Capacitance properties of single wall carbon nanotube/polypyrrole composite films

The electrochemical capacitance properties of composite films prepared from electrically conducting polypyrrole (PPy) and single wall carbon nanotubes (SWNTs) have been investigated for supercapacitor application. The composite films were electropolymerized from homogenous mixture of pyrrole (Py) and raw SWNTs, or suspension of Py and functionalized SWNTs. The SWNTs were functionalized and cut by suspending in concentrated H2SO4/HNO3 solution and sonicating in a water bath. The morphology of composite films was observed by field emission scanning electron microscope (SEM). The electrochemical capacitance properties of the composite films were measured by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques in 1 M KCl aqueous solutions. Due to the mesoporous structure of carbon nanotubes (CNTs), the easily accessible electrode/electrolyte interface allowed very fast charging/discharging process. Moreover, owing to the high electrical conductivity of CNTs and the mesoporous structure, the composite films had very low resistance and almost ideal capacitance behavior even on deeply discharged states. On the contrary, the pure PPy films had much larger resistance and worse capacitance properties due to the low conductance and contracted volume on discharging (reduced) states. On the other hand, in PPy/functionalized CNTs composite films, the PPy was doped by immobile functionalized CNTs. On reduced state, the PPy chains become neutral and the negative charge on functionalized CNTs must be balanced by cations with smaller size. The balancing behavior of cations could diminish ion-transfer polarization further. Therefore, the specific capacitance of PPy/SWNTs and PPy/functionalized SWNTs composite films would reach 144 F g−1 and 200 F g−1 at scanning rate of 200 mV s−1, respectively. All of the above implies that the PPy/SWNTs composite, especially PPy/functionalized SWNTs composite, was a kind of promising electrode material of supercapacitor.