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.     

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.     

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.     

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.     

Improved hydrogen adsorption of ZnO doped multi-walled carbon nanotubes

Hydrogen storage is still one of the most important problems to improve hydrogen energy usage widespread. New materials capable of storing hydrogen with high efficiency must be introduced to overcome this problem. In recent years, addition of metals or inorganic compounds to multiwalled carbon nanotubes (MWCNTs) has been generally used for hydrogen uptake studies to enhance adsorption property of the nanotubes. In this study, Zinc oxide (ZnO) nanoparticles doped MWCNTs (ZnO-MWCNTs) have been produced as new reversible hydrogen storage materials, and we have investigated characterization of ZnO-MWCNTs by XRD, SEM, TGA, TEM and BET analyses. The functionalized MWCNTs and ZnO doped MWCNTs were subjected to hydrogenation step by dynamic gas sorption analyser under pressure of 5–50 bar. The hydrogen uptake capacities of the materials under different pressures were measured gravimetrically. It was indicated that by controlling the pressures for hydrogenation of ZnO-MWCNTs induces the spillover of ZnO nanoparticles in the layer of MWCNTs which in return with high hydrogen adsorption capacity. Consequently, the hydrogen adsorption of the functionalized MWCNTs (f-MWCNTs) and the ZnO-MWCNTs were achieved to be 1.05 wt% and 2.7091 wt% under pressure of 50 bar as maximum.     

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.     

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.     

Low-temperature hydrogen release through LiAlH4 and NH4F react in Et2O

The application of hydrogen energy urgently requires a high-capacity hydrogen storage technology that can release hydrogen at low temperature. The composite of LiAlH₄ and NH₄F has a hydrogen storage capacity of up to 8.06 wt%, but the release of hydrogen requires a reaction temperature of about 170 °C, and the reaction is difficult to control. In this work, the reaction between LiAlH₄ and NH₄F is proposed to be carried out in diethyl ether to improve its hydrogen release performance. It exhibits good hydrogen release performance over a wide temperature range of −40–25 °C, and the hydrogen release capacity at −40 °C, −20 °C, 0 °C and 25 °C can reach 4.41 wt%, 6.79 wt%, 6.85 wt% and 7.78 wt%, respectively. The activation energy of the reaction is 38.41 kJ mol⁻¹, which is much lower than many previously reported catalytic hydrolysis systems that can release hydrogen at room temperature. Our study demonstrates a high-performance hydrogen storage system with very low operating temperature, which may lay the foundation for the development of practical mobile/portable hydrogen source in the north and the Arctic.     

The synthesized nickel-doped multi-walled carbon nanotubes for hydrogen storage under moderate pressures

Hydrogen adsorption capacity of Multiwalled carbon nanotubes (MWCNTs) decorated with Nickel (Ni) nanoparticles has been presented at room temperature and under moderate pressures of 4–20 bar. The functionalization of carbon nanotubes was carried by H₂SO₄-HNO₃ reducing agents and the Ni supported MWCNTs (Ni-MWCNTs) were prepared by wet chemical method. The structure and morphology characterization of samples were performed by XRD, TEM, EDX and SEM analyses. These nanotubes then subjected to hydrogenation step by using Sievert's-like apparatus. The hydrogenation of the Ni-MWCNTs was performed at 298 K and moderate hydrogen pressures of 4–20 bar. The obtained results show that there is a correlation between hydrogen storage capacity and hydrogen pressure that; as the pressure was increased, hydrogen uptake capacity enhanced due to physisorption. In addition, maximum hydrogen storage capacity of Ni-MWCNTs was found to be 0.298 wt % at room temperature and under pressure of 20 bar.     

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.     

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.     

Facile synthesis and hydrogen storage application of nitrogen-doped carbon nanotubes with bamboo-like structure

This paper reports a facile method for the preparation of nitrogen-doped carbon nanotubes (N-doped CNTs) that shows enhanced hydrogen storage capacity. The synthesis method involves simple pyrolysis of melamine using FeCl₃ as catalyst in tube furnace. The materials were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, elemental analysis, Raman spectroscopy, and nitrogen adsorption–desorption analysis. The results indicated that the prepared N-doped CNTs have a bamboo-like structure with thin compartment layers. The nitrogen doping concentration, specific surface area, and total pore volume of the N-doped CNTs were determined to be 1.5 at%, 135 m²/g, and 0.38 cm³/g, respectively. The hydrogen adsorption measurements at 77 K showed that the N-doped CNTs exhibits gravimetric hydrogen uptake of 0.21 wt% at 1 bar and 1.21 wt% at 7 bar. At room temperature, hydrogen uptake as high as 0.17 wt% at 298 K and 19 bar is achieved, which is among the highest data reported for the N-doped carbon materials under the same condition.     

Microstructure and hydrogen storage properties of (Ti0.32Cr0.43V0.25) + x wt% La (x = 0–10) alloys

The composite alloy of Ti_0.32Cr_0.43V_0.25 with x wt% La (where x = 0–10) was prepared by arc melting technique. The effect on hydrogen storage capacity, flatness of the plateau pressure, and residual hydrogen was investigated in La added Ti_0.32Cr_0.43V_0.25. Crystalline phase and microstructure of the prepared composite alloy were investigated and characterized by XRD, SEM and TEM. The crystal structure was refinement using Rietveld analysis. The effective hydrogen storage capacity of the composite alloy was found comparable to the parent alloy, when 5 wt% La was added. The effective hydrogen capacity (∼2.31 wt%) was close to that of the parent alloy (2.35 wt%) and the plateau slope was significantly improved from 30.5 of the parent alloy to 14.6. Appropriate mechanisms associated with the improved flatness by the La addition has been discussed in terms of the refined crystalline structure. Using TG/DTA method we have shown the differences in the interaction of residual hydrogen with the BCC phase of both parent alloy and 5 wt % La mixed alloy.     

Hydrogen uptake of manganese oxide-multiwalled carbon nanotube composites

Hydrogen uptake of pristine multi-walled carbon nanotubes is increased more than three-fold at 298 K and hydrogen pressure of 4.0 MPa, upon addition of hydrogen spillover catalyst manganese oxide, from 0.26 to 0.94 wt%. Simple and convenient in situ reduction method is used to prepare Mn-oxide/MWCNTs composite. XRD, FESEM, and TEM demonstrates nanostructural characterization of pristine MWCNTs and composite. TGA analysis of Mn-oxide/MWCNTs composites showed a single monotonous fall related to MWCNTs gasification. Enhancement of hydrogen storage capacity of composite is attributed to spillover mechanism owing to decoration of Mn-oxide nanoparticles on outer surface of MWCNTs. Hydrogen uptake follows monotonous dependence on hydrogen pressure. Oxide-MWCNTs composite not only shows high hydrogen storage capacity as compared to pristine, but also exhibit significant cyclic stability upon successive adsorption–desorption cycles.     

Investigation of hydrogen storage capacity of multi-walled carbon nanotubes deposited with Pd or V

This work presents the synthesis and characterization of multi-walled carbon nanotubes (multi-walled CNTs) deposited with Pd or V and their hydrogen storage capacity measured by Sievert's volumetric apparatus. The CNTs were grown by the CVD method using LPG and LaNi₅ as the carbon source and catalyst, respectively. Pd was impregnated on the CNTs by the reflux method with hydrogen gas as a reducing agent, while V was embedded on the CNTs by the vapor deposition method. The average metal particle size deposited on the CNTs was around 5.8 nm for Pd and 3.6 nm for V. Hydrogen adsorption experiments were performed at room temperature and at −196 °C under a hydrogen pressure of 65 bar. At −196 °C, the treated CNTs had a maximum hydrogen uptake of 1.21 wt%, while the CNTs deposited with Pd (Pd-CNTs) and CNTs deposited with V (V-CNTs) possessed lower surface areas, inducing lower hydrogen adsorption capacities of 0.37 and 0.4 wt%, respectively. For hydrogen sorption at room temperature, the CNTs decorated with the metal nanoparticles had a higher hydrogen uptake compared to the treated CNTs. Hydrogen adsorption capacity was 0.125 and 0.1 wt% for the Pd-CNTs and V-CNTs, respectively, while the hydrogen uptake of the treated CNTs was <0.01 wt%. For the second cycle, only half of the first hydrogen uptake was obtained, and this was attributed to the re-crystallization of the defect sites on the carbon substrate after the first hydrogen desorption.     

Hydrogen storage properties of V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy

In this paper, we present the synthesis, first hydrogenation kinetics, thermodynamics and effect of cycling on the hydrogen storage properties of a V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 high entropy alloy. It was found that the V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 alloy crystallizes in body-centred cubic (BCC) phase with a small amount of secondary phase. The first hydrogenation is possible at room temperature without incubation time and reaches a maximum hydrogen storage capacity of 3.45 wt%. The pressure composition isotherm (P–C–I) at 298 K shows a reversible hydrogen desorption capacity of 1.78 wt% and a desorption plateau pressure of 80.2 kPa. The capacity loss is mainly due to the stable hydride with the desorption enthalpy of 31.1 kJ/mol and entropy of 101.8 J/K/mol. The hydrogen absorption capacity decreases with cycling due to incomplete desorption at room temperature. The hydrogen absorption kinetics increases with cycling and the rate-limiting step is diffusion-controlled for hydrogen absorption.     

Preheated self-aligned graphene oxide for enhanced room temperature hydrogen storage

This study explored the hydrogen adsorption capacity of self-assembled aligned graphene oxide at room temperature. The characteristics of as-prepared graphene oxide were determined by scanning electron microscopy, Raman spectroscopy, and X-ray diffractometry techniques. Three different temperatures were taken for preheating, i.e., 25, 250, and 400 °C. The maximum adsorption pressure was given to 20 bar, and we evaluated the hydrogen adsorption competency at room temperature (25 ± 2 °C). The maximum hydrogen storage capacity was achieved ~2.5 wt%, which was found for the graphene oxide sample preheated at 400 °C. This hydrogen storage capacity was 67% and 40% more than the graphene oxide samples preheated at 25 and 250 °C, respectively. Such an enhancement of hydrogen storage capacity in the self-aligned graphene oxide samples at room temperature is attributed to reduced interlayer spacing and increased topological defects in preheated graphene oxide samples at 400 °C.     

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.     

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.