Liquid Organic Hydrogen Carriers (LOHCs) are a class of compounds that enable reversible hydrogen storage and transport through chemical bonding. These molecules store hydrogen via catalytic hydrogenation and release it through dehydrogenation, offering a promising alternative to compressed gas or metal hydrides. Key LOHC systems include toluene-methylcyclohexane (MCH) and dibenzyltoluene (DBT), which balance high hydrogen capacity, stability, and compatibility with existing infrastructure.
The hydrogenation-dehydrogenation cycle is central to LOHC functionality. During hydrogenation, an unsaturated organic compound (e.g., toluene or dibenzyltoluene) reacts with hydrogen in the presence of a catalyst, forming a hydrogen-rich molecule (methylcyclohexane or perhydro-dibenzyltoluene). The reverse reaction, dehydrogenation, releases hydrogen when needed. For toluene-MCH, the reaction proceeds as follows:
C7H8 (toluene) + 3H2 ↔ C7H14 (methylcyclohexane).
Similarly, dibenzyltoluene (C21H20) undergoes hydrogenation to form perhydro-dibenzyltoluene (C21H38), storing six hydrogen molecules.
Thermodynamics play a critical role in LOHC systems. Hydrogenation is exothermic, typically occurring at moderate temperatures (100–200°C) and pressures (30–50 bar), while dehydrogenation is endothermic, requiring higher temperatures (250–300°C) and often reduced pressures. The enthalpy of hydrogenation for MCH is approximately 205 kJ/mol H2, necessitating significant energy input for hydrogen release. Catalysts are essential to lower activation barriers; platinum, palladium, and ruthenium on alumina or carbon supports are common for dehydrogenation, while nickel and ruthenium catalysts are used for hydrogenation. Catalyst deactivation due to coking or sintering remains a challenge, driving research into more robust materials.
Energy efficiency of LOHC systems depends on the hydrogenation-dehydrogenation cycle and auxiliary processes. The round-trip efficiency (hydrogen stored vs. hydrogen recovered) ranges from 60% to 75%, accounting for thermal inputs and catalyst performance. This is lower than compressed gas storage (80–90%) but avoids high-pressure risks. Compared to metal hydrides, LOHCs offer superior energy density (6–7 wt% for MCH, 6.2 wt% for DBT) and easier handling at ambient conditions.
Safety is a major advantage of LOHCs. Unlike compressed or liquefied hydrogen, LOHCs are non-flammable in their hydrogenated form and can be transported using conventional fuel infrastructure. Methylcyclohexane and dibenzyltoluene are liquid at room temperature, eliminating boil-off losses and high-pressure hazards. Their toxicity and environmental impact are comparable to diesel, requiring standard handling protocols.
Infrastructure compatibility further enhances LOHC appeal. Existing tanker trucks, pipelines, and storage tanks for petroleum products can be repurposed with minimal modification. This reduces capital costs compared to cryogenic or high-pressure systems. However, dehydrogenation units must be integrated at end-use sites, adding complexity.
Industrial applications of LOHCs are expanding. In Japan, the toluene-MCH system is being tested for hydrogen export, leveraging maritime shipping routes. Germany’s Hydrogenious LOHC Technologies employs dibenzyltoluene for stationary storage and mobility solutions. The chemical industry also uses LOHCs for hydrogen logistics in refineries and ammonia plants, where large-scale storage is critical.
Ongoing research aims to optimize LOHC performance. Key focus areas include:
- Developing cheaper, more active catalysts to reduce dehydrogenation temperatures.
- Identifying novel carrier molecules with higher hydrogen capacity and lower dehydrogenation enthalpy.
- Improving system integration to minimize energy penalties during hydrogen release.
- Enhancing catalyst longevity through advanced supports and doping strategies.
Comparative analysis with other storage methods highlights trade-offs. While LOHCs lag in energy efficiency compared to compressed hydrogen, they excel in safety and scalability. Metal hydrides face weight and cost limitations, whereas LOHCs provide a practical solution for long-distance transport. The absence of high-pressure or cryogenic requirements makes LOHCs particularly suitable for decentralized hydrogen distribution.
Future advancements may unlock broader adoption. Pilot projects in Europe and Asia are validating LOHC feasibility for cross-border hydrogen trade. Innovations in catalyst design and process engineering could further improve efficiency, narrowing the gap with competing technologies. As hydrogen economies mature, LOHCs are poised to play a pivotal role in bridging production and demand centers safely and efficiently.
In summary, LOHCs represent a versatile and scalable approach to hydrogen storage and transport. Their reversible chemistry, compatibility with existing infrastructure, and inherent safety make them a compelling option for industrial and energy applications. Continued research and deployment will determine their role in the evolving hydrogen landscape.