Synthesis And Modification Of MnO₂ Material For Supercapacitors

Manganese oxide?MnO₂?has been considered as the most promising material used as supercapacitor for its low cost,abundance,high theoretical specific capacitance and good environmental compatibility.But the real specific capacitance of MnO₂ is far from the theoretical value.The reason can be due to the poor conductivity of MnO₂ as a kind of semiconductor materials and the low utilization ratio of the active component.Only a very thin surface layer of MnO₂ can take part in the charge/discharge process.But the underlying MnO₂ material can not participate in the reaction,which limit its development and application.Therefore,in order to improve the conductivity and theoretical capacity utilization,MnO₂ electrode materials have been prepared by low-tempreture solid reaction,hydrothermal synthesis and electrochemical deposition.Furthermore,MnO₂/C composites were synthesised by doping with different types of carbon materials.The microstructure,morphology and electrochemical properties of the target composites were characterized by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,transmission electron microscope?TEM?,cyclic voltammetry?CV?,constant current charge/discharge and electrochemical impedance spectrums?EIS?techniques,respectively.Some valuable results were obtained and the main points of this thesis are summarized as follows:1.MnO₂/C composites were prepared by low-temperature solid-state method.It was demonstrated that the molar ratio of KMnO4/MnCl2 has a strong effect on the structure,morphology and electrochemical properties of the MnO₂ product.XRD and SEM images show that the obtained MnO₂ is a-MnO₂,and the average diameter is about 0.5?1.5 ?m,which are constituted of nanoparticles of 20 nm.Electrochemical measurements showed that under 100 mA/g,the discharge specific capacitances of the prepared sample is 259,220,215,199 and 210 F/g at the KMnO4/MnCl2 molar ratio of 3:2,2:1,1:1,1:2 and 2:3,respectively.In addition,the average specific capacitance of the MnO₂ sample?mole ratio of KMnO4/MnCl2 is 3:2?at 100,150,200,250 and 300 mA/g are 259,211,190,172 and 165 F/g,respectively.Thus,the as prepared MnO₂ sample with a KMnO4/MnCl2 molar ratio of 3:2 exhibits the best capacitance performance.Based on the optimized process route,the effects of doping contents of bamboo charcoal?BC?,multi-walled carbon nanotubes?MWNTs?and activated carbon?AC?on the phase structure,surface morphology and electrochemical properties of the composite were investigated.As a result of this study,the adding content of 1 wt.%BC,7 wt.%MWNTs and 7 wt.%AC were defined conclusively as the optimum preparative conditions.And the discharge specific capacitances of MnO₂/BCi wt.%,MnO₂/MWNTs7 wt.%and MnO₂/AC7 wt.%composites at 100 mA/g are 306,349,375 F/g,respectively.2.MnO₂ materials were prepared by hydrothermal method.The influence of preparation conditions on the structure and electrochemical capacitive performances of the materials has been investigated by XRD,SEM,TEM and electrochemical tests.Results show that the as-prepared MnO₂ powders under the optimum reaction conditions have typical ?-MnO₂ structure with hollow/porous micro-nano spheres and micron-rods morphology.The average diameter of ?-MnO₂ micro-nano spheres were about 0.2?0.8 ?m,and the micron-rods with diameters of ca.30 nm and lengths of 5 ?m.The orthogonal and single-factor experiment showed the importance sequence of the various factors to discharge capacitance is material ratio,reaction time,reaction temperature and filling ratio.The results show the optimum reaction conditions were as follows:the reaction temperature is 150?,the molar ratio of KMnO4 to MnCl2 is 2.5:1.0,the reaction time is 3 h and the filling factor was 40%.Under the conditions the product retains 255,170,133,105 and 88 F/g after 5 charge/discharge cycles at 100,150,200,250 and 300 mA/g,respectively.The results of the impedance spectra measurements showed the MnO₂ electrodes had a lower charge transfer resistance?Rct?and good electrochemical capacitance behavior.Both the equivalent series resistance?ESR?and Rct of MnO₂ material presented a gradually increasing trend.Based on the optimized process route,the effects of doping contents of graphite oxide?GO?,graphene?GR?and BC on the phase structure,surface morphology and electrochemical properties of the composite were investigated.The synergy effect reached the best when 1 wt.%GO,5 wt.%GR and 3 wt.%BC were added.When discharged at 100 mA/g,the initial discharge specific capacitances of MnO₂/GOi wt%,MnO₂/GR5wt%and MnO₂/BC3 wt.%composite electrode were 258,257 and 270 F/g.3.MnO₂ electrode materials for supercapacitors were prepared by electroplating method.Orthogonal experiment and single-factor fermentation experiment were employed to inspect the influences of reaction temperature,current density,the type and content of main salt,conductive salt concentration factors on the properties of MnO₂.The optimized conditions are obtained by XRD,SEM,galvanostatic charge-discharge,CV and EIS analyse.The results showed that the optimum reaction conditions were as follows:the reaction temperature was 50? the current density is 15 mA/cm2,the type and content of main salt is 0.2 mol/L MnCl2,the conductive salt?Na2SO4?concentration is 0.05 mol/L.Under the conditions the as-prepared sample exhibit morphology of spheres packed tightly and the average diameter of spheres were about 0.01?10 ?m.The first discharge capacitances of the sample are 260,190,181,169,164 F/g at 400,600,800,1000 and 1200 mA/g,respectively.The maximum discharge specific capacitance was obtained when the as-prepared sample doped with 0.04 g/L GO and 0.06 g/L GR),which delivered 260,190,181,169,164 F/g and 337,121,99,93,90 F/g at 400,600,800,1000 and 1200 mA/g,respectively.