*4.2.3.1 Actuator*

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

pure rGO fiber. The area-specific capacitance is 107.2 mF · cm<sup>−</sup><sup>2</sup>

energy density is between 6.6 and 9.7 μ Wh · cm<sup>−</sup><sup>2</sup>

posite fiber shows excellent conductivity (2.9 × 104

The specific surface area of the fiber is 2210 m2

a supercapacitor with a conductivity of 64.7 S · cm<sup>−</sup><sup>1</sup>

the current density is increased to 100 A · g<sup>−</sup><sup>1</sup>

specific capacitance (586.4 F · cm<sup>−</sup><sup>3</sup>

about 25 S · cm<sup>−</sup><sup>1</sup>

208.2 F · cm<sup>−</sup><sup>3</sup>

of 0.1 A · g<sup>−</sup><sup>1</sup>

can reach 178 F · g<sup>−</sup><sup>1</sup>

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

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

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>

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 com-

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

, and the specific capacity of the fiber is 409 F · g<sup>−</sup><sup>1</sup>

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

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

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].

*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.*

. The specific capacitance of 56 F · g<sup>−</sup><sup>1</sup>

, which has excellent electrochemical performance [99]. In addition,

, the specific capacitance of the assembled solid-state supercapacitor

, and it has good rate performance and cycle stability. Guoxing

rGO fiber after thermal reduction at 220°C, as shown in **Figure 14**.

S · m<sup>−</sup><sup>1</sup>

· g<sup>−</sup><sup>1</sup>

), far exceeding the value of pure GBFs [148].

(73.4 F · g<sup>−</sup><sup>1</sup>

) and ultrahigh-volume-

, and the conductivity is

and a specific capacitance of

[117]. Chen et al. used cellulose

when the

still exists when

[146]. Bingjie et al. synthesized the

), and the

[147].

**80**

**Figure 14.**

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 surface chemical state of graphene [150, 151].

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.

## *4.2.3.2 Solar cell*

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.

#### **Figure 15.**

*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.*

obtained GBFs by wet spinning and then made its surface loaded with Pt metal particles by electrodeposition to the obtained counter electrode. The titanium wire with titanium dioxide microtubules on the surface is used as the working electrode; the dye-sensitized solar cell (DSSC) has an energy conversion efficiency of 8.45%, which is much higher than other linear photovoltaic devices. The continuous collection of energy can be realized by putting linear solar cells into conventional clothes [153]. The high surface properties and good electrical and electrochemical properties of graphene are the important reasons to improve the performance of fiber DSSC.

## **5. Conclusions and perspective**

This chapter mainly summarizes the main preparation methods, properties, and application fields of CNT, CF, and GBF materials. Among them, CNTs have unique one-dimensional nanostructures and excellent mechanical, electrical, and optical properties. Through various methods of modification, researchers continue to prepare CNT composite nanomaterials with excellent performance, which has a good application prospect. Starting from the needs of the application field, it is the trend to study the carbon nanotube composite materials in the future to expect to obtain the high-efficiency structure which is corresponding to the application performance. Although some progress has been made in the preparation and properties of carbon nanotube composites, the mechanism of improving the properties of composites and the dispersion of carbon nanotubes still need to be explored.

CF is a new type of fiber material with high strength and high modulus, which contains more than 95% carbon. Its quality is lighter than that of aluminum, but its strength is higher than that of steel, and it has the characteristics of corrosion resistance and high-temperature resistance. It is an important material in the military industry and civil use. With the rapid development of CF composite and the continuous improvement of molding technology, its application scope is expanding day by day, and it shows good application potential in many fields. However, the physical and chemical properties of CF composites are complex, so it is necessary to study the basic theories of physical and chemical properties, mechanics, and heat, so as to improve the performance of CF composites.

GBFs have achieved great success in functional application, and it is far more amazing than CF. So far, various preparation methods have been studied and used in large-scale production of GBFs, which provides a positive impetus for the future application of GBFs. GBFs have been given new performance and function and provide new opportunities for various applications, including fiber-optic actuators, batteries, super electric containers, dye-sensitized solar cells, and sensors.

GBFs are a kind of graphene nanosheet assembled in one-dimensional space. At present, the structure of GBFs can be regulated in the following aspects: (1) Diameter. Generally, the diameter of GBFs is 10–100 μm. If it is prepared by electrospinning, its diameter can be controlled below 500 μm. (2) Porosity. On the one hand, it can be prepared by self-assembly, rolling, graphitization, and sintering; on the other hand, it can be prepared by freeze-drying, air spinning, and other methods. In addition, graphene hollow fiber can also be prepared. (3) Orientation. The arrangement of graphene sheets has a great influence on the properties of GBFs. The GBFs with a high degree of orientation can be obtained by the stress field orientation effect in the wet spinning process, the self-assembly in the electrochemical deposition process, and the second phase auxiliary orientation effect in the composite fiber. (4) Section morphology. It is difficult to maintain the circular cross section of the fiber, which is generally irregular. At present, the conventional method is to

**83**

*Fiber Composites Made of Low-Dimensional Carbon Materials*

adjust the fiber cross-sectional shape by adjusting the spinneret hole shape, but the

In order to meet the needs of different applications, graphene composite fibers appear. The additive materials include metal, inorganic, and polymer materials, such as silver nanowires, silicon nanoparticles, molybdenum disulfide nanoparticles, polypyrrole nanoparticles, etc. Basically, any nanomaterial can be added to GBFs to get graphene composite fiber. But one of the key problems is to control the structure of the composite fiber. The main control factor is the morphology of the second phase and its distribution in the fiber. For GBFs and its composite fiber, the main problems are as follows: (1) Compared with the graphene nanoflakes, the properties of GBFs are greatly cracked. (2) GBFs are composed of layers, which are very different from the chain structure of the traditional chemical fiber, so its flexibility is poor. (3) It is difficult to realize continuous production. Even with the most suitable wet spinning method for continuous production, its continuous production

Although GBFs are faced with many problems, remarkable achievements have been made. Compared with CF, GBFs have the characteristics of high strength, high modulus, conductivity, and certain flexibility, which have developed into a new type of high-performance fiber. On the other hand, graphene composite fiber is committed to develop into a new type of multifunctional intelligent fiber. This kind of fiber starts from modifying the traditional general-

purpose fiber to improve some aspects of the performance of the general-

The authors thank the support of Stevens startup fund.

The authors declare no conflict of interest.

purpose fiber and to develop new kinds of fiber, such as graphene/nano titanium oxide composite fiber. It can also develop new fiber performance and functions, such as energy storage, and finally realize multiple functions such as perception, judgment, correspondence, information transmission, etc. on the fiber and become a new type of intelligent material. Therefore, GBFs and its composite fiber will be widely used in aerospace, energy sensing, intelligent life, and other

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

is very difficult, and the yield is very low.

research progress is slow.

fields in the future.

**Acknowledgements**

**Conflict of interest**

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

**5. Conclusions and perspective**

so as to improve the performance of CF composites.

be explored.

obtained GBFs by wet spinning and then made its surface loaded with Pt metal particles by electrodeposition to the obtained counter electrode. The titanium wire with titanium dioxide microtubules on the surface is used as the working electrode; the dye-sensitized solar cell (DSSC) has an energy conversion efficiency of 8.45%, which is much higher than other linear photovoltaic devices. The continuous collection of energy can be realized by putting linear solar cells into conventional clothes [153]. The high surface properties and good electrical and electrochemical properties of graphene are the important reasons to improve the performance of fiber DSSC.

This chapter mainly summarizes the main preparation methods, properties, and application fields of CNT, CF, and GBF materials. Among them, CNTs have unique one-dimensional nanostructures and excellent mechanical, electrical, and optical properties. Through various methods of modification, researchers continue to prepare CNT composite nanomaterials with excellent performance, which has a good application prospect. Starting from the needs of the application field, it is the trend to study the carbon nanotube composite materials in the future to expect to obtain the high-efficiency structure which is corresponding to the application performance. Although some progress has been made in the preparation and properties of carbon nanotube composites, the mechanism of improving the properties of composites and the dispersion of carbon nanotubes still need to

CF is a new type of fiber material with high strength and high modulus, which contains more than 95% carbon. Its quality is lighter than that of aluminum, but its strength is higher than that of steel, and it has the characteristics of corrosion resistance and high-temperature resistance. It is an important material in the military industry and civil use. With the rapid development of CF composite and the continuous improvement of molding technology, its application scope is expanding day by day, and it shows good application potential in many fields. However, the physical and chemical properties of CF composites are complex, so it is necessary to study the basic theories of physical and chemical properties, mechanics, and heat,

GBFs have achieved great success in functional application, and it is far more amazing than CF. So far, various preparation methods have been studied and used in large-scale production of GBFs, which provides a positive impetus for the future application of GBFs. GBFs have been given new performance and function and provide new opportunities for various applications, including fiber-optic actuators,

GBFs are a kind of graphene nanosheet assembled in one-dimensional space. At present, the structure of GBFs can be regulated in the following aspects: (1) Diameter. Generally, the diameter of GBFs is 10–100 μm. If it is prepared by electrospinning, its diameter can be controlled below 500 μm. (2) Porosity. On the one hand, it can be prepared by self-assembly, rolling, graphitization, and sintering; on the other hand, it can be prepared by freeze-drying, air spinning, and other methods. In addition, graphene hollow fiber can also be prepared. (3) Orientation. The arrangement of graphene sheets has a great influence on the properties of GBFs. The GBFs with a high degree of orientation can be obtained by the stress field orientation effect in the wet spinning process, the self-assembly in the electrochemical deposition process, and the second phase auxiliary orientation effect in the composite fiber. (4) Section morphology. It is difficult to maintain the circular cross section of the fiber, which is generally irregular. At present, the conventional method is to

batteries, super electric containers, dye-sensitized solar cells, and sensors.

**82**

adjust the fiber cross-sectional shape by adjusting the spinneret hole shape, but the research progress is slow.

In order to meet the needs of different applications, graphene composite fibers appear. The additive materials include metal, inorganic, and polymer materials, such as silver nanowires, silicon nanoparticles, molybdenum disulfide nanoparticles, polypyrrole nanoparticles, etc. Basically, any nanomaterial can be added to GBFs to get graphene composite fiber. But one of the key problems is to control the structure of the composite fiber. The main control factor is the morphology of the second phase and its distribution in the fiber. For GBFs and its composite fiber, the main problems are as follows: (1) Compared with the graphene nanoflakes, the properties of GBFs are greatly cracked. (2) GBFs are composed of layers, which are very different from the chain structure of the traditional chemical fiber, so its flexibility is poor. (3) It is difficult to realize continuous production. Even with the most suitable wet spinning method for continuous production, its continuous production is very difficult, and the yield is very low.

Although GBFs are faced with many problems, remarkable achievements have been made. Compared with CF, GBFs have the characteristics of high strength, high modulus, conductivity, and certain flexibility, which have developed into a new type of high-performance fiber. On the other hand, graphene composite fiber is committed to develop into a new type of multifunctional intelligent fiber. This kind of fiber starts from modifying the traditional generalpurpose fiber to improve some aspects of the performance of the generalpurpose fiber and to develop new kinds of fiber, such as graphene/nano titanium oxide composite fiber. It can also develop new fiber performance and functions, such as energy storage, and finally realize multiple functions such as perception, judgment, correspondence, information transmission, etc. on the fiber and become a new type of intelligent material. Therefore, GBFs and its composite fiber will be widely used in aerospace, energy sensing, intelligent life, and other fields in the future.
