Predictive Control of Catalyst Selectivity in the Electroreduction of Carbon Dioxide

Abstract

The direct electrocatalytic reduction of carbon dioxide to fuel-type chemicals remains a promising utilization pathway to mitigate rising carbon dioxide concentrations in the atmosphere. Copper metal is the most widely studied catalyst used in the reaction due to its unique ability to produce hydrocarbon species. While a lot of focus has been improving the catalysts, the experimental parameters that affect the reaction have not been fully understood. A variety of factors have been shown to affect the activity and selectivity of the CO2 electroreduction reaction. Pre-electrolyzing the electrolyte is a common practice after Hori et al. 1 determined that trace impurities in the salts used to make the electrolyte solutions can deposit on the electrode surface and inhibit the reaction. High purity metal electrodes (ex. Cu 99.999+ %) are used for a similar reason. The identity of the electrolyte also has an effect, leading to different product distributions even by simply using a salt with a different alkali metal cation2. Electrolyte concentration has also been shown to alter the product selectivity3. Hori et al. concluded that the cationic species and electrolyte concentration affect the local pH at the electrode surface, which plays a major role in regulating the reaction. Other researchers have also demonstrated the importance of local pH and its effect on the reaction. Gattrell et al. 4 calculated the surface concentrations of several important species in the electroreduction of CO2 in KHCO3 solutions, showing that local concentrations are drastically different from the bulk. Factors such as current density and boundary layer thickness play a role. To optimize our electrochemical set-up, we investigated how the CO2 bubbling rate and the solution flow rate within an electrochemical flow cell affected the reaction. We found that the selectivity between CH4 and C2H4 can be predictively controlled by varying the bubbling rate and solution flow rate, and that the two are inversely related. A combination of mass transport processes and local pH affects can be used to explain our observations. This work highlights the importance of controlling parameters in the electroreduction of CO2 and demonstrates predictive control of hydrocarbon selectivity. 1. Y. Hori, H. Konishi, T. Futamura, A. Murata, O. Koga, H. Sakurai and K. Oguma, Electrochimica Acta, 2005, 50, 5354-5369. 2. A. Murata and Y. Hori, Bulletin of the Chemical Society of Japan, 1991, 64, 123-127. 3. Y. Hori, R. Takahashi, Y. Yoshinami and A. Murata, Journal of Physical Chemistry B, 1997, 101, 7075-7081. 4. N. Gupta, M. Gattrell and B. MacDougall, Journal of Applied Electrochemistry, 2006, 36, 161-172.