Preparation And Electrochemical Properties Of Graphene-based Electrode Materials For Supercapacitor

Supercapacitors, also called electrochemical capacitors, have been widely applied inmany fields acted as environmentally friendly and good performance energy storage devices. Graphene （GNS）, an emerging two-dimensional nano-materials, has excellent physical,chemical and thermal properties. Graphene has a large specific surface area and excellentelectrical conductivity, making it an ideal electrode material for electrochemical capacitors. Inaddition, graphene is a single layer of graphite materials, thus it can take full advantage of itsentire surface to form electric double layer during the process of electrochemical reaction. Inorder to improve the electrochemical properties of graphene, the graphene-based compositesas the electrode material of supercapacitors are currently an extremely hot research topic.In this dissertation, the corrugated graphene, graphene/MnO₂composite andgraphene/TiO₂/MnO₂composite as supercapacitor electrode materials were prepared bydifferent approaches. The microstructure and morphology of the prepared materials wereexamined by the X-ray diffraction （XRD）, scanning electron microscopy （SEM）, transmissionelectronic microscopy （TEM）, Raman spectroscopy and so on. Electrochemical propertieswere characterized by cyclic voltammetry, galvanostatic charge-discharge andelectrochemical impedance spectroscopy.We used the hot and cold stress generated through the dramatic temperature changes toform corrugations and folds on graphene surface, and the high temperature liquid nitrogenmethod was the optimum process. In addition, considerable mesopores （15⁴0nm） can beobserved on the graphene surfaces owing to the carbon oxidation at elevated temperatureunder air atmosphere. The maximum specific capacitance of349F·g-1at2mV·s-1wasobtained for the corrugated graphene electrode in6mol·L^-1KOH aqueous solution.Additionally, the electrode showed excellent electrochemical stability along with anapproximately8.0%increase of the initial specific capacitance after5000cycle tests.With graphene as a conductive carbon substrate, graphene/MnO₂composites weresynthesized by the redox reaction between potassium permanganate and ethanol. Meanwhile,we investigated the effect of corrugated graphene layers, different MnO₂loading and themicroscopic structure of the substrate on electrochemical properties of graphene/MnO₂composites. With the similar loading of MnO₂, GGM2material showed higher specific capacitance than HGM2, indicating the specific capacitance of GNS/MnO₂composites isrelated to not only the amount of load of MnO₂but also the microstructure of the conductivecarbon substrate. With the increase in the amount of manganese dioxide loading, the specificcapacitance of GNS/MnO₂composites firstly increased to the maximum value and thendecreased, demonstrating MnO₂loading had a great influence on the the specific capacitanceof the composites.With titanium dioxide as an electron transfer medium, graphene/TiO₂/MnO₂compositeswere synthesized by a microwave irradiation method. In addition, we investigated the effectof different test conditions on electrochemical properties of graphene/TiO₂/MnO₂composites.Electrons of titanium dioxide are excited in the continuous irradiation of the ultraviolet （UV）lamp and prouced holes and defects. Due to the uniform coating on titanium dioxide, theconductivity of the manganese dioxide was greatly improved. The electrochemical test resultsshowed that the specific capacitance of GNS/TiO₂increased from116.6F·g-1to216.8F·g-1atscan rate of2mV·s-1when MnO₂was incorporated. In addition, the specific capacitance ofgraphene/TiO₂/MnO₂composites increased nearly by10%from168.5F·g-1to182.1F·g-1at ascan rate of50mV·s-1under UV light irradiation.Finally, we assembled two hybrid electrochemical supercapacitors GGM2//GY-GNS andHGM2//HY-GNS with prepared graphene/MnO₂composites and graphene as positive andnegative electrodes, respectively. The electrochemical properties of supercapacitor wereinvestigated in1mol·L^-1Na2SO4electrolyte between the potential window of0¹.8V. Theobtained power density of hybrid capacitor GGM2//GY-GNS could reach164.93W·kg-1at anenergy density of41.23Wh·Kg-1, and still maintained8205.86W·kg-1when the energydensity was8.21Wh·Kg-1, which was better than the other hybrid electrochemicalsupercapacitors.