Elsevier

Journal of Power Sources

Volume 246, 15 January 2014, Pages 800-807
Journal of Power Sources

Enhanced capacitance and stability of p-toluenesulfonate doped polypyrrole/carbon composite for electrode application in electrochemical capacitors

https://doi.org/10.1016/j.jpowsour.2013.07.121Get rights and content

Highlights

  • pTS doped PPy/C composites has been synthesized for application in supercapacitors.

  • A comprehensive investigation has been performed on these composites.

  • pTS has imparted thermal and electrochemical stability to the composites.

  • Enhancement in conjugation length and electrical conductivity with pTS is observed.

  • pTS doped PPy/C composites show improved specific capacitance and stability.

Abstract

Polypyrrole/carbon (PPy/C) composites have been synthesized using varying concentration of p-toluenesulfonate (pTS) dopant by surface initiated in-situ chemical oxidative polymerization with the purpose to develop an electrode material for supercapacitors. The influence of pTS on the structure of the composite is observed through Fourier transform infrared (FT-IR) and Raman spectroscopy while the morphological features have been examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The dc conductivity shows direct correlation with pTS concentration and follows Mott's three dimensional variable range hopping (3D-VRH) charge transport. The performance of PPy/C composite electrode for charge storage has been analyzed using electrochemical tools such as cyclic voltammetry and electrochemical impedance spectroscopy. The maximum specific capacitance ∼395 F g−1 in 0.5 M Na2SO4 aqueous solution with significant stability (∼500 cycles) is obtained for the material synthesized using equimolar concentration (0.1 M) of pTS to pyrrole.

Introduction

Globally, in the past few decades, there has been an ever increasing demand for environment friendly and efficient energy storage systems. Among all the energy storage systems available, supercapacitors (SCs) are high in demand due to their distinctively high power density, reasonable energy density and longer cycle life [1], [2], [3]. Based on the charge storage mechanism, the SCs can be categorized into two groups; electrical double layer capacitors (EDLC) and pseudo-capacitors (PC). In EDLCs the capacitance comes purely from the electrostatic charge accumulation at the electrode/electrolyte interface, which strongly depends on the surface area of the active material. Carbon based nanostructured materials, which possess high surface area, fall under this category. On the other hand, PC or redox supercapacitors uses fast and reversible active redox reactions for charge storage. Transition metal oxides (RuO2, Fe3O4 and MnO2) and electrically conducting polymers are its typical examples. Furthermore, these two mechanisms can simultaneously work together depending upon the nature of the active material.

Conducting polymers, specifically polypyrrole (PPy) has generated wide interest in the area of energy storage owing to its unique features such as high conductivity, environment friendliness, fast charge–discharge kinetics and low cost [4], [5], [6], [7], [8], [9]. Moreover, its characteristic redox doping–undoping process can be exploited in the charge storage systems, utilizing both the electrochemical double layer at the interface and pseudo-capacitive behavior. Nevertheless, the simultaneous occurrence of swelling and contraction with this doping–undoping process, weakens the materials stability, which affects its long term utilization.

To overcome this, PPy has been used together with various carbon materials such as PPy/graphene [10], [11], [12], PPy/activated carbon [13], PPy/carbon aerogel [5], PPy/single walled carbon nanotube [14], PPy/carbon nanofiber [15] and PPy/carbon black [16], etc. Although most of the porous materials exhibit large capacitance, the electrical conductivity deteriorates due to unavailability of conducting pathways or existence of oxygen containing functional groups [17] which largely limits the power capacity [15]. Recently Yang et al. [16] have demonstrated that a specific capacitance of 366 F g−1 can be achieved in carbon black/PPy nanocomposites in 1.0 M NaNO3 electrolyte solution. Moreover, high conducting and thermally stable PPy can be synthesized using aromatic dopant anions [18], [19], [20]. In a recent investigation [21] it has been found that aromatic dopant such as p-toluenesulfonate (pTS) is resistant to overoxidation and therefore can be used in electrode applications. Hence for the present work, it is thought worthwhile to modify polypyrrole–carbon (PPy/C) composites using aromatic dopant pTS to obtain a high conducting PPy with substantial amount of thermal and electrochemical stability.

The present study reports the synthesis of various PPy/C composites doped with varying concentration of pTS anion using surface initiated in-situ chemical oxidative polymerization. The structural and morphological features of these composites have been correlated with change in their conjugation length. The effect of pTS on inter-chain connectivity, thermal stability and charge transport has been examined. The impact of this dopant on the performance of these composites for electrode applications in electrochemical capacitors has been studied by cyclic voltammetry and electrochemical impedance spectroscopy.

Section snippets

Materials

Pyrrole (Py) monomer was procured from Fluka Chemie and was doubly distilled prior to synthesis. Ammonium peroxodisulfate (APS) (oxidant), sodium sulfate (Na2SO4) and p-toluenesulfonic acid (dopant) were products of Merck, Germany and used as received. The Vulcan-carbon XC-72R (particle size ∼ 50 nm) was a product of Cabot Corporation, Massachusetts. All the solutions used in the present work were made in deionized (D.I.) water (∼18 MΩ cm).

Synthesis of PPy/C composites

The PPy/C composites were prepared using in-situ

Structural characterizations

The morphology and the nature of the active material can impact the charging–discharging process and hence its overall capacitance. In PPy/C composites, the capacitance can come from both the electrochemical double layer at the electrode/electrolyte interface and pseudo-capacitance due to its redox nature.

Fig. 1(a–e) shows the morphological features of PPy/C composites; S1, S2, S3, S4 and S5, respectively. As a representative result, the energy dispersive analysis using X-rays (EDAX) of sample

Conclusion

The electrochemical performance and stability of the PPy/C composites has been improved through doping of pTS anion in the PPy matrix. With increase in dopant concentration the overall structure of PPy/C composites is transformed to a more diffused matrix of inter-connecting networks of conducting domains for efficient charge transport. The enhanced conjugation length and higher conductivity of pTS doped samples impart the stability to the composites. The electrochemical investigations indicate

Acknowledgments

The authors are thankful to the Director, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi for his kind permission to publish this work and Drs. R.B. Mathur, S.R. Dhakate, S.K. Dhawan, R. Pasricha and S.P. Singh for providing their experimental facilities. A. Kumar is thankful to Council of Scientific & Industrial Research (CSIR), New Delhi, for the award of Senior Research Fellowship. R. Singh is thankful to CSIR, New Delhi for the award of Emeritus Scientist Fellowship.

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