Experiments for electrochemical C-Cl bond formation reaction (CClF) were conducted by mixing CO gas (as the carbon source) and Cl2 gas (as the Cl source). Pure CO was provided as the carrier gas. Since Cl2 gas is dangerous and thus not commercially available, it was generated in-house from an electrochemical Cl2 evolution reaction conducted in 0.1 M HClO4 -1.0 M NaCl mixed solution as the electrolyte. CO and Cl2 gases were mixed either internally (noted as "internal CClF" thereafter) or externally (noted as "external CClF" thereafter). A sketch of internal and external CClF experimental configurations can be found in Figure .