Preparation of CoAl layered double hydroxide nanoflake arrays and their high supercapacitance performance

Applied Clay Science 102 (2014) 28–32

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Applied Clay Science journal homepage: www.elsevier.com/locate/clay

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Preparation of CoAl layered double hydroxide nanoflake arrays and their high supercapacitance performance Pan Guoxiang a,⁎, Xia Xinhui b, Luo Jingshan b, Cao Feng a, Yang Zhihong a, Fan Hongjin b,⁎ a b

Department of Materials Chemistry, Huzhou University, Huzhou 313000, PR China School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore

a r t i c l e

i n f o

Article history: Received 13 July 2013 Received in revised form 27 September 2014 Accepted 9 October 2014 Available online xxxx Keywords: CoAl layered double hydroxides Porous film Nanoflake arrays Supercapacitor

a b s t r a c t Porous cobalt aluminum layered double hydroxide (CoAl-LDH) nanoflake arrays on nickel foam were synthesized by a facile hydrothermal synthesis method. The obtained CoAl-LDH film consisted of interconnected nanoflakes with the thicknesses of ~ 20 nm and showed an extended net-like structure. The electrochemical performance of CoAl-LDH nanoflake arrays was tested by cyclic voltammetry and galvanostatic charge/discharge test. The porous CoAl-LDH nanoflake arrays showed good pseudocapacitive performances with high capacitance and good cycle stability. At room temperature, the porous CoAl-LDH nanoflake arrays exhibited a high specific capacitance of 930 F/g at 2 A/g and their specific capacitances were still up to 669 F/g at 16 A/g. A good cycling stability was also observed for the CoAl-LDH nanoflake arrays with a capacitance retention of 88.9% of the highest value after 2000 cycles at 2 A/g. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Supercapacitors, also known as electrochemical capacitors, are emerging electrochemical energy storage devices, which hold fascinating advantages including fast charge/discharge capability, long cycling life, and high power density (Wang et al., 2012). Supercapacitors operate between rechargeable batteries and conventional capacitors with respect to energy and power performance. They possess much higher energy density than conventional capacitors, and higher power density than batteries (Simon and Gogotsi, 2008). According to the energy storage mechanism, supercapacitors can be classified as electric double layer capacitors (EDLCs) and pseudocapacitors (also called Faraday capacitors), which produce higher specific capacitances and higher energy densities compared to electric double layer capacitors. In recent years, the research on pseudocapacitors has gradually become a hot spot in the area of supercapacitor research (Wang et al., 2010; Liu et al., 2011; Xia et al., 2011). Despite great progress, supercapacitors still suffer from a relatively lower energy density than batteries, and this limits their use as a main power source. Electrode material is the key component of a supercapacitor and great efforts have gone into developing high-performance pseudocapacitive materials such as transition metal oxides/hydroxides, conducting polymers, metal sulfides and binary metal oxides/hydroxides (Li and Kaner, 2008; Yu et al., 2010; Yan et al., 2011; Cao et al., 2012; Hastak et al., 2012; Lv et al., 2012; Pan et al., 2012a, 2012b). Recently, layered double hydroxides (LDHs) have evoked great interest for pseudocapacitor ⁎ Corresponding authors. E-mail addresses: [email protected] (P. Guoxiang), [email protected] (F. Hongjin).

http://dx.doi.org/10.1016/j.clay.2014.10.003 0169-1317/© 2014 Elsevier B.V. All rights reserved.

applications (Wang et al., 2005, 2011; Zhang et al., 2012). LDH is a kind of layered functional material composed of metal hydroxide layers and intercalated anions. The general composition of LDH is M2+1 − xM3+x(OH)2(An−)x/n·mH2O, in which M2+ represents a divalent cation (typically Mg2+, Zn2+, Ni2+, Co2+, Cu2+), M3+ designates a trivalent cation (typically Al3 +, Fe3 +, Cr3 +), An− is the interlayer − − anions (typically CO2− 3 , NO3 , OH ), x is 0.2–0.33 and m is the number of interlayer structural water molecules (Cavani et al., 1991). Previously, cobalt aluminum layered double hydroxides (CoAl-LDHs) and their graphene-based composite have been reported as electrode materials for supercapacitor applications (Busca et al., 2009; Pan et al., 2012a, 2012b; Turco et al., 2004). However, these power CoAl-LDH materials need to be further processed into an electrode with binders and additives. This comes with the risk of negating the gains in diffusion length and electronic conductivity associated with the reduced active material particle size. There may also be new penalties arising from the addition of supplementary interfaces (Wang et al., 2005). To preserve the benefits of electrochemistry at the nanoscale, it is significant to directly fabricate free standing CoAl-LDH nanoarrays on a current collector. It is well accepted that pseudocapacitance mainly comes from highly reversible redox reactions of active substance on the surface or bulk phase of the electrode material. Porous nanoarrays obviously improve the contact area between the electrode active material and electrolyte, and shortens the diffusion channel between the electrons and ions with fast kinetics (Lu et al., 2012). In the present work, a facile hydrothermal method for the fabrication of porous cobalt aluminum layered double hydroxide (CoAlLDH) nanoflake arrays on nickel foam substrate is reported. Highly porous CoAl-LDH nanoflakes grew vertically to the substrate forming

P. Guoxiang et al. / Applied Clay Science 102 (2014) 28–32

an integrated electrode. The as-prepared CoAl-LDH nanoflake arrays as a cathode of supercapacitor were thoroughly studied.

29

3. Result and discussion 3.1. Morphology of the porous array film

2. Experimental section 2.1. Reagents In this work, Co(NO3)2·6H2O, Al(NO3)3·9H2O, CO(NH2)2, NH4F and KOH obtained from Sinopharm Chemical Reagent Co., Ltd. were analytical pure and all solutions were prepared with deionized water. Clean nickel foam (size in 4 cm × 2 cm) obtained from Changsha Liyuan New Material Co., Ltd. was used as the substrate. 2.2. Material preparation The CoAl-LDH nanoflake arrays were fabricated as follows (Costantino et al., 1998). In a typical synthesis, 1.2 mmol Co(NO3) 2·6H2O, 0.4 mmol Al(NO3)3·9H2O, 8 mmol urea and 4 mmol NH4F were dissolved in 35 mL deionized water, and the mixture solution was stirred and then transferred to an autoclave. Then a piece of nickel foam with top side protected by an insulation tape was immersed into the reaction solution. The hydrothermal reaction was conducted at 120 °C for 6 h and then cooled down to room temperature. Finally, the sample was taken out and rinsed three times with deionized water. The load weight of CoAl-LDH nanoflakes on the nickel foam substrate was ~2.0 mg cm−2 as measured with an analytical balance. 2.3. Structural characterization The microstructure and morphology of the sample were characterized by a field emission scanning electron microscopy (FESEM, FEI SIRION 200, accelerating voltage was 5.0 kV, the spot size was 3.0, and the work distance was 13.5 mm), JEOL JEM-2010 transmission electron microscopy (TEM, accelerating voltage was 100 kV) and X-ray diffraction (XRD, RIGAKU D/Max-2550 used Cu Kα radiation (40 kV/ 100 mA), with a slit width of 0.5°, 2θ angle ranged from 5° to 80° with a scan speed of 2°/min). Infrared spectroscopy (FT-IR) analysis was conducted using a NICOLET 5700 measurement (measured in a KBr pellet with sample/KBr weight ratio of 1/100 over the range from 400 to 4000 cm−1, spectral resolution was 4 cm−1 and the number of scans was 32). 2.4. Electrochemical performance test The electrochemical measurements were carried out in a threeelectrode electrochemical cell containing a 2 M KOH aqueous solution as the electrolyte. Cyclic voltammetry (CV) measurements were carried out between 0 V and 0.8 V at 25 °C on a CHI660c electrochemical workstation (Chenhua, Shanghai), CoAl-LDH nanoflake arrays as the working electrode, Hg/HgO as the reference electrode and a Pt foil as the counter-electrode. The galvanostatic charge/discharge tests were conducted on a LAND battery program-control test system. The CoAlLDH nanoflake array electrode, together with a nickel mesh counter electrode and an Hg/HgO reference electrode were tested in a threecompartment system. The specific capacitance can be calculated as follows: C ¼ IΔt=mΔV

ð1Þ

with C representing the specific capacitance, F/g; I the discharge current, A; Δt the discharge time, s; m the weight of active substance, mg; and ΔV the difference of discharge voltage, V.

The SEM images showed that the as-prepared film was composed of interconnected nanoflakes with thicknesses of ~ 20 nm and formed a homogeneous three-dimensional network structure with pores ranging from 50 to 500 nm (Fig. 1(a) and (b)). The nanoflake arrays were perpendicular to the substrate and presented a height of 1.2 μm (Fig. 1(c) and (d)). The nanoflakes interconnected with each other with a smooth texture (Fig. 1(d)). The vertical growth of CoAl-LDH nanoflake arrays and the porous structure formed by the interconnected nanoflakes may be beneficial for improving the specific surface area of the array film and effectively increasing the reaction area of the material (Cao et al., 2012; Cavani et al., 1991; Pan et al., 2012a, 2012b). At the same time, this nanoflake array structure may provide shortened ion/electron diffusion distances and improve reaction kinetics in the electrochemical reaction process and thus a high electrochemical performance may be anticipated. 3.2. Structural characterization of CoAl-LDHs The powder XRD pattern of the CoAl-LDH nanoflakes indicated three strong reflections at 11.1°, 22.6° and 34.7° that were indexed as (003), (006), and (009) of LDH, and the corresponding d00l values were 0.78, 0.39, 0.26 nm, respectively (Fig. 2(a)). They were characteristic X-ray reflections of a lamellar structure, indicating that the obtained nanoflakes were the desired CoAl-LDH phase (JCPDS: 51-0045). A strong broad band around 3400 cm−1 shown in Fig. 2(b) was generally interpreted as the CO3–H2O bridging mode, H-bonded interlayer H2O surrounding the interlayer anion and the metal–OH stretching mode as reported by Kloprogge et al. (2004) and Kloprogge (2005). A strong absorption band at 1380 cm−1 was attributed to CO32−, due to the asymmetric stretching vibration of the C–O bond. The weak absorption band near 1600 cm−1 was mainly ascribed to the H–O–H bending vibration of interlayer water molecules. Wide absorption bands at 400–800 cm−1 belonged to the O–M–O, M–O, and M–O–M related vibrational modes of LDHs. The FT-IR result was in agreement with the XRD result. 3.3. Electrochemical performance of CoAl-LDH array film The cyclic voltammetry (CV) curve exhibited a pair of obvious redox peaks, which were caused by the redox reaction of Co(II) in the hydroxide sheets of CoAl-LDHs (Casella and Contursi, 2012; Scavetta et al., 2012) (Fig. 3(a)). The CoAl-LDH nanoflake arrays showed pseudocapacitive properties rather than electric double-layer capacitance characteristics. Specific capacitance of the electrode mainly came from the redox reaction of the active materials. The redox couple in the CV curve corresponded to Faraday reaction as follows: −

−

CoðOHÞ2 þ OH ↔ CoOOH þ H2 O þ e :

ð2Þ

Redox peaks in the CV curve had a good symmetry, meaning that the active material had a good electrochemical reversibility. With the increase of the scanning rate, oxidation and reduction peaks shifted to more positive or negative directions, respectively. The CV curve of nickel foam substrate (not shown in this paper) was also measured. Through the calculation of specific capacitance, the contribution from nickel foam was less than 3% (Cao et al., 2012; Pan et al., 2012a, 2012b). So the specific capacitance contributed by nickel foam can be ignored in this work. Rate characteristic was an important parameter for supercapacitor. The discharge curves and corresponding specific capacitances of CoAlLDH nanoflake arrays were tested when the charge current density was 2 A/g and voltage window was 0–0.55 V (Fig. 3(b) and (c)).

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a

b

20 nm

2µm

100 nm

c

d

Fig. 1. (a) and (b) SEM images of the CoAl-LDH nanosheet array film; (c) and (d) TEM images of individual nanoflakes obtained from nanoarray film.

According to Eq. (1), the specific capacitance of porous CoAl-LDH film electrode was 891 F/g, 829 F/g, 742 F/g and 669 F/g at the discharge current density of 2, 4, 8 and 16 A/g, respectively. Even at the large current density of 16 A/g, its specific capacitance still kept 75% of the highest value. These values were much higher than those of powder material prepared by Wang et al. (2005). The specific capacitance decreased as the discharge current density increased. This was mainly caused by the increase of the voltage drop of electrode materials and inadequate reaction of active substances.

b

Intensity/a.u.

(003)

Intensity/a.u.

a

The cycling stability of CoAl-LDH nanoflake arrays at 2 A/g showed that the arrays needed an initial activation process, and then slowly reached the highest activity value of about 930 F/g (Fig. 3(d)). The improvement of specific capacitance was related to Al3+ extraction from the hydroxide sheets in the alkali solution (Lu et al., 2012). After 2000 times cycle test, the specific capacitance remained at 827 F/g and retained 88.9% of the high value, indicating that porous CoAl-LDH nanoflake arrays had a good cycling stability. The nanoflake array structure remained intact and no collapse occurred after 2000 cycles (Fig. 4).

(006) (009) (110)

10

20

30

40

50

2/degree

60

70

80

4000

3500

3000

2500

2000

1500 -1

Wave number/cm

Fig. 2. Structural characterization patterns of layered double hydroxides nanoflakes. (a) XRD; (b) FT-IR.

1000

500

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b

0.25

2

Current density(mA/cm )

0.20

40mV/s

20mV/s

0.10 10mV/s

0.05 5mV/s

0.00

0.6 0.5

0.15

Potential vs. Hg/HgO(V)

a

31

-0.05

0.4 2A/g 0.3 4A/g 0.2

8A/g 16A/g

0.1

-0.10

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0

50

100

Potential vs. Hg/HgO(V)

1000

d 1000

900

900

800

800

700

700

Specific capacitance(F/g)

Specific capacitance(F/g)

c

150

200

250

Time(s)

600 500 400 300 200

600 500 400 300 200 100

100

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0 0

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4

6

8

10

12

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18

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600

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1200

1500

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2100

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Current density(A/g)

Fig. 3. (a) Cyclic voltammograms at different scanning rates (5, 10, 20, 40 mV/s); (b) discharge curves and (c) corresponding specific capacitances at different discharge current densities (2, 4, 8, 16 A/g); (d) cycling performances at 2 A/g of CoAl-LDH array film.

This good mechanical stability was responsible for the good cycling stability (Cavani et al., 1991). In addition, the Al3 + in combination with the Co2 + in the LDH hydroxide sheets prevented the occurrence of the normally observed phase transfer from α to β phase for the pure cobalt hydroxide during the charging/discharging process and improved its cycling performance.

4. Conclusion In this work, highly porous CoAl-LDH nanoflake arrays were successfully prepared by a facile hydrothermal synthesis method. The CoAlLDH film consisted of uniform nanoflakes with thicknesses of ~20 nm. The CoAl-LDH nanoflake arrays showed excellent supercapacitor performance with a specific capacitance of 930 F/g at 2 A/g. After 2000 cycles, the specific capacitance still maintained 88.9% of the maximum value. The high capacitance mainly came from the porous structure. The good cycling performance was possibly related with the stable LDH nanoflake array structure demonstrated by SEM analysis. The in situ hydrothermal synthesis process for LDH growth on the nickel foam was simple and controllable. It also had a potential application for the preparation of supercapacitor materials with a high specific capacitance and long cycling characteristics. Acknowledgments The authors gratefully acknowledge the financial support to this work by the National Natural Science Foundation of China (No. 31070451).

1µm

Fig. 4. SEM image of CoAl-LDH film after 2000 cycles.

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