Liquid Organic Hydrogen Carriers (LOHCs) represent a promising solution for hydrogen storage and transport, particularly in sectors where energy density and safety are critical. Aviation and maritime industries face unique challenges in adopting hydrogen as a fuel due to the need for high energy density, long-range capabilities, and efficient refueling infrastructure. LOHCs offer a viable pathway to address these challenges by enabling hydrogen to be stored and transported in a liquid form at ambient conditions, similar to conventional fuels. This article examines the potential of LOHCs in aviation and maritime hydrogen propulsion, focusing on energy density requirements and refueling logistics.

Energy density is a critical factor for aviation and maritime applications. Conventional jet fuels and marine diesel have high energy densities, typically ranging between 35-45 megajoules per kilogram (MJ/kg). Pure hydrogen, while having a gravimetric energy density of approximately 120 MJ/kg, faces volumetric energy density challenges due to its low density in gaseous form and the cryogenic requirements for liquid hydrogen storage. LOHCs, however, store hydrogen chemically bound to organic molecules, allowing for liquid-phase storage at near-ambient temperatures and pressures. The effective energy density of LOHCs, accounting for the hydrogen release process, typically ranges between 6-7 wt% hydrogen, translating to an energy density of around 25-30 MJ/kg when considering the entire system. While this is lower than conventional fuels, it is significantly higher than compressed or cryogenic hydrogen storage in terms of volumetric efficiency, making it more practical for space-constrained applications like aircraft and ships.

The chemical stability of LOHCs is another advantage for aviation and maritime use. Unlike cryogenic hydrogen, which requires complex insulation and boil-off management, LOHCs can be handled using existing fuel infrastructure with minor modifications. This reduces the need for entirely new supply chains and simplifies integration into current refueling systems. For example, toluene and dibenzyltoluene are among the most studied LOHC candidates due to their stability, relatively high hydrogen capacity, and compatibility with conventional fuel handling equipment. These characteristics make LOHCs particularly attractive for long-haul maritime shipping, where fuel storage and handling must align with existing port logistics.

Refueling logistics present a significant hurdle for hydrogen adoption in aviation and maritime sectors. Traditional hydrogen refueling requires high-pressure or cryogenic systems, which are impractical for large-scale applications like airports or ports. LOHCs, however, can be refueled using liquid transfer methods similar to those used for diesel or jet fuel. This allows for faster refueling times and reduces the need for specialized infrastructure. In aviation, LOHCs could be integrated into existing airport fuel supply systems, with dehydrogenation units installed at airports to release hydrogen for use in fuel cells or combustion engines. For maritime applications, ships could carry LOHCs in standard fuel tanks, with dehydrogenation occurring onboard or at port facilities. This flexibility simplifies the transition from fossil fuels to hydrogen-based propulsion.

The dehydrogenation process, however, introduces energy penalties that must be addressed. Releasing hydrogen from LOHCs requires thermal energy, typically at temperatures between 200-300 degrees Celsius, depending on the carrier material. This energy demand can reduce the overall system efficiency, but advancements in catalyst development and heat integration are improving the process. Recent research has demonstrated dehydrogenation efficiencies exceeding 80% for certain LOHC systems, making them increasingly competitive with other hydrogen storage methods. For aviation, waste heat from engines or fuel cells could potentially be utilized to drive dehydrogenation, further improving system efficiency. In maritime applications, excess heat from ship engines or industrial processes at ports could serve the same purpose.

Safety considerations are paramount in both aviation and maritime environments. LOHCs offer inherent safety advantages over compressed or cryogenic hydrogen, as they are non-explosive and non-flammable in their hydrogen-loaded state. This reduces risks associated with fuel handling, storage, and transport, particularly in crowded port areas or during aircraft refueling operations. The liquid nature of LOHCs also minimizes leakage risks compared to gaseous hydrogen systems. Regulatory frameworks for LOHC handling are still under development, but their similarity to existing hydrocarbon fuels may facilitate faster certification compared to alternative hydrogen storage methods.

The scalability of LOHC systems is another factor supporting their potential in aviation and maritime applications. Global production capacity for candidate LOHC materials already exists due to their use in other industries, enabling rapid scale-up without significant supply chain bottlenecks. This contrasts with some alternative hydrogen storage materials that require new production facilities. For maritime shipping, which accounts for nearly 3% of global CO2 emissions, the ability to transition existing vessels to LOHC-based hydrogen systems through retrofit solutions could accelerate decarbonization efforts. Similarly, in aviation, where fuel alternatives are limited by weight and volume constraints, LOHCs present a viable medium-term solution while fully electric or other advanced propulsion technologies mature.

Challenges remain in optimizing LOHC systems for aviation and maritime use. The weight penalty from carrier molecules and dehydrogenation equipment must be minimized to avoid compromising payload or range. Research is ongoing to develop lighter-weight LOHC materials and more compact dehydrogenation reactors. Additionally, the life cycle emissions of LOHC systems must be considered, particularly when deriving hydrogen from fossil sources. Pairing LOHCs with green hydrogen production methods will be essential to maximize environmental benefits.

In conclusion, LOHCs offer a practical pathway for hydrogen adoption in aviation and maritime propulsion by addressing key challenges around energy density and refueling logistics. Their liquid form, compatibility with existing infrastructure, and inherent safety advantages make them particularly suited to these demanding applications. While technical hurdles remain, continued advancements in LOHC materials and system integration are enhancing their viability as a bridge technology in the transition to sustainable hydrogen-based transportation. As the aviation and maritime industries seek to reduce their carbon footprints, LOHCs represent a promising solution that balances performance, safety, and practicality.