Fabrication of Carbon-based Supercapacitor Electrode Materials through a C3N4 Template Method and Their Related Electrochemical Performance

Author:Liu Jing Hua

Supervisor:li xin


Degree Year:2019





Supercapacitors(SCs)are new-type energy storage devices with complementary characteristics to secondary batteries and traditional electric capacitors.Due to their high power density,long cycle life,and fast charge/discharge processes,SCs have attracted increasing attentions.The electrochemical performance of SCs is closely related to the electrode materials.In order to improve the energy density and power density of SCs,advanced electrode materials with high performance have been designed and synthesized.Carbon nitride(C3N4)decomposes at temperatures above 700℃and a plenty of nitrogenous substances are released during the decomposition process,which can be served as N dopants to prepare N-doped materials or nitrides.This fabrication method can be summarized to a kind of self-sacrificing template method.Without the introduction of NH3 atmosphere and post-treatment process,a lot of attentions have been focused on this facile and efficient method.Moreover,the space-confinement function of C3N4 makes it helpful to assist the fabrication of various novel structures.This dissertation mainly focuses on the development of facial and effective approach for the fabrication of supercapacitor electrode materials.A two-step process was adopted for the pyrolysis of the mixture containing dicyandiamide(DCDA)and glucose.And then,N-doped carbon nanosheets(NCNs)were collected.C3N4 was formed during the condensation process of DCDA at 550℃,but it underwent severe decomposition at higher temperature.Doping N atom was introduced into the carbon skeleton and NCNs fabricated at different high temperature were formed.NCNs prepared at 800℃exhibit the highest specific capacitance of 170.5 F g-1 at current density of 1 A g-1.In order to improve the electrochemical performance of NCNs,pseudocapacitive materials were introduced into the N-doped carbon materials.FeCl3·6H2O was added into the mixture of DCDA and glucose.A similar two-step pyrolysis process was preformed to fabricate Fe-based N-doped carbon nanomaterials(Fe-NC-Ts)at different high temperatures(T).Due to the catalytic function of Fe subatances and the space-confinement function of C3N4,the composition and morphology of Fe-NC-Ts were greatly changed during the high temperature between 600℃-1000℃.The phase transformation of the Fe-containing substances at different temperature can be ascribed to the multistep reactions(Fe3+→Fe2O3→Fe3O4→Fe3C→α-Fe).At the same time,the morphology was transformed from 2D layered structure to 1D tubular structure.The hybrid prepared at 700℃(Fe-NC-700)was composed of layered carbon materials with Fe3O4 and Fe3C nanoparticles on its surface.Fe-NC-700exhibits a highest specific capacitance of 217.8 F g-11 at current density of 1 A g-1 in a three-electrode system.As for the hybrid prepared at 800℃(Fe-NC-800),it shows a unique peapod-like nanorod-in-nanotube structure with Fe3C nanorods encapsulated into the N-doped carbon nanotubes.Fe-NC-800 possesses an excellent cycling stability.After 10000 cycles,the capacity retention of Fe-NC-800 was about 91.3%.When Fe-NC-700 electrode was assembled to a symmetrical capacitor,it delivers an energy density of 8.94 W h kg-1 at a current density of 1 A g-1.VOSO4 was added into the mixture of DCDA and glucose.After a two-step heat treatment,VN/N-doped carbon nanosheets(VN/NCN-Ts)were in-situ fabricated at different high temperatures.C3N4 was served not only as the N source of NCNs,but also as the N source of VN.Moreover,the carbon materials can protect the VN nanoparticles from oxidation,which can greatly improve the stability of the electrode materials.The hybrid prepared at 700℃(VN/NCN-700)shows the highest VNxOy content and doping N content,which bring out higher specific capacitance for VN/NCN-700 than that of the other VN/NCN-Ts.The specific capacitance of VN/NCN-700 at 1 A g-1 was 285.4 F g-1.When assembled to a symmetrical capacitor,it delivers an energy density of 10.3 W h kg-1 and 7.6 W h kg-1 at a power density of276.3 W kg-1 and 5484.2 W kg-1.Polydopamine@SiO2 spheres were added into the mixture of DCDA and glucose.After a two-step pyrolysis process and following etching of SiO2 cores,porous carbon materials with N-doped hollow spheres and N-doped nanosheets(NHCS/NCN-Ts)were synthesized.NHCS/NCN prepared at 800℃(NHCS/NCN-800)has ultra-high N content(23.2 wt%),moderate pyridinic-N(40.2%),pyrrolic-N(26.6%),and graphitic-N(29.6%)ratio,as well as high specific surface area(701 m2 g-1)and large pore size volume(1.38 cm3g-1),which exhibits excellent capacitive performance and good cycling performance.In a three-electrode system,NCN/NHCS-800 diaplays the highest specific capacitance of 425 F g-1 at a current density of 1 A g-1.When the current density is increased to 50 A g-1,the specific capacitance can maintain a high value of 275 F g-1.After 10000 cycles,the capacity retention of NHCS/NCN-800remains about 90%,indicating a good stability.When assembled to a symmetrical capacitor,it delivers an energy density of 11.9 W h kg-1 at a power density of 499.3W kg-1.