Preparation And Electrochemical Properties Of Hierarchically Porous Nitrogen-doped Carbon Materials

Energy storage and conversion play a crucial role in modern energy systems,and the exploration of advanced electrode materials is vital but challenging.In terms of the selection of electrode materials,carbon materials have attracted great interests as electrode materials for supercapacitors,due to their high surface area,electrical conductivity and low cost.As one of the precursors for preparing carbon materials,polyacrylonitrile（PAN）has gradually attracted wide attention due to its ability to generate nitrogen-rich carbon materials with excellent pseudo-capacitive properties.However,its application in energy storage is usually limited by its uncontrollable pore structure.In order to overcome the above problems,this dissertation composes PAN-based nitrogen-rich carbon with other graphitic carbon materials with large specific surface areas,or uses template method to make it have a hierarchically porous structure.By tunning the experimental conditions such as the ratio of the composite components and the reaction temperature,the composition and structure of the products can be well controlled.By investigating the structure-activity relationship and the synergistic effect,the efficiency enhancement mechanism could also be explored,and finally a series of carbon-based electrode materials with excellent energy storage properties were prepared.This dissertation provides new strategy and approach for the construction of carbon-based electrode materials with unique hierarchical porous structures and the realization of high-performance electrochemical energy storage process.The main research contents are as follows:（1）The three-dimensional graphene network embedded with nitrogen-doped carbon nanoparticles were prepared by using graphene oxide（GO）and polyacrylonitrile（PAN）as precursors and through a modified phase inversion method combined with calcination process.The mild non-solvent exchange process can not only effectively solve the problem of easy agglomeration of PAN-based carbon materials,but also restrain the stack of graphene nanosheets,and thus the controllable construction of the three-dimensional porous structure of carbon materials can be realized.When used as electrode for supercapacitors,the as-prepared material delivered a high capacitance of 431.9 F g^-1 at 0.1 A g^-1 and 156.8 F g^-1 at 20 A g^-1,as well as a stable cyclic behavior without capacitance decay after 5000 cycles.Such a remarkable capacitive performance was attributed to its hierarchically porous structure and proper nitrogen doping content（9.68±0.24 at%）,which facilitated the migration of electrolyte ions and provided abundant redox active sites for the faradaic reactions.The synthetic strategy may provide new idea for the rational design and synthesis of new carbon materials with controlled doping level and three-dimensional porous structure.（2）By using poly（vinylidenefluoride-co-hexafluoropropylene）（PVDF-HFP）as the template,a kind of nitrogen-doped carbon monolith with well-distributed hierarchical pores was prepared through a modified phase-inversion method.Due to the hierarchically porous structure and proper doping level,the synthesized electrode material delivered superior specific capacitances(312.2 F g^-1 at 0.5 A g^-1 and 217.6 F g^-1 at 20 A g^-1).Its high chemical and structural stability make the capacitance stable after 10,000 charge-discharge cycles at 20 A g^-1.Furthermore,the electrode material based symmetric supercapacitor exhibits a remarkable energy density of 7.1 Wh kg^-1at the power density of 250 W kg^-1.The simple yet efficient synthetic strategy and the enhanced electrochemical capacitive performances of the prepared nitrogen-doped carbon make it a promising electrode material for energy storage devices.（3）Through a facile one-step pyrolysis strategy,PAN-derived nitrogen-doped carbon nanoparticles and EDTANa₂Zn salt-derived three-dimensional carbon framework are effectively combined to prepare a hierarchical porous nitrogen-doped carbon matrix material with three-dimensional morphology（HPNC-50）.Benefit from the composite material’s large specific surface area(379.4 m² g^-1),high nitrogen doping content（15.7 at%）and unique three-dimensional hierarchical porous structure,HPNC-50 has a specific capacitance of up to 315.9 F g^-1(at 0.5 A g^-1)and excellent rate performance(capacitance retention rate is 64.8%at 20 A g^-1).In addition,HPNC-50 also has excellent cycle stability.The specific capacitance did not decay after 10,000 charge-discharge cycles at the current density of 20 A g^-1.This research not only effectively solves the agglomeration problem between PAN-derived nitrogen-doped carbon nanoparticles,but also prevents the stacking of EDTANa₂Zn salt-based nitrogen-doped carbon films and improves its low doping content of active nitrogen,which favors the preparation of high-performance composite electrode materials.Based on the above results,it can concluded that HPNC-50 is a supercapacitor electrode material with excellent performance,which expected to be applied in high-performance and more reliable capacitors.