Super-capacitive performance depending on different crystal structures of MnO2 in graphene/MnO2 composites for supercapacitors

In the present study, we synthesize nanoneedle structures of MnO2/graphene nanocomposites (N-RGO/MnO2) and birnessite-type MnO2/graphene nanocomposites (B-RGO/MnO2). The morphologies and microstructures of as-prepared composites are characterized by X-ray diffractometry, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Our characterizations indicate that nanoneedle structures of MnO2 and birnessite-type MnO2 are successfully formed on graphene surfaces. Capacitive properties of the N-RGO/MnO2 and B-RGO/MnO2 electrodes are measured using cyclic voltammetry, galvanostatic charge/discharge tests, and electrochemical impedance spectroscopy in a three-electrode experimental setup using a 1 M Na2SO4 aqueous solution as the electrolyte. The N-RGO/MnO2 electrode displays a specific capacitance as high as 327.5 F g(-1) at 10 mV s(-1), which is higher than that of a B-RGO/MnO2 electrode (248.5 F g(-1)). It is believed that the nanoneedle structure of MnO2 shows excellent electrochemical properties than birnessite-type MnO2 for the electrode materials for supercapacitors.