# A Review of Reactor Designs for Hydrogen Storage in Clathrate Hydrates
###### Abstract
For sII, each unit cell contains 136 water molecules and 24 cages: 16 small 5\({}^{12}\) pentagonal dodecahedral cavities and 8 larger 5\({}^{12}\)6\({}^{4}\) hexakaidecahedrons. physical parameters such as temperature or pressure have to be changed to release the H\({}_{2}\), and (4) the storage of H\({}_{2}\) in clathrate hydrates can be applied anywhere, ideally close to the location of H\({}_{2}\) production, and (5) enormous amounts of H\({}_{2}\) can be released from clathrate hydrates within very short times under controlled conditions. However, it is unfortunate that the formation of pure H\({}_{2}\) hydrate with high H\({}_{2}\)-storage capacity is so far only possible in a diamond-anvil cell under very high pressures (>150 MPa) and only in small quantities ([PERSON] et al., 2002). In recent years, several researchers tried to modify and optimise the formation conditions of hydrates containing H\({}_{2}\). One approach was to use an additional hydrate former which occupies some cavities of the hydrate structure and thus stabilizes it. The formation of a hydrate from water and H\({}_{2}\) only results in a structure II hydrate phase. A unit cell of a structure II hydrate consists of two different cavity types, namely, 16 pentagonal dodecahedrons (5\({}^{12}\)) and 8 hexakaidecahedrons (5\({}^{12}\)6\({}^{4}\)). Large molecules, such as tetrahydrofuran (THF), are also known to form structure II hydrates with the 5\({}^{12}\)6\({}^{4}\) cavities filled and the 5\({}^{12}\) cavities empty at ambient pressure and temperatures <277 K. THF was therefore used to form a binary hydrate with H\({}_{2}\) at moderate pressure (13.8 MPa) and temperature (270 K) conditions with the large cavities filled with THF and the small cavities filled with H\({}_{2}\)[5]. This structure can be used as a'molecular sieve' to separate hydrogen from a gas mixture. [PERSON] et al. [6] reported the first observation of the sieving behavior of a massive hydrate layer toward different gas molecules based on their different diffusion rates in THF hydrate. Hydrogen can penetrate a 5-mm thick THF hydrate layer in 24 h with a diffusion coefficient of 6.1 \(\times\) 10\({}^{-12}\) m\({}^{2}\)/s, while the THF hydrate is'resistant' to CH\({}_{4}\) and CO\({}_{2}\) molecules. For hydrogen-storage applications, unfortunately, the occupancy of the large cavities with a stabilising molecule results in a significant decrease of H\({}_{2}\)-storage capacity, which is limited to 1.06 wt% [7]. In addition, the H\({}_{2}\)-storage capacity varies strongly depending on the THF concentration and the applied pressure and temperature conditions [8]. The same is true for other structure II hydrate formers applied for the formation of H\({}_{2}\) hydrates, such as cyclopentane, furan, 2,5-dihydrofuran, 1,3-dioxlane, tetrahydrofuran, 1,4-dioxane, acetone, cyclohexanone, and methyl-cyclohexane. It turned out that the H\({}_{2}\) storage capacity was similar to that of using THF under identical operating conditions [8]. Slightly higher H\({}_{2}\) capacities were observed for binary H\({}_{2}\) hydrates with structure H when methyl-tert-butyl ether (MTBE) or dimethylcyclohexane (DMCH) were used as stabilisers. In these cases, the H\({}_{2}\) capacities ranged between 1.42 wt% (MTBE) and 1.37 wt% (DMCH), respectively [7]. Another approach used the different, purely synthetic hydrate structure VI for H\({}_{2}\) storage, leading to promising results with regard to H\({}_{2}\) capacities (5.18 wt% H\({}_{2}\)). This structure VI, however, is not stable and transforms into structure II hydrates [7; 9]. Recently, the formation of mixed gas hydrates form a gas mixture containing H\({}_{2}\), CH\({}_{4}\), and C\({}_{2}\)H\({}_{6}\) was reported [10]. The additional hydrocarbons CH\({}_{4}\) and C\({}_{2}\)H\({}_{6}\) enable the formation of a mixed hydrate under moderate pressure and temperature conditions. However, the H\({}_{2}\)-storage capacity of the resulting mixed hydrate phase is in the range of the approaches mentioned above.