Novel Low-dimentional Carbon Materials:Preparation And Electrochemical Capability

Supercapacitors（SCs）,as a new-type energy storage device that can bridge the gap between secondary batteries and traditional physical capacitors,have found extensive applications in various energy storage field,such as portable electronics,hybrid electric vehicles,smart power grids and city rail transit,etc.This is mostly because of the outstanding electrochemical performances of SCs,including relatively high energy density and power density,extra-long cycle life as well as rapid charge and discharge speeds.However,in recent years,the commercial electrode materials for SCs have little significant improvement in structural design and performance optimization,and most of them are still based on the activated carbons（ACs）,which is mainly derived from direct carbonization-activation of natural products.It is lack of rational pathways for accelerating ion diffusion speed and shortening ion transfer length,which caused large ion transfer resistance along with poor high-rate response.In order to overcome these structural defects of ACs,herein,ultrathin hierarchically porous carbon nanosheets（UCNs）with tailorable thickness and 3D superstructured carbon spheres composed of graphene-like ultrathin nanosheets were synthesized through a brand new strategy.The main contents are summarized as follows:1.In this part,we report a brand-new“moulding–demoulding”strategy to prepare 3D hierarchically superstructured carbon spheres（denoted as HSCS）.This“moulding–demoulding”strategy is on the basis of constructing a flower-like hybrid comprised of alternately sandwich-type layered carbon/Zn₂SiO₄,via synchronous hydrothermal reactions between zinc gluconate and SiO₂ nanospheres.After elimination of the Zn₂SiO₄,graphene-like ultrathin carbon nanosheets with a thickness less than 1 nm are released（2³carbon layers）,leading to the formation of HSCS.The carbon nanosheets are connected with each other and assembled into a rigid continuous network,which is helpful to stabilize the 3D skeleton.Moreover,owing to amorphous carbon framework with abundant structural defects,this superstructured carbon material shows a high surface area of 549 m²g^–1.More importantly,it is found that this superstructure can be coated on the surface of any SiO₂-containing substrate,which offers an avenue to design superstructured carbon with various morphologies.In addition,we analyzed the influence of“anion effect”for flower-like structure.The HSCS has optimized nanostructure for ion transfer and electron immigration,then presents ultrafast supercapacitive energy storage.The discharge specific capacity of 1@5-HSCS is 162 F g^-1 at 0.3 A g^-1;even when the current density reach up to20 A g^-1,the specific capacity still remains 117 F g^-1,indicating a superior capacity retention of 72%.2.Herein,a type of novel ultrathin carbon nanosheets（UCNs）with well-developed hierarchical meso-/macropores are synthesized by combining bubbling and templating methods,in which an organic-metal salt（ferrous gluconate）with low melting point is used as carbon source,catalyst and template.The prepared UCNs have exceptional structural controllability,that is,the sheet thickness can be well tailored from 142 to 53 nm,and the embedded pores are adjustable in the range of 20 to 200 nm.Meanwhile,owing to robust carbon skeleton to tolerate the KOH etching,the surface area of UCNs can be raised to3230 m² g^–11 after an activation process.Based on these unique structural merits,the UCNs exhibit attractive supercapacitive performance.The specific capacitance of UCNs can reach235 F g^–1 at 0.1 A g^–1,and even at a very large current density of 50 A g^–1,a high capacitance of 168 F g^–1 is still retained,indicating good retention of 71%.3.Based on the front chapter,a novel 3D crosslinked network composed of graphitic carbon nanofibers（CNFs）were synthesized.In this part,oleic acid was used as carbon source to substitute ferrous gluconate,and ammonium chloride was used for swelling agent.This study shows that CNFs were fabricated via carbonization of liquid oleic acid directly,so the carbon atoms are arranged partially order in CNFs.Mealwhile,the microstructure of CNFs also can be adjusted by changing the temperature of carbonization and the heating rate.The final carbonized product of CNFs-800-10-2h shows some superior structural merits,for example,1D carbon nanofibers can enlarge the accessible surface area between electrolyte and active material,and shorten the transfer distance of Li⁺;Lots of mesopores/macropores can simplify the charge transfer and increase the storage capacity of electrolyte.Based on these unique structural advantages,the 3D crosslinked carbon network composed of 1D canbon nanofibers show impressive lithium storage capability.The intial discharge capacity can reach up to 284 mAh g^-1 at 0.1 A g^-1,even after 400 cycles at the same current density,a stable capacity of 226 mAh g^-11 is still remained,which means the average capacity loss is just 0.05%in each cycle,indicating an superior cycling stability.