| Supercapacitors, representing a class of energy-storage devices with high power density, energy density, long cycle life, low cost and environmental friendly, have drawn considerable attention for their potential applications in mobile electronic devices, hybrid electrical vehicle and uninterruptible power supplies systems. The conductive polypyrrole (PPy) and polyaniline (PANI) have the advantages of environmental stability, easy synthesis, high conductivity, redox reversibility and so on, therefore, have the important applied research value. The electrochemical energy storage performance of electrode of supercapacitor is closely related to the active material of electrode, the microstructure of electrode and the conductivity of substrate material. The ordered nanohybrid of anion doping PPy and proton acid doping PANI can take full advantage of electrolyte ions in the reversible doping-dedoping reactions during charge and discharge process, which can enhance the overall capacitance performance by synergy effect. In this paper, we carried out the research work focused on the electrochemical energy storage of polypyrrole/poyaniline coaxial nanoarray materials. Three kinds of ordered hybrid electrode materials based on TiO2 NTAs, free-standing and TiN NTAs had been synthesized:polypyrrole/titania/polyaniline coaxial nanotube hybrid (PPy/TiO2/PANI), free-standing polyaniline nanorods embedding in polypyrrole nanopores coaxial nanoarray film (PPy/PANI), polypyrrole/titania nitride/polyaniline coaxial nanotube hybrid (PPy/TiN/PANI). The cyclic voltammetry (CV) measurement, the galvanostatic charge-discharge (GCD) measurement, the electrochemical impedance spectroscopy (EIS) measurement were used to study the electrochemical capacitance performance of these materials. The main work are listed as follows.1. The preparation and electrochemical properties of PPy/TiO2/PANI coaxial nanotube hybrid.Highly ordered TiO2 NTAs greatly assist the ion diffusion, which contribute to the capacitance of electrode. Additionally, TiO2semiconductor can produce electron-hole pairs under UV light illumination, which can accelerate the electro-chemical polymerization of PPy and PANI. Therefore, the TiO2 NTAs can be regarded as the substrate material for supercapacitor electrode fabrication. In this study, PPy was selectively coated on the outer surface of TiO2 NTAs to form PPy/TiO2 coaxial nanotube hybrid using a normal pulse voltammetry electrodeposition method. Then, PANI was coated on the inner surface of PPy/TiO2 coaxial nanotube hybrid to form PPy/TiO2/PANI coaxial nanotube hybrid using a photoassisted cyclic voltammetry electrodeposition method. The morphology, microstructure and electrochemical energy storage of PPy/TiO2/PANI coaxial nanotube hybrid have been fully investigated. The SEM characterization indicates that the PPy deposited only in the space between neighboring nanotubes, and the PANI deposited only on the internal nanotube walls to form PPy/TiO2/PANI coaxial nanotube hybrid. The GCD measurement shows that the specific capacitance, power density and energy density of PPy/TiO2/PANI was 646.67 F g-1, 0.2 Wh kg-1 and 57.48 kW kg-1 at a current density of 0.5 A g-1 and potential range from 0 to 0.8 V in 1 M H2SO4 electrolyte. The specific capacitance of PPy/TiO2/PANI was higher than that of PPy/TiO2 (301.6 F g-1) under the same test conditions. It indicate that nanohybrid of anion doping PPy and proton acid doping PANI can take full advantage of electrolyte ions in the reversible doping-dedoping reactions during charge and discharge process, which can enhance the overall capacitance performance by synergy effect.The specific capacitance of PPy/TiO2/PANI dropped from 646.67 to 281.2 F g-1 as the current density increased from 0.5 to 5 A g’1, showing a poor rate capability. The capacitance of PPy/TiO2/PANI declined from 267.9 to 193.7 F g-1 after 500 cycles at a current density of 2 A g-1, showing 72.3% capacitance retention. The internal resistance of PPy/TiO2/PANI increased up to 18.8Ωcm-2, which was due to the semiconductor characteristic of TiO2 NTAs. The EIS measurement showed the charge transfer resistance of PPy/TiO2/PANI up to 0.072Ωcm-2. Therefore, the low conductivity of PPy/TiO2/PANI coaxial nanotube hybrid needs to be improved.2. The preparation and electrochemical properties of free-standing polyaniline nanorods embedding in polypyrrole nanopores coaxial nanoarray film.Conducting polymer films exhibit characteristic properties of flexibility and lightweight, which arouse considerable attention in the area of flexible supercapacitors. The semiconductor characteristic of TiO2 NTAs restricted the capacitance performance of PPy/TiO2PANI, although highly ordered TiO2 NTAs can assist the ion diffusion. In order to solve the problem of conductivity, we adopted the method of chemical etching to remove TiO2 NTAs substrate, preparing free-standing polyaniline nanorods embedding in polypyrrole nanopores coaxial nanoarray film. The morphology, structure and electrochemical energy storage of PPy/PANI have been fully investigated. The thickness of PPy/PANI film was 900 nm, one side was ordered and opened, the other side was partially sealed. The GCD measurement shows that the capacitance, power density and energy density of PPy/PANI was 212.8 F g-1,0.2 Wh kg-1 and 18.92 kW kg-1 at a current density of 0.5 A g-1 and potential range from -0.1 to 0.7 V in 1 M H2SO4 electrolyte. The capacitance of PPy/PANI dropped from 212.8 to 172.4 F g-1 as the current density increased from 0.5 to 2 A g-1,-showing a poor rate capability. The capacitance of PPy/PANI declined from 157.5 to 125.2 F g-1 after 500 cycles at a current density of 2 A g-1, showing 79.5% capacitance retention. The internal resistance was reduced from 18.8Ωcm-2 of PPy/TiO2/PANI to 1.912 cm-2 of PPy/PANI. The charge transfer resistance also had an obvious decrease from 0.016Ω cm-2 of PPy/TiO2/PANI to 0.016Ω cm-2 of PPy/PANI. The improvement of conductivity of PPy/PANI was caused by the better ion diffusion and charge thansfer after the removing of TiO2 NTAs. However, the film had an apparent defect of easy cracking, which would affect the stability performance.An aqueous symmetric supercapacitor was constructed using PPy/PANI electrodes and 1 M H2SO4 electrolyte solution. The capacitance, power density and energy density of the supercapacitor was 48.81 F g-1,0.8 Wh kg-1 and 4.34 kW kg-1 at a current density of 2 A g-1 and potential range from -0.1 to 0.7 V. Thus, the free-standing polyaniline nanorods embedding in polypyrrole nanopores coaxial nanoarray film can be used to fabricate flexible supercapacitors with good capacitance performance.3. The preparation and electrochemical properties of PPy/TiN/PANI coaxial nanotube hybrid.PPy/TiN/PANI coaxial nanotube hybrid can not only provide effective pathways for ion diffusion, but also show much higher conductivity than PPy/TiO2/PANI coaxial nanotube hybrid. Compared with PPy/PANI film, TiN NTAs substrate can work as the framework to prevent structure collapses and easy break up of films. Therefore, the TiN NTAs can be regarded as an promising substrate material for supercapacitor electrode. In this study, PPy was selectively coated on the inner surface of TiN nanotube walls to form PPy/TiN coaxial nanotube hybrid using a normal pulse voltammetry electrodeposition method. Then, the PANI was coated on the inner and upper surface of PPy/TiN coaxial nanotube hybrid to form PPy/TiN/PANI coaxial nanotube hybrid using a cyclic voltammetry electrodeposition method. The morphology, structure and electrochemical energy storage of PPy/TiN/PANI have been fully investigated. The SEM characterization indicates that the PPy deposited on the inner surface of TiN NTAs, and the PANI deposited on surface of PPy/TiN coaxial nanotube hybrid further. The GCD measurement shows that the specific capacitance, power density and energy density of PPy/TiO2/PANl was 1220.9 F g-1,0.2 Wh kg1 and 108.5 kW kg-1 at a current density of 0.5 A g-1 and potential range from -0.2 to 0.6 V in 1 M H2SO4 electrolyte. The capacitance of PPy/TiN/PANI was much higher than that of PPy/TiO2/PANI (646.67 F g-1) under the same test conditions. The capacitance of PPy/TiN/PANI dropped from 1220.9 tol026.4 F g-1 as the cureent density increased from 0.5 to 10 A g-1, showing a good rate capability. The capacitance of PPy/TiN/PANI declined from 506.9 to 437.5 F g-1 after 1000 cycles at a current density of 10 A g-1 in 1 M LiClO4 electrolyte, showing 86.3% capacitance retention. The internal resistance was reduced from 18.8 Ω cm-2 of PPy/TiO2/PANI to 0.19 Ω. cm-2 of PPy/TiN/PANI. The charge transfer resistance was reduced from 0.072 Ω cm-2 of PPy/TiO2/PANI to 0.013Ωcm-2 of PPy/TiN/PANI. Compared with PPy/TiO2/PANI coaxial nanotube hybrid and PPy/PANI film, PPy/TiN/PANI coaxial nanotube hybrid showed higher conductivity, capacitance retention and specific capacitance, indicating that PPy/TiN/PANI coaxial nanotube hybrid are more suitable material for supercapacitors.An aqueous symmetric supercapacitor was constructed using PPy/TiN/PANI coaxial nanotube hybrid electrodes and 1 M H2SO4 electrolyte solution. The specific capacitance, power density and energy density of the supercapacitor was 214.8 F g-1,19.09 Wh kg-1 and 4 kW kg-1 at a current density of 10 A g-1 and potential range from -0.2 to 0.6 V. The specific capacitance of this PPy/TiN/PANI aqueous symmetric supercapacitor was nearly 4 times than PPy/PANI aqueous symmetric supercapacitor. An all-solid-state symmetric supercapacitor was constructed using PPy/TiN/PANI coaxial nanotube. hybrid electrodes and H2SO4-EG-PVA gel electrolyte. The capacitance, power density and energy density of the supercapacitor was 202.2 F g-1,9 kW kg-1 and 91.0 Wh kg-1 at a current density of 10 A g-1 and potential range from 0 to 1.8 V. The power density and energy density of all-solid-state symmetric supercapacitor were much higher than that of aqueous symmetric supercapacitor, which may be due to the use of gel polymer electrolyte with wider potential window. |
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