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Enhanced Specific Capacitance of Self-assembled Threedimensional CNT/Layered Silicate/Polyaniline hybrid sandwiched nanocomposite for Supercapacitor Application Ramesh Oraon, Amrita Adhikari, Santosh Tiwari, and Ganesh Chandra Nayak ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01389 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 20, 2016

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ACS Sustainable Chemistry & Engineering

Enhanced Specific Capacitance of Self-assembled Three-dimensional CNT/Layered Silicate/Polyaniline hybrid Sandwiched nanocomposite for Supercapacitor Application R.Oraona, A.De.Adhikaria, S.K.Tiwaria and G.C.Nayaka* a

Department of Applied Chemistry, ISM Dhanbad, Dhanbad 826004, Jharkhand, India *Corresponding Author Email: [email protected]

Abstract

Plate-like montmorillonite (known as nanoclay), with ~ 1 nm thick aluminosilicate layers, is the most common layered silicate used due to its availability and environment friendly nature. With increasing demand of cheaper and eco-friendly materials for energy storage applications, these layered silicates could be explored as a suitable candidate. In this work, a series of self-assembled three dimensional electrode materials, for supercapacitors, have been synthesized using carbon nanotube (CNT), closite 30B and polyaniline (PANI) by both in-situ and ex-situ approach. The role of layered silicate towards electro-chemical performances of electrode materials has been explored. Morphological studies revealed the successful coating of PANI over CNT and silicate layers of closite 30B and development of a self-assembled three dimensional porous sandwiched like structure. With the incorporation of layered silicate, morphology was observed to be highly porous with enhanced electro active surface which permits the easy access of electrolyte towards improved electrochemical performance. A 28% increase in specific capacitance, with respect to CNT/PANI nanocomposite, was achieved with the ex-situ addition of closite 30B which increased to 110% for the in-situ product. The sample retains about 92% of initial specific capacitance after 2000 cycles. Similar results of capacitance performance were also documented from Galvanostatic charging discharging analysis at 5A/g. All these results demonstrate the promising behavior of developed electrode material for supercapacitor application. Keywords: Supercapacitor, layered silicate, Carbon Nanotube, self-assembly, hybrid nanocomposite. Introduction:

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The search and increasing demand of efficient alternating material has driven a trend towards downsizing and lesser power consumption with greater performance for energy storage.1 Fabrication of such new desirable material within micro and nanoscale confinement with high accuracy and precision has now become an urgent task for future development in energy storage and conversion. Supercapacitors (SCs), also called as electrochemical double layer capacitor or ultracapacitor, has gained tremendous attention in recent years due to their long cycle life, high power pulse supply, high dynamics of charge propagation and easy operational mechanism.2 By the virtue, they are substantially vital to meet the future energy demand in hybrid electric vehicle, memory back-up systems, and industrial energy management.3 Charge storage in SCs relies on the formation of electrochemical double layer capacitance (EDLCs) and pseudocapacitance at electrode electrolyte interface. EDLCs store charges electrostatically via reversible ion adsorption utilizing high surface area carbonaceous material, whereas later type pseudocapacitance stores energy through fast surface redox reaction involving metal oxides, conducting polymer etc.2, 4 In recent times, a great deal of research have been focused to electronically conducting polymers (ECPs) based SCs owing to its high capacitive performance and low material cost.5, 6

Due to high power & energy density and longer cycle life, ECPs are considered for several

advanced application such as electrochromic displays, batteries (as an auxiliary power source), membrane, sensors, anticorrosive coating and as backup power source for computer memory.2, 7 Among various ECPs like polyaniline (PANI)8,9, polypyrrole (PPy)10,11, polyethylenedioxythiophene (PEDOT)

12

, polythiophene (PTh)

13

etc. PANI has been considered as a

promising electroactive material due to its superior property like variable oxidation state, high electronic conductivity, low cost, good processability, environmental stability and mechanical flexibility.2,

6, 8

However, limited cyclability, high self-discharge rate and slow kinetics of ion

transport during charging-discharging process often restricts proper application of PANI as pseudocapacitor material.7,

8

In this context, to overcome the stability problem with cycling,

integration of PANI with suitable nanomaterial possessing high chemical & thermal stability, high specific surface area are highly desirable towards high capacitive performance. Recently, utilization of CNT as an advanced nanomaterial have become trendy for the fabrication of CNT based energy storage devices. Owing to its high specific surface area and


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unique electronic structure it exhibits extraordinary electrical, optical, chemical and mechanical property thus rising up as an attractive alternative for SCs electrode material.14, 15 Yin and Liu et al.

16

reported the well-constructed CNT mesh/PANI nanoporous electrode material for SCs

application synthesized by chemical in-situ approach. Kim et al.

17

reported the improved

mechanical property of PANI/CNT nanosheet nanocomposite which was synthesized by in-situ chemical polymerization. The presence of sp2 hybridized bonds over the structure of both CNT and PANI in coupled form revealed synergistic effects offering an attractive route to create a new breed of multifunctional material. A strong interfacial coupling via donor-acceptor binding and π-π interaction could be ascribed to the synergistic effect between them. The higher electrical conductivity, high accessible surface area and better charge transfer of CNT interconnectivity increases the specific surface area of polymeric material and thereby improves the electrical conductivity and mechanical property.2,

18

However high entangled structure of

CNT often suffers with the challenges of dispersion, stability and processability in solution due to intense agglomeration caused by strong Vander Waals interaction.19, 20 To further improve the capacitive performance of CNT/PANI nanocomposite a homogenous nanometer-sized and wellordered structural morphology must be considered.21 Previously, CNTs/PANI nanocomposite have been enormously used for SCs application. For instance, Kong et al.

22

reported maximum

specific capacitance of 224 F/g of MWCNTs/PANI nanocomposite synthesized by in-situ chemical oxidative polymerization. Similarly, Dong et al.

7

obtained maximum specific

capacitance of 328 F/g with 25 wt% of MWCNTs/PANI nanocomposite. Mi and Zhang et al. 21 reported the MWCNTs/PANI core shell structure with PANI layer synthesized by microwave assisted polymerization with maximum specific capacitance of 312 F/g and 22 Wh/Kg energy density. Polymer nanocomposite based on layered aluminum silicate or montmorillonite have received enormous research interest in various application due to their enhanced mechanical and physical properties than conventional composites.23 At the nanoscale mixing, the combination of these organic and inorganic constituents has proven to be fruitful pathway24 Layered aluminum silicate (here after mentioned as nanoclay) offers an interesting intriguing properties like natural abundance with environmental stability, platelet like geometry with high aspect ratio and well established intercalation chemistry compared to other silicate material.24, 25 Recently, Vellakkat et al.

25

reported the dielectric constant and transport mechanism of novel intercalated



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nanoclay/PANI composite for industrial purpose prepared by mechnochemical method. Rashmi et al.

26

reported the effect of nanoclay wt% loading on the conductivity of epoxy clay based

nanocomposite, where the highest conductivity was obtained with 2 wt% nanoclay loading achieved due to subsequent increase in volume fraction of entire interfacial region coupled with enhanced free charge carrier. Similarly, Basak et al.

27

reported because of large abundance,

environmentally friendliness and intercalation chemistry it can be used in various polymer nanocomposite as reinforcement phase even at lower percent of clay loading which offers wide array of property improvement. In addition, various scientist used this nanoclay material for various application as a catalyst because of its Bronsted and Lewis acidic sites within its intergallery.28, 29- 30. Although both CNT and nanoclay possessed excellent properties, their applications are limited with a common problem of agglomeration which limits their contribution below theoretical calculated values. In this work we have combined these two nanomaterials where they both synergistically decreased agglomeration and increased the surface area, owing to their interpenetrating structure and organic inorganic nature. Thus, the objective of this work is to extend the use of organoclay and shed new light on the influence that exert on the capacitive performance of clay based polymer nanocomposite. In this present work, we are presenting this nanoclay nanomaterial as a dopant for SCs application. We focused on the novel synthesis of insitu and ex-situ nanoclay based CNT/PANI electrode material. The role of nanoclay was explored as dopant to facilitate the charge storage and electrolyte accessibility within the electrode material. We have also explored the effect caused by the sequential addition of nanoclay to CNT/PANI based nanocomposite and their respective contribution towards the capacitive behavior. Experimental section: Material Used: All chemicals used were taken of analytical grade and used without further purification. Aniline was obtained from RFCL limited new Delhi (India). Ammonium persulphate (APS) was supplied from Merck specialist private limited, Mumbai. Multiwalled carbon nanotubes (MWCNTs) (90% C purity for industrial applications) were supplied from Nanocyl 7000 S.A,


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Rue de lessor 4, Belgium. Nanoclay (Closite 30B) was purchased from Mat Web LLC, Blacksburg, USA. Fabrication of PANI: A facile in-situ approach was employed for the fabrication of PANI in the presence of oxidant APS. Initially, 1 ml aniline was mixed with 50 ml distilled water in a 250 ml beaker. This solution was stirred continuously for 15 minutes. In another 250 ml beaker 2.5 g APS was dissolved in 50 ml distilled water. Then APS solution was added dropwise to aniline solution under constant stirring maintaining a temperature around 0-5°C. After the complete addition of APS, slowly the color of entire solution turned to dusty deep green with progression of time. Then, the entire solution was left for 5 hours under refrigeration. After the polymerization, the obtained product was filtered and washed several times with distilled water. The product obtained was further washed with ethanol and dried under vacuum at 60°C for 12 hours. Fabrication of CNT/PANI nanocomposite: Similar procedure was adopted for the fabrication of CNT/PANI nanocomposite. Initially, 0.05 g MWCNT and 1 ml aniline was dispersed in 50 ml distilled water in 250 ml beaker by sonication (using frontline probe type sonicator, 500 W) for 5 minutes. Another solution of APS (2.5 g of APS dissolved in 50 ml distilled water) added dropwise to MWCNT/aniline solution to initiate polymerization and the whole reaction mixture was kept at 0-5 °C. In order to ensure the efficient polymerization of aniline, this was continuously stirred for 4 hours after the complete addition of APS till the color appeared dusty greenish. After filtration, the product was processed through several washing steps with distilled water followed by ethanol. The product was dried under vacuum at 60°C for 12 hours and coded as CA. The in-situ MWCNT/Nanoclay/PANI nanocomposite was prepared by following the same process as mentioned above. In the first step 0.05g of MWCNT and 0.05g of nanoclay was stirred followed by addition of 1 ml aniline. This solution mixture was further sonicated for better dispersion and polymerization in presence of APS. The as obtained sample was designated as CNA (according to the chronological addition of the constituents). Similarly, for ex-situ composite, CA was synthesized with 0.05g of MWCNT and 1ml of aniline by following the above polymerization process. After the purification process of CA, 0.05 g of nanoclay was



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added to the so prepared CA and sonicated in ethanol for 30 minutes. The resulting mixture was filtered and dried to obtain the ex-situ product and designated as CAN. Figure (1) depicts schematic representation of the synthesis of nanocomposite. Figure 1: Schematic diagram of synthesis of Nanoclay based Nanocomposite Characterization: The as synthesized nanocomposite PANI, CA, CNA & CAN were characterized by FTIR spectroscopy using Perkin Elmer RXI within the range from 400 to 4000 cm-1 to investigate the bonding properties and various functionality. Sample for FTIR analysis were prepared by mixing potassium bromide (KBr) of spectroscopic grade and materials in the weight ratio of 10:1 and pelletized. UV analysis of PANI, CA and nanoclay based CNT/PANI (in-situ & ex-situ) nanocomposite were performed using SCHIMADZU spectrophotometer. The UV spectra of all samples were recorded in the wavelength ranging from 200-700 nm. UV analysis was carried out by dispersing each sample in methanol. Investigation of microstructure and surface morphologies of nanocomposite was carried out using Supra 55 Carl Zeiss, Germany at an accelerating voltage of 5 kV. Sample preparation was done by adhering small amount of sample on carbon tape followed by thin layer gold coating by sputtering. The X-ray diffraction analysis of the samples were conducted on the Bruker X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Ǻ) at a scanning rate of 1°/min. To analyze the coating status and uniformity of PANI over the surface of both MWCNTs and Nanoclay Transmission electron microscopy analysis (HR-TEM; JEOL 2100) was performed. Approximately, 1 mg of sample was added to 10-15 ml acetone and sonicated for 30 minutes continuously. From this well dispersed solution, one drop of solution was placed over copper grid with the help of micropipette for TEM analysis. Conventional three electrode system was employed to measure the electrochemical performance like cyclic voltammetry (CV), galvanostatic charging-discharging (GCD) and Electrochemical Impedance spectroscopy (EIS) in electrochemical workstation CHI 660 C, where nanocomposite, Pt and SCE (Ag/AgCl) were working, counter and reference electrode, respectively. All CV and GCD analysis were recorded within the potential window ranging from 0-0.8 V at various scan rates (10, 20, 50, 100 and 200 mV/s) and current density 5 A/g, respectively, in 1M aq. KCl as an electrolyte. For EIS measurement, a frequency range from 1 MHz to 0.01 Hz was applied with applied amplitude of 5 mV versus open circuit potential.


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Results and discussion: FTIR spectroscopy: Figure (2) represents the FTIR spectra of PANI and nanoclay based CNT/PANI nanocomposites. The characteristics absorption bands of PANI appears near 812 cm-1, 1146 cm1

, 1300 cm-1, 1479 cm-1 and 1566 cm-1 corresponds to C-H out of plane bending vibration of a 1,4-

disubstituted aromatic ring, C-H bending mode of quinoid ring, C-N and C-N

+

stretching

vibration mode or C-H bending mode of a benzenoid ring, C=C stretching mode of benzenoid ring and C=C stretching mode of a quinoid ring, respectively.9 Figure 2: FTIR spectra of pure PANI, CA, CAN & CNA

The broad band at 3438 cm-1 can be ascribed to the N-H stretching vibration.2 The absorption peak near 2916 cm-1 corresponds to C-H stretching vibration.2 Peak centered on 1050 cm−1 can be attributed to the asymmetric stretching vibration of the Si−O−Si bond in nanoclay. The FTIR spectra of CA, CNA and CAN shows all the peaks corresponding to PANI which suggest the successful polymerization of aniline in presence of CNT and nanoclay. For all nanocomposites, C=C stretching vibration of benzenoid and quinoid units of PANI, shifted to 1499 cm-1 and 1578 cm-1, from 1479 cm-1 and 1566 cm-1, respectively. These changes in the composites may be associated with the interaction of Si and Al with the nitrogen atom in PANI. Hydrogen bonding between oxygen of nanoclay particles and the PANI molecule can also contribute to the shifting of bands. This shift can also be ascribed to the confinement of PANI chains on CNT and nanoclay (CNA), during in-situ coating process. This growth of PANI over CNT and nanoclay restricted the modes of vibrations in PANI, which led the frequencies of vibrations to move to a relatively high wavenumber. These interaction between nanoclay, CNT and PANI can influence the coating of these nanoparticles with PANI and better electrochemical performance. UV-Visible Spectroscopy Analysis: The electronic properties and interaction between PANI, nanoclay, and CNT were analyzed by UV-visible spectroscopy and shown in figure (3).



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Figure 3: UV spectra of pure PANI, CA, CAN & CNA Two characteristics peak observed near 273 nm and 370 nm corresponding to π-π* and polaron-π* transition in quinoid and benzenoid units of PANI. 9 These two peaks are present in all spectra which confirmed the presence of PANI. A gradual shifting of benzenoid unit peak from 370 nm to 372, 375 and 383 nm is observed for CA, CAN and CNA, respectively. Similarly, a blue shift from 273 nm to 270, 269 and 265 nm is evidenced for CA, CAN and CNA, respectively. The peak shift can be correlated to the interaction between nanoclay and CNT with PANI as discussed in the FTIR section. To further analyze the coating of CNT and nanoclay morphological analysis was carried out and discussed in the following section. Field Emission Scanning Electron Microscopy (FESEM): The FESEM images of neat MWCNT, CA, CNA & CAN are shown in the figure 4(a-d). FESEM image of MWCNTs shows entangled and agglomerated tubes with average outer diameter of 30-40 nm (figure 4(a)) which increase to 80-90 nm, in CA, with the coating of PANI (figure 4(b)). In addition a uniform coating of individual MWCNT with PANI is evidenced. Figure 4(c) shows the change in morphology of CNA with the incorporation of nanoclay. In the FESEM image, an interpenetrated structure of CNT and nanoclay is observed where both coated with PANI. However, ex-situ product, i.e. CAN shown in figure (4d), demonstrates a compact morphology as compared to in-situ product. This image also shows an interpenetrating structure of CNT and nanoclay where CNTs are coated with PANI. The nanoclay sheets are observed to be more transparent as compared to coated nanoclay sheets in CNA. From these two images it is clear that in-situ and ex-situ addition of nanoclay to the CNT/PANI system does not drastically change their morphologies. However, it must be noted that in case of CNA, the coated CNTs are separated by a PANI coated nanoclay which is a conducting sheet due to the coating of PANI whereas in case of CAN, the coated CNTs are separated with only nanoclay layers which lacks conductivity. This distribution of nanoclay, in bulk, can affect the electrochemical performance of the composite material which has been discussed in the electrochemical section.

Figure 4: FESEM image of (a) CNT, (b) CA, (c) CNA and (d) CAN To further analyze the bulk morphology, TEM analysis was carried out and presented in figure 5(a-e). TEM image of pristine CNT (figure 5a) shows the tubular morphology with


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various diameters. An increase in diameter of nanotubes are observed in CA due to the coating of PANI over CNT (figure 5(b)). A closer look shows the clear coating of CNT with PANI (Figure 5(c)). With the incorporation of nanoclay (CNA), apart from coated CNT, TEM image shows sheet like morphology of coated nanoclay (figure 5(d)). Similar morphology is also observed in CAN (figure 5(e)). However, nanoclay in CAN appeared more transparent as compared to CNA which can be attributed to the coating of PANI over nanoclay sheets. In both cases i.e. CAN and CNA the nanoclay sheets and coated CNT are dispersed uniformly. To access the effect of nanoclay on porosity of nanocomposites, surface analysis was carried out as discussed in the following section.

Figure 5: TEM images of (a) Pristine CNT, (b, c) CA, (d) CNA and (e) CAN

BET isotherm: The specific surface area and pore size distribution of CA, CNA & CAN were measured by nitrogen adsorption/desorption analysis following BET and DFT method and presented in figure 6(a-b). Figure 6: (a) BET isotherm and (b) pore size distribution of all samples For comparison we have also performed the surface analysis of PANI and nanoclay/PANI (marked as NA) composite. The surface area of PANI is found to be 55 m2/g which increased to 187 m2/g with the addition of CNT as in CA. However, this value is still lower than the reported BET surface area of CNT/PANI composite. The lower BET surface area may be due to agglomeration (caused due to high entanglement density of CNT, in partial overlapping and coalescing of individual CNT) during the in-situ polymerization process which blocked pores and thereby lowers the surface area. Similar trend was also observed for nanoclay and it’s composite with PANI. Surface area of nanoclay was about 150 m2/g which decreased to 40 m2/g after fabrication with PANI in nanocomposite NA. This decrease in surface area indicates that the nanoclay sheets were entrapped inside the PANI matrix which resulted in blocked pores and hence reduced surface area. In-situ product CNA shows maximum BET surface area of 810 m2/g as compared to ex-situ product CAN (346 m2/g). This substantial increase in surface area of CNA indicates that nanoclay and CNT complement each other for



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both exfoliation of nanoclay and reduction of entanglement density of CNTs and the structure is stabilized by the in-situ coating of PANI. In case of CAN the exfoliation was triggered during the ex-situ addition of nanoclay and sonication. Pore size distribution study shows that all the nanocomposites possesses a mesoporous structure which can enhance the specific capacitance. To confirm the exfoliation of nanoclay XRD analysis was carried out and discussed in the following section. XRD Analysis: All samples were analyzed with XRD and shown in figure 7. Pure nanoclay shows a prominent peak at 2θ value 4.8o which corresponds to the (100) plane and a d-spacing of 1.84 nm.31 This peak shifts to a lower 2θ value on incorporation of nanoclay in CNT/PANI composite. In case of CNA, peak is observed at 2θ value 1.21o which indicates a tremendous increase in inter-planer spacing nanoclay to 7.35 nm. Similar peak shift is observed for CAN to a 2θ value of around 1o with a d-spacing of about 9.8 nm and thus showing that d-spacing in case of CAN is larger as compared to in-situ CNA. This can be attributed to the fact that during synthesis process, the increased average diameter of CNT/PANI nanocomposite tends to exfoliate the intergallery of nanoclay layers to a larger extent under the influence of sonication. Interestingly nanoclay/PANI composite NA also shows similar shifting which indicates that polymerization of aniline inside the intergallery of nanoclay and sonication acting together for layer exfoliation and stabilization. In case of ex-situ CAN composite nanoclay layers were disrupted during the sonication and PANI coated CNTs were intercalated between the sheets which prevented restacking of nanoclay layers. A schematic representation of this exfoliation mechanism is presented in figure 1. Figure 7: XRD plots of pure nanoclay, CA, CNA & CAN Apart from (1 0 0) peak shifting to lower 2θ value, a small peak appeared at around 2θ ~ 6.45° (magnified view of XRD shown in inset) for NA, CNA and CAN which indicates a decrease in d spacing. This peak was observed due to collapse of intergallery spacing. The nanoclay, closite 30B is an organically modified montmorillonite clay where intergallery has been modified with ammonium ion having a long tallow group with the help of ion-exchange reaction. It is expected that during synthesis process, organic moiety might come out of the intergallery and could not be replaced with either aniline, CNT or PANI and remained in an


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agglomerated form resulting in reduced intergallery spacing and thus a peak appeared at higher 2θ value. Similarly a broad peak was also observed at 2θ value of 3.7o (magnified section in inset), for CAN, corresponding to a slight increase in inter layer spacing which might be due to a small fraction of PANI coated CNT intercalated in the intergallery. Cyclic Voltammetry Study: The electrochemical performances of nanoclay based hybrid nanocomposites were analyzed by cyclic voltammetry (CV) using conventional three electrode system in 1M aq. KCl and shown in figure 8 (a-e). Clearly, no redox pairs in CV curves were observed, on both positive and negative sweep, in whole potentio-dynamic investigation and studied at various scan rates.7 It was found that scan rates can significantly affect the current density achieved during the potential scans.2 With the increase in scan rate, current density also increased which can be attributed to diffusion kinetics. For a fixed potential window, slower scan rate will take longer time to record a voltammogram as compared to fast scan rate which in turn alter the size of the diffusion layer above the electrode surface. In a slow voltage scan diffusion layer will grow much further from the electrode in comparison to a fast scan rate and hence the flux to the electrode surface is considerably smaller at slow scan rates than it is at faster rates. As the current is proportional to the flux towards electrode, the magnitude of the current will be lower at slow scan rates and higher at high rates. Figure 8: CV plots at scan rates of (a) 10, (b) 20, (c) 50, (d) 100 and (e) 200 mV/s Among electrode materials, current density of in-situ CNA composite is more than other systems which indicates a higher specific capacitance and hence better electrochemical performance. To quantitatively analyze the electrochemical performances of electrode materials, specific capacitance, energy density and power density of all electrode materials are calculated using following equations and results are shown, at different scan rates, in figure 9 (a-b). Specific capacitance = ∫IdV/m*SR*V --------(1) 32, 33 Where, I = Current; m = Mass of prepared samples; V = potential window; SR = Scan rate Energy density = CV2/2 ------------(2) 2, 34



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Where, C= Specific Capacitance, V= Potential Window Power density = E/∆t--------(3) 2 Where, E= Energy density, ∆t= time (h) From figure 9(a), it is found that specific capacitance decreases with scan rate. The specific capacitance of PANI increased with the incorporation of CNT and nanoclay but the increase is more prominent in case of CA as compared to NA. This increase in specific capacitance in case of CA can be attributed to highly conducting nature of CNT array and surface area which can significantly affect the ion diffusion and storage. In case of NA, the increase can be attributed to the high surface area of the composite as well as electron transfer between nanoclay and PANI through the Bronsted and Lewis acid sites on nanoclay.25 However, combined act of nanoclay and CNT further increases the specific capacitance of both ex-situ CAN and in-situ CNA composites where CNA shows higher specific capacitance as compared to CAN. A maximum specific capacitance of 331 F/g, at the scan rate of 10 mV/s, is achieved for the in-situ CNA. The ex-situ product CAN offers a specific capacitance of 202 F/g, at 10 mV/s, which is higher than pure PANI, CA and NA. Energy and power densities of electrode materials are plotted against each other for different electrode materials and shown in the Ragone plot (figure 9(b)). Figure 9: (a) Specific capacitance and (b) Ragone plots of all samples at different scan rates The Ragone plot shows the similar trend for energy density and power density as that of specific capacitance. These observations indicate that CNA is a better electrode material as compared to other studied systems. This enhancement can be attributed to the synergetic effect of nanoclay and CNT for the exfoliation of nanoclay and dispersion of CNTs leading to high surface area and mesoporous structure which allows easy access of electrolyte during electrochemical process. Apart from high surface area of the electrode materials, conductivity of CNT and electron transfer between nanoclay and PANI chains also contributed towards the enhancement of specific capacitance. While CNT enhanced the surface area of PANI/CNT nanocomposite and creates a conducting path in electrode material for charge distribution and


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storage, contribution of nanoclay is complicated and require further discussion. A probable mechanism of nanoclay role has been discussed in the following text. Montmorillonite clay belongs to a planar 2:1-type phyllosilicate class. The layered structure is composed of silicon based tetrahedral sandwiched between two aluminum based octahedral. The montmorillonite layers carry an overall negative charge arising from isomorphous substitution of Mg and Fe for Al in the octahedral sheet and, to a lesser extent, substitution of Si in the tetrahedral sheet by A1.35 The negative charges on the layers are balanced by hydrated cations in the interlamellar space (usually Na + or Ca 2+ in the natural form) which can be readily exchanged (substituted with a modified ammonium ion in closite30B by ion exchange method).

In addition montmorillonite has some isomorphous substitution in the

tetrahedral sheet and contains about 3% by weight of structural iron which contribute about 15– 50% of the total layer charge in montmorillonite. These substituted sites are the active sites on montmorillonite which can donate and accept electron. In another such report, Atkinson et al.36 proposed that nanoclay has both Bronsted and Lewis acid sites, associated with interlamellar region and edge site, respectively, and can be effectively used as catalyst. Similarly, Kaur et al.29 reported, the use of montmorillonite clay as an efficient, heterogeneous and green catalyst for organic synthesis. In our system, increase in specific capacitance in presence of nanoclay can be attributed to two factors. Firstly, the contribution towards double layer capacitance which depends on surface area of electrode material. As discussed in the BET surface area section, insitu CNA possessed high surface area as compared to CA which can enhance the double layer capacitance and hence the specific capacitance. Similarly, in case of ex-situ CAN composite the increase in surface area can contribute towards the double layer capacitance and hence increases the specific capacitance as compared to CA. However, the increase in surface area of CAN is lesser as compared to CNA which reflected in specific capacitance. Secondly, contribution towards pseudocapacitance by nanoclay/PANI must be considered. As discussed above, the active sites on montmorillonite nanoclay can act both as electron acceptor and donor which can accept or donate electrons to PANI and can enhance the specific capacitance. As suggested by Solomon and Loft Fe present in the octahedral lattice can switch from Fe2+ to Fe3+ by donating one electron which will be transferred to PANI and nanoclay act as a dopant.37 Similarly, Fe present in the tetrahedral lattice sites can switch from Fe3+ to Fe2+ by accepting one electron from PANI. Here, we have considered Fe as both oxidizable and reducible atom in the silicone and



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aluminum lattice although other transition metals can be present and can influence the electron transfer reaction. This electron transfer reaction is in well accordance with the previous studies where Solomon and Loft suggested that montmorillonite has strong electron accepting sites at the crystal edges.38 Both these mechanisms suggest that nanoclay can act both as n-type and ptype dopant, depending on the active sites, which enhances the pseudocapacitance of PANI and specific capacitance of the whole system.25 The CNT present in the system act as transferring media in the electrode which distribute the potential uniformly inside the bulk electrode, during charging, and influence the ion storage and hence the capacity of the electrode. The proposed conduction mechanism of nanoclay based CNT composite is presented schematically in figure (10). Figure 10: Proposed conduction mechanism in nanoclay CNT based nanocomposite Now the next question is why the ex-situ composites is having lesser specific capacitance as compared to in-situ composite, even though the ratio of constituents are same. The reason is the surface area and difference in electron hoping inside the electrode material. Since the role of surface area has already been discussed, the hopping mechanism needs to be concentrated on. Electron transport mechanism in conducting polymers is a result of intra and inter chain electron hopping. In our system, CNA, conduction takes place through intra as well as inter chain hopping process in and between PANI chains. Since, in in-situ CNA composite all CNT and nanoclay sheets are coated with PANI, electron transport in PANI is a result of inter and intra chain electron hopping (in PANI) as well as to and from nanoclay and CNT. Contrarily, for CAN, since the nanoclay sheets are not coated with PANI and present between PANI coated CNT, intra chain electron hopping is restricted resulted in increased internal charge transfer resistance of the system and hence the specific capacitance. This phenomenon is schematically shown in figure 10. Keeping all such fundamental facts about nanoclay and their exerted electrochemical observation on CNT/PANI system in mind, we further performed the cyclic voltammetry test in two electrode configuration with organic electrolyte (1M TEABF4) in two potential window (00.8V & 0-1.6V). ESI figure S1 (a) & (b) shows the comparative CV plots of different sample at scan rate of 10 mV/s. Specific capacitance, energy density and power density were calculated for the two electrode configuration, in organic electrolyte as mentioned in the supporting


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information. Electrochemical results within the potential window (0-0.8V) at 10 mV/s revealed that in-situ CNA and ex-situ CAN possessed maximum specific capacitance of (189F/g) & (127 F/g), respectively which is relatively higher than other related systems PANI (22.38 F/g), NA (52 F/g) and CA (63 F/g). Similar trend was also observed in case of energy density and power density. These values are lower than that in aq. 1M KCl solution which can be attributed to the restricted ionic mobility owing to the larger ionic size of active electroyte ions during electrochemical analysis. To resolve the issue of lower capacitance value, CV analysis was further performed with extended potential range upto 1.6V (presented in ESI figure S1(b)). Similar potentiodynamic response were evidenced for in-situ CNA and ex-situ CAN with highest current density as compared to other systems. At scan rate 10 mV/s, calculation revealed that specific capacitance of PANI and NA increased from 22.38 F/g & 52 F/g to 41.60 F/g & 120 F/g, respectively. Similarly, the specific capacitance value of CA increased from 63 F/g to 133 F/g. Among nanoclay based nanocomposite, in-situ CNA possessed maximum specific capacitance of 275 F/g as compared to CAN having specific capacitance of 140.63 F/g. Similarly, CNA shows the maximum energy density of 97.94 Wh/Kg and power density of 1101.82 W/Kg at 10 mV/s which is comparatively higher than other related systems (ESI figure S1(c) and (d)). The suitability of nanoclay based nanocomposite was further analysed by changing-discharging analysis as discussed in the following section. Galvanostatic charging-discharging (GCD) Analysis: To investigate the long term applicability of electrode material, GCD was performed at current density of 5 A/g for all electrode material from 0-0.8 V, without any further modification, in 1 M aq. KCl solution. Figure 11 represents the comparative charge discharge plots of CA, CAN and CNA. Figure 11: Galvanostatic Charging discharging plots of electrode materials Among all nanocomposite the discharge time duration of CNA is found to be larger as compared to others which is in well accordance with the CV results. Higher discharge time of CNA indicates higher value of specific capacitance. However, deviation from perfect triangular shape (which is the basic feature of the conceptual ideal capacitor) indicating non-ideality due to considerable pseudocapacitance contribution from PANI and nanoclay. As summarized earlier in the CV section, Bronsted and Lewis acidic sites of nanoclay can influence or control the redox



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reaction in the conjugated PANI backbone and the whole system embedded with it. This can be confirmed from the charging discharging plots of PANI and NA (nanoclay/PANI) (shown in inset of figure 11) where non-ideality from ideal behavior signifies the pseudocapacitance contribution from nanoclay and it’s composite with PANI. To further complement the CV calculations, specific capacitance, energy density and power density were calculated from charging-discharging curves which shows a maximum specific capacitance of 312 F/g for in-situ CNA which is relatively higher than ex-situ CAN (237 F/g) and CA (156 F/g) at a current density of 5 A/g (more details in ESI). Further we have also performed the cyclic stability test of electrode material for consecutive 2000 cycles. It is found that samples CA, CAN & CNA retains about 92% of initial capacitance even after 2000 cycles, as shown in figure 12. All these results demonstrates the non-deterioration of electrode material due to the nanoclay inclusion. Figure 12: Specific Capacitance retention after 2000 cycles at the current density of 5 A/g Electrochemical Impedance Spectroscopy: The principle objective of EIS spectroscopy was to obtain information about interfacial properties (capacitance and charge transport behavior) at electrode-electrolyte interface. The most extensively used tool for EIS analysis is the Nyquist plot. This consists of imaginary component (Z//) of the impedance against the real component (Z/). All spectra contains a distorted semicircle in the high frequency region due to porosity of electrode material and a linear part in low frequency region due to mass transfer phenomena during electrochemical process.6 The straight line response at 45° signifies the Warburg resistance which represents the mass transfer parameter of electrochemical process. As can be seen from the EIS plot, shown in inset image figure 13, the charge transfer resistance (RCT) of PANI was much larger as compared to the CA, NA, CAN and CNA. However after the incorporation of nanoclay in PANI RCT value decreases significantly. This can be attributed to the acidic nature of nanoclay which facilitate the charge transport and electrolyte accessibility. This further proves that formation of charge transfer complex between exfoliated nanoclay sheet and conjugated polymer PANI matrix of the composite due to uniform coating of PANI over nanoclay sheet. The charge transfer resistance of CAN and CNA was found to be larger than CA. The lower charge transfer resistance of CA as compared to others is due to the presence of highly conducting nature of CNT and uniform PANI coating on CNT network array. In case of in-situ CNA the relatively lower value of RCT as


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compared to CAN suggested the better charge transfer complex formation demonstrating combined contribution from all components present within electrode material which is in line with the FESEM micrographs of CNA where the well dispersed nanoclay sheets within the CNT array and their coating status with PANI gives highly porous structure with enhanced electroactive surface area access. Impedance behavior in organic electrolyte also shows similar nature (ESI figure S2). Figure 13: EIS analysis of as synthesized nanocomposites Conclusion: Nanoclay based CNT/PANI nanocomposite were successfully synthesized by in-situ & ex-situ approach. The role of nanoclay on electrochemical properties of CNT/PANI nanocomposite was explored by both in-situ and ex-situ addition of nanoclay. Morphological analysis confirmed the coating of PANI over CNT and nanoclay. XRD analysis confirmed the exfoliation of nanoclay in presence of CNT and PANI. It was found that nanoclay can facilitate the charge transfer through hopping mechanism and contributing to total specific capacitance. Among various nanocomposite so synthesized, in-situ CNA showed optimum property with a maximum specific capacitance of 331 F/g which was comparatively higher than CAN and CA. Further, it was found that after 2000 cycle electrode material retains about 92 % of initial specific capacitance. Thus all results revealed that as prepared electrode material can be a promising electrode material for next generation high performance supercapacitors.

Supporting Information: Electrode preparation method and cyclic voltammetry and Impedance analysis in organic electrolyte. Calculation of Specific capacitance, energy density and power density from galvanostatic charge discharge in aqueous medium.

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Figures

Figure 1: Schematic diagram of synthesis of Nanoclay based Nanocomposite



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Figure 2: FTIR spectra of pure PANI, CA, CAN & CNA

Figure 3: UV spectra of pure PANI, CA, CAN & CNA


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(a)

(b)



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(c)

(d) Figure 4: FESEM image of (a) CNT, (b) CA, (c) CNA and (d) CAN


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(a)

(b)



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(c)

(d)


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(e) Figure 5: TEM images of (a) Pristine CNT, (b, c) CA, (d) CNA and (e) CAN

(a)



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(b) Figure 6: (a) BET isotherm and (b) pore size distribution of all samples


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Figure 7: XRD plots of pure nanoclay, CA, CNA & CAN

(a)

(b)



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(c)

(d)


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(e) Figure 8: CV plots at scan rates of (a) 10, (b) 20, (c) 50, (d) 100 and (e) 200 mV/s

(a)



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(b) Figure 9: (a) Specific capacitance and (b) Ragone plots of all samples at different scan rates

Figure 10: Proposed conduction mechanism in nanoclay CNT based nanocomposite


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Figure 11: Galvanostatic Charging discharging plots of electrode materials



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Figure 12: Specific Capacitance retention after 2000 cycles at the current density of 5 A/g


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Figure 13: EIS analysis of as synthesized nanocomposites



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For Table of Contents Use Only

Enhanced Specific Capacitance of Self-assembled Three-dimensional CNT/Layered Silicate/Polyaniline hybrid sandwiched nanocomposite for Supercapacitor Application

R. Oraon, A. De. Adhikari, S. K. Tiwari and G. C. Nayak

This work presents the utilization of ecofriendly naturally available layered silicate to enhance the electrochemical performance of supercapacitor electrodes


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Enhanced Specific Capacitance of Self-Assembled ... - ACS Publications

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