Synthesis and electrochemical capacitance of sheet-like cobalt hydroxide

Abstract

Cobalt hydroxide was synthesized via a new approach, in which the gel was prepared by the reaction of Co(NO₃)₂ solution with citric acid, and then the precursors were obtained after thermal treatment, finally the precursors reacted with KOH solution to produce the samples. The components of the precursors were measured by Fourier transforms infrared (FT-IR) spectroscopy. The morphology and structure of samples were characterized by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The results indicate that the as-prepared materials are β-Co(OH)₂ with sheet-like morphology. The effect of the treatment temperature for the precursor on sheet size was discussed in detail. Cyclic voltammetry and constant current charge/discharge measurements show that the prepared cobalt hydroxide exhibits excellent capacitance performance. The specific capacitance is up to 416.7 F g⁻¹ at a constant current charge/discharge test of 5 mA.

Introduction

In recent years, electrochemical capacitors have attracted growing attention because of their high capacitance, high power density and long cycle life [1], [2], [3], [4]. According to energy storage mechanism, electrochemical capacitors can be classified into two types: the electric double-layer capacitors based on carbon materials and the faradic pseudo-capacitors based on metal oxides and electronically conducting polymers. In the former case, energy storage arises mainly from the separation of charges at the interface between the electrode materials and the electrolyte solution. In the latter case, energy storage is attributed to fast Faradic reactions taking place at the electrode materials at characteristic potentials [3], [5]. On account of the low utilization of carbon-based material [6] and degradation of the conducting polymer material as a result of swelling and shrinkage of electroactive polymers during cycling [7], [8], considerable efforts have been devoted to developing alternative electrode materials for electrochemical capacitors. One promising approach is using transitional metal oxides or hydroxides (e.g. oxides of Mn, Co, Ni, Sn, etc.) for electrochemical capacitors [9]. Initially, hydrated ruthenium oxide was considered as an excellent material because of its remarkable high specific capacitance. However, the high cost and toxic nature of the material have limited its wide applications.

For many years, cobalt hydroxide has been added for the purpose of enhancing the performance of Ni(OH)₂ as a battery active material. Many electrochemical studies of Ni(OH)₂ have been reported, whereas the studies of cobalt hydroxide are quite sparse [10], [11]. Recently, electroactive material Co(OH)₂ as a candidate has spurred research in this field, because it demonstrates to achieve good electrochemical capacitance performance. For example, Liu et al. have successfully synthesized single-crystal hexagonal platelets of β-Co(OH)₂ via homogeneous precipitation [12]. In addition, the so-called deposition transformation has been used to prepare β-Co(OH)₂, and mesoporous Co(OH)₂ has been synthesized by using CH₃(CH₂)₁₀CH₂OSO₂Na as soft template and urea as hydrolysis-controlling agent. However, these capacitive properties studies of cobalt hydroxide showed specific capacitances in the range of 90–341 F g⁻¹ [3], [13], [14]. In order to improve the electrochemical capacitance, many composites of Co(OH)₂ such as Co(OH)₂/USY [15], Co(OH)₂/HY [16], Co(OH)₂/TiO₂ nanotubes [17] were prepared, and the specific capacitance of Co(OH)₂ was improved accordingly.

Since electric double-layer capacitance and pseudo-capacitance due to the redox reaction are both interfacial phenomenon [3], the materials with the sheet-like structure, which is inclined to form layered structure that can provide large inter-sheet spacing for transferring the ions rapidly and increase the electroactive material–electrolyte interface area, is beneficial to the improvement on its electrochemical performances. Recently, many researchers have synthesized Co(OH)₂ via electrochemical methods. Gupta et al. [9] have synthesized α-Co(OH)₂ sheets by electrochemical method, and they demonstrated its specific capacitance was up to 860 F g⁻¹, because the sheet-like structure of the α-Co(OH)₂ promotes effective utilization of the electrode material. In addition, Zhou et al. have successfully synthesized a novel ordered mesoporous cobalt hydroxide film by electrodepositing on titanium substrate, and the electrochemical test results suggest that the ordered H₁-e Co(OH)₂ exhibits a maximum specific capacitance of 1084 F g⁻¹, which is ascribed to the regular nanostructure consisting of mesopores in ordered hexagonal arrays that create the fast electrochemical accessibility of the electrolyte and OH⁻ ions to the bulk of the Co(OH)₂ phase [18]. Although a high specific capacitance can be reached using electrochemical methods, short yield of active materials is a problem in these methods because the product was directly deposited on the surface of conductive substrate and there was only about 10 nm in the thickness generally. Therefore, how to prepare cobalt hydroxide with high specific capacitance on a large scale is an attracting problem for researchers.

In this work, sheet-like cobalt hydroxide was synthesized based on a new route, in which citric acid acted as a chelating reagent and the carbonized citric acid gel played a role of template for forming cobalt hydroxide. Because no strong bonding exists between these sheets, ions and charges may transfer into sheets facilely [19]. The products were improved in terms of electrochemical properties, and their specific capacitance was up to 416.7 F g⁻¹. Due to the low cost, simple procedure, large–scale yield and high specific capacitance, the cobalt hydroxide synthesized by this method can be a promising material for electrochemical capacitors.