graphene composites for high performance supercapacitor electrode

Journal of Power Sources 344 (2017) 185e194

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Amorphous vanadyl phosphate/graphene composites for high performance supercapacitor electrode Ningna Chen a, Jinhua Zhou a, Qi Kang b, Hongmei Ji a, Guoyin Zhu a, Yu Zhang a, Shanyong Chen a, Jing Chen c, Xiaomiao Feng b, Wenhua Hou a, * a

Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, PR China Key Laboratory for Organic Electronics & Information Displays, Institute of Advanced Materials, School of Materials Science & Engineering, Nanjing University of Posts and Telecommunications, Nanjing, 210046, PR China c College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing, 211816, PR China b

g r a p h i c a l a b s t r a c t Amorphous vanadyl phosphate/graphene composites with a unique layer-on-sheet hybrid nanostructure show excellent performances as supercapacitor electrode materials.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 November 2016 Received in revised form 19 January 2017 Accepted 29 January 2017

Amorphous vanadyl phosphate/graphene nanohybrids is successfully synthesized by first exfoliating bulk layered vanadyl phosphate (VOPO4$2H2O) into nanosheets, and then hydrothermal treatment with graphene oxide (GO). The electrochemical properties of the resulted materials are systematically investigated. It is found that a phase transformation from crystalline to amorphous is occurred to VOPO4$2H2O. As supercapacitor electrode material, the amorphous VOPO4/graphene composite exhibits a high specific capacitance (508 F g
1 at 0.5 A g
1), an excellent rate capability (359 F g
1 at 10 A g
1), and a good cycling stability (retention 80% after 5000 cycles at 2 A g
1). Particularly, it simultaneously has a greatly enhanced energy density of 70.6 Wh$kg
1 with a power density of 250 W kg
1. The outstanding energy storage performance mainly originates from the generation of amorphous VOPO4 phase that facilitates ion transport by shortening ion diffusion paths and provides more reversible and fast faradic reaction sites, the hybridization with graphene that greatly improves the electric conductivity and structure stability, and the unique layer-on-sheet nanohybrid structure that effectively enhances the structure integrity. This work not only provides a facile method for the preparation of amorphous VOPO4/ graphene composites, but also demonstrates the enhanced energy density and rate capability of amorphous VOPO4-based materials for potential application in supercapacitors. © 2017 Elsevier B.V. All rights reserved.

Keywords: Vanadyl phosphate Graphene Nanohybrid Supercapacitor Amorphous

1. Introduction

* Corresponding author. E-mail address: [email protected] (W. Hou). http://dx.doi.org/10.1016/j.jpowsour.2017.01.119 0378-7753/© 2017 Elsevier B.V. All rights reserved.

The supercapacitor (also called electrochemical capacitor), as a new type of green energy storage devices, has the advantages of high power density of traditional electrostatic capacitor and high

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energy density of rechargeable batteries, giving rise to a promising application in the fields of mobile communication, electric vehicle, energy storage and so on [1,2]. However, the relatively low energy density and fast attenuation under high rate conditions greatly limit its application. As a main component in the supercapacitor, the electrode material has a great influence on the performance of the supercapacitor. Therefore, much effort has been devoted to the development of novel electrode materials with a high energy and power density simultaneously [3e5]. As a layered material, VOPO4$2H2O, which is formed through vertex-sharing VO6 octahedra and PO4 tetrahedra, is an attractive pseudocapacitive material [6e8]. Because of the existence of V4þ/V5þ redox couple, it is granted with a higher redox potential of 1.0 V (and thus higher pseudocapacitance and energy density) than simple vanadium oxides [9]. Moreover, the exfoliation of layered VOPO4$2H2O into VOPO4 ultrathin nanosheets can be feasibly achieved due to the weak hydrogen bonds between VOPO4 layers [10,11]. The exfoliated 2D VOPO4 ultrathin nanosheets have a high specific surface area and surface atom ratio, being beneficial to the ion diffusion and charge transfer and the maximized contact between electrode and electrolyte. Recently, it has been reported that amorphous materials with nano-architectures, including metal oxides (such as MnO2 [12], MoOx [13], ZnO [14], and NiWO4 [15]) and metal hydroxides (such as Ni(OH)2 [16] and Co(OH)2) [17]), present good pseudo-capacitive performance and environmental benignity. Compared with crystalline materials, amorphous counterparts usually have a large number of structure defects (i.e., vacancies) which can be served as reversible active sites and thus contribute to a high capacitance [18], large channels which can facilitate the diffusion and reaction of electrolyte ions [19], and the isotropic nature which can sustain high strain of volume change along with the redox reaction [20]. It was claimed that non-crystalline VOPO4 had greatly improved catalytic activities compared with crystalline VOPO4 [21e24]. Nevertheless, the electrochemical pseudocapacitor performance of amorphous VOPO4 has not yet been reported. Although transition metal oxides usually have a high theoretical capacitance and a wide voltage window, the poor conductivity prevents them from having a superior cycle stability [6]. Among them, VOPO4 is no exception. In order to prolong the cycle stability, researchers combined VOPO4 with the highly-conductive carbon materials [18,25,26]. For example, Xie's group fabricated a flexible ultrathin-film supercapacitor based on VOPO4/graphene nanosheets [27]. Lee et al. adopted an ice-templated self-assembly process to prepare a three-dimensional (3D) porous nanocomposite of layered VOPO4 and graphene with a high capacitance and a superior capacitance retention [28]. Among the above reports, the existence of graphene greatly improved the conductivity of the integrated material. Moreover, the formation of composite also reduces the need for an adhesive, thereby eliminating the possibility of nanostructure aggregation and other side effects [29]. However, the preparation process of the composites is generally cumbersome and costly. Besides, these methods generally result in the crystalline VOPO4 rather than the amorphous structure. Currently, the preparation methods of amorphous VOPO4 are quite complicated, such as adopting suitable templates [19], using supercritical CO2 as an anti-solvent [30,31], or involving solvent evaporation in vacuum [31]. The controlled synthesis of amorphous VOPO4, especially its nanocomposites with conductive materials for high performance supercapacitors via a simple and facile method, still remains a great challenge. On the other hand, the electrochemical performance of VOPO4-based materials is still not good enough, such as the relatively low energy density and poor rate capability [32,33]. In order to meet the requirements of practical

application, the performance of VOPO4-based materials should be further improved, especially the energy density and rate capability. Here in this work, with the aim of fabricating excellent supercapacitor electrode materials with both a high energy density and a good rate capability, we developed a simple and facile method for the preparation of amorphous VOPO4/graphene nanocomposites. Owing to the generation of amorphous VOPO4 phase and the unique layer-on-sheet nanohybrid structure, the resulted VOPO4/ graphene composites exhibited the outstanding energy storage performance. Based on the experimental results, the corresponding charge-discharge mechanism was also discussed in detail.

2. Experimental 2.1. Materials Graphene oxide (GO) was prepared by the Hummers method, according to the procedure described in the literature [34]. A detailed description of the procedure is provided in supporting information. VOPO4$2H2O was obtained in the light of previously reported [35]. To prepare amorphous VOPO4/graphene composites, the as-prepared VOPO4$2H2O (60 mg) was firstly dispersed in isopropanol (25 mL) by ultrasonic treatment in an ice bath for 30 min, and then GO (30 mg) was added into the solution. For bulk VOPO4$2H2O, the weak hydrogen bonds between VOPO4 layer and H2O molecule are easily destroyed by the applied external force [36]. After sonication for another 30 min, the dispersion was transferred to a Teflon-lined stainless pressure vessel and maintained at 170  C for 20 h. After cooling down to room temperature, the resulting suspension was separated by centrifugation and washing with ethanol for several times. Then the resulting precipitate was dried overnight at 60  C under a vacuum to get amorphous composite VOPO4/graphene (2:1), in which the irregular VOPO4 layers were deposited on graphene nanosheets to give a layer-on-sheet nanohybrid structure. Besides, a phase transformation from the crystalline VOPO4 to amorphous VOPO4 was accomplished. By changing the mass of VOPO4$2H2O (72 mg and 30 mg) and that of GO (18 mg and 60 mg), another two samples with different mass ratios of VOPO4$2H2O and GO were obtained. The resultant samples were denoted as VOPO4/graphene (4:1) and VOPO4/graphene (1:2). For comparison, pure graphene and amorphous VOPO4 were also prepared through a similar process except that only GO (90 mg) or VOPO4 (90 mg) was mixed with isopropanol (25 mL) in the ice bath system.

2.2. Structure and morphology characterization Powder X-ray diffraction (XRD) patterns of the obtained samples were measured on a Philip-X’Pert X-ray diffractometer with Cu Ka radiation (l ¼ 1.5418 Å) at a scanning rate of 0.2 /s in a 2q range of 5e80 . The morphologies of the products were examined by scanning electron microscopy (SEM, JEOL JEM-6300F) and transmission electron microscopy (TEM, JEOL JEM-200CX, operating at an accelerating voltage of 200 kV). For TEM observation, the sample was dispersed in ethanol by ultrasonic treatment and then dropped onto carbon-coated copper grids. FT-IR spectra of products in KBr pellets were recorded using a Bruker model VECTOR22 Fourier transform spectrometer. X-ray photoelectron spectroscopic (XPS) measurements were carried out on an X-ray photoelectron spectrometer (Thermo Fisher Scientific, K-Alpha) equipped with a hemispherical electron analyzer (pass energy of 20 eV) and an Al Ka (hn ¼ 1486.6 eV) X-ray source. A combination of Gaussian and Lorentzian functions was used to fit the curves.

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2.3. Electrochemical measurements The as-prepared samples were mixed with acetylene black and polytetrafluoroethylene (PTFE) emulsion in a weight ratio of 80:10:10, and then a few drops of ethanol were added to form a suspension. After being stirred overnight, the slurry was pressed onto a graphite substrate as the working electrode. The working area of the electrode was set as 1 cm  1 cm and the mass loading of the electrode materials was controlled around 1.2 mg. The thickness of the electrode materials was around 25 mm and made by a doctor blade. The electrode was dried at 120  C under vacuum for 10 h. To test the electrochemical properties of the samples, a classical three-electrode electrochemical cell was used on a CHI 660D electrochemical workstation (Chenhua, Shanghai). In addition, saturated calomel and platinum wire electrodes were used as the reference and counter electrodes, respectively. The electrochemical behaviors of the electrode were investigated by cyclic voltammograms (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS), where the amplitude of the input ac signal was kept at 5 mV and the frequency range was set between 10
1 and 105 Hz. All amperometric experiments were manipulated with the potential windows of
0.1e0.9 V in a 1 M H2SO4 electrolyte. Different scan rates (5, 10, 25, 50 and 100 mV s
1) and constant current densities (0.5, 1, 2, 5 and 10 A g
1) were employed. Pure nitrogen was poured into the electrolyte for 30 min to remove dissolved oxygen in the solvent before test. The test of symmetric supercapacitors based on two electrodes of VOPO4/graphene (2:1) composite separated by a porous nonwoven cloth was performed in a two-electrode cell in 1 M H2SO4 electrolyte. The supercapacitor performance was evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge techniques within the voltage range of 0e1 V on a CHI 660D electrochemical workstation (Chenhua, Shanghai). Different scan rates (5, 10, 20, 50 and 100 mV s
1) and constant current densities (1, 2, 4, 8 and 10 A g
1) were employed.

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and VOPO4/graphene composites. For crystalline VOPO4$2H2O, sharp peaks centered at 12.1 (001), 18.7 (101), 24.2 (002), and 28.8 (200) can be readily indexed to the tetragonal phase of VOPO4$2H2O with a layered structure (PCPDF card No. 84-0111) [27]. After sonication and hydrothermal treatment, a broad diffraction peak located at 28.8 (marked with solid star) and the lack of other diffraction peaks from crystalline VOPO4$2H2O phase demonstrate that the obtained VOPO4$2H2O is amorphous (PCPDF card No. 36-1472) [37]. This result was further confirmed by Raman spectra (see Fig. S1). For graphene, two peaks, which correspond to (002) and (100) planes, can be observed at 2q ¼ 24.5 and 43.5 , respectively [32]. In the case of VOPO4/graphene composites, only two weak peaks of amorphous VOPO4$2H2O and graphene can be observed and there is no distinct peak of crystalline VOPO4$2H2O, confirming the amorphous nature of VOPO4$2H2O in the composites. It is reported that the amorphous phase can ensure abundant surface defects and penetration of ions into more regions of electrode materials, being beneficial for the maximum utilization of the material [38].

3. Results and discussion 3.1. The structure and morphology of the hybrid nanostructures Based on the experimental details, an illustration for the synthesis of amorphous VOPO4/graphene composites is shown in Scheme 1. Fig. 1 shows XRD diffraction patterns of VOPO4$2H2O, graphene,

Fig. 1. XRD patterns of VOPO4$2H2O, graphene and VOPO4/graphene composites.

Scheme 1. An illustration for the synthesis of amorphous VOPO4/graphene composites.

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To observe the morphologies of the as-synthesized samples, the scanning electron microscopy (SEM) was used. As shown in Fig. 2a, the crystalline VOPO4$2H2O sample has a morphology of smooth layered structure with a size of ~5 mm. After sonication and further hydrothermal treatment, the surface of VOPO4$2H2O became rough meanwhile the size became much smaller (Fig. 2b). This stems from the transformation of crystalline VOPO4$2H2O into amorphous phase after hydrothermal treatment [22]. Compared with the folded graphene sheets (Fig. 2f), it can be found in VOPO4/graphene composites that a certain amount of irregular VOPO4 nanosheets are uniformly anchored on graphene (Fig. 2c, 2d, and 2e). Obviously, a low content of VOPO4 leads to a less loading of amorphous VOPO4 layers on graphene. The energy-dispersive X-ray (EDX) analysis was adopted to investigate the elemental composition of amorphous VOPO4/graphene composites. As shown in Fig. S2, the well-defined signals indexed to C, O, V and P elements are clearly visible. The peak intensity ratio of V to O is remarkably decreased as the content of VOPO4 is decreased correspondingly in composites. As an example, the element mappings of VOPO4/graphene (2:1) are also displayed in Fig. S3. Apparently, C, O, V and P elements are evenly distributed in the composite. These results suggest that amorphous VOPO4 layers are successfully hybridized with graphene nanosheets at a nanoscale level. Such a kind of nanohybrid structure will have a favorable effect on the electrochemical properties of the materials [25]. To further reveal the microstructure of the as-prepared samples,

transmission electron microscopy (TEM) was applied. As shown in Fig. 3, VOPO4$2H2O presents distinct lattice fringes and the lattice spacings of 0.31 and 0.74 nm could be ascribed to (200) and (001) crystal planes, respectively [39]. By comparison, the VOPO4/graphene (2:1) composite shows no crystallographic characteristic, revealing an amorphous nature of the composite. This result is consistent with the above-mentioned XRD results. Furthermore, it can be observed that the irregular VOPO4 nanosheets are uniformly distributed on graphene, indicating a rather integrated nanohybrid structure of amorphous VOPO4/graphene composite. To investigate the functional groups of the materials, Fouriertransformed infrared spectroscopy (FT-IR) was utilized. As shown in Fig. 4a, for crystalline VOPO4$2H2O, the band at 688 cm
1 can be attributed to V-O-P bending vibration while those at 920, 977, 1154 and 1623 cm
1 are ascribed to P-O, V-O, V¼O and H-O-H stretching vibrations, respectively [35,40]. By comparison, for amorphous VOPO4$2H2O (Fig. 4b), due to the defects and disorders within the amorphous phase, the V¼O stretching vibration shows a blue shift from 1154 to 1097 cm
1. This blue shift also indicates the significant decrease of hydrogen bonds in amorphous VOPO4$2H2O due to the exfoliation of crystalline VOPO4$2H2O (also see Scheme 1). As shown in Fig. 4g, GO presents strong absorption peaks at 1737, 1623, 1386, 1223 and 1053 cm
1, which are corresponding to the C¼O stretching vibration from the carbonyl and carboxylic groups, the skeletal vibration of aromatic C¼C, carboxy C-OH groups, epoxy C-O group and alkoxy C-O groups, respectively [41]. By contrast, graphene shows dramatically decreased absorption

Fig. 2. SEM images of (a) crystalline VOPO4$2H2O, (b) amorphous VOPO4$2H2O, (c) VOPO4/graphene (4:1), (d) VOPO4/graphene (2:1), (e) VOPO4/graphene (1:2), and (f) graphene.

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Fig. 3. TEM images of crystalline VOPO4$2H2O (a and b) and VOPO4/graphene (2:1) composite (c and d).

Fig. 4. FTIR spectra of (a) crystalline VOPO4$2H2O, (b) amorphous VOPO4$2H2O, (c) VOPO4/graphene (4:1), (d) VOPO4/graphene (2:1), (e) VOPO4/graphene (1:2), (f) graphene, and (g) GO.

peaks of oxygen-containing functional groups (Fig. 4f). In addition, the skeletal vibration of aromatic C¼C domains is blue-shifted from 1623 to 1576 cm
1, demonstrating the formation of graphene due to partial reduction of GO. The characteristic signals of graphene in Raman spectra also confirmed the result (see Fig. S1). For three composites, the characteristic peaks of both amorphous VOPO4 and graphene can be observed, confirming the formation of amorphous

VOPO4/graphene composites after sonication and hydrothermal treatment process. X-ray photoelectron spectroscopy (XPS) was used to assess the binding energy and surface chemical states of the bonded elements. Typical spectra of the as-prepared samples are presented in Fig. 5 and Fig. S4, respectively. For VOPO4$2H2O, there are only three elements, i.e., V, O and P, respectively. However, four signals of C, O, V and P elements could be observed in the composites, further confirming the formation of VOPO4/graphene composites. For the C 1s spectra of three VOPO4/graphene composites (Fig. 5b and Fig. S4b), each spectrum can be deconvoluted into four peaks of OC¼O (288.5 eV), C¼O (287.0 eV), C-O (286.2 eV), and C-C/C¼C (284.7 eV), indicating the successful formation of graphene in the composites [42]. As listed in Table S1, the relative peak areas of C-C/ C¼C in three composites are 62.5%, 63.7% and 71.1%, respectively, being consistent with the content of graphene in composites. Meanwhile, the peak position is almost remained unchanged, indicating the similar carbon nanostructure of graphene in three composites. As shown in Fig. 5c and Fig. S4c, the deconvolution of O 1s peak of the amorphous VOPO4/graphene composites obviously shows three peaks corresponding to O-H (~532.9 eV), O-C¼O (~532.0 eV) and V-O (~531.0 eV), respectively. The O-C¼O and O-H peaks are from graphene while the V-O peak is ascribed to VOPO4 [32]. These results suggest that the oxygenated functional groups on graphene nanosheets are not completely removed, being consistent with the above-mentioned IR results. As the content of graphene is increased in the composite, the relative peak area of V-O bonds is decreased from 74.4% to 59.3% (see Table S2). Meanwhile, the binding energy of V-O also shows a shift from 530.5 eV in VOPO4/ graphene (4:1) to 530.8 eV in VOPO4/graphene (1:2), indicating a close contact and strong interaction between VOPO4 and graphene. As shown in Fig. 5d and Fig. S4d, two peaks are observed at 517.1 and 524.5 eV, which can be attributed to the levels of V 2p3/2 and V

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Fig. 5. XPS spectra of VOPO4/graphene (2:1) composite (a) a survey scan; (b) C 1s region; (c) O 1s region; (d) V 2p region.

2p1/2, respectively [33]. Through the peak deconvolution of the V 2p3/2 and V 2p1/2 core levels, the percentages of surface V4þ and V5þ oxidation states were obtained. As listed in Table S3, it can be seen that a small amount of V4þ and a large amount of V5þ species are co-existed in the samples. In addition, the main intense peak of V5þ 2p3/2 in three composites shows a shift from 517.3 to 517.0 eV with the increment of graphene, further confirming the interaction between VOPO4 and graphene. 3.2. Electrochemical performance of hybrid composites To evaluate the electrochemical performance of the as-prepared materials, the cyclic voltammetry (CV) curves were measured. Fig. 6a show CV curves at a scan rate of 5 mV s
1 for GO, graphene, VOPO4$2H2O, amorphous VOPO4$2H2O and VOPO4/graphene composites. As seen from inset of Fig. 6a, GO exhibits a single redox couple resulting from the reversible oxidation and reduction of oxygen functional moieties [43,44]. After reduction, the CV curve reveals a quasi-rectangle shape, indicating that graphene has a good charge propagation within the electrode [45]. A pair of redox peaks can be observed in all VOPO4-based samples, suggesting the main pseudocapacitance behavior based on the following faradic reaction [33]: VO3þþ Hþ þe
4 HVO3þ The pesuducapacitance process for electrochemical redox

involves the reduction and oxidation of surface metal ions. The oxidation peak is attributable to the oxidation of V4þ to V5þ while the reduction peak is for the reverse process [46]. By comparison, the CV loop area of amorphous VOPO4$2H2O is larger than that of crystalline VOPO4$2H2O as the material with a disordered structure would provide more active sites with a higher redoxing activity [18]. As we know, the number and activity of the redox centers are critical for the electrochemical performance [17]. Among all samples, VOPO4/graphene (2:1) shows the largest CV loop area and thus the highest specific capacitance. Based on CV curves, the specific capacitance values for GO, graphene, VOPO4$2H2O, amorphous VOPO4$2H2O, VOPO4/graphene (1:2), VOPO4/graphene (4:1) and VOPO4/graphene (2:1) composites are 29, 54, 133, 175, 294, 345 and 417 F g
1, respectively. For a better understanding of the electrochemical performance of the supercapacitor electrodes based on the prepared samples, the cyclic voltammetry curves at different scan rates were also recorded. As shown in Fig. 6b, an obvious deviation can be observed in the CV curve of crystalline VOPO4$2H2O when the scan rate is increased from 5 to 100 mV s
1, suggesting a poor electron conduction due to the large interfacial resistance of the electrode [28]. By contrast, amorphous VOPO4$2H2O reveals a relatively slight deformation (Fig. 6c), suggesting a relatively fast interfacial charge transfer which facilitates the diffusion and reaction of electrolyte ions. The CV curves for VOPO4/graphene (4:1) (Fig. S5a) also show a deformation with the increment of scan rates, indicating a leading role of pseudocapacitive behavior. When the content of VOPO4 is

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Fig. 6. Cyclic voltammogram curves of (a) GO, graphene, VOPO4$2H2O and VOPO4/graphene composites at a scan rate of 5 mV s
1, (b) VOPO4$2H2O, (c) amorphous VOPO4$2H2O, and (d) VOPO4/graphene (2:1) electrodes at different scan rates.

decreased, the CV curves for VOPO4/graphene (2:1) (Fig. 6d) show an almost symmetric shape and the deformation is still unobvious even at a high scan rate of 100 mV s
1, indicating a high electron conductivity and a good reversibility. It is well-known that a high electron conductivity and a good reversibility would be beneficial for its high-rate performance [47]. As the content of VOPO4 is further reduced, the redox peaks become rather weak in CV curves of VOPO4/graphene (1:2) (Fig. S5b). To understand why the electrochemical properties of the asprepared materials are different, we conducted the galvanostatic charge-discharge test. Fig. 7a shows the corresponding curves of VOPO4$2H2O and VOPO4/graphene composite electrodes at a current density of 0.5 A g
1. It can be seen that a slight curvature appears in all curves, further conforming the pseudocapacitance as indicated by the appearance of redox peaks in CV curves. By comparison, VOPO4/graphene composites show a higher discharge capability than VOPO4$2H2O samples. Among them, VOPO4/graphene (2:1) exhibits the highest discharge capability with the longest discharge time. The rate capability of the as-prepared materials was tested by galvanostatic charge-discharge at different current densities. As shown in Fig. 7b, the specific capacitances of amorphous VOPO4/ graphene (2:1) composite electrodes are 508, 483, 437, 392 and 359 F g
1 at current rates of 0.5, 1, 2, 5 and 10 A g
1, being much higher than those of VOPO4/graphene (4:1) (367, 333, 313, 283 and 258 F g
1) and VOPO4/graphene (1:2) (225, 208, 192, 171 and 150 F g
1). In addition, VOPO4/graphene (2:1) composite also shows a superior retention (71%) to other two composites. It reveals that an excess or a small amount of VOPO4 content is adverse to the electrochemical performance. To effectively evaluate the cycle stability of electrode materials, three VOPO4/graphene composite electrodes were further tested by

duplicating galvanostatic charge-discharge process over 5000 cycles at a current rate of 2 A g
1. As shown in Fig. 7c, VOPO4/graphene (4:1) and VOPO4/graphene (1:2) electrodes exhibit a final discharge capability of 206 and 162 F g
1, respectively. By comparison, VOPO4/graphene (2:1) composite electrode still remains a high specific capacitance above 350 F g
1 after 5000 cycles (about 80% of its original value). These results demonstrate that VOPO4/ graphene composites have a good cycling stability and a high specific capacitance. Moreover, a comparison of specific capacitance, rate capability and cycling stability of the present work with those reported VOPO4-based composites electrodes has been listed in supporting information (Table S4). To further study the electrochemical properties of VOPO4/graphene (2:1) composite, Fig. 7d presents the galvanostatic chargedischarge curves at different current densities. As the current density is increased, the specific capacitance decreases gradually. At high current densities, the migration of the electrolyte ions is limited and some functional surface areas become inaccessible for charge storage due to the diffusion effect. Note that the amorphous phase is favorable for charge transfers and thus an excellent capacitive behavior can be achieved at high current densities due to the high structure disorder [16]. As shown in inset of Fig. 7d, the slight internal resistance (IR) drop of 0.095 V (10 A g
1) implies a small intrinsic series resistance of amorphous VOPO4/graphene (2:1) composite. This value is much lower than that of VOPO4/ graphene (4:1) (0.134 V, 10 A g
1) and VOPO4/graphene (1:2) (0.125 V, 10 A g
1) (inset of Fig. S6), indicating the lowest internal resistance and thus the fastest transport of electrons of VOPO4/ graphene (2:1) among three composites. The electrochemical impedance spectroscopy (EIS) was used to further understand the transport characteristics in graphene, VOPO4$2H2O, and VOPO4/graphene electrodes. At high frequency,

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Fig. 7. Electrochemical characteristics: (a) Charge-discharge profiles of VOPO4$2H2O and VOPO4/graphene electrodes at a current density of 0.5 A g
1 within a potential range of
0.1e0.9 V, (b) Rate performance of VOPO4/graphene composites, (c) Cycling performance of VOPO4/graphene composites, (d) Charge-discharge profiles of VOPO4/graphene (2:1) electrode at different current densities (inset is the enlarged view of the curves in the relative high voltage), (e) Nyquist plots of graphene, VOPO4$2H2O and VOPO4/graphene composites electrodes, (f) Ragone chart of VOPO4/graphene electrodes and other previously reported data.

the intersection point on the real axis represents the equivalent series resistance (ESR, Rs) of the sample. The higher ESR value, the lower the electrical conductivity of the sample, and vice versa. As shown in Fig. 7e, the corresponding ESR values for graphene, VOPO4$2H2O, amorphous VOPO4$2H2O, VOPO4/graphene (4:1), VOPO4/graphene (2:1) and VOPO4/graphene (1:2) electrodes are 0.96, 14.88, 14.01, 12.81, 1.85 and 5.28 U, respectively. Due to the long-range disorder and short-range order, the amorphous VOPO4$2H2O has a relatively improved electronic conductivity compared with its crystalline form [48]. On the other hand, the introduction of graphene greatly enhances the conductivity of the resulted amorphous VOPO4/graphene composites. By comparison, VOPO4/graphene (2:1) has the lowest ESR value, which is attributed to the close contact and interaction between VOPO4 layers and graphene nanosheets. The diameter of the semicircle on the real axis is linked to the charge-transfer resistance (Rct). Values of Rct for graphene, VOPO4/graphene (4:1), VOPO4/graphene (2:1), and VOPO4/graphene (1:2) are 14.68, 72.97, 43.24, and 59.45 U, respectively. Overall, VOPO4/graphene (2:1) composite shows the lowest Rct and Rs values, indicating a much improved charge transport than other samples. The maximized plane-plane contact between VOPO4 layer and graphene nanosheets leads to a fluent electron transfer from VOPO4 to the highly conductive graphene. Fig. 7f displays the energy and power densities of VOPO4/graphene electrodes on a Ragone diagram along with some previously reported data. It can be seen that amorphous VOPO4/graphene (2:1) composite exhibits a maximum energy density of 70.6 Wh$kg
1 with a power density of 250 W kg
1. Besides, its maximum power density can approach to 5.0 kW kg
1 while maintaining a high energy density of 48.6 Wh$kg
1. These values are superior to other previously reported data [28,32,33]. The electrochemical performance of the assembled symmetrical device based on VOPO4/graphene (2:1) composite electrodes has been studied (see Fig. S7). It can be observed that the CV curves exhibit nearly rectangular shapes and the galvanostatic

chargeedischarge curves present approximately triangular shapes, indicating the superior capacitive behavior. Meantime, the device also shows an excellent cycling performance (94%). 3.3. Charge-discharge mechanism of the hybrid nanostructure Based on the above results, the excellent electrochemical performance of amorphous VOPO4/graphene (2:1) composite can be attributed to the following advantages. First, the graphene in situ formed as the substrate leads to the exposure of more structure units of VOPO4, being beneficial for the adsorption-desorption process of charges and the faradic reactions. Meanwhile, the graphene substrate also provides active centers of EDLC, resulting in a high specific capacitance. Second, the small particle size of amorphous VOPO4 gives a high disorder of structure and many unsaturated atoms resulting from random fluctuations in atomic positions. Hence, more active reaction sites can be provided meanwhile the ion diffusion and charge transfer can be appreciably improved and the permeation of Hþ is more effective [38]. Besides, the nanostructured amorphous VOPO4 nanosheets is conducive to a maximized contact with electrolyte, improving the capacitance significantly. Third, due to the structure similarity of VOPO4 layers and graphene nanosheets, the two components are hybridized at a nanoscale level, resulting in a good structure integrity and a high electronic conductivity and thus a relatively good cycle stability and a high rate capability [26]. A schematic representation for the charge and discharge mechanism is shown in Scheme S1. 4. Conclusions In summary, a facile, cost-effective and potentially scalable technique was proposed for the fabrication of amorphous VOPO4/ graphene composites. On the one hand, the amorphous character of VOPO4 can provide more redox sites and also easier access pathways for the electronic and ionic transports. On the other hand, the

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graphene sheets as a substrate not only lead to the exposure of more structure units of VOPO4 and the improved conductivity, but also provide EDLC and stabilize the hybrid structure to resist the strain release during the cycling process. Finally, the similar structure between amorphous VOPO4 layers and graphene nanosheets contributes to a layer-on-sheet nanohybrid structure. Owing to their unique hybrid nanostructure and amorphous nature, VOPO4/graphene composites exhibit a high capacitance, an excellent rate capability and a good cycling stability, giving rise to a high energy density and power density simultaneously. The present provides a new idea for the design of novel high-performance supercapacitor electrode materials. Acknowledgments The authors greatly appreciate the financial support of Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20130091110010), Natural Science Foundation of Jiangsu Province (BK2011438), National Science Fund for Talent Training in Basic Science (No. J1103310), National Basic Research Program (973 Project) (No. 2009CB623504) and the Modern Analysis Center of Nanjing University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.119. References [1] J.R. Miller, P. Simon, Materials science. Electrochemical capacitors for energy management, Science 321 (2008) 651e652. [2] M. Winter, R.J. Brodd, What are batteries, fuel cells, and supercapacitors? Chem. Rev. 104 (2004) 4245e4270. [3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater 7 (2008) 845e854. [4] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 38 (2009) 2520e2531. [5] W. Wei, X. Cui, W. Chen, D.G. Ivey, Manganese oxide-based materials as electrochemical supercapacitor electrodes, Chem. Soc. Rev. 40 (2011) 1697e1721. , J. Gaubicher, T. Le Mercier, G. Wallez, J. Angenault, M. Quarton, [6] N. Dupre Positive electrode materials for lithium batteries based on VOPO4, Solid State Ionics 140 (2001) 209e221. [7] B.M. Azmi, T. Ishihara, H. Nishiguchi, Y. Takita, Vanadyl phosphates of VOPO4 as a cathode of Li-ion rechargeable batteries, J. Power Sources 119e121 (2003) 273e277. [8] B.M. Azmi, T. Ishihara, H. Nishiguchi, Y. Takita, Cathodic performance of VOPO4 with various crystal phases for Li ion rechargeable battery, Electrochim. Acta 48 (2002) 165e170. [9] Y. Sun, C. Wu, Y. Xie, Sonochemical synthesis of nanostructured VOPO4$2H2O/ carbon nanotube composites with improved lithium ion battery performance, J. Nanopart. Res. 12 (2010) 417e427. [10] N.G. Park, K.M. Kim, S.H. Chang, Sonochemical synthesis of the high energy density cathode material VOPO4$2H2O, Electrochem. Commun. 3 (2001) 553e556. , J. Svoboda, V. Zima, Intercalation chemistry of layered [11] L. Benes, K. Mel anova vanadyl phosphate: a review, J. Incl. Phenom. Macrocycl. Chem. 73 (2012) 33e53. [12] C.C. Hu, C.Y. Hung, K.H. Chang, Y.L. Yang, A hierarchical nanostructure consisting of amorphous MnO2, Mn3O4 nanocrystallites, and single-crystalline MnOOH nanowires for supercapacitors, J. Power Sources 196 (2011) 847e850. [13] B. Liu, X. Zhao, Y. Xiao, M. Cao, High-surface-area F-doped amorphous MoOx with high-performance lithium storage properties, J. Mater. Chem. A 2 (2014) 3338e3343. [14] Y.B. He, G.R. Li, Z.L. Wang, C.Y. Su, Y.X. Tong, Single-crystal ZnO nanorod/ amorphous and nanoporous metal oxide shell composites: controllable electrochemical synthesis and enhanced supercapacitor performances, Energy Environ. Sci. 4 (2011) 1288e1292. [15] L. Niu, Z. Li, Y. Xu, J. Sun, W. Hong, X. Liu, et al., Simple synthesis of amorphous NiWO4 nanostructure and its application as a novel cathode material for asymmetric supercapacitors, ACS Appl. Mater. Interfaces 5 (2013) 8044e8052. [16] H.B. Li, M.H. Yu, F.X. Wang, P. Liu, Y. Liang, J. Xiao, et al., Amorphous nickel hydroxide nanospheres with ultrahigh capacitance and energy density as electrochemical pseudocapacitor materials, Nat. Commun. 4 (2013) 1894.

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