Porous metal-organic frameworks (MOFs) have attracted great attention as potential hydrogen storage materials, [1] because some MOFs can store large amount of H 2 (> 7 wt %) at 77 K and high pressures. [2] However, at room temperature, the H 2 storage capacities of MOFs drop to less than 1 wt % because interaction energies between the frameworks and H 2 are very low (4-8 kJ mol À1 ). There have been efforts to enhance the H 2 storage capacities of MOFs at ambient temperature by methods such as tuning the ligand, [3] generation of open metal sites, [4] and embedding palladium nanoparticles. [5] Despite these modifications, the improvements were not very satisfactory, and none of the materials have yet met the target of the US Department of Energy (DOE) of 2017.In search for a highly efficient material for H 2 storage, we have investigated the possibilities of hybrid materials that would be capable of storing H 2 by both physical adsorption and chemisorption. In particular, we have been interested in composite materials that consist of a MOF and magnesium nanoparticles. Magnesium chemisorbs H 2 at 773 K and 200 atm in the presence of MgI 2 catalyst. [6] Magnesium powder of 50-100 mm size made by ball milling slowly absorbs H 2 at 673 K and 10 bar. [7] The resulting magnesium hydride has a quite high dehydrogenation enthalpy ( % 75 kJ per mol H 2 ), [8] and therefore heavy-metal catalysts should be used to reduce the H 2 desorption temperature and improve kinetics, but they still do not allow effective dehydrogenation. [9] In the presence of heavy-metal catalysts, the H 2 absorption and release temperatures decrease as a result of destabilization of the metal hydride when the particle size of magnesium is reduced. [10] Nanosized magnesium particles have been synthesized by a variety of methods, such as ball-milling, [11] condensation of magnesium metal vapor, [12] plasma metal reaction, [13] infiltration of melted Mg in carbon material, [14] and sonoelectrochemistry [15] or chemical reduction of Mg precursors. [16] Some of the disadvantages of these methods are that they require a long process time, extremely high temperatures (898-1203 K), and electrically or chemically harsh conditions, and yet they provide inhomogeneous size distributions.Herein we report a simple method for fabrication of hexagonal-disk-shaped magnesium nanocrystals (Mg NCs) within a MOF. The fabrication of Mg nanocrystals inside MOFs is unprecedented. Previously, small Ag, Au, Ni, Pd, Ru, or Pt nanoparticles (size, 1.4-10 nm) were prepared in MOFs by immersing redox-active MOFs in the metal-ion solutions [5,17] or by reducing the metal precursors deposited in the MOFs. [18] The present nanocomposite is made by the thermal decomposition of air-sensitive bis-cyclopentadienyl magnesium (MgCp 2 ) vapor in a MOF, leading to the deposition of MG NCs in the MOF, a method t...

Organometallics and Related Molecules for Energy Conversion

“…However, due to their high surface energy and large surface area, the stability of M-NPs is severely decreased, which obstructs the control of size and shape with high uniformity. In particular, the use of M-NPs in porous MOFs with confined void spaces has been proved to be an efficient way of preventing aggregation [1,48,[246][247][248][249][250][251][252]. The nucleation and growth of M-NPs can be averted if the metal or alloy nanoparticles are confined within highly porous scaffold materials, which could be used for the fabrication of metal nanoparticles (NPs) with controlled sizes inside the pores, thereby circumventing a common issue of nanoparticle aggregation [242][243][244][245].…”

Targeted Manipulation of Metal–Organic Frameworks To Direct Sorption Properties

“…Guest incorporation can vary greatly and this includes the introduction of nanoparticles, [126][127][128] and organic molecules [129] such as photoswitches, in particular. Such a kind of modulation of adsorption properties of MOFs may be called guest-assisted adsorption.…”

“…Such a kind of modulation of adsorption properties of MOFs may be called guest-assisted adsorption. Guest incorporation can vary greatly and this includes the introduction of nanoparticles, [126][127][128] and organic molecules [129] such as photoswitches, in particular. [23]…”

“…For example, Mg nanoparticles were introduced to [Zn 4 O(atb) 2 ] n (SNU-90; atb = aniline-2,4,6-tribenzoate) based on the solvent-free organometallic gas-phase infiltration technique. [126] Materials with different loadings were prepared, namely Mg@SNU-90 with 1.26, 6.25, and 10.5 wt % Mg inside the pores. As expected the N 2 adsorption decreases with increasing magnesium inclusion.…”

Immobilization of Aluminum Borohydride Hexammoniate in a Nanoporous Polymer Stabilizer for Enhanced Chemical Hydrogen Storage

“…[8] Recently, nanoconfinement has offered new insights and solutions to current hydrogen-storage technology. [9] Theoretical calculations and preliminary experiments have shown that nanoconfined hydrides feature a significantly improved hydrogen-storage performance with respect to their bulk counterparts. [10] Two routes have been widely used to encapsulate hydrogenstorage materials into nanoscaffolds: solution impregnation and melt infiltration.…”