Elsevier

Journal of Power Sources

Volume 184, Issue 2, 1 October 2008, Pages 682-690
Journal of Power Sources

Fabrication and electrochemical characterization of cobalt-based layered double hydroxide nanosheet thin-film electrodes

https://doi.org/10.1016/j.jpowsour.2008.02.017Get rights and content

Abstract

A continuous cobalt-based layered double hydroxide (LDH) nanosheet thin-film electrode has been fabricated by drying a nearly transparent colloidal solution of cobalt-based LDH nanosheets on an indium tin oxide (ITO)-coated glass plate substrate. The effects of varying the Al content, the film thickness, and the heating temperature on the electrochemical properties of the as-deposited thin-film electrode have been investigated. A thin-film electrode with a Co/Al molar ratio of 3:1, which has a large specific capacitance of 2500 F cm−3 (833 F g−1) and a good high-rate capability, shows the best performance when used as an electrode in thin-film supercapacitors (TFSCs). As the thickness of the thin film was increased from 100 to 500 nm, the specific capacitance of the thin-film electrode remained essentially unchanged, which is due to the porous microstructure generated in the original electrochemical process and the low internal resistance of the thin-film electrode. The specific capacitance of the thin-film electrode showed no observable change after heating at 160 °C, but decreased on further heating to 200 °C, indicating that the electrochemically active Co sites inside the thin-film nanosheet electrode are already essentially fully exposed in the as-prepared material and hence cannot be further exposed through heating. Such a thin-film electrode made up of nanosheets may be a potential economical alternative electrode for use in TFSCs.

Introduction

Interest in micro-scale power sources, such as thin-film batteries (TFBs) and thin-film supercapacitors (TFSCs), has risen in recent years because of their use as power sources in microelectronic mechanical systems (MEMs) and nanoelectronic mechanical systems (NEMs) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. TFBs are suitable for use in most of these devices as a result of their high energy density, rechargeability, and safety in operation. However, their poor ability to deliver high power over limited time intervals limits the applications of TFBs in some special fields such as the startup of some MEMs or NEMs, which require a large burst of power to be delivered in a very short time. To meet this particular demand, the fabrication of high-quality TFSCs with high power density and long cycle life has received considerable attention in recent years [5], [6], [7], [8].

Supercapacitors (also called electrochemical capacitors) that can fill the gap between batteries and conventional dielectric capacitors have considerable potential for use in high power applications [12], [13]. Supercapacitors may be distinguished on the basis of several criteria such as the electrode material utilized, the electrolyte, or the cell design. With respect to electrode material there are three main categories: carbon, (hydrous) metal oxides, and conducting polymers [14]. To date, conducting noble metal oxides like RuO2 or IrO2 are the favored electrode materials for supercapacitors [15], [16], [17], [18], [19]. Although RuO2 gives high specific capacitance (as high as 1300 F g−1 [17], [18]), it has the disadvantages of high cost and toxic nature, which limit its commercial applications. Therefore other transition metal oxides, such as amorphous hydrous cobalt–nickel oxides [20], MnO2 [21], NiO [22], Co3O4 [23], and Fe3O4 [24] have been examined as inexpensive alternatives to RuO2. In addition to good supercapacitor behavior, metal oxides also have the advantage that they can be readily made into thin films. For example, RuO2 and Co3O4 thin-film electrodes have been fabricated for use as the electrodes for TFSCs by a radio frequency (rf) sputtering method [5], [6]. However, sputtering [2], [4], [5], [6], [10] and most of the other techniques for making thin-film electrodes, such as pulse laser deposition (PLD) [11], require high energy, high material consumption, and complicated equipment. Furthermore, using such processes it is difficult to fabricate hydrous metal oxides, which usually have better supercapacitor behavior than crystalline metal oxides [15], [16]. Thus, an alternative route to synthesize hydrous metal oxide thin-film electrodes which is less energy intensive, more economical, and requires simpler apparatus is highly desirable.

We have recently suggested a simple strategy to fabricate a Co/Al LDH nanosheet thin-film electrode with good supercapacitor behavior for use in TFSCs [25]. In this work, our aim is to tailor the supercapacitor behavior of the cobalt-based LDH nanosheet (Co/Al-LDH NS) thin-film electrodes prepared by this procedure. The relationship between the chemical composition and the electrochemical properties of the thin films has been investigated in detail. The effects of varying the thickness and the temperature of formation of the thin film on the electrochemical behavior have also been studied.

Section snippets

Synthesis of nitrate anion-intercalated cobalt-based LDHs with varying Al content

Nitrate anion-intercalated cobalt-based LDHs (Co1−xAlx-LDHs) with varying Al content x were synthesized by a method involving separate nucleation and aging steps (SNAS) [26], [27]. Water used in all preparations was deionized with a conductance below 10−6 S cm−1. Details for a typical synthesis are as follows. Solution A: Co(NO3)2·6H2O and Al(NO3)3·9H2O with a Co/Al molar ratio of 2:1 (i.e. x = 0.33) were dissolved in deionized water (150 mL) to give a solution with a total metal ion concentration

Characterization of the cobalt-based LDH nanosheet thin-film electrodes with varying Al content

The powder XRD patterns of nitrate anion-intercalated Co1−xAlx-LDHs prepared using mixtures with different Al content x are shown in Fig. 1. In each case, the XRD pattern exhibits the characteristic reflections of LDHs with a series of (0 0 l) peaks appearing as narrow symmetric lines at low angle, corresponding to the basal spacing and higher order reflections. No other crystalline phase was present. FT-IR spectra (Fig. 2) of the Co1−xAlx-LDHs with different Al content x showed absorption bands

Conclusions

This study shows that continuous Co1−xAlx-LDH NS thin-film electrodes can be fabricated under mild conditions. Varying the Al content x has a significant effect on the electrochemical properties of the thin-film electrode. The partial isomorphous substitution of Co2+ by Al3+ is the key factor in the improvement of the electrochemical behavior. The Co0.75Al0.25-LDH NS thin-film electrode, which has a large specific capacitance of 2500 F cm−3 (833 F g−1), a good high-rate capability, and low cost, is

Acknowledgements

This work was supported by the National Natural Science Foundation of China, the National 863 Project (Grant No. 2006AA03Z343), the 111 Project (Grant No. B07004), and the Program for Changjiang Scholars and Innovative Research Teams in Universities (Grant No. IRT0406).

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