Clathrate hydrogen hydrate--a promising material for hydrogen storage.

Hydrogen is viewed as a promising clean fuel of the future. A low-cost hydrogen storage technology that provides a high storage capacity and fast kinetics is a critical factor in the development of a hydrogen economy for transportation. The technologies to store hydrogen can be classified into three types: compression, liquefaction, and storage in a solid material. 2] Compressing hydrogen requires a very high pressure to obtain enough hydrogen fuel for a reasonable driving cycle of 400– 500 km, which in turn leads to safety issues related to tank rupture in case of accidents. The large amount of energy consumed during liquefaction and the continuous boil-off of hydrogen limit the possible use of liquid-hydrogen storage technology. Therefore, attention is currently focused on solid storage materials. In general, the storage of hydrogen in solid materials is achieved by one of two processes: chemical reactions, in which the hydrogen reacts with the solid material to form new compounds, and adsorption, in which the hydrogen is adsorbed on the solid material. Materials for the storage of hydrogen through chemical reactions include metals, complex hydrides, and nitrides. Materials with relatively high hydrogen storage capacities usually have a hydrogen-releasing temperature of over 100 8C (some even higher than 200 8C) as a consequence of the high energy needed to break chemical bonds. On the other hand, the release temperature of hydrogen is usually low if hydrogen is stored in a solid material by adsorption, 7] however, such materials have lower storage capacities. A new type of hydrates, namely the clathrate hydrogen hydrates, were recently reported, opening a new direction for hydrogen storage. Storage of H2 in this type of material is carried out by capturing the hydrogen in H2O cages rather than through chemical reaction or adsorption. Hydrogen-bonded H2O frameworks can generate polyhedron cages around guest molecules to form solid clathrate hydrates. There are three common types of gas hydrate structures: 1) the sI hydrate, which consists of 46 water molecules that form two pentagonal dodecahedron (5) and six tetrakaidecahedron (56) cages in a unit cell; 2) the sII hydrate, which consists of 136 water molecules that form sixteen 5 and eight 56 cages in a unit cell; and 3) the sH hydrate, which consists of 36 water molecules that form three 5, two 456, and one 56 cages in a unit cell. The type of crystalline structure that forms depends on the size of the guest molecule; for example, CH4 and C2H6 generate the sI hydrate, C3H8 gives rise to the sII hydrate, and the larger guest molecules, such as cyclopentane in the presence of methane, result in the sH hydrate. In contrast to other gases, the hydrogen molecule with its diameter of 2.72 5 was initially thought to be too small to support a clathrate structure. However, this point of view was challenged by recent experimental results. Mao et al. reported that mixtures of H2 and H2O can crystallize into an sII clathrate with a molar ratio of H2 to H2O of approximately 1:2. When a mixture of H2 and H2O was compressed at a pressure of 180–220 MPa and cooled to 249 K, a single solid compound was formed. Furthermore, energy-dispersive X-ray diffraction (EDXD) measurements indicated that the solid compound has a face-centered cubic unit cell with a= 17.047 0.010 5, in excellent agreement with the archetypal sII clathrate. As the H2/H2O ratio in the hydrate is 0.45 0.05, the 24 cages must be multiply occupied by H2 clusters to accommodate 61 7 molecules of H2. By comparing the size of the H2 clusters and the volume of the cage cavities, Mao et al. proposed that two molecules of H2 were located in each of the 5 cages and four molecules of H2 were located in each of the 56 cages (Figure 1A). This means that the hydrogen hydrate can reversibly store about 5.3 wt% hydrogen (excluding the hydrogen atoms of H2O). [9]