Recent Advances in Graphene and Conductive Polymer Composites for Supercapacitor Electrodes: A Review

: Supercapacitors (SCs) have generated a great deal of interest regarding their prospects for application in energy storage due to their advantages such as long life cycles and high-power density. Graphene is an excellent electrode material for SCs due to its high electric conductivity and highly speciﬁc surface area. Conductive polymers (CPs) could potentially become the next-generation SC electrodes because of their low cost, facile synthesis methods, and high pseudocapacitance. Graphene/CP composites show conspicuous electrochemical performance when used as electrode materials for SCs. In this article, we present and summarize the synthesis and electrochemical performance of graphene/CP composites for SCs. Additionally, the method for synthesizing electrode materials for better electrochemical performance is discussed. spectroscopy analysis (EIS) method. The study indicated that the speciﬁc capacitance of was · − 1 at 0.5 A · − 1 , and the capacitance retention rate was 96% after 3000 cycles. Despite the fact that Co 3 O 4 @NGO/Pind exhibited great electrochemical performance, it was unable to represent the ideal capacitive behavior due to the unsymmetrical proﬁles on CV and non-triangular shapes on GCD. These could be attributed to the addition of multiple compounds and compatibility difference. Hence, further research is necessary to overcome such drawbacks to improve the capacitor properties.


Introduction
Since its discovery in 2004, graphene has attracted extensive interest, and much research has been carried out to study its physical and chemical properties and explore its applications [1][2][3]. The peculiar optical, mechanical, and electrical properties of graphene make it promising in many fields, such as material science [3], biomedicine [4,5], environmental treatment [6,7], and energy [8,9]. In particular, graphene plays an important role in the fight against COVID-19, and it has recently inspired novel technologies to defeat the pandemic [10,11]. Graphene is considered one of the most promising electrode materials for SCs due to its large specific surface area (SSA), outstanding electrical conductivity, flexibility, and excellent chemical stability [12][13][14].
Based on their storage mechanism, SCs can be divided into electrochemical doublelayer capacitors (EDLCs) [15] and pseudocapacitors [16]. EDLCs keep energy through charge accumulation at the electrode/electrolyte interface [17]. Specifically, reversible electrostatic attraction occurs between the ions from electrolytes and the oppositely charged electrode surfaces during the charging progress [18]. By contrast, ions leave the electrode's surface and move back in the electrolyte during the discharging progress. Since no charge transfer takes place during the process, EDLCs exhibit a fast charge/discharge rate and superior power performance compared to conventional capacitors. The SSA and porosity of the electrode are key factors affecting the capacitance of EDLCs [19]; theoretically, the larger the SSA of the electrode material, the higher the capacitance value of the EDLC. Using carbon as the electrode material is the primary method for manufacturing EDLCs, such as carbon nano-onions [20], activated carbon (AC) [21], and carbon nanotubes (CNTs) [22]. The theoretical SSA of graphene is up to 2630 m 2 g −1 [23], making it one of the most promising electrode materials for the manufacture of EDLCs.
In order to overcome the drawbacks of EDCLs and pseudocapacitors, many studies have managed to fabricate new types of hybrid SCs by combining the above electrode materials (both carbonaceous materials and pseudocapacitive materials). In recent years, many CPs have been explored and used to manufacture CP/graphene composites in combination with graphene. The application of CP/graphene composites, such as SCs [42,43], solar cells [44], fuel cells [45], sensing platforms [46], etc., has also been investigated, and the application in SCs has aroused great interest among researchers due to the excellent electrochemical property of CP/graphene composites. These CP/graphene SC electrodes, which are supposed to combine the advantages of both CPs and graphene, show significant improvements on their capacitance performance and cyclic stability. Thus, the disadvantage of CP-only electrodes is effectively ameliorated.
Several reviews [18,47] concerning the synthesis methods and capacitive performance of graphene/CP hybrid SCs are available in the literature; however, most of these papers only focus on the relatively simple binary composites of graphene (and its derivatives) and CPs. In this review, apart from the widely considered binary CP/graphene composite electrodes for SCs, the most recent research progress of more sophisticated ternary and even quaternary composites from the past 5 years is also presented, offering valuable instruction for the selection and combination of promising electrode materials. Specifically, the content is divided into five sections. Section 1 introduces the properties of graphene and CPs and their application in the field of SCs. Section 2 discusses binary graphene/CP composites (PANI, PPy, and Pind) used for electrodes. Section 3 discusses ternary composites synthesized by combining graphene/CP composites with another material. In Section 4, the synthesis strategies and performance of quaternary composites are discussed. Finally, in Section 5, future perspectives on SC electrodes based on CP/graphene composites are also proposed.

Binary Composites
Binary composite refers to a material composed of two elements, parts, or divisions, including metal materials, non-metal materials, organic materials, polymer composite materials, etc. Binary composites, which are composed of CPs and graphene, exhibit higher stability and higher electrical conductivity than individual components due to the positive synergistic affection between CPs and graphene [47].

Graphene/PANI Composites
Polyaniline (PANI) is a promising conductive polymer that exhibits excellent electrochemical activity, environmental stability, and fast Faraday reactivity [48,49]. Many synthesis methods of PANI have been developed in recent decades, such as electrochemical polymerization and oxidative polymerization [50,51]. Apart from the level and type of dopant used, it is well known that the synthesis method adopted also has a significant effect on the electrochemical properties and the electrical conductivity of PANI [52].
PANI/graphene nanocomposites can be synthesized via different procedures such as interfacial polymerization [53], chemical polymerization techniques [51,54], and electrochemical polymerization [55]. In the process of synthesis, graphene can create a suitable environment for PANI growth. Zhao et al. [54] reported the synthesis of PANI/graphene nanocomposites fabricated through in situ high gravity chemical oxidative polymerization means in a rotating packed bed (RPB). Meanwhile, they found that the ammonium persulfate/aniline mole ratio, graphene dosage, reactor type, and aniline concentration play important roles in their morphology and electrochemical performance. A simple in situ chemical oxidative pathway was used to synthesize nanocomposites of PANI with GO in an acidic environment of aqueous sulfuric acid, while dodecylbenzene sulfonic acid (DBSA) was used as a surfactant and dopant [56]. The GO was synthesized from graphite through a modified Hummers method. Gul et al. [56] also studied the influence of different GO concentrations on the composite morphologies (as shown in Figure 1). It can clearly be observed from these images that GO is introduced into composites due to the changes in morphology, and extremely porous nanostructures with distributed nanofibers are developed at 6% of GO ( Figure 1e). DBSA is helpful for the constant growth of PANI on the surface of GO sheets during polymerization. As the GO concentration increases, the specific capacitance of the composite increases, reaching a maximum of 658 F g −1 at 6% of GO; it then decreases further (PANI: 158 F·g −1 , PANI-GO-1: 323 F·g −1 , PANI-GO-2: 345 F·g −1 , PANI-GO-4: 417 F·g −1 , PANI-GO-6: 658 F·g −1 , PANI-GO-8: 387 F·g −1 , PANI-GO-10: 355 F·g −1 ). The largest average pore volume (0.037 cc·g −1 ) and BET surface area (m 2 ·g −1 ), as well as the smallest pore radius (14.561 Å), of PANI-GO-6 seem to be the reasons for the highest specific capacitance.  Many studies have been conducted in recent years using RGO to replace GO as the electrode material of SCs due to the former's greater conductivity [57,58]. In general, RGO Many studies have been conducted in recent years using RGO to replace GO as the electrode material of SCs due to the former's greater conductivity [57,58]. In general, RGO is obtained from GO and then used to compound with PANI. The capacitance properties of RGO/PANI nanocomposites are greatly affected by changing the sequence of compositing or reducing GO and the electrolytes. Moyseowicz et al. [59] synthesized GO from graphite by a modified Hummers method. Then, the GO was reduced via hydrothermal treatment at 180 • C for 12 h in a stainless-steel autoclave while adding PANI into GO compared with the absence of PANI. The specific capacitance values of PANI, RGO-HT (RGO by hydrothermal treatment), and PANI/RGO-HT were 440, 236, and 420 F·g −1 , respectively, at a current density of 0.2 A·g −1 . The specific capacitance value of PANI/RGO-HT was 239 F·g −1 , while PANI was 84 F·g −1 at a current density of 20 A·g −1 . Meanwhile, the composite cyclic stability of PANI/RGO-HT was greatly improved, and the capacity retention rate remained at 80% after 6000 cycles compared with 46% of PANI after just 1000 cycles at a current density of 2 A·g −1 .

Graphene/PPy Composites
Polypyrrole (PPy) is another important CP with a high theoretical capacitance of 620 F·g −1 [60]. Due to its appealing characteristics, such as excellent flexibility, low cost, sizeable energy density [61,62], high cycling stability [63], and quick charge/discharge process [64], PPy can be used to manufacture flexible electronic devices, such as wearable sensors [65], smart textiles [66], and roll-up displays [67]. Therefore, the important role of PPy in energy storage devices and electronic applications has attracted great attention from researchers. However, as with PANI, PPy also has poor cyclic stability. To alleviate this deficiency, researchers have synthesized graphene/PPy composites electrochemically or by polymerization.
Recently, Jia et al. [68] synthesized various PPy/GO films with different mass ratios by the confined polymerization inside the ice method. A prepared frozen pyrrole/GO layer and a mixture of adipic acid and ammonium were added into the mold, where the compound of deionized water and alcohol persulfate was initially added at a temperature of −20 • C. The reaction was carried out at −20 • C for 24 h. Additionally, the PPy/GO film was then obtained after thawing, washing, and drying. 84 Ω). This study showed that, with the increase in GO content, the capacitance and electrical conductivity of SCs increased.
Using hexadecylpyridinium chloride (CPC) as a modifier of GO, Feng et al. [69] synthesized polypyrrole/modified graphite oxide (PPy/MGO) composites via in situ polymerization. CPC contains hydrophobic groups (pyridine rings), which combine with PPy chains and then interact on the surface of GO via π-π stacking. As a result, the specific capacitance of PPy/MGO (202 F·g −1 ) was higher than PPy/GO (137 F·g −1 ), and the capacitance utilization of PPy in PPy/MGO (183 F·g −1 ) was also higher than PPy/GO (129 F·g −1 ), indicating that the modification of GO could improve the performance of PPy/GO composites.
Ghanbari et al. [70] put GO powder into a microwave oven to exfoliate and reduce graphene oxide by microwave irradiation (MRGO), and then in situ polymerized pyrrole onto it. The specific capacitance of the reduced-graphene oxide/polypyrrole nanofiber (RGO/PPy-Nf) nanocomposite was 277 F·g −1 at a 1 A· −1 current density in a solution electrolyte containing 1 M H 2 SO 4 . The study showed that both mechanisms of EDLCs and faradaic reactions played an important role in the RGO/PPy-Nf nanocomposite.

Graphene/Pind Composites
Recently, some researchers have demonstrated that polyindole (Pind) is a promising CP for the manufacture of SCs. The properties of Pind are similar to poly (paraphenylene) and PPy due to its special molecular structure containing a six-membered benzene ring fused in a five-membered pyrrole ring [71]. The synthesis methods of Pind are put into practice via various polymerization routes, such as electrochemical polymerization and chemical oxidative polymerization [72,73]. Pind shows a competitive redox potential compared to PPy and a slower hydrolytic degradation than PANI [74]. However, the conductivity of Pind is much lower than PPy and PANI (two orders of magnitude) [75]. In recent years, the application of Pind in electrode materials combined with graphene for SCs has attracted much attention.
Mudila et al. [76] synthesized various polyindole/graphene oxide (GO) nanocomposites (PINCs) with different concentrations of GO via a dilute solution polymerization method with the assistance of cetyltrimethylammonium bromide. This study showed that, with the increase in GO content, the thermal stability and the specific capacitance of PINCs increased regularly. For example, at a scan rate of 0.001 V/s, the specific capacitance of pure polyindole was 21.89 F·g −1 compared to 399.97 F·g −1 PINCs with 20% w/w GO. In [77],

Graphene/Pind Composites
Recently, some researchers have demonstrated that polyindole (Pind) is a promising CP for the manufacture of SCs. The properties of Pind are similar to poly (paraphenylene) and PPy due to its special molecular structure containing a six-membered benzene ring fused in a five-membered pyrrole ring [71]. The synthesis methods of Pind are put into practice via various polymerization routes, such as electrochemical polymerization and chemical oxidative polymerization [72,73]. Pind shows a competitive redox potential compared to PPy and a slower hydrolytic degradation than PANI [74]. However, the conductivity of Pind is much lower than PPy and PANI (two orders of magnitude) [75]. In recent years, the application of Pind in electrode materials combined with graphene for SCs has attracted much attention.
Mudila et al. [76] synthesized various polyindole/graphene oxide (GO) nanocomposites (PINCs) with different concentrations of GO via a dilute solution polymerization method with the assistance of cetyltrimethylammonium bromide. This study showed that, with the increase in GO content, the thermal stability and the specific capacitance of PINCs increased regularly. For example, at a scan rate of 0.001 V/s, the specific capacitance of pure  [77], the Pind/RGO nanocomposites, synthesized through a typical in situ chemical oxidative polymerization method, presented a high specific capacitance of 322.8 F·g −1 at 1.0 A·g −1 , and the cyclic stability decreased by only 5.5% after 1000 cycles. The above research provides evidence that the combination of Pind and graphene can improve the electrochemical utilization of Pind and the structural stability of graphene/Pind composites in the process of charging and discharging; therefore, specific capacitance and cycle stability have both been improved.

Comparison and Summary
PANI, PPy, and Pind are perceived as highly promising conductive polymers for hybrid SCs due to their large energy density, high conductivity, and eco-friendly and cheap features. There is little difference in environmental and electrochemical stability between these CPs (PANI, PPy, and Pind); however, the conductivity comparison of them follows the order of PPy > PANI > Pind [75,78].
The recent development of graphene and CP (PANI, PPy, and Pind) composites for SCs is summarized in Table 1. The electrochemical properties (i.e., charge/discharge speed, specific capacitance, and cycling stability) of graphene/CP composites are affected by many factors, such as concentrations of graphene and its derivatives, properties of individual CPs, and polymerization methods. Moreover, the combination of CPs and graphene greatly enhances the electrochemical properties of graphene/CP composites, showing excellent synergy between CPs and graphene and its derivatives [79].

Ternary Composites
The binary composites of CPs with graphene and its derivatives have proved to have higher electrochemical properties than their individual components. Inspired by this, researchers have conducted plenty of work to improve the electrochemical performance of graphene/CP composites used in SCs by combining them with another material, such as metal sulfides, metal oxides, and nonmetal oxides, to form a ternary composite material.
Wang and his co-workers [88] fabricated a ternary composite SiO 2 /graphene/PANI (SGP) as the SC electrode material. In the study, hydrophilic SiO 2 was used as an inorganic porous framework layer to bridge PANI and graphene, which not only inhibited the accumulation between graphene and PANI and graphene layers, but also improved ion exchanges and the interactions at the electrolyte/electrode interface. Although the conduc-tivity of SiO 2 is poor, the excellent synergy between SiO 2 , graphene, and PANI improved the specific capacitance of SGP-1 composites (727.0 F·g −1 ) compared with SiO 2 /graphene (88.8 F·g −1 ), graphene (108.0 F·g −1 ), and graphene/PANI (302.1 F·g −1 ); meanwhile, the capacitance retention rate of SGP-4 was 90% after 3500 cycles.
Xu et al. [89] synthesized ZnS/RGO/CP composites by doping with the same mass ratio of CPs (PANI, PPy, polythiophene (PTh), poly 3,4-ethylenedioxythiophene (PEDOT)) on the surface of a ZnS/RGO composite through an in situ polymerization method. Meanwhile, in comparison with other ZnS/RGO/CP composites (PPy, PTh, PEDOT), the capacitance performance and cycle stability of ZnS/RGO/PANI were the best. The specific capacitance of ZnS/RGO/PANI was 722.0 and 1045.3 F·g −1 at 1 A·g −1 in a two-electrode and three-electrode system, higher than 613.8 and 787 F·g −1 of ZnS/RGO/PPy, and the cycle stability of ZnS/RGO/PANI was 76.1% and 160% in a two-electrode and three-electrode system after 1000 loops, which was also higher than 50% and 149% of ZnS/RGO/PPy.
Ramesh et al. [90] synthesized Co 3 O 4 @NGO/Pind composites by hydrothermal treatment after ultrasonication, and the synthesis process is shown in Figure 3. Figure 4 shows that the electrochemical performances of Co 3 O 4 @NGO/Pind composites were researched through galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), and an electrochemical impedance spectroscopy analysis (EIS) method. The study indicated that the specific capacitance of Co 3 O 4 @NGO/Pind was~680 F·g −1 at 0.5 A·g −1 , and the capacitance retention rate was 96% after 3000 cycles. Despite the fact that Co 3 O 4 @NGO/Pind exhibited great electrochemical performance, it was unable to represent the ideal capacitive behavior due to the unsymmetrical profiles on CV and non-triangular shapes on GCD. These could be attributed to the addition of multiple compounds and compatibility difference. Hence, further research is necessary to overcome such drawbacks to improve the capacitor properties.  [89] synthesized ZnS/RGO/CP composites by doping with the same mass ratio of CPs (PANI, PPy, polythiophene (PTh), poly 3,4-ethylenedioxythiophene (PE-DOT)) on the surface of a ZnS/RGO composite through an in situ polymerization method. Meanwhile, in comparison with other ZnS/RGO/CP composites (PPy, PTh, PEDOT), the capacitance performance and cycle stability of ZnS/RGO/PANI were the best. The specific capacitance of ZnS/RGO/PANI was 722.0 and 1045.3 F·g −1 at 1 A·g −1 in a two-electrode and three-electrode system, higher than 613.8 and 787 F·g −1 of ZnS/RGO/PPy, and the cycle stability of ZnS/RGO/PANI was 76.1% and 160% in a two-electrode and three-electrode system after 1000 loops, which was also higher than 50% and 149% of ZnS/RGO/PPy.
Ramesh et al. [90] synthesized Co3O4@NGO/Pind composites by hydrothermal treatment after ultrasonication, and the synthesis process is shown in Figure 3. Figure 4 shows that the electrochemical performances of Co3O4@NGO/Pind composites were researched through galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), and an electrochemical impedance spectroscopy analysis (EIS) method. The study indicated that the specific capacitance of Co3O4@NGO/Pind was ~680 F·g −1 at 0.5 A·g −1 , and the capacitance retention rate was 96% after 3000 cycles. Despite the fact that Co3O4@NGO/Pind exhibited great electrochemical performance, it was unable to represent the ideal capacitive behavior due to the unsymmetrical profiles on CV and non-triangular shapes on GCD. These could be attributed to the addition of multiple compounds and compatibility difference. Hence, further research is necessary to overcome such drawbacks to improve the capacitor properties. The recent development of ternary composites (graphene and its derivatives, CPs, and other materials) for SCs is summarized in Table 2. Compared with binary composites, ternary composites exhibit better electrochemical performance; this is due not only to the synergy between CPs, graphene and its derivatives, and other materials, but also to the increase in SSA that results in a reduction in diffusive path length [91]. In addition, ternary composites of graphene, metal oxides, and CPs can prevent the detachment of metal oxide nanoparticles and improve cycling stability [92]. Ternary composite metals and metal oxides, or sulfides, are the commonly used materials in graphene/CP composites to form ternary composites due to their superior conductivity and pseudocapacitance. Therefore, the improvement in electrochemical performance by adding nonmetallic material (poor conductivity) into graphene/CP composites primarily promotes interfacial interactions [88].  The recent development of ternary composites (graphene and its derivatives, CPs, and other materials) for SCs is summarized in Table 2. Compared with binary composites, ternary composites exhibit better electrochemical performance; this is due not only to the synergy between CPs, graphene and its derivatives, and other materials, but also to the increase in SSA that results in a reduction in diffusive path length [91]. In addition, ternary composites of graphene, metal oxides, and CPs can prevent the detachment of metal oxide nanoparticles and improve cycling stability [92]. Ternary composite metals and metal oxides, or sulfides, are the commonly used materials in graphene/CP composites to form ternary composites due to their superior conductivity and pseudocapacitance. Therefore, the improvement in electrochemical performance by adding nonmetallic material (poor conductivity) into graphene/CP composites primarily promotes interfacial interactions [88].

Quaternary Composites
Based on the above research, it is clear that multiple components can make up for respective deficiencies and improve their comprehensive performance in relation to electrochemistry; quaternary composites of CPs with graphene are supposed to exhibit superior capacity and service life. A recent study showed that, by way of reduction in platinum nanoparticles into a modified PANI with carbon nanotubes (CNTs) and graphene nanosheets (GNS), a quaternary composite of PANI/GNS/CNT/Pt was synthesized [96]. The advantages of four materials-excellent electrical conductivity of GNS and CNTs, large surface area and conductivity of Pt nanoparticles, and high redox activity of PANIwere well presented in the quaternary electrode of PANI/GNS/CNT/Pt, which showed excellent synergy between PANI, GNS, CNTs, and Pt.
Gottam et al. [97] synthesized a quaternary composite of MoO 3 -MC-SiO 2 -PANI through a chemical process using molybdenum oxide (MoO 3 ), mesoporous carbon (MC), silicon dioxide (SiO 2 ), and PANI materials. The study showed that the specific capacitance of the carbon substrate could be improved by adding SiO 2 into it to form MC-SiO 2 , and the combination of the two materials of PANI and MoO 3 could form a dual charge storage redox action, which had the potential for excellent capacitive performance (see Figure 5). Table 3 shows that the specific capacitance and cycle stability of the quaternary composite (PANI/GNS/CNT/Pt) were higher than the ternary composites (PANI/GNS/CNT, PANI/GNT/Pt, and PANI/GNS/Pt) in [96], and the specific capacitance of the quaternary composite (MoO 3 -MC-SiO 2 -PANI) was higher than that of ternary composites (MC-SiO 2 -PANI and MoO 3 -MC-SiO 2 ) in [97]. However, few studies have been performed on quaternary electrodes in recent years, and the study on the mechanism of quaternary composites is unclear, as quaternary composites involve four components. Therefore, more research on quaternary composites is needed to study their synthesis, electrochemical properties, and mechanism, which will provide the possibility for the creation of next-generation energy storage systems.  Table 3 shows that the specific capacitance and cycle stability of the quaternary composite (PANI/GNS/CNT/Pt) were higher than the ternary composites (PANI/GNS/CNT, PANI/GNT/Pt, and PANI/GNS/Pt) in [96], and the specific capacitance of the quaternary composite (MoO3-MC-SiO2-PANI) was higher than that of ternary composites (MC-SiO2-PANI and MoO3-MC-SiO2) in [97]. However, few studies have been performed on quaternary electrodes in recent years, and the study on the mechanism of quaternary composites is unclear, as quaternary composites involve four components. Therefore, more research on quaternary composites is needed to study their synthesis, electrochemical properties, and mechanism, which will provide the possibility for the creation of next-generation energy storage systems.

Summary and Outlook
Carbon materials, metal oxides, and CPs are three generally studied electrode materials for SCs. Metal oxides have a higher specific capacity, but their low conductivity, high cost, and pollution to the environment are limitations for their application in SCs. Graphene is one of the ideal SC materials due to its superior electrochemical properties (such as high electrical conductivity) and highly specific surface area. However, because of the effect of van der Waals interaction, the accumulation of graphene sheets decreased the specific capacitance of graphene electrodes [30,31]. CPs could probably become next-generation SC electrode materials due to their low cost and high pseudocapacitance. However, pure CP electrodes expand and shrink during charging and discharging, resulting in low cycle stability [41]. Nevertheless, with the combination of graphene and CPs, the newly formed graphene/CP composites have the advantages of high specific capacitance and excellent cycle stability, making them a more promising candidate for SC electrode materials.
In this review, we have discussed several graphene/CP composites (PANI, PPy, and Pind) and their potential applications in SCs. The methods of synthesis for electrode materials, including the one-pot synthesis route [56], hydrothermal-assisted synthesis method [59], and ice-interface assisted synthesis method [68], are discussed in this article. Meanwhile, the electrochemical performances of pure, binary, ternary, and quaternary composites have been discussed and compared. EDLCs and pseudocapacitors composed of pure electrode materials have the disadvantage of poor cyclic stability or low specific capacitance, while hybrid SCs composed of binary, ternary, and quaternary composites store energy through Faradic redox reactions and charge accumulation at the electrode/electrolyte interface. The factors affecting SCs are as follows: the interfacial interaction between graphene and CPs, the properties of CPs, the microstructure of composites, etc. [98]. Therefore, researchers have conducted many studies to improve the interfacial interaction between graphene and CPs and the microstructure of composites by adding one more material to form ternary composite electrode materials, and even quaternary composite electrode materials, to further enhance this effect.
In conclusion, graphene/CP composites are superior materials in the application of SCs. Great progress has been achieved with the unremitting efforts of researchers for better electrochemical performance of electrode materials. However, the following questions remain to be addressed by researchers in this field.
First, compared with other carbon materials, such as coconut-shell-activated carbon, the cost of graphene is still high. New techniques for the synthesis of graphene with a low cost and high quality still need to be developed. At present, the electrochemical performance of CPs cannot meet the requirement of large-scale commercial applications, and the synthesis of new CPs also requires development by researchers.
Second, during the processes of synthesizing graphene/CP composites, the high SSA of graphene should be maintained. New techniques for uniformly mixing graphene into CPs need to developed to prevent the restacking of graphene.
Third, the application of binary composites, ternary composites, and quaternary composites of graphene/CP in SCs has proven to be very successful; however, the mechanisms used to explain the synergy between CPs, graphene, and other materials are not clear. Thus, more mechanisms need to be studied to optimize the compositions and morphologies of graphene/CP composites. We believe that graphene/CP composites with higher electrochemical performance can be found thanks to the pursuit of researchers who might finally realize their application in commercial fields.
Author Contributions: X.C. and X.J. conceived the idea. X.C. prepared an initial draft of the manuscript. X.C., Y.Q., K.S. and X.J. have participated in the writing of the final manuscript, and agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.