Sample preparation: The Ru(0001) single crystal was prepared under UHV conditions according to previous reports. The preparation involved Ar ion sputtering (p Ar = 3 × 10 -5 mbar, I = 4 µA cm -2 ) for at least 15 minutes, subsequent cycles of flash annealing to 1600 K and adsorption of 10 L of O 2 (1 L: 10 s p O 2 = 1 × 10 -6 mbar) to remove carbon impurities, and finally three flash annealing cycles to 1600 K (without dosing O 2 ). The quality of the surface of the electrodes was verified by STM measurements under UHV conditions.