|Crystalline sheets produced in covalent organic frameworks (COFs). Credit: Adrien Côté|
Researchers at the University of Michigan have created a class of lightweight, rigid polymers that could serve as a new type of lightweight storage for hydrogen gas for use in fuel cell vehicles. The results appear in a paper in the 17 November issue of Science.
The trick to making the new materials, called covalent organic frameworks (COFs), is coaxing them to assume predictable crystal structures—something that had not been done before with rigid plastics.
Normally, rigid plastics are synthesized by rapid reactions that randomly cross-link polymers. Just as in anything you might do, if you do it really fast, it can get disorganized.—Adrien Côté, first author on the Science paper
Côté and colleagues tweaked reaction conditions to slow down the process, allowing the materials to crystallize in an organized fashion instead of assembling haphazardly. As a result, the researchers can use X-ray crystallography to determine the structure of each type of COF they create, thereby quickly assessing its properties. The methodology allows them then to go back and modify the COF, improving its performance or tailoring it for different applications.
Côté collaborated on the work with Omar Yaghi, who is the Robert W. Parry Collegiate Professor of Chemistry at U-M. Over the past 15 years, Yaghi has taken a similar approach to producing materials called metal-organic frameworks (MOFs).
On the molecular level, MOFs are scaffolds made up of metal hubs linked together with struts of organic compounds. By carefully choosing and modifying the chemical components used as hubs and struts, Yaghi and his team have been able to define the angles at which they connect and design materials with the properties they want.
Like MOFs, COFs can be made highly porous to increase their storage capacity. But unlike MOFs, COFs contain no metals. Instead, they’re made up of light elements—hydrogen, boron, carbon, nitrogen and oxygen—that form covalent bonds with one another.
Using light elements allows you to generate lightweight materials. That’s very important for hydrogen fuel storage, because the lighter the material, the more economical it is to transport around in a vehicle. The strong covalent bonds also make COFs very robust materials.—Adrien Côté
Although the main thrust of the current research is creating materials for hydrogen storage for use with fuel cells, Côté, Yaghi and colleagues also are exploring variations of COFs that might be suitable for use in electronic devices or catalytic applications.
Côté and Yaghi collaborated on the research with assistant professor of chemistry Adam Matzger and graduate students Annabelle Benin and Nathan Ockwig, all of U-M, and Michael O'Keeffe of Arizona State University. The work was funded by the National Science Foundation, the U.S. Department of Energy and the Natural Sciences and Engineering Research Council of Canada.
Porous, Crystalline, Covalent Organic Frameworks, Science 18 November 2005; Vol. 310. no. 5751, pp. 1166–1170; DOI: 10.1126/science.1120411