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Synthesis, characterization, and electrochemical properties of ultrafine β-Ni(OH)2 nanoparticles
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 6 7 4 e8 6 7 9
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Synthesis, characterization, and electrochemical properties of ultrafine b-Ni(OH)2 nanoparticles Mustafa Aghazadeh a,b, Ahmad Nozad Golikand b,*, Mehdi Ghaemi c a
Chemistry Department, Tarbiat Modares University, P.O. Box 13145-185, Tehran, Iran Material Research School, NSTRI, P.O. Box 14395-836, Tehran, Iran c Chemistry Department, Science Faculty, Golestan University, P.O. Box 49138-15739, Gorgan, Iran b
article info
abstract
Article history:
Nickel hydroxide was deposited via cathodic electrodeposition from low-temperature
Received 26 January 2011
0.005 M NiCl2 bath without using any surfactant or template. The cathodic current
Received in revised form
density was 1 mA cm2 and stainless steel was used as the cathode. The XRD pattern
20 March 2011
confirmed that the prepared sample has a pure brucite crystal phase of b-Ni(OH)2 and the
Accepted 26 March 2011
broadening of diffraction peaks showed that the particles size of the prepared b-Ni(OH)2 is
Available online 30 April 2011
extremely small. Thermal behavior and composition of the prepared b-Ni(OH)2 were investigated by DSC-TG and FT-IR analyses. Morphological characterization by SEM and
Keywords:
TEM revealed that b-Ni(OH)2 is composed of well dispersed ultrafine particles with size of
b-Ni(OH)2
about 5 nm. The electrochemical properties of the prepared nanoparticles were studied by
Nanoparticles
means of cyclic voltammetry (CV) and galvanostatic chargeedischarge tests in 1 M KOH.
Cathodic electrodeposition
The prepared b-Ni(OH)2 nanoparticles showed excellent capacitance behavior of 740 F g1
Electrochemical properties
in the potential window of 0e0.55 V vs. Ag/AgCl. These results make the b-Ni(OH)2 nanoparticles as a promising candidate for the supercapacitor electrodes. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Synthesis of metal hydroxides at nanoscale with morphologies such as particle, wire, tube and rod has recently found interesting research area in nanotechnology. This is associated with their nanosize dependent novel physico-chemical properties, enhancing their effective surface area and maximizing their utilization. Among the metal hydroxides, nickel hydroxide (Ni(OH)2) has specific properties including high power density, excellent cyclability and high specific energy. Due to these excellent electrochemical properties, it has found extensive applications like active material in alkaline rechargeable nickel-based batteries [1e12] and has been identified as a very promising material for supercapacitors [13e18].
The electrochemical performance of Ni(OH)2 is directly affected by both its size and morphology, and can be improved by nanoscale. These improvements are attributed to the enhanced specific surface area, fast redox reaction and shortened diffusion path in the solid phase. It has been found that Ni(OH)2 with a smaller crystalline size exhibits better electrochemical properties [3,4,19e21]. For example, Liu and Yu [20] investigated the influence of nanosized Ni(OH)2 addition on the electrochemical performance of nickel hydroxide electrode and found that the 10% addition of nanosized Ni(OH)2 to common micro-sized spherical Ni(OH)2 resulted in a 10% improvement in the utilization of the active material of the Ni(OH)2 electrode and improved its electrochemical performance. Reisner et al. [4] developed nanostructured
* Corresponding author. Tel./fax: þ98 21 8206313. E-mail address: [email protected] (A.N. Golikand). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.144
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 6 7 4 e8 6 7 9
b-Ni(OH)2, a mixture of nanofibers and nanoparticles, which was expected to yield at least a 20% improvement in cathode energy content. Besides crystal structure, morphology of Ni(OH)2 also has a significant influence on its electrochemical properties [3,16e19]. For example, Liu et al. [22] synthesized ribbon- and board-like nanostructures of b-Ni(OH)2 by hydrothermal method. The nanoboards of b-Ni(OH)2 exhibited a specific capacity of as high as 260 mAh/g, which is close to the theoretical capacity of b-Ni(OH)2. Fu et al. [23] performed electrodeposition of a-Ni(OH)2 film on nickel foil and investigated its supercapacity performance. Their results showed that the a-Ni(OH)2 with particle-like morphology has excellent electrochemical performance and its specific capacitance value, as single electrode, is up to 2595 F g1. Cheng et al. [24] reported a specific capacitance of w696 F g1 for the solegel derived Ni(OH)2 xerogels, which is significantly higher than that of the well developed carbon-based materials. Yuan et al. [25] reported a specific capacitance of w710 F g1 for spherical super structured Ni(OH)2. It is well known that nickel hydroxide has two polymorphs: a- and b-Ni(OH)2 [26]. The b-phase exhibits superior stability compared to the a-Ni(OH)2. Further, it is widely used as the classical material in rechargeable batteries, benefiting from its outstanding chemical and thermal stability [27]. b-Ni(OH)2 is usually oxidized to b-NiOOH in a charge process and has a maximum theoretical specific capacity of 289 mAh/g. There are many different methods reported for the synthesis of nanostructured b-Ni(OH)2, such as chemical precipitation [28e30], reverse micelle [30], hydrothermal [31e35], solvothermal [22,36,37], sonochemical [38] and electrodeposition [39,40] methods. However, despite having unique principles and flexibility in controlling the structure and morphology of the products, electrochemical synthesis of nickel hydroxide has rarely been studied. Thus, in this work, we applied a cathodic electrodeposition method for the synthesis of b-Ni(OH)2. By this method, ultrafine b-Ni(OH)2 nanoparticles were successfully prepared by galvanostatic electrodeposition from low-temperature 0.005 M NiCl2 bath without using any additive and template. The electrochemical properties of the prepared b-Ni(OH)2 nanoparticles were investigated by means of cyclic voltammetry (CV) and galvanostatic chargeedischarge tests.
sample was determined by X-ray diffraction (XRD) with a diffractometer (Phillips, PW-1800) using monochromatized Cu Ka radiation. These measurements were conducted in the range of diffraction angles (2q) from 5 to 80 C at 5 min1. The sample morphology was examined using a scanning electron microscopy (LEO 1455VP). Transmission electron microscopy (TEM) images were taken using a Phillips EM 2085 transmission electron microscope with an accelerating voltage of 100 kV. FT-IR spectra were obtained by a Bruker Vector 22 FT-IR spectrometer within the range of 400e4000 cm1 wave numbers. Cyclic voltammetry (CV) was used at different sweep rates to determine the electrochemical properties of the fabricated b-Ni(OH)2 electrode in 1 M KOH and to quantify its specific capacitance. A typical three-electrode experimental cell equipped with a working electrode, a platinum foil counter electrode and an Ag/AgCl reference electrode was used for measuring the electrochemical properties of the working electrode and its performance as a supercapacitor electrode material. The working electrode was formed by mixing 80 wt.% active material, 15 wt.% acetylene black, as the conductive filler and 5 wt.% PTFE, the as binder. The prepared mixture was pressed under 10 MPa into the nickel foam current collectors (1 cm 1 cm) and then dried in oven for 4 h at 80 C. CV and galvanostatic chargeedischarge tests were carried out on a potentiostat (AUTOLAB, Eco Chemie, PGSTAT 30) at room temperature. The cyclic voltammograms (CVs) were recorded in the potential range between 0 and 0.55 V vs. Ag/AgCl at the various scan rates of 2, 5, 10, 25 and 50 mV s1 and the current responses were measured. Galvanostatic charge/discharge tests were performed in the potential range between 0 V and 0.55 V vs. Ag/AgCl at a current density of 0.5 A g1.
3.
Results and discussion
The as-prepared sample can be assigned as nickel hydroxide. This is based on the fact that positively charged nickel (II) species (Ni2þ) are hydrolyzed by electrogenerated base (OH) and accumulate in the form of colloidal Ni(OH)2 particles near the electrode: 2þ
Ni
2.
Experimental
Aqueous NiCl2$6H2O solution (Merck, 0.005 M) was used as the electrolyte in the electrodeposition experiments. The electrochemical cell included a stainless steel cathode (316 L, 100 50 0.5 mm) centered between the two parallel graphite counter electrodes. Prior to each deposition, the steel substrates were given a galvanostatically electropolishing treatment at a current density of 20 A for 5 min in a bath (70 C) containing 50 vol.% phosphoric acid, 25 vol.% sulfuric acid and balanced deionized water. All electrodeposition experiments were performed galvanostatically at a cathode current density of 1 mA cm2. The temperature of bath was fixed at 10 C in the experiments. After drying at room temperature for 48 h, the deposit was scraped from the steel electrodes for further analyses. The phase and crystal structure of the prepared
8675
þ 2OH /Ni OHÞ2 ðsÞ
(1)
Hydroxyl ions are produced by the cathodic reduction of water and/or dissolved molecular oxygen [40]: 2H2 O þ 2e /H2 þ 2OH
(2)
O2 þ 2H2 O þ 4e /4OH
(3)
The XRD pattern of the prepared sample is shown in Fig. 1. All of the diffraction peaks can be indexed entirely to a brucite (space group: P3m1) crystal phase of b-Ni(OH)2, with the ˚ and c ¼ 4.662 A ˚ , which are lattice constants of a ¼ 3.126 A well matched with the reported standard values (JCPDS card 74-2075). No other peaks for the impurities such as a-Ni(OH)2 or other phases are observed in the pattern. This indicates that pure b-Ni(OH)2 was obtained under the current synthetic conditions. All of the diffraction peaks are broad, indicating the nanoscale dimensionality of the crystallites. In general,
8676
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10
20
30
40
50
60
70
80
Fig. 1 e XRD patterns of the prepared b-Ni(OH)2 nanoparticles.
the water content of the as-prepared Ni(OH)2. The amount of water plays an important role in the crystal structure and the electrochemical properties of Ni(OH)2. For example, it has been reported that the water content for b-Ni(OH)2 varies from 0 to 0.3 M fraction (moles of H2O per mole of Ni(OH)2), and for aNi(OH)2, it varies from 0.3 to 0.7 M fraction, depending upon the preparative experimental conditions [41]. For the as-prepared Ni(OH)2, under our electrodeposition conditions, the water molar faction of 0.21 can be calculated from the TG curve. This confirms that the as-prepared sample contains the b phase of Ni(OH)2$0.21H2O. This result confirmed the crystal structure detected from the XRD pattern (Fig. 1). It has been reported that decomposition of Ni(OH)2 into NiO occurs between 298 and 340 C [34,42,43]. Thus the endothermic peak with the maximum located at around 269.7 C corresponds to the endothermic behavior of Ni(OH)2 during the its decomposition into NiO. In fact, this peak is associated with the loss of water produced by dehydroxylation of the hydroxide layers: NiðOHÞ2 /NiO þ H2 O
the broadening of XRD peaks may result from small grain sizes or structural microdistortions in the crystal. Under our experimental conditions, the extremely small particle structure results in a significant broadening of the diffraction peaks in the XRD pattern of b-Ni(OH)2. This can be confirmed by the SEM and TEM images (Fig. 4). Thermal behavior of the as-prepared b-Ni(OH)2 nanoparticles was investigated by DSC analysis. Fig. 2 depicts the typical TGeDTA curve of the as-deposited b-Ni(OH)2 sample. The b-Ni(OH)2 sample underwent a two-step weight loss due to dehydration and decomposition. The two endothermic peaks at 87.3 C and 269.7 C on the DSC curve are indicative of two successive stages of these physico-chemical changes during the heat treatment. The first stage is related to the evaporation of the adsorbed and intercalated water molecules associated with the Ni(OH)2$XadsH2O deposit: NiðOHÞ2 $Xads H2 O/NiðOHÞ2 þX H2 O
(4)
Correspondingly, TG curve shows a relative sharp weight loss with 11.7 wt.%. From this stage on, it is possible to estimate
(5)
For this stage, TG curve shows a sharp weight loss with 19.1 wt.%, which is in good agreement with the theoretical weight loss value (19.4%) caused by the decomposition of b-Ni(OH)2 (Fig. 2). To further support the XRD and TGA results, the quality and composition of the as-prepared Ni(OH)2 were examined by FT-IR spectroscopy in the range of 400e4000 cm1 and the results are shown in Fig. 3. The FT-IR spectrum verifies that the prepared Ni(OH)2 can be characterized as b form, due to the existence of (i) a narrow and strong band at 3640 cm1 relating to the n(OH) stretching vibration, which indicates OH groups in a free configuration, (ii) a strong band at 517 cm1 corresponding to the hydroxyl groups’ lattice vibration, d(OH), and (iii) a weak band around 461 cm1 resulting from the NieO lattice vibration, n(NieO) [26,42e46]. The broad and intense band centered at 3448 cm1 is assigned to the OeH stretching vibration of the interlayer water molecules and of the H-bound OH group. Also the other peak
Transmittance (a. u.)
1055
1640 1540 1400 461
662 517 3448 3640
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm )
Fig. 2 e DCS-TGA curves of the prepared b-Ni(OH)2 nanoparticles.
Fig. 3 e FT-IR spectrum of the prepared Ni(OH)2 nanoparticles.
500
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 6 7 4 e8 6 7 9
20
P1
15
Current / A g
10
5
0
-5
-10
P2 -15 0.0
0.1
0.2
0.3
0.4
0.5
0.6
Potential / V vs. Ag/AgCl
Fig. 5 e Cyclic voltammogram of b-Ni(OH)2 nanoparticles at a scan rate of 2 mV sL1.
Cyclic voltammetry (CV) is considered as a suitable tool to indicate the capacitive behavior of electroactive materials. Fig. 5 presents the CV curve of the b-Ni(OH)2 electrode in 1 M KOH electrolyte at the scan rate of 2 mV s1 at the potential window of 0e0.55 V vs. Ag/AgCl. As shown in Fig. 5, the strong terminal peak deals with the oxidation peaks of water. There is a pair of strong redox peaks as a result of the Faradaic reactions of b-Ni(OH)2. Regarding the b-Ni(OH)2 electrode material, it is well known that the surface faradic reactions will proceed according to the following reaction [46]:
observed at 1640 cm1 is assigned to the bending vibration of water molecules. The peaks located between the 800e1800 cm1 could be assigned to the presence of anions, which have not probably been completely eliminated during the washing stage [44,45]. In this work, the peaks observed at 1540 cm1, 1400 cm1 and 1055 cm1 are assigned to the various vibrational modes of the carbonate groups originating from the adsorption of atmospheric CO2. Morphological characteristics of the prepared b-Ni(OH)2 sample were investigated by SEM and TEM (Fig. 4). The SEM image (Fig. 4a) shows the smooth and uniform particle morphology. Due to the very fine particle texture of the prepared sample, the particle size cannot be detected from SEM image. High magnification imaging by TEM (Fig. 4b) revealed that the prepared b-Ni(OH)2 is composed of well dispersed ultrafine particles with particle size less than 5 nm. To determine the size of the particles, their average crystallite size (D) was calculated from the width of the XRD peak (001) at 2q ¼ 18.3 using the Scherrer equation (D ¼ 0.9l/bcos q), where, l is the wavelength of X-ray radiation, q is the Bragg angle of the peak and b (FWHM) is defined as the full-width at half maximum intensity of the diffraction peak in radians. The average crystallite size of the b-Ni(OH)2 nanoparticles was calculated as equal to 3 nm, close to those observed in the TEM.
b NiðOHÞ2 þOH /b NiOOH þ H2 O þ e
(6)
The anodic peak is due to the oxidation of b-Ni(OH)2 into b-NiOOH and the cathodic peak is for the reverse process. One quasi-reversible electron transfer process is visible in the CV curve, indicating that the measured capacitance is mainly based on redox mechanism [26]. Moreover, the shape of the curves (Fig. 6) displays that the observed capacitance 100
80
60
Current / A g
Fig. 4 e SEM (a) and TEM (b) images of the prepared bNi(OH)2 nanoparticles.
40
20
0
-20
-40 -0.1
0.0
0.1
0.2
0.3
0.4
0.5
Potential / V vs. Ag/AgCl
Fig. 6 e Cyclic voltammograms of the b-Ni(OH)2 nanoparticles at various scan rates.
0.6
0.7
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where, i is the applied current (0.5 A), DV is the potential range (0.55 V), Dt is the time of a discharge cycle and w is the mass of the prepared b-Ni(OH)2 nanoparticles. The specific capacitance value was calculated as equal to 715.3 F g1, which is very close to that calculated from the CV curves (Fig. 6), again confirming the high capacitance of the ultrafine nanoparticles of b-Ni(OH)2.
0.45 0.6
Potential / V vs. Ag/AgCl
0.5
0.4 0.30
0.3
4.
Conclusion
0.2 0.15
0.1
0.00 0.0
0
1000
2000
3000
4000
5000
6000
Time (s)
Fig. 7 e Galvanostatic charge/discharge curve of the bNi(OH)2 nanoparticles at current density of 0.5 A gL1.
characteristic is distinct from that of the electric double layer capacitor, which would produce a CV curve that is usually close to an ideal rectangular shape. Since solution and electrode resistance can distort the current response at the switching potential and this distortion is dependent upon the scan rate [32], as can be seen from Fig. 6, the shape of CV curves of the b-Ni(OH)2 nanoparticles is not significantly influenced by the increasing of the scan rates. This indicates the improved mass transportation and electron conduction within the nanoparticles. The average specific capacitance of b-Ni(OH)2 can be calculated from the CV curves in Fig. 6 by integrating the area under the currentepotential curve:
C¼
1 mnðVa Vc Þ
ZVc IðVÞdV
(7)
Va
The specific capacitances of the prepared b-Ni(OH)2 nanoparticles were calculated to be 1056.1, 834.6, 740.2 and 652.8 F g1 at the scan rates of 2, 5, 10 and 25 mV s1, respectively. The specific capacitance of the b-Ni(OH)2 nanoparticles is more higher than that of micro-size Ni(OH)2 particles (770 F g1 at the scan rate of 2 mV s1 [20]). Thus these values suggest an excellent electrochemical performance and show the nanosize effects on the electrochemical performance of the prepared b-Ni(OH)2 nanoparticles. Electrochemical capacitance behavior of the prepared b-Ni(OH)2 electrode was also investigated by chronopotentiometry in 1 M KOH aqueous solution. Fig. 7 shows the chargeedischarge curves of the b-Ni(OH)2 electrode measured at a current density of 0.5 A g1 within the potential window of 0e0.55 V vs. Ag/AgCl. The shapes of the chargeedischarge curves do not display the characteristic of a pure electric double layer capacitor, but mainly pseudocapacitance, which corresponds with the result of the CV tests. The specific capacitance of bNi(OH)2 electrode was calculated by the following equation: C¼
i$Dt wDn
(8)
Low-temperature cathodic electrodeposition method was applied for the synthesis of Ni(OH)2 nanoparticles. XRD, DSCTG and FT-IR analyses showed that the prepared Ni(OH)2 was a single crystalline pure b phase. The morphological studies by SEM and TEM revealed that the prepared b-Ni(OH)2 was composed of well dispersed ultrafine particles with the size of smaller than 5 nm. Cyclic voltammetry and galvanostatic chargeedischarge tests were performed to evaluate the electrochemical performance of the b-Ni(OH)2 nanoparticles in 1 M KOH electrolyte. CV curves showed a pair of strong redox peaks as a result of the Faradaic redox reactions of b-Ni(OH)2 nanoparticles. The specific capacitance of as high as 740.2 F g1 was achieved within a potential window of 0e0.55 V vs. Ag/AgCl at a scan rate of 10 mV s1. These findings suggest that the ultrafine nanoparticles of b-Ni(OH)2 have excellent electrochemical properties and thus can be recognized as a promising candidate for the supercapacitor applications.
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Synthesis, characterization, and electrochemical properties of ultrafine b-Ni(OH)2 nanoparticles Mustafa Aghazadeh a,b, Ahmad Nozad Golikand b,*, Mehdi Ghaemi c a
Chemistry Department, Tarbiat Modares University, P.O. Box 13145-185, Tehran, Iran Material Research School, NSTRI, P.O. Box 14395-836, Tehran, Iran c Chemistry Department, Science Faculty, Golestan University, P.O. Box 49138-15739, Gorgan, Iran b
article info
abstract
Article history:
Nickel hydroxide was deposited via cathodic electrodeposition from low-temperature
Received 26 January 2011
0.005 M NiCl2 bath without using any surfactant or template. The cathodic current
Received in revised form
density was 1 mA cm2 and stainless steel was used as the cathode. The XRD pattern
20 March 2011
confirmed that the prepared sample has a pure brucite crystal phase of b-Ni(OH)2 and the
Accepted 26 March 2011
broadening of diffraction peaks showed that the particles size of the prepared b-Ni(OH)2 is
Available online 30 April 2011
extremely small. Thermal behavior and composition of the prepared b-Ni(OH)2 were investigated by DSC-TG and FT-IR analyses. Morphological characterization by SEM and
Keywords:
TEM revealed that b-Ni(OH)2 is composed of well dispersed ultrafine particles with size of
b-Ni(OH)2
about 5 nm. The electrochemical properties of the prepared nanoparticles were studied by
Nanoparticles
means of cyclic voltammetry (CV) and galvanostatic chargeedischarge tests in 1 M KOH.
Cathodic electrodeposition
The prepared b-Ni(OH)2 nanoparticles showed excellent capacitance behavior of 740 F g1
Electrochemical properties
in the potential window of 0e0.55 V vs. Ag/AgCl. These results make the b-Ni(OH)2 nanoparticles as a promising candidate for the supercapacitor electrodes. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Synthesis of metal hydroxides at nanoscale with morphologies such as particle, wire, tube and rod has recently found interesting research area in nanotechnology. This is associated with their nanosize dependent novel physico-chemical properties, enhancing their effective surface area and maximizing their utilization. Among the metal hydroxides, nickel hydroxide (Ni(OH)2) has specific properties including high power density, excellent cyclability and high specific energy. Due to these excellent electrochemical properties, it has found extensive applications like active material in alkaline rechargeable nickel-based batteries [1e12] and has been identified as a very promising material for supercapacitors [13e18].
The electrochemical performance of Ni(OH)2 is directly affected by both its size and morphology, and can be improved by nanoscale. These improvements are attributed to the enhanced specific surface area, fast redox reaction and shortened diffusion path in the solid phase. It has been found that Ni(OH)2 with a smaller crystalline size exhibits better electrochemical properties [3,4,19e21]. For example, Liu and Yu [20] investigated the influence of nanosized Ni(OH)2 addition on the electrochemical performance of nickel hydroxide electrode and found that the 10% addition of nanosized Ni(OH)2 to common micro-sized spherical Ni(OH)2 resulted in a 10% improvement in the utilization of the active material of the Ni(OH)2 electrode and improved its electrochemical performance. Reisner et al. [4] developed nanostructured
* Corresponding author. Tel./fax: þ98 21 8206313. E-mail address: [email protected] (A.N. Golikand). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.144
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b-Ni(OH)2, a mixture of nanofibers and nanoparticles, which was expected to yield at least a 20% improvement in cathode energy content. Besides crystal structure, morphology of Ni(OH)2 also has a significant influence on its electrochemical properties [3,16e19]. For example, Liu et al. [22] synthesized ribbon- and board-like nanostructures of b-Ni(OH)2 by hydrothermal method. The nanoboards of b-Ni(OH)2 exhibited a specific capacity of as high as 260 mAh/g, which is close to the theoretical capacity of b-Ni(OH)2. Fu et al. [23] performed electrodeposition of a-Ni(OH)2 film on nickel foil and investigated its supercapacity performance. Their results showed that the a-Ni(OH)2 with particle-like morphology has excellent electrochemical performance and its specific capacitance value, as single electrode, is up to 2595 F g1. Cheng et al. [24] reported a specific capacitance of w696 F g1 for the solegel derived Ni(OH)2 xerogels, which is significantly higher than that of the well developed carbon-based materials. Yuan et al. [25] reported a specific capacitance of w710 F g1 for spherical super structured Ni(OH)2. It is well known that nickel hydroxide has two polymorphs: a- and b-Ni(OH)2 [26]. The b-phase exhibits superior stability compared to the a-Ni(OH)2. Further, it is widely used as the classical material in rechargeable batteries, benefiting from its outstanding chemical and thermal stability [27]. b-Ni(OH)2 is usually oxidized to b-NiOOH in a charge process and has a maximum theoretical specific capacity of 289 mAh/g. There are many different methods reported for the synthesis of nanostructured b-Ni(OH)2, such as chemical precipitation [28e30], reverse micelle [30], hydrothermal [31e35], solvothermal [22,36,37], sonochemical [38] and electrodeposition [39,40] methods. However, despite having unique principles and flexibility in controlling the structure and morphology of the products, electrochemical synthesis of nickel hydroxide has rarely been studied. Thus, in this work, we applied a cathodic electrodeposition method for the synthesis of b-Ni(OH)2. By this method, ultrafine b-Ni(OH)2 nanoparticles were successfully prepared by galvanostatic electrodeposition from low-temperature 0.005 M NiCl2 bath without using any additive and template. The electrochemical properties of the prepared b-Ni(OH)2 nanoparticles were investigated by means of cyclic voltammetry (CV) and galvanostatic chargeedischarge tests.
sample was determined by X-ray diffraction (XRD) with a diffractometer (Phillips, PW-1800) using monochromatized Cu Ka radiation. These measurements were conducted in the range of diffraction angles (2q) from 5 to 80 C at 5 min1. The sample morphology was examined using a scanning electron microscopy (LEO 1455VP). Transmission electron microscopy (TEM) images were taken using a Phillips EM 2085 transmission electron microscope with an accelerating voltage of 100 kV. FT-IR spectra were obtained by a Bruker Vector 22 FT-IR spectrometer within the range of 400e4000 cm1 wave numbers. Cyclic voltammetry (CV) was used at different sweep rates to determine the electrochemical properties of the fabricated b-Ni(OH)2 electrode in 1 M KOH and to quantify its specific capacitance. A typical three-electrode experimental cell equipped with a working electrode, a platinum foil counter electrode and an Ag/AgCl reference electrode was used for measuring the electrochemical properties of the working electrode and its performance as a supercapacitor electrode material. The working electrode was formed by mixing 80 wt.% active material, 15 wt.% acetylene black, as the conductive filler and 5 wt.% PTFE, the as binder. The prepared mixture was pressed under 10 MPa into the nickel foam current collectors (1 cm 1 cm) and then dried in oven for 4 h at 80 C. CV and galvanostatic chargeedischarge tests were carried out on a potentiostat (AUTOLAB, Eco Chemie, PGSTAT 30) at room temperature. The cyclic voltammograms (CVs) were recorded in the potential range between 0 and 0.55 V vs. Ag/AgCl at the various scan rates of 2, 5, 10, 25 and 50 mV s1 and the current responses were measured. Galvanostatic charge/discharge tests were performed in the potential range between 0 V and 0.55 V vs. Ag/AgCl at a current density of 0.5 A g1.
3.
Results and discussion
The as-prepared sample can be assigned as nickel hydroxide. This is based on the fact that positively charged nickel (II) species (Ni2þ) are hydrolyzed by electrogenerated base (OH) and accumulate in the form of colloidal Ni(OH)2 particles near the electrode: 2þ
Ni
2.
Experimental
Aqueous NiCl2$6H2O solution (Merck, 0.005 M) was used as the electrolyte in the electrodeposition experiments. The electrochemical cell included a stainless steel cathode (316 L, 100 50 0.5 mm) centered between the two parallel graphite counter electrodes. Prior to each deposition, the steel substrates were given a galvanostatically electropolishing treatment at a current density of 20 A for 5 min in a bath (70 C) containing 50 vol.% phosphoric acid, 25 vol.% sulfuric acid and balanced deionized water. All electrodeposition experiments were performed galvanostatically at a cathode current density of 1 mA cm2. The temperature of bath was fixed at 10 C in the experiments. After drying at room temperature for 48 h, the deposit was scraped from the steel electrodes for further analyses. The phase and crystal structure of the prepared
8675
þ 2OH /Ni OHÞ2 ðsÞ
(1)
Hydroxyl ions are produced by the cathodic reduction of water and/or dissolved molecular oxygen [40]: 2H2 O þ 2e /H2 þ 2OH
(2)
O2 þ 2H2 O þ 4e /4OH
(3)
The XRD pattern of the prepared sample is shown in Fig. 1. All of the diffraction peaks can be indexed entirely to a brucite (space group: P3m1) crystal phase of b-Ni(OH)2, with the ˚ and c ¼ 4.662 A ˚ , which are lattice constants of a ¼ 3.126 A well matched with the reported standard values (JCPDS card 74-2075). No other peaks for the impurities such as a-Ni(OH)2 or other phases are observed in the pattern. This indicates that pure b-Ni(OH)2 was obtained under the current synthetic conditions. All of the diffraction peaks are broad, indicating the nanoscale dimensionality of the crystallites. In general,
8676
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10
20
30
40
50
60
70
80
Fig. 1 e XRD patterns of the prepared b-Ni(OH)2 nanoparticles.
the water content of the as-prepared Ni(OH)2. The amount of water plays an important role in the crystal structure and the electrochemical properties of Ni(OH)2. For example, it has been reported that the water content for b-Ni(OH)2 varies from 0 to 0.3 M fraction (moles of H2O per mole of Ni(OH)2), and for aNi(OH)2, it varies from 0.3 to 0.7 M fraction, depending upon the preparative experimental conditions [41]. For the as-prepared Ni(OH)2, under our electrodeposition conditions, the water molar faction of 0.21 can be calculated from the TG curve. This confirms that the as-prepared sample contains the b phase of Ni(OH)2$0.21H2O. This result confirmed the crystal structure detected from the XRD pattern (Fig. 1). It has been reported that decomposition of Ni(OH)2 into NiO occurs between 298 and 340 C [34,42,43]. Thus the endothermic peak with the maximum located at around 269.7 C corresponds to the endothermic behavior of Ni(OH)2 during the its decomposition into NiO. In fact, this peak is associated with the loss of water produced by dehydroxylation of the hydroxide layers: NiðOHÞ2 /NiO þ H2 O
the broadening of XRD peaks may result from small grain sizes or structural microdistortions in the crystal. Under our experimental conditions, the extremely small particle structure results in a significant broadening of the diffraction peaks in the XRD pattern of b-Ni(OH)2. This can be confirmed by the SEM and TEM images (Fig. 4). Thermal behavior of the as-prepared b-Ni(OH)2 nanoparticles was investigated by DSC analysis. Fig. 2 depicts the typical TGeDTA curve of the as-deposited b-Ni(OH)2 sample. The b-Ni(OH)2 sample underwent a two-step weight loss due to dehydration and decomposition. The two endothermic peaks at 87.3 C and 269.7 C on the DSC curve are indicative of two successive stages of these physico-chemical changes during the heat treatment. The first stage is related to the evaporation of the adsorbed and intercalated water molecules associated with the Ni(OH)2$XadsH2O deposit: NiðOHÞ2 $Xads H2 O/NiðOHÞ2 þX H2 O
(4)
Correspondingly, TG curve shows a relative sharp weight loss with 11.7 wt.%. From this stage on, it is possible to estimate
(5)
For this stage, TG curve shows a sharp weight loss with 19.1 wt.%, which is in good agreement with the theoretical weight loss value (19.4%) caused by the decomposition of b-Ni(OH)2 (Fig. 2). To further support the XRD and TGA results, the quality and composition of the as-prepared Ni(OH)2 were examined by FT-IR spectroscopy in the range of 400e4000 cm1 and the results are shown in Fig. 3. The FT-IR spectrum verifies that the prepared Ni(OH)2 can be characterized as b form, due to the existence of (i) a narrow and strong band at 3640 cm1 relating to the n(OH) stretching vibration, which indicates OH groups in a free configuration, (ii) a strong band at 517 cm1 corresponding to the hydroxyl groups’ lattice vibration, d(OH), and (iii) a weak band around 461 cm1 resulting from the NieO lattice vibration, n(NieO) [26,42e46]. The broad and intense band centered at 3448 cm1 is assigned to the OeH stretching vibration of the interlayer water molecules and of the H-bound OH group. Also the other peak
Transmittance (a. u.)
1055
1640 1540 1400 461
662 517 3448 3640
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm )
Fig. 2 e DCS-TGA curves of the prepared b-Ni(OH)2 nanoparticles.
Fig. 3 e FT-IR spectrum of the prepared Ni(OH)2 nanoparticles.
500
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20
P1
15
Current / A g
10
5
0
-5
-10
P2 -15 0.0
0.1
0.2
0.3
0.4
0.5
0.6
Potential / V vs. Ag/AgCl
Fig. 5 e Cyclic voltammogram of b-Ni(OH)2 nanoparticles at a scan rate of 2 mV sL1.
Cyclic voltammetry (CV) is considered as a suitable tool to indicate the capacitive behavior of electroactive materials. Fig. 5 presents the CV curve of the b-Ni(OH)2 electrode in 1 M KOH electrolyte at the scan rate of 2 mV s1 at the potential window of 0e0.55 V vs. Ag/AgCl. As shown in Fig. 5, the strong terminal peak deals with the oxidation peaks of water. There is a pair of strong redox peaks as a result of the Faradaic reactions of b-Ni(OH)2. Regarding the b-Ni(OH)2 electrode material, it is well known that the surface faradic reactions will proceed according to the following reaction [46]:
observed at 1640 cm1 is assigned to the bending vibration of water molecules. The peaks located between the 800e1800 cm1 could be assigned to the presence of anions, which have not probably been completely eliminated during the washing stage [44,45]. In this work, the peaks observed at 1540 cm1, 1400 cm1 and 1055 cm1 are assigned to the various vibrational modes of the carbonate groups originating from the adsorption of atmospheric CO2. Morphological characteristics of the prepared b-Ni(OH)2 sample were investigated by SEM and TEM (Fig. 4). The SEM image (Fig. 4a) shows the smooth and uniform particle morphology. Due to the very fine particle texture of the prepared sample, the particle size cannot be detected from SEM image. High magnification imaging by TEM (Fig. 4b) revealed that the prepared b-Ni(OH)2 is composed of well dispersed ultrafine particles with particle size less than 5 nm. To determine the size of the particles, their average crystallite size (D) was calculated from the width of the XRD peak (001) at 2q ¼ 18.3 using the Scherrer equation (D ¼ 0.9l/bcos q), where, l is the wavelength of X-ray radiation, q is the Bragg angle of the peak and b (FWHM) is defined as the full-width at half maximum intensity of the diffraction peak in radians. The average crystallite size of the b-Ni(OH)2 nanoparticles was calculated as equal to 3 nm, close to those observed in the TEM.
b NiðOHÞ2 þOH /b NiOOH þ H2 O þ e
(6)
The anodic peak is due to the oxidation of b-Ni(OH)2 into b-NiOOH and the cathodic peak is for the reverse process. One quasi-reversible electron transfer process is visible in the CV curve, indicating that the measured capacitance is mainly based on redox mechanism [26]. Moreover, the shape of the curves (Fig. 6) displays that the observed capacitance 100
80
60
Current / A g
Fig. 4 e SEM (a) and TEM (b) images of the prepared bNi(OH)2 nanoparticles.
40
20
0
-20
-40 -0.1
0.0
0.1
0.2
0.3
0.4
0.5
Potential / V vs. Ag/AgCl
Fig. 6 e Cyclic voltammograms of the b-Ni(OH)2 nanoparticles at various scan rates.
0.6
0.7
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where, i is the applied current (0.5 A), DV is the potential range (0.55 V), Dt is the time of a discharge cycle and w is the mass of the prepared b-Ni(OH)2 nanoparticles. The specific capacitance value was calculated as equal to 715.3 F g1, which is very close to that calculated from the CV curves (Fig. 6), again confirming the high capacitance of the ultrafine nanoparticles of b-Ni(OH)2.
0.45 0.6
Potential / V vs. Ag/AgCl
0.5
0.4 0.30
0.3
4.
Conclusion
0.2 0.15
0.1
0.00 0.0
0
1000
2000
3000
4000
5000
6000
Time (s)
Fig. 7 e Galvanostatic charge/discharge curve of the bNi(OH)2 nanoparticles at current density of 0.5 A gL1.
characteristic is distinct from that of the electric double layer capacitor, which would produce a CV curve that is usually close to an ideal rectangular shape. Since solution and electrode resistance can distort the current response at the switching potential and this distortion is dependent upon the scan rate [32], as can be seen from Fig. 6, the shape of CV curves of the b-Ni(OH)2 nanoparticles is not significantly influenced by the increasing of the scan rates. This indicates the improved mass transportation and electron conduction within the nanoparticles. The average specific capacitance of b-Ni(OH)2 can be calculated from the CV curves in Fig. 6 by integrating the area under the currentepotential curve:
C¼
1 mnðVa Vc Þ
ZVc IðVÞdV
(7)
Va
The specific capacitances of the prepared b-Ni(OH)2 nanoparticles were calculated to be 1056.1, 834.6, 740.2 and 652.8 F g1 at the scan rates of 2, 5, 10 and 25 mV s1, respectively. The specific capacitance of the b-Ni(OH)2 nanoparticles is more higher than that of micro-size Ni(OH)2 particles (770 F g1 at the scan rate of 2 mV s1 [20]). Thus these values suggest an excellent electrochemical performance and show the nanosize effects on the electrochemical performance of the prepared b-Ni(OH)2 nanoparticles. Electrochemical capacitance behavior of the prepared b-Ni(OH)2 electrode was also investigated by chronopotentiometry in 1 M KOH aqueous solution. Fig. 7 shows the chargeedischarge curves of the b-Ni(OH)2 electrode measured at a current density of 0.5 A g1 within the potential window of 0e0.55 V vs. Ag/AgCl. The shapes of the chargeedischarge curves do not display the characteristic of a pure electric double layer capacitor, but mainly pseudocapacitance, which corresponds with the result of the CV tests. The specific capacitance of bNi(OH)2 electrode was calculated by the following equation: C¼
i$Dt wDn
(8)
Low-temperature cathodic electrodeposition method was applied for the synthesis of Ni(OH)2 nanoparticles. XRD, DSCTG and FT-IR analyses showed that the prepared Ni(OH)2 was a single crystalline pure b phase. The morphological studies by SEM and TEM revealed that the prepared b-Ni(OH)2 was composed of well dispersed ultrafine particles with the size of smaller than 5 nm. Cyclic voltammetry and galvanostatic chargeedischarge tests were performed to evaluate the electrochemical performance of the b-Ni(OH)2 nanoparticles in 1 M KOH electrolyte. CV curves showed a pair of strong redox peaks as a result of the Faradaic redox reactions of b-Ni(OH)2 nanoparticles. The specific capacitance of as high as 740.2 F g1 was achieved within a potential window of 0e0.55 V vs. Ag/AgCl at a scan rate of 10 mV s1. These findings suggest that the ultrafine nanoparticles of b-Ni(OH)2 have excellent electrochemical properties and thus can be recognized as a promising candidate for the supercapacitor applications.
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