Chemical-clathrate hybrid hydrogen storage: storage in both guest and host

Hydrogen storage from two independent sources of the same material represents a novel approach to the hydrogen storage problem, yielding storage capacities greater than either of the individual constituents. Here we report a novel hydrogen storage scheme in which recoverable hydrogen is stored molecularly within clathrate cavities as well as chemically in the clathrate host material. X-ray diffraction and Raman spectroscopic measurements confirm the formation of beta-hydroquinone (beta-HQ) clathrate with molecular hydrogen. Hydrogen within the beta-HQ clathrate vibrates at considerably lower frequency than hydrogen in the free gaseous phase and rotates nondegenerately with splitting comparable to the rotational constant. Compared with water-based clathrate hydrate phases, the beta-HQ+H2 clathrate shows remarkable stability over a range of p-T conditions. Subsequent to clathrate decomposition, the host HQ was used to directly power a PEM fuel cell. With one H2 molecule per cavity, 0.61 wt % hydrogen may be stored in the beta-HQ clathrate cavities. When this amount is combined with complete dehydrogenation of the host hydroxyl hydrogens, the maximum hydrogen storage capacity increases nearly 300% to 2.43 wt %.     

Phase relations and binary clathrate hydrate formation in the system H2-THF-H2O

Experimentally determined equilibrium phase relations are reported for the system H2-THF-H2O as a function of aqueous tetrahydrofuran (THF) concentration from 260 to 290 K at pressures up to 45 MPa. Data are consistent with the formation of cubic structure-II (CS-II) binary H2-THF clathrate hydrates with a stoichiometric THF-to-water ratio of 1:17, which can incorporate modest volumes of molecular hydrogen at elevated pressures. Direct compositional analyses of the clathrate phase, at both low (0.20 mol %) and stoichiometric (5.56 mol %) initial THF aqueous concentrations, are consistent with observed phase behavior, suggesting full occupancy of large hexakaidecahedral (51264) clathrate cavities by THF, coupled with largely complete (80-90%) filling of small dodecahedral (512) cages by single H2 molecules at pressures of >30 MPa, giving a clathrate formula of (H2) < or =2.THF.17H2O. Results should help to resolve the current controversy over binary H2-THF hydrate hydrogen contents; data confirm recent reports that suggest a maximum of approximately 1 mass % H2, this contradicting values of up to 4 mass % previously claimed for comparable conditions.     

Molecular hydrogen occupancy in binary THF-H2 clathrate hydrates by high resolution neutron diffraction

We have determined the time-space average filling of hydrogen molecules in a binary tetrahydrofuran (THF)-d(8) + D(2) sII clathrate hydrate using high resolution neutron diffraction. The filling of hydrogen in the lattice of a THF-d(8) clathrate hydrate occurred upon pressurization. The hydrogen molecules were localized in the small dodecahedral cavities at 20 K, with nuclear density from the hydrogen approximately spherically distributed and centered in the small cavity. With a formation pressure of 70 MPa, molecular hydrogen was found to only singly occupy the sII small cavity. This result helps explain discrepancies about the hydrogen occupancy in the THF binary hydrate system.     

Hydrogen in Porous Tetrahydrofuran Clathrate Hydrate

The lack of practical methods for hydrogen storage is still a major bottleneck in the realization of an energy economy based on hydrogen as energy carrier. ¹ Storage within solid‐state clathrate hydrates, ² – ⁴ and in the clathrate hydrate of tetrahydrofuran (THF), has been recently reported. ⁵ , ⁶ In the latter case, stabilization by THF is claimed to reduce the operation pressure by several orders of magnitude close to room temperature. Here, we apply in situ neutron diffraction to show that—in contrast to previous reports^{[5, 6]}—hydrogen (deuterium) occupies the small cages of the clathrate hydrate only to 30 % (at 274 K and 90.5 bar). Such a D₂ load is equivalent to 0.27 wt. % of stored H₂. In addition, we show that a surplus of D₂O results in the formation of additional D₂O ice Ih instead of in the production of sub‐stoichiometric clathrate that is stabilized by loaded hydrogen (as was reported in ref. ⁶ ). Structure‐refinement studies show that [D₈]THF is dynamically disordered, while it fills each of the large cages of [D₈]THF?17D₂O stoichiometrically. Our results show that the clathrate hydrate takes up hydrogen rapidly at pressures between 60 and 90 bar (at about 270 K). At temperatures above ≈220 K, the H‐storage characteristics of the clathrate hydrate have similarities with those of surface‐adsorption materials, such as nanoporous zeolites and metal–organic frameworks, ⁷ , ⁸ but at lower temperatures, the adsorption rates slow down because of reduced D₂ diffusion between the small cages.     

Hydrogen storage in sH hydrates: a Monte Carlo study

Grand canonical Monte Carlo simulations are performed to evaluate the hydrogen-storage capacity of the recently discovered hydrogen hydrates of the sH type, at 274 K and up to 500 MPa. First, the pure H2 hydrate is investigated in order to determine the upper limit of H 2 content in sH hydrates. It is found that the storage capacity of the hypothetical pure H2 hydrate could reach 3.6 wt % at 500 MPa. Depending on pressure, the large cavity of this hydrate can accommodate up to eight H2 molecules, while the small and medium ones are singly occupied even at pressures as high as 500 MPa. Next, the binary H2-methylcyclohexane sH hydrate is examined. In this case, the small and medium cavities are again singly occupied, resulting in a maximum H2 uptake of 1.4 wt %. Finally, the results from simulations on pure H2 and binary hydrates are utilized to investigate the potential of H2 storage in sH hydrates where the promoter molecules occupy the medium instead of the large cavities.     

The influence of pressure on the stability of clathrate hydrates of hydrogen and tetrahydrofuran

Changes in the Gibbs energy of hydration of molecular hydrogen and tetrahydrofuran (THF) at pressures of 0.1, 6.0, and 12.0 MPa over the temperature range 230–300 K were studied by the molecular dynamics method. The Gibbs energy of hydrogen in water-tetrahydrofuran-hydrogen solutions passed minima over the temperature range 235–265 K, which were indicative of a comparatively stable clathrate hydrate state. The Gibbs energy of the hydrogen molecule at the local minimum at 262 K was ∼4.5 kJ/mol; at atmospheric pressure and room temperature, it was ∼2 kJ/mol. An analysis of the radial distribution function and the coordination number of the THF molecule showed that, at 240–257 K, a clathrate hydrate of THF with the structure close to clathrate sII was predominantly formed.     

Study on thermal properties of TBAB–THF hydrate mixture for cold storage by DSC

In order to study the thermal properties of new type environment-friendly binary hydrate for cold storage in air-conditioning system, tests have been carried out by DSC comprehensively on the phase-change temperature and fusion heat of TBAB hydrate, THF hydrate, and TBAB–THF hydrate mixture. The results show a good trend that TBAB–THF hydrate has the superiority for more proper phase-change temperature and increased fusion heat. A broader and more developed view is that adding appropriate amount of hydrate with lower phase-change temperature to hydrate with higher one can make the hydrate mixture more suitable for cold storage (especially for 278–281 K); some hydrates with lower phase-change temperature can even make the fusion heat of mixture hydrate increased greatly. Several new environmental working pairs for binary gas hydrates have been listed to help to promote the application.     

Water cavities of sH clathrate hydrate stabilized by molecular hydrogen: phase equilibrium measurements

In this experimental phase equilibrium study, we show for the first time that it is possible to stabilize structure sH of hydrogen clathrate hydrate with the help of some selected promoters. It was established that the formation pressures of these systems are significantly higher than that of structure sII of hydrogen clathrate hydrate when tetrahydrofuran (THF) is used as a promoter. Although no experimental evidence is available yet, it is estimated that the hydrogen storage capacity of structure sH can be as high as 1.4 wt % of H2, which is about 40% higher compared to the hydrogen storage capacity in structure sII.     

Water cavities of sH clathrate hydrate stabilized by molecular hydrogen

X-ray diffraction and Raman spectroscopic measurements confirm that molecular hydrogen can be contained within the small water cavities of a binary sH clathrate hydrate using large guest molecules that stabilize the large cavity. The potential increase in hydrogen storage could be more than 40% when compared with binary sII hydrates. This work demonstrates the stabilization of hydrogen in a hydrate structure previously unknown for encapsulating molecular hydrogen, indicating the potential for other inclusion compound materials with even greater hydrogen storage capabilities.     

Tuning the Composition of Guest Molecules in Clathrate Hydrates: NMR Identification and Its Significance to Gas Storage

Gas hydrates represent an attractive way of storing large quantities of gas such as methane and carbon dioxide, although to date there has been little effort to optimize the storage capacity and to understand the trade‐offs between storage conditions and storage capacity. In this work, we present estimates for gas storage based on the ideal structures, and show how these must be modified given the little data available on hydrate composition. We then examine the hypothesis based on solid‐solution theory for clathrate hydrates as to how storage capacity may be improved for structure II hydrates, and test the hypothesis for a structure II hydrate of THF and methane, paying special attention to the synthetic approach used. Phase equilibrium data are used to map the region of stability of the double hydrate in P–T space as a function of the concentration of THF. In situ high‐pressure NMR experiments were used to measure the kinetics of reaction between frozen THF solutions and methane gas, and ¹³C MAS NMR experiments were used to measure the distribution of the guests over the cage sites. As known from previous work, at high concentrations of THF, methane only occupies the small cages in structure II hydrate, and in accordance with the hypothesis posed, we confirm that methane can be introduced into the large cage of structure II hydrate by lowering the concentration of THF to below 1.0 mol %. We note that in some preparations the cage occupancies appear to fluctuate with time and are not necessarily homogeneous over the sample. Although the tuning mechanism is generally valid, the composition and homogeneity of the product vary with the details of the synthetic procedure. The best results, those obtained from the gas–liquid reaction, are in good agreement with thermodynamic predictions; those obtained for the gas–solid reaction do not agree nearly as well.     

钾掺杂多壁纳米碳管储氢性能研究

使用自制的钴催化裂解碳氢气法制备多壁纳米碳管,并对其进行退火、掺杂等一系列预处理,然后使用高压高纯氢源,在中压(12 MPa)和室温条件下,进行钾掺杂多壁纳米碳管的储氢性能实验.结果表明:预处理对纳米碳管的储氢性能有很大影响.实验条件下,经过氮气退火,并在1.0 mol/L硝酸钾溶液中掺杂的多壁纳米碳管吸氢量最大(H/C质量分数为3.2%).上述样品在室温下的放氢量一般不超过其吸氢量的50.8%.     

Improving methane storage on wet activated carbons at various amounts of water

Different mesoporous activated carbons were prepared by both chemical and physical activation processes and were examined for methane uptake in the presence of water.Methane isotherms were obtained at ...     

Intensification of methane and hydrogen storage in clathrate hydrate and future prospect

Gas hydrate is a new technology for energy gas (methane/hydrogen) storage due to its large capacity of gas storage and safe. But industrial application of hydrate storage process was hindered by some problems. For methane, the main problems are low formation rate and storage capacity, which can be solved by strengthening mass and heat transfer, such as adding additives, stirring, bubbling, etc. One kind of additives can change the equilibrium curve to reduce the formation pressure of methane hydrate, and the other kind of additives is surfactant, which can form micelle with water and increase the interface of water-gas. Dry water has the similar effects on the methane hydrate as surfactant. Additionally, stirring, bubbling, and spraying can increase formation rate and storage capacity due to mass transfer strengthened. Inserting internal or external heat exchange also can improve formation rate because of good heat transfer. For hydrogen, the main difficulties are very high pressure for hydrate formed. Tetrahydrofuran (THF), tetrabutylammonium bromide (TBAB) and tetrabutylammonium fluoride (TBAF) have been proved to be able to decrease the hydrogen hydrate formation pressure significantly.     

Intensification of methane and hydrogen storage inclathrate hydrate and future prospect

Gas hydrate is a new technology for energy gas(methane/hydrogen)storage due to its large capacity of gas storage and safe.But industrial application of hydrate storage process was hindered by someproblems.For methane,the main problems are low formation rateand storage capacity,which can be solved by strengthening mass andheat transfer,such as adding additives,stirring,bubbling,etc.Onekind of additives can change the equilibrium curve to reduce the formation pressure of methane hydrate,and the other kind of additivesis surfactant,which can form micelle with water and increase the interface of water-gas.Dry water has the similar effects on the methanehydrate as surfactant.Additionally,stirring,bubbling,and sprayingcan increase formation rate and storage capacity due to mass transferstrengthened.Inserting internal or external heat exchange also canimprove formation rate because of good heat transfer.For hydrogen,the main difficulties are very high pressure for hydrate formed.Tetrahydrofuran(THF),tetrabutylammonium bromide(TBAB) andtetrabutylammonium fluoride(TBAF) have been proved to be able todecrease the hydrogen hydrate formation pressure significantly.     

Theoretical investigation of ozone hydrate formation conditions

Structural, dynamic, and thermodynamic properties of ozone, oxygen, and mixed ozone-oxygen hydrates are investigated. The thermodynamic stability regions of these hydrates are found. Ozone can form hydrates at ambient pressure and temperatures below 230 K. Strong dependence of the binary hydrate formation pressure on the ozone concentration in the gas phase is shown. In the formation of the hydrate, ozone concentrates in the hydrate phase. At an ozone concentration of 5 mol.% in the gas phase, the ozone content in the hydrate reaches 40%.     

Hydrophobic Particle Effects on Hydrate Crystal Growth at the Water–Oil Interface

This study introduced hydrophobic silica nanoparticles (SiNPs) into an interface of aqueous and hydrate‐forming oil phases and analyzed the inhibition of hydrate crystal growth after seeding the hydrate slurry. The hydrate inhibition performance was quantitatively identified by micro‐differential scanning calorimetry (micro‐DSC) experiments. Through the addition of 1.0 wt % of SiNPs into the water–oil interface, the hydrate crystal growth only occurred around the seeding position of cyclopentane (CP) hydrate slurry, and the growth of hydrate crystals was retarded. Upon a further increase in the SiNP concentration up to 2.0 wt %, the SiNP‐laden interface completely prevented hydrate growth. We observed a hollow conical shape of hydrate crystals with 0.0 and 1.0 wt % of SiNPs, respectively, but the size and shape of the conical crystals was shrunken at 1.0 wt % of silica nanoparticles. However, the conical shape did not appear with an increased nanoparticle concentration of 2 wt %. These findings can provide insight into hydrate inhibition in oil and gas delivery lines, possibly with nanoparticles.     

Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation

Storage and transportation of natural gas as gas hydrate (“gas-to-solids technology”) is a promising alternative to the established liquefied natural gas (LNG) or compressed natural gas (CNG) technologies. Gas hydrates offer a relatively high gas storage capacity and mild temperature and pressure conditions for formation. Simulations based on the van der Waals–Platteeuw model and molecular dynamics (MD) are employed in this study to relate the methane gas content/occupancy in different hydrate systems with the hydrate stability conditions including temperature, pressure, and secondary clathrate stabilizing guests. Methane is chosen as a model system for natural gas. It was found that the addition of about 1% propane suffices to increase the structure II (sII) methane hydrate stability without excessively compromising methane storage capacity in hydrate. When tetrahydrofuran (THF) is used as the stabilizing agent in sII hydrate at concentration between 1% and 3%, a reasonably high methane content in hydrate can be maintained (∼85–100, v/v) without dealing with pressures more than 5 MPa and close to room temperature.     

Excess Gibbs potential model for multicomponent hydrogen clathrates

A new thermodynamic calculation procedure is introduced to predict the equilibrium conditions of multicomponent gas hydrates containing hydrogen. This new approach utilizes an excess Gibbs potential term to account for second- or higher-order water-cavity distortions due to the presence of multiple guest species. The excess Gibbs potential describes changes in reference chemical potentials according to different compositions of guest mixtures in the hydrate phase. To determine the equilibrium conditions of multicomponent gas hydrates, the excess Gibbs potential term is incorporated to the Lee-Holder model along with the Zele-Lee-Holder cell distortion model. For binary gas hydrates between hydrogen and the other gas molecule, the predicted equilibrium pressure deviates within 10-20% from the experimental value. For the ternary and quaternary mixture hydrates, the model prediction is reasonably good but its error increases with increasing pressure and temperature under the presence of THF.     

Gas hydrate formation in the system C2H6–H2–H2O at pressures up to 250 MPa

Decomposition curves of gas hydrates formed in the ethane–hydrogen–water system were studied in the pressure interval 2–250 MPa. Gas hydrates synthesized at low (up to 5 MPa) pressures were also studied with use of X-ray powder diffraction and Raman spectroscopy. It was shown that ethane–hydrogen mixtures with hydrogen contents 0–30 mol.% form cubic structure I gas hydrates. Higher hydrogen concentration most probably results in appearance of another hydrate phase. We speculate that the gas mixtures with the hydrogen content above 60 mol.% form cubic structure II double hydrate of hydrogen and ethane at temperatures below ≈280 K and pressures above 25 MPa.     

Investigation of hydrate formation in the system H2-CH4-H2O at a pressure up to 250 MPa

Phase equilibria in the system H2-CH4-H2O are investigated by means of differential thermal analysis within hydrogen concentration range 0-70 mol % and at a pressure up to 250 MPa. All the experiments were carried out under the conditions of gas excess. With an increase in hydrogen concentration in the initial gas mixture, decomposition temperature of the formed hydrates decreased. X-ray diffraction patterns and Raman spectra of the quenched hydrate samples obtained at a pressure of 20 MPA from a gas mixture containing 40 mol % hydrogen were recorded. It turned out that the hydrate has cubic structure I under these conditions. The Raman spectra showed that hydrogen molecules are not detected in the hydrate within the sensitivity of the method, that is, almost pure methane hydrate is formed. The general view of the phase diagram of the investigated system is proposed. A thermodynamic model was proposed to explain a decrease in hydrate decomposition temperature in the system with an increase in the concentration of hydrogen in the initial mixture.