Controlled growth of polypyrrole microtubes on disposable pencil graphite electrode and their supercapacitor behavior

Electrochimica Acta 324 (2019) 134875

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Controlled growth of polypyrrole microtubes on disposable pencil graphite electrode and their supercapacitor behavior Subrata Mondal*, N. Aravindan, M.V. Sangaranarayanan Department of Chemistry, Indian Institute of Technology Madras, Chennai, 600036, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 July 2019 Received in revised form 7 September 2019 Accepted 11 September 2019 Available online 14 September 2019

In the present study, we have reported a simple, facile and robust method for the controlled growth of polypyrrole microtubes (PPy MTs) on pencil graphite electrodes (PGE) via potentiostatic deposition without using any templates or structure-directing agents. The crucial role of the supporting electrolytes in dictating the formation of microtubules is analyzed and their nucleation e growth mechanism has been studied using the well-known Scharifker - Hills and Palomer e Pardove models. These microtubules modified pencil graphite electrodes exhibit a satisfactory supercapacitor behavior which has been demonstrated using voltammetric, charge-discharge and impedance studies. It has been observed that, the tubular-like geometry of PPy is more preferable for supercapacitor behavior than other morphology of PPy in the present context. Moreover, the influences of temperature on the specific capacitance and cycle stability have also been investigated. The effect of supporting electrolytes on the supercapacitive behavior of PPy MTs modified PGE have also been analyzed and a significant enhancement in the specific capacitance value with improved rate capability is observed in acidic medium than that of neutral medium. The profound increase in the magnitude of the specific capacitance of the bare pencil graphite electrodes upon modification with PPy MTs has been ascribed to its enhanced surface roughness, fractal dimension and porosity. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Microtubes Supercapacitors Polypyrrole Pencil graphite Voltammetry Ragone plot

1. Introduction Among various electrochemical energy storage devices, supercapacitors (SCs) are of utmost significance because of their high power densities, fast charge-discharge characteristics, satisfactory storage and cycling stability [1e4]. SCs possess higher power densities and energy densities in comparison with batteries and conventional capacitors respectively. Thus, SCs bridge the gap between conventional capacitors and batteries [5,6]. SCs are of two types viz. electrochemical double layer capacitors (EDLCs) and pseudocapacitors. The capacitance of the EDLCs is due to the charge separation at the electrode/electrolyte interface (electrical double layer) [7,8] while, in the case of pseudo-capacitors it is because of faradaic reactions [9]. In order to enhance the specific capacitance of the capacitors, large surface area, suitable porosity and uniform distribution of active materials are generally preferred [10]. In this regard, conducting polymers can be viable active materials due to their tunable

* Corresponding author. E-mail address: [email protected] (S. Mondal). https://doi.org/10.1016/j.electacta.2019.134875 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

physicochemical properties. Moreover, their impressive ability to store charges in the electrical double layer as well as through faradaic reactions resulted in higher specific capacitance than the EDLCs [11e14]. Various conducting polymers such as polypyrrole (PPy), polyaniline, polythiophene, polyindole etc have been widely employed [15e18] for supercapacitors application in the last few decades. In comparison with other conducting polymers, PPy occupies a pivotal place because of its higher electrical conductivity, environment-friendly nature and facile synthesis. Besides, the choice of electrodes plays a crucial role in the supercapacitor performance as adherence and morphology of polymers depend upon the surface properties of the electrode. Amidst this scenario, it is essential to fabricate electrodes which can be easily commercialized, cost effective and exhibit satisfactory supercapacitor performance. Furthermore, the electrolytes employed for electropolymerization and their influence on the specific capacitances need to be systematically investigated. In this context, the inexpensive disposable pencil graphite electrodes (PGEs) can serve as a suitable base material for the deposition of PPy and the PPy modified electrode is anticipated to be a potential supercapacitor due to the combinations of several

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features such as wide availability, low cost, wide potential window and interesting electrochemical properties [19,20]. Moreover, PGE possesses higher conductivity as all the carbons in graphite are sp2 hybridized rendering it more superior than other carbon based electrodes [21]. It was shown that the physical and electrochemical performance of the PGE is comparable to that of the commonly employed glassy carbon electrodes. Although modified PGEs have been studied in various applications viz., for detection of DNA and RNA [22e24], sensing of metal ions, glucose, hydrogen peroxide etc. [25e27], their efficacy as supercapacitors has not been explored in detail [28]. In the present study, the synthesis of polypyrrole microtubes on PGE (PPy MTs/PGE) using a potentiostatic deposition in HClO4 solution is reported and the efficacy of the modified electrode as a supercapacitor is analyzed. The effect of electrolytes upon the morphologies has been investigated by carrying out electropolymerization in HCl, H2SO4, HNO3, LiClO4 and KClO4. The growth mechanism of PPy MTs on the PGE surface has been studied using Sharifker-Hills and Palomer-Pardave models. Furthermore, the supercapacitor performance of PPy MTs/PGE has been analyzed using cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge studies. The effects of potential, deposition time and temperature on the supercapacitive behavior of PPy MTs/PGE have also been investigated. Moreover, the crucial role of supporting electrolytes on the supercapacitive performance of PPy MTs/PGE has been explored in both acidic (0.5 M H2SO4) and neutral medium (0.1 M KClO4). 2. Experimental section

m¼

Q M  F z

(1)

where Q denotes the charge involved during the polymerization and is obtained by integrating the area under the chronoamperograms. M and z represent respectively the molar mass of pyrrole (M ¼ 67.09 g/mol) and number of electrons (z ¼ 2) involved in the polymerization while F represent the Faraday's constant (F ¼ 96500 Cmol-1). After deposition, the electrodes were washed repeatedly with distilled water and dried at room temperature. The mass deposited on the electrode surface was calculated from the Faraday's empirical formula. In addition, the mass of PPy on the electrode surface was measured using a weighing balance. The calculated mass from the chronoamperogram was 0.138 mg while the measured mass (from the weighing balance) was 0.145 mg. Hence the calculated weight has a well agreement with the measured weight of PPy with an error of ~4%. However, we have used the mass of PPy as 0.138 mg throughout the measurements for convenience. The supercapacitor studies of PPy coated PGE (PPy/PGE) were investigated in 0.1 M KClO4 solution and 0.5 M H2SO4 solution. Cyclic voltammetry and galvanostatic charge-discharge were employed to estimate the specific capacitance of PPy MTs/PGE while the electrochemical impedance spectroscopy (EIS) was carried out for the evaluation of time constant (t). A potential window of 0 Ve0.6 V was chosen for the cyclic voltammetry. Nyquist plots were obtained at different potentials in the frequency range of 105 to 101 Hz with amplitude of 5 mV. The galvanostatic chargedischarge experiments were carried out at various current using two PPy/PGEs which serve as the working and counter electrodes.

2.1. Chemicals Pyrrole (SRL Chemicals, India) was distilled under nitrogen atmosphere at reduced pressure and stored in the refrigerator in an amber bottle. Perchloric acid, sulphuric acid, nitric acid, hydrochloric acid from SRL Chemicals, India was employed without further purification. Lithium perchlorate and potassium perchlorate from Sigma Aldrich were used as received. Triply distilled water was used to prepare the solution and for all the electrochemical measurements.

2.3. Instrumentation The scanning electron microscope (SEM) analysis was carried out using FEI Quanta FEG 200 while the transmission electron microscope (TEM) images were acquired using TECNAI G2F30STWIN. The Fourier transform infrared (FTIR) spectrum of PPy was recorded using PerkinElmer spectrophotometer with the aid of KBr pellet. 3. Results and discussion

2.2. Electrochemical measurements The polymerization of pyrrole was carried out in an one compartment cell with three-electrode configuration wherein the pencil graphite electrode (Faber Castell, 0.5 HB) was the working electrode for the deposition of PPy while the saturated calomel electrode (SCE; Bioanalytical systems, USA) and graphite rod serve as the reference and counter electrode respectively. All the potentials are reported here with respect to SCE. The diameter of the PGE is 0.025 cm, its length being 2 cm. The electrochemical measurements were carried out using CH 660A electrochemical workstation (CH Instruments, USA) at a temperature of 25 ± 1
C. The deposition of PPy was performed at various potentials (0.70, 0.75 and 0.80 V) and durations (ranging from 5 to 180 s) in 0.1 M HClO4 solution containing 0.15 M pyrrole. Furthermore, for a fixed time (30s or 60s) and potential (0.75 V) PPy was deposited from various solutions such as HCl, H2SO4, HNO3, LiClO4 and KClO4 of 0.1 M concentration containing 0.15 M Pyrrole. The amount of PPy deposited (m) on PGE was estimated from the equation [29].

3.1. Characterization of PPy MTs and mechanistic investigation for the formation of MTs Fig. 1 depicts the SEM images of PPy deposited from 0.1 M HClO4 on PGE wherefrom the formation of PPy MTs is inferred. These MTs are vertically well aligned with high spatial density and uniform distribution. Although the uniform distribution of these PPy MTs occurs, their size is not homogeneous and may be attributed to the corrugated nature of the PGE surface (Fig. 1A). The effect of applied potential on the morphology was also investigated and no significant changes are noticed (Fig. 1B, C and D). Essentially, at all the applied potentials (i.e. 0.7 V, 0.75 V and 0.8 V) PPy MTs is generated, however, their length increases with the applied potentials (Fig. 1). In order to further comprehend the crucial role played by HClO4 on the formation of tubular morphologies, the polymerization was carried out using different acids (HCl, HNO3 and H2SO4) and their corresponding SEM images are shown in Fig. 2(A, B and C). The formation of aggregated globular shaped PPy is noticed for all the acids from the SEM images and implies the significant influence of

S. Mondal et al. / Electrochimica Acta 324 (2019) 134875

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Fig. 1. Typical Scanning Electron Micrographs of (A) bare PGE and (B, C and D) PPy deposited on PGE at different applied potential of 0.70, 0.75 and 0.80 V respectively. The PPy MTs in Figure B to D were deposited in 0.1 M HClO4.

Fig. 2. Typical Scanning Electron Micrographs of PPy deposited on PGE using different supporting electrolytes at a constant potential of 0.75 V for 60s. In all the cases, the concentration of supporting electrolyte was 0.1 M.

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supporting electrolytes (counter anions) in dictating the tubular morphology. To elucidate this further, electropolymerization was carried out using the same counteranion (i.e. ClO 4 ) but with different cations. Fig. 2D and E depicts the SEM images of PPy as prepared in the presence of KClO4 and LiClO4 and tubular morphologies along with spheres are observed in both cases. However, the use of HClO4 yields the uniform distribution of PPy tubes as mentioned earlier while KClO4 and LiClO4 lead to the PPy tubes along with spheres. Hence it follows that ClO 4 ions play an active role for the formation of tubular morphologies of PPy. The morphologies of polymeric materials depend upon the rate of polymerization which generally varies with the type of counter anions present in the reaction medium. It has been reported that smaller anions such as SO2 4 ,  NO 3 , Cl etc. have higher mobility and may interact strongly with the polarons of the PPy in comparison with larger anions such as  ClO 4 , CF3COO etc [30]. This leads to the variation in the rate of formation of PPy as well as the mechanism of nucleation. Because of the larger sized ClO 4 , nucleation and growth of PPy is slower in comparison to other electrolytes thereby forming ordered PPy MTs. On the other hand, the faster polymerization kinetics in HCl, H2SO4 and HNO3 results in the formation of compact and globular shapes. The PPy as deposited on PGE was further subjected for transmission electron microscope (TEM) analysis and TEM images as depicted in Fig. 3(A, B and C) also show the tubular morphology of PPy. From the TEM images, it can be clearly seen that the diameter of the PPy MTs are greater than 1 mm while the length being 15e20 mm. However, the diameter of the PPy MTs varies from the top to bottom of the microtube. The spectral characterization of PPy MTs was carried by Fourier transform infrared spectroscopy (FTIR)

measurement with the aid of KBr pellet. Fig. 3D depicts the FTIR spectrum of the PPy in which the broadband appear at around 3377-3150 cm1 is attributed to the NeH stretching vibration of PPy. The peak at 2795 cm1 is due to CeH stretching vibration. The absorption band at 1634 cm1 is corresponding to the C]C ring stretching of pyrrole while the CeN stretching vibration is observed at 1324 cm1. The bands at 835 cm1 and 782 cm1 are due to CeH out of plane ring deformation vibrations and CeN bending deformation vibration respectively. The presence of several bands corresponds to the different functional group of PPy MTs consistent with the reported FTIR studies of PPy [31,32]. In order to elucidate the mechanism of formation of tubular shaped PPy, the time-dependent growth was studied with the aid of SEM analysis. At lower deposition times (5 s), the micro containers like morphologies of PPy is forming as can be seen from Fig. 4. These micro containers act as active sites for further polymerization. The formation of these micro containers can be comprehended by invoking the concept of the bubble formation on the electrode as reported in earlier studies [33,34]. As inferred from the polymerization mechanism, the deposition of PPy occurring at the working electrode is an oxidative reaction. Simultaneously the complimentary reaction is the reduction of Hþ ions at the counter electrode. These electrode reactions can be summarized as follows:

Anode: nPy/ðPyÞn þ 2nHþ þ 2ne Cathode: 2nHþ þ 2ne /nH2 During the deposition of PPy, a higher extent of gas evolution (bubble formation) was observed at the counter electrode. This larger amount of gas evolution is ascribed to higher surface area of

Fig. 3. Transmission electron microscope (TEM) images of PPy Microtubes as deposited on PGE (A, B and C). The FTIR spectra of PPy MTs (D).

S. Mondal et al. / Electrochimica Acta 324 (2019) 134875

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Fig. 4. Schematic depiction of the hierarchical growth of PPy at different deposition times and corresponding SEM images.

the counter electrode and acidic condition. These gas bubbles get dispersed into the solution and assemble at the working electrode due to the applied potential. This leads to the formation of micro container-like shapes at the beginning of the deposition process. As the deposition time increases, polymerization continues on the orifice of the initially formed containers (Fig. 4). However, the diameters of the newly formed containers decrease with time. This can be interpreted as follows: Since the polymerization occurs on the orifice of the initially formed containers, the surface area available for deposition of PPy is lower than for the bare surface. The successive decrease in the cavity diameters of the PPy microcontainers occur due to reducing concentration of pyrrole monomers with time and the active space available for deposition. The stepwise growth of containers on the pre-existing container finally culminates in the formation of micro tubes like morphologies. This interpretation is consistent with the SEM images shown in Fig. 4. In order to gain further insights into the nucleation and growth process of PPy MTs, nucleation and growth models were used. As a preliminary investigation, the three dimensional Sharifker e Hills (SH) model [35] was invoked. According to this model, two nucleation mechanisms (instantaneous and progressive) exist while the growth of polymers occurs in two or three-dimensional pathways. In the instantaneous nucleation mode, the number of nuclei formed remains constant while the growth occurs from their pre-existing nuclei on the bare surface without generation of any new nuclei. Thus, the sizes of the deposits are larger with inhomogeneous distribution. On the other hand, progressive nucleation leads to the growth of the nuclei from their former positions along with the generation of new nuclei, thus resulting in a homogeneous distribution. In the case of three-dimensional growth, the growth rates of nuclei at the parallel as well as perpendicular directions to the electrode are nearly same. But in the case of two-dimensional growth mechanism, the growth is more rapid in the parallel direction in comparison with the perpendicular direction until they encroach each other [36]. In the SH model, the type of nucleation and growth mechanism followed by the system is deduced by comparing the experimental chronoamperograms (Fig. 5) with the dimensionless plots obtained from the theoretical equations for instantaneous and progressive nucleation equations [35].


Instantaneous:

I

2 ¼

Imax

1:9542 t

tmax


2 t 1  exp  1:2564 tmax (2)


Progressive:

I Imax

2 ¼

1:2254 t tmax

(

"

2 #)2
t 1  exp  2:3367 tmax (3)

where I and Imax denote the current at t and tmax respectively. It is observed from Fig. 5A that the polymerization process follows a mixed two- and three-dimensional growth pathway [36] according to SH model. Initially, the nucleation of the oxidized monomer occurs instantaneously within a short duration of time (~1 s) as inferred from Fig. 5A. However, the time required for the completion of nucleation varies with the counter anions present in the reaction system. It can be deduced from Fig. 5A that the nucleation of PPy follows the instantaneous mechanism in the case of HClO4. Subsequently, the growth of PPy occurs through conversion of oligomersinto polymers on the electrode surface. Although the nucleation data pertaining to PPy MTs fits well with the SeH model, the growth pattern does not obey any theoretical predictions. Hence, to account for this variance an alternate approach viz. the Palomer-Pardave model [37,38] was analyzed. According to this model, a hemispherical three-dimensional nucleation and growth process [current density ¼ J3D(t)] occurs in conjunction with the electrochemical oxidation of pyrrole [current density ¼ JPO(t)]. The total temporal current density Jtotal(t) consists of (i) [J3D(t)] i.e. the oxidation of pyrrole on the bare electrode surface, formation of oligomers followed by the nucleation and growth of PPy and (ii)[JPO(t)] i.e. oxidation of the pyrrole on the PPy coated surface. Hence the total current density may be represented as [37,38].

Jtotal ðtÞ ¼ J3D ðtÞ þ JPO ðtÞ i.e

(4)

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Fig. 5. Comparison of the experimental potentiostatic response pertaining to the deposition of PPy in HClO4 solution with that of (A) Sharifker e Hills and (B) Palomer-Pardave models.

1  
 1  expðP3 tÞ 12 A @ 1  exp  P2 t  Jtotal ðtÞ ¼ P1 þ P4 t P3 0

(5) where,


P1 ¼

2c0 M

1=2

pr 0

P2 ¼ N0 pk D;

zPO FkPO

0

k ¼ ð8pc0 =rÞ1=2

P3 ¼ r

P4 ¼

zFD1=2 c0

p1=2

In the above equations, c0 denotes the bulk concentration of pyrrole (0.15 M), D being its diffusion coefficient (1.21  105 cm2 s1), r refers to its density (0.967 g/cc), M is its molar mass (67.09 g/mol-1), zPO denotes the charge pertaining to the oxidation of pyrrole, kPO denotes the corresponding rate constant; r refers to the nucleation rate while N0 is the number density of active sites for the nucleation. Fig. 5B depicts the comparison of experimental current transient with the theoretical estimates of equation (5) obtained from non-linear least squares fitting. A satisfactory agreement is noticed, indicating the validity of the Palomer-Pardave model (Fig. 5B). This indicates that the formation of PPy MTs involves two steps. Initially, the three - dimensional nucleation of pyrrole oligomers occurs followed by growth resulting in the formation of micro containers. Further oxidation of pyrrole onto the PPy micro containers leads to the formation of PPy MTs. This can also be confirmed from the SEM images pertaining to the time-dependent growth of PPy MTs (Fig. 4). From the fitted curve, we can easily get the P1, P2, P3 and P4 values and from these values we can estimate the various important parameters viz. kPO, N0, zPO, r etc related to the polymerization process. A table has been provided in supporting information (Table S1) for various parameter viz. P1, P2, P3 and P4 for the polymerization in different supporting electrolyte solution.

3.2. Feasibility of PPy MTs/PGE as supercapacitors The efficacy of PPy modified PGE towards the supercapacitor performance has been analyzed using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. 3.2.1. Three electrode assembly 3.2.1.1. Cyclic voltammetric analysis. Fig. 6 depicts the cyclic voltammogram of PPy MTs/PGE in 0.1 M KClO4 wherein a substantial increase in current (than the bare PGE) is noticed along with the quasi-rectangular feature. It is well known that a large magnitude of current in conjunction with the rectangular shaped, symmetric voltammetric response is a pre-requisite for desirable Supercapacitor behavior [39]. The voltammetric response of the bare PGE is also provided in which the PPy MTs/PGE exhibits the enhancement in current by ~104 times than the bare electrode (inset of Fig. 6). This drastic enhancement in the current response for PPy MTs/PGE is attributed to: (i) tubular morphology of PPy which offers a greater surface area and (ii) porous nature. These two factors result in the facile diffusion as well as fast redox transitions of counter anions into the polymer matrix thereby enabling PPy MTs as more desirable supercapacitors than other geometries. This is validated in Fig. 6 by comparing the cyclic voltammetric response of PPy deposited from other acids viz. HCl, H2SO4 and HNO3 on PGE surface. It can also be inferred from the same that the PPy deposited in HCl and HNO3 exhibited the quasi rectangular feature, the magnitude of current is found to be lower than that of PPy prepared using HClO4. 3.2.1.1.1. Optimization of the experimental parameters. To obtain maximum specific capacitance from PPy MTs/PGE, it is essential to optimize the experimental parameters related to the polymerization process: (i) deposition potential (ii) concentration of HClO4 and (iii) deposition time. During the optimization of experimental conditions, one parameter was varied while keeping the others constant. The deposition of PPy MTs on PGE was carried out at different potentials (0.70 V, 0.75 V and 0.80 V). It is seen from Fig. 7A that PPy MTs deposited at 0.75 V exhibits higher current response as well as a satisfactory symmetric behavior than other deposition potentials. Hence this was chosen as the optimum potential. Subsequently, the concentration of the supporting electrolyte during the deposition of PPy have also been varied while keeping

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Fig. 6. Cyclic voltammetric response of the PPy-coated PGE (as deposited using different acids) in 0.1 M KClO4 (scan rate ¼ 50 mV/s). For comparison, the voltammogram of the bare PGE is also shown under identical conditions.

Fig. 7. The influence of deposition parameters on the cyclic voltammetric response of PPy MTs/PGE in 0.1 M KClO4 solution at the scan of 50 mV/s: (A) deposition potential (B) concentration of the electrolyte (C) deposition time. (D) Bar diagram for comparing the effect of mass loading on the specific capacitance values of PPyMTs/PGE. (E) Cyclic voltammetric response of PPy MTs/PGE in 0.1 M KClO4 at different scan rates. (F) Comparison in specific capacitances (as a function of scan rates) of different PPy samples as deposited using different acids on PGE. (G) Cyclic voltammetric response of PPy MTs/PGE in 0.5 M H2SO4 at various scan rates. (H) Comparison in voltammetric response of PPy MTs/PGE in different electrolyte medium (e.g. 0.1 M KClO4 and 0.5 M H2SO4) at a scan rate of 200 mV/s. (I) Comparison in specific capacitance of PPy MTs/PGE as measured via cyclic voltammetry study in different electrolyte medium as a function of scan rate.

the deposition potential and time constant at 0.75 V and 60 s respectively. A better capacitive performance has been observed for

PPy MTs/PGE prepared using 0.1 M HClO4 solution (Fig. 7B). At higher concentrations of HClO4, the current response decreases and

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may be attributed to the hindered faradaic processes with the increased PPy mass. At higher concentrations, the extent of polymerization is enhanced leading to increase in mass loading of PPy, thereby decreasing the specific capacitance. Furthermore, the supercapacitor performances of PPy MTs/PGE prepared at various deposition times while keeping the other deposition parameters as constant were also analyzed. The corresponding cyclic voltammograms are provided in Fig. 7C wherefrom we infer that the PPy MTs obtained using 60 s deposition time exhibits a superior behavior. Although the current response or area under the voltammogram is large for higher deposition times, in view of the symmetric and quasi-rectangular response, the deposition time of 60 s has been chosen for subsequent studies to avoid the sluggish redox process at higher loading of polymers. The effect of mass loading on the capacitive behavior was also investigated by measuring the specific capacitance at different amount of PPy MTs deposited on PGE (here the amount of PPy MTs was tuned by varying the deposition time). Fig. 7D shows the bar diagram for comparing the specific capacitance value (estimated at the scan rate of 50 mV/s) wherein, the PPy MTs with mass of ~0.13 gm shows higher capacitance value compared to other. Increase in the mass loading leads to increase in electrode thickness which in turn leads to poor ion transfer due to limited ionic conductivity as well as increased ohmic resistance and thereby decreasing the supercapacitive performance. Therefore, PPy MTs deposited in 0.1 M HClO4 solution at an applied potential of 0.75 V for 60 s of deposition has been chosen as optimal mass loading for further studies. 3.2.1.1.2. Specific capacitance using cyclic voltammetry. Fig. 7E depicts the cyclic voltammetric responses of PPy MTs/PGE in 0.1 M KClO4 solution at different scan rates ranging from 5 mV/s to 200 mV/s. The specific capacitance is calculated using the equation [40].

Csc

1 ¼ m  v  ðVa  Vc Þ

Vða

IðVÞdV

(6)

Vc

where n denotes the scan rate; Va-Vc represents the potential window; !I(V) dV was obtained from the integrated area under the voltammogram. The specific capacitances obtained from the above equation decrease with the increase in the scan rates (Fig. 7F). This behavior is consistent with other reported studies [41,42]. This decrease in capacitance with increase in the scan rate may be interpreted as follows: At higher scan rates, the time required to traverse the entire potential window is lower and hence leads to the incomplete redox transition of the counter ions (i.e. lower extent of the diffusional process) at the PPy MTs/PGE surface. Thus, at higher scan rates, the extent of charge-discharge process diminishes yielding lower values. The decrease in capacitance value at higher scan rate may also be due to various factors such as an increase of the active material resistance, active materialelectrolyte interphase resistance and/or long ion-diffusion length etc [43,44]. It is of interest to compute the specific capacitances of PPy/PGE prepared using other acids. All the corresponding voltammogram of the different PPy/PGE in 0.1 M KClO4 at various scan rates are provided in Fig. S1 of the supporting information. Among different morphologies of PPy/PGE, PPy MTs/PGE exhibits a higher specific capacitance indicating the beneficial role played by the tubular shape of PPy in comparison with the globular or cauliflower morphologies (Fig. 7F). For comprehending the role of supporting electrolytes on the supercapacitor behavior of PPy MTs/PGE, we have also performed the voltammetric studies in 0.5 M H2SO4 medium at different scan rates (Fig. 7G). A significant change is observed both in terms of current response and voltammetric

feature in the acidic medium during the cyclic voltammetric analysis of PPy MTs. The quasi-rectangular feature of PPy MTs/PGE is still persisting even at higher scan rate in acidic medium (0.5 M H2SO4) than that of neutral medium (0.1 M KClO4) as can be observed Fig. 7H. The change in specific capacitance value with scan rate for PPy MTs/PGE in 0.5H2SO4 is provided in Fig. 7I wherein the similar trend is observed as mentioned earlier (Fig. 7F). The calculated specific capacitance value in acidic medium is much higher (657 Fgm1) than that of the specific capacitance as measured in neutral medium (477 Fgm1). The decrease in specific capacitance at higher scan rate (200 mV/s) is only 25% in acidic medium which is much smaller than the neutral medium (i.e. 60% decrease) indicating a good rate capability in acidic medium. In this context, it is worth to mention that due to smaller size of counter ion (SO2 4 ) and better conductivity of acidic medium results in better redox transition which in turn responsible for the enhanced the specific capacitance value. To further comprehend the enhanced supercapacitive behavior exhibited by PPy MTs/PGE (prepared in 0.1 M HClO4), the electrochemical active surface area (ECASA) and fractal dimensional analysis were carried out from the voltammetric response of different PPy/PGE in presence of the standard redox probe i.e. K3[Fe(CN)6]. The ECASAs were calculated using the Randels e Sevcik equation whereas, the fractal dimensions were found using the method proposed by Pajkossy and Nyikos [45,46]. The corresponding cyclic voltammograms and calibration plots (between peak current and scan rate) for PPy/PGE prepared in different acids are given Fig. S2 in the supporting information. It is well known that the mass transfer of the modified surface is significantly influenced by its roughness. Thus, a higher mass transfer i.e. enhanced doping/dedoping of the counter anions is anticipated for the modified electrode surface which is having a higher electrochemical active surface area and roughness. It can also be suggested that conducting polymer modified surface with a higher roughness results enhanced supercapacitor behavior. Herein, the surface properties related to self-similarity is characterized using the fractal geometry approach. Pajkossy and Nyikos [45,46] demonstrated that the fractal dimension of the modified surface is related to the current e voltage curves, wherein, they have observed that there is a linear dependence between the peak current (Ip) in cyclic voltammograms and the corresponding square root of scan rates (n). Thus for a diffusion controlled redox transition such as [Fe(CN)6]3-/4-, which occurred via diffusion of an electroactive species to a target surface, followed by heterogeneouselectron transfer, the peak currents are related to the fractal parameter (a) as follows [47e49]:

Ip fna

(7)

Then, the fractal dimension (Df) can be obtained as:

Df ¼ 2a þ 1

(8)

By plotting the log Ipvs. log n, the fractal parameter can be estimated from the slope of the fitted linear curves. In order to utilize this method, the ohmic losses must be negligible. Df values higher than 2 implies rough three-dimensional electrode surfaces whose macroscopic areas are lower than their microscopic areas [50]. In the case of a flat electrode, the Df value is expected to be 2, corresponding to a fractal parameter of 0.5. Lower values of Df can be attributed to inactive surface regions that lower the electroactive surface area below the equivalent flat area. It can be inferred from Table 1 that PPy-ClO-4 is having higher ECASA which in turn leads to higher roughness and higher fractal dimensional value in comparison with PPy prepared from other supporting electrolytes. Interestingly, there exists a strong correlation between ECASA,

S. Mondal et al. / Electrochimica Acta 324 (2019) 134875 Table 1 The various parameters related to the different PPy modified electrode (prepared from different electrolyte) as deduced from voltammetric response in K3[Fe(CN)6] solution. Sample

ECASA (cm2)

Roughness Factor

Df

F/g

PPy-ClO-4 PPy-SO24 PPy-ClPPy-NO-3

1.22 0.80 0.54 0.44

3.81 2.49 1.70 1.36

2.62 2.47 2.17 1.86

475.4 382.8 375.4 270.9

roughness factor and Df values to that of supercapacitance exhibited by the PPy modified PGE. 3.2.1.2. Electrochemical impedance spectroscopy. Electrochemical Impedance spectroscopy enables a quantitative elucidation of various system parameters such as charge transfer resistance, double layer capacitance, ohmic resistance etc [51]. The impedance data can be analyzed using the Nyquist or Bode'plots. The Nyquist plot depicts the variation in the real (Z0 ) and imaginary (Z00 ) components of the impedance at various frequencies and information contained in the Nyquist plots is easy to decipher. Fig. 8A depicts the Nyquist plots of PPy MTs/PGE in 0.1 M KClO4 solution at potentials ranging from 0.1 V to 0.6 V wherefrom distorted semi-circles are observed at high frequencies along with linear vertical spikes at lower values. The intercept of the semicircle on the X-axis provides the ohmic or solution resistance (RS) while the diameter yields the interfacial charge transfer resistance (RCT). Furthermore, different applied potentials lead to the variation in the diameter of the semicircle vis a vis RCT values. At higher potentials, RCT increases as shown in Fig. 8A. Besides, the increase in applied potential results in the gradual decrease in the angle made by the low frequencies on the X-axis (real part of the impedance) and a gradual change from the capacitance to Warburg behavior occurs. These results are consistent with the previously reported conducting polymers based supercapacitors [52,53]. In order to estimate various system parameters, the classical Randles circuit has been modified here by replacing the double layer capacitance and Warburg impedance with two constant phase elements (CPE1 and CPE2) as shown in Fig. 8B. These constant phase elements represent the capacitance arising from either from inhomogeneity of the electrode surface or interfacial ionic diffusion. Further, the introductions of CPEs reflect (i) a distribution of the relaxation

9

Table 2 System parameters deduced by fitting the Nyquist plots to the equivalent circuit along with the estimated time constants for PPy MTs/PGE in 0.1 M KClO4. Potential (V)

Rs (U) Rct (U) CPE1 (U1 s)

h1

CPE2 (U1 s) h2

t ms

0.1 0.2 0.4 0.6

29.42 29.39 29.12 29.40

0.432 0.436 0.481 0.559

0.0355 0.0307 0.0452 0.0389

1.35 1.08 1.08 2.28

10.89 16.59 20.36 23.27

0.00965 0.00340 0.00197 0.00156

0.85 0.85 0.9 0.9

times as a result of inhomogeneities existing at theelectrodeeelectrolyte interface, (ii) the nature of the electrode and (iii) porosity [54]. The time constants (t ¼ 1/2pf*) are calculated from the frequency (f*) corresponding to the maximum in the Nyquist plotsand shown in Table 2. The mean time constant of ~2 ms indicates the fast charge discharge characteristics which are essential for practical applications. Table 2 provides the circuit parameters and the time constants.

3.2.2. Two electrode assembly It is well known that the use of three-electrode assembly overestimates the specific capacitances [13]. Hence, it is preferable to use a two-electrode system to obtain reliable estimates.

3.2.2.1. Galvanostatic charge-discharge analysis. Galvanostatic charge-discharge experiments were carried out to compute the specific capacitances of PPy MTs/PGE using the two electrodes assembly wherein PPy MTs/PGE serve as both working and counter electrodes. Fig. 9A depicts the typical charge-discharge data for PPyMTs/PGE in 0.1 M KClO4 at different current densities (0.08 mAcm2 to 1.20 mAcm2). The chronopotentiogram representing the charging and discharging behavior are not strictly straight lines indicating the influence of faradaic reactions [55]. However, the symmetric nature of the charging and discharging curve signifies the facile supercapacitor behavior of PPy MTs/PGE. Furthermore, the charge discharge experiment was also carried out in 0.5 M H2SO4 solution and a symmetric behavior was noticed analogously (Fig. S3). The specific capacitance of PPy MTs/PGE in different electrolyte medium is estimated from the charge discharge experiment using the equation [56].

Fig. 8. (A) Nyquist plots of PPy MTs/PGE in 0.1 M KClO4 at different potentials in the frequency range of 105 to 101 Hz with an amplitude of 5 mV and (B) The modified Randles circuit employed for fitting the Nyquist plots. The points denote the experimental data while the line indicates the fitting. The inset in (A) depicts the magnified data at higher frequencies.

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S. Mondal et al. / Electrochimica Acta 324 (2019) 134875

Fig. 9. (A) Galvanostatic charge-discharge studies for the supercapacitor performance of PPy MTs/PGE in 0.1 M KClO4 solution. (B) The variation of the specific capacitance with current densities in different electrolyte medium. (C) and (D) represents the change in specific capacitance with the number of cycles and temperature respectively. The inset in (D) depicts the influence of temperature on chronopotentiograms. The points denote the experimental data while the lines are drawn as guides to the eye.

Csc

I  Dt ¼ 2 m  DV

(9)

where I represent the current, Dt denotes the difference in charging and discharging time, m and DV refer respectively to the mass of PPy and the potential window. Fig. 9B depicts the dependence of the specific capacitance on the current densities in different electrolyte medium. In this case also, the PPy MTs/PGE exhibits higher specific capacitance value in 0.5 M H2SO4 medium in comparison to 0.1 M KClO4 solution. The influence of the current densities may be interpreted as follows [57]. At higher current densities, the ohmic loss is larger as well as insufficient faradic reaction. As the higher applied current density leads to faster discharge (lower discharging time) which in turn decreases the extent of faradic reaction at the PPy MTs coated PGE. Further, the decrease in the specific capacitance with applied current density can also be explained mathematically from equation (9); as the specific capacitance is directly proportional to charging-discharging time interval (Dt) which generally varies with current densities. At higher current densities, Dt is smaller (Fig. 9A) thereby decreasing the specific capacitances. It is noteworthy to mention here that the measured capacitance value from charge-discharge experiment in acidic medium shows a decrease in specific capacitance value of only 18% at higher applied current (i.e. from 316.1 F/g at 0.08 mAcm2 to 259.2 F/g at 1.15 mAcm2) indicating a good rate capability [58,59] of PPy MTs/ PGE. However, in case of neutral medium, the measured

capacitance value decreases to 29% at higher applied current density (i.e. from 212.4 F/g at 0.08 mAcm2 to 151.1 F/g at 1.15 mAcm2). These results clearly emphasize the significant role of electrolyte medium towards the supercapacitor performance of PPy MTs/PGE. Furthermore, this charge-discharge behavior is also employed for deducing the cycle stability of PPy MTs/PGE in 0.1 M KClO4 solution. Fig. 9C depicts the dependence of the specific capacitance on the number of cycles at a typical current density of 0.82 mA cm2. With increase in the number of cycles, the slight change in specific capacitance is observed. While the precise origin of the initial decrease in the specific capacitance is not known [57], it may arise from internal redox transformation of the polymeric materials. It is seen from Fig. 9C that almost 80% stability was retained even after 1200 cycles indicating that PPy MTs/PGE is a suitable supercapacitor [59e61]. Moreover, this is in concordance with other PPy samples (without any composite or other additives) reported wherein the cycling stability of 75e80% was observed after 1000e2000 cycles [60,62,63]. Further improvements in the cycling stability can be easily achieved by employing suitable dopant or composite materials [62,64].

3.2.2.2. Effect of temperature on specific capacitance. We have investigated the influence of temperature on the specific capacitance using charge-discharge experiments. Since temperature influences the kinetics of mass transfer as well as the redox properties of the electrode, the pseudocapacitive behavior gets

S. Mondal et al. / Electrochimica Acta 324 (2019) 134875

altered [65]. The charge-discharge profile for PPy MTs/PGE at different temperatures is shown in the inset of Fig. 9D. The specific capacitance increases with temperature and subsequently reaches a plateau (Fig. 9D). The linear relationship of the specific capacitance with temperature till 300 K is due to the accelerated transport phenomena occurring in the electrolyte solution as well as electrode materials [66]. The diffusion coefficient of the electrolyte (here the anion; ClO 4 ) increases with temperature which enhances the redox transition between the PPy MTs/PGE and electrolyte. The increase in the diffusion coefficient (D) with temperature follows from the Stokes-Einstein equation [67] viz.

D¼

kB T 6phri

(10)

where kB denotes the Boltzmann constant, h is the specific viscosity of the solution and ri denotes the hydrodynamic radius of ClO 4 ions. Furthermore, the viscosity of the electrolytes decreases with increase in temperature enabling a faster diffusion of counter ions thereby facilitates the redox process. Besides at an elevated temperature, the ionic conductivity, solubility parameters, thermal stability changes which may also influenced to the specific conductance value of PPy MTs [68]. However, a temperature of 25
C was chosen here for the synthesis of PPy MTs and analysis of the supercapacitor behavior for convenience.

Fig. 10. Ragone plot describing the energy density and power density of different system along with the supercapacitor behavior of PPy MTs/PGE.

11

For further comprehending the supercapacitor behavior of PPy MTs modified PGE, the Ragone Plot has been constructed by calculating the energy density and power density from the Chargedischarge experiment. The energy density and power density were estimated using the following equations:

E¼

1000Csp DV 2 2  3600

(11)

P¼

3600E Dt

(12)

Ragone Plot as depicted in Fig. 10 shows a satisfactory energy density 10 Whkg1 at the high power density of 1500 Wkg-1 and also maintained the adequate energy density of 8 Whkg1 even at higher power density of 4512 Wkg-1. The present study demonstrates a simple, cost-effective protocol for the growth of PPy Microtubes which found potential candidate towards supercapacitor application. The details investigation by SEM analysis (time dependent growth) and by fitting the experimental data with two different models (Scharifker-Hills and PalomerePardove) discloses the systematic evolution of the PPy MTs. The specific capacitance (Csp) value as obtained in neutral medium from three electrode systems is not very high but the Csp value acquired from acidic medium systems is comparable as well as higher to other reported values as can be observed from Table 3. In this context it is worth to mention that the apart from the morphology or roughness of the PPy, the measured Csp values also depends on the electrolyte medium (i.e. neutral or acidic or basic medium) as the redox transition depend on the solution conductivity, sizes of the counter anions and nature of the electrolytes etc. Here it is essential to emphasize an earlier study on PPy modified PGE wherein the specific capacitance was 780 F/g [28]. However, the present synthetic protocol is simple and robust in view of the preparation since the former study involves the utilization of surfactant and acids in conjunction with non-aqueous medium during the synthesis of PPy. Though the substantial improvement in Csp value is not achieved in the present study, the facile protocol as well as systematic investigation for the mechanism of formation of PPy MTs will surely enrich the literature. Besides we have found a fine correlation between the roughness as well as fractal dimension with the specific capacitance of PPy wherein the microtubes shaped possesses greater fractal dimension and thereby enhancing capacitance values. Furthermore, the influence of morphologies on the specific capacitance hitherto has not been systematically analyzed.

Table 3 Specific capacitances of different PPy based electrodes. System

PPy/SS PPy-CA/Ni PANI/GCE PPy/Ti foil PANI-Graphene/GCE PPy/Pt Plate PPy/Graphite Rod PPy MP/GC PANI/Carbon Paper PPy/SS PPy MTs/PGE

Specific Capacitance (F/g) Cyclic Voltammetry (CV)

Galvanostatic Charge-Discharge (CD)

533 433 e 480 ± 50 e 203 340 e e 200 477 (Three electrode) 657 (Three electrode)

528 e 532 e 257 190 e 413 520 e 212 (Two electrode) 316 (Two electrode)

Electrode assembly

Electrolyte for capacitance studies

Ref

Three Three Three Three Three Two Three Three Three Three CV (Three) CD (Two) CV (Three) CD (Two)

0.5 M H2SO4 6 M KOH 1 M H2SO4 1 M KCl 1 M H2SO4 1 M H2SO4 0.8 M HClO4þ 0.5 M LiClO4 0.05 M LiClO4 1 M H2SO4 0.5 M H2SO4 0.1 M KClO4 0.5 M H2SO4

[53] [41] [69] [70] [71] [72] [65] [73] [74] [75] This work

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S. Mondal et al. / Electrochimica Acta 324 (2019) 134875

4. Conclusions A simple and facile potentiostatically controlled protocol for the growth of PPy MTs on inexpensive PGE was reported. The nucleation and growth mechanism of PPy MTs on PGE surface was elucidated using the Sharifker e Hills and Palomer-Pardave models. Furthermore, the crucial role played by the supporting electrolytes on the formation of PPy MTs was investigated. It was inferred that the formation of tubular morphology was significantly influenced by the presence of ClO 4 ions. The efficacy of the PPy MTs/PGEs for supercapacitor applications was demonstrated using cyclic voltammetry, galvanostatic charge-discharge and impedance spectroscopy studies. The specific capacitances of PPy prepared from various acids was found to follow the trend: Csc (PPy-ClO-4)> Csc (PPy-Cl-)> Csc (PPy-NO-3)>Csc (PPy-SO24 ). The influence of temperature on the magnitude of specific capacitances was also investigated using galvanostatic charge discharge techniques with twoelectrode assembly. Further the supercapacitor behavior of PPy MTs/PGE was also investigated in acidic medium wherein a higher specific capacitance value along with good rate capability was observed. Moreover, the enhanced specific capacitance exhibited by PPy MTs/PGE in comparison with other PPy/PGEs was rationalized through higher surface roughness and fractal dimension. Conflicts of interest There are no conflicts of interest to declare. Acknowledgements We thank the reviewers for their valuable comments and suggestion which help us to improve the manuscript. The financial support by the Science and Engineering Research Board (SERB), India is gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134875. References [1] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845e854, https://doi.org/10.1038/nmat2297. [2] B.W. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum, New York, 1999. [3] J.R. Miller, P. Simon, Materials science. Electrochemical capacitors for energy management, Science 321 (2008) 651e652, https://doi.org/10.1126/ science.1158736. [4] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 38 (2009) 2520e2531, https://doi.org/10.1039/b813846j. [5] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797e828, https://doi.org/ 10.1039/c1cs15060j. [6] G.A. Snook, P. Kao, A.S. Best, Conducting-polymer-based supercapacitor devices and electrodes, J. Power Sources 196 (2011) 1e12, https://doi.org/ 10.1016/j.jpowsour.2010.06.084. [7] C. Largeot, C. Portet, J. Chmiola, P.-L. Taberna, Y. Gogotsi, P. Simon, Relation between the ion size and pore size for an electric double-layer capacitor, J. Am. Chem. Soc. 130 (2008) 2730e2731, https://doi.org/10.1021/ja7106178. [8] H. Jiang, P.S. Lee, C. Li, 3D carbon based nanostructures for advanced supercapacitors, Energy Environ. Sci. 6 (2013) 41e53, https://doi.org/10.1039/ C2EE23284G. [9] B.E. Conway, Transition from “supercapacitor” to “battery” behavior in electrochemical energy storage, J. Electrochem. Soc. 138 (1991) 1539, https:// doi.org/10.1149/1.2085829. [10] J.N. Tiwari, R.N. Tiwari, K.S. Kim, Zero-dimensional, one-dimensional, twodimensional and three-dimensional nanostructured materials for advanced electrochemical energy devices, Prog. Mater. Sci. 57 (2012) 724e803, https:// doi.org/10.1016/j.pmatsci.2011.08.003. [11] C. Arbizzani, M. Mastragostino, L. Meneghello, Polymer-based redox supercapacitors: a comparative study, Electrochim. Acta 41 (1996) 21e26, https://

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