Rethinking liquid hydrogen storage with metal-organic frameworks

Liquid hydrogen (LH₂) is attractive for long-distance energy transport because of its high volumetric energy density. But even well-insulated tanks cannot completely prevent heat from entering. As the liquid warms, hydrogen evaporates and pressure builds, leading to boil-off losses during storage and transport.

A research team led by professor Hyunchul Oh of the Department of Chemistry at UNIST investigated whether porous materials could help limit these losses. The team also included professor Hoi Ri Moon of Ewha Womans University, Dr. Jitae T. Park of the Technical University of Munich (TUM) in Germany, and Dr. Mónica Jiménez-Ruiz of the Institut Laue–Langevin (ILL) in France. The researchers used metal-organic frameworks (MOFs)—highly porous crystalline materials—to capture evaporating hydrogen and delay its release as heat enters the tank. The paper is published in the journal Nature Communications.

MOFs contain networks of nanoscale pores that can adsorb hydrogen onto their internal surfaces. At cryogenic temperatures, interactions between hydrogen molecules and the pore walls help keep the gas confined, slowing pressure buildup. The approach complements conventional insulation—instead of focusing only on limiting heat from entering the tank, it also changes how hydrogen responds once heat enters.

The key question is whether that benefit comes at the expense of storage capacity. A porous material takes up space that would otherwise hold liquid hydrogen. The researchers therefore compared two MOFs with very different structures—IRMOF-20, a rigid framework with a large pore volume, and MIL-53(Al), a flexible framework whose pores expand and contract as hydrogen is adsorbed.

IRMOF-20 offered a particularly favorable balance. When the researchers accounted for the space occupied by the material, the system retained about 97% of the volumetric capacity of neat liquid hydrogen. This was possible because hydrogen confined within the pores packed densely enough to compensate for much of the space taken by the MOF.

The researchers then modeled how the materials would affect boil-off in a transport-scale liquid hydrogen tank. Under the mid-vacuum insulation conditions examined, neat liquid hydrogen was predicted to deplete after about 64 days. With IRMOF-20, that period increased to approximately 221 days—more than three times as long.

MIL-53(Al) behaved differently. It retained hydrogen more strongly as temperature increased, but its volumetric capacity was only about 53% of that of liquid hydrogen. The contrast highlights a central design choice for porous materials in cryogenic hydrogen storage—larger pore volume favors capacity, while stronger confinement can improve thermal retention.

"By considering pore volume, hydrogen density, and desorption behavior together, we found that porous materials could reduce boil-off while preserving much of the storage capacity needed for liquid hydrogen transport. These calculations represent an idealized upper bound, so further work is needed to determine how closely this performance can be reproduced in practical tank systems," says Oh, the leader of the study.

The role of neutrons

Experiments offered clues to why IRMOF-20 performed this way. Adsorption measurements indicated that hydrogen confined within its pores reached an effective density higher than that of bulk liquid hydrogen. Inelastic neutron scattering experiments at IN1-Lagrange at ILL, carried out by Park from the Heinz Maier-Leibnitz Zentrum at TUM and Jiménez-Ruiz from ILL, showed restricted molecular rotation inside the pores, providing indirect evidence of strong interactions between hydrogen and the framework. Together, the results suggest that nanoscale confinement can both pack hydrogen densely and stabilize it against release as temperature rises.

"Neutrons are particularly well suited to studying the quantum rotational excitations of H₂ because hydrogen has a large neutron-scattering cross section, and inelastic neutron scattering directly probes molecular motion without the optical selection rules that constrain infrared and Raman spectroscopy," explains TUM researcher Park.

The rotational spectrum of solid parahydrogen consists of a very sharp rotational line at 14.7 meV. Changes in the position of this rotational line upon interaction with different host materials can be used to probe the nature of the interaction potential. When hydrogen interacts with a surface, both the position and shape of the rotational line depend sensitively on the local environment.

"These characteristics make the H₂ rotational transition a sensitive probe of the interaction between hydrogen and its surroundings. In this context, the IN1-Lagrange spectrometer at the Institut Laue-Langevin is particularly well suited to investigating H₂ confined in porous materials. The instrument combines a broad energy-transfer range, high neutron flux, and good energy resolution, enabling changes in the position, shape, and splitting of the H₂ rotational transition to be measured and used as sensitive probes of the H₂–surface interaction," says Jiménez-Ruiz, an ILL scientist responsible for the IN1-Lagrange instrument.