Synthesis of hydrothermally reduced graphene/MnO2 composites and their electrochemical properties as supercapacitors

Hydrothermally reduced graphene/MnO₂ (HRG/MnO₂) composites were synthesized by dipping HRG into the mixed aqueous solution of 0.1 M KMnO₄ and 0.1 M K₂SO₄ for different periods of time at room temperature. The morphology and microstructure of the as-prepared composites were characterized by field-emission scanning electron microscopy, X-ray diffraction, Raman microscope, and X-ray photoelectron spectroscopy. The characterizations indicate that MnO₂ successfully deposited on HRG surfaces and the morphology of the HRG/MnO₂ shows a three-dimensional porous structure with MnO₂ homogenously distributing on the HRG surfaces. Capacitive properties of the synthesized composite electrodes were studied using cyclic voltammetry and electrochemical impedance spectroscopy in a three-electrode experimental setup using 1 M Na₂SO₄ aqueous solution as electrolyte. The main results of electrochemical tests are drawn as follows: the specific capacitance value of HRG/MnO₂-200 (HRG dipped into the mixed solution of 0.1 M KMnO₄ and 0.1 M K₂SO₄ for 200 min) electrode reached 211.5 F g⁻¹ at a potential scan rate of 2 mV s⁻¹; moreover, this electrode shows a good cyclic stability and capacity retention. It is anticipated that the synthesized HRG/MnO₂ composites will find promising applications in supercapacitors and other devices in virtue of their outstanding characters of good cycle stability, low cost and environmentally benign nature.