Northwestern University chemist Mercouri G. Kanatzidis and postdoctoral research associate Gerasimos S. Armatas, have developed a class of new porous materials, structured like honeycomb, that is very effective at separating hydrogen from complex gas mixtures. Called mesoporous materials, so far as the best of the researchers’ knowledge allows, the materials exhibit the best selectivity in separating hydrogen from carbon dioxide and methane.

Hydrogen needs to be purified before it can be used as fuel for fuel cells, but current methods are not very clean or efficient. That makes cleaning one of the roadblocks to the success of the hydrogen itself to be a viable fuel.

The mesoporous materials offer a new way to separate gases not available before. The scientist’s research paper was published online Feb. 15 by the journal Nature Materials. The materials are from a new family of germanium-rich chalcogenides.

Professor Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry in the Weinberg College of Arts and Sciences and the paper’s senior author said, “A more selective process means fewer cycles to produce pure hydrogen, increasing efficiency. Our materials could be used very effectively as membranes for gas separation. We have demonstrated their superior performance.”

The current method of producing hydrogen first yields hydrogen combined with carbon dioxide or hydrogen combined with carbon dioxide and methane. The technology currently used for the next step — removing the hydrogen from such mixtures, which is difficult to do, separates the gas molecules based on their size.

The Kanatzidis and Armatas research is said to offer a better solution. The basis of the new separation method is the new materials do not rely on size for separation but instead rely on polarization — the interaction of the gas molecules with the walls of the material as the molecules move through the membrane.

Testing one form of the family of materials, one composed of the heavy elements germanium, lead and tellurium showed it to be approximately four times more selective at separating hydrogen from carbon dioxide than conventional methods, which are made of lighter elements, such as silicon, oxygen and carbon.

Kanatzidis says, “We are taking advantage of what we call ‘soft’ atoms, which form the membrane’s walls. These soft-wall atoms like to interact with other soft molecules passing by, slowing them down as they pass through the membrane. Hydrogen, the smallest element, is a ‘hard’ molecule. It zips right through while softer molecules, like carbon dioxide and methane take more time.”

The testing was done on a complex mixture of four gases. Hydrogen passed through first, followed in order by carbon monoxide, methane and carbon dioxide. As the smallest and hardest molecule, hydrogen interacted the least with the membrane, and carbon dioxide, as the softest molecule of the four, interacted the most.

Here is an important second advantage – the process takes place at what Kanatzidis calls a “convenient temperature range” – between zero degrees Celsius and room temperature.

The scientists note that small-molecule diffusion through porous materials is a nanoscopic phenomenon. All the pores in the hexagonal honeycomb structure are ordered and parallel, with each hole approximately two to three nanometers wide. The gas molecules are all at least half a nanometer wide.

This is highly useful research, as hydrogen may become a high potential fuel. That “It zips right through” (practically everything) property has had the hydrogen storage business frustrated for years. Perhaps there is a little insight to be gained from the development of these new materials.


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