Preparation Of Carbon-Based Composite Electrode Materials And Their Applications For Supercapacitors

Supercapacitor,as a new type of electric energy storage device,has many advantages of high power density,excellent stability,rapid charge-discharge function and environmental-friendly,etc.,possesses great application value and huge market potential in electronics,communications,transportation,electricity,industrial energy efficiency,national defense,and many other fields.Electrode materials play an important role in supercapacitors,the common electrode materials for supercapacitors include carbon-based materials,transition metal oxides and conducting polymers.The carbon-based materials present a characteristic of electrochemical double layer capacitance,have excellent charge-discharge cycle stability,and meet the high requirements for the life of device in practical application,however they show a low specific capacitance.Both of the transition metal oxides and conducting polymers exhibit characteristics of pseudocapacitance,which depends on the reversible chemical adsorption/desorption or redox reaction to store electric energy and have high specific capacitances,but shows weak cycle stability.These electrode materials for supercapacitors have their own advantages and disadvantages.Hence,the researches on electrode materials are not limited to single component material,composites usually exhibit more outstanding electrochemical properties.Carbon-based composite electrode materials have gain tremendous attention due to their abundant raw materials,low cost,simple preparation process and excellent electrochemical performance.In this thesis,the preparation,electrochemical properties and applications for supercapacitors of carbon-based composite electrode materials were studied,and the main contents are as follow:1.Preparation and capacitance properties of nitrogen doped porous carbonNitrogen-doped porous carbon(NPC)materials were synthesized by using a kind of natural product as carbon resource and melamine as nitrogen resource,which was carbonized and activated by potassium hydroxide(KOH)at high temperature.The sample of NPC-800 activated at 800℃ presents porous micro morphology,besides,the nitrogen doping content reaches 6.45 wt%while nitrogen is mainly identified as pyridinic N and pyrrolic N.Electrodes were fabricated by using the NPC as active materials,and were assembled to be symmetric supercapacitors.The capacitance performance of the devices were characterized by galvanostatic charge-discharge(GCD)and cyclic voltammetry(CV)tests in two-electrode system while 6 mol L-1 KOH used as electrolyte.The supercapacitor with NPC-800 showed specific capacitance of 145 F g-1 at current density of 0.1 A g-1 and energy density of 20 Wh kg-1 at power density of 50 W kg-1.The capacitance retention of 96.9%as well as high coulombic efficiency of 99%is achieved after 1000 charge-discharge cycles.The above results indicate that NPC exhibit excellent capacitance as well as cycle stability and has great application values for supercapacitors.2.Preparation and capacitance properties of the binary composite of graphene oxide/polypyrroleGraphene oxide(GO)was prepared by improved Hummers method,then the binary composite of graphene oxide/polypyrrole(GO/PPy)was synthesized via a modified in situ polymerization method.The binary composite showed a crumpled nanosheet-like morphology by the characterization of scanning electron microscopy(SEM),which is due to the formation of dense coating of PPy on the surface of GO byπ-π stacking,hydrogen bonding and electrostatic interactions etc.The GCD and CV measurements were performed in three electrode system,the obtained results indicate that a specific capacitance value of GO/PPy can reach to 468 F g-1 at current density of 0.5 A g-1,which is superior to the single component of PPy electrode(267 F g-1).The rate performance and cycle stability of the binary composite are also significantly improved owing to synergistic effect between GO and PPy.Thus it can be seen that the capacitive performance of the binary composite of GO/PPy has been enhanced after the combination of PPy and GO.3.Preparation and capacitance properties of the ternary composite of graphene oxide/carbon quantum dots/polypyrroleCarbon quantum dots(CDs),binary composite of graphene oxide/carbon quantum dots(GO/CDs)and ternary composite of graphene oxide/carbon quantum dots/polypyrrole(GO/CDs/PPy)were synthesized by a microwave-assisted pyrolysis method,a hydrothermal method and an in situ polymerization,respectively.The enhanced electron transportation and reduced internal resistance and charge transfer resistance of the electrode are induced by the addition of CDs sandwiched between GO film and PPy layer.The CDs with large specific surface area can enhance the interfacial property among GO,CDs and PPy as well as increase in the dielectric constant of the ternary composite.GO/CDs/PPy exhibits specific capacitance of 576 F g-1 at current density of 0.5 A g-1 and the supercapacitor fabricated by two symmetric GO/CDs/PPy electrodes owns high energy density of 30.1 Wh kg-1 at power density of 250 W kg-1.More importantly,the ternary composite still has high specific capacitance after 5000 charge-discharge cycles,indicating excellent cycle stability.In order to evaluate the practical value of electrode materials,a simple device assembled by five supercapacitors in series connection could power 59 light-emitting diode indicators for more than 60 seconds after only charging for 17 seconds.The above results demonstrate that CDs can play an important role in improving capacitive performance of the ternary composite of GO/CDs/PPy,the constructed hybrid electric double layer capacitor(EDLC)/pseudocapacitor has high energy density,excellent cycle stability and therefore it possesses a huge potential market prospect for supercapacitor.