Preparation And Properties Of Graphene / MnO <2> Composites For Supercapacitors

With the high power density, high energy density and outstanding cycle stability, the graphene/MnO2 composite is thought to be a potential electrode materials used in industrialized applications. However, the high cost of preparing graphene severely restricts the further development of this material, in which way further reducing the cost of preparing graphene and optimizing the preparation process of the composites has great significance for the commercial application of this electrode material.In this research, the graphene of high oxygen content obtained after the oxidation-thermal expansion was first reduced with NaBH4, hydrazine hydrate and ascorbic acid, followed by the second reduction using H2SO4-H2, hydrazine hydrate and H2. Used the obtained graphene as the structure support and KMnO4 solution as the Mn source, the graphene/MnO2 composites was synthesized at different M/G (the theoretical mass ratio of MnO2 and graphene) conditions by the hydrothermal method. The results of SEM, TEM, XRD, FT-IR and electrochemical performance tests were presented to analysis the effect of the second reduction on the properties of graphene. Furthermore, the effect of M/G on the electrochemical performance of the composites was fully investigated and the best M/G conditions were obtained.(1) The electric conductivity of graphene firstly reduced by NaBH4, hydrazine hydrate and ascorbic acid were significantly improved after the second reduction with H2SO4-H2. The electric conductivity of graphene prepared by the first reduction with hydrazine hydrate as the reducing agent and the second reduction with H2SO4-H2 was as high as 669.68 m-1. The optimum condition of M/G for the synthesis of graphene/MnO2 composites were 7,3 and 3, respectively, when using three kind of graphene reduced firstly by NaBH4, hydrazine hydrate and ascorbic acid, followed by the second reduction using H2SO4-H2. The specific capacitance of the corresponding composites reached 329,380 and 397F/g and the retention rate of its initial capacitance after 5000 charge-discharge cycles at the current density of 30A/g were 78.5%,90.1% and 97.2%.(2) The electric conductivity of graphene firstly reduced by NaBH4 rose from 94.4S·m-1 to 291.38·m-1 after the second reduction with hydrazine hydrate. The graphene/MnO2 composite synthesized when M/G was 14 had a comparatively high capacitance. Its specific capacitance reached 420,239,180 and 116F/g at the current density of 0.3,1,3 and 10A/g. The retention rate of its initial capacitance was 60% after 5000 cycles at the current density of 30A/g.(3) The conjugate structure of graphene firstly reduced by NaBH4 was effectively repaired and its electric conductivity rose from 94.4S·m-1 to 407.1S·m-1 after treated by the second reduction with H2. The graphene/MnO2 composite synthesized when M/G was 2 has the highest specific capacitance which reached 928,742,465 and 325F/g at the current density of 0.3,1,3 and 10A/g. The retention rate of its initial capacitance was as high as 99.5% after 5000 cycles at the current density of 30A/g.