Protecting effect of mass transport during electrochemical reduction of oxygenated carbon dioxide feedstocks

Abstract

Electrochemical CO₂ reduction is a promising path toward mitigating carbon emissions while also monetizing waste gas through chemicals production and storage of surplus renewable energy. However, deploying such a technology for use on industrial CO₂ sources requires an understanding of the effects that gas feed impurities have upon CO₂ reduction reaction (CO₂RR). In this work, we elucidate the impact of molecular oxygen on the network of reactions occurring in a CO₂ reduction system. Our findings indicate that for a planar, polycrystalline Au electrode in an aqueous environment, oxygen reduction current is limited by the transport characteristics specific to the cell geometry and solvent; as a result, mass transport confers a protective effect by mitigating the otherwise thermodynamically and kinetically favorable reduction of oxygen. The presence of oxygen does not appear to have a significant impact on either CO₂RR or hydrogen evolution partial currents, indicating that the mechanisms of reduction reactions involving oxygen are independent of CO₂RR and hydrogen evolution. Further, an electrokinetic mechanistic analysis indicates many feasible candidates for the rate-determining step of CO₂RR; there is no indication that the CO₂RR mechanism at P_CO2 = 0.5 atm is altered by the presence of oxygen, as the Tafel slopes (59 mV dec⁻¹) and reaction orders with respect to bicarbonate (0), CO₂ (∼1.5), and protons (0 from lack of KIE) are consistent between systems with P_O2 = 0 atm and those with P_O2 = 0.5 atm. While this is promising for the robustness of CO₂RR to oxygen impurities in gas feeds, the ultimate design tradeoff when utilizing CO₂ sources containing oxygen is between the cost of separation processes and the corresponding cost of power inefficiency as a result of electrons lost to oxygen reduction. This represents a first step in understanding kinetic and transport considerations in the design of gas-impurity-tolerant CO₂ reduction systems.