Electrochemical Syntheses Of Nanostructured Pseudocapacitive Electrode Materials For Supercapacitors

Supercapacitors,also called electrochemical capacitors,are novel energy storage devices to fill in the gap between traditional capacitors and batteries.They have attracted significant attention due to their superior power density and ultra-long cycle life.However,the energy density of supercapacitors is relatively low in comparison with batteries,which has impeded the wide application of these devices.Further increase of their energy density is the key to expand the practical application areas,while modification on the electrode materials is one of the most efficient solutions.Comparing to carbon based materials,pseudocapacitive electrode materials that mainly function on highly reversible Faradaic reactions usually display higher capacitance.Nevertheless,this advantage would be offset by their poor electrical conductivity.In addition,pseudocapacitive electrode materials usually suffer from self-aggregation during systhesis,especially at high mass loading,making the major part inaccessible for electrolyte ions.This leads to the ineffectiveness utilization of active materials and diminishes their charge-storage capacity.Overcoming those issues to develop high performance supercapacitor electrode materials is still a scientific challenge.This thesis investigates the construction of hierarchical nanostructures as an efficient way to achieve high electrochemical performance.Three-dimensional conductive carbon cloth or hetero-atom doped carbon nanowire arrays were used as the substrate,on which nanostructured pseudocapacitive materials including highly aligned polypyrrole(PPy)nanowire arrays,mixed-valence vanadium oxide nanowires and manganese oxide hierarchical structures were deposited through facile electrochemical methods.The materials were characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),X-ray powder diffraction(XRD)and Raman spectroscopy.The electrochemical performance of these materials was studied by cyclic voltammetry(CV),galvanostatic charging/discharging technique,and electrochemical impedance spectroscopy(EIS).Their formation process and electrochemical performance were studied in detail,while a series of symmetric and asymmetric supercapacitors were assembled to test their practical energy storage applications.The various nucleation sites on the high surface carbon substrate and precise control of the electrodeposition parameters allow uniform growth of pseudocapacitive material and the construction of three-dimensional self-supporting electrodes.The strong ?-? interactions or C-O-Metal bonding between the active material and substrate provide effective electron transport as well as strong mechanical stability,while the hierarchical structure helps to enhance the mass transport.Our strategies demonstrate a new opportunity to fabricate high performance electrode materials for energy storage applications.The main research work and results are described as follows.(1)Highly aligned PPy nanowire arrays(PPy NWAs)were electrochemical fabricated on the surface of the carbon fibers in the carbon cloth substrate.The strong?-? interactions between pyrrole molecules and carbon substrate as well as the "soft"template effect of p-toluenesulfonate acid dopants facilitate the formation of one-dimensional PPy nanowires.Facile ion diffusion and eletron conduction pathways are provided by the hierarchical structure of the three-dimensional conductive carbon skeleton and the vertically aligned PPy NWAs on top.The well-ordered PPy NWAs electrode exhibits a high specific capacitance of 699 F g-1 at 1 A g-1,and excellent rate capability with 81.5%of the capacitance retained at 20 A g-1.The assembled symmetric supercapacitor(SSC)of PPy NWAs//PPy NWAs exhibits an outstanding energy density of 32.9 Wh kg-1 at the power density of 650 W kg-1 and still maintains 26.6 Wh kg-1 energy density at the high power density of 13 kW kg-1.The electrical performance is substantially better than most of the other reported SSCs.(2)Hetero-atom(nitrogen and oxygen)doped carbon nanowire arrays(NOC NWAs)were fabricated by annealing the PPy nanowire arrays under N2 atmosphere.These electrodes store charge based on both electrical double layer capacitance(EDLC)from carbon matrices and pseudo-capacitance(PC)from hetero-atoms,thus showing a significant improvement in the electrochemical properties in comparison to the undoped carbon materials.In addition,balancing between the EDLC and PC processes could further enhance the electrochemical performance of the NOC NWAs.This is achieved by careful control on the annealing temperature,with the sample annealed at 500?(NOC-500)displaying the optimal performance.It yields an outstanding areal capacitance of 324 mF cm-2 at 1 mA cm-2,and retains 57%of value when the current density increases to 100 mA cm-2.More importantly,It exhibits outstanding cycling stability with ca.94.8%capacitance retained after 60 000 cycles.The asymmetric supercapacitor using the NOC-500 anode coupling a MnO2 NS As cathode delivers the maximum volumetric energy density of 1.7 mW h cm-3 at the volumetric power density of 0.014W cm-3.(3)The hetero-atom doped carbon nanowire arrays were functionalized by facile electrochemical oxidation methods,and amorphous mixed-valence vanadium oxide nanowire(VOC NWA)were subsequently electro-deposited on top.The functional groups on the substrate act as the nucleation sites for the VOC NWA,which enhances the interaction between the carbon and the oxide.The oxidation state of vanadium was further adjusted by the following cyclic voltammetric scans.Benefiting from the multiple oxidation states of vanadium,the VOC NWA display a wide charge storage potential window of-1.1?0.9 V,allowing the assembled SSC to function with a large voltage of 2 V.Together with the high capacitance of 1.31 F cm-2 at 1.0 mA cm-2 offered by the VOC NWA electrode,the SSC delivers a high energy density of 3.61 mWh cm-3 at the power density of 0.01 W cm-3.Furthermore,the SSC also shows superior cycling stability,with 91%capacitance retention after 10 000 charge-discharge cycles.(4)MnO2 hierarchical structures were electrochemically deposited on the surface of carbon fiber in carbon cloth with a large mass loading of 10 mg cm-2.The crystal structures and morphologies of the deposited MnO2 show strong dependence on the deposition temperature,which further influence the electrochemical properties.At the deposition temperature of 60?,the optimized sample(MnO2-60)is composed of primary two-dimensional ?-MnO2 nanosheets interconnecting with secondary one-dimensional ?-MnO2 nanorod arrays grown on the surface of carbon fiber.The thick oxide layer show porous architecture,which guarantees electrolyte ion migration throughout entire electrode in order to achieve high utilization of MnO2 active material,as well as ensures its good mechanical stability.The MnO2-60 electrode yields an outstanding areal capacitance of 3.32 F cm-2 at 1 mA cm-2.More importantly,it retains 57.2%of the areal capacitance when the current density increases 30 times to 30 mA cm-2.An asymmetric supercapacitor(ASC)was assembled with the as prepared MnO2-60 as cathode and vanadium oxide nanoflowers(V2O5NF)as anode.The MnO2-60//V2O5NF device exhibits an outstanding volumetric energy density of 8.25 mWh cm-3 at the maximum power density of 0.28 W cm-3 and 5.44 mWh cm-3 at 1.12 W cm-3,substantially higher than most of the other reported ASCs.