**3.2 Evaluation of conductivity and electrochemical behaviour by electrochemical impedance spectroscopy (EIS)**

Electrochemical impedance spectroscopy (EIS) is a measurement technique which allows for a wide variety of coating evaluations. EIS is an effective method to probe the interfacial properties of surface-modified electrodes. EIS has been used to characterize the electrical properties of the electropolymerized films [Kiani et al., 2008a, 2008b]. The electrochemical behaviour of polypyrrole changes in the presence of SNS .In order to choose a suitable electrical equivalent circuit for EIS experimental data fitting, one must take in consideration the physicochemical picture of the system under study. In other words, each element of the equivalent circuit should have a physicochemical aspect attributable to it. In the model circuit chosen, Rs presents the uncompensated resistance of the solution between working and reference electrode. CPE1 and R1 stand for the dielectric and resistive characteristics of the conductive polymer on the GC electrode, respectively. In this case R1 is a reverse measure of polymer conductivity. CPE2 and R2 show the capacitance and resistance of the polymer/GC interface. As is evident from the high values of R2 for all as well as the Nyquis plot of studied polymers, due to the fact that polymer layer is impermeable to the ionic charge carrier species, low frequency behaviour of the polymer/GC interface tends to be of capacitive nature. Again more evidence for this fact is reflected in the values of nCPE2 which for all studied samples is not very far than unity. Our main aim by EIS studies was to determine the polymer layer bulk resistivity (or its reciprocal i.e conductivity). In the selected equivalent circuit R1 corresponds to this parameter. The studied electrical parameters were calculated using Zview(II) software. All fitting results are presented in Table 2.


Rs : uncompensated resistance of the solution

R1 and CPE1 : dielectric and resistive characteristics of the conductive polymer

R2 and CPE2 : capacitance and resistance of the polymer/ GC interface

Y0 : CPE Admittance

n : CPE exponent

Table 2. Impedance parameters obtained by fitting the EIS data of poly(Py), poly(SNS) and poly(Py-SNS) on the GC electrode in 3.5% NaCl

Electrosynthesis and Characterization

**5. Acknowledgment** 

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**6. References** 

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of Polypyrrole in the Presence of 2,5-di-(2-thienyl)-Pyrrole (SNS) 129

the presence of SNS. By considering the fact that decreasing the Rct leads to an increase in conductivity, it is predictable that the film of polypyrrole formed in the presence of SNS will be more conductive. In the presence of SNS, value of electrical double layer capacitance (CPE1) rises, indicating a probable increase in the electrode surface area. There is a good complementary agreement between the results of CV and EIS measurements. From these results it can be concluded that the produced polypyrrole containing small amount of SNS has better performance compared to polypyrrole alone for production of batteries,

The authors acknowledge Mr. I. Ahadzadeh, for his kind help in EIS measurements.

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Fig. 9. Nyquist plots for poly(Py), poly(Py-SNS)(100:1 mole ratio) and poly(SNS) in 3.5% (W/V) NaCl solution: A) Exploded view in the high frequency range, B) Proposed equivalent circuit

According to these results (Table 2), we can notice a decrease in the charge transfer resistance value in the case of the polypyrrole in the presence of SNS systems as compared to polypyrrole alone. The Rct (Rct: charge transfer resistance) values obtained for polypyrrole and poly(SNS) are 5.81 and 2282 *Ω.cm2* respectively. This value decreases in the presence of SNS to 1.46 *Ω.cm2*. The polypyrrole film formed in the presence of SNS is more conductive. On the other hand, in the presence of SNS, value of the capacitance of the double layer, CPE1, rises from 8.0E-4 to 0.67 μF.cm-2 which can be attributed to an increase in the electrode surface area. This change in the capacitance strongly supports the hypothesis of the incorporation of SNS in the polypyrrole film. Also, these results support the results of CV in the Figure 9. In the presence of SNS, the conductivity of polypyrrole is improved. Increased value of CPE1 for polypyrrole in the presence of SNS compared to pure polypyrrole confirmed the easy electron transfer of ferro/ferricyanide redox system for poly(Py-SNS) (Fig. 8). Improvement of the conductivity, electroactivity and redoxability of polypyrrole containing SNS leads it to extensive applications in many fields.

### **4. Conclusions**

The resulted poly(Py-SNS)(100:1 mole ratio) showed a considerable increase in the electroactivity, redoxability, and the rate of polymerization in comparison to polypyrrole alone. The cyclic voltammograms of electron transfer ferro/ferricyanide redox system on different modified GC electrode showed that the rate of charge transfer for polypyrrole in the presence of SNS increased in comparison to pure polypyrrole. In addition, the conductivity of polypyrrole was studied by electrochemical impedance spectroscopy. The obtained Rct value for polypyrrole is 5.81 *Ω.cm2*, whereas the value decreases to 1.46 *Ω.cm2* in the presence of SNS. By considering the fact that decreasing the Rct leads to an increase in conductivity, it is predictable that the film of polypyrrole formed in the presence of SNS will be more conductive. In the presence of SNS, value of electrical double layer capacitance (CPE1) rises, indicating a probable increase in the electrode surface area. There is a good complementary agreement between the results of CV and EIS measurements. From these results it can be concluded that the produced polypyrrole containing small amount of SNS has better performance compared to polypyrrole alone for production of batteries, capacitors, diodes, electrochromic devices, sensors and etc.
