Conducting polymers-based electrochemical supercapacitors

Electrochimica Acta 101 (2013) 109–129

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Conducting polymers-based electrochemical supercapacitors—Progress and prospects R. Ramya, R. Sivasubramanian, M.V. Sangaranarayanan ∗,1 Department of Chemistry, Indian Institute of Technology–Madras, Chennai-600036, India

a r t i c l e

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Article history: Received 11 July 2012 Received in revised form 27 September 2012 Accepted 30 September 2012 Available online 8 October 2012 Keywords: Supercapacitors Polyaniline Polypyrrole Nanocomposites Specific capacitance Electrochemical impedance spectroscopy

a b s t r a c t The progress in the field of electrochemical supercapacitors employing polyaniline and polypyrrole during the past decade is reviewed, with special emphasis on electrochemical characterization techniques. The magnitude of the specific capacitance from the galvanostatic charge–discharge studies, cyclic voltammetry and impedance spectroscopy has been compiled. The merit of the Electrochemical Impedance Spectroscopy in deducing the system parameters is pointed out. The unsolved issues pertaining to the conducting polymers based supercapacitors are highlighted. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction The interfacial region between an electrode and an electrolyte, customarily referred to as the ‘electrical double layer’ (EDL) has always been regarded as an active field of interest for comprehending the behavior of ions and dipoles under the influence of an electric field. Thus, the choice of adsorption isotherms, dipolar states of molecules, discreteness of charge effects, etc. became a focus of attention in this context [1]. Hence, the elucidation of the structure of the electrical double layer using experimental data of differential capacitance and theoretical modeling appeared to be a domain of interest with a singular objective of analyzing the equilibrium properties of electrified interfaces [2]. The estimation of electrostatic and non-electrostatic interaction energies of adsorbed species at diverse electrochemical interfaces is in itself a challenging issue. Amidst this scenario, a patent by Becker [3] demonstrated that this interfacial region, spanning a few Å thickness has the ability to store ‘charges’, the quantification of which leads to the ‘double layer capacitance’. Thus, the notion of Electrical Double Layer Capacitor (EDLC) arises wherein high values of the specific capacitance are anticipated in view of the distance being ∼10−8 cm since the capacitance and interfacial thickness are

∗ Corresponding author. Tel.: +91 44 22574209; fax: +91 44 22570545. E-mail address: [email protected] (M.V. Sangaranarayanan). 1 ISE Member. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.09.116

inversely related. In contrast, the conventional capacitors consist of positive and negative electrodes immersed in a dielectric with the physical separation being a few millimeters. Consequently, the field of supercapacitors (also called ultracapacitors) became a frontier area of research in chemistry, physics and materials science and technology [4]. A practical supercapacitor device consists of two electrodes dipped in an electrolyte solution with a suitable separator and in EDLC, the mechanism of charge storage is through a non-faradaic process. Since the charge storage in EDLC arises from adsorption rather than a redox reaction (‘Faradaic process’), these are also termed as non-Faradaic or true capacitors [5,6]. The systems wherein the capacitance arises from redox processes are often termed as ‘pseudocapacitors’ the associated capacitance being the pseudocapacitance. In the case of pseudocapacitors, the electrodes are metal oxides or polymer-coated metals, the charge storage being associated with faradaic processes. Apart from the pseudocapacitors and true (EDLC) capacitors, there exist hybrid capacitors too wherein pseudocapacitors are combined with EDLC capacitors. The entire supercapacitor assembly is chosen such that it (i) is voltammetrically stable as inferred from the cyclic voltammograms at a wide range of scan rates; (ii) is capable of yielding large power densities and charge–discharge cycles as evinced from the galvanostatic studies and (iii) has lowest Equivalent Series Resistance (ESR). It is this combination of factors in conjunction with the versatility of the synthetic protocols of diverse materials that has led to the rapid progress and excitement in the design of supercapacitors.

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R. Ramya et al. / Electrochimica Acta 101 (2013) 109–129

Fig. 1. Schematic visualization of the conduction mechanism of Ppy.

The field of electrochemical supercapacitors has witnessed phenomenal growth during the past few decades not only on account of the impending energy crisis but also due to rapid strides being witnessed in synthesis of novel functional materials of diverse genre [7,8]. In view of the large surface area, diverse carbon based materials are the preferred substrates for the EDLC. While such EDLC’s were being investigated, pseudocapacitors whose redox behavior is ‘perfectly reversible’ came into prominence in view of their possessing ∼105 charge–discharge cycles and specific capacitance of ∼103 Fg−1 . The essential criteria to be satisfied by these pseudocapacitors are as follows: (i) reversibility of faradaic reactions; (ii) large capacitances and (iii) satisfactory charge–discharge cycles. Diverse metal oxides such as RuO2 , Fe2 O3 , MnO2 , etc. and conducting polymers such as polypyrrole (Ppy), polyaniline (PANI), polythiophene, polyindole etc. are typical prototype materials for pseudocapacitors. In particular, conducting polymers are especially suitable in view of their ease of fabrication and flexibility [9]. The ability to store charges in the case of conducting polymers arises from a doping process. In the case of PANI and Ppy, the conductivity arises from p-doping (also known as oxidative doping) wherein the removal of pi-electrons from the conjugation leads to a net positive charge. It is customary to postulate the existence of ‘polarons’ and ‘bipolarons’ in this context so as to imply the creation of the charge carriers in the polymer chain. A schematic depiction of the charge storage mechanism pertaining to Ppy is provided in Fig. 1 as an illustration. An analogous visualization can be depicted for PANI too. In contrast to the p-doping mentioned above, the mechanism of charge storage may also arise from n-doping as in the case of polyacetylene, wherein net negative charges arise. Since the magnitude of the specific capacitance is influenced by the extent and efficiency of the charge storage, the quantification of doping becomes essential; hence, extensive investigations on the choice of dopants are being carried out. The charge storage in pseudocapacitors may also involve intercalation [4] which increases the specific capacitances. It is appropriate to mention here that in the rapidly advancing field of electrochemical supercapacitors, an exhaustive critical review encompassing various features such as (i) the choice of electrode materials (metals, metal oxides, carbon nanotubes, graphene and related materials, metal-polymer composites, etc.); (ii) their structural characterization using spectroscopic and microscopic studies; (iii) the influence of ionic liquids on specific capacitance;

Fig. 2. Histogram obtained using Thomson Reuters web of knowledge with the search term as ‘Supercapacitors’ for the period 2002–2011.

(iv) the fabrication of supercapacitor devices and their marketability/cost effectiveness; (v) the efficacy of hybrid supercapacitors and (vi) theoretical modeling of supercapacitors is rendered almost impossible. An overview of the literature search using Thomson Reuters web of knowledge with the search term ‘Supercapacitors’ indicates an exponential rise in the number of publications (Fig. 2). Several reviews on electrochemical supercapacitors have appeared during the past few years with different perspectives [10–24]. Consequently, the present critique is aimed at covering the literature on electrochemical supercapacitors based on polyaniline and polypyrrole during the past decade (2002–2011) with particular emphasis on electrochemical synthesis and electrochemical characterization. No critique of conducting polymers based supercapacitors will be complete without analyzing the nanocomposites based materials. The latter has received immense attention recently, in view of their versatility, ability to provide higher power densities and faster charge–discharge responses. A major limitation of the conducting polymers based systems coated on metal substrates consists in the sluggish rate of ion transport during the redox reactions. This weakness can be obviated by employing nanostructures of appropriate shapes. Table 1 provides the specific capacitances of PANI and Ppy-based supercapacitors estimated from various electrochemical techniques. It may be noted from Table 1 that three-electrode designs are commonly employed for these studies and that the specific capacitances span a wide range. 2. Synthesis The synthesis of conducting polymers can be accomplished by chemical, photochemical, electrochemical methods, etc. While the electrochemical synthesis of PANI or Ppy involves the use of electrical field as a driving force, the chemical method for the same makes use of powerful oxidizing agents. An equally versatile technique for the synthesis of nanostructures of conducting polymers, during the past decade involves the use of liquid/liquid interfaces. 2.1. Chemical synthesis 2.1.1. Synthesis of PANI and Ppy at liquid–liquid interfaces Several interesting morphologies of conducting polymers arise at liquid–liquid interfaces. In the case of polyaniline, the following studies are noteworthy, viz.: (i) single crystalline


111

Table 1 The magnitude of specific capacitances obtained from various experimental techniques for PANI and Ppy based supercapacitors. For the sake of comparison, the values have been rounded off to the nearest integer. No.

System

(A) Polyaniline Nanostructured PANI on SS [32] 1 PANI on Alizarin treated Ni [33] 2 PANI nanowhiskers on mesoporous carbon [34] 3 PANI sandwiched between Al foil (current collector) enveloped 4 using Al pauch [35]. PANI nanofibres on graphite using PTFE as binder [36] 5 6 PANI nanorods on ITO via electroless surface polymerization [37] PANI doped with H+ and Zn2+ on Platinum [38] 7 PANI nanofibres on SS using acetylene black mixed with PTFE 8 as binder [39] PANI doped with DMS powder along with carbon black on Al 9 foil using PVDF binder [40] 10 PANI on carbon paper [41] PANI-PAA on ITO [42] 11 12 Dispersed PANI nanofiber on Ni [43] PANI doped with HBF4 on SS [44] 13 14 PANI doped with Li on Activated carbon sheet [45] PANI on carbon paper [46] 15 Porous PANI on SS mesh [47] 16 PANI-SPEEK on Carbon paper [48] 17 PANI on Ni foam [49] 18 19 PANI nanofiber on Au [50] PANI on PVA/FTO coated glass [51] 20 21 PANI on hollow carbon spheres coated SS mesh [52] PANI doped DMS on Al foil [53] 22 PANI doped Li on Al mesh [54] 23 24 PANI on SS mesh [55] Whisker like PANI on mesoporous carbon [56] 25 26 PANI-chitosan on SS [57] PANI nanofiber on SS in (EC-EMC-DMC) solvent [58] 27 PANI on mesoporous carbon disk [59] 28 PANI nanosheet on lyotropic liquid crystal coated graphite [60] 29 PANI nanofibers on sodium alginate modified GC [61] 30 31 Porous PANI on ITO [62] PANI on porous carbon rod, SS, Pt [63] 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

PANI on graphite [64] PANI on SS [65] PANI on Graphite [66] PANI nanorod on ITO [67] PANI nanowire on SS [68] PANI nanofiber on SS [69] PANI on CA coated carbon paper [70] PANI on GC [71] PANI on carbon coated SS [72] PANI implanted on activated carbon on SS [73] PANI (Triton X100) on Ni [74] PANI (PTSA) on Ni [75] PANI on SS [76] PANI (PTSA) on SS [77] Emeraldine form of PANI on SS [78] Hydrophilic PANI nanofiber on SS [79] PANI nanowires on SS [80] PANI nanofiber on SS [81] Self-doped PANI on Pt [82] PANI nanoparticle on GC [83] Fuzzy PANI nanofibers on SS [84] PANI nanowires on Au [85] PANI on electro etched carbon cloth [86] PANI nanobelts on Ti [87] PANI nanorods on ITO [88]

(B) Polypyrrole Ppy on Ni foam using Acid blue as dopant [89] 1 HNT Ppy coated Ni foam [90] 2 Ppy on SS [91] 3 Ppy on graphite [92] 4 Ppy/cellulose composite on pt foil [93] 5 Ppy/VGCF/AC composite on Ni foam [94] 6

Specific capacitance (in F/g) from

Electrode assembly

Galvanostatic charge discharge

Cyclic voltammetry

Impedance spectroscopy

– 36 470 40

503 – – –

– – – –

Three Three Three Two

428 –

– 592

– –

Three Three

369 548

– –

– 400

Three Three

130

–

–

Three

– – 160 301 121 480 837 28 380 235; 125 – 525 115 115 73 900 703 120 70 560 2093 238 Carbon 1100; SS-240; Pt-74 210 450 – 3407 742 609 230 400 180 160 2300 404 805 – – – – 524 480 – – 950 673 – 780

98 5 – 140 – – – 27 – – 571 – – – 273 – – – – – – – –

– – – – – – – – – – – – – – – –

Three Three Three Three Three Three Three Three Three Three Three Three Three Three Three Three Three Three Three Three Three Three Three

– – 400 – – – – – – – – – – 438 258 861 775 608 – 166 839 – 1027 873 –

– – – – – – – 85 – 2450 – – – – – – 445 – – – – – – –

Three Three Three Three Three Three Three Three Three Three Three Three Three Three Two Three Three Three Two Three Two Three Three Three Three

305 522 528 – 32 –

– – 533 – – 300

– – – 282 – –

Three Three Three Three Three Three

– – – – –

112


Table 1 (Continued) No.

System

Specific capacitance (in F/g) from

Electrode assembly

Galvanostatic charge discharge

Cyclic voltammetry

Impedance spectroscopy – – – 57 – – – – –

Three Three Three Three Two Two Three Three Three

0.023a 75–98 – –

Three Three Three Three Three Three

7 8 9 10 11 12 13 14 15

Ppy/CA composite on Ni foam [95] Ppy on mesoporous carbon coated SS [96] Ppy on graphite [97] PANI and Ppy on C using emulsion polymerization [98] Ppy on Ti foil [99] Ppy on ITO using both LiClO4 and LiCF3 SO3 as dopant [100] Ppy on C-MEMS [101] Ppy on SS doped with LiClO4 [102] Ppy-Nf and Ppy-LiClO4 on Pt electrode [103]

– – 158 21 427 200 190.61 ± 17.5a – –

16 17 18 19

Ppy on Ni [104] p-doped Ppy on ITO electrode [105] Ppy on ITO [106] (Ppy)Cl on GC Ppy/CNXL on GC [107]

0.027a 78–136 8.0–15.0a –

433 487 – – 480 ± 50a – 162.07 ± 12.4a 130 344 for Ppy-Nf.; 355 Ppy-ClO4 0.030a – – 258

20

Ppy/Nafion on the gold coated PVDF membrane Ppy/PTS on the gold coated PVDF membrane [108]

380 420

336 –

– –

21

Ppy on Ti [109]

0.096a for Ppy in NaCl 0.008a for Ppy in TOSNa



472 – 3.5 × 10−9 0.40 – – 90 566

476 255a – – 100 100 150 –

– – – – – – – –

22 23 24 25 26 27 28 29

Ppy on SS [110] Ppy on Pt [111] Ppy on Ni-coated AAO [112] Ppy on Pt microelectrode [113] Ppy on carbon fiber paper [114] Ppy using pulse depsosition on graphite [66] Ppy/CA on SS [115] Ppy nanowires on Au coated Pt [116]

Three Two Three Three Three Two Three Three Three

Footnotes: SS—stainless steel, ITO—Indium tin oxide, C-MEMS—carbon microelectrode chemical systems, VGCF—vapor grown carbon fibres, PVDF—polyvinylidene fluoride, CNXL—cellulose nanocrystals, CA—carbon aerogel, AC—activated carbon, PTSA—para toluene sulphonic acid, PTFE—polytetra fluoroethylene, HNT—halloysite nanotubes PAA—polyacrylic acid, DMS—dimethyl sulphate. a Specific capacitance per unit area (in F/cm2 ).

nano-needles at water–dichloromethane interface [25]; (ii) nanofibers at water–toluene interface [26]; (iii) nanobelts, wherein the non-aqueous phase consists of diethyl ether, toluene or dichloromethane [27] and (iv) water-soluble polyaniline at water–chloroform interfaces [28]. Such novel synthetic strategies can then be employed for fabrication of supercapacitors. In this context, the efficacy of PANI nanofibres synthesized at water–dichloromethane interface deserves mention in view of their yielding a large specific capacitance of 548 Fg−1 [29]. Analogously, Ppy consisting of chain-like morphologies has been synthesized at water–chloroform interface using pyrrole monomer and surfactants in the organic phase and Fe(NO3 )·9H2 O at the aqueous phase [30]. 2.1.2. Homogeneous phase synthesis of PANI and Ppy The feasibility of conducting polymers as materials for supercapacitors was first demonstrated by Rudge et al. [31]. Subsequently, a variety of such polymers has been investigated and among them, mention may be made of the following: polyaniline, polypyrrole, polythiophene and their substituted monomers. The pre-requisites for conducting polymers-based supercapacitors are environmental stability, facile doping and de-doping as well as ease of fabrication. In the chemical synthesis of PANI and Ppy pertaining to their feasibility as supercapcitors, various dopants [32–131], initiators [39,44,71], templates and oxidants [32–61,89–98,119–123] have been employed. Soft templates include various surfactants [44,60,62,74,92,95,97,98,102,111,119], seeded growth [36], biotemplates [61,124] etc. Among various hard templates, mention may be made of polycarbonates [42] and Anodic Aluminium Oxide (AAO) [112]. For stability and suitable adhesion on electrodes,

binders such as Poly tetrafluoroethylene (PTFE), Polyvinylidene fluoride (PVDF), N-methyl pyrrolidone (NMP) as well as Nafion have been employed. A novel synthesis of PANI nanowires using dilute polymerization on Au/Cr layers thermally etched on polyethylene terephthalate (PET) films leads to the design of microsupercapacitors [120]. 2.1.3. Nanocomposites of PANI and Ppy The PANI-nanocomposites [126–162,59,163–169] and Ppy-nanocomposites [181,95,182–211] with diverse forms of carbon and oxides have been synthesized using various oxidants FeCl3 [132–134,181,95,182–193], (NH4 )2 S2 O8 [131,135–162,59,163–167,184–192], KMnO4 [168,169,203–206], K2 Cr2 O7 [132], FeCl2 [207], Fe(NO3 )3 [208], under acidic conditions. The synthesized material can then be either pressed into pellets or coated onto electrodes using appropriate binders. Several interesting morphologies arise in chemical synthesis of conducting polymers-nanocomposites if soft templates such as CTAB (cetyl (trimethyl)ammonium bromide) [128,194,195,200], DBSA (dodecyl benzenesulphonic acid) [126], DTAB (dodecyl (trimethyl)azanium bromide) [195], OCTA (noctadecyl(trimethyl)ammonium) [211], SDS (sodium dodecyl sulfate) [137,95,216,98] are employed. Apart from carbon based substrates, other electrodes commonly employed are Ni, SS, Ti, Indium Tin Oxide (ITO), Ta, Glassy Carbon (GC), Au, Pt, Ag, etc. Fig. 3 depicts the growth mechanism of PANI nanowires on graphene oxide sheets along with the SEM images [153]. Fig. 4 depicts the nanotubes of PANI-MoO3 composite wherein the facile nature of the chemical synthesis in yielding diverse shapes is demonstrated [127]. The clever strategy of employing a surfactant (CTAB) and its


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Fig. 3. (i) The growth mechanism of PANI and (ii) SEM image of PANI on Graphene Oxide layers using chemical polymerization. Reprinted with permission from Ref. [153]. © 2010 American Chemical Society.

subsequent removal so as to obtain Ppy nanowires within stacked graphene oxide sheets has been demonstrated schematically in Fig. 5 [195]. 2.1.4. Nanocomposites of copolymer of PANI and Ppy The nanocompoites consisting of the copolymer of PANI and Ppy on graphite electrodes have also been synthesized using methyl orange as a template and FeCl3 as the oxidant so as to form Ppy nanotubes; subsequently, the polymerization of aniline using

Fig. 5. (i) Schematic illustration for the formation of GO/Ppy composite and (ii) a typical SEM image. Reprinted with permission from Ref. [195]. © 2010 American Chemical Society.

(NH4 )2 S2 O8 as the oxidant and H2 SO4 as the dopant takes place on the Ppy nanotubes thereby yielding the copolymer as depicted schematically in Fig. 6 [236]. This system has a specific capacitance of 416 Fg−1 at a current density of 3 mA cm−2 in galvanostatic

Fig. 4. (A) SEM and (B) TEM images of PANI/MoO3 nanotubes. Reprinted with permission from Ref. [127]. © 2011 Wiley VCH.

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Fig. 6. TEM images of (a) Ppy nanotubes and (b) Ppy/PANI nanotube composites. Reprinted with permission from Ref [236]. © 2008 Elsevier.

charge–discharge experiments. If HCl is employed in stead of H2 SO4 , in the above synthetic protocol for Ppy and PANI composites [237] the specific capacitance becomes 827 Fg−1 at 8 mA cm−2 . The emulsion polymerization method using Sodium Dodecyl Sulphate (SDS) for the synthesis of copolymer of Ppy and PANI composite yields the specific capacitance as 66 Fg−1 [98]. 2.2. Electrochemical synthesis and its influence on specific capacitance Although conducting polymers and their composites with various carbon based and oxide materials can be obtained using chemical/electrochemical methods, the removal of templates becomes tedious in chemical synthesis and the polymeric structure may also get destroyed; further, the remaining oxidants may also lead to contamination. Hence the environment-friendly electrochemical method is more preferred. The other notable advantages are as follows: (i) the rate of polymerization and hence the morphology can be controlled effortlessly; (ii) the polymeric films are stable in diverse solvents and (iii) a variety of electrochemical techniques can be employed for the synthesis. 2.2.1. Potentiodynamic method In the potentiodynamic method of which cyclic voltammetry is a special case, a time-dependent external potential is applied as an input and the corresponding current is measured. The resulting cyclic voltammogram has distinct features providing a clue to the redox states. The integration of the appropriate cyclic voltammogram yields the charge consumed which is then a measure of the faradaic efficiency. A. PANI and its composites It has been demonstrated that nanofibres of PANI arise by employing para-toluene sulphonic acid (p-TSA) as the dopant and structure directing agent [77]. On the other hand, PANI nanorods are obtained on glassy carbon electrodes if H2 SO4 is employed as the dopant [251]. Furthermore, PANI formed on PANCA (polyacrylonitrile-carbon) aerogels exhibit fibrous morphology which is beneficial for enhancing the performance of supercapacitors [70]. The crucial role played by the concentrations of the monomer and dopant in influencing the nanostructure has also been demonstrated [63] and the effect of the scan rate analysed [73]. An elegant use of the reverse pulse

voltammetry to generate PANI nanofibres has also been pointed out [82]. In the case of the nanocomposites of PANI, the potential window and number of cycles need to be optimized so as to obtain satisfactory specific capacitances. PANI/MnO2 nanocomposites lead to a specific capacitance of 87 Fg−1 [170] while PANI/CNT composites using H2 SO4 as the dopant has the specific capacitance as 1030 Fg−1 [171,173]. The synthesis of PANI/CNT composites on SS [174] as well as on Ni [177] using H2 SO4 leads to the capacitance as ∼500 Fg−1 . On the other hand, the PANI/TiO2 composite using H2 SO4 provides the specific capacitance of 740 Fg−1 and that of PANI/Graphene composites on ITO yields 640 Fg−1 [179,180]. These studies indicate that both oxides based and CNT based conducting polymers have widely varying capacitances being dictated by the experimental protocols. B. Ppy and its composites The potentiodynamic synthesis on Indium Tin Oxide electrodes using LiClO4 and LiCF3 SO3 [100] lead to Ppy spheres. However, Ppy/CNT composites lead to an uniform thick coating when a wide potential window of ∼−1.5 to 1.5 V vs. SCE is employed. The potentiodynamic polymerization of pyrrole using LiClO4 on ITO [213], KCl (CNT) on Au (Quartz crystal) [214], NaClO4 (carbon surface) on Au plate [215] and HF (CNT) on GC [216] yield, respectively, the specific capacitances of 205 Fg−1 , 240 Fg−1 , 10 Fg−1 and 116 mF cm−2 .The specific capacitance for the copolymer of PANI and Ppy composite on Pt electrodes using H2 SO4 is estimated as 523 Fg−1 [238]. 2.2.2. Galvanostatic method Galvanostatic techniques wherein a constant current density is impressed upon the working electrode for an optimal duration of time, constitute versatile methods not only for the synthesis of diverse materials but also for pre-treatment of electrodes. A. PANI and its composites Both the normal and pulsed galvanostatic methods have been employed whereby the thickness and morphology of the polymer can be altered using the time of deposition and current density as depicted in Figs. 7 and 8. PANI nanowires were obtained with a specific capacitance of ∼950 Fg−1 using a current density of 0.01 mA cm−2 for 60 min [85]. An interesting study of PANI so as to demonstrate the efficiency of the ionic liquids in the


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Fig. 7. (i) SEM image of PANI nanowires grown galvanostatically and (ii) schematic depiction of ionic mobility within PANI nanowires. Reprinted with permission from Ref. [85]. © 2010 American Chemical Society.

context of supercapacitors needs to be pointed out. The perchlorate ions act as dopants in the synthesis of the PANI nanowires on Au sheets [85]. If the ionic liquid ethyl methyl imidazolium trifluorosulphonylimide (EMITFSI) is employed, the mobility of the trifluorosulphonyl (TFSI) anions is facilitated within the nanowire structure while perchlorate ions move ‘in and out’ of the polymer chain as depicted schematically in Fig. 7. The replacement of the smaller ClO4 − ions by the bulky TFSI− ions decreases the capacitance as anticipated. On the other hand, PANI nanowires on carbon cloth can also be obtained if the current density is 5 mA cm−2 [86]. PANI nanorods and coralline-like structures on ITO electrodes arise from the pulsed galvanostatic technique, subsequently yielding a capacitance of 591 Fg−1 [88]. PANI nanobelts were obtained at 70 ◦ C using a current density of 1 mA cm−2 [87]. Porous PANI nanorods and nanofibres were synthesized using a pulsed galvanostatic method at a current density of 2 mA cm−2 [62,69,88]. When PANI nanowires were synthesized on AAO at a current density of 0.65 mA cm−2 , a specific capacitance of 700 Fg−1 is noticed [163]. These studies indicate the importance of the current density and time in influencing the morphology and thereby the specific capacitance. B. Ppy and its composites In the case of Ppy, the efficacy of the pulsed current technique at the current density of 2 mA cm−2 has been investigated on Ta electrodes using p-TSA as the dopant [117]. Ppy horn-like structures may also be obtained on Ta at a current density of 5 mA cm−2 using the same dopant [118]. A commonly observed globular structure [104] of Ppy with a thickness of 2.5 ␮m can be synthesized on Ni electrodes using a current of 1 mA for 30 min. When Ppy is doped with Nafion, the number of charge–discharge cycles was 3000 with a constant specific capacitance of 350 Fg−1 [103]. Ppy nanowire arrays were obtained using a current density of 0.6 mA for 30 min and the specific capacitance is estimated as 566 Fg−1 [116]. Ppy horn-like structures using the pulsed galvanostatic techniques yield a specific capacitance of 403 Fg−1 at a current density of 0.5 mA cm−2 [224]. Galvanostatic synthesis of Ppy using current densities ranging from 0.1 mA cm−2 to 1 A cm−2 has been carried out [217–226,249] wherein the estimated capacitances are ∼60–1500 Fg−1 . A very high specific capacitance of 1510 Fg−1 is deduced for Ppy/graphene composites [218]. The specific capacitance of 620 Fg−1 arises if Ppy/MnO2 based electrodes synthesized at a current density of 4 mA cm−2 for a duration of 200 s [226] are employed. From the

foregoing, it can be inferred that each control variable, viz. the current density, deposition time and the nature of the substrate and the composite, dictates the magnitude of specific capacitance. 2.2.3. Potentiostatic method The potentiostatic techniques seem less popular in the synthesis of PANI and Ppy, presumably on account of the difficulty associated with quantifying the overpotential losses. A. PANI and its composites PANI nanofibres were synthesized using potentials of 0.75 V and 0.80 V vs. SCE [79,81] with respective specific capacitances as 851 Fg−1 and 608 Fg−1 . PANI nanowires on SS electrodes yield specific capacitances as 775 Fg−1 [68] and 742 Fg−1 [80]. Fuzzy PANI nanofibres were synthesized using a constant potential of 0.8 V vs. SCE for five minutes [84]. The polymerization and the nature of the resulting polymer can be controlled by choosing the optimum potentials and deposition time. The potentiostatic polymerization of PANI/CNT on SS [172,175] and PANI on graphene sheets [176] at 0.75 V vs. SCE yield specific capacitances of ∼250 Fg−1 and ∼500 Fg−1 , respectively PANI electropolymerized on AAO [178] yields a specific capacitance of ∼700 Fg−1 . B. Ppy and its composites Ppy was electrodeposited on AAO electrodes using a constant potential of 0.8 V vs. Ag/AgCl so as to obtain 1 ␮m thick films [112]. When Ppy was deposited on a three-dimensional MEMS (Microelectromechanical systems) electrode at 0.8 V vs. SCE for ∼20 min, a capacitance of 56 mF cm−2 [109] was deduced. Porous composites of cellulose and Ppy [108] as well as Ppy nanobricks were obtained [110] while a columnar structure of Ppy is also realizable by a proper control of potential window [111]. If Ppy is deposited at 0.7 V vs. SCE on nanoporous gold (NPG) films from chemical etching of Au/Ag films using H2 SO4 , the capacitance of 270 Fg−1 arises [125]. Electrochemical synthesis of Ppy based composites for supercapacitor applications can also be carried out under constant potential conditions [227–235] and well-defined nanostructures arise if experimental conditions are optimized. For Ppy based composites, the applied potential ranges from 0.7 V to 1.0 V vs. either SCE or Ag/AgCl. Among several studies employing Ppy based composites, the following studies are noteworthy: (i) Well aligned conically shaped Ppy/RuO2 on Au

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Fig. 8. (i) Schematic illustration of the growth and (ii) SEM images of Ppy horns. Reprinted with permission from Ref. [227]. © 2008 American Chemical Society.

electrodes [227], (ii) Ppy/MWCNT nanocomposite on Au using ionic liquids [231], (iii) Ppy nanorods on AAO in conjunction with a composite of RuOx [232]; (iv) Ppy along with a composite of anthraquinone sulphonic acids [233] and (v) Ppy/MWCNT on SS wires as current collectors using a pulsed potentioamperometric method [235]. The specific capacitance from a three-electrode set-up in the above systems yields the specific capacitance as ∼103 Fg−1 . As a typical mechanism for growth of Ppy, Fig. 8 depicts how horn-like structures arise systematically on Au surfaces [227].

Interestingly, the options for synthesis of conducting polymers both chemically and electrochemically have still not been exhausted. It may be pointed out in this context that surfactants in general yield polymer nanostructures with diverse morphologies. A systematic investigation on the role of the surfactants regarding the enhancement of capacitances has not yet been carried out. Analogously, the substrate on which polymerization occurs is also crucial since under identical conditions, stainless steel has proved to be superior than nickel in the case of polyaniline [75,76]. A rational approach, preferably


employing a factorial design of experiments needs to be formulated. 3. Electrochemical characterization of supercapacitors The electrochemical characterization of conducting polymers based supercapacitors is often carried out using (i) Galvanostatic charge–discharge studies; (ii) Cyclic Voltammetry and (iii) Electrochemical Impedance Spectroscopy (EIS). These techniques are in fact not complimentary to each other in the context of design of supercapacitors; rather each one of them has individual merits. For example, in order to obtain reliable preliminary estimates of the specific capacitance, galvanostatic charge–discharge studies using a three-electrode (working, reference and counter electrodes) assembly in a suitable medium are carried out. On the other hand, cyclic voltammetry provides information regarding the reversible nature of the electrode in a suitable electrolyte and the effect of the scan rate on the specific capacitance. The EIS not only yields accurate estimates of specific capacitance but also gives insights into the synergistic effect of each interfacial parameter. Barring a few studies employing two-electrode configurations, the other specific capacitances listed in Table 1 are values arising from a three-electrode assembly. These values per se depend upon the operating variables (scan rate in cyclic voltammetry, current density in galvanostatic charge–discharge experiments and potential in EIS) and hence provide only approximate estimates. In reality, the actual capacitances are significantly lower if two electrode assemblies are employed; hence, fabrication of suitable devices and testing under appropriate discharge conditions is the sole criterion for judging the efficiency and stability of any supercapacitor. Lest one may think that the magnitude of specific capacitance is an essential criteria in the design of supercapacitors, we hasten to add that other quantities such as power density, energy density, number of charge–discharge cycles, time constants etc. are equally crucial. The power density can be obtained from the Equivalent Series Resistance (vide infra) while the energy density is given by E=

1 2 CV 2

(1)

where V denotes the potential while ‘C’ denoting the capacitance can be expressed either gravimetrically or volumetrically or per unit area. The three-electrode assembly employed in such studies overestimates the specific capacitance significantly and hence the correct value should be ascertained from galvanostatic charge/discharge experiments pertaining to two electrodes [132]. Thus, some of the apparently large values reported in Table 1 will show substantial decrease in a practical context during the fabrication of devices. Here too, the current needs to be optimized in order to maximize the number of cycles as well as ensure stability for a satisfactory duration of time. However, investigations using a three-electrode assembly are pre-requisites for the preliminary choice of materials and electrolytes. 3.1. Galvanostatic charge–discharge experiments In these experiments, a constant current density is applied and the potential vs. time response is recorded. A typical potentiodynamic response (chronopotentiogram) is given in Fig. 9. The specific capacitance is evaluated using the equation C=

I × t V × m

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charge–discharge cycles arise without significant decrease in the specific capacitance. Table 1 provides specific capacitances for Ppy and PANI based systems and these estimates often employ three-electrode assemblies, thereby overestimating the magnitude of the specific capacitances. As can be inferred from Table 1, the electrodes used are Ni [33,43,49,74,75,90,94,104], SS [32,39,44,47,52,55,57,58,65,68,69,73,76–81,91,96,102,110,115], ITO [37,42,62,67,88,100,105], Carbon [34,45,46,48,56,59,63,70,86,98,101,114], Graphite [36,61,64,66,92,97], Glassy carbon [61,71,83,107], Au [50,85,108], Pt [38,63,82,111,113,116], Al [35,40,53,54,112], Ti [87,99,109], Ta [117,118], etc., while the morphological structures of PANI are nanowires [68,80,85], nanobelts [87], nanorods [37,67,88], nanowhiskers [34,56], etc. The surfactants seem to have a beneficial effect on the capacitance although no systematic studies have yet been carried out. The surfactants hitherto-employed are TritonX100 [62,74], SDS [44,60,95,98,102], DBSA [111] and CTAB [92,97] for PANI and Ppy based systems. The number of charge–discharge cycles obtainable from the galvanostatic experiments depends upon the magnitude of current while the specific capacitance in general decreases with increase in the number of cycles. In the case of PANI and Ppy nanocomposites, the commonly used electrodes are SS, Ni and graphite electrodes. A few surfactants such as CTAB, DBSA and SDS are used as structure directing agents. p-TSA, naphthalene disulphonic acid (NDA) and naphthalene sulphonic acid (NSA) play the dual role of a surfactant and a dopant. A variety of PANI and Ppy composite nanostructures such as nanowires [136], nanotubes [140,236], thorns [189], horns [118,224], nanorods [142,203,232], nanofibers [136,144,149,155,167], nanoplates [213], nanosheets [127,181,209,230] have been obtained and these yield larger specific capacitances. 3.2. Cyclic voltammetry Cyclic voltammetry is customarily employed for investigations of electron transfer processes in diverse contexts; however, in the analysis of supercapacitors, they are useful for (i) ascertaining the stability of the electrode; (ii) choosing the appropriate electrolyte and (iii) establishing the electrolyte concentrations. As is well-known, the potential window needs to be large in the cyclic voltammetric experiment and hence organic electrolytes are more preferred since the aqueous electrolytes restrict the magnitude of the potential window to ∼1 V. The cyclic voltammograms are rectangular in shape for two-electrode assemblies, while those for three-electrode assemblies exhibit features depicted schematically in Fig. 10. The specific capacitance (in Fg−1 ) from cyclic voltammetry is estimated from C=

i ×m

(3)

where i denotes the current and m is the mass loading, v being the scan rate. If ‘m’ in the above equation is replaced by the area (A) of the electrode, capacitance per unit area is obtained (F cm−2 ). As can be seen from the above equation, the specific capacitance depends upon the scan rate and the latter is an experimental ‘control variable’. 3.3. Electrochemical impedance spectroscopy

(2)

where I indicates the current, t denotes the discharge time, V is the potential range, m being the mass of the active material. The current density and time of discharge are parameters to be chosen such that maximum number of

The analysis of the conducting-polymers based supercapacitors almost invariably employs the impedance analysis so as to delineate the influence of different system components. The representation of the impedance data can be carried out using (i) Nyquist plot, viz. the imaginary vs. real part of the impedance (−Z vs. Z );

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Fig. 9. (i) Cyclic voltammogram at different scan rates and (ii) Chronopotentiometric response at different polymerization times (60, 120 and 360 s) of Ppy/graphene composites. Reprinted with permission from Ref. [228]. © 2011 American chemical society.

Fig. 10. (i) Schematic depiction of cyclic voltammograms for PANI based supercapacitors at different scan rates c > b > a (a) 100 mV s−1 , (b) 300 mV s−1 and (c) 500 mV s−1 and (ii) Chronopotentiometric response [75].

(ii) Bode’ phase angle plot ( versus log ω) and (iii) Bode’ magnitude plot (|Z| versus log ω). However, for estimating the specific capacitances, the variation of the imaginary part of the impedance (−Z ) with the reciprocal frequency (1/f) is constructed in order to calculate the slope from [239] d(−Z ) 1 = 2C d(1/f )

(4)

where f denotes the frequency. By dividing the C value obtained from the above equation by the mass of the deposited species, the specific capacitance in per unit mass arises. While the above equation is satisfactory for estimating the specific capacitance, and has been employed hitherto, the power of the EIS will be lost if all the system parameters (double layer capacitance, various resistances, exchange current density, Warburg impedance, etc.) essential for comprehending the electrochemical behavior, are not reported. Hence, the construction of appropriate equivalent circuits for deducing the system parameters becomes a sine qua non. Table 2 provides illustrative examples of PANI and Ppy based supercapacitors wherein different equivalent circuits have been postulated while Table 3 indicates the same for PANI and Ppy composites. The composites of conducting polymer and oxides/carbon based materials exhibit very low ESR values and hence yield high specific capacitances. Thus, the development of supercapacitors involving metal oxides/carbon materials and conducting polymers composites is poised for phenomenal growth

during the coming years on account of interesting scientific and technological possibilities. 3.3.1. Equivalent circuits and specific capacitances A common feature of most of the circuits of Table 2 is the presence of the solution resistance (Rs ), charge transfer resistance (Rct ), Warburg impedance (W) and double layer capacitance (Cdl ) vis a vis constant phase elements (CPE). The capacitances denoted as Cg [42], CL [105], CF [92] and CC [41] denote, respectively, the geometric capacitance, limiting capacitance, Faradaic capacitance and contact capacitance. Before describing the EIS analysis, it is essential to point out the equivalent circuit involving solution resistance (Rs ), charge transfer resistance (Rct ), double layer capacitance (Cdl ) and Warburg impedance (ZW ) popularly known as Randles circuit [240] depicted in Fig. 11. The Warburg impedance incorporating the diffusional characteristics of the electroactive species, has been extensively employed in the context of supercapacitors [41,42,72,92,105,250–252] in conjunction with other elements.

Fig. 11. Randles equivalent circuit for electrochemical systems [4].


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Table 2 The equivalent circuit parameters for various PANI and Ppy coated electrodes. System

Equivalent circuit

Rs ()

Rct ()

PANI on Carbon paper [41]

1.35

PANI on porous carbon/SS [72]

0.43

1147

PANI on SS [248]

1.00

83

Ppy on ITO [105]

140–330a

Ppy and substituted Ppy on carbon fiber microelectrode [250]

–

PANI on GC [251]

6.36

Ppy nanowires on graphite groove using PTFE binder [92]

2.25a

Ppy on Al [252]

135.5

2.25

92–130a

–

253.5

1.05*

7.05

120


Table 2 (Continued) System

Equivalent circuit

Rs ()

Rct ()

PANI on ITOa [42]

140.2

486.26

PANI on SS [253]

–

–

a in cm2 . ZD denotes the impedance containing the dual line of capacitors and resistors in parallel [229]. Rab denotes the absorption resistance of PANI [253] [a Rct consists of individual charge transfer resistances].

The total impedance for the circuit of Fig. 11 can be derived as, √ √ (Rct + (/ ω)) − (j/ ω) (5) Ztotal = Rs + √ √ (1 + · ωCdl ) + j(ωCdl Rct + ωCdl ) where is the Warburg coefficient, comprising the diffusion coefficient of the species. In the case of the Ppy on ITO [19], the additional limiting capacitance CL is incorporated yielding the total impedance as, √ √ (Rct + (/ ω)) − (j/ ω) j Ztotal = Rs + (6) − √ √ ωCL (1 + ωCdl ) + j(ωRct Cdl + ωCdl ) Interestingly, the double layer capacitance (Cdl ) as well as the Warburg impedance (ZW ) can also be replaced by two constant phase elements CPE1 and CPE2 so as to obtain the circuit depicted in Fig. 12. This circuit has been widely employed in the PANI and Ppy based supercapacitors [38,39,71,74–77,81,82,251,254]. Furthermore, this circuit is useful for the analysis of the Underpotential Deposition of metals [255] and the efficacy of the UPD-based supercapacitors has been pointed out by Conway [4]. The impedance of a constant phase element is, ZCPE =

1 Q (jω)

n

(7)

where the significance of Q is dictated by the exponent n. If n = 1, Q becomes the capacitance. In the case of two constant phase

Fig. 12. Equivalent circuit commonly employed for conducting polymers based supercapacitors.

elements, two exponents n1 and n2 are required. The total impedance for the circuit shown in Fig. 12 is as follows: Ztotal = Rs +

(ZCPE1 ) × (Rct + ZCPE2 ) Rct + ZCPE2 + ZCPE1

(8)

where ZCPE1 and ZCPE2 consist of two exponents n1 and n2 (cf. Eq. (7)). By employing the usual techniques of complex variables, the expressions for the real and imaginary parts of the impedance as well as the phase angle follow effortlessly. Such detailed analyses are useful in estimating all the system parameters characterizing the supercapacitors. In simple RC circuits, the double layer capacitance and charge transfer resistances enable an estimate of the time constant using, = Rct Cdl

(9)

We note that the time constant is also influenced by the geometry of the electrode; in the case of microelectrodes, the time constant increases with increasing radius [262]. In the design of supercapacitors, low time constants are preferred so as to ensure fast charge–discharge characteristics. There are two distinct approaches to the construction of equivalent circuits for any electrochemical system, viz. (i) fitting of the impedance data using the in-built software of the electrochemical work station and (ii) formulating the real and imaginary parts of the impedance with subsequent non-linear regression analysis using any of the computer programming languages. As an illustrative example, Fig. 13 provides the fitting of the Nyquist data using the in-built software provided by the electrochemical workstation [76]. However, abundant choices become available if the fitting is carried out using an appropriate computer program. The impedance equations formulated for a chosen equivalent circuit provide an insight into the role played by each circuit parameter for deciphering its influence. This methodology has been employed in fitting the Nyquist, Bode’ phase angle and Bode’ magnitude plots (Fig. 14) for polypyrrole based stainless steel electrodes [241] with the circuit parameters obtained using MATLAB programming.


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Table 3 The equivalent circuit parameters for various PANI and Ppy composites with oxides and carbon materials. System

Equivalent circuit

Rs ()

Rct ()

Ppy/Graphene on SS [194]

0.27

0.15

WACNP/RuO2 on Au [227]

–

0.20

Ppy/Graphene [200]

1.50

15.08

PANI/Fe2 O3 on SS [126]

0.20

1.00

Ppy/MnO2 on Ni sheet [206]

1.20

3.35

3.3.2. Influence of Ohmic and charge transfer resistances It is imperative to comment upon the parameters of Table 4 pertaining to polyaniline and polypyrrole based supercapacitors. The charge transfer resistance is a measure of the resistance associated with the electron transfer process and is inversely related to the exchange current density. Thus, if Rct is lower, it signifies a facile interfacial electron transfer process and hence a higher specific Faradaic(pseudo)capacitance. The lower Rct values vis a vis higher exchange current densities are indicators of the suitability of the electrode material. The electrode substrates employed are SS [76], Nickel [74,75], Glassy carbon [71,251], Carbon paper [41], Pt [38,82], etc. The magnitude of the solution resistance is dictated by the nature of the electrolyte solution as well as its concentration

and cannot be altered too much without tampering with the number of charge–discharge cycles. The solution resistances too vary over a wide range in systems hitherto-investigated, viz. in studies employing polyaniline, the solution resistance increases from ∼0.05 (if co-doped with Zn2+ and H+ ions) [38] to 6.74 (if p-toluene sulphonic acid is employed) [251]. In the case of Ppy based supercapacitors, the ohmic resistance varies from 0.6 (in the case of Ppy nanosheets on SS) [254] to 330 (in the case of Ppy on ITO) [105]. The value of Rs varies from 0.27 (Ppy/graphene on SS) to 1.5 (Ppy on Graphene) while Rct varies from 0.15 (Ppy/graphene on SS) [194] to 15.08 (Ppy/Graphene) [200]. These values imply that the conducting polymers based composites decrease the ESR and have higher specific capacitances

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Fig. 13. Nyquist plots for PANI on SS at (a) 0.2 V and (b) 0.4 V using the in-built software of the electrochemical work station. The points denote the experimental data while the line denotes the fitting pertaining to the circuit of Fig. 12 [76].

Fig. 14. (a) Nyquist, (b) Bode’ magnitude and (c) Bode’ phase angle plots for polypyrrole coated SS electrode [241]. Table 4 The parameters for various PANI and Ppy systems pertaining to the equivalent circuit shown in Fig. 12. System

Rs ()

Rct ()

n1

n2

PANI co-doped with Zn2+ and H+ on Pt [38] PANI-PDD on SS mesh [39] PANI (Tx-100) on Ni [74] PANI on Ni [75] PANI on SS [76] PANI-PDD on GCE [71] PANI on SS [81] Self-doped PANI/HCl on Pt [82] PANI/PTSA on SS [77] PANI/PTSA on GCE [251] Ppy nanosheets on SS [254]

0.05 1.64 4.00a 4.00a 1.60a 1.70a 4.24 0.84 2.18 6.74 0.60a

0.03 0.76 7.00a 348a 1.70a 0.12a 13.06 1.10a 7.88 5.16 36.06a

0.85 0.69 0.78 0.78 0.92 0.80 0.68 0.61 0.89 0.57 –

0.99 0.95 0.85 0.90 0.92 0.96 0.98 0.94 0.83 0.95 –

a

In cm2 .


Fig. 15. The dependence of specific capacitance on the inverse of the total resistance for a few conducting polymers based electrodes (the numbers denote the corresponding systems in Table 4).

vis a vis power densities. Furthermore, since the resistors Rs and Rct are in series in the circuits of Table 4, the combined resistance becomes, R = Rs + Rct

(10)

Thus, a typical variation of the specific capacitance with the inverse of the combined resistance (1/R) as shown in Fig. 15 arises wherein the specific capacitance increases with the reciprocal of the total resistance. For a more realistic description of the impedance data, incorporation of two charge transfer resistances Rct1 and Rct2 has been advocated as in the case of PANI on ITO [42] on account of ionic and charge transfer resistances. The resistance of a supercapacitor sometimes termed as ‘Equivalent Series Resistance’ arises from that of the electrodes, current collectors and the medium. The maximum power of a supercapacitor is related to ESR and the potential (V) as, Pmax =

V2 4ESR

(11)

The ESR values have been reported for the PANI [39,79,83] and Ppy based [94,111], supercapacitors; these can be obtained from two different experimental techniques, viz. (i) from constant current discharge data using [242] the equation below, ESR =

Echarge − Edischarge 2I

(12)

and (ii) from the Nyquist plot by extrapolation of the impedance to high frequency data. 3.3.3. Influence of the electrical double layer The ability to store charges is manifested by the analysis of EDLC and hence the electrical double layer analysis should play a prominent role in any design of supercapacitors. In the case of real capacitors such as carbon based materials, the double layer thickness and interfacial area are explicitly present in the capacitance expression, viz. C=

ε0 εr A d

(13)

where d denotes the double layer thickness, A being the interfacial area, εr represents the relative permittivity of the medium and ε0 is the permittivity of the free space. This expression originally

123

Fig. 16. Estimation of the charge transfer resistance, solution resistance and double layer capacitance from impedance data [246].

arising from the classical Helmholtz model has undergone much metamorphosis in modern theories of the electrical double layer through the density functional theories of electrode surfaces due to the pioneering works of Schmickler [243], Badiali [244] and Kornyshev [245]. It is essential to realize that the electron densities of the electrode surfaces are crucial parameters in the theory of electrical double layer [256]. A systematic method of investigating the influence of electrode materials on the performance of supercapacitors is still lacking. In the absence of an exhaustive experimental study of chosen polymers on different metal surfaces, a rational choice of substrates using theoretical principles becomes difficult. For example, apart from the cost considerations, there are no other limitations on the choice of electrode materials for a given polymer. It is worthwhile considering the electronic structure of the electrode surfaces on the performance of supercapacitors. Furthermore, the precise values for the interfacial area and thickness in the above equation have interesting connotations as regards the modeling of supercapacitors [257]. In the case of pseudocapacitors based on conducting polymers, the influence of the double layer is somewhat implicit and may arise through two different factors (i) exchange current density and (ii) redox capacitance. An elegant method of deducing the double layer capacitance and charge transfer resistance has been advocated by Sarac et al. [250] using the extrapolation of log Z axis at ω = 1 (log ω = 0) for Ppy on carbon fiber microelectrodes. The frequency corresponding to the maximum value of the phase angle is given by ω( max) = {(1/Cdl Rct )(1 + Rct /Rs )}

1/2

(14)

As shown in Fig. 16, the frequency corresponding to the maximum phase angle (ω(max) ) is noted. Subsequently, the extrapolation to log(ω) = 0, in the log |Z| vs. log(ω) plot yields the double layer capacitance [246]. The ohmic or solution resistance and charge-transfer resistances can also be obtained analogously. The high frequency region of the Bode’ magnitude plot on extrapolation yields the solution resistance Rs , whereas the low frequency region of the Bode’ magnitude plot provides the value of Rs + Rct wherefrom the value of Rct follows. These studies indicate that various components of the electrochemical system can indeed be deduced elegantly from suitable impedance plots, thus obviating the need for a complete equivalent circuit fitting of the data.

124


4. Structural characterization of supercapacitors For structural characterization of supercapacitors, several spectroscopic and microscopic studies in conjunction with the X-ray diffraction techniques are carried out. The Fourier Transform Infrared (FTIR) studies of PANI [34,36,38,42,43,47,51–53,55,56,74–76,78,79,83,84,87] and Ppy [90,91,95,102,110] exhibit characteristic bands corresponding to various vibrational modes and provide information regarding the interaction with the electrode surface and influence of surfactants and dopants. From the X-ray diffraction spectra, the amorphous, semi-crystalline and crystalline nature of PANI [34–36,39,41–43,47,53,55,74–76,78–80,87] and Ppy [90,91,110] are inferred. The SEM and TEM studies provide morphological patterns of various PANI and Ppy based systems indicated in Table 1. Such structural characterization studies are pointers to the need for an interdisciplinary approach for fabrication of supercapacitors, with expertise from diverse disciplines being mandatory. Fig. 17. The variation of the specific capacitance with the exponent of the constant phase element (CPE) pertaining to the equivalent circuit of Fig. 12. The numbers refer to the system mentioned in Table 4. The line is drawn as a guide to the eye.

The nature of the electrical double layer is incorporated in the circuit either through the double layer capacitances or constant phase elements. These constant phase elements take into account, the inhomogeneities existing at the interface, porosity, the specificity of the electrode, and dynamic disorder associated with diffusion. Other constant phase elements associated with the system such as CPEpolymer , etc. become necessary in order to compensate for the porosities and inhomogeneities of the polymeric moiety in case of solid polymer supercapacitors. The two exponents ‘n1 and ‘n2 characterize, respectively, the porosity of the system and capacitive nature of the electrode. The variation of specific capacitance with n2 is given in Fig. 17. 3.3.4. Transmission line models and beyond A corollary to the equivalent circuit representation is the transmission line model formulation wherein the circuit elements are the resistances, capacitances and inductances as before but with incorporation of (i) all the charge carrying species; (ii) nature of electrodes (porous vs. non-porous); (iii) diffusional characteristics and (iv) time evolution. All the governing processes are taken into account while formulating and solving the transport equations in the steady as well as transient states. The mathematical analysis is indeed complex in view of the need to solve the system of nonlinear partial differential equations; but the reward lies in obtaining a complete understanding of the stability of the device. As a typical illustration of the power of the transmission line model in this context, the galvanostatically synthesized polypyrrole on ITO deserves mention [247] in view of its ability to delineate the role of film thickness as well as the diffusion coefficient of the polymer. Such studies involving supercapacitor assemblies in conjunction with the transmission line models will shed more light on the cycle life of the fabricated devices from fundamental principles, partially eliminating the need for ad hoc formulation. It has been pointed out [258] that any model for supercapacitors pertaining to the impedance analysis is a combination of capacitances, resistances and inductances. The inclusion of the inductance component often ensures that all the frequency ranges are taken into account. By incorporating the porous nature of the electrodes, it has been demonstrated elegantly [258] that the simulation of a commercial 1400 F supercapacitor can be accomplished at various temperatures and bias voltages. The extension of this methodology for conducting polymers based systems in a generic manner has hitherto not been achieved.

5. Modeling of electrochemical supercapacitors In contrast to the batteries and fuel cells, the modeling of supercapacitors has remained somewhat passive partially on account of diverse influences of each constituent. Even theoretical correlations governing the performance of supercapcitors do not exist. This limitation is due to the fact that the interfacial parameters such as area and thickness in EDLC are a priori unknown. As pointed out by Brain et al. [257], even the estimate of the maximum theoretical capacitance for a chosen system is not clear. In the case of pseudocapacitors based on conducting polymers, this limitation is further compounded by the fact that there is no systematic study wherein a given conducting polymer is investigated under identical conditions with different metals. One does anticipate a correlation of the experimental specific capacitance with work function of the substrate on the lines of the Hydrogen Evolution reaction on different electrodes [259]. 6. Fabrication of devices The design and fabrication of supercapacitors using carbon based materials in conjunction with various oxides are the potential candidates for commercial exploitation. Nevertheless, by a judicious choice of inexpensive substrates and mild electrolytes, other conducting polymers based supercapacitors too can be envisaged. By employing stacks of PANI coated SS electrodes, a capacitance of 420 F for 1000 cycles has been demonstrated by Prasad et al. [65]. Since the substrate is SS, such supercapacitors are inexpensive and can be commercially exploited. 7. Perspectives The foregoing analysis has provided an overview of the present status of polyaniline and polypyrrole based supercapacitors along with their composites, the emphasis being on the contributions during the past decade. Although these two systems have attracted extensive research, much remains to be done. For example, the influence of the surfactant in enhancing the specific capacitance of conducting polymers based supercapacitors deserves a more thorough analysis. Analogously, the influence of the dopants vis a vis electrolytes has not yet been deciphered systematically. For example, whether the maximum power density is limited by the electrolyte or surface area is not yet clear. Even for simple metal electrodes such as SS, Ni, Ti, etc., the maximum obtainable specific capacitance is not easy to decipher. This limitation arises on account of the Faradaic efficiency vis a vis the nature of the


interaction of the polymer with the electrodes. It is of interest to note that different electropolymerization techniques can yield varying capacitances for a chosen monomer and the most suitable technique for a given polymer needs to be established by ‘trial and error’ methods at present. A variety of nanostructures of conducting polymers at liquid–liquid interfaces has been obtained by different protocols. These can be coated onto electrodes using methods such as spin-coating, drop casting, electrospinning etc. Their usefulness as capacitor materials has not yet been extensively investigated. It will be of interest to investigate the feasibility of these polymers as supercapacitors after coating onto electrodes using suitable binders and adhesives. Analogously, the influence of morphological structures on the magnitude of the specific capacitance requires a systematic investigation. The correlation between the conductivity of polymers and specific capacitance is anticipated but has not yet been elucidated. The efficacy of other conducting polymers such as polyindole has not yet been fully explored. This is particularly surprising since polyindole possesses high redox activity, slow degradation rate and good thermal stability. Another unexplored area is the study of other copolymers (analogous to polyaniline and polythiophene [260]) for supercapacitor applications. At a fundamental level, the influence of ions on the structure of PANI and associated memory effects will throw further light on the intrinsically complex polymerization processes [261]. Several metal-polymer composites can be synthesized in a facile manner and their suitability as supercapacitors need to be studied. While the metal-polymer composites based systems may not enhance the capacitance value significantly, they provide marked stability and processability. The theoretical studies concerning the performance of the supercapacitors using phenomenological/microscopic models are long overdue. In the absence of rigorous criteria regarding the choice of polymers, electrode materials, electrolytes and surfactants, it becomes tedious to predict ab initio the practical performance of the supercapacitors. A combined strategy involving thermodynamic calculations of Faradaic efficiency of the polymers in conjunction with materials chemistry of electrodes is warranted in order to achieve significant progress in the design of supercapacitors. 8. Conclusions The development of the PANI and Ppy based electrochemical supercapacitors during the past decade is reviewed. The diverse protocols for chemical and electrochemical synthetic methodologies have been indicated. The electrochemical characterization of the polymers and their composites is emphasized. The specific capacitances estimated using various techniques are compiled. The efficacy of impedance studies in deducing the system parameters from the fitting of the equivalent circuit data is pointed out. The modeling of supercapacitors from the perspective of electrical double layer theories is indicated. Acknowledgements The helpful comments of the reviewers on an earlier version of the manuscript are gratefully acknowledged. The authors thank V. Divya, Subrata Mondal and T. C. Girija for valuable help during the preparation of this review. This work was supported by the DST and CSIR, Government of India. References [1] P. Delahay, Double Layer and Electrode Kinetics, Interscience-Wiley, New York, 1965.

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[2] W. Schmickler, Electronic effects in the electric double layer, Chemical Reviews 96 (1996) 3177. [3] H.E. Becker, Low Voltage Electrolytic Capacitor, U. S. Patent 2,800, 616 (to General Electric Co.) (1957). [4] B.E. Conway, Electrochemical Supercapacitors, Kluwer Academic, Plenum, New York, 1999. [5] K.R. Prasad, K. Koga, N. Miura, Electrochemical deposition of nanostructured Indium oxide: high-performance electrode material for redox supercapacitors, Chemistry of Materials 16 (2004) 1845. [6] M. Kalaji, P.J. Murphy, G.O. Williams, The study of conducting polymers for use as redox supercapacitors, Synthetic Metals 102 (1999) 1360. [7] C. Largeot, C. Portet, J. Chmiola, P. Taberna, Y. Gogotsi, P. Simon, Relation between the ion size and pore size for an electric double-layer capacitor, Journal of the American Chemical Society 130 (2008) 2730. [8] S. Kandalkar, D. Dhawale, C. Kim, C. Lokhande, Chemical synthesis of cobalt oxide thin film electrode for supercapacitor application, Synthetic Metals 160 (2010) 1299. [9] K. Lota, V. Khomenko, E. Frackowaik, Capacitance properties of ordered porous csarbon materials prepared by a templating procedure, Journal of Physics and Chemistry of Solids 65 (2004) 287. [10] Y. Zhai, Y. Dou, D. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai, Carbon materials for chemical capacitive energy storage, Advanced Materials 23 (2011) 4828. [11] C.D. Lokhande, D.P. Dubal, O. Joo, Metal oxide thin film based supercapacitors, Curr. Appl. Phys. 11 (2011) 255. [12] G. Lota, K. Fic, E. Frackowiak, Carbon nanotubes and their composites in electrochemical applications, Energy Environ. Sci. 4 (2011) 1592. [13] X. Chen, G. Wu, Y. Jiang, Y. Wang, X. Chen, Graphene and graphene-based nanomaterials: the promising materials for bright future of electroanalytical chemistry, Analyst 136 (2011) 4631. [14] G.A. Snook, P. Kao, A.S. Best, Conducting-polymer-based supercapacitor devices and electrodes, Journal of Power Sources 196 (2011) 1. [15] W. Deng, X. Ji, Q. Chen, C.E. Banks, Electrochemical capacitors utilizing transition metal oxides: an update of recent developments, RSC Adv. 1 (2011) 1171. [16] M. Inagak, H. Konno, O. Tanaike, Carbon materials for electrochemical capacitors, Journal of Power Sources 195 (2010) 7880. [17] C. Xu, F. Kang, B. Li, H. Du, Recent progress on manganese dioxide supercapacitors, Journal of Materials Research 25 (2010) 1421. [18] L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chemical Society Reviews 38 (2009) 2520. [19] N.A. Choudhury, S. Sampath, A.K. Shukla, Hydrogel-Polymer electrolytes for electrochemical capacitors: an overview, Energy Environ. Sci. 2 (2009) 55. [20] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Materials 7 (2008) 845. [21] A.G. Pandolfo, A.F. Hollenkamp, Review. Carbon properties and their role in supercapacitors, Journal of Power Sources 157 (2006) 11. [22] W. Qin, L. Jian-ling, G. Fei, L. Wen-sheng, W. Ke-zhong, W. Xin-dong, Activated carbon coated with polyaniline as an electrode material in supercapacitors, New Carbon Mater. 23 (2008) 275. [23] E. Frackowiak, F. Beguin, Carbon materials for the electrochemical storage of energy in capacitors, Carbon 39 (2001) 937. [24] W. Wei, X. Cui, W. Chen, D.G. Ivey, Manganese oxide-based materials as electrochemical supercapacitor electrodes, Chemical Society Reviews 40 (2011) 1697. [25] N. Nuraje, K. Su, N.L. Yang, H. Matsui, Liquid interfacial polymerization to grow singlecrystalline nanoneedles of various conducting polymers, Acs. Nano 2 (2008) 502. [26] X. Zhang, C. King, R. Anil, K.M. Sanjeev, Nanofibers of polyaniline synthesized by interfacial polymerization, Synthetic Metals 145 (2004) 23. [27] H. Ma, Y. Luo, S. Yang, Y. Li, F. Cao, J. Gong, Synthesis of aligned polyaniline belts by interfacial control approach, Journal of Physical Chemistry C 115 (2011) 12048. [28] D. Ekarat, T.D. Stephan, Interfacial polymerization of water-soluble polyaniline and its assembly using the layer-by-layer technique, JOM 19 (2009) 39. [29] G. Hui, F. Li-Zhen, Z. Hongchang, Q. Xuanhui, Polyaniline nanofibers obtained by interfacial polymerization for high-rate supercapacitors, Electrochimica Acta 56 (2010) 964. [30] D. Panagiotis, N. Dimitrios, V. Daniel, B. Nicholaos, P. Stane, T. Christos, P. Dimitrios, Interfacial polymerization of pyrrole and in situ synthesis of polypyrrole/silver nanocomposites, Polymer 48 (2007) 2007. [31] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J.P. Ferraris, Conducting polymers as active materials in electrochemical capacitors, Journal of Power Sources 47 (1994) 89. [32] D.S. Dhawale, A. Vinu, C.D. Lokhande, Stable nanostructured polyaniline electrode for supercapacitor application, Electrochimica Acta 56 (2011) 9482. [33] O. Koysuren, C. Du, N. Pan, G. Bayram, Preparation and comparison of two electrodes for supercapacitors: Pani/CNT/Ni and Pani/Alizarin-treated nickel, Journal of Applied Polymer Science 113 (2009) 1070. [34] Y. Yan, Q. Cheng, G. Wang, C. Li, Growth of polyaniline nanowhiskers on mesoporous carbon for supercapacitor application, Journal of Power Sources 196 (2011) 7835. [35] K.S. Ryu, K.M. Kim, N. Park, Y.J. Park, S.H. Chang, Synthesis and electrochemical properties of V2 O5 intercalated with binary polymer, Journal of Power Sources 103 (2002) 305. [36] H. Mi, X. Zhang, S. Yang, X. Ye, J. Luo, Polyaniline nanofibers as the electrode material for supercapacitors, Materials Chemistry and Physics 112 (2008) 127.

126


[37] C.A. Amarnath, J.H. Chang, D. Kim, R.S. Mane, S. Han, D. Sohn, Electrochemical supercapacitor application of electroless surface polymerization of polyaniline nanostructures, Materials Chemistry and Physics 113 (2009) 14. [38] J. Li, M. Cui, Y. Lai, Z. Zhang, H. Lu, J. Fang, Y. Liu, Investigation of polyaniline co-doped with Zn2+ and H+ as the electrode material for electrochemical supercapacitors, Synthetic Metals 160 (2010) 1228. [39] H. Guan, L. Fan, H. Zhang, X. Qu, Polyaniline nanofiber obtained by interfacial polymerization for high-rate supercapacitors, Electrochimica Acta 56 (2010) 964. [40] K.S. Ryu, Y. Hong, Y.J. Park, X. Wu, K. Kim, Y. Lee, S. Chang, S. Lee, Polyaniline doped with dimethylsulfate as a polymer electrode for all solid-state power source system, Solid State Ionics 175 (2004) 759. [41] P. Sivaraman, S.K. Rath, V.R. Hande, A.P. Thakur, M. Patri, A.B. Samui, Allsolid-supercapacitor based on polyaniline and sulfonated polymers, Synthetic Metals 156 (2006) 1057. [42] B. Gupta, R. Prakash, Interfacial polymerization of carbazole: Morphology controlled synthesis, Materials Science and Engineering C 29 (2009) 1746. [43] H. Zhang, Q. Zhao, S. Zhou, N. Liu, X. Wang, J. Li, F. Wang, Aqueous dispersed conducting polyaniline nanofibers: promising high specific capacity electrode materials for supercapacitor, Journal of Power Sources 196 (2011) 10484. [44] S. Palaniappan, S.L. Devi, Novel chemically synthesized polyaniline electrodes containing a fluoroboric acid dopant for supercapacitors, Journal of Applied Polymer Science 107 (2008) 1887. [45] J. Fang, M. Cui, L. Hai, Z. Zhang, Y. Lai, Hybrid supercapacitor based on polyaniline doped with lithium salt and activated carbon electrodes, J. Cent. South Univ. Technol. 16 (2009) 434. [46] P. Sivaraman, R.K. Kushwaha, K. Shashidahara, V.R. Hande, A.P. Thakur, A.B. Samui, M.M. Khandpekar, All solid supercapacitor based on polyaniline and crosslinked sulfonated poly[ether ether ketone], Electrochimica Acta 55 (2010) 2451. [47] J. Liu, M. Zhou, L. Fan, P. Li, X. Qu, Porous polyaniline exhibits highly enhanced electrochemical capacitance performance, Electrochimica Acta 55 (2010) 5819. [48] P. Sivaraman, V.R. Hande, V.S. Mishra, A.B. Ch Srinivasa rao, Samui, All-solid supercapacitor based on polyaniline and sulfonated poly(ether ether ketone), Journal of Power Sources 124 (2003) 351. [49] J.H. Park, O.O. Park, Hybrid electrochemical capacitors based on polyaniline and activated carbon electrodes, Journal of Power Sources 111 (2002) 185. [50] B.C. Kim, J.S. Kwon, J.M. Ko, J.H. Park, C.O. Too, G.G. Wallace, Preparation and enhanced stability of flexible supercapacitor prepared from Nafion/Polyaniline nanofiber, Synthetic Metals 160 (2010) 94. [51] D.S. Patil, J.S. Shaikh, D.S. Dalavi, S.S. Kalagi, P.S. Patil, Chemical synthesis of highly stable PVA/PANI films for supercapacitor application, Materials Chemistry and Physics 128 (2011) 449. [52] Z. Lei, Z. Chen, X.S. Zaho, Growth of Polyaniline on hollow carbon spheres for enhancing electrocapacitance, Journal of Physical Chemistry C 114 (2010) 19867. [53] K.S. Ryu, S.K. Jeong, J. Joo, K.M. Kim, Polyaniline doped with dimethy sulphate as a nucleophilic dopant and its electrochemical properties as an electrode in a lithium secondary battery and a redox supercapcitor, Journal of Physical Chemistry B 11 (2007) 731. [54] K.S. Ryu, X. Wu, Y. Lee, S.H. Chang, Electrochemical capacitor composed of doped polyaniline and polymer electrolyte membrane, Journal of Applied Polymer Science 89 (2003) 1300. [55] S. Palaniappan, S.B. Sydulu, T.L. Prasanna, P. Srinivas, High temperature oxidation of aniline to highly ordered polyaniline-sulphate salt with a nanofiber morphology and its use as electrode materials in symmetric supercapacitors, Journal of Applied Polymer Science 120 (2011) 780. [56] Y. Wang, H. Li, Y. Xia, Ordered whisker like polyaniline grown on the surface of mesoporous carbon and its electrochemical capacitance performance, Advanced Materials 18 (2006) 2619. [57] Y.A. Ismail, J. Chang, S.R. Shin, R.S. Mane, S. Han, S.J. Kim, Hydrogel-Assisted polyaniline microfibers as controllable electrochemical actuatable supercapacitor, Journal of the Electrochemical Society 156 (2009) A313. [58] J. Li, J. Fang, M. Cui, H. Lu, Z. Zhang, Electrochemical performance of interfacially polymerized polyaniline nanofibers as electrode materials for non-aqueous redox supercapacitors, J. Cent. South Univ. Technol. 18 (2011) 78. [59] H. Tamai, M. Hakoda, T. Shiono, H. Yasuda, Preparation of polyaniline coated activated carbon and their electrode performance for supercapacitor, J. Mater. Sci. 42 (2007) 1293. [60] L. Shi, X. Wu, L. Lu, X. Yang, X. Wang, Intercalated polyaniline nanosheets prepared from lyotropic liquid crystalline solutions and their capacitive performance, Synthetic Metals 160 (2010) 989. [61] Y. Li, X. Zhao, Q. Xu, Q. Zhang, D. Chen, Facile preparation and enhanced capacitance of polyaniline/sodium alginate nanofiber network for supercapacitors, Langmuir 27 (2011) 6458. [62] S. Jiao, J. Tu, C. Fan, J. Hou, D.J. Fray, Electrochemically assembling of a porous nano-polyaniline network in a reverse micelle and its application in a supercapacitor, Journal of Materials Chemistry 21 (2011) 9027. [63] S.K. Mondal, K. Barai, N. Munichandraiah, High capacitance properties of polyaniline by electrochemical deposition on a porous carbon substrate, Electrochimica Acta 52 (2007) 3258. [64] C. Hu, J. Lin, Effects of the loading and polymerization temperature on the capacitive performance of polyaniline in NaNO3 , Electrochimica Acta 47 (2002) 4055.

[65] K. Rajendra prasad, N. Munichandraiah, Fabrication and evaluation of 450 F electrochemical redox supercapacitors using inexpensive and high performance, polyaniline coated, stainless steel electrodes, Journal of Power Sources 112 (2002) 443. [66] R.K. Sharma, A.C. Rastogi, S.B. Desu, Pulse polymerized polypyrrole electrodes for energy density electrochemical supercapacitors, Electrochemistry Communications 10 (2008) 268. [67] B.K. Kuila, B. Nandan, M. Bohme, A. Janke, M. Stamm, Vertically oriented arrays of polyaniline nanorods and their superelectrochemical properties, Chemical Communications 38 (2009) 5749. [68] V. Gupta, N. Miura, Electrochemically deposited polyaniline nanowire’s network: A high-performance electrode material for redox supercapacitor, batteries, fuel cells and energy conservation, Electrochemical and Solid-State Letters, Electrochemical and Solid-State Letters 12 (2005) A630. [69] H. Zhou, H. Chen, S. Luo, G. Lu, W. Wei, Y. Kuang, The effect of polyaniline morphology on the performance of polyaniline supercapacitors, Journal of Solid State Electrochemistry 9 (2005) 574. [70] H. Talbi, P.E. Just, L.H. Dao, Electropolymerization of aniline on carbonized polyacrylonitirle aerogels electrodes: Application for supercapacitors, Journal of Applied Electrochemistry 33 (2003) 465. [71] Z. Mandic, M.K. Rokovic, T. Pokupcic, Polyaniline cathodic material for electrochemical energy sources: the role of morphology, Electrochimica Acta 54 (2009) 2941. [72] W. Chen, T. Wen, H. Teng, Polyaniline-deposited porous carbon electrode for supercapacitor, Electrochimica Acta 48 (2003) 641. [73] W. Chen, T. Wen, Electrochemical and capacitive properties of polyanilineimplanted porous carbon electrode for supercapacitors, Journal of Power Sources 117 (2003) 273. [74] T.C. Girija, M.V. Sangaranarayanan, Polyaniline-based nickel electrodes for electrochemical supercapacitors-influence of Triton X 100, Journal of Power Sources 159 (2006) 1519. [75] T.C. Girija, M.V. Sangaranarayanan, Analysis of polyaniline-based nickel electrodes for electrochemical supercapacitors, Journal of Power Sources 156 (2006) 705. [76] T.C. Girija, M.V. Sangaranarayanan, Investigation of polyaniline coated stainless steel electrodes for electrochemical supercapacitors, Synthetic Metals 156 (2006) 244. [77] G. Xu, W. Wang, X. Qu, Y. Yin, L. Chu, B. He, H. Wu, J. Fang, Y. Bao, L. Liang, Electrochemical properties of polyaniline in p-toluene sulphonic acid solutions, Eur. Polym. J 45 (2009) 2701. [78] V.S. Jamadade, D.S. Dhawale, C.D. Lokhande, Studies of electrosynthesized leucoemeraldine, emeraldine and pernigraniline forms of polyaniline films and their supercapacitive behavior, Synthetic Metals 160 (2010) 955. [79] D.S. Dhawale, R.R. Salunkhe, V.S. Jamadade, D.P. Dubal, S.M. Pawar, C.D. Lokhande, Hydrophilic polyaniline nanofibrous architecture using electrosynthesis method for supercapacitor application, Curr. Appl. Phys. 10 (2010) 904. [80] V. Gupta, N. Miura, High performance electrochemical supercapacitor from electrochemically synthesized nanostructured polyaniline, Materials Letters 60 (2006) 1466. [81] H. Li, J. Wang, Q. Chu, Z. Wang, F. Zhang, S. Wang, Theoretical and experimental specific capacitance of polyaniline in sulphuric acid, Journal of Power Sources 190 (2009) 578. [82] H.R. Ghenaatian, M.F. Mousavi, S.H. Kazemi, M. Shamsipur, Electrochemical investigation of self doped polyaniline nanofibers as a new electroactive material for high performance redox supercapacitors, Synthetic Metals 159 (2009) 1717. [83] R. Ganesan, S. Shanmugam, A. Gedanken, Pulsed sonoelectrochemical synthesis of polyaniline nanoparticles and their capacitance properties, Synthetic Metals 158 (2008) 848. [84] D.S. Dhawale, D.P. Dubal, V.S. Jamadade, R.R. Salunkhe, C.D. Lokhande, Fuzzy nanofibrous network of polyaniline electrode for supercapacitor applications, Synthetic Metals 160 (2010) 519. [85] K. Wang, J. Huang, Z. Wei, Conducting polyaniline nanowire arrays for high performance supercapacitors, Journal of Physical Chemistry C 114 (2010) 8062. [86] Q. Cheng, J. Tang, J. Ma, H. Zhang, N. Shinya, L. Qin, Polyaniline-coated electroetched carbon fiber cloth electrodes for supercapacitors, Journal of Physical Chemistry C 115 (2011) 23584. [87] G. Li, Z. Feng, J. Zhong, Z. Wang, Y. Tong, Electrochemical synthesis of polyaniline nanobelts with predominant electrochemical performances, Macromolecules 43 (2010) 2178. [88] J. Tu, J. Hou, W. Wang, S. Jiao, H. Zhu, Preparation of porous nanorod polyaniline film and its high electrochemical capacitance performance, Synthetic Metals 161 (2011) 1255. [89] Y. Wang, C. Yang, P. Liu, Acid blue AS doped polypyrrole(PPY/AS) nanomaterials with different morphologies as electrode materials for supercapacitors, Chemical Engineering Journal 172 (2011) 1137. [90] C. Yang, P. Liu, Y. Zhao, Preparation and characterization of coaxial halloysite/polypyrrole tubular nanocomposited for electrochemical energy storage, Electrochimica Acta 55 (2010) 6857. [91] D.P. Dubal, S.V. Patil, A.D. Jagadale, C.D. Lokhande, Two step novel chemical synthesis of polypyrrole nanoplates for supercapacitor application, J. Alloy Compd. 509 (2011) 8183.

R. Ramya et al. / Electrochimica Acta 101 (2013) 109–129 [92] Q. Wu, K. He, H. Mi, X. Zhang, Electrochemical capacitance of polypyrrole nanowire prepared by using cetyltrimethylammonium bromide (CTAB) as soft template, Materials Chemistry and Physics 101 (2007) 367. [93] H. Olsson, G. Nystrom, M. Stromme, M. Sjodin, L. Nyholm, Cycling stablility and self-protective properties of a paper-based polypyrrole energy storage device, Electrochemistry Communications 13 (2011) 869. [94] J. Kim, Y. Lee, A.K. Sharma, C.G. Liu, Polypyrrole/carbon composite electrode for high-power electrochemical capacitors, Electrochimica Acta 52 (2006) 1727. [95] H. An, Y. Wang, X. Wang, L. Zheng, X. Wang, L. Yi, L. Bai, X. Zhang, Polypyrrole/carbon aerogel composite material for supercapacitor, Journal of Power Sources 195 (2010) 6964. [96] J. Zhang, L. Kong, J. Cai, Y. Luo, L. Kang, Nano-composite of polypyrrole/modified mesoporous carbon for electrochemical capacitor application, Electrochimica Acta 55 (2010) 8067. [97] Y. Han, X. Qing, S. Ye, Y. Lu, Conducting polypyrrole with nanoscale hierarchical structure, Synthetic Metals 160 (2010) 1159. [98] S. Palaniappan, S.B. Sydulu, P. Srinivas, Synthesis of copolymer of aniline and pyrrole by inverted emulsion polymerization method for supercapacitor, Journal of Applied Polymer Science 115 (2010) 1695. [99] L. Fan, J. Maier, High performance polypyrrole electrode materials for redox supercapacitors, Electrochemistry Communications 8 (2006) 937. [100] A.M.P. Hussain, A. Kumar, Enhanced electrochemical stability of all-polymer redox supercapacitors with modified polypyrrole electrode, Journal of Power Sources 161 (2006) 1486. [101] M. Beidaghi, C. Wang, Micro-supercapacitors based on three dimensional interdigital polypyrrole/C-MEMS electrode, Electrochimica Acta 56 (2011) 9508. [102] M. SelvaKumar, D. Krishna Bhat, LiClO4 -doped plasticized chitosan as biodegradable polymer gel electrolyte for supercapacitors, Journal of Applied Polymer Science 114 (2009) 2445. [103] B.C. Kim, J.M. Ko, G.G. Wallace, A Novel capacitor material based on Nafiondoped polypyrrole, Journal of Power Sources 177 (2008) 665. [104] W. Sun, X. Chen, Preparation and characterization of polypyrrole films for three-dimensional microsupercapacitor, Journal of Power Sources 193 (2009) 924. [105] S.K. Tripathi, A. Kumar, S.A. Hashmi, Electrochemical redox supercapacitors using PVdF-HFP based gel electrolytes and polypyrrole as conducting polymer electrode, Solid State Ionics 177 (2006) 2979. [106] S.A. Hashmi, H.M. Upadhyaya, Polypyrrole and poly(3-methyl thiophene)based solid state redox supercapacitors using ion conducting polymer electrolyte, Solid State Ionics 152–153 (2002) 883. [107] B.C. Kim, C.O. Too, J.S. Kwon, J.M. Ko, G.G. Wallace, A flexible capacitor based on conducting polymer electrodes, Synthetic Metals 161 (2011) 1130. [108] S.Y. Liew, W. Thielemans, D.A. Walsh, Electrochemical capacitance of nanocomposite polypyrrole-cellulose films, Journal of Physical Chemistry C 114 (2010) 17926. [109] W. Sun, R. Zheng, X. Chen, Symmetric redox supercapacitor based on microfabrication with 3-dimensional polypyrrole electrodes, Journal of Power Sources 195 (2010) 7120. [110] D.P. Dubal, S.V. Patil, W.B. Kim, C.D. Lokhande, Supercapacitors based on electrochemically deposited polypyrrole nanobricks, Materials Letters 65 (2011) 2628. [111] M.D. Ingram, H. Staesche, K.S. Ryder, ‘Activated’ polypyrrole electrodes for high power supercapacitor applications, Solid State Ionics 169 (2004) 51. [112] L. Liu, Y. Zhao, Q. Zhou, H. Xu, C. Zhao, Z. Jiang, Nano-polypyrrole supercapacitor arrays prepared by layer-by-layer assembling method in anodic aluminium oxide templates, Journal of Solid State Electrochemistry 11 (2007) 32. [113] M.D. Ingram, H. Staesche, K.S. Ryder, ‘Ladder-doped’ polypyrrole; a possible electrode material for inclusion in electrochemical supercapacitor, Journal of Power Sources 129 (2004) 107. [114] A. Izadi-Najafabadi, D.T. Tan, J.D. Madden, Towards high power polypyrrole/carbon capacitors, Synthetic Metals 152 (2005) 129. [115] M. Selvakumar, D. Krishna Bhat, LiClO4 doped cellulose acetate as biodegradable polymer electrolyte for supercapacitor, Journal of Applied Polymer Science 110 (2008) 594. [116] J. Huang, K. Wang, Z. Wei, Conducting polymer nanowire arrays with enhanced electrochemical performance, Journal of Materials Chemistry 20 (2010) 1117. [117] J. Wang, Y. Lu, J. Wang, X. Du, Towards a high specific power and high stability polypyrrole supercapacitors, Synthetic Metals 161 (2011) 1141. [118] J. Wang, Y. Xu, F. Yan, J. Zhu, J. Wang, Template-free prepared micro/nanostructured polypyrrole with ultrafast charging/discharging rate and long cycle life, Journal of Power Sources 196 (2011) 2373. [119] A. Subramania, S.L. Devi, Polyaniline nanofibers by surfactant-assisted dilute polymerization for supercapacitor applications, Polymers for Advanced Technologies 19 (2008) 725. [120] K. Wang, W. Zou, B. Quan, A. Yu, H. Wu, P. Xiang, Z. Wei, An all solid-state flexible micro-supercapacitor on a chip, Adv. Energy Mater. 1 (2011) 1068. [121] Y.G. Wang, H.Q. Li, Y.Y. Xia, Ordered whiskerlike polyaniline grown on the surface of mesoporous carbon and its electrochemical capacitance performance, Advanced Materials 18 (2006) 2619. [122] I. Kovalenko, D.G. Bucknall, G. Yushin, Detonation nanodiamond and onion like-carbon-embeded polyaniline for supercapacitor, Advanced Functional Materials 20 (2010) 3979.

127

[123] M. Jin, Y. Liu, Y. Li, Y. Chang, D. Fu, H. Zhao, G. Han, Preparation of flexible polypyrrole/polypropylene composite fibrous film for electrochemical capacitors, Journal of Applied Polymer Science 122 (2011) 3415. [124] S. Radhakrishnan, C.R.K. Rao, M. Vijayan, Electrochemical synthesis and studies of polypyrroles doped by renewal dopant carbanol azophenylsulphonic acid derived from cashew nutshells, Journal of Applied Polymer Science 114 (2009) 3125. [125] F. Meng, Y. Ding, Sub-micrometer thick all-solid-state supercapacitors with high power and energy densities, Advanced Materials 23 (2011) 4098. [126] S. Radhakrishnan, C.R.K. Rao, M. Vijayan, Performance of conducting polyaniline-DBSA and polyaniline-DBSA/Fe3 O4 composites as electrode materials for aqueous redox supercapacitors, Journal of Applied Polymer Science 122 (2011) 1510. [127] L. Zheng, Y. Xu, D. Jin, Y. Xie, Polyaniline-intercalated molybdenum oxide nanocomposite: Simultaneous synthesis and their enhanced application for supercapacitor, Chem. Asian J. 6 (2011) 1505. [128] M.d. Moniruzzaman, C.K. Das, Preparation and characterization of insitu polymerized nanocomposites based on polyaniline in presence of MWCNTs, Macromolecular Symposium 298 (2010) 34. [129] L.G. Bulusheva, E.O. Fedorovskaya, A.V. Okotrub, E.A. Maximovskiy, D.V. Vyalikh, X. Chen, H. Song, Electronic state of polyaniline deposited on deposited carbon nanotube or ordered mesoporous carbon templates, Phys. Status Solidi 248 (2011) 2484. [130] V. Branzoi, F. Branzoi, L. Pilan, Electrochemical fabrication and capacitance of composite films of carbon nanotubes and polyaniline, Surface and Interface Analysis 42 (2010) 1266. [131] W. Xing, X. Yuan, S.P. Zhuo, C.C. Huang, Preparation of polyaniline-coated mesoporous carbon and its enhanced electrochemical properties, Polymers for Advanced Technologies 20 (2009) 1179. [132] V. Khomenko, E. Frackowiak, F. Beguin, Determination of the specific capacitance of conducting polymer/nanotube composite electrodes using different cell configurations, Electrochimica Acta 50 (2005) 2499. [133] H. Wang, Q. Hao, X. Yang, L. Lu, X. Wang, Graphene oxide doped polyaniline for supercapacitors, Electrochemistry Communications 11 (2009) 1158. [134] F.E. Amitha, A.L.M. Reddy, S. Ramaprabhu, A non-aqueous electrolyte-based asymmetric supercapacitor with polymer and metal oxide/multiwalled carbon nanotube electrodes, Journal of Nanoparticle Research 11 (2009) 725. [135] M. Satish, S. Mitani, T. Tomai, I. Honma, MnO2 assisted oxidative polymerization of aniline on graphene sheets: superior nanocomposite electrodes for electrochemical supercapacitors, Journal of Materials Chemistry 21 (2011) 16216. [136] J. Xu, K. Wang, S. Zu, B. Han, Z. Wei, Hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage, ACS Nano 9 (2010) 5019. [137] Z. Hu, Y. Xie, Y. Wang, L. Mo, Y. Yang, Z. Zhang, Polyaniline/SnO2 nanocomposite for supercapacitor application, Materials Chemistry and Physics 114 (2009) 990. [138] B. Gao, Q. Fu, L. Su, C. Yuan, X. Zhang, Preparation and electrochemical properties of polyaniline doped with benzenesulphonic functionalized multiwalled carbon nanotubes, Electrochimica Acta 55 (2010) 2311. [139] X. Lu, H. Dou, S. Yang, L. Hao, L. Zhang, L. Shen, F. Zhang, X. Zhang, Fabrication and electrochemical capacitance of graphene/polyaniline/carbon nanotube ternary composite film, Electrochimica Acta 56 (2011) 9224. [140] Y. Zhou, Z. Qin, L. Li, Y. Zhang, Y. Wei, L. Wang, M. Zhu, Polyaniline/multiwalled carbon nanotube composites with core-shell structures as supercapacitor electrode materials, Electrochimica Acta 55 (2010) 3904. [141] M. Yang, B. Cheng, H. Song, X. Chen, Preparation and electrochemical performance of polyaniline-based carbon nanotubes as electrode material for supercapacitor, Electrochimica Acta 55 (2010) 7021. [142] Z. Zhu, G. Wang, M. Sun, X. Li, C. Li, Fabrication and electrochemical characterization of polyaniline nanorods modified with sulfonated carbon nanotubes for supercapacitor applications, Electrochimica Acta 56 (2011) 1366. [143] H. Mi, X. Zhang, S. An, X. Ye, S. Yang, Microwave-assisted synthesis and electrochemical capacitance of polyaniline/multi-walled carbon nanotube composites, Electrochemistry Communications 9 (2007) 2859. [144] C. Bian, A. Yu, H. Wu, Fibriform polyaniline/nano-TiO2 composite as an electrode material for aqueous redox supercapacitors, Electrochemistry Communications 11 (2009) 266. [145] C. Meng, C. Liu, S. Fan, Flexible carbon nanotube/polyaniline paper-like films and their enhanced electrochemical properties, Electrochemistry Communications 11 (2009) 186. [146] V.H. Nguyen, J. Shim, Facile synthesis and characterization of carbon nanotubes/silver nanohybrids coated with polyaniline, Synthetic Metals 161 (2011) 2078. [147] A. Ubul, R. Jamal, A. Rahman, T. Awut, I. Nurulla, T. Abdiryim, Solid-state synthesis and characterization of polyaniline/multi-walled carbon nanotubes composite, Synthetic Metals 161 (2011) 2097. [148] J. Yan, T. Wei, Z. Fan, W. Qian, M. Zhang, X. Shen, F. Wei, Preparation of graphene nanosheet/carbon nanotube/polyaniline composite as electrode material for supercapacitors, Journal of Power Sources 195 (2010) 3041. [149] J. Li, H. Xie, Y. Li, J. Liu, Z. Li, Electrochemical properties of graphene nanosheets/polyaniline nanofibers composites as electrode for supercapacitors, Journal of Power Sources 196 (2011) 10775. [150] Q. Li, M.H. Nayfeh, S. Yau, Brushed-on flexible supercapacitor sheets using a nanocomposite of polyaniline and carbon nanotubes, Journal of Power Sources 195 (2010) 7480.

128


[151] E. Frackowaik, V. Khomenko, K. Jurewicz, K. Lota, F. Beguin, Supercapacitor based on conducting polymers/nanotubes composites, Journal of Power Sources 153 (2006) 413. [152] S.R. Sivakkumar, W. Kim, J. Choi, D.R. Macfarlane, M. Forsyth, D. Kim, Electrochemical performance of polyaniline nanofibers and polyaniline/multiwalled carbon nanotube composite as an electrode material for aqueous redox supercapacitors, Journal of Power Sources 171 (2007) 1062. [153] Q. Li, J. Liu, J. Zou, A. Chunder, Y. Chen, L. Zhai, Synthesis and electrochemical performance of multi-walled carbon nanotube/polyaniline/MnO2 ternary coaxial nanostructures for supercapacitors, Journal of Power Sources 196 (2011) 565. [154] L. Li, Z. Qin, X. Liang, Q. Fan, Y. Lu, W. Wu, M. Zhu, Facile fabrication of uniform core-shell structured carbon nanotube-polyaniline nanocomposites, Journal of Physical Chemistry C 113 (2009) 5502. [155] Q. Wu, Y. Xu, Z. Yao, A. Liu, G. Shi, Supercapacitor based on flexible graphene/polyaniline nanofiber composite films, ACS Nano 4 (2010) 1963. [156] J. Xu, K. Wang, S. Zu, B. Han, Z. Wei, Hierarchical nanocomposite of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage, ACS Nano 4 (2010) 5019. [157] Z. Lei, Z. Chen, X.S. Zhao, Growth of polyaniline on carbon nanospheres for enhancing electrocapacitance, Journal of Physical Chemistry C 114 (2010) 19867. [158] C. Meng, C. Liu, L. Chen, C. Hu, S. Fan, Highly flexible and all solid-state paperlike polymer supercapacitors, Nano Letters 10 (2010) 4025. [159] A.K. Mishra, S. Ramaprahu, Functionalized graphene-based nanocomposite for supercapacitor applications, Journal of Physical Chemistry C 115 (2011) 14006. [160] A.V. Murugan, T. Muraliganth, A. Manthiram, Rapid, facile microwavesolvothermal synthesis of graphene nanosheets and their polyaniline nanocomposites for energy storage, Chemistry of Materials, Chemistry of Materials 21 (2009) 5004. [161] X. Yan, Z. Tai, J. Chen, Q. Xue, Fabrication of carbon nanofiber-polyaniline composite flexible paper for supercapacitor, Nanoscale 3 (2011) 212. [162] L. Kong, J. Zhang, J. An, Y. Lo, L. Kang, MWCNTs/PANI composite materials prepared by in-situ chemical oxidative polymerization for supercapacitor electrode, J. Mater. Sci. 43 (2008) 3664. [163] X. Deyu, J. Qi, F. Guanggang, D. Yi, K. Xiaoming, C. Wei, Z. Yang, Preparation of cotton shaped CNT/PANI composite and its electrochemical performances, Rare Metals 30 (2011) 94. [164] S. Sopcic, M.K. Rokovic, Z. Mandic, G. Inzelt, Preparation and characterization of RuO2 /Polyaniline composite electrodes, Journal of Solid State Electrochemistry 14 (2010) 2021. [165] K. Zhang, L.L. Zhang, X.S. Zhao, J. Wu, Graphene/Polyaniline nanofiber composites as supercapacitors electrodes, Chemistry of Materials 22 (2010) 1392. [166] H. Gomez, M.K. Ram, F. Alvi, P. Villalba, E. Stephanakos, A. Kumar, Grapheneconducting polymer nanocomposite as novel electrode for supercapacitors, Journal of Power Sources 196 (2011) 4102. [167] R.Y. Song, J.H. Park, S.R. Sivakkumar, S.H. Kim, J.M. Ko, D. Park, S.M. Jo, D.Y. Kim, Supercapacitive properties of polyaniline/nafion/hydrous RuO2 composite electrodes, Journal of Power Sources 166 (2007) 297. [168] W. Ni, D. Wang, Z. Huang, J. Zhao, G. Cui, Fabrication of nanocomposite electrode with MnO2 nanoparticles distributed in polyaniline for electrochemical capacitors, Materials Chemistry and Physics 124 (2010) 1151. [169] Z. Zhou, N. Cai, Y. Zhou, Capacitive of characterstics of manganese oxide and polyaniline composite thin film deposited on porous carbon, Materials Chemistry and Physics 94 (2005) 371. [170] L. Chen, L. Sun, F. Luan, Y. Liang, X. Liu, Synthesis and pseudocapacitive studies of composite films of polyaniline and manganese oxide nanoparticles, Journal of Power Sources 195 (2010) 3742. [171] H. Zhang, G. Cao, W. Wang, K. Yuan, B. Xu, W. Zhang, J. Cheng, Y. Yang, Influence of microstructure on the capacitive performance of polyaniline/carbon nanotube array composite electrodes, Electrochimica Acta 54 (2009) 1153. [172] V. Gupta, N. Miura, Polyaniline/single walled carbon nanotube (PANI/SWCNT) composites for high performance supercapacitors, Electrochimica Acta 52 (2006) 1721. [173] H. Zhang, G. Cao, Z. Wang, Y. Yang, Z. Shi, Z. Gu, Tube-covering-tube nanostructured polyaniline/carbon nanotube array composite electrode with high capacitance and superior rate performance as well as good cycling ability, Electrochemistry Communications 10 (2008) 1056. [174] J. Zhang, L. Kong, B. Wang, Y. Luo, L. Kang, In-situ electrochemical polymerization of multi-walled carbon nanotube/polyaniline composite films for electrochemical supercapacitors, Synthetic Metals 159 (2009) 260. [175] V. Gupta, N. Miura, Influence of the microstructure on the supercapacitive behavior of polyaniline/single walled carbon nanotube composites, Journal of Power Sources 157 (2006) 616. [176] D. Wang, F. Li, J. Zhao, W. Ren, Z. Chen, J. Tan, Z. Wu, I. Gentle, G. Lu, H. Cheng, Fabrication of graphene/polyaniline composite paper via in-situ anodic electropolymerization for high performance flexible electrodes, ACS Nano 3 (2009) 1745. [177] J. Liu, J. Sun, L. Gao, A promising way to enhance the electrochemical behavior of flexible single-walled carbon nanotube/polyaniline composite films, Journal of Physical Chemistry C 114 (2010) 19614. [178] Y. Cao, T.E. Mallouk, Morphology of template-grown polyaniline nanowires and its effect on the electrochemical capacitance of nanowire arrays, Chemistry of Materials 20 (2008) 5260.

[179] S.H. Mujawar, S.B. Ambade, T. Battumur, R.B. Ambade, S. Lee, Electropolymerization of polyaniline on titanium oxide nanotubes for supercapacitor application, Electrochimica Acta 56 (2011) 4462. [180] X.M. Feng, R.M. Li, Y.W. Ma, R.F. Chen, N.E. Shi, Q.L. Fan, W. Huang, One step electrochemical synthesis of graphene/polyaniline composite films and its applications, Advanced Functional Materials 21 (2011) 2989. [181] D. Zhang, X. Zhang, Y. Chen, P. Yu, C. Wang, Y. Ma, Enhanced capacitance and rate capability of graphene/polypyrrole composite as electrode material for supercapacitor, Journal of Power Sources 196 (2011) 5990. [182] H. Lee, H. Kim, M.S. Cho, J. Choi, Y. Lee, Fabrication of polypyrrole(PPY)/carbon nanotube (CNT) composite electrode on ceramic fabric for supercapacitor applications, Electrochimica Acta 56 (2011) 7460. [183] J.H. Kim, Y.S. Lee, A.K. Sharma, C.G. Liu, Polypyrrole/carbon composite electrode for high-power electrochemical capacitors, Electrochimica Acta 52 (2006) 1727. [184] M. Mallouki, F.T. Van, C. Sarrazin, C. Chevrot, J.F. Fauvarque, Electrochemical storage of polypyrrole-Fe2 O3 nanocomposites in ionic liquid, Electrochimica Acta 54 (2009) 2992. [185] M. Jin, G. Han, Y. Chang, H. Zhao, H. Zhang, Flexible electrodes based on polypyrrole/manganese dioxide/polypropylene fibrous membrane composite for supercapacitor, Electrochimica Acta 56 (2011) 9838. [186] Q. Xiao, X. Zhou, The study of multiwalled carbon nanotube deposited with conducting polymer for supercapacitors, Electrochimica Acta 48 (2003) 575. [187] A.M. White, R.C.T. Slade, Polymer electrodes doped with heteropolymetallates and their use within solid- state capacitors, Synthetic Metals 139 (2003) 123. [188] J.H. Kim, A.K. Sharma, Y.S. Lee, Synthesis of polypyrrole and carbon nano-fiber composite for the electrode of electrochemical capacitors, Mat. Lett. 60 (2006) 1697. [189] H. Mi, X. Zhang, Y. Xu, F. Xiao, Synthesis, characterization and electrochemical behavior of polypyrrole/carbon nanotube composites using organometallicfunctionalized carbon nanotubes, Applied Surface Science 256 (2010) 2284. [190] K.H. An, K.K. Jeon, J.K. Heo, S.C. Lim, D.J. Bae, Y.H. Lee, High-capacitance supercapacitor using a nanocomposite electrode of single-walled carbon nanotube and polypyrrole, Journal of the Electrochemical Society 149 (2002) A1058. [191] M. Choi, B. Lim, J. Jang, Synthesis of mesostructured conducting polymercarbon nanocomposites and their electrochemical performance, Macromol. Res. 16 (2008) 200. [192] M. Mallouki, F.T. Van, C. Sarrazin, P. Simon, B. Daffos, A. De, C. Chevrot, J. Fauvarque, Polypyrrole-Fe2 O3 nanohybrid material for electrochemical storage, Journal of Solid State Electrochemistry 11 (2007) 398. [193] S. Konwer, R. Boruah, S.K. Dolui, Studies on conducting polypyrrole/graphene oxide composite as supercapacitor electrodes, J. Electronic Mat. 40 (2011) 2248. [194] S. Biswas, L.T. Drzal, Multilayered nanoarchitechture of graphene nanosheets and polypyrrole nanowires for high performance supercapacitor electrodes, Chemistry of Materials 22 (2010) 5667. [195] L.L. Zhang, S. Zhao, X.N. Tian, X.S. Zhao, Layered graphene oxide nanostructures with sandwiched conducting polymers as supercapacitor electrodes, Langmuir 26 (2010) 17624. [196] J. Zhang, L.B. Kong, J.J. Cai, Y.C. Luo, L. Kang, Nano-composite of polypyrrole/modified mesoporous carbon for electrochemical capacitor applications, Electrochimica Acta 55 (2010) 8067. [197] X. Wang, C. Yang, T. Wang, P. Liu, Praseodymium oxide/polypyrrole nanocomposites for electrochemical energy storage, Electrochimica Acta 58 (2011) 193. [198] C. Yang, P. Liu, Polypyrrole/conductive mica composites: preparation, characterization and application in supercapacitor, Synthetic Metals, Synthetic Metals 160 (2010) 768. [199] Y. Han, L. Hao, X. Zhang, Preparation and electrochemical performance of graphite oxide/polypyrrole composites, Synthetic Metals 160 (2010) 2336. [200] S. Sahoo, G. Karthikeyan, G.C. Nayak, C.K. Das, Electrochemical characterization of in-situ polypyrrole coated graphene nanocomposites, Synthetic Metals 161 (2011) 1713. [201] Y.Q. Han, B. Ding, X.G. Zhang, Effect of feeding ratios on the structure and electrochemical performance of graphite oxide/polpyrrole nanocomposites, Chinese Science Bulletin 56 (2011) 2846. [202] B. Zhang, Y. Xu, Y. Zheng, L. Dai, M. Zhang, J. Yang, Y. Chen, X. Chen, J. Zhou, A facile synthesis of polypyrrole/carbon nanotube composite with ultrathin, uniform and thickness tunable polypyrrole shells, Nanoscale Res. Lett. 6 (2011) 431. [203] J. Zhang, X. Li, In-situ synthesis of ultrafine ␤-MnO2 /polypyrrole nanorod composites for high performance supercapacitors, Journal of Materials Chemistry 21 (2011) 10965. [204] R.K. Sharma, A. Karakoti, S. Seal, L. Zhai, Multiwalled carbon nanotubepoly(4-styrene sulphonic acid) supported polypyrrole/manganese oxide nano-composite for high performance electrochemical electrodes, Journal of Power Sources 195 (2010) 1256. [205] S.R. Sivakkumar, J.M. Ko, D.Y. Kim, B.C. Kim, G.G. Wallace, Performance evaluation of CNT/polypyrrole/MnO2 composite electrodes for electrochemical supercapacitor, Electrochimica Acta 52 (2007) 7377. [206] Z.H. Dong, Y.L. Wei, W. Shi, G.A. Zhang, Characterization of doped polypyrrole/manganese oxide nanocomposite for supercapacitor electrodes, Mat. Chem. Phys. 131 (2011) 529. [207] C. Zhou, S. Kumar, C.D. Doyle, J.M. Tour, Funtionalized single walled carbon nanotubes treated with pyrrole for electrochemical supercapacitor membranes, Chemistry of Materials 17 (2005) 1997.

R. Ramya et al. / Electrochimica Acta 101 (2013) 109–129 [208] J.H. Park, J.M. Ko, O.O. Park, D.W. Kim, Capacitance properties of graphite/polypyrrole composite electrode prepared by chemical polymerization of pyrrole on graphite fiber, Journal of Power Sources 105 (2002) 20. [209] J. Oh, M.E. Koslov, B.G. Kim, H.K. Kim, R.H. Baughman, Y.H. Hwang, Preparation and electrochemical characterization of porous SWCNT-PPy nanocomposite sheet for supercapacitor application, Synthetic Metals 158 (2008) 638. [210] G.M. Suppes, C.G. Cameron, M.S. Freund, A polypyrrole/phosphomolybdic acid /poly (3 4-ethylenedioxythiophene)/phosphotungstic acid asymmetric supercapacitor, Journal of the Electrochemical Society 157 (2010) A1030. [211] X. Zhang, W. Yang, Y. Ma, Synthesis of polypyrrole-intercalated layered manganese oxide nanocomposite by a delamination/reassembling method and its electrochemical capacitance performance, Electrochemical and Solid-State Letters 12 (2009) A95. [213] A.M.P. Hussain, D. Saikia, F. Singh, D.K. Avasthi, A. Kumar, Effects of 160 MeV Ni12+ ion irradiation on polypyrrole conducting polymer electrode materials for all polymer redox supercapacitor, Nucl. Instr. Meth. Phys. Res. B 240 (2005) 834. [214] G.A. Snook, G.Z. Chen, D.J. Frey, M. Hughes, M. Shaffer, Studies of deposition of and charge storage in polypyrrole-chloride and polypyrrole-carbon nanotube composites with an electrochemical quartz crystal microbalance, Journal of Electroanalytical Chemistry 568 (2004) 135. [215] A. Faye, G. Dione, M.M. Dieng, J.J. Aaron, H. Cachet, C. Cachet, Usefulness of a composite electrode with a carbon surface modified by electrosynthesized polypyrrole for supercapacitor applications, Journal of Applied Electrochemistry 40 (2010) 1925. [216] Y. Xu, S.Q. Zhuang, X.Y. Zhang, P.G. He, Y.Z. Fang, Configuration and capacitance properties of polypyrrole/aligned carbon nanotube synthesized by electropolymerization, Chinese Science Bulletin 56 (2011) 3823. [217] V. Branzoi, L. Pilan, F. Branzoi, Nanocomposite films obtained by electrochemical codeposition of conducting polymers and carbon nanotubes, Electroanalysis 21 (2009) 557. [218] P.A. Mini, A. Balakrishnan, S.V. Nair, K.R.V. Subramanian, High supercapacitive electrodes made of graphene/polypyrrole, Chemical Communications 47 (2011) 5753. [219] B. Muthulakshmi, D. Kalpana, S. Pitchumani, N.G. Renganathan, Electrochemical deposition of polypyrrole for symmetric supercapacitors, Journal of Power Sources 158 (2006) 1533. [220] R.K. Sharma, A.C. Rastogi, S.B. Desu, Manganese oxide embedded polypyrrole nanocomposites for electrochemical supercapacitors, Electrochimica Acta 53 (2008) 7690. [221] A. Izadi-Najafabadi, D.T.H. Tan, J.D. Madden, Towards high power olypyrrole/carbon capacitors, Synthetic Metals 152 (2005) 129. [222] J. Wang, Y. Xu, X. Chen, X. Sun, Capacitance properties of single walled carbon nanotube/polypyrrole composite films, Comp. Sci. Technol. 67 (2007) 2981. [223] S.A. Hashmi, A. Kumar, S.K. Tripathi, Investigations on electrochemical supercapacitors using polypyrrole redox electrodes and PMMA based gel electrolytes, J. Eur. Polym. 41 (2005) 1373. [224] J. Zhang, L.B. Kong, H. Li, Y.C. Luo, L. Kang, Synthesis of polypyrrole film by galvanostatic method and its application as supercapacitor electrode materials, J. Mater. Sci. 45 (2010) 1947. [225] C. Shi, I. Zhitomirsky, Electrodeposition and capacitive behavior of films for electrodes of electrochemical supercapacitors, Nanoscale Res. Lett. 5 (2010) 518. [226] S.A. Hashmi, H.M. Upadhyaya, MnO2 -polypyrrole conducting polymer composite electrodes for electrochemical redox supercapacitors, Ionics 8 (2002) 272. [227] J. Zhang, S.J. Bao, C.M. Li, H. Bian, X. Cui, Q. Bao, C.Q. Sun, J. Guo, K. Lian, Well aligned cone shaped nanostructure of polypyrrole/RuO2 and its electrochemical supercapacitor, Journal of Physical Chemistry C 112 (2008) 14843. [228] A. Davies, P. Audette, B. Farrow, F. Hassan, Z. Chen, J.Y. Choi, A. Yu, Graphenebased flexile supercapacitors: pulse-electropolymerization of polypyrrole on free-standing graphene films, Journal of Physical Chemistry C 115 (2011) 17612. [229] A. Liu, C. Li, H. Bai, G. Shi, Electrochemical deposition of polpyrrole/sulfonated graphene films, Journal of Physical Chemistry C 114 (2010) 22783. [230] P. Si, S. Ding, S.W. David Lou, D.H. Kim, An electrochemically formed three-dimensional structure of polypyrrole/graphene nanoplatelets for high performance supercapacitors, RSC Adv. 1 (2011) 1271. [231] X. Lin, Y. Xu, Facile synthesis and electrochemical capacitance of composites of polypyrrole/multi-walled carbon nanotubes, Electrochimica Acta 53 (2008) 4990. [232] H. Lee, M.S. Cho, I.H. Kim, J.D. Nam, Y. Lee, RuOX /polpyrrole nanocomposite electrode for electrochemical capacitors, Synthetic Metals 160 (2010) 1055. [233] X. Lang, Q. Wan, C. Feng, X. Yue, W. Xu, J. Li, S. Fan, The role of anthraquinone sulfonate dopants in promoting performance of polypyrrole composites as pseudo-capacitive electrode materials, Synthetic Metals 160 (2010) 1800. [234] Z.T. Zhang, W.H. Song, Electrochemical preparation and electrochemical behavior of polypyrrole/carbon nanotube composite films, Front. Mater. Sci. China 3 (2009) 194.

129

[235] Y. Fang, J. Liu, D.J. Yu, J.P. Wicksted, K. Kalkan, C.O. Topal, B.N. Flanders, J. Wu, J. Li, Self-supported supercapcitor membranes: Polypyrrole-coated carbon nanotube networks enabled by pulsed electrodeposition, Journal of Power Sources 195 (2010) 674. [236] H. Mi, X. Zhang, X. Ye, S. Yang, Preparation and enhanced capacitance of core-shell polypyrrole/polyaniline composite electrode for supercapacitors, Journal of Power Sources 176 (2008) 403. [237] A.Q. Zhang, L.Z. Wang, L.S. Zhang, Y. Zhang, Preparation and electrochemical capacitance of poly(pyrrole-co-aniline), Journal of Applied Polymer Science 115 (2010) 1881. [238] A.Q. Zhang, Y. Zhang, L.Z. Wang, X.F. Li, Electrosynthesis and capacitive performance of polyaniline-polypyrrole composite, Polym. Comp. 32 (2011) 1. [239] D. Belanger, X. Ren, J. Davey, F. Uribe, S. Gottesdeld, Characterization and longterm performance of polyaniline-based electrochemical capacitors, Journal of the Electrochemical Society 147 (2000) 2923. [240] J.E.B. Randles, Kinetics of rapid electrode reactions, Discussions of the Faraday Society 1 (1947) 11. [241] R. Ramya, M.V. Sangaranarayanan, Analysis of polypyrrole-coated stainless steel electrodes – Estimation of specific capacitance and construction of equivalent circuits, J. Chem. Sci. 120 (2008) 25. [242] X. Liu, P.G. Pickup, Ruthenium oxide supercapacitors with high loadings and high power and energy densities, Journal of Power Sources 176 (2008) 410. [243] W. Schmickler, in: A.F. Silvia (Ed.), Trends in Interfacial Electrochemistry, Reidel, Dordrecht, 1986, p. 453. [244] J.P. Badiali, Contribution of the metal to the differential capacitance of the ideally polarizabe electrodes, Electrochimica Acta 31 (1986) 149. [245] A.A. Kornyshev, M.B. Partenskii, W. Schmickler, Self-consistent density functional approach to a metal/electrolyte solution interface, Zeitschrift Fur Naturforschung Section A 39 (1984) 1122. [246] E. Ghali, in: R. Winston Revie (Ed.), Wiley Series in Corrosion, Corrosion Resistance of Al and Mg Alloys, Understanding Performance and Testing, John Wiley and Sons Inc., Hoboken, New Jersey, 2010, p. 584. [247] G.G. Belmonte, Impedence analysis of galvanostatically synthesized polypyrrole films. Correlation of ionic diffusion and capacitance parameters with the electrode morpholog, Electrochimica Acta 47 (2002) 4263. [248] S.K. Mondal, K.V. Prasad, N. Munichandraiah, Analysis of electrochemical impedence of polyaniline films prepared by galvanostatic, potentiostatic and potentiodynamic methods, Synthetic Metals 148 (2005) 278. [249] A. Kumarappa, I. Zhitomirsky, Electropolymerization of polypyrrole films on stainless steel substrates for electrodes of electrochemical supercapacitors, Synthetic Metals 162 (2012) 868. [250] A.S. Sarac, S. Sezgim, M. Ates, C.M. Turhan, Electrochemical impedence spectroscopy and morphological analyses of pyrrole, phenylpyrrole and methoxyphenyl pyrrole on carbon fiber microelectrodes, Surface and Coatings Technology 202 (2008) 3997. [251] W. Zou, W. Wang, B. He, M. Sun, M. Wang, L. Liu, X. Xu, Effects on counter ions on pseuodocapacitance performance of polyaniline in sulphuric acid and ptoluene sulphonic acid electrolyte, Journal of Electroanalytical Chemistry 641 (2010) 111. [252] M.L. Tsai, Preparation and characterization of Ppy/Al2 O3 /Al used as solid-state capacitors for microsystem-effect of amount of electricity passed, Journal of Power Sources 173 (2007) 613. [253] P.R. Deshmukh, S.N. Pusawale, V.S. Jamadade, U.M. Patil, C.D. Lokhande, Microwave assisted chemical bath deposited polyaniline films for supercapacitor applications, J. Alloy Compd. 509 (2011) 5064. [254] D.P. Dubal, S.H. Lee, J.G. Kim, W.B. Kim, C.D. Lokhande, Porous polypyrrole clusters prepared by electropolymerization for a high performance supercapacitor, Journal of Materials Chemistry 22 (2012) 3044. [255] T.C. Girirja, M.V. Sangaranarayanan, Underpotential deposition of Tl on Ag in the presence of bromide ions-estimation of specific capacitance for design of electrochemical supercapacitors, Journal of Applied Electrochemistry 36 (2006) 531. [256] R. Saradha, M.V. Sangaranarayanan, Theory of electrified interfaces: A combined analysis using the density functional approach and Bethe approximation, Journal of Physical Chemistry B 102 (1998) 5468. [257] B. Skinner, T., Chen, M.S., Loth, B.I. Shklovskii. Theory of volumetric capacitance of an electric double layer supercapacitor, ar Xi v: 1101.1064v3, [cond-mat.mtrl-sci] 15 May 2011. [258] D. Riu, N. Retiere, D. Linzen, Proceedings of the 39th IAS Annual Meeting on Industry Applications Conference, Conference Record of the 2004 IEEE, vol. 4, 2004, 2550. [259] S. Trasatti, Work function, electronegativity, and electrochemical behavior of metals: III Electrolytic hydrogen evolution in acid solutions, J. Electroanal. Chem. Interfacial Electrochem. 39 (1972) 163. [260] R. Holze, Co-polymers- A refined way to tailor intrinsically conducting polymers, Electrochimica Acta 56 (2011) 10479. [261] A. Abd-Elwahed, R. Holze, Ion size and size memory effects with electropolymerized polyaniline, Synthetic Metals 131 (2002) 61. [262] R.J. Foster, T.E. Keyes, in: C.G. Zoski (Ed.), Handbook of Electrochemistry, Chapter 6, Elsevier, 2007.