Chemical storage of hydrogen in Liquid Organic Hydrogen Carriers (LOHC)

The use of hydrogen produced by electrolysis from "green" renewable energies is booming: hydrogen can indeed be used as a clean energy carrier, to meet the challenges of decarbonizing industry. However, given the physico-chemical properties of this particular molecule, hydrogen storage is still an obstacle to the mass development of its use. In particular, safe, compact, low-energy and economically viable technological solutions have yet to be found for long-term storage (time-shifted use) and transport (space-shifted use).

Several hydrogen storage technologies have been studied, and are currently being developed or industrially produced. Each storage technology has its own advantages and disadvantages, which vary depending on the hydrogen production process upstream of storage, and the hydrogen consumption process downstream of storage. At present, no single technology has proven to be ideal or universal, so comparing them for the purpose of developing a new application is unavoidable.

The hydrogen storage method of interest here is chemical storage in organic molecules, liquid under ambient conditions of temperature and pressure, known as LOHCs (Liquid Organic Hydrogen Carriers). Numerous molecules are being considered for chemical storage. In addition to hydrogen storage capacity by mass, other selection criteria have been examined: the thermodynamics of the reaction cycle (low energies are preferred), the availability of molecules, the toxicity of the hydrogen-rich and hydrogen-poor molecules involved in the cycle, and their state at ambient temperature (solid, liquid or gaseous). This last point is important because it often distinguishes LOHCs, which are liquids, from other hydrogen-storing molecules, such as ammonia and formaldehyde, which are gaseous under ambient temperature and pressure conditions.

Chemical hydrogen storage in LOHCs is achieved by catalytic hydrogenation of hydrogen-poor compounds, particularly aromatic molecules. The hydrogenation reaction is generally carried out at hydrogen pressures of between 30 and 60 bar, and at temperatures of between 130 and 200°C. Molecular hydrogen is recovered by a catalytic dehydrogenation reaction, at pressures close to atmospheric pressure, and at temperatures of at least 200°C, the reaction being endothermic.

Hydrogen-rich and hydrogen-poor compounds are liquid and chemically stable under standard temperature and pressure conditions. They can be easily stored for long periods, and transported over long distances, using existing petrochemical infrastructures.

Four scientific challenges are linked to this storage option, to meet the sustainability criteria for solutions (safety, low energy and economic costs):

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