Electrochemical Capacitor Electrodes Based on High-Purity Eelectrospun Polyaniline and Polyaniline-Carbon Nanotube Nanofibers

Abstract

Freestanding, binder-free supercapacitor electrodes based on high purity polyaniline (PANI) nanofibers were fabricated using electrospinning. To prevent jet break-up and to enable successful electrospinning of high-purity PANI nanofibers (93 wt%), large molecular weight poly (ethylene) oxide (PEO) was added to the PANI solution as a carrier polymer. The high molecular weight of PEO allowed us to impart adequate entanglements (critical for electrospinning) at a low PEO. Scanning electron microscope (SEM) and Brunner-Emmet-Teller (BET) analysis were used to examine the morphology and porosity of the nanofibers. The PANI nanofibers, with an average diameter of 678±54 nm, were characterized by micro-pores on their surfaces possibly due to evaporative cooling during electrospinning. These pores, with the most common size of 1.6 nm, resulted in a specific surface area of 20 m2 g-1. To enhance electrical conductivity as well as structural stability of the resultant electrodes, carbon nanotubes (CNTs) were added to the PANI/PEO solution. The PANI/CNT nanofibers exhibited an average diameter of 528±50 nm. The slight decrease in diameter is attributed to introduction of highly conductive CNTs in the polymer solution which leads to higher charge density and therefore greater jet elongation. Transmission electron microscope (TEM) showed excellent distribution of CNTs within the nanofibers. The electroactivity of these freestanding PANI and PANI/CNT nanofibers electrodes were examined using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. Specific capacitance of 308 and 385 F g-1 at 0.5 A g-1 were achieved for PANI and PANI/CNT based electrodes, respectively. Both electrodes showed excellent rate capability when cycling voltammetry (CV) scan rate was varied from 5 to 100 mVs-1 and current density was increased to 10 A g-1. Areal capacitances of 1.15 and 1.37 F cm-2 were obtained for PANI and PANI-CNT, respectively, when electrode loading was increased with negligible loss in gravimetric specific capacitance. Capacitance retention of 70 and 82% were observed for PANI and PANI/CNT electrodes after 1000 cycles, respectively, indicating enhanced electrochemical performance with incorporation of only 12 wt% of CNT to the PANI nanofibers. The promising electrochemical performance of the fabricated electrodes, we believe, stems from the porous 3-D electrode structure characterized by nonwoven interconnected nanostructures. The interconnected nanofiber network facilitates efficient electron conduction whereas the inter- and intra-fiber porosity enable excellent electrolyte penetrations within the polymer matrix hence quick ions transport to active sites. Moreover, the freestanding electrodes avoid potential structural damage most likely encountered when preparing paste for electrochemical testing in the “conventional” way. Simotwo, S; Delre, C and Kalra, V. Supercapacitor Electrodes Based on High-Purity Electrospun Polyaniline and Polyaniline-Carbon Nanotube Nanofibers, submitted (2016) Figure 1