Preparation And Supercapacitor Performance Of Carbon Materials With Heterogeneous Doping

With the increasing crisis of energy and environment,supercapacitors have attracted wide attention due to their advantages such as fast charging and discharging speed,high power density and good cycle stability.Carbon material has been widely used as electrode material because of its advantages of environmental protection,wide source and low cost,but its low energy density seriously hinders its practical application.Therefore,it is necessary to explore the formation and doping mechanism of carbon materials and further optimize their electrochemical properties.In this paper,we systematically selected a variety of precursors and prepared a variety of carbon materials by different methods.The effects of biomass components,carbon chain structure and heterogeneous atomic functional groups on the microstructure and properties of carbon materials were investigated in detail.Based on the different properties of carbon materials,the porous carbon materials with layered microstructure and excellent porous structure were selected for further study,and the porous carbon suitable for gel electrolyte was prepared.In addition,biomass-based porous carbon with low cost was prepared for loading mixed metal compounds.Meanwhile,quantum chemical calculation and molecular dynamics（MD）simulation were used to study the formation process,electronic properties and storage mechanism of electrode materials.（1）Nanoscale/microscale graphene-like sheets are synthesized with KOH as micropore-forming agent,Fe（NO₃）3·9H₂O as mesopore-forming agent and graphite catalyst.It is systematically researched to get the effects of biomass components on their properties.Cane sugar can form flat graphene-like nanosheets with high conductivity.Their performance drops sharply at 100 A g^-1,indicating that biocarbons need a support of carbon skeleton to operate normally at high current density.Bagasse pith contains amount of cellulose and hemicellulose,which are good for forming pores.Bagasse pith-derived graphene-like sheets possess large specific surface area(2923.58 m² g^-1),high specific capacitance(514.14 F g^-1 at 0.3 A g^-1 and 372.57 F g^-1 at 100 A g^-1)and high energy density(19.33 W h kg^-1 at 49.95 W kg^-1).Due to homogeneous coated doping with graphene-like nanosheets,sugarcane pith-derived graphene-like sheets possess low impedance（with a solution resistance of 0.02Ω）high rate capability(maintained 82.34%from 0.3 A g^-1 to 100 A g^-1)and high cycling stability（maintained 101.51%after 5000 cycles）,which is better than lots of graphene doping.Sugarcane skin contains more lignin which has hexagonal carbon rings.The graphitization extent of sugarcane skin-derived graphene-like sheets is significantly high.（2）Heteroatom-rich porous carbons（HPCs）are prepared from nontoxic organic crystals,and quantum chemical calculations and ReaxFF MD simulations are performed.Effects of carbon chains and functional groups on HPCs are investigated.In micromorphology,the open-chain compound changes little and is etched into interconnected particles.The compounds containing ring chains change a lot and form 2D thin layers.The dimer forms 3D flower-like structures composed of thin layers.During doping,N in the amino group has a tendency to form pyrrolic-N,and N in the N-heterocycle has a tendency to form quaternary-N.L-lysine-derived HPC shows a large specific surface area(3353.99 m² g^-1).For symmetric supercapacitor,it possesses high specific capacitance(439.11 F g^-1 at 0.3 A g^-1 and 265.57 F g^-1 at 100 A g^-1)and excellent cyclic stability（maintained 96.86%after 5000 cycles）.For asymmetric supercapacitor,it shows more pseudocapacitance（accounted for 33.84%at 20 mV s-1）and higher specific capacitance(586.58 F g^-1 at 0.3 A g^-1 and 457.40 F g’1 at 100 A g^-1).In gel electrolyte,its power density and energy density increase by 39.58%and 133.21%,respectively,with voltage rising from 1 V to 1.4 V.（3）N/O codoped layered porous carbons（NOLPCs）are prepared from L-tyrosine by a simple method.The NOLPCs with layered microstructure can fully contact electrolytes,and have large specific surface area(the highest up to 3221.57 m² g^-1),and show excellent electrochemical performance in both aqueous electrolyte and gel electrolyte.NOLPC-4.5 for the asymmetric supercapacitor with aqueous electrolyte displays high specific capacitance(512.21 F g’1 at 0.3 A g^-1,336.43 F g^-1 at 100 A g^-1)and excellent cyclic stability（95.40%retention after 5000 cycles）.Moreover,it for the asymmetric supercapacitor with gel electrolyte also shows high specific capacitance(285.41 F g^-1 at 0.3 A g^-1,192.24 F g^-1 at 50 A g^-1)and excellent cyclic stability（94.26%retention after 5000 cycles）.The nanoscale NOLPC configuration is constructed by MD simulations.The configuration has similar microporous structure and elemental composition to the NOLPCs,and the electrolyte can adequately diffuse into its microporous structure.（4）O-doped layered porous carbon（OLPC）is prepared from bagasse pith by chemical activation.Its flat and broad surface is ideal for loading metal hydroxide.A simple strategy is devised to synthesize Sr-doped urchin-like NiCo hydroxide（U-SrNiCo-OH）and Sr-doped flower-like NiCo hydroxide@O-doped layered porous carbon（F-SrNiCo-OH@OLPC）.The U-SrNiCo-OH electrode shows high specific capacity(616.69 C g^-1 at 1 A g^-1 and 425.00 C g^-1 at 50 A g^-1)and excellent rate performance(68.92%retention from 1 A g^-1 to 50 A g^-1).Compared with the U-SrNiCo-OH electrode,the F-SrNiCo-OH@OLPC-40 electrode exhibits close specific capacity(584.16 C g^-1 at 1 A g^-1 and 341.66 C g^-1 at 50 A g^-1)and higher cyclic stability（63.59%retention after 5000 cycles）.When used as the positive material,F-SrNiCo-OH@OLPC-60 performs best.In the gel electrolyte,the asymmetric supercapacitor based on F-SrNiCo-OH@OLPC-60 delivers high specific capacitance(93.60 F g^-1 at 0.5 A g^-1 and 47.00 F g^-1 at 10 A g^-1)and high energy density(29.12 Wh kg^-1 at 374.14 W kg^-1).Based on the molecular dynamics（MD）simulations,the OLPC configuration with microporous structure similar to that of OLPC is constructed,and the charging dynamics of asymmetric supercapacitor is studied.（5）The OLPC is further designed to be loaded with mixed metal sulfides.A simple strategy is devised to synthesize granular NiCoFe sulfide（G-NiCoFe-S）and granular NiCoFe sulfide@O-doped layered porous carbon（G-NiCoFe-S@OLPC）.The G-NiCoFe-S electrode shows high specific capacity(548.57 C g^-1 at 1 A g^-1 and 150.00 C g^-1 at 50 A g^-1)and excellent cycle stability（45.34%after 5000 cycles）.In the synergy of G-NiCoFe-S and OLPC,G-NiCoFe-S@OLPC-50 electrode exhibits high specific capacity(445.99 C g^-1 at 1 A g^-1 and 211.67 C g^-1 at 50 A g^-1),higher rate performance(47.46%retention from 1 A g^-1 to 50 A g^-1)and higher cycle stability（48.19%retention after 5000 cycles）.When used as the positive material,G-NiCoFe-S@OLPC-70 performs best.In the gel electrolyte,the asymmetric supercapacitor based on G-NiCoFe-S@OLPC-70 delivers high specific capacitance(56.42 F g^-1 at 0.5 A g^-1,43.48 F g^-1 at 10 A g^-1)and high energy density(16.33 W h kg^-1 at 360.88 W kg^-1).