Room temperature PEDOT:PSS encapsulated MWCNTs thin film for electrochemical supercapacitor

    Room temperature PEDOT:PSS encapsulated MWCNT’s thin film for electrochemical supercapacitor Swapnil S. Karade, Babasaheb R. Sankapal PII: DOI: Reference:

S1572-6657(16)30159-X doi: 10.1016/j.jelechem.2016.04.012 JEAC 2590

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

5 December 2015 2 April 2016 5 April 2016

Please cite this article as: Swapnil S. Karade, Babasaheb R. Sankapal, Room temperature PEDOT:PSS encapsulated MWCNT’s thin film for electrochemical supercapacitor, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.04.012

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ACCEPTED MANUSCRIPT Room temperature PEDOT:PSS encapsulated MWCNT’s thin

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film for electrochemical supercapacitor

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Swapnil S. Karade, Babasaheb R. Sankapal*

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Nano Materials and Device Laboratory, Department of Applied Physics, Visvesvaraya National Institute of Technology, South Ambazari Road, Nagpur, 440010 Maharashtra, India.

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Abstract

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Simple and cost effective ‘dip and dry’ approach has been demonstrated for the coating of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) onto multiwalled carbon nanotubes (MWNCTs) thin film, films are characterized by different

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techniques such as X‒ ray diffraction, Fourier transform infrared spectroscopy and scanning and transmission electron microscopy techniques where uniform coating of amorphous

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PEDOT:PSS on MWCNTs has been confirmed. The electrochemical performance of the PEDOT:PSS/MWCNTs electrode has been evaluated by using cyclic voltammetry and

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galvanostatic charge-discharge techniques. The specific capacitance of 235 Fg-1 at 5 mVs-1 has been achieved within 0 to 0.9 V potential range in 1M aqueous H2SO4 electrolyte. Furthermore, electrochemical impedance spectroscopy (EIS) analysis showed the lower equivalent series resistance and excellent frequency response of PEDOT:PSS/MWCNTs electrode material. Keywords: Multiwalled carbon nanotubes, PEDOT:PSS, Thin Film, Supercapacitor *Corresponding Author: [email protected]; [email protected] Contact No.: +91 (712) 2801170; Fax No.:- +91 (712) 2223230

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ACCEPTED MANUSCRIPT 1. Introduction The growing demand for energy and power sources, supercapacitors are attracting

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great interest because of having high power density than dielectric capacitors and the high

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energy density than rechargeable batteries [1, 2]. Such types of supercapacitors have greater

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advantage in digital communication, electric vehicle, pulse power application and short term power sources for mobile electronic devices [3, 4]. According to the charge storage

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mechanism, supercapacitor electrode materials are divided into two categories. The carbon based materials with high surface area and long cycle life appear under electrical double layer

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capacitors (EDLCs) have storage capacity practically lower than that of batteries. The metal oxides, metal chalcogenides, and conducting polymers with excellent redox activity and

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prominent electrochemical properties show the pseudocapacitive behaviour with high specific

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capacitance compared with EDLCs [5]. The hybrid capacitor is a combination of EDLCs and

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pseudocapacitor [6, 7], means combination of faradic and non-faradic reactions. Multiwalled carbon nanotubes (MWCNTs) are an excellent form of carbon, which have attracted much attention as supercapacitor electrode material due to their unique

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mesoporous network, high electrochemically accessible surface area, outstanding electrical conductivity, and excellent chemical stability. CNTs have a novel carbon nanostructure with a densely packed honeycomb with three dimensional porous nanostructure, due to its unique properties such as large specific surface area, high electrical conductivity, excellent electron transfer rate and high mechanical strength expands a highly promising material for many applications in nanoelectronics, flexible electronics, batteries, supercapacitors, solar cells, gas and chemical sensors [8-12]. Furthermore, MWCNTs provide well directed conducting path to facilitate a fast electrochemical kinetic process during high current density charge/discharge. Increase in supercapacitance of MWCNTs is very hard to achieve due to

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ACCEPTED MANUSCRIPT difficulties in controlling the pore size and electrochemically active surface area. To increase energy as well as power density, transition metal oxides and conducting polymers

to

enhance

electrochemical

performance

[13-16].

To

achieve

better

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MWCNTs

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(polyaniline, polypyrrole, polythiophene, PEDOT:PSS etc.) must have be deposited with

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electrochemical performance, some unique composition of carbon materials including MWCNT, carbon cloth, graphene oxide have been structured [17]. To commercialize the supercapacitor technologies for large scale application, the electrode must be scalable,

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inexpensive, low cost processing, which can have better stability.

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The development of composites for use in supercapacitor electrodes comprising of conducting polymers with the nano-network structure of the CNTs provides enhanced

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electronic and ionic conductivity that can considerably improve the charge storage and

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delivery [18–19]. It was suggested that conducting polymers like polypyrrole, polyaniline and polythiophene can be stored charge through redox reactions but the carbon materials lacks

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this property [20]. Peng et al [20] and Snook et al [21] summarise the work regarding the composites of carbon nanotubes (CNTs) and conducting polymers in their review. They

compared

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concluded that these composites could improve the specific capacitance or conductivity when with

each

individual

constituent.

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poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a versatile and solution-processable polymers useful for making transparent, conductive and antistatic films. Among list of polymers, PEDOT:PSS is a conjugated polymer i.e. a mixture of two ionomers widely used as the active material in flexible and printed electronics because of its good electrical conductivity, high transparency, low redox potential and good process ability [22, 23]. Amongst conducting polymers, PEDOT exhibits not only a high conductivity but also an unusual stability in the oxidized state, being considered as perhaps the most stable conducting polymer currently available and hence many researchers have studied PEDOT as the

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ACCEPTED MANUSCRIPT electrode material for supercapacitors [24]. Lee et al. [25] reported the in-situ polymerization of many polymeric composites with MWCNTs for supercapacitor application. Yang et al.

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[26] reports the PEDOT:PSS/MWCNTs composite at different mass ratios for supercapacitor

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application. Higher molecular weight PSS is to acts a counter ion and keep the PEDOT chain

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segments dispersed in the aqueous medium can be used to make thin film. Hence, combination of PEDOT:PSS with novel carbon nanostructures may be a potential solution to the shortcomings. PEDOT/PSS has a conjugated backbone that allows easy transportation of orbital system. It is interesting as a charge storage

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de-localised electrons through the

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material because of the oxygen atoms that have unfilled valence shells leading to higher doping levels [27].

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In the present investigation, simple, low cost and controlled ‘dip and dry coating’

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method has been used to deposit the composite of multiwalled carbon nanotubes (MWCNTs)

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and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). The synthesis process includes two steps: deposition of MWCNTs on the stainless steel sample followed by coating of PEDOT:PSS shell on MWCNTs by ‘dip and dry coating’ technique. The formed

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film has been used as an active electrode material in supercapacitor and symmetric device with prior structural surface morphology and chemical characterization. 2. Experimental 2.1 Deposition of MWCNTs on stainless steel substrate The commercially available MWCNTs (>95% purity, outer diameter from 15-20 nm and length from 5-15 μm) were purchased from Nano Amor (Housten, TX). The MWCNTs were refluxed in H2O2 at 60 oC for 48 h to remove amorphous carbon and to generate functional groups which provides more reaction sites with conducting channels. These functionalized MWCNTs were thoroughly rinsed with double distilled water and dried in oven at 90oC for

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ACCEPTED MANUSCRIPT 12 h. Further, 0.125 g of functionalized MWCNTs was sonicated in 25 ml aqueous solution with 1 wt. % Triton X-100 for 1 h to obtain stable dispersion. Well cleaned stainless steel

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substrate was immersed in this solution for 10s so that the MWCNTs get adsorbed on the

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was repeated for 15-20 times to get desired thickness [28].

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surface of the substrate and further dried under IR lamp to get rid of solvents. This process

2.2 Coating of PEDOT:PSS on MWCNTs

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The shell formation of PEDOT:PSS on MWCNTs was performed by using ‘dip and dry coating’ technique. The commercially available PEDOT:PSS solution from Baytron (HC

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Starck’s Clevios PH1000) was used to form the shell with the optimized ratio of PEDOT:PSS: H2O (1:4). At each time, each volume of the ratio was ultrasonicated for 1 h.

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Then MWCNTs coated stainless steel substrate was immersed in PEDOT:PSS solution for 9

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sec and dried under the IR lamp for 10 min in air. The ratio (PEDOT:PSS: H2O),

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ultrasonication and dipping times were optimized to get uniform and thin layer coating of PEDOT:PSS on MWCNTs towards better supercapacitive performance. The schematic about

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the shell formation of PEDOT:PSS on MWCNTs is shown in figure 1. 2.3 Characterizations The structural study of PEDOT:PSS/MWCNTs thin film was performed by using X-ray diffractometer (X-Ray Diffraction System Ultima IV of Rigaku Corporation, Japan) operating at 40 kV and 40 mA. Fourier transform infrared spectroscopy (FTIR) was performed by using (JASCO 410) in the spectral range 4000-400 cm-1. The surface morphological study was performed by field emission scanning electron microscope (FESEM, JSM-6360LA). The electrochemical properties of the supercapacitor including chargedischarge and electrochemical impedance were performed by using potentiostat/Galvanostat (Princeton Applied Research, PARSTAT-4000, USA). A typical three electrode cell was

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ACCEPTED MANUSCRIPT employed consisting of Ag/AgCl as reference electrode (RE), platinum wire as counter electrode (CE) and PEDOT:PSS/MWCNTs as working electrode (WE). The mass loading of

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sample PEDOT:PSS/MWCNTs sample on SS substrate was measured by gravimetric weight

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difference method using a sensitive microbalance.

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3. Result and discussion 3.1 Structural studies

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The X-ray diffraction patterns of stainless steel as reference, MWCNTs and

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PEDOT:PSS/MWCNTs thin films are as shown in fig. 2 (a). The peak (002) at 2 = 26.24o gives the clear evidence for the characteristic graphitic peak of MWCNTs [28], whereas no

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any specific peak observed for PEDOT:PSS, so that PEDOT:PSS showing polymeric the

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amorphous nature. The small peak at 21.38o in both MWCNT and composite films arising due to the residual oxygen containing functional groups in the CNT. Where, other peaks

substrate.

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obtained at 43.58, 50.40 and 74.34 degree results from the contribution of stainless steel

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The chemical structures of the synthesized MWCNT and PEDOT:PSS/MWCNT composite films have been confirmed by using FTIR spectra obtained in the range from 2500 cm-1 to 450 cm-1 (Fig. 2 (b)). As shown in the MWCNT curve, the absorption bands at 2315, 1511 and 1219 cm-1 are associated with stretching of the carbon nanotube backbone [12]. The IR bands at 1509, 1310, and 1229 cm-1 are mainly from the C=C and C–C stretching of the quinoidal structure of the thiophene rings and the sulfonic acid groups of PSS. The vibrations at 1143 and 1090 cm-1 may originate from to C–O–C bond stretching in the ethylene dioxy group. The C–S bond stretching in the thiophene ring can also be seen at 980 and 691 cm-1 [29-31]. This results supports well for the formation of PEDOT:PSS shell on MWCNTs.

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ACCEPTED MANUSCRIPT 3.2 Surface morphological study Figure 3 (a, b) shows the FESEM images of MWCNTs and PEDOT:PSS/MWCNTs with

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a scale bar of 300 nm. Uniform coverage of PEDOT:PSS is clearly seen on entire outside

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surface of MWCNTs. The wrapped PEDOT:PSS on MWCNTs keeps the mesoporous

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network of nanotubes which help to better interaction between electrode and electrolyte. This this porous network is highly desirable to maximize the surface area and potentially allows

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the larger values of capacitance which can be obtained during supercapacitor studies [32]. Fig. 3c shows the TEM image of PEDOT:PSS/MWCNTs. TEM image roughly evaluate the

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thickness of PEDOT:PSS deposited on MWCNTs is about 6 to 7 nm.

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3.3 Supercapacitor studies

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The electrochemical properties of PEDOT:PSS/MWCNTs thin films were investigated by employing the cyclic voltammetry (CV) measurements in a 1 M H2SO4 aqueous electrolyte

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within potential window of 0 to +0.9 Vs Ag/AgCl using three electrode system. The maximum anodic and cathodic currents with symmetry, rectangular type nature, and large

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area under the CV curve are the ideal characteristics of CV for SCs. Fig. 4 (a) shows the CV curves for number of dipping cycles of PEDOT:PSS on MWCNTs coated SS substrate. Initially current increases up to 9 dipping cycles then it is decreases. Fig. 4 (b) shows the variation of specific capacitance with number of dipping cycles. The rate of increase in the specific capacitance is nonlinear. Furthermore, slight decrease in specific capacitance is observed could be attributed to the increase in thickness of PEDOT:PSS on MWCNTs, which may develop stress to cause delamination, resulting in peeling off the film after the film reaches at terminal thickness (maximum specific capacitance). In order to find the suitable electrolyte for better electrochemical performance for PEDOT:PSS/MWCNTs electrode, CV curve of the composite were recorded at 100 mVs-1 scan rate using various aqueous

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ACCEPTED MANUSCRIPT electrolytes (Na2SO3, NaOH, KOH, KCl, Na2SO4 and H2SO4) with 0.5M concentration of and are shown in fig. 4 (c). It can be observed that some of the curves exhibit an almost

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rectangular and symmetric shape which indicates non-faradaic charging (electrochemical

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double layer capacitance). Furthermore, the CV curves exhibit prominent redox peaks, which

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indicate faradic charging (pseudocapacitance). The corrosion of SS is possible at high temperature by alkali salts, so that the PEDOT:PSS/MWCNTs layer protective against stainless steel corrosion by the electrolytes at room temperature [33]. The H2SO4 electrolytic

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solution gives higher area under the CV curve due to the higher conductivity of electrolyte.

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The specific capacitance was calculated by using the following equation;

and

denotes

the

response

current.

The

mass

is the potential of

MWCNTs

and

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range

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is the specific capacitance, υ is the potential scan rate,

where,

PEDOT:PSS/MWCNTs thin film on SS substrate is 0.31 and 0.40 mg cm-2, respectively. Inset of fig. 4 (c) shows the values of specific capacitance with respect to variation in

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electrolytes. The fig. 4 (d) shows the comparative CV curve of MWCNTs and PEDOT:PSS/MWCNTs composite thin films at the scan rate of 100 mVs -1 in H2SO4 electrolyte. Inset of fig. 4 (d) shows the plot of specific capacitance for MWCNTs and PEDOT:PSS/MWCNTs thin film samples. The CV cycle indicating identical change in anodic and cathodic current, due to that specific capacitance of MWCNT sample is low (31 Fg-1) compared with composite thin film of PEDOT:PSS/MWCNTs (71 Fg-1). Hence, it is conclude that PEDOT:PSS coated MWCNTs shows superior supercapacitive behavior than the bare MWCNTs.

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ACCEPTED MANUSCRIPT The variation in electrolyte (H2SO4) concentrations was performed from 0.1 M to 1M at constant potential window and scan rate as shown in figure 4 (e). The change in concentration

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gives distinguishable variation of CV curves which might be due to the efficient or deficient

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ion concentration in electrolyte. The higher rectangular shape CV curve was observed for 1M

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H2SO4 electrolyte. Above 1M H2SO4 concentration the pill-off the film from the substrate surface was observed. The plot of specific capacitance with respect to variation in concentration is as shown in inset of fig. 4 (e). The variation of CV curves in 1M H2SO4 at

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different scan rate is shown in the fig. 4 (f). The scan rate was varied from 5 to 200 mVs-1.

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The PEDOT:PSS/MWCNTs film displays rectangular CV curve at lower scan rate. These results indicate that the charge-discharge response of electric double layer (EDLC) is highly

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reversible and kinetically facile [34]. However, the CV curve at higher scan rates deviates

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from its rectangular shape behavior. The specific capacitance values decreased from 235 to 43 Fg-1 with scan rate from 5 to 200 mVs-1 respectively, which are shown in the inset of

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figure 4 (f). At the higher scan rate, the ions on the electrode are reduced with increasing current, while ions in the electrolyte diffuse too slowly to satisfy the need of ions near the

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interface during the charging and discharging process [35]. That means, as the scan rate increases the concentration gradient at the electrode-electrolyte interface increases and the time constant become larger. The transient response is pronounced leading to lag between charging and discharging of the capacitor causing distortion of CV curve [36]. The galvanostatic charge-discharge curves of PEDOT:PSS/MWCNTs composite are shown in Fig 5 (a) at different current densities (1.25 - 5 Ag-1). The low IR drop is observed during charge-discharge process, indicating the good capacitive and fast charge-discharge behavior of the composite thin film. It is also observed that all charge-discharge curves are not ideal straight line, indicating the involvement of pseudocapacitative behavior for PEDOT:PSS/MWCNTs samples. In other words, the curves start to become linear during the

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ACCEPTED MANUSCRIPT charge-discharge at higher current densities which is likely the EDLC behavior. The nonlinear behavior of charge-discharge profile indicates that, PEDOT:PSS/MWCNTs thin films

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exhibit pseudocapacitive behavior for charge storage [37]. The galvanostatic charge-

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discharge curves were used to calculate the specific capacitance by using the following

where,

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equation:

is specific capacitance,

is the discharge time,

is

is the mass of active material. Additionally, the graph of specific

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potential window, and

is discharge current,

capacitance against the current density is shown in the inset of fig. 5 (a). The obtained values

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of specific capacitance (Cs) were used to calculate energy density (E) and further values of

where,

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energy density used to calculate power density (P) by using following equations:

is the specific capacitance,

is the discharging time and

is the potential

window during discharging cycle. The maximum specific capacitance of 232 Fg-1 at the current density of 1.25 Ag-1 is achieved for composite thin film. Also, it is observed that the specific capacitance decreases by increasing in current density. The values presented in this work are comparable with previously reported values based on relative composition (Table 1) which is achieved by ‘dip and dry’ coating technique. The values of energy density and power density for PEDOT:PSS/MWCNTs sample are plotted in Regone plot (fig. 5 (b)). The maximum energy

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ACCEPTED MANUSCRIPT density of PEDOT:PSS/MWCNTs is 26 Whkg-1 with power density 1.3 kWkg-1. At higher current densities, however, our electrodes have stable and much higher power density. This

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type of electrode is useful for several applications where very high power density and

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moderate energy density is needed. As the current density increased, the specific energy

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slightly decreased and the specific power increased. Thus, even at very high current density, our composite thin film of PEDOT:PSS/MWCNTs shows very good specific power without much loss in specific energy. This remarkable capacitor behavior may be due to better

performance,

the

energy

storage

characteristics

of

the

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electrochemical

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utilization of synergy of PEDOT:PSS and CNT composite. According to the above results of

PEDOT:PSS/MWCNTs composites are illustrated in inset of fig. 5 (b). These results imply

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that PEDOT:PSS/MWCNTs sample is ideally suitable for developing the supercapacitor

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device.

Electrochemical impedance analysis is recognized as one of the principle methods the

fundamental

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examining

behavior

of

electrode

materials

for

supercapacitor.

Electrochemical impedance measurements were carried out between 0.1 Hz and 10 kHz with

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AC amplitude of 10 mV under open circuit potential. Fig. 5 (c) shows electrochemical impedance spectrum in the form of Nyquist plot of PEDOT:PSS/MWCNTs composite electrode with inset as an equivalent circuit, where Zr and Zi are the real and imaginary parts of the impedance, respectively. It can be seen in fig. 5 (c), the obtained plot is composed of a semicircle at the high frequencies, which is related to Faradic reactions. The linear curve at the low frequency region can be attributed to the diffusion controlled process in the electrolyte. That means the slope of the straight line in the low frequency range because of the Warburg resistance which is a result of the frequency dependence of ion diffusion from the electrolyte solution to the electrode interface [38-40]. The existence of constant phase element (CPE) may be due to (i) inhomogeneity at the electrode−electrolyte interface causing

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ACCEPTED MANUSCRIPT the distribution of relaxation times, (ii) porosity, (iii) the nature of the electrode and (iv) the active disorder associated with diffusion [38]. The initial non-zero intercept on Zi at the

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beginning of the semicircle have the value of about 1.4 Ω in 1M H2SO4 electrolyte. The

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resistance projected by the semicircle is due to the active electrode material. The resistance

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value of PEDOT:PSS/MWCNTs composite electrode is 18.32 Ωcm-1 which shows that the obtained charge transfer resistance is comparably low. Fig. 5 (d) shows the cyclic retention stability of MWCNTs and PEDOT:PSS/MWCNTs thin film at 100 mVs-1 scan rate. The

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MWCNT and PEDOT:PSS/MWCNTs composite sustains 74 % and 92 % of its retention

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stability after 2500 cycles. The retention stability of PEDOT:PSS/MWCNTs composite thin film is higher. It might be due to degradation of MWCNTs minimises after the coating of on

it.

These

results

demonstrate

the

outstanding

capability

of

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PEDOT:PSS

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PEDOT:PSS/MWCNTs thin film in high energy and high power storage applications. The rate of intercalation/deintercalation of electrolyte ions in the active electrode material

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depends upon the surface morphology of the active electrode material. The nanostructured porous surface morphology is useful in order to enhance the utilization of active electrode

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material by taking sufficient time for intercalation/ deintercalation of electrolyte ions, results in improved electrochemical performance of the active electrode material [41]. 3.4 Electrochemical properties of symmetric supercapacitor device (SSCD): Thin

film

symmetric

supercapacitor

has

been

fabricated

using

two

PEDOT:PSS/MWCNTs films on stainless steel substrate separated by porous mica as spacer with 1M H2SO4 as an aqueous electrolyte. Fig. 6a shows the symmetric supercapacitor device (SSCD) of PEDOT:PSS/MWCNTs thin films. Fig. 6 (b) shows the CV curves of SSCD of PEDOT:PSS/MWCNTs at scan rates of 5-100 mVs-1 within a potential windows of 0-0.9 V. As the scan rate increases, increment in current is observed which reveals the ideal capacitive and fast charge–discharge behavior of PEDOT:PSS/MWCNTs SSCD. The plot of specific

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ACCEPTED MANUSCRIPT capacitance with scan rate is presented in fig. 6 (c). The maximum specific capacitance of 105 Fg-1 is observed at the scan rate of 5 mVs-1. Frackowiak et al. reported PEDOT:PSS and

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multi walled nanotube composites that achieved capacitance values of 100 Fg-1 [42]. The

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galvanostatic charge-discharge curves of PEDOT:PSS/MWCNTs SSCD are shown in fig. 6

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(d) at different current densities (2-8 Ag-1). The low IR drop is observed during chargedischarge process, indicating the good capacitive and fast charge-discharge behavior of SSCD. The maximum specific capacitance of 108Fg-1 is obtained at the specific current of 1

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Ag-1 (fig. 6 (e)). As a comparison, recent work published by Han et al. involving

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electrochemical tests where the electrode comprised of PEDOT:PSS and with highly surface area graphene oxide in 1M H2SO4, yielded capacitance values of 108 Fg-1 [43]. The ED and

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PD values for PEDOT:PSS/MWCNTs SSCD device are calculated using the charge-

maximum

ED

of

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discharges curves and plotted in Fig. 6 (f). As seen from Ragone plot, SSCD exhibits 12.18

Whkg-1

with

PD

of

1100

Wkg-1.

Interestingly,

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PEDOT:PSS/MWCNTs SSCD device demonstrates the PD of 4500 Wkg-1 maintaining ED of 10 Whkg-1 at current density of 8 Ag-1. These results imply that, the PEDOT:PSS/MWCNTs

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composite thin film is ideally suitable for developing the SSCD.

4. Conclusion

A shell of PEDOT:PSS on MWCNTs synthesized by a simple and cost effective ‘dip and dry coating’ method has been successfully used supercapacitor electrode material. The porous network of MWCNTs provides fast electronic and ionic conducting channels in electrolyte and the shell of PEDOT:PSS coating on MWCNTs provides high specific capacitance. The maximum specific capacitance of 235 Fg-1 was achieved for PEDOT:PSS/MWCNTs at the scan rate of 5 mVs-1. Significantly, PEDOT:PSS/MWCNTs show remarkable capacity retention of about 92 % in 1M H2SO4 electrolyte. Furthermore symmetric device exhibits

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ACCEPTED MANUSCRIPT maximum energy density of about 12.18 Whkg-1 with power density of 1100 kWkg-1. Hence, this system creates a platform to design high performance supercapacitor electrodes.

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project (Do. No.: SB/S2/CMP/032/2013, dated 20/12/2013).

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Acknowledgements: BRS is thankful to DST-SERB, Govt. of India, through sanctioned

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ACCEPTED MANUSCRIPT Table Caption Table1. The comparative representation of different traits of PEDOT:PSS/MWCNT and

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corresponding composite for supercapacitor application.

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Figure Captions

Fig. 1 Schematic representation of formed shell of PEDOT:PSS on MWCNTs.

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Fig. 2 (a) X-ray diffraction patterns of stainless steel, MWCNTs and PEDOT:PSS/MWCNTs (b) FTIR spectra of MWCNT and PEDOT:PSS/MWCNTs composite thin films.

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PEDOT:PSS coated MWCNTs.

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Fig. 3 FE-SEM images of a) MWCNTs, b) PEDOT:PSS/MWCNTs and c) TEM image of

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Fig. 4 (a) plot of CV curves with different electrolytes, inset shows the specific capacitance against different electrolytes (b) Comparative CV curve of MWCNT and

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PEDOT:PSS/MWCNTs thin film, inset shows obtained values of specific capacitance. (c) CV curves at different concentrations, inset shows the specific capacitance values at

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different concentrations. (d) CV curve at different scan rates of 5 to 200 mVs-1. Inset shows the specific capacitance values with scan rate. Fig. 5 (a) Galvanostatic charge-discharge at different current densities, inset shows the specific capacitance against different current densities. (b) Ragone plot of PEDOT:PSS/MWCNTs sample, inset shows schematic representation of ion diffusion path in PEDOT:PSS shell on MWCNTs for energy storage. (c) Shows Nyquist plot of PEDOT:PSS/MWCNT thin film with an equivalent circuit as in inset. (d) Capacity retention of PEDOT:PSS/MWCNTs sample for 2500 cycles. Inset shows the CV curves for different cycle numbers at 100 mVs-1 scan rates.

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ACCEPTED MANUSCRIPT Fig. 6 (a) The image of symmetric supercapacitor device (SSCD) of PEDOT:PSS/MWCNTs thin films. (b) The CV curves of SSCD within scan rate of 5–100 mVs-1. (c) The graph of

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capacitance

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discharge curves of SSCD device at different current density. (e) The graph of specific

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PEDOT:PSS/MWCNTs SSCD.

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(f)

Ragone

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Method

Electrolyte

PEDOT-PSS/SWCNT

Polymerization

1M NaNO3

Specific capacitance (Fg-1) 133 Fg-1 at 5 mVs-1

PEDOT/CNT

Polymerization

1M H2SO4

100 Fg-1 at 2 mA

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Graphite oxide (GO)/PEDOT composites

In situ polymerization

1M H2SO4

108 Fg-1 at 0.5 Ag-1

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Graphene-PEDOT

Chemical oxidative polymerization

2M HCl

374 Fg-1 at 0.01 Ag-1

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PEDOT-PSS/MWCNT

In situ polymerization under hydrothermal condition

0.5 M H2SO4

198 Fg-1 at 0.5 Ag-1

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PEDOT:PSS/MWCNTs

Dip and dry coating

1M H2SO4

235 Fg-1 at 5 mVs-1

Present work

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Graphical abstract

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Highlights

Shell of PEDOT:PSS on MWCNTs by simple Dip and dry coating technique.

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MWCNTs covered uniformly with shell of PEDOT:PSS.

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PEDOT:PSS/MWCNTs provide high specific capacitance with excellent stability.

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PEDOT:PSS/MWCNTs shows excellent capacity retention.

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