Boosting the supercapacitive properties of polypyrrole with chitosan and hybrid silver nanoparticles/nanoclusters

Abstract

We report a one-step route to hierarchical polypyrrole/chitosan decorated with hybrid Ag nanoparticles/nanoclusters (Ag@PPy/CS) via electrodeposition. The ternary nanocomposite electrode was applied as an electrode material for a supercapacitor. Experimental results showed that the ternary nanocomposite has a specific capacitance of 513 F g⁻¹ at 0.2 A g⁻¹, which was greatly improved from that of the PPy electrode (273 F g⁻¹). The chitosan provided excellent hydrophilicity to the ternary nanocomposite, at the same time it controlled the growth of metallic Ag. The ultra-small Ag nanoparticles (1–2 nm) enhanced the electrical conductivity of the PPy while the Ag nanoclusters (30–80 nm) acted as spacers to prevent the restacking of the electrode films as well as contributing to the electrical conductivity. The as-assembled symmetric supercapacitor of the ternary nanocomposite delivered a specific capacitance of 183 F g⁻¹ and outstanding cycling stability with 98.3% capacitance retention after 1000 charge/discharge cycles. Considering the facile preparation and moreover the exceptional electrochemical performance, the Ag@PPy/CS nanocomposite electrode could be well-suited for high-performance supercapacitors.

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1. Introduction

The increasing demand for energy has triggered enormous research activities for energy storage and conversion from clean and renewable energy sources. Electrochemical capacitors, also known as supercapacitors, are promising energy storage devices for many applications including portable power systems, hybrid electric vehicles, consumer electronics, and power back-up systems.^1,2

Generally, supercapacitors are classified into two types; (i) electric double layer capacitors (EDLC), which are carbon-based materials that depend on the non-faradaic charge separation at the electrode/electrolyte interface, and (ii) pseudocapacitors, which are made of metal oxides and conducting polymers and undergo faradaic charge transfer across the active material of the electrodes. The conducting polymer, polypyrrole (PPy) is particularly promising and one of the most widely used polymers owing to its good electrical conductivity, ease of synthesis, low cost, its excellent environmental stability and electrochemical processing flexibility.³ The general structure of all PPy consist of a regularly alternating single (C–C) and double (C=C) bonds, forming a region of overlapping p-orbitals, bridging the adjacent single bonds. This will allow a delocalization of π-electrons across all the adjacent aligned p-orbitals, giving rise to a π-conjugated backbone throughout the polymer chains. The π-conjugated backbone is responsible for the generation and propagation of charge carriers, making the polymers intrinsically conducting. The redox reaction associated with doping and dedoping of the PPy chains give rise to its pseudocapacitive properties.⁴

Recently, researches have shifted their focuses on the synthesis of PPy composites electrode in order to reinforce the stability of the PPy, as well as to improve the capacitance of PPy. Hybrids of PPy that combines carbonaceous materials or metal oxides with PPy include graphene,⁵ carbon nanotubes,⁶ activated carbon,⁷ carbon aerogel,⁸ MnO₂,⁹ and CoO¹⁰ have been widely studied. Carbonaceous materials had shown to improve the mechanical strength and electrical conductivity of PPy while metal oxides were employed to increase the energy density of the PPy.¹¹

Another effective way that is worthwhile to explore is by incorporating the highly conductive metallic silver into the structures of PPy to assist the electron transport throughout the conjugated system of PPy. Zhao et al. studied the effect of different morphologies of Ag@PPy core–shell structures on the electrical conductivity of the composite using sodium dodecyl benzyl sulfonate (SDBS) as a template.¹² The concentration of sodium dodecyl SDBS were shown to affect the morphologies of the Ag@PPy core–shell structures. The electrical conductivity increased from 11 ± 2 to 165 ± 5 S cm⁻¹ when the morphology of the Ag@PPy core–shell changed from spherical to nanofibrous. The formation of Ag@PPy composite nanotubes were demonstrated by Yang et al.¹³ In the presence of polyvinylpyrrolidone (PVP), Ag nanoparticles were uniformly adsorbed onto the surface of the PPy nanotubes, introducing new trap sites on the surface of PPy nanotubes. This effectively enhanced its chemiresistor response towards ammonia as a result of the excellent charge transmission in the nanocomposite with the introduction of highly conducting Ag nanoparticles.

There are limited studies on Ag@PPy for supercapacitors applications. In a recent study by Patil et al., PPy/polyacrylic acid (PAA)/Ag nanocomposite electrodes have been synthesized for supercapacitor applications through the polymerization of PPy followed by dipping in a solution of PPy, PAA and silver nitrate.¹⁴ The nanocomposite achieved a maximum specific capacitance of 226 F g⁻¹ at 10 mV s⁻¹. In our previous report, we studied the effect of the concentration of silver nitrate on the electrochemical performance of Ag@PPy nanocomposite electrode. The unique architectures of hybrid Ag nanoparticle/nanocluster-decorated PPy and the reduced resistivity of the electrode gave a specific capacitance of 414 F g⁻¹ at 0.2 A g⁻¹.¹⁵

Besides increasing the electrical conductivity of PPy, excellent electrode wettability is also indispensable for producing outstanding supercapacitors. Many studies have demonstrated that the hydrophilicity of an electrode surface can enhance the supercapacitive properties of an active material.^16–19 An electrode with good wettability enables better infiltration of the electrolyte into the framework of the electrode. The incorporation chitosan (CS) is able to provide good hydrophilic properties to an electrode material due to its ability to form hydrogen bonding from its hydroxyl and amine groups. Furthermore, it is also an attractive biomaterial with many unique properties such as environmental friendly nature, low cost, nontoxicity, high mechanical strength, and antimicrobial activity that have found its niche in a variety of applications.^20–24

In the present investigation we attempt to combine the merits CS and Ag with PPy to further augment the supercapacitive characteristics of PPy. The roles of CS and Ag on the structural and electrochemical properties on the Ag@PPy/CS ternary nanocomposite were discussed. The electrochemical performance was first investigated using a three-electrode configuration followed by the fabrication of symmetric supercapacitor of Ag@PPy/CS.

2. Experimental methods

The Ag@PPy/CS nanocomposite electrode was electrochemically deposited from an aqueous solution placed in a one-compartment cell. The deposition solution contained 0.1 M NapTS, 0.1 M pyrrole, 0.05 M AgNO₃ and 0.75 wt% CS. The deposition solution was stirred for 5 min to allow the redox reaction between the pyrrole and Ag⁺. The ternary nanocomposite was electrodeposited onto a graphite sheet substrate (2 × 1 cm) in a three-electrode system with platinum as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. A constant potential of +0.8 V was applied until an accumulated charge of 4 C was reached and the sample-coated electrodes were rinsed with water remove any unreacted materials from the deposition solution. Similarly, PPy, Ag@PPy and PPy/CS electrodes were also prepared for comparison. The mass of the deposited material was in the range of 2.0–2.4 mg.

Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra were recorded on a Perkin-Elmer FT-IR spectroscope (model 1725x). The morphologies of the prepared electrodes were examined using a field emission scanning electron microscope (FESEM, FEI Quanta SEM Model 400F) and a high resolution transmission electron microscope (HRTEM, Hitachi HT-7700). Water contact angle were measured on a goniometer (Dataphysics, OCA, 15EC).

The electrochemical performances of electrodes were measured using Gamry Reference 600 using 1.0 M H₂SO₄ as the electrolyte to better understand the electrochemical properties of the Ag@PPy/CS nanocomposites. The electrochemical tests were firstly performed with a three-electrode setup, where the as-prepared nanocomposite, platinum wire and SCE was employed as the working electrode, counter electrode and reference electrode, respectively.

3. Results and discussion

3.1 Synthesis and formation mechanism

The Ag@PPy/CS nanocomposite electrode was formed via a facile and easily scalable electrodeposition method. The as-prepared deposition solution was subjected to electrodeposition under a constant potential of +0.8 V to coat the Ag@PPy/CS nanocomposite onto a piece of graphite sheet. It is interesting to note that the reduction of Ag and the deposition of CS are only possible at negative potentials. However, they are also feasible under anodic potentials in the presence of pyrrole. In the former case, pyrrole acted as a reducing agent²⁵ to reduce the Ag⁺ ions to Ag. In the preparation step of the deposition solution, the redox reaction was initiated when the AgNO₃ was added. This causes the pyrrole to oxidize to form radical cations of pyrrole monomers, releasing electrons in the process to reduce Ag⁺ ions to Ag. During the electrodeposition, the release of free electrons from polymerization of pyrrole continues to reduce Ag⁺ ions to Ag. In the latter case, the pyrrole monomers were already entrapped in the CS biopolymer due to the hydrogen bonds formed between the hydroxyl and amine groups of CS with the amine group of pyrrole monomers. During the polymerization of pyrrole on the electrode surface, the attached pyrrole monomers on CS will be attracted to the electrode surface and propagate to form PPy/CS films with the simultaneous doping of Ag on the PPy/CS films, producing the Ag@PPy/CS nanocomposite electrode.

CS has also been reported to act as a reducing agent in either basic condition, heating, or both.^26–28 The basic suspension and the degradation products of CS (e.g. glucosamide) during heating may supply electrons and function as a reducing agent.²⁹ Since no base and heating were involved in this work, the role of CS as a reducing agent was unlikely. However, CS can act as a stabilizing agent, protecting the Ag particles from agglomeration.^30,31

3.2 Material characterization

Fig. 1a shows the FTIR spectra of the CS, PPy, Ag@PPy, PPy/CS and Ag@PPy/CS nanocomposite. For the CS powder, the broad band around 3318 cm⁻¹ is due to the stretching vibrations of the primary amine (–NH₂) and hydroxyl groups (O–H) in CS.²¹ The bands at 2876 cm⁻¹ and 1588 cm⁻¹ were attributed to the aliphatic C–H stretching and –NH₂ bending vibration, respectively.²⁰ C–O stretching mode of –CH₂–OH group was observed at 1381 cm⁻¹ while a broad peak at 1027 cm⁻¹ was attributed to the C–O–C stretching.³²

Fig. 1 (a) FTIR of CS, PPy, Ag@PPy, PPy/CS and Ag@PPy/CS (b) XPS spectra of Ag@PPy and Ag@PPy/CS nanocomposite electrodes at Ag 3d core level spectra.

The spectra of PPy and Ag@PPy showed the typical N–H stretching at 3340 cm⁻¹, the peak located 3000 cm⁻¹ was assigned to aromatic C–H stretching of the pyrrole ring and the peak at 1622 cm⁻¹ was corresponded to the C=C vibration of the pyrrole ring. The peak at 1490 cm⁻¹ was contributed by the C–N stretching vibration while the peak at 1415 cm⁻¹ was due to the pyrrole ring vibration. Absorption bands at 1278, 1094 and 990 cm⁻¹ were corresponded to C–H in-plane deformation, N–H in plane deformation vibration and the C–H out-of-plane vibration, respectively.^33,34

The infrared spectrum of the PPy/CS and Ag@PPy/CS nanocomposites showed a broad absorption band centered around 3313 cm⁻¹ due to the overlapping of N–H and O–H stretching vibration of CS with the amine group of PPy.³⁵ The absorption band corresponded to –NH₂ bending vibration in the CS at 1588 cm⁻¹ was not observed in the PPy/CS and Ag@PPy/CS nanocomposite. Instead, a new peak attributed to the asymmetric NH₃⁺ bending at 1649 cm⁻¹ appeared in the CS nanocomposites, indicating the protonation of the amine group in the CS.^20,36 The CS in the nanocomposites remained protonated because the electrodeposition took place under an anodic potential of +0.8 V in the presence of pyrrole. CS films that are electrodeposited under cathodic potentials will give deprotonated CS films. This is due to the fact that cathodic electrodeposition takes advantage of CS's pH dependent solubility. At negative potentials, the evolution of hydrogen leads to a locally high pH near the cathode surface. The CS becomes deprotonated and is no longer soluble under basic conditions, depositing CS films on the electrode.^20,37

Fig. 1b shows the XPS spectroscopic analysis of core level spectra for Ag 3d of Ag@PPy and Ag@PPy/CS. The Ag 3d_5/2 peak at 368.0 eV and Ag 3d_3/2 peak at 374.1 eV with spin energy separations of 6.1 eV indicate the presence of metallic silver in the nanocomposite electrodes.³⁸ XRD patterns of the PPy nanocomposite electrodes as well as PPy and CS were shown in Fig. S1.† The diffraction pattern of PPy and its nanocomposites showed a broad peak at 21.9°, indicating that the electrodeposited PPy is in the amorphous phase. CS exhibited two peaks at 9.6° and 20.3° which are the typical fingerprints of CS.²³ However, both characteristic peaks of CS were not observed in the nanocomposite (PPy/CS and Ag@PPy/CS) suggesting that the CS did not induce the phase change of PPy. When Ag is present, three diffraction peaks positioned at 38.1°, 44.3° and 64.4° can be indexed to the (1 1 1), (2 0 0) and (2 2 0) diffraction planes of metallic Ag (JCPDS no. 89-3722), respectively. This also confirmed that the hybrid Ag particles are crystalline in nature.³⁹

3.3 Morphological studies

Fig. 2 shows the FESEM morphologies of the as-prepared samples. Fig. 2a shows that the PPy electrode had the typical cauliflower morphology. Upon the addition of CS, the PPy/CS electrode surface appeared to be rougher with more noticeable protrusion of bulbous-like spheres compared to PPy as seen in Fig. 2b. The morphology of Ag@PPy nanocomposite electrode is shown in Fig. 2c. Two distinguishable sizes of Ag were observed; namely Ag nanoparticles and Ag nanoclusters. The bigger Ag nanoclusters (55–100 nm) can be seen decorated evenly alongside the smaller Ag nanoparticles on the PPy film. As for the Ag@PPy/CS nanocomposite electrode, both Ag nanoparticles and Ag nanoclusters can be observed as well. However, the Ag nanoclusters appeared to be smaller in size, with their particle size in the range of 30–80 nm, as shown in Fig. 2d. As mentioned in our previous studies, the Ag nanoparticles were already formed in the deposition solution during the redox reaction between pyrrole and Ag⁺ ions. The oxidation of the pyrrole during electrodeposition at the electrode surface continues to supply electrons to reduce the remaining Ag⁺ ions to Ag nanoclusters.¹⁵ The significantly higher rate of reduction of Ag⁺ during electrodeposition gave rise to the bigger particle size of Ag nanoclusters. The base morphology of Ag@PPy/CS is also similar to that of PPy/CS where bulbous-like spheres can be seen projecting out of the surface.

Fig. 2 FESEM images of (a) PPy, (b) PPy/CS, (c) Ag@PPy and (d) Ag@PPy/CS nanocomposite electrodes electrode. Insets of (c) and (d) show the lower magnification images of the respective nanocomposites.

The structure of the Ag@PPy and Ag@PPy/CS nanocomposite electrodes was characterized by HRTEM and shown in Fig. 3. The Ag nanoclusters in both nanocomposite electrodes can be seen in Fig. 3a and c. An obvious decrease in the size of Ag nanoclusters was observed in Ag@PPy/CS due to the addition of CS. Similarly, the ultra-small Ag nanoparticles also showed reduction in size when CS is present in the nanocomposite. The Ag nanoparticles from Ag@PPy displayed a size range of 2–4 nm (Fig. 3b) while the Ag nanoparticles from Ag@PPy/CS displayed an even smaller size range of 1–2 nm as seen in Fig. 3d. It was found that the addition of CS caused an apparent decrease in the particle sizes of both Ag nanoclusters and Ag nanoparticles. Ag⁺ ions readily form coordinate bonds with the lone pair electrons from the amine groups of PPy. When CS is added, more coordinate bonds can be formed with the Ag⁺ ions due to the presence hydroxyl groups in the CS, providing more sites for the reduction of Ag⁺ ions. Furthermore, the long polymer chains of CS retarded the growth of metallic Ag sterically by protecting the particles from agglomeration and slowing the particle growth, facilitating the particle size reduction of Ag.^40,41

Fig. 3 HRTEM images of (a) Ag nanoclusters, (b) Ag nanoparticles from Ag@PPy and (c) Ag nanoclusters and (d) Ag nanoparticles from Ag@PPy/CS.

The schematic representation for the one-step electrodeposition of Ag@PPy/CS nanocomposite is illustrated in Fig. 4. Color change can be observed in the reaction vessel during the preparation of the deposition solution indicating the initial formation of Ag particles and oligomers of pyrrole.

Fig. 4 Schematic representation for the formation of Ag@PPy/CS nanocomposite via one-step electrodeposition.

3.4 Surface wettability studies

The wettability of a solid with water depends on the interfacial tensions between water/air, water/solid and solid/air. The wettability of the electrode by aqueous electrolyte plays an important role on the specific capacitance of the electrode because a hydrophilic electrode will allow better electrolyte infiltration throughout the internal volume of the material. The wetting results are showed in Fig. 5. A contact angle of 180° means complete non-wetting, on the other hand, a contact angle of 0° corresponds to complete wetting. The water contact angles of PPy and Ag@PPy were measured to be 85.1° and 85.2°, respectively. Both electrodes having similar contact angle implies that the embedded Ag nanoparticles and Ag nanoclusters on the PPy surface did not affect the hydrophilicity of the PPy. In addition, their contact angles were less than less than 90°, indicating that PPy is an intrinsically hydrophilic material, as the amine groups on PPy readily form hydrogen bonds with water.⁴² The hydrophilicity of the CS nanocomposite electrodes were improved tremendously due to the extensive formation of hydrogen bonds with the hydroxyl and amine groups of CS. Hence, PPy/CS and Ag@PPy/CS displayed contact angles of 41.8° and 43.3°, respectively. This revealed that the hydrophilicity of the electrodes was enhanced by the addition of CS.

Fig. 5 Digital images of water droplets on the surfaces of the as-prepared electrodes.

3.5 Electrochemical characterization

CV and GCD tests were performed to evaluate the electrochemical performances of the electrodes in a three-electrode system. The CV curves of the electrodes were shown in Fig. 6a at a scan rate of 10 mV s⁻¹, with 1 M H₂SO₄ as the electrolyte. Clearly, all the nanocomposite electrodes exhibited higher peak current than that of PPy. Two pairs of redox peaks can be observed when Ag nanoparticles and Ag nanoclusters are present in the nanocomposites. It has been reported that the electrochemical redox behavior and the peak separation are dependent on the size of Ag particles.^43,44 The Ag nanoparticles and Ag nanoclusters in the nanocomposite electrodes can be distinguished easily from their redox peak. The anodic and cathodic peaks of Ag nanoparticles in both nanocomposite electrodes are 0.19 and 0.01 V, respectively, having a peak separation of 0.18 V. Their redox peak potentials are the same due to them having very close size ranges. The Ag nanoclusters in the ternary nanocomposite Ag@PPy/CS displayed the anodic and cathodic peaks at 0.44 and 0.21, respectively, with a peak separation of 0.23 V. On the other hand, the Ag nanoclusters in the binary nanocomposite Ag@PPy displayed a pair of redox peaks located at 0.50 and 0.20, owing to the anodic and cathodic peak of Ag, with a peak separation of 0.30 V. As the size of Ag increase, it is observed that the peak separation between the anodic and cathodic peaks increases. This is due to the decrease in the electron transfer with an increase in the size.⁴³ The CV results signify the presence of hybrid Ag nanoparticles and Ag nanoclusters in the Ag@PPy and Ag@PPy/CS nanocomposites. Table 1 summarizes detailed analysis of the electrochemical parameters such as anodic and cathodic peak potentials, peak separation obtained from the cyclic voltammograms at 10 mV s⁻¹ scan rate, along with the size range of Ag particles.

Fig. 6 (a) CV curves at 10 mV s⁻¹ and (b) GCD curves at 0.2 A g⁻¹.

Table 1 Electrochemical data for Ag@PPy and Ag@PPy/CS nanocomposite electrodes

Electrode	Types of Ag particles	Size range (nm)	Anodic peak Ea (V)	Cathodic peak Ec (V)	Peak separation ΔE (V)
Ag@PPy	Ag nanoparticles	2–4	0.19	0.01	0.18
	Ag nanoclusters	55–100	0.50	0.20	0.30
Ag@PPy/CS	Ag nanoparticles	1–2	0.19	0.01	0.18
	Ag nanoclusters	30–80	0.44	0.21	0.23

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The GCD curves of the nanocomposite electrodes were shown in Fig. 6b. Ag@PPy/CS showed the longest discharge time, revealing that it has the highest specific capacitance among the other electrodes. Hence, the specific capacitances calculated from the discharge curve at 0.2 A g⁻¹ were 273, 403, 414 and 513 F g⁻¹ for PPy, PPy/CS, Ag@PPy and Ag@PPy/CS. The Ag@PPy/CS outperformed the other electrodes due to the synergetic contribution from the Ag particles and CS. The Ag nanoparticles increase the electrical conductivity of the PPy by enhancing the electron hopping along the conjugated PPy chains while the bigger Ag nanoclusters prevent the PPy/CS films from restacking. Additionally, the CS promotes the electrolyte affinity of the Ag@PPy/CS, resulting in an upsurge of electrolyte penetration into the surface.⁴⁵ The combined merits of hybrid Ag nanoparticle/nanocluster and CS with PPy led to a high specific capacitance of the ternary nanocomposite.

3.6 Supercapacitor device performance

Symmetric supercapacitor devices were assembled from the as-synthesized nanocomposites electrodes in order to completely determine the electrochemical performances and the potentials applications of the materials. The CV curves of the electrodes scanned at 10 mV s⁻¹ were shown in Fig. 7a. It can be seen that the area of the CV curves for the binary and ternary nanocomposite electrodes were larger than that of PPy, due to the effective enhancement of the capacitance properties from the addition of Ag and CS, with Ag@PPy/CS showing the largest CV curve. Even at a high scan rate of 200 mV s⁻¹, Ag@PPy/CS still maintains a fairly rectangular cyclic voltammogram, demonstrating ideal capacitive behavior (Fig. 7b). The GCD curves of the symmetric supercapacitors operated at 0.2 A g⁻¹ current density were shown in Fig. 7c. The ternary nanocomposite Ag@PPy/CS showed the highest specific capacitance of 183 F g⁻¹ followed by Ag@PPy, PPy/CS and PPy with specific capacitances of 161, 146, and 96 F g⁻¹, respectively.

Fig. 7 (a) CV curves at 10 mV s⁻¹, (b) CV curves of Ag@PPy/CS symmetric supercapacitor at various scan rates, (c) GCD curves at 0.2 A g⁻¹, and (d) comparison of the dependence of specific capacitance on the current density of the symmetric supercapacitors.

Fig. 7d displayed the specific capacitance of the symmetric supercapacitors as a function of current density. Ag@PPy/CS still exhibits the highest specific capacitance at all current densities due to its enhanced electrical conductivity and hydrophilicity. The critical aspect of the electrode wettability becomes more evident when comparing between Ag@PPy and PPy/CS. It is interesting to note that Ag@PPy showed higher specific capacitance than that of PPy/CS at low current densities but its performance rapidly degrades at higher current densities, rendering it to perform poorer than PPy/CS. Generally, the specific capacitance decreases with increasing current density because there is less time for the electrolyte to interact with the electrode. A more hydrophilic electrode will allow rapid infiltration of the electrolyte into the framework of the electrode; hence more capacitance can be retained at high current densities. Therefore, the PPy/CS has higher specific capacitance than Ag@PPy at high current densities.

Electrochemical impedance spectroscopy (EIS) studies were performed to investigate the influence of Ag and CS on the resistance of the nanocomposite. The Nyquist plots of EIS for the symmetric supercapacitors are shown in Fig. 8. At high frequencies region, the equivalent series resistance (ESR) and charge transfer resistance (R_ct) can be obtained from the intercept at the real axis and the diameter of the semicircle, respectively. The ESR represents the contact resistance between the electrolyte, current collector, and active material; the intrinsic resistance of the active material; and the resistance of the electrolyte. The corresponding ESR and R_ct values of the symmetric supercapacitors are summarized in Table 2. The PPy/CS gave the highest R_ct of 4.08 Ω due to the addition of the non-conducting CS into the nanocomposite.⁴⁶ Nonetheless, the increased resistance of the PPy/CS electrode was compensated for its enhanced hydrophilicity, exhibiting higher specific capacitance value than the pure PPy electrode. The electrical conductivity was successfully restored upon the embedment of hybrid Ag nanoparticle/nanocluster into the Ag@PPy/CS nanocomposite electrode. As a result, the R_ct value of Ag@PPy/CS was evidently reduced to 0.89 Ω which is close to that of Ag@PPy (0.82 Ω).

Fig. 8 Nyquist plots of EIS for the as-fabricated symmetric supercapacitors.

Table 2 The ESR and R_ct values of the symmetric supercapacitors extracted from the Nyquist plots

Electrodes	ESR (Ω)	Rct (Ω)
PPy	0.30	2.90
PPy/CS	0.22	4.08
Ag@PPy	0.26	0.82
Ag@PPy/CS	0.28	0.89

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The energy and power densities of the symmetric supercapacitors were calculated from the GCD curves by applying eqn (3) and (4) provided in the ESI.† The Ragone plots of energy density versus power density are shown in Fig. 9a. The Ag@PPy/CS nanocomposite electrode had an energy density of 16.3 W h kg⁻¹ at a power density of 322 W kg⁻¹. The power density improved to 8707 W kg⁻¹, with an energy density of 7.0 W h kg⁻¹. The energy and power density of the Ag@PPy/CS nanocomposite electrode was superior to Ag@PPy, PPy/CS and PPy, proving the active involvement of the hybrid Ag nanoparticle/nanocluster and CS in improving the electrochemical properties of the PPy.

Fig. 9 (a) Ragone plots, and (b) cycling stability plots for the symmetric supercapacitors.

The cycling stability of the electrodes was accessed over 1000 charge/discharge cycle at 0.5 A g⁻¹ and are shown in Fig. 9b. The Ag@PPy/CS and Ag@PPy revealed good capacitance retention, retaining up to 98.3% and 98.9%, respectively. The rapid increment of the specific capacitance in the first 100 cycles of the PPy/CS is believed to be caused by the good water absorbency of CS,⁴⁷ but still able to retained up to 94.0% of its initial capacitance. PPy only retained 88.8% capacitance due to its excessive swelling and shrinking of the polymer chains during the charge/discharge process.⁴⁸ The improved cycling stability of the binary and ternary nanocomposites revealed that the electrochemical stability of PPy has been enhanced.

4. Conclusion

In this study, we demonstrate a facile one-step electrodeposition to produce Ag@PPy/CS nanocomposite. The results show that Ag@PPy/CS nanocomposite electrode is beneficial for the transportation of both electrolyte and electrons, leading to high electrochemical performance. Firstly, the increased accessibility to the electrolyte is due to the enhanced hydrophilicity contributed by CS. Secondly, the coordination of CS with Ag stabilized the growth of metallic Ag, reducing the particle size of the hybrid Ag nanoparticle/nanocluster. Thirdly, the evenly distributed ultra-small Ag nanoparticles (1–2 nm) enhanced the electron hopping system of the conjugated PPy. Finally, the larger Ag nanoclusters in the size range of 30–80 nm acted as spacers, preventing the restacking of PPy/CS films besides improving the electrical conductivity of the nanocomposite. The elegant synergy between CS and Ag boosted the supercapacitive properties of PPy making it a promising candidate for application as electrode for high-performance supercapacitors.

Acknowledgements

This work was financially supported by a University of Malaya Research Grant Program (RP007/13AFR), and the High Impact Research Grant from the Ministry of Higher Education of Malaysia (UM.C/625/1/HIR/MOHE/SC/21).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13697d

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