Researchers from Stanford University and the Technical University of Denmark (DTU) have engineered and demonstrated a solid-oxide electrochemical cell (SOC) with a porous ceria electrode that achieves stable and selective CO2 electrolysis beyond the thermodynamic carbon deposition threshold. A paper on the work is published in Nature Energy. ow electricity and an Earth-abundant catalyst can convert CO2 into energy-rich carbon monoxide (CO) better than conventional methods.
High-temperature CO2 electrolysers offer exceptionally efficient storage of renewable electricity in the form of CO and other chemical fuels, but conventional electrodes catalyse destructive carbon deposition. Ceria catalysts are known carbon inhibitors for fuel cell (oxidation) reactions; however, for more severe electrolysis (reduction) conditions, catalyst design strategies remain unclear. Here we establish the inhibition mechanism on ceria and show selective CO2 to CO conversion well beyond the thermodynamic carbon deposition threshold.
Operando X-ray photoelectron spectroscopy during CO2 electrolysis—using thin-film model electrodes consisting of samarium-doped ceria, nickel and/or yttria-stabilized zirconia—together with density functional theory modelling, reveal the crucial role of oxidized carbon intermediates in preventing carbon build-up. Using these insights, we demonstrate stable electrochemical CO2 reduction with a scaled-up 16 cm2 ceria-based solid-oxide cell under conditions that rapidly destroy a nickel-based cell, leading to substantially improved device lifetime.—Skafte et al.
We showed we can use electricity to reduce CO2 into CO with 100% selectivity and without producing the undesired byproduct of solid carbon.—William Chueh, an associate professor of materials science and engineering at Stanford, one of three senior authors of the paper
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.
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 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.
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.—Christopher Graves, a senior author
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 currently limited to well below 50% CO product concentration in today’s cells. This could also reduce production costs.
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.—Michal Bajdich, a senior author of the paper and an associate staff scientist at the SUNCAT Center for Interface Science & Catalysis
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 project was supported by Haldor Topsoe A/S, Innovation Fund Denmark, the Danish Agency for Science, Technology & Innovation and Energinet.dk., the U.S. Department of Energy, the SUNCAT Center and a National Science Foundation CAREER award.
Theis L. Skafte, Zixuan Guan, Michael L. Machala, Chirranjeevi B. Gopal, Matteo Monti, Lev Martinez, Eugen Stamate, Simone Sanna, Jose A. Garrido Torres, Ethan J. Crumlin, Max García-Melchor, Michal Bajdich, William C. Chueh & Christopher Graves (2019) “Selective high-temperature CO2 electrolysis enabled by oxidized carbon intermediates” Nature Energy doi: 10.1038/s41560-019-0457-4