Comparing Tandem Cell Designs for Electrochemical CO₂ 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.