Towards the Knittability of Graphene Oxide Fibres

Multiscale Structural Design of 2D Nanomaterials-based Flexible Electrodes for Wearable Energy Storage Applications

2D nanomaterials play a critical role in realizing high-performance flexible electrodes for wearable energy storge devices, owing to their merits of large surface area, high conductivity and high strength. The electrode is a complex system and the performance is determined by multiple and interrelated factors including the intrinsic properties of materials and the structures at different scales from macroscale to atomic scale. Multiscale design strategies have been developed to engineer the structures to exploit full potential and mitigate drawbacks of 2D materials. Analyzing the design strategies and understanding the working mechanisms are essential to facilitate the integration and harvest the synergistic effects. This review summarizes the multiscale design strategies from macroscale down to micro/nano-scale structures and atomic-scale structures for developing 2D nanomaterials-based flexible electrodes. It starts with brief introduction of 2D nanomaterials, followed by analysis of structural design strategies at different scales focusing on the elucidation of structure-property relationship, and ends with the presentation of challenges and future prospects. This review highlights the importance of integrating multiscale design strategies. Finding from this review may deepen the understanding of electrode performance and provide valuable guidelines for designing 2D nanomaterials-based flexible electrodes.

High Toughness Combined with High Strength in Oxide Ceramic Nanofibers

Traditional oxide ceramics are inherently brittle and highly sensitive to defects, making them vulnerable to failure under external stress. As such, endowing these materials with high strength and high toughness simultaneously is crucial to improve their performance in most safety-critical applications. Fibrillation of the ceramic materials and further refinement of the fiber diameter, as realized by electrospinning, are expected to achieve the transformation from brittleness to flexibility owing to the structural uniqueness. Currently, the synthesis of electrospun oxide ceramic nanofibers must rely on an organic polymer template to regulate the spinnability of the inorganic sol, whose thermal decomposition during ceramization will inevitably lead to pore defects, and seriously weaken the mechanical properties of the final nanofibers. Here, a self-templated electrospinning strategy is proposed for the formation of oxide ceramic nanofibers without adding any organic polymer template. An example is given to show that individual silica nanofibers have an ideally homogeneous, dense, and defect-free structure, with tensile strength as high as 1.41 GPa and toughness up to 34.29 MJ m-3 , both of which are far superior to the counterparts prepared by polymer-templated electrospinning. This work provides a new strategy to develop oxide ceramic materials that are strong and tough.

PET/Graphene Nanocomposite Fibers Obtained by Dry-Jet Wet-Spinning for Conductive Textiles

The combination of polyethylene terephthalate (PET), one of the most used polymers in the textile industry, with graphene, one of the most outstanding conductive materials in recent years, represents a promising strategy for the preparation of conductive textiles. This study focuses on the preparation of mechanically stable and conductive polymer textiles and describes the preparation of PET/graphene fibers by the dry-jet wet-spinning method from nanocomposite solutions in trifluoroacetic acid. Nanoindentation results show that the addition of a small amount of graphene (2 wt.%) to the glassy PET fibers produces a significant modulus and hardness enhancement (≈10%) that can be partly attributed to the intrinsic mechanical properties of graphene but also to the promotion of crystallinity. Higher graphene loadings up to 5 wt.% are found to produce additional mechanical improvements up to ≈20% that can be merely attributed to the superior properties of the filler. Moreover, the nanocomposite fibers display an electrical conductivity percolation threshold over 2 wt.% approaching ≈0.2 S/cm for the largest graphene loading. Finally, bending tests on the nanocomposite fibers show that the good electrical conductivity can be preserved under cyclic mechanical loading.

Fibre electronics: towards scaled-up manufacturing of integrated e-textile systems

The quest for a close human interaction with electronic devices for healthcare, safety, energy and security has driven giant leaps in portable and wearable technologies in recent years. Electronic textiles (e-textiles) are emerging as key enablers of wearable devices. Unlike conventional heavy, rigid, and hard-to-wear gadgets, e-textiles can lead to lightweight, flexible, soft, and breathable devices, which can be worn like everyday clothes. A new generation of fibre-based electronics is emerging which can be made into wearable e-textiles. A suite of start-of-the-art functional materials have been used to develop novel fibre-based devices (FBDs), which have shown excellent potential in creating wearable e-textiles. Recent research in this area has led to the development of fibre-based electronic, optoelectronic, energy harvesting, energy storage, and sensing devices, which have also been integrated into multifunctional e-textile systems. Here we review the key technological advancements in FBDs and provide an updated critical evaluation of the status of the research in this field. Focusing on various aspects of materials development, device fabrication, fibre processing, textile integration, and scaled-up manufacturing we discuss current limitations and present an outlook on how to address the future development of this field. The critical analysis of key challenges and existing opportunities in fibre electronics aims to define a roadmap for future applications in this area.

Development and Applications of MXene-Based Functional Fibers

The increasing interest toward wearable and portable electronic devices calls for multifunctional materials and fibers/yarns capable of seamless integration with everyday textiles. To date, one particular gap inhibiting the development of such devices is the production of robust functional fibers with improved electronic conductivity and electrochemical energy storage capability. Recent efforts have been made to produce functional fibers with 2D carbides known as MXenes to address these demands. Ti₃C₂T_x MXene, in particular, is known for its metallic conductivity and high volumetric capacitance, and has shown promise for fibers and textile-based devices when used either as an additive, coating or the main fiber component. In this spotlight article, we highlight the recent exciting developments in our diverse efforts to fabricate MXene functionalized fibers, along with a critical evaluation of the challenges in processing, which directly affect macroscale material properties and the performance of the subsequent prototype devices. We also provide our assessment of observed and foreseen challenges of the current manufacturing methods and the opportunities arising from recent advances in the development of MXene fibers and paving future avenues for textile design and practical use in advanced applications.

Conducting materials as building blocks for electronic textiles

ABSTRACT

To realize the full gamut of functions that are envisaged for electronic textiles (e-textiles) a range of semiconducting, conducting and electrochemically active materials are needed. This article will discuss how metals, conducting polymers, carbon nanotubes, and two-dimensional (2D) materials, including graphene and MXenes, can be used in concert to create e-textile materials, from fibers and yarns to patterned fabrics. Many of the most promising architectures utilize several classes of materials (e.g., elastic fibers composed of a conducting material and a stretchable polymer, or textile devices constructed with conducting polymers or 2D materials and metal electrodes). While an increasing number of materials and devices display a promising degree of wash and wear resistance, sustainability aspects of e-textiles will require greater attention.

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

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.

1D Supercapacitors for Emerging Electronics: Current Status and Future Directions

1D supercapacitors (SCs) have emerged as promising candidates to power emerging electronics in recent years because of their unique advantages in energy storage and mechanical flexibility. There are four main research fronts in the development of 1D SCs: 1) enhancing mechanical characteristics, 2) achieving superior electrochemical performance, 3) enabling multiple device integration, and 4) demonstrating multifunctionality. Here, a brief history of 1D SCs is presented and significant research achievements regarding the four fronts identified as the main pillars of the development of 1D SCs are highlighted. The current challenges of the fabrication and utilization of 1D SCs are critically examined and potential solutions are analyzed. Plus, the performance inconsistencies arising from the improper use and extreme diversity of performance evaluation and reporting methods are highlighted. Beyond, perspectives on future efforts are provided and goals regarding the four research fronts are set, to further push 1D SCs toward practical applications. The development of 1D SCs is summarized here, with existing obstacles diagnosed, corresponding solutions proposed, and future directions indicated accordingly.

Janus Graphene Liquid Crystalline Fiber with Tunable Properties Enabled by Ultrafast Flash Reduction

Flash photothermal treatment via Xenon lamp with a broad wavelength spectrum can effectively remove oxygen functionalities and restore sp² domains at graphitic carbon materials. The chemical composition and relevant structure formation of flash reduced graphene oxide liquid crystal (GOLC) fibers are investigated in accordance with flash irradiation conditions. Owing to the spatial controllability of reduction level via anisotropic flash irradiation, the mechanical properties and electrical conductivity of graphene fibers can be delicately counterbalanced to attain desired properties. High sensitivity humidity sensors can be fabricated from the flash reduced fibers demonstrating notably higher sensitivity over the thermally reduced counterparts. This ultrafast flash reduction holds great promise for multidimensional macroscopic GO based structures, enabling a wide range of potential applications, including textile electronics and wearable sensors.

Directed self-assembly of silica nanoparticles in ionic liquid-spun cellulose fibers

The application range of man-made cellulosic fibers is limited by the absence of cost- and manufacturing-efficient strategies for anisotropic hierarchical functionalization. Overcoming these bottlenecks is therefore pivotal in the pursuit of a future bio-based economy. Here, we demonstrate that colloidal silica nanoparticles (NPs), which are cheap, biocompatible and easy to chemically modify, enable the control of the cross-sectional morphology and surface topography of ionic liquid-spun cellulose fibers. These properties are tailored by the silica NPs' surface chemistry and their entry point during the wet-spinning process (dope solution ^DSiO₂ or coagulation bath ^CSiO₂). For ^CSiO₂-modified fibers, the coagulation mitigator dimethylsulphoxide allows for controlling the surface topography and the amalgamation of the silica NPs into the fiber matrix. For dope-modified fibers, we hypothesize that cellulose chains act as seeds for directed silica NP self-assembly. This results for ^DSiO₂ in discrete micron-sized rods, homogeneously distributed throughout the fiber and for glycidoxy-surface modified ^DSiO₂@GLYEO in nano-sized surface aggregates and a cross-sectional core-shell fiber morphology. Furthermore, the dope-modified fibers display outstanding strength and toughness, which are both characteristic features of biological biocomposites.

Facile Solution Processing of Stable MXene Dispersions towards Conductive Composite Fibers

2D transition metal carbides and nitrides called "MXene" are recent exciting additions to the 2D nanomaterials family. The high electrical conductivity, specific capacitance, and hydrophilic nature of MXenes rival many other 2D nanosheets and have made MXenes excellent candidates for diverse applications including energy storage, electromagnetic shielding, water purification, and photocatalysis. However, MXene nanosheets degrade relatively quickly in the presence of water and oxygen, imposing great processing challenges for various applications. Here, a facile solvent exchange (SE) processing route is introduced to produce nonoxidized and highly delaminated Ti3C2T x MXene dispersions. A wide range of organic solvents including methanol, ethanol, isopropanol, butanol, acetone, dimethylformamide, dimethyl sulfoxide, chloroform, dichloromethane, toluene, and n-hexane is used. Compared to known processing approaches, the SE approach is straightforward, sonication-free, and highly versatile as multiple solvent transfers can be carried out in sequence to yield MXene in a wide range of solvents. Conductive MXene polymer composite fibers are achieved by using MXene processed via the solvent exchange (SE) approach, while the traditional redispersion approach has proven ineffective for fiber processing. This study offers a new processing route for the development of novel MXene-based architectures, devices, and applications.

2D Crystal-Based Fibers: Status and Challenges

2D crystals are emerging new materials in multidisciplinary fields including condensed state physics, electronics, energy, environmental engineering, and biomedicine. To employ 2D crystals for practical applications, these nanoscale crystals need to be processed into macroscale materials, such as suspensions, fibers, films, and 3D macrostructures. Among these macromaterials, fibers are flexible, knittable, and easy to use, which can fully reflect the advantages of the structure and properties of 2D crystals. Therefore, the fabrication and application of 2D crystal-based fibers is of great importance for expanding the impact of 2D crystals. In this Review, 2D crystals that are successfully prepared are overviewed based on their composition of elements. Subsequently, methods for preparing 2D crystals, 2D crystals dispersions, and 2D crystal-based fibers are systematically introduced. Then, the applications of 2D crystal-based fibers, such as flexible electronic devices, high-efficiency catalysis, and adsorption, are also discussed. Finally, the status-of-quo, perspectives, and future challenges of 2D crystal-based fibers are summarized. This Review provides directions and guidelines for developing new 2D crystal-based fibers and exploring their potentials in the fields of smart wearable devices.

Hybrid carbon nanostructured fibers: stepping stone for intelligent textile-based electronics

The journey of smart textile-based wearable technologies first started with the attachment of sensors to fabrics, followed by embedding sensors in apparels. Presently, garments themselves can be transformed into sensors, which demonstrates the tremendous growth in the field of smart textiles. Wearable applications demand flexible materials that can withstand deformation for their practical use on par with conventional textiles. To address this, we explore the potential reasons for the enhanced performance of wearable devices realized from the fabrication of carbon nanostructured fibers with the use of graphene, carbon nanotubes and other two-dimensional materials. This review presents a brief introduction on the fabrication strategies to form carbon-based fibers and the relationship between their properties and characteristics of the resulting materials. The likely mechanisms of fiber-based electronic and storage devices, focusing mainly on transistors, nano-generators, solar cells, supercapacitors, batteries, sensors and therapeutic devices are also presented. Finally, the future perspectives of this research field of flexible and wearable electronics are discussed. The present study supplements novel ideas not only for beginners aiming to work in this booming area, but also for researchers actively engaged in the field of fiber-based electronics, dealing with advanced electronics and wide range of functionalities integrated into textile fibers.

Highly Conductive Ti3 C2 Tx MXene Hybrid Fibers for Flexible and Elastic Fiber-Shaped Supercapacitors

Fiber-shaped supercapacitors (FSCs) are promising energy storage solutions for powering miniaturized or wearable electronics. However, the scalable fabrication of fiber electrodes with high electrical conductivity and excellent energy storage performance for use in FSCs remains a challenge. Here, an easily scalable one-step wet-spinning approach is reported to fabricate highly conductive fibers using hybrid formulations of Ti₃ C₂ T_x MXene nanosheets and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate. This approach produces fibers with a record conductivity of ≈1489 S cm^-1 , which is about five times higher than other reported Ti₃ C₂ T_x MXene-based fibers (up to ≈290 S cm^-1 ). The hybrid fiber at ≈70 wt% MXene shows a high volumetric capacitance (≈614.5 F cm^-3 at 5 mV s^-1 ) and an excellent rate performance (≈375.2 F cm^-3 at 1000 mV s^-1 ). When assembled into a free-standing FSC, the energy and power densities of the device reach ≈7.13 Wh cm^-3 and ≈8249 mW cm^-3 , respectively. The excellent strength and flexibility of the hybrid fibers allow them to be wrapped on a silicone elastomer fiber to achieve an elastic FSC with 96% capacitance retention when cyclically stretched to 100% strain. This work demonstrates the potential of MXene-based fiber electrodes and their scalable production for fiber-based energy storage applications.

Dynamic assembly of liquid crystalline graphene oxide gel fibers for ion transport

Colloidal dispersions with liquid crystallinity hold great promise for fabricating their superstructures. As an example, when graphene oxide (GO) sheets are assembled in the liquid crystalline state, they can turn into ordered macroscopic forms of GO such as fibers via the wet spinning process. Here, we report that by reinforcing intersheet interactions, GO liquid crystals (LCs) turn into mechanically robust hydrogels that can be readily drawn into highly aligned fibrillar structures. GO hydrogel fibers with highly aligned sheets (orientation factor, f = 0.71) exhibit more than twice the ionic conductivity compared to those with partially aligned structures (f = 0.01). The hierarchically interconnected two-dimensional nanochannels within these neatly aligned GOLC hydrogel fibers may facilitate controlled transport of charge carriers and could be potentially explored as cables for interconnecting biosystems and/or human-made devices.

High-Performance Biscrolled MXene/Carbon Nanotube Yarn Supercapacitors

Yarn-shaped supercapacitors (YSCs) once integrated into fabrics provide promising energy storage solutions to the increasing demand of wearable and portable electronics. In such device format, however, it is a challenge to achieve outstanding electrochemical performance without compromising flexibility. Here, MXene-based YSCs that exhibit both flexibility and superior energy storage performance by employing a biscrolling approach to create flexible yarns from highly delaminated and pseudocapacitive MXene sheets that are trapped within helical yarn corridors are reported. With specific capacitance and energy and power densities values exceeding those reported for any YSCs, this work illustrates that biscrolled MXene yarns can potentially provide the conformal energy solution for powering electronics beyond just the form factor of flexible YSCs.

Graphene oxide liquid crystals: a frontier 2D soft material for graphene-based functional materials

Graphene, despite being the best known strong and electrical/thermal conductive material, has found limited success in practical applications, mostly due to difficulties in the formation of desired large-scale highly organized structures. Our discovery of a liquid crystalline phase formation in graphene oxide dispersion has enabled a broad spectrum of highly aligned graphene-based structures, including films, fibers, membranes, and mesoscale structures. In this review, the current understanding of the structure-property relationship of graphene oxide liquid crystals (GOLCs) is overviewed. Various synthetic methods and parameters that can be optimized for GOLC phase formation are highlighted. Along with the results from different characterization methods for the identification of the GOLC phases, the typical characteristics of different types of GOLC phases introduced so far, including nematic, lamellar and chiral phases, are carefully discussed. Finally, various interesting applications of GOLCs are outlined together with the future prospects for their further developments.

Dry spinning approach to continuous graphene fibers with high toughness

Graphene fiber (GF) has emerged as a new carbonaceous fiber species since graphene-based liquid crystals were discovered. The growing performances of GFs in terms of their mechanical performance and their functionalities have assured their extensive applications in structural materials and functional textiles. To date, many spinning strategies utilizing coagulation baths have been applied in GF, which necessitates a complicated washing process. Dry spinning is a more convenient and green method for use with fibers in the chemical fiber industry, and should be a good option for GFs; however, this technique has never been used in a system of GF. In this research, first the dry spinning technique was used to fabricate continuous GFs and the dry spun GFs showed good toughness and flexibility. The dry spinnability of graphene oxide liquid crystals was achieved by choosing dispersive solvents with low surface tension and high volatility. The dry spun neat GFs possessed high toughness up to 19.12 MJ m^-3, outperforming the wet spun neat GFs. This dry spinning methodology facilitates the green fabrication of fibers of graphene and graphene-beyond two-dimensional nanomaterials, and it may also be extended to other printing technologies for complex graphene architectures.

Wrinkle-stabilized metal-graphene hybrid fibers with zero temperature coefficient of resistance

The interfacial adhesion between graphene and metals is poor, as metals tend to generate superlubricity on smooth graphene surface. This problem renders the free assembly of graphene and metals to be a big challenge, and therefore, some desired conducting properties (e.g., stable metal-like conductivities in air, lightweight yet flexible conductors, and ultralow temperature coefficient of resistance, TCR) likely being realized by integrating the merits of graphene and metals remains at a theoretical level. This work proposes a wrinkle-stabilized approach to address the poor adhesion between graphene surface and metals. Cyclic voltammetry (CV) tests and theoretical analysis by Scharifker-Hills models demonstrate that multiscale wrinkles effectively induce nucleation of metal particles, locking in metal nuclei and guiding the continuous growth of metal islands in an instantaneous model on rough graphene surface. The universality and practicability of the wrinkle-stabilized approach is verified by our investigation through the electrodeposition of nine kinds of metals on graphene fibers (GF). The strong interface bonding permits metal-graphene hybrid fibers to show metal-level conductivities (up to 2.2 × 10⁷ S m^-1, a record high value for GF in air), reliable weatherability and favorable flexibility. Due to the negative TCR of graphene and positive TCR of metals, the TCR of Cu- and Au-coated GFs reaches zero at a wide temperature range (15 K-300 K). For this layered model, the quantitative analysis by classical theories demonstrates the suitable thickness ratio of graphene layer and metal layer to achieve zero TCR to be 0.2, agreeing well with our experimental results. This wrinkle-stabilized approach and our theoretical analysis of zero-TCR behavior of the graphene-metal system are conducive to the design of high-performance conducting materials based on graphene and metals.

In situ twisting for stabilizing and toughening conductive graphene yarns

Graphene fibres are of great interest for diverse applications such as their use in supercapacitors, batteries, stimulus-responsive actuators and smart textiles. However, previously reported graphene-based fibres commonly have very low toughness and poor stretchability, which can limit many of their possible applications. We demonstrate an efficient approach for preparing super-tough yarns of reduced graphene oxides via the in situ twisting of multiplied graphene oxide fibres during fibre extrusion and coagulation, followed by chemical reduction and drying. These twisted multiplied yarns have a toughness of 46.3 J m^-3, much higher than previous results for graphene-based fibres and films, and benefit from the retention of high strength and a significant increase in the failure strain due to the stably twisted hollow structure of the yarns. These yarns also show very stable electrical resistance when subjected to cyclic stretching at a large tensile strain. We demonstrate the utilization of such yarns for stretchable conducting electronic wires.

Multifunctional, biocompatible and pH-responsive carbon nanotube- and graphene oxide/tectomer hybrid composites and coatings

Here we present a route for non-covalent functionalization of carboxylated multi-walled carbon nanotubes and graphene oxide with novel two-dimensional peptide assemblies. We show that self-assembled amino-terminated biantennary and tetraantennary oligoglycine peptides (referred to as tectomers) effectively coat carboxylated multi-walled carbon nanotubes and also strongly interact with graphene oxide due to electrostatic interactions and hydrogen bonding as the driving force, respectively. The resulting hybrids can be made into free-standing conducting composites or applied in the form of thin, pH-switchable bioadhesive coatings onto graphene oxide fibers. Monitoring of cell viability of pancreatic cell lines, seeded on those CNT hybrids, show that they can be used as two- and three-dimensional scaffolds to tissue engineer tumour models for studying ex vivo the tumour development and response to treatment. This highly versatile method in producing pH-responsive hybrids and coatings offers an attractive platform for a variety of biomedical applications and for the development of functional materials such as smart textiles, sensors and bioelectronic devices.

Tough and Thermosensitive Poly(N-isopropylacrylamide)/Graphene Oxide Hydrogels with Macroscopically Oriented Liquid Crystalline Structures

Bulk graphene oxide (GO) nanocomposite materials with macroscopically oriented GO liquid crystalline (LC) structures exhibit interesting anisotropic properties, but their facile preparations remain challenging. This work reports for the first time the facile preparation of poly(N-isopropylacrylamide) (PNIPAM)/GO nanocomposite hydrogels with macroscopically oriented LC structures with the assistance of a flow field induced by vacuum degassing and the in situ polymerization accelerated by GO. The hydrogel prepared with a GO concentration of 5.0 mg mL(-1) exhibits macroscopically aligned LC structures, which endow the gels with anisotropic optical, mechanical properties, and dimensional changes during the phase transition. The hydrogels show dramatically enhanced tensile mechanical properties and phase transition rates. The oriented LC structures are not damaged during the phase transition of the PNIPAM/GO hydrogels, and hence their LC behavior undergoes reversible change. Moreover, highly oriented LC structures can also be formed when the gels are elongated, even for the gels which do not have macroscopically oriented LC structures. Very impressively, the oriented LC structures in the hydrogels can be permanently maintained by drying the gel samples elongated to and then kept at a constant tensile strain. The thermosensitive nature of PNIPAM and the angle-dependent nature of the macroscopically aligned GO LC structures allow the practical applications of the PNIPAM/GO hydrogels as optical switches, soft sensors, and actuators and so on.