Influence of Dopants on Performance of Polypyrrole Coated Carbon Nanotube Electrodes and Devices

Polypyrrole (PPy) and PPy-multiwalled carbon nanotube (MWCNT) composites were prepared for charge storage application in electrodes of electrochemical supercapacitors (ES). Chemical polymerization of PPy was performed using 4-amino-5-hydroxynaphthalene-2,7-disulfonic acid monosodium salt hydrate (AHDA) and diamine green black (DAGB) as new anionic dopants. It was found that DAGB improved colloidal stability of MWCNT dispersions and allowed the fabrication of PPy coated MWCNT. Testing results provided an insight into the influence of the chemical structure of the dopants on the microstructure of the PPy-MWCNT composites and electrochemical performance. The PPy-MWCNT composites showed significant improvement in capacitance, compared to pure PPy electrodes for active mass loading of 10 mg cm−2. The highest capacitance of 220 F g−1 was achieved at a scan rate of 2 mV s−1 for PPy coated MWCNT, prepared using DAGB. The capacitance retention at 100 mV s−1 was found to be 60.9%. The integral and differential capacitances, calculated from the cyclic voltammetry and impedance spectroscopy data were analyzed at different conditions. The PPy coated MWCNT, prepared using DAGB, were used for the fabrication of symmetric devices, containing two similar PPy-MWCNT electrodes and asymmetric devices containing PPy-MWCNT positive electrode and vanadium nitride (VN)-MWCNT negative electrodes. The asymmetric devices offered advantages of higher capacitance, lower impedance, larger voltage window and improved power-energy characteristics.