Liquid Carriers for Hydrogen Transport

As global energy systems transition towards decarbonisation, hydrogen has emerged as a versatile energy vector capable of replacing fossil fuels in heavy industry, long-haul transport, and seasonal grid balancing. However, the fundamental physical properties of hydrogen present severe logistical obstacles. Being the lightest and smallest of all chemical elements, hydrogen gas possesses an exceptionally low volumetric energy density under ambient conditions. To transport meaningful quantities, it must either be compressed to immense pressures—typically between 350 and 700 bar—or cooled to minus 253 degrees Celsius to achieve liquefaction. Both approaches require substantial energy expenditure; liquefaction alone can consume nearly a third of the energy content of the gas itself. Furthermore, compressed gas requires heavy composite cylinders, while cryogenic liquid hydrogen is prone to continuous boil-off losses and requires specialised vacuum-insulated vessels, limiting its viability for long-distance international trade.

To circumvent these storage bottlenecks, chemical engineers have increasingly focused on Liquid Organic Hydrogen Carriers, commonly referred to as LOHCs. These are pairs of unsaturated and saturated organic compounds that can store and release hydrogen through reversible catalytic reactions. During the initial charging phase, known as hydrogenation, gaseous hydrogen is catalytically bonded to an unsaturated carrier liquid under moderate heat and elevated pressure. This chemical reaction is exothermic, discharging thermal energy that can potentially be captured for district heating or industrial processes. The resulting hydrogenated liquid possesses physical properties remarkably similar to conventional diesel or crude oil. Crucially, it can be handled, stored, and transported across existing global maritime and pipeline infrastructure without boiling off or suffering from gradual pressure degradation.

Several aromatic hydrocarbons have been evaluated as potential carrier media, each offering distinct thermal and chemical advantages. Early research explored toluene, which transforms into methylcyclohexane upon hydrogenation; however, its volatility and relatively low flash point present notable safety concerns during large-scale handling. In recent years, dibenzyltoluene has emerged as one of the most promising candidates. Dibenzyltoluene remains liquid across an extraordinarily broad temperature range, from minus 39 to well over 300 degrees Celsius, and possesses an exceptionally low vapour pressure alongside negligible flammability at room temperature. Another heavily investigated category includes nitrogen-containing heterocycles, such as N-ethylcarbazole, which exhibit a lower reaction enthalpy, meaning less thermal energy is required to extract the bound hydrogen during the unloading phase.

The release of hydrogen from the saturated carrier molecule—termed dehydrogenation—occurs at the point of consumption or regional distribution centres. Unlike the charging step, dehydrogenation is strongly endothermic, necessitating a sustained supply of high-temperature thermal energy, typically between 250 and 320 degrees Celsius. Solid catalysts containing precious metals such as platinum or palladium are traditionally employed to facilitate the cleavage of chemical bonds. To make the entire supply chain economically and environmentally viable, researchers emphasise the importance of heat integration. If the thermal energy required for dehydrogenation is supplied by waste heat, concentrated solar power, or the exhaust heat from high-temperature fuel cells, the overall energy efficiency of the carrier cycle increases substantially.

A critical technical challenge in the dehydrogenation process lies in the purity of the liberated hydrogen gas. Proton-exchange membrane fuel cells, which are widely deployed in passenger vehicles and heavy trucks, are notoriously sensitive to contaminants and require hydrogen of at least 99.97 per cent purity. During high-temperature dehydrogenation, minor quantities of organic vapour and volatile by-products can contaminate the gas stream. If these contaminants enter the fuel cell, they can rapidly poison the electrocatalyst, causing irreversible operational failure. Consequently, robust separation technologies, such as multi-stage condensation traps, activated carbon filters, or palladium-based separation membranes, must be installed downstream to remove any entrained hydrocarbon residues before the gas reaches end users.

Another significant operational consideration is the chemical longevity of the carrier liquid over repeated cycles. Although the hydrogenation and dehydrogenation loop is theoretically infinite, thermal stress and repeated catalytic exposure inevitably trigger minor side reactions. Over hundreds of cycles, these parasitic reactions generate high-boiling polymeric residues or split the carrier into low-boiling fragments. Concurrently, the solid catalysts can suffer from coking, wherein carbon deposits accumulate on the catalyst surface and block active reaction sites. To maintain system performance, operators must incorporate continuous filtration systems and periodically replenish degraded fluid, balancing the cost of chemical replacement against energy throughput.

Despite these technical hurdles, the economic case for LOHCs remains compelling, particularly for intercontinental maritime shipping where the reuse of existing tanker fleets and port facilities avoids the enormous capital expenditure of building dedicated cryogenic terminals. Furthermore, because LOHCs do not self-discharge or pose severe blast hazards, they offer unparalleled safety for long-duration strategic energy reserves. As chemical syntheses improve and catalyst formulations become cheaper, liquid organic carriers are poised to form a vital link in the worldwide distribution of green hydrogen.