Elsevier

Journal of Power Sources

Volume 182, Issue 2, 1 August 2008, Pages 642-652
Journal of Power Sources

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

https://doi.org/10.1016/j.jpowsour.2008.03.090Get rights and content

Abstract

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.

Introduction

Recently, electrochemical capacitors (ECs) have attracted considerable attention for use in high power energy storage devices. Electrochemical capacitors have potential applications including power enhancement and primary or hybrid power sources combined with batteries and fuel cells for the hybrid electric vehicle (HEV) or fuel cell electric vehicle (FCEV). In these ECs, the energy stored is either capacitive or pseudocapacitive in nature. The capacitive or non-Faradaic process is based on charge separation at the electrode/solution interface. On the other hand, the pseudocapacitive process consists of Faradaic redox reactions that occur within the active electrode materials. Carbon, conducting polymers and transition metal oxides are the most widely used active electrode materials [1], [2]. Hydrous RuO2 exhibits the best properties among the many transition metal oxide materials investigated as pseudocapacitor (or supercapacitor) materials thus far. An amorphous phase of RuO2·xH2O formed using a sol–gel method at low temperatures shows a specific capacitance as high as 720 F g−1 in an acidic electrolyte [3]. However, its commercial use is limited by its high cost. Therefore, there has been considerable effort on the search for alternative electrode materials, such as cobalt oxide [4], manganese oxide [5], [6], [7], [8], [9], and nickel oxide [10], [11], [12], [13], [14], [15], [16], [17], which are inexpensive and show similar pseudocapacitive behavior to hydrous RuO2.

Nickel oxide is considered a potential electrode material for supercapacitors in alkaline electrolytes on account of its easy synthesis and relatively high specific capacitance (mass-normalized capacitance, F g−1). However, nickel oxide has the significant drawback of a small operating potential window (∼0.5 V). This can be overcome by adapting an asymmetric EC configuration, which employs nickel oxide as the positive electrode and another material as the negative electrode [18], [19], [20]. Nickel oxides as a supercapacitor electrode material can be prepared using a variety of synthetic routes, such as the thermal treatment of an electrodeposited or sol–gel prepared nickel hydroxide [10], [11], [12], [13], [14], liquid crystal templating electrodeposition [15], simple liquid-phase process [16], and the replication of a mesoporous silica template [17]. The specific capacitance of nickel oxide electrode materials ranges from 50 to 350 F g−1 (from a single electrode) depending on the method of synthesis, which is still far from the theoretical value of 2584 F g−1 within 0.5 V.

A literature review of composite electrodes made from transition metal oxides and carbonaceous materials [21], [22], [23], [24], [25], [26], [27], [28] showed that metal oxides can be produced with a high specific capacitance and rate capability. This is particularly so when a small amount of metal oxide is dispersed uniformly over a conducting and porous carbonaceous material with a very high surface area due to the increased electrical conductivity, electrochemical utilization of the metal oxide and ionic transport throughout the internal volume of the electrode. Based on this concept, a NiOx coating on a carbon nanotube (CNT) film substrate with a 3-dimensionally interconnected network structure was previously synthesized [23]. The NiOx-normalized specific capacitance of the NiOx coated CNT electrode (i.e., NiOx/CNT electrode) was reported to be ∼1000 F g−1, which is approximately three times higher than the ∼350 F g−1 obtained for the NiOx thin film electrode [23]. This high specific capacitance of the NiOx/CNT electrode was attributed to the construction of an electrode with a nanometer-thick NiOx layer on a CNT film substrate, which has a 3-dimensional network structure.

In contrast to the viewpoint of the electrochemical utilization of nickel oxides, the oxide loading in the electrode should also be considered for practical applications, particularly for large capacitor applications, such as power sources for the HEV or FECV. Hence, there is a need for an electrode with a higher volumetric capacitance (volume-normalized capacitance, F cm−3) and geometric capacitance (area-normalized capacitance, F cm−2), which are generally proportional to the loading of the active material, with little sacrifice of the specific capacitance. In the case of the NiOx/CNT nanocomposite electrode, it is believed that the specific, geometric and volumetric capacitances will be affected by the microstructure of the electrode, which is closely related to the oxide loading. Therefore, this study examined the effects of the NiOx loading on the microstructure and corresponding electrochemical properties of NiOx/CNT electrodes, i.e. the geometric, volumetric and specific capacitance. In order to provide a clearer understanding of the electrochemical properties of the nanostructured nickel oxide electrode for potential use in supercapacitors, the detailed electrochemical redox reaction of the NiOx/CNT electrode in a 1 M KOH solution was also examined by X-ray photoelectron spectroscopy (XPS) and deconvolution analysis of the cyclic voltammograms (CVs).

Section snippets

Experimental

CNT films with a 3-dimensional nanoporous network structure were deposited onto a Pt coated Si wafer using an electrostatic spray deposition (ESD) technique [23], [27], [28], [29]. A CNT film coated Pt/Si wafer was used as a substrate for the electrochemical preparation of nickel oxide. The process for preparing the CNT film substrate is described elsewhere [23], [27], [28], [29]. The multi-walled carbon nanotubes (MWNTs) were supplied from ILJIN Nanotech Co., Ltd. The thickness and mass of the

Comparison of the electrochemical properties of the bare CNT film, Ni(OH)2/CNT, and NiOx/CNT electrodes

Fig. 1 shows the XRD patterns of the CNT film substrate, as-deposited Ni(OH)2 coated on CNT film (i.e., Ni(OH)2/CNT electrode) and NiOx coated CNT film (i.e., NiOx/CNT electrode), which was produced by heating the Ni(OH)2/CNT films to 300 °C for 1 h. The characteristic graphitic (0 0 2) peak of the MWNT at 26° was clearly observed on the CNT film substrate [30]. The XRD pattern of the as–deposited Ni(OH)2/CNT electrode corresponded to the well-known layered α-Ni(OH)2 structure with a characteristic

Conclusions

NiOx/CNT electrodes with various wt.% of NiOx were prepared by the electrochemical deposition of Ni(OH)2 onto CNT film substrates followed by heating to 300 °C for 1 h. The detailed electrochemical redox reaction of the NiOx/CNT electrode in a 1 M KOH solution was examined by XPS and deconvolution analysis of the CVs. A comparison of the electrochemical properties with the as-deposited Ni(OH)2/CNT electrode showed that the NiOx/CNT electrode with 300 °C heat treatment has high rate capability

Acknowledgements

This work was supported by Korea Science & Engineering Foundation (KOSEF) through the National Research Lab. Program funded by the Ministry of Science and Technology (No. R0A-2007-000-10042-0). The experiments at Brookhaven National Lab. was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, under the program of “Hybrid and Electric Systems”, of the U.S. Department of Energy under Contract Number DEAC02-98CH10886.

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