Review
Energy efficient capacitors based on graphene/conducting polymer hybrids

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Abstract

Graphene (GRP) and conducting polymers (CPs) are interesting classes of emerging materials for various applications. To date, extensive research effort has been devoted to the investigation of diverse properties of graphene and conducting polymers such as polypyrrole (PPy), polyaniline (PAni), and polythiophene (PTh). The combination of these materials can be very advantageous in terms of practical applications in energy storage/conversion systems. Among various those systems, energy efficient electrochemical capacitors (ECs) have become popular due to the recent need for small and portable devices. Therefore, in this article, the application of GRP/CP hybrids for high performance capacitors is described concisely. In particular, an extensive and concise summary on the previous research activities regarding GRP/CP capacitors is covered. Subsequently, recent patents related to the preparation and application of GRP/CP capacitors are also introduced briefly. It is certain that this article can provide essential information for future study.

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The advantages and synergy of graphene and conducting polymers as electrode candidates for electrochemical capacitors was discussed based on an extensive summary of impressive research activities.

Introduction

GRP is a monolayer of graphite with a hexagonal structure in which carbons have a sp2 hybrid orbital [1]. However, their properties are dramatically different from those of carbon allotropes [1], [2], [3]. Owing to the unprecedented advantages of GRP such as a high carrier mobility and capacity, a tunable band gap, an ambipolar electric field effect, and quantum Hall effect [2], [3], [4], [5], [6], [7], [8], [9], [10], GRP has become a competitive material for various applications such as electronics, batteries, fuel cells, capacitors, photovoltaics, and sensors [11], [12], [13], [14], [15], [16], [17], [18]. In general, GRPs can be produced by two methods, solution and chemical vapor deposition (CVD) [19], [20], [21], [22], [23], [24], [25]. Recently, numerous elegant preparation techniques have been developed, thus the usability of GRP is expanding rapidly.

On the other hand, CPs such as PPy, PAni, PTh, and poly(vinylphenylene) allow intermolecular charge transport, because they contain a large conjugation structure with a lot of sp2 hybridized carbons. Nanocomposites having CPs can utilize their inherent characteristics, thus they have demonstrated excellent performances. CPs become popular for applications such as electronics, energy storage/conversion, environmental monitoring, and sensors since their discovery in 1976 [26], [27], [28], [29], [30]. Fig. 1 displays the most popular conducting polymers.

Considering the positive aspects of GRP and CPs, it is natural that GRP and CPs can be combined to generate more sophisticated structures and materials. In addition, novel components such as nanocrystals and biomolecules can be added to produce hybrid materials [29], [30]. That is, GRP/CP hybrids can provide a mechanically robust and chemically stable flatform for many challenges such as energy devices, optoelectronic architectures, sensors, and air/water purification. To date, GRP/CPs have shown their feasibility for environmental protection and monitoring, for example removal of toxic molecules, oxidation/reduction, photocatalysis, and gas storage [31]. GRP/CPs has also been demonstrated as one of the most competitive candidates for energy storage/conversion systems [32], [33], [34], [35], [36].

Among various energy storage/conversion devices, ECs are attracting a dramatic interest owing to their high energy density and long cycle life [37]. There has been an enormous effort to improve their performance in material perspective [38]. Carbons such as carbon nanotubes and GRPs have been recently most popular due to relatively high energy density [39]. For example, it was calculated that the theoretical capacitance of a single-layer GRP was approximately 20 μF [40]. Although additional components such as metal oxides [41] have been incorporated to improve performance, however, the challenges were hindered by the low electrical conductivity of metal oxides [42]. Therefore, CPs can be a strong alternative candidate material for ECs owing to their excellent electrical conductivity. On the contrary, control over their morphology and physical properties is a difficult task [37].

An EC is a simple electrical component used to store energy electrochemically. ECs have irreplaceable advantages such as high power density, short charge time, a long cycle life, and favorable safety considerations. ECs aim to make up the critical gap between battery (high energy density) and conventional capacitor (high power density).

ECs, also called as supercapacitors can be classified into two broad categories, which are non-faradic (electrical double layer capacitors (EDLCs)) and faradic (pseudo-capacitors) depending on the charge storage mechanism [43], [44]. For non-faradic process, charge adsorption take place between the interfaces of electrodes and electrolyte. Only carbon materials are acceptable as electrode materials because of high specific surface area. As the charge/discharge reactions involve no chemical reactions, they can have a long cycle life theoretically. For faradic capacitors, the process is similar with what occur in a battery. In this case, the charge storage is achieved by electron transfer due to reduction/oxidation reactions in electro-active materials. Thus metal oxides and conducting polymers can be employed as electrode components. In general, pseudo-capacitors show a high energy density while EDLCs are relatively more stable with better cyclability. When a non-faradic capacitor is charged, its conducting plates have charges of equal magnitudes but opposite signs (Fig. 2) [45].

The faradic process occurring in the molecules of conducting polymers is electrochemical doping [46]. It is extraction of electrons from the polymer backbone through the circuit followed by intercalation of an anion into the backbone to balance the positive charge. It is well known that most CPs have been synthesized by oxidative polymerization mechanism employing proper oxidizing agents [47].

The presence of conjugation with alternating single and double bonds is a prerequisite for polymers to become electrically conducting. This is reasonable because CPs must have both charge carriers and an orbital that allows the charge carriers to move. For most cases, polymers cannot possess charge carriers, hence the carriers must be supplied by oxidation/reduction. Electron acceptors such as I2 promote oxidation while electron donors such as Na facilitate reduction reaction. In this way, potential charge carriers, for example polaron, bipolaron, and soliton are introduced (Fig. 3). This process is different from those seen in semiconductors, because a redox reaction happens in a polymer chain and the insulating polymer is converted to an ionic complex [48].

When a charge migrates through an insulating crystal, it will be surrounded by a lattice polarization. A polaron is termed as a moving charge accompanying with the polarization field. On the other hand, by a simplified chemical definition, a polaron is just a radical ion. When a secondary electron is introduced to the polaron, it is called a “bipolaron”. In case a secondary electron is incorporated to anywhere else on the polymer molecule chain, it is designated as a “soliton” [48].

ECs have high power density with relatively low energy density. On the contrary, LIBs show the opposite performance. They share some common features such as the existence of two electrodes that are in contact with the electrolyte, electron and ion conduction in electrodes and electrolyte, separation of electron and ion transport during the charge/discharge [49]. However, differences between ECs and LIBs are remarkable. First, the potential difference between the two electrodes in LIBs is constant ideally, but the voltage of ECs declines linearly as the charge elapses. Second, charge storage occurs by a faradic reaction in the case of LIBs, while it takes place by means of non-faradic reaction for ECs.

Even if a remarkable progress has been made, the practical application of GRP/CPs hybrid materials for high performance capacitors is still a challenging issue. Accordingly, it is meaningful to summarize the previous research activities for future study. Therefore, herein, an extensive and succinct summary on energy efficient capacitors based on GRP/CPs is provided. First, impressive research works on GRP/PPy, GRP/PAni, and GRP/PTh are described. In addition, recent patents on GRP/CPs will be introduced to catch a research trend associated with the application of capacitors. It is expected that this compact article can offer sufficient information for future study in relevant fields.

Section snippets

Graphene

ECs are representative energy storage devices emerging as one of the most lucrative energy storage systems in the future energy technology. To this purpose, rapid progress is not being made for the development of highly efficient capacitors. The electrode materials having unique structural and electrochemical properties such as carbon nanomaterials, metal oxides, multicomponent hybrids, and conducting polymers have shown great performances [50], [51], [52]. To date, carbon nanomaterials have

Graphene/conducting polymer capacitors

As mentioned in the previous section, GRP has been considered as a very promising material for ECs. Therefore, a lot of research activities have been conducted recently. The preparation of GRP/CPs hybrids and demonstration of their performance as an electrode material for ECs has been a mainstream of research for a couple of years. However, several issues have appeared such as control over morphology and properties of materials, use of foreign molecules or additives, introduction of fascinating

Conclusions and outlook

Compared with other alternatives, conducting polymers are inherently competitive materials for electrochemical capacitors. In addition, graphene has unprecedentedly excellent properties, which can be combined synergistically with conducting polymer. Thus, there has been a remarkable progress regarding electrochemical capacitors based on novel graphene/conducting polymer hybrids. To date, polymers such as polypyrrole, polyaniline, and polythiophene and its derivatives have been widely used for

Conflicts of interest

The authors declared none.

Author contribution

JWB collected information and literature and prepared a draft, OSK and CSL organized this manuscript, and JYP, OSK, and CSL reviewed and edited the manuscript extensively.

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1B03934440). This work was also supported by the KRIBB Initiative Research Program (KRIBB, Korea) and the National Research Council of Science & Technology grant (CAP-15-09-KIMS) funded by the MSIP.

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