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Energy, Environmental, and Catalysis Applications
Interfacial Constructing Flexible V2O5@Polypyrrole Core-Shell Nanowire Membrane with Superior Supercapacitive Performance Jian-Gan Wang, Huanyan Liu, Hongzhen Liu, Wei Hua, and Minhua Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05660 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018
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ACS Applied Materials & Interfaces
Interfacial Constructing Flexible V2O5@Polypyrrole Core-Shell Nanowire Membrane with Superior Supercapacitive Performance Jian-Gan Wang, †, ‡* Huanyan Liu, † Hongzhen Liu, † Wei Hua, † Minhua Shao ‡* †
State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of
Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), Xi’an 710072, China ‡
Department of Chemical and Biological Engineering, Hong Kong University of Science and
Technology, Clear Water Bay, Kowloon, Hong Kong 999077, China
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ABSTRACT
Flexible membrane consisting of ultralong V2O5@conducting polypyrrole (V2O5@PPy) core-shell nanowires is prepared by a facile in situ interfacial synthesis approach. The V2O5 is for the first time demonstrated to show versatile function of reactive template to initiate the uniform and conformal polymerization of PPy nanocoating without the need for extra oxidants. The freestanding PPy-encapsulated V2O5 nanowire membrane is of great benefit in achieving strong electrochemical harvest by increasing electrical conductivity, shortening ion/electron transport distance, and enlarging electrode/electrolyte contact area. When evaluated as binder- and additive-free supercapacitor electrodes, the V2O5@PPy core-shell hybrid delivers a significantly enhanced specific capacitance of 334 F g-1 along with superior rate capability and improved cycling stability. The present work would provide a simple yet powerful interfacial strategy for elaborate constructing V2O5/conducting polymers toward various energy storage technologies. KEYWORDS: V2O5; conducting polymer; nanocomposite; supercapacitor; reactive temple synthesis
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INTRODUCTION
The worldwide concern on the energy and environmental problems has driven the scientists and engineers to develop sustainable and renewable power sources and their related energy storage technologies. Among the well-developed energy storage systems, electrochemical capacitors, also called supercapacitors, have triggered significant research activities owing to their prominent characteristics of high power density, fast charge/discharge rate, long cycling lifetime, and low maintenance.1 The unique electrochemical properties make supercapacitors promising in a myriad range of applications ranging from electronic devices, electric vehicles, to large-scale smart electric grid. According to the charge storage mechanism, supercapacitors can be classified into electrochemical double layer capacitors (EDLCs) and pseudocapacitors.2, 3 EDLCs store energy by electrostatic ion adsorption/desorption at the electrode/electrolyte interfaces,4 while the charges stored in pseudocapacitors come from the fast and reversible Faradaic redox reactions occurred on the subsurface of the active materials.5, 6 Generally, compared with porous carbonaceous materials that used in EDLCs,7 pseudocapacitive electrode materials (e.g., metal oxides and conducting polymers) could deliver much higher specific capacitance,8-10 which enables them more attractive for the next-generation supercapacitors.
Among various pseudocapacitive candidates, vanadium pentoxide (V2O5) is considered to be promising electrode materials due to its excellent electrochemical activity, natural abundance in earth, and low cost.11 It is well-known that V2O5 is typical of orthorhombic layered structure that allows easy insertion/extraction of ions accompanying with multiple valence transition between V(II) and V(V) states.12 Pioneering work introduced by Goodenough demonstrated that V2O5 could be used as ideal pseudocapacitive materials in a mild aqueous electrolyte.13 Nevertheless, the electrochemical ACS Paragon Plus Environment
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activity of V2O5 is greatly hindered by its poor electrical conductivity (10-3-10-5 S cm-1), vanadium dissolution in electrolyte, and structural instability caused by a large volume change during cycling.14, 15
To mitigate these limitations, great efforts have been devoted to engineering different V2O5
nanostructures16-18 or their nanocomposites. The most effective strategy up to now is to hybridizing V2O5 nanostructures with conductive materials, such as carbon nanotubes (CNTs),12, 19, 20 graphene,11, 21-24
carbon nanofibers,25, 26 polyaniline (PANI),27 and polypyrrole (PPy)28, 29. Notwithstanding these
advances, the rational design as well as the precisely controlled synthesis of a desired hybrid morphology is still a great challenge to fulfill the rigid requirements of high-performance supercapacitors.
In recent years, ever-increasing research interests have been dedicated to develop flexible smart electronic devices,30 which will inevitably have a prosperous market in future daily life. Flexible energy storage devices play an important role in the development of flexible electronics, and accordingly, electrode materials are critically necessary to meet the mechanical flexibility.31 The design approaches include (i) fabrication of freestanding and flexible materials and (ii) direct growth or casting of active materials on an extra-flexible substrate (such as carbon cloth).26, 32 It should be noted that the former approach is more fascinating because it not only excludes the use of insulating binder and conductive additives that may degrade the electrode performance, but also eliminates the assistance of a current collector which accounts for 30-50 wt.% of the total electrode weight. Therefore, the purpose of this study is to develop a rational fabrication strategy of flexible V2O5– based materials for fully maximizing the electrochemical harvest.
In this work, we propose a totally new versatile synthesis method of fabricating flexible V2O5@PPy core-shell nanowire membranes for high-performance supercapacitors. For the first time, ACS Paragon Plus Environment
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V2O5 is demonstrated as reactive (oxidative) templates to guide in situ interfacial formation of conducting PPy nanocoatings. The strategy allows uniform PPy sheathed on V2O5 matrix, and more notably, avoids the use of additional oxidants, thereby enabling our route easy processing, low cost, and eco-friendly. Benefiting from the core-shell nanoheterostructure, a strong synergistic effect is achieved on the flexible V2O5@PPy electrode with significant improvement in the electrochemical activity and stability of V2O5. EXPERIMENTAL SECTION
Materials Synthesis. All the chemical reagents were purchased from Sinaparm. V2O5 nanowires were prepared by a solvothermal method. In a typical procedure, 0.36 g of commercial V2O5 powders and 5 ml of H2O2 (30 wt.%) were dissolved into 30 ml of deionized water under magnetic stirring for 30 min. Subsequently, the resulting dark red mixture was transferred into a 50 ml Teflon-lined stainless steel autoclave and maintained at 200 °C for 12 h. After natural cooling down to room temperature, orange precipitates were collected, rinsed, and dried. The V2O5@PPy hybrid nanowires were fabricated by in situ interfacial reactive template method using sodium dodecylbenzenesulfonate (NaDBS) as surfactant and dopant. Specifically, 100 mg of V2O5 nanowires and 200 mg of NaDBS were dispersed into 100 ml of deionized water under sonication for 1 h. The pH value of the solution was adjusted to 1 by adding diluted H2SO4. After cooling down to 0-5 °C in an ice bath, 25 µl of purified pyrrole monomers were added into the V2O5 suspension under stirring and maintained for 4 h to complete the polymerization reaction. The solution color evolved from orange to black, indicating the formation of PPy. Finally, the black products were collected followed by rinsing with ethanol and deionized water for three times, respectively. The flexible membranes were fabricated by a simple vacuum filtration technique. Briefly, 30 mg of the resulting nanowires were dispersed into ACS Paragon Plus Environment
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50 ml of deionized water under sonication and then filtered onto a filter paper. Circular membranes with a diameter of 5 cm were peeled from the filter paper and the mass density of the active materials is about 1.5 mg cm-2.
Materials Characterization. The microstructure of the samples were examined by field-emission scanning electronic microscopy (FE-SEM, NanoSEM 450, FEI) and transmission electron microscopy (TEM, FEI TalosF200X). X-ray diffraction (XRD, Shimadzu XRD-7000) was used to determine the crystallographic structure. N2 adsorption/desorption measurement was carried out to determine the Brunauer–Emmett–Teller (BET) specific surface area of the sample. Raman spectra were recorded on a Renishaw inVia Raman Spectrometer under a laser excitation wavelength of 532 nm. Fourier transform infrared spectrum (FTIR) was obtained by ThermoNicolet iS50 FTIR spectrometer. The mass content of PPy is estimated by thermogravimetric analysis (TGA, Mettler Toledo TGA/DSC 3+) in air. The surface compositional states were examined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific).
Electrochemical Measurements. The binder- and additive-free working electrodes were directly punched from the flexible membranes for electrochemical evaluation. The electrochemical measurements of cyclic voltammetry (CV), galvanostatic charge/discharge test, and electrochemical impedance spectrum (EIS) were carried out on a Solartron Electrochemical Station (1460E) using a three-electrode configuration, in which a Pt foil and a saturated calomel electrode (SCE) were served as the counter and reference electrodes, respectively. 1.0 M Na2SO4 aqueous solution was used as the electrolyte. Symmetric cells were also assembled by two identical freestanding electrodes. The specific capacitance is calculated using the discharge curves.
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RESULTS AND DISCUSSION
Figure 1 schematically illustrates our design and synthesis strategy of V2O5@PPy core-shell nanowires. Ultralong V2O5 nanowires are first prepared by a simple hydrothermal method. The ultralong one-dimensional (1D) nanostructures are important to enable the formation of flexible and freestanding membranes, as shown in the digital photo of the orange membrane. To obtain the unique core-shell configuration, the key issue is creating uniform PPy nanolayers surrounding the V2O5 nanowires. In this study, a facile in situ interfacial polymerization method is proposed for the first time to achieve this purpose. The interfacial polymerization is based on the redox reaction between V2O5 and pyrrole monomers. It is worth noting that the oxidation potential (Eo) of V2O5 in an acidic environment is 0.957 V (half reaction: V2O5 + 6H+ + 2e- → 2VO2+ + 3H2O),33 which is higher than that of pyrrole monomers (Eo=0.7 V)34. In other words, V2O5 is capable of serving excellent oxidants to initiate the polymerization of pyrrole monomers. As the formation of PPy occurs only at the pyrrole/V2O5 interfaces, V2O5 nanowires are actually functioning as reactive templates to guide the construction of V2O5@PPy core-shell nanoheterostructure. The use of NaSDBS surfactant herein is of great dual-benefit in dispersing the V2O5 nanowires and enhancing the electronic conductivity of PPy by SDBS-doping during preparation.29,
35
This novel in situ interfacial strategy not only
facilitates the uniform formation of PPy nanolayers, but also eliminates additional oxidants (e.g., FeCl3 or Na2S2O8) that are used in most conventional methods.29 Consequently, the 1D nanowire structure can be well preserved to maintain the membrane (black one) with good mechanical flexibility (Figure 1). The thickness of the membrane is about 60 µm from the cross-sectional image (Figure S1). Furthermore, it is important to note that the present synthesis strategy provides a powerful and versatile approach that may be extended to hybridizing V2O5 with other conducting
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polymers, such as PANI (Eo=0.5 V)36 and poly(3,4-ethylenedioxythiophene) (PEDOT, Eo=0.75 V)37.
Figure 1. Schematic illustration of the in situ interfacial synthesis process of V2O5@PPy nanowire membrane.
Figure 2(a) and S2 show the typical SEM morphology of the as-prepared V2O5 nanowires. It is clearly observed that the freestanding membrane is composed of ultralong and straight nanowires with random orientation and smooth surface. The 1D nanostructures are of hundreds of micrometers in length and ~80 nm in diameter, corresponding to an aspect ratio as high as >1000. The high aspect ratio is beneficial in the nanowire entanglement to form a flexible and porous membrane architecture. As mentioned earlier, the in situ interfacial polymerization of PPy is based on the reactive template of V2O5, and accordingly, PPy nanolayer would intimately coated onto the solid nanowire backbone. The resulting V2O5/PPy hybrid morphology is displayed in Figure 2(b-c). Encouragingly, the panoramic SEM image is identical to the pristine one, revealing the hybrids comply with the 1D
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nanowire morphology and porous architecture of V2O5. In addition, the surface of the hybrids is clean without any granule impurities (Figure 2(c)), suggesting that our in situ interfacial synthesis strategy is advantageous over the conventional methods using additional oxidants. More microstructural information is provided by TEM imaging. As shown in Figure 2(d-e), the PPy nanolayer is uniformly and conformally sheathed onto the V2O5 scaffold, thus constructing a unique core-shell configuration. The thickness of the PPy nanolayer is estimated to be approximately 15 nm. The corresponding SAED pattern (inset) shows well-defined diffraction spots that can be readily assigned to the orthorhombic structure, indicating high-quality single crystalline nature of the V2O5 nanowires. The HRTEM image in Figure 2(f) exhibits distinct lattice fringes with an interplanar spacing of 0.218 nm, which agrees well with the distance of the (020) planes of V2O5.
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Figure 2. SEM images of (a) V2O5 and (b, c) V2O5@PPy nanowires. (d, e) TEM and (f) HRTEM images of V2O5@PPy (SEAD, inset).
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Figure 3. (a) XRD patterns and (b) Raman spectra of V2O5 and V2O5@PPy. (c) FT-IR spectra and (d) TG analysis of pristine V2O5 and V2O5@PPy. The phase structure and component of the as-prepared samples is investigated by XRD, Raman and FTIR analysis. As shown in Figure 3(a), all of the diffraction peaks can be well indexed to the orthorhombic V2O5 (PDF # 41-1426).26, 38 The sharp profile of the diffraction peaks indicates the good crystallinity of the samples. In addition, no other peaks are detected, revealing a high purity of the products. The presence of conducting PPy is further confirmed by Raman spectra and FTIR. As shown in Figure 3(b), in addition to the fundamental V-O Raman scattering peaks of V2O5 phase,18 the broad peaks in the band range of 1200-1600 cm-1 can be ascribed to the characteristic stretching of pyrrole ring and C-C/C=C backbone as well as in-plane deformation of N-H bonds.39 More
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structural details of the products can be obtained by FTIR examination (Figure 3(c)). The pure V2O5 displays stretching absorption modes of V=O bond at 1021 cm-1 and V-O-V bond at 854 and 520 cm-1.21, 40 The adsorption peaks at 1555 and 1461 cm-1 are assigned to the backbone stretching vibrations of C=C/C-C in the pyrrole ring.41 The peaks at 1305 and 1046 cm-1 belong to the in-plane deformations of C-H bond. The peaks at 1191 and 1097 cm-1 can be ascribed to the in-plane deformatins of C-N and N-H bonds, respectively. It is noted that the C-H out-of-plane vibration bands at around 964 cm-1 are induced by doping DBS-anions, which is beneficial for enhancing the electronic conductivity of PPy.35 The mass content of PPy in the hybrid is estimated to be 23.2 wt.% from the TGA measurement (Figure 3(d)). The BET specific surface area of the hybrid is determined to be about 60 m2 g-1 (Figure S3).
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Figure 4. (a) XPS survey spectrum of V2O5@PPy film and the core level spectra of (b) V 2p, (c) C ACS Paragon Plus Environment
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1s (d) N 1s.
The chemical composition and elemental states of the V2O5@PPy are investigated by XPS technique. The survey spectrum (Figure 4(a)) shows the main elemental signals of V, O, C, and N with no trace of other impurities. The core-level spectrum of V 2p (Figure 4(b)) exhibits a V 2p1/2 peak at 525.0 eV and a V 2p3/2 peak at 517.5 eV, which are characteristic of V5+ states.42 The C1s spectrum (Figure 4(c)) can be well fitted by three components centered at 284.2, 285.0, and 286.5 eV, corresponding to C-C, C-N, and C-O bonds, respectively.43-45 The N1s spectrum (Figure 4(d)) with a binding energy at 400.4 eV is indicative of pyrrolic N species in the PPy component.28 The O 1s spectrum (Figure S4) is composed of oxygen species of V-O bonds (531. 1 eV) and –OH (532.6 eV). All these structural and chemical information indicates the successful introduction of PPy nanolayers into the products.
The flexible V2O5@PPy core-shell nanocomposite is evaluated as binder- and conductive additive-free electrodes for supercapacitors. Figure 5(a) shows the CV curves at various scan rates in a mild Na2SO4 solution. The CV curves exhibit quasi-rectangular shape with good mirror symmetry and rapid voltammetric response, which is characteristic of fast and reversible pseudocapacitive behavior of V2O5. The associated charge storage mechanism involves the surface chemisorption or subsurface intercalation of alkaline ions into V2O5 (i.e., V2O5 + xNa+ + xe- → NaxV2O5).13, 16 Notably, as the scan rate increases to 50 mV s-1, the hybrid electrode maintains a good CV shape, indicating excellent rate capability, which can be attributed to the improved electronic conductivity caused by the PPy nanoshells. For comparison, the pristine V2O5 nanowire electrode exhibits poor
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electrochemical performance and large polarization due to the poor conductivity of V2O5 (Figure S5). This suggests that the electrochemical activity of V2O5 is substantially enhanced by introducing conducting PPy nanoshells.
Galvanostatic charge/discharge measurement was carried out to validate the excellent electrochemical performance of V2O5@PPy. Figure 5(b) displays the charge/discharge curves of the hybrid electrode at different current densities. All curves are of triangular profile with good linearity and small resistance drop, again demonstrating that the hybrid electrode possesses superior supercapacitive behavior. In addition, the charge branches are highly symmetric to the discharge counterparts, indicating a high Coulombic efficiency. The specific capacitance is calculated according to the discharge curves. As shown in Figure 5(d), the specific capacitance of the V2O5@PPy nanocomposite is determined to be 344 F g-1 at a current density of 0.2 A g-1, which is much higher than that of pristine V2O5 (226 F g-1), pure PPy (234 F g-1), and also most of previously-reported V2O5-based electrodes (Table S1). More remarkably, when the current density is increased by 50 times to 10 A g-1, the nanocomposite electrode could still deliver a specific capacitance as high as 189 F g-1, indicating its excellent rate performance. The smaller specific capacitance at a higher current rate can be ascribed to the reduced diffusion time. In sharp contrast, the bare V2O5 electrode almost loses the charge storage capability at this current rate. The increase in the utilization efficiency of V2O5 is readily attributable to the improved charge transfer kinetics derived from the conducting PPy pathways. In addition, the presence of PPy could also make pseudocapacitance contribution to the overall capacitance because the PPy itself is highly Faradic in nature through a couple of reversible doping/undoping redox reactions (Figure S6).46 Furthermore, the V2O5@PPy-assembled symmetric supercapacitor exhibits excellent supercapacitive performance
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with a high specific capacitance of 308 F g-1 (Figure S7), demonstrating the potential application of the hybrid materials.
EIS is carried out to gain an in-depth understanding of the supercapacitive behavior of the nanocomposite. The resulting Nyquist plots is present in Figure 5(d). Both the EIS profiles are composed of a depressed semicircle in the high frequency region and a straight line in the low frequency region. The significant difference is the diameter of the semicircle, which represents the charge transfer resistance (Rct) of the electrode.47 Evidently, the Rct of the V2O5@PPy electrode (7.6 Ω) is no more than half of the pristine one (17.8 Ω). The reduced electrode resistance of the nanocomposite is responsible for the high-rate delivery.
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Figure 5. (a) CV curves and (b) charge/discharge profiles of V2O5@PPy electrode. (c) Specific capacitance vs. current density, (d) Nyquist plots, and (e) cycling performance of V2O5@PPy and pure V2O5 electrodes. Long cycling stability is an important criterion for practical application of supercapacitor. V2O5 is vulnerable to structure damage caused by the large volume change during ion insertion/extraction cycling operation. And accordingly, the bare V2O5 electrode suffers from fast electrochemical failure, as shown in Figure 5(e). When a thin nanoshell of PPy is sheathed on the V2O5, the cycling performance of the hybrid electrode is impressively enhanced with only 17.5% capacitance degradation after 2000 cycles. It is worth noting that conducting polymers generally show poor cycling stability attributable to repetitive swelling/shrinkage of the polymer chains.5 The good cycling stability of the hybrid suggests a strong shape synergistic effect of V2O5 and PPy. Specifically, the PPy nanoshells could accommodate the volume excursion of the underneath V2O5, whilst in turn, the V2O5 cores could provide solid backbones to maintain the polymer integrity by interlinking the PPy chains.41 To confirm the robust structural stability, the morphology of the V2O5@PPy electrode after 2000 cycles is examined. As shown in Figure S8, the hybrid electrode has good preservation of the nanowire structure without any cracks and agglomerations, demonstrating
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the excellent electrode integrity.
Figure 6 shows the energy storage characteristics and synergistic advantages of the V2O5@PPy core-shell nanowire electrode. First, the conducting PPy nanoshells greatly assist in accelerating the charge transfer efficiency of V2O5 to achieve maximum electrochemical utilization (high specific capacitance). Second, the nanosized heterostructure shortens the ion/electron transport distance to enable fast electrode kinetics for better rate capability. Third, the highly porous network facilitates electrolyte ingress and penetration throughout the whole electrode and thus enlarges the electrode/electrolyte contact area. Fourth, the interactive mechanical combination of each component collectively improves the electrode stability during long cycling test. And finally, the freestanding membrane eliminates the need of insulating binders and conductive additives that may bring sluggish reaction kinetics and blockage of electro-acitve sites. As a result, the flexible V2O5@PPy core-shell nanowire membrane manifests outstanding supercapacitive performance.
Figure 6. Charge storage characteristics of V2O5@PPy core-shell nanowires.
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CONCLUSIONS
In summary, conducting PPy-encapsulated V2O5 core-shell nanowire membranes have been successfully prepared by a novel in situ interfacial polymerization method. V2O5 is demonstrated to be excellent oxidative templates enabling uniform and conformal formation of PPy nanoshells. The unique core-shell nanoheterostructure renders strong electrochemically synergistic interactions of improved
electrical
conductivity,
reduced
ion/electron
transport
pathways,
enlarged
electrode/electrolyte interfaces, and enhanced structural stability. A high specific capacitance of 334 F g-1 is achieved for the flexible V2O5@PPy electrode together with high-rate and long-cycling performance. We believe that the present design strategy could pave a new feasible avenue of developing V2O5-conducting polymers for various energy storage systems. Supporting information
Cross-sectional SEM image and N2 adsorption/desorption isotherm of V2O5@PPy. SEM and TEM images of pure V2O5 nanowire. Core-level XPS spectrum of O 1s. CV and charge/discharge curves of pure V2O5 PPy, and V2O5@PPy assembled symmetric supercapacitor. SEM and TEM images of V2O5@PPy nanowire electrode after cycling test. A table summary of supercapacitive performance of V2O5-based materials. Corresponding Authors
*E-mail:
[email protected] (J.-G. Wang);
[email protected] (M. Shao)
ACKNOWLEDGMENTS
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The authors thank the research supports from National Natural Science Foundation of China (51772249, 51521061), Fundamental Research Funds for the Central Universities (G2017KY0308), Hong Kong Scholars Program (XJ2017012), and State Key Laboratory of Control and Simulation of Power System and Generation Equipment (Tsinghua University, SKLD17KM02).
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