Gas storage plays an essential role in both carbon sequestration and hydrogen-fuelled cars, two of the most touted green technologies. To date, however, most proposed methods for storing gases in these applications have been dismissed as too inefficient, or impractical because of the extreme physical conditions they require. Now, new research in Europe and the US could pave the way for a promising solution that would involve locking gases away in an ice-like structure known as a "hydrate".

Channelling vast quantities of carbon dioxide into deep underground bunkers may sound like a rather crude way of meeting international emissions targets, but many governments around the world are investing significant resources in trying to develop this technology. Indeed, Steven Chu, the US Energy Secretary, has recently called for carbon capture and storage (CCS) to be ready for widespread deployment within 10 years. Another technology that requires gas storage is the automobile powered by hydrogen.

Current techniques cost too much money and require too much energy to be practical for these applications.

Like (but not the same as) ice

Felix Lehmkühler at the TU Dortmund and his colleagues are exploring an alternative approach to gas storage that could solve some of these problems. They are interested in storing gas by combining it with water to form a hydrate – a solid similar to ice.

Unlike standard ice, a hydrate crystal offers cavities where other atoms or molecules can be stored and then easily released when the substance is heated. These cavities are abundant but limited in size, thus only small molecules such as hydrogen (H2) or carbon dioxide (CO2) can be guests. Researchers have long since noted the potential of hydrates for gas storage but the high pressures and low temperatures required for their formation has prevented their application.

In recent years, Lehmkühler and a number of other researcher groups have come to realize that stability of hydrates can be held at much lower pressures if other substances are added to the water cage. In particular, a hydrate holding both an organic liquid known as Tetrahydrofuran (THF) and hydrogen (H2) can remain stable at 50 bar, whereas a pure hydrogen hydrate can only remain stable when the pressure is 2000 bar.

Take it to the synchrotron

In this latest research, Lehmkühler and his team study the formation process of gas hydrates at the molecular level, as the fundamental science is still poorly understood.

"Despite over 150 years of hydrate research, the microscopic mechanisms of hydrate formation from the gas-aqueous-ice phases are still not clear," says Saman Alavi, a hydrate researcher at the Steacie Institute for Molecular Sciences in Canada, who was not involved in this latest research.

In a series of experiments carried out at the European Synchrotron Radiation Facility (ESRF) in France, they investigated the formation of THF hydrate. The intense X-ray beam provided the researchers with a tool to examine the formation of hydrates using the method of X-ray Raman scattering. What they found is that the hydrate formation process differs significantly from nucleation of standard ice. "A detailed knowledge of the hydrate formation process on molecular length scales can help tune hydrates for storage," says Lehmkühler.

"This research provides fundamental understanding of the processes leading to hydrate formation," says Carolyn Koh, a hydrates researcher at the Colorado School of Mines in the US. "The combination of theory, simulation, and experiment is important in advancing our understanding of hydrate nucleation, which is a key step towards the synthesis and manufacture of hydrate storage materials.

Experiments have already been performed where liquid carbon dioxide – pumped to the ocean floor – forms hydrates but it is not clear how stable these structures are due to our lack of basic understanding. What is more, changes in water temperature – e.g. due to climate change – may accelerate the hydrate breakdown and release of carbon dioxide back into the atmosphere.