Preparation And Electrochemical Property Of Graphene And Its Composite Based On Nickel Oxide

Graphene is a two-dimensional monolayer of carbon atoms packed into a honeycomb lattice which was discovered by Andre Geim and Konstantin Novoselov in 2004. Graphene has a very special electronic structure, which gives rise to its excellent electronic properties. At room temperature, the carrier mobility and thermal conductivity of graphene are up to 200000 cm2 V-1 S-1 and 5300 W m-1 K-1 respectively, which are of great benefit to reduce the internal resistance and improve the thermal performance of supercapacitor. In addition, the theoretical specific surface area of single-layer graphene is as high as 2600 m2 g-1. The specific capacitance of single-layer graphene measured by CV was 21μF cm-2, thus the theoretical specific capacitance of graphene electrode in EDLC is up to 550 F g-1. Therefore, graphene shows widely potential application in supercapacitors. However, the electrochemical performance of graphene in supercapacitors is affected by many factors, such as the oxygen functional groups、morphology、conductive agents and polymer binders, et al. At the same time, the supercapacitors based on graphene, as electrochemical double layer capacitance (EDLC), have high power density but suffer from low specific capacitance and weak energy density. In this paper, we focused on studying the influence of oxygen functional groups、morphology、conductive agents and polymer binders on the electrochemical performance of graphene. We also explored the changes of electrochemical performance of graphene from modification of NiO nano-particles and the synergies between graphene and NiO. The main content included the following parts:1. Thermal expansion grapheme (RGO) was prepared by microwave irradiation reducing garphene oxide. Due to the short time of microwave irradiation, graphene oxide could not reduce completely. FTIR and XPS results show that the oxygen functional groups of graphene include carboxyl、phenol、quinone and carbonyl. And the SEM and TEM images show that the presence of oxygen functional groups would increase the folds of graphene. In neutral solution, electrochemical tests indicated that the charge conversion/storage mechanism of graphene with oxygen functional groups depends on the electrical double layers. While in an acidic or alkaline electrolyte, the pseudocapacitor is raising from fast Faraday redox reaction between the electrolyte and the oxygen functional groups. From the galvanostatically charge/discharge measurements, in 1 M H2SO4 electrolyte, the specific capacitance of thermal expansion graphene (800 W) was 276 F g-1 at the current density of 1 A g-1. However, in the 5M NaOH electrolyte, the specific capacitance at the same current density was 245 F g-1. The specific pseudocapacitance per unit atomic percentage for either carboxyl or phenol group in 5 M KOH was obtained as 22.34 F g1 at.%-1. For carbonyl group and quinone group in 1 M H2SO4, they were a slightly deviated value of 27.585 F g"1 at.%-1 from that of carboxyl or phenol.2. Garphene oxide was 80℃ preheated for 2 h before microwave irradiation reduction. BET results show that after pre-treatment, the specific surface area、pore volume and mesoporous rate of thermal expansion graphene were 304 cm2 g-1、1237.34 cm3 g-1 and 88.52% respectively, which significantly increased comparison to the graphene without pre-treatment. In the Na2SO4 electrolyte, the specific capacitance of RGOp at a scan rate of 1,5 and 10 mV s-1 were 187、177 and 172 F g-1 respectively, which high than that of graphene without pre-treatment at the same scan rate.3. The electrodes of supercapacitors were prepared via a simple and convenient strategy by microwave-assisted synthesis of functionalized graphene on Ni foam (FG-Ni foam) in ambient condition. The functionalized graphene on Ni foam was directly used as electrode for electrochemical double layer capacitance without any conductive agents and polymer binders, such as acetylene black and polyterafluoroethylene. The direct contact between FG and the foam current collector without any polymer binder helps to decrease the resistance of the FG-Ni foam electrode and improve the cycle life of the supercapacitor based on the FG-Ni foam electrodes. FG-Ni foam exhibits good electrochemical performance with a maximum specific capacitance of 265 F g-1 at the charge/discharge current density of 1 A g-1 with 5 M NaOH as electrolyte. After 10000 cycling GCD tests, a high level retaining specific capacitance of 261 F g-1 was obtained which still retained 97.7% of the initial capacity and the charge/discharge efficiency was approximate to 99.9%.4. Graphene/nickel oxide composites were prepared by microwave-assisted in-situ hydrothermal synthesis of nano-particles NiO on the defects of thermal expansion graphene. The nanoparticles NiO with an average diameter of 25 nm evenly and densely coated on the graphene sheet. Simultaneously the weight percentage of NiO in the RGO/NiO composites is estimated to be 19.8 wt%. In 5 M NaOH electrolyte, the specific capacitance of RGO/NiO composites was 608 F g-1, exceeding that of graphene@NiO mixtures (347 F g-1) by a factor of 1.75. And the specific capacitance of NiO in RGO/NiO composites and RGO@NiO mixtures were 1856 F g-1 and 915 F g-1 respectively. At the same time, the specific capacitance of graphene in RGO/NiO composites was 296 F g-1, which was 1.44 time than that of graphene in RGO@NiO mixtures. It is obviously to see the synergies between grapheme and NiO in composites, which leading to make the electrochemical performance of them close to the theoretical value.