Graphene fiber based supercapacitors:Strategies and perspective toward high performances

Modern wearable electronics are thirsty for flexible, lightweight energy storage and supply devices. Flexible fiber-shaped supercapacitors, possess good flexibility, high power density, fast charging capability and long cycle life, becoming a promising option for wearable devices. The past decade has witnessed the emergence of graphene fiber based supercapacitors（GFSCs） as one of the most active vicinity in fiber-supercapactiors, for their excellent properties including high surface area, chemical stability, excellent electrical conductivity, lightweight and mechanical properties. In this perspective, we introduced the basic energy storage mechanisms of GFSCs, followed by the analysis in improving their overall performances, recent advances, and a conclusive discussion on the challenges and opportunities.

Flexible thermocouple using a thermoelectric graphene fiber with a seamless junction

Temperature is an important physical variable that indicates the condition of the human body and artificial systems.Advanced wearable applications require the development of temperature sensors with different form factors.In this study,a fiber-shaped thermoelectric temperature sensor is fabricated using a continuous graphene fiber whose two halves possess different reduction states.A seamless junction is formed by partially reducing a wet-spun graphene oxide fiber with hydroiodic acid(HI)solutions of different concentrations.One-half of the fiber is mildly reduced with 0.97 wt%HI,while the other half is highly reduced with 30.6 wt%HI.The different reduction states of the graphene composite fiber result in different Seebeck coefficients,allowing for the fabrication of a fiber-shaped graphene thermocou-ple without any laborious assembly.The flexible graphene thermocouple exhibits high sensitivity with a thermopower of 12.5μV K^(-1)in the temperature range of room temperature to∼70℃.Furthermore,it exhibits high linearity with a correlation coefficient exceeding 0.995 and fast response with a time constant of 0.24 s.Owing to its mechanical robustness and flexibility,the stand-alone graphene ther-mocouple can be knitted into a cotton fabric glove,which presents a fast response to environmental changes without any external power source.This work offers a unique fabrication method for producing a high-performance,flexible thermocouple that features a seamless and clear junction without the use of additional materials.This alternative method eliminates the complicated assembly processes typically required for conventional thermocouples.

N-Doped graphene fiber anchored Pd nanoparticles as a fixed-bed catalyst for continuous-flow reduction of N-containing unsaturated compounds

Catalytic fixed-bed is an efficient and facile system for scalable organic synthesis due to its continuous and fast flow operation process.As a key unit in the fixed-bed system,catalytically active packing materials are required to possess some properties,such as high activity,excellent stability,and porous packing structure.Herein,we prepare a fibrous fixed-bed catalyst by anchoring Pd nanoparticles on N-doped graphene fiber(NHG)(Pd/NGF).Due to the porous and loose packing structure,the resultant Pd/NGF catalyst can be easily filled into the continuous-flow reactor to construct a fixed-bed system with low flow resistance.The corresponding catalytic fixed-bed system exhibits a favourable flow rate(8 mL/min)and excellent durability toward reduction reactions of N-containing unsaturated compounds to produce aromatic amines.This work provides a new design concept of fibrous fixed-bed catalysts with dual-active components(i.e.,graphene-derived active materials and metal nanoparticles)and catalytic organic synthesis in a continuous-flow process.

Recent Progress of Graphene Fiber/Fabric Supercapacitors:From Building Block Architecture,Fiber Assembly,and Fabric Construction to Wearable Applications

High-performance fiber-shaped power sources are anticipated to considerably contribute to the continuous development of smart wearable devices.As one-/two-dimensional(1D/2D)frameworks constructed from graphene sheets,graphene fibers and fabrics inherit the merits of graphene,including its lightweight nature,high electrical conductivity,and exceptional mechanical strength.The as-fabricated graphene fiber/fabric flexible supercapacitor(FSC)is,therefore,regarded as a promis-ing candidate for next-generation wearable energy storage devices owing to its high energy/power density,adequate safety,satisfactory flexibility,and extended cycle life.The gap between practical applications and experimental demonstrations of FSC is drastically reduced as a result of technological advancements.To this end,herein,recent advancements of FSCs in fiber element regulation,fiber/fabric construction,and practical applications are methodically reviewed and a forecast of their growth is presented.

Porous Graphene-Based Electrodes for Fiber-Shaped Supercapacitors with Good Electrical Conductivity

Graphene fibers(GFs)are ideal electrodes for f ibrous supercapacitors because of their excellent mechanical and electrical properties.However,the actual specific s urface area(SSA)of GFs is low due to the restacking of g raphene sheets,which limits the capacitance enhancement o f GFs.One of the effective ways to increase the SSA of GFs is to construct a porous fiber structure,but it disrupts t he conductive pathway and adversely affects the electrochemical(EC)performance of GFs.Therefore,a wet-spun porous GF electrode coated with polypyrrole(PPy)was reported in this paper.The resulting electrode exhibits an areal specific capacitance of 31.25 mF·cm^(-2)and a good c ycling stability with a capacitance retention rate of 95%after 5000 charge/discharge cycles.These indicate that porous structures and PPy coating can synergistically improve the EC performance of GFs.

Chemically doped macroscopic graphene fibers with significantly enhanced thermoelectric properties

Flexible wearable electronics, when combined with outstanding thermoelectric properties, are promising candidates for future energy harvesting systems. Graphene and its macroscopic assemblies （e.g., graphene-based fibers and films） have thus been the subject of numerous studies because of their extraordinary electrical and mechanical properties. However, these assemblies have not been considered suitable for thermoelectric applications owing to their high intrinsic thermal conductivity. In this study, bromine doping is demonstrated to be an effective method for significantly enhancing the thermoelectric properties of graphene fibers. Doping enhances phonon scattering due to the increased defects and thus decreases the thermal conductivity, while the electrical conductivity and Seebeck coefficient are increased by the Fermi level downshift. As a result, the maximum figure of merit is 2.76 ~ 10~, which is approximately four orders of magnitude larger than that of the undoped fibers throughout the temperature range. Moreover, the room temperature power factor is shown to increase up to 624 btW.m-l.K-2, which is higher than that of any other material solely composed of carbon nanotubes and graphene. The enhanced thermoelectric properties indicate the promising potential for graphene fibers in wearable energy harvesting systems.

Systematic characterization of transport and thermoelectric properties of a macroscopic graphene fiber

Graphene, a two-dimensional material with extraordinary electrical, thermal, and elastic performance, is a potential candidate for future technologies. However, the superior properties of graphene have not yet been realized for graphenederived macroscopic structures such as graphene fibers. In this study, we systematically investigated the temperature （T ）-dependent transport and thermoelectric properties of graphene fiber, including the thermal conductivity （A）, electrical conductivity （o）, and Seebeck coefficient （S）. A increases from 45.8 to 149.7 W·m^-1·K^-1 and then decreases as T increases from 80 to 290 K, indicating the boundary-scattering and three-phonon Umklapp scattering processes. σ increases with T from 7.1 × 10^4 to 1.18 × 10^5 S·m^-1, which can be best explained by the hopping mechanism. S ranges from -3.9 to 0.8 μV·K^-1 and undergoes a sign transition at approximately 100 K.

An efficient chemical reduction-induced assembly of Fe_(3)O_(4)@graphene fiber for wire-shaped supercapacitors with ultrahigh volumetric energy density

Benefiting from high flexibility and weavability,the wire-shaped supercapacitors(SCs)arouse tremendous interests for the applications in wearable/portable electronics.Graphene fiber(GF)is considered as a promising linear electrode for wire-shaped SCs.However,the bottleneck is how to develop the GF-based linear electrode with facile fabrication process while wellmaintaining satisfactory electrochemical performance.Herein,a novel Fe_(3)O_(4)@GF composite linear electrode is proposed via a chemical reduction-induced assembly approach,in which the GO and Fe_(3)O_(4) nanoparticles(NPs)realize the efficient selfassembly owing to the electrostatic and van der Waals interactions,as well as the sufficient reduction of GO during the preparation process.The resultant fiber-shaped architecture shows boosted charge-transfer kinetics,high flexibility and structural integrity.Such Fe_(3)O_(4)@GF linear electrode exhibits excellent electrochemical behaviors including a large volumetric specific capacitance(~250.75 F cm^(−3)),remarkable rate capability and favorable electrochemical kinetics in aqueous electrolyte,superior than previously reported GF-based linear electrodes.For real application,a high-performance wire-shaped SC with excellent flexibility and weavability is fabricated based on such Fe_(3)O_(4)@GF linear electrode and gel electrolyte,demonstrating ultrahigh volumetric energy density(18.8 mWh cm^(−3)),power density(4000 mW cm^(−3))and strong durability(~93.5%retention after 10000 cycles).Prospectively,the fabricated wire-shaped SC can maintain reliable electrochemical behaviors in various deformation states,showing its potentials in future portable and wearable devices.

Characterization and Saturable Absorption Property of Graphene Oxide on Optical Fiber by Optical Deposition

We prepared graphene oxide（GO） saturable absorber（SA） successfully through optical deposition method, which is a simple but effective approach to deposit various materials onto substrate under the effects of light, and investigated several factors that influence the optical deposition result of GO onto optical fiber end, including poly（methyl methacrylate）（PMMA） concentration, light intensity, light mode, and deposition time. The efficient optically deposited GO preserving its nonlinearity guaranteed by GO/PMMA composite formation was also demonstrated. The GO SA prepared by optical deposition shows superior saturable absorption property with modulation depth and nonsaturable loss of 6% and 40%, respectively.

Graphene Quantum Dots Derived from Carbon Fibers for Oxidation of Dopamine

We demonstrated a facile method to prepare photoluminescent graphene quantum dots using commercial polyacrylonitrile（PAN） based carbon fibers（CFs） as the raw material by facile chemical oxidation and exfoliation method. The as-prepared GQDs with uniform size exhibit an excitation-independent photoluminescence behavior, which is similar to other semiconductor quantum dots. Moreover, when acting as catalyst the uniform GQDs have better activity for electrochemical oxidation of dopamine（DA） than graphene oxides（GOs）. The square wave voltammogram（SWV） peak values of GQDs are in good correspondence with DA concentrations and can act as a sensor of DA.

Mode-Locked Thulium Ytterbium Co-Doped Fiber Laser with Graphene Oxide Paper Saturable Absorber

A mode-locked thulium ytterbium co-doped fiber laser （TYDFL） is proposed and demonstrated by using a commercial graphene oxide （GO） paper as saturable absorber （SA）. The GO paper is sandwiched between two fiber ferrules and incorporates a ring laser cavity to generate soliton pulse train operating at 1942.0nm at a threshold multimode pump power as low as 1.8 W. The mode-locked TYDFL has a repetition rate of 22.32 MHz and the calculated pulse width of 1.1 ns. Even though the SA has a low damage threshold, the easy fabrication of GO paper should promote its potentiM application in ultrafast photonics.

Using Reduced Graphene Oxide to Generate Q-Switched Pulses in Er-Doped Fiber Laser

Using the reduced graphene oxide（rGO） as a saturable absorber（SA） in an Er-doped fiber（EDF） laser cavity,we obtain the Q-switching operation. The rGO SA is prepared by depositing the GO on fluorine mica（FM） using the thermal reduction method. The modulation depth of rGO/FM is measured to be 3.2%. By incorporating the rGO/FM film into the EDF laser cavity, we obtain stable Q-switched pulses. The shortest pulse duration is3.53 μs, and the maximum single pulse energy is 48.19 nJ. The long-term stability of working is well exhibited.The experimental results show that the rGO possesses potential photonics applications.

A Mini Review on Nanocarbon-Based 1D Macroscopic Fibers:Assembly Strategies and Mechanical Properties

Nanocarbon-based materials, such as carbon nanotubes(CNTs) and graphene have been attached much attention by scientific and industrial community. As two representative nanocarbon materials, one-dimensional CNTs and twodimensional graphene both possess remarkable mechanical properties. In the past years, a large amount of work have been done by using CNTs or graphene as building blocks for constructing novel, macroscopic, mechanically strong fibrous materials. In this review, we summarize the assembly approaches of CNT-based fibers and graphene-based fibers in chronological order, respectively. The mechanical performances of these fibrous materials are compared, and the critical influences on the mechanical properties are discussed. Personal perspectives on the fabrication methods of CNT-and graphene-based fibers are further presented.

Controllable preparation of graphene glass fiber fabric towards mass production and its application in self-adaptive thermal management

Direct synthesis of graphene on nonmetallic substrates via chemical vapor deposition (CVD) has become a frontier research realm targeting transfer-free applications of CVD graphene.However,the stable mass production of graphene with a favorable growth rate and quality remains a grand challenge.Herein,graphene glass fiber fabric (GGFF) was successfully developed through the controllable growth of graphene on non-catalytic glass fiber fabric,employing a synergistic binary-precursor CVD strategy to alleviate the dilemma between growth rate and quality.The binary precursors consisted of acetylene and acetone,where acetylene with high decomposition efficiency fed rapid graphene growth while oxygencontaining acetone was adopted for improving the layer uniformity and quality.Notably,the bifurcating introducing-confluent premixing (BI-CP) system was self-built for the controllable introduction of gas and liquid precursors,enabling the stable production of GGFF.GGFF features solar absorption and infrared emission properties,based on which the self-adaptive dual-mode thermal management film was developed.This film can automatically switch between heating and cooling modes by spontaneously perceiving the temperature,achieving excellent thermal management performances with heating and cooling power of~501.2 and~108.6 W m-2,respectively.These findings unlock a new strategy for the large-scale batch production of graphene materials and inspire advanced possibilities for further applications.

A Novel Multiscale Reinforcement by In-Situ Growing Carbon Nanotubes on Graphene Oxide Grafted Carbon Fibers and Its Reinforced Carbon/Carbon Composites with Improved Tensile Properties

In-situ growing carbon nanotubes （CNTs） directly on carbon fibers （CFs） always lead to a degraded tensile strength of CFs and then a poor fiber-dominated mechanical property of carbon/carbon composites （C/ Cs）. To solve this issue, here, a novel carbon fiber-based multiscale reinforcement is reported. To synthesize it, carbon fibers （CFs） have been first grafted by graphene oxide （GO）, and then carbon nanotubes （CNTs） have been in-situ grown on GO-grafted CFs by catalytic chemical vapor deposition. Characterizations on this novel reinforcement show that GO grafting cannot only nondestructively improve the surface chemical activity of CFs but also protect CFs against the high-temperature corrosion of metal catalyst during CNT growth, which maintains their tensile properties. Tensile property tests for unidirectional C/Cs with different preforms show that this novel reinforcement can endow C/C with improved tensile properties, 32% and 87% higher than that of pure C/C and C/C only doped with in-situ grown CNTs. This work would open up a possibility to fabricate multiscale C/Cs with excellent global performance.

Reduced Graphene Oxide/Carbon Fiber Composite Membrane for Self-floating Solar-thermal Steam Production

Solar-thermal water evaporation has attracted increasing attention owing to the promising potential to solve the global clean water and energy crisis.But,the development of this strategy is limited by the lack of materials with high solar-thernal conversion efficiency,local heating of superficial water,easy preparation and low cost.Herein,we proposed a facile strategy to prepare a reduced graphene oxide/carbon fiber composite membrane,denoted as RGO/CF membrane.The surface of the RGO/CF membrane was highly hydrophobic,endowing the composite membrane with the self-floating ability on the water without any assistance.The light absorbance ability achieved as high as ca.98%in the wavelength range of 300-1200 nm.The steam evaporation efliciency under the illumination of3-sun was 97%,generating water steam at a rate of 4.54 kg·m^-2·h^-1.Moreover,the solar-thermal steam production rate showed high stability during successive 30 cvcle tests.

Q-switched Er-doped fiber laser with low pumping threshold using graphene saturable absorber

We propose a Q-switched Er-doped fiber laser （EDFL） with a threshold pumping power as low as 7.4 mW, and demonstrate using graphene polyvinyl alcohol （PVA） thin film as a passive saturable absorber （SA）. The SA is fabricated from graphene flakes, which is synthesized by electrochemical exfoliation of graphite at room temperature in 1% sodium dodecyl sulfate aqueous solution. The flakes are mixed with PVA solution to produce a thin film, which is then sandwiched between two ferrules to form a SA and integrated in the EDFL ring cavity to generate a stable Q-switched pulse train. The pulse train operates at 1560 nm with a threshold pump power of 7.4 roW. At maximum 1480 nm pump power of 33.0 roW, the EDFL generates an optical pulse train with a repetition rate of 27.0 kHz and pulse width of 3.56/as. The maximum pulse energy of 39.4 nJ is obtained at a pump power of 14.9 roW. This laser can be used as a simple and low-cost light source for metrology, environmental sensing, and biomedical diagnostics.

Dual-wavelength single-longitudinal-mode fiber laser with switchable wavelength spacing based on a graphene saturable absorber

We propose and demonstrate a dual-wavelength single-longitudinal-mode(SLM) fiber laser with switchable wavelength spacing based on a graphene saturable absorber(GSA) and a Wave Shaper. By virtue of the excellent saturable absorption ability of graphene, the linewidths of the lasing wavelengths can be effectively reduced and eventually SLM operation can be obtained. The linewidths of both wavelengths are measured to be narrower than 7.3 kHz. The obtained results suggest that the graphene would be a good candidate nonlinear optical material for applications in related photonic fields, such as SLM oscillation generation for microwave generation and optical sensing.

Plasma enhanced reduction method for synthesis of reduced graphene oxide fiber/Si anode with improved performance

Silicon(Si)is considered as one of the most promising anode materials for advanced lithium-ion batteries due to its high theoretical capacity,environmental friendliness,and widespread availability.However,great challenges such as volumetric expansion,limited ionic/electronic conductivity properties and complex manufacturing processes hinder its practical applications.Herein,a novel plasma-enhanced reduced graphene oxide fibers/Si(PrGOFs/Si)composite anode is first proposed by using wet-spinning technology followed by plasma-enhanced reduction method.The PrGOFs provide large space to accommodate the volume expansion of Si nanoparticles(SiNPs)by forming a flexible 3D conductive network.Compared to the conventional thermally reduced graphene oxide fibers/Si(TrGOFs/Si)sample,the PrGOFs/Si anodes demonstrate higher conductivity,specific surface area,and superior fabrication efficiency.Accordingly,the Pr GOFs/Si anodes exhibit a reversible capacity of 698.3 mA h/g,and maintain a specific capacity of 602.5m Ah/g at a current density of 200 m A/g after 100 cycles,superior to conventional Tr GOFs/Si counterparts.This research presents a novel strategy for the preparation of high-performance Si/carbon anodes for energy storage applications.

Synthesis, Characterization and Gas Sensing Properties of Graphene Oxide-Multiwalled Carbon Nanotube Composite

Graphene oxide （GO）-multiwalled carbon nanotube （MWCNT） composite was synthesized and characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy, micro Raman, Fourier transform infrared and ultraviolet-visible near infrared spectroscopy techniques. Spectral characteris- tics of cladding modified fiber optic gas sensors were studied for various concentrations of ammonia, ethanol and methanol at 27 ℃. Thickness of the gas sensing layer was controlled by varying the concentration of composite in ethanol medium （0.5 and 1 mg/mL） for three times dipping process. The O.S mg/ mL concentrated GO-MWCNT coated sensor showed 1.20, 1.40 and 1.15 times higher sensitivity than the GO coated sensor for ammonia, ethanol and methanol vapors, respectively. Furthermore, it exhibited 1.50, 1.80 and 1.80 times better sensitivity than 1 mg/mL concentrated GO-MWCNT coated sensor for ammonia, ethanol and methanol vapors, respectively. The presence of functional groups in GO increased the sen- sitivity. This is mainly attributed to the effective electron charge transfer between the composite materials and analytes.