Synthesis of La-doped NiO nanofibers and their electrochemical properties as electrode for supercapacitors

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CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 6973–6977 www.elsevier.com/locate/ceramint

Synthesis of La-doped NiO nanofibers and their electrochemical properties as electrode for supercapacitors Jianfeng Jian, Fangyan Luo, Chaojun Gao, Can Suo, Xinchang Wang, Hongzhang Song, Xing Hu School of Physical Engineering, Laboratory of Material Physics, Zhengzhou University, Zhengzhou 450052, PR China Received 9 August 2013; received in revised form 14 November 2013; accepted 6 December 2013 Available online 17 December 2013

Abstract La-doped NiO nanofibers were synthesized by the electrospinning method. The X-ray diffraction (XRD) pattern showed that La doping does not change the crystal structure up to the doping ratio of La/Ni ¼ 1.5%. Electrochemical properties of La-doped NiO nanofibers were investigated using cyclic voltammetry and galvanostatic charge/discharge. The results showed that the La doping can enhance the charge/discharge specific capacitance and electrochemical stability of the NiO nanofibers. Especially, the sample with doping ratio of La/Ni ¼1.5% could reach a discharge specific capacitance of 94.85 F g  1 at a constant current density of 5 mA cm  2. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: NiO oxide nanofibers; Supercapacitors; Element doping; Electrospinning

1. Introduction Supercapacitor, also known as electrochemical capacitor, is a kind of novel energy storage device [1], which was regarded as an important power source for applications in various fields, such as hybrid electric vehicles and mobile electronic devices [2]. In a supercapacitor system, the electrode material will play a key role [3]. For the positive electrode materials of supercapacitor, RuO2 is regarded as the best one, however, its high cost limits its applications [4,5]. NiO oxide ceramic is generally considered as a promising positive electrode material because of its favorable capacitive characteristics and lower cost as well as environmental friendliness [6,7]. Low-dimensional nanomaterials (such as nanowires, nanofibers, and nanotubes) have attracted much attention in the passed years due to their special properties and interesting behaviors in many aspects [8]. It is believed that the metal oxide nanofibers prepared by the electrospinning method maybe have excellent properties when used in supercapacitors [9]. Moreover, when doping with elements, such as transition or no-transition metal ions, it may enhance the discharge n

Corresponding author. E-mail address: [email protected] (J. Jia).

capacitance and the electrochemical activity of the metal oxide electrode [10]. For instance, the specific capacitance and cycling stability of the La-doped NiO porous microspheres will have a remarkable enhancement as compared with the undoped samples [11]. In this paper, we reported the preparation of La-doped NiO nanofibers by the electrospinning method and the performance of the nanofibers as used as positive electrode materials. A asymmetric supercapacitor was assembled using 2 M KOH aqueous solution as electrolyte, porous activated carbon materials as negative electrode, and La-doped NiO nanofibers as positive electrode (we named the supercapacitor as AC/KOH/ NiOx nanofibers asymmetric supercapacitor). The electrochemical performances of the supercapacitor were investigated. 2. Experimental 2.1. Synthesis of La-doped NiO nanofibers Ni(CH3COO)2  4H2O and La(NO3)3  6H2O with molar ratios of La/Ni¼ 0.1%, 0.5%, 1.0%, 1.5%, 2.3% were added into aqueous poly vinyl alcohol (PVA, Mw 80,000) solution under vigorous stirring at 40 1C for 4 h. Then 5 ml alcohols were

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J. Jia et al. / Ceramics International 40 (2014) 6973–6977

dropped slowly into the solution with stirring. Ultimately, a lucid and viscous sol solution was obtained for electrospinning. In our experiment, a voltage of 10 kV between the cooper plate collector and the syringe needle was applied and the distance from the collector to the syringe needle was 12 cm. The La-doped PVA/ Ni (CH3COO)2  4H2O composite nanofibers were collected on the cooper plate during electrospinning processes. Then the obtained composite nanofibers were calcinated at 650 1C for 3 h in air to remove PVA resulting Ladoped NiO nanofibers.

(200) (111)

(220) (311) (222) 0% La

Intensity (a.u.)

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0.1% La 0.5% La 1.0% La

2.2. Structural and morphologic characterization The phase of nanofibers were examined using an X-ray diffractometer (PANalytical, Netherlands) with Cu/Kα radiation (λ ¼ 1.5418 Ǻ). And the morphologies of nanofibers were observed by a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6700F, Japan) using acceleration voltage of 15.0 kV.

1.5% La 2.3% La * 0 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 2θ /° Fig. 1. XRD patterns of as-prepared La-doped NiO nanofibers.

3. Results and discussion

Scherrer equation according to the XRD pattern [12]) varied from 70 nm to 53 nm with the increase of La doping amount. However, when the La doping ratio is beyond 1.5% impure phase will emerge as the XRD pattern of the sample with La doping ratio 2.3% shown (impurity labeled with n). Fig. 2 shows the SEM images of the NiO nanofibers before and after calcination at 650 1C. It can be seen that before calcination the surface of the fibers is very smooth with fibers diameter about 400 nm, as shown in Fig. 2(a). While, after calcination at 650 1C, due to the decomposition of PVA and the transformation from metal salts into metal oxides, the surface of the nanofibers becomes very coarse, as shown in Fig. 2(b). It can also be seen that the fiber is consisted of small crystal grains. Fig. 2(c) is the SEM image of 1.5% La-doped NiO nanofibers after calcination at 650 1C, which basically have the same appearance as the undoped NiO fibers, but the crystal grains are smaller than that undoped samples, which is consistent with the result of Fig. 1. Surface area analysis was carried out by using the Brunauer–Emmett–Teller (BET) nitrogen adsorption method (Micromeritics Instrument Corporation ASAP2020). The BET (Brunauer–Emmett–Teller) specific surface areas of both undoped NiO nanofibers and 1.5% La-doped NiO nanofibers were found to be 13.1 and 21.8 m2 g  1 respectively.

3.1. Structure and morphology

3.2. The electrochemical performance

The XRD patterns La-doped NiO nanofibers with different La doping amount are shown in Fig. 1. It can be seen that all peaks can be indexed to NiO and no impure phase can be found when the La-doping ratio is less than 1.5%. The crystal phases of those samples are identified to be a cubic structure according to the standard card (JCPDS75-197). The peaks of the La-doped NiO nanofibers shifted slightly to lower angle with the increase of La doping amount. At the same time, the peak intensities decrease with the increase of La doping amount. It indicates that small La doping amount will not change the crystal structure of NiO, but the average crystal grain size (calculated using

Fig. 3 shows the Cyclic Voltammograms (CV) curves of the capacitors prepared with electrodes of different La doping amount NiO nanofibers in the voltage range from 0.0 to 0.5 V. The scan rate of the voltage was 10 mV s  1. For each curve, a pair of redox current peaks at about 0.26 and 0.30 V can be seen clearly, which indicates the character of the Faradic capacitance [13–15]. The potential difference between the anodic and cathodic peaks is used as a measure of the reversibility of the electrochemical redox reaction [16]. It can be seen that the potential difference for the capacitor with 1.5% La-doped NiO as electrodes is the smallest one among these

2.3. Electrochemical characterization For electrochemical studies, the positive electrodes were fabricated by mixing the La-doped NiO nanofibers, acetylene black and binder (polyvinylidene difluoride, PVDF) in the mass ratio: 0.75:0.15:0.10 using N-methyl pyrrolidone (NMP) as solvent to dissolve the PVDF binder. The mixture was spread on a Ni foil and dried at 80 1C and then were densified under 10 MPa pressure. The negative electrodes using the activated carbon (AC) were prepared by the same method as the positive electrode. With the saturated calomel electrode as the reference electrode, platinum electrode as auxiliary electrode, the asprepared electrodes as working electrode and 2 mol/L KOH solution as the electrolyte, the electrochemical workstation (273 A constant potential/constant current instrument) was used for the Cyclic Voltammetry test. Before measurement, the positive and negative electrodes were dipped into 2 mol/L KOH solutions for 24 h to make electrode materials fully infiltrated. In our measurements, constant current charge-discharge cycling test were performed using a battery comprehensive performance test platform (lithium battery BTS test system).

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Fig. 2. SEM images of the NiO nanofibers before and after calcination at 650 1C.

160

1mA.cm-2

140

2mA.cm-2 3mA.cm-2 4mA.cm-2 5mA.cm-2 1.5%La

C /(F.g-1)

120 100 80

1.0%La 60

0.5%La 0.1%La 2.3%La 0%La

40 20 0

100

200

300

400

500

600

Cycle number Fig. 3. Cyclic Voltammograms curves of different La-doped NiO nano-fibers as electrodes.

capacitors, indicating its better reversibility in the electrochemical redox reaction. Moreover, the redox current of this capacitor is also the highest one, implying a high specific capacitance of the capacitor prepared using the 1.5% La-doped NiO nanofibers as electrode. 3.3. Asymmetric supercapacitor studies Fig. 4 shows the specific capacitance of the as prepared asymmetric capacitors as a function of the cycle number measured

Fig. 4. Plots of specific capacitance versus cycle number.

with a constant current density (1 mA cm  2 to 5 mA cm  2 respectively) at room temperature. As shown in Fig. 4, at the lower current density (1 mA cm  2), the specific capacitance increases with the increase of cycle number. This means that when the current density is low, the activation process of the electrode material is slow. With the increase of the cycle number, more and more active substance can be activated, and thus enhance the specific capacitance [17]. However, when the current density raised to 3 mA cm  2, the specific capacitances almost kept the same and did not changed with the increase of the cycle number.

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5mA.cm-2

120 100 80 60

0% La

1000

4mA.cm-2

1mA.cm-2

UR

Ur

800

3mA.cm-2 2mA.cm-2

V / (mV)

Specific capacitance (F.g-1)

140

1.5% La

600 400 200

40

0

20

-200 0%

0.1%

0.5%

1.0%

1.5%

2.3%

0

100

200

Component (La)

300

400

500

600

700

t / (s)

Fig. 5. Specific capacitance as a function of La component at different current density.

Fig. 6. Galvanstatic profiles (voltage vs. time plots) of 1.5% La doped and undoped samples.

This indicates that at a higher current density, the active substance is already activated sufficiently. The effect of the doping of La is evident, all capacitors using the La-doped NiO nanofibers as electrodes show a higher specific capacitance as compared with the undoped one, except the highly 2.3% La-doped sample (should due to its impure phase). Especially, the 1.5% La-doped sample shows a promising specific capacitance. Our results indicate that using suitable La-doped NiO nanofibers as electrodes can enhance the specific capacitance remarkably. As a comparison, Fig. 5 gives the specific capacitance versus La doping amount at different current density. It can be seen clearly that the specific capacitances increase with the increase of La doping amount and La doping amount of 1.5% shows the best performance. Beyond doping amount of 1.5%, the specific capacitance begin decrease, which should attribute the existence of impure phase in the fibers doe to excess La doping. At current density of 5 mA cm  2, the specific capacitance of the 1.5% La-doped sample can reach 94.85 F g  1, which is 5.3 times higher than the value (17.78 F g  1) of the undoped pure NiO sample. This result consists with the previous reported result that the appropriate La doping can significantly improve the specific capacitance of NiO microspheres [11]. Fig. 6 shows the galvanostatic cycling performance of the capacitors with pure NiO nanofiber electrode and 1.5% La doped NiO nanofiber electrode respectively. The cycles were between voltages 0 and 1.2 V at a current density of 5 mA cm  2. It can be seen from Fig. 6, the voltage drop process can be divided into two stages: one is the discharge instantaneous voltage drop; another is the slow voltage drop. The first stage is caused by the internal equivalent series resistance (mainly attributed to the internal resistance of the electrode material, the internal resistance of the electrolyte and contact resistance), the second stage is caused by the electrode pseudocapacitive properties [18]. Obviously, the instantaneous voltage drop (Ur) of the undoped sample is greater than that (UR) of the 1.5% La doped sample. It indicates that the internal equivalent series resistance (ESR) of the 1.5% La doped sample is lower than that of the undoped sample, which indicates the advantages

Table 1 Test data for different rate La-doped samples at current density of 5 mA cm  2. La content (at%)

Specific capacitance (F g  1)

ESR (Ω)

0 0.1 0.5 1.0 1.5 2.3

18.0 33.2 42.6 68.1 94.8 27.4

11.7 11.0 10.6 9.1 8.3 9.2

of La doping. The internal equivalent series resistance can be obtained from the discharge instantaneous voltage drop according to ESR ¼

ΔU 2I

ð1Þ

where ΔU is the discharge instantaneous voltage drop, and I is discharge current. It was found that the ESR decreased slightly with the increase of La doping amount, reaching a minimum of 8.3 Ω when the the La doped content was1.5%, as shown in Table 1. Moreover, the cycles curve is kept almost un-change, indicating a good electrochemical stability and reversibility property of the capacitors. The larger specific capacitance and excellent electrochemical stability for La-doped NiO nanofiber electrode may be caused by the crystal defect once La replaces Ni atom in the lattice. Moreover, compared with the zero-dimensional nanostructures (such as nanospheres or nanoparticles), the onedimensional nanofibers with a larger ratio of length to diameter can obviously reduce the interfacial contact resistance between the active materials and the electrolyte, which results in higher charge transfer rates and significantly faster electronic kinetics. Fig. 7 shows the plots of the specific capacitance and the coulombic efficiency versus time of the capacitor with 1.5% La doped NiO nanofiber electrode at current density of 5 mA cm  2. It can be seen, the coulomb efficiency is maintained at

J. Jia et al. / Ceramics International 40 (2014) 6973–6977 0 20 80 40 60 60 40

efficiency (%)

specific capacitance (F.g-1)

100

80 20 100 0 0

200

400

600

800

1000

cycle number Fig. 7. The coulombic efficiency versus time of 1.5% La doped sample.

more than 90%, which also proves that the electrode has a good reversibility. 4. Conclusions La-doped NiO nanofibers can be synthesized by the electrospinning method. The ideal doping amount of La is at La/Ni ratio 1.5%. When using such La-doped NiO nanofibers to make the positive electrode of supercapacitors, it shows excellent electrochemical properties as compared with the undoped NiO electrode. Both the specific capacitance and the electrochemical stability can be improved greatly. A higher specific capacitance of 94.85 F g  1 can be obtained, which is 5.3 times higher than the capacitors with pure NiO nanofibers as electrode. The supercapacitor with La-doped NiO nanofibers as electrode also has good cycling stability; the coulomb efficiency can keep at more than 90% after 1000 cycles. Our results demonstrate that the La-doped NiO nanofibers can be a potential positive electrode material for the high-performance supercapacitors. References [1] B.E. Conway, Transition from “Supercapacitor” to “Battery” behavior in electrochemical energy storage, J. Electrochem. Soc. 138 (1991) 1539–1548. [2] S. Sarangapani, B.V. Tilak, C.P. Chen, Materials for electrochemical capacitors theoretical and experimental constraints, J. Electrochem. Soc. 143 (1996) 3791–3799.

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