# Hydrates for Cold Storage: Formation Characteristics, Stability, and Promoters
## 4 Conclusions  
The formation characteristics, crystal morphology, and factors promoting the formation of hydrate of R141b, TME, and TBAB/TBAC mixtures under 0.1 MPa and 5 \({}^{\circ}\)C were investigated. The produced hydrates presented high latent heat and could be formed under normal pressure, which indicated that these hydrates have the potential for use in cold-storage technologies. The main conclusions of this study are as follows:  
1. The R141b and TBAB/TBAC solutions did not form hydrates within 48 h when no induction or promotion measures were included. By contrast, hydrate crystals were formed from the TME solution after 3 h of reaction.
2. When a 1 cm\({}^{3}\) ice cube was added to the reactor, the induction time for hydrate formation decreased, and the formation rate increased. When 500 rpm magnetic stirring was used, although the rate of hydrate formation from the TBAB/TBAC mixture was improved, the formation rate was significantly lower than that achieved with the addition of ice. When magnetic stirring at 500 rpm was combined with 0.1 g graphene, the induction time increased slightly. Nevertheless, because of nanostructures' characteristics and the high thermal conductivity of graphene, it could provide a large specific surface area for the contact between the TBAB/TBAC mixture and water molecules while eliminating the heat from the system, which may promote the formation of hydrates and significantly increases the formation rate.
3. Microscopic examination was conducted to obtain the morphological characteristics of the hydrate crystals produced. The R141b hydrate crystals were mainly granular or branched and occurred as local clusters, which were easily separated by free water. The TME hydrate crystals presented an overall ellipsoid shape and a vein-like surface with large crystal grains, which may not be conducive to mass transfer. Many nucleation sites were obtained when TBAB/TBAC hydrates were formed and the crystal structure was dense, which may improve the formation rate.
4. The time for the formation of hydrates from the TBAB/TBAC mixture during the formation cycles gradually decreased with the increasing numbers of cycles and finally stabilized. When the formation process stabilized, the hydrate dissociation process also stabilized. This phenomenon could indicate the potential of these hydrates for application in cold storage owing to their good durability and short process time for heat absorption and release.  
Figure 5: Changes in the formation and dissociation times of TBAB/TBAC in the cyclic hydrate formation experiment, indicating stability with different promotion measures (addition of 1 cm\({}^{3}\) ice cube, stirring at 500 rpm, and stirring at 500 rpm + 0.1 g graphene). (**a**) Relationship between induction time and formation cycles; (**b**) relationship between dissociation time and dissociation cycles.  
**Author Contributions:** Conceptualization, C.L.; data curation, M.W.; funding acquisition, H.C.; methodology, H.C. and M.W.; validation, Z.L. and B.F.; writing--original draft, H.C. and B.H.; writing--review and editing, C.L. and B.H.; investigation, B.H. and C.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China (Grant No. 51734010).  
The authors declare no conflict of interest.