Preparation Of Biomass-based Porous Carbon Nanosheets And Investigation Of Their Supercapacitor Performance

In recent years,biomass precursors have been widely used in many fields such as biosensing and energy storage devices due to their rich sources,low cost,unique structure and abundant heteroatoms.As a new energy storage device,supercapacitors have rapid charge-discharge and high energy density and have received numerous interests in energy application fields such as electric vehicles.According to energy storage mechanisms,supercapacitors can be divided into two types:electric double layer capacitors（EDLC）and pseudocapacitors.Through electrochemical adsorption/desorption of cations and anions at the electrode/electrolyte interface,the EDLC forms a dense electric double layer to store the electric charge,and has the advantages of good chemical stability,fast charge/discharge rate and high energy density.For example,porous carbon is a kind of ideal EDLC the electrode material.However,EDLC generally have some disadvantages such as lower specific capacitance and lower energy density.The pseudocapacitor stores the charge through the surface reversible Faraday reaction,which is famous for its higher specific capacitance.Usually,conductive polymers and transition metal oxides are used as pseudocapacitor electrode materials.However,the application of pseudocapacitors is limited due to their shortcomings such as poor cycle stability and electric conductivity.Therefore,this dissertation proposes the use of biomass as carbon precursors through a variety of preparation methods,such as control of material morphology,change of element doping,increase of specific surface area,to prepare a series of porous carbon and carbon-based composite materials.Moreover,the structure and performance of materials were characterized by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,nitrogen adsorption-desorption technique,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and Raman spectroscopy.Their electrochemical properties were investigated using an electrochemical workstation.The correlative works were summarized as follows:1.Using glucose as a carbon source,oxygen-enriched porous carbon nanosheets（OPCNs）were prepared in one-step synthesis by the assistance of Mg₂（OH）₂CO₃that was decomposed to gases and magnesium oxide,respectively acting as blowing activator and hard template.The effects of different weight ratio and carbonization temperature were systematically investigated,and the comparisons with traditional activations,template synthesis and chemical blowing method were also studied.The sample OPCNs-1 has abundant oxygen content of 13.5 at%and a high specific surface area.Electrochemical tests show that the prepared OPCNs-1 has higher specific capacitance than those porous carbons prepared by other methods,and have good rate performance as well.2.Soybean milk powder was used as the natural nitrogen-doped carbon precursor.Nitrogen-doped porous carbon nanosheets（NPCNS）with broken nano-sheet structure were prepared by using CaCO₃ nanospheres as hard template and KOH as activator and molten salt template.The obtained sample NPCNS-4-1-2 has the largest specific surface area and micro-meso-macroporous hierarchical structure.Electrochemical tests indicate that the specific capacitance of NPCNS-4-1-2 is as high as 240.7 F g^-1(1 A g^-1),and excellent rate performance(the original 92.2%is retained at 20 A g^-1).Using KOH-PVA as a solid electrolyte,the assembled all-solid- state symmetric supercapacitor has high energy density(energy density is 10.2 Wh kg^-1 at a power density of 351 W kg^-1)and good cycleability（a 89.3%retention of the intial capacitance after 5000 cycles）.3.Peach gum with self-template effect was employed as carbon precursor to prepare nitrogen- doped porous carbon nanosheet materials using KOH activation and urea as nitrogen precursorThe prepared SNACNs have the highest specific surface area and high oxy-nitride doping（1.8 at%nitrogen content and 18.1 at%oxygen content）.Electrochemical tests demonstrate that the specific capacitance of SNACNs is as high as 350.6 F g^-1(1 A g^-1),and good rate performance(the initial 71.2%is retained at 40 A g^-1).The assembled symmetrical supercapacitor has a high energy density(energy density of 17.3 Wh kg^-1 at a power density of 449.6 W kg^-1)and good cycleability（90.2%retention after 5000 cycles）.4.Using peach gum as a carbon source and magnesium acetate as a sacrificial template,the Ni（OH）₂/CNS was prepared by a simple ion exchange process.It was shown by HRTEM that nickel hydroxide sheets were tightly covered onto the surface of carbon sheets.By adjusting the amount of magnesium acetate,the optimum material Ni（OH）₂/CNS-4 was obtained,which has a specific capacitance of 2457 F g^-1(1 A g^-1).The assembled asymmetric supercapacitor has a high energy density of 36.9 Wh kg^-1(power density of 400 W kg^-1)and good cycle performance（83.9%retention after 5000 cycles）.