How to store energy -2

This post continues our discussion from the previous post (see How to store energy -2). Let's keep considering how to increase the volmetric energy density in the context of hydrogen and its derivative.

Physically attach hydrgen to other substance = adsorbate

When physically attached to other substances, hydrogen is not converted chemically to something different but is just stored as hydrogen molecule. The substance that makes this adsorption possible in an efficient manner needs to possess high specific surface area such as high porocity.
Among potential candidates under consideration for this purpose is metal-organic framework, MOF.^[1]

Chemically attach hydrogen to other substance = hydride

When hydrogen is attached to metal via some chemical reaction, the product is called metal hydride. Metal hydride relies on the following reaction:
metal + hydrogen <--> metal hydride ...(1)
Thermodynamic characteristics of involved metal and its hydride determine the operation parameters, such as temperature and pressure, of hydrogenation (from left to right in the equation) and dehydrogenation (from right to left in the equation). The representative metal hydride is NaAlH4, for instance.^[2,3]

When hydrogen is attached to other chemicals, the product is called chemical hydride. Chemcial hydride also makes use of similar reaction.^[4]：
chemial + hydrogen <--> chemical hydride ...(2)
Going toward the right in equation is hydrogenation reaction, whereby the hydrogen is stored. Likewise, going toward the left is dehydrogenation to release hydrogen. The representative combination of "chemical" and "chemical hydride" is nitrogen and ammonia, which is dictated by the following equation:
N₂ + 3H₂ <--> 2 NH₃ ...(3)
In this model case, "hydrogen" is carried as ammonia, and its dehydrogenation yields hydrogen back.

Another well-known example in Japan is methyl-cyclohexane and toluene.
C₇H₈ + H₂ <--> C₇H₁₄ ...(4)
As methyl-cyclohexane (C₇H₁₄), hydrogen is carried, and its dehydrogenation reproduceds hydrogen. The by-product, toluene, is collected and reused for hydrogeneation, closing the cycling.

Advanced Hydrogen Energy Chain Association for Technology Development (AHEAD) has been investigating a supply-chain of hydrogen with the methyl-cyclohexane worldwide. The chain considered is: ^[5,6]

1. In Brunei, hydrogen is produced, which is used to hydrogenate toluene into methyl-cyclohexane
2. Methyl-cyclohexane is shipped to Japan
3. In Japan, methyl-cyclohexane is dehydrogenated to produce hydrogen, which yileds toluene as well
4. The toluene is shipped back to Brunei

Thus fas, this and previous posts touched upon the use of hydrogen as is (compression and liquefaction) and attaching/converting to something. The next post will compare these approaches.

References

1. U.S. Department of Energy, “Sorbent Storage Materials”, https://www.energy.gov/eere/fuelcells/sorbent-storage-materials (accessed on 2020/03/27).
2. U.S. Department of Energy, “Metal Hydride Storage Materials”, https://www.energy.gov/eere/fuelcells/metal-hydride-storage-materials (accessed on 2020/03/27).
3. R.C. Bowman, B. Fultz, MRS Bulletin 2002, 688.
4. U.S. Department of Energy, “Chemical Hydrogen Storage Materials”, https://www.energy.gov/eere/fuelcells/chemical-hydrogen-storage-materials (accessed on 2020/03/27).
5. 千代田化工建設，”CHIYODA テクニカル・レビュー” 2020.
6. 千代田化工建設, “SPERA水素®千代田の水素供給事業” , https://www.chiyodacorp.com/jp/service/spera-hydrogen/innovations/ (accessed on 2020/03/27).
7. 次世代水素エネルギーチェーン技術研究組合, https://www.ahead.or.jp/jp/research.html (accessed on 2022/02/03)