# A Review of Reactor Designs for Hydrogen Storage in Clathrate Hydrates
###### Abstract  
Clathrate hydrates are ice-like, crystalline solids, composed of a three-dimensional network of hydrogen bonded water molecules that confines gas molecules in well-defined cavities that can store gases as a solid solution. Ideally, hydrogen hydrates can store hydrogen with a maximum theoretical capacity of about 5.4 wt%. However, the pressures necessary for the formation of such a hydrogen hydrate are 180-220 MPa and therefore too high for large-scale plants and industrial use. Thus, since the early 1990s, there have been numerous studies to optimize pressure and temperature conditions for hydrogen formation and storage and to develop a proper reactor type via optimisation of the heat and mass transfer to maximise hydrate storage capacity in the resulting hydrate phase. So far, the construction of the reactor has been developed for small, sub-litre scale; and indeed, many attempts were reported for pilot-scale reactor design, on the multiple-litre scale and larger. The purpose of this review article is to compile and summarise this knowledge in a single article and to highlight hydrogen-storage prospects and future challenges. hydrogen storage; clathrate hydrate; reactor design; Water-Spraying Reactor; Stirred-Tank Reactor; Bubbling Reactor; Water-In-Oil Emulsion; Fixed-Bed Reactor; unidirectional growth +
Footnote †: journal: _Sci._  
addition, LOHC require catalysts or/and elevated pressures and temperatures [2]. A different, promising approach could be the storage of H\({}_{2}\) in clathrate hydrates. Clathrate hydrates are ice-like, crystalline solids, composed of a three-dimensional network of hydrogen bonded water molecules that confines gas molecules in well-defined cavities [3], as shown in Figure 1. Considerable amounts of gas can be stored in clathrate structures as a solid solution: approximately 164 m\({}^{3}\) of gas can be stored in 1 m\({}^{3}\) gas hydrate at 273.15 K and 0.1 MPa. In general, non-polar gas molecules that have to have a certain size in order to shield the forces of attraction of the water molecules and to stabilize the water cages are encased into the clathrate structures. Depending on the size of the hydrate-forming gas molecules, different clathrate hydrate structures such as structure I, structure II and structure H have been identified. However, for a long time, the H\({}_{2}\) molecule was considered to be too small to stabilize the water lattices in any hydrate structure and was therefore not considered as a hydrate former [3]. Following work of [PERSON] et al. in 1999, [PERSON] et al. confirmed that H\({}_{2}\) is also able to form gas hydrates with structure II [2]. Since, unlike other gas hydrates, several H\({}_{2}\) molecules can occupy a water cage, an approximate H\({}_{2}\)/H\({}_{2}\)O molar ratio of 1:2, corresponding to a H\({}_{2}\) storage capacity of about 5.4 wt% can be achieved as a theoretical upper limit for maximal cage occupancy--double in small and quadruple in large sII-lattice cavities [4]. However, this is controversial, and disputed by many in the neutron-scattering and condensed-matter-theory communities. The advantages of H\({}_{2}\) storage in clathrate hydrates compared to all previously mentioned methods are manifold: (1) within the clathrate hydrate structure, the H\({}_{2}\) molecule is stored in the solid phase, decreasing the mobility and reactivity of the escharotic H\({}_{2}\) molecule, (2) for the formation of H\({}_{2}\) hydrate, only water is needed in addition to H\({}_{2}\), (3) since there are no chemical bonds between the H\({}_{2}\) molecule and the hydrate structure, only  
Figure 1: Schematic of sI and sII polymorphs with host lattice shown in terms of 5\({}^{12}\), 5\({}^{12}\)6\({}^{2}\) and 5\({}^{12}\)6\({}^{4}\) cages. In sI, two small 5\({}^{12}\) pentagonal dodecahedral cavities and six larger tetrakaidecahedral 5\({}^{12}\)6\({}^{2}\) cages define the unit cell of 46 water molecules.