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Synthesis and electrochemical studies of Ta – Graphene nanocomposite film modified platinum electrode
Synthesis and electrochemical studies of Ta – Graphene nanocomposite film modified platinum electrode R. Rajesh, R. Abirami, S.M. Senthil Kumar, K. Rajasekar, K. Balkis Ameen PII: DOI: Reference:
S1572-6657(16)30448-9 doi: 10.1016/j.jelechem.2016.09.003 JEAC 2817
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
6 April 2016 11 August 2016 2 September 2016
Please cite this article as: R. Rajesh, R. Abirami, S.M. Senthil Kumar, K. Rajasekar, K. Balkis Ameen, Synthesis and electrochemical studies of Ta – Graphene nanocomposite film modified platinum electrode, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.09.003
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ACCEPTED MANUSCRIPT Synthesis and Electrochemical studies of Ta – Graphene nanocomposite film modified platinum electrode
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R. RajeshA*, R. AbiramiA, S.M. Senthil KumarB,
Department of Nanotechnology, Anna University Regional Campus Coimbatore,
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K. RajasekarA, K. Balkis AmeenC*
Tamilnadu, India – 641046 B
Electrochemical Materials Science Division, CSIR-Central Electrochemical
Jyoti Ceramic Industries Pvt. Ltd. Satpur, Nashik - 422007, India
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C
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Research Institute, Karaikudi 630003, Tamil Nadu, India
Email: [email protected], [email protected] Abstract:
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Tantalum/Reduced Graphene Oxide composite is presented in this work as an
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electrode material for capacitor application. Reduced graphene oxide material was synthesized by modified Hummer’s method and Tantalum doped graphene oxide electrodes
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are fabricated by an electrophoretic deposition method. The physico-chemical properties of the as-synthesized materials are characterized by X-ray diffraction, N2 sorption analyses, Fourier Transform Infrared and Raman spectroscopic techniques. The structural details are
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elucidated from the scanning and transmission electron microscopic analyses. The X-ray diffraction results confirmed the structural integrity of graphene after the Ta doping process. Raman analysis confirmed the graphitic nature of graphene with a surface area of 281 m2/g. The surface area of the tantalum doped composite decreased to 214 m2/g due to the deposition of Ta ions onto the graphene surface. The Ta doped graphene composite exhibited comparatively higher capacitance value of 1420 µF/cm2 than graphene (980 µF/cm2), indicating that this composite can store more charge in comparison with graphene. This enhancement of capacitance with Ta doped graphene is thus found to be a good candidate for super/ultra capacitor applications
Keywords: Graphene; Graphene/Ta Composite; Supercapacitor; Areal capacitance
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ACCEPTED MANUSCRIPT INTRODUCTION Nanostructured materials for supercapacitor application are drawing more attention recently in revolutionizing the performance of energy storage devices with higher
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energy/power densities1. To date, carbon materials on nanoscale dimensions have proven its capability in achieving high energy/power density, good rate performance and long cycling
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life than other conventional materials2. When compared to the conventional batteries, supercapacitors have high power efficiency, fast charge-discharge time and long cyclic stability. Research has been accelerated towards the development of better electrode material
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to improve the gravimetric capacitance thereby enhancing the energy density of the supercapacitors. Based on the energy storage mechanism, the supercapicitors are classified into two types, electrochemical double layer capacitors (EDLCs) and pseudo-capacitors. In
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EDLCs the charge storage process is non-faradic and the energy storage is electrostatic3 wherein carbon based materials are widely used4. Among the carbon family, graphene based composites are specifically attractive because of its unique electrical and mechanical
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properties5. Pseudocapacitors (metal oxides and conducting polymers) on the other hand
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exhibit capacitance from the faradaic redox reaction occurring on the electrode surface6. Development of supercapacitor with nanostructured materials has been actively
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studied for more than two decades. Electrodes made of metal oxides, conducting polymers, polymer composites possess promising theoretical capacitance, but as these materials tend to be disordered and have low surface area these electrodes suffer less cyclic stability and are
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also relatively expensive. Carbon based materials like activated carbon7, carbon aerogels8, carbon nanotubes9,10 and graphene have been widely examined and showed stable performance characteristics. Especially, metal oxide modified graphene electrodes such as graphene-ZnO11, graphene-MnO212, graphene-RuO213 and graphene/CNT composites14 are gaining much importance for the supercapacitors applications. Even though these materials possess remarkably large surface areas, these materials have not shown promise for highly efficient supercapacitors. The non conformal experimental condition suggests that a systematic study of the nanocarbon structures may provide crucial insights to realise ideal supercapacitors in these systems. In this work, we show that the capacitance behaviour considerately depends on the post synthesis treatment of the graphene layers with tantalum oxide. To understand the interaction of tantalum oxide with graphene layers various physio-chemical characterisation viz. XRD, BET Isotherm, laser Raman spectroscopy and SEM results are studied. The 2
ACCEPTED MANUSCRIPT electrochemical behavior of the electrodes was studied by cyclic voltammetry and AC Impedance technique. Our material exhibited relatively high areal capacitance value
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combined with good cyclic stability.
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MATERIALS AND METHODS Chemicals
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Graphite flakes from Asbury Carbons and tantalum penta chloride (TaCl5) from Sigma-Aldrich are purchased. Potassium Permanganate (KMnO4), Sodium Sulphate (Na2SO4) and ethanol are purchased from Himedia, Hydrogen Peroxide (H2O2) and
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Hydrazine Hydrate (N2H5OH) from LOBACHEMIE are used.
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Exfoliation of graphene layers
Exfoliation of graphite flakes into graphene layers was facilitated by adopting modified hummer’s method15. In a typical synthesis, 1 g of graphite flakes was mixed with 45
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ml of concentrated sulphuric acid and stirred for 30 minutes. As the intercalation process
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starts, the mixture was cooled under ice bath with constant stirring. To this mixture, 6 g of KMnO4 was slowly added in portions and the temperature was maintained at < 20℃ with
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continued stirring. After 3 - 4 hours of stirring, a grey colored viscous solution was obtained. Finally to the reaction mixture, 140 ml of water and 5 ml of H2O2 were added under vigorous stirring for 15 minutes. The yellow color graphene oxide thus obtained was thoroughly washed with 3-4 litres of distilled water followed by 3% H2O2 solution. The
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synthesized graphene oxide was filtered and dried in vacuum oven. Further, graphene oxide was reduced with hydrazine hydrate solution. 50 mg of graphene oxide was mixed with 50 ml of double distilled water followed by few ml of ammonia solution. To this solution, 1ml of hydrazine hydrate (60%) solution was added with continuous stirring. The mixture was refluxed at 100℃ for 12 hours and washed with 3-5 litres of water followed by ethanol and filtered. The black colored reduced graphene oxide was finally air dried and used for the electrode fabrication.
Fabrication of Ta/Reduced Graphene Oxide Capacitor Electrode
The as-synthesized reduced graphene oxide (5 mg) was dispersed in an ethanol solution containing Ta ion (5 mg of TaCl5 dissolved in 20ml of ethanol). This slurry was used
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ACCEPTED MANUSCRIPT to fabricate the electrodes by electrophoretic method16. In a typical procedure, two platinum wires are immersed in the slurry. A D.C voltage of 24V was applied between the electrodes for 2 hours, which forms a smooth coating of Ta doped graphene oxide over the cathodic
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platinum wire. This electrode was further air dried at 80℃ for 12 hours. The blank electrode
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without tantalum ions was also prepared by adopting same procedure. The electrodes prepared from reduced Graphene Oxide and Ta doped reduced Graphene Oxide will be
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hereby denoted as rGO and Ta/rGO throughout this article.
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PHYSICO CHEMICAL CHARACTERIZATION
The physiochemical characteristics of rGO and Ta/rGO composite was studied by X-
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Ray diffraction, Scanning electron microscopy (SEM), Nitrogen adsorption and Raman spectroscopic studies. X-ray diffraction patterns were recorded on a Rigaku Ultima III system.
The surface area analysis of the electrode materials are measured by nitrogen
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physisorption technique with Quantachrome NOVA 1200e instrument. The quality of
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graphene layers and Ta-Graphene composite was examined by HORIBA Jobin Yvon Raman spectromenter equipped with a 543 nm laser. JSM-6390 Scanning electron microscope with
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EDAX and Philips CM20 transmission electron microscope were used to analyze the surface morphology of these materials. XPS analysis was done with Kratos axis ESCA spectrometer with a hemispherical analyzer. The electrochemical characteristics and areal capacitance
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value of both rGO and Ta/rGO electrodes are studied by the CH Instruments CHI601C electrochemical analyzer using three electrode systems. RESULTS AND DISCUSSION X-Ray Diffraction Analysis
The powder X-Ray diffraction patterns of the as-synthesized rGO and Ta/rGO composite are shown in Figure 1. Both the samples showed a well defined pattern with a sharp peak at 2θ = 25° corresponding to the [002] plane and a small peak at 2θ = 44° corresponding to the [100] plane of the graphite structure17,18. We observed that the addition of tantalum did not affect the crystallinity of graphene, as the tantalum ion make a bond with a functional group present on the graphene surface.
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Fig. 1. X-ray diffraction patterns of (a) reduced Graphene Oxide, rGO and (b) Ta/rGO.
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BET- Nitrogen Physisorption Studies
The specific surface area of the as-synthesized rGO and Ta/rGO materials was studied
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by nitrogen adsorption studies (Fig. 2). The samples are degassed at 300℃ for 3 hours in
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nitrogen atmosphere to get rid of adsorbed gases and moisture. The specific surface area was calculated from multipoint BET method and the corresponding value of rGO and Ta/rGO are
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281 and 214 m2/g respectively. The addition of tantalum atoms into the graphene reduced the surface area of graphene material because of the blocking of the graphene surface by Ta atoms still maintaining the layered structure of graphene material19,20. Such a high surface
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area material can be utilized in enhancing the accessible area for the ions during the electrochemical process.
Fig. 2. N2 adsorption isotherms of (a) rGO and (b) Ta/rGO. 5
ACCEPTED MANUSCRIPT Fourier Transform Infra-Red Spectroscopic Studies
The FTIR spectra of rGO and Ta/rGO are shown in Fig. 3. From the spectra, the
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following functional groups are identified. The characteristic O-H stretching vibration peaks
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are observed at 3402 cm-1, the -C=C- non oxidized sp2 bonding peak at 1608 cm-1 and O-H deformation of the C-OH stretching vibration band is observed at 1359 cm-1. The -C=O and 1
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C-O stretching vibration bands of -COOH functional group is observed at 1713 and 1224 cmrespectively21,22. In addition to this, we observed a broad peak at 1000 and 1200 cm-1
indicating the presence of -C–O and -C–C stretching vibrations. In reduced graphene oxide,
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the doublet bands at 1436 and 1372 cm-1 are assigned to an anti-symmetric and symmetric stretching vibration of COO- functional group. After the addition of tantalum, these anti-
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symmetric and symmetric stretching vibration bands shifted to the doublet peaks to 1428 and 1359 cm-1 frequency23. The characteristic stretching vibrations of Ta-O-Ta and Ta-Ox bonds are observed at 907cm-1 and 530 cm-1 indicating the replacement of hydroxyl groups by the
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tantalum ions24. The incorporation of tantalum atom in turn also reduced the height of C-O
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bond stretching vibrations peak as also reported earlier25.
Fig. 3. Fourier Transform IR spectra of (a) rGO and (b) Ta/rGO
Raman Spectroscopic Studies
Raman spectroscopy is a non destructive method to characterize the nanocarbon materials and in this study, samples of rGO and Ta/rGO are analyzed and shown in Fig. 4. 6
ACCEPTED MANUSCRIPT From the spectra, the existence of G band and D band can be seen in both the samples. The G band is assigned to the first order scattering of the E2g mode phonon of C sp2 atoms. The D band arises due to the size reduction in the in-plane sp2 domains by extensive oxidation and is
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assigned to the breathing mode of the A1g symmetry26. In reduced graphene oxide, the G band
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is located at 1585 cm-1, which is very close to the pristine graphite (~1581 cm-1) and the D band at 1359 cm-1 indicating the complete reduction of graphene oxide to graphene. In
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Ta/rGO sample, the G band and D band are observed at 1566 and 1344 cm-1
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addition of tantalum atoms in rGO reduced the peak intensity also shifting the peak position towards the low frequency region. However, in both the cases a strong G-Band indicates the
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graphitized layer structure with the IG/ID value greater than 2 indicating more than 3 layers of graphene29,30. The intensity ratio of G band and D band of rGO and Ta/rGO is 3.59 and 3.75
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inferring the decrease in size of the sp2 domains upon Ta doping26. Further, the addition of tantalum alter the band gap of reduced graphene oxide resulting in the peak shifts31,32. The overtone 2D bands for rGO is located at 2725 cm-1 and for Ta/rGO this band is blue shifted to
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2690 cm-1 accompanied with an increased intensity. This confirmed that Ta transfers charge
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to the graphene layers establishing bonding and thus increasing the conductivity in the resulting hybrid electrode. The metal-graphene bonding and the resulting peak shift is already
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been observed in other metal systems as well with either red/blue shift depending the net charge transfer33. In this study it is thus observed that Ta formed an efficient electrode system with the graphene layers.
1200
-1 1566 cm
a) rGO b) Ta/rGO
1000 900
Intensity (counts)
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-1 1585 cm
800 700 -1 1344 cm
600 500 400
-1 2690 cm
-1 1359 cm
-1 2725 cm
300 200 100 1000
1500
2000
2500
3000
-1
Wavenumber (cm )
Fig. 4. Laser Raman spectra of (a) reduced graphene oxide and (b) Ta/rGO
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ACCEPTED MANUSCRIPT Morphological Characterization
The morphological studies of rGO and Ta/rGO composite materials are studied by
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scanning electron microscope and the images are shown in Fig. 5 (a, b). The SEM studies
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showed the stacked layers of graphene sheets. The presence of tantalum in Ta/rGO was confirmed by the EDAX analysis on the surface scan as shown in Fig. 5c. The EDAX
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analysis showed the concentration of the elements to be 80 wt.% by carbon, 9.8 wt.% by oxygen and 10.2 wt.% by tantalum. Thus the doping of tantalum by 10 wt.% in the rGO matrix was confirmed. The transmission electron microscopy image of Ta doped graphene
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oxide is shown in Fig. 5d. The synthesized reduced graphene oxide is seen as transparent sheet and the presence of tantalum is shown with an arrow. Also, it can be seen that the
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tantalum oxide particles are wrapped within the graphene sheets evidencing an intact bonding
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between Ta and graphene.
Fig. 5. SEM images of (a) reduced graphene oxide, (b) Ta/rGO, (c) EDAX spectrum of Ta/rGO and (d) TEM image of Ta/rGO.
XPS Analysis of Ta Doped Reduced Graphene Oxide The oxidation states of the elements tantalum and carbon are studied by X-ray photoelectron spectroscopy. The C 1s spectrum in Ta/rGO is shown in Fig. 6a. Four components are observed in C 1s spectrum which correspond to non oxidized carbon at 284.3 eV, the oxidized carbon of C-O bond at 286 eV, carbonyl carbon (O-C=O) at 289.1 eV and 8
ACCEPTED MANUSCRIPT carboxylate carbon (O=C-OH) at 289.7 eV. Thus the XPS analysis of C 1s spectrum revealed that the as-synthesized material is partially oxidized also in agreement with our FT-IR results. The narrow scan XPS spectrum of Ta 4f level in Ta/rGO is shown in Fig. 6b along with its
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deconvoluted peak positions. We observed two peaks at lower binding energies at 20.5 and
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23 eV corresponding to the +2 and +1 4f7/2 oxidation states of tantalum. These are assigned to the tantalum in its metallic form34. The next doublet peaks are observed at 27 and 27.4 eV
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corresponding to the 4f7/2 and 4f5/2 splitting which is assigned to the +5 oxidation state tantalum pentoxide35. A small peak corresponding to +4 oxidation state is also observed in
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the scan at the binding energy of 24.7 eV.
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Fig. 6. Narrow scan XPS analyse of (a) C1s spectrum and (b) Ta4f spectrum of Ta/rGO sample.
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Electrochemical studies of Ta doped reduced Graphene Oxide Composite
The electrochemical properties of rGO and Ta/rGO are studied by cyclic voltammetry with 1M Na2SO4 electrolyte. Ta/rGO was deposited on to a platinum wire and used as the working electrode with Pt and saturated KCl filled Ag electrodes as counter and reference electrode. The cyclic voltammetry studies are performed at different scan rates (0.1, 0.2, 0.3, 0.4 and 0.5V/s) and the current – voltage characteristics of rGO and Ta/rGO are shown in Fig. 7(A). and 7(B). The following equation36 is used for the calculation of areal capacitance and are listed in Table.1. CA= ( where,
/ (2S x ΔV x v)
is integrated area of the CV curve, v scan rate in V/s, ΔV and S are
potential window and active surface area of the electrode respectively. The electrode with a surface area of 0.079 cm2 is used. 9
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It can be seen from Table. 1 that Ta/rGO material exhibited a higher capacitance of 1.5 mF/cm2 in comparison with rGO (1 mF/cm2) at the scan rate of 100 mV/sec. This
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enhancement is also observed for all the other scan rates tested in this study.
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Tantalum based electrolytic capacitors are a separate class of capacitors in which Tantalum is used as cathode materials. Research focused on enhancing the specific
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capacitance of the above said class of capacitors exploit the promising conductivity of graphene in developing solid electrolytes37. Hybrid capacitors combining tantalum oxide and nanocarbon materials like graphene are scarce in the literature38 and this recent work will
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provide an insight towards the development of high energy density capacitors. The cyclic stability of rGO and Ta/rGO upto 1000 number of cycles is evaluated and
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are shown in Fig. 8. Bare reduced graphene oxide electrode materials exhibited almost constant capacitance upto 1000 cycles and in the case of Ta doped graphene oxide the areal capacitance is reduced with respect to increasing number of cycles. However, the areal
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capacitance value of Ta doped graphene oxide is higher (1420 µF /cm2) than undoped
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reduced graphene oxide electrode (980 µF /cm2) at all the scan rates. The percentage decrease in the retention was found to be 33% in the case of Ta/rGO composite whereas with only rGO
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electrode it was 11% decrease in the capacitance.
Fig. 7. Cyclic voltammetry curves of (A) reduced Graphene Oxide electrode (B) Ta/rGO electrode at different scan rates in 1M Na2SO4 electrolyte
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ACCEPTED MANUSCRIPT Table. 1. The areal capacitance of reduced graphene oxide and Ta doped graphene oxide at different scan rates.
(mV/s) 100
980
2.
200
720
3.
300
600
4.
400
510
5.
500
480
1420 1070 910 800 640
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Ta doped graphene oxide
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Reduced graphene oxide
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Areal capacitance (µF/cm2)
Scan rate
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S. No
Fig. 8. Cyclic stability of (a) reduced Graphene Oxide and (b) Ta/rGO electrodes at the scan rate of 100 mV/s.
The electrochemical impedance Spectroscopic (EIS) measurements are carried out in the frequency range of 1 Hz to 100 Hz with the applied potential of 0.5 V. We used this EIS technique to elucidate the electrical conductivity, ion diffusion and charge transfer kinetics of the electrode materials. Fig. 9a shows the Nyquist plot and the parameters of series resistance 11
ACCEPTED MANUSCRIPT (Rs), charge transfer resistance (Rct), double layer capacitance (Cdl), Faradaic capacitance (CF) and Warburg resistance (W) are calculated and is presented in Table. 2. The intersect in the X – axis of the Nyquist plot at high frequency region is called series resistance and this value
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decreased from 21 Ω to 18 Ω after adding tantalum to reduced graphene oxide. Similarly, the
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charge transfer resistance (the diameter of the semicircle at high frequency region) also decreased from 7.5 Ω to 6 Ω with tantalum addition. In the present work, semi-circle is not
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observed in the Nyquist plot and this might be due to high ionic conductivity on electrodeelectrolyte interface39,40. The decrease in the resistance and an increase in the electrical conductivity are due to the existence of metallic tantalum complexes which was confirmed by
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XPS analysis. After the addition of tantalum the double layer capacitance (Cdl) and faradaic capacitance (CF) increased from 4.0 to 5 µF/cm2 and 333to 645 µF/cm2 respectively. The Ta
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doped graphene oxide curve is more vertical than reduced graphene oxide indicating the high capacitive nature of the electrode. The Warburg resistance, which is an indication of the ion diffusion in the electrode/electrolyte interface also decreased from 423 Ω to 95 Ω after the
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addition of tantalum.
Fig. 9 (a) Nyquist plot of reduced graphene oxide and Ta doped graphene oxide (b) Admittance spectra of reduced graphene oxide and Ta doped graphene oxide (c) Bode absolute plot of reduced graphene oxide and Ta doped graphene oxide and (d) Bode angle plot of reduced graphene oxide and Ta doped graphene oxide 12
ACCEPTED MANUSCRIPT From the admittance spectra (Fig. 9b.), the Ta doped graphene oxide material showed high admittance than reduced graphene oxide electrode material. The Bode absolute (log f vs Log Z) plot of both electrode materials is shown in Fig. 9c. The slope value at high frequency
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region is close to zero for both electrodes indicating the characteristic behavior of a resistor.
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Similarly the slope value is less than 1 at low frequency region indicating pseudo-capacitive nature of the electrodes. The Bode angle (log f vs – phase angle) of reduced graphene oxide
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and Ta doped graphene oxide is shown in Fig. 9d. The phase angle of both electrodes is close to 45° confirming the pseudo-capacitive nature. Surprisingly, the phase angle values increased to ~60° at mid frequency region. Due to the partial reduction, the reduced graphene
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oxide also showed pseudo-capacitive nature rather than electrical double layer capacitive nature. Thus it is concluded that the as-synthesized Ta/rGO composite is a hybrid electrode
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exhibiting both electrical double layer and pseudo-capacitance.
Table. 2. Impedance parameters calculated from the Nyquist plot.
Conclusion:
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Cdl in µF/cm2 4.0 4.6
20.6 18.2
Rct (Ω)
W (Ω)
7.5 6.2
423.4 94.6
CF (µF/cm2) 332.9 645.4
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Graphene oxide Ta-GO
RS (Ω)
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Electrode
In this work, we have synthesized the reduced graphene oxide and Ta doped graphene oxide electrode materials by modified Hummer’s method and its physico chemical properties
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are well studied by various characterization technique. The Raman spectroscopy and powder X-Ray diffraction techniques confirmed the graphitization with/without Ta ions. N2 adsorption studies showed the surface area of graphene and Ta doped graphene to be 281 and 214 m2/g indicating the large surface area for charge storage in both these electrodes. The oxidation states and structure morphology of tantalum doped graphene oxide material was studied by XPS and TEM analysis. Ta doped graphene composite exhibited relatively a higher capacitance value of 1420 µF/cm2 than reduced graphene of 980 µF/cm2 at 100 mV/sec The high areal capacitance values for Ta/rGO electrode indicate that tantalum doped graphene oxide composites have remarkable effects and found to be a good candidate for supercapacitor applications.
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33. I. S. Esparza, J. Fan, J. M. Michalik, L.A. Rodríguez, M.R. Ibarra, J.M. Teresa: J. Phys. D: Appl. Phys., 49 (2016) 105301 34. K. Prasenjit, K. Wang, H. Liang: Electrochemical and Solid-State Letters, 11, (2008) C13-C17. 35. C.C. Chun, J.S. Jeng, J.S. Chen: Thin Solid Films, 413, (2002) 46-51. 36. Y. Xie, H. Du: RSC Advances, 5, (2015) 89689-89697. 37. L. Zhao, H. Li , M. Li, S. Xu, C. Li, C. Qu, L. Zhang, B. Yang: Applied Energy, 162, (2016) 197–206. 38. N. Njomo, T. Waryo, M. Masikini, C. O. Ikpo, S. Mailu, O.Tovidea,, N. Rossa, A. Williamsb, N.Matinisea, C. E. Sundaya, N. Mayedwaa, P. G.L. Bakera, K. I. Ozoemenac, E. I. Iwuohaa: Electrochimica Acta, 128, (2014) 226–237 39. R. Rajesh, K. Balkis Ameen, K. Rajasekar: Electrochimica Acta, 180, (2015) 53-63. 40. S. Navjot Kaur, A.C. Rastogi: Nanoscale research letters, 9, (2014) 1. 15
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Highlights Nanocarbon electrode based supercapacitors exhibits high efficiency Metal oxide with multiple oxidation states shows remarkable pseudocapacitive nature
The areal capacitance value of Ta doped reduced graphene oxide is 1420 The cyclic stability of metal oxide/nanocarbon composite is higher
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µF/cm2
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S1572-6657(16)30448-9 doi: 10.1016/j.jelechem.2016.09.003 JEAC 2817
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
6 April 2016 11 August 2016 2 September 2016
Please cite this article as: R. Rajesh, R. Abirami, S.M. Senthil Kumar, K. Rajasekar, K. Balkis Ameen, Synthesis and electrochemical studies of Ta – Graphene nanocomposite film modified platinum electrode, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.09.003
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ACCEPTED MANUSCRIPT Synthesis and Electrochemical studies of Ta – Graphene nanocomposite film modified platinum electrode
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R. RajeshA*, R. AbiramiA, S.M. Senthil KumarB,
Department of Nanotechnology, Anna University Regional Campus Coimbatore,
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K. RajasekarA, K. Balkis AmeenC*
Tamilnadu, India – 641046 B
Electrochemical Materials Science Division, CSIR-Central Electrochemical
Jyoti Ceramic Industries Pvt. Ltd. Satpur, Nashik - 422007, India
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C
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Research Institute, Karaikudi 630003, Tamil Nadu, India
Email: [email protected], [email protected] Abstract:
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Tantalum/Reduced Graphene Oxide composite is presented in this work as an
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electrode material for capacitor application. Reduced graphene oxide material was synthesized by modified Hummer’s method and Tantalum doped graphene oxide electrodes
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are fabricated by an electrophoretic deposition method. The physico-chemical properties of the as-synthesized materials are characterized by X-ray diffraction, N2 sorption analyses, Fourier Transform Infrared and Raman spectroscopic techniques. The structural details are
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elucidated from the scanning and transmission electron microscopic analyses. The X-ray diffraction results confirmed the structural integrity of graphene after the Ta doping process. Raman analysis confirmed the graphitic nature of graphene with a surface area of 281 m2/g. The surface area of the tantalum doped composite decreased to 214 m2/g due to the deposition of Ta ions onto the graphene surface. The Ta doped graphene composite exhibited comparatively higher capacitance value of 1420 µF/cm2 than graphene (980 µF/cm2), indicating that this composite can store more charge in comparison with graphene. This enhancement of capacitance with Ta doped graphene is thus found to be a good candidate for super/ultra capacitor applications
Keywords: Graphene; Graphene/Ta Composite; Supercapacitor; Areal capacitance
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ACCEPTED MANUSCRIPT INTRODUCTION Nanostructured materials for supercapacitor application are drawing more attention recently in revolutionizing the performance of energy storage devices with higher
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energy/power densities1. To date, carbon materials on nanoscale dimensions have proven its capability in achieving high energy/power density, good rate performance and long cycling
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life than other conventional materials2. When compared to the conventional batteries, supercapacitors have high power efficiency, fast charge-discharge time and long cyclic stability. Research has been accelerated towards the development of better electrode material
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to improve the gravimetric capacitance thereby enhancing the energy density of the supercapacitors. Based on the energy storage mechanism, the supercapicitors are classified into two types, electrochemical double layer capacitors (EDLCs) and pseudo-capacitors. In
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EDLCs the charge storage process is non-faradic and the energy storage is electrostatic3 wherein carbon based materials are widely used4. Among the carbon family, graphene based composites are specifically attractive because of its unique electrical and mechanical
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properties5. Pseudocapacitors (metal oxides and conducting polymers) on the other hand
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exhibit capacitance from the faradaic redox reaction occurring on the electrode surface6. Development of supercapacitor with nanostructured materials has been actively
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studied for more than two decades. Electrodes made of metal oxides, conducting polymers, polymer composites possess promising theoretical capacitance, but as these materials tend to be disordered and have low surface area these electrodes suffer less cyclic stability and are
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also relatively expensive. Carbon based materials like activated carbon7, carbon aerogels8, carbon nanotubes9,10 and graphene have been widely examined and showed stable performance characteristics. Especially, metal oxide modified graphene electrodes such as graphene-ZnO11, graphene-MnO212, graphene-RuO213 and graphene/CNT composites14 are gaining much importance for the supercapacitors applications. Even though these materials possess remarkably large surface areas, these materials have not shown promise for highly efficient supercapacitors. The non conformal experimental condition suggests that a systematic study of the nanocarbon structures may provide crucial insights to realise ideal supercapacitors in these systems. In this work, we show that the capacitance behaviour considerately depends on the post synthesis treatment of the graphene layers with tantalum oxide. To understand the interaction of tantalum oxide with graphene layers various physio-chemical characterisation viz. XRD, BET Isotherm, laser Raman spectroscopy and SEM results are studied. The 2
ACCEPTED MANUSCRIPT electrochemical behavior of the electrodes was studied by cyclic voltammetry and AC Impedance technique. Our material exhibited relatively high areal capacitance value
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combined with good cyclic stability.
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MATERIALS AND METHODS Chemicals
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Graphite flakes from Asbury Carbons and tantalum penta chloride (TaCl5) from Sigma-Aldrich are purchased. Potassium Permanganate (KMnO4), Sodium Sulphate (Na2SO4) and ethanol are purchased from Himedia, Hydrogen Peroxide (H2O2) and
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Hydrazine Hydrate (N2H5OH) from LOBACHEMIE are used.
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Exfoliation of graphene layers
Exfoliation of graphite flakes into graphene layers was facilitated by adopting modified hummer’s method15. In a typical synthesis, 1 g of graphite flakes was mixed with 45
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ml of concentrated sulphuric acid and stirred for 30 minutes. As the intercalation process
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starts, the mixture was cooled under ice bath with constant stirring. To this mixture, 6 g of KMnO4 was slowly added in portions and the temperature was maintained at < 20℃ with
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continued stirring. After 3 - 4 hours of stirring, a grey colored viscous solution was obtained. Finally to the reaction mixture, 140 ml of water and 5 ml of H2O2 were added under vigorous stirring for 15 minutes. The yellow color graphene oxide thus obtained was thoroughly washed with 3-4 litres of distilled water followed by 3% H2O2 solution. The
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synthesized graphene oxide was filtered and dried in vacuum oven. Further, graphene oxide was reduced with hydrazine hydrate solution. 50 mg of graphene oxide was mixed with 50 ml of double distilled water followed by few ml of ammonia solution. To this solution, 1ml of hydrazine hydrate (60%) solution was added with continuous stirring. The mixture was refluxed at 100℃ for 12 hours and washed with 3-5 litres of water followed by ethanol and filtered. The black colored reduced graphene oxide was finally air dried and used for the electrode fabrication.
Fabrication of Ta/Reduced Graphene Oxide Capacitor Electrode
The as-synthesized reduced graphene oxide (5 mg) was dispersed in an ethanol solution containing Ta ion (5 mg of TaCl5 dissolved in 20ml of ethanol). This slurry was used
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ACCEPTED MANUSCRIPT to fabricate the electrodes by electrophoretic method16. In a typical procedure, two platinum wires are immersed in the slurry. A D.C voltage of 24V was applied between the electrodes for 2 hours, which forms a smooth coating of Ta doped graphene oxide over the cathodic
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platinum wire. This electrode was further air dried at 80℃ for 12 hours. The blank electrode
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without tantalum ions was also prepared by adopting same procedure. The electrodes prepared from reduced Graphene Oxide and Ta doped reduced Graphene Oxide will be
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hereby denoted as rGO and Ta/rGO throughout this article.
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PHYSICO CHEMICAL CHARACTERIZATION
The physiochemical characteristics of rGO and Ta/rGO composite was studied by X-
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Ray diffraction, Scanning electron microscopy (SEM), Nitrogen adsorption and Raman spectroscopic studies. X-ray diffraction patterns were recorded on a Rigaku Ultima III system.
The surface area analysis of the electrode materials are measured by nitrogen
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physisorption technique with Quantachrome NOVA 1200e instrument. The quality of
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graphene layers and Ta-Graphene composite was examined by HORIBA Jobin Yvon Raman spectromenter equipped with a 543 nm laser. JSM-6390 Scanning electron microscope with
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EDAX and Philips CM20 transmission electron microscope were used to analyze the surface morphology of these materials. XPS analysis was done with Kratos axis ESCA spectrometer with a hemispherical analyzer. The electrochemical characteristics and areal capacitance
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value of both rGO and Ta/rGO electrodes are studied by the CH Instruments CHI601C electrochemical analyzer using three electrode systems. RESULTS AND DISCUSSION X-Ray Diffraction Analysis
The powder X-Ray diffraction patterns of the as-synthesized rGO and Ta/rGO composite are shown in Figure 1. Both the samples showed a well defined pattern with a sharp peak at 2θ = 25° corresponding to the [002] plane and a small peak at 2θ = 44° corresponding to the [100] plane of the graphite structure17,18. We observed that the addition of tantalum did not affect the crystallinity of graphene, as the tantalum ion make a bond with a functional group present on the graphene surface.
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Fig. 1. X-ray diffraction patterns of (a) reduced Graphene Oxide, rGO and (b) Ta/rGO.
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BET- Nitrogen Physisorption Studies
The specific surface area of the as-synthesized rGO and Ta/rGO materials was studied
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by nitrogen adsorption studies (Fig. 2). The samples are degassed at 300℃ for 3 hours in
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nitrogen atmosphere to get rid of adsorbed gases and moisture. The specific surface area was calculated from multipoint BET method and the corresponding value of rGO and Ta/rGO are
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281 and 214 m2/g respectively. The addition of tantalum atoms into the graphene reduced the surface area of graphene material because of the blocking of the graphene surface by Ta atoms still maintaining the layered structure of graphene material19,20. Such a high surface
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area material can be utilized in enhancing the accessible area for the ions during the electrochemical process.
Fig. 2. N2 adsorption isotherms of (a) rGO and (b) Ta/rGO. 5
ACCEPTED MANUSCRIPT Fourier Transform Infra-Red Spectroscopic Studies
The FTIR spectra of rGO and Ta/rGO are shown in Fig. 3. From the spectra, the
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following functional groups are identified. The characteristic O-H stretching vibration peaks
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are observed at 3402 cm-1, the -C=C- non oxidized sp2 bonding peak at 1608 cm-1 and O-H deformation of the C-OH stretching vibration band is observed at 1359 cm-1. The -C=O and 1
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C-O stretching vibration bands of -COOH functional group is observed at 1713 and 1224 cmrespectively21,22. In addition to this, we observed a broad peak at 1000 and 1200 cm-1
indicating the presence of -C–O and -C–C stretching vibrations. In reduced graphene oxide,
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the doublet bands at 1436 and 1372 cm-1 are assigned to an anti-symmetric and symmetric stretching vibration of COO- functional group. After the addition of tantalum, these anti-
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symmetric and symmetric stretching vibration bands shifted to the doublet peaks to 1428 and 1359 cm-1 frequency23. The characteristic stretching vibrations of Ta-O-Ta and Ta-Ox bonds are observed at 907cm-1 and 530 cm-1 indicating the replacement of hydroxyl groups by the
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tantalum ions24. The incorporation of tantalum atom in turn also reduced the height of C-O
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bond stretching vibrations peak as also reported earlier25.
Fig. 3. Fourier Transform IR spectra of (a) rGO and (b) Ta/rGO
Raman Spectroscopic Studies
Raman spectroscopy is a non destructive method to characterize the nanocarbon materials and in this study, samples of rGO and Ta/rGO are analyzed and shown in Fig. 4. 6
ACCEPTED MANUSCRIPT From the spectra, the existence of G band and D band can be seen in both the samples. The G band is assigned to the first order scattering of the E2g mode phonon of C sp2 atoms. The D band arises due to the size reduction in the in-plane sp2 domains by extensive oxidation and is
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assigned to the breathing mode of the A1g symmetry26. In reduced graphene oxide, the G band
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is located at 1585 cm-1, which is very close to the pristine graphite (~1581 cm-1) and the D band at 1359 cm-1 indicating the complete reduction of graphene oxide to graphene. In
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Ta/rGO sample, the G band and D band are observed at 1566 and 1344 cm-1
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addition of tantalum atoms in rGO reduced the peak intensity also shifting the peak position towards the low frequency region. However, in both the cases a strong G-Band indicates the
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graphitized layer structure with the IG/ID value greater than 2 indicating more than 3 layers of graphene29,30. The intensity ratio of G band and D band of rGO and Ta/rGO is 3.59 and 3.75
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inferring the decrease in size of the sp2 domains upon Ta doping26. Further, the addition of tantalum alter the band gap of reduced graphene oxide resulting in the peak shifts31,32. The overtone 2D bands for rGO is located at 2725 cm-1 and for Ta/rGO this band is blue shifted to
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2690 cm-1 accompanied with an increased intensity. This confirmed that Ta transfers charge
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to the graphene layers establishing bonding and thus increasing the conductivity in the resulting hybrid electrode. The metal-graphene bonding and the resulting peak shift is already
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been observed in other metal systems as well with either red/blue shift depending the net charge transfer33. In this study it is thus observed that Ta formed an efficient electrode system with the graphene layers.
1200
-1 1566 cm
a) rGO b) Ta/rGO
1000 900
Intensity (counts)
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-1 1585 cm
800 700 -1 1344 cm
600 500 400
-1 2690 cm
-1 1359 cm
-1 2725 cm
300 200 100 1000
1500
2000
2500
3000
-1
Wavenumber (cm )
Fig. 4. Laser Raman spectra of (a) reduced graphene oxide and (b) Ta/rGO
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ACCEPTED MANUSCRIPT Morphological Characterization
The morphological studies of rGO and Ta/rGO composite materials are studied by
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scanning electron microscope and the images are shown in Fig. 5 (a, b). The SEM studies
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showed the stacked layers of graphene sheets. The presence of tantalum in Ta/rGO was confirmed by the EDAX analysis on the surface scan as shown in Fig. 5c. The EDAX
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analysis showed the concentration of the elements to be 80 wt.% by carbon, 9.8 wt.% by oxygen and 10.2 wt.% by tantalum. Thus the doping of tantalum by 10 wt.% in the rGO matrix was confirmed. The transmission electron microscopy image of Ta doped graphene
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oxide is shown in Fig. 5d. The synthesized reduced graphene oxide is seen as transparent sheet and the presence of tantalum is shown with an arrow. Also, it can be seen that the
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tantalum oxide particles are wrapped within the graphene sheets evidencing an intact bonding
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between Ta and graphene.
Fig. 5. SEM images of (a) reduced graphene oxide, (b) Ta/rGO, (c) EDAX spectrum of Ta/rGO and (d) TEM image of Ta/rGO.
XPS Analysis of Ta Doped Reduced Graphene Oxide The oxidation states of the elements tantalum and carbon are studied by X-ray photoelectron spectroscopy. The C 1s spectrum in Ta/rGO is shown in Fig. 6a. Four components are observed in C 1s spectrum which correspond to non oxidized carbon at 284.3 eV, the oxidized carbon of C-O bond at 286 eV, carbonyl carbon (O-C=O) at 289.1 eV and 8
ACCEPTED MANUSCRIPT carboxylate carbon (O=C-OH) at 289.7 eV. Thus the XPS analysis of C 1s spectrum revealed that the as-synthesized material is partially oxidized also in agreement with our FT-IR results. The narrow scan XPS spectrum of Ta 4f level in Ta/rGO is shown in Fig. 6b along with its
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deconvoluted peak positions. We observed two peaks at lower binding energies at 20.5 and
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23 eV corresponding to the +2 and +1 4f7/2 oxidation states of tantalum. These are assigned to the tantalum in its metallic form34. The next doublet peaks are observed at 27 and 27.4 eV
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corresponding to the 4f7/2 and 4f5/2 splitting which is assigned to the +5 oxidation state tantalum pentoxide35. A small peak corresponding to +4 oxidation state is also observed in
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the scan at the binding energy of 24.7 eV.
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Fig. 6. Narrow scan XPS analyse of (a) C1s spectrum and (b) Ta4f spectrum of Ta/rGO sample.
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Electrochemical studies of Ta doped reduced Graphene Oxide Composite
The electrochemical properties of rGO and Ta/rGO are studied by cyclic voltammetry with 1M Na2SO4 electrolyte. Ta/rGO was deposited on to a platinum wire and used as the working electrode with Pt and saturated KCl filled Ag electrodes as counter and reference electrode. The cyclic voltammetry studies are performed at different scan rates (0.1, 0.2, 0.3, 0.4 and 0.5V/s) and the current – voltage characteristics of rGO and Ta/rGO are shown in Fig. 7(A). and 7(B). The following equation36 is used for the calculation of areal capacitance and are listed in Table.1. CA= ( where,
/ (2S x ΔV x v)
is integrated area of the CV curve, v scan rate in V/s, ΔV and S are
potential window and active surface area of the electrode respectively. The electrode with a surface area of 0.079 cm2 is used. 9
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It can be seen from Table. 1 that Ta/rGO material exhibited a higher capacitance of 1.5 mF/cm2 in comparison with rGO (1 mF/cm2) at the scan rate of 100 mV/sec. This
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Tantalum based electrolytic capacitors are a separate class of capacitors in which Tantalum is used as cathode materials. Research focused on enhancing the specific
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capacitance of the above said class of capacitors exploit the promising conductivity of graphene in developing solid electrolytes37. Hybrid capacitors combining tantalum oxide and nanocarbon materials like graphene are scarce in the literature38 and this recent work will
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provide an insight towards the development of high energy density capacitors. The cyclic stability of rGO and Ta/rGO upto 1000 number of cycles is evaluated and
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are shown in Fig. 8. Bare reduced graphene oxide electrode materials exhibited almost constant capacitance upto 1000 cycles and in the case of Ta doped graphene oxide the areal capacitance is reduced with respect to increasing number of cycles. However, the areal
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capacitance value of Ta doped graphene oxide is higher (1420 µF /cm2) than undoped
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reduced graphene oxide electrode (980 µF /cm2) at all the scan rates. The percentage decrease in the retention was found to be 33% in the case of Ta/rGO composite whereas with only rGO
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electrode it was 11% decrease in the capacitance.
Fig. 7. Cyclic voltammetry curves of (A) reduced Graphene Oxide electrode (B) Ta/rGO electrode at different scan rates in 1M Na2SO4 electrolyte
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ACCEPTED MANUSCRIPT Table. 1. The areal capacitance of reduced graphene oxide and Ta doped graphene oxide at different scan rates.
(mV/s) 100
980
2.
200
720
3.
300
600
4.
400
510
5.
500
480
1420 1070 910 800 640
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Ta doped graphene oxide
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Reduced graphene oxide
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Areal capacitance (µF/cm2)
Scan rate
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S. No
Fig. 8. Cyclic stability of (a) reduced Graphene Oxide and (b) Ta/rGO electrodes at the scan rate of 100 mV/s.
The electrochemical impedance Spectroscopic (EIS) measurements are carried out in the frequency range of 1 Hz to 100 Hz with the applied potential of 0.5 V. We used this EIS technique to elucidate the electrical conductivity, ion diffusion and charge transfer kinetics of the electrode materials. Fig. 9a shows the Nyquist plot and the parameters of series resistance 11
ACCEPTED MANUSCRIPT (Rs), charge transfer resistance (Rct), double layer capacitance (Cdl), Faradaic capacitance (CF) and Warburg resistance (W) are calculated and is presented in Table. 2. The intersect in the X – axis of the Nyquist plot at high frequency region is called series resistance and this value
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decreased from 21 Ω to 18 Ω after adding tantalum to reduced graphene oxide. Similarly, the
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charge transfer resistance (the diameter of the semicircle at high frequency region) also decreased from 7.5 Ω to 6 Ω with tantalum addition. In the present work, semi-circle is not
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observed in the Nyquist plot and this might be due to high ionic conductivity on electrodeelectrolyte interface39,40. The decrease in the resistance and an increase in the electrical conductivity are due to the existence of metallic tantalum complexes which was confirmed by
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XPS analysis. After the addition of tantalum the double layer capacitance (Cdl) and faradaic capacitance (CF) increased from 4.0 to 5 µF/cm2 and 333to 645 µF/cm2 respectively. The Ta
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doped graphene oxide curve is more vertical than reduced graphene oxide indicating the high capacitive nature of the electrode. The Warburg resistance, which is an indication of the ion diffusion in the electrode/electrolyte interface also decreased from 423 Ω to 95 Ω after the
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Fig. 9 (a) Nyquist plot of reduced graphene oxide and Ta doped graphene oxide (b) Admittance spectra of reduced graphene oxide and Ta doped graphene oxide (c) Bode absolute plot of reduced graphene oxide and Ta doped graphene oxide and (d) Bode angle plot of reduced graphene oxide and Ta doped graphene oxide 12
ACCEPTED MANUSCRIPT From the admittance spectra (Fig. 9b.), the Ta doped graphene oxide material showed high admittance than reduced graphene oxide electrode material. The Bode absolute (log f vs Log Z) plot of both electrode materials is shown in Fig. 9c. The slope value at high frequency
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region is close to zero for both electrodes indicating the characteristic behavior of a resistor.
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Similarly the slope value is less than 1 at low frequency region indicating pseudo-capacitive nature of the electrodes. The Bode angle (log f vs – phase angle) of reduced graphene oxide
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and Ta doped graphene oxide is shown in Fig. 9d. The phase angle of both electrodes is close to 45° confirming the pseudo-capacitive nature. Surprisingly, the phase angle values increased to ~60° at mid frequency region. Due to the partial reduction, the reduced graphene
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oxide also showed pseudo-capacitive nature rather than electrical double layer capacitive nature. Thus it is concluded that the as-synthesized Ta/rGO composite is a hybrid electrode
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exhibiting both electrical double layer and pseudo-capacitance.
Table. 2. Impedance parameters calculated from the Nyquist plot.
Conclusion:
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Cdl in µF/cm2 4.0 4.6
20.6 18.2
Rct (Ω)
W (Ω)
7.5 6.2
423.4 94.6
CF (µF/cm2) 332.9 645.4
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Graphene oxide Ta-GO
RS (Ω)
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Electrode
In this work, we have synthesized the reduced graphene oxide and Ta doped graphene oxide electrode materials by modified Hummer’s method and its physico chemical properties
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are well studied by various characterization technique. The Raman spectroscopy and powder X-Ray diffraction techniques confirmed the graphitization with/without Ta ions. N2 adsorption studies showed the surface area of graphene and Ta doped graphene to be 281 and 214 m2/g indicating the large surface area for charge storage in both these electrodes. The oxidation states and structure morphology of tantalum doped graphene oxide material was studied by XPS and TEM analysis. Ta doped graphene composite exhibited relatively a higher capacitance value of 1420 µF/cm2 than reduced graphene of 980 µF/cm2 at 100 mV/sec The high areal capacitance values for Ta/rGO electrode indicate that tantalum doped graphene oxide composites have remarkable effects and found to be a good candidate for supercapacitor applications.
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Highlights Nanocarbon electrode based supercapacitors exhibits high efficiency Metal oxide with multiple oxidation states shows remarkable pseudocapacitive nature
The areal capacitance value of Ta doped reduced graphene oxide is 1420 The cyclic stability of metal oxide/nanocarbon composite is higher
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