Composite Nanofibers by Growing Polypyrrole on the Surface of Polyaniline Nanofibers Formed under Free Melting Condition and Shell-Thickness-Dependent Capacitive Properties

An efficient approach was presented to prepare polyaniline/polypyrrole (PANi/PPy) composite nanofibers by growing PPy layers on the surface of PANi nanofibrous seeds as electrode materials for supercapacitors in neutral electrolyte. Core layer of PANi nanofiber was firstly synthesized by the chemical oxidative polymerization of aniline monomers under free melting condition of reaction solutions in fully aqueous system without the assistance of any templates or organic solvents. Then the shell layer of PPy was fabricated by in-situ chemical oxidative polymerization of pyrrole monomers with the above-mentioned PANi nanofiber as a seed, and the PPy shell layer thicknesses were tuned by changing the molar ratio of aniline to pyrrole. The resulting PANi/PPy composites were investigated by field-emission scanning electron microscopy, ultraviolet-visible spectroscopy, Fourier transform infrared and Raman spectrometry. Furthermore, electrochemical behaviors in Na2SO4 electrolyte were tested by cyclic voltammetry, galvanostatic charge-discharge techniques and electrochemical impedance spectroscopy. It turned out that low molar ratio of aniline to pyrrole is helpful to increase the PPy shell layer thicknesses, yield and conductivity of PANi/PPy composite nanofibers. A great improvement on the capacitive properties could be achieved by choosing appropriate PPy shell layer thickness. The results showed that benefiting from strong synergy effect and π-π interaction between PANi core and PPy shell layer as well as low electrochemical impedance, PANi/PPy composite nanofibers prepared with the molar ratio of 1:1 (PPy shell layer thickness of about 12.5 nm) displayed the highest specific capacitance of 1550.2 F g−1 at scan rate of 5 mV s−1 and 758.8 F g−1 at the current density of 1 A g−1 with the best cycling stability of 70.3 % after 500 cycles in 0.5 M Na2SO4 electrolyte, which exhibited a great potential in the development of high-performance electrode materials operated in environmentally friendly electrolyte.