Preparation Of Carbon Dots-based Activated Carbon And Its Application In Supercapacitor

Supercapacitors had the advantages of high power density,long cycle life,relative safety,and no pollution,and were considered to be a very promising energy storage device.As the electrode material of supercapacitor,activated carbon materials had high specific surface area,without chemical reaction in the process of charge and discharge,and relied on the electric double layer mechanism to store charges,which had high reliability and cycle stability.However,their electrochemical performance needed to be further improved.Carbon dots could improve the electrochemical performance of supercapacitors due to their unique quantum size effect,abundant surface functional groups,various defects and edges.Existing research results showed that the use of carbon dots combined with other materials for supercapacitor electrode materials exhibited synergistic effects to improve the specific capacitance and cycle stability of supercapacitors.However,the current research results mainly focused on the regulation and recombination of supercapacitor electrode materials such as metal oxides,conductive polymers and graphene by carbon dots.However,for widely used activated carbon materials for supercapacitors,there were few studies on the use of carbon dots to construct or dope.Based on the above analysis,in order to further improve the electrochemical performance of activated carbon-based supercapacitors,three carbon dots-based activated carbon materials were constructed by different preparation methods and used as electrode materials for supercapacitors.The microstructure and chemical composition of the prepared materials were investigated by transmission electron microscopy（TEM）,field emission scanning electron microscopy（FESEM）,X-ray diffraction（XRD）,Raman spectroscopy（Raman）,X-ray photoelectron spectroscopy（XPS）and Brunauer-Emmett-Teller（BET）,and the electrochemical properties of the materials were studied through electrochemical tests（cyclic voltammetry,galvanostatic charge-discharge,etc.）,and the advantages of carbon dots-based activated carbon as electrode materials for supercapacitors were analyzed.The main research contents were as follows:1.Preparation of carbon dots-based activated carbon/polypyrrole（AC-CDs/PPy）hybrid material:the carbon dots were obtained by mixing citric acid and urea in solid phase and then pyrolyzed to obtain carbon dots.Then,the carbon dots were mixed with KOH and activated at high temperature to obtain carbon dots-based activated carbon（AC-CDs）.The fumigation method was used to controllably polymerize polypyrrole（PPy）on the surface of the carbon dots-based activated carbon to prepare AC-CDs/PPy hybrid materials.The fumigation time of pyrrole on the surface of AC-CDs was investigated,and a series of samples of AC-CDs/PPy-0h,AC-CDs/PPy-1h,AC-CDs/PPy-2h and AC-CD/PPy-3h were obtained and their electrochemical performance were tested.Using 3M KOH as the electrolyte,the AC-CDs/PPy-2h showed a high specific capacitance(481 F g^-1)at a current density of1 A g^-1in a three-electrode system.Furthermore,AC-CDs/PPy-2h was assembled as the electrode material into a two-electrode supercapacitor,and the device exhibited a specific capacitance of 226 F g^-1at a current density of 1 A g^-1.And AC-CDs/PPy aqueous supercapacitors exhibited excellent cycling stability in both three-electrode and two-electrode systems.2.Preparation of in-situ carbon dots-doped activated carbon materials（CD@AC）:three raw materials including small molecules and polymer,2,6-diaminopyridine（DAP）,carboxylated chitosan（CCS）and glucose（GLU）were mixed with KOH and calcined in a tube furnace,the precursors containing carbon dots were generated at a temperature of 20°C higher than the melting point of the raw materials for 30 min respectively,and then heated up and activated at high temperature to realize the in-situ doping of carbon dots into activated carbon to obtain CD@AC materials.Electrochemical tests showed that in two-electrode organic supercapacitor,compared with the activated carbon without CDs,the specific capacitance of CD@AC prepared by DAP,CCS and GLU increased by 55%,33%and 47%,respectively.These results suggested that this simple and scalable strategy for in-situ doping of carbon dots into activated carbon could be used to prepare activated carbon materials for high-performance supercapacitors.3.Preparation of in-situ carbon dots-doped activated carbon/iodine-doped polypyrrole（CD@AC/PPy-I）hybrid material:polyvinylpyrrolidone（PVP）was used as raw material,mixed with KOH and then freeze-dried.The obtained porous precursor was kept at 150°C for 30 min in a tube furnace to generate the precursor containing carbon dots.The precursor was further heated and activated at high temperature to obtain in-situ carbon dots-doped activated carbon（CD@AC）material.Then CD@AC and iodine pyrrole solution were placed in a closed container,and the CD@AC/PPy-I hybrid material was obtained by controllable polymerization of iodine-doped polypyrrole on the surface of CD@AC by fumigation.The effect of different iodine contents in pyrrole（0%,30%,70%,100%）on the material was investigated,and a series of samples of CD@AC/PPy-I-0,CD@AC/PPy-I-0.3,CD@AC/PPy-I-0.7 and CD@AC/PPy-I-1 were obtained,and their microstructure and chemical composition were characterized by TEM,FESEM,XRD,Raman,XPS and BET,and the electrochemical performance of the supercapacitors in the two-electrode organic system was analyzed by electrochemical tests.At a current density of 1 A g^-1,the specific capacitance of the CD@AC/PPy-I-1 supercapacitor reached 183 Fg^-1.