( A similar plot with a lower upper potential limit is shown in Fig. and discussed in the manuscript. S4: Influence of tubes around the plasma counter electrode During aCGDE, H 2 is evolved at the plasma counter electrode. In order to prevent that the H 2 diffuses far into the electrolyte, where it could oxidize the auxiliary electrochemical Pt electrode, a glass tube was placed around the plasma counter electrode. In this way, the H 2 bubbles are directed outside the electrolyte (see Fig. in the manuscript). Figure shows CVs recorded at the Pt AE in 0.01 M KOH at a scan rate of 50 mV s -1 directly after aCGDE (after step B1) and in Fig. after aCGDE and additional 15 min waiting time (after steps B1 and B2). Corresponding measurements in a setup without glass tube are presented by the dashed curves. Overall the curves recorded with or without employing a glass tube look similar. Only when the AE is placed close to the PWE (1.5 cm) (red curves in Fig. ), the positive current density in the negative going scan in the potential region > 0.60 V RHE is larger when a glass tube was employed as if no glass tube was employed. As discussed in the manuscript, the positive current density was addressed to H 2 O 2 oxidation.