*2.3. Characterization*

The as-prepared PO-graphene was characterized by field emission scanning electron microscope (FESEM; Zeiss EVO-50, Zeiss, Germany), Transmission electron microscopy (TEM) images are performed on a Tecnal G2 20STWIN microscope operating (FEI, Hillsboro, OR, USA) at 200 kV. Fourier transform Infrared spectroscopy (FT-IR) analyses were accomplished on Thermal Nicolet 6700 (Thermo Nicolet, Shanghai, China). Laser Raman spectra were carried out using Raman spectrometry (RENISHAW inVia Raman Microscope, London, UK). X-ray diffraction analysis (XRD) was carried out by Bruker D8 Advance (BRUKER AXS, Berlin, Germany), Cu K α = 0.154 nm with a voltage of 40 kV.

The electrochemical measurements were performed by a three-electrode cell. In detail, the working electrode was composed of PO-graphene (90 wt %) and polyvinylidene fluoride PVDF (10 wt %). A Pt foil electrode and a saturated calomel electrode (SCE) (CH Instruments Ins, Shanghai, China) were used as the counter electrode and reference electrode, respectively. The electrolytes were composed of 0.1 M KI and 0.2 M H2SO4. The cyclic voltammetry (CV) tests were performed from −0.5 to 0.7 V (vs. SCE). Electrochemical impedance spectroscopy (EIS) measurements were performed under open circuit conditions over a frequency region from 0.01 Hz to 100 kHz by applying an AC signal of 5 mV in amplitude throughout the test. The galvanostatic charge–discharge (GCD) analyses were performed in a potential range of 0–0.7 V (vs. SCE). The CV, GCD and EIS tests were all performed on a CHI660D electrochemical workstation (CH Instruments Ins, Shanghai, China).
