Properties of the clathrates of hydrogen and developments in their applicability for hydrogen storage

Clathrate Hydrates as a Kind of Promising Ice Nanoreactors

Since their discovery, clathrate hydrates (CHs) have received great attention both from theoretical and experimental aspects due to their great potential for gas storage and prospective applications as icy crystal materials. However, there has been limited research on the decomposition, reduction or other reactions of gases enclosed in CHs. Thanks to their unique hydrogen bonding network and cavity structures, CHs can serve as the promising nanoreactors to achieve chemical conversions, e. g. reducing greenhouse gases. In this review-type article, we characterize the potential performance of such CHs nanoreactors by discussing their multiple functions including important roles of hydrogen bonds in CHs, e. g. the confinement effect and proton source, and then discuss the enhanced electron-binding ability of guest molecules and the structures and properties of trapped electrons in the stacked nanocages, which contribute to our understanding of chemical reactions occurring in CHs. Finally, we provide detailed analyses of representative reaction mechanisms underwent in CH nanoreactors and effective calculational and molecular dynamics simulation methods. This review-type article aims to provide a detailed summary about the functional characteristics of CHs and reactivity in CHs, which make CHs a kind of promising icy nanoreactors.

Large-cage occupation and quantum dynamics of hydrogen molecules in sII clathrate hydrates

Hydrogen clathrate hydrates are ice-like crystalline substances in which hydrogen molecules are trapped inside polyhedral cages formed by the water molecules. Small cages can host only a single H2 molecule, while each large cage can be occupied by up to four H2 molecules. Here, we present a neutron scattering study on the structure of the sII hydrogen clathrate hydrate and on the low-temperature dynamics of the hydrogen molecules trapped in its large cages, as a function of the gas content in the samples. We observe spectral features at low energy transfer (between 1 and 3 meV), and we show that they can be successfully assigned to the rattling motion of a single hydrogen molecule occupying a large water cage. These inelastic bands remarkably lose their intensity with increasing the hydrogen filling, consistently with the fact that the probability of single occupation (as opposed to multiple occupation) increases as the hydrogen content in the sample gets lower. The spectral intensity of the H2 rattling bands is studied as a function of the momentum transfer for partially emptied samples and compared with three distinct quantum models for a single H2 molecule in a large cage: (i) the exact solution of the Schrödinger equation for a well-assessed semiempirical force field, (ii) a particle trapped in a rigid sphere, and (iii) an isotropic three-dimensional harmonic oscillator. The first model provides good agreement between calculations and experimental data, while the last two only reproduce their qualitative trend. Finally, the radial wavefunctions of the three aforementioned models, as well as their potential surfaces, are presented and discussed.

A temperature programmed desorption study of interactions between water and hydrophobes at cryogenic temperatures

It is considered that hydrophobic solutes dissolve in water via the formation of icelike cages in the first hydration shell. However, this conventional picture is currently under debate. We have investigated how hydrophobic species, such as D₂, Ne, Ar, Xe, CH₄, and C₃H₈, interact with water in composite films of amorphous solid water (ASW) based on temperature programmed desorption (TPD). The D₂ and Ne species tend to be incorporated in ASW without being caged, whereas two distinct peaks assignable to the caged species are identifiable for the other solutes examined here. The low-temperature peak is observed preferentially for Ar and CH₄ prior to crystallization. The hydrophobes are thought to be encapsulated in porous ASW films via reorganization of the hydrogen bond network up to 100 K; most of them are released in a liquidlike phase that occurs immediately before crystallization at ca. 160 K. The nature of hydrophobic hydration at cryogenic temperature appears to differ from that in normal water at room temperature because the former resembles crystalline ices in the local hydrogen-bond structure rather than the latter. No ordered structures assignable to clathrate hydrates were identified before and after crystallization.

Path integral simulations of confined parahydrogen molecules within clathrate hydrates: Merging low temperature dynamics with the zero-temperature limit

Clathrate hydrates, or cages comprised solely of water molecules, have long been investigated as a clean storage facility for hydrogen molecules. A breakthrough occurred when hydrogen molecules were experimentally placed within a structure-II clathrate hydrate, which sparked much interest to determine their feasibility for energy storage [Mao et al., Science 297, 2247-2249 (2002)]. We use Path Integral Molecular Dynamics (PIMD) and Langevin equation Path Integral Ground State (LePIGS) for finite temperature and zero-temperature studies, respectively, to determine parahydrogen occupancy properties in the small dodecahedral (5¹²) and large hexakaidecahedral (5¹²6⁴) sized cages that comprise the structure-II unit cell. We look at energetic and structural properties of small clusters of hydrogen, treated as point-like particles, confined within each of the different sized clathrates, and treated as rigid, to determine energetic and structural properties in the zero-temperature limit. Our predicted hydrogen occupancy within these two cage sizes is consistent with previous literature values. We then calculate the energies as a function of temperature and merge the low temperature results calculated using finite temperature PIMD with the zero-temperature results using LePIGS, demonstrating that the two methods are compatible.

Hydrates for Cold Storage: Formation Characteristics, Stability, and Promoters

The potential of hydrates formed from R141b (CH3CCl2F), trimethylolethane (TME), and tetra-n-butylammonium bromide/tetra-n-butylammonium chloride (TBAB/TBAC) to be used as working substances for cold storage was investigated to provide a solution for unbalanced energy grids. In this study, the characteristics of hydrate formation, crystal morphology of hydrates, and the stability of hydrate in cyclic formation under 0.1 MPa and at 5 °C were carried out. It found that the ice had a positive effect on the hydrate formation under same conditions. Upon the addition of the ice cube, the induction time of R141b, TME, and TBAB/TBAC hydrates decreased markedly, and significantly high formation rates were obtained. Under magnetic stirring, the rate at which TBAB/TBAC formed hydrates was significantly lower than that when ice was used. In microscopic experiments, it was observed that the TBAB/TBAC mixture formed hydrates with more nucleation sites and compact structures, which may increase the hydrate formation rate. In the multiple cycle formation of TBAB/TBAC hydrates, the induction time gradually decreased with the increasing number of formation cycles and finally stabilized, which indicated the potential of the TBAB/TBAC hydrates for application in cold storage owing to their good durability and short process time for heat absorption and release.

Gas hydrates in confined space of nanoporous materials: new frontier in gas storage technology

Gas hydrates (clathrate hydrates, clathrates, or hydrates) are crystalline inclusion compounds composed of water and gas molecules. Methane hydrates, the most well-known gas hydrates, are considered a menace in flow assurance. However, they have also been hailed as an alternative energy resource because of their high methane storage capacity. Since the formation of gas hydrates generally requires extreme conditions, developing porous material hosts to synthesize gas hydrates with less-demanding constraints is a topic of great interest to the materials and energy science communities. Though reports of modeling and experimental analysis of bulk gas hydrates are plentiful in the literature, reliable phase data for gas hydrates within confined spaces of nanoporous media have been sporadic. This review examines recent studies of both experiments and theoretical modeling of gas hydrates within four categories of nanoporous material hosts that include porous carbons, metal-organic frameworks, graphene nanoslits, and carbon nanotubes. We identify challenges associated with these porous systems and discuss the prospects of gas hydrates in confined space for potential applications.

High-Pressure Sorption of Hydrogen in Urea

Hydrogen sorption in urea C(NH₂)₂O has been probed by direct measurements in Sievert's apparatus at 7.23 and 11.12 MPa as well as by Raman spectroscopy for the sample compressed and heated in a high-pressure gas-loaded diamond-anvil cell up to 14 GPa. Both these methods consistently indicate the occurrence of small nonstoichiometric sorption of hydrogen in urea phase I. The compression of urea in hydrogen affects the Raman shifts of the C-N bending mode δ and the stretching mode υ_s. The sorption affects the H₂ vibron position too. The sorption of 1.3 × 10^-2 at 11.12 MPa corresponds to a stochastic distribution of H₂ molecules in channel pores of urea. The mechanism leading to this stochastic sorption involves strong correlations between the swollen nanodot regions around the pores accommodating H₂ molecules and the squeezed neighboring pores too narrow to act as possible sorption sites. This study on the hydrogen-bonded framework (HOF) of urea marks the smallest pores capable of absorbing hydrogen documented so far. This observation also reveals a new class of compounds, which is located between those that absorb large stoichiometric amounts of certain guest molecules and those that do not absorb them at all, namely, the group of compounds that absorb the guests in a stochastic manner.

Computational density-functional approaches on finite-size and guest-lattice effects in CO2@sII clathrate hydrate

We performed first-principles computations to investigate guest-host/host-host effects on the encapsulation of the CO₂ molecule in sII clathrate hydrates from finite-size clusters up to periodic 3D crystal lattice systems. Structural and energetic properties were first computed for the individual and first-neighbors clathrate-like sII cages, where highly accurate ab initio quantum chemical methods are available nowadays, allowing in this way the assessment of the density functional (DFT) theoretical approaches employed. The performance of exchange-correlation functionals together with recently developed dispersion-corrected schemes was evaluated in describing interactions in both short-range and long-range regions of the potential. On this basis, structural relaxations of the CO₂-filled and empty sII unit cells yield lattice and compressibility parameters comparable to experimental and previous theoretical values available for sII hydrates. According to these data, the CO₂ enclathration in the sII clathrate cages is a stabilizing process, either by considering both guest-host and host-host interactions in the complete unit cell or only the guest-water energies for the individual clathrate-like sII cages. CO₂@sII clathrates are predicted to be stable whatever the dispersion correction applied and in the case of single cage occupancy are found to be more stable than the CO₂@sI structures. Our results reveal that DFT approaches could provide a good reasonable description of the underlying interactions, enabling the investigation of formation and transformation processes as a function of temperature and pressure.

The potential of hydrogen hydrate as a future hydrogen storage medium

Hydrogen is recognized as the "future fuel" and the most promising alternative of fossil fuels due to its remarkable properties including exceptionally high energy content per unit mass (142 M J / k g ), low mass density, and massive environmental and economical upsides. A wide spectrum of methods in H 2 production, especially carbon-free approaches, H 2 purification, and H 2 storage have been investigated to bring this energy source closer to the technological deployment. Hydrogen hydrates are among the most intriguing material paradigms for H 2 storage due to their appealing properties such as low energy consumption for charge and discharge, safety, cost-effectiveness, and favorable environmental features. Here, we comprehensively discuss the progress in understanding of hydrogen clathrate hydrates with an emphasis on charging/discharging rate of H 2 (i.e. hydrate formation and dissociation rates) and the storage capacity. A thorough understanding on phase equilibrium of the hydrates and its variation through different materials is provided. The path toward ambient temperature and pressure hydrogen batteries with high storage capacity is elucidated. We suggest that the charging rate of H 2 in this storage medium and long cyclic performance are more immediate challenges than storage capacity for technological translation of this storage medium. This review and provided outlook establish a groundwork for further innovation on hydrogen hydrate systems for promising future of hydrogen fuel.

Kinetic Analysis of Methane-Propane Hydrate Formation by the Use of Different Impellers

In the present study, the effect of different kinds of impellers with different baffles or no baffle was investigated. Up-pumping pitched blade turbine (PBTU) and Rushton turbine (RT) were the two types of impellers tested. The reactor was equipped with different designs of baffles: full, half and surface baffles, or no baffles. Single (PBTU or RT) and dual (PBTU/PBTU or RT/RT) use of impellers with full (FB), half (HB), surface (SB), and no baffle (NB) combinations formed two sets of 16 experiments. The first group of experiments was close to the equilibrium line (P = 26.5 bars and T = 8.5 °C), and the second group was deep in the equilibrium line (P = 24.5 bars and T = 2 °C). There was estimation of rate of hydrate formation, induction time, hydrate productivity, overall power consumption, split fraction, and separation factor. In both single and dual impellers, the results showed that RT experiments are better compared to PBTU in the rate of hydrate formation. The induction time is almost the same because we are deep in the equilibrium line while, hydrate productivity values are higher in PBTU compared to RT experiments. As a general view, RT experiments consume more energy compared to PBTU experiments.

Mechanism for H2 diffusion in sII hydrates by molecular dynamics simulations

Among the many different types of molecules that form clathrate hydrates, H₂ is unique as it can easily diffuse into and out of clathrate cages, a process that involves the physical-chemical interactions between guest (H₂) and host (water) molecules, and is unlike any other molecular system. The dynamic and nano-scale process of H₂ diffusion into binary structure II hydrates, where the large cages are occupied by larger molecules, was studied using molecular dynamics simulation. As the H₂ molecules diffused from one cage to another, two types of diffusion processes were observed: (i) when moving between a pair of large cages, the H₂ molecules pass through the central part of the hexagonal rings; (ii) however, when the H₂ molecules move from a large cage to a small one, it requires one of the pentagonal rings to partially break, as this allows the H₂ molecule to pass through the widened space. While the diffusion of H₂ molecules between large cages was found to occur more frequently, the presence of SF₆ molecules in the large cages was found to inhibit diffusion. Therefore, in order to attain higher H₂ storage capacities in binary hydrates, it is suggested that there is an optimal number of large cages that should be occupied by SF₆ molecules.

Two-dimensional hydrogen hydrates: structure and stability

The structure and stability of two-dimensional hydrogen hydrate were investigated in this work using density functional theory. The results are in line with expectations that the occupied cages are more stable after their confinement between two parallel hydrophobic sheets. The four two-dimensional hydrogen hydrate crystals - BLHH-I, BLHH-II, BLHH-III and BLHH-IV - that we predicted were much more stable in a restricted environment than in a free environment, even close to or exceeding conventional hydrogen hydrates. Besides, we found that the stability of two-dimensional hydrates is inversely related to the increase in temperature. Our work highlights that two-dimensional hydrates provide a new research idea in the field of hydrogen storage.

Molecular dynamics simulation of sI methane hydrate under compression and tension

Molecular dynamics (MD) analysis of methane hydrate is important for the application of methane hydrate technology. This study investigated the microstructure changes of sI methane hydrate and the laws of stress–strain evolution under the condition of compression and tension by using MD simulation. This study further explored the mechanical property and stability of sI methane hydrate under different stress states. Results showed that tensile and compressive failures produced an obvious size effect under a certain condition. At low temperature and high pressure, most of the clathrate hydrate maintained a stable structure in the tensile fracture process, during which only a small amount of unstable methane broke the structure, thereby, presenting a free-motion state. The methane hydrate cracked when the system reached the maximum stress in the loading process, in which the maximum compressive stress is larger than the tensile stress under the same experimental condition. This study provides a basis for understanding the microscopic stress characteristics of methane hydrate.

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

An in situ method on kinetics of gas hydrates

Gas hydrate formation is a high-risk and common flow assurance problem in subsea oil production plants. The modern strategies to mitigate hydrate formation have switched from thermodynamic inhibition to risk management. In this new mitigation strategy, hydrate formation is allowed as long as it does not lead to plugging of pipelines. Thus, understanding the growth kinetics of gas hydrates plays a critical role in risk management strategies. Here, we report a new accurate and in situ approach to probe the kinetics of gas hydrate formation. This approach is based on the hot-wire method, which probes the thermal properties of the medium surrounding the hot-wire. As the thermal properties of gas hydrate and its initial constituents are different, variation in these properties is used to probe kinetics of hydrate growth front. Through this in situ method, we determine kinetics of cyclopentane hydrate formation in both mixing and flow conditions. The findings show that at ambient pressure and a temperature of 1-2 °C, the hydrate formation rate under mixing condition varies between 1.9 × 10^-5 and 3.9 × 10^-5 kg m^-2 s^-1, while in flow condition, this growth rate drops to 4.5 × 10^-6 kg m^-2 s^-1. To our knowledge, this is the first reported growth rate of cyclopentane hydrate. This in situ approach allows us to probe kinetics of hydrate formation where there is no optical access and provides a tool to rationally design risk management strategies for subsea infrastructures.

Simulation of Capture and Release Processes of Hydrogen by β-Hydroquinone Clathrate

Using molecular simulation techniques, we investigate the storage capabilities of H₂ gas by the clathrate of hydroquinone (HQ). Quantum mechanics calculations have been used to assess structure and interactions at the atomic scale and molecular dynamics to model the HQ clathrate at successive equilibriums during the processes of capture and release of H₂, as well as the diffusion of H₂ inside the clathrate structure. The thermodynamic conditions of the simulations performed try to reproduce closely the corresponding experimental procedures, with results that are in good agreement with literature observed trends. The results obtained contribute to depict a more complete and better substantiated image of the mechanisms involved in stability and in the processes of capture and release of H₂ by the HQ clathrate.

Reverse micelles enhance the formation of clathrate hydrates of hydrogen

HYPOTHESIS

Clathrate hydrates of hydrogen form at relatively low pressures (e.g., ca. 10 MPa) when a co-former compound is added. In that case, however, the gravimetric amount of stored hydrogen drops to less than 1 wt% from ca. 5.6 wt% without a co-former. Another factor hindering the entrapment of hydrogen into a clathrate matrix appears to be of a kinetic origin, in that the mass transfer of hydrogen into clathrates is limited by the macroscopic scale of the gas-water interfaces involved in their formation. Thus, the enhanced formation of binary (hydrogen + co-former) hydrates would represent a major achievement in the attempt to exploit those materials as a convenient means for storing hydrogen.

EXPERIMENTS

Here, we present a simple process for the enhanced formation of binary hydrates of hydrogen and several co-formers, which is based on the use of reverse micelles for reducing the size of hydrate-forming gas-water interfaces down to tens of nanometers. This reduction of particle size allowed us to reduce the kinetic hindrance to hydrate formation.

FINDINGS

The present process was able to (i) enhance the kinetics of the formation process; and (ii) assist clathrate formation when using water-insoluble coformers (e.g., cyclopentane, tetrahydrothiophene).

Ostwald's rule of stages and metastable transitions in the hydrogen–water system at high pressure

The hydrogen water system has been extensively studied above 0.5 GPa and below 0.2. We present neutron diffraction studies in the intermediate pressure range.

Micro-Tomographic Investigation of Ice and Clathrate Formation and Decomposition under Thermodynamic Monitoring

Clathrate hydrates are inclusion compounds in which guest molecules are trapped in a host lattice formed by water molecules. They are considered an interesting option for future energy supply and storage technologies. In the current paper, time lapse 3D micro computed tomographic (µCT) imaging with ice and tetrahydrofuran (THF) clathrate hydrate particles is carried out in conjunction with an accurate temperature control and pressure monitoring. µCT imaging reveals similar behavior of the ice and the THF clathrate hydrate at low temperatures while at higher temperatures (3 K below the melting point), significant differences can be observed. Strong indications for micropores are found in the ice as well as the THF clathrate hydrate. They are stable in the ice while unstable in the clathrate hydrate at temperatures slightly below the melting point. Significant transformations in surface and bulk structure can be observed within the full temperature range investigated in both the ice and the THF clathrate hydrate. Additionally, our results point towards an uptake of molecular nitrogen in the THF clathrate hydrate at ambient pressures and temperatures from 230 K to 271 K.

Cyanoacetohydrazide under Pressure: Chemical Changes in a Hydrogen-Bonded Material

Cyanoacetohydrazide (CAH, C3H5N3O) has been studied under pressure using diamond anvil cell techniques. CAH was characterized using Raman spectroscopy to 30 GPa and synchrotron X-ray diffraction to 45 GPa. The Raman spectra of CAH show reasonable qualitative agreement with first-principle calculations. The X-ray data reveal that CAH maintains its monoclinic structure to approximately 22 GPa with a density change of 12% over this range. Near 22 GPa, the Raman modes and most of the X-ray diffraction peaks disappear. These pressure-induced changes are irreversible upon the release of pressure, and the transformed sample can be recovered to ambient pressure. The recovered sample is photosensitive and shows reaction even at low laser powers of 10 mW at 532 nm. The paper concludes with observations of the roles of hydrogen bonding, molecular configurations, and the behavior of the cyano group in the pressure-induced changes in CAH.

Impact of the Condensed-Phase Environment on the Translation-Rotation Eigenstates and Spectra of a Hydrogen Molecule in Clathrate Hydrates

We systematically investigate the manifestations of the condensed-phase environment of the structure II clathrate hydrate in the translation-rotation (TR) dynamics and the inelastic neutron scattering (INS) spectra of an H2 molecule confined in the small dodecahedral cage of the hydrate. The aim is to elucidate the extent to which these properties are affected by the clathrate water molecules beyond the confining cage and the proton disorder of the water framework. For this purpose, quantum calculations of the TR eigenstates and INS spectra are performed for H2 inside spherical clathrate domains of gradually increasing radius and the number of water molecules ranging from 20 for the isolated small cage to more than 1800. For each domain size, several hundred distinct hydrogen-bonding topologies are constructed in order to simulate the effects of the proton disorder. Our study reveals that the clathrate-induced splittings of the j = 1 rotational level and the translational fundamental of the guest H2 are influenced by the condensed-phase environment to a dramatically different degree, the former very strongly and the latter only weakly.

Competing quantum effects in the free energy profiles and diffusion rates of hydrogen and deuterium molecules through clathrate hydrates

Depending on the temperature, competing quantum effects are found to accelerate or decelerate the diffusion rate of hydrogen compared to deuterium in clathrates.

Urea and deuterium mixtures at high pressures

Urea, like many network forming compounds, has long been known to form inclusion (guest-host) compounds. Unlike other network formers like water, urea is not known to form such inclusion compounds with simple molecules like hydrogen. Such compounds if they existed would be of interest both for the fundamental insight they provide into molecular bonding and as potential gas storage systems. Urea has been proposed as a potential hydrogen storage material [T. A. Strobel et al., Chem. Phys. Lett. 478, 97 (2009)]. Here, we report the results of high-pressure neutron diffraction studies of urea and D2 mixtures that indicate no inclusion compound forms up to 3.7 GPa.

A molecular dynamics study of model SI clathrate hydrates: the effect of guest size and guest–water interaction on decomposition kinetics

One of the options suggested for methane recovery from natural gas hydrates is molecular replacement of methane by suitable guests like CO₂ and N₂.

Structural transformation and tuning behavior induced by the propylamine concentration in hydrogen clathrate hydrates

The structures and the guest–host distributions of propylamine hydrates provide useful information on the hydrophilic guest–host interactions in clathrate hydrates.

Triple Guest Occupancy and Negative Compressibility in Hydrogen-Loaded β-Hydroquinone Clathrate

The molecular interactions and structural behavior of a previously unexplored clathrate system, hydrogen-loaded β-hydroquinone (β-HQ+H2), were investigated under high pressure with synchrotron X-ray diffraction and Raman/infrared spectroscopies. The β-HQ+H2 system exhibits coupling of two independently rare phenomena: multiple occupancy and negative compressibility. The number of H2 molecules per cavity increases from one to three, causing unit cell volume increase by way of unique crystallographic interstitial guest positioning. We anticipate these occupancy-derived trends may be general to a range of inclusion compounds and may aid the chemical and crystallographic design of both high-occupancy hydrogen storage clathrates and novel, variable-composition materials with tunable mechanical properties.