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electrospun carbon nanofibers: Controlled fabrication and high capacitive property
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One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property Fujun Miao, Changlu Shao*, Xinghua Li*, Yang Zhang, Na Lu, Kexin Wang, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China
article info
abstract
Article history:
Ultra-long one-dimensional (1D) electrospun carbon nanofibers (CNFs) were obtained by
Received 27 September 2013
carbonization treatment of polyacrylonitrile fibers at 1000 C in nitrogen atmosphere.
Received in revised form
Using CNFs as templates, beta-nickel hydroxide (Ni(OH)2) nanoplates were prepared on
30 January 2014
CNFs by hydrothermal reactions. The contents of Ni(OH)2 in the 1D Ni(OH)2/CNFs hetero-
Accepted 3 February 2014
structures could be controlled by adjusting the concentrations of Ni(Ac)2 precursors. The
Available online xxx
1D heterostructures as electrode materials of supercapacitors exhibited high specific
Keywords:
and Ni(OH)2 nanoplates. The excellent properties of the heterostructures could be ascribed
b-Ni(OH)2
to the rapid electron transport along the longitude direction of CNFs, the high specific
Carbon nanofibers
pseudo-capacitance of Ni(OH)2 nanoplates, and the good electrical contacts between CNFs
capacitance compared with CNFs, pure Ni(OH)2 nanoplates, and physical mixtures of CNFs
Electrospinning technique
and Ni(OH)2 nanoplates. And, the specific capacitance retention of the heterostructures
Heterostructures
was over 90% after 500 cycles of charge and discharge at a high current density of 12.5 A/g,
Supercapacitors
suggesting their good cycling stability. It was expected that the 1D Ni(OH)2/CNFs heterostructures as electrode materials with excellent capacitive properties would greatly promote their practical applications to the energy storage for supercapacitors. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, electrochemical capacitors, also known as supercapacitors, have attracted more and more attentions due to their super energy storage performances, such as fast charging and discharging, environment friendly, high power densities, long cycle life, low maintenance cost, etc. [1e6].
Supercapacitors are kinds of energy storage devices between traditional capacitors and batteries, which can be classified as electric double-layer capacitors (EDLCs) and faradaic pseudocapacitors depending on their energy storage mechanisms. For EDLCs, energy storage arises from the accumulation of electrons and ionic charges at the interface between the electrode materials and the electrolyte solution. Carbon materials are usually used as electrode materials of EDLCs because of their
* Corresponding authors. Tel.: þ86 43185098803. E-mail addresses: [email protected] (C. Shao), [email protected] (X. Li). 0360-3199/$ e see front matter Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
Please cite this article in press as: Miao F, et al., One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
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good electrical conductivity, physical and chemical stability [7,8]. Moreover, carbon nanomaterials with large specific surface areas including ultrafine activated carbon [9,10], carbon nanotubes [11], graphene [12], and carbon nanofibers [13] can facilitate the rapid transport of the electrolyte ions from bulk solution to the surface of electrode which is crucial to high specific capacitance and power density supercapacitors. Among numerous carbon nanomaterials, one-dimensional (1D) carbon nanofibers are considered as one of the promising candidates for the electrode materials of supercapacitors. The 1D carbon nanofibers can be prepared by physical methods, chemical methods and electrospinning technique [13,14]. Notably, electrospun carbon nanofibers (CNFs) possess not only unique ultra-long 1D nanostructure but also three dimensional porous networks, which benefits for both electron transport along the longitude directions and constructing the electrodes for supercapacitor devices. To improve the properties of CNFs for supercapacitors with high specific capacitance and energy density, many strategies have been proposed, such as increasing the surface areas by controlling the diameter or pore size of CNFs [15], surface modification of CNFs [16], or constructing composite structures with transitional metal oxides or hydroxides [17]. Among these strategies, constructing composite structures is attractive because it may combine the high power density from CNFs and high energy density from pseudocapacitive materials and bring new hybrid supercapacitors of EDLCs and pseudocapacitors [18]. Till now, many oxides, such as MnO2 [19,20], hydrous RuO2 [21], SnO2 [22] and cobalt oxide [23] have been studied to develop the new hybrid supercapacitors based on CNFs. Among various pseudocapacitive electrode materials, nickel hydroxide (Ni(OH)2) is very attractive due to its high specific capacitance, low cost, and readily available [24e27]. However, as far as we know, nickel hydroxide/CNFs composite materials for the new hybrid supercapacitors have not been reported yet. In the present work, we designed the direct fabrication of hybrid nanostructures by integrating Ni(OH)2 with CNFs, where CNFs served as both the conductivity networks and the scaffold/support which decreased the internal resistance and improved the cycling stability [28,29] and Ni(OH)2 nanoplates provided high specific capacitance relying on faradic redox reactions. The 1D beta-nickel hydroxide nanoplates/electrospun carbon nanofibers (Ni(OH)2/CNFs) heterostructures have been fabricated successfully by hydrothermal reactions. Electrochemical studies demonstrated that 1D Ni(OH)2/CNFs heterostructures possessed high specific capacitance and excellent cycling stability as electrodes for supercapacitors, compared with pure CNFs, pure Ni(OH)2 nanoplates, and physical mixtures of CNFs and Ni(OH)2. Our results suggested that the low specific capacitances of CNFs and the intrinsic insulating properties of Ni(OH)2 could be solved by constructing 1D Ni(OH)2/CNFs heterostructures.
(Mw ¼ 150,000) powder was dissolved in 10 ml of N, N-dimethylformamide (DMF) solvent. After stirring at room temperature for 12 h, the above precursor solution was drawn into a hypodermic syringe for electrospinning. The applied voltage to the syringe tip was about 10 kV and the distance between the syringe tip and the collector was about 10 cm. The as-spun PAN fibers were collected on aluminum foil. For the carbonization process, PAN fiber mats were placed in a tube furnace and stabilized in air for 60 min at 270 C, and then carbonized in nitrogen gas atmosphere at 1000 C at a ramp rate of 5 C min1, and finally cooled down to room temperature. The obtained CNFs were pre-treated with HNO3 (65 wt. %) for 48 h at room temperature, and washed with distilled water to pH ¼ 6, then dried at 70 C. The pretreatment step provided abundant active sites in favor of the formation of heterostructures.
Fabrication of 1D Ni(OH)2/CNFs heterostructures In a typical synthesis of Ni(OH)2/CNFs heterostructures, w15 mg of CNFs were dispersed in 20 ml anhydrous DMF. The suspension with CNFs was heated to 80 C in round bottom flasks with magnetic stirring. After reaching 80 C, 5 ml of 0.2, 0.4 and 0.6 M Ni(Ac)2 aqueous solution was injected into the above suspension, respectively. The mixtures were kept at 80 C with stirring for 1 h. After that, the as-made composites were collected by centrifuge and washed with distilled water, then dispersed in 20 ml of water. The obtained solutions were sealed in 25 ml Teflon lined stainless steel autoclaves for hydrothermal reactions at 180 C for 10 h. The final products were collected by centrifuge, water-washing, and dried at 60 C, labeled as NC1, NC2 and NC3, respectively. Pure Ni(OH)2 nanoplates were synthesized similar to NC1 without the addition of CNFs. The physical mixtures of CNFs and Ni(OH)2 nanoplates were prepared by sonicating pure Ni(OH)2 nanoplates with CNFs in ethanol.
Characterization Field emission scanning electron microscopy (FESEM, FEI Quanta 250 FEG) was used to observe the morphologies of the samples. High-resolution transmission electron microscope (HRTEM) images were acquired using a JEOL JEM-2100 (acceleration voltage: 200 kV). Energy dispersive X-ray (EDX) spectroscopy, being attached to FESEM, was used to analyze the composition of the samples. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-2500 X-ray diffractometer. Fourier transform infrared (FT-IR) spectra were obtained on Magna 560 FT-IR spectrometer with a resolution of 1 cm1. The weight ratio of different components can be obtained through thermogravimetric analysis (TGA). X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB LK II instrument with a Mg Ka-ADES (hn ¼ 1253.6 eV) source at a residual gas pressure of below 108 Pa.
Experimental
Electrochemical measurements
Preparation of carbon nanofibers
The as-prepared 1D Ni(OH)2/CNFs heterostructures were mixed with polytetrafluoroethylene (60 wt.% water suspension, Aldrich) in a ratio of 100:1 by weight and then dispersed in ethanol; the suspension was drop-dried into 1 cm 1 cm
In our experiments, the preparation process of CNFs consisted of two steps. Firstly, 1.5 g of polyacrylonitrile (PAN)
Please cite this article in press as: Miao F, et al., One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
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nickel foam (2 mm thick, 110 ppi, 95% porosity) at 70 C; the foam with sample loaded was compressed before measurements. Electrochemical measurements were carried out on the Electrochemical Analyzer/Workstation (CHI600D chenhua Shanghai) in 1 M KOH solution with a three-electrode configuration. The as-prepared nickel foam electrodes with the samples were used as working electrodes. Ag/AgCl electrode and platinum wire were used as reference electrode and counter electrode, respectively. The cyclic voltammograms were recorded from 0 to 0.5 V at scan rates of 5, 10, 20 and 40 mV/s. The galvanostatic charge and discharge were also studied at the same system.
Results and discussions Material characterization The surface morphologies of CNFs, pure Ni(OH)2 and 1D Ni(OH)2/CNFs heterostructures were investigated by SEM.
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Fig. 1(A) showed that the CNFs exhibited 1D nanostructures with a diameter of 300e400 nm, which allowed a shorter ion diffusion path in favor of improving the rate capability and specific capacitance [30]. Fig. 1(B) represented that the obtained pure Ni(OH)2 by hydrothermal reaction were hexagon nanoplates, which was the typical characteristic of hexagonal crystal system. For 1D Ni(OH)2/CNFs heterostructures (NC1) shown in Fig. 1(C), hexagon Ni(OH)2 nanoplates were uniformly grown on the surfaces of CNFs. However, for NC2 and NC3, the Ni(OH)2 nanoplates aggregated on the surfaces of CNFs with increased Ni(Ac)2 concentrations as shown in Fig. S1. The energy-dispersive X-ray spectrum of NC1 demonstrated that C, O and Ni elements existed in the 1D Ni(OH)2/CNFs heterostructures without other impurity elements. The microstructure of NC1 was also investigated by TEM and HRTEM as shown in Fig. 1(E) and (F). We noted that the heterostructures were retained although they had been ground and ultrasonically dispersed for TEM measurements. It indicated that there might be chemical interactions between Ni(OH)2 nanoplates and CNFs, instead of physical adsorption.
Fig. 1 e SEM images of (A) CNFs, (B) Ni(OH)2, (C) Ni(OH)2/CNFs heterostructures (NC1); (D) EDX spectrum of NC1; (E) TEM and (F) HRTEM images of NC1. Please cite this article in press as: Miao F, et al., One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
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The inset of HRTEM image in Fig. 1(F) showed an interplanar distance of 0.27 nm, which corresponded well to that of the lattice space of (001) for hexagonal crystal system Ni(OH)2 [31]. These results directly demonstrated that the Ni(OH)2/CNFs heterostructures were successfully fabricated. The crystal structures of CNFs, pure Ni(OH)2 and Ni(OH)2/ CNFs heterostructures were studied by XRD as shown in Fig. 2. The bottom curve in Fig. 2 gave the XRD pattern of CNFs which exhibited two broad peaks centered at around 22 and 44 . For the top curve of pure Ni(OH)2, the diffractive peaks, at 2q ¼ 19.2 , 33 , 38.5 , 52.1 , 59 , 62.7 , 70.4 and 72.7 , could be attributed to the diffractions of (001), (100), (101), (102), (110), (111), (103) and (201) planes of hexagonal crystal system Ni(OH)2 (JCPDS card No.3-177), respectively. All diffractive peaks of the 1D Ni(OH)2/CNFs heterostructures (NC1 in Fig. 2, NC2 and NC3 in Fig. S2) could be indexed into Ni(OH)2 and CNFs, and no characteristic peaks of impurities, such as NiO and Ni, were observed, suggesting that the composition of the heterostructures were just Ni(OH)2 and CNFs. The FT-IR spectral analysis was also employed to characterize CNFs, pure Ni(OH)2 and NC1 as illustrated in Fig. 3. For the CNFs, a broad absorption peak at 1200 cm1 (marked with solid diamond) could be ascribed to the CeC stretching vibration. While for pure Ni(OH)2 and NC1, the sharp absorption peak at 3642 and 521 cm1 (marked with solid circles) could be ascribed to the hydroxyl stretching vibration and the hydroxyl stretching mode of non-hydrogen bonds, respectively. The absorption peaks at 1625 cm1 and 3440 cm1 (marked with solid squares) originated from the stretching and bending vibration of adsorbed water. The peak at 1558 cm1 (marked with solid triangle) corresponded to the asymmetric and symmetric stretching band of COOe. Moreover, the lattice vibrations of NieO bonds located at 459 cm1 (marked with solid pentacle) could also be observed in the spectra [32]. And, the FT-IR spectra of NC2 and NC3 were also shown in Fig. S3. The FT-IR results showed that a large number of functional groups formed on the surface of CNFs due to the pretreatment step. The functional groups might serve as the active sites for the growth of Ni(OH)2 nanoplates to form heterostructures, which brought a good stability of the composites. To further investigate the interactions between CNFs and Ni(OH)2, X-ray photoelectron spectroscopy (XPS) was performed. Fig. 4(A) showed the fully scanned spectra of CNFs,
Fig. 3 e FT-IR spectra of CNFs, pure Ni(OH)2, and Ni(OH)2/ CNFs heterostructures (NC1).
pure Ni(OH)2, and Ni(OH)2/CNFs heterostructures (NC1) in the range of 0e1200 eV. It demonstrated that only C, O and Ni elements existed in the 1D Ni(OH)2/CNFs heterostructures. The high resolution XPS spectra of C1s for NC1 and CNFs were shown in Fig. 4(B). The peak at 284.6 eV corresponded to CeC bonds which originated from CNFs. The peaks at 285.9 and 289 eV could be attributed to CeO and OeC]O groups, respectively [33]. It clearly showed that the OeC]O groups nearly disappeared after the growth of Ni(OH)2 nanoplates on CNFs, suggesting that the carboxylic groups might act as nucleation sites during hydrothermal reactions [34,40]. Fig. 4(C) represented the XPS spectra of O1s for CNFs, pure Ni(OH)2, and NC1. For CNFs, the O1s peaks at 531.6 eV, 532.6 eV and 533.3 eV were related to surface hydroxyl groups (OeH), oxygen making single bonds with carbon (CeO) and double bonds with carbon (C]O), which originated from the pretreatment before the growth of Ni(OH)2 [34,35]. After hydrothermal reactions, a new peak at 530.9 eV appeared for NC1. It might be related to NieO bonds. However, for pure Ni(OH)2, the O1s binding energy of NieO bonds at 529.3 eV was much lower than that of NC1. Similar phenomenon could be found for Ni2p of NC1 and pure Ni(OH)2. As shown in Fig. 4(D), the peak of Ni2p for NC1 shifted obviously to a higher binding energy compared to that for pure Ni(OH)2 [36]. Therefore, it further suggested that the interactions between CNFs and Ni(OH)2 nanoplates was not just Van der Waals interactions but complex covalent chemical bonds affording facile electrons transfer between CNFs and Ni(OH)2 nanoplates, which was a key factor to improve the electrochemical properties of 1D Ni(OH)2/CNFs heterostructures for supercapacitors.
Electrochemical properties
Fig. 2 e XRD patterns of CNFs, pure Ni(OH)2, and Ni(OH)2/ CNFs heterostructures (NC1).
The cyclic voltammetry (CV) and the galvanostatic chargedischarge (GCD) were used to demonstrate the capacitive behavior of 1D Ni(OH)2/CNFs heterostructures. Fig. 5(A) showed the CV curves of NC1 at different scan rates from 5 to 40 mV/s. A pair of redox peaks were observed, which corresponded to the reversible reactions of Ni(Ⅱ) 4 Ni(Ⅲ). The redox peaks gradually widened with increased scan rates, reflecting the ideal capacitive behavior of NC1 [37,38]. Therefore, 1D Ni(OH)2/CNFs heterostructures exhibited primary
Please cite this article in press as: Miao F, et al., One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
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Fig. 4 e (A) XPS fully scanned spectra of CNFs, pure Ni(OH)2, and Ni(OH)2/CNFs heterostructures (NC1); XPS spectra of (B) C1s for NC1 and CNFs, (C) O1s for CNFs, pure Ni(OH)2, and NC1, and (D) Ni2p for NC1 and pure Ni(OH)2.
pseudocapacitive behaviors. The average specific capacitance values were calculated from the CV curves according to the following equation: Z C¼
Idt =ðmDVÞ
where the I was the oxidation or reduction current, dt was the time differential, m indicated the mass of the active electrode materials, and DV indicated the voltage range of one sweep segment. As shown in Fig. 5(B), the average specific capacitance of NC1 was calculated to be w115 F/g based on total sample mass and w1050 F/g based on the mass of Ni(OH)2 (very close to the reported value 1267 F/g) [40] at a scan rate of 5 mV/s. At a high scan rate of 40 mV/s, the calculated specific capacitance of NC1 was 76 F/g, w66% of that at 5 mV/s. We also measured the background signal from the Ni foam, and it could be neglected in the present study as shown in Fig. S4. Fig. 5(C) showed galvanostatic discharge curves of NC1 at different current densities. The specific capacitance values were evaluated from discharge curves, according to the equation: C ¼ ðIDtÞ=ðmDVÞ where I, Dt, m, DV represented the discharge currents, the time for a full discharge, the mass of the active materials, and the voltage change after a full discharge, respectively. Fig. 5(D) gave the calculated average specific capacitances at different current densities. The specific capacitance of NC1 was w76.5 F/g (based on total sample mass, w696 F/g based on the mass of Ni(OH)2) at a charge and discharge current density of 4 A/g. The specific capacitance was still as high as 29.7 F/g even at a high charge and discharge current density of 35 A/g.
The cycle ability of NC1 was evaluated by constant current charge-discharge test at a current density of 12.5 A/g as illustrated in Fig. 5(E). The specific capacitance retention was w90% after 500 cycles (Fig. 5(F)). Furthermore, the capacitive behaviors of CNFs, pure Ni(OH)2, and physical mixture of CNFs and Ni(OH)2 were also investigated to understand the capacitive performance of 1D Ni(OH)2/CNFs heterostructures. We noted that CNFs and pure Ni(OH)2 nanoplates showed much lower specific capacitances and inferior rate capabilities, as revealed by CV and GCD measurements and showed in Fig. S5 and Fig. 6, respectively. Moreover, the IR drop for pure Ni(OH)2 in Fig. 6(C) was w0.12 V, indicating the large internal resistance of pure Ni(OH)2. However, the IR drop for NC1 could be negligible (Fig. 5(C)). Thus, the CNFs could significantly improve the electrical conductivity of Ni(OH)2 in the form of heterostructures. It is known that low internal resistance was of great importance in energy storing devices, as less energy was wasted [39]. Therefore, 1D Ni(OH)2/CNFs heterostructures might be much potential for the new hybrid supercapacitors. The physical mixtures of CNFs and Ni(OH)2 were prepared by sonicating pure Ni(OH)2 nanoplates and pure CNFs in ethanol in the weight ratio of CN1 (w11% for the mass percentages of Ni(OH)2 in the composites, obtained from thermogravimetric analysis in Fig. S6). And, the average specific capacitance values were w49 F/g and w33 F/g at scan rates of 5 mV/s and 40 mV/s from CV measurements shown in Fig. 7(A) and (B), and w46.8 F/g and w29 F/g at the discharge current densities of 1 A/g and 8 A/g from GCD measurements shown in Fig. 7(C) and (D), respectively. Obviously, the specific capacitance of the physical mixtures was much lower than that of NC1 (115 F/g at the scan rate of 5 mV/s). Therefore, for NC1, the good electrical contacts
Please cite this article in press as: Miao F, et al., One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
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Fig. 5 e Electrochemical characterizations of Ni(OH)2/CNFs heterostructures (NC1): (A) CV curves and (B) the calculated average specific capacitances at different scan rates; (C) Galvanostatic discharge curves and (D) the calculated average specific capacitances at different current densities; (E) Galvanostatic charge and discharge curves and (F) the calculated average specific capacitances versus cycle number at a current density of 12.5 A/g.
by chemical interactions and uniformly distribution of Ni(OH)2 by the growth method might also contribute to the higher specific capacitance. The above results revealed that the 1D Ni(OH)2/CNFs heterostructures possessed higher specific capacitance and more remarkable rate capability as electrode materials than CNFs, pure Ni(OH)2 and the physical mixtures of CNFs and Ni(OH)2, which could be ascribed to (a) the good conductivity of CNFs which decreased the internal resistance of Ni(OH)2 and enhanced the electrons transport, (b) the high specific pseudo-capacitance of Ni(OH)2 relying on faradic redox reactions, and (c) the chemical interactions between the Ni(OH)2 nanoplates and CNFs which served as “bridges” in favor of rapid charges transferring between Ni(OH)2 and CNFs and good structure stabilities [40]. We also investigated the influence of the loading percentages of Ni(OH)2 on CNFs on the capacitive properties of 1D Ni(OH)2/CNFs heterostructures as shown in Fig. S7 and S8. The specific capacitances decreased obviously along with increasing the percentages of Ni(OH)2, and NC1 presented the highest specific capacitance and best rate capability among all the samples. From the SEM (Fig. S1) and XRD (Fig. S2) results, the crystal size and aggregation of Ni(OH)2 nanoplates were both
increased with increasing the precursor concentrations of Ni(Ac)2. Therefore, the internal resistance of 1D Ni(OH)2/CNFs heterostructures was increased, resulting the inferior capacitive properties.
Conclusions In summary, 1D Ni(OH)2/CNFs heterostructures with potentially high specific capacitance and excellent cycling stability have been fabricated by using a two-step method of electrospinning technology and hydrothermal reaction. Hexagon Ni(OH)2 nanoplates were uniformly grown on the surface of CNFs. The chemical interactions between Ni(OH)2 and CNFs not only brought good structure stability but also contributed to the electron transport during the electrochemical reactions. Therefore, the good conductivity of CNFs, the high specific pseudo-capacitance of Ni(OH)2, and the chemical interactions between the Ni(OH)2 nanoplates and CNFs resulted a higher specific capacitance and more remarkable rate capability for the 1D Ni(OH)2/CNFs heterostructures compared with CNFs, pure Ni(OH)2 and the physical mixtures of CNFs and Ni(OH)2. The specific capacitance and rate capability of 1D Ni(OH)2/
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Fig. 6 e Electrochemical characterizations of pure Ni(OH)2: (A) CV curves and (B) the calculated average specific capacitances at different scan rates; (C) Galvanostatic discharge curves and (D) the calculated average specific capacitances at different current densities.
CNFs heterostructures could be further improved by optimizing the physical and chemical properties of Ni(OH)2 and CNFs. Therefore, the 1D Ni(OH)2/CNFs heterostructures can be a prominent candidate as electrode materials of
supercapacitors for energy storage. Moreover, it is highly desirable to achieve the heterostructures based on flexible CNFs sheets which could serve as electrodes of supercapacitors directly.
Fig. 7 e Electrochemical characterizations of physical mixtures of CNFs and pure Ni(OH)2: (A) CV curves and (B) the calculated average specific capacitances at different scan rates; (C) Galvanostatic discharge curves and (D) the calculated average specific capacitances at different current densities. Please cite this article in press as: Miao F, et al., One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
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Acknowledgments [16]
The present work is supported financially by the National Basic Research Program of China (973 Program) (Grant No. 2012CB933703), the National Natural Science Foundation of China (No. 91233204, 51272041, 11304035, and 61201107), the 111 Project (No. B13013), the Fundamental Research Funds for the Central Universities (12SSXM001), and the Program for Young Scientists Team of Jilin Province (20121802).
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Appendix A. Supplementary data [19]
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.02.008.
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One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property Fujun Miao, Changlu Shao*, Xinghua Li*, Yang Zhang, Na Lu, Kexin Wang, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China
article info
abstract
Article history:
Ultra-long one-dimensional (1D) electrospun carbon nanofibers (CNFs) were obtained by
Received 27 September 2013
carbonization treatment of polyacrylonitrile fibers at 1000 C in nitrogen atmosphere.
Received in revised form
Using CNFs as templates, beta-nickel hydroxide (Ni(OH)2) nanoplates were prepared on
30 January 2014
CNFs by hydrothermal reactions. The contents of Ni(OH)2 in the 1D Ni(OH)2/CNFs hetero-
Accepted 3 February 2014
structures could be controlled by adjusting the concentrations of Ni(Ac)2 precursors. The
Available online xxx
1D heterostructures as electrode materials of supercapacitors exhibited high specific
Keywords:
and Ni(OH)2 nanoplates. The excellent properties of the heterostructures could be ascribed
b-Ni(OH)2
to the rapid electron transport along the longitude direction of CNFs, the high specific
Carbon nanofibers
pseudo-capacitance of Ni(OH)2 nanoplates, and the good electrical contacts between CNFs
capacitance compared with CNFs, pure Ni(OH)2 nanoplates, and physical mixtures of CNFs
Electrospinning technique
and Ni(OH)2 nanoplates. And, the specific capacitance retention of the heterostructures
Heterostructures
was over 90% after 500 cycles of charge and discharge at a high current density of 12.5 A/g,
Supercapacitors
suggesting their good cycling stability. It was expected that the 1D Ni(OH)2/CNFs heterostructures as electrode materials with excellent capacitive properties would greatly promote their practical applications to the energy storage for supercapacitors. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, electrochemical capacitors, also known as supercapacitors, have attracted more and more attentions due to their super energy storage performances, such as fast charging and discharging, environment friendly, high power densities, long cycle life, low maintenance cost, etc. [1e6].
Supercapacitors are kinds of energy storage devices between traditional capacitors and batteries, which can be classified as electric double-layer capacitors (EDLCs) and faradaic pseudocapacitors depending on their energy storage mechanisms. For EDLCs, energy storage arises from the accumulation of electrons and ionic charges at the interface between the electrode materials and the electrolyte solution. Carbon materials are usually used as electrode materials of EDLCs because of their
* Corresponding authors. Tel.: þ86 43185098803. E-mail addresses: [email protected] (C. Shao), [email protected] (X. Li). 0360-3199/$ e see front matter Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
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good electrical conductivity, physical and chemical stability [7,8]. Moreover, carbon nanomaterials with large specific surface areas including ultrafine activated carbon [9,10], carbon nanotubes [11], graphene [12], and carbon nanofibers [13] can facilitate the rapid transport of the electrolyte ions from bulk solution to the surface of electrode which is crucial to high specific capacitance and power density supercapacitors. Among numerous carbon nanomaterials, one-dimensional (1D) carbon nanofibers are considered as one of the promising candidates for the electrode materials of supercapacitors. The 1D carbon nanofibers can be prepared by physical methods, chemical methods and electrospinning technique [13,14]. Notably, electrospun carbon nanofibers (CNFs) possess not only unique ultra-long 1D nanostructure but also three dimensional porous networks, which benefits for both electron transport along the longitude directions and constructing the electrodes for supercapacitor devices. To improve the properties of CNFs for supercapacitors with high specific capacitance and energy density, many strategies have been proposed, such as increasing the surface areas by controlling the diameter or pore size of CNFs [15], surface modification of CNFs [16], or constructing composite structures with transitional metal oxides or hydroxides [17]. Among these strategies, constructing composite structures is attractive because it may combine the high power density from CNFs and high energy density from pseudocapacitive materials and bring new hybrid supercapacitors of EDLCs and pseudocapacitors [18]. Till now, many oxides, such as MnO2 [19,20], hydrous RuO2 [21], SnO2 [22] and cobalt oxide [23] have been studied to develop the new hybrid supercapacitors based on CNFs. Among various pseudocapacitive electrode materials, nickel hydroxide (Ni(OH)2) is very attractive due to its high specific capacitance, low cost, and readily available [24e27]. However, as far as we know, nickel hydroxide/CNFs composite materials for the new hybrid supercapacitors have not been reported yet. In the present work, we designed the direct fabrication of hybrid nanostructures by integrating Ni(OH)2 with CNFs, where CNFs served as both the conductivity networks and the scaffold/support which decreased the internal resistance and improved the cycling stability [28,29] and Ni(OH)2 nanoplates provided high specific capacitance relying on faradic redox reactions. The 1D beta-nickel hydroxide nanoplates/electrospun carbon nanofibers (Ni(OH)2/CNFs) heterostructures have been fabricated successfully by hydrothermal reactions. Electrochemical studies demonstrated that 1D Ni(OH)2/CNFs heterostructures possessed high specific capacitance and excellent cycling stability as electrodes for supercapacitors, compared with pure CNFs, pure Ni(OH)2 nanoplates, and physical mixtures of CNFs and Ni(OH)2. Our results suggested that the low specific capacitances of CNFs and the intrinsic insulating properties of Ni(OH)2 could be solved by constructing 1D Ni(OH)2/CNFs heterostructures.
(Mw ¼ 150,000) powder was dissolved in 10 ml of N, N-dimethylformamide (DMF) solvent. After stirring at room temperature for 12 h, the above precursor solution was drawn into a hypodermic syringe for electrospinning. The applied voltage to the syringe tip was about 10 kV and the distance between the syringe tip and the collector was about 10 cm. The as-spun PAN fibers were collected on aluminum foil. For the carbonization process, PAN fiber mats were placed in a tube furnace and stabilized in air for 60 min at 270 C, and then carbonized in nitrogen gas atmosphere at 1000 C at a ramp rate of 5 C min1, and finally cooled down to room temperature. The obtained CNFs were pre-treated with HNO3 (65 wt. %) for 48 h at room temperature, and washed with distilled water to pH ¼ 6, then dried at 70 C. The pretreatment step provided abundant active sites in favor of the formation of heterostructures.
Fabrication of 1D Ni(OH)2/CNFs heterostructures In a typical synthesis of Ni(OH)2/CNFs heterostructures, w15 mg of CNFs were dispersed in 20 ml anhydrous DMF. The suspension with CNFs was heated to 80 C in round bottom flasks with magnetic stirring. After reaching 80 C, 5 ml of 0.2, 0.4 and 0.6 M Ni(Ac)2 aqueous solution was injected into the above suspension, respectively. The mixtures were kept at 80 C with stirring for 1 h. After that, the as-made composites were collected by centrifuge and washed with distilled water, then dispersed in 20 ml of water. The obtained solutions were sealed in 25 ml Teflon lined stainless steel autoclaves for hydrothermal reactions at 180 C for 10 h. The final products were collected by centrifuge, water-washing, and dried at 60 C, labeled as NC1, NC2 and NC3, respectively. Pure Ni(OH)2 nanoplates were synthesized similar to NC1 without the addition of CNFs. The physical mixtures of CNFs and Ni(OH)2 nanoplates were prepared by sonicating pure Ni(OH)2 nanoplates with CNFs in ethanol.
Characterization Field emission scanning electron microscopy (FESEM, FEI Quanta 250 FEG) was used to observe the morphologies of the samples. High-resolution transmission electron microscope (HRTEM) images were acquired using a JEOL JEM-2100 (acceleration voltage: 200 kV). Energy dispersive X-ray (EDX) spectroscopy, being attached to FESEM, was used to analyze the composition of the samples. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-2500 X-ray diffractometer. Fourier transform infrared (FT-IR) spectra were obtained on Magna 560 FT-IR spectrometer with a resolution of 1 cm1. The weight ratio of different components can be obtained through thermogravimetric analysis (TGA). X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB LK II instrument with a Mg Ka-ADES (hn ¼ 1253.6 eV) source at a residual gas pressure of below 108 Pa.
Experimental
Electrochemical measurements
Preparation of carbon nanofibers
The as-prepared 1D Ni(OH)2/CNFs heterostructures were mixed with polytetrafluoroethylene (60 wt.% water suspension, Aldrich) in a ratio of 100:1 by weight and then dispersed in ethanol; the suspension was drop-dried into 1 cm 1 cm
In our experiments, the preparation process of CNFs consisted of two steps. Firstly, 1.5 g of polyacrylonitrile (PAN)
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nickel foam (2 mm thick, 110 ppi, 95% porosity) at 70 C; the foam with sample loaded was compressed before measurements. Electrochemical measurements were carried out on the Electrochemical Analyzer/Workstation (CHI600D chenhua Shanghai) in 1 M KOH solution with a three-electrode configuration. The as-prepared nickel foam electrodes with the samples were used as working electrodes. Ag/AgCl electrode and platinum wire were used as reference electrode and counter electrode, respectively. The cyclic voltammograms were recorded from 0 to 0.5 V at scan rates of 5, 10, 20 and 40 mV/s. The galvanostatic charge and discharge were also studied at the same system.
Results and discussions Material characterization The surface morphologies of CNFs, pure Ni(OH)2 and 1D Ni(OH)2/CNFs heterostructures were investigated by SEM.
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Fig. 1(A) showed that the CNFs exhibited 1D nanostructures with a diameter of 300e400 nm, which allowed a shorter ion diffusion path in favor of improving the rate capability and specific capacitance [30]. Fig. 1(B) represented that the obtained pure Ni(OH)2 by hydrothermal reaction were hexagon nanoplates, which was the typical characteristic of hexagonal crystal system. For 1D Ni(OH)2/CNFs heterostructures (NC1) shown in Fig. 1(C), hexagon Ni(OH)2 nanoplates were uniformly grown on the surfaces of CNFs. However, for NC2 and NC3, the Ni(OH)2 nanoplates aggregated on the surfaces of CNFs with increased Ni(Ac)2 concentrations as shown in Fig. S1. The energy-dispersive X-ray spectrum of NC1 demonstrated that C, O and Ni elements existed in the 1D Ni(OH)2/CNFs heterostructures without other impurity elements. The microstructure of NC1 was also investigated by TEM and HRTEM as shown in Fig. 1(E) and (F). We noted that the heterostructures were retained although they had been ground and ultrasonically dispersed for TEM measurements. It indicated that there might be chemical interactions between Ni(OH)2 nanoplates and CNFs, instead of physical adsorption.
Fig. 1 e SEM images of (A) CNFs, (B) Ni(OH)2, (C) Ni(OH)2/CNFs heterostructures (NC1); (D) EDX spectrum of NC1; (E) TEM and (F) HRTEM images of NC1. Please cite this article in press as: Miao F, et al., One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
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The inset of HRTEM image in Fig. 1(F) showed an interplanar distance of 0.27 nm, which corresponded well to that of the lattice space of (001) for hexagonal crystal system Ni(OH)2 [31]. These results directly demonstrated that the Ni(OH)2/CNFs heterostructures were successfully fabricated. The crystal structures of CNFs, pure Ni(OH)2 and Ni(OH)2/ CNFs heterostructures were studied by XRD as shown in Fig. 2. The bottom curve in Fig. 2 gave the XRD pattern of CNFs which exhibited two broad peaks centered at around 22 and 44 . For the top curve of pure Ni(OH)2, the diffractive peaks, at 2q ¼ 19.2 , 33 , 38.5 , 52.1 , 59 , 62.7 , 70.4 and 72.7 , could be attributed to the diffractions of (001), (100), (101), (102), (110), (111), (103) and (201) planes of hexagonal crystal system Ni(OH)2 (JCPDS card No.3-177), respectively. All diffractive peaks of the 1D Ni(OH)2/CNFs heterostructures (NC1 in Fig. 2, NC2 and NC3 in Fig. S2) could be indexed into Ni(OH)2 and CNFs, and no characteristic peaks of impurities, such as NiO and Ni, were observed, suggesting that the composition of the heterostructures were just Ni(OH)2 and CNFs. The FT-IR spectral analysis was also employed to characterize CNFs, pure Ni(OH)2 and NC1 as illustrated in Fig. 3. For the CNFs, a broad absorption peak at 1200 cm1 (marked with solid diamond) could be ascribed to the CeC stretching vibration. While for pure Ni(OH)2 and NC1, the sharp absorption peak at 3642 and 521 cm1 (marked with solid circles) could be ascribed to the hydroxyl stretching vibration and the hydroxyl stretching mode of non-hydrogen bonds, respectively. The absorption peaks at 1625 cm1 and 3440 cm1 (marked with solid squares) originated from the stretching and bending vibration of adsorbed water. The peak at 1558 cm1 (marked with solid triangle) corresponded to the asymmetric and symmetric stretching band of COOe. Moreover, the lattice vibrations of NieO bonds located at 459 cm1 (marked with solid pentacle) could also be observed in the spectra [32]. And, the FT-IR spectra of NC2 and NC3 were also shown in Fig. S3. The FT-IR results showed that a large number of functional groups formed on the surface of CNFs due to the pretreatment step. The functional groups might serve as the active sites for the growth of Ni(OH)2 nanoplates to form heterostructures, which brought a good stability of the composites. To further investigate the interactions between CNFs and Ni(OH)2, X-ray photoelectron spectroscopy (XPS) was performed. Fig. 4(A) showed the fully scanned spectra of CNFs,
Fig. 3 e FT-IR spectra of CNFs, pure Ni(OH)2, and Ni(OH)2/ CNFs heterostructures (NC1).
pure Ni(OH)2, and Ni(OH)2/CNFs heterostructures (NC1) in the range of 0e1200 eV. It demonstrated that only C, O and Ni elements existed in the 1D Ni(OH)2/CNFs heterostructures. The high resolution XPS spectra of C1s for NC1 and CNFs were shown in Fig. 4(B). The peak at 284.6 eV corresponded to CeC bonds which originated from CNFs. The peaks at 285.9 and 289 eV could be attributed to CeO and OeC]O groups, respectively [33]. It clearly showed that the OeC]O groups nearly disappeared after the growth of Ni(OH)2 nanoplates on CNFs, suggesting that the carboxylic groups might act as nucleation sites during hydrothermal reactions [34,40]. Fig. 4(C) represented the XPS spectra of O1s for CNFs, pure Ni(OH)2, and NC1. For CNFs, the O1s peaks at 531.6 eV, 532.6 eV and 533.3 eV were related to surface hydroxyl groups (OeH), oxygen making single bonds with carbon (CeO) and double bonds with carbon (C]O), which originated from the pretreatment before the growth of Ni(OH)2 [34,35]. After hydrothermal reactions, a new peak at 530.9 eV appeared for NC1. It might be related to NieO bonds. However, for pure Ni(OH)2, the O1s binding energy of NieO bonds at 529.3 eV was much lower than that of NC1. Similar phenomenon could be found for Ni2p of NC1 and pure Ni(OH)2. As shown in Fig. 4(D), the peak of Ni2p for NC1 shifted obviously to a higher binding energy compared to that for pure Ni(OH)2 [36]. Therefore, it further suggested that the interactions between CNFs and Ni(OH)2 nanoplates was not just Van der Waals interactions but complex covalent chemical bonds affording facile electrons transfer between CNFs and Ni(OH)2 nanoplates, which was a key factor to improve the electrochemical properties of 1D Ni(OH)2/CNFs heterostructures for supercapacitors.
Electrochemical properties
Fig. 2 e XRD patterns of CNFs, pure Ni(OH)2, and Ni(OH)2/ CNFs heterostructures (NC1).
The cyclic voltammetry (CV) and the galvanostatic chargedischarge (GCD) were used to demonstrate the capacitive behavior of 1D Ni(OH)2/CNFs heterostructures. Fig. 5(A) showed the CV curves of NC1 at different scan rates from 5 to 40 mV/s. A pair of redox peaks were observed, which corresponded to the reversible reactions of Ni(Ⅱ) 4 Ni(Ⅲ). The redox peaks gradually widened with increased scan rates, reflecting the ideal capacitive behavior of NC1 [37,38]. Therefore, 1D Ni(OH)2/CNFs heterostructures exhibited primary
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Fig. 4 e (A) XPS fully scanned spectra of CNFs, pure Ni(OH)2, and Ni(OH)2/CNFs heterostructures (NC1); XPS spectra of (B) C1s for NC1 and CNFs, (C) O1s for CNFs, pure Ni(OH)2, and NC1, and (D) Ni2p for NC1 and pure Ni(OH)2.
pseudocapacitive behaviors. The average specific capacitance values were calculated from the CV curves according to the following equation: Z C¼
Idt =ðmDVÞ
where the I was the oxidation or reduction current, dt was the time differential, m indicated the mass of the active electrode materials, and DV indicated the voltage range of one sweep segment. As shown in Fig. 5(B), the average specific capacitance of NC1 was calculated to be w115 F/g based on total sample mass and w1050 F/g based on the mass of Ni(OH)2 (very close to the reported value 1267 F/g) [40] at a scan rate of 5 mV/s. At a high scan rate of 40 mV/s, the calculated specific capacitance of NC1 was 76 F/g, w66% of that at 5 mV/s. We also measured the background signal from the Ni foam, and it could be neglected in the present study as shown in Fig. S4. Fig. 5(C) showed galvanostatic discharge curves of NC1 at different current densities. The specific capacitance values were evaluated from discharge curves, according to the equation: C ¼ ðIDtÞ=ðmDVÞ where I, Dt, m, DV represented the discharge currents, the time for a full discharge, the mass of the active materials, and the voltage change after a full discharge, respectively. Fig. 5(D) gave the calculated average specific capacitances at different current densities. The specific capacitance of NC1 was w76.5 F/g (based on total sample mass, w696 F/g based on the mass of Ni(OH)2) at a charge and discharge current density of 4 A/g. The specific capacitance was still as high as 29.7 F/g even at a high charge and discharge current density of 35 A/g.
The cycle ability of NC1 was evaluated by constant current charge-discharge test at a current density of 12.5 A/g as illustrated in Fig. 5(E). The specific capacitance retention was w90% after 500 cycles (Fig. 5(F)). Furthermore, the capacitive behaviors of CNFs, pure Ni(OH)2, and physical mixture of CNFs and Ni(OH)2 were also investigated to understand the capacitive performance of 1D Ni(OH)2/CNFs heterostructures. We noted that CNFs and pure Ni(OH)2 nanoplates showed much lower specific capacitances and inferior rate capabilities, as revealed by CV and GCD measurements and showed in Fig. S5 and Fig. 6, respectively. Moreover, the IR drop for pure Ni(OH)2 in Fig. 6(C) was w0.12 V, indicating the large internal resistance of pure Ni(OH)2. However, the IR drop for NC1 could be negligible (Fig. 5(C)). Thus, the CNFs could significantly improve the electrical conductivity of Ni(OH)2 in the form of heterostructures. It is known that low internal resistance was of great importance in energy storing devices, as less energy was wasted [39]. Therefore, 1D Ni(OH)2/CNFs heterostructures might be much potential for the new hybrid supercapacitors. The physical mixtures of CNFs and Ni(OH)2 were prepared by sonicating pure Ni(OH)2 nanoplates and pure CNFs in ethanol in the weight ratio of CN1 (w11% for the mass percentages of Ni(OH)2 in the composites, obtained from thermogravimetric analysis in Fig. S6). And, the average specific capacitance values were w49 F/g and w33 F/g at scan rates of 5 mV/s and 40 mV/s from CV measurements shown in Fig. 7(A) and (B), and w46.8 F/g and w29 F/g at the discharge current densities of 1 A/g and 8 A/g from GCD measurements shown in Fig. 7(C) and (D), respectively. Obviously, the specific capacitance of the physical mixtures was much lower than that of NC1 (115 F/g at the scan rate of 5 mV/s). Therefore, for NC1, the good electrical contacts
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Fig. 5 e Electrochemical characterizations of Ni(OH)2/CNFs heterostructures (NC1): (A) CV curves and (B) the calculated average specific capacitances at different scan rates; (C) Galvanostatic discharge curves and (D) the calculated average specific capacitances at different current densities; (E) Galvanostatic charge and discharge curves and (F) the calculated average specific capacitances versus cycle number at a current density of 12.5 A/g.
by chemical interactions and uniformly distribution of Ni(OH)2 by the growth method might also contribute to the higher specific capacitance. The above results revealed that the 1D Ni(OH)2/CNFs heterostructures possessed higher specific capacitance and more remarkable rate capability as electrode materials than CNFs, pure Ni(OH)2 and the physical mixtures of CNFs and Ni(OH)2, which could be ascribed to (a) the good conductivity of CNFs which decreased the internal resistance of Ni(OH)2 and enhanced the electrons transport, (b) the high specific pseudo-capacitance of Ni(OH)2 relying on faradic redox reactions, and (c) the chemical interactions between the Ni(OH)2 nanoplates and CNFs which served as “bridges” in favor of rapid charges transferring between Ni(OH)2 and CNFs and good structure stabilities [40]. We also investigated the influence of the loading percentages of Ni(OH)2 on CNFs on the capacitive properties of 1D Ni(OH)2/CNFs heterostructures as shown in Fig. S7 and S8. The specific capacitances decreased obviously along with increasing the percentages of Ni(OH)2, and NC1 presented the highest specific capacitance and best rate capability among all the samples. From the SEM (Fig. S1) and XRD (Fig. S2) results, the crystal size and aggregation of Ni(OH)2 nanoplates were both
increased with increasing the precursor concentrations of Ni(Ac)2. Therefore, the internal resistance of 1D Ni(OH)2/CNFs heterostructures was increased, resulting the inferior capacitive properties.
Conclusions In summary, 1D Ni(OH)2/CNFs heterostructures with potentially high specific capacitance and excellent cycling stability have been fabricated by using a two-step method of electrospinning technology and hydrothermal reaction. Hexagon Ni(OH)2 nanoplates were uniformly grown on the surface of CNFs. The chemical interactions between Ni(OH)2 and CNFs not only brought good structure stability but also contributed to the electron transport during the electrochemical reactions. Therefore, the good conductivity of CNFs, the high specific pseudo-capacitance of Ni(OH)2, and the chemical interactions between the Ni(OH)2 nanoplates and CNFs resulted a higher specific capacitance and more remarkable rate capability for the 1D Ni(OH)2/CNFs heterostructures compared with CNFs, pure Ni(OH)2 and the physical mixtures of CNFs and Ni(OH)2. The specific capacitance and rate capability of 1D Ni(OH)2/
Please cite this article in press as: Miao F, et al., One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
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Fig. 6 e Electrochemical characterizations of pure Ni(OH)2: (A) CV curves and (B) the calculated average specific capacitances at different scan rates; (C) Galvanostatic discharge curves and (D) the calculated average specific capacitances at different current densities.
CNFs heterostructures could be further improved by optimizing the physical and chemical properties of Ni(OH)2 and CNFs. Therefore, the 1D Ni(OH)2/CNFs heterostructures can be a prominent candidate as electrode materials of
supercapacitors for energy storage. Moreover, it is highly desirable to achieve the heterostructures based on flexible CNFs sheets which could serve as electrodes of supercapacitors directly.
Fig. 7 e Electrochemical characterizations of physical mixtures of CNFs and pure Ni(OH)2: (A) CV curves and (B) the calculated average specific capacitances at different scan rates; (C) Galvanostatic discharge curves and (D) the calculated average specific capacitances at different current densities. Please cite this article in press as: Miao F, et al., One-dimensional heterostructures of beta-nickel hydroxide nanoplates/electrospun carbon nanofibers: Controlled fabrication and high capacitive property, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.02.008
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Acknowledgments [16]
The present work is supported financially by the National Basic Research Program of China (973 Program) (Grant No. 2012CB933703), the National Natural Science Foundation of China (No. 91233204, 51272041, 11304035, and 61201107), the 111 Project (No. B13013), the Fundamental Research Funds for the Central Universities (12SSXM001), and the Program for Young Scientists Team of Jilin Province (20121802).
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Appendix A. Supplementary data [19]
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.02.008.
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