3D Interlaced Networks of VO(OH)2 Nanoflakes Wrapped with

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3D Interlaced Networks of VO(OH)2 Nanoflakes Wrapped with Graphene Oxide Nanosheets as Electrodes for Energy Storage Devices Yifu Zhang, Meng Chen, Tao Hu, and Changgong Meng ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00364 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Applied Nano Materials

3D Interlaced Networks of VO(OH)2 Nanoflakes Wrapped with Graphene Oxide Nanosheets as Electrodes for Energy Storage Devices

Yifu Zhang*, Meng Chen, Tao Hu, Changgong Meng

School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China

*Corresponding authors. E-mail address: [email protected]

1 ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The evolution of newfangled electrode materials with excellent properties has gained tremendous attention to meet their requirements applied to flexible energy devices. Vanadium oxides have attracted attractive attention for the field in energy storage, and the exploration for novel V-based materials applied to energy storage has been brought much focus recently. Vanadium oxyhydroxide VO(OH)2, which has been rarely paid attention, may deserve wider attention owing to the great potential for energy storage. To play their full strengths, it’s an excellent way to hybridize with carbon materials such as rGO, which has superior conductivity and structural stability. Herein, we develop a novel 3D composite constructing from VO(OH)2 nanoflakes wrapped with reduced graphene oxide nanosheets (denoted as 3D VO(OH)2/rGO) as enhanced performance electrodes for symmetric energy storage devices. 3D VO(OH)2/rGO is synthesized by a onestep hydrothermal route and this novel composite possesses a unique architecture and the intimate interaction, which endues VO(OH)2/rGO with remarkable electrochemical performance. VO(OH)2/rGO electrode yields the high specific capacitance of 512 F·g−1 (1024 C·g−1) at 0.5 A·g−1 within −1.2~0.8 V. Impressively and unexpectedly, the assembled VO(OH)2/rGO symmetric supercapacitor exhibits the integration of desirable flexibility, 2.4 V high voltage and high energy density, which reaches 56.26 Wh·kg−1 at 104.16 W·kg−1. After charging for 100 s, two devices in series can light red and blue LED for 12 min and 5 min, respectively. This work not only enriches the research of vanadium oxyhydroxides, but also offers new opportunities for assembling high-performance energy storage devices.

Keywords: VO(OH)2; VO(OH)2/rGO composites; Electrochemical properties; Energy storage devices


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1. Introduction Supercapacitors (SCs) have been widely developed as energy storage devices owing to their unique characteristics in terms of energy and power densities, and the ability to store higher energy densities than traditional capacitors and reach to higher power density than batteries makes SCs ideal for applications requiring quick bursts of energy, for instance, portable electronics, high power electronic devices and hybrid electric vehicles 1-7. On the basis of the formula E=1/2CV2, the energy density (E) of a SC is depended on specific capacitance (C) and cell potential window (V). To improve C and enlarge V are of great essential for achieving a SC with high-performance, which are governed by the intrinsic properties of the electrodes and electrolytes 8. Therefore, designing and fabricating novel electrode materials are of vital significance but enormous challenges

9-11

. The energy storage of SCs comes from two mechanisms 12. One is a non-

Faradic process (double-layer capacitance) and the other is Faradic process (pseudocapacitance). According to the kind of electrode materials applied to SCs, these two mechanisms can work independently or collaboratively 12. Electrode materials for electrochemical double-layer capacitors (EDLCs) compose of carbon materials which possess high surface area and superior electrical conductivity (e.g. carbon films/textiles, graphene, carbon nanotubes, activated carbon and so on)

13

. However, carbonaceous

materials also have some drawbacks like low specific capacitance and poor energy density. Electrode materials for charge-transfer-reaction pseudocapacitor compose of transition metal oxides (e.g.: RuO2, MnO2) and conducting polymers, which usually exhibit excellent specific capacitance and energy density but inferior power density and cycle ability

8,14

. Thus, integrating electrode materials with double-layer

capacitive and pseudocapacitive features in one system can combine their both advantages and even accompany with synergistic effects, which can greatly improve the electrochemical performances of the composites 1. Recently, the intercalation pseudocapacitance (IPC) has been arisen as an energy storage mechanism, which inherits the merits of batteries and SCs 15. The kinetics of the IPC are surface-controlled processes and the integrated behavior exhibits capacitive. The electrodes with IPC properties possess the optimal characteristics of both batteries (superior energy density) and SCs (excellent power density)

15-17

. Just

recently, vanadium oxides and their derivative materials have attracted much attention as promising IPC electrode materials, where the faradaic process is used to transfer ions embedded in the tunnels or layers of redox active materials without ion diffusion restriction and crystal phase transition

17-20

. For years,

vanadium oxides (e.g.: V2O5 and VO2 with multifarious structures and morphologies) have been extensively



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used in energy storage systems, not just owing to the rich sources, low price, and can survive in variable valence states, but also the really large potential window and the high energy density

21-26

. Nevertheless,

vanadium based oxyhydroxides have been rarely investigated which may exhibit strinkingly promising scientific properties in electrochemistry 27. Up to now, only Haggite V4O6(OH)4 28 and Duttonite VO(OH)2 29

has been reported. VO(OH)2 is composed of chains of distorted octahedra VO2(OH)4 sharing edges. To

synthesize VO(OH)2 via the aqueous precipitation, the pH was adjusted to 4.0 which is are very fastidious and unmanageable. Very recently, VO(OH)2 was synthesized with the assistance of PVP 30. Both works 29,30 studied its electrochemical properties and demonstrated it could store energy by the intercalation/deintercalation process (IPC feature). However, its electrochemical property is severely troubled by the dissolution, inferior structure durability and high impedance. Thus, this work will focus on the improvement of the electrochemical performance of VO(OH)2 on its application as IPC materials, which is essentially urgent and valuable to break new ground of VO(OH)2 materials. The coorperation of adding conducting materials with superior stable structure is a feasible strategy to optimize the electrochemical performance of the electrodes

31-33

. Graphene oxide (GO), two-dimensional

macro-molecular sheets of sp2-hybridized carbon atoms with a honeycomb structure, has been considered as a next generation carbonaceous material for SCs

31,34,35

. However, as GO materials are susceptible to

restack in solution because of the strong π–π interactions between the basal planes of graphene sheets and sufficient large pores for easy penetration of electrolyte ions are absent, the direct application of GO to SC applications has been hindered 31. Previous studies show that carbonaceous materials are widely applied for the hybridization with vanadium oxides as SCs’ electrodes 36-38. Hence, the binary composite of VO(OH)2 combing with GO networks GO is expected to emerge, which amalgamates the superiorities of each components. Herein, a 3D VO(OH)2 nanoflakes wrapped with reduced GO (3D VO(OH)2/rGO) composite is synthesized by a one-step hydrothermal strategy. Due to the synergistic effects of different component, the electrochemical properties of 3D VO(OH)2/rGO have been considerable improved. The assembled flexible VO(OH)2/rGO symmetric SC device performs a wide potential range up to 2.4 V with an impressive energy density of 56.26 Wh·kg−1 at 104.16 W·kg−1, which is satisfied and promising for practical application.


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2. Results and Discussion 2.1. Composition, structure and morphology of VO(OH)2/rGO The electrode material 3D VO(OH)2/rGO was synthesized by simple solution processes, as schematically depicted in Fig. 1a. 2D GO nanosheets were synthesized by a modified Hummer’s route 38,39. The synthesized GO contains lots of functional groups, which provides sites for condensation of VO(OH)2 during the addition of NaOH solution

40

. Firstly, VO2+ was anchored on the functional GO surface.

According to the previous exploration 30, the regulation of pH to 4.7 is the decisive condition for synthesis of vanadium oxyhydroxide. Secondly, the hydrothermal treatment further promote the growth and formation of VO(OH)2 nanaflakes on GO surface through Ostwald Ripening. In the meantime, GO is reduced to rGO by a small amount of V4+ discussed below, which ensures the high conductivity of 3D VO(OH)2/rGO composite 39. Thus, VO(OH)2 nanoflakes wrapped with rGO nanosheets are yielded and stack together to form 3D interlaced network. In such an integrated composite, the simultaneous assembly of nanoflakes on rGO nanosheets facilitates to prevent restacking of rGO nanosheets 41. The presence of the rGO nanosheets with good electrical conductivity facilitates the electron transport in 3D porous VO(OH)2/rGO composite, as described in Fig. 1b. The electrons can access VO(OH)2 nanoflakes quickly when they reach rGO nanosheets, and thereby the conductivity of 3D VO(OH)2/rGO is improved 41. The above inference is proved by the improved electrochemical performance of 3D VO(OH)2/rGO composite electrode material.



Fig. 1. (a) Schematic description of the route for 3D VO(OH)2/rGO; (b) The ideal electron-transfer pathway for synthesized VO(OH)2/rGO composite, showing electrons can go everywhere once they reach rGO nanosheets.

Fig. 2 displays the FE-SEM (a-c) and TEM (d-h) pictures of prepared VO(OH)2/rGO composite. The composite is constituted from 2D rGO nanosheets and VO(OH)2 nanoflakes, in which the nanoflakes are supported on rGO. To be specific, in the absence of GO, VO2+ species condensate with settled orientation to synthesize VO(OH)2 nanorods (Fig. S1). However, in the presence of GO, the morphology of VO(OH)2/rGO (sheet-like structure) is dramatically different from VO(OH)2. In Fig. 2a, the VO(OH)2/rGO nanosheets are randomly dispersed and closely connected with each other. Furthermore, VO(OH)2/rGO gives a paper-like feature, suggesting a flexible character. It can be observed from Figs. 2b and 2c that, for sheet-like hybrid composite, rGO nanosheets play a role of substrate, which is decorated with flaky VO(OH)2 structures. VO(OH)2 nanoflakes are densely and uniformly anchored on rGO nanosheets to get an almost completely coated monolayer, where there is strong interaction with each component, according to the analysis of XPS discussed below. And the architecture which rGO nanosheets are densely covered by VO(OH)2 nanoflakes effectively prevent restacking of rGO nanosheets caused by π stacking interactions 42

. VO(OH)2 in hybrid is irregular intermixed flakes with a few hundred square nanometers and they are

intimately stacked together compared with pure VO(OH)2. The above argument is strongly supported by


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TEM observations (Fig. 2d-h). It is visible in Fig. 2d that VO(OH)2 nanoflakes are wrapped up and down by rGO nanosheets. In the magnified TEM images (Fig. 2e-g), it clearly seen that VO(OH)2 nanoflakes are densely and uniformly anchored on rGO to exhibit a sandwich-like composite. Furthermore, from TEM images (Fig. 2e-g), the interior structures of VO(OH)2/rGO is considerably loose because the contrast grade is either overt or covert 26. The deep insight shows that the composite exhibits the hierarchical nanoporous structures, which are expected to lead electrolyte ions quickly and easily infiltrate to the whole electrode materials 43. Fig. 2h shows a HRTEM image of VO(OH)2/rGO from the red square in Fig. 2g, in which the connected structures between VO(OH)2 and 2D rGO are apparently detected. As covered by rGO, the fingerprint between the neighboring planes of VO(OH)2 is not very clear or occasionally observed. As inserted in Fig. 2h, the lattice fringe of VO(OH)2 with 0.20 nm is detected, which corresponds to XRD peak of VO(OH)2 at around 2θ = 45° (Fig. S2). A SAED pattern (Fig. 2h) confirms that the VO(OH)2/rGO has a typical polycrystalline structure and the degree of crystallinity is relatively high. The diffraction rings with the lattice fringes with spaces of 0.36, 0.25, 0.20, 0.18 and 0.15 nm are corresponding to the XRD peaks of VO(OH)2 (Fig. S2) at around 2θ = 24°, 36°, 45°, 50°, and 62°, respectively. Hence, the SAED pattern convincingly prove the existence of VO(OH)2. Therefore, the perfect structural integrity of 3D VO(OH)2/rGO composite constructed from 2D rGO nanosheets and VO(OH)2 nanoflakes is fabricated. This unique architecture suggests that VO(OH)2/rGO composite is rather not oversimplified physical admixture, but there are indeed chemical bonds between VO(OH)2 and rGO. For the change of morphology from VO(OH)2 nanorods to nanoflakes, rGO nanosheets not only provide anchor points for growth of VO(OH)2, but also the structure-directing agents

27

. In general, the structure that VO(OH)2 nanoflakes

tightly anchor on rGO nanosheets not only enhances electrical conductivity, but also improves structural stability, which is beneficial to the electrochemical cycle stability. Moreover, VO(OH)2 nanoflakes overlapped with each other closely interconnect with rGO nanosheets, which assists to bridge the distance of electrons and ions, simultaneously improve their electrochemical performance.



Fig. 2. (a-c) FE-SEM images of VO(OH)2/rGO with different magnifications; (d-h) TEM images and a SAED pattern inset (h) of VO(OH)2/rGO with different magnifications.

Fig. S3 shows the XRD pattern of GO and graphite, which supports the successful synthesis of GO. There is a strong characteristic peak at 10.66° for GO, indicating that graphite sheets (d002 = 0.355 nm) are expanded to 0.860 nm by the introduced oxygen functional groups via the improved Hummers’ method 38,39. Fig. S2 gives XRD patterns of VO(OH)2 and VO(OH)2/rGO. The as-prepared VO(OH)2 is very consistent with the JCPDS, No. 11-0209, demonstrating that VO(OH)2 is successfully prepared by a facile


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hydrothermal method. For VO(OH)2/rGO, most of the peaks are attributed to the VO(OH)2, except few impure peaks indexed to V5+ (V2O5), which is due to the oxidation of GO. On the one hand, V4+ is partially oxidized to V5+, on the other hand, GO is reduced to rGO. The characteristic reflection of GO in VO(OH)2/rGO disappears but an indistinct broad peak at around 22° appears, which is ascribed to the (002) plane of exfoliated rGO. In addition, the peak of rGO is weak and broad, and overlapped by the strong peak of VO(OH)2. It can be speculated that only few rGO nanosheets respond to the XRD diffraction signal, since the surface of rGO is almost covered with VO(OH)2, which can effectively avoid restacking of rGO nanosheets 44. Figs. S4 and S5 respectively show ED and elemental mapping images of VO(OH)2/rGO, revealing the coexistence of carbon, oxygen and vanadium elements. The omnipresent and omnipresent distribution suggests that VO(OH)2 is intimately combined with rGO to form 3D networks (Fig. 2). To further prove the uniform distribution of VO(OH)2 on rGO surface, Raman mapping was carried out (Fig. S6). Fig. S6a shows the optical image of VO(OH)2/rGO, and Fig. S6b displays the corresponding Raman mapping of VO(OH)2/rGO signal at 516 cm−1. As can be seen from the randomly selected areas (10×10 μm2), signal of VO(OH)2/rGO almost responds in the whole area, although the intensity is different. The silent area of the VO(OH)2/rGO respond is due to the unevenness of the surface caused by the concave during pressing

45

. Hence, the Raman mapping of VO(OH)2/rGO proves the rGO sheets are covered

uniformly by the VO(OH)2 nanoflakes. Fig. S7 shows the TEM image of VO(OH)2/rGO and their corresponding elemental mapping images, respectively. Elemental mappings of VO(OH)2/rGO confirm the simultaneous existence of carbon (red), vanadium (yellow and green), and oxygen (blue), indicating the uniform distribution of VO(OH)2.

Fig. S8 shows the full XPS spectra (The C1s in VO(OH)2 was used as charge reference.), revealing the elements in the corresponding samples: VO(OH)2 (V, O), GO (C, O) and VO(OH)2/rGO (V, O, C). Fig. 3a represents a binding energy for V2p peaks in VO(OH)2 and VO(OH)2/rGO. The energy bands at 516.3 eV (V2p3/2) and 523.8 eV (V2p1/2) suggest that vanadium is VⅣ in VO(OH)2 30. For VO(OH)2/rGO, the V2p3/2 binding energy which can be treated as two peaks is contributed to VⅣ (516.5 eV) and VV (517.5 eV) 46, respectively. Although V4+ is partially converted to V5+ because of the oxidation of GO, the relative small peak area of V5+ declares the major vanadium is V4+, indicating that most of the V-based material is still VO(OH)2in the composite. This results supports that GO is reduced to rGO by a small amount of V4+. The C1s peaks of VO(OH)2/rGO and GO are showed in Fig. 3b. They can both be split into three peaks at 284.4



eV, 285.9 eV, and 288.5 eV in the composite (at 284.8 eV, 286.9 eV, and 288.6 eV in GO), which corresponds to C−C/C=C, C−O, and C=O, respectively. For GO, the carbon bond with the strongest relative intensity is C−O, suggesting the graphite is successfully oxidized in the oxidation reaction. However, in VO(OH)2/rGO, the peak of C=C/C−C exhibits complete dominance, while the intensity of C−O and C=O become enormously weaken. All these changes suggest that, with much removal of oxygen functional groups, sp2 carbon network get effectively restored and GO has been great reduced to rGO. Fig. 3c shows the O1s spectrum of GO and VO(OH)2/rGO. The peak can be fitted into two peaks: 532.3 eV and 533.1 eV, which corresponds to O−C and O=C of residual oxygen groups. For VO(OH)2/rGO sample, except the two similar peaks of O−C (531.2 eV) and O=C (532.0 eV), the binding energy arising at 530.1 eV is indexed to V−O. It’s worth noting that the binding energy of V4+ in VO(OH)2/rGO slightly shift to a higher value compared with VO(OH), whereas the binding energies of carbon bonds and oxygen functional groups of VO(OH)2/rGO shift to lower values. These result indicate that there exists a certain intercalation between VO(OH)2 nanoflakes and rGO nanosheets, driven by a probable electrons transfer from VO(OH)2 to rGO, which is the considerable evidence that VO(OH)2 effectively combine with rGO through the chemical interactions 47,48. Commonly, the inner electrons and nucleus can produce strong Coulomb interactions in the atom, and a shielding effect applied by the outer electrons. Therefore, the binding energy (Eb) is calculated as follows: Eb=Vc+Vn where Vc means the Coulomb force and Vn represents the repulsive force. According to the equation, with the decrease of the outer electron concentration of VO(OH)2, the corresponding shielding effect will reduce and the binding energy of the inner electrons will increase due to a penetration effect, then causing V2P peaks of VO(OH)2 shifting to a higher energy direction. In the meantime, the shielding effect is enhanced when density of outer electrons increased, resulting in the corresponding O1s and C1s peaks in rGO shifting to a lower energy direction. According to the above results, the VO(OH)2/rGO has an intimate interface and an electron interaction through V-O-C bonds. Moreover, this intimate interaction of electrons is conducive to the improve the conductivity of VO(OH)2/rGO thereby enhance the electrochemical properties. FIIR spectra of GO, VO(OH)2 and VO(OH)2/rGO are shown in Fig. 3e. In GO, there is a broad peak at 3400 cm−1, which is contributed to O−H of carboxylic acid. And the other peaks at 1720, 1208, 1050, 851 and586 cm−1are ascribed to C=O, C−O in phenolic, C−O in alkoxy, C−H bond and O−C=O in carboxylic acid, suggesting that there are abundant oxygen groups on GO 49. In VO(OH)2, the wavenumbers


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at 3563 cm−1 and 3268 cm−1 are assigned to the stretching vibrations of V−OH and hydrogen-bonded OH groups, and the strong peak at 969 cm−1 is related to VO2+ stretching vibration. The wavenumbers at 866 cm−1 and 799 cm−1 are indexed to the in-plane V−OH deformations. And the wavenumbers at 623 cm−1, 542 cm−1 and 446 cm−1 are assigned to the torsional modes of the OH groups

50

. Most of the peaks in

VO(OH)2/rGO are well consistent of pure VO(OH)2 except three new peaks at 3430 cm−1, 1124 cm−1, and 1050 cm−1. The peak presents at around 3430 cm−1 is related to the O−H bond of rGO. The wavenumber at 1124 cm−1 is owing to the stretching vibration of C−O, which shifts to the lower wavenumber compared with GO. The reason for the blue shift is that oxygen group coordinates to metal ions, which is the prove rGO interconnects with VO(OH)2 51. And the weak peak appears at 1040 cm−1 is ascribed to V5+=O. The existence of V5+ is in consistence with the reslut of XRD and XPS. Raman spectra are employed for investigating the structure of GO, VO(OH)2 and VO(OH)2/rGO. Raman signals from 0 to 1100 cm−1 of VO(OH)2/rGO (Fig. 3f) are attributed to various bending and stretching modes of VO(OH)2. As for GO and VO(OH)2/rGO, two peaks are the characteristic peaks of D- (1351 cm−1) and G- (1599 cm−1) bands, respectively. The intensity ratio of the D- and G-band (ID/IG) is frequently used for the defects and disorder assessment. ID/IG ratios are 1.0 and 1.24 for the GO and VO(OH)2/rGO. Corresponding to the analysis of XPS, VO(OH)2 interact with rGO via V-O-C bonds which usually causes defects such as bonding disorders and vacancies in the graphene lattice, and thus these defects increase the intensity ratio of ID/IG. Moreover, the relatively high ID/IG value of VO(OH)2/rGO also is the result of the growth direction of rGO for VO(OH)2 in the synthesis process, which leads to a high density of open and disordered edges or grain boundaries on the top scattering more Raman signal 52,53. Therefore, higher values of VO(OH)2/rGO proves that VO(OH)2 is effectively interact with rGO materials. In addition, the location of G-band is also a vital proof for the structural change of carbon materials, which can further support the above findings. Compared with GO, the G-band of VO(OH)2/rGO is shifted to lower frequency, demonstrating there are more vacancies in the graphene lattice and compressive strain effect from surface physiochemical interaction between VO(OH)2 and rGO 54.



Fig. 3. V2p (a), C1s (b) and O1s (c) core level XPS spectra of VO(OH)2, GO and VO(OH)2/rGO, repsectively; (d) N2 adsorption/desorption isotherms of VO(OH)2 and VO(OH)2/rGO, insert the BJH pore size-distribution; FTIR (e) and Raman (f) spectra of VO(OH)2, GO and VO(OH)2/rGO.

The porous properties of VO(OH)2 and VO(OH)2/rGO were determined by N2 adsorption/desorption measurements, as depicted in Fig. 3d. The BET specific area of VO(OH)2 and VO(OH)2/rGO measures 14.3 and 34.4 m2·g−1, respectively. The corresponding BJH pore volume respectively reaches 0.049 and 0.115 cm3·g−1. The values of both BET specific area and BJH pore volume of VO(OH)2/rGO are higher than that of VO(OH)2, illustrating that the incorporation of rGO increases the surface area and enhances the porosity. Obviously, both of the samples display a type IV isotherm with a hysteresis loop indicating the existence of mesopores

55

. The pore size-distribution of VO(OH)2 and VO(OH)2/rGO calculated from

adsorption part of nitrogen isotherm is inserted in Fig. 3d. Compared with VO(OH)2, VO(OH)2/rGO shows more apparent meso- and macropores structures. The pore size-distribution of VO(OH)2/rGO is mainly centered at two regions: 2~3.5 nm and 10~75 nm, whereas VO(OH)2 only exhibits the large region. Furthermore, the amount of pores of VO(OH)2/rGO is much more than VO(OH)2. The various porous structures can cooperate well with each other. On the one hand, the macropores act as reservoirs to lead electrolyte to quickly insinuate into the electrodes. On the other hand, the mesopores can offer lots of active sites for faradic process. The co-existence of various porous structure shortens the electron transport path and accelerates the insertion/desertion of the ions, which is expect to improve the electrochemical properties


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of 3D VO(OH)2/rGO 1.

2.2. Electrochemical properties of 3D VO(OH)2/rGO To investigate the electrical properties of 3D VO(OH)2/rGO, the electrodes were sufficiently tested by using a three-electrode system in 1 mol·L−1 LiClO4/PC. Fig. S9 shows the representative CV curves of VO(OH)2/rGO (20%) within various potential windows, which reveals that the highest specific capacity is achieved in the relatively large potential window of −1.2~0.8 V. Thus, the GCD and CV measurements were performed within this potential range. Fig. 4a-d shows GCD curves of VO(OH)2 and 3D VO(OH)2/rGO composites with various rGO contents (10%, 20% and 40%) at different current densities from 0.5 to 10 A·g−1. The shapes of these GCD curves are nonlinear, which are different from the typical EDLC electrode materials whose GCD curves are triangular. There are two platform parts in VO(OH)2/rGO composites, which well corresponds to the positions of the redox peaks in CV curves (Fig. 5). The reason for appearance of plateaus is the charge storage mechanism of vanadium oxides owing to the intercalation/extraction of Li ions, which causes reversible redox reactions 56. Fig. 4e summarizes the GCD curves at 1 A·g−1, which clearly observes that VO(OH)2/rGO (20%) exhibits the best electrochemical performance. The specific capacitances measure 62 F·g−1 (0), 294 F·g−1 (10%), 403 F·g−1 (20%) and 257 F·g−1 (40%), correspondingly, the specific capacities reach 124 C·g−1 (0), 588 C·g−1 (10%), 806 C·g−1 (20%) and 514 C·g−1 (40%). Furthermore, Fig. 4f describes the relationship between specific capacities and current densities of these four samples at different current densities from 0.5 to 10 A·g−1. First, the values of the capacities decline with the increasing current densities. The reasons for the attenuation are not only due to incremental voltage drop 26, but also the inadequate utilization of active material. Second, the capacities of 3D VO(OH)2/rGO composites are considerably improved compared with VO(OH)2, which evidences the perfect cooperation of carbonaceous material and VO(OH)2 in agreement with other composites

57-59

.

Among VO(OH)2/rGO composites, there is too little content of rGO in VO(OH)2/rGO (10%), which can realize the effect of rGO. By the contrary, overdose of rGO in VO(OH)2/rGO (40%) limits the electrochemical performance owing to the poor electrochemical performance of GO (Fig. S10) or causing by agglomeration and then destroying the structural stability

60

. Last, VO(OH)2/rGO (20%) exhibits the

highest capacity within the whole current density ranges. The specific capacities reach 512, 403, 353, 276 and 237 F·g−1 (1024, 806, 706, 552 and 474 C·g−1) at the current densities of 0.5, 1, 2, 5 and 10 A·g−1, respectively. The achieved values are even higher than other V-materials as supercapacitor electrodes



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concluded in Table S1.

Fig. 4. Electrochemical properties of VO(OH)2 and 3D VO(OH)2/rGO composites with various rGO contents: (a-d) GCD curves at different current densities; (e) Summary of GCD curves at 1 A·g−1; (f) Specific capacities vs. current densities.

To further assess the electrochemical performance of VO(OH)2 and VO(OH)2/rGO, their CV profiles were carried out at different scan rates from 5 to 100 mV·s−1 as shown in Figs. 5a and 5b, respectively. The CV curves at various scan rates which still maintains similar shape, suggesting that VO(OH)2 and VO(OH)2/rGO possess an excellent faradic respond as electrode materials. With increasing scan rates, the respond currents also increase, illustrating the rapid ionic and electronic transport

61

. Furthermore, the

currents of anodic and cathodic peaks are almost consistent, which is the evidence that outstanding electrochemical reversibility. Besides, due to the effect of polarization, redox peaks slightly shift at different scan rates, but they are still observed even at 100 mV·s−1, which states the good ionic and electronic conduction and superior rate capability. Intuitively, Fig. S11 compares the CV curves of VO(OH)2 and VO(OH)2/rGO at 10 mV·s−1, which show redox peaks owing to VO(OH)2 surrounded by EDLC 19. Wang et al.

12

mentioned that the relationship between voltammetric and various sweep rates is represented as

follows: i = avb When the b = 0.5, the material is battery behavior which is controlled by diffusion. Whereas b is equal or 14 ACS Paragon Plus Environment

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closed to 1, which means the electrode active material is intercalation pseudocapacitive 12,62. Fig. S12 shows the CV curves from 0.1 to 5 mV·s−1 and plots of log (i) versus log (v) of VO(OH)2/rGO, in which the b value is approximately 1, suggesting a surface-controlled capacitor behavior via a fast Li+ intercalation process. The similar phenomenon is reported in newly published literatures 15,18,48,63. In addition, it’s worth mentioning that the mass-loading of active materials decides the true performance, which was reported by Gogotsi et al 64. In this work, the mass loading is about 3.5 mg/cm2 and VO(OH)2/rGO still retain a high capacitance, indicating that it exhibits superior intercalation pseudocapacitive behavior. Therefore, the capacitance of VO(OH)2/rGO originated from the intercalation pseudocapacitance

20

and the EDLC

associated with rGO. The electrochemical redox reactions of VO(OH)2 may be represented by the following equation: VO(OH)2+xLi+ +xe−↔LixVO(OH)2 (x shows the mole fraction of the intercalated Li ions). Besides, as for VO(OH)2/rGO composite, it exists another pair of redox peak. The reason is that V4+ is oxidized to V5+ due to the oxidation of GO which are proved in above discussion of composition. As well known that vanadium oxides consisting of V5+ are typical battery-type electrode materials, which shows characteristic redox peaks caused by reversible faradaic redox reaction

65,66

. Hence, the store energy of

VO(OH)2/rGO is the combination of the intercalation pseudocapacitive and double-layer capacitive process.

Cycling stability is an important parameter for high-performance SCs. Fig. 5c shows the cycling performance of VO(OH)2and 3D VO(OH)2/rGO. The cycling performance of VO(OH)2/rGO (95% after 5000 cycles) is much remarkable compared with VO(OH)2 (7% after 2000 cycles). The poor cycle performance of VO(OH)2 is caused by its unstable structures during the repeated change-discharge process 29

. For 3D VO(OH)2/rGO composite, the capacitance exhibits a slight decline in the first 100 cycles, due to

the typical problem of vanadium-based materials that active materials are hard to avoid dissolution during constant circulation 67. Nevertheless, after 5000 cycles, the capacitance retention reaches to 95% due to interacted 3D networks of VO(OH)2/rGO. Namely, rGO nanosheets provide many active sites for combination with VO(OH)2, thus VO(OH)2 nanoflakes tightly anchor to rGO. As is well known, rGO possesses excellent stable structure and conduction. Therefore, the cycle stability of VO(OH)2/rGO is greatly improved and superior to other V-based materials (Table S1). In addition, Coulomb efficiency of VO(OH)2/rGO remain almost 100%, suggesting good reversibility of the electrode. But for VO(OH)2, it declines after many cycles corresponding to the poor cycling stability (Fig. S13). Fig. 5d depicts the Nyquist plots of VO(OH)2 and VO(OH)2/rGO, which contains two parts: the higher frequency region (the arc part)



and the lower frequency region (straight line part), which can be fitted by the equivalent circuit. Rs represents the bulk resistance from the electrolyte, electrode materials and the interface between active material and Ni foam, which can be represented by the intercept of X axis. Cdl is related to the constant phases-angle element of double layer capacitance. Rct is the charge-transfer resistance, which is represented by diameter of semicircle. And Zw stands for the Warburg resistance related to the inclined line in the lower frequency region, which means the solid-state diffusion of Li ions. In Fig. 5d, VO(OH)2/rGO shows a more depressed semicircle than VO(OH)2, which is hard to be observed. Rs (axis intercepts) of VO(OH)2 and VO(OH)2/rGO are about 0.5 Ω and 1.5 Ω, respectively. Rs and Rct of VO(OH)2/rGO are considerably reduced compared with VO(OH)2 owing to the larger specific area and abundant porous structure of VO(OH)2/rGO. On the one hand, the pores are like reservoirs for electrolyte to intimately and rapidly infiltrate to the interior of active material. On the other hand, the larger specific area provides more active sites for electrochemical reactions. Thus, electron and Li+ transfer more quickly to promote redox reaction kinetics 60. The line in the low frequency region of VO(OH)2/rGO is more vertical than VO(OH)2, which demonstrates the excellent conduction and the shorter Li+/electronic transmission path. With consociation of rGO, the conductivity of VO(OH)2/rGO has been greatly improved, which supports that rGO effectively facilitate the electron-transfer as described in Fig. 1b.

Fig. 5. Electrochemical characterizations of VO(OH)2 and 3D VO(OH)2/rGO composite: (a) CV 16 ACS Paragon Plus Environment

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curves of VO(OH)2 at different scan rates; (b) CV curves of 3D VO(OH)2/rGO composite at different scan rates; (c) Cycle performance; (d) Nyquist plots over 100 kHz to 0.01 Hz, inserted an equivalent electrical circuit. 2.3. Electrochemical performance of VO(OH)2/rGO To explore the practical application of 3D VO(OH)2/rGO composite, the flexible symmetric SC (SSC) was assembled using this unique composite, denoted as VO(OH)2/rGO SSC device, as schematically shown in Fig. S14. From CV curves of VO(OH)2/rGO SSC device (Fig. 6a), it’s observed that the voltage window can be extended to 2.6 V. For feasibility and optimum, the voltage range is 0~2.4 V, which is larger than many devices in the references (generally less than 2 V, Table S2). As shown in Fig. 6b, the shape of CV curves is quasi-rectangular and maintain well even at 100 mV·s−1. It demonstrates the VO(OH)2/rGO SSC device possesses outstanding capacitive behavior and good rate property. Fig. 6c shows GCD files within the voltage of 0~2.4 V at 0.5, 1, 2, 5, 10 mA·cm−2, and the specific capacitances (Fig. 6d) reach to 70.3, 68.1, 47.9, 30.5 and 20.2 F·g−1 (405, 392, 276, 176 and 116 mF·cm−2) respectively. Notably, the GCD curves are symmetric and quasi-linear, which reveals the VO(OH)2/rGO SSC device owns remarkable reversibility. To explore the cycle stability of VO(OH)2/rGO SSC device in real application, the 2000 cycle stability test was carried out at 50 mV·s−1 (Fig. 6e). During the first 300 cycles, the retention of VO(OH)2/rGO SSC device gradually increases and reach up to 125% of its initial value. This phenomenon is the activation process of the electrode, which is attributed to the electrolyte gradually infiltrate the active materials with repeated charge/discharge process 68. After 2000 cycles, the Coulomb efficiency is almost 100% whereas the capacitance retention is about 96% with little decline. The reason is that the voltage window of 2.2 V is really wide, thus the strong stress introduced by structure expansion during Li+ insertion into VO(OH)2/rGO easily causes its structure to collapse, which lead to promote the dissolution of VO(OH)2/rGO. Moreover, limited to the conditions of devices manufacturing, during the many times charging/discharging process, the ion mobility and kinetic in device is slower than that case in threeelectrode system. The above reasons lead to the decrease of the capacitance retention. To study the flexibility and mechanical stability of VO(OH)2/rGO SSC device, the CV test was carried out at various bending angles at 50 mV·s−1. Under the bending angles from 0 ° to 150 ° (Fig. 6f), the CV curves keep nearly similar shape, which evidences the excellent flexibility and mechanical stability of VO(OH)2/rGO SSC device. Fig. S15 displays the CV curves of nickel foam bending to 90° after 1st, 30th, 50th and 100th cycles. They are gradually deformed from 1st to 100th bending.



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Fig. 6. Electrochemical performance of 3D VO(OH)2/rGO SSC device: (a) CV curves at different voltage windows; (b) CV curves at various scan rates; (c) GCD curves at various current densities; (d) Capacitances vs. current densities; (e) Cycling performance and Coulomb efficiency of the 3D VO(OH)2/rGO SSC device configuration; (f) CV curves at different bending states.

Fig. 7d describes the Ragone plots of the VO(OH)2/rGO SSC device compared with previous reports. The maximum energy density measures 56.26 Wh·kg−1 (3.24 Wh·m−2) at 104.16 W·kg−1 (6 W·m−2), which is comparable or even higher than previous reports (Table S2), such as V2O5 SSC (43 Wh·kg−1 at 900 W·kg−1) 56, V2O5/polyindole SSC (38.7 Wh·kg−1 at 900 W·kg−1) 69, rGO/V2O5 ASS (75.9 Wh·kg−1 at 900 W·kg−1) 39, RG/VO2//RG ASC (22.8 Wh·kg−1 at 425 W·kg−1) 70, graphene/VO2 SSC (21.3 Wh·kg−1)

71

,

Ni0.85Se@MoSe2//GNS (25.5 Wh·kg−1 at 420 W·kg−1) 72, NiCo2O4-MnO2/GF//CNT/GF (55.1 Wh·kg−1 at 187.5 W·kg−1)

73

, V2O5//V2O5 (1.18 W·h·m−2 at 4 W·m−2) and V2O5//C (1.22 W·h·m−2 at 4 W·m−2)

PANI/SWCNTs//PANI/SWCNTs (19.45 Wh·kg−1 at 320.5 W·kg−1)

74

,

75

. According to the above results,

VO(OH)2/rGO SSC device exhibits the excellent electrochemical performance with large voltage window up to 2.4 V, the specific capacitance as high as 70.3 F·g−1 (405 mF·cm−2) at 0.5 mA·cm−2, energy density of 56.26 Wh·kg−1 (3.24 Wh·m−2) at 104.16 W·kg−1 (6 W·m−2). The cycle stability is about 96% after 2000 cycles and the Coulomb efficiency is almost 100%. These features are even higher than, or comparable to some related devices as shown in Table S2. Since a single device limits working potential window, using serial assemblies is a simple and viable way to control over the operating voltage. GCD curves for individual device and two devices connected in 18 ACS Paragon Plus Environment

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series are tested, as shown in Fig. 7a. The charge/discharge voltage of two devices connected in series reaches up to 4.8 V with almost the same discharge time compared with a single device which is approached to the theoretical factor 2, confirming to the theorem of series connections of capacitors. To further assess the practicability of VO(OH)2/rGO SSC device, it was used to power the LED lights. After charging two VO(OH)2/rGO SSC devices at 4.8 V, they can light red and blue LED with working voltage from 1.8~2.0 V and 2.2~2.4 V, respectively. After charge for 100 s, the red light can work for about 12 min (Fig. 7b, c) and the blue lights for about 5 min (Fig. 7e, f). The red LED is lighter and works longer than the blue one, which is due to the higher working voltage of blue LED. Thus, the assembled VO(OH)2/rGO SSC device exhibits the excellent electrochemical performance.

Fig. 7. Electrochemical property of 3D VO(OH)2/rGO SSC device: (a) GCD curves of the device shown in panel; (b, c) Red LED lighted by two SSC devices in series at different times; (d) Ragone plots compared with the previous reports; (e, f) Blue LED lighted by two SSC devices in series at different times. 3.

Conclusion In summary, 3D VO(OH)2/rGO composites with unique architectures were successfully synthesized

through a facile low temperature one-step hydrothermal method. VO(OH)2 nanoflakes wrapped with rGO nanosheets are yielded and stacked together to form 3D interlaced network. Due to the unique 3D networks,



low resistance, and synergistic action of double-layer capacitive and intercalation pseudocapacitive features, VO(OH)2/rGO composites exhibit excellent electrochemical performance with the specific capacitance as high as 512 F·g−1 (1024 C·g−1) at 0.5A·g−1 and prominent cycle stability with 95% of the vital value after 5000 cycles. As for symmetric device, VO(OH)2/rGO SSC device not only delivers high energy density of 56.26 Wh·kg−1 at 104.16 W·kg−1, but also exhibits the outstanding flexibility and wide potential window of 2.4 V. Notably, two connected devices in series can successfully power the commercial red and blue LED with 12 min and 5 min, respectively, supporting the considerable practical applications. Owing to the competitive effect of electrical conductivity rGO and intercalation pseudocapacitors VO(OH)2 on the electrochemical performance of hybrid composites, the optimum content of rGO in a hybrid composite for the high-performance SC is 20%. It is believed that VO(OH)2/rGO can be used in broader applications, such as Li-ion batteries, Na-ion batteries and flexible chemical sensors. This work not only enriches the research of vanadium oxyhydroxides and demonstrates their great potential for energy storage, but also breaks new ground of vanadium oxyhydroxides, which provides new insights into the fabrication of electrode materials for the next-generation SCs. Acknowledgements 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 (C.M., Grant No. DUT18RC(6)008) and the China Sponsorship Council (Y.Z., Grant No. 201806065025).

Conflicts of interest There are no conflicts of interest to declare. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ××××××× Supporting Information including: Experimental section; SEM and TEM images of VO(OH)2 (Figure S1); XRD patterns of graphite, GO, VO(OH)2 and VO(OH)2/rGO composite (Figure S2, S3); EDS and elemental mapping images (Figure S4, S5); Raman mapping spectrum of VO(OH)2/rGO composite (Figure S6); TEM elemental mappings of VO(OH)2/rGO composite (Figure S7); Full XPS spectra (Figure S8); CV


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curves of VO(OH)2/rGO (Figure S9); electrochemical properties of GO (Figure S10); the plots of log (i) versus log (v) (Figure S10); CV curves of VO(OH)2 and 3D VO(OH)2/rGO composite (Figure S11); CV curves and the plots of log (i) versus log (v) of VO(OH)2/rGO (Figure S12); Coulomb efficiency of VO(OH)2/rGO and VO(OH)2 (Figure S13); a schematic diagram illustrating the 3D VO(OH)2/rGO SSC device configuration (Figure S14); The stability of nickel foam with bending 100 times (Figure S15); Comparison of electrochemical properties of 3D VO(OH)2/rGO composite applied to single electrode system (Table S1) and SSC device (Table S2) with the previous literatures.

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Table of Contents Graphic

3D VO(OH)2/rGO SSC device exhibits the excellent electrochemical performance with large potential window up to 2.4 V, the specific capacitance as high as 70.3 F·g−1 (405 mF·cm−2) at 0.5 mA·cm−2, energy density of 56.26 Wh·kg−1 (3.24 Wh·m−2) at the power density of 104.16 W·kg−1 (6 W·m−2) and cycle stability of 96% after 2000 cycles.


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3D Interlaced Networks of VO(OH)2 Nanoflakes Wrapped with

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