Understanding the factors that govern activity and selectivity of the electrochemical carbon dioxide reduction reaction on copper catalysts
Keywords: Chemical Engineering.

Coupling electrochemical CO2 reduction (CO2R) with a renewable energy source to create fuels and chemicals is a promising strategy towards achieving a sustainable global energy economy. By recycling carbon dioxide (CO2) into useful products, not only could this reaction serve as a "green" source of traditionally fossil--derived resources, but it could also enable storage of energy from intermittent electricity generators such as wind and solar. However, copper is the only material that has shown a propensity to make highly desired products such as ethylene and ethanol, and even still, it requires large overpotentials (excess energy input) to drive the reaction and generates a variety of products that would necessitate costly separations. Thus, there are still many remaining challenges in improving catalyst efficiency, selectivity, and long-term stability before commercial realization of this CO2 utilization technology. Understanding the surface reactivity of carbon monoxide (CO), a key CO2R intermediate to hydrocarbons and alcohols, is essential for the development of catalysts that can better meet our needs. Therefore, in this work, a custom-designed electrochemical cell was first used to investigate planar polycrystalline copper for CO reduction (COR) under alkaline conditions. There was a significant decrease in the overpotential for multi-carbon (C2+) products under CO reduction conditions compared to CO2 reduction conditions, which we conclude is primarily the result of a pH effect. Further analysis of the reaction products revealed common trends in selectivity that indicate the production of both oxygenated and multi-carbon products are favored at lower overpotentials. These potential-dependent selectivity trends suggested that an increased roughness factor (RF), defined as the ratio of the electrochemically active surface area (ECSA) to the geometric electrode area, could also be used to improve the reaction selectivity in addition to increasing the total activity. In short, since the total current density over the geometric electrode area at a given potential is directly correlated to the total number of active sites, the RF of a material will shift the potentials where the maximum COR/CO2R reaction rates can be achieved before reaching mass transport limitations, which will have a strong impact on the resulting product distribution. To demonstrate this principle, we then synthesized a 3-D Cu nano-flower morphology, tested it for both COR and CO2R, and compared it to our previous results on planar (2-D) Cu. By tuning the surface area of this material such that the reaction rate is optimized in a potential window where C2+ products are favored by each reaction, we have been able to achieve impressive yields for multi-carbon compounds; these products made up nearly 100% of CO converted at --0.23 V vs. the reversible hydrogen electrode (RHE) and 94% of CO2 converted at --0.85 V vs. RHE. Combined, these experimental findings outline key principles for designing electrolyzers that can utilize CO2 and/or CO to make valuable products with high energy efficiency. There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, the electrochemical cell design, etc. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. In the final part of this dissertation, we take a close, comprehensive look at these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regards to research directions and best practices. First, we suggest several techniques and protocols that can allow for more reliable comparisons, enable a more fundamental understanding of the catalysis, and help drive this technology towards commercialization. Then, we describe the experimental probes that have been used in attempts to discern the mechanisms by which products are formed, and following that, we present our current understanding of the complex reaction networks for CO2R on Cu. We then analyze two key methods that have been used to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook and development of electrochemical CO2R technologies.