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(730h) Electrochemical CO2 Conversion to Formic Acid: Optimization of Production & Efficiency Via Operating Voltage Tuning and pH Regulation

Thompson, J. - Presenter, University of Kentucky
Moreno, D., University of Kentucky
Omosebi, A., Center for Applied Energy Research
Liu, K., University of Kentucky
Electrochemical utilization of CO2 has been a hot topic to target high-value, cost-effective products. Conventional processes for CO2 conversion to fuel sources, such as the Kemira processes or hydrogenation, are challenging and expensive due to their requirement for high temperatures and pressures. Electrochemically, a catalyst can be used at a fixed applied potential to reduce CO2 to form a desired product with less competition at modest operating conditions. Formic acid (FA) has been a product of interest due to its niche market in livestock feeds as well as for hydrogen storage. Furthermore, FA has the added benefits of low thermodynamic input and a favorable atom economy. Electrochemically, FA is produced through a direct two-electron transfer process involving CO2 with a proton source, requiring less energy input and fewer reaction steps than traditional processes. However, there remain obstacles for the broader-scale implementation of CO2 on the market, including competition with unwanted CO and H2, the presence of molecular oxygen, and stability of the electrocatalyst . Additionally, electrode efficiencies with conventional catalysts for FA have rarely been more than 50%.

The unique assets of the UK CAER electrochemical CO2 reduction process include an organic charge carrier to shuttle electrons to the catalyst for more efficient FA production, and novel electrode materials to improve efficiency and stability. The flow system comprises two separate cells to decouple electrochemical reduction of the charge carrier with the FA production via an engineered catalyst. The design and material considerations iterate from previous studies based on appropriate choice of electrode material and electrolyte concentrations.

Recent results show that with appropriate design changes and operating conditions to the flow system, FA production above 25 mM can be achieved along with efficiencies above 50%. Peak FA production is found to occur at an applied voltage of approximately -0.85 V vs. Ag/AgCl reference electrode, which aligns with the expected voltage for peak charge carrier production. Due to the mixing with the bulk vessel solution in the flow cell and transit time delays, a gradient is present which results in a 20% higher measured FA production at the immediate production cell exit, relative to within the mixing vessel. This will indicate a further increase in extracted FA concentration when it is siphoned out from the production cell during system scale-up. One major finding from this study is that there is a tradeoff between faster kinetics to produce FA and system efficiency with the engineered catalyst. System design, selection of operating conditions and their inter-dependence will be discussed.

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  5. Moreno, D., Omosebi, A., Abad, K., Jeon, B.W., Landon, J., Liu, K., Kim, Y. H., and Thompson, J. “Electrochemical utilization of CO2 from coal power plants” 15th International Conference on Greenhouse Gas Control Technologies, GHGT-15, 1-18 Mar 2021. Submitted 15 Feb. 2021.