Design Synthesis And Capacitive Performances Of Carbon Based Doping And Composite Materials

Supercapacitors represent a type of green and efficient electrochemical energy storage devices with superior advantages such as high power densities,long cycling lifetimes,rapid charging-discharging rates,wide operation temperature range,superior safety and non-pollution,which can be potentially applied in power output and charging-discharging rate dependent facilities including auxiliary power sources,start-up device as well as portable electronics.Electrode is the main part that sustains the charge storage and output,the specific surface area,pore feature,hydrophilicity,crystallinity as well as the redox activity of electrode material directly determine the energy storage capacity of a supercapacitor.Hence,the design and synthesis of high capacitance electrode materials,and further the construction of efficient energy storage system to improve the energy storage performance of device is currently the main task in supercapacitor field.Carbons are a significant type of electric double layer?EDL-?electrode materials,the optimization of specific surface area,conductivity and hydrophilicity of carbon material is an efficient way to improve the energy storage performance.Moreover,carbon material can serve as conductive matrix for hetero-element doping and guest materials hybriding to derive a series of doped or composite materials with favorable capacitive and catalytic performances.In this thesis,all of our work starts with porous carbons,a series of activated carbons were synthesized from different precursors.By optimization of the specific surface areas,electrons and ions transport channels and hetero-elements doping,a series of N doped porous carbons with enhanced capacitive performances and catalytic activities were yielded.Furthermore,by construction of supercapacitors based on different electrolyte systems,enhanced energy storage capacities can be achieved by the pseudocapacitance contributions from both electrodes and the electrolytes.Additionally,comprehensively optimized capacitance,rate capability and cycleability were achieved by the formation of graphene-pseudocapacitive metal oxide composite,and widened voltage window was accomplished by construction of asymmetric supercapacitor with activated carbon anode,therefore,balanced energy density,power density and cycleability were achieved.The main contents of this thesis are summarized as follows:1.Activated carbon?a-PFC3?with high specific surface area,hierarchical pore feature with enriched micropores and meso-/macropores,moderate graphitization degree and high surface wettability wasprepared by carbonization and followed alkali activation of a biomass precursor,paulownia flower?PF?.The symmetric supercapacitor based on a-PFC3 offered a high specific capacitance(297 F g^-1at 1 A g^-1),a considerable rate capability and a superior cycleability.The a-PFC3 based symmetric supercapacitors can be further connected in series to afford amplified device array with higher output voltage,showing practical application performance.2.Nitrogen doped activated carbons?a-NCs?with microporous texture,tunable N doping level and forms were synthesized by carbonization and controllable alkali activation of polyaniline?PANI?precursor at different temperatures,which were further employed as electrode materials of supercapacitors and heterogeneous catalysts for hydrogenation of 4-nitrophenol.Experimental results show that the activation temperature plays a crucial role on the specific surface area,N doping level and forms,graphitization degree and therefore the capacitive performance and catalytic activity of the afforded a-NCs,thus allow us to facilely tune the application performances as required through manipulating the synthesis conditions.By activation at lower temperature?600 oC?,the afforded a-NC600 maintains high N and O contents,the more pseudocapacitance active species contributed to the maximized specific capacitance(309 F g^-1 at 1 A g^-1).In contrast,the sample activated at 800 oC,labeled as a-NC800,demonstrated high surface area and increased fraction of graphitic N,the specific capacitance decreased,but it was capable of offering preferential heterogeneous catalytic activity for the hydrogenation of 4-nitrophenol.3.N doped porous carbons?NCs?with high specific surface areas,moderate graphitization degrees and high N fractions were prepared by simple carbonization of melamine sponge-PANI composite precursor,and the energy storage performances of the resultant supercapacitors can be optimized by tuning the carbonization temperature of precursor.Additionally,this type of NCs demonstrated high catalytic activities for the redox reaction of I-/I3-pair in electrolyte,which could substantially enhance the pseudocapacitance by promoting the faradic reaction of electrolyte.Under an optimal KI concentration of0.06 M in H2SO4 electrolyte,the electrode specific capacitance achieved to 616 F g^-1,showing the superior energy storage capacity,and a high cycleability was also achieved simultaneously.This section of work involved the substantial improvement of the energy storage performance of supercapacitor by the catalytic effect between electrode and redox active electrolyte,thus provided a novel concept for design of high specific capacitance supercapacitors.4.Graphene-interconnected porous Co₃O₄ nanofiber array composite was hydrothermally deposited onto Ni foam collector by controlled assembly orientation of Co₃O₄ using graphene oxide and P123 surfactant,thus afforded faradic electrode with high specific capacitance and efficient electrons and ions diffusion channels.In 2 M KOH electrolyte system,a specific electrode capacitance of 2056 F g^-1(1 A g^-1)was achieved,simultaneously,superior rate capability and cycleability were also achieved.Additionally,activated carbon with high EDL-capacitance was also synthesized from fullerene soot.The constructed activated carbon?graphene-Co₃O₄ composite asymmetric supercapacitor exhibited a widened voltage window of 1.6 V,therefore was capable of offering an energy density of 50 Wh kg^-1,as well as superior power density and cycleability,thus can be employed as efficient and long lifetime supercapacitor.