In situ Raman spectroscopy was carried out using a LabRAM HR 800 spectrometer combined with an Olympus BX41 microscope. The spectra were measured from the top, with the focus plane underneath the standard microscope glass slip (covering the cell assembly) and the electrolyte film on top of the carbon electrode. A schematic drawing of the two-electrode electrochemical in situ cell ECC-Opto-Gas from EL-CELL (Hamburg, Germany) is given in Supplementary Fig. . As an objective lens a 40× Olympus LUCPlanFL N (NA=0.6; corrected for the cover thickness) was used. To avoid sample damage, the laser with 532 nm wavelength worked at reduced power (0.5 mW). With the given grating/slit setup (300 mm -1 ; 200 µm) the spectral (pixel) resolution is about 3.6 cm -1 . The acquisition time per spectrum was 30 s (4-fold accumulations) for a total measurement time and time resolution of 120 s/spectrum. In addition, the DuoScan System was applied to continuously scan the laser spot over a 20×20 µm area. With regard to the spectral analysis the band deconvolution of the I3 -/I5 -bands (Supplementary Fig. ) was done using three Lorentz peaks (initial position 110 cm -1 , 143 cm -1 , and 224 cm -1 ) and one Gaussian peak (initial position 165 cm -1 ), this routine is based on the band assignments from Ref. . The deconvolution of the G-and D-band was done according to the method introduced in Ref. .