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Energy storage: hydrogen should be stored in oil

Monday, September 4th, 2017 | Gadgets

A research group from Jülich has been developing liquid carriers that can store hydrogen for several years. They are intended to absorb hydrogen and can later be released only with the aid of a catalyst. They are chemically stable and, like other fuels, can be stored and transported in normal tanks. The latest development is a catalyst that can both store and release hydrogen. To date, two different catalysts have been required.

The carrier reaches an energy density which corresponds to about one sixth of that of gasoline or diesel. Hitherto, hydrogen has been transported in gastanks at about 700 bar pressure. But even at such high pressures the tanks occupy only about 70 grams of hydrogen per liter. In the Power2Gas process, hydrogen can be converted to methane with carbon dioxide (CO2), which has a much higher energy density at the same pressure. But this process is just as CO2-neutral as the source from which the CO2 originates. Otherwise, it must be recovered from the exhaust gas of the combustion at great expense. A liquid, reusable hydrogen carrier would not have any of these problems.
The idea of ​​the researchers is based on the fact that some molecules exist in two forms, which differ only by additional hydrogen atoms. In the conversion from one form to the other, they can absorb or release hydrogen without being burnt or altered in any other way. They are immediately available again for the reverse reaction.
Hydrogen can be attached to ring molecules
Such reactions are possible for ring-shaped hydrocarbons, such as benzene. Benzene consists of six ring-shaped carbon atoms which are each connected to a hydrogen atom outside the ring. There is also a very similar molecule, which is also ring-shaped. Cyclohexane also consists of six carbon atoms, each of which is associated with two hydrogen atoms. It is at least possible to react benzene with hydrogen in order to obtain cyclohexane therefrom and then to reverse the reaction and release the hydrogen again.

In practice, benzene is not suitable for the task. The researchers in Jülich had to find a substance that absorbs hydrogen with as little additional energy as possible. In addition, it must be chemically as stable as possible and can not be converted into other substances by either of the two reactions. Finally, a catalyst has to be developed that makes these reactions possible under acceptable conditions at all. In addition, the substance must also remain liquid at high temperatures so that it does not have to be separated from the hydrogen gas that is generated.
Three rings are ideal
The choice of the researchers fell on dibenzyltoluene (DBT or H0-DBT), a compound of three benzene rings, which can absorb 18 hydrogen atoms, equivalent to 6.2 percent of its own weight. Hydrogen is called perhydrodibenzyltoluene, but it is abbreviated to H18-DBT. DBT has hitherto mainly been used as a heat carrier oil which can be heated up to 350 degrees Celsius without evaporating or decomposing.
Theoretically, from one kilogram of H18-DBT hydrogen, 2.45 kilowatt hours of energy can be obtained, which are then converted into electricity or work in a fuel cell or a combustion engine. This is, after all, about one-fifth of the energy density of conventional fuels. But the chemical prerequisites for the process are not ideal, which is why the researchers still have to do a lot of development work.

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