Au Oxide Formation during aCGDE Optical microscope and SEM images of Au POSCs taken after HV electrolysis at 250 ms, 2 s, and 30 s at 540 V are shown in Fig. . Additional optical microscope images for other electrolysis times can be found in the SI in Section S3. After 30 s the Au POSC is red, clearly indicating Au oxide formation. For shorter electrolysis times, the red color is not as clearly apparent in the optical microscope images.From visual inspection, the red color is clearly apparent for electrolysis times longer than 2 s.The electrochemical characterization above (see Fig. ) suggests that Au oxide also forms at shorter electrolysis times. Thus, the Au oxide films at electrolysis times shorter than two seconds are presumably too thin to be apparent by the red color by the naked eye. More detailed insights on the structural properties and the integrity of the Au oxide films are visualized by SEM images of the entire POSCs after all electrolysis times in Fig. . A magnification of the Au(111*) and Au(100*) regions can also be found in Fig. .