Syntheses Of Pseudocapacitive Materials And Their Electrochemical Performances In Asymmetric Supercapacitors

Supercapacitors,as a kind of novel energy storage devices,have attracted significant attention due to their environmental friendliness,fast charging/discharging rate and good electrochemical cycling stability.However,the energy density of supercapacitors is relatively low in comparison with lithium-ion batteries and fuel cells.The improvement of energy density(E)of supercapacitors can be achieved by maximizing their specific capacitance(C)and/or operating voltage(U)according to the equation of E=1/2CU².Asymmetric supercapacitors(ASCs)usually display high operating voltage as the positive and negative electrodes store charge in different potential ranges.In general,the specific capacitances of supercapacitors are mainly depended on the electrode materials.Compared with carbon-based materials which store charges through electrochemical double-layer,pseudocapacitive materials could show higher specific capacitances as they store charges through fast Faradic reactions.However,the practical electrochemical performances of pseudocapacitive materials can be severely impeded due to the intrinsic poor conductivity,ineffective utilization of active materials and the inferior electrochemical stability.Thus,the design and preparation of advanced pseudocapacitive positive and negative electrode materials for high-performance asymmetric supercapacitors are big challenges.Carbon cloth(CC)woven by carbon fibers can be used as conductive substrate to afford pseudocapacitive materials.Ion transfer channels can be constructed in the afforded electrodes,based on the three-dimensional porous structure of CC.The contact of pesudocapacitive materials with electrolyte can be promoted to improve their energy storage behaviors.In this thesis,highly loaded manganese oxide(MnO_x)and?-Fe₂O₃@PPy core/shell nanoflaks were grown on CC to prepare M-0.625 and F-400@PPy electrodes,respectively.To enhance the interaction of pesudocapacitive materials with the conductive substrate,CC was electrochemically functionalized to obtain functionalized carbon cloth(FCC).Then.polypyrrole(PPy)were electrochemically grown on FCC to prepare PPy/FCC electrode.Scanning electron microscope(SEM).transmission electron microscopy(TEM).X-ray photoelectron spectroscopy(XPS),X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR)were conducted to characterize the electrodes.Cyclic voltammetry(CV),constant current charging/discharging technique and electrochemical impedance spectra(EIS)were used to investigate their electrochemical performances.To further investigate their applications in energy storage,three asymmetric supercapacitors were assembled by using PPy/FCC,M-0.625 and F-400@PPy as electrodes.The main results are listed below:(1)CC was functionalized by a facile electrochemical method,which was helpful to improve its wettability and electrochemical performace.PPy was electrochemically grown on surfaces of fibers in FCC substrate to prepare PPy/FCC electrode.Fast electron transportation avenue was formed facilitated by the interaction of the polymer with the conductive substrate,established through doping PPy by the oxygen containing functional groups in FCC.The contact between PPy and electrolyte can be strengthened through the three-dimensional porous structure of CC to improve the energy storage performance.PPy/FCC electrode delivered a high areal capacitance of 341.2 mF cm^-2 at a current density of 1 mA cm^-2.A capacitive retention of 70%was acheived when the current density increased 20 times from 1 to 20 mA cm^-2,showing its good rate capacity.(2)Highly loaded manganese oxide(MnO_x)was electrochemically deposited on CC substrate.Through simply tuning the concentration of the complexing agents in the plating electrolyte,the morphologies and the contents of the hydrous species(e.g.H-O-H and Mn-O-H)of MnO_x can be optimized.Both factors were helpful for the electrolyte ion transport in the bulk oxide.A highly porous nanostructured electrode of M-0.625,with an appropriate hydrous species content,was prepared in an aqueous solution containing 0.0125 M MnSO₄,0.625 M(NH4)2SO4 and 1.25 M NaAc.Fluent electrolyte ion migration was guaranteed for highly loaded manganese oxide electrode and rate capabilty was improved.The M-0.625 electrode with a high mass loading of 7.02 mg cm^-2 can deliver a high areal capacitance of 1.64 F cm^-2 at the current density of 1 mA cm^-2.The M-0.625 electrode exhibited 69%capacitive retention when the current density increased from 1 to 20 mA cm^-2,displying its good rate capability.(3)?-Fe₂O₃ nanoflakes were prepared on CC substrate via a simple electrodeposition method and the followed annealing process.The effects of annealing temperature on the energy storage capacity and electrochemical cycling stability of iron oxide electrodes were investigated.PPy thin film was electrochemically deposited on F-400 electrode to afford F-400@PPy electrode with core/shell nanoflak structure.The coating of PPy on?-Fe₂O₃ nanoflakes can improve electrical conductivity of the integral electrode and alleviate the volumetric change of ?-Fe₂O₃ during the long-term charging/discharging process,which can enhance the electrochemical cycling stability.The F-400@PPy electrode with a high mass loading of 5.07 mg cm^-2 can deliver a gravimetric capacitance of 186 F g-1(942.9 mF cm^-2)at the current density of 1 mA cm^-2,61.5%of its initial capacitance can be retained when the current density increased from 1 to 50 mA cm^-2.This electrode also shows 92.3%capacitive retention after 10000 charging/discharging cycles,indicating its good cycling stability.(4)To study the application of PPy/FCC,M-0.625 and F-400@PPy electrodes in supercapacitors,MnO₂/CC,PPy/HFCC(highly functional carbon cloth,HFCC)and MnO_x/CC electrodes were prepared to balance charges with PPy/FCC,M-0.625 and F-400@PPy,respectively.A high-performance asymmetric supercapacitor(ASC)was assembled by using PPy/FCC as anode and MnO_x/CC as cathode.This MnO₂/CC//PPy/FCC ASC with a large operating voltage of 1.8 V demonstrated a high volumetric energy density of 0.80 mWh cm^-3 at 12.85 mW cm^-3 and still maintained 0.51 mWh cm^-3 volumetric energy density at the high volumetric power density of 250 mW cm^-3.This device also exhibited outstanding cycling stability with 90.7%capacitance retained after 10000 cycles.An advanced ASC was assembled by using M-0.625 as cathode and PPy/HFCC as anode.This M-0.625//PPy/HFCC ASC with a large operating voltage of 1.8 V delivered a high volumetric energy density of 2.67 mWh cm^-3 at 8.73 mW cm^-3.This device also showed good cycling stability with 91.73%capacitance retention after 10000 galvanostatic charge/discharge cycles.Furthermore,another state-of-the-art ASC was assembled by using F-400@PPy as anode and MnO_x/CC as cathode.This MnO_x/CC//F-400@PPy ASC exhibited a high volumetric energy density of 1.29 mWh cm^-3 at 8.34 mW cm^-3 and still maintained 0.95 mWh cm^-3 volumetric energy density at the high volumetric power density of 166.8 mW cm^-3.This device also showed excellent cycling stability with only 8.9%capacitive decay after 10000 cycles.