Process dependent graphene/MnO2 composites for supercapacitors

• Simple change of reaction procedure for RGO/MnO2 bring enhancement of capacitance.
• Nanoneedle MnO2 is formed on the graphene oxide surface using functional groups.
• GO/MnO2 composite is reduced to RGO/MnO2 via hydrazine reduction by dipping method.
• Functional groups on GO sheets enable the homogeneous dispersion of MnO2.

Two types of graphene/MnO2 composites were synthesized by different reaction procedures. S1 was synthesized as follows: first, nanoneedle MnO2 was formed on the GO sheets using various functional groups (GO/MnO2). In the second stage, GO/MnO2 was reduced to graphene/MnO2 (S1) via the dipping method. S2 was synthesized using a different reaction order: first, graphene oxide was reduced to graphene and nanoneedle MnO2 was formed on graphene sheets. The morphology and microstructure of the as-prepared composites were characterized by X-ray diffractometery, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Characterization indicated that the nanoneedle MnO2 structures in the S1 composite were homogeneously dispersed on graphene sheets, whereas MnO2 in the S2 composite formed aggregates due to absence of functional groups. The capacitive properties of S1 and S2 electrodes were measured using cyclic voltammetry, galvanostatic charge/discharge tests, and electrochemical impedance spectroscopy in a three-electrode experimental setup with an aqueous solution of 1 M Na2SO4 as the electrolyte. The S1 electrode exhibited a specific capacitance as high as 327.5 F g−1at 10 mV s−1, which was higher than that of the S2 electrode (229.9 F g−1). It is anticipated that the formation of nanoneedle MnO2 on the GO surface following the reduction procedure could be a promising fabrication method for supercapacitor electrodes.