*2.1. Materials*

High purity graphite rod was obtained from C Nano Technology Co. Ltd., Beijing, China. Ammonium phosphate, Poly(vinylidene fluoride) (PVDF), Sulfuric acid (H2SO4) and potassium iodide (KI) were obtained from Aladdin Co. Ltd., Shanghai, China.

#### *2.2. Phosphorus Oxide Modified Graphene (PO-Graphene) Preparation*

In a typical procedure, high purity graphite rod and platinum plate (1 × 1 × 0.05 cm3) were used as the anode and cathode, respectively, and the distance was fixed at 1.5 cm. The exfoliation was performed at 10 V in 1 M ammonium phosphate aqueous. After the preparation, the sample was collected by centrifugation at 5000 rpm for 30 min and purified repeatedly with distilled water until PH = 7.
