The Preparation Of Carbon Quantum Dots-based Composite Materials And Their Applications In Symmetric And Hybrid Supercapacitors

Carbon quantum dots(CQDs)as a new class of carbon material have been widely researched in many fields such as biomedicine,sensors and photocatalysis on account of their excellent biocompatibility,chemical stability and nontoxicity.In recent years,CQDs have attracted increasing attention in the field of supercapacitors due to their large specific surface area,rich oxygen-containing functional groups,a large number of edge defects as well as simple preparation process.In this work,CQDs were prepared by a modified chemical oxidation method.Moreover,the CQDs-based composite aerogel electrode materials were constructed,and their application in symmetric and hybrid supercapacitors were further studied.The main contents and results are as follows:(1)CQDs were synthesized by a modified chemical oxidation method utilizing the commercial acetylene black as carbon sources.Then,the nitrogen and boron co-doped CQDs/reduced graphene oxide(rGO)composite aerogel(N,B-CQDs/rGO)was further prepared by hydrothermal method.The research shows that the specific capacitance of N,B-CQDs/rGO composite aerogel is enhanced compared to rGO and N,B-rGO,exhibiting that the specific capacitance of N,B-CQDs/rGO is361.9 F g^-1 at 1 A g^-1,and still retains 230.5 F g^-1 at a high current density of 50 A g^-1.The excellent capacitance properties are attributed to the synergistic effect of the introduction of CQDs,N,B co-doping and unique three-dimensional porous aerogel structure.In addition,the symmetrical supercapacitor was constructed by using N,B-CQDs/rGO composite aerogel as the electrode materials,which shows a high energy density of 8.81 Wh kg^-1 at a power density of 125 W kg^-1,and remain an energy density of 7.15 Wh kg^-1 at a power density of 10 k W kg^-1.Meanwhile,it exhibits an outstanding cycle stability with 90.1%of capacitance retention over 10000 cycles at 3 A g^-1.(2)CQDs were prepared by the modified chemical oxidation method using the commercial micron-sized graphite powders as carbon sources,and the nitrogen and phosphorus co-doped CQDs/rGO composite aerogel(N,P-CQDs/rGO)was further synthesized by hydrothermal method.The results show that the as-prepared composite aerogel exhibits a high specific capacitance(453.7 F g^-1 at 1 A g^-1),an excellent rate performance with a specific capacitance of 315.5 F g^-1 at 50 A g^-1 and an outstanding cyclic stability(93.5%of capacitance retention over 10000 cycles at 10 A g^-1).Furthermore,the symmetrical supercapacitor based on N,P-CQDs/rGO composite aerogel was assembled,which can be noticed that the working voltage window of the device was widened to 1.3 V owing to the introduction of phosphorus-containing functional groups.The energy density of the device is 15.69 Wh kg^-1 at a power density of325 W kg^-1,and still retains 10.84 Wh kg^-1 at an ultra-high power density of 13 k W kg^-1.The device also shows a good cycle stability with 85.5%of capacitance retention after 10000 cycles at 5 A g^-1.(3)In order to further improve the specific capacitance of CQDs-based composite aerogel electrode materials,the nitrogen and sulfur co-doped CQDs/rGO/NiCo₂S₄ composite aerogel(N,S-CQDs/rGO/NiCo₂S₄)was synthesized by hydrothermal method.The results reveal that the composite aerogel exhibits a high specific capacity(162.6 m Ah g^-1 at 1 A g^-1),an excellent rate performance with a specific capacity of 125.7 m Ah g^-1 at 50 A g^-1 and a good cycling stability(87.5%of capacity retention over 5000 cycles at 3 A g^-1).The excellent capacity performance of the N,S-CQDs/rGO/NiCo₂S₄ composite aerogel can be attributed to the synergistic effect of N,S-CQDs/rGO aerogel with excellent electrical conductivity and structural stability and NiCo₂S₄ with rich redox reaction.Moreover,the hybrid supercapacitors were constructed by using N,S-CQDs/rGO/NiCo₂S₄ composite aerogels as cathode,N,S-CQDs/rGO composite aerogels as anode,which exhibits a high energy density of 51.0 Wh kg^-1 at a power density of 287.8 W kg^-1,still remains 17.2 Wh kg^-1 at a high power density of 14.4 k W kg^-1.Meanwhile,it shows an excellent cycle stability with 82.9%of capacitance retention after 10000 cycles at 3 A g^-1.