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 °C. Compared with the as deposited Ni(OH)2 on CNT film substrates (Ni(OH)2/CNT electrodes), the 300 °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 F g−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.