Shaoyang Lin,† Yuval Fishler,† Soonho Kwon, † Annette Böhme, Weixuan Nie, Moon Young Yang, Jesse Matthews, Zachery W. B. Iton, Brian C. Lee, Thomas Jaramillo, Harry A. Atwater, William A. Goddard III*, Wilson A. Smith*, Kimberly A. See*, Chem. Catalysis, 2025, accepted.

Abstract: Increasing the product selectivity and decreasing the cost of product separation is critical for large scale application of electrochemical CO₂ reduction (ECO₂R). We hypothesize that highly concentrated aqueous electrolytes can tune the microenvironment of the catalyst/electrolyte interface and improve product selectivity. Compared to a conventional electrolyte concentration of 1 M HCOOK, the use of a 7.1 M HCOOK electrolyte increases the FE ratio of C₂H₄/CO from 2.2 ± 0.3 to 18.3 ± 4.8 at -1.08 V vs RHE on a Cu gas diffusion electrode. Based on electrochemical analysis and AIMD simulation, the identity and concentration of the cation and anion play more important roles in controlling the CO₂R reaction pathway than the bulk CO₂ solubility and the bulk pH of electrolytes. In-situ ATR-SEIRAS suggests that, unlike 1 M HCOOK, the *CO-bridge binding mode on Cu is dominant in 7.1 M HCOOK electrolyte, which potentially results in less CO release and higher yield of C₂H₄. This study demonstrates that while we can tailor the electrolyte composition to shift product selectivity, the factors that control the product selectivity are numerous and cannot be distilled down into one correlated property-reactivity relationship. Thus, when CO2R conditions are changed, care must be taken to understand their effects on the bulk electrolyte properties and the electrode-electrolyte interface.