The good thing about the global climate crisis we all face is more and more researchers are coming up with new inventions that cut carbon emissions or use CO2 to our advantage. Fuel cells turn chemicals into electricity; now a University of Toronto engineering team has developed a way to harness electricity to make valuable chemicals from waste carbon, or CO2. A kind of reverse fuel cell.
Professor Ted Sargent was a co-author of the research paper recently published in Science Daily:
“For decades, talented researchers have been developing systems that convert electricity into hydrogen and back again. Our innovation builds on that legacy, but by using carbon-based molecules, we can plug directly into existing hydrocarbon infrastructure.”
Hydrogen and oxygen come together on the surface of a catalyst in hydrogen fuel cells. This causes a chemical reaction that releases electrons that are then captured by specialized materials in the fuel cell and pumped into a circuit. A very different family of device, the microbial fuel cell, draws its electrons from bacteria digesting organic waste instead.
The devices performing opposite functions to fuel cells are electrolyzers; they use electricity to drive chemical reactions. The scientists who worked on this project are experts in designing electrolyzers that convert CO2 into different carbon-based materials, like ethylene.
Co-author and Ph.D. candidate Joshua Wicks explains ethylene:
“Ethylene is one of the most widely produced chemicals in the world. It’s used to make everything from antifreeze to lawn furniture. Today it is derived from fossil fuels, but if we could instead make it by upgrading waste CO2, it would provide a new economic incentive for capturing carbon.”
The study explains that the electrolyzers used today do not produce enough ethylene to compete with what is derived from fossil fuels. So, the team was challenged with the unique nature of the particular chemical reaction to transform CO2 into ethylene and other carbon-based molecules.
Adnan Ozden is another Ph.D. candidate who worked under Sargent on the research team who talks about the reaction requirements:
“The reaction requires three things: CO2, which is a gas; hydrogen ions, which come from liquid water; and electrons, which are transmitted through a metal catalyst. Bringing those three different phases — especially the CO2 — together quickly is challenging, and that is what has limited the rate of the reaction.”
During the experiments, the team needed to overcome the challenge of bringing the reactants together. The catalyst in the new electrolyzer is composed of small particles that are embedded in a layer of Nafion.
This ionomer or polymer conducts charged particles known as ions. Nafion is commonly used in fuel cells to transport positively charged hydrogen (H+) ions around within the reactor.
F. Pelayo García de Arquer is a postdoctoral fellow who explains the use of Nafion:
“In our experiments, we discovered that a certain arrangement of Nafion can facilitate the transport of gases such as CO2. Our design enables gas reactants to reach the catalyst surface fast enough and in a sufficiently distributed manner to significantly increase the rate of reaction.”
Once the team was able to successfully speed up the reaction, they were able to transform CO2 into ethylene and other products 10 times faster. They did this without increasing the cost or reducing the efficiency of the reverse fuel cell reactor.
This is a great discovery by the team, but they are still a long way off from being able to produce a reverse fuel cell commercially. Cao-Thang Dinh is a professor of chemical engineering at Queen’s University who helped out on the project.
“We can pump in electrons 10 times faster, which is great, but we can only operate the system for about ten hours before the catalyst layer breaks down,” said Dinh. “This is still far from the target of thousands of hours that would be needed for industrial application.”
The team said these same principles can be applied to the synthesis of valuable chemicals like ethanol.
García de Arquer points out:
“Even if we stop using oil for energy, we are still going to need all of these molecules. If we can produce them using waste CO2 and renewable energy, we can have a major impact in terms of decarbonizing our economy.”
