Hydrogen storage in low silica type X zeolites

Low silica type X zeolites (LSX, Si/Al = 1) fully exchanged by alkali-metal cations (Li(+), Na(+), and K(+)) were studied for their hydrogen storage capacities. Hydrogen adsorption isotherms were measured separately at 77 K and <1 atm, and at 298 K and <10 MPa. It was found that the hydrogen adsorption capacity of LSX zeolite depended strongly on the cationic radius and the density of the cations that are located on the exposed sites. The interaction energies between H(2) and the cations follow the order Li(+) > Na(+) > K(+), as predicted based on the ionic radii. Oxygen anions on zeolite framework were minor adsorption sites. Li-LSX had an H(2) capacity of 1.5 wt % at 77 K and 1 atm, and a capacity of 0.6 wt % at 298 K and 10 MPa, among the highest of known sorbents. The hydrogen capacity in LSX zeolite by bridged hydrogen spillover was also investigated. A simple and effective technique was employed to build carbon bridges between the H(2) dissociation catalyst and the zeolite to facilitate spillover of hydrogen atoms. Thus, the hydrogen storage capacity of Li-LSX zeolite was enhanced to 1.6 wt % (by a factor of 2.6) at 298 K and 10 MPa. This is by far the highest hydrogen storage capacity obtained on a zeolite material at room temperature. Furthermore, the adsorption rates were fast, and the storages were shown to be fully reversible and rechargeable. Further optimization of the bridge building technique would lead to an additional enhancement of hydrogen storage.     

Gas adsorption and storage in metal-organic framework MOF-177

Gas adsorption experiments have been carried out on a zinc benzenetribenzoate metal-organic framework material, MOF-177. Hydrogen adsorption on MOF-177 at 298 K and 10 MPa gives an adsorption capacity of approximately 0.62 wt %, which is among the highest hydrogen storage capacities reported in porous materials at ambient temperatures. The heats of adsorption for H2 on MOF-177 were -11.3 to -5.8 kJ/mol. By adding a H2 dissociating catalyst and using our bridge building technique to build carbon bridges for hydrogen spillover, the hydrogen adsorption capacity in MOF-177 was enhanced by a factor of approximately 2.5, to 1.5 wt % at 298 K and 10 MPa, and the adsorption was reversible. N2 and O2 adsorption measurements showed that O2 was adsorbed more favorably than N2 on MOF-177 with a selectivity of approximately 1.8 at 1 atm and 298 K, which makes MOF-177 a promising candidate for air separation. The isotherm was linear for O2 while being concave for N2. Water vapor adsorption studies indicated that MOF-177 adsorbed up to approximately 10 wt % H2O at 298 K. The framework structure of MOF-177 was not stable upon H2O adsorption, which decomposed after exposure to ambient air in 3 days. All the results suggested that MOF-177 could be a potentially promising material for gas separation and storage applications at ambient temperature (under dry conditions or with predrying).     

Hydrogen adsorption on microporous materials at ambient temperatures and pressures up to 50 MPa

High surface area microporous adsorbents are often proposed as potential hydrogen storage materials, although typically at 77?K and less than 5?MPa. In this study, we focus on conditions more suitable for automotive applications by investigating the storage capacities of microporous materials at 298?K and at pressures up to 50?MPa. In an effort to derive trends within and across material classes, we examined a wide range of materials with varying microstructures including the activated carbons AX-21, KUA-5, and MSC-30; a zeolite templated carbon; a hypercrosslinked polymer; and the Metal Organic Frameworks MOF-177, IRMOF-20, MIL-53, ZIF-8, and Cu₃(btc)₂. The peak excess adsorption of these materials ranged from 0.8–1.8?wt.%, although many did not reach their maximum capacity even at high pressures. However, the total volumetric storage gains over compressed hydrogen gas were quite low and, in many cases, negative. In addressing ambient temperature adsorption at significantly higher pressures than previously reported, our data confirms and extends the range of validity of several existing DFT calculations. Furthermore, our data suggest that, for both activated carbons and MOFs, factors other than specific surface area govern ambient temperature adsorption capacity. Contrary to some reports, the high fractions of sub-nanometer pores in some of the investigated MOFs did not appear to enhance the excess adsorption even at high pressures. For on-board applications with ambient temperature storage, significant enhancements to the attractive force at the materials’ surface are required, beyond merely increasing specific surface area, or for MOFs, tuning of pore sizes.     

Significantly enhanced hydrogen storage in metal-organic frameworks via spillover

The utilization of hydrogen in fuel-cell powered vehicles is limited by the lack of a safe and effective system for hydrogen storage. At the present time, there is no viable storage technology capable of meeting the DOE targets. Porous metal-organic frameworks (MOFs) are novel and potential candidates for hydrogen storage. Until now it is still not possible to achieve any significant hydrogen storage capacity in MOFs at ambient temperature. Here, we report, for the first time, significant amounts of hydrogen storage in MOF-5 and IRMOF-8 at ambient temperature by using a very simple technique via hydrogen dissociation and spillover. Thus, hydrogen uptakes for MOF-5 and IRMOF-8 can be enhanced by a factor of 3.3 and 3.1, respectively (to nearly 2 wt % at 10 MPa and 298 K). Furthermore, the isotherms are totally reversible. These findings suggest that our technique is suitable for hydrogen storage in a variety of MOF materials because of their similar structures as MOF-5 and IRMOF-8.     

Zeolite-templated carbon materials for high-pressure hydrogen storage

Zeolite-templated carbon (ZTC) materials were synthesized, characterized, and evaluated as potential hydrogen storage materials between 77 and 298 K up to 30 MPa. Successful synthesis of high template fidelity ZTCs was confirmed by X-ray diffraction and nitrogen adsorption at 77 K; BET surface areas up to ~3600 m(2) g(-1) were achieved. Equilibrium hydrogen adsorption capacity in ZTCs is higher than all other materials studied, including superactivated carbon MSC-30. The ZTCs showed a maximum in Gibbs surface excess uptake of 28.6 mmol g(-1) (5.5 wt %) at 77 K, with hydrogen uptake capacity at 300 K linearly proportional to BET surface area: 2.3 mmol g(-1) (0.46 wt %) uptake per 1000 m(2) g(-1) at 30 MPa. This is the same trend as for other carbonaceous materials, implying that the nature of high-pressure adsorption in ZTCs is not unique despite their narrow microporosity and significantly lower skeletal densities. Isoexcess enthalpies of adsorption are calculated between 77 and 298 K and found to be 6.5-6.6 kJ mol(-1) in the Henry's law limit.     

A hydrogen sorption study on a Pd-doped CMK-3 type ordered mesoporous carbon

An ordered mesoporous carbon of CMK-3 type was prepared and modified by metal doping with Pd nanoparticles. The hydrogen sorption performance of the pristine and composite materials was studied at different temperature and pressure conditions in order to provide insight into the underlying storage mechanism. It was shown that metal doping can lead to enhanced hydrogen storage at 298 K as a result of a weak chemisorption process initiated by the so-called “spillover” effect.     

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.     

Isotope tracer study of hydrogen spillover on carbon-based adsorbents for hydrogen storage

A composite material comprising platinum nanoparticles supported on molecular sieve templated carbon was synthesized and found to adsorb 1.35 wt % hydrogen at 298 K and 100 atm. The isosteric heat of adsorption for the material at low coverage was approximately 14 kJ/mol, and it approached a value of 10.6 kJ/mol as coverage increased for pressures at and above 1 atm. The increase in capacity is attributed to spillover, which is observed with the use of isotopic tracer TPD. IRMOF-8 bridged to Pt/C, a material known to exhibit hydrogen spillover at room temperature, was also studied with the hydrogen-deuterium scrambling reaction for comparison. The isotherms were reversible. For desorption, sequential doses of H2 and D2 at room temperature and subsequent TPD yield product distributions that are strong indicators of the surface diffusion controlled reverse spillover process.     

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.     

Hydrogen storage in metal-organic frameworks by bridged hydrogen spillover

The possible utilization of hydrogen as the energy source for fuel-cell vehicles is limited by the lack of a viable hydrogen storage system. Metal-organic frameworks (MOFs) belong to a new class of microporous materials that have recently been shown to be potential candidates for hydrogen storage; however, no significant hydrogen storage capacity has been achieved in MOFs at ambient temperature. Here we report substantially increased hydrogen storage capacities of modified MOFs by using a simple technique that causes and facilitates hydrogen spillover. Thus, the storage of 4 wt % is achieved at room temperature and 100 atm for the modified IRMOF-8. The adsorption is reversible, and the rates are fast. That has made MOFs truly promising for hydrogen storage application.     

Thermally modulated multilayered graphene oxide for hydrogen storage

We have obtained high pressure H(2) isotherms with respect to the interlayer distance of multilayered graphene oxide (GO) modulated by thermal annealing. The maximum storage capacity is 4.8 (0.5) wt% at 77 K (298 K) and at 9.0 MPa pressure. We found the optimum GO interlayer distance for maximum H(2) uptake at 6.5 ?, similar to the predicted distances from first-principles calculations for graphite materials. Our results reveal that multilayered GO can be a practical material of choice to allow the use of graphene as a hydrogen storage material, provided that only small amounts of O and OH functional groups exist as spacers on GO sheets.     

Preparation of platinum-decorated porous graphite nanofibers, and their hydrogen storage behaviors

In this work, the hydrogen storage behaviors of porous graphite nanofibers (GNFs) decorated by Pt nanoparticles were investigated. The Pt nanoparticles were introduced onto the GNF surfaces using a well-known chemical reduction method. We investigated the hydrogen storage capacity of the Pt-doped GNFs for the platinum content range of 1.3-7.5 mass%. The microstructure of the Pt/porous GNFs was characterized by X-ray diffraction and transmission electron microscopy. The hydrogen storage behaviors of the Pt/GNFs were studied using a PCT apparatus at 298 K and 10 MPa. It was found that amount of hydrogen stored increased with increasing Pt content to 3.4 mass%, and then decreased. This result indicates that the hydrogen storage capacity of porous carbons is based on both their metal content and dispersion rate.     

Fundamental studies and perceptions on the spillover mechanism for hydrogen storage

In this feature article, the atomic-scale understanding of the hydrogen spillover mechanism for hydrogen storage in metal-doped carbon materials and metal-organic frameworks is discussed by critically assessing recent computational and experimental studies. It is argued that the spillover mechanism involves: (a) the generation and desorption of mobile H atoms on the metal nanoparticles (b) the diffusion of H atoms in weakly-bound states on the support (c) the sticking and immobilization of H atoms at preferential locations of the receptor where barriers to sticking are decreased, and, (d) the Eley-Rideal recombination of the adsorbed H atoms with diffusing mobile H atoms to form H(2). The implications and open questions on the mechanism and effectiveness of hydrogen storage by spillover are critically assessed.     

Effect of nitrogen doping on hydrogen storage capacity of palladium decorated graphene

A high hydrogen storage capacity for palladium decorated nitrogen-doped hydrogen exfoliated graphene nanocomposite is demonstrated under moderate temperature and pressure conditions. The nitrogen doping of hydrogen exfoliated graphene is done by nitrogen plasma treatment, and palladium nanoparticles are decorated over nitrogen-doped graphene by a modified polyol reduction technique. An increase of 66% is achieved by nitrogen doping in the hydrogen uptake capacity of hydrogen exfoliated graphene at room temperature and 2 MPa pressure. A further enhancement by 124% is attained in the hydrogen uptake capacity by palladium nanoparticle (Pd NP) decoration over nitrogen-doped graphene. The high dispersion of Pd NP over nitrogen-doped graphene sheets and strengthened interaction between the nitrogen-doped graphene sheets and Pd NP catalyze the dissociation of hydrogen molecules and subsequent migration of hydrogen atoms on the doped graphene sheets. The results of a systematic study on graphene, nitrogen-doped graphene, and palladium decorated nitrogen-doped graphene nanocomposites are discussed. A nexus between the catalyst support and catalyst particles is believed to yield the high hydrogen uptake capacities obtained.     

Storage of hydrogen at 303 K in graphite slitlike pores from grand canonical Monte Carlo simulation

Grand canonical Monte Carlo (GCMC) simulations were used for the modeling of the hydrogen adsorption in idealized graphite slitlike pores. In all simulations, quantum effects were included through the Feynman and Hibbs second-order effective potential. The simulated surface excess isotherms of hydrogen were used for the determination of the total hydrogen storage, density of hydrogen in graphite slitlike pores, distribution of pore sizes and volumes, enthalpy of adsorption per mole, total surface area, total pore volume, and average pore size of pitch-based activated carbon fibers. Combining experimental results with simulations reveals that the density of hydrogen in graphite slitlike pores at 303 K does not exceed 0.014 g/cm(3), that is, 21% of the liquid-hydrogen density at the triple point. The optimal pore size for the storage of hydrogen at 303 K in the considered pore geometry depends on the pressure of storage. For lower storage pressures, p < 30MPa, the optimal pore width is equal to a 2.2 collision diameter of hydrogen (i.e., 0.65 nm), whereas, for p congruent with 50MPa, the pore width is equal to an approximately 7.2 collision diameter of hydrogen (i.e., 2.13 nm). For the wider pores, that is, the pore width exceeds a 7.2 collision diameter of hydrogen, the surface excess of hydrogen adsorption is constant. The importance of quantum effects is recognized in narrow graphite slitlike pores in the whole range of the hydrogen pressure as well as in wider ones at high pressures of bulk hydrogen. The enthalpies of adsorption per mole for the considered carbonaceous materials are practically constant with hydrogen loading and vary within the narrow range q(st) congruent with 7.28-7.85 kJ/mol. Our systematic study of hydrogen adsorption at 303 K in graphite slitlike pores gives deep insight into the timely problem of hydrogen storage as the most promising source of clean energy. The calculated maximum storage of hydrogen is equal to approximately 1.4 wt %, which is far from the United States Department of Energy (DOE) target (i.e., 6.5 wt %), thus concluding that the total storage amount of hydrogen obtained at 303 K in graphite slitlike pores of carbon fibers is not sufficient yet.     

Metal-organic frameworks—New materials for hydrogen storage

Published data on the physical sorption of hydrogen by new materials with a large specific surface area, crystalline microporous metal-organic frameworks (MOFs), are systematized and analyzed. The hydrogen-accumulating properties of MOFs are compared with those of traditional materials (charcoals and zeolites) and nanocarbon systems. The role of secondary hydrogen spillover in the development of new approaches to increase the adsorption capacity of hydrogen storage materials is separately considered.     

Isosteric enthalpies for hydrogen adsorbed on nanoporous materials at high pressures

A sound understanding of any sorption system requires an accurate determination of the enthalpy of adsorption. This is a fundamental thermodynamic quantity that can be determined from experimental sorption data and its correct calculation is extremely important for heat management in adsorptive gas storage applications. It is especially relevant for hydrogen storage, where porous adsorptive storage is regarded as a competing alternative to more mature storage methods such as liquid hydrogen and compressed gas. Among the most common methods to calculate isosteric enthalpies in the literature are the virial equation and the Clausius–Clapeyron equation. Both methods have drawbacks, for example, the arbitrary number of terms in the virial equation and the assumption of ideal gas behaviour in the Clausius–Clapeyron equation. Although some researchers have calculated isosteric enthalpies of adsorption using excess amounts adsorbed, it is arguably more relevant to applications and may also be more thermodynamically consistent to use absolute amounts adsorbed, since the Gibbs excess is a partition, not a thermodynamic phase. In this paper the isosteric enthalpies of adsorption are calculated using the virial, Clausius–Clapeyron and Clapeyron equations from hydrogen sorption data for two materials—activated carbon AX-21 and metal-organic framework MIL-101. It is shown for these two example materials that the Clausius–Clapeyron equation can only be used at low coverage, since hydrogen’s behaviour deviates from ideal at high pressures. The use of the virial equation for isosteric enthalpies is shown to require care, since it is highly dependent on selecting an appropriate number of parameters. A systematic study on the use of different parameters for the virial was performed and it was shown that, for the AX-21 case, the Clausius–Clapeyron seems to give better approximations to the exact isosteric enthalpies calculated using the Clapeyron equation than the virial equation with 10 variable parameters.     

Predicting adsorption isotherms of low-volatile compounds by temperature programmed desorption: iodine on carbon

Equilibrium adsorption isotherms for low-volatile compounds are extremely difficult to measure. A simple technique using temperature programmed desorption (TPD) is proposed. It is demonstrated that the two parameters needed for constructing the Langmuir isotherm can be derived with data from the TPD technique alone. Thus, the Langmuir isotherms of iodine on AX-21 super-activated carbon were obtained with this technique. A series of TPD experiments for samples with different initial loadings of iodine were carried out by varying the heating rates which resulted in different peak desorption temperatures. The peak desorption temperature decreased as the initial loading was increased because of the re-adsorption effect. The Langmuir constant was derived from kinetic theory with the activation energy for desorption obtained from the experiment. The activation energy for desorption was 12.3 kcal/mol. The Langmuir constants determined by this technique were in comparable order of magnitude to the reported values for iodine on activated carbon. The saturation capacity of AX-21 for iodine could also be determined from the TPD data obtained from samples with different initial loadings. The estimated saturation capacity from the TPD experiment was 2.96 g I(2)/g AX-21, which was close to the experimentally measured saturation capacity of 3.25 g I(2)/g AX-21 for the same system.     

Gas adsorption process in activated carbon over a wide temperature range above the critical point. Part 1: modified Dubinin-Astakhov model

Simulations of the thermal effects during adsorption cycles are valuable tools for the design of efficient adsorption-based systems such as gas storage, gas separation and adsorption-based heat pumps. An analytical representation of the measured adsorption data over the wide operating pressure and temperature swing of the system is necessary for the calculation of complete mass and energy conservation equations. In Part 1, the Dubinin-Astakhov (D-A) model is adapted to model hydrogen, nitrogen, and methane adsorption isotherms on activated carbon at high pressures and supercritical temperatures assuming a constant microporous adsorption volume. The five parameter D-A type adsorption model is shown to fit the experimental data for hydrogen (30 to 293 K, up to 6 MPa), nitrogen (93 to 298 K, up to 6 MPa), and for methane (243 to 333 K, up to 9 MPa). The quality of the fit of the multiple experimental adsorption isotherms is excellent over the large temperature and pressure ranges involved. The model’s parameters could be determined as well from only the 77 K and 298 K hydrogen isotherms without much reducing the quality of the fit.     

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。