carbon nanotube supercapacitors: Technological advances and challenges

Journal of Power Sources 352 (2017) 174e186

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Review article

Polypyrrole/carbon nanotube supercapacitors: Technological advances and challenges Adeel Afzal a, b, *, Faraj A. Abuilaiwi a, b, **, Amir Habib c, Muhammad Awais d, Samaila B. Waje c, e, Muataz A. Atieh f, g a

Department of Chemistry, College of Science, University of Hafr Al Batin, PO Box 1803, Hafr Al Batin, 31991, Saudi Arabia Affiliated Colleges at Hafr Al Batin, King Fahd University of Petroleum and Minerals, PO Box 1803, Hafr Al Batin, 31991, Saudi Arabia Department of Physics, College of Science, University of Hafr Al Batin, PO Box 1803, Hafr Al Batin, 31991, Saudi Arabia d Department of Industrial Engineering, Taibah University, PO Box 344, Medina, Saudi Arabia e Higher Colleges of Technology, Abu Dhabi Women College, Box 41014, Abu Dhabi, United Arab Emirates f Qatar Environment and Energy Research Institute, Qatar Foundation, PO Box 5825, Doha, Qatar g College of Science and Engineering, Hamad Bin Khalifa University, Qatar Foundation, PO Box 5825, Doha, Qatar b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


A review of Polypyrrole/carbon nanotube (PPy/CNT) supercapacitors is presented.
Chemical and electrochemical fabrication of PPy/CNTcomposites is discussed.
Factors influencing the capacitive performance of supercapacitors are emphasized.
Effect of porosity, mass ratio, dopant, and electrolytes on properties is studied.
Current limitations and challenges of PPy/CNT supercapacitors are defined.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 November 2016 Received in revised form 21 March 2017 Accepted 26 March 2017

The supercapacitors are advanced electrochemical energy storage devices having characteristics such as high storage capacity, rapid delivery of charge, and long cycle life. Polypyrrole (PPy) e an electronically conducting polymer, and carbon nanotubes (CNT) with high surface area and exceptional electrical and mechanical properties are among the most frequently studied advanced electrode materials for supercapacitors. The asymmetric supercapacitors composed of PPy/CNT composite electrodes offer complementary benefits to improve the specific capacitance, energy density, and stability. This article presents an overview of the recent technological advances in PPy/CNT composite supercapacitors and their limitations. Various strategies for synthesis and fabrication of PPy/CNT composites are discussed along with the factors that influence their ultimate electrochemical performance. The drawbacks and challenges of modern PPy/CNT composite supercapacitors are also reviewed, and potential areas of concern are identified for future research and development. © 2017 Elsevier B.V. All rights reserved.

Keywords: Carbon nanotube Electrochemistry Energy storage Polypyrrole Supercapacitor

* Corresponding author. Department of Chemistry, College of Science, University of Hafr Al Batin, PO Box 1803, Hafr Al Batin, 31991, Saudi Arabia. ** Corresponding author. Affiliated Colleges at Hafr Al Batin, King Fahd University of Petroleum and Minerals, PO Box 1803, Hafr Al Batin, 31991, Saudi Arabia. E-mail addresses: [email protected] (A. Afzal), [email protected] (F.A. Abuilaiwi). http://dx.doi.org/10.1016/j.jpowsour.2017.03.128 0378-7753/© 2017 Elsevier B.V. All rights reserved.

1. Introduction 21st century is witnessing a global revolution in the pursuit for alternative energy resources such as wind, solar, hydro, geothermal,

A. Afzal et al. / Journal of Power Sources 352 (2017) 174e186

and biomass etc. [1,2]. While the ever-escalating demand for more efficient, greener, and renewable energy technologies has captivated scientists, the need to store this energy has also grown over the past few decades [3,4]. At present, batteries and capacitors are the most common devices for electrical energy storage [5,6]. While batteries have relatively higher energy storing capability, the conventional capacitors deliver greater power [7,8]. An ideal device for electrical energy storage should possess greater storage capacity, high power and energy densities, and excellent cyclability [9]. The electrochemical supercapacitors bridge the performance void between traditional batteries and capacitors in such a way that they can operate at higher specific power as compared to most batteries, and they can reversibly store more energy as compared to conventional capacitors [10,11]. Furthermore, supercapacitors offer rapid charging/discharging and long cycle life, possess high energy density, work in a wide temperature range, and are eco-friendly [12]. According to the charge storage mechanism, supercapacitors are classified into three major types [13]. (a) Electric double-layer capacitors (EDLC): Which involve non-Faradaic processes, i.e. the charge storage is achieved by the adsorption of ionic species and charged particles at the electrode/electrolyte interface [9,14]. EDLC are based on highly porous materials such as activated carbons with substantially high specific surface area that allows greater electrolyte-accessibility and thus, greater energy storage [14e16]. (b) Pseudocapacitors (PSC): Which involve highly reversible Faradaic processes, i.e. charge storage is achieved by superficial redox reactions between the electrochemically active materials and the electrolyte [17,18]. PSC are typically based on metal oxides/hydroxides and exhibit higher specific capacitance as compared to EDLC [17,19]. (c) Asymmetric supercapacitors (ASC) (or hybrid supercapacitors): Which consist of two or more different types of electrode materials with complementary advantages of enhancing the operation voltage and energy density [20,21]. ASC have been developed as high performing electrochemical energy storage systems to overcome the problems of low specific capacitance associated with activated carbons, and low conductivity and poor cyclability with metal oxides/hydroxides [22e24]. Along with the activated carbons and metal oxides/hydroxides, electronic conducting polymer (ECP) and carbon nanotubes (CNT) have been used as the electrode materials for electrochemical supercapacitors [19,22,23]. CNT exhibit high inherent electrical and thermal conductivity, flexibility, excellent chemical, thermal, and mechanical stability, good corrosion resistance, and large surface area polarizability [25e28]. Use of both single-walled (SWCNT) and multi-walled (MWCNT) nanotubes in EDLC result in limited charge storage capacity. Several studies demonstrate that depending on their synthesis and/or fabrication method, pure CNT have low specific capacitance in the range of 4e135 F g1 [29,30]. On the other hand, ECP are attractive materials for supercapacitor electrodes due to low cost, good electrical conductivity, and high pseudo capacitance [31e33]. In addition, ECP have very high theoretical capacitance such as 620 F g1 for polypyrrole [34]. Polypyrrole (PPy) is among the most extensively researched ECP due to its low cost, high capacitance, good environmental stability, excellent mechanical properties, and its aqueous solubility [35e38]. PPy has been deposited on pure and/or functionalized CNT to yield hybrid electrode materials with superior electrochemical performance through a combination of the EDLC and high conductivity of CNT and high pseudo capacitance of redox-active PPy [39,40]. A number of reports confirmed an increase in specific capacitance of PPy/CNT composite based ASC (~150e276 F g1) [25,41e44]. Lin et al. [45] even reported an exceptionally high value of 890 F g1 for PPy/MWCNT composite in 1.0 M KCl as electrolyte. Fang et al. [46] believe that microstructural uniformity, high

175

specific capacitance and conductivity of PPy/CNT composite ASC are derived from strong p-p stacking between the conjugated backbone of PPy and the graphitic sidewall of CNT. However, several other factors such as synthesis/fabrication procedure, porosity and ionic accessibility, nature of electrolyte, scan rate and current density etc. influence the performance of PPy/CNT supercapacitors. In the past few years, PPy/CNT composites emerged as a material of choice for ASC electrodes due to their flexibility, ease of fabrication, and good electrochemical properties. This study reviews recent technological advances ranging from strategies for synthesis and fabrication of PPy/CNT composites to electrochemical performance enhancement factors and challenges. A number of recent papers are reviewed and the data is compared to give readers an overview of the contemporary status of PPy/CNT supercapacitors. Furthermore, limitations of current PPy/CNT composite based ASC and future research directions are also discussed. 2. Strategies for synthesis of PPy/CNT composite supercapacitors PPy/CNT composites are predominantly prepared as PPy coating on CNT surface using different procedures, e.g. chemical oxidative polymerization [47,48], interfacial polymerization [36,49], radiolytic (g-radiation induced chemical) polymerization [50], enzymecatalyzed polymerization [51], electrochemical polymerization [52e54], and layer-by-layer assembly of CNT and PPy [55]. Broadly, we have categorized these methods into two groups based on (a) chemical, and (b) electrochemical processes involved in the polymerization of PPy on CNT surface, which are further discussed in the following sections. 2.1. Chemical methods Chemical methods for synthesis of PPy usually involve the use of oxidants such as ferric chloride, ferric perchlorate, and ammonium peroxydisulfate [56,57]. Through, chemical oxidative polymerization of pyrrole is a straightforward process, the ultimate properties of PPy strongly depend on synthetic conditions and constituents of the reaction mixture [35,58,59]. In a typical chemical oxidative polymerization process, CNT are dispersed into the reaction mixture along with the monomer (pyrrole), the oxidant (e.g. FeCl3), and various additives to form PPy/CNT composites. This procedure is simple and cost effective, and can be easily scaled up for commercial production of PPy/CNT composites. However, dispersion and solubility of CNT in the reaction mixture (i.e. generally an aqueous solution) can be challenging [60]. To address this issue, many researchers have tried to improve the wetting properties of CNT (e.g. in water) through their surface functionalization with hydrophilic groups [61,62]. Consequently, the oxygen-containing functional groups on CNT surface not only allow them to homogeneously disperse in reaction mixtures, but also enhance the interfacial affinity between PPy and intrinsically hydrophobic CNT [62]. CNT surface modification has been achieved by oxidation in concentrated acids or acid mixtures [63e65], which results in the formation of hydroxy-, carboxylic acid-, and other oxygencontaining groups on CNT surface. These surface functional groups electrostatically stabilize CNT in solution and may also act as the nucleation sites for polymer coating on CNT surface [66]. Geng et al. [67] reported that conductivity of acid-treated SWCNT increases by a factor of 4 as compared to pristine SWCNT. However, majority of other reports do not conform with this observation, and state that such functionalization strategies produce defects on CNT sidewalls thus reducing their electronic conductivity [68e70].

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Air plasma treatment is an alternative option for surface modification of CNT, which yields oxygen- as well as nitrogencontaining groups on CNT surface [71]. The electrical conductivity of CNT and graphene are believed to improve through substitutional doping of carbon with heteroelements such as nitrogen, boron, or sulfur [62,72], which means air plasma treated CNT may enhance the electrochemical performance of resulting PPy/CNT composites. Hermas et al. [73] also revealed superior thermal stability of PPy/CNT composites after air plasma activation of CNT. Furthermore, nitrogen-containing pyrrolic groups on plasma treated CNT surface may bind with PPy backbone via conjugated structure along with other non-covalent interactions between PPy and modified CNT [50,62]. Fig. 1 shows a schematic diagram of the air plasma activation and the formation of PPy on the surface of plasma activated CNT. Nonetheless, it is confirmed that concentrated acid and air plasma treatments of CNT remarkably enhance their interfacial compatibility with PPy and supercapacitive performance of the resulting PPy/CNT composite electrodes. Fu et al. [74] fabricated PPy/CNT composite supercapacitors via chemical oxidative polymerization of pyrrole in the presence of acid treated CNT, cetyltrimethylammonium bromide (as surfactant), and ammonium persulfate (as oxidant). They reported gravimetric capacitance of 233.5 F g1 in 1.0 M KCl at 0.5 A g1 current density. Yang et al. [62] chemically polymerized pyrrole on pristine (p-CNT) and air plasma activated (f-CNT) using ammonium peroxydisulfate as oxidant. They achieved reasonably high capacitance for PPy/f-CNT composite (264 F g1) as compared to PPy/p-CNT composite supercapacitor (210 F g1) in 1.0 M H2SO4. 2.2. Electrochemical methods Electrochemical deposition of PPy on CNT has been studied by several researchers due to better control over the morphology of thus formed PPy/CNT supercapacitors [46,75,76]. A typical electrochemical procedure involves potentiostatic or galvanostatic deposition of pyrrole from an electrolyte solution on the surface of CNT that are already assembled on a substrate such as stainless steel, ceramic or cotton fabric in an electrochemical cell [46,77,78]. The electrode potential, current density, and/or deposition time can be varied during the electrodeposition to control the amount of PPy deposited on CNT as well as the microstructure and porosity of resulting PPy/CNT composites.

Chen et al. [79] have developed the robust PPy/CNT composite supercapacitor electrodes by the electrochemical polymerization of pyrrole in a three-electrode configuration with CNT film as the working electrode. They studied the detailed microstructure of PPy/ CNT composite film with transmission electron microscopy (TEM) and X-ray photoelectron microscopy (XPS). Fig. 2 shows the TEM and HRTEM images of PPy/CNT composites along with the XPS survey spectra of pure CNT and PPy/CNT composite. The core-shell structure of electrochemically prepared PPy/CNT composite film is obvious from the HRTEM image (shown in Fig. 2b). In this procedure, a thin layer of amorphous PPy is deposited on the outer surface of crystalline CNT. Recently, it has been suggested that soformed PPy film offers the fast diffusion and migration of ions, thereby enhancing the PPy/CNT supercapacitor's performance [80e82]. The surface elemental analysis of the pure CNT film and CNT/PPy film (shown in Fig. 2c) also confirms the effective deposition of PPy on CNT as indicated by the clearly differentiating N1s signal of N element attributed to the presence of PPy. Hence, electrochemical method is an efficient way to produce core-shell PPy/CNT composite electrodes for supercapacitors. In recent past, pulse electrodeposition of PPy has been studied by several researchers [83e85], and reports suggest that PPy/CNT composites prepared via pulse impregnation technique show higher capacitive performance than continuous galvanostatic or potentiostatic electrodeposition [46,86]. Fang et al. [46] prepared PPY/MWCNT membrane from aqueous solution containing 50 mM pyrrole and 1 M KCl using pulse electrodeposition with varying on (td) and off (tr) times. Fig. 3 shows total PPy deposition charge vs. total deposition time, and the specific capacitance vs. total PPy deposition charge. It is clear that capacitive performance improves substantially (~30 fold) through appropriate PPy deposition. In general, pulse electrodeposition allows more ordered packing of PPy on CNT, i.e. PPy impregnation with short current pulses allows the polymeric backbone to rearrange via stronger p-p interactions with CNT and form an ordered structure [46]. According to Li and Zhitomirsky [86], the higher specific capacitance of pulse electrodeposited PPy/CNT composite electrodes is attributed to the higher surface area and porosity of resulting PPy/CNT films, which allow greater electrolyte accessibility. They developed PPy/MWCNT composites from aqueous 6.7 g L1 pyrrole solution containing 0.1 g L1 MWCNT and 1 g L1 of an organic dye (as the anionic dopant) using continuous galvanostatic or pulse electrodeposition with td and tr of 0.5 s at a current

Fig. 1. A schematic diagram showing preparation of plasma-activated CNT and PPy/CNT composite. Adapted with permission from Ref. [62]. Copyright (2015) Elsevier.

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Fig. 2. (a) TEM image of the PPy/CNT composite film, (b) the corresponding HRTEM image showing the core-shell structure, and (c) XPS survey spectra of the PPy/CNT film and pure CNT film. Reproduced with permission from Ref. [79]. Copyright (2015) Elsevier.

density of 1 mA cm2 [86]. PPy/MWCNT supercapacitors prepared via pulse electrodeposition show high specific capacitance (~390 F g1) as compared to those prepared via galvanostatic deposition (~275 F g1). 3. Performance of PPy/CNT composite supercapacitors In this study, the capacitive performance of PPy/CNT composite supercapacitors is reviewed in terms of specific capacitance, energy density, and cycle life. Table 1 shows the capacitive performance of PPy/CNT composite electrodes under different experimental conditions. The data is obtained from recent selected publications, and compared in order to identify the elements influencing the performance of supercapacitors. It is important to note that a number of factors including fabrication technique, PPy/CNT ratio, surface morphology, and experimental conditions such as the electrolyte, scan rate and current density influence the ultimate capacitive properties of a PPy/CNT composite supercapacitor. These factors are separately reviewed and discussed in the following sections. 3.1. Porosity vs. capacitive properties The structural uniformity, high porosity and surface area are very important to achieve the desired capacitive properties because these structural characteristics can promote the electrolyte ions' accessibility and their rate of diffusion into the bulk of redox-active material [87e91]. Nyholm et al. [92] suggest that abundant pores are critical when PPy is used as the supercapacitor electrode, because they can expedite the uptake of electrolyte ions. Kim et al. [93] report that high gravimetric capacitance and excellent highrate competency of PPy/CNT supercapacitors can be achieved by controlling the pore size. Three-dimensional porous PPy/CNT composite film electrodes exhibit a specific capacitance of 250 F g1 at 10 mV s1 [93]. Highly porous redox-active materials are ideal for supercapacitor applications [88,91], because these pores not only allow the electrolyte to infiltrate but also enhance the transfer of electrons through interconnected network. Zhou et al. [94] recently reported synthesis of highly flexible, free-standing PPy/CNT films via electrodeposition of PPy on high surface area (764 m2 g1)

oxidized CNT. With an average pore diameter of ~13 nm, PPy/CNT solid-state supercapacitors exhibit high capacitance (305 F g1) and energy density (42 Wh kg1). Zhang et al. [11] fabricated PPy/CNT electrode materials with three-dimensional ordered macroporous (3-DOM) architecture to improve their capacitive properties. They used nanosized silica as a sacrificial filler to control pore size of PPy/CNT composites. Fig. 4 shows a schematic illustration of the fabrication process, and the SEM image of macroporous PPy/CNT composite. They achieved remarkably high specific capacitance of 427 F g1 for 3-DOM PPy/ CNT composite electrodes at 5 mV s1 scan rate. They conclude that 3-DOM PPy/CNT films show a greater flux of ions (per unit length) as compared to planar and nanoporous films due to large size of mesopores that leads to higher capacitance [11]. 3.2. PPy: CNT mass ratio vs. capacitive properties The effect of PPy/CNT mass ratio on the capacitive properties of PPy/CNT composite electrodes remains the focus of several studies. A detailed review of literature reveals contrasting facts about optimum PPy loading on CNT that is mainly dependent on synthetic methodology. PPy/CNT composite supercapacitors produced via in situ chemical oxidative polymerization of pyrrole on CNT networks exhibit the highest capacitance at very low (2e10 wt%) CNT content [74,95,96]. There are only a few reports suggesting a higher optimum CNT content (~20 wt%) in chemically polymerized PPy/CNT composites for supercapacitor applications [97,98]. Fig. 5 shows the surface morphology of PPy/CNT composites with different PPy: CNT mass ratios. It is demonstrated that thicker PPy layers are formed on CNT surface at lower CNT content and vice versa [95]. A thick PPy layer can be inefficient for charge accumulation since it may reduce the pore size and possibly obstruct diffusion pathways for electrolyte ions. On the other hand, chemical in situ polymerization methods inherently suffer from poor dispersion of nanotubes at higher (>20 wt%) CNT content. In addition, the presence of more CNT in reaction mixture may also catalyze the production of PPy in bulk leading to the formation of PPy aggregates [42], which in turn decrease the capacitive properties of resulting PPy/CNT composites. Thus, 2e10 wt% CNT in a chemically polymerized PPy/CNT composite is ideal for supercapacitor applications.

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demonstrated the highest capacitive performance of 302 F g1 for electrochemically deposited PPy/CNT composite electrode with 50 wt% CNT content. In view of these reports, 47e50 wt% CNT content in electropolymerized PPy/CNT composites is optimum for supercapacitor applications as compared to 2e10 wt% CNT in chemically polymerized PPy/CNT electrodes.

3.3. The dopants vs. capacitive properties

Fig. 3. (a) Total PPy deposition charge vs. total deposition time at Td ¼ 90 s, Tr ¼ 0 (black line); Td ¼ 10 s, Tr ¼ 300 s (blue line); and Td ¼ 5 s, Tr ¼ 600 s (red line). (b) Specific capacitance vs. total charge of PPy deposition on MWCNT membranes. Both quantities are normalized to the mass of MWCNTs. The PPy was deposited under pulsed electrodeposition conditions at Td ¼ 5 s, Tr ¼ 600 s (red curve); and Td ¼ 10 s, Tr ¼ 300 s (blue curve). Adapted with permission from Ref. [46]. Copyright (2010) Elsevier. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Conversely, electrochemical methods enable the formation of PPy/CNT composite electrodes with CNT content as high as ~81.8 wt %, while achieving reasonably high specific capacitance (249 F g1) [99]. Xu et al. [100] fabricated PPy/CNT sponges as supercapacitor electrodes, and reported an increase in specific capacitance from 224 F g1 to 350 F g1 with an increase in CNT content from 30 wt% to 49 wt%. However, the capacitance dropped to 153 F g1 with a further increase in CNT content (66 wt%). A decrease in capacitance of PPy/CNT composite with increasing CNT content is generally attributed to the lower intrinsic capacitance of CNT and the smaller pseudo-capacitance contribution from PPy [95]. Li et al. [99] also report a similar value (~47.6 wt%) for the optimum CNT content in PPy/CNT composites. The electrochemically deposited PPy/CNT composites exhibit the highest capacitance of 335 F g1 with 47.6 wt% CNT as compared to 249 F g1 at 81.8 wt% CNT and 182 F g1 at 19.1 wt% CNT. Recently, Guo et al. [101] have

Fig. 6 shows the molecular structures of most commonly used dopants in polymerization of PPy/CNT composites for supercapacitor applications. The dopants are primarily incorporated into the polymers to improve their electrical conductivity and capacitive performance. Several researches have been focused on modifying PPy based materials, developing advanced anionic dopants, and optimizing conditions for PPy film deposition [88,102e105]. It has been shown that aromatic anionic dopants increase the electrical conductivity and thermal stability of PPy films [106e108]. Wang et al. [109] report that the conductivity of PPy can be increased by three orders of magnitude with different dopant anions. Sharma et al. [110] describe that negatively charged poly(sodium 4styrenesulfonate) not only allows the dispersion of MWCNT in aqueous solution, but also facilitates the electrostatic interactions between PPy chains and CNT surface during polymerization. Weng et al. [111] reveal that multiple-charged ionic dopants form linkages between individual pyrrole molecules, and facilitate charge transfer. Zhitomirsky and coworkers [24,96,98,112] made use of various organic dyes, e.g. amaranth (AM), cresol purple (CP), indigo carmine (IC), and pyrocatechol violet (PV) as the dopant anions, and malachite green (MG) as doping cation during synthesis and fabrication of PPy/CNT composite electrodes. The use of AM has proved to maintain good cycling stability of PPy/MWCNT single electrodes, i.e. 101.2% capacitance retention after 5000 cycles [112]. IC and PV are adsorbed on MWCNT surface and are involved in polymerization of pyrrole by promoting nucleation and growth of PPy on MWCNT [86,98]. MG-oxalate salt permits the formation of stable PPy and CNT suspensions, and PPy/CNT composites prepared with MG-oxalate salt show ~5% higher specific capacitance as compared to those prepared without it [96]. Chen and Zhitomirsky [113] also employ negatively charged azo-dyes such as tartrazine (TT) and brilliant yellow (BY) containing redox-active azo (eN]Ne) groups, which can be transformed to hydrazo (eHNeNHe) groups during an electrochemical reaction. These redox-active anionic dopants are believed to enhance the charge storage capability of PPy. They demonstrate that TT containing PPy/MWCNT produce higher capacitance (~177 F g1) as compared to BY containing PPy/MWCNT composites (~105 F g1). The lower capacitance resulted from BY is attributed to it bigger mass and lower charge as compared to TT. These authors conclude that charge/mass ratio of the dopant anions is an important feature in controlling the size of PPy particles and the specific capacitance [113]. It is believed that the delocalized p-electrons are removed during the oxidative polymerization, and the electrical neutrality of PPy is reached by doping a suitable amount of anions into PPy [114]. Recently, Liu et al. [78] have studied the effects of varying the molar ratio of p-toluenesulfonic acid (TsOH, the anionic dopant) to pyrrole (the monomer) on capacitive properties of PPy/MWCNT electrodes fabricated on cotton fabric. They determine that minimum chargetransfer resistance (13.9 U cm2) and the highest specific capacitance (535 F g1) of PPy/MWCNT/cotton fabric electrodes are achieved at (TsOH: pyrrole) molar ratio of 1: 2.

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Table 1 A comparison of the capacitive properties of PPy/CNT based composite supercapacitors. Electrode material description

Experimental conditions Electrolyte vs. reference Potential range (V) 0.4e0.6

Scan rate (mV s1) 5

Capacitance (F g1) (Current density)

Electrochemically deposited PPy on randomly stacked MWCNT PPy chemically polymerized with 9.6 wt% MWCNT

1.0 M Na2SO4 (vs. Ag/ AgCl) 1.0 M KCl

0e0.6

1e50

167.2 (0.5 mA cm

PPy chemically polymerized at CNT grown on ceramic fabric PPy/CNT

1.0 M LiClO4/PC

1.0e1.0

25

152.8 (1 mA cm2)

1.0 M KCl (vs. SCE)

0.8e0.5

10

164 (0.2 A g1)

[46]

Electrochemically deposited PPy on acid (H2SO4: HNO3 2.0 M KCl (vs. SCE) e 3: 1) treated MWCNT 1.0 M KCl (vs. SCE) PPy nanowires chemically prepared in situ with acid (16 M HNO3) treated 2 wt% CNT

0e0.6

10

160 (0.5 mA cm2)

0e0.6

10

243 (1 A g1)

0.6e0.6

10

233.5 (0.5 A g1)

) 84.9

15.1

1

)

183.2 (8.0 A g Electrophoretically deposited PPy/MWCNT Enzyme catalyzed in situ PPy/MWCNT PPy nanofibers and 10 wt% MWCNT dispersed in organic dye and deposited Electrochemically prepared 3-dimensional ordered macroporous PPy/MWCNT Chemically prepared PPy with 20 wt% MWCNT dispersed in organic dye Electrochemically designed CNT@PPy core-shell sponge with 52.4 wt% PPy Chemically polymerized PPy with MWCNT dispersed in organic dye Electrochemically deposited PPy on freestanding vacuum-filtered CNT PPy/MWCNT composites chemically prepared using anionic dopants Electrochemically deposited PPy on CNT strip

2

)

211 (0.2 A g 1.0 M KCl

0.5 M Na2SO4 (vs. SCE) 1.0 M NaNO3 0.5 M Na2SO4 (vs. SCE)

0.5e0.4 0e0.9 0.5e0.4

2 50 2

390 (1 mA cm2) 46.2 (1 mA cm2) ~170

1.0 M KCl (vs. SCE)

0.4e0.2

5

427 (0.2 A g1)

0.9

2.0 M KCl

0.45e0.45 2

376 (0.5 A g1)

0.5 M Na2SO4

0.5e0.4

150.8 (1 mA cm2)

2

85.0 (5 mA cm

)

4.5

2

0e0.8

10

0.5e0.4

2

0.28 F cm (1.4 mA cm2) 179

PVA/H2SO4 gel (vs. Ag/ 0e0.8 AgCl) PVA/H2SO4 gel with HQ (vs. Ag/AgCl) Electrochemically deposited PPy (51 wt%) on CNT fiber 1.0 M H2SO4 (vs. SCE) 0.2e0.6

50

36.1 (0.2 A g1)

100

350.5 (5.0 A g

0.5e0.2

10

188 (5 mA cm2)

1.0 M H2SO4 (vs. SCE) Electrochemically deposited PPy on oxidized CNT fibers 1.0 M LiClO4

0e1.0

300

264 (5 mA cm2) 305 (2.5 mA cm2)

42

Electropolymerized PPy on CNT grown over carbon cloth 0.5 M H2SO4

0.2e0.6

5

486.1

3.9

Electrochemically deposited PPy (50 wt%) on CNT fiber e (single electrode) Electrochemically deposited PPy (50 wt%) on CNT fiber e (flexible, solid state SC) Electrochemically deposited PPy on CNT loaded cotton fabric Electrochemically deposited PPy on acid-treated CNT loaded cotton fabric Electrochemically deposited PPy on free standing CNT film Chemically prepared PPy with TiO2/SWCNT complex in methyl orange Chemically prepared PPy: MnO2 nanocomposite with 30 wt% MWCNT Chemically polymerized PPy with rGO/CNT (mass ratio 9/1) Chemically polymerized PPy with rGO/CNT (mass ratio 9/1) plus carbon black Chemically prepared in situ PPy/chlorinated-MWCNT/ GO nanocomposite Hydrothermally deposited MnO2 on electropolymerized PPY@CNT core-shell structure

1.0 M H2SO4

0.2e0.6

5

302 (1.0 A g1)

6.0 M H3PO4/PVA gel

0e0.8

2

69

Chemically polymerized PPy with air-plasma activated CNT

1.0 M KCl (vs. SCE)

10.7

)

4.7

1

)

H3PO4/PVA gel

0.2e0.7

25

202.0 (1.8 A g

2.0 M NaCl (vs. SCE)

0e0.8

1

535 (2 mA cm2)

H3PO4/PVA gel

0e0.9

5

331.4 (2.0 A g1)

1.0 M KCl

0.3e0.3

10

282

0.5 M Na2SO4

0.2e0.8

0.5 M KCl (vs. Ag/AgCl) 0.5e0.2

31.2

3.6 1

)

64.6

1.0 1

5

365 (2 A g

25

178 (1 A g1)

[95] [77] [129]

[97] [61] [74]

89.9% (1000) 66.4% (1000) >60% (5000) 101.2 (5000) 95% (10000) 118.1 (1000)

[11] [98] [100] [112] [79] [24] [130]

1

55.7 (0.2 A g

~97% (500) 85% (5000) 55% (5000) 95% (5000) 83% (1000) 94% (1000) 85% (1000) 72% (3000)

[86] [51] [96]

2

0.5 M Na2SO4

PVA/H2SO4 gel (vs. Ag/ AgCl) 0.5 M Na2SO4 (vs. SCE)

100

Cyclability Ref.

427

1

58 wt% graphene with PPy/CNT PPy chemically polymerized with 20 wt% MWCNT

Energy density (Wh kg1)

)

103% (2000) 87.7% (5000) 89% (1000) 100% (5000) 82% (10000) 96% (5000) 92% (5000) 50% (500) 97.8% (100) 65.6% (10000) 63.9% (1000)

44

[99] [62]

[94] [140] [101]

[135] [78] [12] [144] [145]

10% (275) [118]

199 (1 A g1)

0.5 M Na2SO4 (vs. Ag/ AgCl) 1 M NaNO3

14.3

0.8e0

10

406.7 (0.5 A g

1 M Na2SO4

0e1.0

2

529.3 (0.1 A g1)

1

) 38.4

81% (5250) 92% (1000) 89.7 (2000)

[146] [147]

Table note: Graphene oxide (GO); Hydroquinone (HQ); Poly(vinyl alcohol) (PVA); Propylene carbonate (PC); Reduced graphene oxide (rGO); Saturated calomel electrode (SCE); Supercapacitor (SC).

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Fig. 4. A schematic illustration of the fabrication process of macroporous PPy/CNT composite films: (a) self-assembling of SiO2 colloidal template; (b) immersing the SiO2 colloidal template into the electrolytic solution; (c) electrochemical copolymerization; and (d) removal of the SiO2 colloidal template. (e) SEM image of PPy/CNT composite prepared by cyclic voltammetry for 3000 potential scanning cycles. Adapted with permission from Ref. [11]. Copyright (2013) American Chemical Society.

Fig. 5. FESEM images of (a) pristine MWCNT, and PPy/MWCNT composites with (b) 9.6 wt% CNT, (c) 20.1 wt% CNT, and (d) 30.5 wt% CNT. Adapted from Ref. [95]. Copyright (2010) Korean Chemical Society.

3.4. The electrolyte vs. capacitive properties The supporting electrolyte is not only a means of charge transport, but also plays an essential role in regulating the capacitive performance and cyclability of supercapacitors [115e117]. However, the effects of supporting electrolyte on electrochemical performance of PPy/CNT supercapacitors have not been studied

extensively. A few reports published recently demonstrate that different aqueous electrolytes have a marked influence on specific capacitance and cycling stability of PPy/CNT composites [97,118]. Paul et al. [97] fabricated PPy/MWCNT composites via chemical oxidative polymerization of pyrrole in aqueous FeCl3. They studied the effect of 1.0 M aqueous electrolytes, e.g. H2SO4, Na2SO4, KCl, and KOH on capacitive properties, and reported very high specific

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Fig. 6. Chemical structure of commonly used dopants in polymerization of PPy/CNT composites: (a) p-Toluene sulfonic acid, (b) amaranth, (c) cresol purple, (d) pyrocatechol violet, (e) poly(sodium 4-styrenesulfonate), (f) indigo carmine, (g) malachite green oxalate, (h) brilliant yellow, and (i) and tartrazine.

capacitance (539 F g1) in aqueous KOH. For other electrolytes, the capacitance values were: 176 F g1 in H2SO4, 114 F g1 in Na2SO4, and 165 F g1 in KCl [97]. It has been demonstrated by some researchers that the electrolytes have minimum effect on the cycling behavior of supercapacitors based on activated carbons and carbon nanotubes [119,120], however the life cycle of PPy based supercapacitors are greatly affected by different electrolytes. For instance, the cycling stability of PPy/CNT composite electrodes decreases drastically in 1.0 M KOH due to the perturbation of polymer's conjugated structure by hydroxide (OH) ions [97,121]. Paul et al. [97] achieved the highest cycling stability of PPy/CNT supercapacitors in 1.0 M KCl electrolyte. Peng et al [118] fabricated PPy/CNT/r-GO composite supercapacitors and investigated the effects of six supporting electrolytes: KCl, KNO3, K2SO4, NaCl, NaNO3, and Na2SO4 at 25 mV s1 CV tests. They also noticed that charge/discharge process of PPy is critically influenced by both cations and anions [118]. The mechanisms accountable for the differences in capacitive properties of PPy/CNT composite electrodes in different electrolyte environments are still not clearly understood. However, the size and radius of hydrated cations and anions, their intrinsic ionic conductivities in aqueous solutions are believed to influence the specific capacitance and cyclability of PPy/CNT supercapacitors. Table 2 provides the radii of hydrated ions and their molar ionic conductivities [122e125]. Clearly H3Oþ ion has the smallest hydrated ionic radius and the highest ionic conductivity, which explains why Paul et al. [97] obtained higher specific capacitance in H2SO4 (176 F g1) as compared to Na2SO4 (114 F g1). Similarly, higher specific capacitance of PPy/CNT electrodes in KOH (539 F g1) compared with KCl (165 F g1) can be explained on the basis of higher molar ionic conductivity of the counter ion (OH). A comparison of the KCl and K2SO4 electrolytes however reveals conflicting observations as rGO/PPy/CNT composite electrodes exhibit higher specific capacitance in KCl (178 F g1) compared to K2SO4 (131 F g1) [118]. Although SO2 4 ions have more than twice the ionic conductivity than Cl ions, they are responsible for lower capacitance. This may be attributed to the greater hydrated ionic 2 radius of SO 4 , which makes it difficult for SO4 ions get in and out

of the polymer matrix during the charging and discharging of PPy/ CNT electrodes. Thus, the rate of adsorption and desorption may also play a significant role in determining the capacitive properties of PPy/CNT electrodes. However, the influence of various aqueous electrolytes on the performance of PPy/CNT supercapacitors still needs to be explored further. Furthermore, a disadvantage of the aqueous electrolytes is that water may easily electrolyze to generate gases that is ascribed to relatively narrow electrochemical potential range of aqueous electrolytes. In fact, regardless of the acidity the thermodynamic potential window of water is only 1.23 V [126]. The electrochemical stability window of aqueous electrolytes is essentially important for supercapacitors because it determines the cell voltage and in turn influences the energy and power densities [117]. Several works have focused on expanding the potential window of aqueous electrolytes. For instance, Wessells et al. [127] determined the potential window of 5 M LiNO3 aqueous solutions to be 2.3 V, spanning from 0.55 to 1.75 V with respect to the standard hydrogen electrode. Tomiyasu et al. [128] recently reported the widest potential window of 3.2 V for a saturated aqueous solution of sodium perchlorate and superior supercapacitor performance. However, most works reviewed herein used a narrow potential range of 0.8e1.0 V for various types of aqueous and gel electrolytes. Only, Lee et al. [77] measured the electrochemical characteristics of PPy/ CNT composite (on the ceramic fiber) with LiClO4-propylene carbonate electrolyte solution in the stable potential window of 2.0 V, spanning from 1.0 to 1.0 V at a scan rate of 25 mV s1. Thus, there is a need to determine the effects of different electrolytes on extending the potential window of aqueous electrolytes as well as to explore new electrolytes that are stable in a wider potential range. 3.5. Experimental parameters vs. capacitive properties There are other experimental parameters such as scan rate, current density, and active material loading, which can influence the capacitive properties of PPy/CNT supercapacitors [61,74,86,129].

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Table 2 The radii of common non-hydrated and hydrated cations and anions, and their molar ionic conductivity values in aqueous solutions. Ion

Non-hydrated ionic radii (Å)

Hydrated ionic radii (Å)

Ionic conductivity (S cm2 mol1)

Hþ H3Oþ Naþ Kþ OH Cl SO2e 4

<0.50 1.50 0.95 1.33 1.33 1.81 2.58

e 2.82 3.58 3.31 3.00 3.32 3.79

349.8 50.1 73.5 198.3 76.3 160.0

Although some of the facts regarding the relationship between specific capacitance and the above-mentioned experimental parameters are well-established and well-known, it is important to mention these facts herein to enlighten the readers and to realize the breadth and the scope of this review. Albeit the high-rate capability, improved energy density, and cyclability are desired for practical applications of supercapacitors as electrical energy storage devices, the capacitance of supercapacitor electrodes generally decreases with increasing scan rate and current density [74,79,101,129,130]. Fig. 7a shows the cyclic voltammetric (CV) curves of the fiber shaped PPy/CNT supercapacitor with PVA/H2SO4 gel electrolyte containing 0.3 g hydroquinone (HQ) at various scan rates from 20 mV s1 to 200 mV s1. It is known that scan rate and charge/ discharge current are directly related to each other, i.e. higher scan rate induces higher charge/discharge current. Therefore, redox reactions can occur more thoroughly at lower scan rates in PPy/CNT supercapacitors [79,130]. The shape of CV curves differs at different scan rates with evident redox peaks. The quasi-rectangular shape of CV curves at lower scan rates indicating the best capacitive behavior of PPy/CNT supercapacitors. Fig. 7b exhibits the galvanostatic charge/discharge (GCD) curves of the fiber shaped PPy/CNT supercapacitor at various current densities from 0.2 A g1 to 1.0 A g1. While equal charge/discharge times indicate excellent electrochemical reversibility, the shape of GCD curves at lower current densities implies greater faradaic contribution (redox reactions) to the charge accumulation process [130,131]. According to Fu et al. [74], the rate performance is better evaluated by GCD at higher current densities. They attribute the high rate capability of PPy/CNT composites to rapid charge transport offered by CNT results in the enhancement of faradaic processes in redox-active PPy. In PPy/CNT composites, the embedded CNT improve the rate of electrochemical reactions and decrease the charge transfer resistance [74]. Consequently, the pseudocapacitive

charge storage in PPy is increased providing high specific capacitance (183.2 F g1) at high current density (8 A g1). Zhitomirsky and coworkers [86,96,112,113] use Ni foam or plaque based electrodes and study the effect of PPy/CNT mass loading on the performance of resulting supercapacitors rendering a realistic picture of the capacitive performance per unit mass of PPy/CNT composites. The optimum active mass for carbon bases supercapacitors is shown to be 10 mg cm2 [113,132], while it is in the range of 1 mg cm2 to 1 mg cm2 for PPy based supercapacitors [133,134]. Albeit, some studies claim excellent capacitive properties of PPy/CNT supercapacitors at ~30 mg cm2 active mass loading [96,98,112], it is challenging to fabricate PPy/CNT electrodes with high material loading due to reduced surface area and pore accessibility with increasing PPy/CNT thickness. Li and Zhitomirsky [86] report the highest specific capacitance (390 F g1) for 1 mg cm2 PPy/CNT loaded electrodes at a scan rate of 2 mV s1. There are other experimental parameters such as scan rate, current density, and active material loading, which can influence the capacitive properties of PPy/CNT. 4. Limitations and challenges Considering the data published to date and reviewed herein, there are some aspects of PPy/CNT supercapacitors that need immediate attention of the researchers. The foremost challenge associated with PPy/CNT composite electrodes is their cycling stability (or cyclability) [135]. PPy is known to exhibit poor cyclability, which is primarily due to its large volume transformations during repetitive redox (charge/discharge) cycles [14]. The loss in capacitance during continuous injection/expulsion of electrolyte ions is generally related to the repeated volumetric expansion/contraction of PPy, which deteriorates charge distribution, induces conformational changes in p-conjugated PPy chains, and leads to serious degradation [136]. The embedding of CNT in PPy matrix enhances the mechanical strength and stability of PPy/CNT composites and prevents extensive swelling/shrinking of PPy network during the long-term charge/discharge cycles [74,137]. As given in Table 1, majority of researcher report limited cyclability data, i.e. for 1000e5000 cycles only, which is insufficient to certify the application of PPy/CNT composite electrodes in modern supercapacitors. The highest capacitance retentions achieved after 1000, 2000, and 5000 cycles are 118% [24], 103% [130], and 101% [112] respectively. Thus, an anomalous increase in capacitance has been observed in limited number of charge/discharge cycles, i.e. 1000e5000 cycles. The initial increase in capacitance is usually ascribed to the structural adjustments during repetitive cycling [138,139]. Therefore, it is important to consider reports that discuss

Fig. 7. (a) Cyclic voltammetric curves, and (b) galvanostatic charge/discharge curves of a fiber-shaped PPy/CNT supercapacitor with PVA/H2SO4 gel electrolyte containing 0.3 g hydroquinone at various scan rates and current densities. Adapted with permission from Ref. [130]. Copyright (2015) Royal Society of Chemistry.

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the cycling stability of PPy/CNT composite supercapacitor at 10000 cycles [12,79,140]. These reports suggest that PPy/CNT composite supercapacitors can retain up to 95% capacitance after 10000 cycles [79], which is promising for practical applications. Nevertheless, there is a general need to improve the cyclability of PPy/CNT composite supercapacitors. There is a large variance in capacitive properties of PPy/CNT composites, which can be observed in Table 1, e.g. specific capacitance ranges from 55 to 535 F g1, energy density ranges from 3.6 to 84.9 Wh kg1, and cyclability experiments reveal 55e101% capacitance retention after 5000 cycles. This observation points out other issues related to microstructural uniformity of PPy/CNT composites, which is usually limited by the synthetic methodology [25]. In situ chemical oxidative polymerization of PPy with CNT from aqueous solution may suffer from poor dispersion of CNT, PPy aggregation, and increased resistance owing to weaker CNT interconnection [42,46,141]. To overcome these problems and to achieve better interfacial connections between PPy and CNT, CNT functionalization is required that has been mostly achieved through concentrated acid treatment. Furthermore, chemical methods employ strong oxidants and aqueous acidic solutions for fabrication of PPy/ CNT composites. Some of these strategies may induce CNT surface defects, thus reducing their inherent conductivity, and specific capacitance of resulting PPy/CNT composites [142]. On the other side, electrochemical deposition methods cannot be scaled-up for mass production of PPy/CNT composites due to complicated procedures. According to some reports, the electrodeposition of PPy on pre-assembled CNT may obstruct the electrolyte channels on the surface of resulting composite [85,141]. The electropolymerization of pyrrole on CNT surface is often accompanied with the formation of PPy films on non-noble metal substrates in addition to the oxidation and dissolution of the substrate during anodic electropolymerization [86]. Another drawback of the electrochemical methods is the oxidation and degradation of CNT and corrosion of current collectors in acidic electrolytes such as H2SO4, which can be overcome by using, e.g. Na2SO4 as the electrolyte. Therefore, there is a need to improve the existing synthetic routes, and develop new methods for fabrication of structurally homogeneous PPy/CNT composite electrodes. Recently, air plasma activation [62] to modify CNT surface and pulse electrodeposition [46] of PPy on CNT have emerged as better alternatives to address some of the above-mentioned limitations of chemical and electrochemical methods, but more work should be put in to achieve the true potential of PPy/CNT composite supercapacitors. 5. Summary and outlook This article presents a state-of-the-art review of recent technological advances and their limitations in realization of PPy/CNT composite supercapacitors. PPy/CNT composites have achieved excellent specific capacitance (535 F g1) [78], high energy density (84.9 Wh kg1) [77], and remarkable cycling stability (95% at 10000 cycles) [79]. However, as mentioned earlier, there is a large variance in the reported data primarily due to differences in synthesis and fabrication methods, and differences in experimental conditions to some extent. The optimum experimental conditions and prerequisites for improved capacitive performance of PPy/CNT composites are reviewed and described in detail in separate sections. Herein, we present a few basic necessities for analyzing capacitive performance of PPy/CNT composites and propose future research directions taking into consideration the reviewed literature. Firstly, it has been noted that PPy/CNT composite supercapacitors are mostly arbitrated on the basis of specific capacitance and limited cyclability, while there are other potential characteristics that should be mentioned in modern scientific

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researches to provide a complete picture. For instance, the energy density, i.e. the storage capacity, and the power density, i.e. the speed of charge/discharge, are not always revealed in a scientific paper, which can lead to false conclusion regarding the status and potential of a PPy/CNT supercapacitor. In addition, the cycling stability should be analyzed and reported for a large number of cycles (10000) considering the practical application of supercapacitors. Hence, the researchers should be encouraged to provide detailed analysis of the capacitive performance of PPy/CNT composite supercapacitors. On practical and research forefronts, there is a need to devise new methods and improve the existing routes for fabrication of PPy/CNT composites. A major goal of future research is to augment the interfacial compatibility between PPy and CNT in order to benefit from the high theoretical specific capacitance of PPy (620 F g1) and high conductivity and mechanical stability of CNT. The design of non-covalent CNT functionalization schemes that may allow the attachment of carboxylic acid moieties on CNT surface through, e.g. pp stacking interactions can lead to enhanced capacitive performance of resulting PPy/CNT composites. Such noncovalent functionalization methods can offer defect-free CNT surfaces with exceptional electrical and mechanical properties. Simmons et al. [143], for example, demonstrate the use of pyrenecarboxylic acid to modify CNT surface, thus avoiding any damage to the core CNT structure as an alternative to acid treatments. There are a number of characteristic advantages associated with the appropriate functionalization of CNT surface. This not only improves the interface interactions between PPy and CNT, but allows the scientist to integrate a higher amount (wt%) of CNT in PPy/ CNT composites. At higher CNT content, PPy coating becomes thinner leading to improved ionic accessibility through electrolyte channels on PPy surface, and better capacitive properties of resultant PPy/CNT composites. Moreover, the formation of ternary composites involving, e.g. graphene or metal oxides, with PPy/CNT may yield additional performance benefits [118,144e147]. However, the need to improve interface in binary or ternary PPy/CNT based composite supercapacitors and to devise methods that can yield structurally uniform composite materials remains a challenge. On theoretical forefront, there is a need to investigate and explain the charging mechanisms of PPy/CNT composite supercapacitors. Several researchers try to probe the charging mechanism of PPy using various electrolyte environments [34,129,148,149], but the exact charging mechanism of PPy/CNT supercapacitors is still not fully known. Since PPy/CNT composite makes an asymmetric supercapacitor (ASC), different charging mechanisms may be involved in a reversible sequence, e.g. faradaic processes such as redox reactions, electrosorption, and intercalation or non-faradaic charge accumulation on the surface of PPy/CNT surface [150]. Therefore, it is essential to identify these charging mechanisms and their equilibrium in order to fully understand and exploit the capacitive properties of PPy/CNT composites. In retrospect, we can safely say that PPy/CNT composites make high performing electrochemical energy storage devices, but enough is to be done to meet the commercial requirements of supercapacitors.

Acknowledgements Authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through Science and Technology Unit at King Fahd University of Petroleum and Minerals (KFUPM) for funding this work through project No. 13-NAN467-04 as part of the National Science, Technology, and Innovation Plan.

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