Modeling hydrogen adsorption in the metal organic framework (MOF-5, connector): Zn4O(C8H4O4)3

Density functional theory investigation is performed to understand the underlying mechanism of hydrogen adsorption in the MOF-5 by using for first time the connector structure. The analysis of chemical bonds of the connector's atoms shows a good agreement between experimental and theoretical results. In particular, we show that this material has a desorption temperature of 115 K and an initial hydrogen storage capacity around 1.57 wt% which are close to the experimental values. We consider the coupling-energy mechanism to explore the most stable configurations in multiple adsorption sites namely metallic, carboxylic and cyclic sites. Three orientations which are vertical, horizontal and sloping are taking into account. The results show that the metallic and cyclic sites are more stable for multiple hydrogen molecule storage and the system reaches 4.57 wt% as a gravimetric storage capacity which is located in the interval 4.50–5.20 wt% found experimentally. In addition, the desorption temperature is improved significantly.     

Hydrogen storage in 1D nanotube-like channels metal–organic frameworks: Effects of free volume and heat of adsorption on hydrogen uptake

Metal–organic Frameworks generate significant interest for their potential application as Hydrogen storage materials. Grand Canonical Monte Carlo (GCMC) simulations were performed at two different temperatures 77 and 300 K over a wide range of pressures to describe H₂ adsorption in 7 metal–organic frameworks (MOFs), which all have the same framework topology but different surface chemistry and different pore sizes. DREIDING and UFF force fields were identified to be able to predict adsorption isotherms for H₂ in MOFs in a reasonable agreement with the experimental data from the literature. This work reveals that at 77 K the total amount of H₂ adsorbed correlates mainly with: the heat of adsorption at low pressure and the free volume at high pressure. While at 300 K the amount adsorbed mainly correlates with the available free volume at both low and high pressure. None of the MOFs studied fulfils DOE requirement, this is due to their low heat of adsorption. The required adsorption energy to meet the DOE targets is estimated to be 34 kJ/mol.     

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.     

Interpreting the hydrogen adsorption on organic groups functionalized MOF-5s by statistical physics model

The hydrogen adsorption isotherms at equilibrium on four adsorbents (MOF-5 and three modified MOF-5s named, CH₃-MOF-5, Br-MOF-5 and Cl-MOF-5) were studied using a monolayer model with four adsorption sites energies. The analytical expression of this model was developed using the grand canonical ensemble in statistical physics by taking some working hypotheses involving some physicochemical parameters which can describe the adsorption process. These parameters are: four numbers of hydrogen adsorbed molecules per site (n₁, n₂, n₃ and n₄), four receptor site densities (N_M1, N_M2, N_M3 and N_M4), four saturation adsorbed quantities (Q₁, Q₂, Q₃ and Q₄) and four adsorption energies (??₁, ??₂, ??₃ and ??₄). The evolutions of these parameters in relation with temperature were discussed to understand and interpret the adsorption process at different temperatures. Fitting results revealed that the adsorption of hydrogen on MOF-5 is an exothermic physisorption process. The adsorption surface is inhomogeneous with many site energies. The fitting of the adsorption site is achieved by an aggregate of hydrogen molecules. The adopted model expression is used to derive the thermodynamic potential functions which govern the sorption mechanism such as entropy S_a, free enthalpy of Gibbs G and internal energy E_int.     

Graphene pillared with hybrid fullerene and nanotube as a novel 3D framework for hydrogen storage: A DFT and GCMC study

A new-type 3D pillared graphene framework with hybrid fullerene and nanotube pillars (PGF-hFN) has been created depended on density functional theory (DFT) and first-principles molecular dynamics simulations (MD). It is proved to have excellent thermal structural stability. The average adsorption energy of Li is 2.77 eV much higher than the metal cohesive energy excluding lithium aggregation problem. From DFT calculations, for Li-decorated B-doped PGF-hFN, the hydrogen gravimetric density (HGD) is as high as 12.92 wt% and the according volumetric uptake is 96.4 g/L with an average adsorption energy of 0.195 eV per H₂. Further grand canonical Monte Carlo (GCMC) simulations predict 7.2 wt% in excess HGD and 53.8 g/L in excess volumetric hydrogen density at near ambient temperature (233 K) and 100 bars with the ideal adsorption enthalpy which have exceeded the 2020 the U.S. Department of Energy (DOE) ultimate target for mobile applications. Our multiscale theoretical simulations indicate this new pillared structure should be a promising carrier accessible for sorption of hydrogen molecules.     

Hydrogen adsorption on graphene,hexagonal boron nitride,and graphene-like boron nitride-carbon heterostructures: A comparative theoretical study

The results of DFT and ab initio calculations of the hydrogen physisorption on graphene, hexagonal boron nitride (h-BN), and a graphene-like boron nitride-carbon heterostructure (GBNCH) are discussed. PBE-D3, B3LYP-D3 as well as MP2 methods were employed in calculating the adsorption energies (Ea) of a hydrogen molecule to the appropriate structure and the optimal distances between them. Six adsorption sites were examined, and it is demonstrated that the ‘hollow’ sites are favorable for hydrogen adsorption. It was established that GBNCH exhibits increased Ea values in comparison with graphene and h-BN. Hydrogen adsorption isotherms at different temperatures were obtained using grand canonical Monte-Carlo simulations, and it was shown that GBNCH reveals advanced adsorption properties in comparison with its counterparts. The usage of GBNCHs for hydrogen storage is also discussed.     

Recent advances in covalent organic framework (COF) nanotextures with band engineering for stimulating solar hydrogen production: A comprehensive review

Due to the continuous consumption of fossil fuels, natural reserves are depleting and it has been earnest need for developing new sources of energy. Among the several solar energy conversion techniques, photocatalytic hydrogen (H₂) generation is regarded as one of the most promising routes. Till date, several metal-based semiconductor materials have been investigated, however, H₂ generation is not substantial with the notion of sustainable development. Current research trends show the growing interest in advanced and metal free photocatalyst materials such as covalent organic frameworks (COFs) due to their several benefits such as crystalline porous polymers with pre-designed architectures, large surface area, exceptional stability, and ease of molecular functionalization. By combining COFs with other functional materials, composites may be created that display unique characteristics that exceed those of the separate components. This work provides a comprehensive development on COFs as a photocatalysts and their composites/hybrids for photocatalytic hydrogen generation with a focus on visible-light irradiation. To reduce the dependency on novel metals and overcome the drawbacks of individual material, the creation of composite materials based on covalent-organic frameworks (COFs) are systematically discussed. In addition, advantages in terms of performance, stability, durability of composites/hybrids COFs for photocatalytic hydrogen production in reference to traditional catalysts are investigated. Different composites such as metals loading, morphological development, band engineering, and heterojunctions are systematically discussed. Finally, challenges and opportunities associated with constructing COF-based catalysts as future research prospective for chemistry and materials science are highlighted.     

Hydrogen adsorption on modified activated carbon

The aim of this work is to investigate hydrogen adsorption on prepared super activated carbon (AC). Litchi trunk was activated by potassium hydroxide under N₂ or CO₂ atmosphere. Nanoparticles of palladium were impregnated in the prepared-AC. Hydrogen adsorption was accurately measured by a volumetric adsorption apparatus at 77, 87, 90 and 303 K, up to 5 MPa. Experimental results revealed that specific surface area of the prepared-AC increased according to KOH/char ratio. The maximum specific surface area reached up to 3400 m²/g and total pore volume of 1.79 cm³/g. The maximum hydrogen adsorption capacity of 2.89 wt.% at 77 K and under 0.1 MPa, was obtained on these materials. The hydrogen adsorption capacity of the 10 wt.% Pd-AC was determined as 0.53 wt.% at 303 K and under 6 MPa. This amount is higher than that on the pristine AC (0.41 wt.%) under the same conditions.     

Hydrogen adsorption of Li functionalized Covalent Organic Framework-366: An ab initio study

This work deals with the investigations of hydrogen adsorption energies of the Li functionalized Covalent Organic Framework-366 (COF-366) by using the density functional theory method. Based on total energy calculations, it was found that Li atom is preferentially trapped at the center site of the tetra(p-amino-phenyl) porphyrin and the onN site of a terephthaldehyde chain. Moreover, hydrogen adsorption energies per H₂ for 1–3 H₂ loadings range from 0.03 to 0.22 eV. According to ab initio molecular dynamics simulations, our results found that hydrogen capacities of Li functionalized COF-366 at ambient pressure are 2.06, 1.58, and 1.05 wt% for 77, 150 and 298 K, respectively.     

Modulated synthesis of zirconium-metal organic framework (Zr-MOF) for hydrogen storage applications

A modulated synthesis of Zr-metal organic framework (Zr-MOF) with improved ease of handling and decreased reaction time is reported to yield highly crystalline Zr-MOF with well-defined octahedral shaped crystals for practical hydrogen storage applications. The Zr-MOF obtained from the modulated synthesis showed high thermal and moisture stabilities with enhanced hydrogen storage capacity. Further study suggests that the modulated synthesis of Zr-MOF may lead to the development of a flow-through synthesis process.     

Hydrogen storage in Ca-decorated,B-substituted metal organic framework

The feasibility to store hydrogen in calcium-decorated metal organic frameworks (MOFs) is explored by using first-principles electronic structure calculations. We show that substitution of boron atoms into the benzene ring of the MOF linker substantially enhances the Ca binding energy to the linker as well as the H₂ binding energy to Ca. The Kubas interaction between H₂ molecules and Ca added in the MOF gives rise to a large number of bound H₂'s (8H₂'s per linker) with the binding energy of 20 kJ/mol, which makes the system suitable for reversible hydrogen storage under ambient conditions.     

Hydrogen adsorption on graphane: An estimate using ab-initio interaction

The interaction between hydrogen molecule and graphane, material synthesized when a graphene plane is fully functionalized by hydrogen atoms, is assessable by quantum mechanical ab-initio calculations. Therefore for hydrogen, it is possible to estimate the adsorption properties of a porous material similar to activated carbons, the adsorbent surface of which is made of graphane planes instead of graphene or basal graphitic planes. The calculation realized by Monte-Carlo simulations in the grand canonical ensemble shows that the hydrogen adsorption of graphane stays qualitatively similar to that of graphene.     

Hydrogen storage on graphitic carbon nitride and its palladium nanocomposites: A multiscale computational approach

Hydrogen storage capacity (HSC) of multilayer graphitic carbon nitride, d-g-C₃N₄ (d is interlayer spacing), and its palladium nanocomposite, d-Pd@g-C₃N₄, were investigated using multiscale computational techniques including quantum mechanics calculations and grand canonical Monte Carlo (GCMC) simulation. According to the results, the volumetric HSC of 8-g-C₃N₄ and 8-Pd@g-C₃N₄ can reach to DOE target of 30 gH₂/L at 177 K, 5.7 MPa, and 177 K, 4.0 MPa, respectively. The gravimetric HSC of 10-g-C₃N₄ and 12-Pd@g-C₃N₄ meet the DOE target of 4.5 wt% at 150 K, 3.5 MPa, and 125 K, 4.0 MPa, respectively. The incorporation of Pd atoms enhances the delivery volumetric HSC of 6-, 8-, 10-, and 12-g-C₃N₄ by 49, 55, 129, and 146%, respectively at 177 K and 0.5 MPa. On the other hand, the incorporation of Pd atoms has a negative effect on the delivery gravimetric HSC of 6- and 8-g-C₃N₄ and positive effect for 10- and 12-g-C₃N₄. The estimated isostric heat, Q_st, of adsorption is 5.5–8.5 kJ/mol. The maximum value of Q_st for both nanoadsorbents belong to those with d = 8 Å. The structure of adsorbates and possibility of multilayer adsorption occurrence were also investigated using pair correlation functions and density profiles.     

Hydrogen storage in zeolite imidazolate frameworks. A multiscale theoretical investigation

A multiscale approach is used to investigate the hydrogen adsorption in nanoporous Zeolite Imidazolate Frameworks (ZIFs) on varying geometries and organic linkers. Ab initio calculations are performed at the MP2 level to obtain correct interaction energies between hydrogen molecules and the ZIF structures. Subsequently, classical grand canonical Monte-Carlo (GCMC) simulations are carried out to obtain the hydrogen uptake of ZIFs at different thermodynamic conditions of pressure and temperature.     

Synthesis and hydrogen storage studies of metal−organic framework UiO-66

Metal−organic framework UiO-66 has high chemical and thermal stability. However, it is difficult to produce such Zr-based MOFs with good crystalline morphology. Here, highly pure metal−organic framework UiO-66 has been synthesized at low temperature (50 °C). The as-synthesized sample has been characterized by X-ray diffraction, thermogravimetric analysis, nitrogen adsorption, and scanning electron microscopy. Its hydrogen-storage capacity has been measured by means of an Intelligent Gravimetric Analyser. The results showed that UiO-66 was synthesized in octahedral crystals of well-defined sizes (150−200 nm) and had a high specific surface area (1358 m²/g). The as-synthesized UiO-66 showed a significant hydrogen uptake even at a moderate pressure, which increased to 3.35 wt% at 77 K and 1.8 MPa. A grand canonical Monte Carlo simulation (GCMC) has been employed to calculate the adsorption of hydrogen in UiO-66. The result of this simulation provided a theoretical foundation for the experimental results.     

Enhanced isosteric heat of adsorption and gravimetric storage density of hydrogen in GNP incorporated Cu based core-shell metal-organic framework

The present work explores the hydrogen adsorption potential of graphene nanoplatelet incorporated core-shell metal-organic frameworks (MOFs). Core-shell MOFs [(HKUST-1@Cu-MOF-2 (HM) and Cu-MOF-2@HKUST-1 (MH)] and their graphene nanoplatelets (GNP) incorporated composites [GNP@HKUST-1@Cu-MOF-2 (GHM) and GNP@Cu-MOF-2@HKUST-1 (GMH)] were synthesized by solvothermal method. The core-shell formation and the structural effect of graphene nanoplatelts incorporation was established and studied using X-ray diffraction analysis, thermogravimetric analysis, scanning electron microscopy and X-ray photoelectron spectroscopy. The hydrogen adsorption studies were carried out at maximum pressure of 2 bar and temperature of 100 K using Sievert's apparatus. GHM exhibited the highest storage capacity of 2.3 wt% at 100 K and 2 bar. Significantly, contrary to MH, the average pore size in HM increased from 1.66 nm to 2.58 nm after the addition of graphene nanoplatelets, resulting in Type IV N₂ sorption isotherm as found through BET studies. Additionally, the theoretical isosteric heat of adsorption was estimated using the Clausius Clayperon equation where GHM showed an exceptionally high isosteric heat of adsorption of 14.7 kJ/mol. Therefore these studies on newly developed core-shell MOF/GNP bring out the effect of GNP incorporation on the structure of the MOF, formation of the local active centres and carbon clusters, which are critical to hydrogen adsorption.     

Hydrogen adsorption and storage behaviors of Li-decorated PdS2 monolayer: An extended tight-binding study based on DFT

The hydrogen adsorption and storage of the lithium-decorated PdS₂ monolayer at nano-size has been investigated by using extended tight-binding (GFN1-xTB) based on density functional theory (DFT). The calculation results demonstrate that the average adsorption energies of 1–5H₂ change in 0.47–0.20 eV/H₂ range which decreases with increasing of adsorbed hydrogen molecule number. The gravimetric density for hydrogen storage adsorption with 12Li atom and 60H₂ molecules of Li-decorated PdS₂ monolayer is about 6.98 wt% considered as possible application in hydrogen storage. The examination of the hydrogen store mechanism between the monolayer and hydrogen molecules is presented by polarization between Pd and H₂, which can be effect on the adsorption behavior.     

Hydrogen binding in alkali-decorated iso-reticular metal organic framework-16 based on Zn, Mg, and Ca

Hydrogen adsorption energies were investigated in three different types of iso-reticular Metal Organic Framework-16, Zn-/Mg-/Ca-MOF16, decorated with either Li, Na, or K. Concerning the binding strengths of the alkali metals, the density functional theory calculations reveal that Li is bound strongest to the host framework, followed by K and Na. Decoration with Li also results in the highest hydrogen adsorption energies among the studied alkali metals. Furthermore, Zn-MOF16 exhibits the highest hydrogen adsorption energies near the metal oxide cluster, while hydrogen binding strengths at organic linker sites do not differ substantially between Zn-/Mg-/Ca-MOF16. Based on these results, we conclude that for Metal Organic Framework-16, Li-decorated Zn-MOF16 appears to be the optimal choice for hydrogen storage among the nine combinations.     

Anion exchange composite membrane based on ionic liquid-grafted covalent organic framework for fuel cells

Covalent organic frameworks (COFs) have been considered promising hydroxide-conducting materials for their highly ordered crystalline porous structure and tunable functionality. However, the lack of hydroxyl conduction functional groups on the COFs frameworks restricts their further development in anion exchange membrane fuel cells (AEMFCs). At present, impregnated ionic liquids (ILs) are mainly used to solve this problem, but they still face the challenge of ILs leakage under working conditions. Here, we report a novel IL-functionalized covalent organic framework (IL-COF), which is prepared by grafting guanidinium-based IL onto the channel walls of COF via the Williamson ether reaction and then doped into guanidinium-functionalized poly(2,6-dimethyl-1,4-phenylene oxide) (GPPO) to prepare IL-COF/GPPO composite membranes. The ILs grafted into the COFs nanochannels act as the “active sites” in the membranes to enhance the migration rate of the hydroxide ions and thus improve the conductivity. Accordingly, the hydroxide conductivity of the resultant IL-COF/GPPO composite membrane with IL-COF doping amount of 5 wt% can reach up to 89.93 mS cm^?1 at 80 °C under hydrated condition, 61% higher than that of the pristine GPPO membrane. Meanwhile, its hydroxide conductivity retains 90.31% after alkaline treatment for 14 days. Compared with IL-impregnated COF composite membrane (IL@COF/GPPO), IL-COF/GPPO membrane has superior hydroxyl conductivity and long-term stability since chemical grafting can more firmly immobilize ILs into COF channels than impregnation.     

Hydrogen adsorption properties of in-situ synthesized Pt-decorated porous carbons templated from zeolite EMC-2

To increase the interaction between the adsorbed hydrogen and the adsorbent surface to improve the hydrogen storage capacity at ambient temperature, decorating the sorbents with metal nanoparticles, such as Pd, Ni, and Pt has attracted the most attention. In this work, Pt-decorated porous carbons were in-situ synthesized via CVD method using Pt-impregnated zeolite EMC-2 as template and their hydrogen uptake performance up to 20 bar at 77, 87, 298 and 308 K has been investigated with focus on the interaction between the adsorbed H₂ and the carbon matrix. It is found that the in-situ generated Pt-decorated porous carbons exhibit Pt nanoparticles with size of 2–4 nm homogenously dispersed in the porous carbon, accompanied with observable carbon nanowires on the surface. The calculated H₂ adsorption heats at/near 77 K are similar for both the plain carbon (7.8 kJ mol⁻¹) and the Pt-decorated carbon (8.3 kJ mol⁻¹) at H₂ coverage of 0.08 wt.%, suggesting physisorption is dominated in both cases. However, the calculated H₂ adsorption heat at/near 298 K of Pt-decorated carbon is 72 kJ mol⁻¹ at initial H₂ coverage (close to 0), which decreases dramatically to 20.8 kJ mol⁻¹ at H₂ coverage of 0.014 wt.%, levels to 17.9 at 0.073 wt.%, then gradually decreases to 2.6 kJ mol⁻¹ at 0.13 wt.% and closes to that of the plain carbon at H₂ coverage above 0.13 wt.%. These results suggest that the introduction of Pt particles significantly enhances the interaction between the adsorbed H₂ and the Pt-decorated carbon matrix at lower H₂ coverage, resulting in an adsorption process consisting of chemisorption stage, mixed nature of chemisorption and physisorption stage along with the increase of H₂ coverage (up to 0.13 wt.%). However, this enhancement in the interaction is outperformed by the added weight of the Pt and the blockage and/or occupation of some pores by the Pt nanoparticles, which results in lower H₂ uptake than that of the plain carbon.