The Pt wire used as PWE was protected at both ends with polymer caps (fabricated from Eppendorf pipette tips). The cap around the wire apex is necessary to prevent the wire from melting during aCGDE. The caps were adjusted such that 2 mm of the wire are exposed to the electrolyte (surface area of 0.031 cm 2 ). The PWE and plasma counter electrode were placed between 1.2 cm to 1.5 cm apart from one another. The voltage between these electrodes was applied with a TDK-Lambda Power Supply (630 V / 1.365 A). The power supply was controlled with a LabView program. Unless otherwise mentioned, 580 V were applied for 30 s between the PWE and plasma counter electrode. At this voltage the plasma ignites immediately, as compared to lower voltages. The Pt wire used as AE was immersed 10 mm in the electrolyte solution, where the distance was controlled by putting a protective polymer cap (see above) at the upper end of the wire (surface area of 0.159 cm 2 ). In this case the apex of the wire does not need special protection, since the current densities are much lower compared to those recorded at the PWE. A home-made and freshly prepared reversible hydrogen electrode (RHE) was used as reference electrode. The potential at the AE was controlled with a FHI ELAB potentiostat. All potentials are given on the RHE scale (V RHE ).