COOR with an UPL up to 1.05 V From the COOR onset potential, the current density in the COOR-CV rises sharply, both for the Pt-1 and Pt-3 electrodes, resulting in a distinct peak A1. A similar behaviour is commonly observed for bulk CO electro-oxidation on Pt electrodes. Note that the MS-CV does not show a peak in the peak A1 region, in contrast to earlier observations for the COOR on supported Pt/C catalyst electrodes and rough Pt ATR films. The peak is also absent in further measurements on various Pt electrodes presented in Section S4. The additional peak A1 in the CV must result from the rapid formation / uptake of OH/O species on the surface, which does not show up in the MS-CV signal. This OH/O formation is apparent in a peak A1 in the positive going scan of the redox-CV in Figs. and.