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Electrochemical behavior of polyaniline: A study by electrochemical impedance spectroscopy (EIS) in low-frequency
Solid State Ionics 346 (2020) 115198
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Electrochemical behavior of polyaniline: A study by electrochemical impedance spectroscopy (EIS) in low-frequency João C. Martinsa,b, José C. de M. Netob, Raimundo R. Passosa, Leandro A. Pocrifkaa, a b
T
⁎
GEMATA - LEEN, Departamento de Química, Universidade Federal do Amazonas, CEP 69077-000 Manaus, AM, Brazil Departamento de Engenharia de Materiais, Universidade do Estado do Amazonas, CEP 69065-020 Manaus, AM, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Conductive polymer PANI Electrochemical impedance spectroscopy (EIS) Low-frequency
Conducting polymers are increasingly attracting the interest of many researchers in different areas due to their good conductivity, stability and easy preparation. Polyaniline is heavily studied in sensors, corrosion protection, electrochromic devices, and supercapacitors, however, even with numerous applications, there are still few pieces of literature that report their electrochemical behavior in regions low-frequency of by electrochemical impedance spectroscopy (EIS). In this work, polyaniline was electropolymerized by galvanic synthesis (chronoamperometry) in stainless steel substrate. Its performance was evaluated by cyclic voltammetry (CV) galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). It was observed in its lowfrequency pseudocapacitive analysis, in the benzoquinone and hydroquinone intermediate regions, capacitance values above 100 mF·cm−2, and in the emeraldine state, 295 F·g−1 was obtained. Such results provide a better and detailed understanding of PANI oxidation and reduction processes and could optimize the use of the polymer in various fields of applications.
1. Introduction The idea of the formulation of electrochemical capacitors is important for the development of more efficient capacitors, in view of the expansion of increasingly advanced devices, such as mobile phones, solar cells, notebooks, GPS (Global Positioning System) and hybrid automobiles that need more energy [1–3]. In addition, the development of effective and low-cost devices is a determining factor for the viability of industrial production. In these devices, materials are employed as conducting polymers, carbon and transition metal oxides. Among these materials, polymers have several advantages that make them suitable for application in supercapacitors, such as low-cost, reduction of environmental impacts, high conductivity when doped, high porosity, high storage capacity, reversibility and redox potential adjustable through chemical changes [4–7]. Polyaniline (PANI) is one of the most studied conductive polymers, thus attracting attention as a promising material for application in several areas such as sensors [6,8,9], corrosion protection [10,11], electrochromic devices [12,13] and supercapacitors [14]. Even with a range of applications of PANI, many research groups [15–17] are and have been studyng the influence of different aqueous environments on its conductivity behavior. Electrochemical impedance spectroscopy (EIS) is an excellent tool for studying redox-processes of ⁎
conducting polymers. By EIS the redox processes taking place on the electrode surface can be monitored as a function of the applied dc-potential. With impedance spectroscopy it is possible to get information of charge-transfer and diffusion processes as well as charging of the double-layer and the bulk polymer layer which is not possible by cyclic voltammetry [18]. Toušek [16], observed the mixed electron and proton conductivity of polyaniline films by EIS, at lower frequencies it is related to polarons that are formed by the redistribution of electrons in protonated PANI, and in higher frequencies, it is not affected and can it is attributed to holes released at higher potential. Fiordiponti [17] and Schrebler [19], carried out an impedance study of polyaniline films in aqueous acidic and neutral organic solutions, where the polymer capacitance was associated with the behaviors of its potentials observed by EIS, and so the capacitives properties could be separated from the faradaic a non-faradaic processes in high intermediate and low-frequency regions. In work of Biaggio [20], PANI electrochemically prepared in HClO4 and H2SO4 solutions was studied by EIS. Its behavior was analyzed by low-frequency differential capacity and its redox process, by the EIS studies. The rate of the in PANI is slower in PANI made in HClO4 than that in H2SO4, these behaviors are associated with the diffusion coefficient, which is related to transport processes ionic in the polymers films prepared in different solutions.
Corresponding author at: GEMATA - LEEN, Departamento de Química, Universidade Federal do Amazonas, CEP 69077-000 Manaus, AM, Brazil. E-mail address: [email protected] (L.A. Pocrifka).
https://doi.org/10.1016/j.ssi.2019.115198 Received 27 May 2019; Received in revised form 11 December 2019; Accepted 18 December 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
Solid State Ionics 346 (2020) 115198
J.C. Martins, et al.
Roβberg [21], impedance spectra of electropolymerized polyaniline films were analyzed by the impedance theory, in order to evaluate characteristic of the PANI states. Whose observations on low-frequency capacitance increases linearly with of cyclic voltammetric redox charge of the PANI films. Radoševic [22], rated the polyaniline films by lowfrequency impedance spectra. The specific features of the polymers were controlled by charge transport processes, ion diffusion and migration in the first case, and ionic conductivities in the second case that are Warburg differences in relaxation times of diffusion and migration processes, or by the impedance of ionic channels of EIS in higher and intermediate frequencies. In his work Mondal [23] studied the effect of heating on PANI electrochemical activity by EIS. It was observed by low-frequency a decrease in pseudocapacitive activity attributed to an irreversible loss of water molecules in that system. Žic [24] analyzed polyaniline by EIS measurements in the low-frequency region and showed that the phase shift is under the influence of the applied potential and the response is related to the PANI layer region under influence of the refox process. Electroneutrality of the PANI layer is achieved by the counter-ions fluxes, which balance the electrons that enter/leave the electrode due to the redox process of PANI. The aim of this paper was to evaluate the synthesis of polyaniline by electrodeposition (ED) at different times and to test its pseudocapacitive properties using cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS) methods, to assess the capacitance properties of the material in lowfrequency regions extracted from EIS data, where the frequencies are low. The detailed understanding of the oxidation and reduction processes of PANI could help to optimize the use of PANI in various fields of applications.
Fig. 1. SEM in a) PANIT1and b) PANIT2.
2. Synthesis of PANI
Autolab. After polarization of 300 s, spectra were obtained in the frequency range of 0.01 to 10,000 Hz, where an alternating perturbation alternating from 10 mV peak to peak was applied. Spectra were collected between potentials of −0.20 to 0.70 V.
The substrates used as working electrodes were made of 304 stainless steel with dimensions (1 × 0.5 cm2). Substrates cleaning consisted of washing them with ethanol P.A. and in physical attack by sandblaster, to improve adhesion to PANI electrodeposition. An electrochemical system with four electrodes was adopted, in which the stainless-steel plate, where the materials were electrodeposited, as working electrode (WE); silver/silver chloride (Ag/AgCl) saturated with KCl as reference electrode (RE), and two platinum counters electrodes (CE). The galvanostatic technique was employed for the electrodeposition, at a potential of 0.70 V (700 mV) with a H2SO4 concentration of 0.5 mol L−1 and 0.1 mol L−1 of aniline. The produced materials were PANIT1 (900 s) and PANIT2 (1800 s). This potential was used to form PANI in its emeraldine oxidation state. The syntheses were carried out using a AutoLab potentiostat/galvanostat, model PGSTAT302N.
4. Results and discussions In order to verify the effect of time variation on PANI electrodeposition, SEM was used to characterize the electrode surfaces of the electrodes in order to report the polymers deposition behavior in the protonation level which is between the acid and the monomer, because the deposition time leads to differences in the formation of polymeric chains. In Fig. 1a) and b), PANIT1 and PANIT2 materials are shown, respectively. PANIT1 material image is more uniform and with more spacious cavities between the pores in nanometric dimensions [25,26], such characteristics are more useful for the electrolyte ions penetration into the polymer active sites in the electrochemical analyses compared to the synthesis of PANIT2, because the material formed in greater time, obtained a greater polymer mass, more compact structure due to the stacking of the PANI chains, what thus hinders the electrolyte penetration in the polymer active area [27,28]. PANI electrochemical formation occurs in two stages; first, there is the formation of a more compact and homogeneous layer, as observed in the electrode PANIT1, in second stage a more agglomerated structure of weak and open polymer chains is grown and formed, such behavior might be attributed to the fact that there a polymer side growth rate occurred much faster than the normal one during the initial stage, as perceived at PANIT2, these descriptions are present in the literature [27–29]. There is still another effect that can be explained by the oxidation of aniline on the electrode surface, one is favored being one favoring in comparison with the polymer chains oxidation, this observation was also discussed in another works carried out by the literature [25–29].
3. Characterization techniques Scanning Electron Microscopy (SEM), a Zeiss-Branded Device (DSM940A), was used to get information about the electrode morphology with the polyaniline. For the electrochemical characterization, the same system was used as the one for polymer synthesis, and a solution of 1.0 mol L−1 of H2SO4 was adopted as electrolyte. The electrochemical techniques used in the electrode were: cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. Cyclic voltammetry measurements were performed at different scanning speeds 200, 100, 50 and 25 mV·s−1, and for the galvanostatic charge-discharge tests, the current densities adopted were 1, 3 and 5 mA·cm−2. In both analyses the potential range used was −0.20 to 0.70 V. Before the measurements, the solutions were purged with nitrogen (N2) for 10 min and they were performed at room temperature. The electrochemical impedance spectroscopy tests were carried out in an analyzer module of frequency response (FRA) coupled to the 2
Solid State Ionics 346 (2020) 115198
J.C. Martins, et al.
The potential energy of the oxidation and reduction systems of polymers was determined by GCD measurements with the purpose of check their real capacitive behavior. According to Fig. 3, all the samples were tested in the same potential conditions used for the CV tests, the electrodes presented behavior with curves that are not symmetrical between the charge–discharge processes, thus characterizing pseudocapacitive materials, which are not linear, as is the case with electrodes that store energy by non-faradic processes. The initiation of the discharge curve presented a resistive behavior, due the current collector (i.e., the substrate electrode), which is characteristic of electrodes constituted of polyaniline and it is still noticeable the variable potential drop with time, indicating that there were consecutive electron exchanges on the electrode surface through faradaic reactions [34–37]. Such characteristics of the charge–discharge curves confirm the pseudocapacitive behavior of electrodes of this nature. As the current density increases the voltage drop also increases, which leads to a decrease in the material capacitance. Through parameters of the galvanostatic charge-discharge, the calculations of specific capacitance (Csc) were made through Eq. (1), according to [32].
Csc =
i td ∆Um
(1)
where i is the current, ΔU potential window, td discharge time and m the mass of the electrode. The values achieved by Csc using the current of 1 mA·cm−2 for of PANIT1 and PANIT2 were 295 F·g−1 and 185 F·g−1, respectively. The difference of 110 F·g−1 between the materials with the lowest and the highest deposition time is explained as follows: PANI formed in larger time, obtained a greater polymer mass, not necessarily this electrode (PANIT2) will give a better response, due to its morphological structure it presents polymeric chains stacking the same harder the penetration of electrolyte ions into the active sites of the polymer, although it has a better behavior in terms of GCD when compared to the PANIT1 synthesis, the polymer structure provides a better electrode/ electrolyte interaction, whose solution ions reach more effectively the material's active sites, this structural variation has been previously observed by SEM, these considerations are in line with the literature [27,28,30,34,37,38]. The polymers oxidation and reduction reactions associated with capacitive and resistive processes were evaluated by the electrochemical impedance spectroscopy technique. Fig. 4 shows the Nyquist plots by PANI potential, it is observed that the polymeric materials presented the same electrochemical behavior, however with different frequency magnitudes, the PANIT1 presented frequencies of higher magnitudes when compared to PANIT2, Table 1 exhibit values of behavior of the polymers resistive potentials with reference to the charge transfer resistance (Rct) of (−0.2 V, −0.1 V and 0.0 V), for the material PANIT1 Rct in 5450, 5799 and 1392 Ω (Ohm), while for PANIT2 obtained resistance was 3204, 4177 and 1207 Ω. It was observed in these potentials in the low-frequency regions, a slope of 45°, these behavior is similar in both depositions, indicating that the capacitive process is controlled the Warburg diffusion, this measure corresponds to the leucoemeraldine oxidation state in the two deposition conditions. However, it is important to emphasize that in these analyses in the insulation potential there was no solution resistance (Rs). It was also observed in Table 1 for the 0.1 V potentials in both materials their Rct obtained values of about 143 and 131 Ω, respectively. In these regions, their inclinations are greater than 45°, characterized by a diffusional control. At the potential of 0.2 V there were no Rcts at both times and in the low-frequency regions, its inclinations were close to 90°, which shows that the capacitive process is not controlled by diffusion. Analyzing the most conductive parameters, we see the potential 0.3 V corresponds to the emeraldine phase, since the parameters of 0.4 to 0.6 V correspond to the intermediate states, which are the
Fig. 2. Polyaniline voltammograms at different speed scanning a) PANIT1 and b)PANIT2.
In order to evaluate the various systems in terms of process reversibility, a step distinction analysis in oxidation-reduction processes was made from the CV measurements in Fig. 2. The shape of the voltammograms is characteristic of materials that present faradaic processes, that is, materials that store energy through oxidation and reduction reactions in the electrode surface. Their curves are different from the closed rectangular form of materials that store energy through nonfaradaic processes [30–32]. The chosen region of PANIT1 and PANIT2 for the voltammetry study was the potential window of −0.2 to 0.70 V, which shows the profile of the leucoesmeraldine/emeraldine redox pair (Scheme 1), these redox peaks embedded in a high background current indicate a pseudocapacitive performance of PANI, a slight anodic shift, this behavior is due to the nature of the substrate adopted in the work that is prone to submit such a change [31,33]. However, even with such behaviors observed at all rates analyzed, it still configures reversible reactions, whose control occurs both by the mass transfer step and by the step of transfer of charge, in this case, the Nernst equation is satisfied. What is still noticeable between the two materials is their current density at the scan rates, such behavior is because the material formed in longest time obtained the largest polymer mass and the stacking of the PANIT2 chains, this characteristic was already observed in the morphologies shown by SEM, such observations are also described in the works [14,30–32].
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Scheme 1. Leucoesmeraldine/emeraldine redox pair
Fig. 4. Electrochemical Impedance Spectroscopy Curves de a) PANIT1 and b) PANIT2.
Fig. 3. Load-Discharge Curves of Polyaniline at different current densities a) PANIT1 and b) PANIT2.
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synthesis time (PANIT1), and 40 mF·cm−2 for the PANIT2. For the specific capacitance low-frequency graph got by EIS, one can better see the materials electrochemical behavior, and it was found that the best regions of performance for the PANIT1 and by this analysis, we observed that intermediates potentials this polymer also exhibited capacitance values representative as already reported in the literature [21,22,31]. This behavior is associated with the magnifications of different values from one material to another, such characteristics in terms of electrochemical responses are directly associated to the synthesis method, the protonating agent adopted (H2SO4) and the time of deposition. Because the 0.2 V potential reached the best response at low-frequency capacitance, this parameter imitiates the cathodic reaction in CV for both PANT1 and PANIT2, however, the emerald phase (0.3 V) proved to be the most conductive in the two synthesis times, by GCD analysis the material with the shortest deposition time getting its higher specific capacitance and in the EIS (Nyquist) its inclination is close to 90°, these characteristics are strongly dependent on morphology, as PANIT2 has a more compact structure than PANIT1, this behavior has already been observed by SEM.
Table 1 Values of polyaniline resistive potentials. Resistive potentials of PANI E (V)
−0.2 −0.1 0.0 0.1
Values of Rct extracted by the Nyquist graphic PANIT1
PANIT2
5450 5799 1392 143
3204 4177 1207 141
5. Conclusions In this study, it was observed the PANIT1 and PANIT2 obtained in the potential 0.70 V, provided significant changes in the pseudocapacitive property and much of it is due to the morphology obtained in the materials, which assists the understanding of the electrochemical responses behavior. By the CVs, we observed that there was the formation of the leucoemeraldine/emeraldine redox processes, in of charge-discharge galvanostatic characterization (GCD) the PANIT1 presented a specific capacitance of 295 F·g−1 since the structure of this polymer observed by SEM provided a better electrode/electrolyte interaction. By the electrochemical impedance analyses through the Nyquist diagrams, we observed that the responses of higher amplitudes of frequencies of the better capacity behaviors were in the potential of 0.3 V that corresponds to the emeraldine base oxidation base. In the calculations for low-frequency capacitance by EIS, confirmed that the emeraldine phase is the most conductive, however, the benzoquinone and hydroquinone intermediate phases have capacitance values higher than 100 mF·cm−2. Therefore without concern for the polymer degration state and through the results of low-frequency capacitance we see that the polymer with the shortest deposition time behaves electrochemically similar to the result achieved in the charge–discharge analysis, such obtained results help to better understand and in detail the PANI oxidation and reduction processes.
Fig. 5. Behavior of Polyaniline Film Potentials in Low Frequency Capacitance.
benzoquinone and hydroquinone respectively [33], which also presents the capacitive character, there is no Rs in the described potentials, and the observed lines are close to 90°, which shows that the material is capacitive. It can be observed that in the materials PANIT1 and PANIT2, the value of 0.7 V. At such potential the pernigraniline phase begins, which is associated to degradation of the material. The purpose of the low-frequency specific capacitance study was to evaluate the capacitive potential of materials in regions where the frequencies are small [39], its calculation in this section is presented by Eq. (2), to assemble the low-frequency plots, with the capacitance of low-frequency (Clf), frequency (f) and the imaginary point of the impedance part (Zim).
Clf =
1 2πfZ im
(2)
Fig. 5 shows the potential behavior obtained by the specific capacitance low-frequency calculations, we observed that in the potentials of −0.2 to 0.1 V presented the character of greater resistivity. This aspect is related to the charge transfer, which is observed in the Nyquist impedance graphs, this resistivity behavior related to the reduction part of PANI where the value of 60 mF·cm−2 was reached at the parameter of 0.1 V for PANIT1, while for PANIT2 reached 20 mF·cm−2 at this same potential. For the parameters of 0.2 to 0.7 V, that correspond to the oxidized (conducting) region of the conducting polymer and the homogeneity of the materials are observed, with values above 140 mF·cm−2 for the PANIT1 synthesis and 60 mF·cm−2 for PANIT2 at the potential of 0.2 V and about 120 and 50 mF·cm−2 at the degradation potential of the material in the two polymers synthesis. The emeraldine phase is the most capacitive state of the material, and in 0.3 V obtained a value of 110 and 50 mF·cm−2 in the PANIT1 and PANIT2 materials. In the intermediate phase, which is the benzoquinone and hydroquinone corresponding to the 0.4 to 0.6 V and the values found in these potentials were of 120 mF·cm−2, obtained for the polymer made in a shorter
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Electrochemical behavior of polyaniline: A study by electrochemical impedance spectroscopy (EIS) in low-frequency João C. Martinsa,b, José C. de M. Netob, Raimundo R. Passosa, Leandro A. Pocrifkaa, a b
T
⁎
GEMATA - LEEN, Departamento de Química, Universidade Federal do Amazonas, CEP 69077-000 Manaus, AM, Brazil Departamento de Engenharia de Materiais, Universidade do Estado do Amazonas, CEP 69065-020 Manaus, AM, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Conductive polymer PANI Electrochemical impedance spectroscopy (EIS) Low-frequency
Conducting polymers are increasingly attracting the interest of many researchers in different areas due to their good conductivity, stability and easy preparation. Polyaniline is heavily studied in sensors, corrosion protection, electrochromic devices, and supercapacitors, however, even with numerous applications, there are still few pieces of literature that report their electrochemical behavior in regions low-frequency of by electrochemical impedance spectroscopy (EIS). In this work, polyaniline was electropolymerized by galvanic synthesis (chronoamperometry) in stainless steel substrate. Its performance was evaluated by cyclic voltammetry (CV) galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). It was observed in its lowfrequency pseudocapacitive analysis, in the benzoquinone and hydroquinone intermediate regions, capacitance values above 100 mF·cm−2, and in the emeraldine state, 295 F·g−1 was obtained. Such results provide a better and detailed understanding of PANI oxidation and reduction processes and could optimize the use of the polymer in various fields of applications.
1. Introduction The idea of the formulation of electrochemical capacitors is important for the development of more efficient capacitors, in view of the expansion of increasingly advanced devices, such as mobile phones, solar cells, notebooks, GPS (Global Positioning System) and hybrid automobiles that need more energy [1–3]. In addition, the development of effective and low-cost devices is a determining factor for the viability of industrial production. In these devices, materials are employed as conducting polymers, carbon and transition metal oxides. Among these materials, polymers have several advantages that make them suitable for application in supercapacitors, such as low-cost, reduction of environmental impacts, high conductivity when doped, high porosity, high storage capacity, reversibility and redox potential adjustable through chemical changes [4–7]. Polyaniline (PANI) is one of the most studied conductive polymers, thus attracting attention as a promising material for application in several areas such as sensors [6,8,9], corrosion protection [10,11], electrochromic devices [12,13] and supercapacitors [14]. Even with a range of applications of PANI, many research groups [15–17] are and have been studyng the influence of different aqueous environments on its conductivity behavior. Electrochemical impedance spectroscopy (EIS) is an excellent tool for studying redox-processes of ⁎
conducting polymers. By EIS the redox processes taking place on the electrode surface can be monitored as a function of the applied dc-potential. With impedance spectroscopy it is possible to get information of charge-transfer and diffusion processes as well as charging of the double-layer and the bulk polymer layer which is not possible by cyclic voltammetry [18]. Toušek [16], observed the mixed electron and proton conductivity of polyaniline films by EIS, at lower frequencies it is related to polarons that are formed by the redistribution of electrons in protonated PANI, and in higher frequencies, it is not affected and can it is attributed to holes released at higher potential. Fiordiponti [17] and Schrebler [19], carried out an impedance study of polyaniline films in aqueous acidic and neutral organic solutions, where the polymer capacitance was associated with the behaviors of its potentials observed by EIS, and so the capacitives properties could be separated from the faradaic a non-faradaic processes in high intermediate and low-frequency regions. In work of Biaggio [20], PANI electrochemically prepared in HClO4 and H2SO4 solutions was studied by EIS. Its behavior was analyzed by low-frequency differential capacity and its redox process, by the EIS studies. The rate of the in PANI is slower in PANI made in HClO4 than that in H2SO4, these behaviors are associated with the diffusion coefficient, which is related to transport processes ionic in the polymers films prepared in different solutions.
Corresponding author at: GEMATA - LEEN, Departamento de Química, Universidade Federal do Amazonas, CEP 69077-000 Manaus, AM, Brazil. E-mail address: [email protected] (L.A. Pocrifka).
https://doi.org/10.1016/j.ssi.2019.115198 Received 27 May 2019; Received in revised form 11 December 2019; Accepted 18 December 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
Solid State Ionics 346 (2020) 115198
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Roβberg [21], impedance spectra of electropolymerized polyaniline films were analyzed by the impedance theory, in order to evaluate characteristic of the PANI states. Whose observations on low-frequency capacitance increases linearly with of cyclic voltammetric redox charge of the PANI films. Radoševic [22], rated the polyaniline films by lowfrequency impedance spectra. The specific features of the polymers were controlled by charge transport processes, ion diffusion and migration in the first case, and ionic conductivities in the second case that are Warburg differences in relaxation times of diffusion and migration processes, or by the impedance of ionic channels of EIS in higher and intermediate frequencies. In his work Mondal [23] studied the effect of heating on PANI electrochemical activity by EIS. It was observed by low-frequency a decrease in pseudocapacitive activity attributed to an irreversible loss of water molecules in that system. Žic [24] analyzed polyaniline by EIS measurements in the low-frequency region and showed that the phase shift is under the influence of the applied potential and the response is related to the PANI layer region under influence of the refox process. Electroneutrality of the PANI layer is achieved by the counter-ions fluxes, which balance the electrons that enter/leave the electrode due to the redox process of PANI. The aim of this paper was to evaluate the synthesis of polyaniline by electrodeposition (ED) at different times and to test its pseudocapacitive properties using cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS) methods, to assess the capacitance properties of the material in lowfrequency regions extracted from EIS data, where the frequencies are low. The detailed understanding of the oxidation and reduction processes of PANI could help to optimize the use of PANI in various fields of applications.
Fig. 1. SEM in a) PANIT1and b) PANIT2.
2. Synthesis of PANI
Autolab. After polarization of 300 s, spectra were obtained in the frequency range of 0.01 to 10,000 Hz, where an alternating perturbation alternating from 10 mV peak to peak was applied. Spectra were collected between potentials of −0.20 to 0.70 V.
The substrates used as working electrodes were made of 304 stainless steel with dimensions (1 × 0.5 cm2). Substrates cleaning consisted of washing them with ethanol P.A. and in physical attack by sandblaster, to improve adhesion to PANI electrodeposition. An electrochemical system with four electrodes was adopted, in which the stainless-steel plate, where the materials were electrodeposited, as working electrode (WE); silver/silver chloride (Ag/AgCl) saturated with KCl as reference electrode (RE), and two platinum counters electrodes (CE). The galvanostatic technique was employed for the electrodeposition, at a potential of 0.70 V (700 mV) with a H2SO4 concentration of 0.5 mol L−1 and 0.1 mol L−1 of aniline. The produced materials were PANIT1 (900 s) and PANIT2 (1800 s). This potential was used to form PANI in its emeraldine oxidation state. The syntheses were carried out using a AutoLab potentiostat/galvanostat, model PGSTAT302N.
4. Results and discussions In order to verify the effect of time variation on PANI electrodeposition, SEM was used to characterize the electrode surfaces of the electrodes in order to report the polymers deposition behavior in the protonation level which is between the acid and the monomer, because the deposition time leads to differences in the formation of polymeric chains. In Fig. 1a) and b), PANIT1 and PANIT2 materials are shown, respectively. PANIT1 material image is more uniform and with more spacious cavities between the pores in nanometric dimensions [25,26], such characteristics are more useful for the electrolyte ions penetration into the polymer active sites in the electrochemical analyses compared to the synthesis of PANIT2, because the material formed in greater time, obtained a greater polymer mass, more compact structure due to the stacking of the PANI chains, what thus hinders the electrolyte penetration in the polymer active area [27,28]. PANI electrochemical formation occurs in two stages; first, there is the formation of a more compact and homogeneous layer, as observed in the electrode PANIT1, in second stage a more agglomerated structure of weak and open polymer chains is grown and formed, such behavior might be attributed to the fact that there a polymer side growth rate occurred much faster than the normal one during the initial stage, as perceived at PANIT2, these descriptions are present in the literature [27–29]. There is still another effect that can be explained by the oxidation of aniline on the electrode surface, one is favored being one favoring in comparison with the polymer chains oxidation, this observation was also discussed in another works carried out by the literature [25–29].
3. Characterization techniques Scanning Electron Microscopy (SEM), a Zeiss-Branded Device (DSM940A), was used to get information about the electrode morphology with the polyaniline. For the electrochemical characterization, the same system was used as the one for polymer synthesis, and a solution of 1.0 mol L−1 of H2SO4 was adopted as electrolyte. The electrochemical techniques used in the electrode were: cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. Cyclic voltammetry measurements were performed at different scanning speeds 200, 100, 50 and 25 mV·s−1, and for the galvanostatic charge-discharge tests, the current densities adopted were 1, 3 and 5 mA·cm−2. In both analyses the potential range used was −0.20 to 0.70 V. Before the measurements, the solutions were purged with nitrogen (N2) for 10 min and they were performed at room temperature. The electrochemical impedance spectroscopy tests were carried out in an analyzer module of frequency response (FRA) coupled to the 2
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The potential energy of the oxidation and reduction systems of polymers was determined by GCD measurements with the purpose of check their real capacitive behavior. According to Fig. 3, all the samples were tested in the same potential conditions used for the CV tests, the electrodes presented behavior with curves that are not symmetrical between the charge–discharge processes, thus characterizing pseudocapacitive materials, which are not linear, as is the case with electrodes that store energy by non-faradic processes. The initiation of the discharge curve presented a resistive behavior, due the current collector (i.e., the substrate electrode), which is characteristic of electrodes constituted of polyaniline and it is still noticeable the variable potential drop with time, indicating that there were consecutive electron exchanges on the electrode surface through faradaic reactions [34–37]. Such characteristics of the charge–discharge curves confirm the pseudocapacitive behavior of electrodes of this nature. As the current density increases the voltage drop also increases, which leads to a decrease in the material capacitance. Through parameters of the galvanostatic charge-discharge, the calculations of specific capacitance (Csc) were made through Eq. (1), according to [32].
Csc =
i td ∆Um
(1)
where i is the current, ΔU potential window, td discharge time and m the mass of the electrode. The values achieved by Csc using the current of 1 mA·cm−2 for of PANIT1 and PANIT2 were 295 F·g−1 and 185 F·g−1, respectively. The difference of 110 F·g−1 between the materials with the lowest and the highest deposition time is explained as follows: PANI formed in larger time, obtained a greater polymer mass, not necessarily this electrode (PANIT2) will give a better response, due to its morphological structure it presents polymeric chains stacking the same harder the penetration of electrolyte ions into the active sites of the polymer, although it has a better behavior in terms of GCD when compared to the PANIT1 synthesis, the polymer structure provides a better electrode/ electrolyte interaction, whose solution ions reach more effectively the material's active sites, this structural variation has been previously observed by SEM, these considerations are in line with the literature [27,28,30,34,37,38]. The polymers oxidation and reduction reactions associated with capacitive and resistive processes were evaluated by the electrochemical impedance spectroscopy technique. Fig. 4 shows the Nyquist plots by PANI potential, it is observed that the polymeric materials presented the same electrochemical behavior, however with different frequency magnitudes, the PANIT1 presented frequencies of higher magnitudes when compared to PANIT2, Table 1 exhibit values of behavior of the polymers resistive potentials with reference to the charge transfer resistance (Rct) of (−0.2 V, −0.1 V and 0.0 V), for the material PANIT1 Rct in 5450, 5799 and 1392 Ω (Ohm), while for PANIT2 obtained resistance was 3204, 4177 and 1207 Ω. It was observed in these potentials in the low-frequency regions, a slope of 45°, these behavior is similar in both depositions, indicating that the capacitive process is controlled the Warburg diffusion, this measure corresponds to the leucoemeraldine oxidation state in the two deposition conditions. However, it is important to emphasize that in these analyses in the insulation potential there was no solution resistance (Rs). It was also observed in Table 1 for the 0.1 V potentials in both materials their Rct obtained values of about 143 and 131 Ω, respectively. In these regions, their inclinations are greater than 45°, characterized by a diffusional control. At the potential of 0.2 V there were no Rcts at both times and in the low-frequency regions, its inclinations were close to 90°, which shows that the capacitive process is not controlled by diffusion. Analyzing the most conductive parameters, we see the potential 0.3 V corresponds to the emeraldine phase, since the parameters of 0.4 to 0.6 V correspond to the intermediate states, which are the
Fig. 2. Polyaniline voltammograms at different speed scanning a) PANIT1 and b)PANIT2.
In order to evaluate the various systems in terms of process reversibility, a step distinction analysis in oxidation-reduction processes was made from the CV measurements in Fig. 2. The shape of the voltammograms is characteristic of materials that present faradaic processes, that is, materials that store energy through oxidation and reduction reactions in the electrode surface. Their curves are different from the closed rectangular form of materials that store energy through nonfaradaic processes [30–32]. The chosen region of PANIT1 and PANIT2 for the voltammetry study was the potential window of −0.2 to 0.70 V, which shows the profile of the leucoesmeraldine/emeraldine redox pair (Scheme 1), these redox peaks embedded in a high background current indicate a pseudocapacitive performance of PANI, a slight anodic shift, this behavior is due to the nature of the substrate adopted in the work that is prone to submit such a change [31,33]. However, even with such behaviors observed at all rates analyzed, it still configures reversible reactions, whose control occurs both by the mass transfer step and by the step of transfer of charge, in this case, the Nernst equation is satisfied. What is still noticeable between the two materials is their current density at the scan rates, such behavior is because the material formed in longest time obtained the largest polymer mass and the stacking of the PANIT2 chains, this characteristic was already observed in the morphologies shown by SEM, such observations are also described in the works [14,30–32].
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Scheme 1. Leucoesmeraldine/emeraldine redox pair
Fig. 4. Electrochemical Impedance Spectroscopy Curves de a) PANIT1 and b) PANIT2.
Fig. 3. Load-Discharge Curves of Polyaniline at different current densities a) PANIT1 and b) PANIT2.
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synthesis time (PANIT1), and 40 mF·cm−2 for the PANIT2. For the specific capacitance low-frequency graph got by EIS, one can better see the materials electrochemical behavior, and it was found that the best regions of performance for the PANIT1 and by this analysis, we observed that intermediates potentials this polymer also exhibited capacitance values representative as already reported in the literature [21,22,31]. This behavior is associated with the magnifications of different values from one material to another, such characteristics in terms of electrochemical responses are directly associated to the synthesis method, the protonating agent adopted (H2SO4) and the time of deposition. Because the 0.2 V potential reached the best response at low-frequency capacitance, this parameter imitiates the cathodic reaction in CV for both PANT1 and PANIT2, however, the emerald phase (0.3 V) proved to be the most conductive in the two synthesis times, by GCD analysis the material with the shortest deposition time getting its higher specific capacitance and in the EIS (Nyquist) its inclination is close to 90°, these characteristics are strongly dependent on morphology, as PANIT2 has a more compact structure than PANIT1, this behavior has already been observed by SEM.
Table 1 Values of polyaniline resistive potentials. Resistive potentials of PANI E (V)
−0.2 −0.1 0.0 0.1
Values of Rct extracted by the Nyquist graphic PANIT1
PANIT2
5450 5799 1392 143
3204 4177 1207 141
5. Conclusions In this study, it was observed the PANIT1 and PANIT2 obtained in the potential 0.70 V, provided significant changes in the pseudocapacitive property and much of it is due to the morphology obtained in the materials, which assists the understanding of the electrochemical responses behavior. By the CVs, we observed that there was the formation of the leucoemeraldine/emeraldine redox processes, in of charge-discharge galvanostatic characterization (GCD) the PANIT1 presented a specific capacitance of 295 F·g−1 since the structure of this polymer observed by SEM provided a better electrode/electrolyte interaction. By the electrochemical impedance analyses through the Nyquist diagrams, we observed that the responses of higher amplitudes of frequencies of the better capacity behaviors were in the potential of 0.3 V that corresponds to the emeraldine base oxidation base. In the calculations for low-frequency capacitance by EIS, confirmed that the emeraldine phase is the most conductive, however, the benzoquinone and hydroquinone intermediate phases have capacitance values higher than 100 mF·cm−2. Therefore without concern for the polymer degration state and through the results of low-frequency capacitance we see that the polymer with the shortest deposition time behaves electrochemically similar to the result achieved in the charge–discharge analysis, such obtained results help to better understand and in detail the PANI oxidation and reduction processes.
Fig. 5. Behavior of Polyaniline Film Potentials in Low Frequency Capacitance.
benzoquinone and hydroquinone respectively [33], which also presents the capacitive character, there is no Rs in the described potentials, and the observed lines are close to 90°, which shows that the material is capacitive. It can be observed that in the materials PANIT1 and PANIT2, the value of 0.7 V. At such potential the pernigraniline phase begins, which is associated to degradation of the material. The purpose of the low-frequency specific capacitance study was to evaluate the capacitive potential of materials in regions where the frequencies are small [39], its calculation in this section is presented by Eq. (2), to assemble the low-frequency plots, with the capacitance of low-frequency (Clf), frequency (f) and the imaginary point of the impedance part (Zim).
Clf =
1 2πfZ im
(2)
Fig. 5 shows the potential behavior obtained by the specific capacitance low-frequency calculations, we observed that in the potentials of −0.2 to 0.1 V presented the character of greater resistivity. This aspect is related to the charge transfer, which is observed in the Nyquist impedance graphs, this resistivity behavior related to the reduction part of PANI where the value of 60 mF·cm−2 was reached at the parameter of 0.1 V for PANIT1, while for PANIT2 reached 20 mF·cm−2 at this same potential. For the parameters of 0.2 to 0.7 V, that correspond to the oxidized (conducting) region of the conducting polymer and the homogeneity of the materials are observed, with values above 140 mF·cm−2 for the PANIT1 synthesis and 60 mF·cm−2 for PANIT2 at the potential of 0.2 V and about 120 and 50 mF·cm−2 at the degradation potential of the material in the two polymers synthesis. The emeraldine phase is the most capacitive state of the material, and in 0.3 V obtained a value of 110 and 50 mF·cm−2 in the PANIT1 and PANIT2 materials. In the intermediate phase, which is the benzoquinone and hydroquinone corresponding to the 0.4 to 0.6 V and the values found in these potentials were of 120 mF·cm−2, obtained for the polymer made in a shorter
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