Cr2O3/rGO films were first exfoliated from sliver substrate and broken by probe ultrasonication at 750 W 20% amplitude for 15 min. The mixture was dropped casted on carbon fiber paper with working area of 1 cm 2 . By weighing before and after drop casting the loading was controlled to be ~0.1mg. Then Nafion solution was used to reinforce the sample. 100µL Nafion solution (Sigma Aldrich, 5 wt %) was dispersed in 0.9ml ethanol and DI water (mixed ratio 1:4) to make a stabilizing solution. 100µl stabilizing solution was dripped on the surface of the Cr2O3/rGO with carbon fiber paper. Bulk Cr2O3 (Sigma-Aldrich ≥98% particle size <50 µm) was prepared following the same drop casting and reinforcing method, and all powder distribute well on working area of carbon fiber paper. After drying for 24 hrs, the compound electrode was put back to the same three-electrodes system in 1 M KOH solution.