Internal CClF means that electrochemical Cl2 evolution was conducted in the presence of CO (i.e., in the CO atmosphere), allowing for the potential of electrochemical C-Cl bond formation on the electrocatalyst. External CClF means electrochemical Cl2 evolution was carried out in the absence of CO, and then the produced Cl2 was mixed with CO physically, such that the mixing between Cl2 and CO gases occurred in the absence of electrochemistry. In this way, if C-Cl bond formation is observed, it cannot happen electrochemically, but either chemically or locally in the mass spectrometry ionization chamber (more details are discussed in the Results and Discussion section below).