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.     

Heterofullerene C48B12-impregnated MOF-5 and IRMOF-10 for hydrogen storage: A combined DFT and GCMC simulations study

The H₂ storage properties of isoreticular metal-organic framework materials (IRMOFs), MOF-5 and IRMOF-10, impregnated with different numbers and types of heterogeneous C₄₈B₁₂ molecules were investigated using density functional theory and grand canonical Monte Carlo (GCMC) calculations. The excess hydrogen adsorption isotherms of IRMOFs at 77 K within 20 bar indicate that suitable number and type of C₄₈B₁₂ molecules play a crucial role in improving the H₂ storage properties of IRMOFs. Among the studied pure and nC₄₈B₁₂ (n = 1, 2, 4, 8) in Ci symmetry impregnating into MOF-5, at 77 K under 6 bar, MOF-5-4C₄₈B₁₂ with a 3.5 wt% and 29.9 g/L hydrogen storage density, and at 77 K under 12 bar, the pure MOF-5 with a 4.9 wt% and 31.0 g/L hydrogen storage density has the best hydrogen storage properties. Whereas, among the studied pure and nC₄₈B₁₂ (n = 1, 2, 4, 8) in S6 symmetry impregnating into IRMOF-10, IRMOF-10-8C₄₈B₁₂ always shows the best hydrogen storage properties among the pure and C₄₈B₁₂-impregnated IRMOF-10 at 77 K within 20 bar. IRMOF-10-8C₄₈B₁₂ has a 6.0 wt% and 34.6 g/L hydrogen storage density at 77 K under 6 bar, and has a 7.1 wt% and 41.4 g/L hydrogen storage density at 77 K under 12 bar. The confinement effect of IRMOFs on C₄₈B₁₂ molecules, and steric hindrance effect of C₄₈B₁₂ molecules on IRMOFs mainly affects the H₂ uptake capacity by comparing the absolute H₂ molecules in individual IRMOFs units, C₄₈B₁₂ molecules, and IRMOFs-nC₄₈B₁₂ compounds. The absolute hydrogen adsorption profiles show that eight C₄₈B₁₂ molecules impregnating into MOF-5 can exert obvious steric effects for H₂ adsorption. The saturated gravimetric and volumetric H₂ densities of IRMOF-10-8C₄₈B₁₂ higher than those of MOF-5-8C₄₈B₁₂ due to with larger free volume.     

Multiscale study of the structure and hydrogen storage capacity of an aluminum metal-organic framework

First-principles calculations based on density functional theory and Grand Canonical Monte Carlo (GCMC) simulations are carried out to study the structure of a new Aluminum Metal-Organic Framework, MOF-519, and the possibility of storing molecular hydrogen therein. The optimized structure of the inorganic secondary building unit (SBU) of MOF-519 formed by eight octahedrally coordinated aluminum atoms is presented. The different storage sites of H₂ inside the SBU and the BTB ligand are explored. Our results reveal that the SBU exhibits two different favorable physisorption sites with adsorption energies of ?12.2 kJ/mol and ?1.2 kJ/mol per hydrogen molecule. We have also shown that each phenyl group of BTB has three stable H₂ adsorption sites with adsorption energies between ?6.7 kJ/mol and ?11.37 kJ/mol. Using GCMC simulations; we calculated the molecular hydrogen (H₂) gravimetric and volumetric uptake for the SBU and MOF-519. At 77 K and 100 bar pressure, the hydrogen uptake capacity of SBU is considerably enhanced, reaching 16 wt.%. MOF-519 has a high gravimetric uptake, 10 wt.% at 77 K and 4.9 wt.% at 233 K. It has also a high volumetric capacity of 65 g/L at 77 K and 20.3 g/L at 233 K, indicating the potential of this MOF for hydrogen storage applications.     

A model study of effect of M = Li,Na, Be,Mg, and Al ion decoration on hydrogen adsorption of metal-organic framework-5

The effect of light metal ion decoration of the organic linker in metal-organic framework MOF-5 on its hydrogen adsorption with respect to its hydrogen binding energy (ΔB.E.) and gravimetric storage capacity is examined theoretically by employing models of the form MC₆H₆:nH₂ where M = Li⁺, Na⁺, Be²⁺, Mg²⁺, and Al³⁺. A systematic investigation of the suitability of DFT functionals for studying such systems is also carried out. Our results show that the interaction energy (ΔE) of the metal ion M with the benzene ring, ΔB.E., and charge transfer (Q_trans) from the metal to benzene ring exhibit the same increasing order: Na⁺ < Li⁺ < Mg²⁺ < Be²⁺ < Al³⁺. Organic linker decoration with the above metal ions strengthened H₂-MOF-5 interactions relative to its pure state. However, amongst these ions only Mg²⁺ ion resulted in ΔB.E. magnitudes that were optimal for allowing room temperature hydrogen storage applications of MOF-5. A much higher gravimetric storage capacity (6.15 wt.% H₂) is also predicted for Mg²⁺-decorated MOF-5 as compared to both pure MOF-5 and Li⁺-decorated MOF-5.     

Stabilized Li-decoration and enhanced hydrogen storage on reduced graphene oxides

Hydrogen storage properties of Li-decorated graphene oxides containing epoxy and hydroxyl groups are studied by using density functional theory. The Li atoms form Li₄O/Li₃OH clusters and are anchored strongly on the graphene surface with binding energies of −3.20 and −2.84 eV. The clusters transfer electrons to the graphene substrate, and the Li atoms exist as Li⁺ cations with strong adsorption ability for H₂ molecules. Each Li atom can adsorb at least 2H₂ molecules with adsorption energies greater than −0.20 eV/H₂. The hydrogen storage properties of Li-decorated graphene at different oxidation degrees are studied. The computations show that the adsorption energy of H₂ is −0.22 eV/H₂ and the hydrogen storage capacity is 6.04 wt% at the oxidation ratio O/C = 1/16. When the O/C ratio is 1:8, the storage capacity reaches 10.26 wt% and the adsorption energy is −0.15 eV/H₂. These results suggest that reversible hydrogen storage with high recycling capacities at ambient temperature can be realized through light-metal decoration on reduced graphene oxides.     

Toward hydrogen storage material in fluorinated zirconium metal-organic framework (MOF-801): A periodic density functional theory (DFT) study of fluorination and adsorption

In this study, the structural properties and hydrogen adsorption energy of the fluorinated metal-organic framework (MOF)-801 were evaluated using density functional theory (DFT). We calculated the Zr–F bond distance to be approximately 0.225 nm, which is longer than the bond distance in zirconium fluoride compounds. Due to the electronegativity of F, this site was considered as an adsorption site for hydrogen. We determined the adsorption energy to be ?5 kcal/mol per hydrogen (H₂) molecule, which is higher than that of H₂ in pristine MOF. This value is also slightly lower than the adsorption energy in a metal-decorated MOF. The introduction of F atoms is determined to enhance the binding capacity of MOF-801.     

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.     

Interaction of H2 with fragments of MOF-5 and its implications for the design and development of new MOFs: A computational study

The interaction energies (IEs) of H₂ and various organic ligands have been computed using coupled-cluster method with singles, doubles, and noniterative triples (CCSD(T)) at the complete basis set (CBS) limit. The density fitting-density functional theory-symmetry adapted perturbation theory (DF-DFT-SAPT) approach has been used to probe the nature of interaction between H₂ and organic linkers. It has been found that dispersive interaction predominantly stabilizes the intermolecular complex formation of H₂ on a variety of organic linkers. Furthermore, H₂ binding affinity of inorganic connectors is improved by partial isomorphic substitution of Zn by different metal ions such as Fe, Co, Ni and Cu. A new modified metal-organic framework (MOF-5 M) has been designed based upon the insight from the organic and inorganic fragments. The present study provides valuable information required for the design of novel MOFs with improved affinity for H₂ adsorption.     

Effects of structural modifications on the hydrogen storage capacity of MOF-5

Here, we describe the preparation of four structurally modified MOF-5s and carried out a systematic study of the effects of the structural modifications on the evolution of the crystal structure, pore characteristics, and H₂ capacities of MOF-5s. The structural modifications were found to significantly influence the pore characteristics, and the specific surface areas of the MOF-5s decreased with the evolution of an ultrafine porosity. These changes were correlated with an increase in the H₂ storage capacity of the MOF-5 (from 1.2 to 2.0 wt% at −196 °C and 1 bar). The structural modifications also enhanced the thermal stability of the MOF-5s (the decomposition temperature increased from 438 °C to 510 °C). These results are particularly useful for the design of favorable MOF-based adsorbents with a high H₂ uptake coupled with a high thermal stability.     

Molecular simulation on hydrogen storage properties of five novel covalent organic frameworks with the higher valency

With the methods of density functional theory (DFT) and molecular simulations, we have investigated the structural characteristics and hydrogen storage properties of five new reported boron-phosphorus cube based covalent organic frameworks (BP-COFs) with the higher valency. The structural parameters of five BP-COFs were researched by the numeric Monte Carlo (NMC) method, and the hydrogen adsorption properties were studied with grand canonical Monte Carlo (GCMC) simulations under the pressure of 0.1 bar–100 bar at both 77 K and 298 K. The results reveal that BP-COF-4 and BP-COF-5 possess the higher hydrogen adsorption capacities than BP-COF-1 to BP-COF-3 at both 77 K and 298 K. The possible methods to improve the H₂ adsorption properties of five BP-COFs are also proposed. We hope this study may provide some reference and inspiration for exploring new COFs with the higher valency as high-performance hydrogen storage materials in future.     

A study on the hydrogen storage performance of graphene–Pd(T)–graphene structure

The 1–6 H₂ molecule adsorption energy and electronic properties of sandwich graphene–Pd(T)–Graphene (G–Pd(T)–G) structure were studied by the first-principle analysis. The binding energies, adsorption energies, and adsorption distances of Pd atoms-modified single-layer graphene and bilayer graphene with H₂ molecules at B, H, T adsorption sites were calculated. In bilayer graphene, the adsorption properties at T sites were found to be more stable than those at B and H sites. The binding energy of Pd atoms (4.16 eV) on bilayer graphene was higher than the experimental cohesion energy of Pd atoms (3.89 eV), and this phenomenon eliminated the impact of metal clusters on adsorption properties. It was found that three H₂ molecules were stably adsorbed on the G–Pd(T)–G structure with an average adsorption energy of 0.22 eV. Therefore, it can be speculated that G–Pd(T)–G is an excellent hydrogen storage material.     

A density functional theory and grand canonical Monte Carlo simulations study the hydrogen storage on the Li-decorated net-τ

To find ideal hydrogen storage media, hydrogen storage performance of Li decorated net-τ has been investigated by first-principles calculations. Maximum 6 Li atoms are adsorbed on net-τ, with the average binding energy of 2.15 eV for per Li atom. Based on 6Li-decorated net-τ, up to twenty H₂ molecules are adsorbed, with a high H₂ storage capacity of 12.52 wt% and an appropriate adsorption energy of 0.21 eV/H₂. Finally, H₂ uptake performance is measured by GCMC simulations. Our results suggest that Li-decorated net-τ may be a promising hydrogen storage medium under realistic conditions.     

Hydrogen adsorption of O/N-rich hierarchical carbon scaffold decorated with Ni nanoparticles: Experimental and computational studies

Hierarchical carbon scaffold (HCS) with multi-porous structures, favoring hydrogen diffusion and physisorption is doped with 2–10 wt % Ni for storing hydrogen at ambient temperature. Due to N- and O-rich structure of melamine-formaldehyde resin used as carbon precursor, homogeneous distribution of heteroatoms (N and O) in HCS is achieved. 2 wt % Ni-doped HCS shows the highest hydrogen capacity up to 2.40 wt % H₂ (T = 298 K and p (H₂) = 100 bar) as well as excellent reversibility of 18.25 g H₂/L and 1.25 wt % H₂ (T = 298 K and p (H₂) = 50 bar) and electrical production from PEMFC stack up to 0.7 Wh upon eight cycles. Computations and experiments confirm strong interactions between Ni and heteroatoms, leading to uniform distribution small particles of Ni. This results in enhancing reactive surface area for hydrogen adsorption and preventing agglomeration of Ni nanoparticles upon cycling. Ni K-edge XANES spectra simulated from the optimized structure of Ni-doped N/O-rich carbon using DFT calculations are consistent with the experimental spectra and suggest electron transfer from Ni to hydrogen to form Ni–H bond upon adsorption. Considering low desorption temperature (323 K), not only chemisorbed hydrogen is involved in adsorption mechanisms but also physisorption and spillover of hydrogen.     

Investigation of cryo-adsorption hydrogen storage capacity of rapidly synthesized MOF-5 by mechanochemical method

Comparisons were made between the samples mechanochemically (MOF-5(M)) and solvothermally (MOF-5(S)) prepared for the development of efficient hydrogen storage medium. Synthesized samples were undergone structural characterization as well as adsorption equilibrium measurements of hydrogen at temperature-pressure range 77 K–87 K and 0.1–10 MPa. Grand Canonical Monte Carlo (GCMC) simulations were further conducted to study the behaviors of hydrogen molecules adsorbed on MOF-5. It shows that, besides the advantage of large scale synthesis and a lower cost, mechanochemical method respectively brings about 207% and 90.5% increments in the specific surface area and the maximum excess adsorption capacity of hydrogen at 77 K within pressure range 0–10 MPa. Results also reveal that the crystal within MOF-5(M) is regular and distributing uniformly with a mean size only one tenth of that of the MOF-5(S); at 77 K within pressure range 0–10 MPa, Toth equation can predict the adsorption equilibrium data of hydrogen on two MOF-5 samples with a mean relative error less than 1.5%. It suggests that MOF-5(M) is more promising for hydrogen storage by adsorption for practical applications.     

Lithium decoration of boron-doped hybrid fullerenes and nanotubes as a novel 3D architecture for enhanced hydrogen storage: A DFT study

A three dimensional (3D) dumbbell-like nanostructure composed by interconnected fullerenes and nanotubes with Lithium decoration and boron-doping (37Li@C₁₃₉B₃₁) has been proposed in virtue of density functional theory (DFT) and first-principles molecular dynamics (MD) simulations which shows excellent geometric and thermal stability. First-principles calculations are performed to investigate the hydrogen adsorption onto the 37Li@C₁₃₉B₃₁. The results indicate that B substitution can improve the metal binding and the average binding energy of 37 adsorbed Li atoms on the C₁₃₉B₃₁ (2.79 eV) is higher than the cohesive energy of bulk Li (1.63 eV) suppressing the clustering. Meanwhile, the H₂ storage gravimetric density of 178H₂@37Li@C₁₃₉B₃₁ reaches up to 15.9 wt% higher than the year 2020 target from the US department of energy (DOE). The average adsorption energy of H₂ molecules falls in a desirable range of 0.18–0.27 eV. Moreover, grand canonical ensemble Monte Carlo (GCMC) simulations reveal that at room temperature the hydrogen gravimetric density (HGD) adsorbed on 37Li@C₁₃₉B₃₁ reaches up to 11.6 wt% at 100 bars higher than the DOE 2020 target. Our multiscale simulations indicate that our proposed nanostructure provides a promising medium for hydrogen storage.     

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.     

Ti decorated heterocyclic rings for hydrogen storage

This study uses first-principles calculations to investigate and compare the hydrogen storage properties of Ti doped benzene (C₆H₆Ti) and Ti doped borazine (B₃N₃H₆Ti) complexes. C₆H₆Ti and B₃N₃H₆Ti complex each can adsorb four H₂ molecules, but the former has a 0.11 wt% higher H₂ uptake capacity than the latter. Ti atoms bind to C₆H₆ more strongly than B₃N₃H₆. The hydrogen adsorption energies with Gibbs free energy correction for C₆H₆Ti and B₃N₃H₆Ti complexes are 0.17 and 0.45 eV, respectively, indicating reversible hydrogen adsorption. The hydrogen adsorption properties of C₆H₆Ti have also been studied after boron (B) and nitrogen (N) atom substitutions. Several B and N substituted structures between C₆H₆Ti and B₃N₃H₆Ti with different boron and nitrogen concentration and at different positions were considered. Initially, one boron and one nitrogen atom is substituted for two carbon atoms of benzene at three different positions and three different structures are obtained. Seven structures are possible when four carbon atoms of benzene are replaced by two boron and two nitrogen atoms at different positions. The hydrogen storage capacity of the C₆H₆Ti complex increases as boron and nitrogen atom concentrations increases. The positions of substituted boron and nitrogen atoms have less impact on H₂ uptake capacity for the same B and N concentration. The position and concentration of B and N affects the H₂ adsorption energy as well as the temperature and pressure range for thermodynamically favorable H₂ adsorption. The H₂ desorption temperature for all the complexes is found to be higher than 250 K indicates the stronger binding of H₂ molecules with these complexes.     

Computational screening of metal-organic frameworks with open copper sites for hydrogen purification

With the increasing demand for environmental protection worldwide, metal-organic frameworks (MOFs) have been pivotal in the clean energy domain. Due to the high surface areas, large porosities and structural tunability, they are promising for the adsorption separation of H₂/CH₄ mixtures. High-throughput computational screening was adopted to identify the optimal adsorbents for hydrogen purification from 502 MOFs with open copper sites. Firstly, the adsorption performance of H₂/CH₄ mixture in 440 MOFs, which exhibit non-zero surface area and over -3.8 Å largest cavity diameter (LCD), was calculated using grand canonical Monte Carlo (GCMC) simulations at 300 K and various pressures. Secondly, we identified the top 9 high-performance MOFs by evaluating the ranking of candidate adsorbent performance according to a combination metric of adsorption performance score (APS, the product of adsorption capacity of CH₄ and selectivity of CH₄ over H₂) and percent regenerability (R%). PCN-39 and MOF-505 exhibit high APS of 101 mol kg⁻¹ and 67.9 mol kg⁻¹, respectively, promising for hydrogen purification. Subsequently, the breakthrough curves of H₂/CH₄ mixture through the fixed bed packed with some optimal MOFs were predicted to evaluate their effects in practical hydrogen purification. UMODEH08 or UMOBEF04 exhibits the long dimensionless residence time over 30 of CH₄ for the H₂/CH₄ separation. Finally, we also explored the behaviors of the radial distribution functions (RDF) and adsorption equilibrium configurations to further demonstrate how the selected MOFs differentiate CH₄ from H₂. The investigation on all these observations at molecular level will pave the way for the development of new materials for clean energy applications.     

Improved design of metal-organic frameworks for efficient hydrogen storage at ambient temperature: A multiscale theoretical investigation

A multiscale theoretical technique is used to examine the combination of different approaches for hydrogen storage enhancement in metal-organic frameworks at room temperature and high pressure by implementation lithium atoms in linkers. Accurate MP2 calculations are performed to obtain the hydrogen binding sites and parameters for the following grand canonical Monte Carlo (GCMC) simulations. GCMC calculations are employed to obtain the hydrogen uptake at different thermodynamic conditions. The results obtained demonstrate that the combination of different approaches can improve the hydrogen uptake significantly. The hydrogen content reaches 6.6 wt% at 300 K and 100 bar satisfying DOE storage targets (5.5 wt%).     

A DFT study of H2 adsorption on lithium decorated 3D hybrid Boron-Nitride-Carbon frameworks

Based on density functional theory (DFT) and first-principles molecular dynamics (MD),a new 3D hybrid Boron-Nitride-Carbon–interconnected frameworks (BNCIFs) consisting of organic linkers with Li decoration is created and optimized. Firstly, Li adsorption behaviors on several BNC_xcomplexes are investigated and compared systematically. The results indicate C substitution of N atom in pure BN layer could improve the metal binding energy effectively. Secondly, the BNC layer (BNC_NN) is chosen to model the frameworks of BNCIFs. The average binding energy of adsorbed Li atoms on BNCIFs is 3.6 eV which is much higher than the cohesive energy of bulk Li and avoids the Li clustering problem. Finally, we study the H₂ adsorptions on the Li decorated BNCIFs by DFT. Every Li atom could adsorb four H₂ molecules with an average binding energy of 0.24 eV. The corresponding gravimetric and volumetric storage capacities are 14.09 wt% and 126.2 g/L respectively overpassing the published 2020 DOE target. The excellent thermal stability of 160H₂@40Li@BNCIFs is also proved by MD. This nanostructure could be served as a promising hydrogen storage medium at ambient conditions.