Preparation Of Bimetallic Oxide/Carbon Nanotube Composites And Its Application In Flexible Electrode

With the development of electronic devices in the direction of portable,foldable and wearable,the flexible supercapacitor(FSCs)used as the driving power source has aroused people's attention and become the research hotspot.At present,the development of flexible supercapacitors is still facing the challenge of relatively low capacitance.Thus,the research of electrode material which determines the properties of FSCs is the key to solve the problem.Among the electrode active materials,carbon nanotubes(CNT)have large specific surface area,good electrical conductivity and high chemical and electrochemical stability.However,CNT are limited by the double-layer energy storage mechanism and the increase of specific capacitance.Compared with single metal oxides,bimetallic oxides can reduce the optical band gap and improve the electrical conductivity due to the diversification of their components,and the synergistic effect between different components makes them have higher specific capacitance and cycle stability.The bimetallic oxide is energy storage with pseudocapacitance.The specific capacitance is several orders of magnitude higher than that of the carbon material,but the conductivity and stability are relatively poor,and the agglomeration is easy and the specific surface area is low,which greatly reduces the utilization ratio.The bimetallic oxide/CNT composite is formed by combining the bimetallic oxide and the CNT.The specific surface area of the bimetallic oxide is improved effectively,and the electrode material with high specific capacitance and good cycling stability can be obtained.On the other hand,bimetallic oxide/CNT is prepared by solvothermal method.The process is simple and the yield is relatively high.It is convenient for large-scale production and application.It is very important to realize the application of supercapacitor.Firstly,a-MWCNT as carrier,CoCl₂·6H₂O and NiCl₂·6H₂O as mother salts,NiCo₂O₄/MWCNT composites were prepared by solvothermal method.The effects of carrier specification and concentration,main salt species and concentration,surfactant type and reaction time on the discharge performance of the electrode material were investigated.X-ray diffraction(XRD),field emission scanning electron microscopy(FESEM),field emission projection electron microscopy(TEM),and X-ray energy dispersive spectroscopy(EDS)were used to characterize the samples.(CV),Cyclic voltammetry(CV),constant current charge-discharge curve(GCD),alternating current impedance spectroscopy(EIS)were used to characterize the electrochemical performance of the composites.The results showed that NiCo₂O₄/MWCNT composites prepared with 1.6 g/L a-S1020 MWCNT,0.075 mol/L metal ions of Co²⁺:Ni²⁺=2:1(molar ratio),Tween-80 as additive and reacting 14 h have a relatively high specific capacitance.NiCo₂O₄/MWCNT composites showed a spinel-like,nearly hexagonal lamellar shape with a specific surface area of 7.8 times higher than that of NiCo₂O₄(193.9 and 24.8 m²/g,respectively).At a current density of 5 m A/cm²,the discharge time is about 2440 s and the mass ratio is as high as 3873 F/g,which is 15.3 times of NiCo₂O₄ specific capacitance(discharge time 159.6 s).The introduction of carbon material in the composites can greatly improve the stability of bimetallic oxide electrode materials.The capacitance retention rate of NiCo₂O₄/MWCNT is about 84%after 5,000 times of CV scanning,which is equal to the stability of MWCNT and 30%higher than that of NiCo₂O₄.Secondly,NiCo₂O₄/MWCNT was combined with polypyrrole(PPy)to form a composite film composed of carbon material,metal oxide and conductive polymer on the surface of cotton fabric by inorganic particle dispersion and organic monomer in-situ polymerization,which fabricated PPy/NiCo₂O₄/MWCNT fabric electrode.PPy/NiCo₂O₄/MWCNT fabric electrode were characterized by XRD and FESEM.The electrochemical properties of the fabric-based flexible electrode materials were compared by using the pulse potential method and the chemical polymerization method.The results show that the composite films with PPy/NiCo₂O₄/MWCNT composite films retain their flexibility and can be used as flexible supercapacitors.The morphology of EL-PPy/NiCo₂O₄/MWCNT prepared by pulsed-potential method and CH-PPy/NiCo₂O₄/MWCNT fabricated by chemical method composite films show"cauliflower-like"composite with porous 3D network structure and irregular membrane structure without obvious pore,respectively.Their area specific capacitance of 1.301F/cm² and 2.464 F/cm² were 2.65 times and 4.92 times higher than those of the EL-PPy/MWCNT and CH-PPy/MWCNT fabric electrodes,respectively.The results showed that the EL-PPy/NiCo₂O₄/MWCNT fabric electrode had better cycling stability than the CH-PPy/NiCo₂O₄/MWCNT fabric electrode with the capacity retention rate of90.96%and 45.5%after 1500 CV scans(the scan rate was 500 mV/s).