The selection of Liquid Organic Hydrogen Carriers (LOHCs) is a critical aspect of hydrogen storage and delivery systems, particularly for applications requiring safe, efficient, and reversible hydrogenation and dehydrogenation. The choice of an optimal LOHC depends on a combination of chemical, physical, and economic factors, including hydrogen storage capacity, reversibility, thermal stability, toxicity, and cost. Each of these properties plays a significant role in determining the suitability of an LOHC for specific applications, such as industrial hydrogen supply, energy storage, or mobility solutions.
Hydrogen storage capacity is a primary consideration, as it directly impacts the energy density of the carrier. The theoretical maximum hydrogen storage capacity is determined by the number of hydrogen atoms that can be bound and released per molecule of the carrier. For example, dibenzyltoluene (DBT) can store up to 6.2 wt% hydrogen, while N-ethylcarbazole (NEC) has a higher capacity of around 5.8 wt%. Toluene, another common LOHC, stores approximately 6.1 wt% hydrogen. However, practical storage capacities are often lower due to incomplete hydrogenation or dehydrogenation. The trade-off between high hydrogen content and other properties, such as stability, must be carefully evaluated.
Reversibility is another crucial factor, as it determines the feasibility of repeated hydrogenation and dehydrogenation cycles without significant degradation of the carrier. A good LOHC should maintain its structural integrity over multiple cycles to ensure economic viability. For instance, DBT exhibits excellent reversibility, with minimal degradation even after hundreds of cycles, making it suitable for long-term use. In contrast, some carriers, such as certain heterocyclic compounds, may suffer from side reactions or polymerization over time, reducing their lifespan. The ability to fully dehydrogenate the carrier at moderate temperatures is also essential to avoid energy penalties during hydrogen release.
Thermal stability is closely related to reversibility and determines the temperature range in which the LOHC can operate without decomposition. High thermal stability is necessary to prevent degradation during storage or transportation, especially in applications where the carrier may be exposed to elevated temperatures. DBT is known for its robust thermal stability, with a decomposition temperature above 300°C, making it suitable for high-temperature applications. NEC, while stable at moderate temperatures, may require careful temperature control to avoid unwanted side reactions. Toluene, though stable, has a relatively low boiling point, which can limit its use in high-temperature environments.
Toxicity and environmental impact are critical for ensuring safe handling and compliance with regulations. The ideal LOHC should have low toxicity, minimal environmental persistence, and safe byproducts upon dehydrogenation. DBT is considered environmentally benign, with low toxicity and negligible vapor pressure, reducing inhalation risks. NEC, while generally safe, requires careful handling due to its potential irritant properties. Toluene, though widely used, poses higher toxicity and flammability risks, necessitating stringent safety measures. The choice of carrier must align with the intended application’s safety requirements, particularly in densely populated or sensitive environments.
Cost considerations encompass both the initial price of the carrier and the overall system economics, including hydrogenation and dehydrogenation expenses. The raw material cost, synthesis complexity, and energy requirements for hydrogen release all influence the total cost of ownership. DBT is relatively inexpensive and derived from abundant feedstocks, contributing to its widespread adoption. NEC, while more costly, offers advantages in specific applications due to its higher hydrogen capacity and faster dehydrogenation kinetics. Toluene is cost-effective but may incur higher processing costs due to its volatility and lower energy density. Balancing these economic factors with performance requirements is essential for selecting the most suitable LOHC.
The application context further refines the selection criteria. For stationary energy storage, where weight and volume are less critical, DBT’s stability and low cost make it a preferred choice. In mobility applications, where energy density and rapid dehydrogenation are prioritized, NEC or similar carriers may be more appropriate despite their higher cost. Industrial hydrogen supply chains may favor toluene or DBT due to their established infrastructure and handling protocols. Each application demands a tailored approach to LOHC selection, weighing the trade-offs between capacity, reversibility, stability, safety, and cost.
Examples of LOHCs illustrate these trade-offs. Dibenzyltoluene is widely used in large-scale energy storage due to its excellent thermal stability, low toxicity, and moderate hydrogen capacity. Its main limitation is the relatively high temperature required for dehydrogenation, which can increase energy consumption. N-ethylcarbazole offers faster dehydrogenation kinetics and higher hydrogen capacity but suffers from higher costs and potential degradation over cycles. Toluene, while inexpensive and easy to handle, has lower energy density and higher flammability risks, limiting its use in certain applications.
In summary, the selection of LOHCs is a multifaceted decision that requires careful evaluation of hydrogen storage capacity, reversibility, thermal stability, toxicity, and cost. Each property influences the carrier’s performance and suitability for specific applications, from industrial hydrogen supply to mobile energy systems. By understanding the advantages and limitations of carriers like dibenzyltoluene, toluene, and N-ethylcarbazole, stakeholders can make informed choices that balance technical requirements with economic and safety considerations. The ongoing development of new LOHC materials and optimization of existing ones will further enhance their role in the hydrogen economy.