Comparing Tandem Cell Designs for Electrochemical CO ₂ Reduction to Ethylene

Electrochemical carbon dioxide reduction (CO2R) is a promising approach for the decentralized production of fuels such as ethylene (C2H4). However, the use of Cu, the most efficient metal CO2R catalyst for the generation of C2H4 known to date, generally yields a product stream with poor selectivity. In an effort to increase selectivity, the reaction from CO2 to C2H4 can be broken down into two steps using tandem CO2R electrolyzers: formation of CO from CO2 and subsequent reduction of CO to C2H4. Here, we present two novel tandem electrolyzer architectures that closely integrate two cathodes, one for CO generation and one for conversion to C2H4, while still enabling independent electrical control of the cathodic surfaces. Cathode segmentation in each of these designs also permits the controlled sequencing of mass flow of chemical intermediates in the order of Au to Cu cathode catalysts, in contrast to earlier work relying on uncontrolled, passive diffusion to facilitate the flow of chemical intermediates between catalysts. When comparing the performance of the newly developed electrolyzer cell designs with a dual electrolyzer system, we found that the dual electrolyzer system yields the highest C2H4 faradaic efficiencies (FEs) of 31% and C2H4 concentrations (∼8 mol %). However, a single Cu-containing electrolyzer outperformed all three tandem systems in terms of C2H4 FE (34%). Our findings, enabled by independent control of the two tandem cathode surfaces, indicate that tandem CO2R systems need to be evaluated carefully by testing them at various relevant current densities.