Investigation of polyaniline-coated stainless steel electrodes for electrochemical supercapacitors

Synthetic Metals 156 (2006) 244–250

Investigation of polyaniline-coated stainless steel electrodes for electrochemical supercapacitors T.C. Girija, M.V. Sangaranarayanan ∗ Department of Chemistry, Indian Institute of Technology, Madras 600036, India Received 19 July 2005; received in revised form 24 November 2005; accepted 2 December 2005 Available online 30 January 2006

Abstract The specific capacitance of polyaniline (PANI) deposited potentiodynamically on a stainless steel (SS) substrate, in the presence of p-toluene sulphonic acid (PTS) is estimated. Cyclic voltammetric experiments, galvanostatic charge–discharge studies and impedance analysis are carried out in order to investigate the applicability of the system as an electrochemical supercapacitor. Fourier transform infrared (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques are employed for characterization of the electrode. © 2006 Elsevier B.V. All rights reserved. Keywords: Capacitance; Polyaniline; Potentiodynamic deposition; p-Toluene sulphonic acid; Stainless steel

1. Introduction The studies pertaining to the design and development of electrochemical supercapacitors continue to be a frontier area of research in energy conversion and storage devices, on account of significant enhancement in power densities [1–4]. Among a variety of supercapacitors hitherto investigated [5], mention may be made of: (i) metal oxide systems such as RuO2 , (ii) high surface area materials such as carbon and (iii) conducting polymers-based substrates. It is of interest to note that each of the three types possesses desirable characteristics and the essential touch-stone lies in a satisfactory balance between the magnitude of specific capacitance and cost of the materials vis a vis fabrication. Conducting polymers are suitable systems for supercapacitors on account of their ease of processability and excellent reversibility behaviour [6]. These polymers utilise their ␲conjugated backbones to transfer electrical charge from the current collector to the electrolyte. Conducting polymers are referred to as either n-doped (reduced state) or p-doped (oxidised state) relative to their neutral state [7]. The mechanism of charge storage is three-dimensional, since the amount stored is proportional to the extent of electroactive species absorbed on

∗

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0379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.12.006

the electrode [8]. This leads to larger values of specific energy (Wh kg−1 ) and specific capacitance (F g−1 ) in contrast to processes solely dependent on adsorption phenomena. Polymers such as polypyrrole [9,10], polythiophene [11–13], polyaniline (PANI) [14–16], poly(ethylene dioxythiophene) [17], etc., have been investigated on carbon, nickel, stainless steel (SS), etc. Among the conducting polymers, polyaniline has distinct advantages, viz. higher environmental stability, electrical conductivity and easy processability [18,19]. In the present investigation p-toluene sulphonic acid (PTS) is used as a dopant for PANI on SS. It has been demonstrated that the electrodeposition of PANI on stainless steel, reduced significantly the corrosion rate in H2 SO4 media [20]. The mechanism of the protection of corrosion indicates that even before the electrodeposition process, a layer of passivating oxide film is formed on the steel surface and the PANI film in contact with the surface leads to stabilization of the oxide against dissolution and/or reduction process. Sekine et al. [21] and Troch-Nagels et al. [22] found that PANI coatings electrochemically deposited from H2 SO4 , HNO3 , etc., were not able to prevent the corrosion since the resulting PANI films were brittle and powdery on the surface. It was also inferred [23] that PTS medium is convenient for PANI electrodeposition when compared to oxalic acid since the extent of corrosion occurring before deposition of the film can be controlled and corrosion rate diminishes with increase in the current density. Further, from thermal stability point of view – an important feature for polymer-based elec-

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trodes – PTS-doped PANI shows the most stable redox activity even at 180 ◦ C due to the thermally stable structure of PTS. The objectives of this communication are: (i) to estimate the specific capacitance of polyaniline coated on a stainless steel substrate, (ii) to analyse the stability of the PANI-based stainless steel substrates employing Tafel polarization studies, (iii) to construct the equivalent circuit pertaining to the system, for comprehending the mechanism of charge storage and (iv) characterize the morphology of the polymer employing X-ray diffraction (XRD), Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) studies. 2. Experimental Electrochemical measurements were made in a onecompartment cell with a three-electrode configuration using Ag/AgCl (Bioanalytical Systems, USA) as the reference electrode, while a commercially available stainless steel foil (1.5 cm × 1.5 cm, 1 mm thickness) served as the working electrode, Pt foil (Bioanalytical Systems) being the counter electrode. SS foil was polished with emery paper to a rough finish, washed free of emery particles and then air-dried. The composition of SS as deduced from X-ray fluorescence spectroscopy is 25.7% chromium, 4.1% nickel, 0.38% manganese and 0.02% sulphur. Analar grade aniline (SRL Ltd., India) employed in the present work was vacuum-distilled at 120 ◦ C and analar grade p-toluene sulphonic acid (SRL Ltd.) was used without pretreatment. Double-distilled water was used for the preparation of solutions. The electrolyte solution of 0.1 M aniline and 0.5 M PTS was employed for electropolymerisation. The deposition of polyaniline was carried out potentiodynamically wherein the working electrode was subjected to potential cycling between −0.2 and 1.2 V versus Ag/AgCl at a scan rate of 300 mV s−1 for 100 cycles. After deposition, the coated PANI films were rinsed with 0.5 M PTS in order to remove soluble monomeric species. Cyclic voltammetry, galvanostatic charge/discharge experiments, impedance analysis and Tafel studies were performed using an electrochemical workstation CHI 660A (CH Instruments, USA). The impedance measurements were recorded in the frequency range 105 to 10−3 Hz with an excitation signal of 5 mV. All the experiments were carried out at a temperature of 32 ± 1 ◦ C. FTIR spectra of the PTS-doped PANI was obtained using a Perkin-Elmer spectrophotometer. Scanning electron microscopy images were recorded using JEOL JSM-840A instrument. X-ray diffraction pattern of the sample was recorded using a Shimadzu XD-D1 powder X-ray diffractometer using Cu K␣ source.

Fig. 1. Multi cycle voltammogram depicting the electropolymerisation of aniline on SS from 0.1 M aniline + 0.5 M p-toluene sulphonic acid at a scan rate of 300 mV s−1 . Number of cycles = 100. Inset depicts the cyclic voltammogram obtained after first 25 cycles.

layer of PANI is deposited in each sweep between 0.9 and 1.2 V. In the potential range between −0.2 and 0.9 V, the deposited PANI undergoes reversible redox reactions, namely leucoemeraldine/emeraldine transition at 0.2 V and emeraldine/pernigraniline at 0.7 V. Thus, the growth of the deposit takes place layer by layer and each layer becomes electrochemically active before the next layer is deposited. As the sweep rate increases, the amount of PANI deposited at faster sweep rates is activated more efficiently than the polymer synthesized at lower sweep rates. Hence, a scan rate as high as 300 mV s−1 is employed. In Fig. 1, the peak at 0.2 V corresponds to the leucoemeraldine/emeraldine transition and it shifts in the positive direction as the number of cycles is increased. The second anodic peak at 0.55 V arises from the redox behaviour of the reaction intermediates. During the cathodic scans, the reduction peaks are well resolved in the initial cycles. On repeated cycling, however, there is a merging of cathodic peaks. Fig. 2 depicts the cyclic voltammogram of SS/PANI electrode in 0.5 M PTS at different scan rates. A pseudocapacitive current arising essentially from the redox transitions of the PANI molecular chain is noticed in Fig. 2. At lower scan rates, viz. 0.5 and 1 mV s−1 , the reversibility behaviour is noticed. The peaks at 0.27 V corre-

3. Results and discussion 3.1. Cyclic voltammetric studies Fig. 1 depicts the cyclic voltammograms recorded for 100 continuous cycles during the polymerization of aniline on SS substrate in the presence of 0.5 M PTS. A mass loading of 0.38 mg cm−2 of PANI on SS is estimated from the cyclic voltammogram of Fig. 1. In the potentiodynamic method, a

Fig. 2. Cyclic voltammogram for SS/PANI at sweep rates: (a) 0.5 mV s−1 and (b) 1.0 mV s−1 .

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Fig. 3. Infrared spectra of PTS-doped PANI grown by potentiodynamic deposition on SS substrate. Table 1 Characteristic frequencies of potentiodynamically synthesized PANI doped in the presence of p-toluene sulphonic acid Wave number (cm−1 )

Band characteristics

565 806 1033 1123 1482 3232 3453

CH out-of-plane bending vibration para-Disubstituted aromatic rings Due to SO3 − group of the PTS CH in-plane bending vibration CN stretching of benzenoid rings Aromatic CH stretching NH stretching of aromatic amines

sponding to the lecucoemeraldine/emeraldine transition of PANI charge/discharge currents are typically capacitive-like. 3.2. Infrared spectra The FTIR spectra of PTS-doped PANI is presented in Fig. 3 and the assignment of the bands is listed in Table 1.The presence of the two bands in the range of 1500–1600 cm−1 are assigned to the non-symmetric C6 ring stretching modes. The higher frequency vibration at 1600 cm−1 has a major contribution from the quinoid rings while the lower frequency mode at 1500 cm−1 depicts the presence of benzenoid ring units. The presence of these two bands indicates that the polymer is composed of the amine and imine units. The band of the dopant ion and other

Fig. 5. X-ray diffraction pattern of PTS-doped PANI on SS. Radiation: Cu K␣.

characteristic bands confirm the presence of conducting emeraldine salt phase in the polymer. The sharpness of the peak at 1033 cm−1 in PTS-doped PANI refers to the efficient doping of the PANI. 3.3. SEM characterisation SEM measurements were carried out in order to deduce the morphology of the electropolymerised PANI which is strongly dependent upon the polymerization conditions, the nature of the solvent and the method of polymerisation. Fig. 4 depicts the SEM micrograph of the PANI deposited SS electrode. PANI doped with PTS exhibits a mixed fibrous and tubular morphology (highly porous) which is similar to that reported for PANI grown from acidic media and with aromatic sulphonic acids [24,25]. A high porosity of the material is essential for obtaining satisfactory power densities [26]. PANI particles of size ∼100 nm is formed on SS in the presence of 0.5 M PTS. Such nanostructured PANI materials would be ideal candidates as electrode materials for supercapacitors since these possess high surface area. 3.4. XRD studies Fig. 5 depicts the XRD pattern of the SS specimen coated with PTS-doped PANI. In the XRD pattern of the sample, reflections at 2θ values of 45◦ , 52◦ and 75◦ corresponding to the SS metal

Fig. 4. Scanning electron micrographs of PANI deposited potentiodynamically on SS substrate in the presence of 0.5 M PTS at: (a) 2000× and (b) 10,000× magnification.

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occur as reported in earlier studies [27]. The reflections at 2θ values of ∼9◦ and ∼10◦ is attributed to the PTS-doped PANI layer and are in agreement with previous PTS-doped PANI investigations [28]. The sloping background in the XRD pattern arises on account of the SS substrate [29]. 3.5. Galvanostatic charge–discharge experiments In order to obtain information regarding the practical feasibility of SS/PANI electrode galvanostatic charge–discharge cycles were recorded in 0.5 M PTS. Typical chronopotentiograms at current densities of 3.0, 5.0 and 7.0 mA cm−2 over a potential window of 0.7 V is shown in Fig. 6. Ideally, the positive potential limit should not rise above 0.75 V in order to avoid oxidative degradation of the polymer [30] and the negative potential limit should not fall below 0 V in order to prevent the decrease of capacitance [31–33]. The voltage variation is nearly linear during both charging and discharging. Further, the anodic charging curves are symmetric to the corresponding cathodic discharging counterparts indicating that PANI-based films exhibit capacitive-like behaviour. The specific capacitance is customarily deduced from these data using the relation [34] Ccp =

i t vm

(1)

Fig. 7. Variation of capacitance with number of cycles at different current densities.

where i, t, v and m denote, respectively, current density, discharge time, potential range and the active weight of the electrode material (0.38 mg cm−2 ). Specific capacitances (in F g−1 ) of the order of 8.05 × 102 , 5.38 × 102 and 3.21 × 102 are obtained at current densities 3.0, 5.0 and 7.0 mA cm−2 , respectively. The decrease in specific capacitance with increase in current density is due to the enhancement in ohmic drop with increase in current density. For low current densities, the specific capacitance is maximum on account of the lower ohmic drop (related to the electronic and ionic resistance), the whole porosity in the electrode depth can be fully accessed. By increasing the current density, potential distribution arises across the inter electrode spacing due to porous structure of the electrode. In order to evaluate the stability of the electrodes, the charge–discharge cycling tests were conducted for 1000 cycles. Fig. 7 shows the variation of specific capacitance with the number of cycles for SS/PANI electrodes at different current densities. The specific capacitance of the SS/PANI electrodes was as high as 8.05 × 102 F g−1 at the early stage of cycling for 3.0 mA cm−2 discharge as seen from Fig. 7. However, the capacitance decayed from 8.05 × 102 to 7.83 × 102 F g−1 after 1000 cycles. This decrease in capacitance after 1000 cycles is consistent with the performance of other PANI-based electrodes such as PANI/carbon [35], LiPF6 salt-doped PANI [36], etc. 3.6. Impedance analysis

Fig. 6. Galvanostatic charge–discharge curves of SS/PANI electrode at various current densities: (a) 3.0 mA cm−2 , (b) 5.0 mA cm−2 and (c) 7.0 mA cm−2 .

Impedance spectroscopy is a powerful tool for mechanistic analysis of interfacial processes and for evaluation of rate constants, double layer capacitance, etc. In addition, the frequency dependence of the constant phase angle yields information about the geometry of the electrode surface [37]. In the present investigation on PANI-coated SS substrate, impedance data is employed to obtain an estimate of the specific capacitance. The data analysis is similar to that discussed in the case of PANI coated on Ni substrate [16]. Fig. 8 depicts the typical Nyquist diagrams pertaining to the SS/PANI electrode in 0.5 M PTS solution at different potentials. A distorted semi-circle in the high frequency region and a vertically linear spike in the low frequency regime is noticed. The high frequency intercept of the semi-circle on the real axis yields the ohmic resistance (Rsol ) while the diameter provides

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Fig. 8. Impedance spectrum in the range 100 kHz–1 mHz of SS/PANI at various potentials: (a) 0.20 V, (b) 0.40 V, (c) 0.60 V and (d) 0.75 V. Squares denote experimental values while the line represents the fitting of the data to the equivalent circuit of Fig. 10 using the parameters of Table 2.

the charge transfer resistance (Rct ) of the PANI/electrolyte interface. However, the value of Rct increases with the potential as inferred from the diameter of the semi-circle. The angle made by low frequency data on the real axis decreases gradually from ∼90◦ to ∼45◦ on increasing the applied potential from 0.20 to 0.75 V. As the potential is increased, a gradual transition to Warburg behaviour occurs and at 0.75 V, a diffusion controlled doping–dedoping of anions occur resulting in Warburg behaviour. The frequency corresponding to the maximum (f* ) of the imaginary component (−Z ) in the semi-circle yields the time constant τ as τ=

1 2πf ∗

charge–discharge experiments (cf. Section 3.5). An equivalent circuit consistent with the above behaviour is shown in Fig. 10. This circuit is identical with that proposed earlier for PANI/Ni [16]. In the equivalent circuit of Fig. 10, two constant phase elements (CPEs) replace the customary double layer capacitance and Warburg impedance. These two constant phase elements denote respectively a capacity at inhomogeneous electrode surfaces and ionic diffusion process at the electrode. The constant phase element arises on account of the following causes: (a) a

(2)

τ ∼ 9 × 10−5 s and this indicates fast charge–discharge characteristics. The low frequency differential capacitance (Ct ) is calculated from the variation of the imaginary component of the impedance with the reciprocal of the frequency (−Z versus 1/f plot). The slope is equal to 1/(2πCt ). A typical plot of Z versus 1/f for an applied potential of 0.2 V is shown in Fig. 9. The capacitance values (in F g−1 ) at applied potentials of 0.20, 0.40 and 0.60 V are 8.1 × 102 , 7.7 × 102 and 7.4 × 102 , respectively. These values are consistent with that obtained from

Fig. 9. Variation of imaginary component of impedance (−Z ) with reciprocal of frequency (1/f) at an applied potential of 0.2 V. Points denote the experimental data while the line is obtained from the linear regression analysis. Correlation coefficient ∼0.99.

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Table 2 System parameters deduced by fitting the Nyquist plots to the equivalent circuit of Fig. 10 Potential (V)

Rct ( cm−2 )

0.2 0.4 0.6 0.75

1.5 1.7 2.35 5.8

± ± ± ±

0.05 0.1 0.2 0.2

CPE1 (in 10−4 −1 cm−2 s) 9.2 8.5 8.2 0.25

± ± ± ±

0.2 0.2 0.5 0.05

distribution of the relaxation times as a result of inhomogenities existing at the electrode/electrolyte interface, (b) porosity, (c) nature of the electrode and (d) dynamic disorder associated with diffusion. The equivalent circuit of Fig. 10 is also analogous to nickel hexacyanoferrate composite electrode [38] wherein the cations from the electrolyte medium intercalate in and out of the electrode. For SS/PANI system, the p-toluenesulphonate anions on account of their large size get immobilized in the PANI and the redox process occurs due to the diffusion of H+ ions through the polymer material. The equivalent circuit parameters are obtained from AC impedance simulator of the electrochemical workstation. Their values are shown in Table 2.The magnitude of the solution resistance is ∼1.6  cm2 . The exponents n1 and n2 from the definition [39,40] ZCPE1 = [Q(jω)n1 ]−1 and ZCPE2 = [Q(jω)n2 ]−1 with each exponent lying in the range −1 to +1. The parameter Q is composed of properties related to the surface and the electroactive species, ω is the angular frequency (ω = 2πf) while the exponent n arises from the slope of the log Z versus log f plot. In general, the slope of log Z versus log f plot (denoted as n) is a manifestation of the electrode nature. n1 values denote the porosity of the electrode which almost remains a constant for all the applied potentials studied whereas n2 values denote the capacitive nature of the system. A value of unity for n2 indicates the capacitance while n = 0 and 0.5 denotes, respectively, the resistance and Warburg behaviour. From Table 2, it follows that in the potential range of 0.20–0.60 V (half-oxidized conducting emeraldine state) the value of n1 deduced as 0.95 for the entire potential range refers to the porous nature of the electrode while the value of n2 at ∼0.9 (between 0.2 and 0.6 V) indicates the highly capacitive nature of the system. At 0.75 V (completely oxidized state) the value of n2 = 0.17 implies the resistive nature of the SS/PANI electrode. 3.7. Tafel studies Electrodeposition of PANI is generally carried out on inert substrates such as Pt, Au, glassy carbon, etc., due to the requirement that they should be stable in acidic electrolytes during polymerization. However, attempts have also been made to elec-

CPE2 (−1 cm−2 s) 0.25 0.21 0.27 0.15

± ± ± ±

0.05 0.02 0.05 0.05

n1 0.95 0.92 0.90 0.85

n2 ± ± ± ±

0.05 0.01 0.03 0.02

0.95 0.92 0.90 0.17

± ± ± ±

0.01 0.02 0.05 0.02

Fig. 11. Tafel polarization data of: (a) bare SS and (b) PANI-coated SS in 0.5 M PTS.

trodeposit PANI on non-noble metals such as Ni, Al, Ta, SS, etc. [41]. Since SS is used as the substrate for PANI deposition in the present study, its stability and corrosion resistance during an extended period assumes significance. For this purpose, the open circuit potential (OCP) of a PANI-coated SS was measured at several time intervals till a duration of 2 h. The initial values measured for bare SS and PANI/SS were 0.26 and 0.47 V, respectively. This positive shift in potential suggests surface passivation [42]. Tafel data of SS in 0.5 M PTS is shown in Fig. 11. The Tafel plots of the PANI (PTS) covered SS electrode are different from the corresponding data of the bare SS electrode. Neither the cathodic nor the anodic polarization data are linear. The corrosion potential becomes more positive when the SS surface is covered by PANI films while the corrosion current increases from 7.8 × 10−5 A cm−2 for bare SS to 3.9 × 10−3 cm−2 for SS/PANI. The shift in the corrosion potential indicates the protection of the metal surface by the PANI film due to the formation of a passivated layer which restricts the occurrence of corrosion. The shift in OCP to a more positive value in conjunction with a concomitant increase in the corrosion current has been observed in related investigations [42,43]. 4. Conclusions

Fig. 10. Equivalent circuit for SS/PANI electrode. Rsol , solution resistance; Rct , charge transfer resistance; CPE1 and CPE2 denote the two constant phase elements.

The feasibility of PANI formed on SS in the presence of p-toluene sulphonic acid as electrode material for supercapacitor is demonstrated. A specific capacitance of 8.05 × 102 F g−1 is estimated and a good cycling behaviour (∼103 cycles) has been demonstrated for PANI-coated SS electrodes. Tafel studies indicate that SS/PANI electrodes offer satisfactory resistance to corrosion.

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Acknowledgement The valuable comments of the reviewer are gratefully acknowledged. This work was supported by the DRDO, Government of India. References [1] [2] [3] [4]

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