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DOI: 10.1039/C4RA01719F (Paper) RSC Adv., 2014, 4, 19476-19481

A simple approach to prepare nickel hydroxide nanosheets for enhanced pseudocapacitive performance

Anjon Kumar Mondal*a, Dawei Suab, Shuangqiang Chena, Bing Suna, Kefei Lia and Guoxiu Wang*a
aCentre for Clean Energy Technology, School of Chemistry and Forensic Science, University of Technology, Sydney, Broadway, Sydney, NSW 2007, Australia
bInstitute of Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia. E-mail: anjonmondal@yahoo.com; Guoxiu.wang@uts.edu.au; Fax: +61 02 9514 1656; Tel: +61 02 9514 8244

Received 27th February 2014 , Accepted 11th April 2014

First published on 15th April 2014

Abstract

Nickel hydroxide nanosheets were synthesized by a simple microwave assisted heating method and investigated as electrochemical pseudo-capacitive materials for supercapacitors. The crystalline structure and morphology of the as-obtained Ni(OH)2 nanosheets were characterized by X-ray diffraction, nitrogen adsorption–desorption isotherms, field emission scanning electron microscopy and transmission electron microscopy. The electrochemical properties of the Ni(OH)2 nanosheets were evaluated by cyclic voltammetry and chronopotentiometry technology in 2 M KOH solution. The nickel hydroxide nanosheet electrode shows a maximum specific capacitance of 2570 F g−1 at a current density of 5 A g−1 and exhibits superior cycling stability. These results suggest its potential application as an electrode material for supercapacitors.

1. Introduction

Electrical energy storage and conversion from alternative energy sources have been attracting more attention due to the rising demand for energy and growing concern about air pollution and global warming. In the recent developments of different efficient energy storage devices, supercapacitors are among the promising next-generation most investigated electrochemical power sources which offer extremely high power density, fast energy delivery, and long life cycling behaviour.1–4 Supercapacitors are playing very important roles as energy buffers and uninterruptible power sources for hybrid electric vehicles and short-term power sources for mobile electronic devices.5,6 On the basis of different charge storage mechanisms and the use of electrode materials, supercapacitors are divided into electrical double-layer capacitors (EDLC) and pseudocapacitors. EDLCs are usually based on the charge separation at the electrode/electrolyte interface while the pseudocapacitors involve reversible faradic redox reactions on the surface of electroactive materials.7,8 In general, the energy density based on pseudo-faradic process is usually many times greater than that of EDLC.9,10 Since surface properties play vital role in charge storage mechanisms, an active electrode material with improved surface properties and high capacitive performance is essential to develop an advanced supercapacitor device.11–13

As inexpensive and low toxic energy storage materials metal oxides and hydroxides are interesting for electrochemical capacitor applications. Although the ruthenium oxide-based supercapacitors exhibit much higher specific capacitance and better electrochemical stability, the high material cost has limited it from commercialization.14 Many effort to date has been aimed for alternative low cost electrode materials with good capacitive properties, such as Co3O4,15 MnO2,16,17 NiO,18 and Co(OH)2.19 In recent years, Ni(OH)2, an active transition metal hydroxide has received an increasing scientific and technological attention because it is extensively used as the electrode material in nickel-based alkaline secondary batteries such as nickel–metal hydride (Ni–MH), nickel–iron (Ni–Fe), nickel–zinc (Ni–Zn), and nickel–cadmium (Ni–Cd).20–23 In view of its superior theoretical electrochemical properties, relatively low cost, and well-defined electrochemical redox activity, Ni(OH)2 is also an attractive alternative electrode material for supercapacitors.24–30 It is well-known that nickel hydroxide has two polymorphs: α-Ni(OH)2 and β-Ni(OH)2.31,32 Both α and β-Ni(OH)2 with various structures and morphologies have been synthesized, such as nanoparticles,33 nanoflakes,34 nanowires,35 nanosheets36 and nano/microspheres37,38 using different methods. The electrochemical tests suggest that the performance of Ni(OH)2 is directly affected by both its size and morphology. These improvements are ascribed to the enhanced specific surface area, first redox reaction and shorten diffusion path in the solid phase.

Among the different processing technologies being adopted for the synthesis of materials, the microwave processing has drawn much attention owing to its inherent advantages, including short heating time and homogeneous thermal transmission.39,40 Microwave method does not produce major thermal gradients and also involve accelerated kinetics that can lead to produce material with uniform sizes and high purity.41 In the present work, we report a simple and large-scale production strategy for the synthesis of Ni(OH)2 nanosheets by a microwave heating method and investigated its electrochemical performances for supercapacitors.

2. Experimental

2.1. Synthesis of Ni(OH)2 nanosheets

In the typical synthesis of Ni(OH)2 nanosheets, 0.4 g of Ni(NO3)2·6H2O and 0.165 g urea (molar ratio: 1[thin space (1/6-em)]:[thin space (1/6-em)]2) were dissolved in 15 ml of de-ionized (DI) water. 20 ml ethylene glycol (EG) was added into the above solution and stirred the solution for 2 h to form transparent solution. The final solution was then heated to 140 °C for 30 min in a microwave synthesizer (Model: NOVA-II), then allowed to cool to room temperature. The material was centrifuged, rinsed with DI water and absolute ethanol several times to remove soluble impurities. The precipitate was dried in an oven overnight at 60 °C.

2.2. Structure characterization

The crystallographic phases of as-synthesized Ni(OH)2 nanosheets were investigated using the GBC MMA X-ray diffractometer (λ = 0.15405 nm). The specific surface area and the porosity distributions were calculated using Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) method, respectively by using a Micromeritics 3 Flex™ surface characterization analyzer at 77 K. Thermogravimetric analysis (TGA) was carried out on a 2960 SDT thermal analyzer with a heating rate of 10 °C min−1 from room temperature to 700 °C in air. AFM measurements were performed on Dimension 3100 SPM with a tapping mode. The morphology and structure of the sample were examined by field emission scanning electron microscopy (FESEM, Zeiss Supra 55VP) and transmission electron microscopy (TEM, JEOL 2011 TEM facility).

2.3. Fabrication of electrodes and electrochemical measurements

The working electrodes were fabricated by mixing 80 wt% of as-synthesized Ni(OH)2 nanosheets as an active material with 15 wt% acetylene black and 5 wt% polyvinylidene fluoride as conducting material and binder, respectively. A small amount of N-methyl-2-pyrrolidinone was added to the mixture to form slurry. The slurry was coated (area of coating: 1 cm2) on a pre-treated Ni foam for electrical conductivity and vacuum-dried at 100 °C overnight. Cyclic voltammetry (CV) and chronopotentiometry studies were performed for evaluating electrochemical properties using a CHI 660C electrochemistry workstation in a three electrode configuration with Ni(OH)2 loaded Ni foam as the working electrode, platinum foil as the counter electrode and saturated calomel electrode (SCE) as the reference electrode in 2 M KOH solution.

3. Results and discussion

Fig. 1 shows X-ray diffraction (XRD) pattern of the as-synthesized Ni(OH)2 nanosheets. The diffraction peaks can be readily indexed as (001), (002), (110), (111) and (300) crystal planes and assigned to the α-Ni(OH)2 structure according to the JCPDS card no. 22-0444. No diffraction peaks from impurities are found in α-Ni(OH)2, indicating that the sample is pure α-Ni(OH)2.
image file: c4ra01719f-f1.tif
Fig. 1 XRD pattern of α-Ni(OH)2 nanosheets.

The Ni(OH)2 nanosheets were characterized by nitrogen adsorption and desorption isotherm and corresponding pore size distribution (as shown in Fig. 2). The isotherm of the Ni(OH)2 nanosheets can be classified as type IV isotherm with a distinct hysteresis loop, reflecting the presence of mesoporous structure.42 The mesoporous structure was further confirmed by the BJH pore size distribution data. The pore size distribution reveals with a mean pore diameter of 2.9 nm (the inset of Fig. 2). The Ni(OH)2 nanosheets show high BET surface area of 82 m2 g−1. Generally, increased surface area of electrode/electrolyte interface can enhance the charge transport and improve the electrochemical properties of the electrode. The high BET surface area of Ni(OH)2 nanosheets provide the possibility of effective transportation of electrons and ions, which is very important for electrode materials in practical applications for supercapacitors.


image file: c4ra01719f-f2.tif
Fig. 2 Nitrogen adsorption/desorption isotherm curve of Ni(OH)2 nanosheets. The inset is the corresponding BJH pore size distributions.

Thermal behaviour of the as-prepared Ni(OH)2 nanosheets was examined by TGA. Fig. 3 depicts a typical TGA curve of Ni(OH)2 nanosheets. The initial weight loss from room temperature to 250 °C is about 5% which is expected to the removal of absorbed water. A very sharp weight loss from 250 °C to 310 °C suggests the decomposition of Ni(OH)2 to NiO. At higher temperature there is no major weight loss observed, indicating the absence of additional phase or structural changes.


image file: c4ra01719f-f3.tif
Fig. 3 TGA curve of Ni(OH)2 nanosheets.

The morphology of the Ni(OH)2 nanosheets were observed by FESEM. Fig. 4 shows the different magnification SEM images of the as-prepared Ni(OH)2 nanosheets. At the low magnification SEM images (Fig. 4(a) and (b)) the materials are curely and wrinkled thin sheets of Ni(OH)2. Higher magnification SEM images (Fig. 4(c) and (d)) reveal that the Ni(OH)2 nanosheets are flexible, interconnected with each other and have an abundant surfaces similar to silk waves.


image file: c4ra01719f-f4.tif
Fig. 4 FESEM images of Ni(OH)2 nanosheets (a) and (b) low magnification and (c) and (d) high magnification.

We used transmission electron microscopy (TEM) to analyse the detailed crystal structure of Ni(OH)2 nanosheets as shown in Fig. 5. Fig. 5(a) and (b) shows the low magnification TEM images of Ni(OH)2 nanosheets, from which it can be seen that the as-prepared Ni(OH)2 present wrinkled paper shape thin sheets. The corresponding selected area electron diffraction (SAED) pattern (Fig. 5(c)) shows the disperse rings were well indexed to the crystal planes of (110), (111), (210), and (221), confirming well crystalline feature of the hexagonal Ni(OH)2. Through the high magnification TEM image (Fig. 5(d)) we can observe small pores and the nanosheets demonstrated thin thickness. Fig. 6 shows the atomic force microscopy (AFM) images of Ni(OH2) nanosheets obtained by tapping mode. According to the AFM the thickness of the nanosheets is about 6 nm.


image file: c4ra01719f-f5.tif
Fig. 5 (a) and (b) Low magnification TEM images of Ni(OH)2 nanosheets. (c) Corresponding selected area electron diffraction pattern (SAED) (d) high magnification TEM image of Ni(OH)2 nanosheets.

image file: c4ra01719f-f6.tif
Fig. 6 Atomic force microscopy images of Ni(OH)2 nanosheets obtained by tapping mode. The topography, amplitude and height profile in (a–c) are included, respectively. The height profiles obtained from the topography images are marked by white line with a red cross. Mica discs were used as the substrate.

The formation mechanism of Ni(OH)2 nanosheets in microwave-mediated process, involves well-known hydrolysis-precipitation process. In this process, urea generates OH− ions in the reaction medium, which react with Ni+ to form Ni(OH)2. The generation of OH− ions mainly take place at the controlled rate which influences the crystal growth process. The reactions involved in the growth of Ni(OH)2 nanosheets are as follow:50

 
NH2CONH2 → NH4+ + NCO− (1)
 
NCO− + 3H2O → HCO3− + NH4+ + OH− (2)
 
Ni+ + 2OH− → Ni(OH)2 (3)

With the existence of ethylene glycol, the product offered nanosheet structure.

In this study, we investigated the electrochemical properties of the as-synthesized Ni(OH)2 nanosheets by CV and galvanostatic charge–discharge. Fig. 7(a) shows the CV curves of Ni(OH)2 nanosheet electrode at different scan rates in the potential range of 0–0.5 V in 2 M KOH solution. All CV curves comprise a pair of strong redox peaks, which indicates that the capacitance is interrelated with faradic pseudocapacitor. It is apparent that the electron transfer is reversible.43 The anodic peaks can be observed in the range of 0.3–0.45 V, which are attributed to the reduction of Ni(OH)2 to NiOOH, while the cathodic peaks at 0.1–0.2 V is associated with the conversion of NiOOH back to Ni(OH)2 according to the following reaction mechanism:44

 
Ni(OH)2 + OH− ↔ NiOOH + H2O + e− (4)


image file: c4ra01719f-f7.tif
Fig. 7 (a) CV curves of the Ni(OH)2 nanosheet electrode at different scan rates and (b) galvanostatic charge–discharge curves at the current density of 10 A g−1 in 2 M KOH electrolyte.

The shape of the CV curves does not change expressively, which demonstrates a high electrochemical reactivity.45 The area under the CV curves can be used to evaluate the capacitance of the system. The specific capacitance values can be calculated from the CV curves by using the following equation:

 
C = Q/υmΔV (5)
where Q is the charge obtained by integrating the CV curves, υ is the scan rate, m is the mass of active material and ΔV is the potential range of each scan. The corresponding specific capacitance is 2351, 2159, 1982, 1742 and 1324 F g−1 at the scan rates of 2, 5, 10, 20 and 50 mV s−1, respectively, indicating a high rate capability. The capacitance of the Ni(OH)2 nanosheet electrode was also measured using galvanostatic charge and discharge tests. Fig. 7(b) shows the charge–discharge curves of Ni(OH)2 electrode at the current density of 10 A g−1 within the voltage range of 0–0.4 V. The shapes of the charge–discharge curves are non-linear, which also clearly demonstrates the pseudo-capacitance of the material. Fig. 8 shows the galvanostatic charge and discharge curves at different current densities. The non-linear charge–discharge curves further verify the pseudo-capacitive properties of Ni(OH)2 nanosheet electrode. The specific capacitance (C) of the Ni(OH)2 electrode was also calculated from the charge–discharge curves according to the equation:
 
C = IΔt/ΔVm (6)
where I is the discharge current (A g−1), Δt is the discharge time (sec), ΔV is the potential window (V), m is the mass of active material (mg). The specific capacitance calculated from the charge and discharge curves are 2570, 2281, 2105 and 1805 F g−1 at the current density of 5, 10, 20 and 40 A g−1, respectively. This indicating the stable reversible features of the nanosheet structure of Ni(OH)2. It is noticeable that the Ni(OH)2 nanosheets can be electrochemically utilized even at high current densities. The amazing improvement of the specific capacitance of Ni(OH)2 nanosheet structure can be ascribed to the effective utilization of layered Ni(OH)2. The interconnected layered thin nanosheet structure (6 nm in thickness) provides adequate space and not only facilitate the penetration of electrolyte into the whole matrix, but also shortens the ion diffusion paths.


image file: c4ra01719f-f8.tif
Fig. 8 Galvanostatic charge and discharge curves at different current densities (5 to 40 A g−1).

The specific capacitance calculated from both CV and charge–discharge is higher than the previously reported Ni(OH)2 nanoflakes,46 Ni(OH)2 nanotube arrays47 and mesoporous Ni(OH)2.48

Fig. 9(a) shows the specific capacitances (determined from the CV curves) at different scan rates. The specific capacitance decreased with the increase of scan rates. This can be attributed to the existence of internal active sites that are not able to continue the entire redox conversion at high scan rates, which indicating that some part of the electrode surfaces are not accessible at high scan rates.49 The long-term cycle stability is the key parameter for pseudocapacitors. As shown in Fig. 9(b) the cycling stability was examined up to 2000 charge–discharge cycles at the scan rate of 50 mV s−1 in 2 M KOH electrolyte. The 90% capacitance is retained after 2000 cycles. The nanosheet structure has many interspaces which can take place the volume expansion easily during charge–discharge cycling, demonstrating an excellent stability of the electrode. In aqueous electrolyte, a little decrease of specific capacitance is observed during long-term charge–discharge cycling, which can be ascribed to the deterioration of active material caused by the partial dissolution in electrolyte and the detachment of active material from the current collector.51


image file: c4ra01719f-f9.tif
Fig. 9 (a) Specific capacitance as a function of scan rates and (b) cycling performance of Ni(OH)2 nanosheet electrode.

The above results show that the thin Ni(OH)2 nanosheets has high specific capacitance and good cycling stability owing to its thin nanosheet structure with high surface area, indicating an attractive electrode material for supercapacitors.

4. Conclusions

In this work, we synthesized Ni(OH)2 nanosheets through a simple one-step microwave-mediated hydrothermal method. The as-synthesized material has high specific surface area and narrow pore size distribution, which provide more active sites for fast energy storage and also beneficial for the transportation of electrolyte ions. The XRD, BET, SEM, AFM and TEM shows the material is layered thin nanosheet morphology of Ni(OH)2 with mesoporous character. The electrochemical measurements showed that the Ni(OH)2 nanosheets exhibit a high specific capacitance of 2570 F g−1 at the current density of 5 A g−1 with good cycling stability, suggesting a promising electrode material for supercapacitors.

Acknowledgements

We gratefully acknowledge the financial support provided by the Australian Research Council (ARC) through the ARC discovery project (DP1093855).

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