Graphene Fiber-Based Wearable Supercapacitors: Recent Advances in Design, Construction, and Application

Facile Preparation of High-Performance Free-Standing Micro-Supercapacitors by Optimizing Oxygen Groups on Graphene

Free-standing micro-supercapacitor (MSC) devices without substrate or current collectors are promising for practical applications. However, it is still difficult to prepare high-performance free-standing MSC devices because of the requirement of optimized active sites, conductivity, ion diffusion, controlled patterns, moisture susceptibility, etc. Here, it is proposed that the optimization of oxygen level on graphene is promising to solve these requirements because of the balance of sp2 and sp3 hybridization. Using the medium-oxidized graphene, the flexible, conductive, hydro-stable, easy-processing film can be facilely obtained, which facilitates the preparation of free-standing MSC electrodes. After constructing with gel electrolyte, the free-standing MSC device shows a high capacitance of 898.4 mF cm-2 using aqueous-gel electrolyte and 383.6 mF cm-2 using ion-gel electrolyte with mass loading of ca. 10 mg cm-2. Correspondingly, the MSC device can achieve a landmark energy density of 42.6 µWh cm-2 at 0.85 mW cm-2 (7.1 mWh cm-3 at 141.7 mW cm-3). The advantages of high performance, facile preparation, and low inactive components make the free-standing MSC device promising for practical applications.

Smart Green Cotton Textiles with Hierarchically Responsive Conductive Network for Personal Healthcare and Thermal Management

Whereas cotton as an abundant natural cellulose has been widely used for sustainable and skin-friendly textiles and clothes, developing cotton fabrics with smart functions that could respond to various stimuli is still eagerly desired while remaining a great challenge. Herein, smart multiresponsive cotton fabric with hierarchically copper nanowire interwoven MXene conductive networks that are seamlessly assembled along a 3D woven fabric template for efficient personal healthcare and thermal comfort regulation is successfully developed. The robust hierarchically interwoven conductive network was "glued" and protected by organic conductive polymer poly(3,4-ethylenedioxythiophene) along a 3D interconnected fabric template to enhance interfacial adherent and environmental stability. Benefiting from the robust multiresponsive hierarchically interwoven conductive network, smart cotton fabric exhibits real-time response to various external stimuli (light/electrical/heat/temperature/stress), and the details of human activities can be accurately recognized and monitored. Furthermore, the porous structure of 3D smart fabric induced strong capillary force and confinement to phase change materials PEG, which exhibits a wide range of phase transition temperatures for efficient thermal comfort regulation. After further encapsulation with transparent fluorosilicone resin, the smart cotton fabric exhibits excellent self-cleaning performance with water/oil repellent. The smart multiresponsive cotton fabrics hold great promise in next-generation wearable systems for efficient personal healthcare and thermal management.

Recent Development on Transition Metal Oxides-Based Core-Shell Structures for Boosted Energy Density Supercapacitors

In recent years, nanomaterials exploration and synthesis have played a crucial role in advancing energy storage research, particularly in supercapacitor development. Researchers have diversified materials, including metal oxides, chalcogenides, and composites, as well as carbon materials, to enhance energy and power density. Balancing energy density with electrochemical stability remains challenging, driving intensified efforts in advancing electrode materials. This review focuses on recent progress in designing and synthesizing core-shell materials tailored for supercapacitors. The core-shell architecture offers advantages such as increased surface area, redox active sites, electrical conductivity, ion diffusion kinetics, specific capacitance, and cyclability. The review explores the impact of core and shell materials, specifically transition metal oxides (TMOs), on supercapacitor electrochemical behavior. Metal oxide choices, such as cobalt oxide as a preferred core and manganese oxide as a shell, are discussed. The review also highlights characterization techniques for assessing structural, morphological, and electrochemical properties of core-shell materials. Overall, it provides a comprehensive overview of ongoing TMOs-based core-shell material research for supercapacitors, showcasing their potential to enhance energy storage for applications ranging from gadgets to electric vehicles. The review outlines existing challenges and future opportunities in evolving TMOs-based core-shell materials for supercapacitor advancements, holding promise for high-efficiency energy storage devices.

Core-Sheath Heterogeneous Interlocked Conductive Fiber Enables Smart Textile for Personalized Healthcare and Thermal Management

Whereas thermal comfort and healthcare management during long-term wear are essentially required for wearable system, simultaneously achieving them remains challenge. Herein, a highly comfortable and breathable smart textile for personal healthcare and thermal management is developed, via assembling stimuli-responsive core-sheath dual network that silver nanowires(AgNWs) core interlocked graphene sheath induced by MXene. Small MXene nanosheets with abundant groups is proposed as a novel "dispersant" to graphene according to "like dissolves like" theory, while simultaneously acting as "cross-linker" between AgNWs and graphene networks by filling the voids between them. The core-sheath heterogeneous interlocked conductive fiber induced by MXene "cross-linking" exhibits a reliable response to various mechanical/electrical/light stimuli, even under large mechanical deformations(100%). The core-sheath conductive fiber-enabled smart textile can adapt to movements of human body seamlessly, and convert these mechanical deformations into character signals for accurate healthcare monitoring with rapid response(440 ms). Moreover, smart textile with excellent Joule heating and photothermal effect exhibits instant thermal energy harvesting/storage during the stimuli-response process, which can be developed as self-powered thermal management and dynamic camouflage when integrated with phase change and thermochromic layer. The smart fibers/textiles with core-sheath heterogeneous interlocked structures hold great promise in personalized healthcare and thermal management.

Enhancing Electron/Ion Transport in SnO2 Quantum Dots Decorated Polyaniline/Graphene Hybrid Fibers for Wearable Supercapacitors with High Energy Density

Fiber-based supercapacitors are the potential power sources in the field of wearable electronics and energy storage textiles due to their unique advantages of electrochemical properties and mechanical flexibility, but achieving high energy density and practical energy supply still presents some challenges. In this study, we reported an approach of microfluidic assisted wet-spinning to fabricate SnO2 quantum dots encapsulated polyaniline/graphene hybrid fibers (SnO2 QDs@PGF) by incorporating uniformly polyaniline into graphene fibers and covalently bridging SnO2 quantum dots. The assembled SnO2 QDs@PGF fiber-typed flexible supercapacitors exhibit an ultralarge specific areal capacitance of 925 mF cm-2 in PVA/H2SO4, superior rate capabilities, and capacitance retention of 88% after 8000 cycles, indicating that the SnO2 QDs@PGF possess near-ideal capacitance properties, efficient ion transfer rate, and good cycling stability. In the EMITFSI/PVDF-HFP electrolyte system, SnO2 QDs@PGF realize a wide operating potential window of 2.5 V, a specific areal capacitance of 678.4 mF cm-2, and an energy density of 147.2 μWh cm-2 at 500 μW cm-2, which can be utilized to power an alarm clock, an electronic timer, and a desk lamp with a requirement of a 3 V battery. The exceptional performance of the SnO2 QDs@PGF can be attributed to the molecular-level homogeneous composite of granular polyaniline and graphene nanosheets and the interfacial C-O-Sn covalent coupling strategy employed between SnO2 QDs and PGF. These avenues not only effectively prevent the undesirable restacking of graphene nanosheets but also increase the interlayer electroactive sites, ordered ion diffusion channels, and strong interfacial charge transfer.

Multifunctionality Analysis of Structural Supercapacitors- A Review

Structural supercapacitors (SSCs) are multifunctional energy storage composites (MESCs) that combine the mechanical properties of fiber-reinforced polymers and the electrochemical performance of supercapacitors to reduce the overall mass in lightweight applications with electrical energy consumption. These novel MESCs have huge potentials, and their properties have improved dramatically since their introduction in the early 2000's. However, the current properties of SSCs are not sufficient for complete energy supply of electrically driven devices. To overcome this drawback, the aim of the current study is to identify key areas for enhancement of the multifunctional performance of SSCs. Critical modification paths for the SSC constituents are systematically analyzed. Special focus is given to the improvement of carbon fiber-based electrodes, the selection of structural electrolytes and the implementation of separators for the development of more efficient SSCs. Finally, current SSCs are compared in terms of their multifunctionality including material combinations and modifications.

High Electrochemical Capacity MnO2/Graphene Hybrid Fibers Based on Crystalline Regulatable MnO2 for Wearable Supercapacitors

Fiber-based supercapacitors (FSCs) exhibit desirable application potential and development prospects in wearable energy storage devices because of their flexibility and wearability. However, the low capacity in the unit volume and insufficient fiber strength hinder their further development in practical application. Herein, the MnO2 nanomaterials with regulatable crystalline structure were synthesized by one-step hydrothermal strategy. The formation of the MnO2 crystalline structure involved the "crimp-phase transition" process. Among them, the 2 × 2 tunnel type α-MnO2 nanowires exhibited excellent electrochemical capacitance (43.8 F g-1), high rate performance (61%, 0.25 to 6 A g-1), and remarkable cyclic stability (99%), which can be attributed to their good symmetry in space and high shared vertices proportion. On this basis, the α-MnO2 nanowires were coblended with GO to construct MnO2/rGO hybrid fibers by scalable continuous wet spinning and in situ acid reduction. Noteworthily, in MnO2/rGO hybrid fibers, the doping amount of MnO2 nanowires as high as 50 wt % could be achieved, while the strength reached 11.73 MPa, which can be ascribed to the superior surface morphology of MnO2 nanowires and the unique cement wall structure of hybrid fibers. Finally, the obtained hybrid fiber electrodes were assembled into symmetrical FSCs. Notably, the FSCs delivered remarkable volume specific capacitance (129.5 F cm-3) and impressive energy density (18 mWh cm-3) at 1.75 A cm-3. In addition, the assembled all-solid-state FSCs indicated excellent deformability and application potential. This work offers some insight for promoting the continuous preparation of fiber electrodes, the development of FSCs, and practical application in wearable energy textile.

High-Voltage MXene-Based Supercapacitors: Present Status and Future Perspectives

As an emerging class of 2D materials, MXene exhibits broad prospects in the field of supercapacitors (SCs). However, the working voltage of MXene-based SCs is relatively limited (typically ≤ 0.6 V) due to the oxidation of MXene electrode and the decomposition of electrolyte, ultimately leading to low energy density of the device. To solve this issue, high-voltage MXene-based electrodes and corresponding matchable electrolytes are developed urgently to extend the voltage window of MXene-based SCs. Herein, a comprehensive overview and systematic discussion regarding the effects of electrolytes (aqueous, organic, and ionic liquid electrolytes), asymmetric device configuration, and material modification on the operating voltage of MXene-based SCs, is presented. A deep dive is taken into the latest advances in electrolyte design, structure regulation, and high-voltage mechanism of MXene-based SCs. Last, the future perspectives on high-voltage MXene-based SCs and their possible development directions are outlined and discussed in depth, providing new insights for the rational design and realization of advanced next-generation MXene-based electrodes and high-voltage electrolytes.

Understanding and Optimizing Capacitance Performance in Reduced Graphene-Oxide Based Supercapacitors

Reduced graphene-oxide (RGO)-based electrodes in supercapacitors deliver high energy/power capacities compared to typical nanoporous carbon materials. However, extensive critical analysis of literature reveals enormous discrepancies (up to 250 F g^-1 ) in the reported capacitance (variation of 100-350 F g^-1 ) of RGO materials synthesized under seemingly similar methods, inhibiting an understanding of capacitance variation. Here, the key factors that control the capacitance performance of RGO electrodes are demonstrated by analyzing and optimizing various types of commonly applied electrode fabrication methods. Beyond usual data acquisition parameters and oxidation/reduction properties of RGO, a substantial difference of more than 100% in capacitance values (with change from 190 ± 20 to 340 ± 10 F g^-1 ) is found depending on the electrode preparation method. For this demonstration, ≈40 RGO-based electrodes are fabricated from numerous distinctly different RGO materials via typically applied methods of solution (aqueous and organic) casting and compressed powders. The influence of data acquisition conditions and capacitance estimation practices are also discussed. Furthermore, by optimizing electrode processing method, a direct surface area governed capacitance relationship for RGO structures is revealed.

Wet Spinning of Graphene Oxide Fibers with Different MnO2 Additives

We present the fabrication of graphene oxide (GO) and manganese dioxide (MnO₂) composite fibers via wet spinning processes, which entails the effects of MnO₂ micromorphology and mass loading on the extrudability of GO/MnO₂ spinning dope and on the properties of resulted composite fibers. Various sizes of rod and sea-urchin shaped MnO₂ microparticles have been synthesized via hydrothermal reactions with different oxidants and hydrothermal conditions. Both the microparticle morphology and mass loading significantly affect the extrudability of the GO/MnO₂ mixture. In addition, the orientation of MnO₂ microparticles within the fibers is largely affected by their microscopic surface areas. The composite fibers have been made electrically conductive via chemical or thermal treatments and then applied as fiber cathodes in Zn-ion battery prototypes. Thermal annealing under an argon atmosphere turns out to be an appropriate method to avoid MnO₂ dissolution and leaching, which have been observed in the chemical treatments. These rGO/MnO₂ fiber cathodes have been assembled into prototype Zn-ion batteries with Zn wire as the anode and xanthan-gum gel containing ZnSO₄ and MnSO₄ salts as the electrolyte. The resulted electrochemical output depends on the annealing temperature and MnO₂ distribution within the fiber cathodes, while the best performer shows stable cycling stability at a maximum capacity of ca. 80 mA h/g.

All-solid-state Ti3C2Tx neutral symmetric fiber supercapacitors with high energy density and wide temperature range

All-solid-state Ti₃C₂T_x neutral symmetric fiber supercapacitors (PVA EGHG Ti₃C₂T_x FSCs) with high energy density and wide temperature range are constructed by using polyvinyl alcohol (PVA)-ethylene glycol hydrogel (EGHG)-sodium perchlorate (NaClO₄) as electrolyte and separator, and Ti₃C₂T_x fiber as electrodes. Ti₃C₂T_x fiber is prepared using 130 mg mL^-1 Ti₃C₂T_x nanosheet ink as an assembly unit in a coagulation bath of isopropyl alcohol (IPA) and distilled water with 5 wt% CaCl₂ by a wet spinning method. The prepared Ti₃C₂T_x fiber exhibits a specific capacity of 385 F cm^-3 and a capacitance retention of 94 % after 10,000 cycles in 1 M NaClO₄ electrolyte. The assembled PVA EGHG Ti₃C₂T_x FSCs deliver a specific capacitance of 41 F cm^-3, a volumetric energy density of 5 mWh cm^-3, and a capacitance retention of 92 % after 500 times continuous bending. Furthermore, it shows good flexibility and excellent capacitance over a wide temperature range of -40 to 40 °C and maintains its electrochemical performance under varying degrees of bending. This study provides a viable strategy for designing and assembling all-solid-state neutral symmetric fiber supercapacitors with high energy density and wide temperature range.

The Rising Aerogel Fibers: Status, Challenges, and Opportunities

Aerogel fibers garner tremendous scientific interest due to their unique properties such as ultrahigh porosity, large specific surface area, and ultralow thermal conductivity, enabling diverse potential applications in textile, environment, energy conversion and storage, and high-tech areas. Here, the fabrication methodologies to construct the aerogel fibers starting from nanoscale building blocks are overviewed, and the spinning thermodynamics and spinning kinetics associated with each technology are revealed. The huge pool of material choices that can be assembled into aerogel fibers is discussed. Furthermore, the fascinating properties of aerogel fibers, including mechanical, thermal, sorptive, optical, and fire-retardant properties are elaborated on. Next, the nano-confining functionalization strategy for aerogel fibers is particularly highlighted, touching upon the driving force for liquid encapsulation, solid-liquid interface adhesion, and interfacial stability. In addition, emerging applications in thermal management, smart wearable fabrics, water harvest, shielding, heat transfer devices, artificial muscles, and information storage, are discussed. Last, the existing challenges in the development of aerogel fibers are pointed out and light is shed on the opportunities in this burgeoning field.

A high-performance and flexible electrode film based on bacterial cellulose/polypyrrole/nitrogen-doped graphene for supercapacitors

With the development and popularity of portable electronic devices, there is an urgent need for flexible energy storage devices suitable for mass production. We report freestanding paper electrodes for supercapacitors fabricated via a simple but efficient two-step method. Nitrogen-doped graphene (N-rGO) was first prepared via a hydrothermal method. This not only obtained nitrogen atom-doped nanoparticles but also formed reduced graphene oxide. Pyrrole (Py) was then deposited on the bacterial cellulose (BC) fibers as a polypyrrole (PPy) pseudo-capacitance conductive layer by in situ polymerization and filtered with nitrogen-doped graphene to prepare a self-standing flexible paper electrode with a controllable thickness. The synthesized BC/PPy/N₁₅-rGO paper electrode has a remarkable mass specific capacitance of 441.9 F g^-1, a long cycle life (96 % retention after 3000 cycles), and excellent rate performance. The BC/PPy/N₁₅-rGO-based symmetric supercapacitor shows a high volumetric specific capacitance of 244 F cm^-3 and a max energy density of 67.9 mWh cm^-3 with a power density of 1.48 W cm^-3, suggesting that they will be promising materials for flexible supercapacitors.

Facile Access to Fabricate Carbon Dots and Perspective of Large-Scale Applications

Carbon dots (CDs), fluorescent carbon nanoparticles with particle sizes < 10 nm, are constantly being developed for potential large-scale applications. Recently, methods allow CD synthesis to be carried out on large-scale preparation in a controlled fashion are potentially important for multiple disciplines, including bottom-up strategy, top-down method. In this review, the recent progresses in the research of the methods for large-scale production of CDs and their functionalization are summarized. Especially, the methods of CD synthesis, such as large-scale preparation, hydrothermal/solvothermal, microwave-assisted, magnetic hyperthermia microfluidic and other methods, along with functionalization of CDs, are summarized in detail. By promising applications of CDs, there are three aspects have been already reported, such as enhancing mechanical properties, flame retardancy, and energy storage. Also, future development of CDs is prospected.