The fully reduced VGM 200 with a loading mass of 2 mg was soaked into 50 mL of DI water for 1 h and then ultrasonicated for 30 min to obtain the VC reduced GO material (VG-M) aqueous suspension. For electrodes fabrication, the ink was prepared using 200 μL of VG-M suspension (1 mg/mL) mixing with 20 μL of Nafion ethanol solution (1 mg/mL) and ultrasonicated for 1 h. The ink was then drop-casted on strictly 1 cm 2 working area of a piece of 2 cm 2 carbon fiber paper. As prepared carbon paper was dried in an oven at 60 ℃ for 1 h to fabricate the working electrode. The VC reduced GO using conventional stirring method (VG-S) was prepared following a previously reported method for comparison, generally, 50 mg of VC was added to 50 mL of GO aqueous suspension (0.1 mg/mL) with vigorous stirring under ambient conditions for 48 h. The obtained VG-S aqueous suspension was vacuum filtrated, washed and dispersed in ethanol with a concentration of 1 mg/mL. The VG-S working electrode was fabricated with the same method as the VG-M working electrode. The active material loading mass of the working electrode was controlled to be around 0.2 mg by weighing before and after drop-casting.