Liquid organic hydrogen carriers for hydrogen storage: advances, challenges, and technoeconomic and sustainability perspectives.

Liquid Organic Hydrogen Carriers (LOHCs) have emerged as one of the most promising chemical-based options for hydrogen storage and transport. The review critically analysed peer-reviewed literature, primarily published recently, with emphasis on recent development, challenges and mitigating strategies. The review finds that Toluene-Methylcyclohexane (Toluene/MCH), benzyl toluenes (BT), dibenzyl toluenes (DBT), and N-ethylcarbazole (NEC) are technically advanced LOHC families with commercial applications. For hydrogenation reactions, noble metals, primarily Pt, Pd, and Ru, supported on oxide surfaces are widely used. The reaction temperature for the Toluene/MCH and NEC systems ranges from 180 to 225 °C, whereas BT and DBT require around 300 °C. For dehydrogenation, major catalyst/support systems include Pt/KIT-6 mesoporous silica, Pt/TiO2, Pt/Al2O3, Ru/Al2O3, Pt/V2O5, Pt/Y2O3, and Pt-Re/Al2O3. The dehydrogenation reaction critically suffers from a high-temperature requirement of ∼250-450 °C, due to its endothermic nature. Various types of reactors are used for the LOHC process, including fixed-bed, fluidised-bed, and Trickle/slurry-bed reactors. Significant challenges of LOHC systems include the high cost of catalysis, especially with noble catalysts; catalyst deactivation; high temperature/energy requirements, especially during dehydrogenation; carbon coking; slow reaction rates; instability under cycling; and by-product formation. From a technoeconomic perspective, the LOHC system is expensive, with a range of $2.0-7.0/kgH2 depending on the carrier and catalyst used. Therefore, LOHC technology necessitates a comprehensive, system-wide strategy rather than molecular-level advancements. One such strategy might be efficient heat utilisation in low-carbon hydrogen logistics, including integrated methods such as microwave and exhaust heat recovery. The management of carriers must align with chemical properties and logistics, while system-level techno-economic analysis (TEA) and life cycle assessment (LCA) are essential. Innovations in catalysts focus on inductively heatable structures and durability during cycles, while reactor advancements aim to improve temperature regulation and address transmission constraints.