Preparation Of Hierarchically Porous Carbon Derived From Biomass And Application In Supercapacitors

Electrode material is the most critical factor in determining the electrochemical performance of supercapacitors.Waste biomass has been widely used in supercapacitors due to its good electrical conductivity and large specific surface area.As a new type of carbon material,biomass carbons own many advantages such as wide source,low price,simple preparation process,eco-friendly,and attracted much attention in the field of energy storage.In this thesis,two biomass materials?nanocellulose and wood?were selected as precursors and the electrochemical properties of biomass carbons were improved by nitrogen doping or loading some transition metal hydroxides.The morphology,chemical composition,characterization test of structure,electrochemical performance of composite electrode are investigated.The main contents are as follows:?1?One piece of natural poplar as a precursor,after carbonized in nitrogen atmosphere at1000 ^oC for 2 h,a 3D structure wood-based biomass carbon material with hierarchically porous was prepared and used as a self-supporting electrode.The CW electrode has a large number of micropores and mesopores with a high surface area of 568.13 m²/g.The electrochemical performance of the CW was tested.The results show that the energy storage mode of CW is the electric double layer energy storage mode,which exhibited a high area specific capacitance?area specific capacitance is 3.274 F/cm² at the current density of 1mA/cm²?and good rate performance?capacitance retention is 59%when the current density increased from 1 mA/cm² to 20 mA/cm²?.?2?The Co?OH?₂ nanoflakes were grown in situ on the surface of the porous CW?including the inside walls of the channels?using a typical electrodeposition process.The areal specific capacitance is about 3.723 F/cm² at a current density of 1.0 mA/cm²,even at a high current density of 30 mA/cm²,a high areal capacitance of 1.568 F/cm² can be still achieved,indicating outstanding rate capability of the Co?OH?₂@CW electrode.An ASC was constructed by stacking a CW electrode,a cellulose paper separator,and a CW@Co?OH?₂electrode in a sandwich-like structure,each cell component was infiltrated with the PVA-KOH gel electrolyte.The CW/PVA-KOH/CW@Co?OH?₂ capacitor shows wide potential of 1.5 V,a high capacitance of 2.2 F/cm² at the current density of 1.0 mA/cm².Even at 20 mA/cm²,the capacitance can also achieve 1.3 F/cm² and the capacitance retention is59%.The maximum energy density and power density of ACS are 0.69 mWh/cm² and 15.447W/cm²,respectively.All-solid-state ASC also exhibited excellent capacitance retention,retaining 85%of the initial value after 10 000 GCD cycles,and powered an LED?2.5 V,1.0W?for 30 min by two in series.?3?Hierarchically porous nitrogen-doped carbon?HPNC?with a helically arranged rodlike structure has been synthesized by in situ growth of ZIF-8?zeolitic imidazolate frameworks?nanoparticles on cellulose nanocrystals?CNC?,which simultaneously acts as both a self-template and a carbon source,and controls the growth of ZIF-8 coated CNC,and a subsequent carbonization.The HPNC exhibits helically ordered structure with a large specific surface area.The HPNC950 electrodes has a high specific capacitance of 172 F/g at a current density of 0.1 A/g.Moreover,the maximum energy and power densities were 23.75 Wh/kg and 5000 W/kg.Further,a symmetric supercapacitor is constructed and displays significantly increased electrochemical performances,demonstrating high specific capacity??100 F/g at0.2 A/g,and?74 F/g at 5.0 A/g?,superior rate performance??70 F/g at 10 A/g?,while maintaining excellent cycling stability??81%of the initial capacitance is achieved after 5000 cycles?.