Prevention of Graphene Restacking for Performance Boost of Supercapacitors—A Review

Graphene is a promising electrode material for supercapacitors mainly because of its large specific surface area and high conductivity. In practice, however, several fabrication issues need refinement. The restacking of graphene flakes upon being packed into supercapacitor electrodes has become a critical challenge in the full utilization of graphene’s large specific surface area to further improve the device performance. In this review, a variety of recent techniques and strategies are overviewed for the prevention of graphene restacking. They have been classified into several categories to improve and facilitate the discussion on the underlying ideas. Based on the overview of the existing techniques, we discuss the trends of future research in the fields.


Introduction
Energy storage systems have been playing an important role in our lives.There are a great deal of applications, including portable electronics, hybrid electric vehicles and large industrial equipment [1].At present, batteries are the most widely used systems which may store energy at densities as high as 180 Wh/kg [1,2].However, batteries suffer from many limitations, such as low power density (only 50-200 W/kg), long charging time (0.3-3 h), limited cycle life, abrupt failure, poor low-temperature kinetics, and safety concerns caused by the usage of lithium [3,4].In many applications, supercapacitors are expected to complement or even replace batteries.Supercapacitors, also called electrochemical capacitors or ultracapacitors, typically comprise two electrodes immersed in an electrolyte with a thin layer of separator in between, and two current collectors (metal) connecting the electrodes [3].They store energy using the simple charge separation at the electrochemical interface (double layer) between the high-surface-area electrode and the electrolyte, and/or using pseudo-capacitance resulting from fast surface redox reactions.Consequently, supercapacitors are often of high power density (1-10 kW/kg), short charge/discharge time (in seconds), and long life cycles (over 10 5 times).As compared with batteries, the main shortcoming for supercapacitors is their relatively low energy density (1-10 Wh/kg).Further improvement of their performance mainly relies on advanced electrode materials, which should possess high electrical conductivity, large specific surface area, and long-term electrochemical stability [3].
As a matter of fact, a flood of papers have demonstrated the promise of graphene in supercapacitor applications.They cover almost all aspects of the performance boost of supercapacitors.As discussed above, supercapacitors store energy mainly relying on two kinds of mechanisms.One is the electrochemical double-layer capacitance (EDLC), which results from charges absorbed in the interfaces between the electrodes and the electrolytes, i.e., the double layers.A high EDLC requires electrodes with high electrical conductivity and large surface area.In theory, supercapacitors based on graphene electrodes can achieve EDLC as high as ~550 F/g [16].The other is the pseudo-capacitance, which is produced by highly reversible redox (faradic) reactions in electrode surfaces.Pseudo-capacitive materials typically include conducting polymers and a variety of transition metal oxides (e.g., RuO 2 , MnO 2 and IrO 2 ).In general, pseudo-capacitive materials can achieve higher capacitance than EDLC, despite their poor cycling stability [2,17].Although graphene itself is not pseudo-capacitive, composites incorporating graphene with pseudo-capacitive materials represent an important possibility to fabricate high-performance and low-cost supercapacitors [11].
Recently, more attention has been paid to supercapacitors with asymmetric electrodes [1,2,10,18], one of which is a capacitor-like electrode (as the power source), whereas the other is a battery-like faradaic electrode (as the energy source).An appropriate combination of electrodes in asymmetric supercapacitors can offer the advantages of both supercapacitors (charging rate and cycle life) and batteries (energy density).In particular, asymmetric supercapacitors may have evidently increased operation voltage that further improves the energy and power densities.Advanced asymmetric supercapacitors based on graphene electrodes have also been demonstrated to be of high energy density [18,19].
Consequently, the high electrical conductivity, large specific surface area, great incorporation with various pseudo-capacitive materials, and suitability for asymmetric supercapacitors make graphene a highly suitable material for supercapacitors.These points have been well demonstrated in many other review papers [8][9][10][11][12][13][14][15].However, in practice, people still have not been able to utilize the full potential of graphene for supercapacitors.The specific EDLC (typically around 100-200 F/g) for practical graphene-ba challenge is strongly red sheet-to-she have been d we do not at echniques w even further

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A near-i oriented, or efficiently u of vertically ultrathin sup 2.1.(d) SEM image of the cross-section of a free-dried solvated graphene film; (e) XRD patterns of as-prepared and free-dried solvated graphene films.Reprinted with permission from [32].Copyright 2011 John Wiley and Sons.

Pseudo-Capacitive Metal Oxide Nanoparticles
Pseudo-capacitive metal oxide nanoparticles are an important sort of spacers for graphene.Not only can they separate graphene nanosheets for more assessable surface area, but they also provide extra pseudo-capacitance to further improve the capacitance.A shortcoming of metal oxide is their poor conductivity.Usually, these metal oxides include RuO 2 , MnO 2 , Ni(OH) 2 , NiO, Fe 3 O 4 , CeO 2 , Co 3 O 4 , ZnO, SnO 2 , and so on [10,15].Recent studies concerning the involvement of these metal oxides in more complicated graphene composites are introduced in this review, as discussed below.

Conducting Polymers
Conducting polymers, such as polyaniline (PANI), polypyrrole (PPy), and poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), have attracted great interest in studies on supercapacitors because of their very high pseudo-capacitance.Note that pseudo-capacitance relies on fast reversible redox reactions in the electrode surfaces, differing from EDLC, which arises from direct charge separation at the electrode/electrolyte interface.Conducting polymers often possess various oxidation states and the redox reactions involve transitions between different oxidation states.For example, redox reactions in PANI-based supercapacitors often involve two transitions: one is between a semiconducting state (leucoemeraldine) and a conducting state (polaronicemeraldine), and the other is between the emeraldine and pernigraniline forms [33,34].Both of them can be identified by the peaks in cyclic voltammograms.A shortcoming of conducting polymers is their relatively poor cycling stability.Composites of conducting polymers with graphene [35] have been demonstrated as ideal electrode materials for supercapacitors because they can work synergistically to achieve both high energy density and improved cycling stability.Recently, advanced techniques [33][34][35][36][37][38] have also been developed to utilize conducting polymers as effective spacers for graphene electrodes to further increase the EDLC.The techniques include electrostatic interaction leading to PPy-sandwiched layered graphene [38], and grafting [33,34], direct coating [37], and vertically planting conducting polymers onto graphene [36].
A general problem for conducting polymer spacers is their weak interaction (van der Waals force) with graphene, which easily causes phase separation and hence degrades electrochemical performance.An effective way to enhance the graphene/polymer interaction is to graft conducting polymers into graphene sheets [33,34].As illustrated in Figure 11, one strategy is that 4-aminophenol [33] is first introduced into GO sheets by covalent functionalization and subsequently polymerized in the presence of aniline monomer.Due to the thionyl chloride vapor used in the graft process, GO is reduced concomitantly, thereby yielding highly conducting networks.Another strategy [34] may involve a first graft of amide groups into GO sheets by covalent functionalization, followed by reaction of the GO derivative with PANI nanofibers and reduction of GO by hydrazine.The covalent functionalization ensures the compatibility of graphene with conducting polymer (PANI) and minimizes phase separation.As a result, a high specific capacitance over 600 F/g has been attained [34].Zhang et al. [38] proposed another general route to fabricate layered graphene oxide with sandwiched conducting polymer.It is mainly based on electrostatic interactions between negatively charged GO sheets and positively charged surfactant micelles.Consequently, the surfactant micelles are adsorbed onto the graphene surface to form sandwiched structures (Figure 12).In addition, the hydrophobic cores of the surfactant micelles can attract conducting polymer monomer and after polymerization, layered GO structures pillared with the conducting polymers can be obtained.The demonstrated GO/PPy composite can provide a specific capacitance over 500 F/g. Figure 12.Schematic illustration of the formation process of the sandwich-structured GO/PPy composites.Reprinted with permission from [38].Copyright 2010 American Chemical Society.
Like carbon nanotube, one attractive strategy for effective spacers is the vertical growth of conducting polymers onto graphene sheets.Xu et al. [36] introduced a facile method to fabricate such a composite (Figure 13).The aligned PANI nanowires were produced by dilute polymerization of aniline monomer in the GO aqueous solution.The morphologies of PANI nanowires were strongly affected by the aniline concentration and the polymerization temperature.Under an optimal concentration of 0.05 M aniline and 0.36 mg/mL GO in aqueous solution and with a low temperature of -10 °C, nucleation and vertical growth of PANI can uniformly take place on GO sheets.These composites of vertically oriented PANI on GO sheets can provide a high specific capacitance of 555 F/g and good cycling stability.
Finally, further discussions are still necessary in order to fully understand the systems of graphene/conducting polymers and graphene/metal oxides.Although in this review we emphasize the function of conducting polymers and metal oxides as the spacers for graphene, they actually work synergistically, which means graphene actually also helps to disperse conducting polymers better or prevent metal oxides from agglomeration.

Figure 13. SEM images of vertically grown polyaniline on graphene oxide sheets.
Reprinted with permission from [36].Copyright 2010 American Chemical Society.

Ternary Graphene Structure
Although carbon nanotubes, metal oxides and conducting polymers are all excellent spacers for graphene electrodes, each of them has a significant shortcoming.Carbon nanotubes cannot provide pseudo-capacitance, metal oxides have low conductivity and conducting polymers are poor in cycling stability.Recent research tends to adopt the ternary structure which involves two kinds of the above spacers in addition to graphene so as to overcome the shortcoming and enhance the performance of the supercapacitors.

Performance Overview
Table 1 reviews the electrochemical performance for most of the supercapacitors fabricated from techniques included in Section 2. The listed values for all the parameters stand for the best performance reported for the devices.The electrolytes are usually aqueous solutions, unless otherwise specified.The cycling stability is characterized by the capacitance retention after a certain number of cycles.Note that for some devices, the capacitance may increase after cycling.Table 1.Review of specific capacitance (C s ), specific power (P s ), specific energy (E s ), cycling stability, equivalent series resistance (ESR) and resistor-capacitor time constant (t 0 ) for a variety of graphene-based supercapacitors.In several references, some parameters are not available, but instead their corresponding parameters are listed with explicitly indicated units.The electrolyte is ionic liquid.b The electrolyte is organic solution.c The unit is W/cm 3 ; d The unit is mW h/cm 3 ; e The unit is mF/cm 2 .
From Table 1, it is clear that most techniques have significantly improved the specific capacitance, among which the ternary structures (spaced graphene with carbon nanotubes and pseudo-capacitive materials [43,44]) and hierarchical structure (graphene foam with metal oxide [55]) even have specific capacitance exceeding 1000 F/g.These devices also exhibit excellent cycling stability and relatively low ESR.However, one should note that both high specific power and high specific energy can be achieved even for low-specific-capacitance supercapacitors if they are compatible with ionic liquid as the electrolytes [23,28,32].Therefore, it can be inferred that the combination between prevention of graphene restacking and introduction of pseudo-capacitance should represent a promising direction for design of supercapacitor electrodes, while the integration with ionic liquid as the electrolytes, which also benefits from the prevention of graphene restacking [23], is an important strategy to further improve the specific power/energy densities.

Conclusions and Outlook
We have reviewed recent research on preventing flake restacking for performance improvement in graphene-based supercapacitors.A variety of techniques and/or strategies have been demonstrated to make better use of the large specific surface area of graphene, ranging from proper placement of graphene (vertically oriented graphene), to modification of graphene (deformed graphene and spaced graphene), to construction of 3D graphene networks.These techniques have significantly improved the electrochemical performance of graphene-based supercapacitors.The performance boost can be clearly seen from the review in Table 1, where most techniques achieve a high specific capacitance over 200 F/g (or equivalent).It further confirms that graphene is really a promising material for supercapacitor applications and that suppressing flake restacking provides an effective way to achieve higher performance for the supercapacitors.
Because of more and more demanding requirements from emerging applications, a further performance boost is still indispensable for graphene-based supercapacitors.Based on the attained performance among all the techniques included in this review, we predict several trends for future researches in this field: (1) The development of novel techniques to prevent graphene restacking is of continuous interest.A technique is especially favorable if it can bring a thorough improvement of all the parameters, rather than only one or a few of them, including specific capacitance, specific power, specific energy, cycling stability and frequency response, and so forth; (2) Incorporation of the existing techniques for preventing graphene restacking with other techniques/strategies may represent another important direction for future research.For example, many of the above techniques may be incorporated with ionic liquid electrolyte, and/or asymmetric-electrode architecture for further performance improvement; (3) Demonstration of new ternary structures for spaced graphene and hierarchical structures for 3D graphene network will continue to attract great interest since they are promising to synergistically take full advantage of the large surface area and excellent electrochemical stability of graphene, and the pseudo-capacitance of conducting polymers or metal oxides to simultaneously achieve high power/energy density, excellent cycling stability, and high rate.However, for such composite structures, the issues of co-existence and potential windows among different active materials should be carefully addressed; (4) More attention will be paid to research on application-oriented supercapacitors, such as current filters, compressible supercapacitors, flexible supercapacitors, and micro-supercapacitors.We hope this review can provide useful information and inspiration for future studies to expedite the development of graphene-based supercapacitors.

Figure 10 .
Figure 10.(a) and (b) Photographs of the as-formed flexible self-stacked solvated graphene films; (c) Schematic illustration of the cross-section of a solvated graphene film; (d) SEM image of the cross-section of a free-dried solvated graphene film; (e) XRD patterns of as-prepared and free-dried solvated graphene films.Reprinted with permission from [32].Copyright 2011 John Wiley and Sons.

Figure 11 .
Figure 11.Schematic illustration of the formation process of polyaniline-grafted graphene composites.Reprinted with permission from [33].Copyright 2012 American Chemical Society.