**4.4 Supercapacitor**

PPy offers a greater degree of flexibility in electrochemical processing than most conducting polymers, and consequently the material has been the subject of much research as a super capacitor or battery electrode (Snook et al., 2011). Due to greater density, polymer had a high capacitance per unit volume (400-500 Fcm-3). Combining PPy with polyimide (a dopant of high molecular weight) is claimed to improve the charge storage property (Iroh & Levine, 2003), due to the polyimide matrix protecting the PPy from oxidative degradation. Polyimide is cathodically electroactive, whereas PPy is anodically electroactive.

MWCNT coated with PPy has also been used in supercapacitor. The maximum value of 163 F g-1 has been obtained for MWCNTS prepared at 600°C and modified by a PPy layer of 5 nm, whereas it is only 50 F g-1 for the pristine nanotube (Jurewicz et al., 2001). PPy/GNS composite is a promising candidate for supercapacitor to have higher specific capacitance, better rate capability and cycling stability than those of pure PPy. The specific capacitance of PPy/GNS composite based on the three electrode cell configuration is as high as 482 F g-1 at a current density of 0.5 A g-1 (Zhang et al., 2011).

PPy doped with nafion ions or ClO4¯ exhibits a specific capacitance of 344 or 355 F g-1. Cycle life experiments revealed that the nafion material retains 98% of the original capacitance after 3000 cycles whereas that doped with ClO4¯ retains only 70% over this period (Kim et al., 2008b).The PPy active films doped with ClO4¯ ion in NaCl solution demonstrates an ideal supercapacitor behavior, i.e. rectangle-like CV shape at scan rates from 5-200 mV s-1, linear galvanoststic charge/discharge curves at currents loads from 0.5 to 2 mA and stable cyclic property. However, dopings with Cl¯ ions give rise to nonideal property of supercapacitor (Sun et al., 2009).

PEDOT/PPy composite can be used as an electrochemical supercapacitor electrode material. This composite electrode produced the specific capacitance as 230 Fg-1 in 1M lithium perchlorate aqueous solution and 290 F g-1 in 1M KCl aq solution (Wang et al., 2007). A pulsed polymerized Py film obtained on the polished graphite plate has been reported to exhibit very high capacitance (400 Fg-1) and high energy density (250 Wh h kg-1) and can be used as supercapacitor. Stability tests performed on this pulse polymerized PPy electrode yield long cycle life upto 10,000 cycles, the charge/discharge current density being 5mA cm-2 (Sharma et al., 2008).
