Synthesis and Characterisations of TiO 2 Coated Multiwalled Carbon Nanotubes / Graphene / Polyaniline Nanocomposite for Supercapacitor Applications

Nowadays with ever increasing demand of energy, developing of alternative power sources is an important issue all over the world. In this respect we have prepared nanocomposites based on metal oxide (titanium oxide) coated multiwalled carbon nanotubes (MWCNTs)/polyaniline (PANI) with graphene and without graphene and studied their electrochemical performance. The formation of the polymer in the nanocomposites was confirmed by the Fourier Transform Infrared Spectroscopy (FTIR) study. The morphological characterisations were carried out by the Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM). To characterize the prepared nanocomposites electrode, a cyclic voltammetry test for measuring specific capacitance, and an impedance test were conducted. The highest value of specific capacitance obtained for the TiO2 coated MWCNTs/PANI nanocomposite was 443.57 F/g at 2 mV/s scan rate. Upon addition of graphene nanosheet to the TiO2 coated MWCNTs in a weight ratio of (9:1) the specific capacitance value increased to 666.3 F/g at the same scan rate, also resulting in an increase in energy density and power density.


Introduction
Supercapacitors, also known as electrochemical capacitors or ultracapacitors are of interest in terms of their high energy density and high power density as well as pollution free long term energy supply source.Conventional capacitor has the property of high power density but suffers from low energy density, whereas, conventional battery has got the property of high energy density but low power density.Supercapacitors form a bridge between the two by combining the high energy density and high power delivery status.Supercapacitors store energy either by the formation of electrical double layer at the electrode electrolyte interface, typically known as Electric Double Layer Capacitor (EDLC) or by the pseudocapacitance mechanism or by both.The charge storage mechanism in EDLC is non-faradaic i.e. no electron transfer reaction occurs and the process is directly electrostatic [1].EDLC increases the rate of response but suffers from comparatively less amount of charge storage.In case of redox supercapacitor, the active species undergoes fast and reversible oxidation and reduction.The pseudocapacitance, which ar due to electron transfer ises redox process, causes typically 10 times greater charge storage comparable to that of EDLC.Typically, designing of a supercapacitor requires three essential components, the electrodes, the electrolyte and the separator.The supercapacitor electrode plays the role of charge storage/delivery and determines energy density and power density [2].For the supercapacitor electrode materials, carbon materials, conducting polymer and metal oxides are mostly used [3].Carbon materials such as activated carbon, nano porous carbon materials were used previously but now these are replaced mostly by CNT and graphene.The charge storage mechanism in pristine CNT is due to electrical double layer formation and it has got very good absorption characteristic due to the accessible mesopores formed by the entangled individual CNTs [4].However the specific capacitance of pristine CNT is low and due to their bundle structure, their effecttive surface area decreases.This results in their restricted use in many devices.Graphene is an outstanding material for supercapacitor electrode owing to its very high surface area (2675 m 2 /g), high thermal conductivity, extreme electrical conductivity and very high mechanical strength.Use of metal oxide as electrode materials such as RuO 2 , MnO 2 , SnO 2 , NiO [5][6][7][8] have been studied in detail.The combination of CNT or graphene with the metal oxide also has been investigated in the recent years.A. L. M. Reddy and S Ramaprabhu obtained specific capacitance of 138 and 93 F/g for RuO 2 /MWNT and SnO 2 / MWNT nanocomposite electrodes respectively [9].For the graphene metal oxide nanocomposites, Cheng et al. obtained a specific capacitance of 328 F/g for MnO 2 / graphene at 1mA charging current [10], whereas, Wang et al reported high specific capacitance of 855 F/g for Ni(OH) 2 /graphene nanocomposites at 5mV/s scan rate and 367 F/g for RuO 2 /graphene at 2 mV/s scan rate [11].Conducting polymers such as, polypyrrole, polyaniline (PANI), polythiophene are used as another part of electrode material.Among the various conducting polymers, PANI has been studied extensively due to its low cost, easy synthesis procedure, redox reversibility as well as good environmental stability and moderated electrical conductivity [1,12].Graphene or CNT combines with the conducting polymer and stores energy by the electronic and ionic charge separation as well as by the faradaic charge transfer across the electrode electrolyte interface [1, [13][14][15][16].CNT and graphene have free surface pi-electrons.The surface pi-electrons can interact strongly with PANI through the quinoid ring and thereby facilitate the functionalization of CNT and graphene.LI.Fang et al. found a specific capacitance of 305.3 F/g for CNT/PANI composite with a 50 nm thick coating of PANI over CNT.Similarly, L. B. Kong et al. obtained the highest specific capacitance value of 224 F/g for the MWCNTs/PANI nanocomposite materials containing MWCNTs of 0.8 wt% [17].In case of graphene based PANI composites A. V. Murugan reported highest specific capacitance of 408 F/g for (1:1) weight of PANI and graphene [18].TiO 2 is a non-toxic, low cost transition metal oxide and available in abundance.Various composite materials for supercapacitor applications containing TiO 2 have been investigated in detail.Reddy et al. obtained a specific capacitance of 166 F/g for TiO 2 /MWCNTs nanocrystalline composite by chemical reduction method [9].C. Bian et al. studied the fibriform polyaniline/nano-TiO 2 composite containing 80% conducting polyaniline by mass which showed maximum specific capacitance of 330 F/g at a constant current density of 1.5 A/g [19].Further, A. K. Mishra and S. Ramaphrabu found a maximum specific capacitance of 265 F/g for TiO 2 decorated functionalised graphene [20].However in our present work, we have prepared three nanocomposites based on MWCNTs/PANI, TiO 2 -MWCNTs/PANI and graphene/TiO 2 -MWCNTs/PANI, where the composition of graphene: TiO 2 -MWCNTs is 9:1 by weight.The samples have been prepared by in situ oxidative polymerisation method and their efficiency as supercapacitor electrode material has been studied extensively.

Preparation of Nanocomposites
Titanium oxide (TiO 2 ) coated MWCNTs have been synthesised by following a sol-gel process, already reported by Siu-Ming Yuen et al. [21].In brief, MWCNTs were dispersed in iso-propanol and it was followed by addition of Titanium(IV) n-butoxide and distilled water in a weight ratio of MWCNT:titanium(IV) n-butoxide:iso-propanol: water (1:0.3:50:25).The whole reaction mixture was kept at room temperature for 48 hours and then the isopropanol was evaporated by heat treatment to obtain TiO coated MWCNTs.Nanocomposites based on unmodified MWCNTs/PANI and TiO 2 modified MWCNTs/PANI were prepared by insitu oxidative polymerisation of aniline, reported elsewhere.60 mg MWCNTs were dispersed in water by using cetyltrimethyl ammonium bromide (CTAB) as dispersing agent by ultrasonication for minutes using ultrasonic processor, Sonapros PR-250.To that suspension, 0.6 ml aniline monomer was added and sonicated for further 10 minutes.Subsequently, ammonium persulphate (APS) solution containing 2.04 gm APS was added and the sonication was continued for another 20 minutes.Then the whole suspension was kept at 0˚C -5˚C for 12 hours, filtered and washed with water ethanol solution to remove the unreacted monomer and the residue was dried to get MWCNTs/PANI nanocomposites.The same procedure was followed for the TiO 2 -MWCNTs/PANI and graphene/TiO 2 -MWCNT/PANI composites preparation.For preparation of the graphene/TiO 2 -MWCNTs/PANI nanocomposites, the weight ratio initially taken for the graphene to TiO 2 -MWCNTs was (9:1).

Fabrication of Electrode
For fabrication of the electrode for electrochemical characterisations the nanocomposites were dispersed in 1% nafion solution via sonication.The nafion solution was prepared by mixing 10 µl nafion in 1ml dehydrated ethanol.The well dispersed samples were casted onto the glassy carbon (GC) electrode surface (diameter -3 mm) and completely dried in air.Here the GC electrode acted as working electrode.

Fourier Transform Infrared Spectroscopy (FTIR) Study
FTIR of the MWCNTs and graphene based nanocomposites was performed using a NEXUS 870 FTIR (Thermo Nicolet) to investigate the formation of PANI in presence of TiO 2 coated MWCNTs and TiO 2 coated CNT/graphene nanocomposites and the plot is shown in Figure 1.The peak at 1575 cm -1 indicates the C=C stretching frequency of the quinoid ring of the PANI unit, whereas the peak at 1464 cm -1 indicates the C=C stretching of the benzenoid ring [22] that shifts to 1485 cm -1 for the graphene/TiO 2 -MWCNTs/PANI.The peaks at 1299 cm -1 and 801 cm -1 denote the C-N stretching and C-C or C-H stretching of benzenoid unit.The peak at 3433 cm -1 can be attributed to the N-H stretching frequency [23].For the TiO 2 coated MWCNTs/graphene/PANI composite the peak gets broadened near 3435 cm -1 probably due to some moisture absorption thereby O-H stretching frequency contributing to some extent.The peaks at 2855 cm -1 and 2923 cm -1 are associated with the symmetric and asymmetric vibrations of C-H bond [23,24].All these FTIR data indicate the formation of PANI for both the composites.The peak at 608 cm -1 indicates the Ti-O stretching frequency of the TiO 2 unit thus confirming the formation of TiO 2 over MWCNT [25].

XRD Analysis
The modification of MWCNTs surface by TiO 2 was confirmed by the XRD analysis using a Rigaku difractometer with a Cu Kα radiation (λ = 1.54056 cm -1 ). Figure 2 shows the XRD pattern of pure MWCNTs, TiO 2 coated MWCNTs and graphene/TiO 2 -MWCNTs/PANI nanocomposite.For pure MWCNTs, the strong and sharp   200), ( 105), ( 211), (204) crystalline plane respectively [26].This indicates the crystalline nature of the TiO 2 coated MWCNTs.However in case of the composite, the corresponding peak at 25.24˚, which is a characteristic peak of pure PANI is absent which means that most of the PANI is deposited on the surface of the TiO 2 coated MWCNTs [19].This is an advantage as it shortens the diffusion path of the electrolyte and provides a larger electrochemical surface.

Morphological Characterizations
The surface morphologies of the as prepared nanocomposites were analysed by FESEM using Carl Zeiss-SU-PRATM 40 with an accelerating voltage of 5 kV.The samples were first gold coated to make the surface well conducting.Electrochemical characterisations such as Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) of the prepared nanocomposites were carried out by GAMRY Ref 3000 instrument by using a three electrode system where platinum and saturated calomel electrode (SCE) were used as counter and reference electrodes respectively.For the electrochemical characterizations potential window was chosen from  -0.7 V to +0.7 V) under which the nanocom ( posites Specific capacitance: showed excellent electrochemical behaviour and the electrolyte used was 1 M KCl.The specific capacitance (Cs in F/g) of the nanocomposites from the CV measurement are calculated by using the following equation [27].
where, I(V) is the instantaneous current in CV, is the potential window and m is However, the better activity of the TiO 2 -MWCNTs/PANI is the area the mass of the electroactive material.is due to the better synergism between the PANI and the TiO 2 modified MWCNTs surface.TiO 2 is an n-type semiconductor and the surface charge is more than the other region due to the effective contribution of the positively charged depletion region [28].Incorporation of graphene causes an increase in the surface area on which PANI can grow.This improves the area of contact with the electrolyte and an improved conductivity in the redox pseudocapacitive composite structure results in an increased supercapacitance.The CV plot of graphene/TiO 2 -MWCNTs/ PANI at various scan rates are given in Figure 5(b).The non-rectangular nature of the CV curve and presence of peaks at low scan rate indicate deviation from ideal behaviour and the considerable contribution of pseudocapacitance to the total specific capacitance.The pseudocapacitance can arise due to both TiO 2 and PANI. Figure 5(c) represents the variation of specific capacitance with scan rate for the two composites.The energy density and power density of the composites are calculated by using the following equations [29] Energy density Power density is the specific capacitance in F/g, V is the operating voltage range in Volt and T is the discharge time.The highest value of energy density for TiO 2 -MWCNTs/ PANI and graphene/TiO 2 -MWCNTs/PANI was respecttively 120.75 Wh/kg and 181.3 Wh/kg at 2 mV/s scan rate whereas highest power density was respectively 3152 W/kg and 5142.85W/kg at 200 mV/s scan rate.given in Tables 1 and 2.

Electrochemical Im
Electrochemical Impedance Spectroscopy (EIS) was c ried out by using GAMRY ref.3000 using an ac volta amplitude 5 mV between the frequency range 0.1 Hz to1 MHz.EIS is a very important and useful technique to formulate a hypothesis when the EIS experimental data is fitted with an suitable equivalent circuit model.Thus it is a convenient tool to obtain many a electrochemical information such as, electrolyte resistance, charge transfer resistance, double layer capacitance etc. [30].The Nyquist plots of the nanocomposites for analysis the EIS data are shown in Figure 6(a).The equivalent circuit to which the EIS data was fitted is shown in Figure 6(b).
For an ideal capacitor the impedance plot should be a vertical line and parallel to the imaginary impedance axis.However our as prepared nanocomposites show deviation from ideality.The non-ideal behaviour of the nano-

. Conclusion
In this study, the hene/ TiO 2 -MWCNTs/PANI nanocomposites were successfully prepared by in situ oxidative polymerisation method.The two nanocomposites showed maximum specific capacitances of 443.57F/g and 666.3 F/g respectively at 2 mV/s scan rate in 1M KCl.The TiO 2 -MWCNTs acted as current collector as well as conducting wire interconnected among the graphene and PANI.The graphene/ TiO 2 -MWCNTs/PANI composite showed high energy density of 181.3 Wh/kg and very high power density of 5142.85W/kg which were higher than that of TiO 2 -MWCNTs/PANI.The reasonably high value of energy density and power density ensure the nanocomposite functions as an efficient supercapacitor electrode material.

Figure 5 (
a) represents the CV plots of MWCNTs/PANI, TiO 2 -MWCNTs/ PANI and graphene/TiO 2 -MWCNTs/PANI composites at 10 mV/s scan rate which showed specific capacitance respectively, 114 F/g, 158.09F/g, and 269.46 F/g.The highest specific capacitance value obtained for the TiO 2 -MWCNTs/PANI composite was 443.57F/g at 2 mV/s scan rate.Upon addition of graphene at a weight percentage of (9:1) to that of TiO 2 coated MWCNTs, the specific capacitance value increases tremendously.The highest specific capacitance obtained for the nanocomposite, graphene/TiO 2 -MWCNTs/PANI was 666.3 F/g at 2 mV/s scan rate.For MWCNTs/PANI composite the combination of MWCNTs and PANI shows superior performance than the individual component (already reported) possibly due to the combination of easy electrolyte accessibility and a reduction in diffusion distance.

Table 1 . Various values of specific capacitance (F/g), energy d WCNTs/PANI nanocomposites at different scan rate.
sity (Wh/kg) and Power density (W/kg) obtaine M