Process dependent graphene/MnO₂ composites for supercapacitors

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 1M Na₂SO₄ as the electrolyte. The S1 electrode exhibited a specific capacitance as high as 327.5Fg^-1at 10mVs^-1, which was higher than that of the S2 electrode (229.9Fg^-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.