Electrochemical procedure: The electrochemical and -catalytic measurements were performed in a dual thin layer flow cell equipped with a Pt collector electrode in a second compartment at the electrolyte outlet. The potential was controlled by a bipotentiostat (Pine Instruments, AFCBP-1). A Au wire served as a counter electrode, and a home-made reversible hydrogen electrode served as a reference electrode. Throughout this work, all potentials are referred to the RHE scale. During the ORR measurements, the potential of the collector electrode was set to 1.2 V versus the reversible hydrogen electrode (RHE), allowing for almost fully selective oxidation of H 2 O 2 formed during the ORR. A detailed description of the cell design can be found elsewhere. For the electrochemical measurements, the freshly prepared Ru(0001) single crystals were transferred into a load lock chamber that was flushed with N 2 until atmospheric pressure was reached. Subsequently, the electrode was mounted on the electrochemical flow cell, by pressing the electrode against the O-ring of the cell. The deaerated electrolyte was flown through the cell. Possible bubbles were removed from the electrolyte capillaries. Potential control can only be guaranteed after the complete removal of all bubbles. Consequently, the electrode was left at open circuit potential during this process, which took ca. 30 s. Then, several BCVs were recorded until the residual O 2 level in the electrolyte decreased so that the current in the double layer region (within region III) was almost symmetric on the current axis. Next, several BCVs with different scan rates and potential limits were recorded (the number varied between different measurements). Finally, the electrolyte was purged for 15 min with O 2 , followed by several potential cycles (ORR-CVs) with different potential limits and scan rates.