Note that during CClF, phosgene is one possible product. Considering its potential danger, we have calculated its highest possible productivity from the conducted reactions and confirmed everything was operated far below safety limits. Details can be found in the SI. Additionally, since the reaction was conducted under an anodic potential, CO (carrier gas) oxidation to CO2 and O2 evolution (from water splitting in aqueous electrolyte) were both inevitable side reactions. However, this is simply a proof-ofconcept reaction that allows for analysis in terms of any Chem-I issues, and thus analyzing these reactions was not within the scope of this work. Figure ) compares the mass spectra in the CO atmosphere with and without electrochemically formed Cl2 (i.e., internal CClF). The produced Cl2 showed IMX increase at M35, M37, M70, M72, and M74, which were induced by 35 Cl+, 37 Cl+, [ 35 Cl 35 Cl]+, [ 35 Cl 37 Cl]+, and [ 37 Cl 37 Cl]+, respectively (the crustal abundance of 35 Cl is ~76% and 37 Cl is ~24%). M36 and M38 also showed a noticeable increase. Previous Cl2 evolution studies also demonstrated an increase in IM36 and IM38, but were vague about exactly how this was being produced , .