Electrochemical characterisation: In Ulm, the electrochemical characterisation of the Ptmodified Ru(0001) electrodes was performed in a dual thin-layer flow cell in a DEMS configuration which is described elsewhere. Before the experiments, the flow cell, including all inlet capillaries, was thoroughly rinsed with hot MilliQ water. For this work, we used a homebuilt RHE as a reference electrode (RE) and a Au wire as a counter electrode (CE). The potential was controlled with a Pine AFRDE5 bipotentiostat, and the data were acquired with an in-house programmed software.The different measurement procedures are described in Section. S2. A detailed description of the calibration of the DEMS signals is provided in Section S3.