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Stanford team develops copper catalyst for increased selectivity of production of ethanol via electroreduction of CO2

Researchers at Standford University have designed large-format, thin-film copper catalysts for the electroreduction of CO2 to ethanol. The results are published in Proceedings of the National Academy of Sciences.

“One of our long-range goals is to produce renewable ethanol in a way that doesn’t impact the global food supply. Copper is one of the few catalysts that can produce ethanol at room temperature,” he said. “You just feed it electricity, water and carbon dioxide, and it makes ethanol. The problem is that it also makes 15 other compounds simultaneously, including lower-value products like methane and carbon monoxide. Separating those products would be an expensive process and require a lot of energy,” said study principal investigator Thomas Jaramillo, an associate professor of chemical engineering at Stanford and of photon science at the SLAC National Accelerator Laboratory.

The electrochemical reduction of CO2 (CO2R) is a process that could couple to renewable energy from wind and solar to directly produce fuels and chemicals in a sustainable manner. However, developing catalysts is a major challenge for this reaction, and significant advances are needed to overcome the issues of poor energy efficiency and product selectivity. One reason for these issues is that there are a limited number of catalysts that can effectively convert CO2 to products that require more than two electrons (>2e products), e.g., methane, methanol, ethylene, etc.. Therefore, developing catalysts that are effective for CO2R to >2e products would greatly improve prospects for utilization, and such an endeavor requires a deeper understanding of the relevant surface chemistry.

Out of the polycrystalline metals, Cu is the only one that has shown a propensity for CO2R to >2e products at considerable rates and selectivity. To date, its uniqueness is reflected by how nearly all work on catalysts with improved activity and selectivity for >2e products is based on Cu. However, poly-crystalline Cu is not particularly selective toward any one >2e reduction product. Thus, it is critical to understand what active site motifs lead to this unique selectivity for further reduced products and to apply this knowledge to develop new materials with this electrocatalytic behavior.

… In this study, we use electron-beam (e-beam) deposition to epitaxially grow large-format single-crystal analogous Cu thin films on Si and Al2O3 single crystals. After growth, a combination of X-ray pole figures and electrochemical scanning tunneling microscopy are used to correlate the bulk and in situ surface structures. After physical characterization, we use our previously reported electrochemical flow cell design with high product detection sensitivity to examine the dependence of CO2R activity and selectivity on surface structure. Using these results, we confirm the dependence of C–C coupling on surface structure, and provide insights on surface motifs that govern selectivity between >2e oxygenates and hydrocarbons.

—Hahn et al.

For the PNAS study, the Stanford team chose three samples of crystalline copper, known as copper (100), copper (111) and copper (751). Scientists use these numbers to describe the surface geometries of single crystals.

Copper (100), (111) and (751) look virtually identical but have major differences in the way their atoms are arranged on the surface. The essence of our work is to understand how these different facets of copper affect electrocatalytic performance.

—Christopher Hahn, co-lead lead author of the study

In previous studies, scientists had created single-crystal copper electrodes just 1-square millimeter in size. For this study, Hahn and his co-workers at SLAC developed a novel way to grow single crystal-like copper on top of large wafers of silicon and sapphire. This approach resulted in films of each form of copper with a 6-square centimeter surface, 600 times bigger than typical single crystals.

To compare electrocatalytic performance, the researchers placed the three large electrodes in water, exposed them to carbon dioxide gas and applied a potential to generate an electric current.

The results were clear. When the team applied a specific voltage, the electrodes made of copper (751) were far more selective to liquid products, such as ethanol and propanol, than those made of copper (100) or (111); Cu(751) had the highest oxygenate/hydro-carbon ratio of the three Cu orientations.

Ultimately, the Stanford team would like to develop a technology capable of selectively producing carbon-neutral fuels and chemicals at an industrial scale.

The eye on the prize is to create better catalysts that have game-changing potential by taking carbon dioxide as a feedstock and converting it into much more valuable products using renewable electricity or sunlight directly,” Jaramillo said. “We plan to use this method on nickel and other metals to further understand the chemistry at the surface. We think this study is an important piece of the puzzle and will open up whole new avenues of research for the community.

—Tom Jaramillo

Funding was provided by the US Department of Energy and the Stanford Global Climate and Energy Project.

Resources

  • Christopher Hahn, Toru Hatsukade, Youn-Geun Kim, Arturas Vailionis, Jack H. Baricuatro, Drew C. Higgins, Stephanie A. Nitopi, Manuel P. Soriaga, and Thomas F. Jaramillo (2017) “Engineering Cu surfaces for the electrocatalytic conversion of CO2: Controlling selectivity toward oxygenates and hydrocarbons,” PNAS 114 (23) 5918-5923 doi: 10.1073/pnas.1618935114

Comments

SJC

Excellent work, CO2 to ethanol could be VERY useful.

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