Conducting Polymer /Graphene composite electrodes for supercapacitors

Graphene, as a new emerging carbonaceous material has gained a lot of attention in the past decade due to its extraordinary intrinsic properties. Conducting polymers exhibit high potential in supercapacitors because of their advantages over other electrode materials including good conductivity, flexibility, ease of synthesis etc. But both materials have some drawbacks when they merely used as electrodes. Therefore, research community has move towards composite electrodes to avoid disadvantageous realities and accomplish the best performance. This review summarizes recent development of graphene and conducting polymer (Polypyrrole and Polyaniline) based composite electrodes for supercapacitors and the comparison of their performance.


INTRODUCTION
From the past over several decades, due to industrialization, increase in global population, rapid development of the global economy, depletion of fossil fuels, and increasing environmental pollution, there is an urgent need for efficient, clean and sustainable sources of energy, as well as new technologies associated with energy conversion and storage devices, such as batteries, fuel cells and supercapacitors 1 . Due to its high power density over batteries fuel cells (which have high energy storage), high energy density compared to the conventional capacitors (which have high power output), and long lifecycle, supercapacitors or ultracapacitors become prominent. For these supercapacitors to realize their promise in the outcome, it is important that their energy and power densities must be maximized 2 . To increase the performance of supercapacitors, different materials and various methods were used in recent years. However, the performance of supercapacitors depends mainly on the electrode material and the properties of the electrode/electrolyte interface. Therefore, it is important that the electrode materials should demonstrate high capacitance, good mechanical and structural stability to reach long cycling life 3 . At present, the electrode materials of supercapacitors can be mainly divided into two categories. One is carbon-based materials with electric double-layer feature including graphene, carbon nanotubes, porous carbon, carbon foam, activated carbon, carbon fiber and so on 4 . Second electrode material type is mainly conducting polymers (e.g., polyaniline, polypyrrole) and/or metal oxides which has pseudocapacitive feature through redox reactions to achieve the electron transfer and charge storage 3 . These standard supercapacitors have two identical electrodes which aligned symmetrically and the charges are stored electrostatically in electric double layer capacitors (EDLCs) and faradaically in pseudocapacitors or redox capacitors at the electrochemical double layer situated in the electrode/ electrolyte interfaces 5 . EDLCs utilize the high surface area of carbonaceous materials and exhibit a high power density and excellent cycle life. However, they have lower energy density due to the low areal capacitance of electrode materials and narrow operating potentials 4 . Redox capacitors are also suffering from some drawbacks, such as lower energy and power densities and poor cycling stabilities. Therefore, to avoid these weaknesses and improve performances recent research works have moved towards to merge the electrodes of EDLCs and redox capacitors. However, supercapacitors can be much more complex devices when combined different materials for electrode fabrication. These hybrid capacitors try to develop the relative advantages and diminish the relative disadvantages of EDLCs and redox capacitors to comprehend better performances by utilizing both faradaic and electrostatic processes to store charge and have achieved energy and power densities greater than EDLCs. EDLCs consist of carbon-based materials which have large surface area, high porosity, controllable morphology and high electrical conductivity. Moreover, these carbon materials can be treated to modify its structural properties and thereby to accumulate chemical and physical properties for numerous applications 6 . Scientists believed that the increasing specific surface area will increase the capacitance of EDLCs 7 . Therefore, from its discovery in 2004, graphene with one atom thickness and large specific area has attracted the major focus of supercapacitor industry 8 .
Thermal conductivity and mechanical stiffness of these graphene sheets (one-atom-thick two-dimensional layers of sp 2 -bonded carbon) may rival the remarkable in-plane values for graphite (3,000 W m -1 K -1 and 1,060 GPa, respectively); and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties which got the attention for energy storage applications 9 . Two-dimensional (2D) graphene has exhibited unusual and intriguing physical, chemical and mechanical properties due to its honeycomb lattice structure. The use of graphene has overtaken the use of carbon nanotubes (CNTs) because CNTs restricted from achieving electric double-layered capacitance for industrial devices 10 . Today, most research works on graphene-based supercapacitors concentrate on using graphene nano-platelets, graphene nano-powders and other graphene derivatives such as graphene oxide, reduced graphene oxide, chemical modified graphene, etc. Even though the highest quality and least defects in graphene sheet were obtained via chemical vapor deposition (CVD) which cannot compete against activated carbon due to its high manufacturing cost and hard scalability 8,11 . To obtain better contact in the current collector, polymer binders, specifically, fluoropolymers like polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) were used. The low conductivity of these polymer binders could lead to a decrease in energy density in supercapacitors. Therefore, rather than using only a binder, scientists have tested conducting polymer (CP) to comprehend both factors.
These conducting polymers are electrically conductive and can undergo redox reactions to fulfill pseudo-capacitance in addition to the electric double layer capacitance 8 . Polypyrrole (PPy), polyaniline (PANI), and Poly(3,4-ethylenedioxythiophene) (PEDOT) are the three most commonly used CPs in supercapacitors due to their excellent conductivity, high capacitance, ease of synthesis, low cost, good flexibility and lightweight 12 . However, it is a challenge to build high power supercapacitors using CPs because they exhibit poor stabilities during the charge-discharge processes 10 . In recent times, activated carbon and CNTs have been used to fabricate supercapacitors due to their good stability, but these microstructures limit the value of the capacitance. Therefore CNT-PANI has been tested in supercapacitor electrodes to improve the capacitance. While doing so the PANI stability increased as well 13 . In this review we mainly focused on the recent supercapacitor electrodes fabricated from composite materials such as graphene-polyaniline (G-PANI) and graphene-polypyrrole (G-PPy), for energy applications. Thereby, the review allows to comprehending the technology of using graphene and conducting polymer composite electrodes in supercapacitor industry and mitigates the disadvantages of using sole graphene electrodes or conducting polymer electrodes.

GRAPHENE BASED ELECTRODES FOR SUPERCAPACITORS
One atom thick, 2D layer graphene is a unique carbon material that has potential for energy storage device applications 14 . Recently, it was proposed that graphene can be used as a material for supercapacitor applications due to its outstanding characteristics like high electrical conductivity, chemical stability, large surface area and mainly because it doesn't depend on the distribution of pores at solid state, when compared with other carbon materials such as activated carbon, CNT etc. 15 . Among all carbon materials used as electrochemical double layer capacitor electrodes, newly developed graphene has the highest specific surface area (SSA) which can be larger than 2630 m 2 g -1 16 . This graphene is capable of achieving an ultra-high specific capacitance if the entire SSA is fully utilized 14 . According to Xia et al. single-layer graphene has the theoretical specific capacitance of ~21 uF cm -2 and the corresponding specific capacitance is ~550 F g -1 when the entire surface area is fully utilized 17 . However, in the practical situation due to serious agglomeration during both preparation and application processes, expected values were not observed 18 . Therefore, different graphene synthesis methods and various electrode preparation processes were considered to obtain the maximum capacitance values. Nowadays, there are many different approaches being tested for the production of graphene varieties such as CVD, micromechanical exfoliation, arch discharge method, unzipping of CNTs, epitaxial growth, electrochemical and chemical methods and intercalation methods in graphite 19 . Another major advantage of using graphene as electrode material is that the both major surfaces of graphene sheet are exterior and therefore can be readily accessible by electrolyte material 20 . Even though mechanical exfoliation of graphite using scotch tape method gives high quality graphene, this method is not suitable for mass production 19  developed highly corrugated graphene sheets using thermal reduction of graphite oxide at a high temperature followed by a rapid cooling in liquid nitrogen which gives high specific capacitance of 349 F g -1 in 6 M KOH aqueous solution 23 . Graphene oxide (GO) same as graphite oxide is another significant graphene derivative, which can readily made by graphite. Due to the disruption of its sp 2 bonding networks, GO is often described as an electrical insulator. But in reality, this could vary due to incomplete oxidization 24 . These GOs can be synthesized using modified Hummers method from powdered flake graphite 25 .
Depending on the GO synthesis techniques, different functional groups and their various distributions on the surface can be seen. Stankovich et al. suggested an alternative method for creating single sheets starting from GO exfoliation to create stable aqueous dispersions of individual sheets 26 . Insulating GO can be converted into graphene by a controllable reduction of GO along with the reducing agent hydrazine hydrate which is an efficient and a low cost method. Reduction of GO is the removal of oxygen containing functional groups in GO such as hydroxyl, carboxyl and epoxy groups giving reduced GO (rGO) 16 . Ruoff et al. introduced the chemically modified graphene (CMG) as electrode materials to obtain the specific capacitances of 135 and 99 F g -1 in aqueous and organic electrolytes, respectively.
These CMG materials are made from 1-atom thick sheets of carbon and functionalized as required 27 . Li et al. prepared graphene nanosheets by chemical modification with KOH. The specific capacitance was checked using cyclic voltammetry and the value reach up to about 136 F g -1 , which is an increase of 35% with that of pristine graphene nanosheets 28 . Table 01 summarizes the electrochemical performances of electrodes based on graphene and its derivatives.

CONDUCTING POLYMER BASED ELECTRODES FOR SUPERCAPACITORS
Conducting polymers (CPs) are realized as promising materials for the high-performance supercapacitors. They have high specific capacitances because of their charge processes concern the whole polymer mass not only limits to the surface as in the case of carbon-based materials. Moreover, they exhibit high conductivities in the charged states, while their charge-discharge processes are generally fast. This review will focus on the use of two conducting polymers, polyaniline (PANI), polypyrrole (PPy) composites with graphene as supercapacitor electrodes.

PANI
PANI has been studied and experimented in a wide range as an electrode material for supercapacitors due to its high conductivity, electroactivity, specific capacitance, good stability in air and ease of synthesis 8 . PANI to be used as supercapacitor electrode material, a protic solvent (acidic medium/ protic ionic liquid) is required 34 . PANI can be synthesized by various techniques, and by these different methods, properties of PANI will vary. Most common two synthesis methods are oxidative polymerization and electrochemical polymerization. In addition to that, interfacial polymerization, electrospinning, seeding polymerization and templated polymerization also play a major role in PANI preparation [35][36][37][38]  Hence, scientists try to overview different approaches while researching for the preparation of PANI based composites (with carbon materials and/or metal oxides) to overcome the challenging situation which will be discuss later on in this review.

PPy
PPy, as one of the major conducting polymer, has become a promising electrode material for supercapacitors due to its intrinsic properties such as high electrical conductivity and

CONDUCTING POLYMER/GRAPHENE COMPOSITES
Although pure conducting polymers (PANI, PPy) possess a lot of unique properties, they might not appropriate as electrodes active materials alone in the supercapacitor due to the drawbacks mentioned earlier. In order to improve the electrochemical performances of CPbased supercapacitors, researchers have tried to synthesize binary and even ternary composites with other active materials, mainly including carbon materials (which in this case: graphene-based materials) will be reviewed hereinafter.

Graphene-Polyaniline (G-PANI) composites
Various G-PANI nanocomposites have been studied, such as PANI and nitrogen-dopedgraphene (NG), PANI/nitrogen doped GO or rGO, PANI/graphene hydrogel (GH) 10 constructed an electrode using highly conductive GH combined with PANI which exhibited excellent electrochemical properties. It showed specific capacitance value of 710 F g -1 at 2 A g -1 and 73% capacitance retention upon increasing current to 100 A g -1 . In addition to that, GH/PANI electrode gave a maximum energy density of 24 Wh kg -1 and power density of 30 kW kg -1 , and also exhibits 86% capacitance retention after 1000 cycles 54 . In summary, there are various practices to synthesize G-PANI composites which have better electrochemical performances, thus moving towards to comprehend practical application requirements.

Graphene-Polypyrrole (G-PPy) composites
Novel approaches have been made to develop the nanostructured conductive polymer, polypyrrole (PPy) with electrically conductive graphene and its derivatives to achieve higher specific capacitances and good cycling stability. Drzal et al. created 100% binder free composite electrode with multilayered graphene sheets and PPy nanowires which could reach to specific capacitance of 165 F g -1 upon increasing scan rate. Further its cyclic voltammograms showed nearly ideal rectangular shape when rising of the scanning rate implying high electrochemical cyclic stability 65 74 . As a conclusive summary, G-PPy composite supercapacitors have given a lot of benefits in the supercapacitor industry, suggesting that they can be further used as a modification of both graphene and PPy.

COMPARISON AND SUMMARY
Graphene and its derivatives have a great influence on the CPs by improving its morphologies, electrical properties and structural stabilities, thus when they are combined together to create composites, great development of their electrochemical properties can be seen 75 .