A group of chemists led by Dr. John Ripmeester, Program Leader of the Materials Structure and Function Group at the Canadian National Research Council’s Steacie Institute for Molecular Sciences (NRC-SIMS), have developed a technique to pack twice as many hydrogen molecules into a gas hydrate framework than was previously possible.
Gas hydrates that contain hydrogen require pressures of 25 atmospheres and temperatures at 0 degrees Celsius to remain stable. In that state, the typical gas hydrate contains approximately 2% hydrogen per volume of hydrate.
“The target storage capacity is 5 to 8% hydrogen to make a useful fuel storage medium,” said Dr. Ripmeester. “We made it up to 5%. We’re not completely there, but we are approaching the standard.”
Despite their minimal storage capacity, gas hydrates are an attractive hydrogen source for fuel cells. They are not explosive and require less refrigeration and pressure to store than liquid hydrogen, making them easier to transport. Hydrates are mostly water, an inexpensive alternative to most other hydrogen-containing chemicals.
To pump up the volume of hydrogen in gas hydrates, the NRC-SIMS researchers added small amounts of tetrahydrofuran (THF) a common solvent in the manufacturing of adhesives, plastics, magnetic tape and pharmaceuticals. The THF molecules stabilise the hydrate structure enabling it to carry twice as many hydrogen molecules.
Now that they are close to the recommended 5 to 8% hydrogen content, the NRC-SIMS team is working to speed up the chemical reaction. Loading and unloading that much hydrogen into the hydrate structure can take days to weeks.
The group is experimenting with coating tiny silica beads with hydrate to increase surface area and speed up the chemical reaction. So far, they’ve managed to reduce hydrogen loading time down to a few hours. Not yet feasible for real-world applications, but they’re getting there.
Although hydrates were first synthesized in the lab in the early 1800’s, hydrate deposits were not discovered in nature until the 1960’s. Natural gas hydrates, most of which contain methane, form under permafrost and in the ocean on continental margins where water temperature and pressure are just right.