Research Progress of Graphene-Based Materials on Flexible Supercapacitors

: With the development of wearable and ﬂexible electronic devices, there is an increasing demand for new types of ﬂexible energy storage power supplies. The ﬂexible supercapacitor has the advantages of fast charging and discharging, high power density, long cycle life, good ﬂexibility, and bendability. Therefore, it exhibits great potential for use in ﬂexible electronics. In ﬂexible supercapacitors, graphene materials are often used as electrode materials due to the advantages of their high speciﬁc surface area, high conductivity, good mechanical properties, etc. In this review, the classiﬁcation of ﬂexible electrodes and some common ﬂexible substrates are ﬁrstly summarized. Secondly, we introduced the advantages and disadvantages of ﬁve graphene-based materials used in ﬂexible supercapacitors, including graphene quantum dots (GQDs), graphene ﬁbers (GFbs), graphene ﬁlms (GFs), graphene hydrogels (GHs), and graphene aerogels (GAs). Then, we summarized the latest developments in the application of ﬁve graphene-based materials for ﬂexible electrodes. Finally, the defects and outlooks of GQDs, GFbs, GFs, GHs, and GAs used in ﬂexible electrodes are given. RGO / MWCNTcompositeﬁberstoachievebetterelectrochemicalperformances. ThePPy / RGO / MWCNT composite ﬁbers were twisted into springs and assembled with solid electrolyte. The stretchable and self-healing (polyurethane) PU shell was coated on the ﬁbers. The PU shell ensures the stretchability of the supercapacitor and self-healing properties by reconnecting the broken ﬁber electrodes when they are brought together. Reprinted with permission from [91]. Copyright 2017 ACS Nano.


Introduction
In recent years, people have begun to focus on clean and renewable energy such as solar, wind, and hydrogen, due to the energy shortage and environmental damage caused by the continuous consumption of fossil energy and other non-renewable resources [1,2]. In order to use energy sources more effectively in real life, energy storage equipment needs to improve the efficiency of storage and conversion [3]. Electrochemical energy storage devices such as lead batteries and lithium ion batteries are relatively mature energy storage technologies, but their power density is low and their cycle stability is poor [4]. Therefore, it is very important to develop an energy storage device with high performance [5]. As a type of energy storage equipment, supercapacitors have the advantages of long cycle life [6], high power density [7], and good safety [8]. What is more, the rapid charge and discharge characteristics of supercapacitors show a huge application in starting power. In the last decade, they have occupied a certain share of the market, especially in automotive applications [9]. Since the concept of supercapacitors was put forward, the overall performance of supercapacitors has been greatly improved [10]. However, the low energy density and high cost of electrode materials seriously restrict its commercial application [11,12]. With the development of science and technology, portable and wearable electronic equipment has become an inevitable development trend. To match with the portable and wearable electronic equipment, the flexible supercapacitors can be prepared into various sizes and shapes to meet different application environments, and they can also maintain the stability of capacitance performance under different bending and deformation conditions, showing great development prospects [13]. According to the principle of energy storage, supercapacitors can   [40]. Copyright 2011 ACS Nano.
In this review, we first briefly summarize various flexible electrode materials for flexible supercapacitors. Secondly, five graphene-based materials including GQDs, GFbs, GFs, GHs, and GAs are introduced in detail, especially their advantages and disadvantages in the preparation and application of flexible electrodes. After that, some latest developments in their application to flexible   In this review, we first briefly summarize various flexible electrode materials for flexible supercapacitors. Secondly, five graphene-based materials including GQDs, GFbs, GFs, GHs, and GAs are introduced in detail, especially their advantages and disadvantages in the preparation and application of flexible electrodes. After that, some latest developments in their application to flexible  [40]. Copyright 2011 ACS Nano.

Flexible Supercapacitors
With the innovation and development of micro and nanotechnology such as microelectronics and nanoengineering, coupled with the needs of different application scenarios such as wearable and high temperature resistance, new requirements have been put forward for the size, mechanical performance, and electrochemical performance in energy storage devices [45,46]. Flexible supercapacitors can achieve both great mechanical and electrochemical properties in the preparation process. At the same time, they can also design different structures and sizes according to different application scenarios, such as a one-dimensional fiber structure, two-dimensional plane structure, and three-dimensional laminated structure [47]. The one-dimensional fiber structure has good flexibility, so it can be deformed through large-scale bending and knotting. However, the active material load is low and the utilization rate is not high, which greatly affects the cycle life and energy density [9]. The two-dimensional planar structure exhibits good flexibility, and it is relatively light, thin, and compact, which is very suitable for integration in microelectronic devices. However, the active material loading of the planar structure electrode is not high, resulting in a small specific capacitance and low energy density [13,14]. The three-dimensional laminated structure has a large volume and a large load of active materials, enabling full contact between the electrode material and the electrolyte, accelerating the charge transfer rate and achieving high power density and energy density. Nonetheless, due to the larger volume and size, the three-dimensional laminated structure is not suitable for integration, and it cannot be deformed through twisting and knotting [21,22,25]. From the above three structures, the flexible electrode with different structures is prepared according to different application scenarios. Certainly, specific capacitances including the length-specific capacitance, area-specific capacitance, and volume-specific capacitance will be represented the performance of different electrode structures [32][33][34]. Common preparation methods for flexible supercapacitors include laser engraving, 3D printing, coating, screen printing, etc. [48][49][50][51].
For flexible supercapacitors, the choice of electrode materials is particularly important [52]. Flexible electrodes include substrate flexible electrodes and self-supporting flexible electrodes [53]. Self-supporting flexible electrodes are often prepared by chemical vapor deposition (CVD), vacuum filtration, electrodeposition, etc. [38,42,43,54]. The self-supporting electrodes prepared by these methods can simplify the electrode structure, alleviate the electrode quality, and improve the overall performance of the device. However, the self-supporting flexible electrodes cannot work under stretching, limiting the scope of their application [55]. Compared with self-supporting flexible electrodes, substrate flexible electrodes have excellent tensile and compressive properties due to the good mechanical properties of the substrate materials [56]. With the continuous research and innovation, many materials with excellent mechanical properties are used as substrates. The substrates can be divided into conductor substrates and insulating substrates [57]. The conductor substrate is mainly represented by metal substrates and carbon substrates, such as copper, titanium, nickel, carbon cloth, carbon fiber, etc. [29,32,40]. After stretching for a long time, the deformation of a metal substrate is irreversible, while there are some cracks in carbon substrates. Insulating substrates include common paper, sponge, fabric, and polymer substrates [38]. As the most common flexible substrates, paper has the advantages of its cheapness and easy availability. While sponge is compressible, it is very suitable for the preparation of high-density compressible supercapacitors; the fabric has good flexibility, light weight and low price, so it has unique advantages in flexible wearable devices. The polymer substrate has not only good flexibility, but also certain corrosion resistance, so it is suitable for application in highly corrosive environments [58][59][60]. The gel electrolyte has both the high conductivity of the liquid electrolyte and the mechanical strength of the solid electrolyte, so it is often used as the electrolyte of the flexible supercapacitors [44]. It is essential for high-performance flexible supercapacitors to obtain a gel electrolyte with a small thickness and a large electrode contact interface area. Therefore, it is key to prepare a suitable electrolyte to combine with the electrodes [61].

Preparation of GQDs
GQDs are composed of graphene with a high crystallinity of sp 2 and small atomic thickness less than 100 nm. The thickness of a single atom GQDs structure is close to the "limit size" [40,62]. Therefore, GQDs have unique characteristics derived from the quantum confinement and edge effects of powerful quantum dots, such as photoluminescence conductivity [41], chemical inertness [63], excellent stability [64], and environmental friendliness [65]. In addition, GQDs with nanoscale size can be well dispersed in various organic solvents, which can promote the chemical reactions in solution and improve electron mobility [66]. Considering the special physical and chemical characteristics of GQDs, it is an ideal electric double layer electrode material with high capacitance and stability [67].

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As shown in Figure 3, the methods for preparing GQDs include top-down and bottom-up methods [68]. For the top-down method, it uses graphene as a raw material, and then it utilizes chemical etching, electrochemical oxidation stripping, plasma treatment, and other methods in sequence to cut large-size sheet graphene into small-size GQDs [63,64]. For the bottom-up method, small organic molecules are used as the carbon source, and the structure contains a certain number of conjugated carbon atoms, which are first synthesized by the cage fullerene method and the solution chemistry method. Among them, the bottom-up method can control the size of GQDs, but the preparation processes are complicated and require strict and complex reaction conditions for organic materials [65]. Compared with the bottom-up method, the top-down method can control the size distribution under a simple experimental environment without complicated and demanding experimental conditions [69]. It was reported that there are many methods for preparing GQDs, including the hydrothermal method, acid oxidation method, oxygen plasma etching method, microwave method, and pyrolysis method [70]. With the excellent properties of GQDs, they exhibit a great potential in energy storage, biosensing, and light-emitting devices [71][72][73][74][75][76]. Table 1 shows the electrochemical performance for a variety of representative GQD supercapacitors.

Preparation of GQDs
GQDs are composed of graphene with a high crystallinity of sp 2 and small atomic thickness less than 100 nm. The thickness of a single atom GQDs structure is close to the "limit size" [40,62]. Therefore, GQDs have unique characteristics derived from the quantum confinement and edge effects of powerful quantum dots, such as photoluminescence conductivity [41], chemical inertness [63], excellent stability [64], and environmental friendliness [65]. In addition, GQDs with nanoscale size can be well dispersed in various organic solvents, which can promote the chemical reactions in solution and improve electron mobility [66]. Considering the special physical and chemical characteristics of GQDs, it is an ideal electric double layer electrode material with high capacitance and stability [67]. As shown in Figure 3, the methods for preparing GQDs include top-down and bottom-up methods [68]. For the top-down method, it uses graphene as a raw material, and then it utilizes chemical etching, electrochemical oxidation stripping, plasma treatment, and other methods in sequence to cut large-size sheet graphene into small-size GQDs [63,64]. For the bottom-up method, small organic molecules are used as the carbon source, and the structure contains a certain number of conjugated carbon atoms, which are first synthesized by the cage fullerene method and the solution chemistry method. Among them, the bottom-up method can control the size of GQDs, but the preparation processes are complicated and require strict and complex reaction conditions for organic materials [65]. Compared with the bottom-up method, the top-down method can control the size distribution under a simple experimental environment without complicated and demanding experimental conditions [69]. It was reported that there are many methods for preparing GQDs, including the hydrothermal method, acid oxidation method, oxygen plasma etching method, microwave method, and pyrolysis method [70]. With the excellent properties of GQDs, they exhibit a great potential in energy storage, biosensing, and light-emitting devices [71][72][73][74][75][76]. Table 1 shows the electrochemical performance for a variety of representative GQD supercapacitors.  [68]. Reprinted with permission from [68]. Copyright 2018 Elsevier. Reprinted with permission from [68]. Copyright 2018 Elsevier.

Electrodeposited GQDs for Flexible Electrode Materials
Depositing GQDs on a flexible substrate is a common method in flexible supercapacitors. Electrodeposition generally uses a two-electrode system, in which a flexible substrate can be used as a working electrode, platinum foil can be used as a counter electrode, and graphene quantum dots solution can be used as an electrolyte [78]. This method can attach GQDs to the flexible substrate evenly and strongly. As shown in Figure 4, Li et al. [79] deposited nitrogen-doped (N-doped) GQDs on GHs and carbon fiber (CF) composite materials. The electrode exhibited a good volume specific capacitance (93.7 F/cm 3 ) and an excellent capacitance retention. Using it as the positive electrode and GHs and CFs as the negative electrode, an asymmetric flexible supercapacitor was assembled. Since the N-GQD/GH/CF electrode exhibited very high volumetric capacitance and the assembly of an asymmetric supercapacitor also achieved a potential window of up to 2 V, the supercapacitor exhibited an energy density of up to 3.6 mWh/cm 3 at a power density of 35.6 mW/cm 3 . As shown in Figure 5, Lee et al. [77] deposited graphene staggered patterns on polyethylene terephthalate (PET) substrate by CVD and then electrochemically deposited GDQs on the graphene staggered patterns. The assembled micro-flexible supercapacitor with this electrode exhibited high transparency and high storage capacity (9.09 µF/cm 2 ). The doped GQDs have many functional groups, so they can be deposited on a flexible substrate to increase the electrochemical performance. Li et al. [80] electrochemically deposited GQDs co-doped with N and O atoms onto a CNT/CC substrate, resulting in a highly active, high specific surface area. The NO-GQD/CNT/CC electrode had an area specific capacitance of up to 461 mF/cm 2 .   [79]. Reprinted with permission from [79]. Copyright 2019 Elsevier.  . Schematic illustration of the preparation process of the graphene hydrogel/carbon fiber (GH/CF) negative electrode and N-GQD/GH/CF positive electrode and their assembly into an asymmetric flexible fiber supercapacitor [79]. Reprinted with permission from [79]. Copyright 2019 Elsevier.

GQDs for the Modification of Flexible Electrode Materials
As a single layer of graphene nanomaterials, GQDs have good conductivity, solubility, and easy functionalization, which can be used to modify other electrode materials to improve electrochemical performance. The feature is easy to implement, and it can combine the advantages of GQDs with other materials [61,72]. Ligninsulfate (Lig) is a kind of surface containing carbonyl, methoxy, phenolic hydroxyl, and other functional groups. Therefore, the preparation of Lig into nano-sized materials can well increase the active sites of the electrode material, and it can also enhance the electron transfer efficiency between the electrolytes. Xu et al. [81] used Lig to prepare GQDs to modify GHs. The porous structure of GHs and the active quantum dots from GQDs can accelerate electronic transfer efficiency. The obtained GHs-GQDs electrode had a specific capacitance of 451.7 F/g, and the retention rate of the capacitance was 93.3% after 180 • bending, which showed good mechanical flexibility. As is shown in Figure 6, Islam et al. [82] directly grafted GQDs onto CFs in situ via covalent ester linkages at a temperature of 90 • C. The grafted materials provide sufficient active sites from GQDs, which produce surface defects and prevent the recombination of GQDs. The specific capacitance of GQDs grafted on CFs was 5.5 times larger than that of CFs, and the capacitance retention rate was 97% after 5000 cycles.
Coatings 2020, 10, 892 8 of 25 retention rate of the capacitance was 93.3% after 180° bending, which showed good mechanical flexibility. As is shown in Figure 6, Islam et al. [82] directly grafted GQDs onto CFs in situ via covalent ester linkages at a temperature of 90 °C. The grafted materials provide sufficient active sites from GQDs, which produce surface defects and prevent the recombination of GQDs. The specific capacitance of GQDs grafted on CFs was 5.5 times larger than that of CFs, and the capacitance retention rate was 97% after 5000 cycles.

Fiber Supercapacitors
GFbs, the macroscopic whole formed by the integration of a single graphene sheet, have many extraordinary characteristics such as electrical characteristics, elasticity, rigidity, and strong stability [83]. Therefore, they have great potential in the application of flexible supercapacitors, as shown in

Fiber Supercapacitors
GFbs, the macroscopic whole formed by the integration of a single graphene sheet, have many extraordinary characteristics such as electrical characteristics, elasticity, rigidity, and strong stability [83]. Therefore, they have great potential in the application of flexible supercapacitors, as shown in Table 2. In particular, when they are used in micro electronic devices, textile electronic products, and implantable medical devices, they have unique advantages due to the small size, high flexibility, good braid ability, and easy integration into small-sized or various-shaped devices [84,85]. Compared with traditional planar supercapacitors, the energy density of fiber supercapacitors is lower, and the mechanical performance also faces great challenges. Since the fiber electrodes are solid and the interface areas between the electrode surface and electrolyte are low, the electron storage and transfer ability decline. Although the GFbs electrodes have the advantages of good flexibility and strong braid ability, they are still many challenges to maintain good electrochemical performance under the action of external forces such as compression and bending [86]. To improve the energy density, GFbs can be made into hollow shapes to obtain an additional internal interface area. Qu et al. [87] prepared a fiber supercapacitor with hollow graphene oxide (HGO) and a conductive polymer. The good mechanical properties of the conductive polymer and the good conductivity of the GFbs have a good mutual promotion effect, resulting in the high flexible and electrochemical properties of fiber supercapacitors. Graphene-carbon nanotube composite fibers are proposed to prevent graphene intermediate layer π-π stacking and increase the specific surface area [88]. However, the application of GFbs in supercapacitors will cause Coatings 2020, 10, 892 9 of 25 irreparable damage due to the corrosion and accidental cutting. Therefore, self-healing materials emerged for supercapacitors to prevent structural fracture from mechanical damage [89,90]. As shown in Figure 7, Wang et al. [91] prepared a fiber spring with self-healing ability based on GO fibers. The assembled supercapacitor had a capacity retention rate of 82.4% in the first large stretch and a capacity retention rate of 54.2% after the third healing, which provided a direction for the next generation of stretchable self-healing electronic devices. Coatings 2020, 10, x FOR PEER REVIEW 9 of 28 Table 2. In particular, when they are used in micro electronic devices, textile electronic products, and implantable medical devices, they have unique advantages due to the small size, high flexibility, good braid ability, and easy integration into small-sized or various-shaped devices [84,85]. Compared with traditional planar supercapacitors, the energy density of fiber supercapacitors is lower, and the mechanical performance also faces great challenges. Since the fiber electrodes are solid and the interface areas between the electrode surface and electrolyte are low, the electron storage and transfer ability decline. Although the GFbs electrodes have the advantages of good flexibility and strong braid ability, they are still many challenges to maintain good electrochemical performance under the action of external forces such as compression and bending [86]. To improve the energy density, GFbs can be made into hollow shapes to obtain an additional internal interface area. Qu et al. [87] prepared a fiber supercapacitor with hollow graphene oxide (HGO) and a conductive polymer. The good mechanical properties of the conductive polymer and the good conductivity of the GFbs have a good mutual promotion effect, resulting in the high flexible and electrochemical properties of fiber supercapacitors. Graphene-carbon nanotube composite fibers are proposed to prevent graphene intermediate layer π-π stacking and increase the specific surface area [88]. However, the application of GFbs in supercapacitors will cause irreparable damage due to the corrosion and accidental cutting. Therefore, self-healing materials emerged for supercapacitors to prevent structural fracture from mechanical damage [89,90]. As shown in Figure 7, Wang et al. [91] prepared a fiber spring with selfhealing ability based on GO fibers. The assembled supercapacitor had a capacity retention rate of 82.4% in the first large stretch and a capacity retention rate of 54.2% after the third healing, which provided a direction for the next generation of stretchable self-healing electronic devices. (B) GO/(multiwalled carbon nanotubes) MWCNT-based mixture solution was injected into pipes, followed by sealing at both ends. After GO was reduced to RGO at 90 • C in an oven, RGO/MWCNT composite fibers were prepared. A layer of (polypyrrole) PPy was electrodeposited on partially dried RGO/MWCNT composite fibers to achieve better electrochemical performances. The PPy/RGO/MWCNT composite fibers were twisted into springs and assembled with solid electrolyte. The stretchable and self-healing (polyurethane) PU shell was coated on the fibers. The PU shell ensures the stretchability of the supercapacitor and self-healing properties by reconnecting the broken fiber electrodes when they are brought together. Reprinted with permission from [91]. Copyright 2017 ACS Nano.

New Strategies for Preparing GFbs
The main approaches for preparing GFbs are solution spinning, wet-spinning, and dry-spinning. Among these methods, simple wet spinning is the most ideal method for preparing GFbs [97]. Pure GFbs have a low specific surface area, and they are difficult to obtain a satisfactory specific capacitance. Therefore, new strategies for preparing GFbs have to be emerged. In traditional wet spinning, the spinneret is a key component for the synthesis of pure GFbs with a porous surface. Cai et al. [98] found that the metal needle extracted from a medical syringe could be used as a spinneret to synthesize porous pure GFbs, which exhibited a specific surface area of 839 m 2 /g and a specific capacitance of 228 mF/cm 2 , far exceeding the specific capacitance (47.2 mF/cm 2 ) of GFbs prepared with polytetrafluoroethylene (PTFE). Chen et al. [99] proposed manufacturing high-performance reduced graphene oxide (RGO)/clay hybrid fiber. It exhibited good mechanical properties, hydrophilicity, and conductivity. Flexible all-solid-state supercapacitors with a power density of 28.33 mwh/cm 3 at an energy density of 6.14 mwh/cm 3 had shown great potential in flexible devices. In addition to the innovation in the preparation method, Zhao et al. [100] prepared a new type of coaxial fiber supercapacitor by improving the structure of supercapacitors. Compared with the twisted GFbs supercapacitors, the internal resistance of coaxial fiber supercapacitors was lower, and the specific capacitance (205 mF/cm 2 , 182 F/g) and energy density (17.5 µWh/cm 2 , 15.5 Wh/kg) of coaxial fiber supercapacitors were also higher. The retention rate of the coaxial fiber supercapacitors was 92% after bending 180 • through 100 cycles, demonstrating excellent performance under bending conditions. As shown in Figure 8, Yang et al. [61] obtained core-sheath GFbs with a coaxial three-channel structure by direct wet spinning. The gel electrolyte of proper thickness was prepared by using different concentrations of sodium carboxymethyl cellulose (CMC) solution and fibers with different extrusion rates. The core-sheath GFbs exhibited a high specific capacitance of 249 mF/cm 2 and 96% capacitance retention rate after 10,000 cycles. At the same time, the capacitance was almost unaffected when bent by 180 • .

Composite GFbs
Compounding with other materials such as carbon materials, transition metal oxides, and conductive polymers, the electrochemical performance of GFbs can be greatly improved, obtaining electrode materials with excellent flexibility, conductivity, and cycle stability [92,93]. Carbon materials with a large specific surface area can be used to increase the area specific capacitance and energy density [94]. Carbon nanotubes (CNTs) have excellent mechanical properties and electrical conductivity, and GFbs have strong loading and electrocatalytic capabilities, so CNTs and GFbs are ideal flexible electrode materials. The combination of CNTs and GFbs can obtain excellent electrochemical performance [83,95]. Jia et al. [101] used a simple and time-saving method to prepare nanocomposite fibers with a porous structure. They used the different mass ratios of graphene and CNTs (10:1; 5:1; 1:1; 1:0), which were expressed as G 10 /CNTs, G 5 /CNTs, G 1 /CNTs, and GFs, respectively. As shown in Figure 9, the tensile strength of GFs, G 1 /CNTs, G 5 /CNTs, and G 10 /CNTs were 281, 71, 146, and 197 MPa, and they could be bent into a variety of shapes, achieving excellent mechanical properties in electronic equipment. The combination of metal oxides can well increase the pseudocapacitance effect of carbon materials and energy density. The CNTs/Co 3 O 4 composite fibers were twisted to fabricate a fiber supercapacitor, exhibiting an area-specific capacitance of 52.6 mF/cm 2 [102]. Zhang et al. [96] prepared MnO 2 /holey reduced graphene oxide (HRGO) fiber electrodes, demonstrating a mass specific capacitance of 245 F/g. The symmetric all-solid supercapacitor assembled by using H 3 PO 4 polyvinyl alcohol (PVA) gel electrolyte not only showed good flexibility, but also a specific capacitance far exceeding a single electrode material. Conductive polymers such as polyaniline, poly (3,4-ethyldioxythiophene), and pseudo-active inorganic groups are often used in the composite of carbon materials [103]. Poly ionic liquid (PIL) is a new type of polymer that possesses macromolecular-ionic liquid moieties and multifunctional properties. Gopalsamy et al. [104] obtained PIL and graphene composite fibers by wet spinning with areal energy density (9.31 µWh/cm 2 ) and volume energy density (8.28 mWh/cm 3 ).

Composite GFbs
Compounding with other materials such as carbon materials, transition metal oxides, and conductive polymers, the electrochemical performance of GFbs can be greatly improved, obtaining electrode materials with excellent flexibility, conductivity, and cycle stability [92,93]. Carbon materials with a large specific surface area can be used to increase the area specific capacitance and energy density [94]. Carbon nanotubes (CNTs) have excellent mechanical properties and electrical conductivity, and GFbs have strong loading and electrocatalytic capabilities, so CNTs and GFbs are ideal flexible electrode materials. The combination of CNTs and GFbs can obtain excellent electrochemical performance [83,95]. Jia et al. [101] used a simple and time-saving method to prepare nanocomposite fibers with a porous structure. They used the different mass ratios of graphene and CNTs (10:1; 5:1; 1:1; 1:0), which were expressed as G10/CNTs, G5/CNTs, G1/CNTs, and GFs, respectively. As shown in Figure 9, the tensile strength of GFs, G1/CNTs, G5/CNTs, and G10/CNTs were 281, 71, 146, and 197 MPa, and they could be bent into a variety of shapes, achieving excellent mechanical properties in electronic equipment. The combination of metal oxides can well increase the pseudocapacitance effect of carbon materials and energy density. The CNTs/Co3O4 composite fibers were twisted to fabricate a fiber supercapacitor, exhibiting an area-specific capacitance of 52.6 mF/cm 2 [102]. Zhang et al. [96] prepared MnO2/holey reduced graphene oxide (HRGO) fiber electrodes, demonstrating a mass specific capacitance of 245 F/g. The symmetric all-solid supercapacitor assembled by using H3PO4 polyvinyl alcohol (PVA) gel electrolyte not only showed good flexibility, but also a specific capacitance far exceeding a single electrode material. Conductive polymers such as polyaniline, poly(3,4-ethyldioxythiophene), and pseudo-active inorganic groups are often used in the composite of carbon materials [103]. Poly ionic liquid (PIL) is a new type of polymer that possesses macromolecular-ionic liquid moieties and multifunctional properties. Gopalsamy et al. [104] obtained PIL and graphene composite fibers by wet spinning with areal energy density (9.31 μWh/cm 2 ) and volume energy density (8.28 mWh/cm 3 ).

Problems in Preparing GFs
GFs are compact materials obtained by stacking graphene face-to-face. GFs with different thicknesses have different applications. For example, the films with a thickness of less than 100 nm

Problems in Preparing GFs
GFs are compact materials obtained by stacking graphene face-to-face. GFs with different thicknesses have different applications. For example, the films with a thickness of less than 100 nm are generally used for transparent electrode materials, while films with a thickness of more than 100 nm are used for electrode materials for energy storage devices, such as supercapacitors or lithium batteries [105]. The preparation methods of GFs mainly include the suction filtration method, coating method, solution casting method, interface self-assembly method, and so on [106]. Although the GFs assembled by suction filtration can obtain extremely high density due to layer-to-layer stacking, the specific surface area accessible to electrolyte and transmission channels between ions is reduced [75,76]. At a low scan rate, the electrochemical performance of supercapacitors with GFs electrodes is stable, and high density-specific capacitance performance can be obtained. However, at a high scan rate or high current densities, the contact area of the electrolyte ion and GF surface is small due to the lack of pore channels, which increases the diffusion resistance between ions and reduces the diffusion efficiency of ions [107]. Therefore, the accumulation of GFs is the key problem to be solved for their use in flexible supercapacitors. As is shown in Table 3, GFs supercapacitors exhibit excellent electrochemical performance.

Porous/Embedded GFs
Researchers have made great efforts in solving the accumulation of GFs. One method is to prepare GFs with a porous structure, which can construct ion channels and accelerate ion migration. The other is to embed nanoparticle spacers between the GFs layers, alleviating layer-to-layer accumulation [117,118]. Shao et al. [119] prepared cell GFs with a porous three-dimensional structure as electrode materials, providing a rich ion library for the gel electrolyte. The assembled quasi-solid micro-supercapacitor had excellent electrochemical performance. In addition, laser reduction can be also used to produce thin films with an internal pore structure [120]. The embedding of MXene could alleviate the accumulation of GFs, and the symmetric supercapacitor assembled by the composite electrode showed an ultra-high volume energy density of 32.6 Wh/L, which was the highest reported values of carbon and MXene-based materials in aqueous electrolytes [116]. In addition to metal oxides, carbon spheres can also be embedded into GFs as spacers to enhance ion transport capabilities. Liu et al. [121] added carbon spheres modified by MnO 2 to the design of GFs to improve the specific surface area. The sandwich porous structure of carbon spheres provided a large number of paths for electrolyte ion penetration. In Figure 10, Xu et al. [112] used a simple hydrothermal method to coat Fe and Co on the surface of carbon spheres and then embed them in GFs to obtain metal@carbon sphere/GFs (Fe@C/G and Co@C/G). Fe@C/G and Co@C/G electrodes exhibited area-specific capacitances up to 2.19 and 0.80 F/cm 2 . The all-solid asymmetric flexible supercapacitor assembled by Fe@C/G and Co@C/G showed excellent volume energy density of up to 5.99 mWh/cm 3 at a volume power density of 0.013 W/cm 3 , and the retention rate of the capacitor was 80.5% after being folded 500 times. These results illustrate its huge potential in flexible supercapacitors. In addition to the above two methods, a vertically oriented graphene structure grown by plasma deposition has attracted great interest from scientific researchers. Due to the special discrete open structure of vertically-oriented graphene, it solves the problem of graphene accumulation and achieves a high specific surface area [45]. Le et al. [122] used electrochemical quartz crystal microbalance and the electrochemical gravimetric method to reveal the ion exchange mechanism of vertically oriented graphene supercapacitor electrodes, and they demonstrated that vertically oriented graphene nanosheets promote the rapid transmission of electrolyte ions and shorten the diffusion path. Ma et al. [123] used plasma-enhanced chemical vapor deposition to prepare vertically oriented graphene, which was used as a flexible electrode to exhibit a surface capacitance of 2.45 mF/cm 2 . The capacitance remained unaltered even under 100,000 times of bending or 180 • folding, showing excellent capacitance retention.
Researchers have made great efforts in solving the accumulation of GFs. One method is to prepare GFs with a porous structure, which can construct ion channels and accelerate ion migration. The other is to embed nanoparticle spacers between the GFs layers, alleviating layer-to-layer accumulation [117,118]. Shao et al. [119] prepared cell GFs with a porous three-dimensional structure as electrode materials, providing a rich ion library for the gel electrolyte. The assembled quasi-solid micro-supercapacitor had excellent electrochemical performance. In addition, laser reduction can be also used to produce thin films with an internal pore structure [120]. The embedding of MXene could alleviate the accumulation of GFs, and the symmetric supercapacitor assembled by the composite electrode showed an ultra-high volume energy density of 32.6 Wh/L, which was the highest reported values of carbon and MXene-based materials in aqueous electrolytes [116]. In addition to metal oxides, carbon spheres can also be embedded into GFs as spacers to enhance ion transport capabilities. Liu et al. [121] added carbon spheres modified by MnO2 to the design of GFs to improve the specific surface area. The sandwich porous structure of carbon spheres provided a large number of paths for electrolyte ion penetration. In Figure 10, Xu et al. [112] used a simple hydrothermal method to coat Fe and Co on the surface of carbon spheres and then embed them in GFs to obtain metal@carbon sphere/GFs (Fe@C/G and Co@C/G). Fe@C/G and Co@C/G electrodes exhibited areaspecific capacitances up to 2.19 and 0.80 F/cm 2 . The all-solid asymmetric flexible supercapacitor assembled by Fe@C/G and Co@C/G showed excellent volume energy density of up to 5.99 mWh/cm 3 at a volume power density of 0.013 W/cm 3 , and the retention rate of the capacitor was 80.5% after being folded 500 times. These results illustrate its huge potential in flexible supercapacitors. In addition to the above two methods, a vertically oriented graphene structure grown by plasma deposition has attracted great interest from scientific researchers. Due to the special discrete open structure of vertically-oriented graphene, it solves the problem of graphene accumulation and achieves a high specific surface area [45]. Le et al. [122] used electrochemical quartz crystal microbalance and the electrochemical gravimetric method to reveal the ion exchange mechanism of vertically oriented graphene supercapacitor electrodes, and they demonstrated that vertically oriented graphene nanosheets promote the rapid transmission of electrolyte ions and shorten the diffusion path. Ma et al. [123] used plasma-enhanced chemical vapor deposition to prepare vertically oriented graphene, which was used as a flexible electrode to exhibit a surface capacitance of 2.45 mF/cm 2 . The capacitance remained unaltered even under 100,000 times of bending or 180° folding, showing excellent capacitance retention. Figure 10. Schematic illustration of the preparation process of Fe@C/G and Co@C/G film. Reprinted with permission from [112]. Copyright 2019 Elsevier. Figure 10. Schematic illustration of the preparation process of Fe@C/G and Co@C/G film. Reprinted with permission from [112]. Copyright 2019 Elsevier.

Doped GFs
The accumulation problem of graphene can also be improved by doping [79]. The reason for using nitrogen atom doping is that the difference in atomic radius between nitrogen and carbon atoms is small, and nitrogen is more electronegative than carbon, so nitrogen atoms are more easily doped into the carbon network for the alternative doping of carbon atoms [59,63]. After doping nitrogen, carbon atoms were partially replaced, which can effectively adjust the electronic properties, surface catalytic properties, and local chemical properties of graphene [66,80,124]. Compared with the undoped graphene, doped graphene can increase the active site of the specific surface area as well as enhance the solubility and accessibility in the electrolyte solution, so the density of the free charge carriers and conductivity increased [125]. Jin et al. [125] fabricated porous self-supporting N-doped GFs using hydrogels, as shown in Figure 11. N-doping ensures the sufficient pseudocapacitance and conductivity of the film, and the macroporous structure facilitates rapid ion adsorption. Using the porous self-supporting N-doped GFs as a negative electrode and macroporous graphene/polypyrrole composite films as a positive electrode, an asymmetric supercapacitor was assembled. The assembled supercapacitor exhibited an energy density of 34.51 Wh/kg at a power density of 849.77 W/kg. Wen et al. [126] designed and fabricated self-supporting N-doped GFs with a lightweight 3D porous structure by CVD, which has excellent conductivity and a high specific surface area. It not only promoted the rapid transfer of electrons, but also provided a large surface area for the deposition of active substances. In addition to N atoms, S, P, B, and other atoms have been also proven to be doped with GFs [127]. Heteroatomic double doping can achieve the synergistic effect of different functional groups and obtain more excellent capacitance [128,129]. Yu et al. [130] prepared the steam-assistant heteroatoms of sulfur and phosphorus dual-doped graphene films (s-SPG) through the ice template and thermal activation method. The flexible supercapacitors synthesized by s-SPG films exhibited a good pseudocapacitance with a specific capacitance of 169 F/g and a cyclic stability of 92.5%. In Figure 12, the specific capacitance of GFs doped by S and P atoms was much higher than that of undoped GFs and single atom-doped GFs. At the same time, GFs doped by S and P atoms exhibited excellent flexibility in bending.
nitrogen, carbon atoms were partially replaced, which can effectively adjust the electronic properties, surface catalytic properties, and local chemical properties of graphene [66,80,124]. Compared with the undoped graphene, doped graphene can increase the active site of the specific surface area as well as enhance the solubility and accessibility in the electrolyte solution, so the density of the free charge carriers and conductivity increased [125]. Jin et al. [125] fabricated porous self-supporting N-doped GFs using hydrogels, as shown in Figure 11. N-doping ensures the sufficient pseudocapacitance and conductivity of the film, and the macroporous structure facilitates rapid ion adsorption. Using the porous self-supporting N-doped GFs as a negative electrode and macroporous graphene/polypyrrole composite films as a positive electrode, an asymmetric supercapacitor was assembled. The assembled supercapacitor exhibited an energy density of 34.51 Wh/kg at a power density of 849.77 W/kg. Wen et al. [126] designed and fabricated self-supporting N-doped GFs with a lightweight 3D porous structure by CVD, which has excellent conductivity and a high specific surface area. It not only promoted the rapid transfer of electrons, but also provided a large surface area for the deposition of active substances. In addition to N atoms, S, P, B, and other atoms have been also proven to be doped with GFs [127]. Heteroatomic double doping can achieve the synergistic effect of different functional groups and obtain more excellent capacitance [128,129]. Yu et al. [130] prepared the steam-assistant heteroatoms of sulfur and phosphorus dual-doped graphene films (s-SPG) through the ice template and thermal activation method. The flexible supercapacitors synthesized by s-SPG films exhibited a good pseudocapacitance with a specific capacitance of 169 F/g and a cyclic stability of 92.5%. In Figure  12, the specific capacitance of GFs doped by S and P atoms was much higher than that of undoped GFs and single atom-doped GFs. At the same time, GFs doped by S and P atoms exhibited excellent flexibility in bending. Figure 11. Schematic illustration of the fabrication process of N-doped graphene aerogel films [125]. Reprinted with permission from [125]. Copyright 2019 Elsevier. Figure 11. Schematic illustration of the fabrication process of N-doped graphene aerogel films [125]. Reprinted with permission from [125]. Copyright 2019 Elsevier.

Problems of Graphene Gels Used in Flexible Electrodes
Graphene can be randomly stacked from side to side or between sides to form a three-

Problems of Graphene Gels Used in Flexible Electrodes
Graphene can be randomly stacked from side to side or between sides to form a three-dimensional graphene with well-developed pores [27,40]. The three-dimensional graphenes have many new advantages such as a large specific surface area, porous structure, high mechanical properties, and good electron transport capabilities [131]. The common three-dimensional graphenes are sponge-like, foam-like, and gel-like [132]. Graphene gels are an ideal material for flexible electrodes, which can be divided into graphene hydrogels (GHs) and graphene aerogels (GAs) [133]. For GHs, there are two main drawbacks when it was used as an electrode material. Firstly, the force between the sheets is not uniform, so that some are strong and some are weak, resulting in the low overall strength of the material. Besides, the structural defect and disordered stack of the GHs lead to a poor electrical conductivity [134]. GAs not only have the advantages of graphene's high specific surface area and great conductivity, but also aerogel's characteristics of low density and high porosity [44]. The unique structure of GAs can bring out the physical and chemical properties of a single piece of graphene, overcoming the shortcomings of stacking between graphene sheets [135]. Furthermore, they have the characteristics of rich aerogel pores and uniform distribution [136]. GAs have strong adsorption capacity and can be recovered in an aqueous solution, avoiding the secondary damage to the environment and expanding the scope of application of graphene materials [137]. Nevertheless, GAs are the carbon material, and low specific capacitance is an unavoidable shortcoming [138]. As shown in Table 4, many studies have shown that the three-dimensional graphene gels can be used as flexible electrodes for supercapacitors to obtain high mass specific capacitance.

GHs Flexible Supercapacitors
In order to improve the conductivity and flexibility of GHs, they are helpful to compound with carbon nanomaterials. An et al. [93] constructed a three-dimensional N-doped carbon nanosheet framework into the GHs structure, achieving a high specific capacitance of 242 F/g at a current density of 1A/g. The energy density in the ionic liquid electrolyte was as high as 60.4 Wh/kg. As shown in Figure 13, 60 red light-emitting diodes (LED, 2.2 V) could be easily lit after charging for only 10 s in a single device filled only with ionic liquid electrolyte. The addition of transition metal oxide greatly improves the energy density of GHs [134]. Wang et al. [139] prepared NiOOH nanosheets/GHs by solvothermal and hydrothermal mixing reactions. They exhibited a good preformance with a high capacitance of 1162 F/g at a current density of 1 A/g and an excellent rate capability. The assembled asymmetric supercapacitor exhibited a significant energy density of 66.8 Wh/kg at a power density of 800 W/kg and an excellent cycle stability with an 85.3% capacitance after 8000 cycles. In addition, the easy functionalization of GHs can fill the gap between carbon materials such as carbon cloth and graphene sheets, improving the overall electrochemical performance [140]. As shown in Figure 14, Wu et al. [141] used composite GHs functionalized with polyaniline (PANI) and Lig to fill the blanks of carbon cloth (CC). The pseudocapacitance provided by PANI and the functional group provided by Lig brought many active sites and high specific capacitance to the entire electrode.

GHs Flexible Supercapacitors
In order to improve the conductivity and flexibility of GHs, they are helpful to compound with carbon nanomaterials. An et al. [93] constructed a three-dimensional N-doped carbon nanosheet framework into the GHs structure, achieving a high specific capacitance of 242 F/g at a current density of 1A/g. The energy density in the ionic liquid electrolyte was as high as 60.4 Wh/kg. As shown in Figure 13, 60 red light-emitting diodes (LED, 2.2 V) could be easily lit after charging for only 10 s in a single device filled only with ionic liquid electrolyte. The addition of transition metal oxide greatly improves the energy density of GHs [134]. Wang et al. [139] prepared NiOOH nanosheets/GHs by solvothermal and hydrothermal mixing reactions. They exhibited a good preformance with a high capacitance of 1162 F/g at a current density of 1 A/g and an excellent rate capability. The assembled asymmetric supercapacitor exhibited a significant energy density of 66.8 Wh/kg at a power density of 800 W/kg and an excellent cycle stability with an 85.3% capacitance after 8000 cycles. In addition, the easy functionalization of GHs can fill the gap between carbon materials such as carbon cloth and graphene sheets, improving the overall electrochemical performance [140]. As shown in Figure 14, Wu et al. [141] used composite GHs functionalized with polyaniline (PANI) and Lig to fill the blanks of carbon cloth (CC). The pseudocapacitance provided by PANI and the functional group provided by Lig brought many active sites and high specific capacitance to the entire electrode.  Reprinted with permission from [141]. Copyright 2019 Royal Society of Chemistry.

GAs Flexible Supercapacitors
Metal oxides are also often added to GAs to enhance the pseudocapacitance [136]. Li et al. [142] reported self-supporting GAs electrodes reinforced by NiCo2S4 nanoparticles, reducing the contact resistance and effectively shortening the electron and ion transmission paths. Therefore, NiCo2S4/GAs electrodes showed a high specific capacitance of 704.34 F/g at the current density of the 1 A/g. In addition, the solid asymmetric supercapacitor device based on NiCo2S4/GAs electrodes had an energy density of 20.9 Wh/kg and an ideal cycle stability at a power density of 800.2 W/kg, showing a great potential for storage devices in the energy field. However, the compound between metal oxides and GAs may have poor adhesion and stability. In response to this issue, Lv et al. [143] used carbon quantum dots (CQDs) to serve as a connecting bridge between MnO2 and GAs, improving the stability. As shown in Figure 15, the MnO2/CQDs/GA electrodes exhibited more excellent electrochemical performance than that of MnO2/GAs and MnO2/grapheme (MnO2/G) without added CQDs. The combination of graphene-based materials and conductive polymers exhibit remarkable properties [130,131]. PANI is a polymer commonly used in compounding with GAs, which is very effective for improving the specific capacitance of GAs [144]. However, the non-covalent connection between graphenes and conductive polymers results in poor stability of the composite material. This Reprinted with permission from [141]. Copyright 2019 Royal Society of Chemistry.

GAs Flexible Supercapacitors
Metal oxides are also often added to GAs to enhance the pseudocapacitance [136]. Li et al. [142] reported self-supporting GAs electrodes reinforced by NiCo 2 S 4 nanoparticles, reducing the contact resistance and effectively shortening the electron and ion transmission paths. Therefore, NiCo 2 S 4 /GAs electrodes showed a high specific capacitance of 704.34 F/g at the current density of the 1 A/g. In addition, the solid asymmetric supercapacitor device based on NiCo 2 S 4 /GAs electrodes had an energy density of 20.9 Wh/kg and an ideal cycle stability at a power density of 800.2 W/kg, showing a great potential for storage devices in the energy field. However, the compound between metal oxides and GAs may have poor adhesion and stability. In response to this issue, Lv et al. [143] used carbon quantum dots (CQDs) to serve as a connecting bridge between MnO 2 and GAs, improving the stability. As shown in Figure 15, the MnO 2 /CQDs/GA electrodes exhibited more excellent electrochemical performance than that of MnO 2 /GAs and MnO 2 /grapheme (MnO 2 /G) without added CQDs. The combination of graphene-based materials and conductive polymers exhibit remarkable properties [130,131]. PANI is a polymer commonly used in compounding with GAs, which is very effective for improving the specific capacitance of GAs [144]. However, the non-covalent connection between graphenes and conductive polymers results in poor stability of the composite material. This problem can be solved by covalent modification between graphenes and conductive polymers [145,146]. Li et al. [147] realized the covalent functionalization of reduced graphene hydrogel (RGOA) by grafting PANI onto the surface of RGOA. The RGOA-PANI electrode exhibited good rate performance and cycle stability. In addition, doped graphene hydrogel is also a good way to obtain high energy density and power density [148]. As shown in Figure 16, the specific capacitances of GAs with different doping ratios of N and S atoms were superior to those of undoped GAs.

Conclusions
Supercapacitors have the advantages of fast charge and discharge, high power density, and long cycle life. Common electrode materials include carbon materials, metal oxide materials, and conductive polymer materials. Among them, graphene-based materials are the most favored because of their advantages of high specific surface area and high conductivity. In this review, we mainly introduced five graphene-based electrode materials, which are GQDs, GFbs, GFs, GHs, and GAs, respectively. GQDs have a nano-size and good solubility in solvents; GFbs and GFs have excellent flexibility and conductivity; the three-dimensional pore structures of GHs and GAs provide a very large specific surface area. In the past few years, flexible supercapacitors based on graphene-based materials have made great progress. Nevertheless, there are still many challenges hindering the development of flexible supercapacitors. For example, the preparation method of GQDs are complex, which needs to be simple and efficient. The specific surface area of GFbs and GFs is low, and it is difficult for GHs and GAs to form a stable and controllable pore structure. In general, it is very important to obtain graphene-based materials with stable structures, because the stable graphene can exert its own advantages and maximize the synergistic effect of compounding with other materials. We believe that flexible supercapacitors will have a bright prospect after these problems are solved.
Author Contributions: Y.D. and P.X. conceived the idea; Y.D. and P.X. wrote the paper; J.Y. and J.C. advised the paper. All authors have read and agreed to the published version of the manuscript.

Conclusions
Supercapacitors have the advantages of fast charge and discharge, high power density, and long cycle life. Common electrode materials include carbon materials, metal oxide materials, and conductive polymer materials. Among them, graphene-based materials are the most favored because of their advantages of high specific surface area and high conductivity. In this review, we mainly introduced five graphene-based electrode materials, which are GQDs, GFbs, GFs, GHs, and GAs, respectively. GQDs have a nano-size and good solubility in solvents; GFbs and GFs have excellent flexibility and conductivity; the three-dimensional pore structures of GHs and GAs provide a very large specific surface area. In the past few years, flexible supercapacitors based on graphene-based materials have made great progress. Nevertheless, there are still many challenges hindering the development of flexible supercapacitors. For example, the preparation method of GQDs are complex, which needs to be simple and efficient. The specific surface area of GFbs and GFs is low, and it is difficult for GHs and GAs to form a stable and controllable pore structure. In general, it is very important to obtain graphene-based materials with stable structures, because the stable graphene can exert its own advantages and maximize the synergistic effect of compounding with other materials. We believe that flexible supercapacitors will have a bright prospect after these problems are solved.
Author Contributions: Y.D. and P.X. conceived the idea; Y.D. and P.X. wrote the paper; J.Y. and J.C. advised the paper. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.