a review on graphene fibers: expectations, advances, and...

REVIEW

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A Review on Graphene Fibers: Expectations, Advances, and Prospects

Bo Fang, Dan Chang, Zhen Xu,* and Chao Gao*

Dr. B. Fang, D. Chang, Dr. Z. Xu, Prof. C. GaoMOE Key Laboratory of Macromolecular Synthesis and FunctionalizationDepartment of Polymer Science and EngineeringKey Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang ProvinceZhejiang University38 Zheda Road, Hangzhou 310027, P. R. ChinaE-mail: [email protected]; [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201902664.

DOI: 10.1002/adma.201902664

1. Introduction

Graphene fiber (GF), a new breed of carbonaceous fiber with high expectations in science and technology, was first created by the wet spinning of graphene oxide (GO) liquid crystals (LCs) in the lab by Gao and Xu in 2011.[1] Starting from low-cost graphite, GFs are directly assembled from graphene and graphene derivatives. As summarized in Figure 1, publications about and citations of GF have increased steadily from the year of its development, indicating that the high expectations of GF have received growing interest from the scientific community. The consistent contributions have also boosted the mechan-ical and conducting performance of GF with a continuously rising trend. GFs represent a brand-new horizon for fabricating carbonaceous fibers, and a new branch to the development of structural–functional integrated materials.

Graphene fiber (GF) is a macroscopically assembled fibrous material made of individual units of graphene and its derivatives. Beyond traditional carbon fibers, graphene building blocks consisting of regulable sizes and regular orientations of GF are expected to generate extreme mechanical and transport properties, as well as multiple functions in smart electronic fibrous devices and textiles. Here, the features of GF are presented along four lines: preparation, morphology, structure–performance correlations, and state-of-the-art applications as flexible and wearable electronics. The principles, experiments, and keys of fabricating GF from graphite with different methods, focusing on the industrially viable mainstream strategy, wet spinning, are introduced. Then, the fundamental relationship between the mechanical and transport properties and the structure, including both highly condensed structures for high-performance and hierarchical structures for multiple functions, is presented. The advances of GF based on structure–performance formulas boost its functional applications, especially in electronic devices. Finally, the possible promotion methods and structural–functional integrated applications of GF are discussed.

Graphene Fibers

The history of carbonaceous fibers can be traced back to the nineteenth century (Figure 2). In 1860, J. Swan used carbon filament as the emitter of an electric lamp. Soon after, T. A. Edison realized the vacuum sealing of electric lamps, enabling carbon filaments to emit vis-ible light for a record 45 h. The working principle of carbon filaments in electric lamps is electrically driven Joule heating, mainly depending on the electrically con-ductive property of the filaments, rather than the mechanical performance. One century later, A. Shindo initiated the research and development of carbon fibers (CFs). Commercial CFs are usually fabricated by the carbonization and pyrol-ysis of organic precursor fibers, mainly including three prevailing species: poly-acrylonitrile (PAN), mesophase pitch, and rayon fibers. The building blocks in CFs are controlled by the chemical constitu-

tion of the organic precursors, the original condensed struc-tures of the small and polymer molecules, and the pyrolysis process conducted under tight control. Through a series of processes including cyclization and high-temperature carboni-zation above 1000 °C, polymers or small molecular segments fuse into nanoscale graphene dots. These graphene dots stack into graphite nanocrystals and are interconnected by covalent sp2-hybridized carbon bonding and sp3-hybridized topological defects, such as loops and kinks.[3] Such small building blocks endow CFs with unprecedented mechanical strength but lim-ited conducting performance.[4–11] Carbon nanotubes (CNTs) discovered in 1991 are another suitable basic unit to construct carbonaceous fibers.[12] Through the covalent connection of sp2-hybridized carbon atoms in a 1D direction, CNTs exhibit both high mechanical performance and conducting properties. After being first proposed by Fan and co-workers in 2002,[13] the assembly of CNTs into CNT fibers has been achieved by two strategies, i.e., the dry spinning of CNT forests and the wet spinning of CNT fluids.[14–17] In 2011, Gao and Xu invented a new type of carbonaceous fiber, GF, by macroscopic wet-spin-ning assembly of GO LC dope followed by chemical reduction, opening the avenue to strong yet multifunctional carbonaceous fibers with resourceful carbon raw material (i.e., graphite).[1]

Although CFs have been widely used, boosting their overall properties, which mainly encompass the mechanical strength for structural uses and the functional electric/thermal trans-port merits, is a ceaseless pursuit in both fundamental science and industrial technology. However, the advancement of CFs

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by the traditional pyrolysis method has been retarded because of the following aspects: 1) the sizes of their building blocks, graphite nanocrystals, are difficult to enlarge, which dete-riorates their transport properties;[18] 2) fixed condensed states limit their development toward rich structures and multifunc-tional uses;[19] 3) it is a big challenge to handle molecular-level modification in CFs; and 4) high-temperature carbonization and pyrolysis is costly and requires high energy consumption, further resulting in a cross-linked carbon network with a very limited flexibility.[20] In terms of CNT fibers, highly porous microstructures result in low physical densities and weak con-ducting properties.[21–31] Therefore, new philosophies for pre-paring carbonaceous fibers are urgently needed, especially as the demand for both structural and functional uses increases.[32]

A new philosophy can provide solutions by selecting pow-erful building blocks to construct carbonaceous fibers. This conjecture can possibly be realized over time, especially after the successful exfoliation of individual graphene sheets from graphite crystals,[33–44] which was first achieved through hand peeling by Geim and co-workers and later by mechanical and chemical exfoliation at the large scale.[45] Individual graphene sheets are testified to possess many extraordinary properties and some of the merits exceed the extremes of other known materials, such as the mechanical strength, thermal conduc-tivity, and electron transport. The planar sp2-hybridized carbon bonding promotes graphene to have the highest mechanical strength of 130 GPa and modulus of 1100 GPa in the planar direction.[46] The unique zero-bandgap semimetal attribute leads to a high electron mobility beyond 200 000 cm2 V−1 s−1. In consort with microscale ballistic transport, such high room-temperature electron mobility gives rise to an in-plane electrical conductivity of 1 × 108 S m−1.[47] Additionally, the long phonon mean free path and high phonon group velocity allow the in-plane thermal conductivity of suspended graphene to reach a record high value of 5300 W m−1 K−1.[48,49]

Using graphene as building blocks, assembled GFs have been demonstrated to have aggressive advantages from prefabrication to state-of-the-art developed applications.[50–52] We summarize these advantages in the eye diagram in Figure 3, as follows:

1. Four advantages (4A) of fabrication: i) The raw materials, especially GO, can be prepared facilely at a large scale in advance;[53] ii) the as-prepared spinning dopes can sponta-neously exhibit highly ordered liquid crystalline behaviors, which result in a 3D architecture of GFs;[54] iii) wet-spun graphene oxide fibers (GOFs) possess a self-fusing/healing ability, similar to that of self-healing polymers and metals; and iv) low-cost repair methods, including low-temperature chemical reduction and electrothermal reduction, are avail-able to reduce GOFs into GFs.[55]

2. Six types (6T) of morphology: Mainstream GF is a solid cy-lindrical fiber, composed of highly condensed and ordered graphene building blocks. To satisfy different working sur-roundings, the control of spinning technology and spinneret channels enables GFs to have a high degree of freedom to display various microstructures. The reported morpholo-gies include, at least, five types, i.e., belt-like, hollow, helical, porous, and core–sheath.[56,57]

3. Four merits (4M) of structures: The structures in GFs are de-termined by the condensed state of the graphene units, which can be divided into highly condensed states for high perfor-mance and loosely stacked states for multiple functions. Freely condensed states drive GFs to be equipped with 4M

Bo Fang received his B.S. degree in materials science and engineering from Zhengzhou University in 2013. Then he joined Prof. Chao Gao’s lab at the Department of Polymer Science and Engineering of Zhejiang University and became a Ph.D. candidate in 2016. His current research involves high-performance

graphene fibers for energy conversion and multifunction applications. He is broadly interested in carbon nano-materials for electronics, optoelectronics, energy, and biotechnology.

Zhen Xu received his Ph.D. degree in chemistry in Zhejiang University in 2013. He did postdoctoral research at Zhejiang University in 2013–2015 and at Cambridge Graphene Center at Cambridge University from 2015 to 2016. In 2017, he joined the Department of Polymer Science and Engineering, Zhejiang

University, as a Research Professor. His research interests cover liquid crystals of 2D nanomaterials, graphene macro-scopic materials, and 2D macromolecular behavior.

Chao Gao received his Ph.D. degree from Shanghai Jiao Tong University (SJTU) in 2001. He was appointed as an Associate Professor at SJTU in 2002. He did postdoctoral research at the University of Sussex with Prof. Sir Harry W. Kroto and AvH research at the Bayreuth University with Prof. Axel H. E. Müller. He joined the Department

of Polymer Science and Engineering, Zhejiang University, in 2008 and was promoted as a Qiushi Distinguished Professor. He leads a group of researchers working on graphene chemistry, macroscopic self-assembly, and energy storage.

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of structures, which is superior to traditional carbonaceous fibers. In terms of highly condensed states, i) the adjustable sizes of the graphene building blocks break through the lim-itation of the size of graphite nanocrystals in conventional CFs, and offer a method by which to enhance the conduct-ing properties;[58] and ii) defect management to optimize structural factors, such as the porosity, diametral size, axial orientation, radial arrangement, and interlayer interactions, promotes GFs to show superb mechanical performance. In terms of loosely stacked states, iii) the rich wrinkles of stacked graphene generate strong interface bonding with other materials and superb flexibility;[59] and iv) the graphene building blocks in GFs are able to combine heteromolecules in various dimensions ranging from 0D to 3D.[60]

4. X-uses and applications: Last but not least, the tunable con-densed state of graphene units promotes GFs to fulfill the on-demand design of multifunction capabilities in smart electronic fibrous devices and textiles.[61] State-of-the-art de-veloped flexible electronics include multifunctional textiles, power cables, energy harvesters, wearable energy storage devices, sensors, and neural recording microelectrodes.

Herein, we depict a panoramic review of the features of GF. We retrospectively examine the emerged fabrication methods of GFs, focusing on the industrially viable wet-spinning method.

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Figure 1. The annual publication and citation numbers in ISI Web of Science, and key issues surrounding GF since its discovery in 2011.

Figure 2. The evolution of carbonaceous fibers: carbon filaments, carbon fibers, CNT fibers, and GFs. At the earliest, the uses of carbon filaments in electric lamps mainly depended on their electrical conducting property. Carbon fibers are renewed as a specific material with a high strength and modulus. CNT fibers are a fibrous material with flexibility and electrical conductivity. Currently, GFs feature the integration of high strength, modulus, and electrical and thermal conductivities. CNT fiber image: Reproduced with permission.[2] Copyright 2012, Royal Society of Chemistry. Graphene fiber image: Reproduced with permission.[1] Copyright 2011, Springer Nature. The carbon filament image is from Pixabay.



Then, we review the diverse morphology of GFs, and the recent exploration of the fundamental relationship between the struc-ture and properties is also discussed. The development of multi-functional GFs is categorized by coupling their functional uses, especially in flexible and wearable electronics. As the prospects in the roadmap of GF, we look at possible promotion methods and potential structural–functional integrated applications.

2. Preparation of GFs

Graphene with a 2D topology is extremely anisotropic and most of its favorable properties are exhibited along the planar direction, such as tensile strength and thermal conducting properties.[62–67] From the fundamental principle, a well-ordered arrangement of building blocks is a general perquisite for the high-performance targets of macroassembled materials. This common rule of thumb has been testified by extensive examples ranging from the organization of biological tissues to the fabrication of conventional CFs. In the arduous journey of pitch-based high-modulus CFs, initial pitch-based fibers are too weak to be used, because of the isotropic state of the precursor pitches with chaotic structures and the resultant ill

alignment of fibers. This problem has been resolved by using mesophase pitches with nematic liquid crystallinity as precur-sors to achieve a high degree of molecular orientation in the as-spun pitch fibers, and a further ultrahigh modulus of CFs currently.[68] This experience gives inspiration for the macroas-sembly of graphene sheets into GFs, that is, the fashioning of graphene sheets into a continuous compact state with a regular alignment. Liquid crystal is the fourth matter state with the fluidity of a liquid and the structural order of crystals, and it is usually observed for anisotropic molecules, colloids, and col-loidal giant molecules, such as CNT and rigid polymers.[69–72] Considering the huge aspect ratio (usually beyond 1000) of gra-phene sheets, they naturally form lyotropic LCs when dispersed into solvents at low concentrations.

The finding of LCs from graphene and graphene derivatives, especially GO, guides an important preparation method: liquid crystalline wet spinning. This method has prevailed in the fabrication of GFs and has greatly extended to the production of neat, composite, hybrid GFs, in a continuous and scalable manner. Additionally, other typical methods have emerged, for example, dry spinning, the confined hydrothermal strategy, and film twisting. By virtue of these methods, GFs with controllable composite natures and structures and desired performances

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Figure 3. Eye diagram of GF from prefabrication to state-of-the-art applications developed in flexible electronics.



have been fabricated to form new carbonaceous fiber species for rich applications. Thus, a multilevel hierarchy of the GF industry has been established, incorporating the scalable pro-duction of GO, stable wet spinning, superb GFs, and struc-tural–functional integrated uses (as introduced in Section 5). As listed in the box in Figure 4, the operation of the GF industry requires bridging the gap between achievements and expec-tations and conquering the challenges of every step. Efforts including preparing high-quality raw materials, tightly control-ling wet-spinning procedures, optimizing the performance, and enriching the applications of GFs have to be made for the GF industry to flourish.

2.1. Wet-Spinning Method

2.1.1. From Graphite to GO

A few layers of pristine graphene can be fabricated through micromechanical cleavage and chemical vapor deposi-tion (CVD). Unfortunately, these methods generally yield

an extremely small amount of sample, failing to satisfy the demands of large-scale production.[75] Wet chemical routes (Figure 5A), mainly including chemical exfoliation[76] and electrochemical exfoliation,[77–79] have been proposed to pro-duce GO from natural graphite at a large scale. In a chemical exfoliation route, Hummers and revised Hummers methods have been widely used.[80–90] As displayed in Figure 5B, potassium permanganate and sulfuric acid are introduced to delaminate and oxidize graphite flakes at low temperatures, which are converted into easily dispersed monolayer GO sheets. In an electrochemical exfoliation method (Figure 5C), graphite is connected to an electrical field. The injection of electrons and holes makes graphite either positively or nega-tively intercalated, and the graphite has further been exfoli-ated into a few layers of graphene and into GO sheets by gas expansion in solvent electrolysis.[91–96] With the aggressive advantages of mass production (beyond 10 tons per year at GaoxiTech) and rich functionalization, the chemical exfolia-tion method addresses tremendous concerns, and can realize the bulk production of GO limited only by the size of reac-tion vessels[97] (Figure 5D), and the lateral dimensions of GO

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Figure 4. Expectations, achievements, and challenges in the multilevel hierarchy of the GF industry. The pictures of GO, superb GFs, and GF-based gloves in the top row are reproduced with permission.[73] Copyright 2017–2018, Hangzhou Gaoxi Technology Co., Ltd. The picture of the wet-spinning schematic is reproduced with permission.[74] Copyright 2016, Wiley-VCH.



sheets can be freely controlled from hundreds of micrometers to sub-micrometers.[98]

2.1.2. GO LCs

The breakthrough in the pursuit of GF started from the iden-tification of GO LCs in 2010.[99–103] Because of their rich-ness of oxygen-containing functional groups, GO sheets are highly soluble in a variety of polar solvents, e.g., water, dimethylformamide, and N-methyl-2-pyrrolidone, up to high concentrations even beyond 10% in weight. Due to an entropy-change-induced volume exclusion effect, GO sheets spontaneously form well-ordered lyotropic LCs in solvents at low concentrations, exhibiting orientation or quasi-positional orders (Figure 6A,B).[104] Isotropic GO dispersions were found to spontaneously form nematic, lamellar, and even chiral phases at different concentrations and size distribu-tions (Figure 6C–E). Due to the dynamic nature of the fluidity, the ordered structure of GO LCs can also be adjusted by pH values,[105] electrical/magnetic field, and shear flow. The con-trollable order of GO LCs sets the basis for the wet spinning of GF, at the same time extending their uses in photonic crystals and electro-optical devices.[106–112]

2.1.3. Wet Spinning

Starting from GO LCs, the LC wet-spinning method has been initiated to prepare continuous GFs by Gao and Xu in 2011, making directly assembling graphene into fibers possible.[1] This seminal method also generates a new carbonaceous fiber species different from conventional CFs and CNT fibers. During the wet-spinning experiment, the continuous axial flow of the GO LC spinning dope homogenized the powder crystallinity of the free LCs into a uniform orientational order of GO sheets, which were immobilized by gelation in coagula-tion baths.

The wet-spinning process of GO fibers encompasses a series of procedures involving homogenization in spinning channels, solvent exchange in a coagulation bath, and poststretching by drawing–collecting and air drying. As shown in Figure 7A, the uniaxial shear flow occurring at the inner wall of the spinning pipe compels the GO sheets to achieve high regularity. In the

coagulation bath, solvent exchange between the spinning dopes and the coagulation agent leads to the phase transition from a homogeneous solution to a gel state. Suitable solvating species and binders are conductive to the oriented alignment of GO sheets in this process, resulting in freestanding and robust gel GOFs that can endure continuous pulling and stretching. After being removed from the coagulation bath, the gel GOFs are dried into thin fibers with compact microstructures by capillary shrinkage forces.

After forming the fibers, the precursor GOFs can be con-verted into GFs by chemical and/or thermal reduction to eliminate oxygen-containing functional groups and restore the graphene lattice. Diverse treatments have been applied to con-vert GOFs into GFs, ranging from low-temperature chemical reductions with common reducing reagents, e.g., hydroiodic acid, hydrazine hydrate, aqueous alkali, and sodium citrate, to electrothermal treatment under a low bias and thermal treatment at 1000 K. Chemical reduction partially removes oxygen-containing groups in GOFs, turning insulating GOFs into reduced GFs with an electrical conductivity approaching 5 × 104 S m−1. Residual functional groups can be fully elimi-nated through high-temperature (up to 3000 K) graphitization, accompanied by decreased interlayer spacing, and improved mechanical strength and conducting properties.

Due to the elaborate design used to maximize the sheet arrangement by shear flow and poststretching, the wet-spin-ning method has been frequently used to fabricate high-perfor-mance GFs. The initial wet-spun GF presents good flexibility, and is capable of being woven into fabric (Figure 7B). Some other fascinating properties, such as a high electrical conduc-tivity of 2.5 × 104 S m−1, fracture elongations of 6.8–10.1%, a modulus of 7.7 GPa, and a mechanical strength of 140 MPa, facilitate the performance translation from graphene sheets to macroscopic GFs.[1] So far, in terms of mechanical perfor-mance, neat GF has attained a mechanical strength of 2.2 GPa and a stiffness of 400 GPa.[74] In terms of transport properties, an electrical conductivity of 8 × 105 S m−1 and a thermal con-ductivity of 1290 W m−1 K−1 have been achieved.[113]

LC wet spinning is easily scaled up to prepare strands beyond single fibers, by using multifilament spinning nozzles. The preparation of GF strands shortens the distance to real applications of GF with promising industrial efficiency. Wallace and co-workers replaced the 1-hole spinneret with a 50-hole spinneret (with each orifice having a diameter of 100 µm) and

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Figure 5. A–C) Mass production of GO from graphite (A) by a chemical exfoliation method (B) and an electrochemical route (C). B) Reproduced with permission.[75] Copyright 2015, Elsevier B.V. C) Reproduced with permission.[77] Copyright 2013, American Chemical Society. D) GO dispersion in water with a volume of 10 L, prepared from a chemical exfoliation method. Reproduced with permission.[97] Copyright 2017, American Chemical Society.



simply increased the production efficiency by 50 times.[114] Xu et al. further improved the 50-filament efficiency to 100-fila-ment efficiency (with diameters of single GF filaments from 1.6 to 20 µm). Combined with lengthening of the wet-spinning line and production rates up to 300 m h−1, this fabrication

technology reached a considerable scale comparable to that of the industrial manufacture of CFs and polymeric fibers (Figure 7C). Ascending this work, the obtained macroscopic materials have varying dimensions and unlimited sizes from the millimeter level to the kilometer level.

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Figure 6. A,B) The nematic volume and polarizing microscopy (POM) images of GO dispersions with different mass fractions ranging from 5 × 10−4 to 5 × 10−3 in succession. C) POM images, D) schematic, and E) scanning electron microscopy (SEM) images of the nematic, lamella, and chiral phases of GO LCs. A,B and the left and middle parts of C–E): Reproduced with permission.[104] Copyright 2011, American Chemical Society. Right-hand-side images in C–E) Reproduced with permission.[1] Copyright 2011, Springer Nature.



2.1.4. Keys to Wet Spinning

Some factors need to be tightly controlled when using the wet-spinning method to fabricate continuous GOFs. These keys involve the preparation of GO dopes, and the choices of nozzles and coagulation baths.[97] Every detail may exert an influence on the performance of the obtained fibers.

1. Spinning-grade GO dopes: To boost the quality of GO dopes to spinning grade, some pretreatments to the GO dopes, e.g., filtration, centrifugation, and deaeration, are required. i) Insufficient oxidation of graphite in the chemical exfolia-tion procedure leads to the yield of graphite particles and a high C/O ratio, discounting the dispersibility of the GO dopes in solvents. To remove unlaminated graphite parti-cles, GO dopes need to be purified through a filtering mesh. ii) Generally, their concentration can be adjusted throughout a wide range starting from 1 mg mL−1, depending on the per-formance requirements of the fibers. iii) GO sheets with a uniform size distribution and few debris are desirable, since debris can result in grainy edges and structural discontinu-ity in the GFs. The uniform size distribution of GO can be acquired using a centrifugation method. Centrifugation at high/low speeds can remove small-/large-sized GO sheets, respectively. iv) Air microbubbles in dopes are another factor disrupting the continuity of wet-spun GOFs. Processing GO dopes with a deaeration device can effectively suppress the production of microbubbles.

2. Suitable spinning nozzle: The frequently used internal diam-eter sizes of spinning nozzle are 60–250 µm. A finer spin-ning nozzle generally guides the formation of GOFs with smaller diameters and compact microstructures. GO dopes with low concentrations can be considered when utilizing

a fine spinning nozzle. For example, at a suitable pumping pressure, the concentration is usually limited to <3 mg mL−1 when choosing a nozzle with an internal diameter of 60 µm.

3. Selecting a coagulation bath: In coagulation baths, solvent exchange between the GO dopes and the precipitation agents promotes the curing of the gel GOFs. Two kinds of coagu-lation agents can be used, i.e., aqueous salt solutions and organic baths. In aqueous salt solution baths, the solvating species of the GO dopes is water. Mixtures of alcohol and water with volume ratios ranging from 3:1 to 5:1 are avail-able. To enhance the solidification of GOFs, metal ions (such as Ca2+) with a mass fraction below 10 g mL−1 are added to generate an enhanced cross-linking network between adja-cent GO sheets. Redundant metal ions adsorbed onto the GOFs can be removed by washing and thermal treatments. Due to a high vaporization heat, water is difficult to remove during air drying or infrared heating. Thus, aqueous co-agulation is not suitable for the continuous processing of wet spinning. To resolve this problem, organic coagulation baths have been developed. For example, GO dispersions in dimethylformamide act as dopes, while ethyl acetate is used as coagulation agent. Under mild heating in air, the removal of organic solvents facilitates the shrinkage of the GOFs, which can be continuously collected.

2.1.5. Self-Fusing/Healing Behaviors of Wet-Spun GOFs

Self-healing materials can spontaneously restore their shapes or properties after being damaged. Rich examples of this ability can be found in self-healing polymers, ceramics, and metals.[115–119] Due to brittle attributes and stiff carbon–carbon bonding, it is difficult to produce the capacity of self-fusing

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Figure 7. A) The schematic showing the evolution of GO fibers during wet-spinning and thermal treatment processes. B) The SEM image of a GF-based fabric. Reproduced with permission.[1] Copyright 2011, Springer Nature. C) A 50-filament GOF produced in a continuous manner. B,C) Reproduced with permission.[74] Copyright 2016, Wiley-VCH.



or self-healing in carbon-based materials. However, the easy operation required to detrude and rebuild the hydrogen bond networks in GO assemblies makes it possible for them to pre-sent self-healing behaviors. Recently, macroscopic GO mate-rials, such as GOFs, have been demonstrated to exhibit such abilities under the driving force of moisture or solvating species. In humid surroundings, fractured GO sheets will rearrange to join with each other under the wetting of solvents. The self-healing of GOFs occurs under both drying and wetting condi-tions. In terms of drying GOFs (Figure 8A), the infiltration of solvents into the damaged interface leads to the physical fitting of broken GO sheets, then facilitating the contact and interdif-fusion of GO sheets.[120] In the drying process, the decreased interlayer spacing brings the enhancement and reconstruction of the hydrogen bond networks (Figure 8B,C), further causing the self-healing of the GOF. In wet conditions, GOFs exhibit a gel state. Rich hydrogen bonding on the surface of swollen GOFs directly prompts strong fiber interactions and interfusion (Figure 8D).[121] The self-healing and self-fusing abilities render fascinating applications, such as in multifunctional nonwoven fabrics, smart walker actuators, and IR-transformable graphene architectures[122] (Figure 8E).

2.2. Other Methods

The dry-spinning approach is another industrially viable strategy and can be used to fabricate continuous GFs. Different from the wet-spinning protocol, GO LCs were extruded from a dry-spun spinneret, directly shaped into fibers without a coagu-lation bath and collected in the air, avoiding the trouble of using a coagulation bath. To ensure the mechanical strength required for self-standing GO fibrous dopes without cross-linking in coagulation baths, GO dopes with much high concentrations (e.g., >8 mg mL−1) are needed because of two reasons. First, at high concentrations, GO dispersions exhibit a gel-like behavior with a high elastic modulus, easily forming viscoelastic LC gels. Second, the domain size of GO LCs increases with concentra-tion, which can lead the LCs to align themselves spontaneously in one direction under shear stress.[123] The solvent of the spin-ning dope is another important factor. High surface tension sol-vents aggravate the shrinkage tendency of GO sheets, and low saturated vapor pressure solvents retard the solidification of gel fibers. Thus, high-concentration GO LC dispersions in solvents with a low surface tension and high saturated vapor pressure, such as methanol, ethanol, acetone, and tetrahydrofuran, have

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Figure 8. Self-fusing and self-healing behaviors of GOFs. A–C) Optical pictures and mechanisms of moisture triggered self-healing of GOFs. Repro-duced with permission.[120] Copyright 2017, Wiley-VCH. D) Self-fusing process of swollen GOFs in water. Reproduced with permission.[121] Copyright 2016, The Authors, published by Springer Nature. E) An IR-transformable GOF fabricated by the fusion and healing of GO sheets. Reproduced with permission.[122] Copyright 2018, Wiley-VCH.



been employed for the dry spinning of GFs.[124] Dry-spun GFs generally show poor mechanical strength due to the existence of substantial microvoids and core–shell structures. However, a high toughness of up to 19.12 MJ m−3 and favorable flexibility can be achieved.

Twisting aligned CNT forests, films, or polymer fibers into yarn is a relatively common process in industry and labs.[14,125–128] Recently, this intuitive approach has been used to fabricate twisted GFs (TGFs) by twisting GO films and single GO gel fibers. The spiral-arranged textures along TGFs offer good tear resistance and high toughness. Researchers have usually prepared isolated gel fibers[129] and GO films,[130] and twisted them into individual TGFs by applying a normal torque from motors (Figure 9A,B). After being chemically reduced, the self-coiled TGFs were used as soft conductors, stretchable sensors, fabric-based heaters, and electrocapil-lary suckers.[131,132] The discontinuity of the twisting method for TGFs was solved by constructing a continuous spinning–twisting technique by Fang et al.[133] This continuous pro-duction line connected the LC spinning of GO belts and

the twisting step. Continuous GO belts were first prepared through a microfluidic system by an LC spinning method. Then, the GO belts were twisted by a roller acting as both the collector and the normal torque generator (Figure 9C). The chiral texture of the TGFs can be stably controlled as designed in the continuous spinning–twisting process, and contin-uous TGFs with high flexibility can realize many functional applications in smart textiles, which will be discussed in the Section 5.3 on the functional uses of GFs.

Some other encouraging strategies are also available. Qu and co-workers sealed GO LCs into a hollow pipe, which acted as a mold for the formation of GFs.[134] GO LC dope was shrunked into gel fibers due to a hydrothermal effect that precipitated the GO sheets by partly eliminating the oxygen-containing func-tional groups. The diameters of the GFs could be controlled using the pipe mold. The shrinkage in the capillary tubes by the hydrothermal effect caused random wrinkles of the gra-phene sheets inside the fibers, which lowered the alignment order. Therefore, the mechanical strength of GFs obtained by confined hydrothermal method was lower than that of GFs

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Figure 9. The development of TGFs from an inconsecutive mode to a consecutive mode. The schematic showing TGFs obtained by twisting A) isolated gel GOFs and B) GO films. C) The scheme of the drawing–twisting process used to obtain continuous TGFs. D,E) SEM images of TGFs and F) the cross-sectional observation. A) Reproduced with permission.[129] Copyright 2014, Wiley-VCH. B) Reproduced with permission.[131] Copyright 2016, Royal Society of Chemistry. C–F) Reproduced with permission.[133] Copyright 2019, Royal Society of Chemistry.



obtained by LC wet spinning. Alternatively, the wrinkled struc-ture with high porosity enhanced the electrochemical activity and favored functional uses in energy storage.[135]

The CVD method has been generally used to grow single-layered graphene or few-layered graphene films on catalyst sub-strates, which has been extended to make flexible and porous GFs.[136] With the rapid evaporation of solutions, few-layer gra-phenes (FLGs) detached from the catalyst substrates formed a fibrous material with a high porosity. Without further reduction or purification, the as-prepared GFs showed high electrical con-ductivity beyond 1000 S m−1. This design has been further used to prepare a graphene-based woven fabric by growing FLGs on copper woven mesh.[137] The removal of copper in the ferric salts led to the formation of hollow GFs, which were easily pro-cessed into fabrics by combination with polymers.

3. Morphology

As it is not restricted to the highly condensed state, the mor-phology of GFs could be freely adjusted to satisfy different operation conditions. We have collected all the reported mor-phologies of GFs, mainly including porous, hollow, belt-like, helical, and core–sheath. Remarkably, all the GFs with different morphologies were fabricated by the wet-spinning method, and were collocated through modified procedures. For example, porous GFs were obtained by directly extruding GO LCs into liquid nitrogen followed by freeze drying (Figure 10A).[57] The obtained porous GFs, or graphene aerogel fibers, showed uni-form alignment of the graphene building blocks, a high spe-cific tensile strength of 188 kN m kg−1, and a high electrical conductivity of 4.9 × 103 S m−1. Moreover, the interconnected pores make the porous GFs have a low volume density of 56 mg cm−3, a high specific surface area of up to 884 m2 g−1, and an elongation beyond 5%. The good stretchability and elec-trical conductivity permit porous GFs to be used as lightweight and flexible electronics, while the characteristic of a large specific area facilitates applications in which high mass load is needed, such as in electrochemical catalysts, high-absorption materials, and multifunctional textiles. Continuous hollow

GFs were obtained by adopting a coaxial two-capillary spinning strategy (Figure 10B).[56] Hollow GFs have favorable mechan-ical and conducting properties, and their internal walls can be easily decorated with heteromolecules, such as functional nano-particles. By modifying tubular spinning channels into cuboid microfluidic channels, belt-like GFs can be continuously col-lected (Figure 10C).[138] Compared to fibrous GFs, belt-like GFs have better knittability, and can be knotted into graphene tex-tiles, and they have a higher capacity for use as dye-sensitized fiber solar cells and fabric-based supercapacitors. Helical GFs are twisted from gel GOFs or GO films, which are introduced in Section 2.2. Core–sheath structures are ubiquitous in the aforementioned five morphologies, which are produced by the graded distribution of the shear force in the spinning channels, as discussed in Section 4.1.2.

4. Structures and Properties

The overall properties of macroscopic GFs are determined by the condensed state of their basic graphene units, which is controlled by procedures (e.g., the size of the GO sheets, defect control, and the heat treatment temperature) in the assembly process (Figure 11 and Table 1). Recent efforts concerning GFs have jointly depicted the relationship between the multiscale structure and the properties. The multiscale structure of GFs can be analyzed as a series of sections, including the composi-tion, orientation in the distance along the fiber axis, laminated stacking, interlayer interactions, and atomic structure of the graphene units. Two trends have been determined: one is that ordered compact structures result in high performance and conducting properties, and the other is that hierarchical winkles can be designed to provide flexibility, strong interface bonding, and high chemical reactivity. The former is aimed at obtaining GFs with high mechanical performances and outstanding con-ducting properties, and the products of the latter are suitable for multifunctional uses in electronic fibers and textiles. In this section, we will unfold the analysis of these aspects to establish a guiding map to design and control the structure and proper-ties of GFs.

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Figure 10. A–C) The fabrication of porous GFs (A) , hollow GFs (B), and belt-like GFs (C). A) Reproduced with permission.[57] Copyright 2012, American Chemical Society. B) Reproduced with permission.[56] Copyright 2013, American Chemical Society. C) Reproduced with permission.[138] Copyright 2013, American Chemical Society.



4.1. Mechanical Strength

4.1.1. Axial Orientation

The axial orientation of graphene sheets is important in the mechanical strength of the GFs and can be optimized by flow control, the coagulation choice, and stretching during the wet-spinning process. The axial orientation of 2D GO sheets extruded through a confined space has been confirmed in the first reported GFs. Xu and Gao used a tubular wet-spinning channel to enhance the orientation of GO sheets by applying a shear flow.[1] However, the shear flow control of the orientation only occurs inside the tubular channel. A freedom degree in other directions will emerge once the GO sheets are extruded, which may induce sheets’ curvature and distortion. Trebbin et al. first investigated the flow orientation of anisotropic par-ticles, such as poly(ethylene-co-butylene)-b-poly(ethylene oxide) (PEB–PEO) cylindrical micelles, when passing through a T-junction channel section.[158–160] They found that cylindrical micelles would be oriented parallel to the flow in the channel section close to the narrow walls, but reorientation in the per-pendicular direction occurred when the cylindrical micelles were in a channel expansion zone (Figure 12A). Without any

modulation of the expansion zone, this perpendicular align-ment was very stable, propagating downstream along the remaining part of the process. Using microparticle image velocimetry and computational fluid dynamics simulations, researchers determined that shear thinning behaviors occurred in the cylindrical micelle-like non-Newtonian fluids, which led to high extension rates perpendicular to the flow direction. A threshold value of the ratio of the shear rate to the expansion rate was further calculated to quantitatively reproduce the dis-tribution of the perpendicular alignment and parallel align-ment. The research by Trebbin et al. provides a useful tool to control the flow in the wet spinning of GFs. In the concentra-tion range at which stable LCs form, GO dispersions are non-Newtonian fluids,[102,161] so reorientation to the perpendicular direction happens when the GO sheets pass through spinning channel. Park et al. found this phenomenon when they tried to assemble ionic conductive gel fibers from GO dispersions.[162] When the GO sheets were extruded from narrow channel, the perpendicular alignment relative to the flow direction was identified at a low collecting rate. Xin et al. also achieved the vertical arrangement of graphene sheets by pumping GO solution from a contracted channel into an expanded channel, where the expansion rates were largely amplified.[157]

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Figure 11. The structure, performance, and properties of GFs. The SEM image of flexible electronics is reproduced with permission.[137] Copyright 2015, Wiley-VCH. The SEM image of the interface bonding is reproduced with permission.[139] Copyright 2014, Springer Nature. The schematic of the dot–sheet structure is reproduced with permission.[140] Copyright 2018, Royal Society of Chemistry. The SEM image of CNTs is reproduced with permission.[141] Copyright 2012, Springer Nature. The SEM image of the polymer is reproduced with permission.[142] Copyright 2013, Springer Nature. The schematic of the nanosheets is reproduced with permission.[143] Copyright 2017, American Chemical Society.



To pursue the regular alignment of graphene in GFs, the perpendicular orientation should be suppressed or diminished, since graphene assemblies with irregular microstructures

generally exhibit poor mechanical strength. Two strategies have been proposed. First, contracted spinning channels were devised to control the expansion rates and enhance the

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Table 1. The performance of GFs is associated with their preparation methods, including the heat treatment temperature, microstructure design, and tight control of the spinning procedures.

Study Method Treatment temperature [K]

Strength [MPa] Modulus [GPa] Electrical conductivity [S m−1]

Thermal conductivity [W m−1 K−1]

Xu and Gao (2011)[1] Wet spinning 353 in HI 140 7.7 ≈2.5 × 104

Cong et al. (2012)[144] Wet spinning 353 in HI 182 8.7 ≈3.5 × 103

Dong et al. (2012)[134] Hydrothermal 1073 in vacuum 420 ≈1.0 × 103

Chen et al. (2013)[145] Wet spinning 353 in HI 192 9.8 ≈2.3 × 104

Xu et al. (2013)[146] Ag doping 363 in HI or Vitamin C 360 ≈9.1 × 104

Jalili et al. (2013)[147] Chitosan cross-linking 412 20.1

Xu et al. (2013)[148] Giant GO sheets 353 in HI 501.5 11.2 4.1 × 104

Xiang et al. (2013)[149] Large GO flakes 1273 in Ar 214 47 2.94 × 104

Kim et al. (2014)[150] Diamine cross-linking 384.3 26.6

Xin et al. (2015)[113] Blending large and small sheets 3013 in Ar 1150 135 2.21 × 105 1290

Zhang et al. (2016)[151] Synergistic toughening 353 in HI 864.2 2.9 × 104

Liu et al. (2016)[152] K doping 3273 in Ar 2.24 × 107

Xu et al. (2016)[74] Defect control 3273 in Ar 2200 400 8 × 105

Sheng et al. (2017)[153] Ribbon–sheet structure 503 in hydrothermal reactor 223 1.2 × 103

Liu et al. (2017)[154] Ca doping 3273 in Ar Superconductive at 11 K

Zhang et al. (2018)[155] Sequential interfacial

interactions

363 in HI 740.1 4.33 × 104

Ma et al. (2018)[156] Interface reinforcement 1473 in Ar 724 ≈40 6.6 × 104

Xin et al. (2019)[157] Microfluidic design ≈3000 in Ar 1900 309 1.04 × 106 1575

Figure 12. A) Orientation and X-ray diffraction patterns of PEB–PEO cylindrical micelles in the wide and narrow channel sections as determined by scanning microfocus X-ray diffraction. Reproduced with permission.[158] Copyright 2013, National Academy of Sciences. B) In situ small-angle X-ray scattering (SAXS) patterns and order parameters measured from GO fluid at different locations during an elongational flow. Reproduced with permission.[157] Copyright 2019, Nature Publishing Group. C,D) Structural analysis of GFs without and with defect management. After defect man-agement, GFs exhibit much fewer defects from the macro-scale to the atomic scale. C,D) Reproduced with permission.[74] Copyright 2016, Wiley-VCH.



elongation flow. Second, strong poststretching was used to limit the expansion of the GO fluids in the radial direction. Xin et al. designed a contracted channel with a gradually shrinking diameter to extrude GO fluids from a wide inlet to a narrow outlet.[157] To cope with the shear thinning behaviors of the GO solution, the decreasing cross-sectional area generated a positively increasing elongation rate along the flow direction, largely improving the axial orientation and mechanical strength of the GFs (Figure 12B). Xu et al. exerted strong poststretching and defect engineering on the as-spun GO fibers.[74] Together with other processing methods, the poststretching largely improved the orientation degree (up to 81%) and enhanced the mechanical strength and modulus to 2.2 and 400 GPa, respec-tively (Figure 12C,D). Park et al. also adjusted the orientation degree by employing enhanced stretching.[162] When the collec-tion rate was increased to a high level, the orientation of the obtained gel fibers could be converted from the perpendicular direction to the completely axial direction, which also brought about an improved ionic conducting ability.

4.1.2. Radial Alignment

Controlling the axial orientation of GFs has been extensively investigated but the control of the alignment in the radial direc-tion has rarely been explored. We found some hints in previous studies on how to implement full control of the GF structure. Disorder of the radial alignment is generated by distortion of the GO sheets during the assembly process. Strategies by which to enhance the axial orientation are also available to address this problem, since a high-speed shear rate and elonga-tional flow can compel the GO sheets to be well arranged. Post-stretching during the whole wet-spinning process is effective in reducing random wrinkles and loose stacking of the graphene laminates. Xu et al. chose building blocks with a greater aspect ratio to remit the disorder in the radial direction.[148] The distor-tion of graphene sheets is also ascribed to the mismatching of the geometries of the 2D GO sheets and the tubular channel. When GO plates flow through the tubular channel, a randomly oriented organization and defective voids are produced due to the dimensionally constrained morphology. Xin et al. chose a flat channel to realize good matching between the GO sheets and the inner wall (Figure 13).[157] The configuration of a flat channel is difficult to use to achieve solid cylindrical GFs, but it tends to yield hollow or belt-like GFs. The obtained belt-like GFs exhibited a largely improved orientation order in the radial direction, inhibiting the generation of pinholes and micro-voids. Increasing the aspect ratio of the flat channel can fur-ther optimize the orientation degree, leading to a high tensile strength of ≈1.9 GPa and a modulus of ≈309 GPa.

The skin effect, a phenomenon widely found in the produc-tion of polymers and inorganic fibers,[163–165] is another factor decreasing the radial orientation of GFs. During the wet-spin-ning process, the skin effect will lead to a core–sheath structure, with well-ordered graphene sheets on the surface but a random sheet orientation and a turbostratic arrangement of the gra-phene crystal domains along the transverse direction. The gen-eration of the skin effect is mainly attributed to the flow shear stress gradient along the radial direction of the microchannel,

wherein a large shear stress close to the wall induces the GO sheets to be well aligned and a small shear stress in the core causes the GO sheets to be poorly aligned. Using flat or con-tracted channels is instrumental in the disturbance of the skin effect. Another method is to fine tune the radial dimension of the GFs by lowering the concentration of the spinning dope and decreasing the diameter of the tubular channel. Xu et al. found that ultrafine GFs successfully dispelled the defects of core–sheath structures.[74]

4.1.3. Interlayer Interactions

In tensile failure testing, the tension–shear deformation behavior of GFs suggests that interlayer interactions greatly influence their mechanical properties.[50] During the evolution of GFs from GOFs, three kinds of interactions exist and domi-nate, i.e., hydrogen bonds, coordinative cross-linking, and van der Waals interactions.[51] The mechanical strength of as-spun GOFs is mainly determined by the hydrogen bonds between oxygen-containing groups. After undergoing chemical/thermal treatment, GOFs are converted into reduced GOFs through the partial removal of oxygen-containing functional groups and the decrease of the interlayer space. At this stage, coordina-tive cross-linking and van der Waals interactions contribute to improving the mechanical strength of GFs, while the influence of hydrogen bonds is largely weakened. The coordinative cross-linking between two adjacent graphene layers can be strength-ened by adding polyvalent cations to bridge the redundant oxygen groups. For example, the introduction of Ca2+ improved the mechanical strength of GFs by 65–100%. In addition, the introduction of polymer guests can also strengthen interlayer links by creating covalent cross-linking or reinforcing hydrogen bonding and π–π van der Waals interactions.[166] Redundant oxygen groups are completely removed after undergoing gra-phitized treatment at high temperature. Coordinative cross-linking loses efficacy, and the interlayer interactions are only decided by the van der Waals forces, which boost the mechan-ical strength of the GFs to the gigapascal level. For graphitized GFs, a closer distance between graphene sheets leads to stronger π–π van der Waals interactions. Thus, the overall per-formance of GFs is largely affected by the crystallite sizes.

4.1.4. Crystallite Sizes

Crystallite graphite domains are formed during the high-temperature carbonization of graphene-based assembles. Graphite crystals are highly stiff and thermally/electrically conductive. Small crystallite domain sizes result in substantial scattering centers to reduce the transport properties, and gen-erate many grain boundaries to decrease the modulus. There-fore, the enlargement and growth of the crystalline graphitic domains enables simultaneously high modulus and transport properties in CFs.[4] Compared to the crystallite sizes of CFs (several tens of nanometers), GFs have much larger domain sizes. This is attributed to the direct stacking of large-sized graphene sheets as building blocks. Previously reported GFs have generally exhibited a small domain size, since the raw

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GO sheets were small (<20 µm) and the treatment tempera-ture was limited to below 1300 °C. Xu et al. used large-sized GO sheets as the building blocks to construct GFs. During the increase of the graphitization temperature from 1300 to 3000 °C, the GFs showed simultaneous increases in the mechanical strength and modulus.[74] Xin et al. adjusted the domain sizes of GFs by using small-sized graphene sheets to fill the space and microvoids produced by large-sized gra-phene sheets.[113] Upon increasing the annealing temperature from 1400 to 2850 °C, the domain sizes in the axial and radial directions were improved substantially from 40–50 to 783 and 423 nm, respectively, which were orders of magnitude larger than those of nanocrystalline CFs. The Young’s modulus was also increased from 20 to 135 GPa. However, the trend of the mechanical strength was different, as it reached the highest value of 1080 GPa at 1700 °C. They further improved the domain size in the radial direction to 612 nm by assembling highly oriented graphene belts.[157]

4.1.5. Density

Carbon-based materials feature advantageous lightness because of their ultralow volume densities. For example, conventional

CFs have a volume density of 1.7–1.9 g cm−3, and ideal graphite crystals present a volume density of 2.2 g cm−3.[8] Using this advantage, to achieve a high physical density is helpful for attaining a high mechanical performance, since a higher physical density contributes to more compact microstructures. GFs directly collected from coagulation baths and low-temper-ature treatment environments have generally exhibited loosely stacked building blocks and volume densities below 1 g cm−3. Some modification to procedures, such as using stretching and high-temperature graphitization treatments, can largely improve the density to >1 g cm−3. GFs composed of pure small-sized graphene sheets are very compact, due to their low porosity. This assists the GFs in achieving a high volume den-sity of 1.8–1.9 g cm−3, approaching that of CFs.[113]

4.1.6. Porosity

The pores in GFs are mainly generated during the post-treatment of partially/completely reduced precursor fibers. During thermal or chemical reduction, the removal of oxygen-containing functional groups causes gaseous produc-tions, such as CO, CO2, and H2O, which inevitably damage the compact graphene layers and weaken the mechanical

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Figure 13. Sheet alignment with different degrees in the solution leading to the microstructure and mechanical property differences between the annealed graphene assemblies. A–C) Reproduced with permission.[157] Copyright 2019, Springer Nature.



performance of the GFs.[52] In the case of CFs, there is a clear transition from substantial small pores to a small amount of large pores with increasing treatment temperatures. Thus, graphitization treatments at high temperatures are likely to control the porosity of GFs. Another method is to fill the large pores between large building blocks by interlacing small-sized graphene sheets, which can effectively reduce the porosity from 35% to 15%. Actually, micropores may help GFs function in some specific materials, which perform fast and efficient adsorption and desorption tasks in gas- and liquid-phase systems, similar to activated CFs. Moderate micropores can also act as a buffer retarding the propagation of a crack.[4]

4.1.7. Size Effect

According to Griffith’s size-scaling law, the mechanical strength (σ) of fibers obeys an inverse relationship with their diametral size (D), σ ∼D−1/2.[167] The background of this quan-titative relationship is that downsizing fibers along the trans-verse direction contributes to the regular orientation of the basic constituent units and the decrease of intrinsic flaws. Xu et al. proposed a relationship of σ ∼D−0.47 for GFs, which con-forms to Griffith’s size scaling theory.[74] This suggests that a smaller size can assist GFs in becoming more mechanically strong. Actually, thin fibers are capable of averting the gen-eration and growth of defects at all levels, including disorder in the radial and axial directions and core–sheath structures, thus showing a higher orientation degree, linear density, and mechanical strength, as displayed in Figure 14A–C.

4.2. Transport Properties

Due to an extremely uniform orientation of graphene building blocks along the planar direction, multilayer graphene sys-tems, such as GFs, exhibit anisotropic conducting properties, in which the heat and electron transport in the axial direction is much higher than that in the radial direction.[168–170] This exceptional property has great prospects for applications in

thermal and electronic management devices. The transport behaviors and properties of neat GFs are inherited from single- or few-layer graphene. For GFs, phonon transport from the lat-tice vibration of the covalent sp2 carbon network dominates the thermal conduction, and a delocalized π-bond over the whole graphene sheet determines the electron transport.[16] However, there is still a large performance gap between today’s neat GFs and ideal graphene. The gap of the transport properties is partly caused by structural defects, such as oxygen-containing groups and lattice vacancies, which become phonon- and electron-scattering centers. Residual functional groups and large lattice defects can be eliminated by graphitization treatment at high temperatures. However, the negative influence of grain bound-aries is insurmountable. Grain edges constrain and block the transport of electrons and phonons between graphene sheets. Finding solutions for the interlayer transport may be an option, but it should be mentioned that the interlayer transport of gra-phene is much weaker than intralayer transport.[171–173] The lateral crystal size is a controllable factor, which can lower the relative amount of grain edges. Since the crystalline size of GFs is much larger than that of CFs, by two orders of magnitude at the most, GFs are expected to present much better transport abilities than those of CFs. Other optimizing strategies, such as injecting extra carrier densities or compositing with other con-ductors, are also available.

4.2.1. Electron Transport

Due to the semimetal characteristics of graphene, neat GFs behave as semiconductors. At the earliest, Xu et al. studied the temperature-dependent conductive behavior of GFs at low temperatures.[146] They found that the electrical conductivity of GFs decreased from 4.1 × 104 S m−1 at 299 K to 1.8 × 104 S m−1 at 5 K. This result was also reproduced by Liu et al.[152] The valid cause of this conducting behavior has not been system-atically investigated, but it is probably ascribed to a hopping mechanism. By using the refining strategies mentioned earlier, the room-temperature electrical conductivities of neat GFs have been improved to 8 × 105 S m−1, which is one order of magni-tude higher than that of PAN-based CFs.

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Figure 14. A) SEM images and 2D SAXS patterns, B) the orientation degree, and C) the corresponding specific strength of GFs with different diameters. A–C) Reproduced with permission.[74] Copyright 2016, Wiley-VCH.



Ideal graphene has a high mobility beyond 200 000 cm2 V−1 s−1, but the carrier density is relatively low. This inspires a solution to improve the electrical ability of GFs. Gao and co-workers added silver nanowires into GO dopes.[146] Shear flow in the spinning tube induced the axial arrangement of the nanowires in the obtained GFs. Finally, the prepared silver-doped GFs showed an electrical conductivity of 9 × 104 S m−1 and a current capacity of 7.1 × 103 A cm−1, which were the highest values at that time (Figure 15A). By choosing different chemical reduc-tion agents, a molecular doping mechanism involving hydroi-odic acid and a mixing mechanism involving vitamin C were proposed. The doping strategy was upgraded to a mole cular level by improving the carrier density (Figure 15B). Using a two-zoned vapor transport method, Liu et al. realized the p-type doping of GFs with donor-type potassium, and n-type doping with acceptor-type FeCl3 and Br2.[152] The doped GFs exhibited an improvement of the carrier density by two orders of mag-nitude, boosting the electrical conductivity to a metallic level (≈2.24 × 107 S m−1). Furthermore, the specific electrical con-ductivity of doped GFs transcended benchmark metals. Ma et al. demonstrated that Br-doped GFs showed highly enhanced thermoelectric properties compared to those of undoped GFs.[174] While doping brought phonon scattering to decrease the thermal conductivity, the increased electrical conductivity and Seebeck coefficient still permitted the enhancement of the thermoelectric properties (Figure 15C). Chemical doping was so effective that an attempt to prepare superconductors from GFs was also reported.[154] Liu et al. doped GFs with calcium, and the resultant intercalated GFs exhibited a superconducting

transition at ≈11 K, which was much higher than that of doped graphite superconductors and approached that of commercial NbTi conductors (Figure 15D). Although chemically doped GFs show favorable performance, their operation stability and weather fastness are not satisfying, since the doped molecules are easily oxidized in air. To address this problem, Fang et al. proposed an electrodeposition method.[175] By replacing atomi-cally dispersed doping molecules in GFs, they obtained highly crystalline and densely stacked metal layers on the surface, which could resist oxidation by active oxygen molecules and sustain long-term operation in air. The electrical conductivi-ties of GFs were largely improved to a metallic level with a thin metal layer, which was quite close to the results in a previously reported work (Figure 15E). Moreover, due to the negative tem-perature coefficient of resistance (TCR) of graphene and the positive TCR of metals, metal–GFs with a zero TCR at a wide temperature range were achieved when the thickness ratio of the GF to the metal layer was 0.2 (Figure 15F).

4.2.2. Heat Conduction

For graphene assemblies, it is widely recognized that the thermal conductivity of multilayer graphene exhibits an almost linear decrement versus the number of graphene layers, in comparison to that of single-layer graphene. The main reason for this effect is that the interlayer interactions and vibra-tional restrictions limit the free vibration of graphene sheets, thus retarding phonon transport.[176] This effect can also be

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Figure 15. A,B) Improvement of the electrical conductivity of GFs by doping with silver nanowires (A) and heteroatoms (B). C) The enhanced thermo-electric properties of chemically doped GFs. D) Superconducting behaviors of GFs doped with calcium atoms. E,F) The high conductivities, flexibilities, and zero temperature coefficients of resistance of metal-coated GFs. A) Reproduced with permission.[146] Copyright 2013, Wiley-VCH. B) Reproduced with permission.[152] Copyright 2016, Wiley-VCH. C) Reproduced with permission.[174] Copyright 2016, Springer Nature. D) Reproduced with permis-sion.[154] Copyright 2017, American Chemical Society. E,F) Reproduced with permission.[175] Copyright 2017, Royal Society of Chemistry.



attributed to the influence of grain boundaries. The reduction of phonon scattering and vibrational restrictions can be addressed by achieving large-sized graphitized crystals in GFs, which facilitate more efficient phonon transport. High-temperature treatment can heal the injured graphene sheets and promote the growth of graphitized crystalline domains. Xin et al. found that the domain sizes of GFs were improved substantially from 40–50 to 783 nm upon increasing the annealing temperature from 1400 to 2850 °C.[113] Thus, the thermal conductivity was also increased from ≈300 to ≈1290 W m−1 K−1. Optimizing the sheet alignment and orientation of GFs could further improve the thermal conductivity to ≈1575 W m−1 K−1.

Wallace and co-workers suggested that the intralayer thermal conductivity should not dominate the heat transport of GFs, since most graphene sheets are tangled and curved during the wet-spinning process.[147] The redundancy of multifunctional groups might enhance the interlayer bonding and limit the vibration of the graphene sheets, thus allowing a higher thermal conductivity of GFs compared to graphite.

4.3. Wrinkles and Hybrid GFs

During wet spinning, the drying of GO gel fibers can be described as a liquid–solid transition in which the GO sheets undergo a compaction process due to capillary force, similar to compacting a piece of paper. Evidence indicates that com-pacting 2D sheets generates wrinkles of varying conforma-tions and the wrinkle texture determines the compactness. As a result, GFs feature rich wrinkles on the surface and folding textures in the sections. With amplitudes ranging from nano-meters to micrometers, multiscale wrinkles can be largely found in loosely stacked GFs. Multiscale wrinkles permit GFs to show favorable flexibility. Chen and Dai used a CVD-assisted strategy to grow graphene sheets on a copper wire.[137] After etching away the copper substrates, they obtained GFs with rich wrinkles and pores. With a high electrical conductivity of 127.3 S cm−1, the GFs exhibited a stable electron transport ability over thousands of bending cycles. Li et al. also employed CVD-assisted technology to fabricate GFs.[136] They removed suspended 2D graphene sheets from solvents, and surface ten-sion induced the direct formation of porous GFs with aggres-sive wrinkles on the surface. Aside from flexibility, multiple wrinkles also contribute to GFs presenting interface bonding with polymers and metals, as well as remarkable electrochem-ical, optoelectrical, and energy-storing properties.

Introducing heteromolecules into GFs can not only optimize their specific properties, but also bring extended functions in a variety of areas. Generally, the introduction of guest mole-cules does not damage the 3D order in hybrid GFs. During the process of fluid assembly, GO LCs can play a templating role to arrange the ordering of hierarchical structures over a long range. The guest molecules have included a wide range of com-positions, extending from ions to organic polymers, oxides, and carbon allotropy. The topology of the guests has extended from 0D quantum dots and 1D polymers or CNTs to 2D colloids. In terms of 0D molecules, Li et al. fabricated a dot–sheet structure by mixing carbon quantum dots with GFs.[140] The dot–sheet microstructures prompted the GFs to show a large specific

surface area of 435.1 m2 g−1 for the induced ionic channels and a high mechanical strength. The aggressive merits made the GFs exhibit a large area specific capacitance of 607 mF cm−2, long-term bending durability, and a high energy density of 67.37 µWh cm−2.

The encompassment of polymer guests in GFs yields a “brick and mortar” layered architecture, resembling the micro-structures of nacre shells. In such a biomimetic fiber, graphene panels act as bricks and polymers function as the mortar. Poly-mers can be individually absorbed or grafted onto graphene sheets. In the first case, polymers improve the strength and toughness of GFs by cross-linking the basal plates or edges of graphene sheets through van der Waals interactions and hydrogen bonding. Hu et al. used sodium alginate (SA) as the guest macromolecule and prepared a GO–SA composite with a strength of 784.9 MPa and a modulus of 58 GPa.[177] Cheng and co-workers cross-linked GO sheets with metal ion and 10,12-pentacosadiyn-1-ol, enhancing the strength and tough-ness of the GFs to 842.6 MPa and 15.8 MJ m−3, respectively.[155] This family of guest polymers can be extended to a variety of species, such as hyperbranched polyglycerol, glutaraldehyde, polyacrylonitrile, and poly(vinyl alcohol) (PVA), which can reinforce graphene-based macroscopic materials with different topologies.[178–186] In the second case, covalent links between graphene sheets are built by direct grafting of guest molecules, including linear and hyperbranched polymers.[187–189] In this polymer–graphene biomimetic system, the mechanical tough-ness can be further heightened by adding dispersed CNTs.[141] Shin et al. combined CNTs and reduced GO sheets in PVA fibers, obtaining a high-performance fiber with a gravimetric toughness approaching 1000 J g−1, which is beyond those of natural spider silk (165 J g−1) and Kevlar fiber (78 J g−1). This improvement can be ascribed to the formation of an intercon-nected network of CNTs and reduced GO sheets, functioning to deflect the propagation of cracks under mechanical failure. Other promotions, such as a specific capacitance enhancement of ≈39% and a superb stretchability of 500%, can also be made in this ternary blending system.[190]

Atomically thin 2D nanosheets, e.g., metal oxide, graphene-like 2D crystals, and clay, show similar surface morphologies to that of graphene, as well as much richer electrical, thermal, and electrochemical properties. Incorporating 2D nanosheets into GFs can, at least, produce three benefits: 1) realizing the macroscopic and continuous assembly of 2D nanosheets, which has become highly achievable after the scalable produc-tion of 2D crystal suspensions and developing the template role of GO LCs; 2) translating the fascinating attributes of 2D crystals into macroscopic materials with remarkable mechan-ical endurance; and 3) remedying the mechanical or trans-port defects of GFs. For example, Hoshide et al. constructed a fibrous lithium-ion battery with graphene/titania composite fibers as current collectors.[143] Fang et al. fabricated a fire-resistant conductor with graphene/montmorillonite (MMT) hybrid fibers.[191] Wan et al. reinforced the fatigue resistance of GFs by introducing tungsten disulfide nanosheets, which played the role of a lubricant.[192] Peng et al. realized the nonenzymatic detection of hydrogen dioxide and glucose by combining graphene and gold nanosheets in a fiber-shaped microelectrode.[193]

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5. Flexible Electronics

Upon combining the increasingly improved mechanical per-formance and transport abilities, neat and hybrid GFs have extensive applications in a myriad of areas. The state-of-the-art applications have been mainly focused on the field of flexible and wearable electronics, by virtue of the electrical conductivity, flexibility, toughness, and strength of GFs.

5.1. Multifunctional Fabrics

Efforts to bring individual GFs to textile grade have led to extensive merits since the inherently scalable production and multifunction advantages could be harnessed. As illustrated in Figure 16, three kinds of fabrics have been manufactured from GFs: nonwoven fabrics, neat GF knitted fabrics, and co-knitted fabrics. Li et al. proposed a wet-fusing assembly technique to fabricate nonwoven GF fabrics, where GO fibers were ran-domly oriented and interfused with each other by strong inter-bonding.[121] This nonwoven bonding was highly porous and lightweight. The electrical conductivity and thermal conductivity reached ≈2.8 × 104 S m−1 and ≈301.5 W m−1 K−1, respectively, and the fabric was used as in ultrafast-responding electro-thermal heaters and durable oil-adsorbing felts (Figure 16A). Electrothermal heaters, especially those for wearable clothing, are of great use in healthcare. Wang et al. employed a twist-spinning method to scroll graphene films into highly stretch-able GFs.[132] The elongation at the tensile breakage of twisted GFs reached up to 70%, with a toughness of 22.45 MJ m−3 and a high electrical conductivity of 6 × 105 S m−1. The ultra-fast heating and cooling rates at a low bias enabled the GFs to work as electrothermal fabrics (Figure 16B). Favorable

wearability is a precondition to the fabrication of GF fabrics. GFs obtained from traditional wet-spinning technology usually suffer from a poor flexibility, due to the regular axial orienta-tion and high compactness of graphene sheets. To resolve this problem, Seyedin et al. designed a dry-jet wet-spinning method, leaving an air gap between the as-extruded GO solutions and the coagulation bath.[194] This air gap effectively released the constrained GO sheets, overcoming the skin effect. As a result, the obtained GO fibers exhibited a modulus of ≈7.9 GPa, a strength of ≈135.8 MPa, a tensile strain of ≈5.9%, and a tough-ness of ≈5.7 MJ m−3. Such good mechanical properties allowed the GO fibers to be cowoven with commercial nylon fibers (Figure 16C). The textiles woven from the individual fibers are limited by the weak interactions between fiber units. Addition-ally, applications of GF fabrics in other fields, such as in energy storage textiles, thermal management devices, transparent con-ductors, and wearable electronics, have also been explored.

5.2. Power Cables

A basic function of electrically conductive fibers is to transmit power with high efficiency and low cost. Benefitting from much lower physical density than those of metal conductors and much higher electrical conductivity than those of graphite or carbon fibers, GFs or modified GFs are ahead of the compe-tition among traditional conductors. Moreover, modified GFs with specific conducting behaviors are able to work at different temperatures from ≈0 to 650 K (Figure 17). At temperatures below 11 K, Ca-doped GFs can work as superconductors,[154] and metal-coated GFs show near-zero TCR coefficients from 15 to 300 K.[175] In 2013, Xu et al. composited reduced GOFs with silver nanowires, improving the electrical conductivity of the

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Figure 16. GF-based nonwoven and woven fabrics. A) Preparation of nonwoven fabric from GO solution via a wet-fusing assembly approach. B) Wear-able heater fabricated by twisting graphitized graphene films. C) The knittability of GO fibers, which are co-knitted with a commercial nylon yarn with multiple strands. A) Reproduced with permission.[121] Copyright 2016, The Authors, published by Springer Nature. B) Reproduced with permission.[132] Copyright 2015, Wiley-VCH. C) Reproduced with permission.[194] Copyright 2015, Springer Nature.



GFs by 3–15 times.[146] The integration of mechanical robust-ness and high conductivity rendered the doped GFs with the ability to act as stretchable conductors, which were used in soft circuits. Through a full-scale synergetic defect engineering pro-tocol, Xu et al. boosted the electrical conductivity of neat GFs to 8 × 105 S m−1, as well as the tensile strength to 2.2 GPa, and the modulus to 282 GPa.[74] Such high electron transportation and mechanical robustness facilitate the application of GFs in lightweight rotator coils to assist in the operation of electrical motors. By replacing copper wires, GFs promoted a motor to rotate at a high speed of ≈350 rpm at a bias of 8 V. Sub-sequently, Liu et al. enhanced the electrical conductivities of GFs by a chemical doping method. The high-strength electron transportation enables direct uses in practical applications.[152] The researchers used 1 m long Br-doped GF filaments to replace traditional copper conductors in a lamp. The lamp worked well for a long time at a standard alternating voltage. Carbon atoms are easily attacked and damaged by oxygen in high-temperature air. To ensure the operation of GFs at high temperatures, Fang et al. enhanced the fire-resistance ability of GFs by adding MMT nanosheets.[191] MMT nanosheets are highly thermostable, protecting graphene sheets from the action of oxidative etching. Thus, GFs could work as light-weight and fire-resistant conductors in high-temperature air.

5.3. Energy Harvesters

Energy harvesters are important energy conversion devices that harvest electrical energy from thermal energy, mechanical strain energy, light, and other sources. Generally, the conversion in energy harvesters occurs through the response of energy har-vesters to some stimulus. Pure graphene is difficult to transform

under outside stimuli. However, graphene derivatives, such as GO, are capable of generating responses. Cheng et al. twisted GO gel fibers into TGFs with helical microstructures.[129] In this process, a large strain energy was stored (Figure 18A). The TGFs released the strain energy by interacting with hydrophilic oxygen functional groups. As a result, they showed a rapid revo-lution of 5190 rpm when exposed to moisture (Figure 18B). This actuation was strong. The peak power output reached 71.9 W kg−1, bringing an open-circuit voltage of 1 mV and a short-circuit current of 40 µA upon connecting the TGFs with an electromagnetic induction device (Figure 18C). Actually, TGFs could also respond to many other solvents. Fang et al. discovered that many polar solvents could drive TGFs to yield a fast response, such as acetone, methanol, and ethanol.[133] Acetone-enabled TGFs were used to provide a large start-up torque of 2.7 × 10−7 N m and a record rotor kinetic power of 89.3 W kg−1. Furthermore, the energy harvesting of TGFs could be precisely controlled by combining different-handed units together (Figure 18D). Upon programming the arrangement of handed TGFs, the electrical power output obtained was from 9.8 to 3 W kg−1, with an adjustable frequency from 10 to 5 Hz.

5.4. Wearable Supercapacitors

Supercapacitors are promising energy storage devices for use in miniaturized flexible electronics, which feature high capaci-tance, fast charge–discharge rates, and reliable cycling stability. The rapid development of science and technology requires supercapacitors to have the merits of portable operation, light-ness, and being easy to wear. One way to realize these prop-erties is to use fibrous conductors as the electrodes.[195–206] Achievements in the employment of modified GFs as electrodes

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Figure 17. GFs as high-performance power lines operating at different temperatures. The image of Ca-doped GFs is reproduced with permission.[154] Copyright 2017, American Chemical Society. The picture of metal-coated GFs is reproduced with permission.[175] Copyright 2017, Royal Society of Chem-istry. The picture of GFs bundles is reproduced with permission.[74] Copyright 2016, Wiley-VCH. The picture of the Br-doped GFs bundle is reproduced with permission.[152] Copyright 2016, Wiley-VCH. The photograph of MMT-GFs is reproduced with permission.[191] Copyright 2015, American Chemical Society.



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have pushed wearable supercapacitors to a user-friendly level (Figure 19A).[207,208] Huang et al. directly used two as-prepared GFs in parallel as the electrodes.[209] This supercapacitor rep-resents the first type in Figure 19A, exhibiting an area-specific capacity of 3.3 mF cm−2. Meng et al. grew a layer of a porous gra-phene framework on GF, constructing a core–sheath GF@3D-G structure.[210] Due to the high-exposed surface areas, such struc-tures rendered the GF@3D-G-based supercapacitors capable of exhibiting an area-specific capacitance of 1.2–1.7 mF cm−2. More importantly, the excellent flexibility and fibrous shape enabled them to be woven into textiles. After coating with gel electrolytes (the second type in Figure 19A), a GF@3D-G-based supercapacitor sustained unchanged performance under bending tests (Figure 19B). Yu et al. ameliorated the micro-structures of GFs by combining aligned CNTs with interposed N-doped reduced GO sheets.[211] Such hierarchical design induced the fibers to form an interconnected mesoporous struc-ture, exhibiting a large specific surface area of 396 m2 g−1 and a high electrical conductivity of 10 200 S m−1. Upon loading the fibers into a microsupercapacitor, high specific volumetric capacities of 305 F cm−3 in sulfuric acid and 300 F cm−3 in a PVA/H3PO4 electrolyte were achieved. During the progress of fibrous supercapacitors, a crisis has emerged since most reported GFs were naked, which could result in short-circuiting operation. Kou et al. proposed a coaxial wet-spinning assembly

strategy to fabricate polyelectrolyte-coated GFs.[139] The polymer coatings acted as both the electrolyte and the protecting layer, guarding the GF-based supercapacitors to allow them to work safely, as well as providing excellent energy storage properties and wearability (the third type in Figure 19A,C). Through a similar strategy, Cai et al. prepared a ternary coaxial fiber by con-structing a graphene/CNT/poly(3,4-ethylenedioxythiophe ne): poly(styrenesulfonate) (PEDOT:PSS) core and a carboxymethyl cellulose sheath.[212] The concerted effort of this ternary system made the coaxial fibrous supercapacitors exhibit an advanced area specific capacitance of 396.7 mF cm−2 at 0.1 mA cm−2 and a high energy density of 13.8 µWh cm−2.

5.5. Flexible Batteries

Fibrous batteries are another major application area of GFs in the field of energy storage.[213–224] Chong et al. prepared a fibrous lithium–sulfur battery using composite GFs.[225] Using the wet-spinning method, they fabricated a composite fiber containing graphene, CNTs, and sulfur, which acted as a posi-tive fiber. Lithium fiber was used as the negative electrode (Figure 20A). After adding a separating layer in the middle, the positive and negative fibers were compounded together and sealed in a plastic pipe to obtain a fibrous lithium–sulfur

Figure 18. GFs act as energy harvesters by converting rotor kinetic energy into electrical energy. A–C) Moisture-driven GF motor. D) Handedness-con-trolled energy harvesting system under the driving of polar solvents. A–C) Reproduced with permission.[129] Copyright 2014, Wiley-VCH. D) Reproduced with permission.[133] Copyright 2019, Royal Society of Chemistry.



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Figure 19. A) Three types of GF-based supercapacitors. Initially, GFs were directly used as electrodes, and were separated with an electrolyte. In today’s recognized fibrous supercapacitors, a gel electrolyte is coaxially coated onto the GF electrodes. B) Textile-grade supercapacitor embedded with copper wires and GFs, which maintains a stable performance under flat and bending conditions. A,B) Reproduced with permission.[210] Copyright 2013, Wiley-VCH. C) Supercapacitor device based on coaxial fibers prepared from GFs and poly(methyl methacrylate), exhibiting unchanged electrochemical curves with different bending angles. Reproduced with permission.[139] Copyright 2014, Springer Nature.



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battery. Its initial capacity reached 1255 mAh g−1, with a spe-cific capacity of 2.49 mAh cm−2 at C/20, and the discharge performance remained stable after 30 cyclic bending tests (Figure 20B). Yang et al. electroplated platinum nanoparticles onto the surface of GFs, which worked as a pair of electrodes. Titanium fibers coated with vertically arranged titanium dioxide nanotubes functioned as working electrodes.[226] The batteries not only captured incident photons from any angle, but also exhibited high flexibility, mechanical strength, and electrical conductivity (Figure 20C,D). They realized an energy conver-sion efficiency of 8.45% using GF cells, which was the highest value on record for a fibrous photovoltaic device at the time. Hoshide et al. used a similar wet-spinning method to pre-pare fibrous lithium-ion batteries.[143] The uniqueness of this strategy is that the cathode active material is layered titanium dioxide. After exfoliating the titanium dioxide into nanosheet dispersions, and subjecting to wet spinning with GO, they obtained a composite fiber constructed from pure 2D crystals, in which the reduced GO and titanium dioxide nanosheets were self-assembled layer by layer. Using composite fibers as anode materials brings two advantages: the substantial loading of the active material and close contact between the active materials and the positive collector fluid. After winding the composite fibers with lithium fibers, they fabricated a fibrous lithium-ion battery with a high capacity and cycling stability.

5.6. Sensing Devices

If the elongation of GFs can be improved to some extent, they are suitable for use as flexible resistance sensors. In 2014,

Cheng et al. wrapped a gel-like GOF on a glass rod to form a helical texture, which then formed a helical GF spring after air drying and chemical reduction.[227] The elongation of the GF spring reached 480%, with a strong mechanical durability. They doped the GF spring with ferric oxide, a magnetic par-ticle, which allowed the hybrid GFs to be controlled to stretch and compress in a magnetic field. This magnetically con-trolled strain sensor can be applied in magnetically controlled circuits. Li and co-workers used the technology of 3D printing to design a cross-linking network of GFs.[228] In this process, the mutual penetration of GO sheets caused the formation of fused junctions. This fusion structure allowed the GF network to maintain structural integrity under large deformations, and to further function as a stress sensor. In 2015, Hu et al. used CVD technology to grow a graphene layer on copper wires, and then etched copper to obtain neat GFs.[229] They coated GF with PVA to obtain a composite fiber with a core–sheath structure. The introduction of polymers largely improved the stretchability of the GF. The elongation at break exceeded 15% when the content of PVA reached 10%. In a cyclic bending and stretching test, the resistance change rate of the composite fiber maintained a constant value, which could be accurately regu-lated by the values of the bending radius and tensile strain.

5.7. Neural Recording Microelectrodes

Recording the neural activity of humans is of great importance in healthcare. To realize effective bidirectional communica-tion between a neural system and a machine, it is necessary to develop a cheap microelectrode with the following characteristics:

Figure 20. GFs as flexible batteries. A) Lithium–sulfur cable battery prepared from graphene/CNT/sulfur composite fiber and lithium wire. B) Power changes of a cable battery under cyclic bending tests. A,B) Reproduced with permission.[225] Copyright 2017, Wiley-VCH. C) Dye-sensitized photovoltaic cell prepared from graphene/Pt composite fiber and Ti wire. C,D) Independence of the energy conversion efficiency from the incident angle for the wire cell. C,D) Reproduced with permission.[226] Copyright 2013, Wiley-VCH.



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microscale size comparable to that of individual neurons (<50 µm), low impedance, high surface area, high charge injection capacity, high flexibility, high strength, and agreeable biocompatibility. GF is a suitable candidate. The diameter of GF can be controlled to below 30 µm by adjusting the wet-spinning apparatus. The elec-tronic conductivities can be boosted to a metallic level through sev-eral available methods. Wrinkles in the designs allow GF to exhibit a very high surface area and high flexibility, further matching the morphology of human tissues. The mechanical strength of GF has also been promoted to 2 GPa. Wallace and co-workers employed the favorable attributes of GF to obtain a high-performance micro-electrode to record and detect neuronal activity.[230] They coated polymers and Pt onto the surface to reduce the impedance and enhance the biocompatibility of GF (Figure 21A). The modified GF showed much better charge injection capacity, lower specific impedance, and larger geometrical area than those of previously reported microfibers (Figure 21B). Using the modified GFs as microelectrodes, they conducted in vivo cortical neural recording experiments (Figure 21C,D). The GF microelectrodes were dem-onstrated to reach a high signal-to-noise ratio of 9.2 dB, success-fully transmitting the signals of rat neurons.

6. Prospects

From graphite to CFs, CNTs, CNT fibers, graphene, and today’s GFs, renewed focus on the development of new carbon

materials is persisting, and efforts to translate the new dis-covered properties of carbon into macroscopic structures are never-ending. Through our retrospective analysis of the evo-lution of GFs, we can conclude that their transport properties have been well developed. However, there is still a lot of room for the development of the mechanical properties. In the past few years, some available clues bridging the gap between theo-retical and practically realizing mechanical performance have emerged. These may be summarized as follows:

1. Tailoring the rheological behavior of GO: As a kind of aniso-tropic nanoparticle and non-Newton fluid, GO exhibits obvi-ous shear thinning characteristics under shear flow. This flu-id behavior probably gives rise to the irregular arrangement of the building blocks and the emergence of the skin effect, which reduce the mechanical properties of GFs. Although some solutions, such as the design of a contracted flow, and matching the geometries of the GO sheets and spinning channel, have been suggested, great effort is needed to final-ize the optimal project matching between the wet-spinning apparatus and shear thinning behaviors of GO dopes.

2. Reducing disorder and wrinkles: The distortion of graphene sheets brings disorder and wrinkles within GFs, which lead to the compromise of the mechanical performance. To ad-dress the production of disorder and wrinkles of GFs and perfect the manufacturing flow, the tight control of key at-tributes introduced in Section 2.1.4 is needed. Additionally,

Figure 21. GF-based microelectrodes for neural recording. A) Schematic presenting the fabrication of a GF–Pt microelectrode, and the intracortical implantation process. B) Comparison of the charge injection capacity, specific impedance, and geometrical area of the microelectrodes with those of previously reported neural interfacing electrodes. C) Optical photograph of an inserted GF-based microelectrode showing the in vivo cortical neural recording. D) Signals obtained from the implanted microelectrodes. A–D) Reproduced with permission.[230] Copyright 2019, Wiley-VCH.



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poststretching is effective in suppressing the growth of wrinkles and improving the physical densities. Previously reported work has generally applied one or two rounds of room-temperature poststretching during the wet spinning of GOFs. Applying more rounds of poststretching and exerting a tensile load of sufficient magnitude in the high-tempera-ture graphitization stage may be conductive to reducing the disorder and wrinkles in GFs.

3. Scalable fabrication of GF strands: The fabrication of GF strands is conductive to boosting the state-of-the-art ap-plications of GFs to a promising industrial scale. As intro-duced in Section 2.1.3, LC wet spinning is easy to scale up by preparing strands using multifilament (up to 100-fila-ment) spinning nozzles. However, GF strands still face some challenges when starting the journey, as follows: i) in guaran-teeing persistent continuity and superb mechanical perfor-mance of every filament without breakage; ii) in maintaining the mutual separation of individual GF filaments to conquer the self-fusion behavior of GOFs in wet-spinning production; and iii) in avoiding adhesion to the substrate when collecting wet GOF filaments. Successful examples of wet-spun poly-mer fibers can be used for reference. For example, adding surfactants, such as oily solvents, into coagulation baths may help overcome the self-fusion and interface adhesion of GOF filaments.

With the increasingly explicit correlations between struc-ture and performance, and the exceptional mechanical and transport properties, GF has emerged as an important func-tional material.[231] To extend the application fields of GFs from electronics to broad domains, it is necessary to begin the study of the compressive strength, a factor used to evaluate the

anti-fatigue ability of materials under compression. In carbon materials, the compression strength is dominated by the shear slippage between graphite crystals. The high compression strength of CFs promotes their substantial applications in CF composites and other structural–functional integrated fields. Given their much larger crystalline size and optimized micro-structures compared to those of CFs, GFs might have a high compression strength, which, if it is not so good, can be easily improved by interlayer cross-linking. Some other unestablished fields, including use with phonons, optoelectronics, and elec-tromagnetics, also wait to be developed. Attempts to translate the fascinating properties of few-layer graphene into macro-scopic GFs have yielded a new breed of carbon materials with various properties. In terms of science, achievements in this progress may inspire the research of advanced GFs and other 2D crystals, guiding the creation of a new discipline. In terms of engineering, the marvelous and multifaceted performance of GFs promotes technological refreshing in many areas, and finds promising structural–functional integrated applications in many fields.

As displayed in Figure 22, GFs are ideally suited for applica-tions in which strength, thermal conductivity, stiffness, light-weight, and reinforcement are critical requirements. These applications comprise general engineering and transportation tools, including ships and vehicles, and aerospace engineering instruments, including airplanes, rockets, and satellites. In addition to the concerns of mechanical and functional issues, the merits of flexibility, transport properties, and multifunc-tionality make GFs able to be used in the clothing industry. For example, the superb electrical conductivities allow GF-based fabrics to play a part in healthcare, through specific properties such as electromagnetic shielding, Joule heating, and antistatic

Figure 22. GFs for structural–functional integrated applications in the future.



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electricity. Wearable energy storage devices, such as batteries and supercapacitors, will also receive great attention. Attempts to develop photoelectric properties enable GFs to function as wearable optoelectronics and even to be used to create fabric-based communications.

AcknowledgementsB.F. and D.C. contributed equally to this work. This work was supported by the National Natural Science Foundation of China (nos. 51533008 and 51703194), the National Key R&D Program of China (no. 2016YFA0200200), the Fundamental Research Funds for the Central Universities (no. 2017XZZX008-06), the Hundred Talents Program of Zhejiang University (188020*194231701/113), the Key Research and Development Plan of Zhejiang Province (2018C01049), the Fujian Provincial Science and Technology Major Projects (no. 2018HZ0001-2), and the Foundation of National Key Laboratory on Environment Effects (no. 614220504030717).

Conflict of InterestThe authors declare no conflict of interest.

Keywordscarbonaceous fibers, flexible electronics, graphene fibers, macroscopic assmebly, structural–functional integrated uses

Received: April 25, 2019Revised: May 31, 2019

Published online:

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