Preparation And Application Of Hierarchical Porous Carbon-based Electrode Materials

With the excessive consumption of fossil energy and the aggravation of environmental pollution,it is urgent to develop green,efficient and sustainable energy storage conversion technology and equipment.Owing to their merits of good conductivity,high specific surface area,well-developed nanopore channels and stable chemical and mechanical properties,porous carbon materials have wide application prospects in the fields of supercapacitors and lithium ion batteries,and so on.It is undoubtably that porous carbon materials have made great progress in morphology control and structural design.However,carbon materials with single pore structure are far from enough to meet the practical application requirements due to the continuous expansion of application fields and deepening of research.In recent years,hierarchically porous carbons?HPCs?with two or more than two kinds of different pore structure have received much attention in materials science because of their combination of the advantages of different pore structures.As a result,their performances in various technology fields are better than that of carbon materials with single pore structure.In this paper,several kinds?heteroatom-doping,SnO₂/C composite?of hierarchical porous carbon-based electrode materials were obtained based on the choice of appropriate precursors and rational preparation methods.At the same time,the morphology,structure,composition and electrochemical properties of as-prepared materials were characterized and analyzed in detail by different characterization and test methods.The research results are as follows:1.Preparation of fluorine and oxygen co-doped porous carbons?FOPCs?and the study of their electrochemical properties.Biomass is considered as the most promising carbon precursor due to its advantages of low-cost,renewable,easily available and environmental friendly.In this work,fluorine and oxygen co-doped porous carbons derived from third-class red dates were successfully prepared by a facile pre-carbonization and KOH-activation approach,which showed large specific surface area(1229 m² g^-1),high micropore ratio?⁹3%?,rich oxygen-content?²2.8wt.%?and moderate fluorine doping?¹.0 wt.%?.Owing to the aforementioned advantages,the as-prepared FOPCs exhibited a high specific capacitance of 261 F g^-1at 1 A g^-11 in three-electrode system,with a capacity retention ratio of 64.3%at 20 A g^-1.Moreover,the symmetric supercapacitor exhibited a high energy density of 23.2Wh kg^-1,with excellent capacitance retention capability(about 93.5%retention over 5,000 charge/discharge cycles at 10 A g^-1).2.Synthesis of ultrahigh surface area hierarchical porous carbons?HPCs?and its application in advanced supercapacitors.In this work,we have demonstrated a facile phytic acid?PA?assisted approach through freeze-drying,carbonization and activation processes for the preparation of graphitized HPCs by employing“noncarbonizable”synthetic polymer polyvinylpyrrolidone?PVP?as carbon precursor.The as-obtained sample shows hierarchical porous nanostructure with ultrahigh surface area(up to 3090 m² g^-1),large pore volume(1.62 cm³ g^-1),high-developed microporosity?⁸8%?,moderate nitrogen-doping?².4 wt.%?and abundant oxygen contents?¹9.5 wt.%?.As promising electrode material for supercapacitor,the sample exhibits a high specific capacitance of 406 F g^-11 at 1 A g^-1,with a capacity retention ratio of 61.1%at 20 A g^-1.Additionally,the as-assembled symmetric supercapacitor display a high energy density of 28.9 Wh kg^-1,with outstanding cycling stability of98%capacitance retention after 10,000 cycles.These electrochemical properties are among the highest level in the recently reported carbon-based supercapacitors and have a bright application prospect.More importantly,the facile stabilization strategy endowed by phytic acid proposed in this chapter would provide a new avenue for the preparation of high-quality porous carbons from non-carbonizable precursors.3.The preparation of sodium alginate derived nitrogen-doped hierarchical porous carbons and their electrochemical performances in supercapacitors.In this work,we have demonstrated a freeze-drying assisted carbonization approach for the preparation of high-quality nitrogen-doped hierarchical porous carbons?NHPCs?using natural polymer sodium alginate?SA?and urea as carbon precursor and nitrogen source,respectively.The porous textures and nitrogen contents of NHPCs can be facile modulated by altering the carbonization temperature and the concentration of urea in SA solution.These results provide a valuable reference for the controllable preparation of nitrogen-doped carbon materials.The optimal sample exhibits large specific surface areas(1179 m² g^-1)with hierarchical porous structure and abundant heteroatom doping?Nitrogen:⁶.8 wt.%,Oxygen:¹3.8 wt.%?.As the electrode material of supercapacitor,the NHPC-2-800 electrode showed a high gravimetric capacitance of 301 F g^-11 at 1 A g^-1,with excellent capacitance retention of 73.8%at 20A g^-11 in three-electrode system.In addition,the symmetric supercapacitor displayed a high energy density of 23.8 W h kg^-11 as well as an excellent cycling stability?96.5%specific capacitance retention over 5,000 cycles?.Moreover,the preparation process doesn't need any other activator?KOH,ZnCl₂,etc.?,in which urea was served as both pore-forming agent and nitrogen source.Thus,the pore-forming and the nitrogen doping can be achieved simultaneously by a one step,which can greatly save cost and time.4.In-situ preparation of porous carbon/SnO₂ composite via ion exchange and hydrothermal method and its application as advanced anode materials.In this work,tin alginate formed after ion exchange between sodium alginate and SnCl₂,and then the porous carbon/SnO₂ composite with unique structure?in which SnO₂ nano-crystals with uniform size were anchored within the 3D interconnected mesoporous carbon network?was prepared by hydrothermal carbonization and high-temperature calcination.The resultant product consists of SnO₂ nanocrystals with uniform size?2.2³.8 nm?and continuous mesoporous carbon network,which exhibits a large surface area(257 m² g^-1)and high content of SnO₂?⁷0 wt.%?.Due to the good synergistic effect between the porous carbon coating structure and zero-dimensional SnO₂ nanocrystals,the as-prepared composite delivered a high reversible capacity of1024.6 mAh g^-11 at 200 mA g^-11 even after 300 cycles as anode material for LIBs.In addition,the ion exchange and hydrothermal method?IEHM?proposed herein is expected to be applied to prepare other M_x O_y/C nano-composite materials?where M is polyvalent metals,such as Fe,Co,Cu,Mn,etc.?.These M_x O_y/C composites will have important application prospects in the anodes of lithium ion batteries.