Concepts for improving hydrogen storage in nanoporous materials

Hydrogen storage in nanoporous materials has been attracting a great deal of attention in recent years, as high gravimetric H₂ capacities, exceeding 10 wt% in some cases, can be achieved at 77 K using materials with particularly high surface areas. However, volumetric capacities at low temperatures, and both gravimetric and volumetric capacities at ambient temperature, need to be improved before such adsorbents become practically viable. This article therefore discusses approaches to increasing the gravimetric and volumetric hydrogen storage capacities of nanoporous materials, and maximizing the usable capacity of a material between the upper storage and delivery pressures. In addition, recent advances in machine learning and data science provide an opportunity to apply this technology to the search for new materials for hydrogen storage. The large number of possible component combinations and substitutions in various porous materials, including Metal-Organic Frameworks (MOFs), is ideally suited to a machine learning approach; so this is also discussed, together with some new material types that could prove useful in the future for hydrogen storage applications.     

Efficient synthesis for large-scale production and characterization for hydrogen storage of ligand exchanged MOF-74/174/184-M (M = Mg2+, Ni2+)

A semitechnical route (optimized by BASF SE) to synthesize MOF-74/174-M (M = Mg²⁺, Ni²⁺) efficiently in ton-scale production is presented with the goal of mobile and stationary gas storage applications especially for hydrogen as future energy carrier. In addition, a new member of these series of materials, MOF-184-M (M = Mg²⁺, Ni²⁺) is introduced using ligand exchange strategy in order to produce a more porous analogue (possessing large aperture) without loss of crystallinity. This family comprising MOF-74/174/184 are characterized systematically for hydrogen adsorption properties by volumetric measurements with a Sieverts’ apparatus. Replacing the linker by a longer one results in an increase of the BET area from 984 to 3154 m²/g and an enhancement of the excess cryogenic (77 K) hydrogen storage capacity from 1.8 to 4.7 wt%. The heat of adsorption of linker exchanged MOF-174/184 (as a function of uptake) shows similar values to the parent MOF-74, indicating successful construction of expanded MOFs in large scale production. Finally, a usable capacity of these MOFs is investigated for mobile application, revealing that the increasing surface area without strong binding metal sites through longer linker exchange is one of important parameters for improving usable capacity.     

Hydrogen storage in hybrid nanostructured carbon/palladium materials: Influence of particle size and surface chemistry

Hydrogen adsorption on porous materials is one of the possible methods proposed for hydrogen storage for transport applications. One way for increasing adsorption at room temperature is the inclusion of metal nanoparticles to increase hydrogen–surface interactions. In this study, ordered mesoporous carbon materials were synthesized by replication of nanostructured mesoporous SBA-15 silica. The combination of different carbon precursors allowed to tailor the textural, structural and chemical properties of the materials. These carbons were used for the synthesis of hybrid nanostructured carbon/palladium materials with different sizes of metal nanoparticles. The hydrogen sorption isotherms were measured at 77 K and 298 K between 0.1 and 8 MPa. Hydrogen storage capacities strongly correlate with the textural properties of the carbon at 77 K. At room temperature, Pd nanoparticles enhance hydrogen storage capacity by reversible formation of hydride PdH_x and through the spillover mechanism. The hydrogen uptake depends on the combined influences of metal particle size and of carbon chemical properties. Carbons obtained from sucrose precursors lead to the hybrid materials with the highest storage capacities since they exhibits a large microporous volume and a high density of oxygenated surface groups.     

Enhanced hydrogen storage performance of three-dimensional hierarchical porous graphene with nickel nanoparticles

Three-dimensional hierarchical porous graphene with nickel nanoparticles (3DHPG-Ni) was synthesized through electrostatic assembly method with the assistance of poly (methyl methacrylate) (PMMA) template and subsequent removal of PMMA template by calcination. The morphology, microstructure and hydrogen adsorption properties of 3DHPG-Ni nanocomposites were examined in detail. The obtained 3DHPG-Ni nanocomposite exhibited hierarchical porous structure composed of macro-, meso- and micropores, high specific surface area (925 m² g^?1), large pore volume (0.58 cm³ g^?1) and excellent hydrogen storage capacity. Under the pressure of 5 bar, 3DHPG-Ni nanocomposite showed a maximum hydrogen capacity of 4.22 wt% and 1.95 wt% at 77 K and 298 K, respectively, demonstrating that the as-prepared 3DHPG-Ni nanocomposite was supposed to be a promising material with outstanding properties for practical applications in the field of hydrogen storage. The three-dimensional hierarchical porous structure, evenly distributed Ni nanoparticles and hydrogen spillover effect were responsible for the enhanced hydrogen storage capacities.     

Investigation of hydrogen storage capacity of various carbon materials

Hydrogen storage capacity of various carbon materials, including activated carbon (AC), single-walled carbon nanohorn, single-walled carbon nanotubes, and graphitic carbon nanofibers, was investigated at 303 and 77 K, respectively. The results showed that hydrogen storage capacity of carbon materials was less than 1 wt% at 303 K, and a super activated carbon, Maxsorb, had the highest capacity (0.67 wt%). By lowering adsorption temperature to 77 K, hydrogen storage capacity of carbon materials increased significantly and Maxsorb could store a large amount of hydrogen (5.7 wt%) at a relatively low pressure of 3 MPa. Hydrogen storage capacity of carbon materials was proportional to their specific surface area and the volume of micropores, and the narrow micropores was preferred to adsorption of hydrogen, indicating that all carbon materials adsorbed hydrogen gas through physical adsorption on the surface.     

Hydrogen adsorption and kinetics in MIL-101(Cr) and hybrid activated carbon-MIL-101(Cr) materials

Since the last 15 years, porous solids such as Metal–Organic Frameworks (MOFs) have opened new perspectives for the development of adsorbents for hydrogen storage. Among all MOF materials, the chromium (III) terephthalate-based MIL-101(Cr) is a very stable one which exhibits a good uptake capacity of hydrogen (H₂). In this study, syntheses were carried out in soft conditions without hydrofluoric acid as usually reported in literature. Moreover, activated carbon (AC) was introduced in the preparation of the MOF-based adsorbents to create hybrid materials with large specific surface areas (AC-MOF). Hydrogen storage capacities were assessed at 77 K up to 100 bar, and the measurements of adsorption isotherms were performed using both volumetric and gravimetric methods. The experimental data were shown to be in good agreement. A maximal excess hydrogen uptake of 67.4 mol kg^?1 (13.5 wt.%) has been reached by the hybrid AC-MOF adsorbent at 77 K under 100 bar. The hydrogen storage capacity was so shown to be greatly enhanced by AC addition, as a maximal value of only 41.1 mol kg^?1(8.2 wt.%) was reported for the pristine MIL-101(Cr), under the same conditions. Finally, hydrogen adsorption kinetics were examined at 77 K using experimental transient adsorption curves obtained using volumetric method, and the Linear Driving Force (LDF) model was tested for their interpretation. According to this model, diffusion coefficients could be correctly estimated only in a very low pressure range. However, for high pressures, the quasi-equilibrium assumption is not valid at the initial adsorption times, making the LDF model no more applicable for accurate determination of the average effective diffusivities. To our knowledge we present the first measurement of the adsorption kinetics of hydrogen in a hybrid carbon MOF composite material. Moreover, the adsorption performances reported in this work are the best ones achieved until now by MIL-101(Cr) doping using carbonaceous materials.     

Hydrogen physisorption in high SSA microporous materials - A comparison between AX-21_33 and MOF-177 at cryogenic conditions

The two most promising materials for a hydrogen cryo-adsorption tank, activated carbon AX-21_33 and metal-organic framework MOF-177, have been investigated in the pressure range up to 2 MPa and at temperatures from 77 K to 125 K and at room temperature. The total hydrogen storage, including adsorbed hydrogen and gaseous hydrogen, has been determined for both samples. The results were evaluated with respect to the operating conditions of a tank system at cryogenic conditions, assuming a maximum tank pressure of 2 MPa and a minimum back pressure for the hydrogen consumer of 0.2 MPa. AX-21_33 shows a usable capacity of 3.5 wt.% in the case of isothermal operation at 77 K and 5.6 wt.%, if the tank is loaded at 77 K and the temperature is increased by 40 K during unloading. Under the same conditions, MOF-177 has a usable capacity of 6.1 wt.% and 7.4 wt.%, respectively. The results show that the heat of adsorption has a high impact on the amount of hydrogen remaining in a tank after unloading and that the heat management plays a crucial role for the design of a cryogenic tank system.     

Hydrogen storage comparison of M doped vanadium oxide nanotubes (M = Mo,Zr and W): A molecular simulation study

Zr, Mo and W doped Vanadium oxide nanotube were considered as remarkable materials for hydrogen storage applications. Monte Carlo molecular simulation was performed to study the adsorption behavior of hydrogen molecules on Vanadium oxide nanotubes (VONTs). The effects of temperature, pressure and mole percent of hydrogen on adsorption capacity of VONTs were investigated to provide deep insight of adsorption behavior. The results represented that hydrogen adsorption is an increasing function of pressure and at about 50 MPa all three metal doped VONT has maximum hydrogen capacity. At 5 MPa and room temperature, the hydrogen capacities of Mo, W and Zr doped VONTs were 1.39, 0.88 and 1.43 w% respectively. With temperature increment up to room temperature, more reduction in initial hydrogen capacity were observed in Mo and Zr doped VONTs.Evaluating hydrogen adsorption of Zr doped VONT from pure and hydrogen /nitrogen mixtures at 300 K indicated that under 2 Mpa, modifications in adsorption capacities were insignificant after N2 addition to the environment. Therefore, Zr doped VONT in hydrogen /nitrogen mixture environment can act as a capable adsorbent for Hydrogen storage system in comparison with Mo and W doped VONTs.     

Different effects of temperature on supramolecular protein and non-protein materials in hydrogen storage

The storage of large quantities of hydrogen at ambient temperature is a key factor in establishing a hydrogen-based economy. One strategy for hydrogen storage is to exploit the interaction between H₂ and a solid material by physisorption of hydrogen on porous materials. However, physisorption materials containing MOF, porous carbons, zeolites, clathrates, and synthesized organic polymers physisorb only about 1 wt% of H₂ at ambient temperature. One approach to solving this problem is to prepare new classes of physisorption materials which exhibits a mechanism different from the reported materials in hydrogen storage. Here we report the synthesis of apo cross-linked ferritin supramolecules by disulfide bonds, and their holo form. Unlike non-protein adsorbents, the hydrogen storage capacity of these protein materials increases as a function of temperature over the range of 20–40 °C. The holo supramolecules enable the adsorption of hydrogen up to 3.51 wt% at 40 °C and 40 bar H₂. In contrast, non-protein physisorption materials such as activated carbon and nano Fe₂O₃ marginally adsorb hydrogen, and, as reported, their ability to adsorb hydrogen decreases with increasing temperature under the same experimental condition. These results demonstrate that protein materials have a unique hydrogen storage mechanism which offers new opportunities in exploration of physisorption materials at ambient temperature.     

Molecular simulation of copper based metal-organic framework (Cu-MOF) for hydrogen adsorption

Metal organic framework (MOF) are widely used in adsorption and separation due to their porous nature, high surface area, structural diversity and lower crystal density. Due to their exceptional thermal and chemical stability, Cu-based MOF are considered excellent hydrogen storage materials in the world of MOFs. Efforts to assess the effectiveness of hydrogen storage in MOFs with molecular simulation and theoretical modeling are crucial in identifying the most promising materials before extensive experiments are undertaken. In the current work, hydrogen adsorption in four copper MOFs namely, MOF-199, MOF 399, PCN-6′, and PCN-20 has been analyzed. These MOFs have a similar secondary building unit (SBU) structure, i.e., twisted boracite (tbo) topology. The Grand Canonical Monte Carlo (GCMC) simulation was carried at room temperature (298 K) as well as at cryogenic temperature (77 K) and pressures ranging from 0 to 1 bar and 0–50 bar. These temperatures and pressure were selected to comply with the conditions set by department of energy (DOE) and to perform a comparative study on hydrogen adsorption at two different temperatures. The adsorption isotherm, isosteric heat, and the adsorption sites were analyzed in all the MOFs. The findings revealed that isosteric heat influenced hydrogen uptake at low pressures, while at high pressures, porosity and surface area affected hydrogen storage capacity. PCN-6′ is considered viable material at 298 K and 77 K due to its high hydrogen uptake.     

High pressure cryo-storage of hydrogen by adsorption at 77 K and up to 50 MPa

In this paper, we focused on hydrogen adsorption on large surface area solids, combining optimal extreme conditions i.e. very high pressure and low temperature for gas storage process purpose. Therefore, a new volumetric device is elaborated to obtain excess adsorption isotherms at 77 K up to 500 bar. Two activated carbons with different micro-porosities are analysed in the view of hydrogen storage investigation. Also, the results are compared to zeolite adsorption properties. Based on these results, the total mass and volumetric storage capacity are calculated using the bulk density relationship. Thereby, we obtained high storage in situ capacities equal to 5.2 wt% and 54.5 kgH₂/m³. Further, we also considered practical application aspects related to hydrogen storage process in highly porous packed materials.     

An evolving energy solution: Intermediate hydrogen storage

With the perpetual depletion of non-renewable sources of energy, the need of the hour is to look for alternative greener sources of energy. Hydrogen has emerged as an efficient, safe and versatile energy carrier. The storage and transportation of hydrogen is a problem that needs to be addressed in order to use hydrogen for practical applications. This review focuses on intermediate hydrogen storage which provides a balance between physical and chemical hydrogen storage. An optimum hydrogen adsorption energy at near ambient temperatures (0.2–0.3 eV), reversible hydrogen storage and fast adsorption and desorption kinetics are the advantages offered by intermediate hydrogen storage materials. The review discusses the recent synthetic and computational approaches for storage of hydrogen in 2D materials and inorganic nanostructured compounds.     

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.     

Lithium decoration characteristics for hydrogen storage enhancement in novel periodic porous graphene frameworks

Hydrogen storage in porous materials by physical adsorption is being discussed to provide widespread usage of hydrogen energy systems. One of the recent hydrogen storage media that store hydrogen physically is Porous Graphene Frameworks (PGFs). In the study, three different PGFs were constructed by using Benzene-1,3,5-tricarboxylic acid (BTC), 4,40,400-Benzene-1,3,5-triyltribenzoate (BTB) and 4,40,400-(benzene-1,3,5triyl-tris (benzene-4,1-diyl))tribenzoate (BBC) organic linkers. The geometries of the structures were optimized and lithium atoms were dispersed inside. Then, thirty-three different structures were derived. Finally, hydrogen storage capacities and surface areas of each structure were computed. It was found out that 160 lithium dispersed Graphene-BBC structure has the highest hydrogen storage capacity with 4.26 wt % at 298K and 100 bars while 70 lithium dispersed graphene-BTB structure store 9.81 wt % hydrogen at 77K and 4 bars, and lithium free graphene-BBC structure store 20,68 wt % hydrogen at 77K and 100 bars. Lithium dispersion enabled extra surfaces for Graphene-BTB and Graphene-BBC structures to the limits. But surface area of Graphene-BTC structure decreased with lithium dispersion. The number of limits for Graphene-BTB and Graphene-BBC named structures were 70 and 200 lithium atoms, respectively. At the final it is pointed out that constructed novel PGFs could store comparable and relatively high hydrogen in various conditions. The existence of lithium atoms played a minor role to enhance hydrogen storage capacity but the limits are critically important to reach maximum capacity.     

Transition metal decorated covalent triazine-based frameworks as a capacity hydrogen storage medium

From ab initio density functional theory (DFT) calculations, the structural stability and hydrogen adsorption capacity of transition metal (TM, TM = Sc, Ti, V, Cr, Mn) decorated covalent triazine-based framework (CTF) are discussed. It is found that by calculation, these TM atoms can adsorb on the CTF sheet without clusters. The Sc, Ti, V, Cr and Mn decorated CTF are predicated to bind five, four, three, three and two of hydrogen molecules. We found that Sc and Ti decorated CTF are suitable candidates for effective reversible hydrogen storage at near ambient condition, whereas V, Cr and Mn decorated CTF are not promising materials due to too large average bind energies per hydrogen molecule.     

Hydrogen storage ability of porous carbon material fabricated from coffee bean wastes

The hydrogen storage ability at 298 and 77 K of porous carbon materials with microporous structures fabricated from coffee bean wastes through KOH activation was investigated regarding pore structure. The dependence of hydrogen storage ability on the pore structure of porous carbon materials was investigated at 298 and 77 K to clarify the storage mechanism of carbon materials. Hydrogen storage ability at 298 K was increased linearly with increasing of specific surface area increasing. The maximum amount of stored hydrogen was 0.6 wt.% on porous carbon material with 2070 m²/g specific surface area. The hydrogen storage ability at 77 K was 4.0 wt.% on the same sample. The hydrogen storage ability showed a linear relationship with the micro-pore volume size. These changes in the dependence of the hydrogen storage ability on pore size suggested that the storage configuration changed from two- to three-dimensional. The stored hydrogen densities in porous carbon materials calculated from these values were 5.7 and 69.6 mg/cm³ at 298 and 77 K, respectively. The change in density indicated that the state of stored hydrogen in porous carbon materials was filled up aggregational state, which is extremely close to the liquid state, and suggested the realizing of high hydrogen storage ability on carbon materials fabricated from agricultural waste.     

Scale-up activation of carbon fibres for hydrogen storage

In a previous study, we investigated, at a laboratory scale, the chemical activation of two different carbon fibres (CF), their porosity characterization, and their optimization for hydrogen storage [1]. In the present work, this study is extended to: (i) a larger range of KOH activated carbon fibres, (ii) a larger range of hydrogen adsorption measurements at different temperatures and pressures (i.e. at room temperature, up to 20 MPa, and at 77 K, up to 4 MPa), and (iii) a scaling-up activation approach in which the obtained activated carbon fibres (ACF) are compared with those from laboratory-scale activation. The prepared samples cover a large range of porosities, which is found to govern their ability for hydrogen adsorption. The hydrogen uptake capacities of all the prepared samples have been analysed both in volumetric and in gravimetric bases. Thus, maximum adsorption capacities of around 5 wt% are obtained at 77 K, and 1.1 wt% at room temperature, respectively. The packing densities of the materials have been measured, turning out to play an important role in order to estimate the total storage capacity of a tank volume. Maximum values of 17.4 g l⁻¹ at 298 K, and 38.6 g l⁻¹ at 77 K were obtained.     

Hydrogen storage properties of the novel crosslinked UiO-66-(OH)2

Metal organic frameworks (MOF) are a type of nanoporous materials with large specific surface area, which are especially suitable for gas separation and storage. In this work, we report a new approach of crosslinking UiO-66-(OH)₂ to enhance its hydrogen storage capacity. UiO-66-(OH)₂ was synthesized using hafnium tetrachloride (HfCl₄) and 2, 5-dihydroxyterephthalic acid (DTPA) through a canonical modulated hydrothermal method (MHT), followed by a post-synthesis modification, which is to form a crosslinking structure inside the porous structure of UiO-66-(OH)₂. During the modification process, the phenolic hydroxyl groups on the UiO-66-(OH)₂ reacted with methanal, and HCl aqueous solution and triethylamine served as catalyst (the products denoted as UiO-66-H and UiO-66-T, respectively). Powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), ¹³C nuclear magnetic resonance spectroscopy (¹³C NMR) proved that the crosslinking was formed. The BET specific surface area and the average adsorption pore size of UiO-66-H and UiO-66-T significantly increased after modification. The hydrogen storage capacity of UiO-66-H reached a maximum of 3.37 wt% (16.87 mmol/g) at 77 K, 2 MPa. Hydrogen adsorption enthalpy of UiO-66-T was 0.986 kJ/mol, which was higher than that of UiO-66-(OH)₂ (0.695 kJ/mol). This work shows that UiO-66-(OH)₂ is a promising candidate for potential application in high-performance hydrogen storage.     

Ambient temperature hydrogen storage in porous materials with exposed metal sites

Cross-linked porous polymeric complexes with exposed metal sites are synthesized for room temperature hydrogen storage via physisorption. At 298 K and 100 atm, PTF-Cr exhibits high excess hydrogen storage capacity up to 1.5 wt% with Qst of 11.5 kJ mol^?1 while PTF-Mg exhibits 0.5 wt% with Qst of 8 kJ mol^?1. The result provides insight for development of future storage materials with exposed transition metals.     

Boron-doping effect on the enhanced hydrogen storage of titanium-decorated porous graphene: A first-principles study

The exploitation of solid hydrogen storage materials is an important part of the large-scale application of hydrogen energy. However, Metal agglomeration is one of the main reasons that restrict the hydrogen storage performance of carbon-based hydrogen storage materials. Herein, we develop Ti-decorated boron doped porous graphene as a novel hydrogen storage material based on first-principles calculations. The geometry and electronic structure of Ti-decorated porous graphene with and without boron doped are calculated. Doping boron in porous graphene (PG) can significantly increase the metal-substrate interaction and prevents the formation of Ti-metal clusters. The Ti atom-decorated boron-doped porous graphene (Ti–B/PG) system can stably adsorb sixteen hydrogen molecules with a gravimetric hydrogen uptake of 8.58 wt%. The thermodynamic calculations prove a high usable capacity of the material, at the adsorbing and desorbing conditions of 25 °C, 30 atm and 100 °C, 3 atm. The excellent hydrogen capacity, good recyclability, and desirable desorption capacity of Ti–B/PG make it a very prospective material for hydrogen storage.