GCMC investigation into adamantane-based aromatic frameworks with diamond-like structure as high-capacity hydrogen storage materials

A new class of 3D adamantane-based aromatic framework (AAF) with diamond-like structure was computationally designed with the aid of density functional theory (DFT) calculation and molecular mechanics (MM) methods. The hydrogen storage capacities of these AAFs were studied by the method of grand canonical Monte Carlo (GCMC) simulations. The calculated pore sizes of three AAFs reveal that AAF-1 and AAF-2 belong to microporous materials, while AAF-3 is a member of mesoporous materials. The GCMC results reveal that at 77 K and 100 bar, AAF-3 exhibits the highest gravimetric hydrogen uptake of 29.50 wt%, while AAF-1 shows the highest volumetric hydrogen uptake of 63.04 g L(-1). In particular, the gravimetric hydrogen uptake of AAF-3 reaches the Department of Energy's target of 6 wt% at room temperature. The extraordinary performances of these new AAFs in hydrogen storage have made them enter the list of top hydrogen storage materials up to now.     

Hydrogen storage capacity of catalytically grown carbon nanofibers

In 1996, R. T. K. Baker, and N. M. Rodriguez claimed to have synthesized a new type of carbon nanofiber material capable of storing large amounts of hydrogen at room temperature and pressures above 100 bar, thus making it a powerful candidate for a very efficient energy storage system in mobile applications. Consequently, many scientists all over the world tried to test and verify these findings, however, with partly inconsistent results. We present here for the first time independent hydrogen storage measurements for several types of nanofibers, both synthesized by our group following precisely the specifications given in the literature as well as original samples supplied by Rodriguez and Baker for this study. The hydrogen storage capacities at room temperature and pressures up to 140 bar were quantified independently by gravimetric and volumetric methods, respectively. No significant hydrogen storage capacity has been detected for all carbon nanofibers investigated.     

Influence of quantum effects on the physisorption of molecular hydrogen in model carbon foams

The physisorption of molecular hydrogen in model carbon foams has been investigated from 50 K to room temperature. The study is carried out within the framework of the density functional theory for quantum liquids at finite temperatures. Calculations are performed in the grand canonical ensemble, i.e., the adsorbed fluid is assumed to be in equilibrium with an external gas of hydrogen molecules with concentrations ranging from 8×10(-4) kg m(-3) to n=71 kg m(-3). It is shown that, while strong zero-point energy effects are present even at room temperature, the adsorption isotherms exhibit only a weak dependence on the explicit incorporation of the bosonic exchange symmetry of hydrogen molecules. The increase of the average particle density prevents the deviations from the Maxwell-Boltzmann statistics to become noticeable if the system is cooled down. The volumetric storage capacity of these materials at low temperatures is about one half of the U. S. Department of Energy goal, while the gravimetric capacity is still far from the standards required by mobile applications. The relation between the microscopic structure of the hydrogen fluid and the calculated adsorption properties is also addressed.     

Molecular screening of metal-organic frameworks for CO2 storage

We report a molecular simulation study for CO2 storage in metal-organic frameworks (MOFs). As compared to the aluminum-free and cation-exchanged ZSM-5 zeolites and carbon nanotube bundle, IRMOF1 exhibits remarkably higher capacity. Incorporation of Na(+) cations into zeolite increases the capacity only at low pressures. By variation of the metal oxide, organic linker, functional group, and framework topology, a series of isoreticular MOFs (IRMOF1, Mg-IRMOF1, Be-IRMOF1, IRMOF1-(NH2)4, IRMOF10, IRMOF13, and IRMOF14) are systematically examined, as well as UMCM-1, a fluorous MOF (F-MOF1), and a covalent-organic framework (COF102). The affinity with CO2 is enhanced by addition of a functional group, and the constricted pore is formed by interpenetration of the framework; both lead to a larger isosteric heat and Henry's constant and subsequently a stronger adsorption at low pressures. The organic linker plays a critical role in tuning the free volume and accessible surface area and largely determines CO2 adsorption at high pressures. As a combination of high capacity and low framework density, IRMOF10, IRMOF14, and UMCM-1 are identified from this study to be the best for CO2 storage, even surpass the experimentally reported highest capacity in MOF-177. COF102 is a promising candidate with high capacity at considerably low pressures. Both gravimetric and volumetric capacities at 30 bar correlate well with the framework density, free volume, porosity, and accessible surface area. These structure-function correlations are useful for a priori prediction of CO2 capacity and for the rational screening of MOFs toward high-efficacy CO2 storage.     

Hydrogen storage capacity characterization of carbon nanotubes by a microgravimetrical approach

An accurate gravimetric apparatus based on a contactless magnetic suspension microbalance was developed. This unit was used to measure the hydrogen storage capacity for a variety of carbon nanotubes (CNTs) at room temperature and hydrogen pressures up to 11.5 MPa. The results show that regardless of their synthesis methods, purities, and nanostructures all investigated CNT products possess relatively low hydrogen storage capacities (<0.2 wt %). For comparison, the adsorption characteristics of theses samples were also measured at a pressure of 0.1 MPa and liquid nitrogen temperature (approximately 77 K) by a conventional volumetric approach. The methodological aspects related to the accuracy of the hydrogen uptake measurements are also discussed.     

嵌入配位不饱和金属位对多孔芳香骨架材料储氢性能的影响

基于密度泛函理论(DFT)和巨正则蒙特卡洛(GCMC)模拟方法,系统地研究了引入配位不饱和金属位(CUS)对PAF-30n (n = 1–4)材料储氢性能影响的规律。结果表明,77 K下PAF-302MgO₂_PBE100的最大过量质量储氢量达到7.97% (w);77 K、10 MPa下100%醇镁功能化改性PAF-302和PAF-303的绝对储氢量分别达到9.9% (w) (65.9 g∙L^-1)和15.0% (w) (50.5 g∙L^-1),分别超过美国能源部(DOE)标准80% (64.8%)和173% (26.3%),均超过在相同条件下目前储氢性能最佳的NU-1101 (9.1% (w), 46.6 g∙L^-1)。即使在243 K、10 MPa下,其绝对质量和绝对体积储氢量也能分别达到5.13% (w)和34.19 g∙L^-1,占DOE质量与体积储氢标准的93.3%和85.5%,是目前为止常温储氢性能较为均衡的多孔材料之一。结合等量吸附热(Q_st)、径向分布函数(RDF)和质心几率密度分布(MCPD)方法进一步分析,发现有机链长度增加导致孔隙率增加和体积比表面积减小,是引起多孔材料绝对质量和绝对体积储氢量此消彼长的根本原因。另外,引入CUS能提高PAFs材料对H₂分子亲和力,显著增强其体积储氢量。     

氢气在单壁碳纳米管束的吸附的密度泛函研究

作者利用密度泛函理论（DFT）计算了氢气在单壁碳纳米管束（SWNTs)中管内 和管间的吸附。考察了温度,孔径以及压力对吸附的分子数密度,重量百分比,单 位体积储存能力以及超额吸附量的影响。DFT计算发现,较大的孔径有利于氢气在 SWNTs中的吸附且氢气在管隙中的吸附不可忽略。计算表明在77 K和6 MPa时,氢气 在2.719 mm的SWNTs的总的吸附的重量百分比分别可达到13.2 wt%,这约是美国能 源部（DOE）目标值的两倍,而单位体积储存能力在DOE目标值附近,而在300 K和 6 MPa时,氢气在2.719 nm的SWNTs的总的吸附的重量百分比仅为1.5 wt%。通过实 验结果与计算结果的比较表明,密度泛函理论的计算结果支持SWNTs有较高的吸附 储氢能力的实验结论。     

Computational design of tetrahedral silsesquioxane-based porous frameworks with diamond-like structure as hydrogen storage materials

A novel type of three-dimensional (3D) tetrahedral silsesquioxane-based porous frameworks (TSFs) with diamond-like structure was computationally designed using the density functional theory (DFT) and classical molecular mechanics (MM) calculations. The hydrogen adsorption and diffusion properties of these TSFs were evaluated by the methods of grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The results reveal that all designed materials possess extremely high porosity (87–93 %) and large H₂ accessible surface areas (5,268–6,544 m² g^?1). Impressively, the GCMC simulation results demonstrate that at 77 K and 100 bar, TSF-2 has the highest gravimetric H₂ capacity of 29.80 wt%, while TSF-1 has the highest volumetric H₂ uptake of 65.32 g L^?1. At the same time, the gravimetric H₂ uptake of TSF-2 can reach up to 4.28 wt% at the room temperature. The extraordinary performances of these TSF materials in hydrogen storage made them enter the rank of the top hydrogen storage materials so far.     

Hydrogen storage in porous structures of adamantane‐based nitrogen‐heterocyclic ring with diamond‐like structure

Using the density functional theory and molecular mechanics methods, we calculated the binding energy and parameters about the primitive cell designed by us with the adamantane and the nitrogen heterocyclic ring, the vibrational frequencies about the small complexes. Grand canonical Monte Carlo simulations were performed to predict the capacities for the hydrogen storage and adsorption isotherms. The results show the positive effects of bigger specific surface area and pore volume on hydrogen storage and isosteric heat. The gravimetric hydrogen uptake of adamantane‐based nitrogen‐heterocyclic ring of quaterpyridyl can reach 9.02 wt % at room temperature and 100 bar. But the volumetric H₂ capacities of the four materials are low at T = 298 K because of weak interaction between the materials and H₂ molecule. © 2014 Wiley Periodicals, Inc.     

Hydride-induced amplification of performance and binding enthalpies in chromium hydrazide gels for Kubas-type hydrogen storage

Hydrogen is the ideal fuel because it contains the most energy per gram of any chemical substance and forms water as the only byproduct of consumption. However, storage still remains a formidable challenge because of the thermodynamic and kinetic issues encountered when binding hydrogen to a carrier. In this study, we demonstrate how the principal binding sites in a new class of hydrogen storage materials based on the Kubas interaction can be tuned by variation of the coordination sphere about the metal to dramatically increase the binding enthalpies and performance, while also avoiding the shortcomings of hydrides and physisorpion materials, which have dominated most research to date. This was accomplished through hydrogenation of chromium alkyl hydrazide gels, synthesized from bis(trimethylsilylmethyl) chromium and hydrazine, to form materials with low-coordinate Cr hydride centers as the principal H(2) binding sites, thus exploiting the fact that metal hydrides form stronger Kubas interactions than the corresponding metal alkyls. This led to up to a 6-fold increase in storage capacity at room temperature. The material with the highest capacity has an excess reversible storage of 3.23 wt % at 298 K and 170 bar without saturation, corresponding to 40.8 kg H(2)/m(3), comparable to the 2015 DOE system goal for volumetric density (40 kg/m(3)) at a safe operating pressure. These materials possess linear isotherms and enthalpies that rise on coverage, retain up to 100% of their adsorption capacities on warming from 77 to 298 K, and have no kinetic barrier to adsorption or desorption. In a practical system, these materials would use pressure instead of temperature as a toggle and can thus be used in compressed gas tanks, currently employed in the majority of hydrogen test vehicles, to dramatically increase the amount of hydrogen stored, and therefore range of any vehicle.     

Mg-based nanocomposites with high capacity and fast kinetics for hydrogen storage

Magnesium and its alloys have shown a great potential in effective hydrogen storage due to their advantages of high volumetric/gravimetric hydrogen storage capacity and low cost. However, the use of these materials in fuel cells for automotive applications at the present time is limited by high hydrogenation temperature and sluggish sorption kinetics. This paper presents the recent results of design and development of magnesium-based nanocomposites demonstrating the catalytic effects of carbon nanotubes and transition metals on hydrogen adsorption in these materials. The results are promising for the application of magnesium materials for hydrogen storage, with significantly reduced absorption temperatures and enhanced ab/desorption kinetics. High level Density Functional Theory calculations support the analysis of the hydrogenation mechanisms by revealing the detailed atomic and molecular interactions that underpin the catalytic roles of incorporated carbon and titanium, providing clear guidance for further design and development of such materials with better hydrogen storage properties.     

An International Laboratory Comparison Study of Volumetric and Gravimetric Hydrogen Adsorption Measurements

In order to determine a material's hydrogen storage potential, capacity measurements must be robust, reproducible, and accurate. Commonly, research reports focus on the gravimetric capacity, and often times the volumetric capacity is not reported. Determining volumetric capacities is not as straight-forward, especially for amorphous materials. This is the first study to compare measurement reproducibility across laboratories for excess and total volumetric hydrogen sorption capacities based on the packing volume. The use of consistent measurement protocols, common analysis, and figure of merits for reporting data in this study, enable the comparison of the results for two different materials. Importantly, the results show good agreement for excess gravimetric capacities amongst the laboratories. Irreproducibility for excess and total volumetric capacities is attributed to real differences in the measured packing volume of the material.     

Hydrogen storage in nanoporous carbon materials: myth and facts

We used Grand canonical Monte Carlo simulation to model the hydrogen storage in the primitive, gyroid, diamond, and quasi-periodic icosahedral nanoporous carbon materials and in carbon nanotubes. We found that none of the investigated nanoporous carbon materials satisfy the US Department of Energy goal of volumetric density and mass storage for automotive application (6 wt% and 45 kg H(2) m(-3)) at considered storage condition. Our calculations indicate that quasi-periodic icosahedral nanoporous carbon material can reach the 6 wt% at 3.8 MPa and 77 K, but the volumetric density does not exceed 24 kg H(2) m(-3). The bundle of single-walled carbon nanotubes can store only up to 4.5 wt%, but with high volumetric density of 42 kg H(2) m(-3). All investigated nanoporous carbon materials are not effective against compression above 20 MPa at 77 K because the adsorbed density approaches the density of the bulk fluid. It follows from this work that geometry of carbon surfaces can enhance the storage capacity only to a limited extent. Only a combination of the most effective structure with appropriate additives (metals) can provide an efficient storage medium for hydrogen in the quest for a source of "clean" energy.     

Hydrogen adsorption in carbon nanostructures: comparison of nanotubes, fibers, and coals

Single-walled carbon nanotubes (SWNT) were reported to have record high hydrogen storage capacities at room temperature, indicating an interaction between hydrogen and carbon matrix that is stronger than known before. Here we present a study of the interaction of hydrogen with activated charcoal, carbon nanofibers, and SWNT that disproves these earlier reports. The hydrogen storage capacity of these materials correlates with the surface area of the material, the activated charcoal having the largest. The SWNT appear to have a relatively low accessible surface area due to bundling of the tubes; the hydrogen does not enter the voids between the tubes in the bundles. Pressure-temperature curves were used to estimate the interaction potential, which was found to be 580+/-60 K. Hydrogen gas was adsorbed in amounts up to 2 wt % only at low temperatures. Molecular rotations observed with neutron scattering indicate that molecular hydrogen is present, and no significant difference was found between the hydrogen molecules adsorbed in the different investigated materials. Results from density functional calculations show molecular hydrogen bonding to an aromatic C[bond]C that is present in the materials investigated. The claims of high storage capacities of SWNT related to their characteristic morphology are unjustified.     

Design of new materials for methane storage

One of the strategic goals of the modern automobile manufacturing industry is to replace gasoline and diesel with alternative fuels such as natural gas. In this report, we elucidate the desired characteristics of an optimal adsorbent for gas storage. The U.S. Department of Energy has outlined several requirements that adsorbents must fulfill for natural gas to become economically viable, with a key criterion being the amount adsorbed at 35 bar. We explore the adsorption characteristics of novel metal-organic materials (IRMOFs and molecular squares) and contrast them with the characteristics of two zeolites, MCM-41, and different carbon nanotubes. Using molecular simulations, we uncover the complex interplay of the factors influencing methane adsorption, especially the surface area, the capacity or free volume, the strength of the energetic interaction, and the pore size distribution. We also explain the extraordinary adsorption properties of IRMOF materials and propose new, not yet synthesized IRMOF structures with adsorption characteristics that are predicted to exceed the best experimental results to date by up to 36%.     

Tailoring of nanoscale porosity in carbide-derived carbons for hydrogen storage

The poor performance of hydrogen storage materials continues to hinder development of fuel cell-powered automobiles. Nanoscale carbons, in particular (activated carbon, exfoliated graphite, fullerenes, nanotubes, nanofibers, and nanohorns), have not fulfilled their initial promise. Here we show that carbon materials can be rationally designed for H2 storage. Carbide-derived carbons (CDC), a largely unknown class of porous carbons, are produced by high-temperature chlorination of carbides. Metals and metalloids are removed as chlorides, leaving behind a collapsed noncrystalline carbon with up to 80% open pore volume. The detailed nature of the porosity-average size and size distribution, shape, and total specific surface area (SSA)-can be tuned with high sensitivity by selection of precursor carbide (composition, lattice type) and chlorination temperature. The optimum temperature is bounded from below by thermodynamics and kinetics of chlorination reactions and from above by graphitization, which decreases SSA and introduces H2-sorbing surfaces with binding energies too low to be useful. Intuitively, pores of different size and shape should not contribute equally to hydrogen storage. By correlating pore properties with 77 K H2 isotherms from a wide variety of CDCs, we experimentally confirm that gravimetric hydrogen storage capacity normalized to total pore volume is optimized in materials with primarily micropores ( approximately 1 nm) rather than mesopores. Thus, in agreement with theoretical predictions, a narrow size distribution of small pores is desirable for storing hydrogen, while large pores merely degrade the volumetric storage capacity.     

A single-component liquid-phase hydrogen storage material

The current state-of-the-art for hydrogen storage is compressed H(2) at 700 bar. The development of a liquid-phase hydrogen storage material has the potential to take advantage of the existing liquid-based distribution infrastructure. We describe a liquid-phase hydrogen storage material that is a liquid under ambient conditions (i.e., at 20 °C and 1 atm pressure), air- and moisture-stable, and recyclable; releases H(2) controllably and cleanly at temperatures below or at the proton exchange membrane fuel cell waste-heat temperature of 80 °C; utilizes catalysts that are cheap and abundant for H(2) desorption; features reasonable gravimetric and volumetric storage capacity; and does not undergo a phase change upon H(2) desorption.     

Lithium-doped MOF impregnated with lithium-coated fullerenes: a hydrogen storage route for high gravimetric and volumetric uptakes at ambient temperatures

We theoretically demonstrated that by the impregnation of Li-decorated IRMOF-10 with Li-coated C(60), the hydrogen storage capacity is improved to be 6.3 wt% and 42 g L(-1) at 100 bar and 243 K. Both the gravimetric and volumetric hydrogen uptakes reach the 2015 DOE target at near ambient conditions.     

Hydrogen storage in mesoporous titanium oxide-alkali fulleride composites

Mesoporous titanium oxide-alkali fulleride composites were synthesized and characterized by X-ray diffraction, nitrogen adsorption, Raman spectroscopy, and elemental analysis. The hydrogen sorption properties of these composites were investigated at 77 K, room temperature, and 200 degrees C. A maximum overall volumetric uptake of 27.35 kg/m(3) was obtained for the lithium fulleride composite at 77 K and 100 atm, compared with 25.48 kg/m(3) for the pristine unreduced material under the same conditions. This value was less than those previously reported for bis(toluene)titanium- and bis(benzene)vanadium-reduced materials (40.46 and 33.42 kg/m(3), respectively) and also less than those found for the fulleride-free Li- and Na-reduced materials in this study (28.10 and 28.19 kg/m(3), respectively). At room temperature and 100 atm, the maximum gravimetric storage and adsorption values of fulleride-impregnated composites were 0.99 and 0.11 wt %, respectively, while the corresponding amounts for unreduced material were 0.94 and 0.10 wt %. At 200 degrees C and 100 atm, the maximum gravimetric storage and adsorption capacities of fulleride composites were less than those of the unreduced material, which were 0.62 and 0.06 wt %, respectively. Thus, inclusion of fulleride units in the pores lowered the overall gravimetric and volumetric storage relative to the fulleride-free Na- and Li-reduced counterparts. Like other reduced composites studied in our group, the enthalpies of the reduced composites showed an unusual increasing trend with surface coverage, with the greatest value (6.55 kJ/mol) measured for the Na-reduced fulleride composite. This suggests that the reduced titanium oxide surface provides the majority of the binding sites in these materials.     

H2 storage in carbon materials

In this work a series of commercial carbons with different structural and textural properties were characterised and evaluated for their application in hydrogen storage. The results showed that temperature has a greater influence on the storage capacity of carbons than pressure. The highest H₂ storage capacity at 298 K and 90 bar was 0.5 wt%, while at 77 K and atmospheric pressure it was 2.9 wt%. It is also showed that, in order to predict the hydrogen storage capacity of carbon material both at cryogenic and ambient temperature, the only use of BET surface area or total micropore volume obtained from N₂ adsorption isotherm may be insufficient, the characterization of the narrow microporosity is needed due to its high contribution to hydrogen adsorption capacity. The process involved in hydrogen storage in pure carbon materials seems to be physisorption. Morphological or structural characteristics have no influence, at least on gravimetric storage capacity.