PANI)

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Non-covalent interactions and supercapacitance of pseudo-capacitive composite electrode materials (MWCNT—COOH/MnO2/PANI) Md Moniruzzaman Sk *, Chee Yoon Yue **, Rajeeb Kumar Jena School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 June 2014 Received in revised form 17 October 2014 Accepted 21 October 2014 Available online xxx

We present the in situ oxidative synthesis and electrochemistry of novel pseudo-capacitive nanostructured composite electrode material based on multi-walled carbon nanotube (MWCNT), manganese dioxide (MnO2) and polyaniline (PANI), namely, MWCNT—COOH/MnO2/PANI (PCNAM) for high performance supercapacitor applications. The composite shows about six-fold improvement of electrochemical response compared to MWCNT. The maximum specific capacitance, energy density, and power density of PCNAM were 517.13
15.25 F/g, 71.88
2.12 W h/kg and 10.08
0.26 kW/kg, respectively. The high capacitance of the composite is due to the combination of the electrical double layer capacitance of MWCNT (in MWCNT—COOH) and the gradual introduction of pseudo-capacitance through the redox processes of PANI, —COOH (in MWCNT—COOH) and MnO2. We have also demonstrated the charge transfer phenomena through non-covalent supramolecular interactions (i.e., p–p, n–p, and metal–p) between the components of PANI, MWCNT—COOH, and MnO2 due to the presence of double bonds, availability of lone pair electrons, free charges on nitrogen/oxygen atoms, and vacant metal d orbitals. The existence of such non-covalent interaction was supported by data from Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analysis. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Cabon nanotubes Manganese dioxide Polyaniline Composite Supercapacitors

1. Introduction Supercapacitors (or electrochemical capacitors) [1–4] have attained excellent charge storage ability and have attracted much interests for applications in many advanced power devices due to their high specific capacitance, high power density, and long cyclic stability. Electrochemical capacitors store energy primarily by utilizing the electrical double layer (EDL) and/or redox reactions (pseudo-capacitance), which provide high power and long cycle life but lower energy as compared to rechargeable batteries. Therefore, the choice of suitable active electrode materials for supercapacitors to meet the requirements of high energy and power is a promising field of research. Various active materials which have been studied include carbon [3,5–8], conducting polymers [9–12], metal oxides [13–17], and their hybrid composite materials [18–20]. The carbon based materials that have been studied include activated carbon, carbon fiber, and carbon nanotube (CNT).

* Corresponding author. Tel.: +65 86986971; fax: +65 67924062. ** Corresponding author. Tel.: +65 67906490; fax: +65 67924062. E-mail addresses: [email protected] (M.M. Sk), [email protected] (C.Y. Yue).

Recently, CNTs and their composites [6,7,21,22] have received much attention for their possible application in fabricating a new class of composite electronic materials due to their unique structural, mechanical, and electronic properties. There are primarily two types of nanotube, namely; single-walled carbon nanotubes and MWCNT. Furthermore, it has been reported that electronically conducting polymers (ECPs) have advantages over carbon based electrode material for supercapacitor because in addition to the high pseudo-capacitance arising due to the fast and reversible oxidation and reduction processes [23], it also provides a lower percentage of EDL capacitance. A wide variety of conducting polymers which include PANI, polypyrrole (PPy), polythiophene (PTh), poly(ethylenedioxythiophene) (PEDOT), and their corresponding substituted polymers have been studied for supercapacitor applications. Among the conducting polymers, PANI has strong potential features for energy storage due to its good environmental stability and reversible control of conductivity by doping/de-doping and charge-transfer processes. Metal oxides [13–15] with various oxidation states have also been attracted growing interest for electrodes with pseudocapacitive properties for the electrochemical supercapacitors. Among these, ruthenium oxide (RuO2) presents the best pseudocapacitive properties; however, its high cost, limited availability, and polluting effect on the environment discourage it from large

http://dx.doi.org/10.1016/j.synthmet.2014.10.026 0379-6779/ ã 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: M.M. Sk, et al., Non-covalent interactions and supercapacitance of pseudo-capacitive composite electrode materials (MWCNT—COOH/MnO2/PANI), Synthetic Met. (2014), http://dx.doi.org/10.1016/j.synthmet.2014.10.026

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scale use. In this regard, other metal oxides have been investigated as possible substitutes. In particular, the MnO2 [24–26] is very attractive due to its wide availability and low cost compared to RuO2, as it has no-polluting effects. Thus, MnO2 is actively being evaluated in order to understand how the structural, morphological, and oxidation characteristics affect its electrochemical behavior. It has been shown that MnO2 can deliver higher specific capacitance as it can store charges through redox capacitive mechanisms through several oxidation states. The reported values of specific capacitance of pristine MnO2 and graphene/MnO2 based electrode are about 103 and 310 F/g, respectively [27]. Also, it is reported that the specific capacitance of the MWCNT/MnO2 composite electrode reaches 250.5 F/g [28] which is significantly higher than that of a pure MnO2. Yan et al. [27] have shown that the mechanism involved for storing charges in MnO2 is the switching of its oxidation state between Mn4+ and Mn3+ upon intercalation/ de-intercalation of ions during electrochemical processes. However, PANI like other ECPs also suffers with a serious problem of volumetric swelling and shrinkage during the doping/ de-doping of counter ions. In order to solve this problem, a composite comprising of PANI and CNT and/or transition metal oxides has been considered as the electrode materials for supercapacitor where a synergistic effect [29,30] of the composite in enhancing the electrochemical and mechanical properties can be utilized. The oxidative synthesis of MWCNT/PANI (PCN) composite has enabled supercapacitor electrodes with excellent specific capacitance to be obtained. The improved electrochemical behavior of such a composite electrode is governed by the combined synergistic effects of the system rather than its individual constituents, and this provide opportunities for a smarter design of charge-storage devices. Hence, there is a possibility that the double layer and faradaic redox mechanisms can be combined synergistically in a composite electrode system to increase both the energy density and power density of supercapacitors. This is the focus of the present work. Although, there is some work reported in the literature on the performance of PANI coated MWCNT composites, the improvement of capacitance by pseudo-capacitive components and its charge transport phenomenon in the composites have not been investigated thoroughly. We have synthesized a MWCNT—COOH/ PANI (PCNA) composite because it has been reported in the literature that surface functionalization of MWCNT by introducing acid groups can further increase the capacitance. Additionally, for the enhancement of capacitance through effective charge transport, we have introduced MnO2 into the PCNA composite. Oxidative synthesis of MWCNT—COOH/MnO2/PANI (PCNAM) composite was done by in situ chemical oxidative polymerization of aniline to form a ternary composite. A thorough investigation on type of non-covalent interactions and the relative influence of pseudo-capacitance over double layer capacitance on the overall capacitance is the aim of this work.

2.2. Functionalization of MWCNT and composite preparation process The MWCNT was treated with a solution of concentrated H2SO4 and HNO3 (H2SO4:HNO3 = 3:1 w/w) in the ratio of MWCNT to mixed acids solution of 1:200 to develop the acid functionalized MWCNT. Then, the mixture was stirred at 80  C for 4 h and washed with deionized water several times. Next, the specimen was centrifuged and dried in the oven at 110  C for 8 h. For the preparation of MWCNT based PANI composite, 360 mg MWCNT was treated in 400 ml deionized water, and sonicated for over 30 min to obtain well dispersed suspension. Then, a solution of 1.2 ml aniline monomer and 12 ml of 1 M HCl were added to the above system and again sonicated for 10 min. Next, 50 ml distilled water containing 3.005 g APS (mole ratio of aniline: APS = 1:1) was added slowly as an initiating agent to this solution and the mixture was sonicated for another 10 min. Then, the reaction mixture was left standing in refrigerator at 1–5  C for 24 h. The resulting precipitate was filtered, and washed with distilled water several times, and dried at 60  C for 12 h. The synthesized composite is PCN. The similar process was also used to synthesize PCNA with 360 mg MWCNT—COOH, and PCNAM composites with 180 mg MWCNT—COOH, and 180 mg MnO2 keeping the amount of aniline (0.91 ml) fixed for all the composites. 2.3. Characterization techniques The field emission scanning electron microscopy (FESEM) study was performed using the JEOL JSM 7600 FESEM to observe the surface morphology of the coated polymer onto the MWCNT and MnO2. The transmission electron microscopy (TEM) study was carried out using the JEOL JEM 2010 TEM to observe the bulk morphology of the coated polymer onto the carbon nanotubes and MnO2 in the composite. Fourier transform infrared spectroscopy (FTIR) study was done using the NICOLET 6700 FTIR (Thermo scientific) to investigate the chemical structure of acid functionalized MWCNT and composites. The X-ray photoelectron spectroscopy (XPS) study was performed for all the samples using the Kratos Axis-ULTRA XPS analyzer. The electrochemical study of the composites was investigated using the three-electrode cell by using a galvanostat/potentiostat (Gamry Reference 3000). The electrochemical performance was determined for each composite in a 1 M aqueous KCl solution using cyclic voltammetry (CV), cyclic charge-discharge (CCD), and electrochemical impedance spectroscopy (EIS) experiments. The composite materials were tested using the glassy carbon electrode as the working electrode, platinum wire as the counter electrode, and a silver–silver chloride (Ag/AgCl) as the reference electrode. Thermogravimetric analysis (TGA) for each composite was accomplished using the TGA 2950 over the temperature range from room temperature up to 800 C at a heating rate of 10 C/min under nitrogen atmosphere. 2.4. Electrode preparation and electrochemical measurements

2. Experimental 2.1. Materials MWCNTs (99%, 10–30 nm diameter, and 5–15 mm length) were obtained from Nanostructured and Amorphous Materials Inc., USA. The carbon black was purchased from Age D’or Pte Ltd., Singapore. The sulfuric acid (H2SO4), nitric acid (HNO3), aniline, ammonium persulphate (APS), manganese dioxide (MnO2) (99.99%, 4– 12 nm  50–110 nm  10–30 nm), potassium chloride (KCl), and nafion solution (5wt% in ethanol) were supplied by Sigma–Aldrich, Singapore. The electrochemical cell including the reference electrode, counter electrode, and working electrode were purchased from Technoscience, USA.

The electrodes were prepared with 2 mg of composite in 5 ml nafion solution (5 wt% in ethanol) and 10 wt% carbon black to make a homogeneous paste and then the paste was applied on to the glassy carbon electrode (diameter 3 mm). Then, all the electrodes were dried for 1 h in the oven at 90 C. The cyclic voltammetry characteristics of the composite electrodes were recorded using the three electrode measurement system at different scan rate within the potential range of 0–1.0 V. A similar potential range was also used in the cyclic charge–discharge measurement at the constant current of 1 mA. For the constant current charging/ discharging curve, the specific capacitance (Csp) was calculated [31–33] using the relationship Csp = (I  t)/(V  m), where I is the applied constant current, t = discharging time, V = discharge

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Table 1 EDL capacitive and pseudo-capacitive components in the individual composite. Notation

Sample

EDL capacitive component

Pseudo-capacitive component

MWCNT PCN PCNA PCNAM

MWCNT MWCNT/PANI MWCNT—COOH/PANI MWCNT—COOH/MnO2/PANI

MWCNT MWCNT MWCNT MWCNT

– PANI PANI—COOH PANI—and COOHMnO2

voltage (excluding ohmic drop), and m is the mass of the active electrode material. The energy density (Emax) of a supercapacitor was calculated [31,34] from the relationship Emax = 1/2 CspVi2, where Csp = specific capacitance and Vi = operating voltage range (1.0 V). The impedance spectroscopy measurements were performed at the frequency range of 100 kHz–1 Hz. Power density (Pmax) was calculated [35,36] from the relationship Pmax = Vi2/4Ri, where Vi = operating voltage range, and Ri = equivalent series resistance. 3. Results and discussion The influence of redox conductive polymers, chemical functional groups, or metal oxides to enhance the overall capacitance by contributing the amount of pseudo-capacitance (also known as redox capacitance) is known to be higher than that of capacitance obtained by double layer capacitive materials [37]. The double layer capacitance is related to the surface mechanism whereby electrolyte ions accumulated on the surface of the electrode to form EDL whereas pseudo-capacitance originates from the surface as well as bulk oxidation of the electrode materials. Thus, for any given supercapacitor, the contribution of pseudo-capacitance to the overall capacitance is expected to be larger than that of double layer capacitance. In order to demonstrate this in the current work, several samples with varying relative amounts of double layer capacitance and pseudo capacitance as listed in Table 1 were prepared and analyzed. The new ternary composite PCNAM which possesses high pseudo-capacitive components was synthesized. The electrochemical performance of an EDL capacitive material MWCNT was analyzed, and this was then compared with

progressively more pseudo-capacitive materials that contained components like PANI, —COOH (in MWCNT—COOH) and MnO2. In our study, the relative contribution of pseudo-capacitance to the total capacitance increases in the order PCA < PCNA < PCNAM and the MWCNT sample only possesses pure double layer capacitance. In all the above three composites, the formation of a good interface is critical as it will greatly improve the electronic conductive channels and hence promote excellent interfacial charge transfer between the components. In the present work, the property of the various interfaces will be evaluated in terms of the existence of any operative non-covalent interactions (e.g., p–p, n– p, and metal–p) that can be determined from a thorough analysis of the shifts in the FTIR and XPS peaks. This will provide valuable insights into the enhancement of overall electrochemical performance due to the existence of pseudo-capacitance. 3.1. Morphological and structural characterizations 3.1.1. Surface and bulk characterizations using FESEM and TEM Fig. 1 shows the typical FESEM images of pure MWCNT, and the PCN, PCNA, and PCNAM composites. The coating of the PANI was confirmed from a comparison of the diameter of the pristine nanotube (10–30 nm) and the increased diameter (40–60 nm) of the nanotube in the composite. Fig. 2 exhibits the TEM images of pure MWCNT, and the PCN, PCNA, and PCNAM composites, and pure MnO2. It is clear from Fig. 2 that the PANI has coated uniformly onto the MWCNT, MWCNT—COOH, and MnO2. It can be seen from Fig. 2e that the pristine MnO2 existed as irregular thin platelets.

Fig. 1. FESEM images of (a) pure MWCNT, (b) PCN composite, (c) PCNA composite, and (d) PCNAM composite.

Please cite this article in press as: M.M. Sk, et al., Non-covalent interactions and supercapacitance of pseudo-capacitive composite electrode materials (MWCNT—COOH/MnO2/PANI), Synthetic Met. (2014), http://dx.doi.org/10.1016/j.synthmet.2014.10.026

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Fig. 2. TEM images of (a) pure MWCNT, (b) PCN composite, (c) PCNA composite, (d) PCNAM composite, and (e) pure MnO2.

3.1.2. Molecular characterizations using FTIR and XPS FTIR studies were carried out to confirm the chemical structure of all the composites such as PCN, PCNA, and PCNAM and the attachment of carboxylic acid group (—COOH) to the MWCNT (Fig. 3). The characteristic peaks at about 1583 and 1504 cm1 in all the composites were attributed to the nonsymmetric C6 vibration bands. The peak at about 1583 cm1 was assigned to the C¼C stretching for quinoid ring and at about 1504 cm1 was assigned to the C¼C stretching vibration mode for benzenoid rings indicating the presence of PANI in all the composites [38,39]. The aromatic C—H bending in the plane (1130 cm1) and out of plane (824 cm1) for a 1,4 disubstituted

aromatic rings indicates the presence of linear structure of PANI [39]. The characteristic peak appearing at about 1295 cm1 was due to the C—N stretching in the PANI. Also, the frequency region of 770–730 cm1 and 715–685 cm1 are due to the C—H bending of the mono substituted terminal aromatic ring in the PANI backbone. The peak at 1690 and 1062 cm1 are attributed to the C¼O and C—O stretching, respectively. The FTIR spectra of MWCNT—COOH (Fig. 3e) has peaks at 1718, 1159, and 3445 cm1 which are due to the C¼O, C—O, and O—H stretching vibration, respectively, that are attributed to the —COOH functionalization in the MWCNT. The peak at 1627 cm1 is due to the C¼C stretching in the nanotube.

Fig. 3. Typical IR spectra of (a) pure MWCNT, (b) PCN composite, (c) PCNA composite, (d) PCNAM composite, and (e) MWCNT—COOH.

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Fig. 4A. XPS spectra of PCNAM (a) wide scan and (b) Mn2p.

XPS analysis was carried out between 0 and 1200 eV to evaluate the molecular characteristics of the composite. The wide scan XPS spectra of PCNAM composite in Fig. 4A.(a) contained the C1s, O1s, Mn2p, Mn3p, and N1s spectra. The narrow scan spectra of MnO2 in Fig. 4A.(b) showed two peaks at 641.38 and 652.8 eV for Mn2p3/2 and Mn2p1/2, respectively. 3.1.2.1. Analysis of peak shifting in FTIR and XPS. Comparison of the FTIR peaks due to the C¼C, C¼O, and C—O bonds in the composites to those in the pristine MWCNT—COOH indicates that the peak shifting occurred (see Fig. 3 and Table 2). Such peak shifting clearly indicates the presence of good interaction by polarity changes between the PANI and MWCNT or MWCNT—COOH or MnO2 in the composites which led to lowering of vibrational energy. A comparison of the XPS peak shifting for C¼C, C¼O bonds, and MnO2 in the composites is shown in Fig. 4B. The spectra in Fig. 4B. can reveal the extent of interaction between the components in the composite due to the presence of p and n electrons in C¼C, C¼O, and C—O bonds. Fig. 4B.(a) illustrates the comparison of XPS spectra of C¼C bonds of pristine MWCNT—COOH and C¼C bonds in the composites. It is clear that the peak positions were shifted (Table 3) from 284.08 eV in MWCNT—COOH to lower binding energy values of 282.32, 282.3, and 282.08 eV in PCN, PCNA, and PCNAM composites, respectively. This shift is due to the p–p interactions between the p electrons of the C¼C bond of MWCNT and p electrons of PANI chains in the composite. Similarly, it can be seen that the peak of —COOH group in Fig. 4B.(b) also shifted (Table 3) toward a lower binding energy from 288.41 eV in MWCNT—COOH to 286.57 eV in the PCNA and PCNAM composites. This shift is due to the p–p and n–p interactions between the p electrons of C¼O bond and lone pair electrons (n) of O in the MWCNT—COOH, and p electrons in aromatic rings of PANI chains. Another interaction between MnO2 and PANI was also detected by the XPS spectra as shown in Fig. 4B.(c). The peak of Mn2p1/2 and Mn2p3/2 (Table 4) in the pure MnO2 are located at 653.51 and 642.0 eV whereas, in the composite, these peaks were shifted to

652.8 and 641.38 eV, respectively indicating the existence of good metal–p interaction. 3.2. Electrochemical characterizations 3.2.1. CV study The electrochemical behavior of electrode materials was investigated using a three-electrode electrochemical cell within the potential range of 0–1.0 V in an aqueous 1 M KCl electrolyte. The CV curves of PANI composites at the scan rate of 5, 10, 20, 50, 100 mV/s are shown in Fig. 5A. It can be seen from Fig. 5A.(a) that the CV curves for the pure MWCNT were nearly rectangular shape which indicated that MWCNT exhibits pure double layer behavior. The lack of symmetry (i.e., deviation from rectangularity) in the CV curves in the PCN, PCNA, and PCNAM composites is due to combined contribution of double-layer and pseudo-capacitive behavior to the total capacitance. No peaks were observed for all scan rates for all the composites (PCN, PCNA, and PCNAM) indicating that the electrode is highly reversible and stable [28]. In all the composites (PCN, PCNA, PCNAM), the CV profiles retain a relatively similar shape (without obvious distortion) with gradual increase in current values at increasing scan rates (see Fig. 5A.(b)– (d)). This clearly indicates that the above mesoporous composite electrodes have good charge storage behavior. 3.2.2. CCD study The galvanostatic charge/discharge profile at a constant current of 1 mA in an aqueous 1 M KCl electrolyte for all the specimens are shown in Fig. 5B. It can be seen that all the charge–discharge curves are near-triangular in shape (Fig. 5B.) which implied that the electrode has good electrochemical reversibility which was also confirmed by symmetrical CV curves with no peaks (Fig. 5A.). The ohmic drops (at the beginning of the discharge curve) found are due to the internal resistances (IR) which were much lower (8, 13, 14, and 20 mV for PCNAM, PCNA, PCN, and MWCNT, respectively) for all the composites. The low internal resistance is of great

Table 2 Analysis of peak shifting in FTIR spectra. Peak assignment

Peak value in pristine MWCNT—COOH Peak value in the composite PCN (cm1) (cm1)

Peak value in the composite PCNA Peak value in the composite PCNAM (cm1) (cm1)

C¼C

1627

C¼O C—O

1718 1159

1580 1504 1690 1062

1583 1504 – –

1572 1503 1690 1062

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Fig. 4B. XPS comparison study of peak shifting of (a) C¼C bonds, (b) C¼O bonds in COOH, and (c) Mn2p peaks in MnO2.

importance in all energy storage devices because that will produce unwanted heat during charge–discharge process. However, this lowering of ohmic drops may be due to the non-covalent interactions which greatly compensated the resistance and promoted the electrochemical activity in MWCNT—COOH, MnO2, and PANI through the utilization of the pseudo-capacitive process. The specific capacitances of all specimens were calculated (see Table 5) from the discharging phase of the curve in Fig. 5B. It was observed that the pure MWCNT shows the specific capacitance of 83.67
13.4 F/g, whereas the PCNAM composite exhibits a six-fold higher specific capacitance of 517.13
15.25 F/g at 1 mA constant current. Some previous works reported other PANI based composites like activated carbon/PANI, porous carbon/PANI, carbon nanofibers/PANI, SWCNT/PANI, and MWCNT/PANI which exhibited capacitance values of 380, 180, 264, 485, and 322 F/g, respectively [40]. Herein, in our work the new PCNAM composite shows significant improvement in specific capacitance

(517.13
15.25 F/g), which is higher than the previously reported values indicating a possible application of the composite as a supercapacitor electrode. A maximum energy density of 71.88
2.12 W h/kg has been obtained for PCNAM composite electrode, while an energy density of 11.63
1.86 W h/kg has been obtained for pristine MWCNT based electrode. This higher energy density of PCNAM may be attributed to the fast charge–discharge process in presence of MnO2 in the composite. Also, the PCNAM electrode gives a maximum power density of 10.08
0.26 kW/kg, whereas for pure MWCNT electrode it falls to 4.8
0.19 kW/kg. The low power density of MWCNT over PCNAM can be understood from the obtained equivalent series resistance (ESR) (calculated from the intercept along X-axis) values from the EIS graph (Fig. 5C.) of pristine MWCNT electrode that showed a large ESR of 26 V and PCNAM that showed a lower ESR of 13.6 V. This large improvement in energy and power density for PCNAM composite electrode compared to pristine MWCNT electrode could be attributed to low

Table 3 Analysis of peak shifting of C¼C bond and COOH group in XPS spectra. Peak assignment

Peak value in pristine MWCNT—COOH Peak value in the composite PCN Peak value in the composite PCNA Peak value in the composite PCNAM (eV) (eV) (eV) (eV)

C¼C COOH

284.08 288.41

282.32 –

282.3 286.57

282.08 286.57

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Table 4 Analysis of peak shifting of Mn2p3/2 and Mn2p1/2 in XPS spectra. Peak assignment

The observed value of BE of pure MnO2 (eV)

The observed value of BE of MnO2 in the composite (eV)

Mn2p3/2 Mn2p1/2

642.0 653.51

641.38 652.8

contact resistance arising from the well-adhered interface due to various kinds of non-covalent interactions. 3.2.3. Analysis of proposed mechanisms The pure MWCNT exhibits nearly rectangular CV curves which indicates that the charge storage process in MWCNT is through the formation of EDL. In all the composites (PCN, PCNA, PCNAM), the electrochemical behavior arises from both the mechanisms, namely; double layer phenomenon due to the MWCNT, and

pseudo-capacitance phenomenon due to the presence of PANI, —COOH group, and MnO2. It is known that when the EB (emeraldine base) form of PANI is doped by electrolyte ions that form emeraldine salt (ES), it becomes highly electrically conducting. In PCNA composite, the faradaic reactions occur between the PANI and the ions of the electrolytes, in this case reactions occur between the K+ ions and the nitrogen atoms of the PANI backbone [41,42] since nitrogen functional groups have basic characters (Fig. 6).

Fig. 5A. Cyclic voltammograms of (a) pure MWCNT, (b) PCA composite, (c) PCNA composite, (d) PCNAM composite, and (e) comparison of cyclic voltammograms of MWCNT, PCN, PCNA, and PCNAM at a scan rate of 100 mV/s in 1 M KCl electrolyte.

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1.0

MWCNT PCN

PCNA

100

PCNAM

90 80

MWCNT PCA PCNA PCNAM

70 -Zimag (ohm)

Potential (E/V)

0.8

0.6

0.4

60 50 40 30 20

0.2

10 0

500

1000

1500

2000

0

2500

Time (t/s)

20

Fig. 5B. Typical cyclic charge–discharge graphs of (a) pure MWCNT, (b) PCN composite, (c) PCNA composite, and (d) PCNAM composite at a constant current of 1 mA in an aqueous 1 M KCl electrolyte.

Further improvement in electrochemical properties can be obtained from pseudo-capacitance contribution that is induced by the functional groups. In the case of the PCNA composite, in addition to the charge storage mechanism that is operative in PCN composite, the MWCNT—COOH dissociates to produce H+ ions, and MWCNT—COO ions which then absorb the K+ ions from the electrolytes (KCl) undergoing redox reactions. The released H+ ions (in addition to the ions from electrolyte) are absorbed by PANI thereby increasing conduction. The reverse process probably occurred during the discharging step whereby the PANI chains translated into emeraldine form upon de-doping of the ions (Fig. 6). Furthermore, the increased hydrophilicity due to the presence of polar —COOH functional groups in the PCNA composite improved the electrode/electrolyte contact which facilitated ion accumulation at the electrode. In the PCNAM composite, in connection to the above mechanism, the dissociated H+ ions (from MWCNT—COOH) and electrolyte ions can react and interact with MnO2 and PANI in PCNAM resulting in pseudo-capacitance. As the concentration of H+ ions is much lower than the electrolyte ions, it is the electrolyte ions that will play the prime role in the charging– discharging processes. In the pristine MnO2-based electrodes, we believe that the mechanism for the charge-storage process can be ascribed to the intercalation of alkali metal cations (such as K+) or H+ within the electrode during charging step and de-intercalation during discharging step. The proposed mechanism is supposed to involve a redox transition between the Mn4+ and Mn3+ states that is known [41,42] to be present in MnO2. In our study, the enhancement of capacitance in PCNAM composite can be attributed to the strong attachment between MnO2 and PANI chains by metal–p interaction. This leads to a reduction in the oxidation state through the sharing of charges by a fraction d (i.e., Mn(4d)+ reduced to Mn(3d) + ) which facilitates subsequent redox reactions. This lowering of oxidation state increases the basicity of MnO2, which will enhance the rate of redox reactions with the H+/electrolyte ions producing large pseudo-capacitance. The mechanism is shown below:

40

60

80

100

120

140

160

Zreal (ohm) Fig. 5C. Impedance spectroscopy study of pure MWCNT and PCN composite, PCNA composite and PCNAM composite.

MnO2 þ y

Basicityincreasing

!

Mnð4dÞþ O2 þ ydþ

where y = C¼C bonds or N atoms in PANI and d = amount of charges being shared by MnO2. Mnð4dÞþ O2 þ Xþ þ e $MnOOX; h ih i where; X ¼ Kþ ; Hþ Mnð4dÞþ Mnð3dÞþ

From the above study, it is apparent that the following charge storage mechanisms are involved in the electrode materials: (a) In pristine MWCNT, the capacitance originates through the

double layer formation. (b) In PCN composite, the MWCNT mainly confers double layer

capacitance while PANI mainly produces pseudo-capacitance through faradaic reactions between PANI and the ions of the electrolytes. (c) In PCNA composite, along with the mechanisms presented in PCN composite, further improvement in overall capacitance was obtained through increased pseudo-capacitance induced by the ionizable —COOH groups. Moreover, the increased hydrophilicity as a result of the polar —COOH functional group also significantly improved the electrode/electrolyte contact facilitating the accumulation of ions. (d) In addition to the above mechanisms, in PCNAM composite which contained MnO2, it is believed that interaction between PANI and MnO2 will caused the oxidation state of MnO2 to be reduced by a certain fraction d (due to sharing of some charges), i.e., Mn(4d)+ will reduce to Mn(3d)+, so that subsequent redox reactions will occur readily. Furthermore, this lowering of the oxidation state will increase the basicity of MnO2 and hence will expedite the rate of redox reactions.

Table 5 Specific capacitance, specific energy density, and specific power density values of MWCNT, PCN composite, PCNA composite, and PCNAM composite. Sample

Specific capacitance (Csp) (F/g)

Maximum specific energy density (Esp) (W h/kg)

Maximum specific power density (Psp) (kW/kg)

MWCNT PCN PCNA PCNAM

83.67
13.40 177.3
11.69 299.18
22.64 517.13
15.25

11.63
1.86 24.64
1.62 41.58
3.14 71.88
2.12

4.8
0. 19 6.41
0.07 7.62
0.28 10.08
0.26

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Fig. 6. Charge–discharge mechanism in PANI component in the composite.

3.2.4. EIS Study EIS analysis which has been acknowledged as one of the principal methods to examine the impedance behavior in the mesoporous electrode materials for supercapacitors [1,43]. Fig. 5C. shows the Nyquist plots obtained in an aqueous 1 M KCl electrolyte at the frequency range from 100 kHz to 1 Hz. The Nyquist impedence plots contain two segments, namely; a lower frequency region on the upper right portion, and a high frequency region on the lower left portion of the plot. All the curves (see Fig. 5C.) display almost similar low frequency impedance spectra but exhibit different profiles in the high frequency region. The impedance spectrum of the PCNAM composite which did not exhibit a semicircle in a high frequency region and has a spike in a low frequency region demonstrates good electrode/electrolyte contact. The intercept at Zreal along the X-axis at very high frequencies is known as the ESR. This resistance originates from a combination of: (a) ionic resistance of the electrolyte within the electrode, (b) intrinsic resistance of the material, and (c) contact resistance at the active material/current collector interface [44]. It is apparent that the PCNAM sample had the lowest ESR which indicates that there is efficient charge transfer in this composite. The slope of the middle portion (i.e., spike) of the curve which is known as the Warburg resistance (W) is a reflection of the frequency dependence of ion diffusion/transport process [44]. Ideal supercapacitors have a vertical impedance curve with respect to the Y-axis. The greater the deviation from the vertical axis, the larger is the Warburg resistance. It can be seen from Fig. 5C. that the PCNAM sample has slightly lower Warburg resistance which indicates shorter ion diffusion path lengths in this composite. Hence, the PCNAM composite shows better overall impedance characteristics which are crucial for high performance supercapacitor. 3.2.5. Cyclic stability study Excellent capacitance retention over a number of charge– discharge cycles is a crucial parameter for the long lasting powerdelivery supercapacitors. It has been shown that porous carbon has excellent cycling stability because of the predominance of EDLCs, and the non-existence of the pseudo-capacitance effect [45]. The

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pseudo-capacitive components in the composite electrodes impart poor cycling stability [46] due to the occurrence of reversible redox reactions which partially seals the pores in the nanostructures caused by the swelling/disintegration of materials. Such behavior results in an increased series resistance and deterioration of capacitance. The cycle life of both conducting polymers and metal oxides (both are pseudo-capacitive materials) is shorter than carbon based materials (double layer capacitive material). This is because the structure or volume changes during redox reactions leads to the continuous disintegration of active pseudo-capacitive materials. Therefore, a combination of pseudo-capacitive materials and double layer capacitive material is a very effective way to improve their cyclic performance [45–47]. In pure MWCNT, the retention of capacitance was 92.24% after 1000 cycles whereas for the composites PCN and PCNA, the capacitance retention after 1000 cycles were 90.63 and 90%, respectively. The slightly higher cyclic stability in MWCNT was due to the involvement of EDL mechanism and absence of redox mechanism. In the PCNAM composite, the capacitance reduced by about 10% of the initial capacitance after 1000 cycles (Fig. 5D.), demonstrating good electrochemical stability of this electrode material. It is apparent that the cycling stabilities in all the composites are nearly similar. The reduction of capacitance after 1000 cycles in the composite is probably due to the loss of adhesion of some active material with the current collector or swelling of polymer during charge/discharge cycling [48]. Over the cyclic study, some volume expansion/contraction occurs due to swelling/ de-swelling of the PANI that led to the collapse of the PANI backbone during long-term charge/discharge cycles. This makes the insertion/de-insertion of ions involved in the charge/discharge process more difficult at higher number of cycles which eventually may lead to the capacity loss with cycling. 3.3. TGA study In order to study the thermal stability of the novel composites, TGA study was carried out. Fig. 7 shows the TGA thermograms for all the specimens over a temperature range of 100–800  C. The weight loss for pure MWCNT is almost negligible over the entire temperature range. From 100–200  C, it can be seen from Fig. 7 that there was little or no weight loss in the composite specimens indicating excellent stability in this region. From 200–300  C, the percent weight loss of the composite samples started to increase drastically due to thermal degradation. The rate of percentage weight loss in all the composite specimens (PCN, PCNA, and

Fig. 5D. Cyclic stability study of MWCNT, PCN, PCNA, and PCNAM composites.

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100 a

Weight (%)

90 80 a) MWCNT b) PCN c) PCNA d) PCNAM

70 60

b c d

50 100

200

300

400

500

600

700

800

900

o

Temperature ( C) Fig. 7. TGA graphs of (a) pure MWCNT, (b) PCN composite, (c) PCNA composite, and (d) PCNAM composite.

PCNAM) was similar. From 300–800  C, the PCNAM composite exhibited slightly poorer thermal stability compared to the other two composite specimens probably due to the presence of MnO2 which possess a lower thermal stability than MWCNT. 4. Conclusions In conclusion, it has been shown that the pseudo-capacitive components in the composite can play a significant role in enhancing overall capacitance. It was established that significant interactions through p–p, n–p, and metal–p interactions between the constituent components in the composite greatly assist to enhance the electrochemical performance. Among all the composites studied, the PCNAM composite showed the best electrochemical performance due to the presence of the pseudocapacitive components in the composite. The existence of significant interactions (viz. p–p, n–p, and metal–p) in the composite was confirmed by the FTIR and XPS analysis. This study has shown that the utilization of non-covalent interactions provides an important aspect for the fabrication of high energy based electrochemical energy storage devices. Therefore, the PCNAM composite can be considered as a potential candidate for the high performance supercapacitor applications. References [1] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Springer, 1999. [2] M. Winter, R.J. Brodd, What are batteries, fuel cells, and supercapacitors? Chem. Rev. 104 (2004) 4245–4270. [3] A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role in supercapacitors, J. Power Sources 157 (2006) 11–27. [4] F. Wang, S. Xiao, Y. Hou, C. Hu, L. Liu, Y. Wu, Electrode materials for aqueous asymmetric supercapacitors, RSC Adv. 3 (2013) 13059–13084. [5] F. Picó, J.M. Rojo, M.L. Sanjuán, A. Ansón, A.M. Benito, M.A. Callejas, W.K. Maser, M.T. Martínez, Single-walled carbon nanotubes as electrodes in supercapacitors, J. Electrochem. Soc. 151 (2004) A831–A837. [6] K.H. An, W.S. Kim, Y.S. Park, Y.C. Choi, S.M. Lee, D.C. Chung, D.J. Bae, S.C. Lim, Y. H. Lee, Supercapacitors using single-walled carbon nanotube electrodes, Adv. Mater. 13 (2001) 497–500. [7] E. Frackowiak, K. Metenier, V. Bertagna, F. Beguin, Supercapacitor electrodes from multiwalled carbon nanotubes, Appl. Phys. Lett. 77 (2000) 2421–2423. [8] D. Qu, Studies of the activated carbons used in double-layer supercapacitors, J. Power Sources 109 (2002) 403–411. [9] H.S. Nalwa, Handbook of Organic Conductive Molecules and Polymers: Conductive Polymers: Spectroscopy and Physical Properties, Wiley, 1997. [10] Y.-Z. Long, M.-M. Li, C. Gu, M. Wan, J.-L. Duvail, Z. Liu, Z. Fan, Recent advances in synthesis, physical properties and applications of conducting polymer nanotubes and nanofibers, Prog. Polym. Sci. 36 (2011) 1415–1442.

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