Pt/C reference -In the case of Pt/C benchmark (TKK, TEC10E50E-HT) electrocatalyst measurements, the electrode was placed in an Ar saturated electrolyte under potential control at 0.05 VRHE. The electrocatalyst was electrochemically activated (PCA) for 200 cycles between 0.05 and 1.2 VRHE with a scan rate of 300 mV s -1 at 600 rpm. After PCA, the electrolyte was exchanged with a fresh one and the electrode was no longer under any external potential control (i.e. the conditions corresponded to the open circuit potential -OCP). ORR polarization curves were measured in an oxygen saturated electrolyte at 1600 rpm in the potential window 0.05-1.0 VRHE with a scan rate of 20 mV s -1 . Ohmic resistance of the electrolyte was determined and compensated for as reported in reference . At the end of ORR polarization curve measurement, the electrolyte was purged with CO under potentiostatic mode (0.05 VRHE) in order to ensure successful CO adsorption. Afterwards the electrolyte was saturated with Ar. CO-electrooxidation was performed using the same potential window and scan rate as in ORR, but without rotation and in an Ar saturated electrolyte. After subtraction of background current due to capacitive currents, kinetic parameters were calculated at 0.9 VRHE by using Koutecky-Levich equation . Electrochemically active surface area (ECSACO) was determined by integrating the charge in CO electrooxidation ('stripping') experiments as described in reference . All potentials are given against the reversible hydrogen electrode (RHE), which was measured at the start of the experiment.