Facile Fabrication of Three-Dimensional Hierarchical

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Facile Fabrication of Three-Dimensional Hierarchical Nanoarchitectures of VO2/Graphene@NiS2 Hybrid Aerogel for High-Performance AllSolid-State Asymmetric Supercapacitors with Ultrahigh Energy Density Hsieh-Chih Chen, Yi-Cih Lin, Yan-Lin Chen, and Chi-Jen Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01486 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Applied Energy Materials

Facile Fabrication of Three-Dimensional Hierarchical Nanoarchitectures of VO2/Graphene@NiS2 Hybrid Aerogel for HighPerformance All-Solid-State Asymmetric Supercapacitors with Ultrahigh Energy Density Hsieh-Chih Chen,* Yi-Cih Lin, Yan-Lin Chen, and Chi-Jen Chen Department of Fiber and Composite Materials, Feng Chia University, Taichung 407, Taiwan KEYWORDS: asymmetric supercapacitors, graphene aerogel, vanadium oxide, nickel sulfide, hierarchical structure.

ABSTRACT

In this study, a hierarchical mesoporous VO2/graphene@NiS2 hybrid aerogel is synthesized through the sol–gel and hydrothermal growth approaches. The resultant VO2/graphene@NiS2 hybrid aerogel electrode offers a high specific capacitance up to 1280.0 F g-1 (142.22 mAh g-1) at a current density of 1 A g-1 and exhibits an outstanding cycling performance. The distinguished

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electrochemical performance of VO2/graphene@NiS2 electrode is attributable to the synergistic effect among VO2 particles, graphene networks, and NiS2 nanoflakes, which not only increases the specific surface area and conductivity but also gives extra pseudocapacitance for VO2/graphene@NiS2 hybrid aerogel electrode. In addition, a high-performance all-solid-state asymmetric supercapacitor (SC) is successfully assembled through the novel 3D interconnected VO2/graphene@NiS2 hybrid aerogel as the positive electrode, graphene aerogel as the negative electrode and PVA/KOH gel as the electrolyte. The assembled VO2/graphene@NiS2 hybrid aerogel//graphene aerogel device achieves a superior energy density of 60.2 Wh kg-1 at a power density of 350.0 W kg-1 and sustains 86.2% of its initial capacitance after 10,000 charge/discharge cycles. These results reveal that the multiple composite of VO2/graphene@NiS2 hybrid aerogel is a promising candidate material for high-performance SCs in virtue of its high capacitance, conspicuous energy density and long-term cycling stability.

INTRODUCTION Nowadays, supercapacitors (SCs) with merits of high power density, fast charging-discharging characteristics, excellent reversibility, large capacity, long-term stability and good operational safety have been recognized as the future portable energy storage devices.1-10 Two kinds of SCs through either accumulation of electrons at the electrode-electrolyte interface (namely electrical double-layer capacitors, EDLCs) or fast faradaic redox reactions of the electrode with electrolyte (namely pseudocapacitors) are emerging as the most promising energy storage devices for the future. In addition, all-solid-state asymmetric SCs with advantages of flexibility, safety and light weight have gained great attention for smart and efficient energy storage devices because of the incessant increasing requirements for wearable and portable electronics.11-15 However, most SCs


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still suffer from relatively low energy densities as compared with the rechargeable batteries, which restrict their further development in widespread applications.5,8,16,17 For realistic applications, both of the working voltage and energy density of as-fabricated SCs ought to ameliorate to fulfill the developing demands of emerging technologies for the future. Notably, the electrochemical performances of SCs are primarily determined by not only the compositions and electrical conductivity of the active materials but the assembled configuration of electrodes as well, which are crucial for simultaneously boosting the efficiency of ion and electron transfer. For EDLCs, carbon materials like graphene sheets, a two-dimensional (2D) structure of covalently bonded carbon atoms, exhibit high and stable double layer capacitance because of the remarkable large surface area, high electrical conductivity and good electrochemical stability.18 However, graphene sheets tend to restack themselves owing to the existence of strong π–π stacking interactions between graphene sheets, leading to decreasing the accessible surface area, obstructing the rapid electrolyte diffusion, and raising the electrical and diffusion resistance, and thereby diminish the performances of graphene-based electrodes.19 Nevertheless, selfassembling 2D graphene sheets into 3D interconnected graphene networks such as aerogels and sponges possess advantages of high conductivity, low density, porous structure, and high surface area, which could boost the ions transport between the electrode and electrolyte.20-23 Therefore, these promising characteristics motivate researchers to further design and synthesize novel 3D network-based composites to improve the specific capacitance. Although the 3D porous graphene structure offers high power density, excellent rate capacitance and cycling stability, the energy density is still limited on account of the electrical double-layer charge storage mechanism. Conversely, transition metal oxides/hydroxides,24-26 and conducting polymers27,28 with multiple oxidation states deliver larger specific capacitances and higher energy densities for



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pseudocapacitors in comparison with EDLCs due to the fact that pseudocapacitors store energy through a fast reversible multielectron faradaic reactions on the surfaces of electrode materials.29,30 Consequently, 3D graphene networks coupled with pseudocapacitive materials demonstrate an efficient strategy for improving the electrochemistry performances.31,32 In terms of transition metal oxides, vanadium oxides are known to be a good candidate as the pseudocapacitive materials because of their abundant resources, low cost, multiple oxidation states, and substantial specific capacitance, which deliver higher charge storage capability and wider working potential window.33-35 In spite of the greatly enhanced specific capacitance achieved by means of introduction of the transition metal oxide for raising the pseudocapacitance, graphene/transition metal oxide composites usually endure low specific surface area, fast capacity dropping, inferior rate performance, and poor stability. The results can be attributed to the self-aggregation, dissolution, and the fast-increased charge transfer resistance during cycling.36 Recently, nanostructured transition metal dichalcogenides such as MoS2, WS2 and CoS2 as active materials for SCs have been attracting more and more attention because of their unique advantages, such as 2D nanosheet structure, high conductivity, sufficient surface area, multivalent oxidation states and outstanding capacitance.37-39 Indeed, combining the transition metal dichalcogenide with the conductive matrix like graphene could ameliorate the SC’s capacitance. Unfortunately, this hybrid composite has poor cycling ability due to the structure deteriorates in the redox process.37-39 In this context, multiple component electrode with 3D hierarchical porous yet densely packed nanostructures ought to be designed so as to solve the inherent drawbacks of low energy density and stability of cycling performance. We report herein, for the first time, a straightforward synthetic methodology for preparing a novel electrode based on VO2/graphene@NiS2 aerogel for


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SCs with simultaneously having superior conductivity associated with power density of the asmade electrode and high mass loading related to energy density of the as-made electrode. The results demonstrated that VO2/graphene@NiS2 aerogel electrode presented prominent electrochemical performance, high capacitance and long-term cycling performance than those of the VO2/graphene and graphene@NiS2 counterparts. Moreover, the assembled all-solid-state VO2/graphene@NiS2 hybrid aerogel//graphene aerogel asymmetric SCs possessed an output voltage of 1.4 V, an energy density of 60.2 Wh kg-1 at a power density of 350.0 W kg-1, and a cycling stability of 86.2% capacitance retention after 10,000 charge/discharge cycles. All these results demonstrated that our all-solid-state asymmetric SC with both high voltage and remarkable energy density could potentially meet the demands for the future portable electronics. RESULTS AND DISCUSSION The synthesis procedure for VO2/graphene@NiS2 hybrid aerogel is schematically illustrated in Figure 1. The VO2/graphene@NiS2 hybrid aerogel was prepared through facile sol–gel and hydrothermal procedures with graphene nanosheets as the deposition substrate. Firstly, a small amount of the VO2 powder coupled with vitamin C as the reductant were added into the graphene oxide (GO) suspension to reduce the GO, and then the reaction was executed at 80 °C for 3 h. The color of the suspension became darker and a cylindrical hydrogel of VO2/graphene was obtained. In this process, VO2 particles could insert in the space between graphene sheets through the covalent bonding with the oxygen functional groups of GO nanosheets.40,41 Afterwards, the asprepared hydrogel was converted to the aerogel by a simple freeze-drying process. Subsequently, VO2/graphene aerogel composite was immersed into the NiCl2, thiourea, isopropanol, and ammonia reaction solution at 180 °C for 24 h, followed by the freeze-drying for 24 h to form the VO2/graphene@NiS2 hybrid aerogel.



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Figure 1. Fabrication process of VO2/graphene@NiS2 hybrid aerogel. Field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM) were employed to investigate the morphology and detailed structure of the resultant composites. The surface of graphene aerogel shows a typical wrinkle morphology as shown in Figure S2a, Supporting Information. The FE-SEM image of graphene@NiS2 aerogel reveals many sheet-like morphology of NiS2 with a thickness of 30-50 nm and length of about 500 nm (Figure S2b, Supporting Information). In sharp contrast with graphene@NiS2 aerogel, VO2/graphene@NiS2 aerogel (see Figure 2a) comprises polygonal particles with a size of 450-650 nm embedded between graphene networks to construct an interconnected loosely packed 3D porous structure, which provides not only more active sites for the electrolyte ions diffuse into the deep interfacial area during the charge/discharge process, but also diminish the aggregation of graphene to make good contact with the outlying NiS2 nanoflakes (vide infra). HR-TEM (Figure 2b) further confirms the existence of VO2 particles and NiS2 nanoflakes on graphene by revealing clear lattice spacings of 0.52, 0.61, 0.31, and 0.10 nm, which


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are in accord with the (201) and (001) planes for VO2, and the (200) and (511) planes for NiS2, respectively. Also, the corresponding selected area electronic diffraction (SAED) pattern (inset of Figure 2b) corroborates the polycrystalline nature of VO2/graphene@NiS2 aerogel composite. The homogeneous distribution of Ni, S, V, O, and C elements was further confirmed by EDX element mappings of VO2/graphene@NiS2 hybrid aerogel (Figure 2c), suggesting that NiS2 nanoflakes are uniformly grown to coat the surface of VO2/graphene composite. The specific surface area and porous property analysis were further performed by measuring the nitrogen adsorption/desorption isotherms shown in Figure S3, Supporting Information. The hysteresis loops at relative pressure (P/P0) between 0.2 and 1.0 demonstrate the presence of mesoporous structure. Without VO2, graphene@NiS2 aerogel revealed a specific surface area of 127.8 m2 g-1 with an average pore size of 17.1 nm. In contrast, VO2/graphene@NiS2 aerogel displayed an augmented specific surface area of 141.4 m2 g-1 with an average pore size of 17.3 nm. Indeed, VO2/graphene@NiS2 hybrid aerogel with higher surface area can assist with decreasing resistance for electrolyte ions into the inner interface during the charge/discharge process as well as reducing aggregation of graphene to yield uniformly connected with NiS2 nanoflakes.



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Figure 2. (a) FE-SEM image of the as-prepared VO2/graphene@NiS2 hybrid aerogel. (b) HRTEM (with SAED inset) image of VO2/graphene@NiS2 hybrid aerogel. (c-h) Corresponding element mappings of various elements. The crystallinities of graphene, VO2/graphene, graphene@NiS2, and VO2/graphene@NiS2 hybrid aerogels were identified by XRD patterns (Figure 3a). Only an obvious diffraction peak at around 25° can be seen, which can be assigned to the (002) plane of graphene nanosheets.42 The diffraction peaks at 14.4°, 29.0°, 33.9°, 45.1° and 49.5° can be assigned to the (001), (002), (311), ( 511 ) and (312) planes of VO2 (JCPDS 31-1438), suggesting that monoclinic VO2 was successfully embedded in the VO2/graphene aerogel composite.43 Moreover, the diffraction peaks at 31.5°, 35.3°, 38.8°, 45.1°, 48.0°, and 53.5° can be ascribed to the (200), (210), (211), (220), (221) and (311) planes of NiS2 (JCPDS 89-1495).44 All the characteristic peaks observed in VO2/graphene@NiS2 hybrid aerogel revealed the successful coexistence of graphene, VO2 and NiS2 in the composite frameworks. Raman spectroscopy was conducted to evaluate the carbon structure and quality (Figure 3b). It can be noted that all the samples exhibit two significant peaks


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located at 1336 and 1602 cm-1, according with the typical D and G bands of carbonaceous materials, respectively. The relative intensity ratios of D and G bands (ID/IG) for graphene, VO2/graphene, graphene@NiS2, and VO2/graphene@NiS2 hybrid aerogels have been estimated to be 1.27, 1.16, 1.21, and 1.08, respectively, indicating that VO2/graphene@NiS2 hybrid configuration could effectively restore the conjugated sp2 carbons in graphene aerogel.45 That is, VO2 particles embedded in the graphene could reduce the defects through the covalent bonding with the oxygenincluding functional groups of the graphene aerogel networks.40,41

Figure 3. (a) XRD spectra collected for as-prepared graphene, VO2/graphene, graphene@NiS2, and VO2/graphene@NiS2 hybrid aerogels. (b) Raman spectra of as-prepared graphene, VO2/graphene, graphene@NiS2, and VO2/graphene@NiS2 hybrid aerogel composites. The chemical composition and oxidation state of aerogels were characterized by X-ray photoelectron spectroscopy (XPS) measurement. The Gaussian fitting of the high-resolution Ni 2p spectrum (Figure 4a) depicts two principal peaks, Ni 2p3/2 and Ni 2p1/2 at 854.8 and 872.6 eV, respectively, along with a spin-energy separation of 17.8 eV and two shakeup satellite peaks (denoted as Sat.).46 The S 2p spectrum (Figure 4b) could be fitted with three main peaks at binding energies of 162.4, 163.6 and 168.4 eV. Meanwhile, the peaks centered at 162.4 and 163.6 eV can be attributed to the signals for S 2p3/2 and S 2p1/2 of metal-sulfur bonds, respectively.47 Moreover, the peak located at 168.4 eV can be attributable to the oxidation of S atoms and formation of SO42-



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byproducts.48 Regarding the high-resolution V 2p spectrum (Figure 4c), a doublet structure at ~517 and ~524 eV can be observed, which can be ascribed to the V 2p3/2 and V 2p1/2 core level binding energies, respectively, resulting from the spin orbit splitting. The V 2p3/2 (V 2p1/2) can divided into three separate signals with binding energies of 515.4 (522.8 eV), 516.2 eV (523.6 eV) and 517.2 eV (524.6 eV), which are correlated with the V3+, V4+ and V5+ oxidation states, respectively.49 In fact, the presence of different oxidation states of vanadium in the aerogel can be observed after hydrothermal treatment. A small amount of lower oxidation state of V3+ can be discovered owing to the over-reduction. Nevertheless, V4+ was found to be the dominating oxidation state. Moreover, the existence of higher oxidation state of vanadium is due to the inevitable oxidation of vanadium when exposed to air.50 The high-resolution C 1s spectrum shown in Figure 4d can be categorized into four peaks centered respectively at 284.4 (C−C), 284.8 (C=C), 285.8 (C−O), and 287.4 eV (C=O), suggesting the existence of oxygen-including functional groups on the surface of graphene aerogel networks.51


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Figure 4. XPS spectra of VO2/graphene@NiS2 hybrid aerogel composite: (a) Ni 2p spectrum, (b) S 2p spectrum, (c) V 2p spectrum, and (d) C 1s spectrum. The electrochemical properties of the as-made aerogel composites were probed by means of cyclic voltammograms (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements in a three-electrode cell using 6 M KOH as the aqueous electrolyte with Ag/AgCl as the reference electrode and Pt plate as the counter electrode. Figure 5a shows a comparative study of the CV curves for aerogel electrodes at a scan rate of 5 mV s-1 within a potential window of 0 to 0.6 V. It is worth noting that all the CV curves exhibited a couple of prominent redox peaks in the same voltage range, suggesting the presence of faradaic redox reactions and pseudocapacitive properties of the resultant electrodes. The corresponding redox reactions are given as following.



VO2 + OH− ⇄ VO2OH + e−

(1)

NiS2 + K+ + e− ⇄ NiS2-K+

(2)

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The pseudocapacitive reaction could involve electrochemical charge transfer which couples with the insertion and deinsertion of K+ ions and is faradically stored in the electroactive sites of NiS2. Moreover, compared to the VO2/graphene and graphene@NiS2 counterparts, the oxidation peak potential of the VO2/graphene@NiS2 hybrid aerogel electrode is shifted towards positive, whereas the reduction peak potential of the VO2/graphene@NiS2 hybrid aerogel electrode is moved to negative, which could be ascribed to the lower conductivity of the VO2/graphene@NiS2 electrode, resulting in a lower reversibility of redox processes.52,53 In addition, VO2/graphene@NiS2 aerogel electrode illustrated a large integrated area and considerable intensity than those of the VO2/graphene and graphene@NiS2 aerogel counterparts, indicating a higher specific capacitance for VO2/graphene@NiS2 aerogel owing to the excellent synergistic effect among VO2, graphene networks, and NiS2 nanoflakes. Consequently, it is obvious that a small amount of VO2 particles embedded in the graphene aerogel could be beneficial to improve the capacitance. Additionally, when the scan rate increases (Figure 5b), the current density of VO2/graphene@NiS2 aerogel electrode raises accordingly and the shapes of CV curves are well-preserved, indicating the good rate capability and excellent redox reaction reversibility of the electrode. Furthermore, when the scan rate increased from 2 to 100 mV s-1, the anodic and cathodic peaks shifted toward the positive and negative voltages, respectively, because of the polarization effect. Furthermore, the capacitive property of the targeted aerogel electrodes was investigated by the GCD measurement, and the results are shown in Figure 5c. It can be seen that all curves exhibit a voltage plateau, which agree well with the redox peak in the CV curves (Figure 5a). Additionally, VO2/graphene@NiS2 aerogel electrode possesses a highest capacitance than those of the


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VO2/graphene and graphene@NiS2 aerogel electrodes owing to its longest discharge time. Furthermore, VO2/graphene@NiS2 has a specific capacitance of 1280.0 F g-1 (142.2 mAh g-1), which is around 3.1 times and 1.1 times higher than those of the VO2/graphene 413.2 F g-1 (46 mAh g-1) and graphene@NiS2 1114.5 F g-1 (123.8 mAh g-1) counterparts, respectively. The remarkable electrochemical performance of VO2/graphene@NiS2 electrode could be attributable to the synergistic effect among VO2 particles, graphene networks, and NiS2 nanoflakes, which expanded the specific surface area, uplifted the electrical conductivity and provided additional pseudocapacitance for VO2/graphene @NiS2 hybrid aerogel electrode as well (vide supra). The typical GCD curves of VO2/graphene@NiS2 hybrid electrode measured at different current densities are depicted in Figure 5d. Again, nonlinear charge/discharge curves can be observed, suggesting the pseudocapacitive nature of the electrodes. The specific capacitance values of VO2/graphene@NiS2 electrode calculated from the GCD curves are 1280.0, 1224.8, 1181.3, 1176.5, and 1047.5 F g-1 (142.2, 136.1, 131.3, 130.6, 116.4 mAh g-1) at the current density of 1, 2, 3, 5, and 10 A g-1, respectively. Apparently, VO2/graphene@NiS2 aerogel electrode demonstrated superior capacitive performances at various current densities in comparison with VO2/graphene and graphene@NiS2 aerogel counterparts (see Figure 5e, Figure S4, Supporting Information, and Table S3, Supporting Information). The schematic illustration of VO2/graphene@NiS2 for energy storage is depicted in Figure S5, Supporting Information. The graphene networks provide a continuous electronic conduction path by bridging the VO2 particles, and NiS2 nanoflakes, which can not only reduce the internal resistance, but also facilitate the ionic transport.2 Furthermore, the specific capacitance of VO2/graphene and graphene@NiS2 electrodes significantly decayed with the enhancement of current density due to the fact that only the outer pores were utilized at high current densities, and the inner mesopores were not capable of conducting further redox reactions.



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Conversely, the large accostable surface area of the mesoporous hierarchical structures of VO2/graphene@NiS2 electrode with good electrical conductivity compelled the electrolyte ions to easier penetrate the active regions of the electrode, and thereby diminished the capacitance dropping at high current density. Moreover, the capacitance retention of VO2/graphene, graphene@NiS2, and VO2/graphene@NiS2 aerogel electrodes was about 51.4%, 67.4%, and 81.8%, respectively, indicating that VO2/graphene@NiS2 electrode exhibited good rate capability (Figure 5e).


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Figure 5. (a) Comparison of the CV curves of VO2/graphene, graphene@NiS2, and VO2/graphene@NiS2 hybrid aerogel electrodes at a scan rate of 5 mV s-1. (b) CV curves of VO2/graphene@NiS2 hybrid aerogel electrode at different scan rates. (c) Comparison of the GCD curves of VO2/graphene, graphene@NiS2, and VO2/graphene@NiS2 hybrid aerogel electrodes at a current density of 1 A g-1. (d) GCD curves of VO2/graphene@NiS2 hybrid aerogel electrode at varied current densities. (e) Specific capacitance of different hybrid aerogel electrodes at various current densities. (f) Cycling performance of different hybrid aerogel electrodes over 5,000 cycles at 10 A g-1.



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The charge transfer and electrolyte diffusion in the electrode/electrolyte interface were studied via electrochemical impedance spectroscopy (EIS) measurements. Figure S6a in the Supporting Information depicts the Nyquist plots of these targeted electrodes, which were measured in a frequency range of 10 kHz to 0.1 Hz. The corresponding equivalent circuit (Figure S6b, Supporting Information) was used to fit the impedance curves. Generally, the solution resistance (Rs) represents the internal resistance of the electrolyte. The diameter of the semicircle illustrates the charge transfer resistance (Rct) of the electrochemical reactions on the electrode, arising from the electrode/electrolyte interface. The slope of the oblique line describes the diffusion resistance (Warburg impedance, W) of the OH− ions within the electrode.54 The impedance shape of these electrodes was similar, comprising a semicircle in the high frequency region and an oblique line in the low frequency region. Moreover, these electrodes have very low Rs, indicating the excellent conductivity of the electrodes. It is worthy to note that VO2/graphene@NiS2 hybrid aerogel electrode has a more vertical straight line and a smallest radius of the semicircle, suggesting the faster ionic diffusion behavior and lower charge-transfer resistance.55 Accordingly, VO2/graphene@NiS2 aerogel electrode has fast electron transport and good charge-transfer character, which are in accordance with the CV and GCD measurements (vide supra). In addition, long-term cycle capability is a crucial factor for actual applications in energy-storage systems. The final specific capacitance of VO2/graphene, graphene@NiS2, and VO2/graphene@NiS2 aerogel electrodes is about 23.5%, 45.7%, and 63.5% as compared with the initial values after 5,000 charge/discharge cycles at a current density of 10 A g-1 (Figure 5f). The higher capacitance retention of VO2/graphene@NiS2 aerogel electrode may be attributed to the fact that VO2/graphene@NiS2 electrode could be closely related to the electrochemical charge transfer, which combines with the insertion and deinsertion of K+ ions and is faradically stored in the


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electroactive (defect) sites of NiS2 nanoflakes. Through the synergy with graphene networks and VO2 particles, the overall cycling stability of VO2/graphene@NiS2 could be superior than that of the VO2/graphene counterpart.52,53

Figure 6. (a) Comparative CV curves of VO2/graphene@NiS2 hybrid aerogel and graphene aerogel, tested in a three-electrode configuration at 10 mV s-1. (b) Specific capacitances of allsolid-state VO2/graphene@NiS2 hybrid aerogel//graphene aerogel asymmetric SC calculated from CV and GCD curves with different current densities of 1, 2, 3, 5, and 10 A g-1 and various scan rates of 5, 10, 20, 50, and 100 mV s-1. (c) Cycling performance of VO2/graphene@NiS2 hybrid aerogel//graphene aerogel asymmetric SC collected at 10 mV s-1. The inset displays a commercial red LED could be lighted up via the asymmetric SC. (d) Ragone plots related to the power and energy densities of all-solid-state VO2/graphene@NiS2 hybrid aerogel//graphene aerogel asymmetric SC device. Asymmetric SC was assembled with VO2/graphene@NiS2 hybrid aerogel as the positive electrode and graphene aerogel as the negative electrode. Figure 6a displays that graphene aerogel electrode was tested at a scan rate of 10 mV s-1 with a potential window of -1.0 to 0 V (vs. Ag/AgCl),



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while VO2/graphene@NiS2 hybrid aerogel electrode was examined with a potential window of 0 to 0.4 V (vs. Ag/AgCl). Therefore, a theoretical operational voltage of 1.4 V can be achieved for our study asymmetric SC, which is higher than those of 0.8-1.0 V of the conventional carbonbased symmetric SCs. A series of CV measurements with increasing the potential windows to explore the electrolyte stability region was carried out (Figure S7, Supporting Information). Indeed, a stable potential window of 0-1.4 V was fulfilled in consequence of the soothing enhancement of the current density by increasing the potential. Additionally, the CV curves of the all-solid-state VO2/graphene@NiS2 hybrid aerogel//graphene aerogel asymmetric SC device display a close-torectangular shape at different scan rates from 2 to 100 mV s-1 (Figure S8a in the Supporting Information), indicating that the capacitance originated from the combined contribution of both pseudocapacitance and electric double-layer capacitance, which resulted from the synergistic effect of the two electrodes. The as-fabricated asymmetric SC device exhibits a high specific capacitance of 222.2 F g-1 (86.4 mAh g-1) at 2 mV s-1 and still maintains a moderate specific capacitance of 18.3 F g-1 (7.1 mAh g-1) even at a high scan rate of 100 mV s-1 (Figure 6b). Furthermore, the assembled device from the GCD curves indicate that the highest specific capacitance of the device was up to 221.1 F g-1 (86 mAh g-1) at the current density of 0.5 A g-1 (see Figure 6b and Figure S8b, Supporting Information). With increasing the current density to 10 A g1,

a specific capacitance of 83.57 F g-1 (32.5 mAh g-1) still can be obtained, revealing the good rate

capability of the device. Moreover, the cycling stability of the assembled SC cell is another crucial factor for practical applications. As shown in Figure 6c, the SC device has the capacitance retention of 86.2% after 10,000 cycles, indicating the stable operation within the suggested potential window. To further demonstrate its actual application, the as-made VO2/graphene@NiS2 hybrid aerogel//graphene aerogel asymmetric SC device could be capable of lighting up a commercial red


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LED (inset of Figure 6c). Figure 6d exhibits the Ragone plots (energy density vs. power density) of the assembled all-solid-state asymmetric SC device. The device shows a superior energy density of 60.2 Wh kg-1 at a power density of 350.0 W kg-1. In addition, outstanding energy density of 22.8 W h kg-1 was also preserved even at an extreme high power density of 7000.0 W kg-1 due to the combination of both pseudocapacitance and electric double-layer capacitance. The above results confirm that the mesoporous hierarchical structures of VO2/graphene@NiS2 hybrid aerogel electrode with good electrical conductivity is promising as a high-performance asymmetric SCs. CONCLUSION In summary, we report a novel and durable all-solid-state asymmetric SC device based on 3D interconnected VO2/graphene@NiS2 hybrid aerogel as the positive electrode and graphene aerogel as the negative electrode by using the sol–gel and hydrothermal growth techniques for the first time. The hierarchical mesoporous structures of VO2/graphene@NiS2 hybrid aerogel provided not only a larger contact area between the electrode and the electrolyte but also multiple paths for the electrolyte ions diffusion. In light of the unique advantages, the resultant VO2/graphene@NiS2 hybrid aerogel electrode delivered a high specific capacitance up to 1280.0 F g-1 (142.2 mAh g-1) at a current density of 1 A g-1. Furthermore, the assembled VO2/graphene@NiS2 hybrid aerogel//graphene aerogel SC exhibited a wide potential window along with a remarkable energy density of 60.2 Wh kg-1 at a power density of 350.0 W kg-1. Also, the assembled device demonstrated excellent endurance performance with a capacitance retention of 86.2% after 10,000 charge/discharge

cycles.

These

findings

suggest

the

promising

applicability

of

VO2/graphene@NiS2 hybrid aerogel composite for developing high-performance of energy storage systems.



Page 20 of 24

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H.-C. C.). ORCID Hsieh-Chih Chen: 0000-0003-4060-9489 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully appreciate the financial support from the Ministry of Science and Technology, Taiwan (MOST 105-2113-M-035-003-MY2). The authors would also like to acknowledge Prof. Shiang-Jiuun Chen, Department of Life Science, National Taiwan University, for the highresolution transmission electron microscopy (HR-TEM) support. REFERENCES 1 Yang, P.; Mai, W. Flexible solid-state electrochemical supercapacitors. Nano Energy 2014, 8, 274-290.


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