Investigating the Impact of CO₂ Dilution and Oxygen on the Electrochemical Reduction of CO₂

The consumption of fossil fuels has significantly increased in recent centuries, driven by global technological and industrial advancements. According to recent observations, this has resulted in a considerable rise in emissions of climate-altering gases, particularly carbon dioxide (CO₂), with atmospheric concentrations increasing from the pre-industrial level of 300 ppm to over 400 ppm. Among the most promising strategies for reducing CO₂ emissions is capturing and converting CO₂ using electrolysers. The use of electrochemical cells for CO₂ conversion is particularly intriguing because it allows the utilization of electrical energy from renewable sources, even those inherently intermittent and unpredictable. Moreover, this approach enables the storage of renewable energy in the chemical bonds of the produced chemicals. The capture and conversion processes are traditionally conducted in two separate, independent phases. The intermediate steps, such as storage and transportation, can generate additional greenhouse gas emissions and require further energy consumption. To address these inefficiencies, a promising approach involves using exhaust gases directly from their source as feed gas for the electrochemical CO₂ conversion. Flue gases, characterized by low CO₂ concentrations and various impurities depending on their industrial origin, can significantly impact the performance of electrochemical conversion processes. This thesis presents a comprehensive overview of the state-of-the-art technologies for electrochemical CO₂ reduction and their integration with CO₂ capture processes. It reviews studies examining the effects of CO₂ dilution and typical contaminants in flue gases on electrochemical CO₂ reduction, like O₂, NOx, and SOx. Most importantly, an experimental study was conducted using a three-chamber electrolytic flow cell equipped with an airbrushed copper and tin oxide-based gas diffusion electrode as the cathode, a commercial Ir-MMO as the anode, and a leak-free Ag/AgCl (3.4M KCl) as the reference electrode. Galvanostatic measurements were conducted using aqueous electrolytes, specifically a 0.1 M KHCO₃ solution as the catholyte and a 0.1 M KOH solution as the anolyte. A bipolar ion exchange membrane separated the cathodic and anodic compartments. Herein, feed streams with varying concentrations of CO₂, N₂, and O₂ were used to identify the minimum requirements for any potential upstream separation operation. The dilution effect significantly impacted the Faradaic efficiency. For the system under investigation, faradaic efficiencies towards carbon products greater than 50% were observed starting from a CO₂ molar concentration of 25% at a current density value up to 20 mA cm-2. Furthermore, the results demonstrated a clear shift in selectivity with varying CO₂ concentrations, favoring the production of formate at lower CO₂ concentrations. On the other hand, oxygen had a detrimental effect on performance even at low concentrations due to its higher reduction tendency compared to CO₂. However, these early studies demonstrate that the presence of oxygen is not necessarily incompatible with CO₂ reduction reactions, and a potential strategy is provided to reduce the process sensitivity to oxygen.