External CClF was conducted in a standard H-type cell with a three-electrode configuration and two-chamber separated by a Nafion® 117 membrane (Fuel Cell Store). A Pt foil (>99.999%, GoodFellow), IrO2-coated carbon paper (Dioxide Materials) and a miniature leak-free Ag/AgCl/KCl (sat.) electrode (Innovative Instruments Inc.) were used as the working, counter, and reference electrode, respectively. Cl2 formation was conducted at 200 mA/cm 2 for 10 min in order to create dissolved Cl2 in the anolyte. After the reaction, the Cl2-dissolved anolyte was taken out with a syringe and then injected into the EC-MS cell (with carrier gas flowing through), where no working electrode was installed and thus no electrochemical reaction was running either. In this case, the EC-MS cell was only employed as the electrolyte reservoir. More specifically, the space for the working electrode was sealed with PFTE film. Other headpieces were kept identical as they were in regular electrochemical reactions. In this way, Cl2 and CO gases were mixed physically.