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carbon nanocomposites for high-performance and long-life supercapacitors
Journal Pre-proof Hydrothermal synthesis of coralloid-like vanadium nitride/carbon nanocomposites for high-performance and long-life supercapacitors Qingqing Sun, Yan Lv, Xueyan Wu, Wei Jia, Jixi Guo, Fenglian Tong, Dianzeng Jia, Zhipeng Sun, Xingchao Wang PII:
S0925-8388(19)34141-6
DOI:
https://doi.org/10.1016/j.jallcom.2019.152895
Reference:
JALCOM 152895
To appear in:
Journal of Alloys and Compounds
Received Date: 5 September 2019 Revised Date:
31 October 2019
Accepted Date: 1 November 2019
Please cite this article as: Q. Sun, Y. Lv, X. Wu, W. Jia, J. Guo, F. Tong, D. Jia, Z. Sun, X. Wang, Hydrothermal synthesis of coralloid-like vanadium nitride/carbon nanocomposites for high-performance and long-life supercapacitors, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152895. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
A facile method is designed to obtain coralloid-like vanadium nitride/carbon nanocomposites for high-performance and long-life supercapacitors.
Hydrothermal synthesis of coralloid-like vanadium nitride/carbon nanocomposites for high-performance and long-life supercapacitors Qingqing Sun, Yan Lv, Xueyan Wu, Wei Jia, Jixi Guo,* Fenglian Tong, Dianzeng Jia* Zhipeng Sun, Xingchao Wang Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key Laboratory of Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Urumqi, 830046, Xinjiang P. R. China. Email: [email protected]; [email protected]; Fax: +86-991-8588883; Tel: +86-991-8583083.
Abstract Vanadium nitride is highly desirable for their good electrochemical performance and wide work potential window, but they are cramped practical applications in supercapacitors due to poor cycling stability. Herein a combined strategy is exploited to fabricate coralloid-like nanostructure vanadium nitride/carbon (VN/C) via a facile hydrothermal method. The structural characterization displayed that nano-VN was uniformly distributed in the carbon scaffold materials to improve the electrode behavior. Those composites exhibited a wide voltage window of −1.1 to 0 V with a specific capacitance of 385 F g−1 at 1 A g−1, and exceptional cycling stability (88.9% retention after 10000 cycles). The two-electrode system based on VN/C electrodes obtains a high energy density of 24.2 Wh kg−1 at a power density of 1050 W kg−1. The device also shows perfect cycling stability with 93.3% capacitance retention after 10000 cycles. The coralloid-like VN/C electrode materials can be used in practical applications of the high-performance energy storage device.
Keywords Vanadium nitride/Carbon, Supercapacitors, Electrode materials, Long cycle life
1. Introduction The clean and renewable energy has become an increasing interest in many
countries as the environmental costs and limited supply of fossil fuels are clearly understood. It has enormous internal demand for energy storage devices because of an efficient using of clean and renewable energy [1-4]. The supercapacitor is one of the most potential energy storage devices in current markets, which has many advantages such as high-power density, fast charge-discharge capability, and good cycling stability compared to other secondary batteries [5]. Therefore, supercapacitors, which have high power and energy density, have been attracted increasing attention in the field of electrochemical energy storage devices [6, 7]. According to the electric charge storage mechanism, supercapacitors can be typically classified into electrical double-layer capacitors (EDLCs) and pseudocapacitors (PCs) [8, 9]. EDLCs generally store energy by the electrostatic accumulation of charges at the electrode/electrolyte interface. Carbon materials as the typical electrode materials for EDLCs have long cycle life, but it was still constrained by the poor theoretical specific capacitance [10-14]. PCs chemically store charge through the Faradaic reactions of redox species coated on the electrode surface. Most electrode materials show excellent pseudocapacitance and a wide electrochemically potential window, such as transition metal oxides [15-20], transition metal nitrides [21-24] and transition metal sulfides [25-30]. Among transition metal nitrides, vanadium nitride (VN) has attracted considerable
attention
owing
to
its
large
pseudocapacitance
and
wide
electrochemically potential window [31, 32]. However, these reported vanadium nitride materials generally have poor cycle life because of electrochemical oxidation in alkaline media [33, 34]. To increase cycling stability and energy storage, combining vanadium nitride with carbon-based materials is an effective strategy [12, 35, 36]. However, it is still a challenge how the nano-VN was uniformly distributed in carbon materials to manufacture the efficient composite electrode material, which can improve the electrochemical stability and the electrical conductivity of the electrode material [37, 38]. Ran et al. synthesized nano vanadium nitride/interconnected porous carbon with a specific capacitance of 284.0 F g−1 [39], but intricate fabrication methods limit its application. Liu et al. reported carbon nanosphere@vanadium nitride with a maximum specific capacitance of 300.4 F g−1 at the current density of 1.0 A g−1
[40], but its cyclic stability is poor. Recently, He fabricated VN nanoparticle/GO composites by electrochemical surface-initiated atom transfer radical polymerization method, which showed capacitance above 109 F g−1 in 2 M KOH [41]. However, the synthesis of vanadium nitride/carbon nanocomposites was greatly limited due to its intricate fabrication methods and low capacitance. Herein we highlighted the coralloid-like nanostructure vanadium nitride/carbon electrode materials for high-capacitance supercapacitors, which were synthesized by a facile hydrothermal reaction. Synergistic effect of VN and carbon significantly improves the performance of the negative electrode materials to ensure the electrochemical behavior of the charge-discharge process goes smoothly. The well-dispersed nano-VN and pillared coralloid-like structure provide abundant electroactive sites and the ion diffusion pathway. Simultaneously, this structure that VN encapsulated in carbon impedes electrochemical oxidation of the intimate between VN and electrolytes to achieve long-term cycle life and high specific capacitance [21, 37]. The electrochemical performances based on nanocomposites material electrode and supercapacitor devices were investigated, including specific capacitance, rate capability and energy density. As a result, this composite shows an excellent specific capacitance of 385 F g−1 at 1 A g−1 and 88.9% capacitance retention after 10000 cycles. The two-electrode system based on VN/C electrodes boasts perfect cycling stability with 93.3% capacitance retention after 10000 cycles, suggesting VN/C electrodes have a large potential in high-performance energy storage device for practical applications.
2. Experimental 2.1 Chemicals Vanadyl acetylacetonate and melamine were obtained from Aladdin (Beijing, China). Formaldehyde was analytical reagents received from Sichuan Xi Long Chemical Reagent Factory, China. All chemicals (analytical grade) were used without further purification. 2.2 Synthesis Typically, melamine (1.00 g) and formaldehyde (4.76 g) were dissolved in 50.00
mL deionized water and stirred at room temperature until a clear solution formed. The vanadyl acetylacetonate (VO(acac)2, 0.35 g) was added, then the above mixture stirring continuously for 12 h. After that, the mixture was transferred to a Teflon-lined stainless-steel autoclave and heated at 150 oC for 10 h with a heating rate of 2 oC min−1. The obtained products were collected, centrifuged and washed with deionized water and ethanol several times, then dried at 80 oC for 12 h. Finally, the powder was calcined at 700 oC for 2 h in N2 with a heating rate of 5 oC min−1. For comparison, VN/C-0.25, VN/C-0.35, VN/C-0.45, and pure carbon were also prepared using different contents of VO(acac)2 and without VO(acac)2 as the same condition. Simultaneously the precursor was annealed in Ar atmosphere (referred to as V2O3/C). 2.3 Materials characterization The structural information of samples was performed using XRD (Bruker D8, using filtered Cu Kα radiation, λ = 0.15405 nm) and Raman spectra (Bruker Senterra R200-L spectrometer, 532 nm laser) to identify the crystalline structures. The morphology of coralloid-like nanostructure vanadium nitride/carbon was conducted by using field emission scanning electron microscopy (FESEM Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM-2010F). X-ray photoelectron spectroscopy (XPS, Escalab 250, USA) analysis was used for the sample structure analysis. SSA and pore size distribution of coralloid-like VN/C was determined on Autosorb-IQ,
Quantachrome
by
nitrogen
adsorption-desorption
(Brunauer-Emmett-Teller). 2.4 Electrochemical measurements All electrochemical measurements of VN/C were evaluated by using an electrochemical working station (CHI660D, Shanghai CH Instruments, China) with a Pt plate and saturated calomel electrode (SCE), which can be seen the counter electrode and the reference electrode respectively. The working electrode of a three-electrode system was prepared by using the obtained materials, acetylene black, and poly (trafluoroethylene) emulsion at a mass ratio of 8:1:1 in absolute ethanol, then the mixture was coated on the nickel foam, and dried at 60 oC for 10 h. The electrochemical
tests
including
cyclic
voltammetry
(CV),
galvanostatic
charging/discharging (GCD), and electrochemical impedance spectroscopy (EIS) were carried out in 6 M KOH aqueous electrolyte. The specific capacitance of the electrode can be calculated based on the discharging curves at different current densities by the equation as follow: C = I∆t/ (m∆V)
(1)
Where C (F g−1), I (A), ∆t (s), m (mg), ∆V (V) are the specific capacitance, constant discharge current, discharge time, voltage change during the discharge. The energy density and power density of the device were calculated by according to following equation: E = CV2/7.2
(2)
P = 3600E/∆t
(3)
Where E (Wh kg−1), C (F g−1), V (V), P (W kg−1), ∆t (s) are the energy density of device, specific capacitance, voltage change during the discharging process, power density of device, discharging time, respectively.
3. Results and discussion The detailed fabrication strategy and process of coralloid-like nanostructure VN/C was illustrated in Scheme 1. Briefly, vanadyl acetylacetonate was added to the melamine solution, as it showed uniform dispersion at a suitable hydrothermal temperature. Through thermotreatment under a N2 atmosphere, coralloid-like nanostructure VN/C networks were obtained.
Scheme 1. Schematic illustration of a strategy for the fabrication of VN/C.
The morphologies and microstructures of VN/C were examined by scanning electron microscopy (SEM) analysis. Fig. 1a-b show the representative SEM images with different magnifications of the VN/C with uniform shape and similar diameter. Some branches are interconnected to form a nanostructure with a coralloid-like morphology. As shown in Fig. 1c-d, the microstructure of VN/C nanocomposites was further evidenced by transmission electron microscopy (TEM) and high-resolution TEM image. The image indicates that the VN with a diameter of 5-10 nm is uniformly dispersed in the carbon matrix. Fig. S1 presents the HRTEM images of different proportions of VN/C composites, displaying the thickness of the carbon decreases with the increase of vanadyl acetylacetonate. The SAED pattern shows highly distinguished diffraction rings, which indicate the polycrystalline structure of VN in the carbon framework. The HRTEM image shows lattice fringes with a spacing of 0.21 nm which is corresponding to the (200) crystal planes of cubic VN phase (JCPDS No. 35-0768) and agreement with X-ray diffraction (XRD) data (Fig. 1e) [42]. All the signals can be quickly indexed to the standard card to demonstrate the crystal phase of the composite. It revealed that the characteristic peaks at 37.4, 43.6, 63.4 and 76.0o, which attributed to the (111), (200), (220) and (311) lattice planes of VN/C, respectively. It also exhibited a broad peak approximately at 22o, denoted the presence of amorphous carbon derived from the melamine. It would contribute to the electrical conductivity and augment the stability of the material [22, 43]. The precursor was annealed at 700 oC in Ar atmosphere to confirm the source of the N element in the nanocomposite. The XRD pattern of the sample annealed in Ar gas exhibits the peak of V2O3 (Fig. S2), indicating the N element of vanadium nitride might originate from the N2 atmosphere [44]. The structural information is also confirmed by Raman spectrum (Fig. 1f), the composite has two broad peaks attributed to graphite sp2 carbon band (G band) and the defect sp3 carbon band (D band) of VN/C. The overlapping Raman peaks between the two signals (D and G) and asymmetrical tailing of the peak extending to about 1000 cm−1 demonstrated that the carbon bulk with nitrogen doped [45, 46]. The ratio of the peak intensity of the D−band to that of the G−band (ID/IG) for the VN/C-0.25, VN/C-0.35 and VN/C-0.45 are 1.000, 1.029
and 1.031 respectively, indicating that the disorder of VN/C electrode material structure augments with the increased precursor of vanadium contents. These results unambiguously confirmed that the product is a composite of VN and carbon. The energy dispersive X-ray spectroscopy (EDS) shows the mapping images of VN/C (Fig. 1g), which revealed that the elements of C, N, O and V have a uniform distribution on the material which further identified the ingredients of the compound. The unique structure rich in carbon with doped nitrogen heteroatom would improve the capacitance of the material and enhance the wettability between electrode and electrolyte [47, 48].
Fig. 1 Morphology, structure, and composition of the VN/C-0.35: (a-b) SEM images, (c-d) TEM and HR-TEM image (the inset displays selected-area electron diffraction), (e) XRD pattern, (f) Raman scattering spectrum of the VN/C-0.25, VN/C-0.35 and VN/C-0.45, (g) EDX maps of C, N, O and V revealing uniform distribution of all elements.
To further explore the element valence and bonding detail, XPS of the VN/C showed in Fig. 2. The overall XPS spectra indicate that the prepared VN/C mainly consisted of C, N, O, and V elements (Fig. S3). The signals obtained at 284.7 eV, 285.6 eV, 286.2 eV, 289.3 eV correspond to the high-resolution C 1s spectrum shows three chemical states of C−C, C−N, C−O, C=O groups, respectively (Fig. 2a). It indicates the presence of N-doped carbon material on the surface of the material,
which can also progress the wettability between material and electrolyte, and further improve the electrochemical properties of the product [47, 48]. The high-resolution of the N 1s spectrum (Fig. 2b) showed four fitting peaks: 403.2, 401.1, 400.0, and 398.3 eV. Oxygenated N (N−O) and graphitic N (N−Q) produced by nitrogen-doped carbon section corresponds to characteristic the broad peak at 403.2 and 401.1 eV [49]. Furthermore, other two forceful characteristic peaks were observed at 400.0, and 398.3 eV expected for a metal nitride inconsistent with the data of VN in the document, which corresponding to pyrrolic N (N−5) and pyridinic N (N−6). The peaks of V 2p1 and V 2p3 demonstrate the sum of the designated lines of several different valence states of vanadium. The peaks showed two signals at 514.1 and 521.6 eV corresponding to the vanadium element of VN. Besides, peaks at approximately 517.1 and 524.4 eV arose from the V−O which bonds on the surface of the material (Fig. 2d) [22]. The O 1s XPS spectrum of VN/C (Fig. 2c) showed three signals at 532.6, 531.1 and 529.6 eV, corresponding to the C=O/N−O groups, V−N−O groups and oxygen in vanadium oxide respectively [50, 51]. The peak at 532.6 eV demonstrated the existence of −OH which groups on the VN/C surface and can improve the contact area between the electrolyte and electrode. The other peak at 531.1 eV evidenced a complex mixture of different vanadium oxides on the surface of the product, which includes different valence states of V that cannot transform to nitrides completely [52]. And the presence of different kinds of oxide state will further enhance the electrochemical specific capacitance of material [53]. The XPS spectra of the VN/C-0.25 and VN/C-0.45 are shown in Fig. S4. As illustrated in Table S1, the V/N atomic ratio is smaller than 1, indicating that the N also doped into carbon during the reaction process.
Fig. 2 X-ray photoelectron spectra of the VN/C-0.35.
To evaluate the supercapacitive performance of the VN/C, the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and Electrochemical impedance spectroscopy (EIS) were conducted using a three-electrode configuration in 6 M KOH electrolyte. As shown in Fig. 3a, the CV curves of all the materials were obtained at a scanning rate of 100 mV s−1 in the potential window ranging from −1.1 to 0 V. Compared with the rectangular shape of pure carbon, the faradaic redox peaks were observed from the VN/C materials, which suggests that the dominant charge-storage mechanism in VN/C electrode materials was the pseudocapacitance from reversible redox reactions. Also, the VN/C showed larger peak currents than that of pure carbon, which illustrate the enhanced reversible redox reactions owing to the large quantity of exposed pseudocapacitive active sites of VN nanoparticles. The CV curves of VN/C prepared with different vanadium contents represent almost the same type of curve shape. The VN/C-0.35 exhibited the largest curve area in all samples at 100 mV s−1, suggesting the highest specific capacitance. The CV curves of VN/C-0.25, VN/C-0.45 and pure carbon at different scanning rates were shown in Fig. S5. The
GCD plots of the VN/C sample with different vanadium contents exhibited an inconspicuous curvature and symmetrical triangular shape when it measured at a current density of 1 A g−1 (Fig. 3b). The values of specific capacitance were 282, 385, 218 and 199 F g−1 for VN/C-0.25, VN/C-0.35, VN/C-0.45 and pure carbon, respectively. The VN/C-0.35 showed high stability reflected in the high cycling reversibility and coulombic efficiency without obvious voltage drop with increasing the current density from 0.5 to 20.0 A g−1. The GCD curves of VN/C-0.25, VN/C-0.45 and pure carbon at different current densities were also provided in Fig. S5. We investigated the ion diffusion and electronic transfer properties of all samples using EIS measurement (Fig. 3c). The impedance curves of all the materials include a vertical curve in the low frequency and a semicircle in the high frequency. These curves of the VN/C had a similar semicircle, indicating similar behavior of charge transfer resistance. However, as can be seen from the oblique line at the low-frequency region, the slash line of VN/C-0.35 was almost perpendicular to the X-axis, which indicated that the electrolyte ions had the best diffusion ability in electrode structure. And it disclosed that the equivalent series resistance of VN/C-0.35 was smaller than other samples owing to the rich carbon content from nitrogen-doping promotes the diffusion and penetration of electrolyte ions and improved the electrical conductivity [54]. The specific capacitances of the VN/C sample with different vanadium contents and pure carbon at different current densities were presented in Fig. 3d. The capacitance retention values for VN/C-0.25, VN/C-0.35, VN/C-0.45 and pure carbon were 67.35, 60.00, 64.97 and 59.29% when the current density increasing from 1 to 10 A g−1, respectively. The VN/C-0.35 shows higher specific capacitances at different current densities compared to the other samples. For comparison, the capacitive properties of VN/C at different carbonization temperature were also determined (Fig. S7).
Fig. 3 (a) CV curves, (b) GCD curves, and (c) EIS curves of the pure carbon, VN/C-0.25, VN/C-0.35 and VN/C-0.45, (d) the specific capacitances of the pure carbon, VN/C-0.25, VN/C-0.35 and VN/C-0.45 at different current densities (in 6 M KOH aqueous solution).
Based on the above-revealed results, VN/C-0.35 exhibits exceptional capacitance behavior, thus the CV curves of VN/C-0.35 at scanning rates from 5 to 100 mV s−1 were also investigated. As shown in Fig 4a, it showed similar CV curves shape and virtually no deformation at high scanning rates, indicating a high rate of capability. The GCD curves of the VN/C-0.35 (Fig. 4b) at various current densities of 0.5-20.0 A g−1 exhibited a nearly linear and symmetrical triangular shape indicative of good electrode-reaction reversibility. The large capacitance and cycling stability of VN/C-0.35 are very important for the application of negative electrode materials. The long-term cycling stability of the VN/C-0.35 was further evaluated at a current density of 5 A g−1 (Fig. 4d). The sample exhibited excellent cycling stability with 88.9% capacity retention after 10000 cycles. This excellent cycle stability of VN/C-0.35 resulted from the protective effect of carbon encapsulated vanadium nitride in the composite. The different vanadium oxides on the product surface can improve the capacitance performance of VN/C and prevent VN surface from further oxidation,
while different vanadium oxides could lead to electrochemical instability during charge-discharge cycling performance [38]. We further probed this completion by the SEM images of VN/C-0.35 after cycling (Fig. S8). It maintained the original morphology after 10000 cycles further indicating the excellent electrochemical stability of VN/C-0.35 nanocomposites.
Fig. 4 (a) CV curves of the VN/C-0.35 at different scan rates, (b) GCD curves of the VN/C-0.35 at various current densities., (c) EIS curves of the VN/C-0.35, (d) cycling performance at 5 A g−1 of the VN/C-0.35.
To accurately evaluate the practical application of VN/C, the symmetric SCs featuring a two-electrode system was assembled using the product in 6 M KOH as the electrode. Figure 5a shows the CV curves of VN/C//VN/C over the voltage range of 0−1.1 V at various scanning rates between 5 and 300 mV s−1. The CV curves display a similar rectangle without conspicuous deformation even at the scanning rate of 300 mV s−1, corroborating the good capacitive behavior and reversibility. Fig. 5b showed slight capacitance fading of the GCD curves of the two-electrode system at different current densities from 0.5 to 20 A g−1 at the working potential window of 1.1 V. The specific capacitance measured was 144 F g−1 at the current density of 1 A g−1, and the
retained capacitance measured at a current density to 5 A g−1 was counted to be 118 F g−1. As shown in Fig. 5c, the EIS of the two-electrode system was measured from 0.01 Hz to 100 kHz at room temperature. The two-electrode system had a small intercept at the real axis of 0.75 Ω, indicating a lower intrinsic resistance in the electrochemical system [55]. The electrochemical stability of the two-electrode system at the scan rate of 5 A g−1 was represented in Fig. 5d. It demonstrated excellent cycle stability with 93.3% retention of the initial capacitance after 10000 cycles. We further probed this completion by the SEM images of the two-electrode system after cycling (Fig. S10). As shown in Fig. S9, the specific capacitance was retained about 81.9% with the current density from 1 to 5 A g−1. A large energy density of 24.2 Wh kg−1 was attained at a power density of 1050 W kg−1, and they were two important parameters in the evaluation of the supercapacitors practical application. Table S2 showed the electrochemical performances of the diverse VN-based electrodes reported in the literature. Notably, the coralloid-like VN/C electrode material reveals the application potential in the energy storage device.
Fig. 5 The electrochemical performance of the two-electrode system in 6 M KOH: (a) CV, (b) GCD, and (c) EIS curves, (d) cycling performance at 5 A g−1.
4. Conclusion In summary, the coralloid-like VN/C composite electrode materials were designed and synthesized by hydrothermal process. The novel structure endowed VN/C with high specific capacitance and good cyclic stability. The coralloid-like VN/C exhibited a wide voltage window of −1.1 to 0 V with a specific capacitance of 385 F g−1 at 1 A g−1 and 88.9% capacitance retention after 10000 cycles. Notably, the symmetric device fabricated with VN/C//VN/C exhibited a high energy density of 24.2 Wh kg−1 at a power density of 1050 W kg−1. Besides, excellent cycling stability (93.3%) was obtained at a current density of 5 A g−1 after 10000 cycles. The combinations of VN and carbon should also inspire the design and fabrication of high-performance electrode materials for supercapacitors and provide more detail information for the performance of VN.
5. Acknowledgments This work is supported by the Open Fund of the Key Laboratory of Xinjiang Uygur Autonomous Region (2017D04014), the National Natural Science Foundation of China (U1703251 and 21571152), Program for Tianshan Innovative Research Team of Xinjiang Uygur Autonomous Region (2018D14002), Scientific Research Program of the Higher Education Institution of Xinjiang (XJEDU2017A001).
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Highligts: A facile hydrothermal method was used to fabricate coralloid-like nanostructure vanadium nitride/carbon (VN/C) nanocomposites. The
supercapacitor
negative
electrode
based
on
coralloid-like
VN/C
nanocomposites exhibit a high specific capacitance of 385 F g−1 at 1 A g−1. The electrode exhibits extraordinary cycling stability with 88.9 % capacitance retention after 10000 cycles.
There are no conflicts to declare.
S0925-8388(19)34141-6
DOI:
https://doi.org/10.1016/j.jallcom.2019.152895
Reference:
JALCOM 152895
To appear in:
Journal of Alloys and Compounds
Received Date: 5 September 2019 Revised Date:
31 October 2019
Accepted Date: 1 November 2019
Please cite this article as: Q. Sun, Y. Lv, X. Wu, W. Jia, J. Guo, F. Tong, D. Jia, Z. Sun, X. Wang, Hydrothermal synthesis of coralloid-like vanadium nitride/carbon nanocomposites for high-performance and long-life supercapacitors, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152895. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
A facile method is designed to obtain coralloid-like vanadium nitride/carbon nanocomposites for high-performance and long-life supercapacitors.
Hydrothermal synthesis of coralloid-like vanadium nitride/carbon nanocomposites for high-performance and long-life supercapacitors Qingqing Sun, Yan Lv, Xueyan Wu, Wei Jia, Jixi Guo,* Fenglian Tong, Dianzeng Jia* Zhipeng Sun, Xingchao Wang Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key Laboratory of Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Urumqi, 830046, Xinjiang P. R. China. Email: [email protected]; [email protected]; Fax: +86-991-8588883; Tel: +86-991-8583083.
Abstract Vanadium nitride is highly desirable for their good electrochemical performance and wide work potential window, but they are cramped practical applications in supercapacitors due to poor cycling stability. Herein a combined strategy is exploited to fabricate coralloid-like nanostructure vanadium nitride/carbon (VN/C) via a facile hydrothermal method. The structural characterization displayed that nano-VN was uniformly distributed in the carbon scaffold materials to improve the electrode behavior. Those composites exhibited a wide voltage window of −1.1 to 0 V with a specific capacitance of 385 F g−1 at 1 A g−1, and exceptional cycling stability (88.9% retention after 10000 cycles). The two-electrode system based on VN/C electrodes obtains a high energy density of 24.2 Wh kg−1 at a power density of 1050 W kg−1. The device also shows perfect cycling stability with 93.3% capacitance retention after 10000 cycles. The coralloid-like VN/C electrode materials can be used in practical applications of the high-performance energy storage device.
Keywords Vanadium nitride/Carbon, Supercapacitors, Electrode materials, Long cycle life
1. Introduction The clean and renewable energy has become an increasing interest in many
countries as the environmental costs and limited supply of fossil fuels are clearly understood. It has enormous internal demand for energy storage devices because of an efficient using of clean and renewable energy [1-4]. The supercapacitor is one of the most potential energy storage devices in current markets, which has many advantages such as high-power density, fast charge-discharge capability, and good cycling stability compared to other secondary batteries [5]. Therefore, supercapacitors, which have high power and energy density, have been attracted increasing attention in the field of electrochemical energy storage devices [6, 7]. According to the electric charge storage mechanism, supercapacitors can be typically classified into electrical double-layer capacitors (EDLCs) and pseudocapacitors (PCs) [8, 9]. EDLCs generally store energy by the electrostatic accumulation of charges at the electrode/electrolyte interface. Carbon materials as the typical electrode materials for EDLCs have long cycle life, but it was still constrained by the poor theoretical specific capacitance [10-14]. PCs chemically store charge through the Faradaic reactions of redox species coated on the electrode surface. Most electrode materials show excellent pseudocapacitance and a wide electrochemically potential window, such as transition metal oxides [15-20], transition metal nitrides [21-24] and transition metal sulfides [25-30]. Among transition metal nitrides, vanadium nitride (VN) has attracted considerable
attention
owing
to
its
large
pseudocapacitance
and
wide
electrochemically potential window [31, 32]. However, these reported vanadium nitride materials generally have poor cycle life because of electrochemical oxidation in alkaline media [33, 34]. To increase cycling stability and energy storage, combining vanadium nitride with carbon-based materials is an effective strategy [12, 35, 36]. However, it is still a challenge how the nano-VN was uniformly distributed in carbon materials to manufacture the efficient composite electrode material, which can improve the electrochemical stability and the electrical conductivity of the electrode material [37, 38]. Ran et al. synthesized nano vanadium nitride/interconnected porous carbon with a specific capacitance of 284.0 F g−1 [39], but intricate fabrication methods limit its application. Liu et al. reported carbon nanosphere@vanadium nitride with a maximum specific capacitance of 300.4 F g−1 at the current density of 1.0 A g−1
[40], but its cyclic stability is poor. Recently, He fabricated VN nanoparticle/GO composites by electrochemical surface-initiated atom transfer radical polymerization method, which showed capacitance above 109 F g−1 in 2 M KOH [41]. However, the synthesis of vanadium nitride/carbon nanocomposites was greatly limited due to its intricate fabrication methods and low capacitance. Herein we highlighted the coralloid-like nanostructure vanadium nitride/carbon electrode materials for high-capacitance supercapacitors, which were synthesized by a facile hydrothermal reaction. Synergistic effect of VN and carbon significantly improves the performance of the negative electrode materials to ensure the electrochemical behavior of the charge-discharge process goes smoothly. The well-dispersed nano-VN and pillared coralloid-like structure provide abundant electroactive sites and the ion diffusion pathway. Simultaneously, this structure that VN encapsulated in carbon impedes electrochemical oxidation of the intimate between VN and electrolytes to achieve long-term cycle life and high specific capacitance [21, 37]. The electrochemical performances based on nanocomposites material electrode and supercapacitor devices were investigated, including specific capacitance, rate capability and energy density. As a result, this composite shows an excellent specific capacitance of 385 F g−1 at 1 A g−1 and 88.9% capacitance retention after 10000 cycles. The two-electrode system based on VN/C electrodes boasts perfect cycling stability with 93.3% capacitance retention after 10000 cycles, suggesting VN/C electrodes have a large potential in high-performance energy storage device for practical applications.
2. Experimental 2.1 Chemicals Vanadyl acetylacetonate and melamine were obtained from Aladdin (Beijing, China). Formaldehyde was analytical reagents received from Sichuan Xi Long Chemical Reagent Factory, China. All chemicals (analytical grade) were used without further purification. 2.2 Synthesis Typically, melamine (1.00 g) and formaldehyde (4.76 g) were dissolved in 50.00
mL deionized water and stirred at room temperature until a clear solution formed. The vanadyl acetylacetonate (VO(acac)2, 0.35 g) was added, then the above mixture stirring continuously for 12 h. After that, the mixture was transferred to a Teflon-lined stainless-steel autoclave and heated at 150 oC for 10 h with a heating rate of 2 oC min−1. The obtained products were collected, centrifuged and washed with deionized water and ethanol several times, then dried at 80 oC for 12 h. Finally, the powder was calcined at 700 oC for 2 h in N2 with a heating rate of 5 oC min−1. For comparison, VN/C-0.25, VN/C-0.35, VN/C-0.45, and pure carbon were also prepared using different contents of VO(acac)2 and without VO(acac)2 as the same condition. Simultaneously the precursor was annealed in Ar atmosphere (referred to as V2O3/C). 2.3 Materials characterization The structural information of samples was performed using XRD (Bruker D8, using filtered Cu Kα radiation, λ = 0.15405 nm) and Raman spectra (Bruker Senterra R200-L spectrometer, 532 nm laser) to identify the crystalline structures. The morphology of coralloid-like nanostructure vanadium nitride/carbon was conducted by using field emission scanning electron microscopy (FESEM Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM-2010F). X-ray photoelectron spectroscopy (XPS, Escalab 250, USA) analysis was used for the sample structure analysis. SSA and pore size distribution of coralloid-like VN/C was determined on Autosorb-IQ,
Quantachrome
by
nitrogen
adsorption-desorption
(Brunauer-Emmett-Teller). 2.4 Electrochemical measurements All electrochemical measurements of VN/C were evaluated by using an electrochemical working station (CHI660D, Shanghai CH Instruments, China) with a Pt plate and saturated calomel electrode (SCE), which can be seen the counter electrode and the reference electrode respectively. The working electrode of a three-electrode system was prepared by using the obtained materials, acetylene black, and poly (trafluoroethylene) emulsion at a mass ratio of 8:1:1 in absolute ethanol, then the mixture was coated on the nickel foam, and dried at 60 oC for 10 h. The electrochemical
tests
including
cyclic
voltammetry
(CV),
galvanostatic
charging/discharging (GCD), and electrochemical impedance spectroscopy (EIS) were carried out in 6 M KOH aqueous electrolyte. The specific capacitance of the electrode can be calculated based on the discharging curves at different current densities by the equation as follow: C = I∆t/ (m∆V)
(1)
Where C (F g−1), I (A), ∆t (s), m (mg), ∆V (V) are the specific capacitance, constant discharge current, discharge time, voltage change during the discharge. The energy density and power density of the device were calculated by according to following equation: E = CV2/7.2
(2)
P = 3600E/∆t
(3)
Where E (Wh kg−1), C (F g−1), V (V), P (W kg−1), ∆t (s) are the energy density of device, specific capacitance, voltage change during the discharging process, power density of device, discharging time, respectively.
3. Results and discussion The detailed fabrication strategy and process of coralloid-like nanostructure VN/C was illustrated in Scheme 1. Briefly, vanadyl acetylacetonate was added to the melamine solution, as it showed uniform dispersion at a suitable hydrothermal temperature. Through thermotreatment under a N2 atmosphere, coralloid-like nanostructure VN/C networks were obtained.
Scheme 1. Schematic illustration of a strategy for the fabrication of VN/C.
The morphologies and microstructures of VN/C were examined by scanning electron microscopy (SEM) analysis. Fig. 1a-b show the representative SEM images with different magnifications of the VN/C with uniform shape and similar diameter. Some branches are interconnected to form a nanostructure with a coralloid-like morphology. As shown in Fig. 1c-d, the microstructure of VN/C nanocomposites was further evidenced by transmission electron microscopy (TEM) and high-resolution TEM image. The image indicates that the VN with a diameter of 5-10 nm is uniformly dispersed in the carbon matrix. Fig. S1 presents the HRTEM images of different proportions of VN/C composites, displaying the thickness of the carbon decreases with the increase of vanadyl acetylacetonate. The SAED pattern shows highly distinguished diffraction rings, which indicate the polycrystalline structure of VN in the carbon framework. The HRTEM image shows lattice fringes with a spacing of 0.21 nm which is corresponding to the (200) crystal planes of cubic VN phase (JCPDS No. 35-0768) and agreement with X-ray diffraction (XRD) data (Fig. 1e) [42]. All the signals can be quickly indexed to the standard card to demonstrate the crystal phase of the composite. It revealed that the characteristic peaks at 37.4, 43.6, 63.4 and 76.0o, which attributed to the (111), (200), (220) and (311) lattice planes of VN/C, respectively. It also exhibited a broad peak approximately at 22o, denoted the presence of amorphous carbon derived from the melamine. It would contribute to the electrical conductivity and augment the stability of the material [22, 43]. The precursor was annealed at 700 oC in Ar atmosphere to confirm the source of the N element in the nanocomposite. The XRD pattern of the sample annealed in Ar gas exhibits the peak of V2O3 (Fig. S2), indicating the N element of vanadium nitride might originate from the N2 atmosphere [44]. The structural information is also confirmed by Raman spectrum (Fig. 1f), the composite has two broad peaks attributed to graphite sp2 carbon band (G band) and the defect sp3 carbon band (D band) of VN/C. The overlapping Raman peaks between the two signals (D and G) and asymmetrical tailing of the peak extending to about 1000 cm−1 demonstrated that the carbon bulk with nitrogen doped [45, 46]. The ratio of the peak intensity of the D−band to that of the G−band (ID/IG) for the VN/C-0.25, VN/C-0.35 and VN/C-0.45 are 1.000, 1.029
and 1.031 respectively, indicating that the disorder of VN/C electrode material structure augments with the increased precursor of vanadium contents. These results unambiguously confirmed that the product is a composite of VN and carbon. The energy dispersive X-ray spectroscopy (EDS) shows the mapping images of VN/C (Fig. 1g), which revealed that the elements of C, N, O and V have a uniform distribution on the material which further identified the ingredients of the compound. The unique structure rich in carbon with doped nitrogen heteroatom would improve the capacitance of the material and enhance the wettability between electrode and electrolyte [47, 48].
Fig. 1 Morphology, structure, and composition of the VN/C-0.35: (a-b) SEM images, (c-d) TEM and HR-TEM image (the inset displays selected-area electron diffraction), (e) XRD pattern, (f) Raman scattering spectrum of the VN/C-0.25, VN/C-0.35 and VN/C-0.45, (g) EDX maps of C, N, O and V revealing uniform distribution of all elements.
To further explore the element valence and bonding detail, XPS of the VN/C showed in Fig. 2. The overall XPS spectra indicate that the prepared VN/C mainly consisted of C, N, O, and V elements (Fig. S3). The signals obtained at 284.7 eV, 285.6 eV, 286.2 eV, 289.3 eV correspond to the high-resolution C 1s spectrum shows three chemical states of C−C, C−N, C−O, C=O groups, respectively (Fig. 2a). It indicates the presence of N-doped carbon material on the surface of the material,
which can also progress the wettability between material and electrolyte, and further improve the electrochemical properties of the product [47, 48]. The high-resolution of the N 1s spectrum (Fig. 2b) showed four fitting peaks: 403.2, 401.1, 400.0, and 398.3 eV. Oxygenated N (N−O) and graphitic N (N−Q) produced by nitrogen-doped carbon section corresponds to characteristic the broad peak at 403.2 and 401.1 eV [49]. Furthermore, other two forceful characteristic peaks were observed at 400.0, and 398.3 eV expected for a metal nitride inconsistent with the data of VN in the document, which corresponding to pyrrolic N (N−5) and pyridinic N (N−6). The peaks of V 2p1 and V 2p3 demonstrate the sum of the designated lines of several different valence states of vanadium. The peaks showed two signals at 514.1 and 521.6 eV corresponding to the vanadium element of VN. Besides, peaks at approximately 517.1 and 524.4 eV arose from the V−O which bonds on the surface of the material (Fig. 2d) [22]. The O 1s XPS spectrum of VN/C (Fig. 2c) showed three signals at 532.6, 531.1 and 529.6 eV, corresponding to the C=O/N−O groups, V−N−O groups and oxygen in vanadium oxide respectively [50, 51]. The peak at 532.6 eV demonstrated the existence of −OH which groups on the VN/C surface and can improve the contact area between the electrolyte and electrode. The other peak at 531.1 eV evidenced a complex mixture of different vanadium oxides on the surface of the product, which includes different valence states of V that cannot transform to nitrides completely [52]. And the presence of different kinds of oxide state will further enhance the electrochemical specific capacitance of material [53]. The XPS spectra of the VN/C-0.25 and VN/C-0.45 are shown in Fig. S4. As illustrated in Table S1, the V/N atomic ratio is smaller than 1, indicating that the N also doped into carbon during the reaction process.
Fig. 2 X-ray photoelectron spectra of the VN/C-0.35.
To evaluate the supercapacitive performance of the VN/C, the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and Electrochemical impedance spectroscopy (EIS) were conducted using a three-electrode configuration in 6 M KOH electrolyte. As shown in Fig. 3a, the CV curves of all the materials were obtained at a scanning rate of 100 mV s−1 in the potential window ranging from −1.1 to 0 V. Compared with the rectangular shape of pure carbon, the faradaic redox peaks were observed from the VN/C materials, which suggests that the dominant charge-storage mechanism in VN/C electrode materials was the pseudocapacitance from reversible redox reactions. Also, the VN/C showed larger peak currents than that of pure carbon, which illustrate the enhanced reversible redox reactions owing to the large quantity of exposed pseudocapacitive active sites of VN nanoparticles. The CV curves of VN/C prepared with different vanadium contents represent almost the same type of curve shape. The VN/C-0.35 exhibited the largest curve area in all samples at 100 mV s−1, suggesting the highest specific capacitance. The CV curves of VN/C-0.25, VN/C-0.45 and pure carbon at different scanning rates were shown in Fig. S5. The
GCD plots of the VN/C sample with different vanadium contents exhibited an inconspicuous curvature and symmetrical triangular shape when it measured at a current density of 1 A g−1 (Fig. 3b). The values of specific capacitance were 282, 385, 218 and 199 F g−1 for VN/C-0.25, VN/C-0.35, VN/C-0.45 and pure carbon, respectively. The VN/C-0.35 showed high stability reflected in the high cycling reversibility and coulombic efficiency without obvious voltage drop with increasing the current density from 0.5 to 20.0 A g−1. The GCD curves of VN/C-0.25, VN/C-0.45 and pure carbon at different current densities were also provided in Fig. S5. We investigated the ion diffusion and electronic transfer properties of all samples using EIS measurement (Fig. 3c). The impedance curves of all the materials include a vertical curve in the low frequency and a semicircle in the high frequency. These curves of the VN/C had a similar semicircle, indicating similar behavior of charge transfer resistance. However, as can be seen from the oblique line at the low-frequency region, the slash line of VN/C-0.35 was almost perpendicular to the X-axis, which indicated that the electrolyte ions had the best diffusion ability in electrode structure. And it disclosed that the equivalent series resistance of VN/C-0.35 was smaller than other samples owing to the rich carbon content from nitrogen-doping promotes the diffusion and penetration of electrolyte ions and improved the electrical conductivity [54]. The specific capacitances of the VN/C sample with different vanadium contents and pure carbon at different current densities were presented in Fig. 3d. The capacitance retention values for VN/C-0.25, VN/C-0.35, VN/C-0.45 and pure carbon were 67.35, 60.00, 64.97 and 59.29% when the current density increasing from 1 to 10 A g−1, respectively. The VN/C-0.35 shows higher specific capacitances at different current densities compared to the other samples. For comparison, the capacitive properties of VN/C at different carbonization temperature were also determined (Fig. S7).
Fig. 3 (a) CV curves, (b) GCD curves, and (c) EIS curves of the pure carbon, VN/C-0.25, VN/C-0.35 and VN/C-0.45, (d) the specific capacitances of the pure carbon, VN/C-0.25, VN/C-0.35 and VN/C-0.45 at different current densities (in 6 M KOH aqueous solution).
Based on the above-revealed results, VN/C-0.35 exhibits exceptional capacitance behavior, thus the CV curves of VN/C-0.35 at scanning rates from 5 to 100 mV s−1 were also investigated. As shown in Fig 4a, it showed similar CV curves shape and virtually no deformation at high scanning rates, indicating a high rate of capability. The GCD curves of the VN/C-0.35 (Fig. 4b) at various current densities of 0.5-20.0 A g−1 exhibited a nearly linear and symmetrical triangular shape indicative of good electrode-reaction reversibility. The large capacitance and cycling stability of VN/C-0.35 are very important for the application of negative electrode materials. The long-term cycling stability of the VN/C-0.35 was further evaluated at a current density of 5 A g−1 (Fig. 4d). The sample exhibited excellent cycling stability with 88.9% capacity retention after 10000 cycles. This excellent cycle stability of VN/C-0.35 resulted from the protective effect of carbon encapsulated vanadium nitride in the composite. The different vanadium oxides on the product surface can improve the capacitance performance of VN/C and prevent VN surface from further oxidation,
while different vanadium oxides could lead to electrochemical instability during charge-discharge cycling performance [38]. We further probed this completion by the SEM images of VN/C-0.35 after cycling (Fig. S8). It maintained the original morphology after 10000 cycles further indicating the excellent electrochemical stability of VN/C-0.35 nanocomposites.
Fig. 4 (a) CV curves of the VN/C-0.35 at different scan rates, (b) GCD curves of the VN/C-0.35 at various current densities., (c) EIS curves of the VN/C-0.35, (d) cycling performance at 5 A g−1 of the VN/C-0.35.
To accurately evaluate the practical application of VN/C, the symmetric SCs featuring a two-electrode system was assembled using the product in 6 M KOH as the electrode. Figure 5a shows the CV curves of VN/C//VN/C over the voltage range of 0−1.1 V at various scanning rates between 5 and 300 mV s−1. The CV curves display a similar rectangle without conspicuous deformation even at the scanning rate of 300 mV s−1, corroborating the good capacitive behavior and reversibility. Fig. 5b showed slight capacitance fading of the GCD curves of the two-electrode system at different current densities from 0.5 to 20 A g−1 at the working potential window of 1.1 V. The specific capacitance measured was 144 F g−1 at the current density of 1 A g−1, and the
retained capacitance measured at a current density to 5 A g−1 was counted to be 118 F g−1. As shown in Fig. 5c, the EIS of the two-electrode system was measured from 0.01 Hz to 100 kHz at room temperature. The two-electrode system had a small intercept at the real axis of 0.75 Ω, indicating a lower intrinsic resistance in the electrochemical system [55]. The electrochemical stability of the two-electrode system at the scan rate of 5 A g−1 was represented in Fig. 5d. It demonstrated excellent cycle stability with 93.3% retention of the initial capacitance after 10000 cycles. We further probed this completion by the SEM images of the two-electrode system after cycling (Fig. S10). As shown in Fig. S9, the specific capacitance was retained about 81.9% with the current density from 1 to 5 A g−1. A large energy density of 24.2 Wh kg−1 was attained at a power density of 1050 W kg−1, and they were two important parameters in the evaluation of the supercapacitors practical application. Table S2 showed the electrochemical performances of the diverse VN-based electrodes reported in the literature. Notably, the coralloid-like VN/C electrode material reveals the application potential in the energy storage device.
Fig. 5 The electrochemical performance of the two-electrode system in 6 M KOH: (a) CV, (b) GCD, and (c) EIS curves, (d) cycling performance at 5 A g−1.
4. Conclusion In summary, the coralloid-like VN/C composite electrode materials were designed and synthesized by hydrothermal process. The novel structure endowed VN/C with high specific capacitance and good cyclic stability. The coralloid-like VN/C exhibited a wide voltage window of −1.1 to 0 V with a specific capacitance of 385 F g−1 at 1 A g−1 and 88.9% capacitance retention after 10000 cycles. Notably, the symmetric device fabricated with VN/C//VN/C exhibited a high energy density of 24.2 Wh kg−1 at a power density of 1050 W kg−1. Besides, excellent cycling stability (93.3%) was obtained at a current density of 5 A g−1 after 10000 cycles. The combinations of VN and carbon should also inspire the design and fabrication of high-performance electrode materials for supercapacitors and provide more detail information for the performance of VN.
5. Acknowledgments This work is supported by the Open Fund of the Key Laboratory of Xinjiang Uygur Autonomous Region (2017D04014), the National Natural Science Foundation of China (U1703251 and 21571152), Program for Tianshan Innovative Research Team of Xinjiang Uygur Autonomous Region (2018D14002), Scientific Research Program of the Higher Education Institution of Xinjiang (XJEDU2017A001).
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Highligts: A facile hydrothermal method was used to fabricate coralloid-like nanostructure vanadium nitride/carbon (VN/C) nanocomposites. The
supercapacitor
negative
electrode
based
on
coralloid-like
VN/C
nanocomposites exhibit a high specific capacitance of 385 F g−1 at 1 A g−1. The electrode exhibits extraordinary cycling stability with 88.9 % capacitance retention after 10000 cycles.
There are no conflicts to declare.