Three-Dimensional Network of Vanadium Oxyhydroxide Nanowires

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3D Network of Vanadium Oxyhydroxide Nanowires Hybridize with Carbonaceous Materials with Enhanced Electrochemical Performance for Supercapacitor Meng Chen, Yifu Zhang, Yanyan Liu, Qiushi Wang, Jiqi Zheng, and Changgong Meng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01109 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 20, 2018

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ACS Applied Energy Materials

3D Network of Vanadium Oxyhydroxide Nanowires Hybridize with Carbonaceous Materials with Enhanced Electrochemical Performance for Supercapacitor Meng Chen, Yifu Zhang*, Yanyan Liu, Qiushi Wang, Jiqi Zheng, Changgong Meng School of Chemistry, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, PR China *Corresponding author. E-mail address: [email protected]

Abstract It is always worthwhile to develop novel composite electrode materials for supercapacitors, especially the combination of stable conductive structures and metal oxides with high electrochemical performance. Comparing with vanadium oxides, vanadium oxyhydroxide has been paid rare attention as the energy storage materials. Herein, 3D hierarchical VOx(OH)y/CNT/rGO composite is synthesized by a facile and self-assembly method. CNT/rGO networks act as structure-directing agent and provide excellent conductivity. VOx(OH) y anchored to CNT/rGO networks is a novel vanadium oxyhydroxide with a mixed-valence V+4/+5, composed of major V3O5(OH)4 and minor V2O5·H2O. VOx(OH)y/CNT/rGO composite electrode exhibits a high specific capacitance of 414 F·g−1 at 0.5 A·g−1 within the potential range of −1.2~0.6 V in 1 mol·L−1 LiClO4/PC. The symmetrical supercapacitor can deliver a high energy density of 60.90 Wh·kg−1 at a power density of 81.85 W·kg−1 with a large voltage window of 2.2 V. The preeminent electrochemical properties are owing to the synergistic effects from VOx(OH)y nanowires and conductive CNT/rGO networks. The impressive results illustrate the symmetric supercapacitor is expected and promising for energy storage devices.

Keywords: Vanadium oxyhydroxide nanowires; CNT/rGO networks; Electrochemical properties; Supercapacitor electrode; Symmetric supercapacitor

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1. Introduction Energy crisis has been a serious problem owing to rapid growth rate of population, environmental pollution and waste of resources as rapid development of the globalization 1-3. Therefore, the demands for inexpensive, natural abundance and renewable resources and energies are vital and extremely urgent. Rechargeable electrochemical energy storage devices are great choices to solve the energy crisis 4. Among them, supercapacitors, batteries and electrolytic capacitors are three main kinds of electrochemical energy storage devices

5-7

. Supercapacitors (SCs), also called as ultracapacitors or electrochemical capacitors,

have gained increasing and considerable attention in recent years owing to their longer cycling life, greater power density and faster charge-discharge process compared with batteries 8-12. Very recently, the rapid development of electric vehicles and flexible electronic devices has put forward higher requirements for SCs, which are not satisfied by traditional capacitors. Therefore, designing and fabricating novel SCs with excellent energy density and broad voltage window are of great significance and huge challenges 13-16

. The energy density (E) is governed by specific capacitance (C) and potential window (V) according

to the equation E=1/2CV2. Thus, improving the specific capacitance or enlarging potential window is of great essential, which is largely determined by the properties of the electrode materials 17. In the past decades, vanadium based materials have been widely applied to energy storage, not only due to their various oxidation states and structures, but also the high energy density and wide potential window. The common electrode materials among vanadium based material are VO2 and V2O5 with multifarious structures and morphologies

18-22

. However, vanadium oxyhydroxides, which may bear

scientific importance and promising chemical/physical properties in electrochemistry, have been rarely studied. So far, several vanadium oxyhydroxides have gradually got reputations, for example HaggiteV4O6(OH)423, Duttonite VO(OH)2 and Gain’s hydrate V2O4(H2O)224. They have complex structures and compositions, which leads to their various properties, such as the application for energy storage

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and metal-to-insulator (MIT) transition23. However, the immaturity and difficulty of structural

control and selective synthesis are challenges to vanadium oxyhydroxides. Wu et al. firstly synthesized Haggite in more than 50 years and corrected its molecular formula by V4O6(OH)4 instead of the long-standing known V4O4(OH)6 23. Julie et al.

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synthesized Duttonite and Haggite relying on aqueous

precipitation by adjusting pH at 4.0 and 3.6-3.8, respectively. The synthetic conditions of vanadium oxyhydroxides are very demanding and difficult to control. Julie et al. also first studied the electrochemical properties of Duttonite andHaggite for Li-ion and Na-ion batteries, however, the capacity

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and cycle stability were poor because of the dissolution of electrode materials

24

. Just recently, in our

group, VO(OH)2 has been firstly synthesized with the assistance of PVP 25, and VO(OH)2 indeed exhibits great potential for energy storage. Nevertheless, the electrochemical property of VO(OH)2 is seriously limited by the poor structure durability and low electrical conductive. Therefore, it is essentially necessary and meaningful to improve the electrochemical performance of vanadium oxyhydroxides. Obviously, it’s a wise decision for VO(OH)2 to innovatively hybridize with other conductive and stable materials. Carbonaceous materials not only possess superior structural stability and extraordinary chemical stability, but also are typically used as the electric double-layer capacitors (EDLCs) electrodes 26. Up to now, carbon materials such as carbon nanotubes and graphene are widely used to hybrid with vanadium oxide as SCs’ electrode materials. Compared with pure vanadium materials, the electrochemical performance of composite electrodes is significantly improved, including specific capacitance, rate properties and cycling stability

27-31

. Whereas, in these studies, most of the composites are binary

materials, in other words, it’s more common that vanadium oxides hybridize along with one kind of carbon material. Instead of traditional combination of carbon materials, metal oxide/CNT/graphene, a novel ternary composite has gained more attention and showed surprised performance

32-35

. The GO

nanosheets are well integrated with CNTs owing to their similar structures through π-π attractions, and forming hierarchical conductive networks. This brilliant hybrid architecture not only exhibits fast ion and electron channels and long-term stable structure, but also acts as the structure-directing agent and provides appropriate sites for the growth of V-based materials. Therefore, it’s expected that vanadium oxyhydroxides combining with hierarchical conductive CNT/rGO networks would fuse the advantages of different components and become promising candidates for energy storage devices. Herein, a novel vanadium oxyhydroxide VOx(OH)y nanowires are firstly synthesized on the CNT/rGO networks via a one-step hydrothermal strategy. Abundant defects of GO and CNTs are offered to VOx(OH)y nanowires for hydrothermal assembly, resulting in intimate integration with the networks. Owing to the synergistic effects of each component, the electrochemical performance of ternary composite electrodes has been remarkable improved, leading to superior specific capacitance and stability. The assembled VOx(OH)y/CNT/rGO symmetric SC device shows broad potential window up to 2.2 V and high energy density of 60.90 Wh·kg−1.

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2. Experimental section 2.1. Materials Crude flake graphite was purchased from Aladdin Inc. (99.95% metals basis, ≥ 325 mesh). Multi-walled carbon nanotubes (CNTs) used in this study are bought from TIME NANO Co. with diameters ranging from 20 nm to 30 nm and lengths varying from 10 to 30 µm and purity of 98 wt.%. All other chemicals with analytical grade are purchased from Sinopharm Chemical Reagent Co., Ltd and used without any further purification. The synthesis of functionalized CNTs and graphite oxide (GO) was shown in Supporting Information. 2.2. Synthesis of 3D VOx(OH)y/CNT/rGO ternary composite The fabrication of 3D VOx(OH)y/CNT/rGO ternary composite is illustrated in Scheme 1. In detail, 7.5 mL GO suspension (0.15 g) and 0.05 g functionalized CNTs were dispersed into 25 mL deionized water underultrasonication for 1 h until the GO/CNT homogenous dispersion was obtained. Subsequently, 0.82 g VOSO4 was added to above suspension with vigorously stirring for 20 minutes. NaOH solution (0.5 M) was slowly dropped to adjust pH above at 4.7. After that, the slurry was transferred to a 50 mL Teflon-lined stainless steel autoclave for hydrothermal process, which was heated at 100 ℃ for 48 h. After the reaction, the resulted precipitate was filtered, washed and freeze dried for 12 h. In chemical reactions, oxidants usually reduced themselves 36.Moreover the reactantwas V4+, leading that GO was converted into rGO. VOx(OH)y/CNT/rGO ternary composites were respectively synthesized with various GO (0.015, 0.03, 0.06, 0.12, 0.15, 0.18 g). For comparison, the VO(OH)2/CNT binary composite and pure VO(OH)2 were also synthesized under the similar conditions. To determine the effect of CNT content, VO(OH)2/CNT with varied quality (0.01 g, 0.05 g, 0.1 g, 0.2 g) were prepared.

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Scheme 1. Schematic illustration of synthesis of 3D VOx(OH)y/CNT/rGO ternary composite. 2.3. Structure characterization The morphology and dimensions of the products were observed by field emission scanning electron microscopy (FE-SEM, NOVA NanoSEM 450, FEI) and transmission electron microscopy (TEM, FEITecnai F30, FEI). The samples were dispersed in absolute ethanol with ultrasonication before TEM characterization. The phase and composition of the products was identified by X-ray powder diffraction (XRD) on a PanalyticalX’Pert powder diffractometer at 40 kV and 40 mA with Ni-filtered Cu Kα radiation. The chemical composition of as-obtained samples was revealed by use of an energy-dispersive X-ray spectrometer (EDS) and elemental mapping attached to a scanning electron microscope (SEM, QUANTA450). X-ray photoelectron spectroscopy (XPS) was used to investigate the composition of the products and confirm the oxidation state of vanadium performed on ESCALAB250Xi, Thermo Fisher Scientific. Fourier transform infrared spectroscopy (FTIR) pattern of the solid samples was measured using KBr pellet technique (About 1 wt% of the samples and 99 wt% of KBr were mixed homogeneously, and then the mixture was pressed to a pellet) and recorded on a Nicolet 6700 spectrometer from 4000 to 400 cm−1 with a resolution of 4 cm−1. Raman spectra were obtained using a Thermo Scientific spectrometer, with a 532 nm excitation line. Surface area was determined by Brunauer-Emmet-Teller 5



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(BET) method using Micromeritics ASAP-2020 and the samples were degassed at 150 °C for several hours. 2.4. Electrochemical characterization The working electrodes were prepared using a mixture of 80 wt% of the as-synthesized active materials, 10 wt% of polyvinylidenedifluoride (PVDF) and 10 wt% of carbon black, and N-methyl-2-pyrrolidone (NMP) was used as a solvent. The mixed slurries were coated onto Ni foils and heated at 80 °C overnight to remove the organic solvent. Then these foils were pressed onto Ni-grids at a pressure of 10 MPa. The mass loading was about 3.5 mg·cm−2. The electrolytes were used 1 mol·L−1 LiClO4/PC (lithium perchlorate in propylene carbonate). Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical performance of the electrode materials. The typical three electrode experimental cell was equipped with platinum wire and saturated calomel electrode (SCE) as the counter electrode and reference electrode, respectively. The electrochemical measurements were carried out on a CHI-660D electrochemical work station. The specific capacitance (C, F·g−1) of the active material in the electrode determined using charge-discharge curves can be calculated from the following equations:

=

∙∆

(1)

∙∆

Where C (F·g−1) is the specific capacitance, I (A) is the discharge current, ∆t (s) is the discharge time, m (g) is the mass of the active material in the working electrode, ∆V (V) represents the potential drop during the discharge process. The symmetric SCs (SSCs) were assembled using two pieces of working electrodes and a separator (NKK-PF30AC) sandwiched in between. The electrolyte was used 1 mol·L−1 LiClO4/PC. After dipping in the electrolyte, the entire device was sealed in plastic sheet to avoid the evaporation of electrolyte. The energy density E (W·h·kg−1) and power density P (W·kg−1) of the device can be calculated according to the equation (2) and (3), respectively. Please note the conversion of the unit during the calculation.

E = C ∙ (∆V)

P = ∆

(2) (3)

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3. Results and discussion 3.1. Morphology, structure and composition of the 3D VOx(OH)y/CNT/rGO composite The synthetic process of VOx(OH)y/CNT/rGO-x (x=0.015, 0.03, 0.06, 0.12, 0.15, 0.18 g) hybrid materials is shown in Scheme 1. Firstly, the functional CNTs are well dispersed in amphiphilic GO solution through π-π interaction, and it’s easy to obtain homogeneous suspension after ultrasonication37. Secondly, pH is the key to the formation of vanadium oxyhydroxide, and the ideal value is about 4.7 through the previous exploration 25. Finally, the 3D network of VOx(OH)y/CNT/rGO is synthesized by the hydrothermal treatment combination of freeze-drying. With the addition of CNTs and GO, the architectures of VOx(OH)y/CNT/rGO are different from pure VO(OH)2 and VO(OH)2/CNT composite, supporting by FE-SEM and TEM images, as depicted in Figure S1 (Supporting Information) and Figure 1. The morphologies of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO are observed by FE-SEM. The FE-SEM image (Figure S1a) shows that pure VO(OH)2 consists of short nanorods. With the addition of CNTs, VO(OH)2/CNT has the similar morphology with VO(OH)2, as shown in Figure S1b. CNTs are occasionally observed on the edge, which is probably due to that CNTs are low in content and mostly wrapped by VO(OH)2. Figure 1a-d depicts the FE-SEM images of VOx(OH)y/CNT/rGO, which displays that plate-like structures of rGO with VOx(OH) y nanowires and CNTs appearing on the surface and border are formed 3D network. At high magnification (Figure 1d), it’s clear observed that VOx(OH)y nanowires densely cover on the surface of the hierarchical structures. CNTs are occasionally observed (inset Figure 1c). Besides, CNTs are not easily observed in the visual field, and the reason can be explained that CNTs and rGO close combine to form hierarchical conductive networks and are hidden under nanowires. The networks structure is not only the foothold for VOx(OH)y growing, but also the structure-directing agents from nanorods to nanowires. The changed architectures of VOx(OH)y indicates that the ternary material is not a naive physical mixture, and there is indeed interactivity between each component, which can be improved the electrochemical properties of VOx(OH)y38,39. The structures of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO are further examined by TEM. As shown in Figure S1c-d, pure VO(OH)2 and VO(OH)2/CNT exhibit the similar nanorods. In addition, CNTs are mixed with the VO(OH)2nanorods in Figure S1d, which is the supplement to FE-SEM image of VO(OH)2/CNT. These results are very consistent with FE-SEM observations (Figure S1). In the ternary composite (Figure 1e-f), it can be seen that VOx(OH)y nanowires intimately intertwine with CNT/rGO networks. Meanwhile, VOx(OH)y 7



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nanowires and CNTs anchor densely on the rGO nanosheets and almost overlapped the entire surface, which effectively prevents the restacking of hybrid architectures and forms the 3D network in agreement with FE-SEM images. HRTEM image (insert in Figure 1f) displays that the lattice fringes of CNT and VOx(OH)y. The distance between neighboring planes is about 0.34nm and 0.214 nm, which is corresponding to the CNT (002) and common peak of V3O5(OH)4 and V2O5·H2O at around 2θ = 42°. Overall, the synthesis of 3D architectures of VOx(OH)y/CNT/rGO are due to the attendance of CNT/rGO networks, and the interconnected structure will shorten the transmission path of electrons and ions, as well as boost the fast electron transfer, and enhance the structural stability. This unique framework will facilitate the combination of VOx(OH)y and CNT/rGO, thus giving rise to satisfactory synergistic effects to improve their electrochemical performance 20,40,41.

Figure 1. FE-SEM (a-d) and TEM (e-f) images of VOx(OH)y/CNT/rGO, a FE-SEM image with an edge view inserted in (c) and HRTEM images of CNT and VOx(OH)y inserted in (f). The

specific

surface

area

and

porous

properties

of

VO(OH)2,

VO(OH)2/CNT

and

VOx(OH)y/CNT/rGO were determined by N2 adsorption/desorption measurements, as shown in Figure 2. Based on nitrogen isotherms (Figure 2a), the surface area calculated by BET equation and BJH pore volume of VOx(OH)y/CNT/rGO reach 24.6 m2·g−1 and 0.072 cm3·g−1. The specific area of pure VO(OH)2/CNT and VO(OH)2 are 20.8 m2·g−1 and 14.3 m2·g−1, and their pore volume are 0.095 cm3·g−1 and 0.049 cm3·g−1, respectively. The specific surface area of VOx(OH)y/CNT/rGO is higher than VO(OH)2 and VO(OH)2/CNT, suggesting that the addition of CNT and GO indeed increases the surface 8


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area and enhances the porosity compared with the pure VO(OH)2. At high relative pressure, the sharp capillary condensation step demonstrates that the isotherm of VOx(OH)y/CNT/rGO is corresponded to classical type IV with lots of mesopores structures. Figure 2b depicts the BJH pore size-distribution calculated from adsorption part of nitrogen isotherm. Compared with pure VO(OH)2, it can be observed that VO(OH)2/CNT and VOx(OH)y/CNT/rGO show more obvious porous structures consisting of mesoand macropores. Thepore size-distribution of VO(OH)2/CNT and VOx(OH)y/CNT/rGO are mainly centeredat 22 nm and 11 nm, respectively. The pore size of VOx(OH)y/CNT/rGO is more smaller than that of VO(OH)2/CNT, suggesting that the 3D interconnected VOx(OH)y/CNT/rGO networks are formed in agreement with FE-SEM and TEM observations (Figure 1).The mesopores provide more active sites for the redox reaction and lead the electrolyte to infiltrate rapidly into the material and contact closely, while the macropores are equivalent to reservoirs, which can shorten the path of ionic diffusion. The various porous structure is beneficial to the electrochemical performance of hybrid composites 1,42.

Figure 2. (a) N2 adsorption/desorption isotherms and (b) Barrett–Joyner–Halenda (BJH) pore size-distribution of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO. 9



The composition of VOx(OH)y/CNT/rGO was further evaluated by various tests including EDS, element mapping, XRD, FTIR, Raman, and XPS. EDS spectrum and elemental mappings of VOx(OH)y/CNT/rGOare shown in Figure S2 (Supporting Information), which reveals the coexistence of C (yellow), O (green), and V (red) elements. The ubiquitous and homogeneous distribution is other proof that VOx(OH) y combines intimately with CNT/rGO to form 3D networks, which is well in agreement with FE-SEM and TEM observations. The XRD patterns (Figure S3, Supporting Information) show the crystal phase and structure information of functional CNT, GO, VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO. As shown in Figure S3A, the peak at ca. 25° is corresponding to the characteristic peak of CNT (002)27. XRD pattern of GO is predominated by a single strong peak at 10.66°. This peak indicates a layered structure with a basal spacing of 0.83 nm, suggesting that the nature graphite flakes (d002=0.355nm) are expanded by the typical chemical oxidation 43. Figure S3B shows the XRD patterns of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO hybrid composite. It can be observed that VO(OH)2 and VO(OH)2/CNT has the similar patterns, in which the diffraction peaks can be indexed to VO(OH)2 (JCPDs, No. 11-0209), suggesting VO(OH)2 is synthesized without GO. However, with adding GO, the sample is the mixture of V3O5(OH)4 (JCPDs, No. 11-0210) and V2O5·H2O (JCPDs, No. 11-0673), denoted as VOx(OH)y. It can be inferred that VOx(OH)y/CNT/rGO contains the mixed valance oxidation state of vanadium (+4/+5), which is beneficial for insertion/extraction of Li ions44. The FIIR spectra of GO and CNT are shown in Figure S4 (Supporting Information), and similar stretches of –OH, C=O, aromatic C=C, C-O and C-C are identified at 3400 cm−1, 1722 cm−1, 1620 cm−1, 1222 and 1052 cm−1 respectively, suggesting CNT and GO are successfully functionalized45.Figure 3a shows the FIIR spectra of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO. The spectra of pure VO(OH)2 and VO(OH)2/CNT are similar, which consists with the result of XRD and well implies the successful synthesis of VO(OH)2. The peaks at 3570 and 3520 cm−1 of VO(OH)2 and VO(OH)2/CNT are ascribed to the stretching vibrations of V−OH. The strong band at 967 cm−1 is assigned to the typical VO2+ stretching vibration and the peaks at 868 and 801 cm−1 are contributed to the in-plane V−OH deformations. The peaks at 620, 536 and 447 cm−1 are related to torsional modes of −OH groups 46. As for VOx(OH)y/CNT/rGO, the two peaks of the stretching vibrations of V−OH disappear and a broad peak attends at 3447 cm−1, which is attributed to O−H of carboxylic acid, whereas the hydrogen-bonded OH group of VO(OH)2 and VO(OH)2/CNT is located at 3270 cm−147. The peak at 1568 cm−1 is referred to vibration of the graphene sheets, and the peak at 1140 cm−1 is due to the blue shift of phenolic groups

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after sonication 28,48. It’s worth raising that the strong peak of the typical VO2+at 967 cm−1 is weaker and a new higher frequency peak at 1000 cm−1 appearsin the VOx(OH)y/CNT/rGO, which is due to the significant appearance of V5+=O

46

, in accordance with the analysis of XPS discussed in the following.

Raman spectra are used to investigate the information about the structure of CNT, GO, VO(OH)2/CNT and VOx(OH)y/CNT/rGO. It’s an effective method for studying defect density and disorder degree of carbon containing materials. In Figure 3b, Raman signals in the extend region from 1100 to 1950 cm−1, and two main peaks at around 1350 and 1598 cm−1 represent the D and G bands respectively. The D-band is related to defects and disorder of sp3 hybridization carbon atomsthe in-plane terminations, while the G-band represents the vibration of sp2 carbon atom in plane which associating with order bond structure.The intensity ratioofthe D-band and G-band (ID/IG) is a vital parameter to assess the defects and disorderof CNT and GO.The calculated ID/IG ratios are 1.01, 1.24, 1.38 and 1.31 for the GO, CNT, VO(OH)2/CNT and VOx(OH)y/CNT/rGO. The higher values of VO(OH)2/CNT and VOx(OH)y/CNT/rGO signify that more defects are introduced to the CNTs and rGO, which can prove VO(OH)2 is effectively compounded with carbon materials. These defects will expose more active sites for electron transfer and storage. By the way, the ID/IG of VOx(OH)y/CNT/rGO is slightly lower than VO(OH)2/CNT, indicating that more surface functional groups may be reduced in the ternary composite, which corresponds to the analysis of XPS discussed in the following.

Figure 3. (a) FTIR spectra of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO; (b) Raman spectra of 11



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CNT, GO, VO(OH)2/CNT and VOx(OH) y/CNT/rGO; (c, e) XPS spectra of VO(OH)2/CNT; (d, f) XPS spectra of VOx(OH)y/CNT/rGO. To analyze the chemical composition and valence state, the pure VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO are studied by XPS analysis, as depicted in Figure 3c-f and Figure S5 (Supporting Information). The full XPS spectra(Figure S5a) shows that VO(OH)2 consists of V and O,and VO(OH)2/CNT and VOx(OH)y/CNT/rGOare composed of V, O and C. With the increase of carbon content, the relative intensity of C1s is stronger. Figure 3c shows the core level binding energy for V−O peaks of VO(OH)2/CNT and an energy band at 516.0 eV and 523.6 for V2p3/2 and V2p1/2 respectively, suggesting that vanadium is +4 in the binary composite. For VOx(OH)y/CNT/rGO, the binding energy of V2p3/2 in Figure 3d is deconvoluted into two peaks at 515.7 eV and 517.0 eV, which are contributed to V4+ and V5+49. The presence of V5+ suggests that vanadium is partially oxidized during synthesis, confirming the reduction of GO to rGO. As shown in Figure S5b, the O1s spectrum of VOx(OH)y/CNT/rGO can be fitted into three different oxygen species, which are attributed to V−O (529.9 eV), OH (531 eV)of carboxyl and hydroxyl groups of CNT/rGO networks, and H2O (532.9 eV) of physical residual water near or at the surface, respectively . The C1s peaks of VO(OH)2/CNT (Figure 3e) and VOx(OH)y/CNT/rGO (Figure 3f) can be fitted by a superposition of four peaks at 284.7, 286.2, 287.3, and 289 eV, assigned to C-C/C=C, C-O, C=O and O-C=O, respectively 48. Compared with VO(OH)2/CNT, the percentage of sp2 carbon in VOx(OH)y/CNT/rGO is significantly higher than sp3 group, indicating oxygen functional groups are reduced in large quantities and recovery of sp2 carbon network

47

. The existence of V5+ and the

oxygenated carbon peaks become weaker, which consists well with the Raman and FTIR results that the much oxygen-containing groups of GO are effectively reduced by V4+.Thus, The above results demonstrate the strong coordination between the vanadium and carbon materials, which may be the assistance for thesynergistic effect, indicating VOx(OH)y/CNT/rGO is a promising material with excellent electrochemical performance. GO determines that the obtained vanadium oxyhydroxide is mixed valence, which maybe more convenient for insertion/extraction of Li ions44. 3.2. Electrochemical properties of 3D VOx(OH)y/CNT/rGO as a SC electrode The electrochemical behavior of 3D VOx(OH)y/CNT/rGO composite was studied using a three-electrode system vs. SCE and in 1 mol·L−1 LiClO4/PC (propylene carbonate). For comparison, the electrochemical properties of VO(OH)2 and VO(OH)2/CNT were also investigated. Figure S6 (Supporting

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Information) depicts CV curves of VOx(OH)y/CNT/rGO at various potential limits a scan rate of 50 mV·s−1, indicating that the potential range of −1.2~0.6 V displays clear redox peaks and high capacity. To determinate optimal content of CNTs and GO, GCD curves (Figure 4) was used to evaluate the electrochemical properties of VO(OH)2/CNT composites (the weight (g) of CNT was defined as 0.01, 0.05, 0.1 and 0.2 g, respectively) and 3D VOx(OH)y/CNT/rGO composites (the weight of CNT was 0.05 g; the weight (g) of GO was defined as 0.015, 0.03, 0.06, 0.12, 0.15 and 0.18 g, respectively) at the current density of 1 A·g−1 in the voltage window of −1.2~0.6 V, respectively. VO(OH)2/CNT (0.05) has the longest charge/discharge time compared with the other three samples. The specific capacitances measure 185 F·g−1 (0.01), 253 F·g−1 (0.05), 200 F·g−1 (0.1) and 65 F·g−1 (0.2). Therefore, the sample with the content of 0.05 g CNTs was chosen to synthesize 3D VOx(OH)y/CNT/rGO composite. Figure 4b shows the GCD curves of 3D VOx(OH)y/CNT/rGO with different GO contents. The specific capacitances measure 174 F·g−1 (0.015), 176 F·g−1 (0.03), 198 F·g−1 (0.06), 284 F·g−1 (0.12), 351 F·g−1 (0.15) and 289 F·g−1 (0.18). Thus, VOx(OH)y/CNT/rGO (0.15) possesses the highest specific capacitance. VOx(OH)y/CNT/rGO (0.18) shows the inferior capacitive performance, which may be owing to the aggregation of overmuch GO

34

. Thus, VOx(OH)y/CNT/rGO (0.15) was selected to systematically study

owing its best electrochemical properties. In the following, if it is not stated specially, VOx(OH)y/CNT/rGO represents VOx(OH) y/CNT/rGO (0.15).

13



Page 14 of 30

Figure 4. GCD curves of VO(OH)2/CNT composites with various content of CNTs (a) and VOx(OH)y/CNT/rGO composites with various content of GO (b) at the current density of 1 A·g−1.

Figure 5a shows the CV curves of VOx(OH)y/CNT/rGO composite with the scan rates from 5 mV·s−1 to 100 mV·s−1. Each CV curve displays two pairs of redox peaks. These peaks are contributed to the Li ion insertion/extraction reaction of VOx(OH) y/CNT/rGO composite, which demonstrates that the charge storage mechanism of VOx(OH)y/CNT/rGO composite is based on faradaic battery-typed mechanism 50 in agreement with vanadium oxides

51,52

. As is well known, the charge storage mechanism of the carbon

materials such as CNTs and GO is ascribed to EDLCs mechanism. Thus, VOx(OH)y/CNT/rGO composite store energy by both faradaic process and double-layer capacitive process. Based on other energy storage mechanism of other vanadium oxide

52-54

, the electrochemical insertion/extraction of Li ion in

VOx(OH)y/CNT/rGO composite can be inferred as Equation (4): VOx(OH)y + aLi+ +ae- ↔ Lia VOx(OH)y

(4)

The cathodic peaks at negative current directs the reduction routs of VOx(OH)y to LiaVOx(OH)y

14


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owing to Li ion intercalation, while the anodic peaks at positive current correspond to the oxidation process of LiaVOx(OH)y to VOx(OH)y which is accredited to Li+ ions extraction. As depicted in Figure 5a, similar shapes of CV curves at different scan rates reveal an excellent faradaic response. With the scan rates increasing, the current peaks also increase, suggesting the excellent kinetics and reversibility and the good ionic and electron conduction of VOx(OH)y/CNT/rGO composite

20

. Meanwhile, the anode and

cathode peaks slightly move to the opposite direction due to the polarization effect of VOx(OH)y/CNT/rGO electrode

55

. The near mirror symmetry of the redox peaks suggests a good

electrochemical reversibility. Moreover, the redox peaks still appear even at 100 mV·s−1, revealing good rate capability of VOx(OH)y/CNT/rGO56. Figure 5b shows the GCD curves of VOx(OH)y/CNT/rGO between −1.2 and 0.6 V, which are used to assess its specific capacitance and rate capability. As shown in Figure 5b, two charge-discharge plateaus are observed, which are corresponding to the redox peaks in the CV curves (Figure 5a). Based on the equation (1), the specific capacitances reach 414, 351, 298, 273 and 220 F·g−1 at the current densities of 0.5, 1, 2, 5 and 10 A·g−1, respectively. The specific capacitances decrease with the current densities increasing. The reason could be the insufficient and low utilization rate of active material during GCD process at high current densities. Besides, the decline of capacity may also cause by the incremental voltage drop

52

. The attenuation of the specific capacitance slowly drops from

the low current density (0.5 A·g−1) to the high current density (10 A·g−1). Comparing with the value at 0.5 A·g−1 (414 F·g−1), the retention values are 85 %, 72 %, 66 % and 53 % at 1, 2, 5 and 10 A·g−1. The results suggest the good rate capability of VOx(OH)y/CNT/rGO in agreement with CV results (Figure 5a). To detect the effect of current collector for capacitance, CV and GCD of Ni foils are carried out in Figure S7 (Supporting Information), which confirms that contribution of Ni foils for capacitance is too tiny to be neglected. Furthermore, GCD curves of 3D VOx(OH)y/CNT/rGO with different rGO contents are measured, and the relationship between specific capacitances and current densities are summarized in Figure 5c. It is obviously shown that VOx(OH)y/CNT/rGO (0.15) exhibits the highest specific capacitance, in agreement with the results in Figure 4b. Therefore, the specific capacitance of the as-prepared VOx(OH)y/CNT/rGO reaches as high as 414 F·g−1 at 0.5 A·g−1 in the large voltage window (−1.2~0.6 V), and this value means that the capacity reaches to 745 C·g−1, suggesting 3D VOx(OH) y/CNT/rGO exhibit the excellent charge storage performance. In addition, the electrochemical properties of rGO in LiClO4/PC are performed at −0.8~0 V in Figure S8 (Supporting Information). The CV curves (Figure S8a) at different scan rates all exhibit the perfect shape of rectangle and the GCD curves (Figure S8b) at

15



various current densities show the shape of symmetric tangle, which are the typical characteristics of EDLCs. The values of capacitance calculated from GCD curves are shown in the Figure S8c, which are 47, 39, 34, 27 and 22 F·g−1 at the current densities of 0.5, 1, 2, 5 and 10 A·g−1, respectively.

Figure 5. Electrochemical properties of 3D VOx(OH)y/CNT/rGO composites: (a) CV curves at different scan rates; (b) GCD curves at different current densities; (c) Specific capacitances vs. current densities of various content of GO.

16


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To further compare the electrochemical properties of 3D VOx(OH)y/CNT/rGO composite with VO(OH)2 and VO(OH)2/CNT composite, some tests were carried out and the corresponding results were summarized in Figure 6. The CV curves of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO at the scan rate of 50mV·s−1 are shown in Figure 6a. The area of the CV files is in proportion to the specific capacitance. It is obvious that the area under the CV curve of VO(OH)2 is much smaller than hybrid composites, indicating that the capacitance has been greatly improved by the attendance of carbon materials. 3D VOx(OH)y/CNT/rGO composite exhibits the highest specific capacitance. It is noteworthy that the current of VOx(OH)y/CNT/rGO fast responds to switching potential at 0.6 V, suggesting a lower equivalent series resistance (ESR), which is the key to superior rate capability and power density 30. GCD is used to complement and demonstrate similar results. Figure 6b shows the GCD profiles of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO at the current density of 1 A·g−1. 3D VOx(OH)y/CNT/rGO composite has the longest charge/discharge time among them. The calculated specific capacitances of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO are 61, 253, and 351 F·g−1, respectively. Moreover, Figure 6c describes the relationship between various current densities and specific capacitances of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO. At the same current density, VOx(OH)y/CNT/Rgo exhibits the superior specific capacitance from beginning to end. From 0.5 A·g−1 to 10 A·g−1, the capacitance retention of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO are 39% (from 71 F·g−1 to 28 F·g−1),46% (from 290 F·g−1 to 133 F·g−1) and 53% (from 414 F·g−1 to 220 F·g−1).

17



Figure 6. Comparison of the electrochemical properties of VO(OH)2, VO(OH)2/CNT composite and VOx(OH)y/CNT/rGO composite: (a) CV curves at 50 mV·s−1; (b) GCD curves at 1 A·g−1; (c) Specific capacitances vs. current densities; (d) Cycle performance; (e-f) Nyquist plots over the frequency range of 100 kHz to 0.01 Hz, inserted an equivalent electrical circuit in (e). Cycling performance is another required issue for SCs electrode materials. Figure 6d indicates the cycling stability of the VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO at a scan rate of 50 mV·s−1. In the early 50 cycles, the capacitance retention of VOx(OH)y/CNT/rGO is improved, which may be owing to the electrochemical activation of active material

57

. After 2000 cycles, the capacitance for VO(OH)2,

VO(OH)2/CNT and VOx(OH)y/CNT/rGO retain about 5%, 18%, and 67% of their initial values. There are main two reasons for the reduction of capacitance: On one hand, vanadium oxyhydroxides have poor durability caused by dissolution of electrode material, which is a common phenomenon for vanadium-based materials used as electrodes

31,55

. On the other hand, the architectures are lightly

damaged after suffering from long time repeatedly charge-discharge possess. Nevertheless, VOx(OH)y/CNT/rGO exhibits better cycle performance than VO(OH)2 and VO(OH)2/CNT. The presence of CNT/rGO networks leads to superior structural stability and effectively prevent the reduction of capacitance because VOx(OH)y nanowires assemble on the suitable combination points of CNT/rGO networks to form tight interactivity, which is commendably corroborated by FE-SEM and TEM analyses. In addition, Coulomb efficiency of the three samples remain almost 100% (Figure S9a), indicating good reversibility of electrodes. To reveal why 3D VOx(OH)y/CNT/rGOcomposite displays the best electrochemical properties, EIS was carried out over a frequency range of 100 kHz to 0.01 Hz. As shown in Figure 6e, the Nyquist plots consist of an arc at the higher frequency region and a straight line at the lower frequency region, fitted by an equivalent electrical circuit (inserted in Figure 6e). Rs is related to the internal resistance of the electrodes, Rct is the charge-transfer resistance, and Wo denotes the Warburg impedance and the Cdl represents the double layer-capacitance. From the axis intercepts (Figure 6f), the Rs values of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO are about 1.5 Ω, 1.0 Ω and 0.5 Ω, respectively. For 3D VOx(OH)y/CNT/rGO composite, the semicircle at the high frequency region is hardly observed and the line in the low frequency region is more vertical than VO(OH)2 and VO(OH)2/CNT. This is caused by the faster ion transport and diffusion, indicating the superior conductivity of VOx(OH)y/CNT/rGO. Rs and Rct are greatly reduced benefiting from the ion-transfer channels and the admirable conductivity, which can 18


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attribute to the following reasons: (1) The introduction of carbon materials greatly improved the conduction of electrons; (2) The abundant pores lead electrolyte to infiltrate intimately with the electrode materials, and the coexistent of mesopore and macropore accelerate the ionic diffusion. (3) VOx(OH)ynanowires are mixed valence of V4+ and V5+, which expedites the insertion/extraction of Li ions. Based on the above results, the specific capacitance of 3D VOx(OH)y/CNT/rGO composite reach as high as 414 F·g−1 at 0.5 A·g−1, which is much higher than the values of VO(OH)2 (71 F·g−1) and VO(OH)2/CNT (290 F·g−1), and is even higher than, or comparable to V-based materials as summarized in Table S1, for example, 3D rGO coated V2O5 nanoribbon (437 F·g−1, 1 A·g−1) free-standing (220 F·g−1, 1 A·g−1)

58

36

, Graphene/V2O5

, RG(1.5)/ VO2(B) hybrid composite (104 F·g−1, 0.25 A·g−1)

59

,

hexangular starfruit-like VO2 (216 F·g−1, 1 A·g−1) 60, VO2(B)/ordered mesoporous carbon composite (131 F·g−1, 0.2 A·g−1) 61, interconnected V2O5 network (316 F·g−1) 62, V2O5 nanowires (351 F·g−1, 2 A·g−1) 56, et al. Compared with CNT, CNT/rGO networks provide more proper growth points to anchor VOx(OH) y, which effectively improves the electrochemical utilization of V-based materials for sufficient reaction 34. Moreover, the increase of contact area is conducive to the diffusion of electrolyte to enhance the accessibility of Li ions. Hence, the synergistic effects from the composite electrodes make the best use of the desire function of each component. 3.3. Electrochemical performance of 3D VOx(OH)y/CNT/rGO applied to the SSC device For assessing the practical applications, symmetrical SCs (SSCs) were assembled by using VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO, respectively. Figure 7a shows the CV curves of the VOx(OH)y/CNT/rGO SSC device at different voltage windows at the same scan rate of 50 mV·s−1, indicating that the potential window can reach 2.6 V and the operational and optimal value is 2.2 V, in which shows good symmetry and high capacity. Figure 7b shows the CV curves ofVOx(OH)y/CNT/rGO SSC device at different scan rates (5~100 mV·s−1) at this optimal voltage, which exhibit quasi-rectangular shapes and no deformations even at a high scan rate of 100 mV·s−1. The finding suggests an excellent capacitive behavior with good rate performance of the designed VOx(OH)y/CNT/rGOSSC device. Figure 7c represents the GCD files of VOx(OH)y/CNT/rGO SSC device and the specific capacitances can reach 90.6, 82.3, 49.3, 37.8 and 28.2 F·g−1 at the current density of 0.5, 1, 2, 5, 10 mA·cm−2. The discharge time is shorter than charge time at low current densities, which can be explained that the extraction of Li ions is not fully released due to the intrinsic properties of the prepared materials. The capacitance of VO(OH)2 19



Page 20 of 30

SSC, VO(OH)2/CNT SSC and VOx(OH)y/CNT/rGO SSC devices is compared at the different current densities in Figure 7d. It is clearly observed that VOx(OH)y/CNT/rGO SSC device exhibits the highest capacitance among them. The above results jointly demonstrate VOx(OH)y/CNT/rGO SSC device has the superior electrochemical performance in agreement with the performance of single electrode (Figure 6). To investigate the long term cycling performance of the devices in real application, the cycle performance of VO(OH)2SSC, VO(OH)2/CNT SSC andVOx(OH)y/CNT/rGO SSC devices was studied by CV at the scan rate of 50 mV·s−1, as shown in Figure 7f. VOx(OH)y/CNT/rGO SSC device still retains 87% of initial specific value after 500 cycles, while VO(OH)2/CNT SSC and VO(OH)2SSC devices only remain 21% and 28%, respectively. The cycling stability of VOx(OH)y/CNT/rGO SSC device is better than other two compared devices, in agreement of the results in three-electrode system. And the Coulomb efficiency of SSC devices are all remained close to 100% (Figure S9b), which confirms the decrease of capacitance is mainly caused by the dissolution, and the redox reaction of vanadium oxyhydroxides is a highly reversible process. Figure S10 (Supporting Information) reveals the Nyquist plots of VO(OH)2 SSC, VO(OH)2/CNT SSC and VOx(OH)y/CNT/rGO SSC devices. The sloped portion of VOx(OH)y/CNT/rGO SSC device in the low-frequency region is slightly shorter and steeper than other two devices, indicating there are more paths for ion diffusion in VOx(OH)y/CNT/rGO SSC device, which is a result of the frequency of ion diffusion in the electrolyte to the electrode interface. According to the axis intercepts inserted curves, the values of intrinsic resistance of VO(OH)2 SSC, VO(OH)2/CNT SSC and VOx(OH)y/CNT/rGO SSC devices are estimated to be around 2.8 Ω, 3.0 Ω and 3.8 Ω, respectively. Consisted with the EIS test of electrode materials in the three-electrode system, VOx(OH)y/CNT/rGO SSC device shows the lowest contact resistance at the interface of the active material and the collector. It demonstrates that VOx(OH) y/CNT/rGO SSC device has advantages in practical application. The energy density (E) and power density (P) are two essential parameters to characterize the electrochemical performance of SCs. The energy densities at different power densities are derived from GCD tests. Figure 7e shows the Ragone plots of the VOx(OH)y/CNT/rGO SSC device and other SCs. Based on the equation (2) and (3), the maximum energy density of VOx(OH)y/CNT/rGO SSC device is 60.90 Wh·kg−1 at a power density of 81.85 W·kg−1 (4.09 Wh·m−2 at 5.5 W·m−2), which is not only higher than the values of VO(OH)2/CNT SSC (3.38 Wh·m−2) and VO(OH)2 SSC (1.37 Wh·m−2) devices, but also comparable to, or even higher than other SC devices in the previous reports, such as VO2(A)//AC ASC (0.714 Wh m−2 at 3.75 W m−2)

20

, RG/VO2//RG ASC (22.8 Wh·kg−1 at 425 W·kg−1)

20


59

, VA-CNTs ASS

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(17.34 Wh·kg−1 at 710 W·kg−1) 27, VN10//AC ASC (12.8 Wh·kg−1 at 317 W·kg−1) (29.5 Wh·kg−1 at 800 W·kg−1) Wh·kg−1 at 900 W·kg−1)

64

66

, V2O5 SSC (43 Wh·kg−1 at 900 W·kg−1)

65

63

, V2//ODCSC ASC

, rGO/ V2O5 NS ASS (75.9

, V2O5/polyindole SSC (38.7 Wh·kg−1 at 900 W·kg−1)

67

, 3D

V2O5@PPynetwork//3D rGO ASC (21.0 Wh·kg−1 at 19.3 kW·kg−1) 68. To prove the practicability of the assembled VOx(OH)y/CNT/rGO SSC, the devices units are connected in series to drive the red LED. After charging of two series connected VOx(OH)y/CNT/rGO SSC devices at 4.4 constant potential, the devices can power up red LED for 5 min (as shown in Figure S11), which strongly demonstrates the application potential of VOx(OH)y/CNT/rGO SSC device.

Figure 7. Electrochemical performance of VOx(OH)y/CNT/rGO SSC device: (a) CV curves on various potential limits a scan rate of 50 mV·s−1; (b) CV curves at different scan rates; (c) GCD curves at different current densities; (d) Specific capacitances vs. current densities of VO(OH)2 SSC, VO(OH)2/CNT SSC and VOx(OH)y/CNT/rGO SSC devices; (e) Ragone plot of VOx(OH)y/CNT/rGO SSC device and other reported SC devices in the literatures;(f) Cycle performances of the three devices at a scan rate of 50 mV·s−1. 4. Conclusion In conclusion, a novel vanadium oxyhydroxideVOx(OH)y hybridize with CNT/rGO networks via a facile hydrothermal method. Due to the oxidation of GO, VOx(OH)y is mixed valence of V4+ and V5+, which is mainly made up of V3O5(OH)4 with some V2O5·H2O. Under the optimal CNT and GO content, 21



3D VOx(OH)y/CNT/rGO shows the outstanding electrochemical performance with the specific capacitance as high as 414 F·g−1 at 0.5A·g−1 at the voltage window of −1.2~0.6 V. The symmetric device with 3D VOx(OH)y/CNT/rGO exhibits a high energy densities of 60.90 Wh·kg−1 at a power density of 81.85 W·kg−1 with a broad voltage window up to 2.2 V. The excellent electrochemical properties are owing to the synergistic effects of VOx(OH)y and CNT/rGO networks, which indicates the satisfying cooperation between faradaic battery-typed capacity and double-layer capacitors. Furthermore, the appearance of VOx(OH)y enriches the study of the vanadium oxyhydroxides, not only the guiding significance for its synthesis and composites, but also the enlightenment for the research of electrochemical properties, which is of great significance to their further research. Acknowledgement This work was partially supported by the National Natural Science Foundation of China (Y.Z.; Grant No. 21601026. C.M.; Grant No. 21771030), Fundamental Research Funds for the Central Universities (Y.Z.; Grant No. DUT16RC(4)10) and Doctoral Research Foundation of Liaoning Province (Y.Z.; Grant No. 201601035). Conflicts of interest There are no conflicts of interest to declare. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX Synthesis of functionalized carbon nanotube (CNT) and graphite oxide (GO); FE-SEM and TEM images of VO(OH)2 and VO(OH)2/CNT-0.05 (Figure S1); EDS spectrum and elemental mappings of 3D VOx(OH)y/CNT/rGO composite (Figure S2); XRD patterns of various materials (Figure S3); FTIR spectra of GO and CNT (Figure S4); XPS spectra of VO(OH)2, VO(OH)2/CNT and VOx(OH)y/CNT/rGO (Figure S5); CV curves of VOx(OH)y/CNT/rGO on various potential limits (Figure S6); CV curves and GCD curves of VOx(OH)y/CNT/rGO and Ni foil (Figure S7); Electrochemical properties of rGO (Figure S8); Coulomb efficiency (Figure S9); Nyquist plots of three devices (Figure S10); Brightness changes of red LED in 5 min (Figure S11); Comparison of the electrochemical performance with work from

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(42) Yang, W.; He, L.; Tian, X.; Yan, M.; Yuan, H.; Liao, X.; Meng, J.; Hao, Z.; Mai, L. Carbon-MEMS-Based

Alternating

Stacked

MoS2@rGO-CNT Micro-Supercapacitor

with

High

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Table of Contents Graphic

A novel 3D hierarchical VOx(OH)y/CNT/rGO composite is synthesized through a low temperature one-step hydrothermal method and exhibits a high specific capacitance of 414 F·g−1 at 0.5 A·g−1 at the voltage window of −1.2~0.6 V in 1 mol·L−1 LiClO4/PC. The symmetric device assembled by 3D VOx(OH)y/CNT/rGO delivers a high energy density of 60.90 Wh·kg−1 at a power density of 81.85 W·kg−1 with a large voltage window of 2.2 V.

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Three-Dimensional Network of Vanadium Oxyhydroxide Nanowires

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