John Kevin Gana,
Yee Seng Lima,
Nay Ming Huang*a and
Hong Ngee Limbc
aLow Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail: huangnayming@um.edu.my
bDepartment of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
cFunctional Device Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
First published on 7th September 2016
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.
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 (CC) 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.
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.
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
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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 CC 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 3d5/2 peak at 368.0 eV and Ag 3d3/2 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
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
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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.
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Fig. 4 Schematic representation for the formation of Ag@PPy/CS nanocomposite via one-step electrodeposition. |
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 |
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.
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 (Rct) 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 Rct values of the symmetric supercapacitors are summarized in Table 2. The PPy/CS gave the highest Rct 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 Rct value of Ag@PPy/CS was evidently reduced to 0.89 Ω which is close to that of Ag@PPy (0.82 Ω).
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 |
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.
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.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13697d |
This journal is © The Royal Society of Chemistry 2016 |