Graphene quantum dots hold promise in reducing waste CO2

Graphene quantum dots may offer a simple way to recycle waste carbon dioxide into valuable fuel rather than release it into the atmosphere or bury it underground, according to new research published by an international collaboration of scientists.

“If we can convert a sizable fraction of the carbon dioxide that is emitted, we could curb the rising levels of atmospheric CO2 levels which have been linked to climate change,” said Paul Kenis, William H. & Janet G. Lycan Professor and head of the Department of Chemical and Biomolecular Engineering at the University of Illinois at Urbana-Champaign.

Nitrogen-doped graphene quantum dots seen under a transmission electron microscope prove to be adept at catalyzing carbon dioxide into complex hydrocarbons, according to University of Illinois and Rice University scientists. (Credit: Ajayan Group/Rice University)
The findings are detailed this week in Nature Communications.

The project brought together the expertise of Kenis, an authority in electrochemical systems for carbon dioxide conversion and fuel cells, and Pulickel Ajayan of Rice University, a pioneer in the development of nanostructured materials for applications in energy storage, nanoelectronics, imaging, and sensors. The team also included graduate and undergraduate students, plus researchers from the Institute for Carbon-Neutral Energy Research in Japan, the Shanghai Institute of Microsystem and Information Technology and the Saudi Basic Industries Corp.’s U.S. offices.

Nitrogen-doped graphene quantum dots (NGQDs) are an efficient electrocatalyst to make complex hydrocarbons from carbon dioxide, they reported. At Ajayan’s and Kenis’ labs, researchers turned the greenhouse gas into small batches of ethylene and ethanol. Although they don’t entirely understand the mechanism yet, the researchers found NGQDs were as efficient as copper, which has been widely tested as a catalyst to reduce carbon dioxide into liquid fuels and chemicals. NGQDs keep their catalytic properties longer, they found.

“It is surprising that the activity of the metal-free catalyst is as good as the state-of-the-art copper catalyst. This is a good example of how research into CO2 utilization, converting carbon dioxide into usable products, has benefitted from a collaboration involving researchers from all over the world,” said Sichao Ma, a member of Kenis’ research group who received his PhD in chemistry in July 2016. Ma is currently a senior scientist at Opus 12, Inc.

In lab tests, the material proved able to reduce carbon dioxide by up to 90 percent, and convert 45 percent into either ethylene or alcohol, comparable to copper electrocatalysts.

“I think what we found is fundamentally interesting, because if we can extract and convert carbon dioxide at the source, it may be a good way to solve some of our problems,” said Ajayan, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering. Atmospheric carbon dioxide rose above 400 parts per million earlier this year, the highest it’s been in at least 800,000 years, as measured through ice-core analysis.

Graphene quantum dots are atom-thick sheets of carbon atoms that have been split into particles about a nanometer thick and just a few nanometers wide. Nitrogen atoms introduced to the dots attach at various points, enabling varying chemical reactions when an electric current is applied and a feedstock like carbon dioxide is introduced.

“Carbon is typically not a catalyst,” Ajayan said. “One of our questions is why this doping is so effective. When you put nitrogen into the graphitic lattice, there are multiple positions. Each of these positions, depending on where nitrogen sits, should have different catalytic activity. So it’s been a puzzle, and though people have written a lot of papers in the last five to 10 years on doped and defective carbon being catalytic, the puzzle is not really solved.”

“Our findings suggest that the pyridinic nitrogen sitting at the edge of graphene quantum dots leads the catalytic conversion of carbon dioxide to hydrocarbons,” Wu said. “The next task is further increasing nitrogen density to help increase the yield of hydrocarbons.”

In related research published in ChemSusChem, Kenis and colleagues reported the characterization of a N-doped carbon nitride catalyst for electrochemical reduction of carbon dioxide to carbon monoxide (which is used by the chemical industry to manufacture a variety of chemicals) in an electrochemical flow cell. The pyrolyzed carbon nitride and multiwall carbon nanotube composite outperformed silver, which has been the catalyst of choice for CO production. That project involved Andrew Gewirth, professor of chemistry at Illinois, former UI graduate students Huei-Ru Molly Jhong and Claire Tornow, as well as Stephen Lyth and Bretislav Smid from the Institute for Carbon-Neutral Energy Research, Fukuoka, Japan.

Research into the electrochemical reduction of CO2 into useful chemicals should be of interest to a variety of industries, most notably energy companies, which produce CO2, but also chemical companies looking to become less reliant on fossil fuels in their processes.

“To test the various catalysts, we use electrolysis cells at lab scale. Electrolysis is already being performed at scale for other purposes, for example for the production of chlorine from sodium chloride. It has just not been done for CO2 reduction yet,” Kenis said.

Co-lead authors of the Nature Communications paper are Jingjie Wu of Rice University, Sichao Ma of the University of Illinois and the Institute for Carbon-Neutral Energy Research, and Jing Sun of the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. Co-authors include Paul Kenis and graduate student Byoungsu Kim of the University of Illinois and the Institute for Carbon-Neutral Energy Research; Jake Gold, Raymond Luo and Aaron Yu, undergraduates in Chemical and Biomolecular Engineering at the University of Illinois; Lingyang Zhu, spectroscopist at the University of Illinois; Pulickel Ajayan and Chandra Sekhar Tiwary of Rice; Nitin Chopra and Ihab Odeh of the Saudi Basic Industries Corp., Sugar Land, Texas; Robert Vajtai, a senior faculty fellow in materials science and nanoengineering at Rice; Jun Lou a professor of materials science and nanoengineering at Rice, and Guqiao Dingof the Chinese Academy of Sciences.

Contact: Paul Kenis, Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, [email protected]

Written by the University of Illinois and Rice University.

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