Improved supercapacitive performance in electrospun TiO2 nanofibers through Ta-doping for electrochemical capacitor applications

Accepted Manuscript Title: Improved supercapacitive performance in electrospun TiO2 nanofibers through Ta-doping for electrochemical capacitor applications Authors: Ankit Tyagi, Narendra Singh, Yogesh Sharma, Raju Kumar Gupta PII: DOI: Reference:

S0920-5861(18)30750-8 https://doi.org/10.1016/j.cattod.2018.06.026 CATTOD 11515

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

27-3-2018 25-5-2018 14-6-2018

Please cite this article as: Tyagi A, Singh N, Sharma Y, Gupta RK, Improved supercapacitive performance in electrospun TiO2 nanofibers through Ta-doping for electrochemical capacitor applications, Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.06.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Improved supercapacitive performance in electrospun TiO2 nanofibers

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Ankit Tyagi,a Narendra Singh,a,b Yogesh Sharmac and Raju Kumar Gupta*a,b

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through Ta-doping for electrochemical capacitor applications

a

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Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur208016, UP, India

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b

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Center for Nanosciences and Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, UP, India c

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Department of Physics, Indian Institute of Technology Roorkee, Roorkee-247667, India

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Graphical Abstract

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1.2

Potential (V)

-1 Specific Capacitance (F g )

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Corresponding author. Tel: +91-5122596972; Fax: +91-5122590104. E-mail address: [email protected]

30 0

-30

O

Ti

Ta

Symmetric Supercapacitor

0

1000

1 A g-1 2 A g-1

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3 A g-1 4 A g-1

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5 A g-1

0.3 0.0 0

20

40

Time (s)

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2000 3000 4000 Number of Cycle

5000

Highlights

Synthesis of Ta-doped TiO2 nanofibers using electrospinning technique

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Ta doping in TiO2 increases the conductivity of TiO2 nanofibers

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2% Ta-doped TiO2 nanofibers exhibited twice the specific capacitance than undoped

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

TiO2 nanofibers

Symmetric devices based on 2% Ta-doped TiO2 nanofibers were fabricated

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Abstract

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Here, we report a facile, cost effective, and potentially scalable electrospinning technique to

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synthesize TiO2 nanofibers and Ta-doped TiO2 nanofibers. The nanofibers were characterized

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through various characterization techniques such as FESEM, TEM, XRD, FTIR, Raman spectroscopy, BET surface area analysis and XPS. The specific capacitance values for TiO2

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nanofibers, 2% Ta doped TiO2 nanofibers and 5% Ta doped TiO2 nanofibers at scan rate of 5 mV s-1 were found to be 111 F g-1, 199 F g-1 and 146 F g-1, respectively in 1 M H2SO4 aqueous

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electrolyte. TiO2 nanofibers and Ta doped TiO2 nanofibers exhibit excellent cycling stability (100% retention in specific capacitance up to 3000 cycles). The superior charge storage

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performance of 2% Ta doped TiO2 nanofibers was found due to enhanced electrical conductivity of material, and facile charge transport. The 2% Ta doped TiO2 nanofibers based symmetric supercapacitor device was fabricated and showed specific capacitance of 81 F g-1 at current density of 0.1 A g-1 which remained 46 F g-1 when current density increased to 5 A g-1 in 1 M H2SO4 aqueous electrolyte. The energy density of symmetric supercapacitor was found

to be 11.25 W h kg-1 at power density of 100.49 W kg-1, which remained as 6.32 W h kg-1 at higher power density of 6504.3 W kg-1. Further, 2% Ta doped TiO2 nanofibers based symmetric supercapacitor also showed an excellent cycling stability up to 5000 charge-discharge cycles. This study indicates the potential of Ta doping in achieving high energy density symmetric

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supercapacitor device.

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Keywords:

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Electrospinning, Supercapacitors, Doping, Energy storage,Nanofibers

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1. Introduction

Environmental pollution is increasing with an alarming rate due to consumption of fossil fuels,1, 2 which leads to increase in the demand of renewable energy sources such as solar energy from Sun, wind energy, hydrothermal energy etc.3 In order to store this renewable energy, the frequently used energy storage devices are battery and supercapacitors (also called electrochemical capacitors or ultracapacitors).4-6 Supercapacitors are superior to batteries in

terms of higher power density (>105 W kg-1), longer cycles life (>100,000 cycles), and safe working, which make them unique for the development of advanced hybrid electric vehicles.79

There are two kinds of supercapacitors; electric double layer capacitors (EDLCs) and

pseudocapacitors, the former one is based on electrostatic adsorption of electrolyte ions at the electrode electrolyte interface10, 11 while the other one is based on faradic charge transfer 11, 18

Generally, carbon based materials i.e.

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between electrolyte ion and active material.

graphene,12 carbon nano tubes,13 carbon nano onions,14, 15 and activated carbons16, 17 are used

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for the development of EDLCs. Transition metal oxide and hydroxides, mixed metal oxides, composite of carbon materials with metal oxides and conducting polymers i.e. polyaniline,

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polypyrrole, and polythiophene materials exhibit pseudocapacitance.17, 19 Due to higher energy density and specific capacitance of pseudocapacitors, they are of great interest for the past few

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years in the energy storage field.20, 21 However, conducting polymer based supercapacitors are

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not suitable due to their poor cycling capability.22, 23 RuO2 is the most popular material used for supercapacitor because of its excellent electrochemical stability and higher specific

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capacitance (~2000 F g-1). However, being a rare earth metal, it is costly and toxic to the environment.24 MnO2,25, 26 NiO,27 Co3O4,28 Co(OH)2,29 Ni(OH)2,30 TiO2,31 and Fe2O332 etc.

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have been used as an alternate to RuO2.

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TiO2 is considered as most attractive pseudocapacitive material because of its higher working voltage window, excellent pseudocapacitive behaviour and higher electronic conductivity.21 Apart from this, its commercial viability, non-toxicity to the environment and humans, good

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chemical stability, and abundance in nature makes it further attractive material for pseudocapacitors.33 Electrochemical properties of TiO2 have been shown differently depending on its different morphologies, crystal phase structure, and particle size.33, 34 Nanostructured morphology of the electrode materials reduces the diffusional length and hence increases the power density of the material.35, 36

One dimensional nanostructured materials are attractive candidates in the energy storage field because of their advantages like large specific surface area, facile electron transport along one direction and easy device construction.36, 37 Electrospinning is most facile, cost effective and highly versatile technique to synthesize one dimensional nanofibers mats with controlled diameter, tunable composition, low density, high porosity, as well as high surface area

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inorganic and polymeric mats.38, 39

TiO2 nanoparticles,40 TiO2 composite with reduced graphene oxide,41 TiO2 nanorods,42 TiO2

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nanotubes,43 TiO2 multi walled carbon nanotubes composite,44 TiO2-PANI composite,23 TiO2PPY composite,45 and TiO2 nanofibers37 have already been used for energy storage. TiO2

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electrospun nanofibers shows specific capacitance of 40 F g-1 at current density of 1 A g-1.46

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To increase the specific capacitance of electrospun nanofibers Ni doping was done by Jose and

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co-workers, Ni doping increased the electronic conductivity of the TiO2 nanofibers and hence

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increase the specific capacitance ~4.5 times (40 F g-1 to 179 F g-1 at current density of 1 A g-1) compared to undoped TiO2 nanofibers.46 The same group also studied the doping effect of Nb

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and Zr to the electrospun TiO2 nanofibers and reported that specific capacitance increases in case of Nb from 40 F g-1 to 280 F g-1 but decreased in case of Zr to 30 F g-1 at current density

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of 1 A g-1. Specific capacitance fading after Zr doping was due to the mismatch in size of Ti+4

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(0.605 Å) and Zr+4 (0.72 Å).47 Ta doping is preferred over Nb doping because of its higher equilibrium solubility within the TiO2 network due to lower energy requirements. Due to similar ionic radius of Ta+5 (0.640 Å) with Ti+4 ions, it is dissolved in to the matrix of TiO2.48

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In the present work, we have demonstrated the synthesis of TiO2 nanofibers (TN) through electrospinning and studied the effect of Ta+5 doping on their electrochemical performance. Due to similarity in size of Ta+5 and Ti+4, Ta doping increases the electronic conductivity of TN and hence increased the specific capacitance. The specific capacitance values for TN, 2% Ta doped TiO2 nanofibers (Ta-TN-2) and 5% Ta doped TiO2 nanofibers (Ta-TN-5) were reported

as 111 F g-1, 199 F g-1 and 146 F g-1 respectively through cyclic voltammetry (CV) technique at scan rate of 5 mV s-1 in 1 M H2SO4 aqueous electrolyte. The Ta-TN-2 nanofibers based SSC device was constructed using 1 M H2SO4 aqueous electrolyte, showed specific capacitance of 81 F g-1 at current density of 0.1 A g-1 which remained 46 F g-1 when current density increased to 5 A g-1. The energy density of SSC was found to be 11.25 W h kg-1 at power density of

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100.49 W kg-1 and which remained 6.32 W h kg-1 at higher power density of 6504.28 W kg-1. Ta-TN-2 nanofibers based SSC also exhibited the excellent cycling stability up to 5000 charge-

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discharge cycles. 2. Experimental Section

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2.1. Materials

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Titanium isopropoxide (TIP, 97%), Tantalum ethoxide (TE, 99.98%), and Poly(vinylidene

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fluoride) (PVDF, average MW = 5,34,000) were purchased from Sigma Aldrich, India.

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Polyvinylpyrrolidone (PVP, MW = 1,300,000) were purchased from Alfa Aesar. Ethanol,

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acetic acid, N-Methyl-2-pyrrolidone (NMP) and sulfuric acid (H2SO4, 98%) were purchased from Merck India. Conducting carbon black (Super P) was purchased from MTI Corporation.

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Graphite sheet for making electrodes were purchased from Nickunj Eximp Entp P. Ltd., India.

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The chemicals as received were used without any additional purification. 2.2. Synthesis of TiO2 and Ta doped TiO2 nanofibers TN were prepared through cost effective electrospinning technique.49 In a typical synthesis,

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0.45 g of PVP were dissolved in 7.5 mL of ethanol and stirrer continuously at 55 °C for 30 min. TIP (1.5 g) were dissolved separately in 3 mL of ethanol and 3 mL of acetic acid under continuous stirring at room temperature for 30 min. Solutions of TIP and PVP were mixed together and stirred for another 10 h. Such prepared electrospinning solution were electrospun with the help of plastic syringe with needle (10 mL). Aluminium (Al) foil wrapped rotating

collector drum was maintain at 10 cm from needle tip. Rotary drum was grounded which collects the nanofibers. The voltage between needle and rotary drum was 13 kV and flow rate of electrospinning solution was maintain at 20 µL min-1 with the help of a syringe pump. Collected nanofibers were dried at 60 °C to get freestanding mat which was subjected to calcination in a muffle furnace for 2 h at 500 °C. The Ta doped TiO2 nanofibers were prepared

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by following the exact procedure but the difference is mixing 2 and 5 mole percentage of TE

into the TIP solution. 2% Ta doped TiO2 nanofibers after calcination are named as Ta-TN-2

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and 5% Ta doped TiO2 nanofibers after calcination are named as Ta-TN-5.

2.3. Material Characterization The surface morphology of the TN, Ta-TN-2 and Ta-TN-5

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were observed by field emission scanning electron microscope (FESEM, Quanta 200, Zeiss,

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Germany). The elemental mapping of the samples was carried out by energy dispersive X-ray

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spectroscopy (EDX linked to FESEM, Oxford Instrument, UK). The transmission electron

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microscopy (TEM, Tecnai 20G2, USA) was further used to observe the morphology of nanofibers. The crystal structures of the samples were analysed through X-ray diffraction

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patterns (XRD) obtained from X-ray diffractometer (X’Pert Pro, PAN analytical, Netherlands) using Cu Kα wavelength (λ = 1.5406 Å). KBr pellet method was employed to collect the

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Fourier transform infrared spectra (FTIR, Perkin Elmer, USA) for TN, Ta-TN-2, and Ta-TN-5

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nanofibers. Raman spectrometer (WiTec, Germany) was used to collect Raman spectra for all the samples. Laser light of 532 nm wavelength was used to collect the Raman spectra. X-ray photoelectron spectroscopic (XPS) measurements were obtained with PHI 5000

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Versa Probe II, FEI Inc. Spectrometer to know the Ta doping in TN. The XPS binding energy values for all the samples were referenced to C 1s hydrocarbon peak at 284.6 eV. N2 adsorption-desorption method (Brunauer-Emmett-Teller, BET) was used to analyse BET surface area and BJH (Barrett-Joyner-Halenda) method was used to analyse pore size distribution of the nanofibers (Quantachrome Instruments, USA). The current-voltage (I-V)

characteristic of TN, Ta-TN-2, and Ta-TN-5 nanofibers were carried out using conducting atomic force microscopy (c-AFM) (MFP-3D Origin, Asylum Research, Oxford instruments). Conducting tip of 28±10 nm with Ti/Ir coating was used to measure local current voltage (I-V) curves.

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2.4. Electrochemical Characterization From electrochemical studies, the performance of active material (TN, Ta-TN-2 or Ta-TN-5)

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were analysed through three electrode configuration, where active material was used as

working electrode, whereas Platinum rod and Ag/AgCl/KCl were used as counter and reference electrode respectively. Active material, super P and PVDF were mixed thoroughly in the

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weight ratio of 70:15:15 in NMP solvent to make slurry. The pieces (1 cm x 3 cm) of graphite

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sheet were cut and coated with the slurry on 1 cm x 1 cm and dried at 80 °C for 12 h to evaporate

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NMP. The material loading were kept in the range of 1-2 mg for all the samples.

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Symmetric supercapacitor device (SSC) was constructed by taking two similar Ta-TN-2

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electrodes as positive and negative electrode with same mass loading. Both electrode were placed over each other and separated by a polypropylene separator (Scheme 1). The

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electrochemical performance of active material and SSC was carried out in potential range of 0 to 1 V at various scan rate (5 – 200 mV s-1) and current densities (0.1 to 5 A g-1) by cyclic

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voltammetry (CV) and galvanostatic charge-discharge (GCD) studies respectively. Electrochemical impedance spectroscopy (EIS) analysis was performed at open circuit voltage

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over a frequency range of 0.01 Hz to 100 kHz. CV, GCD and EIS were performed using Potentiostat/Galvanostat (Autolab 302N, Metrohm, Netherlands). Aqueous 1 M H2SO4 was used as electrolyte throughout the study. Specific capacitance (Cs) of active material were calculated through CV and GCD curves using following equations:

Cs

IdV ∫ =

(1)

mυΔV

IΔt C = s mΔV

(2)

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Here C s is the specific capacitance (F g-1), I is current (A), m is mass of active material (g), υ

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is scan rate (V s-1), Δt is discharging time (s) and ΔV is applied potential window (V).

Following equations were used to calculate the energy density and power density of SSC:

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E Δt

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P=

(3)

A

1 Cs V 2 2

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E=

(4)

A

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Here, E is the energy density of SSC (W h kg-1) and P is the power density of SSC (W kg-1).

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Graphite Sheet

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Polypropylene Separator

1 M H2SO4 electrolyte

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Active material coating

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3. Result and discussion

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Scheme 1: Schematic for SSC

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3.1. Morphological, structural, and elemental characterization Morphological and microstructural details of calcined TN and Ta doped TN were examined

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through FESEM images shown in Figure 1. FESEM images of all the samples shows randomly oriented high aspect ratio nanofibers. The average diameter was found to be 40-60 nm in all nanofibers samples. Morphology of the nanofibers does not change after Ta doping. It is further confirmed by the TEM studies. TEM images shows that small particles arranged in such a way to form a porous, high aspect ratio and aligned nanofibers in all the three samples.50 EDX images of TN and Ta doped TN shows the distribution of Ti, O, Ta elements throughout the

nanofibers, which confirms the uniform distribution of Ta in TN (Figure S1). BET surface area was found to be 36.24 m2 g-1 for TN whereas BET surface area for Ta-TN-2 and Ta-TN-5 was measured as 47.09 m2 g-1 and 46.05 m2 g-1 respectively. BET surface area increased for Ta doped TN as compared to un-doped TN because surface of the fibers become more porous after doping, which is also in agreement with the FESEM and TEM images (Figure1). The pore size

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distribution for all three samples is shown in Figure S2. The pore size is near to 10 nm in all three samples.

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XRD spectra of TN, Ta-TN-2, and Ta-TN-5 nanofibers is shown in Figure 2 (a). The XRD spectra shows that all the TiO2 nanofibers samples are crystallized in anatase phase (JCPDS,

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No. 01-084-1285). The peaks at 25.30°, 36.95°, 37.80°, 48.03°, 53.89°, 55.06°, 62.69°, 68.76°,

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70.29°, and 75.05° in TiO2 nanofibers corresponds to the (hkl) planes of (101), (103), (004),

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(200), (105), (211), (204), (116), (220), and (215) respectively.39 The inset of Figure 2(a) also

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shows that the main anatase peak at 25.30° in TN become broaden and shifts slightly in lower angle side upon Ta doping in Ta-TN-2 and Ta-TN-5, but there is no extra peak present due to

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Ta or Ta oxide. Thereby XRD signify that Ta is doped uniformly in the matrix of TiO2 nanofibers. The lattice parameters are increased after Ta doping due to larger size of Ta+5

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(0.640 Å) as compared to the Ti+4 (0.605 Å). This also indicates the Ta doping in TN.46, 47 FTIR

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spectra shows the peaks at 3423 cm-1 due to -O-H stretching mode of H2O, and peak at 1635 cm-1 due to –O-H bending of physisorbed water molecules (Figure 2(b)). The peak at 710 cm1

is due to stretching mode of Ti-O in anatase in TN, and peak at 504 cm-1 is due to Ti-O-Ti

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vibration band. Ti-O-Ti peak intensity is decreased in Ta-TN-2 and Ta-TN-5 because less TiO-Ti bonds are present due to the interference of doped Ta. Peaks at 2923 cm-1, and 2853 cm1

are assigned to C-H stretching band and peak at 2358 cm-1 is due to C-O stretching mode of

atmospheric CO2.50-53

Raman Spectra of TN, Ta-TN-2 and Ta-TN-5 are shown in Figure 2(c). Raman spectra shows the characteristic Eg peak of anatase TiO2 at 144 cm-1 and 640 cm-1. The peak at 517 cm-1 is due to A1g and B1g vibrational mode of TiO2. The Peak at 396 cm-1 is assigned to B1g mode of TiO2, after Ta doping this peak become broaden and shifts slightly in lower wavenumber direction as shown in Figure 2(c) with a vertical line.54 XPS survey scan spectra for TN, Ta-

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TN-2, and Ta-TN-5 are shown in Figure 3(a) which confirms the Ta in Ta-TN-2, and Ta-TN-

5 samples, whereas peaks corresponding to Ta are absent in TN sample. Figure 3(b), (c) and

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(e) shows the Ti 2p spectra for TN, Ta-TN-2 and Ta-TN-5 samples respectively, peaks

corresponding to Ti+4 2p1/2 and Ti+4 2p3/2 are at 458.6 eV and 464.3 eV respectively. Ti+3 2p3/2 peak at 457.1 eV appears in Ta-TN-2 and Ta-TN-5 sample but this peak is absent in TN sample.

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From the XPS study, it is evidenced that the intensity of Ti+3 peak increased linearly as the doping concentration of Ta increased. Figure 3 (d) and (f) shows the Ta 4f spectra for Ta-TN-

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2 and Ta-TN-5 respectively, and exhibits the peaks at 27.7 eV for Ta+5 4f5/2 and 25.7 eV for

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Ta+5 4f7/2 respectively. As the concentration of Ta doping increased in TiO2 nanofibers the

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lattice structure of TiO2 distorts as Ta+5 replaces the Ti+4 in the lattice after doping due to the larger size of Ta+5 as compared to Ti+4. This finding is also supported by XRD results. Because

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of this reason, it is understood that there is one hopping mechanism exist between Ta+5 and

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reduced Ti+3 sites which may be the reason for higher conductivity of the doped samples.55 I-V characteristic of TN, Ta-TN-2 and Ta-TN-5 nanofibers was measured by spin coated their respective samples over a Si wafer to form a film. The scanning area of 5 µm х 5 µm was used

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to obtain topographic images in contact mode and corresponding mapped current images were obtained. Figure S3 shows that topographic images and current images for TN, Ta-TN-2 and Ta-TN-5 samples are showing reasonable correlation. The dark regions in the current images corresponds to less resistive region and the bright spots corresponds to relatively more resistive domains.56-58 The local I-V curves were obtained by choosing a specific point on the dark

region of corresponding current images of TN, Ta-TN-2 and Ta-TN-5 samples. The current was measured at these points by applying voltage sweep from -2 V to +2 V to obtain local I-V curves for all the samples. Figure S4 shows I-V characteristic for TN, Ta-TN-2 and Ta-TN-5 samples. I-V characteristics for TN, Ta-TN-2 and Ta-TN-5 samples shows the formation of Schottky junction between c-AFM tip and TN, Ta-TN-2 and Ta-TN-5 nanofibers. I-V curves

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exhibited that maximum current was found for Ta-TN-2 sample compares to TN and Ta-TN-5 samples, which corresponds to higher electrical conductivity of Ta-TN-2 compare to TN and

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d)

e)

c)

f)

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a)

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Ta-TN-5 samples.59, 60

Figure 1: FESEM images of a) TN, b) Ta-TN-2, c) Ta-TN-5; TEM images of d) TN, e) Ta-

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TN-2, and f) Ta-TN-5.

Intensity (a.u.)

TN Ta-TN-2 Ta-TN-5 24

20

25

26

2(degree)

40 60 2(degree)

27

80

TN Ta-TN-2 Ta-TN-5

b)

3423 O-H

4000

Normalized Intensity

Transmittance (a.u.)

a)

1635 O-H 710 Ti-O

2358 C-O 2923 2853 C-H 3200

504 Ti-O-Ti

2400

1600

Wavenumber (cm-1)

800

1.0

c)

TN Ta-TN-2 Ta-TN-5

Eg

A1g B1g

B1g

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Eg

0.0 200

400

600

800

Wavenumber (cm-1)

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Figure 2: a) XRD spectra for TN, Ta-TN-2, and Ta-TN-5, inset shows the peak corresponding to (101) plane, b) FTIR spectra of TN, Ta-TN-2, and Ta-TN-5, c) Raman spectra of TN, Ta-

400

200

466

Ta-TN-2 Ta 4f

28

26

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460

458

24

Ti+4 2p3/2

Ta-TN-5 Ti 2p

Ti+4 2p1/2

466

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30

462

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Ta+5 4f5/2

e)

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Intensity (a.u.)

Intensity (a.u.)

Ta+5 4f7/2

d)

464

466

Binding Energy (eV)

464

462

460

458

Binding Energy (eV)

456

Ti+3 2p3/2

Ti+4 2p1/2

464

462

460

458

456

Binding Energy (eV)

Binding Energy (eV)

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Binding Energy (eV)

Ta-TN-2 Ti 2p

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Intensity (a.u.)

0

Ti+4 2p1/2

Intensity (a.u.)

600

TNs Ti 2p

Ti+4 2p3/2

c)

N

800

Ti+4 2p3/2

b)

Ti+3 2p3/2

1000

C 1s Ta 4d3 Ta 4d5 Ti 3s Ti 3p Ta 4f

TNs Ta-TN-2 Ta-TN-5

Ta 4p3

Ti 2s

Intensity (a.u.)

O KLL

a)

Ti 2p3

O 1s

TN-2, and Ta-TN-5.

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f)

Ta+5 4f7/2 Ta+5 4f

5/2

Ta-TN-5 Ta 4f

28

26

24

Binding Energy (eV)

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Figure 3: a) XPS survey scan for TN, Ta-TN-2, and Ta-TN-5, b) Ti 2p spectra for TN, c) Ti 2p spectra for Ta-TN-2, d) Ta 4f spectra for Ta-TN-2, e) Ti 2p spectra for Ta-TN-5, and f) Ta 4f spectra for Ta-TN-5.

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3.2. Electrochemical characterization 3.2.1 Three electrode testing In order to investigate the electrochemical performance of the prepared material TN, Ta-TN2, and Ta-TN-5 samples, CV characterization was done in potential range of 0 to 1 V using 3

electrode set up in 1 M H2SO4 as aqueous electrolyte. Figure 4(a) present the CV curves of TN, Ta-TN-2, and Ta-TN-5 at scan rate of 5 mV s-1.

2 0 2 4

4 0 4

8

6

0.0

0.2

0.4

0.6

0.8

(b)

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4

8

5 mV/s 10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s 100 mV/s 200 mV/s

1.0

0.0

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6

(a) -3

TN Ta-TN-2 Ta-TN-5

Current (A)  10

Current (A)  10

-4

8

0.5

1.0

Potential (V, vs Ag/AgCl)

Potential (V, vs Ag/AgCl)

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b) CV curves of Ta-TN-2 at different scan rates.

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Figure 4: a) Comparison of CV curve for TN, Ta-TN-2, and Ta-TN-5 at scan rate of 5 mV s-1

Figure 4(a) shows rectangular CV curves with small oxidation and redox peak for all three

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samples, indicating the combined response of pseudocapacitance and electric double layer capacitance of the TN and Ta doped TN. The oxidation and reduction peak arises due to the

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intercalation-deintercalation of electrolyte ion (H3O+) into the surface of nanofibers.50 It is

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evidenced from the figure 4 (a) that the maximum area of CV curve for the 2% Ta doping reveals the highest specific capacitance for Ta-TN-2 compared to TN and Ta-TN-5. The specific capacitance values for TN, Ta-TN-2 and Ta-TN-5 were found to be 111 F g-1, 199 F

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g-1 and 146 F g-1 respectively (equation 1). This is because of improved faradic reaction in TaTN-2 due to its enhanced electrical conductivity after Ta doping.46 In case of 5% Ta doping specific capacitance value is less because of larger crystal defects.46 Figure 4(b) presents the CV curves of Ta-TN-2 at various scan rates. Curve suggests that as the scan rates increases redox current increases. It is also clear that as the scan rate increases anodic and cathodic peaks

shifts towards more positive and more negative potential respectively, suggesting the reversible faradic process. The CV curves for TN and Ta-TN-5 at different scan rates are given in Figure S5 and S6. The specific capacitance values for TN, Ta-TN-2 and Ta-TN-5 at several scan rates are shown in Table S1. Figure 5 (a) shows the GCD curves for TN, Ta-TN-2, and Ta-TN-5 in

0.6 0.4 0.2

0.8

1.43 A g-1

0.6 0.4 0.2

0

500

1000

1500

2000

1.43 A g-1 2.86 A g-1 4.29 A g-1 5.71 A g-1 7.14 A g-1

(c) 0.8

0.4

0.0

0.0

0.0

1.2

0.14 A g-1 0.71 A g-1

(b)

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0.8

1.0

Potential (V)

Potential (V)

TN Ta-TN-2 Ta-TN-5

Potential (V)

a)

1.0

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aqueous 1 M H2SO4 electrolyte at current density of 0.14 A g-1 in potential range of 0 to 1 V.

0

500

1000

1500

2000

0

40

80

120

160

Time (s)

Time (s)

U

Time (s)

N

Figure 5: a) Comparison of GCD curves for TN, Ta-TN-2, and Ta-TN-5 at current density of

A

0.14 A g-1, b) GCD curve of Ta-TN-2 at lower current densities, c) GCD curves of Ta-TN-2 at

M

higher current densities.

ED

It is expected that the shape of GCD curve should be triangular for EDLCs but in present study it is triangular for all the sample with the appearance of small kink. The non-linearity in the

PT

discharge part of GCD curves again an indication of pseudocapacitive behaviour of the TiO2 and Ta doped TiO2 nanofibers at the electrode-electrolyte interface.61 The presence of

CC E

oxidation-reduction peaks in the CV curves and non-linear shape of GCD curves indicates the deep intercalation of electrolyte ions into the surface of nanofibers. The discharge time was

A

higher for Ta-TN-2 and Ta-TN-5 as compare to undoped TN at the same current density indicates the enhancement in specific capacitance after Ta doping. The specific capacitance was found to be 67 F g-1, 133 F g-1 and 110 F g-1 at the same current density of 0.14 F g-1 for TN, Ta-TN-2 and Ta-TN-5 nanofibers respectively. Figure 5(b) and 5(c) shows the GCD curves for Ta-TN-2 nanofibers at lower and higher current densities.

The specific capacitance values for Ta-TN-2 were found to be 133 F g-1, 119 F g-1, 113 F g-1, 107 F g-1, 104 F g-1, 101 F g-1 and 98 F g-1 at current density of 0.14 A g-1, 0.71 A g-1, 1.43 A g-1, 2.86 A g-1, 4.29 A g-1, 5.71 A g-1 and 7.14 A g-1 respectively. The specific capacitance retained more than 73% after current density increased from 0.14 A g-1 to 7.14 A g-1. The specific capacitance values of TN, Ta-TN-2 and Ta-TN-5 at various current densities are shown

IP T

in Table S2. The GCD curves for TN and Ta-TN-5 at various current densities are given in Figure S7 and S8.

SC R

The potential drop observed in the GCD curve is minimum for Ta-TN-2 nanofiber samples compares to TN and Ta-TN-5 at the same current density which implies that Ta-TN-2

U

nanofibers have lesser internal resistance compare to TN and Ta-TN-5 nanofibers. The lower

N

internal resistance is attribute of higher electrical conductivity of Ta-TN-2 nanofibers than TN

A

and Ta-TN-5 nanofibers. The values of potential drop were found to be 10.38 mV, 8.24 mV

M

and 9.76 mV for TN, Ta-TN-2 and Ta-TN-5 nanofibers respectively at current density of 0.14 A g-1. The potential drop increases as the current density increases for all three samples and

ED

become 112 mV for Ta-TN-2 at higher current density of 2.86 A g-1. Cycling study was done in by GCD measurement in 1 M H2SO4 aqueous electrolyte at current density of 1.43 A g-1 for

PT

3000 cycles for all three samples. It is clear from the Figure 6(a) that after 3000 cycles all the

A

CC E

samples are having more than 100% capacitance retention.

a)

b)

TN Ta-TN-2 Ta-TN-5

250 200

8 6

150 100

50 TN Ta-TN-2 Ta-TN-5

500

2

50

0 0

0

0 0

4

0

1000 1500 2000 2500 3000

50

100

150

2

4

Z' () 200

Z' ()

Number of cycle

6

250

8

300

IP T

-Z'' ()

100

-Z'' ()

Specific capacitance (F g-1)

300

150

SC R

Figure 6: a) Variation of specific capacitance with number of cycles for TN, Ta-TN-2, and TaTN-5 at current density of 1.43 A g-1, b) EIS curves for TN, Ta-TN-2, and Ta-TN-5, inset

U

shows the EIS curves for TN, Ta-TN-2, and Ta-TN-5 at higher frequency.

N

The coulombic efficiency calculated by the ratio of discharging time to charging time in all the

A

samples was found to be almost 100% after 3000 cycles for all the samples. It was observed

M

that there was no peeling off the active material from the electrode surface even after 3000 cycles, which indicates the good physical adhesion of the active material with the electrode

ED

surface. We observed a small increase in the specific capacitance during cycling this might be due to the fact that during continuous charging and discharging more active sites will be created

PT

that helps in enhancing the specific capacitance.61, 62 FESEM images and XRD pattern of Ta-

CC E

TN-2 electrode before and after 3000 charge-discharge cycles are shown in Figures S10 and S11.

The EIS measurements was done to see comparative resistances of TN, Ta-TN-2 and Ta-TN-

A

5 nanofibers in the frequency range of 0.01 Hz to 100 kHz at open circuit potential in 1 M H2SO4 as aqueous electrolyte. Figure 6(b) shows the Nyquist plot obtained for TN, Ta-TN-2 and Ta-TN-5 nanofibers during EIS. Normally, equivalent series resistance (ESR) is the combined resistance including electrolyte resistance, contact resistance between active material and current collector, and resistance due to electro active material, which are calculated by

considering offset in high frequency region of Nyquist plot. It is clear from the Nyquist spectra that ESR, is minimum for the Ta-TN-2 nanofibers compare to TN and Ta-TN-5 nanofibers. The lower value of ESR for Ta-TN-2 nanofibers is attributed due to higher electrical conductivity of it compare to TN and Ta-TN-5 nanofibers. Higher electrical conductivity of Ta-TN-2 sample was also suggested by c-AFM results. The morphology plays an important

IP T

role in ESR of the material, as one dimensional nanofibers having lower ESR than the particles

shaped materials. Hence, fiber shape morphology of our samples helps in reducing ESR of the

SC R

electrodes during electrochemical testing.63 The Nyquist plot is parallel to the vertical axis in

the lower frequency region called Warburg region, which attributes the pseudocapacitive

U

behaviour of the nanofibers.64 It is clear from the figure that Ta-TN-2 nanofibers having higher

N

pseudocapacitive behaviour than TN and Ta-TN-5 nanofibers.

A

3.2.2 Symmetric supercapacitor testing

M

Electrochemical characterization of Ta doped TN in 3 electrode configuration showed that 2%

ED

Ta doping in TN is more suitable for storing charge due to its higher conductivity and less distorted crystal lattice. SSC was assembled by taking two similar weight electrodes of Ta-TN-

PT

2 separated by separator and tested for supercapacitive performance in 1 M H2SO4 aqueous electrolyte. The CV measurement was done in the potential window of 0 – 1 V at various scan

A

CC E

rate from 5 mV s-1 to 200 mV s-1 (Figure 7(a)).

4 2

1.2

(a)

0 -2

0.3 A g-1 0.4 A g-1

0.8

0.5 A g-1 1 A g-1

0.6 0.4

1 A g-1 2 A g-1

c)

0.9

3 A g-1 4 A g-1

0.6

5 A g-1

0.3

0.2 0.0

d) 10

5

0 0

1500 3000 4500 6000 -1 Power density (W kg )

7500

500 Time (s)

100

-1 Specific Capacitance (F g )

15

0.0 0

0

1000 600

e)

20

40 Time (s)

f)

60 40

60

Before cycling After 5000 Cycles

80 400

200

20

SC R

0.2 0.4 0.6 0.8 1.0 Potential (V, vs Ag/AgCl)

-Z'' ()

0.0

IP T

-4

-1 Energy density (W h kg )

1.2

0.1 A g-1 0.2 A g-1

(b)

1.0

Potential (V)

5 mV/s 10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s 100 mV/s 200 mV/s

Potential (V)

Current (A)  10

-3

6

0

0 0

1000

2000 3000 4000 Number of Cycle

5000

0

200

Z' ()

400

600

U

Figure 7: a) CV curves for SSC at different scan rates, b) GCD curves of SSC at lower current

N

densities, c) GCD curves of SSC at higher current densities, d) Ragone plot for SSC, e)

A

Variation of specific capacitance verses number of cycles for SSC, f) EIS curve of SSC before

M

and after cycling.

ED

The shape of CV curve is rectangular with redox peaks present in anodic and cathodic part of the CV curves, revels the pseudocapacitive behaviour of the SSC device. 50 The shape of CV

PT

curve is not changed significantly and remain rectangular and symmetric even after applying higher scan rate of 200 mV s-1, point toward higher rate capability and electrochemical stability

CC E

of Ta-TN-2. The electrochemical reaction at the surface of electro active material can be understood more deeply through Figure S9. Figure S9 shows the linear relationship between

A

√ν vs peak current during CV, which indicate that electrolyte ions intercalate deep into the surface of Ta-TN-2 fibers and bulk redox reaction takes place. The GCD curves of SSC at varies current densities (0.1 A g-1 to 5 A g-1) are shown in Figure 7(b) and 7(c) in 1 M H2SO4 aqueous electrolyte. The shape of GCD curve is triangular with slight deviation in the linear shape it is because of pseudocapacitive charge storage in the SSC device.62

The shape of GCD curve remain intact even at higher current density of 5 A g-1 reveals higher degree of reversibility during redox process. The specific capacitance values were found to be 81 F g-1, 75 F g-1, 72 F g-1, 71 F g-1, and 70 F g-1 at current density of 0.1 A g-1, 0.2 A g-1, 0.3 A g-1, 0.4 A g-1 and 0.5 A g-1 respectively. The specific capacitance values at higher current density of 1 A g-1, 2 A g-1, 3 A g-1, 4 A g-1 and 5 A g-1 were found to be 66 F g-1, 62 F g-1, 58 F

IP T

g-1, 50 F g-1 and 46 F g-1 respectively. The specific capacitance retained 56% of its initial value after increasing current density 50 times from 0.1 A g-1 to 5 A g-1. The potential drop is 11 mV

1

SC R

at current density of 0.1 A g-1 which increased upto 304 mV at higher current density of 5 A gfor the constructed SSC device. The small potential drop is due to the facile charge transport

U

during charging-discharging and higher conductivity of the active material.

N

Ragone plot for SSC device are shown in Figure 7(d), represents the energy density values at

A

various power densities. The Ragone plot shows that energy density does not change drastically

M

with increase in the power density. This indicates the higher power capability of the SSC device, which finally proves that the as constructed SSC device can be excellent candidate for

ED

the commercial application of supercapacitor devices. The energy density was found to be 11.25 W h kg-1 at power density of 100.49 W kg-1 and remained 6.32 W h kg-1 at higher power

PT

density of 6504.28 W kg-1.

CC E

The specific capacitance values with number of charge-discharge cycles are shown in Figure 7(e) at current density of 1 A g-1.There is no capacitance fading noticed after 5000 cycles. During cycling more number of active pores are formed due to continuous intercalation-

A

deintercalation of electrolyte ions into the Ta-TN-2 fibers crystal lattice. This can be further cleared through EIS spectra in Figure 7(f). Figure shows the EIS spectra of device before and after 5000 charge discharge cycles. It is clear from the EIS spectra that ESR of the device decreases slightly after cycling, this might be due to the fact that after continuous charging and

discharging cycles more active pores are formed, hence charge transport become facile into the pores. Ultimately it provides path for the performance enhanced energy storage supercapacitor. Conclusion In the present study, we have demonstrated a simple and cost effective way to synthesize TN,

IP T

Ta-TN-2 and Ta-TN-5 nanofibers having uniform morphology and diameter in the range of 40-60 nm. The effect of Ta doping on the crystal structure and electrochemical performance of

SC R

TiO2 nanofibers has been studied. The specific capacitance values for TN, Ta-TN-2 and Ta-

TN-5 were found to be 111 F g-1, 199 F g-1 and 146 F g-1, respectively at scan rate of 5 mV s-1. The specific capacitance were found to be 67 F g-1, 133 F g-1 and 110 F g-1 at current density

U

of 0.14 A g-1 for TN, Ta-TN-2 and Ta-TN-5 nanofibers, respectively. Ta-TN-2 nanofibers

N

showed higher electrical conductivity, higher rate capability and less resistance to electrolyte

A

ion transport. SSC device based on Ta-TN-2 nanofibers demonstrated the energy density of

M

11.25 W h kg-1 at power density of 100.49 W kg-1 which remained 6.32 W h kg-1 at higher power density of 6504.28 W kg-1. SSC device showed an excellent cycling stability, 100%

PT

ED

specific capacitance retention after 5000 cycles at current density of 1 A g-1.

CC E

Acknowledgements

A

RKG acknowledges financial assistance from Department of Science and Technology (DST), India, through the INSPIRE Faculty Award (Project No. IFA-13 ENG-57) and Grant No. DST/TMD/CERI/C140(G). DST support to the Center for Nanosciences is acknowledged.

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N

52.

A

64.