ULSAN — Researchers have developed a technology that sharply reduces the loss of hydrogen that evaporates when liquid hydrogen is stored or transported over long periods as outside heat seeps in. Filling a tank with a new porous material more than tripled the storage period while keeping the reduction in storage capacity to a minimum.
A team led by Professor Oh Hyun-chul of the Department of Chemistry at the Ulsan National Institute of Science and Technology (UNIST) said on the 29th that it had identified a way to curb evaporative losses of liquid hydrogen using metal-organic frameworks (MOFs), in joint work with Professor Moon Hoi-ri of Ewha Womans University and Dr. Park Ji-tae's team at the Technical University of Munich in Germany. The findings were published in the international journal Nature Communications.
Turning hydrogen into a liquid at minus 253 degrees Celsius greatly reduces its volume, allowing it to be shipped in large quantities. But heat that penetrates the storage tank causes the liquid hydrogen to evaporate into gas, and as pressure inside the tank rises, hydrogen must be vented outside, resulting in losses. Inserting porous materials with nanopores has been discussed as a way to prevent this, but the materials themselves take up space, reducing the tank's total hydrogen storage capacity.
The team overcame this trade-off using a MOF material packed with ultra-fine pores. The researchers compared two types: IRMOF-20, which has a large pore volume and a rigid structure, and MIL-53, whose structure flexes when it adsorbs hydrogen.
In simulations, a virtual tank using IRMOF-20 retained about 97% (96.6%) of the hydrogen storage capacity of an empty tank with no material inside. Meanwhile, based on a vacuum-insulated transport tank of about 56.6 cubic meters, the time required for hydrogen to evaporate and be completely depleted stretched from 64 days to 221 days, more than a threefold increase. Even under low-vacuum conditions, where insulation performance is somewhat weaker, the depletion time also more than tripled, from seven days to 22 days.
Storage capacity barely declined even though the material occupies space inside the tank because hydrogen packs together more densely inside the pores than it does as a liquid. The team estimated the hydrogen density inside the pores at about 82 grams per liter, higher than that of ordinary liquid hydrogen. Neutron scattering experiments also confirmed that the rotational motion of hydrogen inside the pores is restricted, supporting the interpretation that the hydrogen is tightly packed. The team also demonstrated a buffering effect in which hydrogen detaching from the pore walls absorbs heat coming in from outside, delaying the rise in temperature and pressure.
MIL-53, tested alongside it, saw storage capacity fall to about 53%, but showed an ability to hold hydrogen stably up to higher temperatures.
"When you consider pore volume, the ability to pack hydrogen densely and the way adsorbed hydrogen is released together, we showed that inserting a porous material can cut evaporative losses during transport while keeping storage capacity almost intact," Oh said. "But because these results were calculated based on the theoretical maximum performance the material can deliver, further verification is needed to see whether the same effect appears in actual tanks."
UNIST researcher Park Jae-woo took part as first author, with the Institut Laue-Langevin in France and the Helmholtz-Zentrum Berlin in Germany also participating. The work was supported by the National Research Laboratory (NRL 2.0) program run by the Ministry of Science and ICT and the Ministry of Education, as well as the BrainLink program of the National Research Foundation of Korea.
