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Researchers Create Highly Porous Material to Store Energy

Many people have envisioned a future in which we produce less carbon dioxide by using different energy sources or capturing the byproducts of old ones, like driving hydrogen cars or capturing CO₂ from coal-fired power plants.

But before this reality is possible, researchers must find an efficient way to store these materials. Hydrogen, for example, is an extremely light gas, so it’s difficult to store enough on a vehicle to power it 300 miles at a time between fill ups.

One answer is storing energy at the molecular level, in materials called metal-organic frameworks (MOFs). Researchers at Northwestern University, in collaboration with researchers at the University of California Los Angeles and Soongsil University in South Korea, have created new kinds of MOFs that have the highest surface area per unit mass – or highest amount of space able to store energy – ever produced. Their results were recently published in the journal Science.

Randy Snurr, professor of chemical and biological engineering at Northwestern’s McCormick School of Engineering and co-author of the paper, describes MOFs as similar to Tinker Toys – they have corners made up of metallic ions and organic “linkers” in between. Using this framework, researchers can vary the material that acts as the linker and make different shapes and structures – all of which are nearly empty inside.

These materials show promise in everything from energy storage to sensors to drug delivery, but for Snurr, who is interested in energy storage, creating MOFs with a high surface area is important because energy molecules like to adsorb on the surface.

“The material acts like a sponge, allowing you to store more gas in the tank,” he says.

To create this new kind of MOF, researchers at UCLA increased the size of the linkers. The process is tricky, though, since MOFs are made in solutions, and in order to make them ready for storage, researchers must remove all of the solvent molecules from inside the MOFs. So when the UCLA researchers measured the surface area of their new MOF, they found it was good, but not great.

So Snurr and his research group performed computational modeling that showed the surface area could theoretically be much higher. To do this, Snurr uses both geometric calculations and computationally intensive calculations based on statistical mechanics that can predict both the surface area and the amount of gas that could be adsorbed.

Using Snurr’s calculations as a guideline, UCLA researchers were able to adjust their experiments and create an MOF with a surface area of over 6000 meters squared per gram, which is “enormous,” Snurr says.  For context, this is about the area of a soccer field in something the weight of a paper clip.
“It’s a nice illustration of the role that modeling can play in developing these materials,” Snurr says.

Next Snurr and his group hope to introduce new linkers into the MOFs that make them more functional – perhaps to more easily bind to CO₂.

Hydrogen cars and capturing CO₂ with MOFs are not yet ready for commercial use – hydrogen binds easily to the MOFs at low temperatures, but not so easily at room temperatures, and to capture CO2, the MOF has to be able to selectively adsorb it out of exhaust without also adsorbing water, nitrogen, and other impurities.

Nevertheless, Snurr and his group are focused on making MOF energy applications a reality.

“I like working on problems like this that are important,” he says. “It provides real motivation for graduate students who feel that they can make a difference in the world.”

Read about it in Chemical & Engineering News.