Schema-Energiespeicherung ueber chemischer H2-Speicherung

Liquid organic hydrogen carriers (LOHC) are organic compounds that can absorb and release

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

through

chemical reactions A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...

. LOHCs can therefore be used as storage media for hydrogen. In principle, every unsaturated compound (organic molecules with C-C

double A double is a look-alike or doppelgänger; one person or being that resembles another. Double, The Double or Dubble may also refer to: Film and television * Double (filmmaking), someone who substitutes for the credited actor of a character * Th ...

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond order o ...

s) can take up hydrogen during

hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a Catalysis, catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or S ...

. The sequence of endothermal

dehydrogenation In chemistry, dehydrogenation is a chemical reaction that involves the removal of hydrogen, usually from an organic molecule. It is the reverse of hydrogenation. Dehydrogenation is important, both as a useful reaction and a serious problem. At ...

followed by hydrogen purification is considered as the main drawback which limits the overall efficiency of the storage cycle.G. Sievi, D. Geburtig, T. Skeledzic, A. Bösmann, P. Preuster, O. Brummel, ... & J. Libuda (2019). ''Towards an efficient liquid organic hydrogen carrier fuel cell concept''. In: ''

Energy & Environmental Science ''Energy & Environmental Science'' is a monthly peer-reviewed scientific journal publishing original (primary) research and review articles. The journal covers work of an interdisciplinary nature in the biochemical and biophysical sciences and c ...

'', 12(7), 2305-2314. In 2020,

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

built up the world's first international hydrogen supply chain between

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and Kawasaki City utilizing

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) at ...

-based LOHC technology.

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invests in the development for stationary and on-board LOHC-systems.

Principle of LOHC-based hydrogen storage

To absorb hydrogen, the dehydrated form of LOHC (an unsaturated, mostly aromatic compound) reacts with the hydrogen in a

reaction. The hydrogenation is an exothermic reaction and is carried out at elevated pressures (approx. 30-50 bar) and temperatures of approx. 150-200°C in the presence of a

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

. The corresponding saturated compound is thereby formed, which can be stored or transported under ambient conditions. If the hydrogen is needed again, the now hydrogenated, hydrogen-rich form of the LOHC is

dehydrogenated In chemistry, dehydrogenation is a chemical reaction that involves the removal of hydrogen, usually from an organic molecule. It is the reverse of hydrogenation. Dehydrogenation is important, both as a useful reaction and a serious problem. At ...

, with the hydrogen being released again from the LOHC. This reaction is

endothermic In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. p. ...

and takes place at elevated temperatures (250-320°C) again in the presence of a catalyst. Before the hydrogen can be used, it may have to be cleaned of LOHC steam. To increase efficiency, the heat contained in the hot material flow exiting the release unit should be transferred to the cold material flow consisting of hydrogen-rich LOHC entering the release unit in order to keep the energy requirement for preheating it before the reaction low. In particular, the heat released by the hydrogenation reaction when the hydrogen is absorbed can in principle be used for heating purposes or as process heat.

Requirements for LOHC materials

Determination of the degree of hydrogenation

Direct LOHC fuel cell

An alternative, innovative and highly promising approach to convert LOHC-bound hydrogen into electricity is proposed recently. The new unloading sequence consists of an almost thermoneutral catalysed

transfer hydrogenation In chemistry, transfer hydrogenation is a chemical reaction involving the addition of hydrogen to a compound from a source other than molecular . It is applied in laboratory and industrial organic synthesis to saturate organic compounds and reduce ...

step converting

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...

(

acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscib ...

) to secondary alcohol (

2-propanol Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable organic compound with a pungent alcoholic odor. As an isopropyl group linked to a hydroxyl group (chemical formula ) it is the sim ...

) by contacting hydrogen-rich carrier (H18- DBT), and the secondary alcohol is then directly consumed in a

PEMFC Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable ...

(direct isopropanol fuel cell; DIPAFC). It is a CO₂ emission-free, external energy input-free, and safe sequence with no molecular hydrogen at any point during hydrogen releasing. The "direct LOHC fuel cell" based on the LOHC-DIPAFC coupling concept is a very attractive solution for the on-board generation of electric energy in mobile applications, and it's driving researchers to focus on the topic.

Examples of LOHC materials

Toluene / methylcyclohexane

As early as the 1980s there were attempts with

, which is converted to

methylcyclohexane Methylcyclohexane (cyclohexylmethane) is an organic compound with the molecular formula is CH3C6H11. Classified as saturated hydrocarbon, it is a colourless liquid with a faint odor. Methylcyclohexane is used as a solvent. It is mainly converte ...

by hydrogenation. The basic idea of this variant came from the USA in 1975 and was further developed in 1979 at the Paul Scherrer Institute in Switzerland together with the

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. Even then, the prototype of a truck was built that was powered by hydrogen from the dehydrogenation of methylcyclohexane. The entire circuit is known as the Methylcyclohexan-Toluol-H2 system (MTH).

N-ethyl carbazole

Dibenzyltoluene

To circumvent the high melting temperature of ''N''-ethylcarbazole and the high vapor pressure of toluene, dibenzyltoluene can be used. This substance is currently being used as a heat transfer oil. Temperatures of approx. 300°C are necessary for dehydrogenation. However, dibenzyltoluene is superior to other carrier substances in many physico-chemical properties.C. Krieger, K. Müller, W. Arlt: ''Energetische Analyse von LOHC-Systemen als thermochemische Wärmespeicher.'' In: ''Chemie Ingenieur Technik.'' 86, 2014, S. 1441, .

Other potential LOHCs

Implementation

References

{{Reflist Hydrogen technologies