Hydrogen carriers play a critical role in enabling the widespread adoption of hydrogen as an energy vector. Among the most prominent carriers are ammonia, liquid organic hydrogen carriers (LOHCs), and liquid hydrogen. Each has distinct properties influencing leakage risks, energy density, safety, and environmental impact. Understanding these trade-offs is essential for minimizing the climate impact of hydrogen supply chains.
Ammonia is a widely studied hydrogen carrier due to its high hydrogen density and established production infrastructure. It contains 17.6% hydrogen by weight and can be stored as a liquid at moderate pressures or refrigerated conditions. However, ammonia is toxic and poses risks to human health and ecosystems if leaked. When released into the atmosphere, ammonia reacts with other compounds to form particulate matter, indirectly contributing to air quality degradation. Additionally, ammonia has a short atmospheric lifetime, meaning its direct global warming potential (GWP) is relatively low compared to other greenhouse gases. However, its role in secondary aerosol formation complicates its climate impact assessment.
LOHCs, such as toluene-methylcyclohexane or dibenzyltoluene-perhydro-dibenzyltoluene, offer a safer alternative due to their liquid state at ambient conditions. These carriers chemically bind hydrogen, reducing leakage risks during storage and transport. However, LOHCs require energy-intensive dehydrogenation processes to release hydrogen, increasing their lifecycle emissions. Leakage of LOHCs themselves is less hazardous than ammonia or hydrogen, but their decomposition products may still pose environmental risks. The carbon content in LOHCs also means that any leakage contributes to volatile organic compound (VOC) emissions, which can influence air quality and climate through secondary chemical reactions.
Liquid hydrogen (LH2) provides the purest form of hydrogen storage, eliminating the need for reconversion. However, it must be maintained at cryogenic temperatures (below -253°C), leading to significant boil-off losses if insulation is insufficient. Hydrogen leakage is particularly concerning due to its high GWP when released into the atmosphere. Molecular hydrogen interacts with tropospheric hydroxyl radicals, reducing their availability to break down methane, a potent greenhouse gas. This indirect effect means that hydrogen leakage can prolong methane’s atmospheric lifetime, amplifying its warming impact. Studies estimate that hydrogen’s 100-year GWP ranges between 5 and 12, depending on atmospheric modeling assumptions.
Comparing leakage risks across different stages of the supply chain reveals further distinctions. Ammonia leakage is most likely during storage and transport due to its corrosive nature and the need for pressurized or refrigerated containment. Small leaks can be detected by odor, but large-scale releases pose immediate health hazards. LOHCs exhibit lower leakage risks in transit because they remain stable at ambient conditions, but spills during handling could lead to soil and water contamination. Liquid hydrogen faces the highest evaporative losses, particularly during prolonged storage or transfer operations, where even advanced insulation cannot eliminate boil-off entirely.
Energy density is another critical factor. Liquid hydrogen has the highest gravimetric energy density but requires significant energy for liquefaction. Ammonia offers a compromise with reasonable energy density and easier handling than LH2, though it demands additional energy for cracking back into hydrogen. LOHCs have the lowest hydrogen content by weight but benefit from compatibility with existing fuel infrastructure.
Safety considerations further differentiate these carriers. Ammonia requires strict handling protocols due to its toxicity, while LOHCs present fewer immediate hazards but may involve flammable organic compounds. Liquid hydrogen poses risks of embrittlement and requires specialized materials to prevent structural failures at cryogenic temperatures.
Lifecycle assessments (LCAs) provide a quantitative basis for ranking carriers by climate impact. When accounting for production, transport, storage, and reconversion, ammonia often shows a lower overall carbon footprint than LOHCs if derived from renewable sources. However, its toxicity and secondary environmental effects complicate its sustainability profile. LOHCs exhibit higher emissions due to dehydrogenation energy demands but offer logistical advantages. Liquid hydrogen, while clean at the point of use, suffers from high energy penalties in liquefaction and significant leakage-related GWP contributions.
A simplified comparison of key metrics:
| Carrier | Hydrogen Content | Storage Conditions | Leakage Risk | GWP Impact | Energy Penalty |
|-----------------|------------------|----------------------|--------------|------------|----------------|
| Ammonia | 17.6% by weight | Pressurized/refrigerated | Moderate | Low (direct) | Moderate |
| LOHCs | 5-7% by weight | Ambient | Low | Moderate | High |
| Liquid Hydrogen | 100% by weight | Cryogenic (-253°C) | High | High | Very High |
In conclusion, no single carrier is universally superior; each presents trade-offs between energy efficiency, safety, and environmental impact. Ammonia is advantageous for large-scale transport but requires rigorous safety measures. LOHCs offer ease of handling but incur higher energy costs. Liquid hydrogen is optimal for purity but suffers from intrinsic leakage risks. Future advancements in containment technologies and renewable-derived production pathways will be crucial in mitigating the climate impact of hydrogen carriers. Policymakers and industry stakeholders must prioritize leakage prevention and lifecycle emissions reduction to ensure hydrogen fulfills its role in a sustainable energy future.