The COOR on the Pt-modified Ru(0001) electrodes was characterized by cyclic voltammetry at a scan rate of 50 mV s -1 in CO-saturated 0.5 M H 2 SO 4 . The measurements were performed in a dual thin-layer electrochemical flow cell coupled with a DEMS set-up, allowing for operando CO 2 (m/z=44) product detection. 65 During the COOR measurements, the upper potential limit (UPL) was gradually increased in steps of 0.05 V, recording two potential cycles for each UPL. For better identification of differences, we only show a single potential cycle for UPLs of 1.05 V and 1.35 V in Fig. , respectively, for the Pt-1 electrode (left column) and the Pt-3 electrode (right column). Details of the experimental procedure and the respective parameters can be found in the Experimental section and the SI in Section S2, which also includes data for additional UPLs not shown here. The COOR-CVs are shown in black in Figs. and. The mass spectrometric ion current of the CO 2 product formation is converted into a Faraday current and normalized to the Faraday current recorded in the COOR-CV. A detailed description of the conversion and normalization procedure and the data evaluation is provided in the SI in Section S3. The resulting curves are denoted as mass spectrometric cyclic voltammograms (MS-CV), and are depicted in red in Figs. and. For direct comparison, the COOR-CVs and MS-CVs are plotted on top of each other in Figs. and. Since only CO 2 is formed during the COOR, subtracting the MS-CV from the COOR-CV (black minus red curve in Figs. and), yields a CV that represents other surface redox processes, as they are usually found in CVs recorded in CO-free elec-trolytes, plus contributions from CO 2 formation by reaction with preadsorbed OH/O species, which release less than the two electrons per CO 2 assumed in the calibration of the K-factor (see Section S3). We denote this curve as redox-CV, shown in green in Figs. and (plotted along with the COOR-CV for comparison).