Activated Carbon Nanotube Fiber Fabric as a High-Performance Flexible Electrode for Solid-State Supercapacitors

MXene Key Composites: A New Arena for Gas Sensors

With the development of science and technology, the scale of industrial production continues to grow, and the types and quantities of gas raw materials used in industrial production and produced during the production process are also constantly increasing. These gases include flammable and explosive gases, and even contain toxic gases. Therefore, it is very important and necessary for gas sensors to detect and monitor these gases quickly and accurately. In recent years, a new two-dimensional material called MXene has attracted widespread attention in various applications. Their abundant surface functional groups and sites, excellent current conductivity, tunable surface chemistry, and outstanding stability make them promising for gas sensor applications. Since the birth of MXene materials, researchers have utilized the efficient and convenient solution etching preparation, high flexibility, and easily functionalize MXene with other materials to prepare composites for gas sensing. This has opened a new chapter in high-performance gas sensing materials and provided a new approach for advanced sensor research. However, previous reviews on MXene-based composite materials in gas sensing only focused on the performance of gas sensing, without systematically explaining the gas sensing mechanisms generated by different gases, as well as summarizing and predicting the advantages and disadvantages of MXene-based composite materials. This article reviews the latest progress in the application of MXene-based composite materials in gas sensing. Firstly, a brief summary was given of the commonly used methods for preparing gas sensing device structures, followed by an introduction to the key attributes of MXene related to gas sensing performance. This article focuses on the performance of MXene-based composite materials used for gas sensing, such as MXene/graphene, MXene/Metal oxide, MXene/Transition metal sulfides (TMDs), MXene/Metal-organic framework (MOF), MXene/Polymer. It summarizes the advantages and disadvantages of MXene composite materials with different composites and discusses the possible gas sensing mechanisms of MXene-based composite materials for different gases. Finally, future directions and inroads of MXenes-based composites in gas sensing are presented and discussed.

Synthesis of MXene-based nanocomposite electrode supported by PEDOT:PSS-modified cotton fabric for high-performance wearable supercapacitor

The rapid development of wearable and portable electronic devices prompts the ever-growing demand for wearable, flexible, and light-weight power sources. In this work, a MXene/GNS/PPy@PEDOT/Cotton nanocomposite electrode with excellent electrochemical performances was fabricated using cotton fabric as a substrate. Poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) was coated on the cotton fabric to obtain a conductive substrate through a controllable dip-drying coating process, while a nanocomposite consisting of MXene, Graphene nanoscroll (GNS), and polypyrrole (PPy) was directly synthesized and deposited on the PEDOT:PSS-modified cotton fabric via a one-step in situ polymerization method. The resultant MXene/GNS/PPy@PEDOT/Cotton electrode delivers excellent electrochemical performances including an ultra-high areal capacitance of 4877.2 mF·cm-2 and stable cycling stability with 90 % capacitance retention after 3000 cycles. Moreover, the flexible symmetrical supercapacitor (FSC) assembled with the MXene/GNS/PPy@PEDOT/Cotton electrodes demonstrates a prominent areal capacitance (2685.28 mF·cm-2 at a current density of 1 mA·cm-2) and a high energy density (322.15 μWh·cm-2 at a power density of 0.46 mW·cm-2). In addition, the application of the FSC for wearable electronic devices was demonstrated.

Regenerated silk protein based hybrid film electrode with large area specific capacitance, high flexibility and light weight towards high-performance wearable energy storage

Planar wearable supercapacitors (PWSCs) have sparked intense interest owing to their hopeful application in smart electronics. However, current PWSCs suffered from poor electrochemical property, weak flexibility and/or large weight. To relieve these defects, in this study, we fabricated a high-performance PWSC using silk protein derived film electrodes (PPy/RSF/MWCNTs-2; RSF, PPy and MWCNTs represent regenerated silk film, polypyrrole and multi-walled carbon nanotubes, respectively, while 2 is the mass ratio of silk to MWCNTs), which were developed by 'dissolving-mixing-evaporating' and in situ polymerization. In three-electrode, PPy/RSF/MWCNTs-2 showed a superb area specific capacitance of 8704.7 mF cm-2 at 5 mA cm-2, which surpassed numerous reported PWSC electrodes, and had a decent durability with a capacitance retention of 90.7 % after 5000 cycles. The PPy/RSF/MWCNTs-2 derived PWSC showed a largest energy density of 281.3 μWh cm-2 at 1660.1 μW cm-2, and a power density as high as 13636.4 μW cm-2 at 125.6 μWh cm-2. Furthermore, impressive capacitive-mechanical stability with a capacitance retention of 92 % under bending angles from 0 to 150 was depicted. Thanks to the rational and affordable preparation, our study for the first time prepared RSF electrode that had great capacitive property, high mechanical flexibility and light weight, simultaneously. The encouraging results can not only open up a new path to manufacture high-performance flexible electrodes, but may also help to realize the high-value-added utilization of silk.

Current Trends and Promising Electrode Materials in Micro-Supercapacitor Printing

The development of scientific and technological foundations for the creation of high-performance energy storage devices is becoming increasingly important due to the rapid development of microelectronics, including flexible and wearable microelectronics. Supercapacitors are indispensable devices for the power supply of systems requiring high power, high charging-discharging rates, cyclic stability, and long service life and a wide range of operating temperatures (from -40 to 70 °C). The use of printing technologies gives an opportunity to move the production of such devices to a new level due to the possibility of the automated formation of micro-supercapacitors (including flexible, stretchable, wearable) with the required type of geometric implementation, to reduce time and labour costs for their creation, and to expand the prospects of their commercialization and widespread use. Within the framework of this review, we have focused on the consideration of the key commonly used supercapacitor electrode materials and highlighted examples of their successful printing in the process of assembling miniature energy storage devices.

Nickel cobaltite nanowire arrays grown on nitrogen-doped carbon nanotube fiber fabric for high-performance flexible supercapacitors

Flexible supercapacitors have received considerable attention for their potential application in flexible electronics, but usually suffer from relatively low energy density. Developing flexible electrodes with high capacitance and constructing asymmetric supercapacitors with large potential window has been considered as the most effective approach to achieve high energy density. Here, a flexible electrode with nickel cobaltite (NiCo₂O₄) nanowire arrays on nitrogen (N)-doped carbon nanotube fiber fabric (denoted as CNTFF and NCNTFF, respectively) was designed and fabricated through a facile hydrothermal growth and heat treatment process. The obtained NCNTFF-NiCo₂O₄ delivered a high capacitance of 2430.5 mF cm^-2 at 2 mA cm^-2, a good rate capability of 62.1 % capacitance retention even at 100 mA cm^-2 and a stable cycling performance of 85.2 % capacitance retention after 10,000 cycles. Moreover, the asymmetric supercapacitor constructed with NCNTFF-NiCo₂O₄ as positive electrode and activated CNTFF as negative electrode exhibited a combination of high capacitance (883.6 mF cm^-2 at 2 mA cm^-2), high energy density (241 μW h cm^-2) and high power density (80175.1 μW cm^-2). This device also had a long cycle life after 10,000 cycles and good mechanical flexibility under bending conditions. Our work provides a new perspective on constructing high-performance flexible supercapacitors for flexible electronics.

Enhanced Hydro-Actuation and Capacitance of Electrochemically Inner-Bundle-Activated Carbon Nanotube Yarns

Recently, several attempts have been made to activate or functionalize macroscopic carbon nanotube (CNT) yarns to enhance their innate abilities. However, a more homogeneous and holistic activation approach that reflects the individual nanotubes constituting the yarns is crucial. Herein, a facile strategy is reported to maximize the intrinsic properties of CNTs assembled in yarns through an electrochemical inner-bundle activation (EIBA) process. The as-prepared neat CNT yarns are two-end tethered and subjected to an electrochemical voltage (vs Ag/AgCl) in aqueous electrolyte systems. Massive electrolyte infiltration during the EIBA causes swelling of the CNT interlayers owing to the tethering and subsequent yarn shrinkage after drying, suggesting activation of the entire yarn. The EIBA-treated CNT yarns functionalized with oxygen-containing groups exhibit enhanced wettability without significant loss of their physical properties. The EIBA effect of the CNTs is experimentally demonstrated by hydration-driven torsional actuation (∼986 revolutions/m) and a drastic capacitance improvement (approximately 25-fold).

Molecular Modification of Lignin-Based Carbon Materials: Influence of Supramolecular Bonds on the Properties

For the application of lignin-based materials, it is necessary to develop simple and efficient chemical modification strategies for lignin. In this work, the iodization modification strategy is selected to improve the specific surface area and graphitization degree of lignin-based carbon fibers. The introduction of an iodine atom can effectively increase the π electron cloud density of the lignin aromatic hydrocarbon structure. High π electron cloud density can effectively enhance the π-π interaction force between lignin molecules (the supramolecular bonds). The biomass precursors with this intermolecular microstructure exhibit good thermal stability and can maintain the original fibrous morphology during high-temperature treatment, which is beneficial for increasing the specific surface area of biomass-based carbon materials. Furthermore, this intermolecular microstructure also contributes to the graphitization of biomass precursor materials and reduces the spacing of graphite micro-lamellae. The obtained lignin-based carbon fibers with iodization modification exhibit a specific capacitance of 333 F/g at a current density of 1 A/g in the three-electrode tests in 6 M KOH solution. As the assembled supercapacitor, the specific capacitance of lignin-based carbon fibers reaches 87 F/g in 1 M Na₂SO₄ solution. Compared to other modification processes for raw materials, this strategy is simple and efficient and has reference value for the synthesis of other high-performance biomass-based materials.

Two-Dimensional Hybrid Nanosheet-Based Supercapacitors: From Building Block Architecture, Fiber Assembly, and Fabric Construction to Wearable Applications

Fiber-based supercapacitors (F-SCs) have inspired widespread interest in the fields of wearable technology, energy, and carbon neutralization due to their highly deformable flexibility, fast charging/discharging capability, long-term stability, and energy conservation ability. In this review, we summarize the latest developments for fabricating fibrous electrodes of F-SCs where advanced micro two-dimensional (2D) building blocks (e.g., MXene and graphene) are chemically assembled and constructed into ordered mesofibers and multifunctional macrofabrics. Diverse fundamental principles of 2D hybrid nanosheets with respect to surface controls, pseudocapacitive modifications, and microstructural manipulations, promoting rapid electron transfer and charge conduction, are introduced. Additionally, various spinning methods for assembling and fabricating sophisticated fibers with advanced nano/microstructures, including hierarchical skeletons, anisotropic backbones, surface/entire porous frameworks, and vertical-aligned networks, for boosting ionic kinetic transport/storage are presented. Likewise, the structure-activity relationships between the porous structure and electrochemical performance are clarified. Moreover, multifunctional fabrics in terms of high flexibilities/strengths, superior electrical conductivities, and stabilized operations, which realize large energy density, deformable capability, and robust stability under harsh conditions, are emphasized. In particular, the potential power-supply applications, including flexible electronic devices, self-powered functions, and energy-sensor systems, are highlighted. Finally, a short conclusion and outlook, along with the current challenges and future opportunities of next-generation F-SCs, are proposed.

Advances in the Robustness of Wearable Electronic Textiles: Strategies, Stability, Washability and Perspective

Flexible electronic textiles are the future of wearable technology with a diverse application potential inspired by the Internet of Things (IoT) to improve all aspects of wearer life by replacing traditional bulky, rigid, and uncomfortable wearable electronics. The inherently prominent characteristics exhibited by textile substrates make them ideal candidates for designing user-friendly wearable electronic textiles for high-end variant applications. Textile substrates (fiber, yarn, fabric, and garment) combined with nanostructured electroactive materials provide a universal pathway for the researcher to construct advanced wearable electronics compatible with the human body and other circumstances. However, e-textiles are found to be vulnerable to physical deformation induced during repeated wash and wear. Thus, e-textiles need to be robust enough to withstand such challenges involved in designing a reliable product and require more attention for substantial advancement in stability and washability. As a step toward reliable devices, we present this comprehensive review of the state-of-the-art advances in substrate geometries, modification, fabrication, and standardized washing strategies to predict a roadmap toward sustainability. Furthermore, current challenges, opportunities, and future aspects of durable e-textiles development are envisioned to provide a conclusive pathway for researchers to conduct advanced studies.