Stanford University researchers have found a new way to convert carbon dioxide into the building block for sustainable liquid fuels. The new process was very efficient in tests and did not have the reaction that destroys a conventional device.

The Stanford researchers have discovered a practical starting point for converting carbon dioxide recycling it back into sustainable liquid fuels, including fuels for heavier modes of transportation that may prove very difficult to electrify, like airplanes, ships and freight trains.

From left: Christopher Graves, Michal Bajdich and Michael Machala in front of the pulsed laser deposition machine that Machala used to fabricate the electrodes. Image credit: Mark Golden, Stanford University. Click image for the largest view.

Carbon neutral re-use of CO2 has emerged as an alternative idea to sequestering the CO2 underground.

In a new study published in Nature Energy, researchers from Stanford University and the Technical University of Denmark (DTU) show how electricity and an Earth-abundant catalyst can convert CO2 into energy-rich carbon monoxide (CO) better than conventional methods.

The catalyst – cerium oxide – is much more resistant to breaking down. Stripping oxygen from CO2 to make CO gas is the first step in turning CO2 into nearly any liquid fuel and other products, like synthetic gas and plastics. The addition of hydrogen to CO can produce fuels like synthetic diesel and the equivalent of jet fuel. The team envisions using renewable power to make the CO and for subsequent conversions, which would result in carbon-neutral products.

William Chueh, an associate professor of materials science and engineering at Stanford, one of three senior authors of the paper said, “We showed we can use electricity to reduce CO2 into CO with 100 percent selectivity and without producing the undesired byproduct of solid carbon.”

Chueh, aware of DTU’s research in this area, invited Christopher Graves, associate professor in DTU’s Energy Conversion & Storage Department, and Theis Skafte, a DTU doctoral candidate at the time, to come to Stanford and work on the technology together.

Skafte, lead author of the study, who is now a postdoctoral researcher at DTU said, “We had been working on high-temperature CO2 electrolysis for years, but the collaboration with Stanford was the key to this breakthrough. We achieved something we couldn’t have separately – both fundamental understanding and practical demonstration of a more robust material.”

One advantage sustainable liquid fuels could have over the electrification of transportation is that they could use the existing gasoline and diesel infrastructure, like engines, pipelines and gas stations. Additionally, the barriers to electrifying airplanes and ships – long distance travel and the high weight of batteries – would not be problems for energy-dense, carbon-neutral fuels.

Although plants reduce CO2 to carbon-rich sugars naturally, an artificial electrochemical route to CO has yet to be widely commercialized. Among the problems: Devices use too much electricity, convert a low percentage of CO2 molecules, or produce pure carbon that destroys the device. Researchers in the new study first examined how different devices succeeded and failed in CO2 electrolysis.

With the insights gained, the researchers built two cells for CO2 conversion testing: one with cerium oxide and the other with conventional nickel-based catalysts. The ceria electrode remained stable, while carbon deposits damaged the nickel electrode, significantly shortening the catalyst’s lifetime.

DTU’s Graves, a senior author of the study and visiting scholar at Stanford at the time said, “This remarkable capability of ceria has major implications for the practical lifetime of CO2 electrolyzer devices. Replacing the current nickel electrode with our new ceria electrode in the next generation electrolyzer would improve device lifetime.”

Eliminating early cell death could significantly lower the cost of commercial CO production. The suppression of carbon buildup also allows the new type of device to convert more of the CO2 to CO, which is limited to well below 50 percent CO product concentration in today’s cells. This could also reduce production costs.

Michal Bajdich, a senior author of the paper and an associate staff scientist at the SUNCAT Center for Interface Science & Catalysis, a partnership between the SLAC National Accelerator Laboratory and Stanford’s School of Engineering explained, “The carbon-suppression mechanism on ceria is based on trapping the carbon in stable oxidized form. We were able to explain this behavior with computational models of CO2 reduction at elevated temperature, which was then confirmed with X-ray photoelectron spectroscopy of the cell in operation.”

The high cost of capturing CO2 has been a barrier to sequestering it underground on a large scale, and that high cost could be a barrier to using CO2 to make more sustainable fuels and chemicals. However, the market value of those products combined with payments for avoiding the carbon emissions could help technologies that use CO2 overcome the cost hurdle more quickly.

The researchers hope that their initial work on revealing the mechanisms in CO2 electrolysis devices by spectroscopy and modeling will help others in tuning the surface properties of ceria and other oxides to further improve CO2 electrolysis.

This is very good news indeed. It should be considered a breakthrough. The prospect of the energy economy entering into a fast carbon cycle is a true long term goal of immense importance. The adoption, production and marketing has a couple problems though. Research is pretty thin in carbon capture technologies. But carbon capture would seem to be an easier target than hydrogen storage. At least CO2 can be compressed, transported and stored long term, something hydrogen simply defies. The other problem is “payments for avoiding the carbon emissions” that seems to be an indirect way to say “carbon tax”. Tax equals bureaucrats, regulations and rules that would drive up all carbon fuel costs. The other result is once a support is obtained the motive for continued research ceases to exist. More fuel looks possible here, better and cheaper needs more work.

Other Stanford co-authors are PhD alumnus Zixuan Guan, postdoc Michael Machala, former postdocs Matteo Monti and Chirranjeevi B. Gopal, and SLAC postdoc Jose A. Garrido Torres. Other DTU co-authors are PhD candidate Lev Martinez, nanolab group leader Eugen Stamate and staff researcher Simone Sanna. The other co-authors are Ethan J. Crumlin, a research scientist at Lawrence Berkeley National Laboratory, and Max Garcia Melchor, an assistant professor at Trinity College, Dublin.

This project was supported by Haldor Topsoe A/S, Innovation Fund Denmark, the Danish Agency for Science, Technology & Innovation and, the U.S. Department of Energy, the SUNCAT Center and a National Science Foundation CAREER award.


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