Mesoporous Hybrids of Reduced Graphene Oxide and Vanadium

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Mesoporous Hybrids of Reduced Graphene Oxide and Vanadium Pentoxide for Enhanced Performance in Lithium-ion Batteries and Electrochemical Capacitors Gaind P. Pandey, Tao Liu, James Emery Brown, Yiqun Yang, Yonghui Li, Xiuzhi Susan Sun, Yueping Fang, and Jun Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02372 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on March 27, 2016

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ACS Applied Materials & Interfaces

Mesoporous Hybrids of Reduced Graphene Oxide and Vanadium Pentoxide for Enhanced Performance in Lithium-ion Batteries and Electrochemical Capacitors Gaind P. Pandeya, †, Tao Liua, †, Emery Browna, Yiqun Yanga, Yonghui Lib, Xiuzhi Susan Sunb, Yueping Fanga, c, and Jun Lia,* a b

Department of Chemistry, Kansas State University, Manhattan, KS 66506, United States

Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66502, United States c

The Institute of Biomaterial, College of Materials and Energy, South China Agricultural University, Guangzhou, Guangdong 510642, China.

ABSTRACT Mesoporous hybrids of V2O5 nanoparticles anchored on reduced graphene oxide (rGO) have been synthesized by slow hydrolysis of vanadium oxytriisopropoxide using a two-step solvothermal method followed by vacuum annealing. The hybrid material possesses a hierarchical structure with 2030 nm V2O5 nanoparticles uniformly grown on rGO nanosheets, leading to a high surface area with mesoscale porosity. Such hybrid materials present significantly improved electronic conductivity and fast electrolyte ion diffusion, which synergistically enhance the electrical energy storage performance. Symmetrical electrochemical capacitors with two rGO-V2O5 hybrid electrodes show excellent cycling stability, good rate capability, and a high specific capacitance up to ~466 F g-1 (regarding the total mass of V2O5) in a neutral aqueous electrolyte (1.0 M Na2SO4). When used as the cathode in Lithium-ion batteries, the rGO-V2O5 hybrid demonstrates excellent cycling stability and power capability, able to deliver a specific capacity of 295 mAh g-1, 220 mAh g-1, and 132 mAh g-1 (regarding the mass of V2O5) at a rate of C/9, 1C, and 10C, respectively, matching the full theoretical capacity of V2O5 for reversible 2 Li+ insertion/extraction between 4.0 and 2.0 V (vs Li/Li+). It retains ~83% of the discharge capacity after 150 cycles at 1C rate, with only 0.12% decrease per cycle. The enhanced performance in electrical energy storage reveals the effectiveness of rGO as the structure template and more conductive electron pathway in the hybrid material to overcome the intrinsic limits of single-phase V2O5 materials. 1 ACS Paragon Plus Environment


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KEYWORDS: Reduced graphene oxide, vanadium pentoxides, hierarchical hybrids, Li-ion batteries, supercapacitors, electrical energy storage INTRODUCTION Electrochemical energy storage (EES) systems have been widely used for powering today's portable electronic devices and will play a crucial role in supporting future electric vehicles and technologies that utilize energy harvested from intermittent renewable sources such as sunlight and wind.1-4 The increasing energy consumption demands EES systems with higher energy density and higher power capability. Lithium ion batteries (LIBs) and electrochemical capacitors (also known as supercapacitors) are two important EES devices that possess distinct characteristics with high energy and high power (i.e. high charge-discharge rates), respectively5-6. An active research topic is to develop novel electrode materials with combined high performance in energy capacity, charge-discharge rates, and cycling stability. Currently, the capacity and rate capability of cathode materials are the main limiting factors of LIBs, and hence, new cathode materials have been extensively studied in recent years.7-8 Vanadium oxides have been extensively studied for several decades and have been identified as potential electrode materials for LIB cathodes and supercapacitors.9-16 Particularly, vanadium pentoxide (V2O5) has emerged as an attractive sustainable supercapacitor material with fast charge-discharge rates and high specific capacity due to its low cost, easily accessible layered structure for electrolyte ions, variable oxidation states (V3+, V4+, and V5+), ease in synthesis, and nontoxic chemical properties.17 Several promising 3-D porous structures have demonstrated high specific capacitances (up to ~400 F g1

) at low scan rates (≤ 5 mV s-1).9-10 However, the performance at higher rates and the long-time cycling

stability remain to be improved. Vanadium pentoxide is also attractive cathode materials for LIBs due to its high specific capacity and the ease to accommodate Li+ ions in its layered structure.18 Theoretically, it can provide a specific 2 ACS Paragon Plus Environment

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capacity of 442 mAh g-1 (for three Li+ ion insertion/extraction) or 294 mAh g-1 (for two Li+ ion insertion/extraction),19 significantly higher than the maximal values for today’s commercial cathode materials such as LiCoO2 (140 mAh g-1), LiMn2O4 (148 mAh g-1) and LiFePO4 (170 mAh g-1).20 In addition, vanadium is inexpensive and abundant in the crust of the earth, which makes it sustainable for large-scale production. However, it has been very challenging to achieve the theoretical properties of V2O5 at reasonable rate and cycle life in practical EES devices.12, 21-22 The performance is limited by its intrinsic materials properties such as low electronic conductivity (~ 10-3-10-5 S cm-1), slow ion insertion rate as defined by the very low Li+ ion diffusion coefficient (~10-15-10-12 cm2 s-1), and irreversible phase transitions upon deep Li+ intercalation.21, 23 It is known that the Li+ intercalation processes in crystalline V2O5 are accompanied by multiple phase transitions. Trace amounts of Li+ intercalation result in αLixV2O5 (x < 0.01), which is transformed into ε-LixV2O5 (0.35 < x < 0.7) after further lithiation. Insertion of exactly one Li+ leads to the formation of δ-phase LixV2O5 (x = 1), but further lithiation converts it into γ-LixV2O5 (1 < x < 2). At more than two Li+ insertion, irreversible transformation of γ-LixV2O5 to rocksalt type ω-LixV2O5 (2 < x < 3) phase occurs.24 Such phase transition processes induce large lattice strains due to coexistence of different phases within single particles. The lattice mismatch-induced mechanical strain can cause irreversible structural damage resulting in poor battery lifetime and irreversible capacity loss.21 Extensive attempts have been made to overcome these intrinsic limits of V2O5 materials. Two main strategies are enhancing the overall diffusion rate of Li+ ions and improving electrical conduction of the electrode. The first strategy is through the synthesis of V2O5 materials with low dimensional nanostructures such as nanofibers25, nanobelts26, nanosheets14, nano/microspheres24, 27-28 and 3D porous nanosheets assembly.9 This strategy provides larger specific surface area (SSA) and short ion diffusion pathways in solids, which offers more electrochemically active sites and alleviates the concentration polarization of electrode materials. However, the poor electrical contact between V2O5 nanoparticles 3 ACS Paragon Plus Environment


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makes the electronic conductivity even lower than bulk materials. Moreover, these low dimensional nanostructures have a tendency to aggregate after ~100 cycles, rapidly decreasing the capacity and resulting in a shortened life span of the electrodes. The second strategy is mixing V2O5 with electronically conductive carbonaceous materials such as multi-walled carbon nanotubes29-30, carbon nanofiber31, graphene22, 32, and reduced graphene oxide (rGO)12, 20-21, 33-35, which has shown to improve the overall electrical conductivity of the electrode, provide structural stability, and suppress the irreversible loss associated with phase transitions in vanadium oxides. Among all carbonaceous materials, graphene has attracted tremendous attention. Graphene is a single atomic layer of sp2 bonded carbon atoms arranged in a honeycomb crystal structure which provides the highest theoretical SSA (2600 m2 g-1), excellent in-plane electrical conductivity (104 S cm1

), and remarkable mechanical strength (1060 in Young’s modulus)36. The highly conductive graphene

sheets could provide a fast electron conduction path to collect currents from V2O5. In addition, randomly stacked graphene sheets possess a mesoporous structure with a large electroactive surface, which facilitates formation of a good interface with the electrolyte.22 To improve the processing capability, many composite materials used graphene prepared by chemical or thermal reduction of graphene oxide (GO), i.e. rGO. GO is an oxidized graphene sheet with various oxygenated functional groups, which can be easily dispersed in aqueous solution as single sheets and form stronger interface with oxides or polymers. Even though the electrical conductivity of rGO is not as high as the pristine graphene, being 0.05 to 500 S cm-1 depending on the degree of reduction,37 it is substantially higher than V2O5. Significantly enhanced rate capability and long-term stability of EES devices have been observed by incorporating rGO with V2O512, 33-35 and other oxides38. Here, we report the synthesis of a mesoporous rGO-V2O5 hybrid material by an improved solvothermal method involving two-step hydrolysis of vanadium (V) oxytriisopropoxide (VTIP) precursor on GO nanosheets. A following vacuum annealing process coverts the hydrated product into 4 ACS Paragon Plus Environment

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a mesoporous rGO-V2O5 hybrid with uniform V2O5 nanoparticles (~20-30 nm in size) strongly anchored on the surface of rGO nanosheets. Electrochemical measurements show enhanced performance in longcycling stability and high-power capability with the rGO-V2O5 hybrid material as the supercapacitor electrode and LIB cathode.

EXPERIMENTAL SECTION Synthesis of Reduced Graphene Oxide-V2O5 Composites In synthesis, single-layer graphene oxide (GO) dispersed in ethanol (5 mg/mL) was purchased from ACS Material LLC (Medford, MA, USA) and vanadium (V) oxytriisopropoxide (VTIP) was purchased from Sigma-Aldrich (St. Louis, MO, USA). The GO was prepared by modified Hummer's method with an average flake size of ~ 0.5-2.0 µm and average thickness of ~ 0.6-1.2 nm (with > 80% of single-layer ratio). The synthesis of rGO-V2O5 was carried out by a two-step solvothermal method followed with an annealing process. In the first step, 20 mg of GO was diluted in 50 mL isopropanol and ultrasonicated for 1 h to obtain a homogeneous light brown solution. Then VTIP was added dropwise into the above GO solution to a concentration of 1 mM under magnetic stirring for 2 h. After that, 1 mL deionized water in 30 mL of isopropanol was added slowly in dropwise fashion into the solution under continuous stirring for another 30 min. This step ensures uniform adsorption of sufficient partially hydrolyzed VTIP precursor on the GO surface. In the second step, the resulting mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and kept in an electric oven at 180 oC for 18 h. After that, the autoclave was allowed to cool naturally to room temperature. The resulting composite was precipitated by centrifugation at 5000 rpm, washed several times with absolute ethanol, and dried in a vacuum oven at 60 oC for 12 h. During the solvothermal process, the reduction of GO into rGO and growth of VxOy nanoparticles by slow hydrolysis of VTIP were achieved simultaneously. The final rGOV2O5 hybrids were obtained by further thermal annealing under vacuum in a tube furnace at 300 oC for 5 ACS Paragon Plus Environment


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2 h. The thermal annealing under vacuum played two roles: (1) facilitating further reduction of rGO by desorbing oxygen-containing functional groups attached to the carbon backbones39 and (2) improving V2O5 crystallinity by removing hydrated water. The porosity of the rGO-V2O5 hybrids was found slightly increased by this process. Materials Characterization The morphology of rGO-V2O5 hybrids was characterized with a FEI Nova NanoSEM 430 field emission scanning electron microscope (FESEM) at 3.5 kV and a Philips CM-100 transmission electron microscope (TEM) with an accelerating voltage of 100 kV. Raman spectra were taken by a Thermo Scientific DXR Raman microscope with laser excitation at 532 nm at 5 mW power. X-ray diffraction (XRD) spectra were obtained through a Bruker D8 ADVANCE diffractometer (Germany) using Cu Kα (1.5406 Å) radiation. Thermal gravimetric analysis (TGA) was carried out from room temperature to 700 oC under a dynamic dry nitrogen atmosphere at a heating rate of 10 oC min-1 using a Pyris 1 TGA system (PerkinElmer, USA). The N2 adsorption/desorption isotherms were obtained by a Micromeritics analyzer (Model: Gemini VII 2390t, USA) at 77 K. The Brunauer-Emmett-Teller (BET) specific surface area was calculated using adsorption data at the relative pressure range of 0.05-0.3. The Barrett-JoynerHalenda (BJH) pore size distribution was calculated based on the desorption branch of the isotherm. Electrode Fabrication and Electrochemical Test For electrochemical tests, rGO-V2O5 hybrid electrodes were fabricated by coating an n-methyl2-pyrrolidone (NMP) based slurry with a mixture of 75 wt% of active materials, 10 wt% of super P carbon black (Alfa Aesar, Ward Hill, MA, USA) and 5 wt% of poly(vinylidene fluoridehexafluoropropylene) (PVdF-HFP, MW = 40,000) binder onto a current collector. The rest of the chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA). For supercapacitor electrodes, the slurry was coated on nickel foil whereas for Li-ion battery electrode aluminum foil was used as current collector, both from Alfa Aesar (Ward Hill, MA, USA). The electrode was dried under vacuum 6 ACS Paragon Plus Environment

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at 80 oC overnight. The electrode area was ~1.75 cm2 with active material mass loading of ~1.0 mg for supercapacitor electrodes and ~1.9 mg for Li-ion battery electrodes. Symmetric supercapacitor cells were fabricated in 2032 coin cells with 1.0 M Na2SO4 aqueous liquid electrolyte soaked in a fiberglass separator (El-cell, Germany). Electrochemical data were collected in a two-electrode system by cyclic voltammetry (CV), galvanostatic charge-discharge curves and electrochemical impedance spectroscopy (EIS) using a CHI 760D electrochemical workstation (CH Instruments, Austin, TX). For LIB cathode tests, the electrodes were assembled into 2032 coin cells in an Ar-filled glovebox (MBraun LabStar50, < 0.5 ppm of H2O and < 0.5 ppm of O2). A lithium foil was used as the counter electrode and an electrolyte soaked fiberglass as the separator. An electrolyte solution of 1.0 M lithium hexafluorophosphate (LiPF6) in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (1:1:1 v/v) plus 2% vinylene carbonate (Novolyte, Ohio, USA) was used. The cell performance such as charge-discharge capacity at varied C-rates (a rate of 1C corresponding to completing charge or discharge in one hour) and long-cycling stability under a voltage range of 2.0 - 4.0 V (versus Li/Li+) was examined using an 8-channel battery analyzer (BST8-MA, MTI Corporation, Richmond, CA). CV was performed in the potential range of 2.0 - 4.0 V (versus Li/Li+) at varied scan rates of 0.1, 0.5 and 1 mV s-1. EIS was performed using a CHI 760D electrochemical work station with a 10 mV AC amplitude over a frequency range of 100 kHz to 0.01 Hz at different insertion/extraction potentials (2.2 to 3.5 V versus Li/Li+).

RESULTS AND DISCUSSION Characterization of rGO-V2O5 Composites The rGO-V2O5 hybrids were prepared by two-step hydrolysis of VTIP in isopropanol at room temperature to 180 oC onto the GO sheets as schematically shown in Figure 1. In the first step, the 7 ACS Paragon Plus Environment


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functional groups (e. g. hydroxyl, carboxyl and carbonyl) of GO provide centers for condensation of [VO(OH)3] during the initial slow hydrolysis of VTIP.40 A small amount of [VO(OH)3] was anchored onto the GO surface through the nucleophilic addition of a hydroxyl group followed by a coordination expansion and condensation process resulting in the uniform dispersion of VxOy nanoparticles on GO surface. This VTIP precursor was used because it leads to a higher dispersion of oxide domains compared to other precursors such as ammonium metavanadate.41 The use of isopropanol slowed down the hydrolysis of VTIP, ensuring a uniform dispersion instead of rapid bulk aggregation.20, 33 In the 2nd step, the high temperature and high pressure conditions during the solvothermal process further promote the hydrolysis of VTIP to grow larger VxOy nanoparticles on GO surface. At the meantime, isopropanol is capable of reducing GO to rGO,42 which is necessary to ensure the high conductivity of the composites. To confirm this, GO was subjected to the solvothermal method under similar conditions (at 180 oC) without VTIP. The as-prepared rGO was characterized by TGA, Raman spectroscopy and XRD, and discussed in the supporting information (Figure S1). Clearly, the solvothermal process converted GO into rGO with much better thermally stability, higher ratio between D band and G band Raman peak intensities, and reduced interlayer spacing (3.71 Å vs. 7.08 Å), in good agreement with other rGOs in literature.43-44 The solvothermal-prepared rGO-V2O5 hybrids were annealed in vacuum at 300 oC leading to the formation of uniformly coated V2O5 nanocrystals over further reduced rGO with enhanced surface area and porosity. The morphology of the annealed rGO-V2O5 composite was characterized by SEM and TEM. As shown in Figures 2a-b and S2a-c, V2O5 nanoparticles are uniformly anchored on the surface of rGO nanosheets to form a nearly fully covered monolayer. The morphology is drastically different from the solid lamellar V2O5 structure12 or 3D hierarchical V2O5 nanosheets20 on rGOs in previous studies. The stack of V2O5-coated rGO nanosheets formed a hierarchical mesoporous structure (Figure S2b) which makes it much easier for electrolyte ions to access the whole electrode material. TEM images 8 ACS Paragon Plus Environment

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of this hybrid material in Figures 2c-d further confirm that the uniformly coated V2O5 nanoparticles have the diameter ranging from 20 to 30 nm and these nanoparticles remain anchored on rGO sheets even after long-time sonication (Figure S2d-f), indicating a strong interaction between V2O5 nanoparticles and rGO sheets. This unique structure enabled by rGO templates provides a large V2O5 surface directly accessible by the electrolyte and thus reduces the diffusion pathlength of ions in solid V2O5 down to < 30 nm, which is expected to enhance the charge-discharge rate for both supercapacitors and LIBs. In contrast, the as-prepared rGO-V2O5 hybrids (Fig. S2g & h) show a large content of disordered phases in correlation with Raman spectra. The Raman spectra of as-prepared and annealed rGO-V2O5 are shown in Figure 3a. Almost all the peak positions are the same, but the peak intensities have increased after annealing. The two characteristic peaks of carbon material (the D band at 1340 cm-1 and the G band at 1600 cm-1) and the typical peaks of V2O5 (405, 475, 524, 685, and 993 cm-1) were observed in the rGO-V2O5 hybrids. It should be noted that the Raman peaks of V-O (generally observed at 700 and 530 cm-1) shifted slightly to a lower wavenumber (685 and 524 cm-1), indicating a strong interaction between V2O5 nanoparticles and rGO.22, 45 The peak at 993 cm-1 is characteristic of α-V2O5 corresponding to the stretching mode of the vanadyl V=O double bond, which does not exist in the lower oxidation states of vanadium oxides because they do not possess the vanadyl bond.46-47 The peaks located at 475 and 405 cm-1 are assigned to the bending vibration of V2-O, and the V=O bonds of α-V2O5, respectively. There should be a peak at 305 cm-1 due to the bending vibration of the triply bonded V3-O bond in V2O5 and a peak at 286 cm-1 assigned to the bending vibration of V=O bonds of α-V2O5,47 but they overlapped into one peak. The absence of peaks in the range 800-950 cm-1 confirms that the VO2 phase is not present in the hybrid material. The XRD pattern of rGO-V2O5 hybrid in Figure 3b can be attributed to the crystalline orthorhombic phase of V2O5,22 which is very consistent with the JCPDS: 41-1426 database. A very weak 9 ACS Paragon Plus Environment


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and broad diffraction peak from rGO nanosheets is observed at ~24°, which is partially overlapped with the tail of the strong (110) peak of V2O5. It is possible that the V2O5 nanoparticles on the rGO nanosheets prevented them from restacking as in bare rGO or graphene nanosheets.48 Thus only the few layers of rGO at the core of a small portion of hybrids generate XRD signals. Figure 3c shows the TGA curves of the bare GO and the vacuum-annealed rGO-V2O5 hybrid from room temperature to 700 oC. The weight loss before 200 oC in both samples is attributed to evaporation of the absorbed water. The weight loss between 250 and 500 oC in the bare GO sample is ascribed to the thermal reduction of GO (by reducing the amount of oxygenated groups bound to the GO flakes).43-44 About 35% of the mass remains at 700 oC, likely due to graphitized carbon. In the case of the vacuum-annealed rGO-V2O5 hybrid, the thermal reduction of rGO is pushed up to around 450 oC and a nearly constant mass of 87.6% is retained up to 700 oC. As shown in Figure S1a, the bare rGO sample (produced as a control with the similar hydrothermal process) has much higher thermal stability than GO, retaining ~ 80% of mass at 700 oC, very similar to literature.43-44 Assuming that the thermal property of rGO in rGO-V2O5 hybrid is the same as bare rGO and the mass loss in the hybrid between 200 oC (~93.5%) and 700 oC (~87.6%) is purely from rGO, we can estimate that there are ~29.5% of rGO and ~64.0% of V2O5 in the hybrid at room temperature (see the calculation in SI). Thus, the main component in the hybrid material is V2O5 nanoparticle. The surface area and pore size distribution of the as-prepared and annealed rGO-V2O5 samples are characterized by N2 adsorption and desorption measurements. Figure 3d shows the isotherms for the vacuum-annealed sample. The BET surface area of the annealed rGO-V2O5 hybrid is 24 m2 g-1, which is slightly smaller than that of 3D hierarchical V2O5 nanosheets (40.01 m2 g-1)20 but is higher than that of as-prepared rGO-V2O5 sample (18 m2 g-1). The BJH pore size distribution, shown in the inset of Figure 3d, confirms that most of the pore diameter is in the range of 2-30 nm with the highest abundance at ~20 nm. The large mesoporous surface area of the annealed V2O5 hybrid is beneficial to the electrolyte ion 10 ACS Paragon Plus Environment

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transport and surface reactions. Thus fast kinetics for both Li-ion batteries and supercapacitors is expected. Electrochemical Characterization of rGO-V2O5 Hybrids for Supercapacitors The capacitive performances of rGO-V2O5 electrodes are investigated by with CV, galvanostatic charge-discharge and EIS measurements in a two-electrode configuration in 1.0 M aqueous solution of Na2SO4. CV was carried out in form of symmetric supercapacitors in coin cells over a voltage window of 0 - 0.8 V. Figure 4a shows the CV curves at different scan rates. A striking characteristic of the CV curves is the featureless, nearly ideal rectangular shape, indicating the fast faradaic reactions associated with Na+ (and some H+) ions intercalating into or extracting from V2O5 solids. This property generates an ideal pseudocapacitance that is almost independent of the applied voltage similar to the state-of-theart pseudocapacitive electrodes made of RuO2 and V2O3/VOx.49-50 The nearly ideal rectangular shape of CV curves at low scan rates (Figure 4a) indicates a small equivalent series resistance (ESR). However, at higher scan rates (100 - 500 mV s-1), it deviates from the rectangular shape, showing tilt baseline current and larger rounded corners (Figure S3), which is attributed to larger RC factors and a diffusionlimited faradic reactions at the interface between the active V2O5 and the electrolyte. The specific capacitance C0 (in unit of F g-1, regarding the mass of active material) of the electrode can be calculated from the CV curves according to the following equation: 2

𝐶0 = 𝑚𝜐∆𝑉 ∮|𝐼(𝑉)|𝑑𝑉

(1)

where m is the mass of V2O5 in one electrode, ΔV is the potential window, I(V) is the instantaneous current at a given potential, and  is the scan rate. Figure 4b exhibits a plot of the specific capacitance as a function of the scan rate. As expected, the specific capacitance decreases as the scan rate is raised. It has an overall specific capacitance of 466 F g-1 (per mass of the V2O5 in the composite) at 2 mV s-1 and 253 F g-1 at 100 mV s-1. Even at the highest scan rate of 500 mV s-1 (i.e. completing the chargedischarge process in only 1.6 s), ~ 132 F g-1 can be obtained. 11 ACS Paragon Plus Environment


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Figure 4c presents the charge-discharge curves of the symmetrical supercapacitor cell at different constant current densities. All the charge-discharge curves show highly symmetric and nearly linear features in the applied voltage window with a very small “IR drop”, indicating an ideal supercapacitor behavior.51 The specific capacitance C0 can be also calculated using charge-discharge data by the following equation: 2𝐼∆𝑡

𝐶0 = 𝑚∆𝑉

(2)

where I is the applied charge-discharge current and Δt is the charge-discharge time. The specific capacitance at varied current density is presented in Figure 4d. The rGO-V2O5 composite electrode has a specific capacitance of 448 F g-1 at 0.75 A g-1 (per mass of the V2O5 in the composite) and slightly drops to 296 F g-1 when the current density is increased by 20 times to 15.5 A g-1 (i.e. completing the charge-discharge in 15 s). These two techniques are very consistent with each other and both indicate a good rate capability of rGO-V2O5 hybrids. EIS measurements provided further information regarding the kinetics and charge-carrier transport. The Nyquist plot of the symmetrical supercapacitor cell is presented in Figure 4e with an enlarged high frequency region in the inset. In the high frequency region, the exhibited semicircle implies the existence of charge-transfer resistance (Rct). The small Rct value (~ 0.6 Ω for 1.75 cm2 cell area) indicates a very fast charge-transport rate, which is in good agreement with CV and high current chargedischarge studies. At the low frequency region (down to 10 mHz), the impedance response is a straight line close to the imaginary axis (Z”), which indicates a high capacitance and a very small resistance associated with ion diffusion from solution to the electrode surface.50-51 These properties are all resulted from the mesoporous rGO-V2O5 hybrid structure. Associated with the above properties, the rGO-V2O5 hybrid shows excellent cycling stability. Figure 4f shows the discharge capacitance as a function of cycle number during charge-discharge tests up to 1000 cycles at a constant current density of 3 A g-1 of V2O5. The capacitance decreases slightly in 12 ACS Paragon Plus Environment

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the initial 100 cycles but is quickly stabilized. After that, it remains nearly constant in the remaining cycles. By far, the highest specific capacitance of V2O5 was reported to be 1308 F g-1 by Ghosh et al.52 based on the study of an ultrathin V2O5 layer (3 nm) coated on a self-standing carbon nanofiber paper electrode at very slow scan rate (5 mV s

−1

). While it demonstrates the potential of the high specific

capacitance of V2O5, the low V2O5 mass loading in the electrode (

Mesoporous Hybrids of Reduced Graphene Oxide and Vanadium

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