The electrochemical synthesis of Cr2O3 was performed in a three-electrode system at room temperature with 0.1 M CrCl3 solution as electrolyte and a Potentiostat (ZIVE SP1, WonATech Co., Ltd.). A platinum foil (20mm × 20mm × 0.1mm, 99.99%) was used as counter electrode and an Ag/AgCl electrode as reference electrode. Figure . S1 shows the custom-designed working electrode which is described in more detail in SI1. The distance between the working and counter electrode was maintained at 30 mm. Voltage and time of applied voltage was varied as described in the main text. After finishing the synthesis, the surface of Cr2O3/rGO films was carefully rinsed with deionized water.