In conclusion, it was shown that, independent of the Ag thickness, the plasma treatment enhances both the current density, the stability and the selectivity towards CO. However, a minimum Ag thickness of ~200 nm is required to maintain high FE values over large operation durations. The question therefore arises as to the reasons for this performance improvement for the plasma treated electrodes and whether these are related to the chemical modification of the electrode, the modification of the surface morphology, or both. Also, the influence of the substrate needs to be considered. With respect to the latter, 200 nm of Ag where also deposited on the carbon-based GDE and treated with plasma for 36 s, similarly to the electrodes on PTFE. For the Ag200 and Ag200P36 on the carbon-GDE, current densities of 15 -18 mA/cm 2 were achieved (Fig. ). Both samples exhibited an initial FE for CO near 90% at -1.18 V vs. RHE. However, a substantial drop in FECO (to ~60%) was observed after 6 h on stream for both samples. This drop in selectivity can be attributed to the flooding of the carbon-based GDE (i.e. filling of the GDE pores with aqueous electrolyte), and hence hindering the CO2 molecules from reaching the active Ag sites and favoring the hydrogen evolution reaction.