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A new, simple and green method was proposed for preparation of the S-doped graphene based pencil graphite electrodes (S-GPGEs) in sulfuric acid solution by cyclic voltammetric method in this work. The obtained S-GPGEs were investigated by the measurements of scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical performance. The results show that the S-GPGEs were composed of many nano flakes. -C-S-C-and -C-SOx-C-(x:2, 3) groups were determined by XPS. Raman spectra were taken to determine the optical properties of the electrodes. Thus, the results show that S-doped graphene was obtained in situ. S-GPGEs were used as positive electrode components of a vanadium redox flow battery (VRB). The electrodes showed great electrochemical activity in the positive electrolyte of VRB. This novel method is easy, cheap and environmentally friend for production of S-doped graphene for many application such as sensor, electro-optics, energy storage devices and etc.

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Summary Graphene‐based electrodes have great potential for using as positive electrode material of vanadium redox flow batteries. However, production of heteroatom doped graphene oxide in classical methods had many steps and time‐consuming procedure. In this work, binder‐free sulfur‐doped graphene oxide electrodes (S‐GOEs) were obtained from graphite by the chronoamperometric method in eco‐friendly media (sulphuric acid/water) and one step at room temperature. The effects of functional groups ratio on the positive electrolyte performance of vanadium redox battery was investigated via electrochemical methods such as cyclic voltammetry (CV), chrono charge‐discharge, and electrochemical impedance spectroscopy (EIS). Microscopic methods were also used for investigation of the surface morphology of the modified electrodes. Detailed chemical composition of modified electrodes surfaces was investigated by X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR‐ATR) spectroscopy and energy‐dispersive X‐ray spectroscopy (EDS) techniques. CSOxC(x = 2, 3), CSO, hydroxyl, carboxyl and epoxy groups were formed on the modified electrodes during the preparation of graphene oxide based electrodes from the graphite. According to cyclic voltammetry analysis, the electrodes prepared over 3 minutes by chronoamperometric method at 1.9 V (S‐GOE3) showed the best performance as a positive electrode of the vanadium redox battery. The cyclic charge‐discharge test demonstrated that the discharge capacity considerably increased from 171.9 mA h to 179 mA h at 3.2 mA cm−2 discharge current density. Energy efficiency of cell was also climbing by 5% in S‐GOE3 as positive electrode to bare electrode.

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