The Study Of Manganese Oxides Electrode Materials For Supercapacitors

Supercapacitors have been recognized as unique energy storage devices whichfilled the gap between convention dielectric capacitors and batteries. Supercapacitorshave higher energy density than conventional capacitors, higher power density andlonger cycle life than batteries. Supercapacitors, as advance and clean storage, canrelease large discharge current instantly, decrease maintenance cost and offer highcharge-discharge efficience. Based on the above, the promising application ofsupercapacitors in the fields such as electric vehicle, information technology,consumer electronics, aviation＆aerospace etc, have been attracting more and moreattention throughout the world. But existing supercapacitors can’t replace batteries insome fields entirely, because which have the restriction of low energy density. Thedeveloping direction of supercapacitors is that increase energy density,but can’tsacrifice high power density.Advance electrode materials for capacitors is vital to elevate electrochemicalproperties of supercapacitor. Transition-metal oxides, i.e. RuO2,MnOx,NiO, Co3O4,Fe3O4,VOx,etc, have been the most intensively studied. Manganese oxides havedrown much attention due to their high theoretical value of1370F/g, abundance, lowcost, wide electrochemical window, various oxidation value states and less harmfulnature, which have been considered to be the most promising electrode material forsupercapacitors. But current manganese oxide-based supercapacitors are constrainedby poor manganese oxides electrical conductivity. The limitation result to don’tincrease energy density through adding mass of manganese oxides active materials.Nowadays, the problem have been partially solved, through high conductive carbonmaterials(carbon nano tube, graphene) and manganese oxides compose compositeelectrode materials. This method is very intricate and don’t product and applypractically. In our work, we adopt a easy, low-cost chemical synthetic methodpreparing nano-structure manganese oxides at three dimensional, high conductivecarbon materials. The microstructure, morphology and chemical composition werecharacterized by scanning electron microscopy, X-ray diffraction,etc. Electrochemical capacitance of electrodes were investigated by cyclic voltammetry and galvanostaticcharge-discharge technique.In the first part, we select three-dimensional, net and conductive carbon fiberpaper(CFP) as substrate to grow nano-structure manganese oxides. The resultsindicated that synthesized manganese oxides is multivalent including Mn4+and Mn3+and grow in the form of nanowire at surface of CFP. Expressed as a-MnOx·nH2O.Changing deposition times increase active materials mass of manganese oxides atCFP substrates and synthesized MnOx/CFP composite electrodes annealed at differenttemperatures. The result of experiment show that MnOxdeposition mass increasecontinually with adding of deposition times and3D conductive structure with CFPretain commendably. This structure not only elevate specific surface area, but alsofacilitated to provide a quick and easy path for the ions diffusion and electrontransfer.The structure enhance utilization percentage of MnOxactive materials too.The composite electrode with loading active materials~0.25mg/cm2exhibit highspecific capacitance(271.5F/g) at the current density of0.5A/g. Further, the MnOxelectrode displayed remarkable long-term cycling stability,charge-discharge at thecurrent density of15A/g after1000cycles showed little change, or so retention95.8%.The sample that deposition mass of1.04mg/cm2have203.6F/g at the current densityof0.5A/g. The result show that we can increase energy density by elevating MnOxactive materials. The samples annealed at different temperature(200,400℃) act outbetter cycling stability than unannealed samples. Specific capacitance remain131.4%after1000cycles at the current density of6A/g with400℃annealed sample. Butspecific capacitance drop visibly.In the second part, we select other carbon materials as substrate, such as carboncloth(CC), carbon felt(CF). These carbon materials have the same3D conductivestructure. We obtain good results in these substrates. The specific capacitance ofMnOx/CF composite electrode is240.4F/g and MnOx/CC composite electrode is249.7F/g at the current density of0.5A/g, while specific capacitance don’t decreaseremarkably when active materials mass increase. Cycling stability are both nice,respectively retention82.9%and94.1%after1000cycles. The method that water bath reduce potassium permanganate growing MnOxactive materials at different carbon materials is simple.we don’t use conductiveadditive and binder. The carbon fiber paper as substrate grow MnOxactive electrodematerials have higher electrochemical performance through contrasting differentcarbon substrates. But the price of carbon cloth and carbon felt are inexpensive andthese substrates are significant to study and practical application. This simple methodis benefit to practical production and utilization of electrode composite materials forECs.