Storage solutions: Reversible hydrogen

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  • Published: Mar 15, 2019
  • Author: David Bradley
  • Channels: NMR Knowledge Base
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Liquid chemistry

Chemical Hydrogen Storage System Reversible liquid organic hydrogen carrier system made of simple organic chemicals Credit: Milstein et al/Wiley/Angewandte

Proton nuclear magnetic resonance (NMR) spectroscopy was used in studying novel materials for reversible hydrogen storage.

In 2015, David Milstein of the Weizmann Institute of Science in Israel and his colleagues developed a new concept of LOHCs, liquid organic hydrogen carriers. These were based on amide bond formation and hydrogenation wherein dehydrogenation of 2-aminoethanol to form the cyclic peptide diketopiperazine and hydrogenation of the latter, form the basis of such a system with, what they explain has a theoretical 6.56 weight percent hydrogen capacity.

Hydrogen economics

Hydrogen has been touted for many years as the most attractive energy carrier. It, obviously, contains no carbon, unlike fossil fuels, such as oil, coal, and methane, and so could be the currency of a clean energy economy in the future. Generated by sustainable means it could be essentially a zero carbon emissions fuel. Unfortunately, hydrogen is highly explosive. As such, it needs some way to confine the gas that is safe, lightweight, and inexpensive. There has been much effort in developing highly porous storage materials that could adsorb hydrogen gas. However, Milstein and his team have taken a different tack in developing a chemical storage system based on simple and abundant organic compounds. Writing in the journal Angewandte Chemie, the team describes their liquid hydrogen carrier system and point out that it exploits the same catalyst for the charging and discharging reactions.

In essence, the chemical storage of hydrogen takes inspiration from biology where living cells have evolved fine-tuned chemical systems that bind and release hydrogen as needed by those cells. The biological processes are accelerated by nature's catalysts, the enzymes. Chemists have already developed potent synthetic catalysts that can mediate the conversion of hydrogen and a co-reactant into other chemicals in the laboratory. One example is the ruthenium pincer catalyst, a soluble complex of ruthenium with an organic ligand, developed by Milstein's team. They have now used their catalyst to explore the ability of a reaction system of simple organic chemicals to store and release hydrogen.

Storage system

“Finding a suitable hydrogen storage method is an important challenge toward the ‘hydrogen economy,’” is how the authors of the publication explained their motivation. Among the conditions that have to be fulfilled are safe chemicals, easy loading and unloading schemes, and as low a volume as possible. Milstein's team identified a system consisting of ethylenediamine and methanol, which they explain react under catalytic control to release pure hydrogen. The byproduct of the process is ethylene urea. The reverse process sees the ethylene urea taking on the hydrogen gas to form ethylenediamine and methanol. The researchers have observed a 100% efficiency in this conversion with their ruthenium pincer catalyst.

Unfortunately, the hydrogen release reaction is not quite 100 percent but close to it. The reaction seems to proceed via several intermediate stages and reaches an equilibrium of products. That said, they were able to nudge it to completion, that is re-hydrogenation. The team concludes that their system does represent the development of a fully rechargeable system for hydrogen storage. The next step will be to reduce overall reaction time and perhaps more importantly the heat input needed to push the reaction to completion. This might be possible with a modified version of the pincer catalyst or an entirely new design.

Related Links

Angew Chem Int Edn 2019, online: "A Reversible Liquid Organic Hydrogen Carrier System Based on Methanol-Ethylenediamine and Ethylene Urea"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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