Elsevier

Polymer

Volume 54, Issue 3, 5 February 2013, Pages 1033-1042
Polymer

Graphene/polypyrrole nanofiber nanocomposite as electrode material for electrochemical supercapacitor

https://doi.org/10.1016/j.polymer.2012.12.042Get rights and content

Abstract

The present work explores a facile route to synthesize nanocomposite based on graphene and Polypyrrole nanofiber using a biopolymer, sodium alginate. The synthesis procedure of composite is simple, inexpensive and ecofriendly. The possible interaction between graphene and PPy nanofiber has been characterized by FTIR analysis. Morphological study confirmed the fiber like morphology of PPy and the presence of graphene in the nanocomposite. The composite achieved high electrical conductivity of 1.45 S/cm at room temperature and also showed nonlinear Current–Voltage characteristics, which indicates it's potential to be used in various device applications. A maximum capacitance value of 466 F/g has been obtained for this composite at 10 mV/s scan rate in 1 M KCl solution. The composite also showed highest energy density of 165.7 Wh/Kg at 10 mV/s scan rate. Noticeable improvements in other electrochemical properties allow its possible application as electrode material for Supercapacitors.

Introduction

Conducting polymers are the polymers which have the capability to conduct electricity. Due to their unique properties and wide range of application potential in different electronics and device application, these kinds of polymers have attracted the scientists tremendously. Great deals of researches have been done for the processing and fabrication of these polymers. Among different polymers, PolyPyrrole (PPy), Polyaniline (PANI) and Polythiophene (PTh) and their derivatives are widely used because of their easy synthesis, enhanced thermal stability, high electrical conductivity, excellent chemical stability etc. [1], [2], [3]. However, modern researches have been devoted to the fabrication of nanoarchitectures of these conducting polymers for further improvement of different properties. One dimensional nanostructures of conducting polymers have offered several potential applications in different fields such as chemical sensor, gas-separation membranes, neuron devices etc. [4], [5], [6]. PPy are one of the most studied conducting polymers owing to their high electrical conductivity, easy synthetic procedure and cost effectiveness. However, PPy in their nanoscale exhibits unique electrical and electrochemical properties, which allow their application in fabrication of various types of devices. Different kinds of nanoarchitectures of PPy have been investigated by the researchers – like (1) PPy nanoparticles, (2) PPy nanowire, (3) PPy nanotube and (4) PPy nanofiber. Jeon et al. synthesized spherical PPy nanoparticles (with uniform diameter of ∼85 nm) by chemical oxidative polymerization and reported them as an efficient counter electrode for dye-sensitized solar cells [7]. PPy nanoparticles were also synthesized by microemulsion polymerization process [8]. Cheng et al. fabricated highly uniform and ordered PPy nanowire and nanotube arrays by chemical oxidation polymerization [9]. Enzymatic biofuel cells based on PPy nanowire were fabricated by Kim et al. [10]. Composite based on PPy nanowire, nanotube, nanoparticles were investigated for various electronic applications [11], [12], [13], [14]. Many researchers used template based synthesis for the preparation of PPy nanofiber by using zeolite, alumina and other nanostructured templates [15], [16]. Apart from template based synthesis, electrospinning have been used for nanofiber synthesis [17], [18], [19]. Template free techniques like chemical, electrochemical and sonochemical methods were also been reported for the preparation of nanofibers [20], [21]. Although the preparation of PPy nanofibers were studied extensively by many research groups, its composite with carbon nanotube and graphene are not yet explored fully for the electrochemical applications.

The present study focused on the synthesis and properties of graphene and PPy nanofiber (GrPPyN) for the supercapacitor application. To the best of our knowledge, only two research groups studied the PPy nanofiber based graphene nanocomposite. Mahmoudian et al. reported superior electrochemical performance of PPy nanofibers, sandwiched between two electrodeposited reduced graphene oxide layers, which were prepared by electrochemical method [22]. Tien et al. proposed the one-step synthesis of graphene-PPy nanofiber composite, which showed improved NO2 gas sensitivity [23].

Graphene is the two dimensional allotropy of carbon. It is actually the transparent single layer of carbon atoms, arranged in a honeycomb fashion. It has countless exceptional properties like extremely high electrical carrier mobility (two order higher than Si), unique optical property (97% Optical transmission), excellent electrical conductivity (high upto 2 × 103 S/cm), enhanced thermal conductivity (5000 W/mK), extraordinary mechanical strength, endurance of high current density (six order of magnitude higher than Cu) etc., which make it the miracle material of this century [24], [25], [26], [27]. After its discovery in 2004 by A.K. Geim, graphene has been extensively investigated as distinctive component for various applications like aerospace, automotive, electronics, energy storage, solar cell, bio-sensor [28], [29], [30]. Now, graphene has moved from research laboratory to market in order to fulfill huge demand of advanced material in today's world. Owing to its high aspect ratio, surface area, extraordinary electrical and electrochemical properties, it has been considered as an efficient component for the construction of supercapacitor electrode.

Supercapacitors are the furthermost promising energy storage device of this century, which has the ability to reduce the pollution as well as fulfill the growing demand of energy in today's world. It offers high power and energy than conventional capacitor and batteries. Supercapacitors are now used vastly in electronic devices like computers, PC Cards, photographic flash, flashlights, portable media players, automated meter reading equipment and for many more applications. It can store charges in a highly reversible pathway, without any chemical reaction. Based on the charge storage mechanism, supercapacitors can be divided into two classes –

  • 1.

    Electric Double Layer capacitor (EDLC) – Mainly the charge stored through the formation of electric double layer at the electrode–electrolyte interface. The charge storage mechanism is of non-faradic type. Carbon based materials like activated carbon, carbon nanofiber, carbon nanotubes are used as electrode material for this type of supercapacitors.

  • 2.

    Pseudocapacitor – Psuedocapacitors generally store charge faradically, which allows them to achieve higher capacitance properties and enhanced energy density than EDLCs. Different types of conducting polymers, metal oxides (MnO2, RuO2, NiO, SnO2 etc.) and metal hydroxides [Ni(OH)2, Ca(OH)2 etc.] are used as electrode material in Pseudocapacitors.

These two capacitor concepts can be merged together, for the fabrication of a hybrid capacitor, where both faradic and non-faradic processes can be utilized for charge storage and enhanced electrochemical properties [31], [32], [33], [34], [35], [36], [37]. To accomplish this goal, we have synthesized and studied the graphene and PPy nanofiber nanocomposite as an electrode material. Due to its high surface area, graphene can enhance the interaction between deposited pseudocapacitive materials i.e. PPy nanofiber and electrolyte and hence facilitate the electric double layer generation. In addition to the electric double layer, PPy nanofiber also has the potential to enhance the capacitance through faradic reaction, which can enhance the overall capacitance of the nanocomposite.

Section snippets

Materials

Graphene was obtained from Sinocarbon Materials Technology Co. Ltd., China. Pyrrole was supplied by E. Merck Ltd, India. Potassium Chloride (KCl, E. Merck Ltd., India) was used as electrolyte for the electrochemical characterizations. The chemicals, Sodium Alginate (SA, Food Grade), Sodium Hydroxide (NaOH) and Ammonium Persulfate (APS) [(NH4)2SO4], were also obtained from Loba Chemie Pvt. Ltd. Mumbai, India. Before use, the graphene sheets were treated with mixed acid (H2S04 and HNO3) to remove

Fourier transform infrared spectroscopy (FTIR)

FTIR analysis of PPy, graphene, SA, GrPPyN was performed by IR spectrometer (NEXUS 870, Thermo Nicolet). Samples for FTIR analysis were prepared by mixing potassium bromide (KBr) and material in the weight ratio of 10:1 and pelletized.

Field emission scanning electron microscopy (FESEM)

A Carl Zeiss-SUPRA™ 40 FESEM with an accelerating voltage of 5 kV were employed to observe the morphology of GrPPyN. A thin layer of gold was sputtered over the sample before the experiment.

Transmission electron microscopy (TEM)

The TEM analysis of GrPPyN was performed by JEOL 2100 instrument. A small

Mechanism of composite preparation

SA is the sodium salt of alginic acid, an anionic polysaccharide. It is a linear copolymer with homopolymeric blocks of (1-4)-linked β-d-mannuronate (M) and its C-5 epimer α-l-guluronate (G) residues (Fig. 1) and can act as polyelectrolyte. The advantages of using SA for the synthesis of PPy nanofiber are as follows:

  • (1)

    The strong electrostatic repulsions among the carboxylate anions of SA (–COOˉ) allows it (SA) to form an expanded network structure [45].

  • (2)

    It has the ability to form fibrillar

Conclusions

In summary, a simple sodium alginate-assisted in-situ polymerization of pyrrole has been adopted to synthesize graphene/PPy nanofiber nanocomposite. Morphological analysis confirmed the formation of nanofibers on the graphene sheets. We have demonstrated that incorporation of graphene during the in-situ polymerization process enhanced the charge transformation as well as ion transportation throughout the material leading to high electrical conductivity, low internal resistance and better

Acknowledgments

The first author acknowledges CSIR, New Delhi (Grant No. 09/081(1018)/2010-EMR-I) of India for their financial support.

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