Martin J. Dendy Sloan, Carolyn A. Koh, Amadue K. A lot of work has been carried out to investigate hydrates as a medium for the storage of hydrogen.
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No downtime is expected, but site performance may be temporarily impacted. Refworks Account Login. Open Collections. Strobel, Yongkwan Kim, Carolyn A. Koh, E. We have determined that the common clathrate structures may not suffice as H2 storage materials, although these findings will aid in the design and production of enhanced hydrogen storage materials and in the understanding of structure- stability relations of guest-host systems.
In view of current storage limitations, we propose a novel chemical — clathrate hybrid hydrogen storage concept that holds great promise for future materials. All structures are composed of polyhedral water cavities; however, the size, shape and distribution of these cavities vary between structures. The type of cavities comprising each structure is denoted by A x B y , where x and y represent the number of A and B sided faces per cavity.
The type of structure that forms largely depends on the size of the guest molecule . Storing hydrogen with an energy density comparable to current fossil fuel based media has proved to be a great challenge and it is generally accepted that novel approaches must be taken to develop materials that can meet hydrogen storage Proceedings of the 6th International Conference on Gas Hydrates ICGH , Vancouver, British Columbia, CANADA, July , Due to the inherent ability of clathrate hydrates to concentrate gas molecules, these materials have been proposed as an alternative storage material.
In Dyadin et al. In Mao et al. The simple H2 clathrate hydrate structure was confirmed to be sII with a lattice parameter of Additionally, Raman spectroscopic measurements confirmed the presence of H2 in the hydrate phase. In , Lokshin et al. They determined that one hydrogen molecule occupied the small cavities of the sII hydrate. The large cavity occupancy was found to vary between two and four molecules depending on pressure and temperature conditions.
Additionally, Lokshin et al. This distance suggests unique intermolecular forces, as the distance between molecules in solid hydrogen is 3. This fascinating feature suggests potential for clathrates as hydrogen storage materials, as the local density of H2 molecules can be unusually high. Simple hydrogen hydrate has several advantages as a material for H2 storage.
Firstly, the storage material is pure water. When the hydrogen is released and the structure decomposes, the only byproduct is pure water. Secondly, the formation and decomposition kinetics can be very fast. Hydrogen hydrate has been shown to form from powdered ice on the order of minutes, and from one gram of solid ice on the order of hours . Additionally, the hydrogen is stored in molecular form.
This means that, no chemical reactions are required for the hydrogen release. Finally, water is abundant and cheap.
In Florusse et al. Although Udachin et al. Florusse et al. Reducing the formation pressure conditions of hydrogen hydrate is a critical initial step towards realization of a practical storage material. In , Lee et al. Lee et al. Following this approach, Lee et al. In contrast to the double-occupation suggestion by Lee at al.
Strobel et al. Hester et al. Anderson et al. Rovetto et al. Additionally, molecules like propane have also been used . This second molecule is thought to add stabilization through interactions with the small cavity to help off-set unfavorable distortions to the large cavity caused by the presence of the large molecule .
This has many implications to the formation of other types of clathrate structures with hydrogen. Table 1. This suggests that there may be an ideal cavity size for hydrogen in order to maximize hydrogen content while minimizing the required formation pressure. By calculating the small cavity volumes though geometrical considerations , the 1.
This shows that small perturbations in cavity size can have a large impact on cavity filling. Tailoring hydrate cavities to the ideal size for increased hydrogen interactions may provide a valuable path for clathrate based hydrogen storage materials closer to ambient conditions. The formation of sH hydrate with hydrogen demonstrates that hydrogen can be stored within all three of the common clathrate hydrate structures.
Additionally, experimental evidence for hydrogen contained within the small cavities of a semi-clathrate with TBAB has been reported [25, 26]. Depending on the formation conditions and available components, hydrogen may multiply occupy large sII cavities, or singly occupy the small cavities of other structures, providing several different hydrogen storage materials.
Table 2. Hydrogen content in common structures. It is apparent that a balance exists between the hydrogen content and formation pressure. Over the past six years the paradigm has shifted as to the role that hydrogen plays in clathrate science and technology.
Going from a molecule that was thought not to enter the clathrate phase to a molecule that is capable of stabilizing all three common structures as well as semi-clathrates certainly redefines the perception of the interactions between small molecules like hydrogen and clathrate cavities.
Although the hydrogen storage values listed in table 2 appear too low, and the pressure conditions are too high to be considered as functional hydrogen storage materials, the diversity of clathrate structures formed with hydrogen suggests potential for other inclusion compound structures. In order for clathrates to be considered for hydrogen storage, compounds with increased hydrogen content and improved formation and operating conditions must be developed.
In many current technologies this storage substance is comprised of metal atoms, carbon lattices and other types of molecules. For clathrate phases, molecular hydrogen is trapped within cavities formed from a host lattice. This host lattice has not yet been thought capable of contributing to the energy content of the storage material.
The concept of chemical — clathrate hybrid technology is to combine molecular hydrogen contained within the clathrate phase with the hydrogen that is chemically bound to the host lattice.
A schematic illustration of this process is given in Figure 4. Regeneration 3 1 2 High density hydrogen storage material Molecular hydrogen clusters removed Chemical dehydrogenation of host material Molecular H2 clusters Atomic H contained on host material H2 clusters to fuel cell Production of additional H2 Figure 4.
Representation of chemical — clathrate hybrid storage. Proof-of-principle of this concept has been demonstrated using hydroquinone HQ clathrate. HQ is a known clathrate former with guest molecules such as methane or xenon [27, 28]. These cavities have a radius of about 4.
Figure 5. Hydroquinone clathrate. XRD pattern 90 K, 0. At these cold temperatures hydrogen exhibits two Raman peaks in the region for H2 vibration. The frequencies of both the Q1 0 and Q1 1 bands are significantly red-shifted, about 50cm -1 , from that of the free fluid phase. This decrease in H2 stretching frequency is induced by interactions with the clathrate cavity lattice, and is consistent with the enclathration of hydrogen molecules.
The large cages of sII clathrate hydrate have a radius of about 4. With four H2 molecules per cavity this novel clathrate would contain about 2. For the next step of the chemical-clathrate hybrid process, the host material must be chemically dehydrogenated. For our proof-of-principle hydroquinone system, we have targeted the hydroxyl hydrogens as they may be removed in solution by a mild oxidation reaction to form benzoquinone and aqueous protons.
These aqueous protons may then be reduced to form gaseous hydrogen. This simple redox reaction scheme is shown in equations 1 and 2. In the second reaction step Eq.
We have successfully demonstrated the feasibility of dehydrogenation of the hydroquinone molecule by the above reaction scheme Eq. Figure 8 left shows large hydrogen bubbles produced from a 1M hydroquinone solution in a test tube. Figure 8 right depicts a gas chromatogram of the gas in the head-space of the test tube. The head space was not purged so a large fraction of air remains present.
However, H2 gas was undoubtedly produced. Figure 8. Large hydrogen bubbles produced from hydroquinone solution left. Gas chromatogram showing hydrogen productions right. The proof-of-principle H2-hydroquinone chemical- clathrate hybrid system is not yet optimized for ideal hydrogen storage. Nevertheless, this is the first direct evidence that molecular and chemical hydrogen storage can be used concurrently to raise the capacity of a hydrogen storage material.
Table 3 shows the hydrogen storage values for this clathrate at a given cavity occupancy, as well as the storage values combined with the chemical hydrogen storage. Table 3. Compared with the maximum storage of 3. Although the small dodecahedral cavity may only contain one hydrogen molecule, the degree of filling is highly dependent on the cage radius. An ideal radius may exist where lower pressures are required for complete occupancy.
The proposed novel chemical — clathrate hybrid hydrogen storage concept holds great promise for future materials.
Hydrogen Storage in Nanoporous Clathrate Materials
The goal of the proposed research is to develop a new material for on board hydrogen storage for automobile transportation that meets the demands of the International Energy Agency IEA. The research will be focused on the use of clathrate hydrates, which are crystalline inclusion compounds composed of a water host lattice and one or more types of guest molecules. The guest molecules are contained in cages of different sizes. The so-called sII hydrate has cages of two different sizes, and the sH hydrate of three different sizes.
Hydrogen storage in clathrate hydrates
Structure, stability, and reactivity of clathrate hydrates with or without hydrogen encapsulation are studied using standard density functional calculations. Conceptual density functional theory based reactivity descriptors and the associated electronic structure principles are used to explain the hydrogen storage properties of clathrate hydrates. Different thermodynamic quantities associated with H 2 -trapping are also computed. The stability of the H 2 -clathrate hydrate complexes increases upon the subsequent addition of hydrogen molecules to the clathrate hydrates. The efficacy of trapping hydrogen molecules inside the cages of clathrate hydrates in an endohedral fashion depends upon the cavity sizes and shapes of the clathrate hydrates. Computational studies reveal that 5 12 and 5 12 6 2 structures are able to accommodate up to two H 2 molecules whereas 5 12 6 8 can accommodate up to six hydrogen molecules. Adsorption and desorption rates conform to that of a good hydrogen storage material.