University of Nebraska-Lincoln chemist Jian Zhang and his colleagues have synthesized new materials that show promise for energy applications aimed at reducing greenhouse gas emissions.
What makes them special?
They belong to an emerging family of materials called metal-organic frameworks, or MOFs, which feature metallic atom-containing clusters that are linked by carbon atoms. Because their orderly, three-dimensional arrangements have many pores and a large surface area, MOFs are becoming prime candidates for storing gases that include hydrogen, methane and carbon dioxide.
So the pores allow the gases in… But why does the surface area matter?
It’s crucial to overcoming maybe the greatest challenge of harnessing a gas such as hydrogen, which has an energy-per-volume output roughly three times lower than gasoline’s. Under normal conditions, this means that a hydrogen tank must be about three times the size of a gas tank to fuel a vehicle the same distance, for example.
Though high-pressure tanks help address this concern, they’re also heavy and expensive. MOFs are pointing the way toward a better solution by employing a phenomenon called adsorption, whereby hydrogen or other gaseous molecules adhere to their surfaces. The larger the framework’s surface area, the more molecules it can accommodate – and the more molecules it can accommodate, the more densely it can store a gas.
One of the UNL team’s materials, which it named Nebraska Porous Framework 200, or NPF-200, boasts the third-highest surface area of any zirconium-based MOF. And the extremely small diameter of NPF-200’s pores – less than two nanometers, the length that fingernails grow in two seconds – might further boost its storage capacity.
“With a larger pore, you kind of waste some of the void space,” said Zhang, assistant professor of chemistry. “I think that is one of the potential advantages of NPF-200.”
Is there anything else that sets it apart?
Zhang’s team also found that NPF-200 exhibits a never-before-seen MOF topology, which essentially describes the geometric configuration of its building blocks.
“The topology is an important factor to dictate the stability and performance of MOFs in terms of gas adsorption,” Zhang said. “You can imagine two different materials: Although they have the same surface area, the gas adsorption behavior can be completely different (depending on the topology).
“New topologies are not very easy to discover right now for zirconium MOFs, but they’re a fundamental and important factor for potential applications.”
Why did the team use zirconium?
Like most emerging technologies, MOFs face some challenges of their own. Those include stability: Many MOFs are sensitive to water, breaking down when exposed to too much or for too long.
“That is actually, in my opinion, the bottleneck in the development of MOF materials,” Zhang said.
Zirconium atoms have an especially large positive charge, allowing them to bond more strongly with a MOF’s organic links than do other metallic options such as copper. Consequently, the team found that NPF-200 held up well against water, along with both acidic and basic liquids.
“Our MOFs are much, much more stable (than many others),” Zhang said.
Where can I get more information?
Check out the Journal of the American Chemical Society, which recently published a study describing the team’s work. Zhang authored that study with UNL doctoral students Xin Zhang and Jacob Johnson, postdoctoral researcher Xu Zhang, and the University of Chicago’s Yu-Sheng Chen.