Fullerene-impregnated IRMOFs for balanced gravimetric and volumetric H2 densities: A combined DFT and GCMC simulations study

This paper aimed to study the effects of fullerene (C₆₀) impregnation on the isoreticular metal-organic framework (IRMOF) materials MOF-650 (ZnO₄ nodes were connected to azulenedicarboxylate linkers), MOF-5(ZnO₄ nodes were connected to benzenedicarboxylate linkers), and IRMOF-10 (ZnO₄ nodes were connected to biphenyldicarboxylate linkers) for H₂ storage, these IRMOFs had similar structures but different pore volumes and organic linkers. Density functional theory (DFT) and grand canonical monte Carlo (GCMC) calculations indicated that C₆₀ plays an important role in balancing the gravimetric and volumetric H₂ densities of the IRMOFs. The C₆₀@IRMOFs revealed improved volumetric density when H₂ was undersaturated but reduced gravimetric density under H₂ saturation. The saturated gravimetric H₂ density of the IRMOFs was decided by the free volume. At 77 K, C₆₀@MOF-650 had a gravimetric H₂ density of 5.3 wt% and volumetric H₂ density of 42 g/L under 10 bar, and C₆₀@IRMOF-10 had a gravimetric H₂ density of 7.4 wt% and volumetric H₂ density of 43 g/L under 18 bar. These values nearly meet the United States Department of Energy (DOE) gravimetric and volumetric H₂ density ultimate targets (gravimetric H₂ density, 6.5 wt%; volumetric H₂ density, 50  g/L) under ambient pressures. Among the studied IRMOFs, C₆₀@MOF-650 and C₆₀@IRMOF-10 demonstrated the best H₂ storage properties at 233 and 298 K.     

Tuning the hydrogen storage properties of MOF-650: A combined DFT and GCMC simulations study

Combined density functional theory and grand canonical monte Carlo (GCMC) calculations were performed to study the electronic structures and hydrogen adsorption properties of the Zn-based metal-organic framework MOF-650. The benzene azulenedicarboxylate linkers of MOF-650 were substituted by B atoms, N atoms, and boronic acid B(OH)₂ linkers, and the Zn atoms were substituted by Mg and Ca atoms. The calculated electronic densities of states (DOSs) of MOF-650 showed that introduction of B atoms reduces the band gap but damages the structure of MOF-650. Introduction of single N bonds cannot provide active electrons to attract H₂ molecules. Thus, substitutions of B and N into MOF-650 are not suggested. B(OH)₂ substitute in MOF-650 decreased its band gap, slightly improved its hydrogen storage ability and made H₂ molecules more intensively distributed besides organic linkers. GCMC calculations were carried out by estimating the H₂ storage amount of the pure and modified MOFs at 77 and 298 K and from 1 bar to 20 bar. B(OH)₂ linker and Mg/Ca co-doped MOF-650 showed increased H₂ adsorption by approximately 20 wt%. The adsorption of H₂ around different bonds showed the order N–C < C = C < B–C < C–O < B–O.     

Catenated metal-organic frameworks: Promising hydrogen purification materials and high hydrogen storage medium with further lithium doping

Based on the first-principles derived force fields and grand canonical Monte Carlo simulations, we find that the catenated metal-organic frameworks outperform the noncatenated structures, in terms of H₂ separation from other gases (CH₄, CO and CO₂) and H₂ adsorption by Li doping. A system utilizing IRMOF-11 (or IRMOF-13) for hydrogen separation and Li-doped IRMOF-9 for hydrogen storage is therefore proposed, with hydrogen uptake of 4.91 wt% and 36.6 g/L at 243 K and 100 bar for Li-doped IRMOF-9, which is close to the 2017 DOE target. It is promising to find appropriate microporous materials for hydrogen purification and storage at ambient conditions with structure catenated.     

Synthesis and hydrogen-storage performance of interpenetrated MOF-5/MWCNTs hybrid composite with high mesoporosity

Metal-organic frameworks (MOFs) exhibiting high surface area and tunable pore size own broad application prospects. Compared with existing MOFs, MOF-5 [Zn₄O(bdc)₃] is a promising hydrogen storage material due to high H₂ uptake capacity and thermostability. However, further wider applications of MOF-5 have been limited because atmospheric moisture levels cause MOF-5 instability. MOF-5 and multi-walled carbon nanotubes (MWCNTs) hybrid composite (denoted MOFMC) can enhance stability toward ambient moisture and improve hydrogen storage capacity. In this paper, the MOFMC, which has an interpenetrated structure with high mesoporosity, was synthesized. The MOFMC is denoted as Int-MOFMC-meso. It stored 2.02 wt% H₂ at 77 K under 1 bar, which is higher than the MOF-5 with similar structure and the earlier reported MOFMC material. Moreover, the Int-MOFMC-meso can also show more excellent performance of thermostability and moisture stability than the MOF-5 with similar structure.     

Strain and defect engineering of graphene for hydrogen storage via atomistic modelling

The adsorption of hydrogen molecules on monolayer graphene is investigated using molecular dynamics simulations (MDS). Interatomic interactions of the graphene layer are described using the well-known AIREBO potential, while the interactions between graphene and hydrogen molecule are described using Lennard-Jones potential. In particular, the effect of strain and different point defects on the hydrogen storage capability of graphene is studied. The strained graphene layer is found to be more active for hydrogen and show 6.28 wt% of H₂ storage at 0.1 strain at 77 K temperature and 10 bar pressure. We also studied the effect of temperature and pressure on the adsorption energy and gravimetric density of H₂ on graphene. We considered different point defects in the graphene layer like monovacancy (MV), Stone Wales (SW), 5-8-5 double vacancy (DV), 555–777 DV, and 5555-6-7777 DV which usually occur during the synthesis of graphene. At 100 bar pressure, graphene with 1% concentration of MV defects leads to 9.3 wt% and 2.208 wt% of H₂ storage at 77 K and 300 K, respectively, which is about 42% higher than the adsorption capacity of pristine graphene. Impact of defects on the critical stress and strain of defected graphene layers is also studied.     

Li decorated heteroborospherene C4B32 as high capacity and reversible hydrogen storage media: A DFT study

In this work, adsorption of H₂ molecules on heteroborospherene C_2v C₄B₃₂ decorated by alkali atoms (Li) is studied by density functional theory calculations. The interaction between Li atoms and C₄B₃₂ is found to be strong, so that it prevents agglomeration of the former. An introduced hydrogen molecule tilts toward the Li atoms and is stably adsorbed on C₄B₃₂. It is obtained that Li₄C₄B₃₂ can store up to 12H₂ molecules with hydrogen uptake capacity of 5.425 wt% and average adsorption energy of ?0.240 eV per H₂. Dynamics simulation results show that 6H₂ molecules can be successfully released at 300 K. Obtained results demonstrate that Li decorated C₄B₃₂ is a promising material for reversible hydrogen storage.     

Doping the transition metal atom Fe,Co, Ni into C48B12 fullerene for enhancing H2 capture: A theoretical study

The feasibility of transition metal coated fullerene cages M₁₂C₄₈B₁₂(M = Fe, Co, and Ni) for hydrogen storage is investigated by the pseudopotential density functional theory. Fe₁₂C₄₈B₁₂(Co₁₂C₄₈B₁₂ and Ni₁₂C₄₈B₁₂) adsorbs 60(48 and 48) H₂ with moderate average adsorption energy of 0.50(0.45 and 0.32) eV/H₂. The gravimetric hydrogen density of Fe₁₂C₄₈B₁₂(Co₁₂C₄₈B₁₂ and Ni₁₂C₄₈B₁₂) can reach 8.7(6.8 and 6.8) wt%. The Dewar–Kubas interaction dominates the adsorption of H₂ on the outer surface of Fe₁₂C₄₈B₁₂(Co₁₂C₄₈B₁₂ and Ni₁₂C₄₈B₁₂). Therefore, the stable M₁₂C₄₈B₁₂(M = Fe, Co, and Ni) cages can be applied as candidates for hydrogen storage under near-ambient conditions.     

Enhancement of hydrogen storage properties of metal-organic framework-5 by substitution (Zn,Cd and Mg) and decoration (Li,Be and Na)

Two strategies of decoration by three elements Z = Li, Be and Na in cyclic site, and substitution of Zn by Mg and Cd in unit cell were used in the framework of functional density theory to tune the hydrogen storage properties of metal-organic framework-5 (MOF-5) based on Zn whose decomposition temperature and initial gravimetric capacity are 300 K and 1.57 wt% respectively.Based on the adsorption of hydrogen molecules in the crystal surface at three different adsorption sites with three orientations of H₂, we show that our system may reach a maximum gravimetric storage capacity of 4.09 wt% for multiple hydrogen molecules. Moreover, the functionalization of Z combined to the substitution, shows an exceptional improvement of hydrogen storage properties. For example, Mg-MOF-5 decorated with Li₂ has a capacity up to 5.41 wt% and 513 K as desorption temperature.     

Hydrogen permeation and diffusion in densified MOF-5 pellets

The metal-organic framework Zn₄O (BDC)₃ (BDC = 1,4-bezene dicarboxlate), also known as MOF-5, has demonstrated considerable adsorption of hydrogen, up to 7 excess wt.% at 77 K. Consequently, it has attracted significant attention for vehicular hydrogen storage applications. To improve the volumetric hydrogen density and thermal conductivity of MOF-5, prior studies have examined the hydrogen storage capacities of dense MOF-5 pellets and the impact of thermally conductive additives such as expanded natural graphite (ENG). However, the performance of a storage system based on densified MOF-5 powders will also hinge upon the rate of hydrogen mass transport through the storage medium. In this study, we further characterize MOF-5 compacts by measuring their hydrogen transport properties as a function of pellet density (ρ = 0.3–0.5 g cm⁻³) and the presence/absence of ENG additions. More specifically, the Darcy permeability and diffusivity of hydrogen in pellets of neat MOF-5, and composite pellets consisting of MOF-5 with 5 and 10 wt.% ENG additions, have been measured at ambient (296 K) and liquid nitrogen (77 K) temperatures. The experimental data suggest that the H₂ transport in densified MOF-5 is strongly related to the MOF-5 pellet density ρ.     

Yttrium-decorated C48B12 as hydrogen storage media: A DFT study

We report a density functional theory calculation dedicated to analyze the behavior of hydrogen adsorption on Yttrium-decorated C₄₈B₁₂. Electron deficient C₄₈B₁₂ is found to promote charge transfer between Y atom and substrate leading to an enhanced local electric field which can significantly improve the hydrogen adsorption. The analysis shows that Y atoms can be individually adsorbed on the pentagonal sites without clustering of the metal atoms, and each Y atom can bind up to six H₂. molecules with an average binding energy of −0.46 eV/H₂, which is suitable for ambient condition hydrogen storage. The Y atoms are found to trap H₂ molecules through well-known “Kubas-type” interaction. Our simulations not only clarify the mechanism of the reaction among C₄₈. B₁₂, Y atoms and H₂ molecules, but also predict a promising candidate for hydrogen storage application with high gravimetric density (7.51%).     

Enhanced hydrogen storage capacity of Pt-loaded CNT@MOF-5 hybrid composites

We report on an easy synthesis method for the preparation of a hybrid composite of Pt-loaded MWCNTs@MOF-5 [Zn₄O(benzene-1,4-dicarboxylate)₃] that greatly enhanced hydrogen storage capacity at room temperature. To prepare the composite, we first prepared Pt-loaded MWCNTs, which were then incorporated in-situ into the MOF-5 crystals. The obtained composite was characterized by various techniques such as powder X-ray diffractometry, optical microscopy, porosimetry by nitrogen adsorption, and hydrogen adsorption. The analyses confirmed that the product has a highly crystalline structure with a Langmuir specific surface area of over 2000 m²/g. The hybrid composite was shown to have a hydrogen storage capacity of 1.25 wt% at room temperature and 100 bar, and 1.89 wt% at cryogenic temperature and 1 bar. These H₂ storage capacities represent significant increases over those of virgin MOF-5s and Pt-loaded MWCNTs.     

Study on catalytic effect and mechanism of MOF (MOF = ZIF-8, ZIF-67, MOF-74) on hydrogen storage properties of magnesium

Using a deposition-reduction method, Mg/MOF nanocomposites were prepared as composites of Mg and metal-organic framework materials (MOFs = ZIF-8, ZIF-67 and MOF-74). The addition of MOFs can enhance the hydrogen storage properties of Mg. For example, within 5000 s, 0.6 wt%, 1.2 wt%, 2.7 wt%, 3.7 wt% of hydrogen were released from Mg, Mg/MOF-74, Mg/ZIF-8, Mg/ZIF-67, respectively. Activation energy values of 198.9 kJ mol⁻¹ H₂, 161.7 kJ mol⁻¹ H₂, 192.1 kJ mol⁻¹ H₂ were determined for the Mg/ZIF-8, Mg/ZIF-67, Mg/MOF-74 hydrides, which are 6 kJ mol⁻¹ H₂, 43.2 kJ mol⁻¹ H₂, and 12.8 kJ mol⁻¹ H₂ lower than that of Mg hydride, respectively. Moreover, the cyclic stability characterizing Mg hydride was significantly improved when adding ZIF-67. The hydrogen storage capacity of the Mg/ZIF-67 nanocomposite remained unchanged, even after 100 cycles of hydrogenation/dehydrogenation. This excellent cyclic stability may have resulted from the core-shell structure of the Mg/ZIF-67 nanocomposite.     

Sc/Ti-decorated and B-substituted defective C60 as efficient materials for hydrogen storage

Doping heteroatoms and producing defects are perfect methods to improve the hydrogen storage property of TM-decorated carbon materials. In this view, four novel Sc/Ti-decorated and B- substituted defective C₆₀ fullerenes (B₂₄C₂₄) are explored. The special stability, large specific surface, uniform distribution of the metal and positively charged states make these four fullerenes have high hydrogen storage capacities. Especially, each Sc atom in Sc₆B₂₄C₂₄(B₄) can adsorb up to five H₂ molecules with a storage capacity of 6.80 wt %. The adsorbed H₂ molecules in Sc₆B₂₄C₂₄(B₄)–30H₂ begin to relax at 190 K and are 100% released at 290 K. Moreover, a comparative study is carried out for hydrogen storage properties of Sc-decorated B₄, C₄, or N₄ coordination environments. These results provide a new focus on the nature of B-, and N-substituted defective carbon nanomaterials.     

Mg@C60 nano-lamellae and its 12.50 wt% hydrogen storage capacity

The design and synthesis of new hydrogen storage materials with high capacity are the prerequisite for extensive hydrogen energy application which can be achieved by multi-site hydrogen storage. Herein, a Mg@C₆₀ nano-lamellae structure with multiple hydrogen storage sites has been prepared through a simple ball-milling process in which Mg nanoparticles (∼5 nm) are homogeneously dispersed on C₆₀ nano-lamellae. The as-obtained C₆₀/Mg nano-lamellae displays an excess hydrogen uptake of 12.50 wt% at 45 bar, which is far higher than the theoretical value (7.60 wt%) of metal Mg and the US Department of Energy (DOE) target (5.50 wt%, 2020 year), also the experimental values reported by now. The enhanced hydrogen storage mainly comes from several storage sites: MgH₂, H_x–C₆₀ (CH chemical bonding), H₂@C₆₀ (the endohedral H₂ in C₆₀). Interestingly, the hybridization of Mg and C₆₀ not only facilitate the dissociation of H₂ molecules to form CH bonding with C₆₀, but also promote the deformation of C₆₀ and access H₂ molecules into the cavity of C₆₀. This work provides new insight into the underlying chemistry behind the high hydrogen storage capacities of a new class of hydrogen storage materials, fullerene/alkaline-earth metals nanocomposites.     

Hydrogen storage properties of Li-decorated B2S monolayers: A DFT study

Hydrogen storage properties of Li functionalized B₂S honeycomb monolayers are studied using density functional theory calculations. The binding of H₂ molecules to the clean B₂S sheet proceeds through physisorption. Dispersed Li atoms on the monolayer surface increase both the hydrogen binding energies and the hydrogen storage capacities significantly. Additionally, ab initio molecular dynamics calculations show that there is no kinetic barrier during H₂ desorption from lithiated B₂S. Among the studied B₈S₄Li_x (x = 1, 2, 4, and 12) compounds, the B₈S₄Li₄ is found to be the most promising candidate for hydrogen storage purposes; with a 9.1 wt% H₂ content and 0.14 eV/H₂ average hydrogen binding energy. Furthermore, a detailed analysis of the electronic properties of the B₈S₄Li₄ compound before and after H₂ molecule adsorption confirms that the interactions between Li and H₂ molecules are of electrostatic nature.     

Selective decorating of BC3 and C3N nanosheets with single metal atom for hydrogen storage

Employing first-principles calculations, we have studied the structure, stability and hydrogen storage efficiency of pristine and defective BC₃ and C₃N monolayer functionalized by a variety of single metal adatoms. It is found that single Sc adatom, acting as an optimal dopant on perfect BC₃ monolayer, is able to adsorb up to nine H₂ molecules as strongly as around 0.24 eV/H₂, which allows for a hydrogen storage capacity of 7.19 wt% for Sc atoms stably adsorbing on double sides of BC₃ monolayer with eighteen H₂ molecules (18H₂@2Sc/BC₃). Moreover, the desorption temperature and thermodynamical stability of multiple H₂ adsorbed Sc-decorated BC₃ sheet have been addressed and the saturate configuration of 18H₂@2Sc/BC₃ is predicted to be stable at mild temperatures and pressures, i.e. less than 250 K at 1 bar, or larger than 24 bar at room temperature. This study indicates that the Sc-decorated BC₃ monolayer could be a potential H₂ storage candidate, and provides an instructive guidance for designing metal-functionalized carbon-based sheets in hydrogen storage.     

The performance of green carbon as a backbone for hydrogen storage materials

We investigate the use of carbonized bamboo, which has an organic porous structure, as a hydrogen storage material. Bamboo samples were thermally treated at 800, 900, 1000, and 1100 °C for 24 h. The pore size and hydrogen storage capacity of each sample were measured by N₂ and H₂ gas sorption up to 1.13 bar at 77 K. The maximum hydrogen storage was exhibited by the sample treated at 900 °C, which reached 1.35 wt% at 1.13 bar/77 K. The results showed that the bamboo, one of the green carbons, has the potential to be used as an environmental-friendly carbon backbone for hybrid hydrogen storage materials.     

Effect of platinum doping of activated carbon on hydrogen storage behaviors of metal-organic frameworks-5

In this work, we prepared platinum doped on activated carbons/metal-organic frameworks-5 hybrid composites (Pt-ACs-MOF-5) to obtain a high hydrogen storage capacity. The surface functional groups and surface charges were confirmed by Fourier transfer infrared spectroscopy (FT-IR) and zeta-potential measurement, respectively. The microstructures were characterized by X-ray diffraction (XRD). The sizes and morphological structures were also evaluated using a scanning electron microscopy (SEM). The pore structure and specific surface area were analyzed by N₂/77 K adsorption/desorption isotherms. The hydrogen storage capacity was studied by BEL-HP at 298 K and 100 bar. The results revealed that the hydrogen storage capacity of the Pt-ACs-MOF-5 was 2.3 wt.% at 298 K and 100 bar, which is remarkably enhanced by a factor of above five times and above three times compared with raw ACs and MOF-5, respectively. In conclusion, it was confirmed that Pt particles played a major role in improving the hydrogen storage capacity; MOF-5 would be a significantly encouraging material for a hydrogen storage medium as a receptor.     

Synthesis and hydrogenation properties of lithium magnesium nitride

Li-Mg-N-H systems have been focused on as one of the most promising hydrogen storage systems owing to their high hydrogen contents and binary nitride, LiMgN, has a high gravimetric storage density of 8.2 wt% H₂. We synthesized LiMgN by a hydriding thermal reaction between Mg and LiNH₂ under various H₂ pressures and a subsequent dehydrogenation reaction, and optimized conditions for the formation of pure LiMgN. The results demonstrate that pure LiMgN can be produced using hydriding thermal synthesis at 80 bar H₂ pressure and 723 K. The hydriding and dehydriding characteristics of as-synthesized LiMgN were investigated by a Sievers’ type instrument. The reaction product LiMgN can be rehydrogenated by reacting with H₂ under 80 bar of hydrogen pressure at 573 K, and then released under less than 0.5 bar at 573 K. The measured H₂ capacity is about 6.8 wt% during the hydrogenation process.     

Density functional studies on the hydrogen storage capacity of boranes and alanes based cages

The hydrogen storage (H-storage) capacity of various boranes and alanes have been investigated using density functional theory (DFT) based M05-2X method employing 6–31+G** basis set. The changes in the H-storage capacities of borane and alane upon substitution of antipodal atoms in the cages by C, Si, and N have also been investigated. It is found from the calculations that a maximum of 20 H₂ molecules can be adsorbed on the deltahedron faces of these cages. The maximum gravimetric density has been observed for boranes when compared to alanes. The H-storage capacity of closo-borane dianion [B₁₂H₁₂]²⁻, monocarborane [CB₁₁H₁₂]¹⁻, dicarborane [C₂B₁₀H₁₂], and closo-azaborane [NB₁₁H₁₂] cages is almost similar (∼22 wt.%). Among these cages, B_BB dianion show higher binding energy (BE) and BE per H₂ molecule (BE/nH₂) which are 181.06 and 9.03 kJ/mol, respectively. In the case of alanes, dicarbalane [C₂Al₁₀H₁₂] has maximum H-storage capacity of 11.6 wt.%. Based on these findings, a new MOF with carborane (MOF-5_CC) as linker has been designed. The calculation on the new MOF-5B_CC reveals that it has H-storage capacity of 6.4 wt.% with BE/nH₂ of 3.02 kJ/mol.