Controlled Synthesis And Supercapacitive Properties Of Polyimide-Derived Nitrogen-Doped Carbon-Based Electrode Materials

Carbon-based supercapacitors（SCs）have attracted numerous attentions in the realm of energy storage as supplement or even replacement of batteries,by virtue of their high power,long life and good safety.Nevertheless,the low energy density of carbon-based SCs has obstructed their utility in many emerging strategic applications such as electric vehicles.This should be credited to the intrinsically limited capacitance of carbon based on double-layer capacitor,and significantly decreased specific surface area（SSA）caused by agglomeration during the fabrication process.Therefore,this work has focused on the performance optimization of carbon-based supercapacitors from the following aspects:devising high SAA and/or porous structure to increase the active sites;introducing heteroatoms for electrical properties modulation and extra pseudocapacitance;hybridization with high-capacity active materials.The main research content of this thesis is depicted as follows:1.The structure and properties of nitrogen-doped carbon electrode regulated by polymerization.A series of N-doped nanocarbons with distinct morphologies are obtained by pyrolysis of polyimides（PIs）prepared through hydrothermal/solvothermal/conventional reflux polymerization processes.Among them,the N-doped carbon nanobelts（NCB）present abundant surfaces accessible for charge accommodation,interconnected 3D conductive networks favorable for electron/ion transportation,and robust structural stability,as well as high N-doping levels and electroactivity-enriched N configurations for enhanced charge storage capability and surface wettability.Remarkably,the symmetric supercapacitor assembled by such NCB electrodes delivers large specific capacitance(193 F g^-1 at 1 A g^-1),high-rate capability（the capacity retention rate is 91%when the current density is increased by 40 times）,and long cycling stability(nearly 100%retention over 10,000 cycles at 20 A g^-1).The energy density is as high as 6.75 Wh Kg^-1 at the power density of 250 W Kg^-1,and retains 6 Wh Kg^-1at the power density up to 10,000 W Kg^-1.Therefore,this study would provide new insights for exploring nitrogen-doped carbon-based electrode materials with controllable structure,scalable production and well-balanced power and energy output.2.The structure and properties of nitrogen-doped carbon electrode regulated by macromolecules.Based on the above-established synthesis strategy,linear PIs with varied chain structures are obtained via hydrothermal polymerization of diamines containing different flexible linking groups and the identical anhydride as monomers.The incorporation of flexible linkers between redox-active anhydrides results in the facile movement of polyimide chains and the formation of porous 3D networks with high specific surface area.In contrast,rigid aromatic linkers with strongπ–πconjugation tend to assemble into the orderly-stacked sheets,enabling the efficient electron transfer.Due to the excellent structural and thermal stability of PIs,the derived N-doped nanocarbons can well inherit their parental structure merits.Further electrochemical measurements reveal their potential as electrode materials for supercapacitor application.This work demonstrates a simple,efficient and environmentally friendly universal method for the synthesis of PIs,and also provides a new idea for the rational design of the nanostructured carbon electrode materials.3.The structure and properties of nitrogen-doped carbon regulated by surfactant.Hierarchically porous heteroatom–doped carbons（HDCs）are prepared through direct carbonization of polyimide precursors obtained in the presence of cetyltrimethylammonium bromide as a structure-directing agent.The resulting morphologies and porosities of the HDCs can be readily tailored by tuning the dosage of CTAB.Such exquisite hierarchically porous nanostructures guarantee a large ion accessible surface area,a short ion diffusion distance and a large number of ion diffusion channels.Multiple heteroatom-doping in the carbon skeleton can further improve its surface wettability and electrical conductivity,and contribute extra pseudocapacitance as well.The symmetric supercapacitor assembled by the HDC₂ electrodes presents the best overall electrochemical performance,including excellent rate capability(83.3%retention from 0.5 to 20 A g^-1),high energy density(8.4 Wh kg^-1 at 150 W kg^-1),and exceptional durability.This work establishes a simple,green,and reproducible strategy to convert the low-cost polymers into high-performance carbon materials for potential electrochemical energy storage applications.4.Nitrogen doped carbon coated Mo₂C nanocrystals for lithium-ion capacitors.Ultrasmall Mo₂C nanocrystals supported on N-doped carbon nanobelts/nanosheets（Mo₂C@NCB and Mo₂C@NCS）are fabricated by direct carbonization of the composite precursor obtained through the hydrothermal polymerization of PIs in the presence of phosphomolybdic acid(PMo₁₂),showing capacitive-dominant storage mechanisms.The as-assembled lithium-ion capacitor（LIC）using Mo₂C@NCB or Mo₂C@NCS as anode and NCB as cathode delivers a high operating voltage,superior power delivery and energy output,and long-term cycling stability,thus bridging the gap between batteries and SCs.Particularly,the Mo₂C@NCS//NCB device exhibits an energy density of 126 Wh kg^-1 at the power density of0.2 k W kg^-1 and 27 Wh kg^-1 at 40 k W kg^-1,and retains 80.4%of initial capacity after running10,000 cycles at 5 A g^-1,achieving an excellent energy-power-lifespan combination.The LIC assembly strategy established in this work provides new insights into narrowing the kinetics and capacity discrepancy between anode and cathode.