[Reading Science] Maintained 97% Liquefied Hydrogen Storage, Tripled Retention Time

UNIST, Ewha Womans University, and European Researchers Validate MOF-Based Storage Strategy
Storage Duration Extended from 64 to 221 Days
Further Empirical Testing Required for Real-World Tank Application

A new study has found that inserting a porous material filled with countless microscopic holes into liquefied hydrogen tanks can sharply reduce evaporation loss while maintaining nearly the same amount of stored hydrogen. In simulation experiments, about 97% of the storage capacity was retained compared to tanks filled only with liquefied hydrogen, while the time for complete hydrogen depletion increased from 64 days to 221 days, more than tripling. This demonstrates a novel way to address 'evaporation loss,' a major challenge in liquefied hydrogen transportation.

On September 29, UNIST (Ulsan National Institute of Science and Technology) announced that Professor Hyun-Chul Oh from the Department of Chemistry, Professor Hweeri Moon from Ewha Womans University, and researchers from the Technical University of Munich had validated, through adsorption experiments and simulations, a strategy to reduce evaporation loss of liquefied hydrogen using metal-organic frameworks (MOFs). The study was published in the international journal Nature Communications.

Principle of Reducing Liquefied Hydrogen Evaporation Loss Using MOF. The porous material MOF adsorbs hydrogen at high density, slowing down vaporization. In a model assuming a transport medium vacuum tank, the storage duration increased from 64 days with liquefied hydrogen alone to 221 days with the application of IRMOF-20. Provided by the research team

Liquefying hydrogen dramatically reduces its volume, making it advantageous for large-scale storage and transport. The challenge, however, is that liquefied hydrogen must be kept at an ultra-low temperature of around minus 253 degrees Celsius. When external heat enters the tank, it causes the liquid hydrogen to vaporize, raising the pressure and ultimately forcing some to be vented, resulting in losses.

The researchers believed that MOFs could serve as a kind of "thermal buffer" in this process. MOFs are materials with innumerable nanopores. Hydrogen trapped within these pores requires additional heat input to be released. Thus, part of the incoming external heat is used to detach hydrogen from the MOF, which in turn delays the immediate vaporization of liquefied hydrogen.

The research team compared 'IRMOF-20,' a MOF with large pore volume and robust structure, with 'MIL-53,' a MOF whose pore size and shape change according to hydrogen adsorption.

Storage Capacity Maintained at 97% Even with MOF

The key issue was the space occupied by the MOF itself. If the material considerably decreased the amount of hydrogen that can be stored in the tank, practical application for storage and transport would be difficult, even if evaporation was reduced.

In a hypothetical tank applying IRMOF-20, it was found that about 96.6% of the hydrogen could be stored compared to a tank filled only with liquefied hydrogen. Despite the MOF occupying some space, the reduction in storage capacity remained in the 3% range. According to calculations based on hydrogen adsorption and pore volume, the internal hydrogen density within the pores was about 82g/L, higher than that of liquefied hydrogen itself.

The effect in delaying evaporation loss was even more significant. In a simulation with a medium vacuum insulated transport tank of about 56.6㎥, under medium vacuum conditions, a tank filled only with liquefied hydrogen saw complete depletion in about 64 days. With IRMOF-20, this extended to roughly 221 days—a more than threefold increase. Even in low-vacuum conditions, the time to depletion increased from 7 days to 22 days.

Research team photo. (From left) Hyuncheol Oh, Professor at UNIST; Jitae Park, Ph.D. at Technical University of Munich; Hoeri Moon, Professor at Ewha Womans University; Jaewoo Park, Ph.D. at UNIST. Courtesy of UNIST

On the other hand, MIL-53 was more advantageous for retaining hydrogen at higher temperatures, but its storage capacity dropped to around 52.8% of that of a tank filled only with liquefied hydrogen. The time to complete hydrogen depletion under medium vacuum conditions was about 128 days. The researchers analyzed that, beyond the hydrogen adsorption capability of MOFs, pore volume, material density, and the temperature at which hydrogen is released must also be considered.

Neutron inelastic scattering experiments further confirmed that hydrogen inside the MOF pores could not freely rotate. The research team presented this as indirect evidence supporting their interpretation that hydrogen is densely clustered and stably adsorbed inside the pores.

However, these findings do not mean that the storage duration of actual liquefied hydrogen tanks can be immediately increased threefold. The tank analysis was based on theoretical maximum material performance in simulations, and further validation is required to confirm whether the same effect can be achieved in real large-scale tanks.

Professor Oh stated, "Our work demonstrates that considering the pore volume, the capacity to densely store hydrogen, and the behavior of adsorbed hydrogen release shows that porous materials can be used to maintain nearly full storage capacity while reducing evaporation loss during transport. However, as these findings are based on the theoretical maximum performance of the materials, further empirical validation is required to see if this effect can be realized in actual tanks."

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