CDs in hydrogen storage

Hydrogen as clean and renewable energy source could replace fossil fuels, but the lack of method for efficient storage inhibits its application. Carbon nanotubes and other carbon-based nanostructures together with metal-organic frameworks (MOFs) have great potential but still need further development to get material, which can store hydrogen in high density. The porous materials for hydrogen storage usually need high pressure and low temperature (100 bar and 77 K).

It is well known for long that CDs can complex small gas molecules. The first studies of Cramer and Henglein with hydrocarbon gases, carbon dioxide and noble gases did not include hydrogen [1].

Although there is a Japanese patent application claiming that CDs solutions and crown ethers sprayed into a gaseous hydrogen atmosphere can capture hydrogen [2], it was proved by density functional theory (DFT) calculations only recently that the smallest CD (ACD) can bind as much as 48 hydrogen molecules corresponding to 9.8 wt% storage capacity at 77K [3]. Theoretically, two H₂ molecules will be adsorbed on O-2 and O-3,

respectively, one H₂ molecule can be adsorbed on O-1 and O-5, while O-6 can even uptake three H₂ molecules. In summary, about 54 H₂ molecules can be totally adsorbed onto the α-CD molecule.

The maximum adsorption capacity of BCD at 77 K calculated by Monte Carlo simulations can reach 6.82 wt%, while that of titanium modified BCD is increased to 8.31 wt% under similar conditions [4].

Optimized structure of ACD with 48 H₂ molecules [3]

Nobel-prize winner Fräser Stoddart has published nanoporous carbohydrate-metal organic frameworks able to sorb various gases such as nitrogen, carbon dioxide, methane and also hydrogen [5]. They found that hydrogen storage ability was ~100 cm³/g at 77 K at 1 bar.

Recently published CD polymers obtained by crosslinking with trimesoyl chloride (CD-TMC) contain ultrafine micropores with narrow size distribution and in high amount [6]. The gas adsorption capability follows the order of α-CD-TMC > β-CD-TMC > γ-CD-TMC at 35 ^oC and 10 bar. Although γ-CD-TMC has the highest fraction free volume, its pore sizes are too large to keep hydrogen clusters in its cages, small hydrogen molecules can easily fly in and flee out of its large cages. On the other hand, in the case of α-CD-TMC both the inner cavities and the interstitial pores are small enough to capture gas molecules even under moderate pressure and ambient temperature. These conditions are realistic (-40–60 ^oC and max, 12 bar) and the hydrogen storage capacity (2.1 wt%) is near to the requirement (3–4.5 wt%) for application in hydrogen vehicles.

Scheme of the porous CD polymer [6]

References

1. Cramer, F., Henglein, F. M. (1957) Inclusion compounds XII. complexes of alpha-cyclodextrin with gases. Chem. Ber. 90, 2572
2. Yagi, M. (2006) Method for storing hydrogen. JP 2006096609
3. Zhu, H., Liu, Y., Chen, Y., Wen, Z. (2010) Cyclodextrins: Promising candidate media for high-capacity hydrogen adsorption. Applied Physics Letters 96, 054101. doi: http://dx.doi.org/10.1063/1.3294631
4. Zhou, C., Guo, Y., Zhu, H., Fan, G. (2016) Effect of titanium on adsorption of hydrogen in β-cyclodextrin, Computational & Theoretical Chemistry, doi: http://dx.doi.org/10.1016/j.comptc.2016.07.013
5. Forgan, R. S., Smaldone, R. A., Gassensmith, J. J., Furukawa, H., Cordes, D. B., Li, Q., … Stoddart, J. F. (2011). Nanoporous Carbohydrate Metal–Organic Frameworks. Journal of the American Chemical Society, 134(1), 406–417. doi:http://dx.doi.org/10.1021/ja208224f
6. Liu, J., Japip, S., Chung, T.S. (2018) Hydrogen storage in molecular clathrate cages under conditions of moderate pressure and ambient temperature. International Journal of Hydrogen Energy 4(43), 19998-20003. https://doi.org/10.1016/j.ijhydene.2018.09.044