(307b) CO2 Reduction on Copper: Mechanistic Insights By Controlling Heat and Mass Transport

The electrochemical CO₂ reduction has attracted attention as one of the promising strategies to achieve carbon neutrality and scaling-up of this reaction is required for social implementation. Copper has a unique property in the electrochemical reduction of CO₂ which can generate up to 16 different products under the same condition. Controlling selectivity is a key challenge in scale-up. However, copper is not a selective catalyst and the complexity of the mechanism of CO₂ reduction does not allow to optimize the reaction easily. Temperature is an important parameter to modify production distributions as well as applied potential but is not generally taken into consideration in the study of the electrochemical CO₂ reduction. Temperature has effects mainly on kinetics (i.e. Butler-Volmer kinetics), thermodynamics (i.e. solubility), and mass transfer (i.e. diffusivity, and viscosity). To simplify the discussion on temperature-dependency, we decoupled effects of heat and mass transfer by utilizing the gastight rotating cylinder electrode (RCE) cell. This cell has well-defined transport properties and allowed us to study convoluted effects systematically. We explored the range of 5-35 °C for temperatures at 100 and 800 rpm of electrode rotation speeds on crystalline copper. Increased rotation speeds enhance mass transport, decreasing the duration of CO reaction intermediates at the electrode surface. The higher temperature and shorter intermediate residence time promote the generation of H₂ and CO. C-C coupling occurs more readily at room temperature with long CO residence time. Results at lower temperatures show no significant impact of CO residence time on its further reduction, as electrochemical surface reactions are primarily limited by sluggish CO desorption kinetics. Conversely, at ambient and higher temperatures, the dominance shifts towards Eley-Rideal insertion of unbound CO for the subsequent reduction process.