Pseudocapacitive properties of electrochemically prepared nickel oxides on 3-dimensional carbon nanotube film substrates

Nickel oxides on carbon nanotube electrodes (NiOx/CNT electrodes) are prepared by depositing Ni(OH)(2) electrochemically onto carbon nanotube (CNT) film substrates with subsequent heating to 300 degrees C. Compared with the as deposited Ni(OH)(2) on CNT film substrates (Ni(OH)(2)/CNT electrodes), the 300 degrees C heat treated electrode shows much high rate capability, which makes it suitable as an electrode in supercapacitor applications. X-ray photoelectron spectroscopy shows that the pseudocapacitance of the NiOx/CNT electrodes in a 1 M KOH solution originates from redox reactions of NiOx/NiOxOH and Ni(OH)(2)/NiOOH. The 8.9 wt.% NiOx in the NiOx/CNT electrode shows a NiOx-normalized specific capacitance of 1701 Fg(-1) with excellent high rate capability due to the 3-dimensional nanoporous network structure with an extremely thin NiOx layer on the CNT film substrate. On the other hand, the 36.6 wt.% NiOx/CNT electrode has a maximum geometric and volumetric capacitance of 127 mF cm(-2) and 254 F cc(-1), respectively, with a specific capacitance of 671 F g(-1), which is much lower than that of the 8.9% NiOx electrode. This decrease in specific capacitance of the high wt.% NiOx/CNT electrodes can be attributed to the dead volume of the oxides, high equivalent series resistance for a heavier deposit, and the ineffective ionic transportation caused by the destruction of the 3-dimensional network structure. Deconvolution analysis of the cyclic voltammograms reveals that the rate capability of the NiOx/CNT electrodes is adversely affected by the redox reaction of Ni(OH)(2), while the adverse effects from the reaction of NiOx is insignificant. (C) 2008 Elsevier B.V. All rights reserved.