Graphene Hybrid

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C: Physical Processes in Nanomaterials and Nanostructures

Combined Experimental and Theoretical Insights on Energy Storage Application of VO2(D)-Graphene Hybrid Swagatika Kamila, Brahmananda Chakraborty, Suddhasatwa Basu, and Bikash Kumar Jena J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03563 • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019

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The Journal of Physical Chemistry

Combined Experimental and Theoretical Insights on Energy Storage Application of VO2(D)-Graphene Hybrid Swagatika Kamila,a,b Brahmananda Chakraborty,c* Suddhasatwa Basu,a,b Bikash Kumar Jena,a,b* a.

Materials Chemistry Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar-751013, India b Academy of Scientific & Innovative Research, New Delhi-110001, India c High Pressure & Synchrotron Radiation Physics Division, BARC, Mumbai-85

__________________ *Corresponding author. E-mail: [email protected] (Bikash Kumar Jena), [email protected] (Brahmananda Chakraborty)

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ABSTRACT In this work, a rare VO2(D) phase plate-like structures and integrated with graphene (rGO/VO2(D)) has been developed by facile hydrothermal route, explored their activity towards supercapacitor application and validated by the extensive ab-initio simulations using Density Functional Theory (DFT) study. After successful synthesis, the samples have been characterized by various techniques to know its crystal phase, surface morphology and elemental composition. The energy storage performance of these electrode materials was studied by both symmetric and asymmetric supercapacitor device. In symmetric supercapacitor device, the hybrid material shows high specific capacitance of 737 Fg-1 at a scan rate of 1 mVs1

and 244 Fg-1 at a current density 1 Ag-1 with excellent cycle life over 5000 cycles without any

capacitance loss. Further, an asymmetric coin cell supercapacitor device has been fabricated by using rGO as a negative electrode and rGO/VO2(D) hybrid as a positive electrode. The energy storage performance was measured at a wide potential of 2V, and demonstrated towards the powering of LED has been demonstrated. The DFT simulations predict that the significant synergistic effect towards enhanced capacitance attributes to orbital interactions and enhancement of electronic states near Fermi level due to additional C 2p states from graphene.


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1. INTRODUCTION Metal Oxides such as vanadium oxide has garnered substantial attention due to their diverse chemical structure, excellent physicochemical properties, abundant in nature, and low cost.1,2 Among them VO2 is a promising material in energy application, and it exhibits different polymorphs/phases of monoclinic VO2(M) and rutile VO2(R) along with some metastable phases such as tetragonal VO2(A), monoclinic VO2(B), VO2(C), VO2(D) and VO2(P).3 Most of the phases exhibit different crystal structures due to various connectivity of octahedral site of vanadium ions. In particular, VO2(D) phase is a newly discovered metastable phase having a monoclinic structure. The metastable VO2 (D) phase consists of two chains of VO6 octahedra and is connected via the corner-sharing oxygen atom.4,5 The next-generation energy storage device, the supercapacitor is broadly used in consumer electronics, hybrid energy vehicles, and memory back up in some electronic devices etc.6,7 Till now so many electrode materials are developed for supercapacitor applications, and they are classified according to their energy storage principles such as EDLC and pseudocapacitive types.8 Generally, various carbonaceous materials are treated as EDLC electrode materials and graphene having a larger surface area, and higher conductivity received tremendous attention among all.9,10 Some transition metal oxides are used as pseudocapacitive electrode material due to the presence of various oxidation state. The metal atom shuttles electron transfer reaction between the electrode material and electrolyte and gives better capacitance performance 11,12 Therefore; a wider scope is lying to explore the performance and properties of new metal oxide materials and different hybrid structures for supercapacitor application. Metal oxides like MoO 3, WO3, and CrVO4 have been reported to exhibit higher capacitance performance when hybridized with reduced graphene oxides.13–15 3 ACS Paragon Plus Environment


There are some reports has been established of different phases of VO2 for energy storage. Among them, metastable phase of VO2(B) are extensively studied for energy storage supercapacitor application.16–18 Also some reports highlighted the use of pure phase of VO2(M) and VO2(A) phase towards supercapacitor application.19, 20 However, very few reports have been documented on the synthesis of VO 2(D) phase and its applications.5 Though few works have been reported on the energy storage application of other phases of VO2 and their carbon-based composites, the potential of VO2(D) phase is not established so far. As a part of our continuing effort,21–26 here a new method for synthesis of rare phase VO2(D) and its graphene hybrid (rGO/(VO2(D)) were developed and explored their potential supercapacitor application. As a proof-ofconcept an asymmetric supercapacitor device was fabricated to power the LED. To the extent of our knowledge, no such report has been documented on VO 2(D) and rGO/(VO2(D) for supercapacitor application. Further, to know the cause of capacitance contribution in rGO/VO2(D) hybrid, theoretical support has been provided by analyzed extensive ab-initio simulations using Density Functional Theory (DFT). 2. Experimental 2.1 Chemical reagents Ammonium metavanadate (NH4VO3, 99%), Thioacetamide (C2H5NS, 99%) were purchased from Himedia, India. Sodium hydroxide (NaOH, 97%) was purchased from Qualigens. The Nafion was procured from Sigma-Aldrich. Here all the chemicals are used without further purification. The deionized water was obtained by Millipore milli-Q water purification system (18MΩ) and used for the preparation of all aqueous solution. 2.2 Synthesis of VO2(D) plates A rare phase of VO2(D) has been synthesized by a simple hydrothermal process. In this reaction, first 25 mM ammonium metavanadate was taken in a 30 ml Deionised (DI) H2O 4 ACS Paragon Plus Environment

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accompanied by vigorous stirring for complete dissolve. Then the solution pH was maintained to 10 by addition of NaOH. After that 125 mM thioacetamide was added to it and keep it stirring for another 30 min. Finally, the above solution was taken in a 50 ml capacity Teflon lined stainless steel autoclave and hydrothermally heated at 180°C for 24 hours. After synthesis, the material was washed with DI H2O several times and with ethanol. Then it was dried at 60°C overnight in oven and keep it for future use. 2.3 Synthesis of graphene oxide (GO) and reduced graphene oxide (rGO) The synthesis of GO was carried out by modified Hummer’s method.27 The rGO has been synthesized by followed our previous reported work.24 2.4 Synthesis of rGO/VO2(D) hybrid nanostructures The rGO/VO2(D) hybrid was synthesized by taking 15mg of GO into 30ml DI H2O and then sonicated properly to make a good dispersion. After that, the above procedure for the synthesis of VO2(D) was followed up taking this GO solution. 2.5 Characterization The morphology of the samples was characterized by field emission scanning electron microscope (LEO 1525, Carl Zeiss, Oberkochen, Germany) with an accelerating voltage of 5kV. The nanoscale morphology, selected area electron diffraction pattern and hi-resolution images were carried out by using hi-resolution transmission electron microscope (HRTEM, JEOL JEM-2010) operated at 200kV. Powder diffraction analysis was carried out by Ultima IV X-ray diffractometer well equipped with a Ni filtered Cu-Kα (λ=0.154nm) over a 2θ range of 10°-80°. Raman spectroscopy was performed using Renishaw in Via Raman microscope at a laser wavelength of 514nm. The elemental composition of the materials was analyzed by Xray photoelectron spectroscopy (Thermo Fisher Scientific ESCALAB Xi+) with Al-Kα 5 ACS Paragon Plus Environment


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(1486.6eV) as the X-ray source for excitation. The bandgap has been analyzed by UV-Visible DRS technique using Shimadzu 2600 UV-Visible spectrophotometer. 2.6 Electrode Preparation and electrochemical measurements The supercapacitor testing was carried out by design a symmetric swage-lock type stainless steel electrodes. For fabrication of this cell, a homogenous dispersion and of rGO/VO2(D) hybrid in ethanol was deposited on both electrodes and allowed it to dry. The cellulose nitrate membrane (diameter 13 mm) soaked in 1M Na2SO4 electrolyte solution used as a separator between the two electrodes. The electrode materials deposited on stainless steel electrodes and a membrane were fitted by swage-lock type set up. For the asymmetric supercapacitor, a coin-cell setup was used for electrochemical measurements. The coin cell setup (CR2025) was used for the fabrication of asymmetric supercapacitor by taking rGO as cathode material and rGO/VO2(D) hybrids as anode material, and two 15.8 mm diameter (0.5mm thick) stainless steel as a spacer. Before the fabrication of the device, the charge and mass balance was carried out, and the mass ratio of anode to cathode material is estimated. Then, the required quantity of homogenous dispersion of the rGO and rGO/VO2(D) hybrid was drop casted on stainless steel surface, separately. These two electrodes are separated by cellulose nitrate membrane (diameter 18 mm) soaked in 1M Na2SO4 electrolyte solution as a separator. The coin cell setup was assembled by using a pressure of 70 kg/cm2 by hydraulic crimping machine. In both symmetric and asymmetric supercapacitors, the electrochemical measurements were carried out with Biologic Science instrument (VSP-300). 2.7 Simulation Details The First Principles calculations have been carried out using VASP

28–31

code, which is

the plane-wave DFT package. Here we have used well-trusted PBE-GGA exchange correlation


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functional 32 for the description of V, O and C orbitals. We have taken large cut-off energy of 600 eV to make the simulations more accurate. The energy and force convergence was set to 10-6 eV and 0.01 eV/Ǻ respectively. Monkhorst pack 33 k-points grid of 9x7x9 for simulating bulk structures and 7x5x1 for surface structures were taken for integration of Brillouin zone. As the interaction of rGO with VO2(D) surface may involve weak Van der Waals forces, dispersion correction scheme of Grimme DFT-D2

34

has been used where a semi-empirical

dispersion potential has been included to the conventional Kohn-Sham DFT energy using a pair-wise force field. 3. Result and Discussion The bulk structure of VO2(D) with the optimized geometry and lattice parameters from DFT with GGA exchange-correlation functional is presented in Figure 1(a). Simulated lattice parameters are a= 4.50, b=5.52 and c=4.853 Å, matches nicely with the reported theoretical value4of a= 4.584, b=5.580 and c=4.956 Å and experimental value of a= 4.613, b=5.645 and c=4.869 Å. The structure consists of VO6 octahedrons, as shown in Figure 1(a). It is a layered material with open channel in one direction as shown in Figure 1(b). This open space favors the charge transport which may be one of the structural features for enhanced supercapacitance. Computed V-O bond length in the octahedron are 1.93 Å, 1.93 Å, 1.85 Å, 1.85 Å, 2.04 Å and 2.04 Å. The Density of States (DOS) of bulk VO2(D) presented in Figure 1 (c) signifies that the material is semiconducting nature having finite gap at the Fermi level. We have computed the band gap using GGA+U method. Computed bandgap with U=3.7 eV and J=0.8 eV (Ueff=UJ=2.9 eV) comes out to be 0.80 eV, in good agreement with theoretical reported value of 0.9 eV.4 We can see that although VO2(D) phase is semi-conducting, the bandgap is not very large. As we did not get any reference for experimental band gap of VO2(D), the UV-Vis absorption spectroscopy of VO2(D) has been studied (Figure S1a). The experimental band gap value for VO2 (D) has been estimated to be 1.86eV (Figure S1b). Here we mention that the DFT



predicted bandgap could not produce the experimental band gap of 1.86 eV, as DFT underestimates the band gap. We have also computed the band gap by using Ueff=5.0 eV and computed band gap comes to 1.35 eV, close to the experimental band gap. The electronic structure with lower band gap enables the capacitance enhancement due to the reversible rate of chemical reaction to store charge. The partial density of states (PDOS) of V 3d orbital and O 2p orbital are plotted in Figure 1(d). We can see that the conduction band is dominated by V 3d orbitals, and O 2p orbitals have more contribution in the valence band. There is hybridization between V 3d orbitals and O 2p orbitals.

Figure 1. DFT optimized structures for (a) Bulk VO2(D), (b) (011) surface of VO2(D);(c) Total Density of States of bulk VO2(D), computed band gap is 0.80 eV for Ueff=2.9 eV and 1.35 eV for Ueff=5.0 eV, and (d) Partial density of states of V d orbital and O p orbital of bulk VO2(D). The surface morphology and size of VO2(D) plates are analyzed by FESEM and TEM analysis (Figure 2(a-d)). The FESEM image reveals the growth of plate-like structure 8 ACS Paragon Plus Environment

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stacking each other, and TEM analysis corroborates the same. The spotty images of the selected area electron diffraction (SAED) data reveal that VO2 (D) is polycrystalline (Figure 2c inset). The HRTEM image of VO2 gives the d-spacing of 0.38 nm of 011 planes (Figure 2d). Similarly, rGO/VO2(D) hybrids were examined by FESEM and HRTEM analysis (Figure 2(e,f)). From FESEM analysis, it is observed that the VO2(D) and rGO nanosheets are embedded with each other. The stacking of VO2(D) plates disappears in the presence of graphene. That may be due to the interaction between rGO with Vanadium precursor that cause the uniform growth of VO2(D) plates rather on its surface. The TEM analysis of rGO/VO2(D) hybrids supports the above observation that the VO2 plates are well embedded with the rGO sheets.

Figure 2. (a,b) FESEM and (c,d) TEM of VO2 (D). (e) FESEM and (f) TEM of rGO/VO2(D).



The composition and crystal structure of the as-synthesized materials was examined by XRD analysis (Figure 3a). The peaks observed at different diffraction angles are well matched with the literature pattern for the VO2(D) phase. 30 In comparisons to other phases of VO2, it shows different peak positions and matching with the monoclinic phase of NiWO4 (JCPDS 150755).35 In the case of rGO/VO2(D) hybrid, similar peaks have been observed with an extra small hump at ~26°. This peak is due to presence of rGO, confirming the reduction of GO at

Figure 3. (a) XRD and (b) Raman spectrum of VO2 (D) and rGO/VO2 (D) the hydrothermal condition.36 The growth of rGO/VO2(D) hybrids were further analyzed by the Raman spectroscopy (Figure 3b). For VO2(D), the characteristic Raman peaks observed for V-O-V bending, V-O-V stretching and V=O stretching modes.37 Similarly, the Raman spectra of rGO/VO2(D) hybrid exhibits the characteristic peaks of VO2(D) along with two additional peaks at 1342 cm-1 and 1585 cm-1 corresponding to the G and D bands of rGO.38 Further the XPS analysis has been carried out to know surface chemistry of VO2(D) and the interaction between rGO and VO2(D) in the rGO/VO2(D) hybrid. The full scan of the spectra of VO2(D) gives the characteristic V2p, O1s peaks at their respective binding energy (Figure S2). The core-level XPS spectra of V2p is showing characteristic peaks at 516.7 eV and 523.8 eV owing to V2P3/2 and V2P1/2 of V state.39 The deconvoluted peak of V2P3/2 and V2P1/2 at 517.8 eV and 525 eV are corresponding to V5+ state. It may be due to the few partial surface oxidation of


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VO2 by atmospheric oxygen.40 Again the O 1s spectra of VO2(D) is deconvoluted into two peaks. The peaks centered at 530.4 eV, 532.8 eV corresponding to V-O and H2O molecule. The full scan XPS spectrum of rGO/VO2(D) gives the characteristic V2p, C1s and O1s peaks (Figure S3). Here, the V2p peak showing characteristic peaks at 516.5 eV and 523 eV owing to V2P3/2 and V2P1/2. The deconvoluted peak of V2P3/2 and V2P1/2 at 517.3 eV and 524.6eV correspond to V5+ state. There is absent of V-C bond at binding energy of 513.9 eV, confirms there is no possible interaction of vanadium with carbon in the hybrid material.41 Again the deconvoluted peaks of C1s give signals at 284.2 eV is attributed to SP2 carbon atom. The weak peaks observed at 286.7 and 287.8 eV are due to the oxygen functional groups of C-O and OC=O.42 Here the decrease in oxygen functional groups reveal that GO has been reduced successfully by the hydrothermal process. The O1s spectra of rGO/VO2(D) are shifted lower binding energy, and peak broadening happens compared to the O1s peak of VO2(D).43 Here, the deconvoluted O1s spectra shows four peaks. The peaks centered at 529.8 eV, 531 eV, 531.9 eV and 533 eV corresponding to V-O, V-O-C, C-O, and O-C=O respectively.42,43 The existence of 531 eV peak (M-O-C) at higher binding energy confirms the interfacial interaction of VO2(D) with rGO surface through V-O-C bond which, enables the electron transfer pathways of the hybrid material.43,44,45 The synthesis of a rare VO2(D) phase with plate-like structure and its rGO hybrid inspired to explore the energy storage properties towards the supercapacitor application. The capacitance performance was measured by cyclic voltammetry (CV) and galvanostatic chargedischarge (GCD) techniques. The CV at different scan rates and GCD at different current densities of rGO/VO2(D) hybrid are presented (Figure 4(a,b)). The energy storage performance of VO2(D) electrode was studied by CV and GCD techniques at same condition (Figure S4(a,b)). From CV measurements, the specific capacitance of rGO/VO2(D) and VO2(D) electrodes were estimated using equation 1 and found to be 737 Fg-1 and 271 Fg-1, respectively



Figure 4. (a) CV and (b) GCD of rGO/VO2(D). Overlapped (c) CV, (d) GCD, (e) Ragone plot and (f) stability data of VO2(D) and rGO/VO2(D) at a scan rate of 1 mVs-1. The overlapped CV plot of rGO/VO2(D) hybrid and VO2(D) electrodes at a scan rate of 10 mVs-1 is presented in (Figure 4c). The area under the CV curve of rGO/VO2(D) hybrid was found to be larger than the VO2(D) electrode. This observation signifies that the good capacitive behavior of the hybrid is due to a synergistic effect of both VO2(D) and rGO. Here the embedded structure of VO2(D) with the rGO sheets prevent the agglomeration of rGO as well provides a platform for uniform growth of VO2(D) results in the 12 ACS Paragon Plus Environment

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enhancement of both EDLC and pseudocapacitive behavior to the overall capacitance properties.17 The GCD of rGO/VO2(D) hybrid has been shown in Figure 4(b) at different current densities. From GCD technique, the specific capacitance of the hybrid material was calculated using equation 2 and found to be 244 Fg-1 at a current density of 1 Ag-1, Whereas the bare VO2(D) phase gives specific capacitance of 46 Fg-1 at 1Ag-1. The Ragone plot on energy density (ED) and power density (PD) obtained from the CD response is presented in Figure 4e. The ED and PD values of the rGO/VO2(D) were estimated using the equation 6 and 7, and calculated to be 102 Whkg−1 and 368 Wkg−1, respectively at a scan rate of 1 mVs-1. Whereas, the VO2(D) produces the ED and PD of 37.5 Whkg−1 and 135.5 Wkg−1 respectively at similar condition. Further, the ED and PD values have been estimated from GCD technique using equation 6 and 8, respectively, at a current density of 1Ag-1. The rGO/VO2(D) produces the ED and PD of 33.7 Whkg-1 and 997 Wkg-1 whereas VO2(D) gives 6.3 Whkg-1 and 986 Wkg-1 respectively. The operational stability is another important factor for energy storage device. The cycle life of VO2(D) and rGO/VO2(D) has been studied over 5000 cycles at scan rate of 50 mVs-1 (Figure 4f). It was observed that the VO2 (D) shows capacitance retention of 59%, whereas the rGO/VO2(D) electrode shows no capacitance loss over 5000 cycles. The capacitance loss of the material depends upon the change in structure and morphology of the metal oxide during the repeated charge-discharge cycles. However, we attribute that the interaction of graphene with the metal oxide provides sufficient voids spaces in the hybrid material and reduce the strain associated with volume changes occur during charge discharge cycle, which increases the cycle life of the hybrid material rGO/VO2(D).46,47 The performance of present materials has been compared with previous reported different phases of VO2 and their hybrids with other materials, and summarized in Table S1. It reveals the better and comparable capacitance behavior and robust cycle life of rGO/VO2(D).



Further, to know the effect of rGO concentration in the capacitance performance of assynthesized hybrid materials, the variation of graphene in the hybrid has been carried out. Here, the hybrid materials were synthesized by introducing different contents of GO, e.g. 5 mg, 10 mg, 15 mg, 30 mg, and 60 mg to the reaction mixture and keeping all other conditions same. The optimized hybrid material, rGO/VO2(D) contains the 15 mg GO in the reaction. The other hybrids are denoted as rGO/VO2(D)-5, rGO/VO2(D)-10, rGO/VO2(D)-30 and rGO/VO2(D)-60 based on the content of GO as 5 mg, 10 mg, 30 mg, and 60 mg during the synthesis, respectively. The overlapped CV and GCD response are presented in Figure S5. From this plot we observed that the rGO/VO2(D) that content 15 mg of GO during the syntheses shows better capacitance properties than other hybrids. At higher concentration of graphene in the hybrid, showing less capacitance may be due to the restacking nature of graphene nanosheets. Again to validate the practical application, an asymmetric coin cell supercapacitor has been fabricated with increase in operating potential taking rGO and rGO/VO2(D) hybrid as the cathode and anode electrode material respectively (Figure 5(a)). Before the design of the device, the operating potential was optimised by recording the CV performance of the

Figure 5. (a) scheme and image of as-fabricated asymmetric coin cell device. (b) image showing the powering of LED 14 ACS Paragon Plus Environment

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individual cathode and anode materials. The rGO electrode is stable at a potential range of 0.0 to -1.0V, whereas the rGO/VO2(D) hybrid is stable at 0.0 to +1.0V (Figure S6). So the present device can operate at a potential window of 2V. The charge and mass balance were carried out to fabricate the asymmetric coin cell device (Equation 3), and the mass ratio of anode to cathode material is estimated to be 0.6 at scan rate of 4 mVs-1. The CV and GCD techniques have been carried out to evaluate the performance of the fabricated asymmetric coin cell device (Figure S7). From CV, the specific capacitance of the asymmetric device is estimated using equation 4 and found to be 147 Fg-1 at a scan rate of 1 mVs-1. Whereas, from the GCD (Equation 5), it gives a specific capacitance of 17 Fg-1 at a current density of 1 Ag-1. The Ragone plot (ED vs. PD) of the device obtained from CD response is presented in Figure 8 (a). The device gives ED and PD value of 81 WhKg-1 and 147 W kg-1 estimated at a scan rate of 1 mVs-1. Also, the ED and PD values of the device are calculated form the GCD technique, found to be 9.5 Whkg1

and 996 Wkg-1 at a current density of 1 Ag-1. Further, the cycle life of this asymmetric coin

cell device has been checked over 5000 cycles at a scan rate of 50 mVs-1 (Figure S8b)) and that reveals up to 79% of capacitance retention performance. The Electrochemical Impedance Spectroscopy (EIS) measurement was performed to know the resistance of the as-fabricated asymmetric device and the Nyquist plot is presented in Figure S9. The Nyquist plot consists a semi-circular arc at higher frequency region known as charge transfer resistance (RCT), and, an inclined line is Warburg Impedance(ZW) at lower frequency region.48 The RCT value of the device was estimated to be ~39 ohms. The determination of leakage current during selfdischarge is one of the important parameters for practical application. The leakage current was determined using the equation 9.49 The coin cell device was charged by applying 1mA current to reach the potential of 2 Volt and then the self-discharging potential was monitored at different time intervals. It was observed that at initial time, the self-discharging rate is more with higher leakage current of 600 µA at 4 minute and it decreases to 83 µA with due course



of time 1 hour. To know the practical usefulness of the asymmetric coin cell device, a simple application to power a red light emitting diode (LED) has been demonstrated. It was observed that after charging the device at 0.4 Ag-1, it could successfully power the LED (Figure 5b). Also the long term LED performance was checked by charging the device and the video was recorded. It successfully powers the LED for a period one minute (Video S1). A similar observation has been documented elsewhere.50–52 Also the performance of 3V digital watch has been tested by powering with tandem two coin cell device. It was observed that after charging device at 0.4 Ag-1, it could successfully run the digital watch up to 10 minutes. (Video S2 for starting one minute and Figure S10 snapshots at different time intervals). To validate the above experimental observation and to support theoretical explanations for the enhanced capacitance of rGO/VO2(D) hybrid, we have performed Density Functional Theory (DFT) simulations. As we are concerned on the electrode properties of VO2(D) and hybrid structures of rGO with VO2(D), rest of the simulations were done on the surface of VO2(D) and surface of hybrid structures. Using the optimized bulk structure, we have generated (011) surface of VO 2(D) and allowed the system to relax. Then we have constructed the hybrid structure by adding a layer of rGO on (011) surface of VO2(D). To model rGO, we have included epoxy groups(O) in the graphene which is the dominant functional group present in the graphene. The oxygen of epoxy group makes bond having bond length of 1.46 Ả with two adjacent C atoms as shown in (Figure S11(a)). The area of both layers has been adjusted in such a way to minimize the lattice mismatch at the boundaries. Optimized structures of (011) surface of VO2(D) and the hybrid surface is displayed in Figure S11(b) and Figure 11(c), respectively. Figure 6 depicts the total Density of States of (011) surface of VO 2(D) (lower panel) and hybrid structure of rGO and (011) surface of VO2(D) (upper panel). Fermi levels are indicated by magenta lines. We can notice that the DOS for hybrid


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structure contains more electronic states around Fermi level compared to that for pristine (011) surface of VO2(D). Also Fermi level is at higher energy for hybrid structure compared to virgin VO2(D). We observe from Figure 6 that Fermi level shifts to the higher energy in the hybrid structure, from -2.479 eV for VO2(D)surface to -1.765 eV for hybrid structure. But the magnitude of the Fermi energy as well as magnitude of the

Figure 6. Total Density of States of (011) surface of VO2(D) and rGO/VO2(D); corresponding Fermi level Fermi velocity reduces. It has been reported53 that for the graphene-like thin layered system, quantum capacitance is inversely proportional to Fermi velocity. So here shift in Fermi energy support increase in quantum capacitance for the system. Also, for semiconducting system, shifts in Fermi energy towards higher energy implies that occupied electronic states in the system increase.Using the DOS value, we have computed Quantum capacitance using the following formula. 53 17 ACS Paragon Plus Environment


∞

𝐂𝐐 = ⅇ𝟐 ∫∞ 𝐃(𝐄)𝐅𝐓 (𝐄 − ⅇ𝛗𝐆 ) ⅆ𝐄

(1)

Here the parameters are D(E)= DOS of the surface concerned, ϕ G=electrode potential and FT(E)= thermal broadening function. 𝐅𝐓 (𝐄) = (𝟒𝐊 𝐁 𝐓)−𝟏 𝐬ⅇ𝐜𝐡𝟐 (𝐄 ∕ 𝟐𝐊 𝐁 𝐓)

(2)

Figure 7a presents the variation of Quantum capacitance with electrode potential for hybrid and pristine electrodes. We can see that the Quantum capacitance is higher for the hybrid structure for all electrode potential. Please note that Quantum capacitance is significant for the low dimensional system. We can’t compare Quantum capacitance with experimental capacitance as the experimental capacitance also contains doublelayer capacitance (CEDL) as given by the following expression.53 𝟏 𝐂𝐓

𝟏

=𝐂 +𝐂 𝐐

𝟏

(3)

𝐄𝐃𝐋

To bring some insight from orbital interactions for the enhanced capacitance of hybrid structures, we have plotted the PDOS of V d orbital (lower panel), O p orbital and C p orbital of hybrid structure in Figure 7b. We have also compared the PDOS of carbon 2p orbital for reduced graphene oxide with that for hybrid structure in Figure 7b. For reduced grapheme oxide, C 2p orbital resembles the almost graphene-like semimetallic character, whereas in the hybrid structure it shows some enhanced states around Fermi level. The interaction mechanism for the attachment of VO2(D) with rGO is a critical point to address. Here, we mention that the enhancement of capacitance performance in the hybrid structure not only depends on the charge transfer but may be due to several factors, such as; (a) increase in conductivity of hybrid rGO/VO2(D) compared to bare VO2(D) and (b)increase in structural integrity of the hybrid structure to withstand large cycle of operation. We have investigated this issue through:(i) Experimental XPS 18 ACS Paragon Plus Environment

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analysis as discussed in the characterization section, (ii)XPS simulation with VASP and from literature support for a similar system. Experimental XPS analysis predicts that the interfacial interaction of VO2(D) with rGO surface is through V-O-C bond which,

Figure 7. (a)Variation of Quantum capacitance with electrode potential for VO2(D) and rGO/VO2(D) (b) Partial density of states of V d orbital (lower panel), O p orbital and C p orbital for of hybrid structure of VO2(D) surface plus rGO; upper panel shows C p orbital for reduced graphene oxide; Fermi level is shown by magenta line enables the electron transfer pathways of the hybrid material. We have also performed the XPS simulations using DFT and taking the core-hole screening effect into consideration. To model reduced graphene oxide, we have considered epoxy group in graphene where O makes bond having bond length of 1.46 Ả with two adjacent C atoms as shown in Figure 6a. We have computed core level binding energy of V2p, O1s, C 1s of VO2(D) as well as hybrid structure. We have observed a shift in core level binding energy of V2p states. The core-level binding energy of V2P3/2 and V2P1/2 shift by -0.4 eV and -0.9 eV towards lower energy, consistent with the experimental observations. The core-level binding energy of O 1s shifts to lower binding energy by -0.45 eV, consistent with the literature value43 These observations points towards attachment of VO2 structure with reduced graphene oxide. There are reports where bonding and charge 19 ACS Paragon Plus Environment


transfer between metal oxide/sulphides and rGO have been mentioned. The formation of C-Ti bonds and charge transfer have been reported for TiO-graphene composites54 and TiO2-rGO composites.55 Also the charge transfer between vanadium oxides and rGO have been reported.56 So from experimental and theoretical XPS analysis and literature support, we can infer that the orbital interactions and enhancement of electronic states near Fermi level for the hybrid structure results in higher capacitance. 4. Conclusion

In summary, we have demonstrated a new method for the synthesis of a rare phase VO2(D)and its graphene hybrids and demonstrated the energy storage application towards excellent supercapacitor performance. The ab-initio simulations using Density Functional Theory (DFT) validates the experimental observation. An asymmetric coincell type supercapacitor device has been fabricated and powered the LED light for practical application. Supporting Information. Electrochemical Calculations, UV-Visible absorption spectra, XPS data, CV and GCD data, Table showing performance comparison, DFT optimized structure, images of powering to digital watch and etc. Acknowledgments BKJ acknowledges CSIR, New Delhi, India for financial support (OLP-65, OLP-95). SK thanks DST (Government of India) to Inspire fellowship and lab mates for help. BC acknowledges Dr. R. C. Rannot and Dr. A.K. Mohanty for support and encouragement, staff of BARC computer division for supercomputing facility and A. Gangan for his timely help. The authors' thanks Prof. P.V. Satyam and Dr. P. Guha, IoP, Bhubaneswar for TEM and FESEM analysis, and CCC, CSIR-IMMT for characterization facility. References (1)

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Oxide

Nanosheet

Composites

as

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TOC Graphic


Graphene Hybrid

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