Electrochemical impedance spectroscopy of homo and copolymer of aniline and 2,4- dinitrophenol in a binary electrolyte system

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1 Indian Journal of Chemistry Vol. 55A, November 2016, pp Electrochemical impedance spectroscopy of homo and copolymer of aniline and 2,4- dinitrophenol in a binary electrolyte system Ali Parsa a, *, Ebtesam Sobhani a, Azadeh Shakeri a & Farnoush Fridbod b a Department of Chemistry, College of Science, Yadegar -e- Imam Khomeini (RAH) Shahre-rey Branch, Islamic Azad University, Tehran, Iran aliparsa@iausr.ac.ir/iausr.parsa@yahoo.com b Department of Analytical Chemistry, University of Tehran, Iran Received 12 June 2016; re-revised and accepted 2 November 2016 Electropolymerization of aniline and 2,4-dinitrophenol on composite graphite electrode in phosphoric acid medium has been investigated by cyclic voltammetry. The copolymer has been synthesized in a mole ratio of 1:1 of aniline and 2,4-dinitrophenol in aqueous medium with para-toluene sulfonic acid and KCl as the binary supporting electrolytes. The properties of the modified electrodes have been characterized and compared with each other by FTIR spectroscopy. The inductive behavior of polyaniline and copolymers of aniline with 2,4-dinitrophenol has been identified via electrochemical impedance spectroscopy by determining parameters such as impedance and capacitance. Keywords: Electrochemistry, Electropolymerization, Impedance spectroscopy, Dinitrophenol, Inductive behavior, Polyaniline In the last two decades, there has been an unprecedented increase in the demand for conducting composite materials, which are now available as materials with a unique combination of properties 1,2. Among such materials, polyaniline (PAni) is a particularly fascinating material for commercially viable conducting polymers because of its unique proton durability, exceptional redox recyclability, chemical constancy, electrical transmission ability, inexpensiveness and easy synthesis 3-9. Consequently, the composites based on conducting polymers and insulating polymers have been studied as materials for industrial products, for example as light producing diodes, renewable batteries, gas sensors, conductive coatings and anti-static materials However, the major problem with this polymer is that it has a poor solubility in solvent and requires multiple processing techniques. Several methods have been developed to overcome the insolubility challenge associated with the use of this polymer. Post-treatment of the base form of PAni or chemical homopolymerization of aniline (Ani) derivatives and copolymerization of Ani with ring or N-substituted Ani derivatives have been successfully carried out for this purpose 15,16. Unfortunately, the electronic conductivity of the resulting PAni derivatives/copolymers is much less than that of PAni alone, owing to the existence of torsional angle from substituents in the monomeric units that destroys the orbital overlap of the π-electrons and nitrogen lone pair electrons, resulting in a decrease in the extent of conjugation and conductivity. In recent years, copolymers composed of Ani and substituted Ani have received wide attention The use of PAni is challenging because of its conductivity and solubility in polar organic solvents, which depend not only on the oxidation state but also on the degree of protonation and nature of dopants. The incomplete oxidation of PAni causes the bonds to be rearranged, producing an apparent increase in electronic conductivity. PAni has two voltammetric redox pairs and three constant oxidation conditions, with the half-oxidized condition (emeraldine) being extremely conductive in its protonated condition. PAni can be employed as an electrochemical capacitor due to the presence of diverse oxidation conditions 25. The higher oxidation states of PAni have been reported to be involved in hydrolysis and also produces benzoquinone (BQ) 26. The formation of BQ has been well identified by cyclic voltammetry (CV), rotating ring disk electrode (RRDE), FTIR and UV visible spectroscopy. BQ formation during polymerization is usually circumvented by using an appropriate upper potential limit (below 0.80 V vs. Ag/AgCl) 27. The presence of BQ in PAni may enable it to become an inductor in

2 1326 INDIAN J CHEM, SEC A, NOVEMBER 2016 electric circuits. In addition, both the electrochemical changes, of emeraldine to the oxidized form of PAni as well as the BQ to hydroquinone (HQ), occur probably in a similar range of potential. As organic raw materials and intermediates, dinitrophenol (DNP) compounds are widely used in the synthesis and manufacture of dyes, textile, pesticides, pharmaceutical and explosives 28,29. Copolymers of Ani with 2,4-dinotrophenol (poly(anico-2,4-dnp)) have been synthesized by electrochemical polymerization. In the present study, a copolymer comprising Ani and 2,4-DNP units has been deposited on the backbone of the polymer under electropolymerization conditions during which benzoquinone (BQ) is expected to be formed (if Ani alone is polymerized). The inductive behavior of PAni and poly(ani-co-2,4-dnp) has been monitored by electrochemical impedance spectroscopy, which can be effectively used to obtain important electrochemical information about conducting polymer 30. A close comparison of the electrochemical impedance spectroscopy (EIS) results of PAni and the deposited copolymer, revealed the differences in electrochemical characteristics between the copolymer and PAni. 0.5 M para-toluene sulfonic acid (PTSA) (as binary supporting electrolytes) by sweeping potential from 0.2 to +1.0 V at a scan rate of 100 mv s -1. The EIS experiments were conducted in a three-compartment cell with a cell geometry that is similar to that described for the polymerization of Ani/copolymerization of Ani with 2,4-DNP at a potential range of 0.2 to +1.0 V, and a frequency range of 100 khz to 100 mhz. Results and Discussion Electropolymerization of Ani and redox transition in PAni CVs for the growth of PAni using a 50 mmol L -1 Ani solution in phosphoric acid medium containing 0.5 mol L -1 PTSA and 2 mol L -1 KCl (with different mole fractions) on composite 2B pencil graphite are shown in Fig. 1. There were three pairs of redox peaks 31 observed at the potential range of 0.2 to +1.0 V (Fig. 1a). The first pair of redox peaks at the potential range of 0.2 to +0.3 V (denoted as O3/R3) corresponds to the interconversion of leucoemeraldine Materials and Methods Aniline (Merck) was purified by distillation under an atmosphere of nitrogen at reduced pressure. The resulting colorless liquid was kept at 5 C in the dark. 2,4-DNP and the other chemicals used in this research study were obtained from Merck, and used without further purifications. Electrochemical deposition of PAni and copolymer films, as well as electrochemical impedance analysis were carried out by compactstat potentiostat/galvanostat (Ivium Technologies, Netherlands), using a generalpurpose electrochemical system software. FTIR was recorded on a Perkin Elmer 2000 spectrophotometer to probe the structural analysis of the homo and copolymers chains. A three-electrode cell was assembled using the composite 2B pencil graphite (Staedtler Lumograph, Germany) as the working electrode; a pseudo Ag/AgCl as the reference electrode and a platinum wire with a diameter of 0.5 mm as the counter electrode. Procedure Electropolymerization of 50 mmol L -1 of Ani and 2,4-DNP (1:1 mol L -1 ratio) was performed in 1 M H 3 PO 4 containing 10 ml of 2 mol L -1 KCl and Fig. 1 CVs of PAni in 1 mol L -1 H 3 PO 4. [PTSA (0.5 mol L 1 ): KCl (2 mol L 1 ):: (a) 1:1; (b) 1:0].

3 PARSA et al.: ELECTROPOLYMERIZATION OF HOMO & COPOLYMER OF ANILINE+2,4- DNP 1327 (LE) to emeraldine salt (ES). The second pair of redox peaks at potentials above +0.6 V (denoted as O1/R1) is due to the oxidation of ES to pernigraniline (PN) (fully oxidized form) and vice versa 32,33. Those in the potential range of +0.3 to +0.6 V (denoted as O2/R2) are attributed to the degradation of products from HQ to BQ. The first and third redox peaks in the CV of PAni in the presence of KCl and PTSA are sharper as compared to those in absence KCl 34,35. The role of KCl was probably to precipitate an increase in the dielectric constant of the solvent and subsequently increase the acidity of the medium 35. Consequently, the presence of KCl increased the acidity of PTSA in the aqueous medium and affected the reaction of the redox peaks as well as assisted in the mobility of radical cations to the surface of the electrode for the formation of PAni. Usually, solutions with restricted ion pairing and adept mass transport are used. There was an appearance of a green film of PAni on the electrode in the electrolyte solution that contained both the supporting electrolytes, KCl and PTSA after the initiation of polymerization. However, the green film was not observed in the solution without KCl. Consequently, the first and third redox peaks that existed in the CVs indicated that the electrosynthesized PAni was of the best quality 33,36. The FTIR-band spectra of the PAni films are shown in Fig. 2. The vibrational bands at 1580 and 1490 cm 1 are attributed to the quinoid (Q) and benzenoid (B) rings respectively, while the bands at 1300 and 1237 cm 1 are attributed to the C N stretching of ES 3,37. The vibrational bands at 1156 and 819 cm 1 are assigned to the in-plane and out-of-plane C H bending of the aromatic ring, respectively 38. It has been reported that the ratio of the intensity of bands of Q and B is a measure of the degree of oxidation of the PAni film 39. The ratio was zero (0) for the fully reduced form, for the protoemeraldine form, 1 for the partially oxidized form, and 1 for the fully oxidized form. The medium with inorganic salt (Fig. 2a, Q/B~1) contains the partially oxidized form, which probably was B as its most stable form. Figure 2b shows that the medium without inorganic salt (KCl) had PAni in the oxidized form (Q/B 1). This implies that PAni may have a small amount of non-conducting half-oxidized emeraldine base (EB) or the PN form (i.e., more Q structures in the backbone of the polymer) 36. Copolymerization of Ani and 2,4-DNP The CVs of the growth copolymer on composite 2B pencil graphite using 50 mmol L -1 of Ani and 2,4-DNP (1:1 mol L -1 ratio) are shown in Fig. 3. The anodic and cathodic potential peaks were observed to have been shifted when compared with PAni CVs (Fig. 1a, b), indicating the formation of a copolymer. As shown in Fig. 3, the CV revealed two clear redox Fig. 2 Baseline corrected FTIR spectra of PAni electropolymerized in [PTSA (0.5 mol L 1 ):KCl (2 mol L 1 ):: (a) 1:1; (b) 1:0]. Fig. 3 CVs of poly(ani-co-2,4dnp) in 1 mol L -1 H 3 PO 4. [PTSA (0.5 mol L 1 ):KCl (2 mol L 1 ):: (a) 1:1; (b) 1:0].

4 1328 INDIAN J CHEM, SEC A, NOVEMBER 2016 peaks, indicating that PAni was in the EB conducting form, which has the ability to undergo oxidation to produce the PN form that can provide electrons for the reduction of 2,4-DNP and conjugation of the polymeric chains of Ani with 2,4-DNP, which eventually culminate in the formation of the copolymer. The IR spectrum of poly(ani-co-2,4dnp) was similar to that of the respective homopolymers. Although the FTIR spectrum of the copolymer prepared from the mixture of Ani and 2,4-DNP exhibited the characteristic bands of both homopolymers, it was not a simple combination of their spectra. In Fig. 4, the characteristic peaks of the individual homopolymers i.e., (PAni and 2,4-DNP) are found in the copolymer. The peaks at 1580 and 1490 cm -1 indicate the existence of Q and B rings, respectively, with Q/B 1. This implies that PAni may have a small amount of non-conducting, half-oxidized (EB) or (PN) form. The broad band at 3400 cm -1 is similar to the stretching vibration of the amine groups Fig. 4 Baseline corrected FTIR spectra of poly-(ani-co-2,4dnp) electropolymerized. [PTSA (0.5 mol L 1 ):KCl (2 mol L 1 ):: (a) 1:1; (b) 1:0]. Fig. 5 Nyquist diagrams for PAni film. [PTSA (0.5 mol L 1 ):KCl (2 mol L 1 ):: 1:1, measured at: (a) 0.3 V (b) 0.50 V (c) 0.60 V (d) 0.70 V (e) 0.80 V (f) 0.90 V; the solid lines represent the best fitting results according to the equivalent circuits represented in the inset].

5 PARSA et al.: ELECTROPOLYMERIZATION OF HOMO & COPOLYMER OF ANILINE+2,4- DNP 1329 Fig. 6 Nyquist diagrams for PAni film. [PTSA (0.5 mol L 1 ):KCl (2 mol L 1 ):: 1:0, measured at: (a) 0.3 V (b) 0.50 V (c) 0.60 V (d) 0.70 V (e) 0.80 V (f) 0.90 V; the solid lines represent the best fitting results according to the equivalent circuits represented in the inset]. in PAni. The peaks ranging from 1345 to 1330 cm -1 and 1545 to 1520 cm -1 are attributed to the symmetrical and unsymmetrical stretching vibration of NO 2 group that was conjugated to the phenolic rings. The aromatic C-N stretching band of PAni at 1250 cm -1 was not absorbed by the copolymer; instead, it was shifted to a higher wave number 40,41 (1291 cm -1 ). This suggests that not only Ani Ani or 2,4-DNP 2,4-DNP linkages were formed but also some Ani 2,4-DNP linkages. Electrochemical impedance characteristics of PANI/copolymer films Electrochemical characteristics of PAni and copolymer (poly(ani-co-2,4-dnp)) films were examined by studying the impedance measurements at different applied direct current potentials. Typical Nyquist diagrams for PAni films in 1 mol L -1 H 3 PO 4 aqueous solution in 0.5 mol L -1 PTSA and 2 mol L -1 KCl at different applied direct current potentials (0.3 V 0.9 V) are presented in Fig. 5. The impedance spectra recorded at a DC potential of 0.7 V (Fig. 5d) revealed a fascinating phenomenon; an inductive behavior, in contrast to the other conditions (presence and absence of KCl). In the present study, the possible occurrence of simultaneous redox reactions for PAni (EB to PN conversion) and BQ (conversion of BQ/HQ transportations) in the potential region of V could lead to an inductive response. Figures 5 (d, e, f) and 6 (d, e, f) show that all impedance arcs have a semicircle in the high frequency region. This arc was due to the process that occurred at the polymer electrolyte interface, which could be the double layer capacitance in parallel with the ionic charge transfer resistance (R ct ) induced by the ion exchange for charge compensation at the polymer solution interface 30.

6 1330 INDIAN J CHEM, SEC A, NOVEMBER 2016 Scheme 1 Fig. 7 Nyquist diagrams for poly(ani-co-2,4dnp) film. [PTSA (0.5 mol L 1 ):KCl (2 mol L 1 ):: 1:1, measured at: (a) 0.3 V (b) 0.50 V (c) 0.60 V (d) 0.70 V (e) 0.80 V (f) 0.90 V; the solid lines represent the best fitting results according to the equivalent circuits represented in the inset]. The ionic charge transfer resistance reaches a minimum in the EB (50% oxidized) form as the impedance in the main part reaches the minimum at the potential of 0.30 V. This phenomenon may be connected to the change in conductivity of PAni, since the conversion of EB (50% oxidized) form to the LB (fully reduced) or to the PN (fully oxidized) form culminated in a decrease in conductivity 42. A linear plot with a slope of 45 in the middle frequency region (1.58 Hz) (Figs 5 and 6) shows the Warburg diffusion region (Z w or W), which is due to the semiinfinite diffusion of protons at the polymer electrolyte interface. The double layer capacitance and the faradaic pseudo-capacitance are expressed as CPE (Constant Phase Element) because of the irregular geometry of the polymer films. Scheme 1 shows the association between BQ and PAni in the production of an inductive behavior as perceived by the electrochemical impedance responses. The protons released during the electrochemical

7 PARSA et al.: ELECTROPOLYMERIZATION OF HOMO & COPOLYMER OF ANILINE+2,4- DNP 1331 oxidation of the EB form to the PN form may be employed in the transformation of BQ to HQ. The reverse process i.e., the reduction of PAni in PN form to the EB form, requires protons to complete the process of electrochemical transformation of HQ to the BQ form. Consequently, the inductive characteristics of PAni are anticipated to be exhibited only within this range of potential. It would be interesting to analyze the impedance data of the copolymer film based on the above proposed equivalent circuit (inset of Fig. 5d). The impedance responses for copolymer (poly(ani-co-2,4-dnp)) at different applied potentials are shown in Figs 7 and 8. A comparison of the impedance response of PAni (Fig. 5) and copolymer (Fig. 7) at V shows the differences between the two in the middle frequency region. It is obvious that the inductive characteristics are negligible for copolymer at the applied potential of V (Fig. 7). This may be due to the generation of lower amount of BQ under these conditions. The copolymer possesses diphenosemiquinone and diphenoquinone imine structures, which does not favor the formation of BQ from hydrolysis. The incorporation of 2,4-DNP units in the copolymer reduces the process of hydrolysis; hence, a lower inductive behavior was observed. The impedance spectra recorded at 0.7 V potential shows different characteristics in contrast to other conditions (with and without KCl). The shape of the impedance response to this change shows that the electrochemical properties of PAni and poly(ani-co- 2,4 DNP) films were distinctly different in terms of the applied potential and supporting electrolytes. Consequently, a single equivalent circuit could not be used to fit the EIS data under all the studied conditions. Table 1 shows the values of the Randles equivalent circuit elements obtained by fitting the Fig. 8 Nyquist diagrams for poly(ani-co-2,4dnp) film. [PTSA (0.5 mol L 1 ):KCl (2 mol L 1 ):: 1:0,] measured at: (a) 0.3 V (b) 0.50 V (c) 0.60 V (d) 0.70 V (e) 0.80 V (f) 0.90 V; the solid lines represent the best fitting results according to the equivalent circuits represented in the inset.

8 1332 INDIAN J CHEM, SEC A, NOVEMBER 2016 Table 1 The best fitting values of the Randles equivalent circuit elements in Figs 5-8 from the simulation of the impedance data for PAni and poly(ani-co-2,4 DNP) In presence of PTSA In presence of PTSA and KCl PAni Poly(Ani-co-2,4 DNP) PAni Poly(Ani-co-2,4 DNP) R s (ohm) R ct (ohm) W 1/(Ohm sqrt(hz) CPE (F) α α: Degree of electrode surface roughness or cell geometry; R s : The bulk electrolyte solution resistance. Fig. 9 Capacitance vs. potential diagram for PAni film. [PTSA (0.5 mol L 1 ):KCl (2 mol L 1 ):: (a) 1:1; (b) 1:0] (in the frequency range of 100 mhz to 100 khz). experimental results for the Nyquist plots in PAni and poly(ani-co-2,4 DNP) in a binary electrolyte system, and in the presence of PTSA. A more accurate comparison of other EIS parameters such as capacitance of electrochemical solution (C s ) under the two different conditions of electrosynthesis of PAni and poly(ani-co-2,4 DNP) (in the presence and absence of KCl) was made in the potential range of 0.2 to 1.0 V. Figures 9 and 10 shows the plots of capacitance against the potential of Fig. 10 Capacitance vs. potential diagram for poly(anico-2,4dnp) film. [PTSA (0.5 mol L 1 ):KCl (2 mol L 1 ):: (a) 1:1; (b) 1:0] (in the frequency range of 100 mhz to 100 khz). PAni and poly(ani-co-2,4 DNP) in the potential range of 0.2 to 1.0 V and frequency region of 100 mhz to 100 khz. Figures 9a and 10a indicate that in the presence of KCl, the capacitance reaches a maximum at 0.7 V potential; this is not seen in the absence of KCl. Figures 9b and 10b indicate that the potential at 0.7 V was the same for the conductive configuration of PAni as well as poly(ani-co-2,4 DNP) in the presence of KCl and PTSA.

9 PARSA et al.: ELECTROPOLYMERIZATION OF HOMO & COPOLYMER OF ANILINE+2,4- DNP 1333 Conclusions The electropolymerization of PAni and poly(anico-2,4 DNP) was carried out by cyclic voltammetry in aqueous medium using two supporting electrolytes, i. e., KCl and PTSA. The binary supporting electrolyte system with KCl as the secondary electrolyte to PTSA significantly improved the homopolymerization and copolymerization of Ani and 2,4-DNP. The PAni film exhibited an unusual inductive behavior in the range of applied potential of V. The simultaneous occurrence of the interconversion of BQ to HQ, and emeraldine to pernigraniline form of PAni within the potential limits of V induced such an inductive behavior. The presence of BQ in the PAni film that exhibited an inductive behavior was evident from the vibrational bands at 1580 and 1490 cm 1 (attributed to quinoid (Q) and benzenoid (B) rings) in the IR spectrum of the PAni film. The incorporation of 2,4-DNP units in the copolymer due to the generation of lower amount of BQ, precipitated the lower inductive behavior. The presence of KCl also influenced the structural and electrical conductivities as well as the morphological properties of the resulting polymers. It is proposed that KCl can help to increase the dielectric constant of solvent and subsequently increases the acidity of the medium. It also helps in the mobility of the radical cations to the surface of the electrode during the formation of homo and copolymers. Acknowledgement The authors are indebted to the Yadegar-e-Imam Khomeini (RAH) Shahre-rey Branch, Islamic Azad University, Tehran, Iran for its financial support through the Research University (RU) grant (1395/1742:91/07/27). References 1 Ramírez García S, Céspedes F & Alegret S, Electroanalysis, 13 (2001) Singh K, Mishra U & Tiwari A, Indian J Chem, 40A (2001) Ćirić-Marjanović G, Synth Met, 177 (2013) 1. 4 Diaz A & Logan J, J Electroanal Chem Interfacial Electrochem, 111 (1980) MacDiarmid A, Chiang J, Richter A, Epstein & AJ, Synth Met, 18 (1987) Huang W, Yangm F & Zhu C, Faraday Trans, 1 (1986) MacDiarmid A G & Epstein A J, Faraday Discuss Chem Soc, 88 (1989) Shahhosseini L, Nateghi M R, Kazemipour M & Zarandi M B, Mater Chem Phys, 177 (2016) Patel B V, Patel H S & Patel K C, Indian J Chem, 47B (2008) Han Y-K, Chang M-Y, Ho K-S, Hsieh T-H, Tsai J-L & Huang P-C, Sol Energy Mater Sol Cells, 128 (2014) Syed A A & Dinesan M K, Talanta, 38 (1991) Zhou H, Jiao S, Chen J, Wei W & Kuang Y, Thin Solid Films, 450 (2004) Gong J, Hua R-N, Xie Z-W, Wang S-G & Qu L-Y, Polym J, 33 (2001) Su B, Tong Y, Bai J, Lei Z, Wang K, Mu H & Dong N, Indian J Chem, 46A (2007) Chen C, Sun C & Gao Y, Electrochim Acta, 53 (2008) Mu S, Zhang Y & Zhai J, Electrochim Acta, 54 (2009) Melad O & Esleem M, Open J Org Polym Mater, 5 (2015) Li T, Yuan C, Zhao Y, Chen Q, Wei M & Wang Y, High Perform Polym, 25 (2013) Huang L-M, Chen C-H, Wen T-C & Gopalan A, Electrochim Acta, 51 (2006) Wen T-C, Huang L-M & Gopalan A, J Electrochem Soc, 148 (2001) D9. 21 Simoes F, Pocrifka L, Marchesi L & Pereira E, J Phys Chem B, 115 (2011) Chen W-C, Wen T-C & Gopalan A, J Electrochem Soc, 148 (2001) E Thota A, Arukula R, Narayan R, Rao C R & Raju K, RSC Adv, 5 (2015) Chen W-C, Wen T-C & Gopalan A, Synth Met, 130 (2002) Gorey B, Smyth M R, White B & Morrin A, J Mater Chem C, 2 (2014) Stilwell D E & Park S M, J Electrochem Soc, 135 (1988) Genies E, Lapkowski M & Penneau J, J Electroanal Chem Interfacial Electrochem, 249 (1988) Wang X-G, Fan Y-J, Hao Z-X & Gan L-H, Russ J Electrochem, 46 (2010) Lin X, Ni Y & Kokot S, Anal Chim Acta, 765 (2013) Hu C-C & Chu C-H, J Electroanal Chem, 503 (2001) Eftekhari A & Afshani R, J Polym Sci, Part A: Polym Chem, 44 (2006) MacDiarmid A G, Synth Met, 84 (1997) Parsa A & Ab Ghani S, Electrochim Acta, 54 (2009) Yang H & Bard A J, J Electroanal Chem, 339 (1992) Orata D & Buttry D A, J Am Chem Soc, 109 (1987) Parsa A & Ab Ghani S, J Electrochem Soc, 156 (2009) E Yağan A, Pekmez N Ö & Yıldız A, Electrochim Acta, 51 (2006) Sbaite P, Huerta-Vilca D, Barbero C, Miras M C & Motheo A J, Eur Polym J, 40 (2004) Shah K & Iroh J, Synth Met, 132 (2002) Yusof N A, Zakaria N D, Maamor N A M, Abdullah A H & Haron M J, Int J Mol Sci, 14 (2013) Holze R, J Solid State Electrochem, 11 (2007) Gazotti W, Matencio T & De Paoli M-A, Electrochim Acta, 43 (1998) 457.