*4.1.6 Conductive substrate-assisted spontaneous reduction and assembly*

The conductive substrate-induced spontaneous reduction and self-assembly of GO generally proceeds by putting metal substrates (e.g., Al, Fe, Cu) into GO solution for GBF preparation. As shown in **Figure 11**, Junjie et al. take copper wire as the substrate and adopt the three-electrode method to make the GO sheet continuously deposit on the surface of copper wire under the double induction of electrochemistry and template. Both GO and copper are simultaneously restored. Then, they etch and remove the copper wire in the FeCl3 solution to obtain the graphene hollow fiber with an oriented structure. The controllable preparation of the hollow fiber can be realized by controlling the diameter, length of the substrate, and the time of electrochemical deposition. The graphene hollow fiber has excellent flexibility and conductivity and can be used as the electrode material of supercapacitor [121].

**Figure 11.** *Scheme of spontaneous reduction and assembly of graphene hollow fiber on active metals substrates [121].*

#### *4.1.7 Electrophoresis self-assembly method*

The electrophoretic phenomenon occurs in a colloidal solution because charged particles can move under the action of electric field. Lianlian et al. developed a method for preparing GBFs with electrophoretic self-assembly. The graphite probe was used as a positive electrode to invade the GO dispersion. Under constant potential, the graphite probe was extracted slowly and uniformly, and self-assembled GO fibers were formed at the tail of the cathode. After drying and heating, GBFs with a smooth surface and circular cross section can be obtained [122]. Because the electrode moving speed is only 0.1 mm · mm<sup>−</sup><sup>1</sup> , it takes 1 week to get 1-m-long fiber. The yield of GBFs obtained by this method is too low to scale production.

#### **4.2 Applications of GBFs**

Thanks to graphene's superior electrical, mechanical, and thermal properties and good flexibility, GBFs have great potential in sensor, energy storage, energy conversion, and other fields.

#### *4.2.1 Sensor*

With the continuous development of flexible equipment, intelligent devices, including electricity, humidity, force, and temperature, can rapidly make structural changes in the environment and be increasingly concerned by people. The GBFs shows excellent performance in this regard.

Zhao et al. successfully developed a graphene-based multifunctional optical fiber sensor, which can respond to three different stimulations. They deposited GCN on GF (GF and GCN) and twisted it with another GF to form a double helix GBFs. In the twisted structure, the contact interface of the two fibers has a sandwich-like graphene/GCN/graphene structure. Under different external voltage controls, GF and GCN can show three different stimulus modes. Each mode can respond to temperature fluctuation, mechanical interaction, and humidity change and has a high sensitivity to specific stimulation [123]. Yanhong and his team electroplated polypyrrole on half of the surface of GBFs, which changes the current transmission rate on both sides of the fiber. With different types of current, the fiber has different bending states. The prepared electric GBFs are expected to be applied in the multiarm tweezers and mesh driver [124]. Chunfei et al. used twisted GBFs to realize temperature sensing. With the increase of temperature, the fiber resistance decreases. This is mainly due to the transition of semiconductor characteristics between graphene sheets. The fiber has similar sensing characteristics for temperature under different stretching conditions and has a wide application prospect [125].

In addition, GO fiber is partially restored by laser method, which is sensitive to humidity. By changing the position, the fiber can be transformed into various shapes. Taking advantage of the hydrophilic characteristics of GO in a humid environment, the distance between sheets is increased, while graphene is non-hydrophilic. Hence, the bending degree of the fiber changes with the humidity. Meanwhile, the fiber is woven into fabric shape, which still has sensitive response performance [126]. After twisting the spinning GO fiber, the twisted fiber will rotate repeatedly as the humidity changes periodically. When the humidity increase, a large number of oxygen-containing functional groups on the surface of GO will absorb water, and the distance between layers will increase. Otherwise, the distance between layers will decrease. A magnet is added at the lower end of the fiber to prepare a humidity sensing electric motor. The speed of the motor reaches 5190 r · min<sup>−</sup><sup>1</sup> . The motor can convert the change of environmental humidity into electric energy and realize the collection of energy [127].

**77**

*Fiber Composites Made of Low-Dimensional Carbon Materials*

range of 0.005–3 V under the current density of 100 mA · g<sup>−</sup><sup>1</sup>

performance, and the capacity of 100 cycles remains 766 mAh · g<sup>−</sup><sup>1</sup>

excellent battery performances of high linear capacity of 168 mAh · g<sup>−</sup><sup>1</sup>

able equipment, as shown in **Figure 12a** and **b** [133].

capability, and outstanding cyclic behavior. Woon et al. used wet spinning to construct graphene/carbon tube/sulfur electrode as positive material of Li-S battery. Graphene has high conductivity and can transfer electrons rapidly. Meanwhile, GO fiber as a matrix can obtain light fiber with certain mechanical strength for wear-

Compared with wet spinning, the diameter of the nanofiber film obtained by electrospinning is smaller. As the electrode material of lithium battery, it can significantly reduce the migration distance of lithium-ion and increase the specific surface area of the electrode material and improve the electrochemical performance

The GBFs and the GBFs coated with a layer of carbon nitride on the surface are wound together. The middle carbon nitride layer is equivalent to a buffer layer. Its conductivity is related to the layer spacing. With the pressure increase, the distance decreases and the conductivity is, in turn, to increase, which can realize

With the development of science and society, a portable energy storage device is becoming smaller and more flexible. Lithium-ion batteries are a new type of energy storage device, which has the advantages of high energy density, environmental friendliness, long cycle life, and high working voltage. However, the traditional LIBs cannot meet the needs of wearable electronic devices due to its large usage, rigidity, and weight. Therefore, it is necessary to develop new batteries with small volume, lightweight, and high flexibility. GBFs maintain the unique characteristics of the graphene nanosheet. When GBFs are used in the fiber lithium battery, it can realize the series connection with flexible electronic devices and drive them to work stably, achieving high energy density and holding a good commercial prospect [128, 129]. Jung et al. of the Korea Institute of Chemistry used pure GBFs as the negative electrode material of lithium-ion batteries. The battery circulates 100 times in the

[130]. Minsu et al. obtained hollow GBFs by coaxial spin-

in the range of 0.005–1.5 V for 100 cycles under the current density

. Minsu et al. filled the inner space with Si/Ag

ning and increased specific surface area and active site, and its capacity remained

of 0.2C [131]. Due to the low capacity of pure GBF battery, Jong et al. added MnO2 active material in graphene; the addition of MnO2 increased the distance between graphene sheets and gave lithium-ion fast transfer channel. Moreover, the battery made by MnO2 coating of graphene has good cycle stability, and the cycle capacity

nanoparticles, and the outer graphene well controlled the volume expansion of the inner silicon during charging and discharging, providing a smooth electronic channel. Compared with the simple mixing process, it has better cycle stability and rate

The GBFs prepared by the above method have low strength, and it is difficult to form a macroscopical fiber battery. In one report, a fiber battery electrode comprised of 2D/2D layered titania sheets/rGO sheets (titania/rGO) composites was prepared through wet spinning method [132]. By assembling the cathode of titania/ rGO fiber with the anode of lithium wire in parallel, a fiber-shaped half-cell was fabricated. This hybridized fiber electrode had an ordered stacking structure, high linear density of active materials, and abundance of exposed active sites, which endows the fiber electrode with prominent mechanical flexibility combined with

, and the capacity

[131].

, good rate

*DOI: http://dx.doi.org/10.5772/intechopen.92092*

the stress sensing [123].

*4.2.2.1 Lithium-ion batteries*

*4.2.2 Energy storage*

is still 224 mAh · g<sup>−</sup><sup>1</sup>

of 100 times remained 560 mA · g<sup>−</sup><sup>1</sup>

196 mAh · g<sup>−</sup><sup>1</sup>

The GBFs and the GBFs coated with a layer of carbon nitride on the surface are wound together. The middle carbon nitride layer is equivalent to a buffer layer. Its conductivity is related to the layer spacing. With the pressure increase, the distance decreases and the conductivity is, in turn, to increase, which can realize the stress sensing [123].

#### *4.2.2 Energy storage*

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

The electrophoretic phenomenon occurs in a colloidal solution because charged

Thanks to graphene's superior electrical, mechanical, and thermal properties and good flexibility, GBFs have great potential in sensor, energy storage, energy

With the continuous development of flexible equipment, intelligent devices, including electricity, humidity, force, and temperature, can rapidly make structural changes in the environment and be increasingly concerned by people. The GBFs

Zhao et al. successfully developed a graphene-based multifunctional optical fiber sensor, which can respond to three different stimulations. They deposited GCN on GF (GF and GCN) and twisted it with another GF to form a double helix GBFs. In the twisted structure, the contact interface of the two fibers has a sandwich-like graphene/GCN/graphene structure. Under different external voltage controls, GF and GCN can show three different stimulus modes. Each mode can respond to temperature fluctuation, mechanical interaction, and humidity change and has a high sensitivity to specific stimulation [123]. Yanhong and his team electroplated polypyrrole on half of the surface of GBFs, which changes the current transmission rate on both sides of the fiber. With different types of current, the fiber has different bending states. The prepared electric GBFs are expected to be applied in the multiarm tweezers and mesh driver [124]. Chunfei et al. used twisted GBFs to realize temperature sensing. With the increase of temperature, the fiber resistance decreases. This is mainly due to the transition of semiconductor characteristics between graphene sheets. The fiber has similar sensing characteristics for temperature under

different stretching conditions and has a wide application prospect [125].

humidity into electric energy and realize the collection of energy [127].

In addition, GO fiber is partially restored by laser method, which is sensitive to humidity. By changing the position, the fiber can be transformed into various shapes. Taking advantage of the hydrophilic characteristics of GO in a humid environment, the distance between sheets is increased, while graphene is non-hydrophilic. Hence, the bending degree of the fiber changes with the humidity. Meanwhile, the fiber is woven into fabric shape, which still has sensitive response performance [126]. After twisting the spinning GO fiber, the twisted fiber will rotate repeatedly as the humidity changes periodically. When the humidity increase, a large number of oxygen-containing functional groups on the surface of GO will absorb water, and the distance between layers will increase. Otherwise, the distance between layers will decrease. A magnet is added at the lower end of the fiber to prepare a humidity sensing electric motor. The speed of

. The motor can convert the change of environmental

, it takes 1 week to get 1-m-long fiber. The yield of GBFs

particles can move under the action of electric field. Lianlian et al. developed a method for preparing GBFs with electrophoretic self-assembly. The graphite probe was used as a positive electrode to invade the GO dispersion. Under constant potential, the graphite probe was extracted slowly and uniformly, and self-assembled GO fibers were formed at the tail of the cathode. After drying and heating, GBFs with a smooth surface and circular cross section can be obtained [122]. Because the electrode moving

*4.1.7 Electrophoresis self-assembly method*

obtained by this method is too low to scale production.

speed is only 0.1 mm · mm<sup>−</sup><sup>1</sup>

**4.2 Applications of GBFs**

conversion, and other fields.

shows excellent performance in this regard.

*4.2.1 Sensor*

**76**

the motor reaches 5190 r · min<sup>−</sup><sup>1</sup>

## *4.2.2.1 Lithium-ion batteries*

With the development of science and society, a portable energy storage device is becoming smaller and more flexible. Lithium-ion batteries are a new type of energy storage device, which has the advantages of high energy density, environmental friendliness, long cycle life, and high working voltage. However, the traditional LIBs cannot meet the needs of wearable electronic devices due to its large usage, rigidity, and weight. Therefore, it is necessary to develop new batteries with small volume, lightweight, and high flexibility. GBFs maintain the unique characteristics of the graphene nanosheet. When GBFs are used in the fiber lithium battery, it can realize the series connection with flexible electronic devices and drive them to work stably, achieving high energy density and holding a good commercial prospect [128, 129].

Jung et al. of the Korea Institute of Chemistry used pure GBFs as the negative electrode material of lithium-ion batteries. The battery circulates 100 times in the range of 0.005–3 V under the current density of 100 mA · g<sup>−</sup><sup>1</sup> , and the capacity is still 224 mAh · g<sup>−</sup><sup>1</sup> [130]. Minsu et al. obtained hollow GBFs by coaxial spinning and increased specific surface area and active site, and its capacity remained 196 mAh · g<sup>−</sup><sup>1</sup> in the range of 0.005–1.5 V for 100 cycles under the current density of 0.2C [131]. Due to the low capacity of pure GBF battery, Jong et al. added MnO2 active material in graphene; the addition of MnO2 increased the distance between graphene sheets and gave lithium-ion fast transfer channel. Moreover, the battery made by MnO2 coating of graphene has good cycle stability, and the cycle capacity of 100 times remained 560 mA · g<sup>−</sup><sup>1</sup> . Minsu et al. filled the inner space with Si/Ag nanoparticles, and the outer graphene well controlled the volume expansion of the inner silicon during charging and discharging, providing a smooth electronic channel. Compared with the simple mixing process, it has better cycle stability and rate performance, and the capacity of 100 cycles remains 766 mAh · g<sup>−</sup><sup>1</sup> [131].

The GBFs prepared by the above method have low strength, and it is difficult to form a macroscopical fiber battery. In one report, a fiber battery electrode comprised of 2D/2D layered titania sheets/rGO sheets (titania/rGO) composites was prepared through wet spinning method [132]. By assembling the cathode of titania/ rGO fiber with the anode of lithium wire in parallel, a fiber-shaped half-cell was fabricated. This hybridized fiber electrode had an ordered stacking structure, high linear density of active materials, and abundance of exposed active sites, which endows the fiber electrode with prominent mechanical flexibility combined with excellent battery performances of high linear capacity of 168 mAh · g<sup>−</sup><sup>1</sup> , good rate capability, and outstanding cyclic behavior. Woon et al. used wet spinning to construct graphene/carbon tube/sulfur electrode as positive material of Li-S battery. Graphene has high conductivity and can transfer electrons rapidly. Meanwhile, GO fiber as a matrix can obtain light fiber with certain mechanical strength for wearable equipment, as shown in **Figure 12a** and **b** [133].

Compared with wet spinning, the diameter of the nanofiber film obtained by electrospinning is smaller. As the electrode material of lithium battery, it can significantly reduce the migration distance of lithium-ion and increase the specific surface area of the electrode material and improve the electrochemical performance

**Figure 12.**

*(a) Schematic of fiber-shaped lithium-ion battery. (b) Schematic illustration of synthetic route of rGO/CNTs/S fiber [133].*

of the battery [134–136]. Xiaoxin et al. obtained the Si-graphene-C structure which is similar to the coronary artery based on bionics. Graphene can effectively control the volume expansion of Si, and high conductivity is also conducive to the rapid transfer of ions. Meanwhile, the inclusion of graphene also avoids direct contact between Si and electrolyte and avoids the formation of a large number of SEI films. After 200 cycles, the capacity retention rate is still 86.5% [137]. Jian et al. continued to wrap a layer of graphene outside SnO2 and GO nanofibers with a double-layer protection method to inhibit the volume expansion and agglomeration of active materials. This method is applicable to almost all oxide and graphene nanofiber electrodes obtained by electrospinning, with good universality [138].

At present, there are few researches on the application of GBFs in LIB and the assembly of woven fiber batteries. Compared with the traditional button batteries, the assembly process of GBFs is relatively complex, so it is unable to achieve continuous production.

#### *4.2.2.2 Supercapacitor*

In addition to the application in LIB, GBFs are also widely used in the field of supercapacitors. Supercapacitor, also known as a double electric layer capacitor or electrochemical capacitor, is a new energy storage device that uses the rapid adsorption–desorption of electrolyte ions with electrode materials or the reversible oxidation–reduction reaction on the surface of electrode materials to realize electric energy storage [139, 140]. With the continuous development of wearable devices, flexible supercapacitors have become the preferred energy source for various electronic devices due to their fast charge and discharge ability and long cycle life. Among them, fiber supercapacitors have attracted much attention due to their lightweight, small size, high flexibility, and good wearability. GBFs have excellent conductivity and super high specific surface area, so it has been widely used in the field of fiber supercapacitor [141].

Chen et al. prepared pure GBFs with a non-liquid crystal method and further assembled the fibers into flexible supercapacitors. The capacitance of the supercapacitor is 39.1 F · g<sup>−</sup><sup>1</sup> when the current density is 0.2 A · g<sup>−</sup><sup>1</sup> . At the same time, it is found that the electrochemical performance of GBFs can be greatly improved by immersing it into 6 M KOH for 10 min before the electrochemical performance test. At the current density of 0.2 A · g<sup>−</sup><sup>1</sup> , the specific capacitance is 185 F · g<sup>−</sup><sup>1</sup> (226 F · cm<sup>−</sup><sup>3</sup> ), and the energy density is 5.76 Wh · kg<sup>−</sup><sup>1</sup> (power density is 47.3 W · kg<sup>−</sup><sup>1</sup> ) [91]. The capacitor has good toughness and can be woven into fabric and light LED after charging. Hu and Zhao integrated two electrodes (the upper and lower part of rGO) and separator

**79**

**Figure 13.**

*cycles [142].*

*Fiber Composites Made of Low-Dimensional Carbon Materials*

When the scanning rate was increased to 1 and 10 V · s<sup>−</sup><sup>1</sup>

excellent cycle stability and bending durability [145].

electrochemical performance of the composite fibers.

specific surface area can reach 396 m2

fiber electrode is 305 F · cm<sup>−</sup><sup>3</sup>

(the middle part of GO) into the GO optical fiber, as shown in **Figure 13a**, and made a kind of all-in-one fiber graphene supercapacitor (rGO-GO-rGO) without any adhesive. The diameter of the rGO-GO-rGO fiber is 50 μm, and the rGO part is about 1/4 of the fiber width. The rGO-GO-rGO fiber supercapacitor shows remarkable mechanical flexibility, which can bend to various curvature while maintaining high

At present, the specific capacitance of pure GBFs is far less than the theoretical capacitance of graphene. How to improve the capacitance of GBFs is still a big challenge. Currently, an effective method that has been proven and widely used is the hybridization strategy, including doping and compounding with other substances. Doping increases the active region on the surface of graphene and further improves its catalytic activity for a redox reaction. Among all kinds of atom doping, nitrogen atom doping is the most common. Doping nitrogen atoms with extra valence electrons into graphene will introduce new energy into the low energy region of the carbon conduction band. The introduction to this new energy can improve the catalytic activity and electrochemical performance of graphene materials. Yunzhen et al. extruded the GO dispersion into the substrate of hydroxylamine ethanol solution as a network, dried it, and heat it to obtain the nitrogen-doped rGO network fabric. Then, the PT foil was used as the collector to assemble the supercapacitor. The specific

when the scanning rate was 5 mV · s<sup>−</sup><sup>1</sup>

kept at 74.2 and 48.4%, respectively, showing very excellent rate performance [144]. Guan et al. constructed nitrogen-doped porous GBF supercapacitors with high energy density output, large-scale weaving, and flexible wearable application prospects by means of self-assembly of the liquid–liquid interface and molecular functional doping pore formation in the micro-reaction system. The area-specific capacitance of the

Graphene can be compounded with other carbon nanomaterials, conducting polymers, metal oxides/sulfides, and other materials to form graphene composite fibers. The high specific capacitance of the additives can be used to improve the

Yu et al. constructed a graphene/CNT composite fiber. Due to the high conductiv-

fiber supercapacitor prepared by this method is as high as 1132 mF · cm<sup>−</sup><sup>2</sup>

ity of CNTs, the conductivity of the composite fiber can reach 102 S · cm<sup>−</sup><sup>1</sup>

· g<sup>−</sup><sup>1</sup>

Yuning et al. mixed GO and pyrrole monomers as spinning solution and extruded

*(a) Scheme of supercapacitor supported by two electrodes. (b) Capacity decrease with increasing bending* 

in 25% KOH electrolyte.

, which has

, and the

[106].

, the specific capacity was

. The volume-specific capacitance of the

, and the mass-specific capacitance is 508 F · g<sup>−</sup><sup>1</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.92092*

capacitance (**Figure 13b**) [142, 143].

capacity was 188 F · g<sup>−</sup><sup>1</sup>

#### *Fiber Composites Made of Low-Dimensional Carbon Materials DOI: http://dx.doi.org/10.5772/intechopen.92092*

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

of the battery [134–136]. Xiaoxin et al. obtained the Si-graphene-C structure which is similar to the coronary artery based on bionics. Graphene can effectively control the volume expansion of Si, and high conductivity is also conducive to the rapid transfer of ions. Meanwhile, the inclusion of graphene also avoids direct contact between Si and electrolyte and avoids the formation of a large number of SEI films. After 200 cycles, the capacity retention rate is still 86.5% [137]. Jian et al. continued to wrap a layer of graphene outside SnO2 and GO nanofibers with a double-layer protection method to inhibit the volume expansion and agglomeration of active materials. This method is applicable to almost all oxide and graphene nanofiber

*(a) Schematic of fiber-shaped lithium-ion battery. (b) Schematic illustration of synthetic route of* 

At present, there are few researches on the application of GBFs in LIB and the assembly of woven fiber batteries. Compared with the traditional button batteries, the assembly process of GBFs is relatively complex, so it is unable to achieve

In addition to the application in LIB, GBFs are also widely used in the field of supercapacitors. Supercapacitor, also known as a double electric layer capacitor or electrochemical capacitor, is a new energy storage device that uses the rapid adsorption–desorption of electrolyte ions with electrode materials or the reversible oxidation–reduction reaction on the surface of electrode materials to realize electric energy storage [139, 140]. With the continuous development of wearable devices, flexible supercapacitors have become the preferred energy source for various electronic devices due to their fast charge and discharge ability and long cycle life. Among them, fiber supercapacitors have attracted much attention due to their lightweight, small size, high flexibility, and good wearability. GBFs have excellent conductivity and super high specific surface area, so it has been widely used in the

Chen et al. prepared pure GBFs with a non-liquid crystal method and further assembled the fibers into flexible supercapacitors. The capacitance of the supercapac-

that the electrochemical performance of GBFs can be greatly improved by immersing it into 6 M KOH for 10 min before the electrochemical performance test. At the

tor has good toughness and can be woven into fabric and light LED after charging. Hu and Zhao integrated two electrodes (the upper and lower part of rGO) and separator

, the specific capacitance is 185 F · g<sup>−</sup><sup>1</sup>

(power density is 47.3 W · kg<sup>−</sup><sup>1</sup>

. At the same time, it is found

(226 F · cm<sup>−</sup><sup>3</sup>

) [91]. The capaci-

), and

when the current density is 0.2 A · g<sup>−</sup><sup>1</sup>

electrodes obtained by electrospinning, with good universality [138].

continuous production.

**Figure 12.**

*rGO/CNTs/S fiber [133].*

*4.2.2.2 Supercapacitor*

field of fiber supercapacitor [141].

current density of 0.2 A · g<sup>−</sup><sup>1</sup>

the energy density is 5.76 Wh · kg<sup>−</sup><sup>1</sup>

itor is 39.1 F · g<sup>−</sup><sup>1</sup>

**78**

(the middle part of GO) into the GO optical fiber, as shown in **Figure 13a**, and made a kind of all-in-one fiber graphene supercapacitor (rGO-GO-rGO) without any adhesive. The diameter of the rGO-GO-rGO fiber is 50 μm, and the rGO part is about 1/4 of the fiber width. The rGO-GO-rGO fiber supercapacitor shows remarkable mechanical flexibility, which can bend to various curvature while maintaining high capacitance (**Figure 13b**) [142, 143].

At present, the specific capacitance of pure GBFs is far less than the theoretical capacitance of graphene. How to improve the capacitance of GBFs is still a big challenge. Currently, an effective method that has been proven and widely used is the hybridization strategy, including doping and compounding with other substances.

Doping increases the active region on the surface of graphene and further improves its catalytic activity for a redox reaction. Among all kinds of atom doping, nitrogen atom doping is the most common. Doping nitrogen atoms with extra valence electrons into graphene will introduce new energy into the low energy region of the carbon conduction band. The introduction to this new energy can improve the catalytic activity and electrochemical performance of graphene materials. Yunzhen et al. extruded the GO dispersion into the substrate of hydroxylamine ethanol solution as a network, dried it, and heat it to obtain the nitrogen-doped rGO network fabric. Then, the PT foil was used as the collector to assemble the supercapacitor. The specific capacity was 188 F · g<sup>−</sup><sup>1</sup> when the scanning rate was 5 mV · s<sup>−</sup><sup>1</sup> in 25% KOH electrolyte. When the scanning rate was increased to 1 and 10 V · s<sup>−</sup><sup>1</sup> , the specific capacity was kept at 74.2 and 48.4%, respectively, showing very excellent rate performance [144]. Guan et al. constructed nitrogen-doped porous GBF supercapacitors with high energy density output, large-scale weaving, and flexible wearable application prospects by means of self-assembly of the liquid–liquid interface and molecular functional doping pore formation in the micro-reaction system. The area-specific capacitance of the fiber supercapacitor prepared by this method is as high as 1132 mF · cm<sup>−</sup><sup>2</sup> , which has excellent cycle stability and bending durability [145].

Graphene can be compounded with other carbon nanomaterials, conducting polymers, metal oxides/sulfides, and other materials to form graphene composite fibers. The high specific capacitance of the additives can be used to improve the electrochemical performance of the composite fibers.

Yu et al. constructed a graphene/CNT composite fiber. Due to the high conductivity of CNTs, the conductivity of the composite fiber can reach 102 S · cm<sup>−</sup><sup>1</sup> , and the specific surface area can reach 396 m2 · g<sup>−</sup><sup>1</sup> . The volume-specific capacitance of the fiber electrode is 305 F · cm<sup>−</sup><sup>3</sup> , and the mass-specific capacitance is 508 F · g<sup>−</sup><sup>1</sup> [106]. Yuning et al. mixed GO and pyrrole monomers as spinning solution and extruded

**Figure 13.**

*(a) Scheme of supercapacitor supported by two electrodes. (b) Capacity decrease with increasing bending cycles [142].*

them into FeCl3 solution to solidify and polymerize pyrrole in situ, and the PPy/GO composite fiber was obtained after reduction by hydroiodic acid. The fiber has a skin core structure, and its capacitance performance is greatly improved compared with pure rGO fiber. The area-specific capacitance is 107.2 mF · cm<sup>−</sup><sup>2</sup> (73.4 F · g<sup>−</sup><sup>1</sup> ), and the energy density is between 6.6 and 9.7 μ Wh · cm<sup>−</sup><sup>2</sup> [146]. Bingjie et al. synthesized the graphene/molybdenum disulfide composite fiber electrode with the one-step hydrothermal method. The electrode has a new intercalation nanostructure, which effectively combines the excellent conductivity of the graphene sheet layer with the high pseudocapacitance of molybdenum disulfide. The final assembled fiber-like super electric container shows a volume-specific capacitance of up to 368 F · cm<sup>−</sup><sup>3</sup> [147]. Qiuyan et al. overcame the problem of poor interaction between MXene layers and prepared MXene/graphene composite fiber. The orientation distribution of MXene sheets among GO liquid crystal templates realized high load (95 w/w%). The composite fiber shows excellent conductivity (2.9 × 104 S · m<sup>−</sup><sup>1</sup> ) and ultrahigh-volumespecific capacitance (586.4 F · cm<sup>−</sup><sup>3</sup> ), far exceeding the value of pure GBFs [148].

In addition, the structure optimization of GBFs is also an effective way to improve the performance of GBF supercapacitor, which mainly lies in the improvement of specific surface area and the regulation of the layer arrangement structure. The porous GO fiber reported by Seyed et al. in 2014 was transformed into porous rGO fiber after thermal reduction at 220°C, as shown in **Figure 14**.

The specific surface area of the fiber is 2210 m2 · g<sup>−</sup><sup>1</sup> , and the conductivity is about 25 S · cm<sup>−</sup><sup>1</sup> , and the specific capacity of the fiber is 409 F · g<sup>−</sup><sup>1</sup> when the current density is 1 A · g<sup>−</sup><sup>1</sup> . The specific capacitance of 56 F · g<sup>−</sup><sup>1</sup> still exists when the current density is increased to 100 A · g<sup>−</sup><sup>1</sup> [117]. Chen et al. used cellulose nanocrystals (CNC) to adjust the structure of GBFs. CNC nanorods can not only improve the serious accumulation of graphene sheets in GBFs but also inhibit the possible bending and folding of graphene sheets in the process of fiber-forming, so as to form ordered nanopore structure. The composite GBFs were assembled into a supercapacitor with a conductivity of 64.7 S · cm<sup>−</sup><sup>1</sup> and a specific capacitance of 208.2 F · cm<sup>−</sup><sup>3</sup> , which has excellent electrochemical performance [99]. In addition, they also use graphene hollow fiber prepared by the electrochemical method as the electrode of fiber-like supercapacitor [121], and the additional inner surface of hollow fiber can provide more contact area with electrolyte. Under the current density of 0.1 A · g<sup>−</sup><sup>1</sup> , the specific capacitance of the assembled solid-state supercapacitor can reach 178 F · g<sup>−</sup><sup>1</sup> , and it has good rate performance and cycle stability. Guoxing et al. prepared graphene/conductive polymer composite hollow fiber with the hydrothermal method. The combination of hollow structure and pseudocapacitance provided by conductive polymer greatly improved the capacity of the capacitor and provided a new idea for the improvement of supercapacitor capacitance [149].

**Figure 14.**

*Porous graphene fiber and its supercapacitor. (a) SEM image of porous fibers. (b) Schematic illustration of the structure of supercapacitor. (c) CV curves of graphene fibers prepared in different coagulation baths.*

**81**

**Figure 15.**

*Fiber Composites Made of Low-Dimensional Carbon Materials*

Actuators are a kind of stimuli-sensitive device that can respond to external stimuli, such as humidity, temperature, and electrical changes, and transfer the stimulus into deformation or motion [126, 127]. Due to quantum mechanics and electrostatic double-layer effect, graphene may cause space warping or plane expansion under the charge injection. In addition, the intercalation or removal of ions or molecules in graphene products under external stimulation will also lead to the bending, twisting, and even reversible change of the interlayer spacing. In this way, the type and degree of deformation can be controlled by the composition and

Jia et al. showed an electrochemical fiber driver with high driving activity and durability based on GF/polypyrrole (GF/PPY) double-layer structure, as shown in **Figure 15**. Because of the asymmetry of the structure, GF/PPY fiber shows reversible bending deformation under the condition on positive and negative charges. As shown in **Figure 15**, when a positive voltage is applied to GF/PPY fiber, graphene will shrink and expand due to anion discharged from PPY, and the fiber will bend to the left. When a negative voltage is applied, GF/PPY fiber can bend to the right [152]. Compared with rGO, GO has more oxygen functional groups, so it is more sensitive to water. Based on this principle, Huhu et al. fabricated an asymmetric rGO/ GO fiber by region-selective laser reduction along the GO fiber. When exposed to humid air, the rGO/GO fiber can bend to the rGO side and then return to its original state after air moisture dispersion. After that, they made a twisted GO fiber by rotating the GO hydrogel fibers in the direction of rotation. The spiral geometry

inside them was the main reason for the reversible rotation in the moist air.

Wearable solar cells can supply power to flexible smart devices at any time, while

GBFs can be used as electrode materials to achieve this new function. Peng et al.

*Schematic illustration of the expansion-contraction mechanisms of the GF/PPY bilayer structure. Charges in* 

*each electrode are completely balanced by ions from the electrolyte.*

*DOI: http://dx.doi.org/10.5772/intechopen.92092*

surface chemical state of graphene [150, 151].

*4.2.3 Energy conversion*

*4.2.3.1 Actuator*

*4.2.3.2 Solar cell*

### *4.2.3 Energy conversion*
