Preparation of vertically aligned carbon nanotubes and their electrochemical performance in supercapacitors

Vertically aligned carbon nanotubes (VACNTs) were grown on highly n-doped silicon substrates by direct current plasma enhanced chemical vapor deposition to be used as supercapacitor electrodes from a gas mixture of acetylene and hydrogen on nickel islands as the catalyst particles. Scanning electron microscopy and transmission electron microscopy have been exploited to characterize the VACNTs. Electrochemical properties of electrodes were studied by cyclic voltammetry, galvanostatic charge–discharge and impedance spectroscopy technique. According to SEM and TEM images growth mechanism was tip-initiated. Capacitances for four different growth durations of 20, 40, 60 and 120 min were measured to be 1.5, 5.8, 7.5 and 15 mF cm−2, respectively, at 1 mA discharge current. VACNT electrodes fabricated by this method contributed to better electrode capacitance as compared with results of similar previous studies. Furthermore the rate capability of electrodes was found to be excellent. The specific capacitances of a fabricated VACNT electrode and an entangled carbon nanotube (ECNT) electrode in 0.5 M KCl aqueous solution electrolyte were 70 and 8 Fg−1, respectively. Compared with ECNT electrode, VACNT electrode achieved higher specific capacitance. The higher specific capacitance of VACNT was attributed to the high surface area as well as to defects on the VACNTs, which may be formed by the H2-plasma during growth. The maximum specific energy and power of the VACNT electrode were 3.5 Wh kg−1 and 22 kW kg−1, respectively. The results indicate that the VACNTs are promising candidates as electrode materials in supercapacitors.