Construction Of Three-dimensional Nanocarbon Based Electrode Materials And Their Application In Supercapacitors

With the rapid development of economy,many environmental problems were caused by excessive consumption of oil and fossil fuels.Energy crisis and environmental pollution have become two major problems that people face in modern society and need to be solved urgently.It is imperative to develop new type of green energy for solving the energy crisis and environmental problems.Many new types of energy sources(wind power,hydrogen power,solar energy and so on)obtain development and utilization.However,the suitable energy storage device is the important foundation for the new energy development and renewable energy utilization.Compared with other energy storage devices,supercapacitors are a kind of potential new green energy storage device due to their high power density,rapid charging and discharging,long cycle life,low maintenance cost and environmental friendly.Current research work on supercapacitors has become a research hotspot in recent years.However,the energy density of supercapacitors of commercialization is relatively low,and it is difficult to meet the needs of practical application.Therefore,it is imperative to develop supercapacitors with high energy density.In this paper,we construct three-dimensional electrode materials by template and electrostatic assembly method using wheat flour,graphene oxide as the precursor of nanocarbon materials.The microstructures and surface chemistry of the materials were characterized by X-ray diffraction,scanning and transition electron microscopy,Raman spectra and X-ray photoelectron spectroscopy.Cyclic voltammetry,galvanostatic charge-discharge method and electrochemical impedance method were further employed to investigate the electrochemical performance.In additation,our efforts have focused on assembling symmetric//asymmetric system to achieve the purpose of improving the energy density.The main researches are as follows:(1)Three-dimensional functional graphene(TRGN)was successfully synthesized by depositing graphene oxide onto nickel foam and subsequent low temperature reduction process using Ni foam as template and current collectors.Tight integration of graphene with nickel foam and three-dimensional graphene network structure can effectively reduce the internal resistance of the electrode material,which is conducive to the electrons/ions rapid transfer.Moreover,there are a large number of oxygen-containing functional groups on the surface of graphene by the low temperature reduction of GO possess.Oxygen containing functional groups can generate pseudocapacitance in alkaline electrolyte,and produce higher specific capacitance than the electric double layer.As a result,the TRGN electrode delivers a high specific capacitance of 442.8 F·g-1 at 2 mV·s-1 in 6 mol·L-1 KOH electrolyte.Moreover,symmetric supercapacitor based on TRGN exhibits a maximum energy density of 30.4 Wh·kg-1 as well as excellent electrochemical stability with 118%of its initial capacitance after 5000 cycles,showing that TRGN material have excellent electrochemical stability.(2)Three-dimensional interconnected honeycomb-like porous carbon(HPC)has been fabricated through one-step carbonization and activation using KOH as activating/template agent and wheat flour as carbon precursor.Due to its interconnected porous structure(macroporous,mesopores,micropores)with narrow pore size distribution,high specific surface area(1313 m2·g-1),and high heteroatom doping(N:1.1 at%,O:11.2 at%),the HPC electrode exhibits a high specific capacitance of 473 F·g-1 at 0.5 A·g-1 in 6 mol·L-1 KOH electrolyte and outstanding electrochemical stability with capacitance retention up to 94.5%after 10,000 cycles,showing that HPC electrode materials possess high specific capacitance and good electrochemical stability.(3)Nanowire-like manganese dioxide were deposited onto the surface of the HPC under microwave irradiation conditions(HPC/MnO2).Three dimensional interconnected porous structures are conducive to fast ion diffusion in the electrode material,and the smaller nanowires are conducive to shorten the ion diffusion pathway.An electrode made of this material exhibits exhibits a maximum specific capacitance of 259 F g-1 at 2 mV·s-1 and excellent rate performance(68%retention ratio at 200 mV·s-1)in 1 mol·L-1 Na2SO4 electrolyte.Moreever,the HPC//HPC/MnO2 asymmetric supercapacitor exhibits a high energy density of 63.5 Wh·kg-1 and good electrochemical stability(93.4%of initial capacitance retention after 5000 cycles)using HPC as negative electrode and HPC/MnO2 as positive electrode.(4)Graphene/Co-Al hydroxide composite materials(GSP-LDH)with layer by layer structure were synthesized by dodecyl sulfate anions intercalate into the interlayer of Co-Al hydroxide nanosheets as the support structure during NaOH etching of Co-Al hydroxide nanosheets and subsequent electrostatic assembly with graphene oxide.Because the graphene and dodecyl sulfate ion serves as the structural support of the porous LDH and graphene nanosheets have excellent electrical conductivity,resulting in GSP-LDH possess fast ion/electron transport and good structural stability.The GSP-LDH electrode exhibits remarkably improved electrochemical characteristics such as high specific capacitance(1043 F·g-1 at 1 A·g-1)and excellent rate performance capability(912 F·g-1 at 20 A·g-1)in 6 mol·L-1 KOH electrolyte.In addition,the assembled graphene/porous carbon(SGC)//GSP-LDH asymmetric supercapacitor using SGC as the negative electrode and GSP-LDH as the positive electrode delivers a high energy density up to 20.4 Wh·kg-1 at a very high power density of 9.3 kW ·kg-1 and good electrochemical stability with 84%initial capacitance retained after 2000 cycles,showing that good electrochemical stability.