Preparation，Modification And Electrochemical Capacitance Of MnO₂and Graphene Aerogels Electrode Materials

Supercapacitors are charge-storage devices that have been attracting tremendous attentiondue to their high power density, excellent reversibility and long cycle life. The studiesconcentrated on the charge storage mechanisms of MnO₂capacitors, graphene aerogelspreparation, graphene aerogels with MnO₂coating and their application as high-peformancesupercapacitors. The main results are as followed：1. The electrochemical performances of the-, γ-,-MnO₂with different crystallographicstructures was systematically investigated in0.5mol L^-1Li₂SO₄,0.5mol L^-1Na₂SO₄,1molL^-1Ca（NO₃）₂and1mol L^-1Mg（NO₃）₂electrolytes. The results shown that the electrochemicalperformances of the manganese dioxides are depended strongly on the crystallographicstructures of MnO₂as well as the cation in the electrolytes. Due to the-MnO₂consists withlayers structure and the interlayer separation is7, which is suitable for insertion/extractionof some alkaline and alkaline-earth cations, the-MnO₂electrode shown the highest specificcapacitance than that of-MnO₂and γ-MnO₂. We also found that the-, γ-and-MnO₂electrodes in the Mg（NO₃）₂electrolyte shown a higher specific capacitance, while all the, γand-MnO₂electrodes in the Li2SO4electrolyte exhibited a better cycle life. The reason ofthe different behaviour of Li+and Mg2+during the charge/discharge process can be ascribedto the charge effect of the cations in the electrolytes. The ex situ XRD and long-time cyclicvoltammograms measurements was used to systematically study the energy storagemechanism of MnO₂based electrodes. A progressive crystallinity loss of the materials are alsoobserved upon potential cycling at the oxidized states. A reasonable charge/dischargemechanism is proposed in this work.2. Homogeneously distributed self-assembling hybrid graphene aerogels with3Dinterconnected pores, employing three types of carbohydrates （glucose, β-cyclodextrin andchitosan）, have been fabricated by a simple hydrothermal route. Using three types of carbohydrates as morphology oriented agents and reductants can effectively tailor themicrostructures, physical properties and electrochemical performances of the products. Theeffects of different carbohydrates on graphene oxide reduction to form graphene aerogels withdifferent microcosmic morphologies and physical properties were also systemically discussed.The electrochemical behaviors of all graphene aerogel samples showed remarkably strong andstable performances, which indicated that all the3D interpenetrating microstructure grapheneaerogel samples with well-developed porous nanostructures and interconnected conductivenetworks could provide fast ionic channels for electrochemical energy storage.3. We developed an in situ reduction method to form three kinds of hybrid materials ofMnO₂wrapped graphene aerogels （GAs） based on three kinds of GAs for supercapacitor. TheGAs obtained in our work hold well-defined porous distribution, which not only boosted ionand electron movement in electrochemical processes but also could serve as a3D skeleton forcombining with MnO₂. Due to the buffer effect of the porous GAs, it have excellent cyclingperformance with high specific capacity. The synergic effects of the two components lead to acomposite material with high, fast, and stable energy storage.