Vanadium based materials as electrode materials for high performance supercapacitors

Vanadium based materials as electrode materials for high performance supercapacitors

Journal of Power Sources 329 (2016) 148e169 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/lo...

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Journal of Power Sources 329 (2016) 148e169

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Review article

Vanadium based materials as electrode materials for high performance supercapacitors Yan Yan a, Bing Li a, Wei Guo b, Huan Pang a, *, Huaiguo Xue a, ** a b

College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, Jiangsu, PR China College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang, 455002, Henan, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Vanadium based materials for high performance supercapacitor were reviewed.  The advantages and disadvantages were discussed in details.  Perspectives as to the future directions of vanadium based materials were provided.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 April 2016 Received in revised form 4 August 2016 Accepted 8 August 2016

As a kind of supercapacitors, pseudocapacitors have attracted wide attention in recent years. The capacitance of the electrochemical capacitors based on pseudocapacitance arises mainly from redox reactions between electrolytes and active materials. These materials usually have several oxidation states for oxidation and reduction. Many research teams have focused on the development of an alternative material for electrochemical capacitors. Many transition metal oxides have been shown to be suitable as electrode materials of electrochemical capacitors. Among them, vanadium based materials are being developed for this purpose. Vanadium based materials are known as one of the best active materials for high power/energy density electrochemical capacitors due to its outstanding specific capacitance and long cycle life, high conductivity and good electrochemical reversibility. There are different kinds of synthetic methods such as sol-gel hydrothermal/solvothermal method, template method, electrospinning method, atomic layer deposition, and electrodeposition method that have been successfully applied to prepare vanadium based electrode materials. In our review, we give an overall summary and evaluation of the recent progress in the research of vanadium based materials for electrochemical capacitors that include synthesis methods, the electrochemical performances of the electrode materials and the devices. © 2016 Elsevier B.V. All rights reserved.

Keywords: Vanadium based materials High performance Supercapacitors

1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected], (H. Pang), [email protected] (H. Xue). http://dx.doi.org/10.1016/j.jpowsour.2016.08.039 0378-7753/© 2016 Elsevier B.V. All rights reserved.

[email protected]

According to the energy crisis and greenhouse effect in the past few decades, many countries have not only been developing the strategies and rules but also have been developing a lot of high technologies to deal with the global warming effect [1e3]. Many

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energy storage devices, such as the solar energy, wind power, fuel cell, and biofuel have been taking an important role in the high tech industries recently. The most important goal for the energy storage device is to make them thin, small, and easy to carry for everyday use. The supercapacitors (SCs), also called ultracapacitors and electrochemical capacitors (ECs), compared with other storage devices, such as dielectric capacitor, secondary cell, fuel cells, lithium-ion batteries, have higher power density and broader range of working temperature [4,5]. Supercapacitors have not only a large power density of thousands of watts per kilogram, a long cycle life but also an enhanced efficiency of energy utilization. If a supercapacitor integrates with secondary batteries in electronic devices, it will extend the lease and reduce the volume of devices. Take the mobile device for example, the mobile devices will be served by secondary batteries for common use [6,7]. However, when the devices need high-transient power which is the special characteristic of supercapacitors they will be provided by supercapacitors. Thus, if the supercapacitor links up with the secondary batteries in the mobile devices, the devices will not only lessen the volume of device but also prolong the batteries' life, especially in stabilizing the batteries' voltage [8,9]. Supercapacitors can store more energy, because the mechanism of double-layer gets larger interfacial area and the reaction between two electrodes involves the ions' transfer in the atomic range [10,11]. Dielectric capacitors get high power density but low energy density. However, supercapacitor can store higher energy density than traditional capacitor. The fuel cells have high energy density but low power density. The supercapacitor is in the middle of dielectric capacitor and fuel cell. Because of the ostensible characteristics of materials, the energy density is higher than the ceramic capacitors and electrolytic capacitors thousands to tens of thousands of times. The advantages of supercapacitors include [12e17], (1) The long life cycle (more than 50000 times of charge and discharge); (2) The wide application of temperature range (40  Ce60  C); (3) The wide range of voltage: The terminal voltage is 2.5 V for the only one supercapacitor; (4) Possessed of coulombic efficiency: the lost charges in charging and discharging process are close to zero in the supercapacitor; (5) The equivalent series resistance (ESR) is very tiny in the supercapacitor. Since the input and output current is very high, the supercapacitor can be charged and discharged very fast. Selection of electrode materials plays a crucial role in determining the electrical properties of a supercapacitor. Most of the metal compound electrodes are transition metals, because they have changeable valence, which can provide ideal pseudocapacitance. The metal compounds must get three characteristics for applying on supercapacitor [18e20]. First, it is large conductivity. Second, there are more than two valences and the crystalline phase won't be changed with valence change. Third, the proton can intercalate into the lattice of metal compounds, especially for transition metal compounds [21,22]. The cyclic voltammogram of noble metal oxides (MOs) such as RuO2 and IrO2 electrodes have an almost rectangular shape and exhibit excellent capacitor behavior [23,24]. A very high specific capacitance of up to 750 F g1 was reported for RuO2 prepared at relatively low temperatures. Conducting metal oxides like RuO2 or IrO2 were ideal electrode materials in early ECs used for space or military applications [25]. The high specific capacitance in combination with low resistance resulted in very high specific powers. These capacitors, however, turned out to be too expensive. A rough calculation of the capacitor cost showed that 90% of the cost resides in the electrode material. In addition, these capacitor materials are only suitable for aqueous electrolytes, thus limiting the nominal cell voltage to 1 V. Several attempts were undertaken to keep the advantage of the material properties of such metal oxides at

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reduced cost. The dilution of the costly noble metal by forming perovskites was investigated by Lou et al. [26]. Other forms of metal oxides such as nickel oxides, cobalt oxides, manganese oxides, vanadium oxides and iron oxides are actively studied.16. In the periodic table of the elements, vanadium (V) is element twenty-third, and is located in the position of VB, which belongs to 3d Sub family of transition metal elements, the structure of the valence electron layer is 3d34s2, the outermost layer has five valence electrons, and The five valence electrons can participate in bonding, therefore, valence of vanadium rich, is a multivalent metal element, such as þ5, þ4, þ3, þ2, which can provide excellent pseudocapacitance. Among the many compounds formed by vanadium, the valence of þ5 is the most stable. The stability of þ4 and þ 2 are the worst, so the mostly studied vanadium compounds are VOx, VN and vanadium bronze [27e29]. With flexible doping modification methods, the materials type and preparation methods are diverse. The material structure, conductivity, electrolyte, and material loading mass on the electrode have a crucial influence on the pseudocapacitance of vanadium-based materials. We could control the materials construction, form composites, and even develop new vanadium based materials to improve the supercapacitance performance. So as to promote future breakthroughs in this field, our review is dedicated to this important material group, providing a timely effort to comprehensively and critically evaluate the development. 2. Vanadium pentoxide Vanadium oxide as a transition metal oxide (þ2 to þ5), has been widely applied to the electrode material of lithium batteries and sodium ion batteries [30e36]. It can not only produce the oxidation reduction reaction on the surface, but also can occur in the interior. However, because of the poor electrical conductivity and cyclic stability of vanadium oxide, the electrode material of electrochemical capacitor is limited. It is a main way to improve the electrical conductivity and cyclic stability of the electrode materials for electrochemical capacitors. At present, there are more studies on the vanadium oxide with V2O5, V2O3, VO2 and so on [37]. 2.1. Vanadium pentoxide As one of the most representative vanadium oxide, vanadium pentoxide (V2O5) plays an important role in the field of electrochemical energy storage. David Lou's group have carried out extensive and in-depth research of V2O5, owing to their unique structure-determined physical and chemical properties, which can provide for facile Li ions insertion and good cycling stability for lithium-ion batteries and supercapacitors [38e42]. Lee et al. firstly used vanadium pentoxide as an electrode material that can be applied for the supercapacitors. They used meltquenching method to prepare amorphous-V2O5$nH2O that can be an excellent electrode for a faradaic electrochemical capacitor [43]. Cyclic voltammograms versus SCE gave ideal capacitor behavior between 0.0 and 10.8 V at pH 6.67 and between 20.2 and 10.8 V at pH 2.32 with, respectively, a constant specific capacitance over 100 cycles of ca. 350 and 290 F g1, respectively. On short-circuit, aV2O5$nH2O in 2 M KCl aqueous solution at pH 2.32 gave an initial current density of 0.28 A cm2 and a total released charge of 4.5 C cm2, which was to be compared with 0.32 A cm2 and 11.1 C cm2 for RuOOH$nH2O in 5.3 M H2SO4. Moreover, half the stored charge was released 1.6 times faster from the a-V2O5$nH2O electrode. Chemically pure vanadium pentoxide (V2O5) powders can be a good electrode material for supercapacitors [44]. Lao et al. found that the materials which was prepared by co-precipitation and

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further calcination at 300  C were agglomerated in sub-micron particles, and BET analysis showed that the as-prepared V2O5 powders had a high specific surface-area of 41 m2 g1. The V2O5 powders showed a maximum specific capacitance of 262 F g1. That same year, Reddy et al. reported that nanoporous V2O5 which showed good electrochemistry performance can be prepared by sol-gel method [45]. The preparation involved elutriation of aqueous sodium meta vanadate over a cation exchange resin. A maximum capacitance of 214 F g1 was obtained at a scan rate of 5 mV s1 in 2 M KCl. The effect of different electrolytes and the effect of concentration of KCl on the specific capacitance of V2O5 were studied. Specific capacitance faded rapidly over 100 cycles in 2 M KCl at a 5 mV s1 scan rate. As an upgraded version of nanoporous vanadium oxide, Saravanakumar et al. prepared V2O5 nanoporous network (VNN) through simple capping-agent-assisted precipitation technique and it is further annealed at different temperatures [46]. A representative scanning electron microscopy (SEM) image of as synthesized V2O5 powder showed an intimately interconnected network-like structure. The SEM image confirmed that the surface texture of the material had porous structure. They achieved highest specific capacitance of 316 F g1 for interconnected V2O5 nanoporous network. This interconnected nanoporous network created facile nanochannels for ion diffusion and facilitates the easy accessibility of ions. Moreover, after six hundred consecutive cycles the specific capacitance decayed only 24%. In recent years, electrodeposition has been used in the preparation of energy storage nanomaterial. Wei et al. reported the first successful application of an ordered bicontinuous double-gyroid vanadium pentoxide network in an electrochromic supercapacitor [47]. The freestanding vanadia network was fabricated by electrodeposition into avoided block copolymer template that had selfassembled into the double-gyroid morphology. The highly ordered structure with 11.0 nm wide struts and a high specific surface to bulk volume ratio of 161.4 mm1 was ideal for fast and efficient lithium ion intercalation/extraction and faradaic surface reactions,

which are essential for electrochemical energy storage devices with high energy and high power density. Two-dimensional materials have become an important research direction for constructing flexible ultrathin-film supercapacitors, by virtue of their flexibility, ultra-thinness and even transparency. Zhu et al. developed an efficient approach for large-scale production of V2O5 nanosheets (as shown in Fig. 1c), which have a thickness of 4 nm and was utilized as building blocks for constructing 3D architectures via a freezedrying process [48]. The resulting highly flexible V2O5 structures possessed a surface area of 133 m2 g1, ultrathin walls, and multilevel pores. Such unique features are favorable for providing easy access of the electrolyte to the structure when they were used as a supercapacitor electrode, and they also provide a large electroactive surface that advantageous in energy storage applications. As a consequence, a high specific capacitance of 451 F g1 was achieved in a neutral aqueous Na2SO4 electrolyte and the 3D architectures were utilized for energy storage. Remarkably, the capacitance retention after 4000 cycles was more than 90%, and the energy density was up to 107 Wh kg1 at a high power density of 9.4 kW kg1 (as shown in Fig. 1d). Electrospinning has been widely used in the preparation of nanomaterials in recent years. Wee et al. synthesized V2O5 nanofibers (VNF) through a simple electrospinning method, and their application as supercapacitor electrodes are demonstrated [49]. The highest specific capacitance was achieved for VNF annealed at 400  C, which yielded 190 F g1 in aqueous electrolyte (2 M KCl) and 250 F g1 in organic electrolyte (1 M LiClO4 in PC) with promising energy density of 5 Wh kg1 and 78 Wh kg1 respectively. On the basis of Wee's idea, Lala et al. prepared tubular nanofibers (TNFs) of V2O5 via electrospinning technique using a single spinneret for the first time by controlling the properties of the precursor solution [50]. A partially miscible polymeric solution of vanadium oxytrihydroxide [VO(OH)3] was produced by hydrolysis of vanadyl acetylacetonate in Poly (vinylpyrrolidone) (PVP). The phase-separated polymer solution formed the core of the

Fig. 1. SEM images of the V2O5 (a) nanoflowers; (b) nanoballs; (c) nanowires, and (d) nanorods. Reprinted with permission from Ref. [57]. Copyright 2015, Elsevier.

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electrospun fibers whereas the VO(OH)3 formed the shell; the core PVP had been removed by controlled heat treatment. The TNFs had an inner diameter 60 nm and wall thickness ±100 nm. The development of asymmetric supercapacitor is also a trend in recent research. Qu et al. prepared V2O5$0.6H2O nanoribbons and investigated their electrochemical behaviors in K2SO4 aqueous solution [51]. Results showed for the first time that Kþ ions could intercalate/deintercalate reversibly in the V2O5$0.6H2O lattice. An asymmetric supercapacitor with the structure of activated carbon/ 0.5 M K2SO4/V2O5$0.6H2O was successfully assembled, which could be cycled reversibly in the voltage region of 0e1.8 V. This supercapacitor presented an energy density of 29.0 Wh kg1 based on the total mass of the active electrode materials, a very good rate behavior with energy density of 20.3 Wh kg1 at power density of 2 kW kg1. Recently, Lin et al. developed a high-performance asymmetric supercapacitor by using porous vanadium pentoxide (V2O5) nanotubes as positive electrode and activated carbon nanorods as negative electrode in an aqueous 2 M LiNO3 electrolyte [52]. To maximize the energy density of the asymmetric supercapacitor, the initial potentials of work electrodes were tuned to different values (0 V, 0.1 V, 0.2 V, and 0.3 V vs. SCE), and the influence of the electrode potential on the electrochemical properties of the obtained asymmetric supercapacitor has been investigated in depth. The results showed that 0.2 V is the optimal initial electrode potential. At this initial electrode potential, the built V2O5//C asymmetric supercapacitor could be cycled reversibly in the voltage region of 0e1.8 V, and exhibited high energy and power density (46.35 Wh kg1 at 1.8 kW kg1 and 18 kW kg1 at 28.25 Wh kg1). Furthermore, the supercapacitor showed excellent cycling stability, with an almost 100% specific capacitance retention after 10000 cycles. Nanowires are considered to have good electrochemical properties. Wang et al. successfully prepared ultrahigh-aspect-ratio V2O5 nanowires, which used [VO(O2)2(OH)2] as the starting material by a template-free hydrothermal route without the addition of organic surfactant or inorganic ions [53]. The results revealed that the peroxovanadium (V) complexes can be easily transformed to V2O5 nanowires by this hydrothermal route. The uniform nanowires had width about 50 nm and length about dozens of micron. The BET analysis showed the V2O5 nanowires had a high specific surface area of 25.6 m2 g1. The synthesized V2O5 nanowires performed a high capacitance of 351 F g1 when used as supercapacitor electrode in 1 M LiNO3. Since spin coating sol-gel is a good method for the preparation of thin film materials, Jeyalakshmi et al. prepared vanadium pentoxide thin films via this method [54]. The films coated on Fluorine doped Tin Oxide (FTO) and glass substrates were treated at different temperatures ranging from 250  C to 400  C. The vanadium pentoxide films annealed at 300  C for an hour exhibited a maximum specific capacitance of 346 F g1 at a scan rate of 5 mV s1. Surface active agents often play an important role in the morphology control of nanomaterials. Nair et al. synthesized vanadium pentoxide (V2O5) nanoparticles via an anionic, cationic and non-ionic surfactant assisted hydrothermal method in which Ammonium metavanadate (NH4VO3) was used as precursor [55]. In order to further study the effect of different surfactants on the synthesis of V2O5, Qian et al. successfully used three kinds of surfactants, including polyethyleneglycol 6000 (PEG-6000), sodium dodecylbenzene sulfonate (SDBS), and Pluronic P-123 (P123), to prepare nanolayered V2O5$H2O through a simple hydrothermal process, and this resulted in different morphologies, including flower-like flakes, linear nanowires, and 3D networks connected with curly bundled nanowires [56]. The electrochemical performance of these powders showed that the nanowires, which are

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electrodes mediated by PEG-6000, exhibited the highest capacitance of 349 F g1 at a scan rate of 5 mV s1 of all the surfactants studied. However, a symmetric P123 electrode comprising curly bundled nanowires with numerous nanopores showed an excellent and stable specific capacitance of 127 F g1 after 200 cycles. From the above discussion, it is evident that V2O5 nanostructures have a variety of forms, Mu et al. synthesized four types of typical morphologies of differently dimensional pure V2O5 nanostructures including nanoflowers, nanoballs, nanowires and nanorods (shown in Fig. 1) through a simple hydrothermal method and compared their electrochemical properties [57]. The morphology of the product depends on the types of solvent and acid. Hierarchical nanoflowers and zero-dimensional (0D) nanoballs of V2O5 nanocrystals were obtained when using C2H5OH as solvent. One dimensional (1D) nanowires and nanorods were obtained if H2O was used to participate in the reaction. The electrochemical test results indicated that the rod-like structure leads to a significant improvement of storage capacity, electrochemical kinetics and rate capability. And 1D V2O5 nanorods showed the largest specific capacitance of 235 F g1 at the current density of 1 A g1 when used as supercapacitor electrode in 1 M Na2SO4 electrolyte. 2.2. Vanadium pentoxide/compound-carbon material composites Carbon materials (including activated carbon, carbon nanotubes and graphene) are the most frequently used conductive substrates because of their good electronic conductivities, high specific surface areas, and great chemical stabilities. Nevertheless, it is very difficult to directly grow metal oxides/hydroxides on carbon materials, because their surfaces are not compatible. To improve their surface compatibility, oxidative treatments of carbon materials are always necessary, which could introduce oxygen-containing groups facilitating the growth of metal oxides/hydroxides and numerous structural defects on the surfaces of carbon materials. In order to improve the electrical conductivity and cyclic stability of V2O5, the researchers tried to introduce into the material systems the high conductive carbon materials such as activated carbon or carbon fiber, carbon nanotubes, graphene in order to prepare the V2O5 based composites [58e61]. 2.2.1. Vanadium pentoxide/activated carbon or carbon fiber material composites The activated carbon has been widely used, because of the advantages of simple preparation, low cost, large specific surface area and good electrical conductivity. Activated carbon, with extremely high surface areas, controllable pore size, narrow pore size distribution, ordered pore structures and interconnected pore channels, have been studied for applications in electrochemical energy storage. Beside directly being used as electrode materials for EDLCs, activated carbon can also be used as 3D supports for pseudocapacitive materials. Kudo et al. first developed V2O5/carbon composite for supercapacitors [58]. They prepared V2O5 sol by a reaction of metallic vanadium with a hydrogen peroxide solution. Then, they added acetylene black powder into the sol with acetone to yield a homogeneous suspension. The sample electrode which was constituted of a composite of amorphous V2O5 and carbon, was loaded on a macroporous nickel current collector, and heat-treated at 120  C to obtain a sample electrode. The experimental results verified that the composite electrode with the V2O5/carbon ratio of 0.7 in weight showed 54% of the ideal capacity, 360 mAh g1 (4.2e2.0 V) based on V2O5, even at a very high rate discharge at 54 A g1 V2O5. In order to further study the effect of sol condition on the electrochemical performance of V2O5/carbon composite cathode,

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Watanabe et al. used vanadium oxide sol with acetone to develop V2O5/carbon composite cathodes, which can acquire high rate charge/discharge capacity [59]. As flexible supercapacitor has a broad application prospect, 3D nanoarchitectures on flexible current collectors has emerged as an effective strategy for preparing advanced portable and wearable power sources [60]. Li et al. developed a flexible and efficient electrode based on electrospun carbon fibers substrate (ECF) with elaborately designed hierarchical porous V2O5 nanosheets (V2O5ECF) via a simple solvothermal method. The unique configuration of V2O5-ECF composite film fully enables utilization of the synergistic effects from both high electrochemical performance of V2O5 and excellent conductivity of ECF, endowing the films to be an excellent electrode for flexible and lightweight electrochemical capacitors (ECs). Benefiting from their intriguing structural features, V2O5-ECF and ECF films, directly used as electrodes for flexible asymmetric quasi-solid-state electrochemical capacitors, achieve superior flexibility and reliability, enhanced energy/power density, and outstanding cycling stability. Spray pyrolysis is a valid method to synthesize large quantities of high-purity oxide powders that have homogeneous nanosized crystals. Via the spray pyrolysis technique, Wang et al. synthesized nanostructured vanadium pentoxide/carbon (V2O5/carbon) composite powders with enhanced specific capacitance [61]. Electrochemical properties were examined by the cyclic voltammetry technique. Following analysis of powders sprayed at different temperatures, composite powders obtained at an optimum temperature of 450  C yielded a maximum specific capacitance of 295 F g1 in 2 M KCl electrolyte at a 5 mV s1 scan rate. Electrodeposition is one of the most active fields in the preparation of nanomaterials. Electrodeposition of an ultrathin metaloxide layer on carbon-nanofiber can provide a high surface area and improved conductivity of the electrode. Ghosh et al. developed an ultrathin V2O5 layer that was electrodeposited by cyclic voltammetry on a self-standing carbon-nanofiber paper, which was obtained by stabilization and heat-treatment of an electrospun polyacrylonitrile (PAN)-based nanofiber paper [62]. A very-high capacitance of 1308 F g1 was obtained in a 2 M KCl electrolyte when the contribution from the 3 nm thick vanadium oxide was considered alone, contributing to over 90% of the total capacitance (214 F g1) despite the low weight percentage of the V2O5 (15 wt%). By a simple electrospinning method, Kim et al. prepared the vanadium pentoxide (V2O5)/carbon nanofiber composites (CNFCs) from polyacrylonitrile/V2O5 in N,N-dimethylformamide, and investigated their electrochemical properties as supercapacitor electrodes [63]. Different loadings of V2O5, the microstructures of the CNFCs (e.g., nanometer-size diameters, high specific surface areas, narrow pore size distributions, and tunable porosities) were changed, and the textural parameters significantly affected the electrochemical properties of the composites. The CNFC capacitors delivered the high specific capacitances of 150.0 F g1 for the CNFCs in an aqueous electrolyte, with promising energy densities of 18.8 Wh kg1, over a power density range of 400e20000 W kg1. Their group also developed mesopore-enriched activated carbon nanofiber (ACNF) mats which were produced by incorporating vanadium(V) oxide (V2O5) into polyacrylonitrile (PAN) via electrospinning, and their electrochemical properties are investigated as an electrode in supercapacitors [64]. The microstructures of the ACNFs (e.g., nanometer-size diameter, high specific surface area, narrow pore size distribution, and tunable porosity) were changed, and the textural parameters are found to affect the electrochemical properties significantly through the different V2O5 loadings and activation process. The V2O5/PAN-based ACNF electrodes with wellbalanced micro/mesoporosity having an optimal pore range for effective double layer formation in an organic medium are

expected to be useful electrode materials for supercapacitor applications. Guo et al. found that the electrochemical performance and stability of V2O5 nanowires can be significantly improved by coating a thin carbon layer as shell [65]. Their study showed that the [email protected] core shell nanowires achieve a remarkable areal capacitance of 128.5 F cm2 at 10 mV s1 with excellent rate capability. More than 94.4% of the initial capacitance was retained after 10,000 cycles for [email protected] core shell nanowires, which was much higher than the pristine V2O5 nanowires (13.3%). Atomic layer deposition is a method that can be coated on the surface of the substrate by a single atomic film. Daubert et al. used atomic layer deposition (ALD) to grow V2O5 on the surface of activated carbon materials [66]. The V2O5 ALD process was characterized at various temperatures to confirm saturated ALD growth conditions. Capacitance and electrochemical impedance analysis of subsequently constructed electrochemical capacitors showed improved charge storage for the ALD coated electrodes, but the extent of improvement depended on initial pore structure. The ALD of V2O5 onto mesoporous carbon increased the capacitance by up to 46% after 75 ALD cycles and obtained a maximum pseudocapacitance of 540 F g1 (V2O5) after 25 ALD cycles, while maintaining low electrical resistance, high columbic efficiency, and a high cycle life. As a kind of carbon black, Ketjin black is widely used in the preparation of electrode material. Peng et al. prepared V2O5/Ketjin black (VK) nanocomposites with mesoporous mica-like structure via a facile sol-gel method. Through a dip-dry process, the VK nanocomposites were successfully assembled on nickel foams with controllable mass loadings [67]. The as-prepared electrode (VK2) shows high areal capacitance (3.9506 F cm2 at 5 mA cm2) and good cycling stability (90% after 8000 cycles) in a LiCl/PVA gel electrolyte. Furthermore, the VK nanocomposite-based all-solidstate symmetric supercapacitor can provide a maximum energy density of 56.83 Wh kg1. Carbon coating on metal oxide significantly modify the surface chemistry which provides protection layer to active sites and improve the electronic conductivity. Recently, Balasubramanian et al. synthesized carbon coated V2O5 with flowery architecture via co-precipitation method followed by thermal treatment [68]. The carbon coated flowery V2O5 exhibited maximum specific capacitance of 417 F g1 with 100% capacitance retaining even at 2000 continuous charge-discharge cycles. 2.2.2. Vanadium pentoxide/carbon nanotubes composites Carbon nanotubes (CNT) are the most representative nanostructured carbons with one dimensional tubular structures and exhibit outstanding physicochemical properties such as high electrical conductivity, high mechanical strength, high chemical stability, and high activated surface areas. The potential of CNT as electrodes in ECs has been exploited extensively in the last decade. The unique tubular structure of CNT is believed to be able to produce consistent composites and an interconnected conducting network with high porosity for enhanced electron transfer as well as electrolyte accessibility. V2O5 can be coupled with CNT by diverse routes, including electrodeposition, in situ precipitation/ thermolysis/thermal decomposition, solid state reaction and so on. Kim et al. first prepared V2O5$xH2O on the CNT film substrate. The V2O5$xH2O/CNT film electrode showed that V2O5$xH2O was heterogeneously nucleated and uniformly deposited on the CNT film substrate [69]. The V2O5$xH2O/CNT film electrode showed not only a high specific capacitance of 1230 F g1, but also a high rate capability. The maximum specific energy of 851 Wh kg1 and specific power of 125 kW kg1 were obtained from the discharging curves of the V2O5$xH2O/CNT film electrode.

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Hybrid nanocomposites containing CNT have attracted much attention when each constituent component provides different functions for specific applications. Fang developed Vanadium Oxide/Carbon Nanotube Composites [70]. They found that crosssectional scanning electron microscope images show that CNT provide good support for uniform distribution of vanadium pentoxides and in capacitive behaviors, CNT covered with uniformly dispersed oxides lead to a significantly improved capacitive performance, as compared with bare oxide films. Currently, hydrothermal process is the most classic and most commonly used method to prepare nanomaterials. Chen et al. synthesized nanocomposites of interpenetrating CNT and V2O5 nanowires networks via a simple in situ hydrothermal process [71,72]. These fibrous nanocomposites are hierarchically porous with high surface area and good electric conductivity, which made them excellent material candidates for supercapacitors with high energy density and power density. Nanocomposites with a capacitance up to 440 and 200 F g1 were achieved at current densities of 0.25 and 10 A g1, respectively. Asymmetric devices based on these nanocomposites and aqueous electrolyte exhibited an excellent charge/discharge capability. It showed high energy densities of 16 Wh kg1 at a power density of 75 W kg1 and 5.5 Wh kg1 at a high power density of 3750 W kg1. Flexible VNW-CNT nanocomposite papers can be used as electrode material for supercapacitor. Perera et al. develop a simple method for preparing freestanding carbon nanotube (CNT)-V2O5 nanowire (VNW) composite paper electrodes without using binders [73]. Coin cell type (CR2032) supercapacitors are assembled using the nanocomposite paper electrode as the anode and high surface area carbon fiber electrode as the cathode. The supercapacitor with CNT-VNW composite paper electrode exhibited a power density of 5.26 kW kg1 and an energy density of 46.3 Wh kg1. The VNWs and CNT composite paper electrodes showed improved overall performance with a power density of 8.32 kW kg1 and an energy density of 65.9 Wh kg1. Carbon nanotubes are one-dimensional nanomaterials with a huge aspect ratio. By controlled hydrolysis of vanadium alkoxide, Sathiya et al. developed a functionalized CNT that were coated with a 45 nm thin layer of V2O5 [74]. The synthetic processes are clearly demonstrated in Fig. 6. The resulting V2O5/CNT composite had been investigated for electrochemical activity, and the capacity value showed both faradaic and capacitive (nonfaradaic) contributions. At high rate (1 C), the capacitive behavior dominated the intercalation as 2/3 of the overall capacity value out of 2700 C g1 is capacitive, while the remaining is due to Li-ion intercalation. The growth of V2O5 on the surface of multi-walled carbon nanotube can improve the specific capacitance. Shakir et al. developed a simple, low-cost, safe and broadly applicable hierarchical bottom-up assembly route for the formation of ultrathin (3 nm) vanadium oxide (V2O5) film on conducting multi-walled carbon nanotube (MWCNT) [75]. The ultrathin V2O5 showed a very high capacitance of 510 F g1 and possess excellent cyclic stability with negligible decrease in specific capacitance after 5000 cycles. The supercapacitor device based on this hierarchical bottom-up assembly exhibited an excellent charge/discharge capability, and energy densities of 16 W h kg1 at a power density of 800 W kg1. By printing the active material onto an ITO glass current collector, Yilmaz prepared V2O5/CNT gel composites for solid-state supercapacitors [76]. V2O5/CNTs at a 0.5:1 wt ratio was found to achieve the highest capacitance among the various V2O5/CNT ratios investigated. The composite architecture was hierarchically porous, owing to interpenetration of the CNTs with the V2O5 nanosheet. The V2O5/CNT electrodes exhibited high energy density (1.47 mWh cm3) and power density (0.27 W cm3). The capacitance retention

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after 5000 cycles was more than 91% at a chargeedischarge current density of 5 A g1. Atomic layer deposition allows one to uniformly deposit metal oxides on porous CNT electrodes. Lee et al. used ALD technology to build nanostructured vanadium oxide coatings on the surface of MWCNT electrodes, thus offering a novel route for the formation of binder-free flexible composite electrode fabric for supercapacitor applications with large thickness, controlled porosity, greatly improved electrical conductivity and cycle stability [77]. Electrochemical measurements revealed stable performance of the selected MWCNT-vanadium oxide electrodes and remarkable capacitance of up to 1550 F g1 per active mass of the vanadium oxide and up to 600 F g1 per mass of the composite electrode, significantly exceeding specific capacitance of commercially used activated carbons (100e150 F g1). In search of an efficient and effective methodology to prepare 3D V2O5/CNT composite electrode materials for enhanced supercapacitors, Wu et al. synthesized V2O5/MWCNT core/shell hybrid aerogels with different MWCNT contents via a facile mixed growth and self-assembly methodology [78]. V2O5 coated MWCNT raised from the in situ growth of V2O5 on the surface of acid-treated MWCNT, incorporated with V2O5 nanofibers from the preferred orientation growth of V2O5 in a one-step sol-gel process, which is clearly demonstrated in Fig. 8(a). These two kinds of onedimensional fibers self-assemble into a three-dimensional monolithic porous hybrid aerogel. Owing to its high specific surface area, favorable electrical conductivity and unique three-dimensional core-shell structures, the light-weight hybrid aerogel (about 30 mg cm3) exhibited excellent specific capacitance (625 F g1), high energy density (86.8 Wh kg1) and outstanding cycle performance (>20,000 cycles, shown in Fig d). And the optimal content of MWCNT inhybrid aerogels for the highest-performance supercapacitor is 7.6%. Functionalized carbon nanotube has higher surface area, low resistivity and high stability. Using simplified solution based approach, Saravanakuma et al. developed V2O5/functionalized multiwall carbon nanotube (f-MWCNT) hybrid nanocomposite [79]. The addition of f-MWCNT with V2O5 significantly improved the surface area and conductivity, which lead to the high energy and power densities. This nanocomposite showed highest specific capacitance up to 410 F g1 and 280 F g1 at current densities of 0.5 and 10 A g1 respectively. Moreover, this nanocomposite provided excellent energy density (57 Wh kg1), better rate capacity, and a good retention of capacity (86%) up to 600 cycles of chargedischarge. Further a symmetric supercapacitor was fabricated using V2O5/f-MWCNT nanocomposite as electrodes. It showed a specific capacitance of 64 F g1 at a current density of 0.5 A g1. As mentioned above, electrodeposition and atomic layer deposition techniques have been used for the deposition of a thin layer of V2O5 on CNTs. However, the high cost of ALD and the corrosive nature of the electrolyte deposition have limited their practical applications. To overcome these defects, Shakir et al. developed a layer by layer assembly (LBL) technique in which a graphene layer was alternatively inserted between MWCNT films coated with ultrathin (3 nm) V2O5 [80]. The insertion of a conductive spacer of graphene between the MWCNT films coated with V2O5 not only prevents agglomeration between the MWCNT films but also substantially enhances the specific capacitance by 67%, to as high as 2590 F g1. Furthermore, the LBL assembled multilayer supercapacitor electrodes exhibited an excellent cycling performance of >97% capacitance retention over 5000 cycles and a high energy density of 96 Wh kg1 at a power density of 800 W kg1. By a supercritical fluid CO2 adsorption-calcination method, Do et al. fabricated electrochemical capacitor electrodes by depositing an ultra-thin layer of vanadium oxide on a high conducting, large

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specific surface area (SSA) materials (substrates) [81]. The high SSA materials included binder free single walled carbon nanotubeactivated carbon (SWCNT-AC) composites and the traditional electrodes of activated carbon-carbon black-polymer binder (ACCB-binder). The uptake of the organometallic precursor for the oxide (vanadium (III) acetylacetonate) on the substrates was investigated and related to their SSA. Precursor uptakes of up to 54.7 wt% of the initial carbon substrate was achieved. Calcination conditions for converting the precursor to oxide and electrochemical properties of the electrodes were thoroughly investigated. The V2O5 greatly enhanced the overall electrode performances, which showed extremely high specific pseudocapacitance (>1000 F g1 at 100 mV s1). 2.2.3. Vanadium pentoxide/graphene composites As a new kind of carbon material, graphene has become an attractive low cost alternative for CNTs with characteristics of high specific surface area, high electrical conductivity, good mechanical properties. Graphene, due to its high conductivity (26 000 S cm1) and high specific surface area (2630 m2 g1, theoretical value), provides good electron transfer paths and ensures the direct contact of V2O5 with graphene, i.e., reliable electrical contact. Graphene is strictly a two-dimensional single-atom-thick planar sheet of sp2 bonded carbon atoms. Due to the strong sp2 bonds between the carbon atoms, graphene exhibits high mechanical strength in the in-plane direction, which have the potential to built flexible supercapacitors. Chemically derived graphene, such as graphene oxide and reduced graphene oxide, offer advantage in terms of chemical processability to form various composites with V2O5. By solvothermal treatment and a subsequent annealing process, Li et al. first developed V2O5/reduced graphene oxide (rGO) nanocomposites as electrode materials for supercapacitors [82]. In this method, the reduction of graphene oxide can be achieved in a costeffective and environmentally friendly solvent, without the addition of any other toxic reducing agent. Importantly, this solvent can control the formation of the uniform rodlike V2O5 nanocrystals on the surface of rGO. Compared to pure V2O5 microspheres, the V2O5/ rGO nanocomposites exhibited a higher specific capacitance of 537 F g1 at a current density of 1 A g1 in neutral aqueous electrolytes, a higher energy density of 74.58 Wh kg1 at a power density of 500 W kg1, and better stability even after 1000 charge/ discharge cycles. Balkus's group also carried out the research in this field earlier [83,84]. They developed V2O5 nanowires (VNW)-graphene composite flexible paper electrodes which were prepared without using binders. The composite electrode showed balanced EDL and pseudocapacitance as well as an energy density of 38.8 Wh kg1 at a power density of 455 W kg1. The maximum power density of 3.0 kW kg1 was delivered at a constant current discharge rate of 5.5 A g1. The device prepared using rGO-VNW120 anode showed a specific capacitance of 80 F g1. On this basis, they developed an asymmetric supercapacitor with MnO2 nanorods (MNR) on rGO electrodes and V2O5 nanowire on rGO electrodes as anodes. The VNW-rGO anode and MNR-rGO cathode were combined to form a novel hybrid supercapacitor. The hybrid supercapacitor exhibited excellent electrochemical performance reflecting the synergistic effect of combining the MNR-rGO electrode and VNW-rGO electrode. This novel hybrid supercapacitor delivered an energy density of 15 Wh kg1 with a specific capacitance of 36.9 F g1. By dint of their ultra-flexibility and high safety, all-solid-state thin-film supercapacitors (ASSTFSs) have attracted tremendous attention. Bao et al. developed a nanocomposite electrode combined with pseudocapacitive vanadium pentoxide and highly conductive graphene with ultrathin thickness for the application of ASSTFSs [85]. The novel structure of the nanocomposite achieved a

maximal integration of both the merits of each component with high conductivity and ultrathin thickness, which enhanced the electron transfer, shortened the ion diffusion paths and increased the electrode-electrolyte contact in ASSTFSs, leading to high electrochemical performance. The as-fabricated ASSTFS achieved a high areal capacitance of 11,718 mF cm2, a remarkable energy density of 1.13 mWh cm2 at a power density of 10.0 mW cm2 and longterm cycling stability for 2000 cycles, demonstrating the superior electrochemical performance and rendering it a promising candidate for portable electronics. Using the electrospinning technique, Thangappan et al. prepared graphene oxide/vanadium pentoxide nanofibers [86]. They found that graphene oxide-V2O5 composite nanofibers exhibited the better capacitive behavior with better reversible charging/discharging ability and higher capacitance values, compared to pure V2O5 electrodes. 2D heterostructures often have superior electrochemical performance. Nagaraju et al. synthesized the electrode materials based on two-dimensional (2D) heterostructures of V2O5 nanosheets (V2O5 NS) and reduced graphene oxide electrodes for asymmetric supercapacitor applications [87]. Specifically, the 2D V2O5 and rGO/ V2O5 nanosheet electrodes showed a specific capacitance of 253 F g1 and 635 F g1, respectively at a current density of 1 A g1. The capacitance of the heterostructures is almost 2.5 times higher than the 2D V2O5 nanosheets alone. The corresponding energy density of 39 Wh kg1 and 79.5 Wh kg1 were achieved for the two electrodes at a power density of 900 W kg1 in an asymmetric supercapacitor configuration. As the fabricated free-standing electrode was flexible and demonstrated good mechanical properties, Foo et al. developed vanadium pentoxide-reduced graphene oxide free-standing electrodes through a facile and low temperature synthesis approach, which can eliminate the need for current collectors and reduce resistance [88]. The effective exfoliation of rGO allows improved electrolyte ions interaction, achieving high areal capacitance (511.7 mF cm2) coupled with high mass loadings. A fabricated asymmetric flexible device based on rGO/V2O5-rGO (VGO) consisted of approximately 20 mg of active mass and still delivered a low equivalent series resistance (ESR) of 3.36 U with excellent cycling stability. Wu et al. successfully first fabricated a V2O5/graphene hybrid aerogel at ambient pressure through a simple solegel method from commercial V2O5 powder [89]. The V2O5/graphene hybrid aerogel was synthesized through the in situ growth of V2O5 nanofibers on graphene sheets. The V2O5/graphene hybrid aerogel-based supercapacitors exhibited enhanced specific capacitance (486 F g1), high energy density (68 Wh kg1) and outstanding cycle performance. Using a low-temperature hydrothermal method in a single step, Lee et al. synthesized graphene-decorated V2O5 nanobelts (GVNBs) [90]. V2O5 nanobelts (VNBs) were formed on the surface of graphene oxide, a mild oxidant, which also enhanced the conductivity of GVNBs. From the electron energy loss spectroscopy analysis, the reduced graphene oxide was inserted into the layered crystal structure of V2O5 nanobelts, which further confirmed the enhanced conductivity of the nanobelts. The electrochemical energy-storage capacity of GVNBs was investigated for supercapacitor applications. The specific capacitance of GVNBs was evaluated by cyclic voltammetry (CV) and charge/discharge (CD) studies. The GVNBs having V2O5-rich composite, namely, V3G1 (VO/GO5351), showed superior specific capacitance in comparison to the other composites (V1G1 and V1G3) and the pure materials. Moreover, the V3G1 composite showed excellent cyclic stability and the capacitance retention of about 82% was observed even after 5000 cycles. A novel nanohybrid material composed of vanadium pentoxide

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nanofibres (VNFs) and exfoliated graphene were prepared by insitu growth of VNFs onto graphene nanosheets, and explicated as electrode material for supercapacitor applications [91]. The existence of non-covalent interactions between VNFs and graphene surfaces was confirmed by Raman and Fourier transform infrared (FTIR) spectroscopes. Morphological analysis of the nanohybrid revealed that the VNF layer was uniformly grown on the grapheme surfaces, producing high specific surface area and good electronic or ionic conducing path. Compared to pristine VNF, the VNF/graphene nanohybrid exhibited higher specific capacitance of 218 F g1 at current density of 1 A g1, higher energy density of 22 Wh kg1 and power density of 3594 W kg1. Asymmetric supercapacitor devices were prepared by the Spectracarb 2225 activated carbon cloth and VNF/graphene nanohybrid as positive and negative electrode, respectively. The asymmetric device exhibited capacitance of 279 F g1 at 1 A g1, energy density of 37.2 Wh kg1 and power density of 3743 W kg1, which were comparable and superior to reported asymmetric devices consisting of carbon material and metal oxide as electrode components. In order to shorten the synthesis time and improve the capacitance properties, Geng et al. synthesized V2O5$nH2O/graphene composites by self-assembly in a graphite oxide solution under hydrothermal condition [92]. They found that the V2O5$1.6H2O nanobelts were in width of 90 nm and length of 1.5 mm. The hybrid capacitance of this composite reached 579 F g1 at current density of 1 A g1 and decreased 21% after 5000 cycles at 4 A g1. Compared with other conventional methods, microwave reduction has the following advantages: speed, low cost, energy efficiency, homogeneous heating. By a microwave assisted facile route, Ramadoss et al. synthesized reduced graphene oxide/vanadium pentoxide (GV) composites and reported the application of this material towards the supercapacitor application [93]. The asfabricated reduced graphene oxide/vanadium pentoxide (GV-3) composite electrodes displayed outstanding electrochemical performance with a maximum specific capacitance of 250 F g1 at 5 mV s1 and excellent cycling stability, retaining 95% of the initial capacitance, even after 5000 cycles. Asymmetric supercapacitors have been obtaining great attention, owing to their significantly increased energy density. Li et al. report the in situ growth of V2O5 nanorods on highly conductive graphene sheets as anode materials for asymmetric supercapacitors with high specific capacitance and excellent rate capability, which is mainly attributed to the intimate contact between the nanorods and graphene sheets [94]. The asymmetric supercapacitor based on graphene/V2O5 composites and activated carbon was fabricated and evaluated, so that a high energy density of approximately 50 Wh kg1 can be achieved at a power density of 136.4 W kg1, as well as long cycling stability. 2.3. Vanadium pentoxide/conducting polymer composites As another important kind of pseudocapacitive materials, recently, an increasing number of researchers start to study the conducting polymer based composites and its application for the supercapacitors. Pentoxide/poly(3,4-ethylenedioxythiophene) (PEDOT) has the advantages of simple structure, small energy gap and high electrical conductivity, which is widely used in energy storage materials [95e97]. Mai et al. designed the heterostructured nanomaterial with PEDOT as the shell and MnO2 nanoparticles as the protuberance and synthesized the novel cucumber-like MnO2 nanoparticles enriched vanadium pentoxide/PEDOT coaxial nanowires [95]. This heterostructured nanomaterial exhibited enhanced electrochemical cycling performance with the decreases of capacity fading during 200 cycles from 0.557 to 0.173% over V2O5 nanowires at the

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current density of 100 mA g1. Subsequently, Reddy et al. prepared Poly(3,4ethylenedioxypyrrole) (PEDOP)/V2O5 nanobelt hybrid films. The V2O5 nanobelts with a monoclinic structure which was grown by a hydrothermal route and a PEDOT layer was coated onto V2O5 nanobelts by electropolymerization to yield the PEDOT/V2O5 hybrid [96]. The synergistic effects of PEDOP (high electrical conductivity) and V2O5 nanobelts (Large surface area) and their ability to store/ release charge by undergoing reversible Faradaic reactions were reflected in a high specific capacitance of 224 F g1 delivered by the hybrid, higher by 83% and 69% relative to pristine V2O5 and PEDOP. The hybrid showed an energy density of 223 Wh kg1 at a power density of 3.8 kW kg1, and an acceptable cycling performance with 90% capacitance retention after 5000 cycles. Recently, Guo et al. developed a tandem redox reaction strategy for building layered V2O5 (LVO), conducting polymerpoly(3,4ethylenedioxythiophene) (PEDOT), and layered MnO2 (LMO) into a sandwich structured LVO\PEDOT\LMO [97]. Asymmetric supercapacitors built from LVO\PEDOT\LMO cathode and active carbon (AC) anode (LVO\PEDOT\LMOjjAC) using Na2SO4 aqueous electrolyte showed an energy density of 39.2 Wh kg1 (based on active materials), which was among the highest reported for supercapacitors with neutral aqueous electrolytes. The LVO\PEDOT\LMOjjAC supercapacitors also offered high rate capability (21.7 Wh kg1 at 2.2 kW kg1) and good cycle stability (93.5% capacitance retention after 3000 cycles). PPy is an ideal electrode material for supercapacitor, which has high electrical conductivity, good environmental stability, reversible electrochemical redox properties and strong charge storage capacity [98e102]. Bai et al. used electrochemical co-deposition to prepare vanadium oxide (V2O5) and polypyrrole (PPy) that was conducted from vanadyl sulfate (VOSO4) and pyrrole in their aqueous solution to get V2O5-PPy composite, during which onedimensional growth of polypyrrole (PPy) was directed.98 Due to the organic-inorganic synergistic effect, V2O5-PPy composite exhibited good charge-storage properties in a large potential window from 1.4e0.6 V vs SCE, with a specific capacitance of 412 F g1 at 4.5 mA cm2. A model supercapacitor assembled by using the V2O5-PPy composite as the electrode materials displayed a high operating voltage of 2 V and a high energy density of 82 Wh kg1 (at the power density of 800 W kg1). It is well-known that the continuous 3D network can create channels for better ion transport, and the high degree of pore connectivity in the network enhance the mass transport. Qian et al. synthesized 3D V2O5 network with PPy uniformly decorated onto each nanowire that were fabricated to enhance their pseudocapacitive performance [99]. The PPy shell could enhance the electric conductivity and prevent the dissolution of vanadium. These merits together with the ideal synergy between V2O5 and PPy lead to a high specific capacitance of 448 F g1, which was three times higher than that of the stacked V2O5. The all-solid-state symmetric supercapacitor device assembled by the V2O5/PPy core/shell 3D network exhibited a high energy density (14.2 Wh kg1) at a power density of 250 W kg1 and good cyclic stability (capacitance retention of 81% after 1000 cycles). In order to study the optimal ratio of PPy to V2O5, Sun et al. conducted a number of controlled trials and found that the optimal ratio of PPy to V2O5 to be 40% where the sum of the polymer's electric double-layer (EDL) capacitance and the layered oxide's subsurface faradic capacitance could be maximized [100]. By a combined hydrothermal, freeze-drying and nanocasting process, Cao et al. developed a new 3D [email protected] network which was built from numerous ultrathin, flexible and single-crystalline nanoribbons [101]. The unique network can not only provide a high surface area for enhancement of electrolyte/electrode interactions,

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and reduce the diffusion length of ions, but also efficiently maintain the high electrical conductivity. As a result, this network exhibited high capacitance, excellent rate capability and good chargedischarge stability for energy storage. An asymmetric supercapacitor based on a 3D [email protected] network as the cathode material further delivered high energy density and high power density. The addition of surfactant can not only improve the conductivity of the polymer, but also increase the yield of the polymer. Qu et al. developed a core-shell structure of PPy grown on V2O5 nanoribbons fabricated using SDB as surfactant [102]. Benefiting from the nanoribbon morphology of V2O5, the improved charge-transfer and polymeric coating effect of PPy, [email protected] nanocomposite exhibited high energy density, excellent cycling behavior, and good rate capability. Compared with the PEDOT and PPy, the vanadium pentoxide/ polyaniline composites is not much reported. Bai et al. used an electro-codeposition method to synthesize a high performance negative electrode composed of a vanadium oxide (V2O5) and polyaniline (PANI) composite [103]. Scanning electron microscopy revealed that the composite film was composed of onedimensional polymer chains. Significantly, the V2O5-PANI composite nanowires exhibited a wide potential window of 1.6 V (between 0.9 and 0.7 V vs. SCE) and a maximum specific capacitance of 443 F g1 (664.5 mF cm2). The flexible symmetric supercapacitor assembled with this composite film yielded a maximum energy density of 69.2 Wh kg1 at a power density of 720 W kg1, and a maximum power density of 7200 W kg1 at an energy density of 33.0 Wh kg1. These values were substantially higher than those of other pure V2O5 or PANI based supercapacitors. Moreover, the assembled symmetric supercapacitor device showed an excellent stability with 92% capacitance retention after 5000 cycles. Polyindole (PIn) is an intrinsically conducting polymer as a pseudocapacitor material because of its superior conductivity, excellent thermal stability, electrochemical reversibility and storage ability. Zhou et al. fabricated a bamboo-like nanomaterial composed of V2O5/polyindole (V2O5/PIn) decorated onto the activated carbon cloth supercapacitors [104]. The PIn could effectively enhanced the electronic conductivity and prevent the dissolution of vanadium and the activation of carbon cloth with functional groups was conductive to anchoring the V2O5 and improving surface area, which resulted in an enhancement of electrochemical performance and lead to a high specific capacitance of 535.5 F g1. Moreover, an asymmetric flexible supercapacitor based on V2O5/[email protected] carbon cloth and reduced graphene [email protected] carbon cloth exhibited a high energy density (38.7 W h kg1) at a power density of 900 W kg1 and good cyclic stability (capacitance retention of 91.1% after 5000 cycles). 2.4. Other element doped vanadium pentoxide composites Doped metal element have been the subject of interest for enhancing the capacitive behavior of supercapacitor such as maximum current density, good reversibility, good ion storage capacity and cyclic stability. Moreover to achieve fast faradic reaction for electron transport in the electrode and ion transport in the solution, doped metal element electrodes are preferred. Jeyalakshmi et al. prepared interesting thin film electrodes of nickel doped vanadium pentoxide with different levels of doping (2.5e10 wt%) on FTO and glass substrate at 300  C using so-gel spin coating method [105]. The doping of nickel with b-V2O5 had led to enhanced intercalation and deintercalation of ions. b-V2O5 films with 5 wt% of Ni exhibited the maximum specific capacitance of 417 F g1 at a scan rate of 5 mV s1, with a good cyclic stability. Sn4þ doping could alter the microstructure and the morphology of V2O5. Wang et al. prepared Sn4þ-doped V2O5 cathode materials

via a sol-gel method [106]. The results showed that the modified cathode material was mixture of V4þ and V5þ. It was a kind of typical mesopore material with pores of 2e4 nm diameter. Symmetrical curves were obtained by cyclic voltammetry (CV) tests performed at different scanning rates and voltage ranges. In particular, the CV curve showed more obvious rectangle property and better redox properties when the scanning rate was 5 mV s1. At the current density of 200 mA g1, the maximum specific energy, specific power, and coulomb efficiency of the material were 27.25 mA hg1, 494.87 W kg1, and 97%, respectively. Due to its porous Structures, sodium-doped vanadium oxide is widely used in energy storage materials. Khoo et al. successfully synthesized a nanostructured oxide pseudocapacitor electrode utilizing a sodium-doped vanadium oxide (b-Na0.33V2O5) nanobelt network with a three dimensional framework crystal structure via mild hydrothermal conditions and heat treatment [107]. A high specific capacitance of 320 F g1 at 5 mV s1 scan rate has been achieved with two sets of redox peaks being identified, corresponding to the half occupancy at M3 and M2 intercalation sites along the tunnel in the b-Na0.33V2O5 crystal lattice. The bNa0.33V2O5 nanobelt electrode was able to deliver a high energy density of 47 Wh kg1 at a high power density of 5 kW kg1. Superior cycling stability, with only 34% degradation in specific capacitance, was observed in the b-Na0.33V2O5 nanobelts after 4000 cycles. Chang's group reported that Na-doped V2O5 was successfully synthesized using an anodic deposition technique in a plating solution containing VOSO4 and NaCH3COO [108]. The Na doping significantly improved the oxide capacitance. The optimum specific capacitance is about 180 F g1. 2.5. Other vanadium pentoxide composites In recent years various mixed oxides composites such as NiOeMnO2, CoOeMnO2 and a-Fe2O3/MnO2 core-shell nanowire hetero structure arrays were fabricated to improve the capacitive performance of the materials through intruding synergistic effects into an electrode system. Yang et al. prepared highly ordered mixed V2O5eTiO2 nanotubes via self-organizing anodization of Ti-V alloys with vanadium content of up to 18 wt% [109]. In the resulting oxide nanotube arrays, the vanadium is electrochemically switchable leading to a specific capacitance that can reach up to 220 F g1 and the energy density of 19.56 Wh kg1 with perfect reversibility and long-term stability. As a common pseudocapacitance material, MnO2 has been extensively studied. Saravanakumar et al. prepared MnO2 grafted V2O5 nanostructures, which exhibited elevated specific capacitance (450 F g1 at 0.5 A g1), good rate capacity (251 F g1 at 5 A g1) and provided better cycling stability (retaining 89% of capacitance after 500 cycles) [110]. They developed an asymmetric supercapacitor, which used MnO2 grafted V2O5 and activated carbon (AC) as electrodes and exhibited a specific capacitance of 61 F g1 with an energy density of 8.5 Wh kg1. 2.6. Vanadium dioxide Although vanadium pentoxide is one of the most widely used vanadium oxides in the field of electrochemistry, vanadium dioxide is also favored by some researchers. Recent research of vanadium dioxide showed that it also had great potential in the field of electrochemistry. As an important functional vanadium oxide, vanadium dioxide possesses excellent physical and chemical properties. VO2, has been found to show better performance compared to the well known V2O5. This is due to its higher electronic conductivity arising from a mixed-valence V3þ/5þ and structural stability arising from increased edge sharing and the consequent

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resistance to lattice shearing during cycling. Nevertheless, VO2 do not usually deliver higher specific capacitance behavior than other transition metal oxides (such as Co3O4, NiO, MnO2) because of its poor electrical conductivity and the poor structural stability, resulting in limited long-term cycling stability. Shao et al. prepared homogenous hexangular starfruit-like vanadium dioxide for the first time by a one-step hydrothermal method [111]. The assembly process of hexangular starfruit-like structure was observed from SEM images (as shown in Fig. 2a). The electrochemical performance of starfruit-like vanadium oxide was examined by cyclic voltammetry and galvanostatic charge/discharge. The obtained starfruitlike vanadium oxide exhibited a high power capability (19 Wh kg1 at the specific power of 3.4 kW kg1) and good cycling stability for supercapacitors application. The same as V2O5, a strategy for improving the electrochemical property of VO2(B) as the supercapacitors material by designing the material structure is eagerly required. Pan et al. reported that reduction of VO2 resistance by nearly 3 orders of magnitude through H2 treatment that can improve its conductivity for supercapacitor application [112]. A specific discharge capacitance of 300 F g1 and a specific energy density of 17 W h kg1 at a rate of 1 A g1 were demonstrated with long-term cycling stability, which was 4 times higher than the untreated samples. The traditional method of improving the conductivity of VO2 is to prepare the carbon based composite material. Liang et al. synthesized a coaxial-structured hybrid material of vanadium dioxide (VO2(B)) and MWCNT by using a facile sol-gel method assisted with freeze-drying process [113]. A few layers of VO2(B) sheath are firmly coated on the CNTs surfaces (as shown in Fig. 2b), resulting in the formation of network morphology with abundant pores and good electric conductivity. This VO2(B)/CNTs composite for the first time was employed as supercapacitor electrode material, demonstrating better specific capacitance and superior rate capability than the individual components alone. A specific capacitance of 250 F g1

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was obtained in 1 M Na2SO4 solution at a current density of 0.5 A g1, with a capacitance retention up to 71% when the current density was increased to 10 A g1. By one-step simultaneous hydrothermal reduction technology, Deng et al. prepared graphene/VO2 (RG/VO2) hybrid materials with different RG amounts in a mixture of ammonium vanadate, formic acid and graphite oxide (GO) nanosheets [114]. The hydrothermal treatment made the reduction of GO into RG and the formation of VO2 particles with starfruit morphology (as shown in Fig. 2c). The starfruit-like VO2 particles were uniformly embedded in the hole constructed by RG nanosheets, which made the electrodeelectrolyte contact better. A high specific capacitance of 225 F g1 had been achieved for RG(1.0)/VO2 electrode with RG content of 26 wt% in 0.5 M K2SO4 electrolyte. An asymmetrical electrochemical capacitor was assembled by using RG(1.0)/VO2 as positive electrode and RG as negative electrode, and it can be reversibly charged-discharged at a cell voltage of 1.7 V in 0.5 M K2SO4 electrolyte. The asymmetrical capacitor can deliver an energy density of 22.8 Wh kg1 at a power density of 425 W kg1. Moreover, the asymmetrical capacitor preserved 81% of its initial capacitance over 1000 cycles at a current density of 5 A g1. Using commercial V2O5 and graphene oxide as precursors, Wang et al. developed a facile one-step strategy to prepare 3D graphene/VO2 nanobelt composite hydrogels, which can be readily scaled-up for mass production [115]. In the two-electrode configuration, the graphene/VO2 nanobelt composite hydrogel exhibited a specific capacitance of 426 F g1 at 1 A g1 in the potential range of 0.6 to 0.6 V, which greatly surpassed that of each individual counterpart (119 F g1 and 243 F g1 at 1 A g1 for VO2 nanobelt and graphene hydrogel, respectively). Without using any toxic organic solvent, Xiao et al. adopted a hydrothermal method assisted with freeze drying process to form composite supercapacitor materials of metastable vanadium dioxide nanobelts on reduced graphene oxide (RG) layers [116]. The

Fig. 2. (a) SEM images of hexangular starfruit-like vanadium dioxide; (b) VO2(B)/CNTs composite; (c) 3D graphene/VO2 nanobelt composite hydrogel; (d) VO2/HMB core-shell arrays on GF. (a) reprinted with permission from Ref. [111]. Copyright 2012, Elsevier; (b) reprinted with permission from Ref. [113]. Copyright 2013, Elsevier; (c) reprinted with permission from Ref. [114]. Copyright 2013, Elsevier; (d) reprinted with permission from Ref. [118]. Copyright 2015, The Royal Society of Chemistry.

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initial specific capacitance of the composites reached 290.4 F g1 at 0.2 A g1 and maintained 82.3% of the initial value after 1000 cycles at 2 A g1, 37.9% higher than the pure VO2 (B). To examine the effect of graphene lateral size on the electrochemical performance of a hybrid supercapacitor composed of VO2/ GO electrodes, Lee et al. developed a flexible hybrid supercapacitor electrode composed of UGO (ultralarge graphene oxide) sheets (average lateral size of 47 ± 22 mm) and VO2 nanobelts [117]. Thermal treatment converts UGO/VO2 to URGO/VO2 (denoted VURGO). The VURGO hybrid electrode showed a specific capacitance of 769 F g1. Recently, Xia et al. reported that hydrogen molybdenum bronze (HMB) was electrochemically deposited as a homogeneous shell on VO2 nanoflakes grown on graphene foam (GF), forming a GF þ VO2/ HMB integrated electrode structure (Fig. 2d) [118]. Asymmetric supercapacitors based on the GF þ VO2/HMB cathode and neutral electrolyte are assembled and enhanced performance with weaker polarization, higher specific capacitance and better cycling life than the unmodified GF þ VO2 electrode. Capacitances of 485 F g1 (2 A g1) and 306 F g1 (32 A g1) were obtained because of the exceptional 3D porous architecture and conductive network. Various mixed oxides composites can increase the capacitance of active electrode materials. Electrostatic spray deposition can prepare ideal film material. Hu et al. synthesized VO2/TiO2 nanosponges via electrostatic spray deposition with easily tailored nanoarchitectures and composition as binder-free electrodes for supercapacitors [119]. Benefiting from the unique interconnected pore network of the VO2/TiO2 electrodes and the synergistic effect of high capacity VO2 and stable TiO2, the as-formed binder-free VO2/TiO2 electrode exhibited a high capacity of 86.2 mF cm2 (548 F g1) and satisfactory cyclability with 84.3% retention after 1000 cycles. 2.7. Vanadium trioxide V2O3 possesses a 3D VeV framework and its V 3d electrons can itinerate along the V-V chains, leading to metallic behavior. In addition, tunneled structures exist in V2O3, which facilitates ion intercalation/deintercalation. Therefore, V2O3 is particularly suitable as an electrode material in supercapacitors. However, V2O3 have relatively low electric conductivity compared to RuO2, which decreases the charge transfer rate during the charging/discharging process and limits their specific capacitance as electrode materials of supercapacitors. Interestingly, there are only limited reports concerning the synthesis of V3þ based nanostructured materials and their functional applications. It is relatively hard to prepare pure V2O3 nanomaterials, to a certain extent due to their sensitivity

to temperature and atmosphere. Liu et al. prepared a novel 3D hierarchical flowerlike vanadium sesquioxide (V2O3) nano/microarchitecture (as shown in Fig. 3a) consisting of numerous nanoflakes via a solvothermal approach followed by an appropriate heating treatment [120]. When used as the cathode material of pesudocapacitors in Li2SO4, the flowerlike oxide displayed a very high initial capacitance of 218 F g1 at a current density of 0.05 A g1. On this basis, Li et al. developed a micelle-anchoring method for the in situ synthesis of V2O3 nanofl[email protected] core-shell composites (as shown in Fig. 3b) as the electrode materials in supercapacitors [121]. Hexadecyltrimethylammonium bromide (CTAB) micelles assembled to solubilize activated carbon and anchor vanadate ions of the precursor, NH4VO3, onto the carbon surface. During drying and calcination, CTAB and NH4VO3 decompose to produce V2O5, which were carbon-thermally reduced to V2O3 in situ. In the asobtained composites, monodisperse V2O3 nanoflakes stand edgeon the carbon surface, forming a carbon core with a shell layer of edge-on standing V2O3 nanoflakes. Because of the increased electric conductivity and high specific surface area, V2O3 nanofl[email protected] composites exhibited a specific capacitance of 205 F g1 at 0.05 A g1 over a potential range of 0.4e0.6 V, which surpassed those of individual counterparts (67 F g1 and 159 F g1 at 0.05 A g1 for activated carbon and bulk V2O3, respectively). 2.8. Mixed valence vanadium oxide and its composite The multiple stable oxidation states (IIIeV) of vanadium in its oxides and typical layered structures enable VOx to have an even higher charge storage capability than most of other inexpensive transition-metal oxides. Mixed-valence vanadium oxides are promising electrode materials for supercapacitors, as they have multiple oxidation states (V2þ, V3þ, V4þ, V5þ) available for charge storage in a wide range of potential windows. However, the poor electron transport in VOx and the poor cycle stability of VOx both hinder its applications. Core-shell nanocomposite materials have been intensively investigated as electrode for supercapacitor application. Pan et al. dispersed quasi-metallic V2O3 nanocores which were graphene sheets for electrical connection of the whole structure, while a naturally formed amorphous VO2 and V2O5 (called as VOx here) thin shell around V2O3 nanocore acts as the active pseudocapacitive material [122]. This high rate was attributed to the largely enhanced conductivity of this unique structure and a possibly facile redox mechanism. Such an EC can provide 1000 kW kg1 power density at an energy density of 10 Wh kg1. The synthesis of VOx$nH2O using anodic deposition from

Fig. 3. (a) Panoramic SEM image of the flowerlike V2O3 nano/micromaterial; (b) FESEM images of V2O3 nanofl[email protected] composites. (a) reprinted with permission from Ref. [120]. Copyright 2010, Wiley; (b) reprinted with permission from Ref. [121]. Copyright 2014, The Royal Society of Chemistry.

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aqueous VOSO4 solutions was considered as an effective method [123,124]. Hu et al. used anodic deposition to develop a new type vanadium oxide(VOx$nH2O) with porous, three-dimensional (3-D) network architecture plated at 0.7 V (vs. Ag/AgCl) from 25 mM VOSO4 with 5 mM H2O2 showed capacitive-like behavior at 250 mV s1 and CS z 167 F g1 at 25 mV s1 in 3 M KCl for pseudocapacitor applications [122]. On the basis of Hu's work, Huang et al. reported that a three-dimensional porous vanadium oxide was anodically deposited onto graphite substrates denoted as VOx$nH2O/G [123]. Through annealing at temperatures up to 350  C, the thermal stability of VOx$nH2O preserved its porous morphology and excellent capacitive performances in 3 M KCl at pH of 2.4. A maximal specific capacitance of ca. 150e160 F g1 measured at 250 mV s1 was obtained for this porous VOx$nH2O annealed between 150 and 250  C. Only 9e17% loss in specific capacitance was found for these VOx$nH2O when the scan rate of the cyclic voltammetry was increased from 25 to 250 mV s1, demonstrating a typical high power property. The researchers then developed other methods for the preparation of VOx$nH2O. Li et al. synthesized VOx$nH2O with long cyclelife for Li-ion supercapacitors successfully by means of the microwave-assisted hydrothermal synthesis (MAHS) method, which was a faster and more energy-saving method than the conventional hydrothermal synthesis [125]. Without using any surfactant or capping agent, Cheng et al. prepared 3D interconnected porous vanadium oxide network VOx$nH2O via a facile and effective method through controlling solution polarity at room temperature [126]. The experimental results indicated that the microstructure of the as-prepared samples can be effectively tailored by the polarity of reaction solution and the obtained 3D interconnected network delivered a high specific capacitance of 280 F g1. Lai's group carried out a systematic study of V6O13 [127,128]. As shown in Fig. 4a, they used thermal decomposing and quenching method to prepare V6O13 sheet with morphology and mixed valence of V(V) and V(IV) [127]. The products exhibited the sheet morphology with an average size of 2 mm and a thickness of about 200 nm. In the voltage range of 0.2e0.8 V (vs. SCE) in 1 M NaNO3 electrolyte, V6O13 electrode exhibited obvious capacitance performance. At the current density of 50 m Ag1, the material delivered a specific capacitance value of 285 F g1. At the current of 200 mA g1, the electrode exhibited an initial specific capacitance of 215 and 208 F g1 after 300 cycles, which is shown in Fig. 4 (b). Subsequently, they successfully synthesized hollow flowers-like V6O13 with an average size of 3 mm through a facile sol-hydrothermal approach in a short time [128]. Experimental results indicated

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that hollow flowers-like V6O13 can deliver a capacitance of 417 F g1 at a scan rate of 5 mV s1. Different from the V6O13 and VO2, Li et al. successfully synthesized novel hierarchical vanadium oxide microspheres that contain V6O13 and VO2 forming from hyperbranched growth of nanoribbons via a solvothermal method [129]. The as-prepared hierarchical microspheres have a diameter of 5 mm, in which 400 nm long nanoribbons grow as hyperbranches on the clustering nanobelt backbones. These hierarchical microspheres contain 86.2 mass % V6O13 with metallic conductivity and 13.8 mass% VO2. The hierarchical microspheres exhibited a specific capacitance as remarkably high as 456 F g1, with a corresponding volumetric specific capacitance of 3.09 F cm3, at 0.6 A g1 in the potential range of 0e1.2 V. The maximal energy density and power density achieved were up to 22.8 W h kg1 (0.16 mWh cm3) and 1.2 kW kg1 (8.14 mW cm3). To further explore the effect of the valence state of vanadium on the electrochemical performance of VOx. Through tuning the valence state of vanadium, Yu et al. reported an innovative and effective method to significantly boost the durability and capacitance of VOx [130]. The valence state of vanadium was optimized through a very facile electrochemical oxidation method. A superior electrochemical performance and an ultralong cyclic stability of 100,000 cycles were obtained for these electrodes. Zhai et al. first demonstrated the sulfur-doped, oxygen-deficient V6O13-x as an anode electrode for Asymmetric supercapacitors (ASCs) [131]. Significantly, this new electrode achieved a benchmark capacitance of 1353 F g1 (0.72 F cm2) at a current density of 1.9 A g1 (1 mA cm2) in 5 M LiCl solution. The availability of multiple oxidation states in the V6O13-x electrode led to the significant pseudocapacitance in the negative potential window between 0 V and 1 V vs SCE. Chemical vapor deposition (CVD) is a chemical process that is usually used to produce high quality, high-performance, nano materials. Through a CVD technique, Jampani et al. reported the supercapacitance behavior of titanium doped vanadium oxide films grown on vertically aligned carbon nanotubes (VOx:Ti-VACNT) [132]. The capacitance of CVD derived titanium doped vanadium oxide-carbon nanotube composites was measured at different scan rates to evaluate the charge storage behavior. In addition, the electrochemical characteristics of the titanium doped vanadium oxide thin films synthesized by the CVD process were compared to substantiate the propitious effect of the carbon nanotubes on the capacitance of the doped vanadium oxide. Considering the overall materials loading with good rate capability and excellent charge retention up to 400 cycles, it can be noted that attractive

Fig. 4. (a) SEM image of V6O13 sheet; (b) Cycle tests of V6O13 electrode at 200 mA g1 in the potential range of 0.2e0.8 V (vs. SCE) in 1 M NaNO3 electrolyte. a: Cycle number vs. specific capacitance and; b: cycle number vs. chargeedischarge efficiency. Reprinted with permission from Ref. [127]. Copyright 2009, Elsevier.

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capacitance values as high as 310 F g1 were reported. Zhao et al. synthesized hierarchical or micro/nano-structured porous [email protected] composites via a one-step method using phenolic resin as the carbon precursor and ammonium metavanadate as the source of vanadium oxides [133]. They found that vanadium oxides greatly enhanced the electrochemical performance of the materials, due to the faradic capacitance generated from vanadium oxide nanoparticles. A maximum specific capacitance of 171 F g1 was obtained from [email protected] composite with vanadium loading of 44 wt%. Through the hydrothermal process, Fu et al. successfully synthesized graphene/vanadium oxide nanotubes (VOx-NTs) composite in which acetone as solvent and 1-hexadecylamine (HDA) as structure-directing template were used [134]. The composite with the VOx-NTs amount of 69.0 wt% can deliver a specific capacitance of 210 F g1 at a current density of 1 A g1 in 1 M Na2SO4 aqueous solution, which is nearly twice as that of pristine graphene (128 F g1) or VOx-NTs (127 F g1), and exhibited a good performance rate. Hu and co-workers prepared graphene/vanadium oxide (RG/VOx$nH2O) hybrid electrodes with different graphene (RG) amounts by one-step simultaneous hydrothermal-reduction technology in a suspension of NH4VO3, NH2CSNH2 and graphite oxide (GO) nanosheets [135]. RG/VOx$nH2O electrode with RG content of 10 wt% exhibited not only high specific capacitance of 384 F g1 at a scan rate of 5 mV s1 in 0.5 M K2SO4 electrolyte, but also relatively good cycle stability at a current density of 5 A g1 (60% capacitance retention after 1000 cycles). In order to investigate the effects of the graphene content and the treatment temperature on the supercapacitive properties of VOx/graphene nanocomposites, Li et al. prepared the vanadium oxides (VOx)/graphene hybrid materials constructed from 2D graphene nanosheets (GNS) and VOx through a simple two-step procedure including solvothermal method and subsequent thermal treatment [136]. Importantly, the electrochemical properties of asprepared composites are systematically investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy, which are highly dependent on the content of GNS in composite and the annealing temperature. Furthermore, the VOx-7.4% GO-300 composite electrode exhibited the largest specific capacitance and the most excellent rate capability among these composites. Wang et al. found that the electrochemical instability of V3O7 was mainly due to the chemical dissolution in aqueous electrolyte and the structure pulverization during charging/discharging cycling [137]. They had demonstrated a novel strategy to address these limitations by replacing the aqueous electrolyte with a neutral pH LiCl/PVA gel electrolyte. The vanadium oxide nanowire pseudocapacitors with gel electrolyte achieved a maximum areal capacitance of 236 mF cm2 at a current density of 0.2 mA cm2 and an excellent capacitance retention rate of more than 85% after cycling for 5000 cycles. As a kind of chemical deposition methods, electroless deposition method is also an effective method for the preparation of nanomaterials. Wu et al. synthesized the vanadium oxide which consisted of a mixture of amorphous V2O5 and VO2 as a thin film via a simple electroless deposition method [138]. Electrochemical characterizations of the synthesized vanadium oxide showed ideal capacitive behavior with good cycle life. 3. Vanadium nitride Transition metal nitrides are of particular interest by virtue of their synergic advantages of superior electrical conductivity, excellent environmental durability and high reaction selectivity. VN and TiN are considered as the promising pseudocapacitive

candidates for next-generation high performance SCs owing to their excellent electrical conductivity and high specific pseudocapacitance [139e142]. However, the capacitance would significantly degrade at high voltage scanning rates/current densities. 3.1. Vanadium nitride Choi et al. first used a low-temperature route based on a twostep ammonolysis reaction of VCl4 in anhydrous chloroform to synthesize nanocrystalline VN [143,144]. The nanometer-sized crystals increase the susceptibility for surface oxidation, while the high surface area of the nitrides provides more redox-reaction sites. The specific capacitance improved with reduced material loading, and the highest specific capacitance of 1340 F g1 was recorded at a scan rate of 2 mV s1, which decreases to 554 F g1 when tested at 100 mV s1, showing a logarithmic trend. In addition, an impressive specific capacitance of 190 F g1 is obtained at a very high scan rate of 2 V s1. The semi-logarithmic behavior of the specific capacitance versus scan rate was characteristic of all electrochemical capacitors derived from highly porous powders. The VN obtained at 1000  C, however, exhibited a capacitance of only 58.3 F g1 at a scan rate of 2 mV s1. In order to further evaluate the structural characteristics and electrochemical properties of vanadium nitride which was prepared by temperature-programmed ammonia reduction of V2O5 powder, Glushenkov et al. synthesized vanadium nitride and have a systematic performance test of it [145]. The large volume of pores in VN is represented by the range of 15e110 nm (Shown in Fig. 5a). The material has an acceptable rate capability in all electrolytes, showing about 80% of its maximal capacitance at a current load of 1 A g1 in galvanostatic charging/discharging experiments. The capacitance of 186 F g1 was observed in 1 M KOH electrolyte at 1 A g1. By calcining V2O5 xerogel in a furnace under an anhydrous NH3 atmosphere at 400  C, Zhou et al. synthesized vanadium nitride (VN) powder (shown in Fig. 5b). SEM images showed the homogeneous surface of the obtained VN [146]. The CV diagrams illustrated the existence of fast and reversible redox reactions on the surface of VN electrode. The specific capacitance of VN was 161 F g1 at 30 mV s1. Furthermore, the specific capacitance remained 70% of the original value when the scan rate increased from 30 to 300 mV s1. Lu and co-workers developed an effective strategy to stabilize VN nanowire anode without sacrificing its electrochemical performance by using LiCl/PVA gel electrolyte [147]. By suppressing the oxidation reaction and structural pulverization, the VN nanowire electrode exhibited remarkable cycling stability in LiCl/PVA gel electrolyte with a capacitance retention of 95.3% after 10,000 cycles. The VN nanowire anode achieved a high energy density of 0.61 mWh cm3 at current density of 0.5 mA cm2 and a high power density of 0.85 W cm3 at current density of 5 mA cm2. VN thin film electrodes have several advantages compared with other kind of electrodes, because there is no need to add additives to increase the electronic conductivity, nor binder to improve the mechanical stability. Lucio-Porto et al. prepared thin films of VN with different thickness by D.C. reactive magnetron sputtering. Crystalline films with a preferential growth in the direction (111) were obtained [148]. Thin films with a thickness of 25 nm showed the highest specific capacitance (422 F g1) in 1 M KOH electrolyte. Using a combination of electrostatic spinning and high temperature calcination in ammonia, Xu et al. prepared onedimensional vanadium nitride nanofibers [149]. The cross-linked nanofibers composed of nanoparticles construct a facile transport path for charge and electrolyte ion. Moreover, vanadium nitride nanoparticles encapsulated into carbon prevent grain growth and

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Fig. 5. (a) porous VN nanorods; (b) SEM image of VN powders; (c) VN/CNT 3D array; (d) VN-MWCNT composite; (e) [email protected] NWs; (f) Mesoporous Coaxial Titanium NitrideVanadium Nitride Fibers. (a) reprinted with permission from Ref. [145]. Copyright 2010, American Chemical Society; (b) reprinted with permission from Ref. [146]. Copyright 2009, Elsevier; (c) reprinted with permission from Ref. [154]. Copyright 2011, American Chemical Society; (d) reprinted with permission from Ref. [156]. Copyright 2014, Elsevier; (e) reprinted with permission from Ref. [158]. Copyright 2015, Wiley; (f) reprinted with permission from Ref. [165]. Copyright 2011, American Chemical Society.

aggregation, which provided more active sites for electrolyte ion. Owing to this unique structure, vanadium nitride nanofibers exhibited high specific capacitance of 291.5 F g1 at 0.5 A g1 and rate capability with a capacitance of 105.1 F g1 at 6 A g1. Similar to Xu's electrostatic spinning and high temperature calcination in ammonia, Zhao et al. successfully synthesized porous vanadium nitride (VN) hollow fibers via using low-cost starting materials [150]. The VN hollow fibers retained their 1D texture, and their side walls consisted of numerous porous nanoparticles. The electrochemical performance of VN hollow nanofibers was investigated, and the specific capacitance was 115 F g1 at a current density of 1 A g1 in 2 M KOH electrolyte. Recently, Xie's group synthesized mesocrystal nanosheets (MCNSs) of vanadium nitride (VN) via a confined-growth route from thermally stable layered vanadium bronze, representing the first two-dimensional (2D) metallic mesocrystal in inorganic compounds. Benefiting from their single-crystalline-like longrange electronic connectivity, VN MCNSs delivered an electrical

conductivity of 1.44  105 S m1 at room temperature, among the highest values observed for 2D nanosheets [151]. Coupled with their unique pseudocapacitance, VN MCNS-based flexible supercapacitors afford a superior volumetric capacitance of 1937 mF cm3. Nitride MCNSs should have wide applications in the energy storage and conversion fields because their intrinsic high conductivity is coupled with the reactivity of inorganic lattices. Through direct oxidation of metallic vanadium in vacuo, Bondarchuk et al. synthesized pure oxygen-free vanadium nitride films with thickness from 1 nm to 400 nm [152]. These films can deliver a surface capacitance of 3 mF cm2 at a potential scan rate of 3 mV s1 and 2 mF cm2 at a potential scan rate of 1 V s1 in basic (1 M KOH, 1 M LiOH) or neutral (Li2SO4, K2SO4) electrolytes while no redox reactions can be assigned to the charge/discharge process. 3.2. Vanadium nitride/compound-carbon material composites The poor electrochemical stability of VN causes severe

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capacitance loss during charging/discharging cycling process. Eventually, this problem can be solved by the formation of a nanocomposites, most favorably with carbon from a concurrent synthesis approach that generates and controls all synthetic components at the same time. From previous experience, the presence of high purity CNTs in the active electrode could improve the electrochemical properties of VN in supercapacitors. Through the sol-gel synthesis of organic or inorganic vanadium oxide precursors followed by temperature programmed ammonia reduction, Ghimbeu et al. developed nanostructured vanadium nitride/multiwalled carbon nanotubes (VN/CNTs) composites for pseudocapacitor applications [153]. Nitrogen adsorption and impedance spectroscopy measurements showed that the incorporation of CNTs during VN synthesis allows VN/CNTs nanocomposites to be obtained with higher porosity, narrower pore size distribution, better conductivity and improved electrochemical properties compared to VN without CNTs. In particular, cyclic voltammetry using three-electrode cells in KOH showed that the contribution of the redox peaks was increased when VN is associated with the carbon nanotubes. As a consequence, a capacitance increase was measured in the two-electrode system. Another important advantage of using VN/CNTs composites was their high capacitance retention (58%) at high current density (30 A g1) compared with VN (7%). Carbon nanotube arrays have attracted great interest as threedimensional current collector materials for promoting supercapacitive. Zhang et al. demonstrated a simple, direct synthesis methodology to achieve three-dimensional arrays of carbon nanotube vanadium nitride nanostructures, consisting of multiwalled carbon nanotubes covered by nanocrystalline vanadium nitride, firmly anchored to glassy carbon or inconel electrodes (shown in Fig. 5c) [154]. These nanostructures demonstrated a respectable specific capacitance of 289 F g1, which was achieved in 1 M KOH electrolyte at a scan rate of 20 mV s1. The well-connected highly electrically conductive structures exhibited a superb rate capability; at a very high scan rate of 1000 mV s1 there is less than a 20% drop in the capacitance relative to 20 mV s1. Binder-free films composed of active materials and CNTs lead to flexible hybrid electrodes, which demonstrated the potential for SCs. Xiao and co-workers fabricated light weight, thin, and flexible freestanding mesoporous VN nanowires (MVNNs)/CNT hybrid electrodes via a simply vacuum-filtering method, which sufficiently utilized the synergistic effects from the high electrochemical performance of MVNNs and the high conductivity and mechanical consolidation of the CNTs [155]. High performance all-solid-state flexible SCs were constructed based on freestanding MVNN/CNT hybrid electrodes with H3PO4/poly (vinyl alcohol) (PVA) as electrolyte. The whole device (Including electrodes, separator and electrolyte) was only 15 mg, exhibiting a high volume capacitance of 7.9 F cm3 and energy and power density of 0.54 mWh cm3 and 0.4 W cm3 at a current density of 0.025 A cm3. Zhitomirsky et al. prepared composite materials, containing fibrous VN nanoparticles and multi-walled carbon nanotubes (MWCNT) (shown in Fig. 5d) via a chemical method for application in electrochemical supercapacitors [156]. They demonstrated for the first time that VN-MWCNT electrodes exhibited good capacitive behavior in 0.5 M Na2SO4 electrolyte in a negative voltage window of 0.9 V. Quartz crystal microbalance studies provide an insight into the mechanism of charge storage. Composite VN-MWCNT materials showed significant improvement in capacitance, compared to individual VN and MWCNT materials. Testing results indicate that VN-MWCNT electrodes exhibited high specific capacitance at high mass loadings in the range of 10e30 mg cm2, good capacitance retention at scan rates in the range of 2e200 mV s1 and good cycling stability. The highest specific capacitance of 160 mV s1 was

achieved at a scan rate of 2 mV s1. Increasing the surface areas of the nanomaterials can improve his capacity. Using simple salts as porogens, Fechler et al. prepared composites of highly porous nitrogen-doped carbons with functional vanadium nitride nanoparticles with tunable surface area, pore size, pore volume and nanoparticle size was presented [157]. Cesium acetate as a porogen at low concentrations resulted in microporous materials with small VN nanoparticles with a surface area of around 1000 m2 g1, while increasing salt amounts promote small mesopores with bigger nanoparticles and surface areas of up to 2400 m2 g1. By virtue of their potential applications for flexible supercapacitors, the researches of flexible 3D nano-architectures have attracted tremendous interest. Gao et al. developed 3D intertwined nitrogen-doped carbon encapsulated mesoporous vanadium nitride nanowires ([email protected] NWs) (shown in Fig. 5e), which are investigated as thin, lightweight, and self-supported electrodes for flexible supercapacitors (SCs) [158]. The MVN NWs have abundant active sites accessible to charge storage, and the N-doped carbon shell suppresses electrochemical dissolution of the inner MVN NWs in an alkaline electrolyte, leading to excellent capacitive properties. The flexible [email protected] NWs film electrode delivered a high areal capacitance of 282 mF cm2 and exhibited excellent long-term stability with 91.8% capacitance retention after 12,000 cycles in a KOH electrolyte. All-solid-state flexible SCs assembled by sandwiching two flexible [email protected] NWs film electrodes with alkaline poly(vinyl alcohol) (PVA), sodium polyacrylate, and KOH gel electrolyte boasted a high volumetric capacitance of 10.9 F cm3, an energy density of 0.97 mWh cm3, and a power density of 2.72 W cm3 at a current density of 0.051 A cm3 based on the entire cell. Recently, Wang et al. prepared 3D Porous VN nanowiresgraphene composite as a superior anode for high-performance hybrid supercapacitors [159]. The d 3D VN-RGO composite exhibits the large Li-ion storage capacity and fast charge/discharge rate within a wide working widow from 0.01 to 3 V (vs Li/Liþ), which could potentially boost the operating potential and the energy and power densities of Li-ion hybrid capacitors (LIHCs). By employing such 3D VN-RGO composite and porous carbon nanorods with a high surface area of 3343 m2 g1 as the anode and cathode, respectively, a novel LIHCs is fabricated with an ultrahigh energy density of 162 Wh kg1 at 200 W kg1, which also remains 64 Wh kg1 even at a high power density of 10 kW kg1. Balamurugan et al. fabricated novel vanadium nitride/nitrogendoped graphene (VN/NG) composite for stable high performance anode materials for supercapacitors [160]. The VN/NG composite anode material exhibited excellent rate capability, outstanding cycling stability, and superior performance. The NG provided a highly conductive network to boost the charge transport involved during the capacitance generation and also aided the dispersion of nanostructured VN within the NG network. The synergetic VN/NG composite exhibited an ultra-high specific capacitance of 445 F g1 at 1 Ag1 with a wide operation window (1.2 to 0 V) and showed outstanding rate capability (98.66% capacity retention after 10 000 cycles at 10 Ag1). The VN/NG electrode offered a maximum energy density (~81.73 Wh kg1) and an ultra-high power density (~28.82 kW kg1 at 51.24 Wh kg1). By the ammonification process of ionic amphiphilic triblock copolymer micelles/vanadium-contained ions system in NH3/N2 atmosphere, Liu et al. reported an in-situ preparation approach of vanadium nitride nanoparticles on porous carbon nanospheres ([email protected]) [161]. The prepared [email protected] material had a wide operating potential of 1.2 V, and a capacitance of 229.7 F g1. A hybrid supercapacitor device of [email protected]//NiO exhibited a high energy density of 16 Wh kg1 at the power density of 800 W kg1.

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3.3. Vanadium nitride/titanium nitride composites Binary metal nitrides have been studied as EC electrodes and show a good capacitive behavior in aqueous electrolytes. Among nitride materials, TiN holds great promise as an electrode material for SCs, due to its superior electrical conductivity and mechanical stability. Incorporating VN and TiN into an efficiently fast mixed (electron and ion) transportation nanocomposites can be expected to deliver the ingredients for efficient charge transportation and electrochemical energy storage [162e164]. By the coaxial electrospinning, and subsequently annealed in the ammonia, Zhou et al. prepared titanium nitride-vanadium nitride fibers of core-shell structures (Fig. 5f) for supercapacitor applications [165]. These core-shell (TiN-VN) fibers incorporated mesoporous structure into high electronic conducting transition nitride hybrids, which combined higher specific capacitance of VN and better rate capability of TiN. These hybrids exhibited higher specific capacitance (2 mV s1, 247.5 F g1) and better rate capability (50 mV s1, 160.8 F g1). That same year, Dong et al. prepared TiN/VN core-shell composites by a two-step strategy involving coating of commercial TiN nanoparticles with V2O5$nH2O sols followed by ammonia reduction [166]. The highest specific capacitance of 170 mV s1 was obtained when scanned at 2 mV s1 and a promising rate capacity performance is maintained at higher voltage sweep rates. Pang and co-workers hybridized core-shell-structured metalnitride ([email protected]) nanowires with a 3D carbon substrate [167]. This hybrid electrode demonstrates high volumetric capacitance and good cycling stability The high capacitance is attributed to the exceptional electrochemical properties of TiN and VN, as well as the vast surface area and conductive network provided by the microporous 3D carbon structure. The improved stability results from preventing transformation of TiN into poorly conductive TiO2 by the thin shell of VN.

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Recently, Zhang's group fabricate ternary vanadium titanium nitride/carbon (VTiN/C) nanofibers through a facile electrospinning strategy and investigate their electrochemical performance for the first time [164]. The obtained well-interconnected VTiN/C nanofibers with VTiN nanoparticles embedded into carbon ensure rapid electron/ion transfer and offer a highly ion-accessible surface. Appealingly, the VTiN-4/C nanofibers exhibited a greatly improved performance, with a high specific capacitance (430.7 F g1, 0.5 A g1) and a good rate capability (141.7 F g1, 10 A g1). 4. Vanadium sulfide Nowadays, As one of the most popular vanadium based materials in recent studies, Due to the vanadium sulfides having an even higher electrical conductivity and Li ion diffusion rate than vanadium oxides, people's focus is mainly on VS2, V3S4 and their composites. 4.1. Vanadium disulfide As a kind of layered transition-metal dichalcogenides (TMDs), VS2 has been successfully established as a new paradigm in the chemistry of nanomaterials especially for nanotubes and fullerenelike nanostructures as well as the graphene analogues during the past decades. Feng et al. developed new 2D VS2 graphene (shown in Fig. 6a) analogue with less than five SVS atomic layers can be applied directly to the assembly of highly c-oriented VS2 thin films with synergic advantages of high conductivity and 2D permeable channels, thereby opening the door to design practical in-plane supercapacitors for the power sources in advanced ultrathin electronics [168]. Electrochemical characterization revealed a considerable specific electric capacitance of 4760 mF cm2 and an excellent cycling behavior even after 1000 charge/discharge cycles for this inplane supercapacitor.

Fig. 6. (a) SEM image of VS2 nanosheets; (b) V3S4/3DGH electrode; (c) FESEM images of SVS nanospheres anchored by PANI matrix NCs. (a) reprinted with permission from Ref. [168]. Copyright 2011, American Chemical Society; (b) reprinted with permission from Ref. [169]. Copyright 2015, American Chemical Society; (c) reprinted with permission from Ref. [170]. Copyright 2013, The Royal Society of Chemistry.

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4.2. Vanadium tetrasulfide Among vanadium sulfides, V3S4 has a unique distorted Ni-As type structure with ordered metal vacancies in alternate metal layers, which enhances the electrical conductivity of V3S4. Zhai et al. demonstrated that V3S4 was an exceptional pseudocapacitive anode material, which serves not only as a good power source (high electrical conductivity) but also as a good energy source (high capacitance) for the assembled AEC device [169]. By coupling the V3S4/3DGH (shown in Fig. 6b) anode and MnO2/3DGH cathode, it delivered a remarkable energy density of 7.4 Wh kg1 (based on the weight of entire device) at the average power density of 3000 W kg1. 4.3. Silver vanadium sulfide Diggikar et al. synthesized silver vanadium sulfide (SVS) anchored by PANI matrix nanocomposite (NC) (shown in Fig. 6c) via in situ polymerization of aniline [170]. For the preparation of NC, aniline, silver nitrate, ammonium metavanadate and thiourea (TU) were used as the precursors. SVS NPs of size 10e20 nm were anchored in the PANI matrix. The capacitance of PANI-anchored SVS was 440 F g1 and that of PANI is found to be 128 F g1. 5. Mixed metal vanadates Recently, Double metal oxides of vanadium as supercapacitor and hydrogen storage material have shown encouraging results. Mixed metal vanadates (MmV) is one of the most important families of nanomaterials with various intriguing properties such as optical, catalytic, magnetic, LIB material and supercapacitors.

Using a facile and template free method, Butt et al. synthesized novel hierarchical nanospheres (NHNs) of ZnV2O4 (shown in Fig. 7a) [171]. The electrochemical measurements were performed in 2 M KOH solution. The measured specific capacitance of ZnV2O4 electrode was 360 F g1 at 1 A g1 with good stability and retention capacity, which was still 89% after 1000 cycles. Vijayakumar et al. Zn3V2O8 synthesized nanoplatelets using a hydrothermal method [172]. The prepared Zn3V2O8 nanoplatelets were further studied for their potential application in supercapacitors. The Zn3V2O8 nanoplatelets exhibited a maximum specific capacitance of 302 F g1 at a scan rate of 5 mV s1. Furthermore, a Zn3V2O8 electrode retained about 98% of its initial specific capacitance after 2000 cycles. In order to compare the electrochemical performance of different mixed metal vanadates, Liu et al. designed and synthesized Ni3V2O8, Co3V2O8, and the Ni3V2O8/Co3V2O8 nanocomposite (shown in Fig. 7b) as a new class of high performance electrode material for supercapacitors [173]. Ni3V2O8 and Co3V2O8 showed a structure consisting of nanoflakes and nanoparticles, respectively. The Ni3V2O8/Co3V2O8 nanocomposite was prepared by growing Co3V2O8 nanoparticles on the surface of Ni3V2O8 nanoflakes. The composite inherited the structural characteristics and combines the pseudocapacitance's benefited of both Ni3V2O8 and Co3V2O8, showing higher specific capacitance than Co3V2O8 and superior rate capability as well as better cycle stability to Ni3V2O8. Zhang et al. firstly described Co3V2O8 thin nanoplates (shown in Fig. 7c) as a kind of electrode material for supercapacitors [174]. More importantly, from electrochemical measurements, the obtained Co3V2O8 nanoplate electrode showed a good specific capacitance (0.5 A g1, 739 F g1) and cycling stability (704 F g1 retained after 2000 cycles).

Fig. 7. (a) SEM image of ZnV2O4 hierarchical nanospheres; (b) Ni3V2O8/Co3V2O8 nanocomposite; (c) FESEM images of Co3V2O8 nanoplate; (d) FESEM images of Ni3(VO4)2 nanospheres. (a) reprinted with permission from Ref. [171]. Copyright 2014, American Chemical Society; (b) reprinted with permission from Ref. [172]. Copyright 2014, The Royal Society of Chemistry; (c) reprinted with permission from Ref. [174]. Copyright 2014, Nature; (d) reprinted with permission from Ref. [177]. Copyright 2016, The Royal Society of Chemistry.

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Using a combination of a hydrothermal strategy and subsequent annealing treatment, Kong's group synthesized a novel selfsupported electrode of nickel vanadate and nickel oxide nanohybrid on nickel foam with excellent pseudocapacitive properties [175]. The electrode had an energy density of 46 W h kg1 at a power density of 101 W kg1, demonstrating the importance and great potential of nickel vanadate in the development of supercapacitors. Among the mixed metal vanadates, sodium-vanadate-doped material is believed to be a promising candidate for supercapacitor. Zhang et al. prepared sodium-vanadate-doped ordered mesoporous carbon foams (V-MCFs) via an evaporation-induced self-assembly strategy [176]. The resultant V-MCFs exhibited highly ordered mesostructure with specific surface areas of 714 m2 g1 and uniform pore sizes of 4.1 nm. By a hydrothermal method without using any template, Kumar et al. synthesized urchin-shape Ni3(VO4)2 hollow nanospheres (shown in Fig. 7d) [177]. The as-fabricated porous urchin-shaped Ni3(VO4)2 nanosphere electrode exhibited a specific capacity of 402.8 C g1 at 1 A g1 with enhanced rate capability and an excellent capacity retention of 88% after 1000 cycles. An asymmetric supercapacitor was fabricated using Ni3(VO4)2 nanospheres as the cathode and activated carbon as the anode and the electrochemical properties were studied at various scan rates in the potential range of 0.0e1.6 V. The as-fabricated asymmetric supercapacitor (Ni3(VO4)2//AC) achieved a high specific capacity (114 C g1), energy density (25.3 Wh kg1) and power density (240 W kg1). Moreover, this asymmetric supercapacitor displayed an excellent life cycle with 92% specific capacity retention after 1000 consecutive charge-discharge cycles. Recently, Yan et al. reported the synthesis of amorphous aluminum vanadate hierarchical microspheres via a simple hydrothermal approach with polyvinylpyrrolidone as a surface directing agent [178]. The measured specific capacitance of the amorphous aluminum vanadate electrode was 497 F g1 at 1 A g1 with good stability and a retention capacity of 89% after 10,000 cycles. In addition, the fabricated asymmetric supercapacitor device delivered better performance with an extended operating voltage window of 1.5 V, excellent cycle stability (10,000 cycles, 85% capacitance retention), high energy density (37.2 Wh kg1 at 1124.4 W kg1) and high power density (11,250 W kg1 at 25 Wh kg1). 6. Vanadyl phosphate As a kind of layered materials with wide application prospect, vanadyl phosphate (VOPO4) has good electrochemical performance. Due to the enhanced inionicity of (VeO) bonds when (PO4)3- anion is introduced, V4þ/V5þ redox couple of VOPO4

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possesses the higher potential than that for simple vanadium oxide. However, it is seldom used in pseudocapacitors because VOPO4 also has an intrinsic high electrical resistance (~3.0  107 U cm1) and the layered structure of bulk VOPO4 has limited surface area, which lowers the power density of electrochemical devices [179e181]. Ultrathin two-dimensional graphene and graphene analogue material showed a strong advantage in the construction of a flexible supercapacitor. Wu et al. developed an inorganic graphene analogue, two-dimensional vanadyl phosphate ultrathin nanosheets (shown in Fig. 8a) with less than six atomic layers, as a promising material to construct a flexible ultrathin-film pseudocapacitor in all-solid-state [179]. The material showed a high potential plateau of 1.0 V in aqueous solutions, approaching the electrochemical potential window of water (1.23 V). The asestablished flexible supercapacitor had a high redox potential (1.0 V) and a high areal capacitance of 8360.5 F cm2, leading to a high energy density of 1.7 mWh cm2 and a power density of 5.2 mW cm2. Subsequently, Lee et al. developed an a simple ice-templated self-assembly process which is used to prepare a threedimensional (3D) and vertically porous nanocomposite of layered vanadium phosphates (VOPO4) and graphene nanosheets (shown in Fig. 8b) with high surface area and high electrical conductivity [180]. The resulting 3D VOPO4-graphene nanocomposite had a much higher capacitance of 527.9 F g1 at a current density of 0.5 A g1, compared with 247 F g1 of simple 3D VOPO4, with solid cycling stability. It exhibited a wide cell voltage of 1.6 V and a largely enhanced energy density of 108 Wh kg1. In order to study the influence of different preparation methods on the properties of VOPO4$2H2O, Luo et al. carried out a systematic comparison of VOPO4$2H2O prepared by reflux and hydrothermal methods [181]. They found that the material synthesized by reflux method had better performance in the capacitance than that by hydrothermal method, and its specific capacitance was up to 202 mV s1 at 2 mV s1. A total of 67.4% of capacitance was maintained for the VOPO4$2H2O synthesized by reflux method when current density changed from 0.2 to 2 A g1, much higher than those obtained from the hydrothermal-synthesized VOPO4$2H2O supercapacitor of 42.5%. The energy density of VOPO4$2H2O supercapacitor synthesized by reflux method was 18.7 W kg1 at 290 W kg1 and maintained 6.2 Wh kg1 at 1.421 W kg1, much higher than the hydrothermal-synthesized VOPO4$2H2O supercapacitor. He et al. prepared the VOPO4/GO layered hybrid material by a controllable nanosheet reassemble technology between VOPO4 nanosheets and GO nanosheets at room temperature on the basis of the investigation of VOPO4 nanosheet structural stability, then it was calcinated in a tubular furnace at 400  C for 3 h under N2 atmosphere, GO was successfully converted into RGO while

Fig. 8. (a) SEM image of 2D vanadyl phosphate ultrathin nanosheets; (b) SEM image of 3D VOPO4-graphene nanocomposite. (a) reprinted with permission from Ref. [179]. Copyright 2013, Nature; (b) reprinted with permission from Ref. [180]. Copyright 2015, Nature.

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VOPO4$2H2O into VOPO4, and VOPO4/RGO layered hybrid material was obtained [182]. The VOPO4/RGO hybrid electrode with a mass ratio of VOPO4/RGO ¼ 1 exhibited a high specific capacitance of 378 F g1 at a scan rate of 5 mV s1 with a good rate capability. 7. Conclusions and outlooks All in all, this paper mainly introduces the application of vanadium based compounds including vanadium oxide, vanadium nitride, vanadium sulfide, mixed metal vanadates, vanadyl phosphate, and their composite materials in the supercapacitors. Their synthesis methods, micro structure and electrochemical properties are introduced. As a kind of pseudocapacitive materials vanadium based compounds have a wide range of applications, but their poor electrical conductivity, poor cycling stability, low specific capacitance and low energy limit its application. In order to overcome these defects, controlling to prepare micro/nano-structure materials, the development of new type of control vanadium based composite materials and the development of asymmetric supercapacitors will become the focus of future research. The details are as follows: (1) The research shows that some vanadium based compounds with special micro/nano-structure have good surface area, high electrical conductivity and good cycling stability. But the method of synthesizing these compounds is too complicated and expensive, and it is not high in the synthesis of the material. Therefore, it is a trend to adopt a simple and controllable method to synthesize vanadium based pseudocapacitor materials. (2) Developing composite materials is an important way to improve the electrical conductivity and the stability of the vanadium based compounds. At present, vanadium based materials and electric double layer capacitor (activated carbon, carbon nanotubes, graphene, etc.), other materials, such as conducting polymer (polyaniline, polypyrrole), other metal oxides (manganese oxide, cobalt oxide, etc.) has become a very hot research area. (3) Achieving high energy and power density synchronously can be achieved by constructing asymmetric capacitor. The electrochemical performance of asymmetric supercapacitor, such as high energy density, is obviously better than that of the symmetric supercapacitor, which is the hot spot and direction of the study of the supercapacitor. It is promising that such high-rate capability electrodes are of great interest when coupled with a capacitive carbon negative electrode to design asymmetric (hybrid) devices with improved rate capability [183e188]. Acknowledgements This work was supported by the Program for New Century Excellent Talents of the University in China (grant no. NCET-130645) and the National Natural Science Foundation of China (NSFC21201010, 21173183, 21505118 and 51202106), Innovation Scientists and Technicians Troop Construction Projects of Henan Province164200150018, Program for Innovative Research Team (in Science and Technology) in University of Henan Province (14IRTSTHN004, 16IRTSTHN003), the Science & Technology Foundation of Henan Province (122102210253 and 13A150019), the Science & Technology Foundation of Jiangsu Province (BK20150438), 333 Project of Jiangsu Province (Grant BRA2011188), the Six Talent Plan (2015XCL-030), and the China Postdoctoral Science Foundation (2012M521115). We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and

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VK: Ketjin black PEDOT: Poly(3,4-ethylenedioxythiophene) PEDOP: Poly(3,4-ethylenedioxypyrrole) LVO: Layered V2O5 LMO: Layered MnO2 EDL: Electric double-layer RG: Graphene UGO: Ultralarge graphene oxide HMB: Hydrogen molybdenum bronze GF: Graphene foam CTAB: Hexadecyltrimethylammonium bromide ASCs: Asymmetric supercapacitors CVD: Chemical vapor deposition MCNSs: Mesocrystal nanosheets PVA: Poly(vinyl alcohol) LIHCs: Li-ion hybrid capacitors VACNT: Vertically aligned carbon nanotubes PVP: Polyvinylpyrrolidone MAHS: Microwave-assisted hydrothermal synthesis GNS: Graphene nanosheets HDA: 1-hexadecylamine TMDs: Transition-metal dichalcogenides NC: Nanocomposite MmV: Mixed metal vanadates MWNT: Multi-walled nanotube CNTs: Carbon nanotubes NWs: Nanowires 1D: 1-dimensional 2D: 2-dimensional 3D: 3-dimensional 3DGN: Three-dimensional graphene network V-MCFs: Sodium-vanadate-doped ordered mesoporous carbon foams

Yan Yan is now a Ph.D. candidate under Professor Huan Pang's and Huaiguo Xue's supervision, Yangzhou University of chemistry and chemical engineering, China. His research mainly focuses on the field of electrochemistry, including inorganic semiconductors nanostructures, conducting polymer and their applications for energy devices.

Abbreviation index SCs: Supercapacitors EDLCs: Electric double layer capacitors ECs: Electrochemical capacitors ESR: Equivalent series resistance MOs: Metal oxides 1D: One-dimensional LBL: Layer by layer assembly CFP: Carbon fiber paper CV: Cyclic voltammetry RGO: Reduced graphene oxide GO: Graphene oxide SCE: Saturated calomel electrode VNN: V2O5 Nanoporous Network TNFs: Tubular nanofibers VNF: V2O5 nanofibers GVNBs: Graphene-decorated V2O5 nanobelts FTO: Fluorine doped Tin Oxide PEG-6000: Polyethyleneglycol 6000 SDBS: Sodium dodecylbenzene sulfonate P123: Pluronic P-123 0D: Zero-dimensional 1D: One-dimensional CNT: Carbon nanotube ECF: Electrospun carbon fibers MNR: MnO2 nanorodes VNW: V2O5 nanowires PAN: Polyacrylonitrile PANI: Polyaniline PPy: Polypyrrole VNBs: V2O5 nanobelts rGO: Reduced graphene oxide ASSTFSs: All-solid-state thin-film supercapacitors f-MWCNT: Functionalized multiwalled carbon nanotube CNFCs: Carbon nanofiber composites Specific surface area: SSA Single walled carbon nanotube-activated carbon: SWCNT-AC ALD: Atomic layer deposition ACNF: Activated carbon nanofiber VNW: V2O5 nanowire

Bing Li is now a graduate student under Professor Huan Pang's supervision, Yangzhou University of chemistry and chemical engineering, China. Her research mainly focuses on the field of inorganic semiconductors nanostructures and their applications for supercapacitors.

Wei Guo is an Assistant Professor at Anyang Normal College. She obtained her B.E. (2004) in Chemical Engineering and M.S. (2007) in Analytical Chemistry from Zhengzhou University, and Ph.D. (2014) in Inorganic Chemistry from Nankai University in China. Her research interests are in the area of materials for Li-ion batteries and thermoelectric devices, including novel synthesis approaches for nanomaterials. She has authored 13 journal articles.

Y. Yan et al. / Journal of Power Sources 329 (2016) 148e169 Huan Pang received his Ph. D. degree from Nanjing University in 2011. He then founded his research group in Anyang Normal University where he was appointed as a distinguished professor in 2013. He has now jointed Yangzhou University as a university distinguished professor. He has published more than 90 papers in peerreviewing journals including Chemical Society Reviews, Advanced Materials, Energy Environ. Sci., with 2800 citations (H-index ¼ 30). His research interests include the development of inorganic nanostructures and their applications in flexible electronics with a focus on energy devices.

169 Huaiguo Xue received his Ph.D. degree in polymer chemistry from the Zhejiang University in 2002. He is currently a professor of physical chemistry and the dean of the College of Chemistry and Chemical Engineering at the Yangzhou University. His research interests focuses on electrochemistry, functional polymer and biosensors.