For the measurements performed in Ulm, we used H 2 SO 4 Merck Suprapure 98% and methanol Merck EMSURE ® . The electrolytes were purged with N 2 (Westfalen 6.0). For the measurements performed at the ESRF, the solutions were prepared from H 2 SO 4 93-98%, VWR ARISTAR ® ULTRA and methanol (Fluka Analytical, LC-MS Ultra Chromasolv ® ). The electrolytes were purged with Ar (Air Liquide, BIP grade). The Ru(0001) single crystal electrodes were purchased from MaTecK GmbH (purity 99.99%, orientation accuracy <0.1°). The glassware was cleaned before each experiment by storage in highly concentrated KOH (Ulm) or Piranha etch (ESRF) overnight. Subsequently, it was thoroughly boiled and rinsed with hot MilliQ water.