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 MnO ₂/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.