Twice-stuffed permafrost

So far, so good, but this is just hydrogen in the lattice, not hydrogen placed inside it. “This is an important distinction,” Xantheas says. “One kind of hydrogen is very strongly bound to oxygen as part of the water molecule that forms the cage and it remains this way. Then there is the molecular form, H2, the guest molecule inside the cage: two hydrogen atoms connected that goes in and out of the cages. This can be used as a fuel.”

Xantheas and Willow built computer models packing this hydrogen gas inside the 3-D lattice. The simulations showed hydrogen escapes, reducing the cages’ storage capacity.

The explanation lies in the cages’ soccer ball shapes. Ones made of 20 water molecules have 12 pentagonal faces; the 24-water cage has 12 pentagons and two hexagons stitched together. The simulations showed molecular hydrogen mainly leaks through the hexagonal faces “because they are just too big,” Xantheas says. Other researchers tried using a larger molecule, such as tetrahydrofuran (THF), to keep hydrogen in, but that reduces storage capacity. “THF is a body by the door, blocking the exit, but it takes up space that ideally could go to hydrogen molecules.”

The PNNL team spurned THF. Since most clathrates contain methane the researchers wanted to swap for CO2, they instead added a methane molecule and found it kept hydrogen from escaping.

But they also were surprised to observe another low-energy way molecular hydrogen could hop between adjacent cages: a single hydrogen bond in the lattice breaks and allows one water molecule to rotate, opening a gate that closes as soon as the hydrogen passes through.

“The gate closes and the whole lattice is reformed and not destroyed,” Xantheas says. A computational experiment found the “water gate” also opens when the cage holds methane, suggesting it, too, can hop between adjacent clathrates without destroying the scaffold.

“We weren’t aware of this at all coming into the simulations,” he says. “This is a significant finding to see that this occurs in the natural world, and hopefully it puts other researchers on the path of future discoveries.” The PNNL team will follow that path by next trying to simulate exchanges between carbon dioxide and methane in the cages.

The discoveries are intriguing, but Xantheas says big, difficult challenges must be addressed before hydrogen becomes a viable fuel, including achieving system stability near ambient conditions while attaining higher storage capacity. “Even if you find a viable storage system and infrastructure, the economics will take over. But gas hydrates address two of the most environmentally important issues: alternative fuels and developing a safe system to sequester carbon dioxide. They are fascinating systems that show a lot of promise.”