# A Review of Reactor Designs for Hydrogen Storage in Clathrate Hydrates
###### Abstract
In summary, the approach of using stabilising molecules has been successful in view of moderate pressure and temperature for formation and storage conditions. Unfortunately, the decrease of H\({}_{2}\) capacity associated therewith, or the instability of the resulting hydrate structure (sVI), has reduced the attractiveness of the approach. A different approach for hydrogen storage is the formation of semi-clathrates. These are typically ionic compounds, in which the cationic part occupies the cavities of the hydrate structure like a guest molecule, whereas the anionic part is included in the lattice structure along with the water molecules. Alkylammonium and alkylphosphonium salts, such as tetrabutylammonium bromide (TBAB), tetrabutylammonium chloride (TBAC), and tetrabutylphosphonium bromide (TBPB), are typical representatives used in the studies, because they facilitate hydrate formation close to room temperature and at moderate pressures. However, it turns out that the H\({}_{2}\)-storage capacity in TBAB semi-clathrate is lower vis-a-vis the occupancy of H\({}_{2}\) in small cavities of sII hydrates. H\({}_{2}\)-storage capacity in TBPB and TBAC mixed hydrates was reported to be 0.16 wt% and 0.14 wt% H\({}_{2}\), respectively [8]. In addition to the issues related to formation of the hydrate, the hydrogen-insertion mechanism plays also a crucial role not only at a fundamental level, but also in view of potential applications. In a study reported by [PERSON] et al., the enhancement of the H\({}_{2}\) insertion within the THF hydrate achieved with the co-inclusion of acidic additive [11]. In gas hydrate structure, the acidic additive acts as a \"flexibilizer\" of the water cage through the addition of chemical water H-bond defects - promoting H\({}_{2}\) inter-cage diffusion [12]. It should also be noted that sample size in most of the experiments reported in literature was smaller than 20 g. [PERSON] et al., as well as [PERSON] and [PERSON], conducted experiments with larger sample volumes of about 100 g using TBAB, TBAF, and THF, respectively [13; 14; 15]. The observed significantly lower H\({}_{2}\)-storage capacities, i.e., of the order of only 0.1 wt%. Obviously, and problematically, this indicates serious mass- and heat-transfer issues which could not be resolved by continuous stirring of the sample. To render H\({}_{2}\) storage in clathrate hydrates more economically feasible, the following aspects must be addressed:  
1. Pressure and temperature conditions for the formation and storage of H\({}_{2}\) hydrates have to be optimised. Ideally, the pressure should be less than 10 MPa and the temperature around 0 \({}^{\circ}\)C.
2. The H\({}_{2}\)-storage capacity should be as high as possible (ideally approaching \(\sim\)5 wt%). 3. Any hydrate promoters should be eco-friendly and cheap. To explore, and ultimately address, these issues, a greater understanding of the modes of action of promoter-additives is needed, as well as technical advances in the design of hydrate-formation reactors, embracing new concepts in hydrate formation. Both promoter and reactor have to optimise mass and heat transfer to achieve the abovementioned goals. According to the existing literature, the promoting additive has to reduce the gas-liquid interfacial tension, and thus increase mass transfer of the H\({}_{2}\) and water molecules. In addition, the promotor should enhance crystallisation by 'pre-structuring' the water molecules. It should also be hydrophilic to be soluble into the aqueous phase, as well as eco-friendly. In addition to on the usage of promoting substances, the development of proper reactors through optimisation of the heat and mass transfer for hydrate formation is also essential. So far, the construction of such hydrate-formation reactors has been targeted towards small, sub-litre scale; and indeed, many attempts have been reported for pilot-scale reactor designs, on the multiple-litre scale and larger.