All experiments were performed in a rectangular glass cell (20 × 5 × 5 cm) described previously. A schematic including the camera configuration (see below) is shown in the SI in Fig. . As electrolyte, 200 mL of 0.01 M KOH were used. The electrolyte solution was prepared from KOH pellets (≥ 85 %, Carl Roth) and deionized water (≤ 700 µS cm -1 ). The Au wire electrode (purity at least 99.99 %) with a diameter of 0.5 mm was purchased from MaTecK. Before each experiment, the electrode was annealed for 3 min in a propane torch (CAMPINGAZ) to remove organic contaminations and flatten the electrode surface. Electrode holders were 3D-printed using chemically resistant PVDF purchased from 3Dogg.