Two-step electrochemical reduction of CO2 towards multi-carbon products at high current densities

Abstract Two-step electrochemical reduction of CO2 is investigated as an alternative to increase selectivity towards C2 and C3 products. In this type of proposed cascade electrocatalytic operation, CO is produced in a first step and subsequently reduced to multi-carbon products in a second step with significantly higher Faradaic efficiencies compared to a one-step process. Research efforts have been focused on the feasibility of the isolated second step with pure CO as reactant, however the interdependencies of both steps need to be considered. Accordingly, two-step electrochemical reduction of CO2 is studied in this work as an integrated system. Taking into account that the study of this technology at high current densities is crucial for industrial applicability, gas diffusion electrodes and flow-cells were used for operation at current densities above −200 mA cm−2 . Firstly, each step was characterized separately, the first using a silver gas diffusion electrode to generate a mixture of humidified CO, H2, and unreacted CO2; the second step using copper nanoparticles on a carbon-based gas diffusion structure to obtain C2 and C3 products. This step was studied using synthetic mixtures of CO2 and CO with different ratios. Furthermore, experiments with isotope labeled 13CO2 and 13CO were performed in order to obtain some insights on the (electrochemical) reaction path of gas mixtures containing CO2 and CO. Subsequently, the two units were integrated into a system, where the full gas output of the first unit was directly fed to the second unit. The total Faradaic efficiency towards multi-carbon products of this initial system was limited to 20% at total current density of −470 mA cm−2. These initial results together with the isotopic labeling studies indicate that the presence of significant amounts of unreacted CO2 from the first step is detrimental for the second step. A significant improvement was achieved by introducing a CO2 absorption column between the two units and after splitting the overall charge flow applied in each cell in accordance with the main reaction at each step. With this set-up a total Faradaic efficiency towards C2 and C3 products of 62% at a total current density of −300 mA cm−2 was achieved. The results confirm the need for a gas separation technique between the two steps for a feasible two-step electrochemical reduction of CO2.