Electrochemical study of supercapacitor performance of polypyrrole ternary nanocomposite electrode by fast Fourier transform continuous cyclic voltammetry

Abstract

The supercapacitive behavior of polypyrrole/reduced graphene oxide/Au nano particles (Ppy/rGO/AuNPs) as a ternary composite electrode was studied by cyclic voltammetry (CV), galvanostatic charge/discharge (CD), impedance spectroscopy (EIS) and finally fast Fourier transform continuous cyclic voltammetry (FFTCCV) techniques. A composite electrode was synthesized electrochemically in a 1 M KCl solution containing the pyrrole monomer (0.1 M), rGO/AuNPs (0.2% wt) and 5 mM sodium dodecyl sulfate (SDS). Based on the CV study, the specific capacitances of the Ppy and Ppy/rGO/AuNPs electrodes were calculated to be 190 and 310 F g⁻¹, respectively. CD studies showed that using rGO/AuNPs in the structure of the composite electrode decreases the equivalent series resistance of composite film. FFTCCV is a novel and modern electrochemical technique that presents useful data for studying the performance of active materials. The results showed that after 1200 cycles the Ppy/rGO/AuNPs electrode can retain its capacitance compared with the Ppy electrode.

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1 Introduction

Supercapacitors are one of the energy storage systems that are attracting the attention of researchers. Due to their long cycle ability and high energy density, supercapacitors are under focus against dielectric capacitors and batteries.^1–4 Two categories are designed for classification of supercapacitors regarding the charge storage mechanism, electrical double layer capacitors (EDLCs) and pseudocapacitors. The first uses a carbon compound as the active material^5–7 and a redox material acts as the active material in the latter.^8–10 Conductive polymers (CPs) are one of the active materials that are used in pseudocapacitors due to some suitable electrical properties, such as high conductivity, and some chemical properties, such as easy synthetic procedure, good thermal and chemical stability, low cost and environmental friendliness.^11,12 Polyaniline (PANI), polypyrrole (Ppy), polythiophene and its derivatives are the most useful CPs that are applied for supercapacitors.^13–15 Among the CPs, Ppy has some benefits above others, such as the medium that is used for the charge storage process, longer stability than others and wider potential window.¹⁶ Nonetheless, CPs have some defects: low specific capacitance and low time life are their negative points, and the latter happens due to mechanical collision of counter ions with polymer filaments during the charge/discharge process.^17–21 To overcome these points, researchers suggest combining CPs with nano materials. The process of this phenomenon is that nano materials can connect CP filaments together and increase the mechanical stability of composite electrodes. Furthermore, using nano materials in a polymer matrix can enhance the capacitance of the polymer electrode.^{2,13,17,22–25} Carbonaceous and metal compounds are materials that can be combined with CPs. Carbon materials increase the surface of the composite electrode and metal compounds with a redox reaction join in the charge storage mechanism and enhance the capacitance of supercapacitors.^26–29

Graphene is a carbon material that created a new aspect in electrochemistry. The good electrical conductivity, high surface area and capability of this material to form a composite with CPs are some of graphene's features.^16,30–36 By using some highly conductive metals, such as Au nanoparticles, in the structure of graphene sheets, one can promote the electron transfer properties of graphene, which can increase the capacitance of this material and therefore CPs–graphene composites.^33,35,37 In previous work we reported the super capacitive behavior of PANI/nano structural manganese oxides using fast Fourier transform consecutive cyclic voltammetry (FFTCCV).¹¹ The results showed that this electrochemical method was very suitable for studying the performance of composite materials. In this work, we present the supercapacitive study of polypyrrole/reduced graphene oxide/Au nanoparticles (Ppy/rGO/AuNPs) in acidic media using FFTCCV technique. We synthesized the composite by electrochemical method and after characterization the electrochemical performance of this composite electrode in 0.1 M H₂SO₄ solution was assessed.

2 Experimental

All the chemical materials used in this work were Merck products of analytical grade and were used without further purification. Double distilled water was used throughout all experiments.

2.1 Materials characterization and electrochemical evaluation

Electrochemical experiments were carried out by an Autolab General Purpose System PGSTAT 30 (Ecochemie, Netherlands). A conventional three electrode cell with a glassy carbon electrode with an area of 0.03 cm² as the working electrode, platinum wire and an Ag/AgCl reference electrode (Argental, 3 M KCl) were used as the counter and reference electrodes, respectively. The EIS experiments were conducted in the frequency range between 100 kHz and 15 mHz with perturbation amplitude of 5 mV. Morphological investigations of the polymeric films were carried out by using SEM (Philips XL 30). X-ray diffraction patterns were obtained from an X-ray diffractometer (PANalytical X'Pert-Pro) with a Cu-K^α monochromatized radiation source and a Ni filter.

2.2 FFTCCV technique

The FFT experimental data collection was performed with the help of the following equipment: a setup of a computer, equipped with a data-acquisition board (PCL-818HG, Advantech Co.) and a custom-made potentiostat described in our previous works.^3,4,11 A computer program that was developed in Delphi6® environment was used for data acquisition and data processing. The signal calculation in this method was established based on the integration of net current changes over the scanned potential range. It must be noted that, in this case, the current changes at the voltammograms can be caused by various processes, which take place at the electrode surface or matrix of the CP and the composite film. In detail, a CV of the electrode was firstly recorded. Then, the existing high frequency noises were indicated by applying FFT on the collected data. With the help of this information, the cutoff frequency of the analog filter was set at a certain value where the noises were removed from the CV.

2.3 Preparation of graphene oxide (GO) and rGO/AuNP

rGO/AuNPs was synthesized as described in our previous report.³⁷ Graphite (10 g) and concentrated H₂SO₄ (230 mL) were stirred at a constant temperature. In this method, the temperature must be kept below 20 °C. Simultaneously, KMnO₄ was added to suspension such that the temperature of the mixture was fixed during the addition. After that, the reaction temperature was changed to 40 °C and the mixture was stirred for 1 h. Then, 500 mL of deionized water was added to the reaction cell by increasing the temperature to 100 °C. After this step H₂O₂ (2.5 mL) was added slowly, followed by addition of deionized water (500 mL). Then, the solution was washed with HCl (200 mL) and deionized water until the filtrate became neutral and the remaining impurities were removed. The product, graphite oxide, was exfoliated in deionized water in an ultrasonic bath to form graphene oxide (GO) nanosheets.

The GO powder was added to water-dispersed Au nanoparticles and uniformly dispersed by sonication for 1 h. This suspension was then stirred for 24 h at 40 °C. Afterwards, the mixture was filtered using a Buchner funnel and washed with deionized water three times. The final product was dried at 50 °C for 12 h. The XRD pattern and EDS spectrum of the rGO/AuNPs are shown in Fig. 1. The diffraction peaks at 38.1°, 64.5° and 77.5° correspond to the (111), (220) and (311) planes of AuNPs, respectively. Moreover, an additional peak is observed at about 43.6°, which seems to be due to the overlapping (200) reflection plane of Au at 2θ = 44.4° with the (100) plane of graphite.^38,39 Furthermore, Table 1 shows the results that were obtained from EDX analysis of rGO/AuNPs. As can be seen, the weight percentage of AuNPs is low.

Fig. 1 XRD pattern and EDX spectrum of rGO/AuNPs.

Table 1 Results for EDX analysis of rGO/AuNPs

Element	Line	W%	A%
C	Ka	61.92	68.72
O	Ka	37.5	31.24
Au	La	0.58	0.04
		100	100

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2.4 Synthesis of Ppy and Ppy/rGO/AuNPs composite electrodes

Ppy/rGO/AuNPs composite was synthesized by CV technique in 1 M KCl solution containing pyrrole monomer (0.1 M), rGO/AuNPs (0.2% wt) and sodium dodecyl sulfate (0.005 M), which were dispersed in solution by sonication. Ppy electrode was synthesized in the same solution without rGO/AuNPs and sodium dodecyl sulfate. Electro polymerizations were conducted by 10 cycles at a sweep rate of 50 mV s⁻¹ at a potential of 0 to 1 V. The mass of the Ppy films was approximated assuming a current efficiency for the electropolymerization process of 100%, using the equation where n_e = 2.30 accounts for 2.30 electrons per pyrrole during electrodeposition to form the partially oxidized Ppy (0.30 e more than the required 2 e for forming native Ppy).^40,41

Fig. 2 provides the TEM and SEM graphs of rGO/AuNPs, Ppy and Ppy/rGO/AuNPs composite film. As evident, the rGO sheets exhibit rippled and crumpled morphology and have a structure consisting of very thin layers. From Fig. 2(c and d), one can determine distinguishable differences between the Ppy and Ppy/rGO/AuNPs film. As illustrated, rGO/AuNPs distributed in the Ppy network to a form composite film and the Au nanoparticles that are presented in the TEM graph are very small in size.

Fig. 2 TEM graph of rGO/AuNPs (a). SEM images of rGO/AuNPs (b), Ppy (c) and Ppy/rGO/AuNPs electrodes (d).

3 Results and discussion

CV is an electrochemical technique that can provide very useful information about the nature of various electrodes in different media.^21,42,43 Fig. 3 presents the CVs of the Ppy and Ppy/rGO/AuNPs electrodes in 0.1 M H₂SO₄ solution at a scan rate of 25 mV s⁻¹. As can be seen, the shape of the CVs for the two electrodes are different in the area that is surrounded by the CV curve for the ternary composite electrode is much larger than that for the Ppy electrode. The SC of two electrodes can be calculated by using the following equation:where I is the current, m is the mass of reactive material and ν is the potential scan rate. The SC of Ppy and Ppy/rGO/AuNPs electrodes were found to be 190 and 310 F g⁻¹, respectively. There are two types of contribution in the composite structure that lead to enhancement in the capacitive behaviors of the electrode: the electric double-layer capacitance produced by graphene and the pseudo capacitive behavior of Ppy that is attributed to the structure of the ternary film electrodes. In comparison with the electrochemical performance of the individual materials, using rGO/AUNPs at very low amounts can enhance the SC of the composite electrode very distinctively, which shows the synergistic effect of rGO/AuNPs in the Ppy network.^37,44,45 Furthermore, the stability of the current is another feature of the ternary composite electrode. As can be seen in Fig. 3 for the Ppy electrode, the current in the anodic sweep reaches a maximum at 0.2 V and then decreases, but this phenomenon did not occur for Ppy/rGO/AuNPs and the current in the CV of the composite electrode retained its symmetrical shape compared with the CV for Ppy.

Fig. 3 CVs of Ppy and Ppy/rGO/AuNPs electrodes in 0.1 M H₂SO₄ solution at a scan rate of 25 mV s⁻¹.

One of the electrochemical properties of supercapacitor electrodes that shows their kinetic performance is the behavior at various scan rates. Fig. 4 shows the CV curves of the Ppy/rGO/AuNPs electrode at various scan rates in 0.1 M H₂SO₄ media. As presented, the excellent capacitive performance of the Ppy/rGO/AuNPs electrode is verified from these curves. As can be seen, CV curves of Ppy/rGO/AuNPs retain their rectangular shape with increasing scan rate. This behavior can be related to an ideal capacitive performance of the Ppy/rGO/AuNPs electrode.⁴⁶ The Ppy/rGO/AuNPs electrode retains its rectangular CCV shape until the scan rate of 100 mV s⁻¹. The deviation from rectangularity of the CVs becomes obvious as the scan rate increases. This phenomenon can be attributed to the electrolyte and film resistance, and this distortion is dependent on the scan rate. By increasing the sweep rate, deeper active sites in the composite material will not have enough time for reaction with ions from solution.

Fig. 4 CVs of Ppy/rGO/AuNPs electrodes at various scan rates in 0.1 M H₂SO₄ solution.

Fig. 5 presents the relation between the specific capacitance (SC) of two electrodes and scan rate in 0.1 M H₂SO₄ solution. The Ppy/rGO/AuNPs composite electrode shows specific capacitances of 290 and 85 F g⁻¹ at the scan rates of 2 and 300 mV s⁻¹, respectively, whereas the specific capacitance of the Ppy electrode decreased from 195 to 45 F g⁻¹ at the scan rates of 2 and 300 mV s⁻¹, respectively. As observed, the capacitance of two electrodes decreased over the entire range of scan rate, because in fast sweep rates just the outer porosities are used and the deeper ones are not accessible for the dope/undope process.¹¹ In Fig. 5, there are two slopes for decreasing the capacitance for the two electrodes. This phenomenon creates the idea that there are two types of active sites in both matrix electrodes. One of the important parameters that can describe the type of active site is the slope of the change in SC vs. scan rate diagram. As can be seen from Fig. 5, the SC for the ternary electrode changed by a lower slope than that for the Ppy electrode, which shows that there are more accessible active sites for the composite electrode than for the Ppy film due to the independency of SC to scan rate.

Fig. 5 Specific capacitance of Ppy and Ppy/rGO/AuNPs composite electrodes as a function of scan rate in 0.1 M H₂SO₄ solution.

To highlight the capacitance characteristic of the Ppy/rGO/AuNPs ternary composite electrode, the galvanostatic charge/discharge technique has been used. Fig. 6 shows the charge/discharge behavior of the Ppy and Ppy/rGO/AuNPs electrodes in the potential range from 0 to 0.75 V at the current density of 0.9 A g⁻¹. The triangular shape between this potential range indicates the good coulombic efficiency and ideal capacitive behavior of Ppy/rGO/AuNPs as an electrode for supercapacitors. Furthermore, using rGO/AuNPs decreased the equivalent series resistance (ESR) for Ppy/rGO/AuNPs composite electrode. This is related to the internal resistance and appears in the curves during the change of current sign and vice versa.

Fig. 6 Galvanostatic charge and discharge measurements of Ppy and Ppy/rGO/AuNPs electrode in 0.1 M H₂SO₄ solution at a current density of 0.9 A g⁻¹.

The specific capacitance of the two electrodes can be calculated from Fig. 6 by the following equation:

In this equation, is the slope of the discharge curve after the voltage drop at the beginning of each discharge (ESR) and m is the mass of the composite electrodes. The calculated SC were 120 and 190 F g⁻¹ for the Ppy and Ppy/rGO/AuNPs electrodes, respectively.

Fig. 7 presents the charge–discharge curves of the Ppy/rGO/AuNPs electrode at various specific currents. As can be seen, the SC magnitudes of the ternary composite electrode decreased by enhancing the specific current due to intercalation of ions at the surface of the active materials in the electrode/electrolyte interface. Another description is that in low specific currents there is enough time for the insertion and deinsertion of ions into all the porosities of the active materials in the electrode/electrolyte interface. The highest SC for composite electrode is obtained when the current density for the charge/discharge process is 0.9 A g⁻¹.

Fig. 7 Galvanostatic charge–discharge curves of Ppy/rGO/AuNPs electrode at 0.9, 2.2, 4.4, 6.6, 8.8, 11 and 16 A g⁻¹ in 0.1 M H₂SO₄ solution.

To investigate some electrochemical supercapacitive and conductivity behaviors of the two electrodes, the EIS technique was performed.

Nyquist plots of Ppy and Ppy/rGO/AuNPs electrodes at open circuit potential (OCP) are illustrated in Fig. 8. As can be seen, the solution resistance (R_s) can be found by reading the real axis value at the high frequency intercept for both electrodes. The difference between the magnitudes of R_s can be attributed to differences in the ohmic resistance of the two electrodes or the amount of OCP that was applied for the electrodes. As presented in the figure, both plots have a semicircle in high frequencies, which is related to the charge transfer resistance caused by the faradic reactions and the double-layer capacitance (C_dl) at the contact interface between the electrode and the electrolyte solution. A resistance with a slope of 45° in the curve, called Warburg resistance (Z_W), is a result of the frequency dependence of ion diffusion/transport from the electrolyte to the electrode surface.^31,47–49

Fig. 8 Nyquist plots recorded from 10 kHz to 0.01 Hz with an AC amplitude of 5 mV for Ppy and Ppy/rGO/AuNPs electrodes in 0.1 M H₂SO₄ solution.

As observed from Fig. 8, the magnitude of R_ct in the Ppy/rGO/AuNPs electrode was smaller than that in the Ppy film, which shows that the presence of rGO/AuNPs in the polymer matrix enhanced the conductivity and charge transfer performance of the composite electrode. The low frequency capacitance (C_lf) of each film was determined from eqn (3).

In this equation, Z′′ is the imaginary component of impedance at the lowest frequency in the Nyquist diagram (f).

From this equation, the SC of the Ppy and Ppy/rGO/AuNPs electrodes were calculated to be 160 and 280 F g⁻¹, respectively. These results showed that the SC of the ternary composite electrode is nearly two times more than that of the polymer electrode. Furthermore, CV and CD results confirmed the data obtained by the EIS method.

FFTCCV technique could be considered as the best tool for examining the changes in the CVs and charge storage of a capacitor over time. Furthermore, by using this technique one can study the behavior of an electrochemical system momentarily.¹¹ The change in electrochemical behavior of the composite electrode can be observed in the 3D CVs over time. CVs of composite electrode in the range of 0–40 000 s are presented in Fig. 9. As illustrated, over the whole range of time the composite electrode retains its symmetrical shape and just at the beginning of the experiment the CVs improve and get wider. This phenomenon can be attributed to the reason that in the primary CVs there are some impurities that block some active sites of composite electrode and, after sweeping, these impurities are removed from the composite matrix and therefore those active sites can join the reaction process.

Fig. 9 3D cyclic voltammograms of Ppy/rGO/AuNPs ternary electrode as a function of time in 0.1 M H₂SO₄ at a scan rate of 50 mV s⁻¹.

Fig. 10 presents 3D differential CVs of the composite electrode obtained from Fig. 9. Some useful information about the performance of the electrode can be obtained from this figure. As can be seen, by increasing the time, the oxidation part of the diagrams changed and the area that is surrounded by the curves decreases. Although the composite electrode can retain its whole shape, after some time, the CVs of the composite electrode lose their shape a little. The change in the shape of the diagram is related to the drop in current in the oxidative half cycles of the composite electrode.¹¹

Fig. 10 3D differential voltammograms of Ppy/rGO/AuNPs electrode measured at 50 mV s⁻¹.

Another feature of CPs for using as an active material in supercapacitors is the stability of these electrodes after continuous cycles. Fig. 11 presents the stability of two electrodes using consecutive CVs in 0.1 M H₂SO₄ at a scan rate of 50 mV s⁻¹. As can be seen, the Ppy electrode in the time range between 0 and 35 000 s loses its capacitance near 50% but this phenomenon doesn't occur for the ternary composite electrode. As shown, the Ppy/rGO/AuNPs electrode retained its capacitance in this time range. The pattern shows that at the initial cycles the SC of the composite electrode increased from 180 to 204 F g⁻¹, which can be attributed to the fact that in the early cycles there are some active sites and porosities in the composite electrode that are occupied by some impurities and maybe solvent molecules. By conducting the cycles, these materials exited from those sites and therefore the number of active sites of the composite electrode increased. This modification of the composite film results in the increased SC of composite film. After growth, the SC of the Ppy/rGO/AuNPs electrode remained constant until 30 000 s and then decreased. The sharp decrease in the stability percentage of the composite electrode can be attributed to the blocking of some of active sites with negative ions or the degradation of polymer chains due to a change in the polymer volume in the dope/undope process. These results showed that using rGO/AuNPs in the Ppy matrix can enhance the stability of the composite electrode through consecutive cycles.

Fig. 11 Stability of the two electrodes as a function of time in 0.1 M H₂SO₄ at a scan rate of 50 mV s⁻¹.

4 Conclusion

In this paper, some electrochemical features of Ppy/rGO/AuNPs as a ternary composite electrode for use in supercapacitors were studied. The data showed that using rGO/AuNPs in the structure of the Ppy electrode results increases the capacitance of the composite electrode, decreases the ESR of the composite electrode and enhances the stability of the composite film through consecutive CVs. FFTCCV is an applicable and useful technique for studying the performance of supercapacitive materials. Studying the electrochemical performance and stability of electrodes are the advantages of using this technique.

Acknowledgements

The authors are grateful to the Research Council of the University of Shahid Beheshti and the Iranian Nano Council for the financial support of this work.

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