Factors affecting the performance of supercapacitors assembled with polypyrrole/multi-walled carbon nanotube composite electrodes
Introduction
The limited gasoline resources and hydro-electric projects and the portability issues have catalyzed the scientific community to develop efficient and cost effective alternative energy conversion technologies and storage devices. A great deal of attention has been given to supercapacitors due to their high specific capacitance (SC), long-term cycleability and high power density. The materials studied for use in supercapacitors are generally divided into three classes: (i) activated carbons, (ii) metal oxides and (iii) conducting polymers [1]. Conducting polymers and some metal oxides are promising candidates for supercapacitor applications owing to their high pseudocapacitance [2], [3], [4], [5]. The major advantages of conducting polymers over metal oxides include: (i) ease and versatility in synthesis, (ii) high inherent conductivity and (iii) low cost [6], [7]. Among the conducting polymers, polypyrrole (PPy) and polyaniline [8], [9] composites were studied in detail and show promise. Polypyrrole (PPy) is an ideal electroactive material which is environmental friendly and demonstrates good stability [10]. However, virgin PPy shows poor cycling performance due to the volume change caused by the doping–dedoping of ions during charging and discharging [11], [12], [13]. There have been attempts to improve the stability and specific capacitance of PPy synthesized with various morphologies and techniques, but the methods primarily involve electrochemical synthesis [14], [15], [16]. Chemical synthesis is thought to be a more preferred method, because it provides facile synthesis with the advantages of large-scale production of the material and low cost. A high specific capacitance was reported for PPy-based systems in a 3-electrode cell configuration [17], [18]. However, the systems lacked good stability when assembled as a 2-electrode cell (full cell). Rapid or gradual loss of the SC usually associated with cycling is the major shortcoming of PPy-based systems [19], [20]. The challenge remaining to overcome is addressing the stability issues of the full cell assembled with PPy. One of the solutions to address the stability problem of the cells is to synthesize the composites with multi-walled carbon nanotubes (MWCNT), as the nanotubes have a high inherent conductivity, flexibility, chemical and mechanical stability, and large surface area polarizability [21], [22], [23], [24]. It has previously been shown that PPy adheres strongly onto the nanotube surfaces during the polymerization reaction and provides the major capacitance as compared to the negligible contribution from the MWCNTs [25].
In this study, we focused on improving the cycleability of the 2-electrode cell assembled with PPy/MWCNT composite electrodes. We demonstrated the necessity of adequate modification of the PPy/MWCNT composite electrode material in order to exploit the system to obtain good cycling performance. The mechanistic aspects of the ionic compatibility of the doped PPy and the anions of the electrolytes were also investigated in our work.
Section snippets
Experimental
PPy/MWCNT composites were synthesized in water, dichloromethane (DCM), and hexane–dioctylsulfosuccinate (AOT). All chemicals were purchased from Sigma–Aldrich Co. Ltd. The MWCNTs were purchased from Hanwha Nanotech. In order to synthesize the composites, we used a simple oxidative polymerization method.
FT-IR analysis
The FT-IR (Perkin-Elmer–Spectrum GX instrument) analysis of the MWCNT and PPy composites synthesized in water are shown in Fig. 1. The CO functional group is evident from the IR analysis of the MWCNT. The IR peaks observed at 1632 cm−1 and the sharp peak observed at 1261 cm−1 are assigned as the CO stretching vibrations. In addition to the above, the two prominent peaks observed at 1091 cm−1 and 1024 cm−1 are assigned as the CO stretching vibrations. The presence of carbonyl groups commonly seen on
Conclusions
PPy/MWCNT composite materials were synthesized for application to supercapacitors. In terms of stability and therefore, superior cell performance, the composites preferred a neutral electrolyte system over acidic and alkaline media. A high SC (165 F g−1) with excellent stability (1000 cycles) of the cells in a 1.0 M KCl electrolyte was achieved in our study. The linear shape of the charge–discharge curves under a high current density demonstrates the ideal supercapacitor behavior of the cells. The
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Present address: Department of Chemistry, Sacred Heart College Chalakudy, Thrissur, Kerala - 680307, India. Tel.: +91 9497 249 176; fax: +91 480 270 1159.