Ammonia, liquid organic hydrogen carriers (LOHCs), metal hydrides, and liquid hydrogen represent four leading approaches to hydrogen storage and transport, each with distinct trade-offs in energy density, cost, and safety. These carriers address the challenges of hydrogen’s low volumetric energy density and the need for efficient, scalable solutions in a hydrogen economy.

**Energy Density Comparison**
Liquid hydrogen boasts the highest gravimetric energy density, storing nearly 100% hydrogen by weight. However, its volumetric energy density is low, requiring cryogenic temperatures at -253°C to remain liquid, which complicates storage and transport. Ammonia, in contrast, carries 17.6% hydrogen by weight but has higher volumetric energy density than liquid hydrogen due to its ability to remain liquid at milder conditions (-33°C at atmospheric pressure or room temperature under modest pressure). Ammonia’s energy density is further enhanced by its compatibility with existing infrastructure, such as tanks designed for propane.

LOHCs typically store 6-8% hydrogen by weight, with volumetric energy densities comparable to ammonia but lower than liquid hydrogen. Their advantage lies in compatibility with conventional liquid fuel infrastructure, avoiding cryogenic requirements. Metal hydrides offer even lower gravimetric storage, usually below 5% hydrogen by weight, but provide excellent volumetric storage due to the dense packing of hydrogen atoms within metal lattices. However, their energy density is offset by the weight of the metal matrix.

**Cost Considerations**
Liquid hydrogen is expensive due to energy-intensive liquefaction, which consumes 30-40% of the stored hydrogen’s energy content. Cryogenic storage and transport further escalate costs, making it viable primarily for aerospace and niche applications. Ammonia production via the Haber-Bosch process is mature and cost-effective at scale, but cracking ammonia back into hydrogen adds complexity and expense. Despite this, ammonia benefits from existing global production and distribution networks, reducing capital expenditure for new infrastructure.

LOHCs incur costs from hydrogenation (loading) and dehydrogenation (unloading) processes, which require significant heat and catalysts. While LOHCs avoid cryogenics, their cycle efficiency is lower than ammonia or liquid hydrogen. Metal hydrides face high material costs, particularly for rare-earth or complex alloys, and require energy input for hydrogen release. Their long-term durability and cycling stability also impact lifecycle costs.

**Safety Profiles**
Liquid hydrogen poses significant safety risks due to extreme cold, high flammability (4-75% concentration in air), and potential for embrittlement in storage materials. Its low boiling point also leads to rapid vaporization and pressure buildup if containment fails. Ammonia is toxic and corrosive, requiring strict handling protocols, but its detectable odor aids leak identification. It is less flammable than hydrogen, with a narrower combustion range (15-28% in air), reducing explosion risks.

LOHCs are generally safer, as they are non-toxic, non-flammable in their hydrogenated state, and stable at ambient conditions. However, dehydrogenation requires elevated temperatures, introducing thermal hazards. Metal hydrides are inherently safe for stationary storage, with low flammability and no high-pressure or cryogenic risks, but their powder forms can pose dust explosion hazards if mishandled.

**Infrastructure and Scalability**
Ammonia leverages existing global infrastructure for production, shipping, and storage, making it highly scalable for international trade. Its use in fertilizers ensures a robust supply chain, though cracking plants must be developed for hydrogen recovery. LOHCs integrate seamlessly with liquid fuel logistics, but their reliance on dehydrogenation units limits decentralized applications.

Liquid hydrogen infrastructure is limited to high-value sectors like rocketry and requires specialized cryogenic equipment. Metal hydrides are best suited for fixed storage applications, such as stationary power, due to weight and cycling constraints.

**Environmental Impact**
Ammonia production is carbon-intensive unless green hydrogen feeds the Haber-Bosch process. LOHCs and metal hydrides have minimal direct emissions but depend on clean hydrogen sources for sustainability. Liquid hydrogen’s energy penalty in liquefaction raises its carbon footprint unless renewable energy powers the process.

**Conclusion**
The choice between ammonia, LOHCs, metal hydrides, and liquid hydrogen hinges on application-specific priorities. Ammonia excels in energy density and scalability for industrial and transport uses, while LOHCs offer safety and compatibility with liquid fuel systems. Metal hydrides provide compact, safe storage for stationary applications, and liquid hydrogen remains unmatched in gravimetric density for mobility and aerospace. Cost, safety, and infrastructure readiness will dictate their adoption as the hydrogen economy evolves.