Supercapacitive Performance Of MnO₂and MnO₂-graphene Oxide Composites

Supercapacitor is a new potential energy storage device because of its high powerdensity, long cycle life as well as environment-friendly feature. Tremendous attentionhas been paid to SCs owing to their wide and growing range of potential applications,such as electronic communications, heavy machinery and electric vehicles, etc. Theproperty of the electrode is a key factor affecting the performance of capacitor, so theresearch focus of supercapacitors is on the synthesis of high-performance electrodematerials.In this paper, carbon materials and manganese dioxide were chosen as electrodematerials. Graphite oxide was prepared by Hummers’ method and graphene oxidewith different oxidization extents were prepared by the chemical reduction ofgraphene oxide. Simple and efficient redox deposition precipitation method waschosen to prepare the composites of MnO₂-graphene oxide based on a comparingbetween redox deposition precipitation method and hydrothermal method. SEM, TEM,XRD, XPS and FTIR analyses were used to characterize the surface morphology,structure and the oxygen functional groups on the surface of graphene oxide.Electrical conductivity of the graphene with different oxidation degrees was measuredby a four-probe method. Load mass of the manganese dioxide in the composites wereestimated by TG. Capacitance performance were tested by the cyclic voltammetry andthe galvanostatic charge discharge measurement.The layered structure in original flake graphite disappeared after oxidation, thelayers of GO are very thin with a large number of-OH,-C=O,-COOH and otheroxygen-containing functional groups on the surface, as a result the interlayer spacingincreases from0.336nm to0.875nm. On the surface of graphene oxide, obtainedafter hydrazine reduction and freeze-drying, appears a large number of folds and holes.The sheet layers only have3to10carbon layers and the graphitization degreedecreases with the reduction of the surface oxygen functional groups. Conductivity isinfluenced by the reduction time and the conductivity of the sample after8hreduction is4orders of magnitude higher than the original graphite oxide. Manganese dioxide samples prepared by the two methods are both-MnO₂withthe tetragonal crystal phase. The crystallization degree of the-MnO₂produced by theredox deposition precipitation method is weak, and the product composed ofelongated-MnO₂-nanoneedles with a diameter less than10nm disorderedlyoverlapping each other. The crystallization degree of the sample prepared by thehydrothermal method is better, with a morphology of sea urchins composed ofnanorods with a diameter about20nm. After1000charge-discharge cycles, theformer’s capacitance maintained at about85F/g, the retention rate was90.04%; thelatter’s capacitance decreased after increasing and then maintained at74.2F/g, theretention rate was112.77%.The capacitance performances of the composites are mainly influenced by theconductivity of graphene oxide, and the loading of manganese dioxide is the minorimpact factor. Oxygen functional groups on the surface of graphene oxide providedsites of nucleation for manganese dioxide, ensured the uniform dispersion and goodcontact of the manganese dioxide, and the insertion of the MnO₂also promoted theseparation of the graphene oxidation layers.8hRGOM-3showed the best capacitanceperformance with a specific capacitance of124F/g, and the capacitance retention ratewas78.7%after1000charge-discharge cycles.