Preparation Of MnO₂/Graphene Composites And Their Application To Supercapacitor Electrodes

Supercapacitors （Electric Double Layer Capacitors, EDLC） are novel energy storage systems between the battery and the ordinary capacitors. Because of their fast charge and discharge, high efficiency, good cycle performance, environmentally friendly, more safety, supercapacitors have been applied in many fields, such as national defense, aerospace, telecommunications, electricity and railways. Electrode material is the key material impact supercapacitor performance. Therefore it is very important for us to prepare high-performance electrode material. In this paper, we researched the high-performance electrode material by graphene （graphene -like） and MnO₂/graphene （graphene-like） composite. The main research contents include:（1） Graphite oxide was prepared by the improved Hummers method. Influence factors which affect the oxidation degree of graphite, including H₂SO₄ and H₃PO₄ volume ratio, KMnO₄ amount and reaction time were researched. The results show that it is good for the oxidation degree of graphite intercalation and oxidation of 1g graphite, and layer spacing of graphite oxide could reach 7.91 nm, when infulence factors were H₂SO₄:H₃PO₄= 9:1 （90 mL:10 mL） （volume ratio）,6 g KMnO₄ and reaction time 24 h.（2） Two graphene materials were prepared by two different methods which one is hydrazine hydrate and the other one is high temperature reduction in inert atmosphere. The crystal structure and the surface topography of all materials were characterized by means of XRD, FT-IR, Raman, SEM, TEM and TG. Photographs of SEM and TEM showed that two graphene materials are folding and curling. Moreover, the graphene material is thinner and less layers using high temperature reduction in inert atmosphere than using hydrazine hydrate. The average capacitance of using hydrazine hydrate and using high temperature reduction in inert atmosphere are 94.6 F/g and 122.3 F/g at 0.5 A/g in 1 mol/L Na₂SO₄ electrolyte.（3） Two different MnO₂/graphene nanocomposites were synthesized by two different reaction procedures. M1 was synthesized as follows:first, honeycomb MnO₂ was synthesized on the graphene oxide via various functional groups （MnO₂/graphene oxide）. Second, MnO₂/graphene composite was obtained using hydrazine reduction of MnO₂/graphene oxide. M2 was formed as follows:first, honeycomb MnO₂ was synthesized on the graphene nanosheet which was reduced by hydrazine reduction of graphene oxide. The results showed honeycomb MnO₂ of M1 homogeneously deposited onto the surface of graphene and found sesame cake structure. However, the honeycomb MnO₂ of M2 formed aggregates because of absence of functional groups. The capacitive properties of the M1 and M2 were evaluated by cyclic voltammetry, galvanostatic charge/discharge tests and electrochemical impedance, respectively. The M1 have a good electrochemical performance and the average capacitance is as high as 250.6 F/g at 1 Ag in 1 mol L-1 Na₂SO₄ electrolyte, which was higher than that of the M2 （206.8 F g-1）. Therefore, these results demonstrate more exciting potentials of the M1 for high performance supercapacitors or other power source system than M2.（4） Graphene-like was prepared using liquid-polyacrylonitrile （LPAN） as the carbon source. The influence of different carbonization temperature（1200,1300,1400 and 1500℃） on the structure and performance of the graphene-like was investigated. The structure and the surface topography of all materials have been investigated by X-ray diffraction （XRD）, Fourier transform infrared spectrum （FT-IR）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and Raman microscopy, respectively. The results show that graphene-like has a typical graphene-like layer carbon structure. And the graphene layer structure is more and more obvious with the increase of carbonization temperature. Otherwise, different content of MnO₂ were prepared in situ on graphene-like （1500℃）. The influence of the mass ratio of graphene and MnO₂ on the structure and performance of the composite was investigated. At the mass fraction of graphene-like of 30%, the composite has a good electrochemical performance and the average capacitance is as high as 302 F/g at 1 A/g in 1 mol/L Na₂SO₄ electrolyte. After 1000 times cycle, the specific capacitance equal to 265.8 F/g, attenuation rate of 12%.