Many believe that hydrogen is the eventual replacement for gasoline and that future vehicles will be fuel cell-based plug-in hybrids. However, in order to transition to such a hydrogen-based economy, many key challenges remain. The biggest challenges are devising and implementing means to make, store, and ship hydrogen to distribution centers.
One of the key challenges in making hydrogen is the need for purification. Many chemical reactions that produce hydrogen also produce a mixture of hydrocarbon gases and water vapor. In the past separating these substances has been a tricky and inefficient process.
Now chemists at Northwestern University have developed a class of porous materials that may solve this problem by letting hydrogen gas through selectively, while impeding other gases. According to the researchers, the materials exhibit the best known selectivity towards hydrogen over methane and carbon dioxide of any known material.
Mercouri G. Kanatzidis, a professor of chemistry at the university and co-developer of the material, states, "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."
While current separation techniques rely on separating molecules by size, the new porous membrane material separates them by polarizability. The new membrane, composed of germanium, lead and tellurium, lets hydrogen through faster, as it is a hard, small molecule which interacts little with the charged walls. The membrane is a hexagonal nanoporous structure, with parallel tubes about two to three nanometers wide. The gas molecules are at least half a nanometer wide. The membrane selects hydrogen at a rate approximately four times higher than the current best methods.
Professor Kanatzidis describes the material stating, "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 membrane operates within a "convenient temperature range" of zero degrees Celsius and room temperature.
Professor Kanatzidis worked closely with postdoctoral research associate Gerasimos S. Armatas on developing and testing the material. The pair has published a paper entitled "Mesoporous Germanium-Rich Chalcogenido Frameworks with Highly Polarizable Surfaces and Relevance to Gas Separation". It is published online at the journal Nature Materials.