Polypyrrole-based Nanocomposites As Electrode Materials For Supercapacitor

Due to its special structure and excellent physical chemical properties, the polypyrrole as one kind of conducting polymers has attracted great interests in many fields, such as energy storage, molecular device and metal corrosion. Polypyrrole as electrode materials in supercapacitor exhibits good electrical conductivity, strong charge storage capacity and excellent environmental stability. However, compared with those prepared by electrochemical polymerization, polypyrrole prepared by chemical polymerization shows poor electrochemical behavior. It has been demonstrated that polypyrrole-based nanocomposites can exhibit excellent performance as electrode materials in supercapacitor. In this work, we prepared two kinds of polypyrrole-based nanocomposites by in-situ chemical polymerization of pyrrole monomers in the present of multi-walled carbon nanotubes (MWNTs) and nickel hydroxide (Ni(OH)2) nanoparticles, respectively. In addition, PPy-MWNTs-Ni(OH)2 ternary nanocomposites were prepared by mixing PPy-MWNTs and PPy-Ni(OH)2 nanocomposites. Our main results were listed as follows:(1) Effects of PPy shell thickness and the nature of electrolytes on electrochemical properties of PPy-MWNTs:By adjusting the weight ratio of the MWNTs and pyrrole monomers, the PPy shell thickness can be controlled. It is found that when the weight ratio of the MWNTs and pyrrole monomers was 40%, the nanocomposite exhibited best electrochemical performance in 0.1 M Na2SO4 electrolytes, and the maximum specific capacitance of 602 F/g could be obtained at the scan rate of 1 mV/s. Moreover, the nanocomposite showed good cycle stability, and the capacity retention decayed to 77.10% after 500 continuous charge/discharge cycles at 10 mV/s scan rate. Furthermore, the electrochemical properties of PPy-MWNTs nanocomposite were studied in various electrolytes, such as acidic electrolyte (0.1 M H2SO4), alkaline electrolyte (0.1 M NaOH, 0.1 M KOH), neutral solution (0.1 M Na2SO4, K2SO4), and mixed electrolyte (0.05 M Na2SO4+0.05 M NaOH), respectively. The effects of electrolytes on the electrochemical properties were further discussed based on the results from AC impedance spectroscopy. (2) PPy-Ni(OH)2 nanocomposites:β-Ni(OH)2 particles were first prepared through a simple hydrothermal route by using hydrazine hydrate as the alkali and nickel nitrate as nickel source, and no surfactant was applied. The influence of hydrothermal condition on the morphologies and crystal structures was discussed. It is found that the average size of the particles decreased with the decrease the content of nickel source, and the shape changed from hexagonal lamellar crystal to ellipse-shaped crystal with the increase of the pH value. However, the similar crystal structures ofβ-Ni(OH)2 for all as-prepared samples were observed. Then PPy-Ni(OH)2 nanocomposites were prepared by in-situ chemical polymerization of pyrrole monomers in the present of theβ-Ni(OH)2 particles with different shapes and sizes. It is found that the nanocomposite exhibited better electrochemical properties for the nanocomposites including the Ni(OH)2 nanoparticles with a smaller size. Moreover, electrochemical analysis showed that the maximum capacitance of PPy-Ni(OH)2 of 475.3 F/g at 1 mV/s in 1 M KOH electrolytes was obtained for the nanocomposites with a weight ratio of Ni(OH)2 and pyrrole monomers of 1:4, and its capacity retention decayed to 59.5% after 500 continuous charge/discharge cycles.(3) Ternary PPy-MWNTs-Ni(OH)2 nanocomposites:The ternary nanocomposites were prepared by mixing above-mentioned PPy-MWNTs and PPy-Ni(OH)2 nanocomposites with different weight ratios. Electrochemical analysis showed that the enhancement on electrochemical properties of ternary nanocomposites could be achieved, compared with that for the binary nanocomposites. The maximum capacitance of PPy-MWNTs-Ni(OH)2 of 789.4 F/g at the scan rate of 1 mV/s can be achieved with a weight ratio of PPy-MWNTs and PPy-Ni(OH)2 of 1:3, and the capacity retention decayed to 29.1% after 500 continuous charge/discharge cycles in the 0.5 M KOH+0.5 M Na2SO4 electrolyte. The enhanced electrochemical properties of ternary nanocomposites benifited from the combination of the advantage for each components and their synergistic effect. In addition, muti-walled carbon nanotubes in the nanocomposites can provide a three-dimensional conductive channel, and a strong interfacial adhesion can be achieved by the introduction of polypyrrole into the nanocomposites.