Au Oxide Formation during NE From visual inspection and optical microscope images of Au POSCs after HV electrolysis at 300 V, the distinct red color for the formation of a Au oxide is visible after 2 s and becomes more clear after 15 s (see Figs. and and Fig. ). Here, also additional electrochemical measurements clearly indicate Au oxide formation for all electrolysis times (see Fig. red curve). SEM images of the resulting electrodes are shown in Fig. and. Additional SEM images of the whole Au POSCs at all electrolysis times studied at 300 V, including a magnification of the Au(111*) region are shown in the SI in Fig. . Up to 15 s the SEM images of the entire Au POSC in Fig. do not suggest any kind of restructuring of the Au oxide. A closer inspection of, for example, the Au(111*) facet shows a slightly roughened surface in the range from 2 s to 15 s. The surface structures look similar to those observed at 540 V (compare Figs. S6e to g and Figs. ). After 30 s the formation of cracks within the Au oxide film is observed (see Fig. or Figs. and).