Nacre-Inspired Strong MXene/Cellulose Fiber with Superior Supercapacitive Performance via Synergizing the Interfacial Bonding and Interlayer Spacing

Oxygen-Deficient TiO2-Based Dual-Functional Electrochromic Smart Windows: Achieving High Coloration Efficiency and Energy Storage Through Oxygen Defect Engineering

Electrochromic smart windows (ECSWs) have yet to achieve widespread adoption, primarily due to the high cost associated with the exorbitant material used presently. In this study, we explore titanium dioxide (TiO2), a more abundant and cost-effective alternative, as an electrochromic (EC) material with potential for enhanced performance. We address the issue of TiO2's low coloration efficiency (CE) by introducing oxygen defects. Using reactive sputtering, we varied the oxygen flow rate during TiO2 deposition, creating oxygen defect concentrations between 29.9% and 41% and interaction of film with different electrolytes, EC, and energy performance was studied. This led to significant improvements in opacity and EC performance, with only 338 nm thick film, the device shows 55%, 47%, and 44% modulation in solar, luminous, and NIR transmittance and the film shows high CE (22.79 and 26.99 cm2 C-1 at 550 and 632 nm, respectively). The film also exhibited excellent dual-functionality, with an areal capacitance (Ca) of 34 mF cm-2, demonstrating its energy storage capability. The scalability of the process was confirmed by powering a timer display for 11 minutes with a large-area (25 cm2) ECSW. This work paves the way for affordable, dual-functional ECSWs, offering a sustainable solution for modern infrastructure applications.

Nacre-like MXene/Polyacrylic Acid Layer-by-Layer Multilayers as Hydrogen Gas Barriers

MXenes are a promising class of 2D nanomaterials and are of particular interest for gas barrier applications due to their high aspect ratio. However, MXene nanosheets naturally bear a negative charge, which prevents assembly with negatively charged polymers, such as polyacrylic acid (PAA), into gas barrier coatings. Here, we present MXene- and PAA-based layer-by-layer (MXene/PAA LbL) multilayers formed by leveraging hydrogen bonding interactions. When assembled in acidic conditions, MXene/PAA LbL multilayers exhibit conformal, pinhole-free, nacre-like structures. The MXene/PAA LbL multilayers yield high blocking capability and low permeability (0.14 ± 0.01 cc·mm·m-2·day-1·MPa-1) for hydrogen gas which is over 9000 times lower than uncoated niobium (Nb) substrate. These nacre-like structures are also electronically conductive (σDC, up to 370 ± 30 S cm-1). Because these multilayers utilize hydrogen bonding, their properties are highly sensitive to the pH of the assembly and its external environment. Specifically, the reversible deconstruction of these multilayers under basic conditions is experimentally verified. This study shows that hydrogen bonding interactions can be leveraged to form MXene LbL multilayers as gas barriers, electronically conductive coatings, and deconstructable thin films via pH control.

Biomimetic Interfacial Manipulation in Ti3C2Tx Fibers Toward Strengthened Interlayer Cross-linking, Regulated Ion Diffusion and Suppressed Self-Discharge

Simultaneous improvements in mechanical and electrochemical properties of MXene (Ti3C2Tx) fibers, especially tensile strength, output capacitance, and self-discharge suppression remain challenging, yet are critical important for promoting the practical applications of fiber supercapacitors (FSCs) as advanced power supplier in future wearable electronics. Inspired by a tree trunk that not only provides enough structure stability against external attacks but also plays a vital role in nutrient transportation through luxuriant micro-conduits, the strategy of interfacial engineering is proposed for designing Ti3C2Tx fibers. 3-aminopropyltriethoxysilane (APTES) is employed as a reinforcer and spacer that is anchored on Ti3C2Tx surfaces via covalent bonds for modulating stress transfer and increasing accessible active sites, while tannic acid (TA) with abundant phenolic hydroxyl groups is introduced to construct a biomimetic interface for tuning ion diffusion kinetics and further improving tensile strength. The optimized fiber exhibits a high specific capacitance of 1573 F cm-3 at 1 A cm-3 with 83.8% retained at 15 A cm-3 (1318 F cm-3), an improved tensile strength of 152 MPa, and more sustainable self-discharge time twice as long as that of bare MXene fiber. The assembled symmetric FSCs deliver a volumetric energy density of 26.8 mWh cm-3 at a power density of 418.7 mW cm-3.

Ultra-Compact MXene/Alginate/PVA Composite Fibers by Intercalation and Chelation for Enhanced Flame Retardancy and Energy Harvesting

MXene fibers with electro-conductivity and electrochemical properties have drawn growing research interest for its promising applications in wearable electronics, flexible electrodes, and smart textiles. However, producing MXene fibers with high strength keeps challenging because loose MXene sheets are hard to compact tightly due to electrostatic repulsion. Herein, ultra-compact MXene-based fibers are produced by intercalating alginate and polyvinyl alcohol (PVA) layers into MXene nanosheets and chelating via metal ions (i.e., Ca2+). The hydrogen and ionic bond are beneficial to compact MXene nanosheets and decrease the interplanar spacing, which improves the tensile strength. These result in MXene-based fibers with low porosity (0.2 vol%) and a high orientation factor of 0.877 exhibiting high electrical conductivity (1006 S cm‒1). In addition, flame retardancy is enhanced without smoldering owing to the synergistic effect of MXene and metal ions. Moreover, these compact MXene-based fibers with electromagnetic interference shielding, mechanical stability, acid, and alkali-resistant properties, and photo-thermal effect can be achieved for scale production. This strategy paves the way for the continuous production of compact functional fibers, applicable in flame retardant fabric, wireless communication, energy harvesting, and wearable flexible textiles.

Surface Tension-Driven Self-Planarization of MXene Liquid Crystalline Fiber for High-Performance Energy Storage

2D MXene-based liquid crystalline (LC) systems have emerged as promising precursors for constructing highly ordered functional materials, such as fibers, films, and aerogels via solution-based processing. In this study, we demonstrate surface tension-mediated self-planarization of MXene LC fibers by adjusting the solvent composition during wet-spinning, targeting improved electrochemical performance. Ethanol, a poor solvent for MXene, induced spontaneous parallel alignment of MXene platelets and facilitated densification into a ribbon-like geometry during coagulation. The resulting fibers featured a pore volume of 0.11 cm3 g-1 and an average pore diameter of 34 nm, enabling a volumetric capacitance of 1721.7 F cm-3 and an electrical conductivity of 9211.66 S cm-1. The mechanism underlying the self-planarization was investigated by using a range of solvents with varying physicochemical properties to identify key processing parameters. The MXene fibers were successfully implemented into LED-powered supercapacitor prototypes, demonstrating potential applicability for wearable energy applications.

Thermally Conductive Ti3C2Tx Fibers with Superior Electrical Conductivity

High-performance Ti3C2Tx fibers have garnered significant potential for smart fibers enabled fabrics. Nonetheless, a major challenge hindering their widespread use is the lack of strong interlayer interactions between Ti3C2Tx nanosheets within fibers, which restricts their properties. Herein, a versatile strategy is proposed to construct wet-spun Ti3C2Tx fibers, in which trace amounts of borate form strong interlayer crosslinking between Ti3C2Tx nanosheets to significantly enhance interactions as supported by density functional theory calculations, thereby reducing interlayer spacing, diminishing microscopic voids and promoting orientation of the nanosheets. The resultant Ti3C2Tx fibers exhibit exceptional electrical conductivity of 7781 S cm-1 and mechanical properties, including tensile strength of 188.72 MPa and Young's modulus of 52.42 GPa. Notably, employing equilibrium molecular dynamics simulations, finite element analysis, and cross-wire geometry method, it is revealed that such crosslinking also effectively lowers interfacial thermal resistance and ultimately elevates thermal conductivity of Ti3C2Tx fibers to 13 W m-1 K-1, marking the first systematic study on thermal conductivity of Ti3C2Tx fibers. The simple and efficient interlayer crosslinking enhancement strategy not only enables the construction of thermal conductivity Ti3C2Tx fibers with high electrical conductivity for smart textiles, but also offers a scalable approach for assembling other nanomaterials into multifunctional fibers.

Interlayer Manipulation for Accelerating Ion Diffusion Kinetics in Ti3C2TX MXene Fiber toward Enhanced Supercapacitance with High Rate Capability

Polymer incorporation has been proven effective to enhance the mechanical strength of MXene fibers via interfacial cross-linking, yet the simultaneous improvement in electrochemical performance, particularly output capacitance and high rate capability, remains a challenge, and the major obstacle is identified as the sluggish ion diffusion kinetics. Herein, interlayer manipulation in Ti3C2TX fiber is proposed, and the roles of substitutional groups in celluloses are examined. The addition of cellulose can obviously improve the spinnability of MXene dope and effectively bridge the adjacent Ti3C2TX nanosheets via hydrogen bonds. Moreover, hydroxyethyl cellulose with a suitable group size and moderate adsorption ability is preferred for diminishing the steric effect and facilitating rapid proton transport. Simultaneous improvements in capacitance (1531 F cm-3 at 2 A cm-3) and strength (∼76 MPa) are achieved for the optimized M-HEC-1.0% fiber together with a superior high rate capability retaining 89.2% at 15 A cm-3.

Confined Hydrothermal Assembly of Hierarchical Porous MXene/RGO Composite Fibers with Enhanced Oxidation Resistance for High-Performance Wearable Supercapacitors

MXene fibers have emerged as a promising material for energy storage applications due to their exceptional electrical conductivity and surface chemistry. However, the inherent challenges of oxidation instability and severe restacking of MXene nanosheets significantly limit their practical applications in flexible energy storage devices. Here, an innovative confined hydrothermal strategy combined with chemical crosslinking to construct 3D hierarchical porous MXene/reduced graphene oxide (RGO) composite fibers is reported. This rationally designed architecture effectively prevents MXene stacking while creating efficient ion transport channels. Notably, thiourea serves dual roles as both a chemical cross-linker and selective reducing agent, while RGO nanosheets act as physical barriers, synergistically enhancing the composite's oxidation resistance. The optimized composite fiber exhibits outstanding electrical conductivity (862.2 S cm-1), mechanical strength (93.1 MPa), and remarkable oxidation resistance (90.5% conductivity retention after 60 days). The assembled solid-state supercapacitor achieves excellent mechanical flexibility, superior cycling stability, and impressive energy density (13.5 mWh cm-3) and power density (1.9 W cm-3). This work provides a novel and effective strategy for developing high-performance flexible wearable energy storage devices.

Interfacial Confinement Derived High-Strength MXene@Graphene Oxide Core-Shell Fibers for Electromagnetic Wave Regulation, Thermochromic Alerts, and Visible Camouflage

Although electrically conductive Ti₃C₂Tx MXene fibers are promising for wearable electronics, the poor inter-sheet interactions and the random stacking structure of MXene sheets seriously hinder electron transport and load transfer of the fibers. Herein, mechanically strong and electrically conductive MXene@graphene oxide (GO) core-shell fibers are fabricated with a coaxial wet-spinning methodology for electromagnetic wave regulation, thermochromic alerts, and visible camouflage. During the coaxial wet-spinning, the trace-carboxylated GO sheets in the shell align readily because of the spatial confinement of the coaxial needle, while the MXene sheets in the core are progressively oriented and flattened because of the spatial confinement of the GO shell. The positively charged chitosan in the coagulating solution enhances the interfacial interactions between the GO and MXene sheets and facilitates the sheet's orientation inside the fibers. Consequently, the highly aligned core-shell fibers exhibit an ultrahigh tensile strength of 613.7 MPa and an outstanding conductivity of ≈7766 S cm-1. Furthermore, fiber-woven textiles not only offer excellent electromagnetic interference shielding performance but also achieve quantitative regulation of electromagnetic wave transmission by adjusting the angle of the double-layered textiles. The textiles can combine with thermochromic coatings for thermotherapy alerts, visual thermochromic warnings, and visible camouflage.

Tradeoff between Mechanical Strength and Electrical Conductivity of MXene Films by Nacre-Inspired Subtractive Manufacturing

Traditional strategies, by additive manufacturing, for integrating monolayer Ti3C2Tx nanosheets into macroscopic films with binders can effectively improve their mechanical strength, but the electrical conductivity is often sacrificed. Herein, inspired by the aligned nano-compacted feature of nacre, a flexible subtractive manufacturing strategy is reported to squeeze the interlayer 2D spacings by removing the nanoconfined water and interface terminations, leading to the improvement of mechanical strength and stability of Ti3C2Tx layered films without sacrificing the electrical conductivity. After the vacuum annealing of Ti3C2Tx films at 300 °C (A300), the interlayer 2D spacing decreased ≈0.1 nm with the surface functional groups (═O, ─OH, ─F) and interlayer water molecules greatly removed. The tensile strength (95.59 MPa) and Young's modulus (9.59 GPa) of A300 are ≈3 and ≈2 times improved, respectively. Moreover, the A300 films maintain a metallic electrical conductivity (2276 S cm-1) and show greatly enhanced stability. Compared to the original films, the mechanical strength of the A300 films is enhanced by increasing the interlayer friction and energy dissipation with the decrease of interlayer 2D spacings. This work provides a new way for engineering the self-assembled films with more functions for broad applications.

Inkjet-printed flexible V2CTx film electrodes with excellent photoelectric properties and high capacities for energy storage device

Energy storage devices are progressively advancing in the light-weight, flexible, and wearable direction. Ti3C2Tx flexible film electrodes fabricated via a non-contact, cost-effective, high-efficiency, and large-scale inkjet printing technology were capable of satisfying these demands in our previous report. However, other MXenes that can be employed in flexible energy storage devices remain undiscovered. Herein, flexible V2CTx film electrodes (with the low formula weight vs Ti3C2Tx film electrodes) with both high capacities and excellent photoelectric properties were first fabricated. The area capacitances of V2CTx film electrodes reached 531.3-5787.0 μF⋅cm-2 at 5 mV⋅s-1, corresponding to the figure of merits (FoMs) of 0.07-0.15. Noteworthy, V2CTx film electrode exhibited excellent cyclic stability with the capacitance retention of 83 % after 7,000 consecutive charge-discharge cycles. Furthermore, flexible all solid-state symmetric V2CTx supercapacitor was assembled with the area capacitance of 23.4 μF⋅cm-2 at 5 mV⋅s-1. Inkjet printing technology reaches the combination of excellent photoelectric properties and high capacities of flexible V2CTx film electrodes, which provides a new strategy for manufacturing MXene film electrodes, broadening the application prospect of flexible energy storage devices.

"Bridge" interface design modulates high-performance cellulose-based integrated flexible supercapacitors

Flexible supercapacitors offer significant potential for powering next-generation flexible electronics. However, the mechanical and electrochemical stability of flexible supercapacitors under different flexibility conditions is limited by the weak bonding between neighboring layers, posing a major obstacle to their practical application. In this paper, natural coniferous pulp cellulose was successfully modified with ethylenediamine and NiSe2/Cell-NH2/MoS2 cellulose flexible electrodes (NCMF) were fabricated by phase transfer and hydrothermal methods. The amino-modified cellulose (Cell-NH2) acts as a "bridge" between NiSe2 and MoS2, significantly enhancing the interfacial bonding strength of the flexible electrode. The integrated flexible electrode exhibited a high area capacitance (2475 mF/cm2), strong tensile strength (10.3 MPa), and excellent cycling stability (92.1 % capacitance retention after 2500 cycles). The sandwich-structured monolithic supercapacitor achieved both a high electrode capacitance (56.78 F/cm2) and a high energy density (1971.53 μWh/cm2), while maintaining long cycling life (72.73 % capacitance retention after 2000 cycles) and large deformation capability. This makes it suitable for stable power supplies in electronic products and opens new possibilities for the flexible wearable industry. This technology enables stable power supplies for electronic products and paves the way for advancements in the flexible wearable industry.

Revealing the Structural Architecture of Anions Confining Mo2CTx MXene Layers for Robust Li+ Storage

Controllable cation preintercalation enables enhancing the electrochemical activity and kinetics of MXenes. However, the electrostatic repulsion between cations and electrolyte ions induces deteriorative electrolyte ion transport kinetics. Herein, by shifting perceptions from the cation to anion strategies, we successfully preintercalate Cl-, SO42-, and PO43- anions into Mo2CTx MXene via the utilization of diverse etching agents. Due to a smaller ionic radius and low charge, more Cl- ions can be intercalated into Mo2CTx MXene and induce higher dislocation density, larger interlayer spacing, and more negative Zeta potential value. Relying on in situ X-ray diffraction, we monitored the interlayer evolution. The lower lithium-ion concentration gradient in the Mo2CTx MXene delivers a lower concentration polarization, a fast charge and ion transfer kinetics, and an excellent lifespan, holding 540.49 mAh g-1 after 400 cycles at 200 mA g-1. The effect of anion preintercalation provides new insights into the function-oriented design of MXene materials.

Harnessing Holey MXene/Graphene Oxide Heterostructure to Maximize Ion Channels in Lamellar Film for High-Performance Capacitive Deionization

2D Ti3C2Tx MXene-based film electrodes with metallic conductivity and high pseudo-capacitance are of considerable interest in cutting-edge research of capacitive deionization (CDI). Further advancement in practical use is however impeded by their intrinsic limitations, e.g., tortuous ion diffusion pathway of layered stacking, vulnerable chemical stability, and swelling-prone nature of hydrophilic MXene nanosheet in aqueous environment. Herein, a nanoporous 2D/2D heterostructure strategy is established to leverage both merits of holey MXene (HMX) and holey graphene oxide (HGO) nanosheets, which optimize ion transport shortcuts, alleviate common restacking issues, and improve film's mechanical and chemical stability. In this design, the nanosized in-plane holes in both handpicked building blocks build up ion diffusion shortcuts in the composite laminates to accelerate the transport and storage of ions. As a direct outcome, the HMX/rHGO films exhibit remarkable desalination capacity of 57.91 mg g-1 and long-term stability in 500 mg L-1 NaCl solution at 1.2 V. Moreover, molecular dynamics simulations and ex situ wide angle X-ray scattering jointly demonstrate that the conductive 2D/2D networks and ultra-short ion diffusion channels play critical roles in the ion intercalation/deintercalation process of HMX/rHGO films. The study paves an alternative design concept of freestanding CDI electrodes with superior ion transport efficiency.

Mechanically robust and electrically conductive nanofiber composites with enhanced interfacial interaction for strain sensing

Electrically conductive fiberfibre/fabric composites (ECFCs) are competitive candidates for use in wearable electronics. Therefore, it is essential to develop mechanically robust ECFC strain sensors with sensing performance. In this study, MXene assembly and hot-pressing were combined to prepare strong yet breathable ECFCs for strain and temperature sensing. Hydrogen bonding between MXene and polyurethane (PU) and ultrasonication-induced interfacial sintering were responsible for MXene nanosheets assembly on the PU nanofibers. MXene decoration made PU nanofibers electrically conductive, resulting in a conductive network. Hot-pressing improved interface adhesion among the conductive nanofibers. Thus, the mechanical properties of the nanofiber composites, including tensile strength, toughness and fracture energy, were enhanced. The nanofiber composites exhibited surface stability and durability. When the nanofiber composites were used as strain sensors, they showed breathability with a linear resistance response ranging from 1 % to 100 % and cycling stability. In addition, they produced stable sensing signals over 1000 cycles when a notch was present. They could also monitor temperature variations with a negative temperature coefficient (-0.146 %/°C). This study provides an interfacial regulation method for the preparation of multi-functional nanofiber composites with potential applications in flexible and wearable electronics.

Interfacial modulation of Ti3C2Tx MXene using functionalized cellulose nanofibrils for enhanced electrochemical actuation

Electrochemical actuators (ECAs) with low voltage actuation and large deformation ranges generally require electrode materials with high ion kinetic energy transport, high charge storage, and excellent electrochemical-mechanical properties. However, the fabrication of such actuators remains a major challenge. In the present work, hybrid electroactive films were fabricated by self-assembling one-dimensional functionalized cellulose nanofibrils (CNFs) with two-dimensional MXene (Ti3C2Tx). The obtained ECA actuators fabricated by carboxymethylated cellulose nanofibrils (consisting of -CH2COO-surface groups) with Ti3C2Tx integrate excellent curvature (0.1041 mm-1), mechanical strength (21.68 MPa), a bending strain of 0.50 %, and a good actuation displacement of 9.3 mm at a low voltage range of -0.6 to 0.3 V. This may be attributed to the enlarged layer spacing (15.34 Å), which makes the embedding and transport of H+ easier, and excellent adaptivity of mechanical properties achieved by molecular-scaled strong hydrogen bonding, leading to better actuation performance. This study provides a potential research direction for the preparation of ECAs with large actuation deformation.

Interfacially Ordered NiCoMoS Nanosheets Arrays on Hierarchical Ti3C2Tx MXene for High-Energy-Density Fiber-Shaped Supercapacitors with Accelerated Pseudocapacitive Kinetics

Balancing electrochemical activity and structural reversibility of fibrous electrodes with accelerated Faradaic charge transfer kinetics and pseudocapacitive storage are highly crucial for fiber-shaped supercapacitors (FSCs). Herein, we report novel core-shell hierarchical fibers for high-performance FSCs, in which the ordered NiCoMoS nanosheets arrays are chemically anchored on Ti3C2Tx fibers. Beneficial from architecting stable polymetallic sulfide arrays and conductive networks, the NiCoMoS-Ti3C2Tx fiber maintains fast charge transfer, low diffusion and OH- adsorption barrier, and stabilized multi-electronic reaction kinetics of polymetallic sulfide. Consequently, the NiCoMoS-Ti3C2Tx fiber exhibits a large volumetric capacitance (2472.3 F cm-3) and reversible cycling performance (20,000 cycles). In addition, the solid-state symmetric FSCs deliver a high energy density of 50.6 mWh cm-3 and bending stability, which can significantly power electronic devices and offer sensitive detection for dopamine.

Self-Thermal Management in Filtered Selenium-Terminated MXene Films for Flexible Safe Batteries

Li-ion batteries with superior interior thermal management are crucial to prevent thermal runaway and ensure safe, long-lasting operation at high temperatures or during rapid discharging and charging. Typically, such thermal management is achieved by focusing on the separator and electrolyte. Here, the study introduces a Se-terminated MXene free-standing electrode with exceptional electrical conductivity and low infrared emissivity, synergistically combining high-rate capacity with reduced heat radiation for safe, large, and fast Li+ storage. This is achieved through a one-step organic Lewis acid-assisted gas-phase reaction and vacuum filtration. The Se-terminated Nb2Se2C outperformed conventional disordered O/OH/F-terminated materials, enhancing Li+-storage capacity by ≈1.5 times in the fifth cycle (221 mAh·g-1 at 1 A·g-1) and improving mid-infrared adsorption with low thermal radiation. These benefits result from its superior electrical conductivity, excellent structural stability, and high permittivity in the infrared region. Calculations further reveal that increased permittivity and conductivity along the z-direction can reduce heat radiation from electrodes. This work highlights the potential of surface groups-terminated layered material-based free-standing flexible electrodes with self-thermal management ability for safe, fast energy storage.

Ultrahigh Sensitivity for Thermographic Human-Machine Interface via Precious Metals Atomic Layer Deposition on V-MXene: Computational and Experimental Exploration

Global healthcare based on the Internet of Things system is rapidly transforming to measure precise physiological body parameters without visiting hospitals at remote patients and associated symptoms monitoring. 2D materials and the prevailing mood of current ever-expanding MXene-based sensing devices motivate to introduce first the novel iridium (Ir) precious metal incorporated vanadium (V)-MXene via industrially favored emerging atomic layer deposition (ALD) techniques. The current work contributes a precise control and delicate balance of Ir single atomic forms or clusters on the V-MXene to constitute a unique precious metal-MXene embedded heterostructure (Ir-ALD@V-MXene) in practical real-time sensing healthcare applications to thermography with human-machine interface for the first time. Ir-ALD@V-MXene delivers an ultrahigh durability and sensing performance of 2.4% °C-1 than pristine V-MXene (0.42% °C-1), outperforming several conventionally used MXenes, graphene, underscoring the importance of the Ir-ALD innovative process. Aberration-corrected advanced ultra-high-resolution transmission/scanning transmission electron microscopy confirms the presence of Ir atomic clusters on well-aligned 2D-layered V-MXene structure and their advanced heterostructure formation (Ir-ALD@V-MXene), enhanced sensing mechanism is investigated using density functional theory (DFT) computations. A rational design empowering the Ir-ALD process on least explored V-MXene can potentially unfold further precious metals ALD-process developments for next-generation wearable personal healthcare devices.

Regulating Functional Groups Enhances the Performance of Flexible Microporous MXene/Bacterial Cellulose Electrodes in Supercapacitors

Ultrathin MXene-based films exhibit superior conductivity and high capacitance, showing promise as electrodes for flexible supercapacitors. This work describes a simple method to enhance the performance of MXene-based supercapacitors by expanding and stabilizing the interlayer space between MXene flakes while controlling the functional groups to improve the conductivity. Ti3C2Tx MXene flakes are treated with bacterial cellulose (BC) and NaOH to form a composite MXene/BC (A-M/BC) electrode with a microporous interlayer and high surface area (62.47 m2 g-1). Annealing the films at low temperature partially carbonizes BC, increasing the overall electrical conductivity of the films. Improvement in conductivity is also attributed to the reduction of -F, -Cl, and -OH functional groups, leaving -Na and -O functional groups on the surface. As a result, the A-M/BC electrode demonstrates a capacitance of 594 F g-1 at a current density of 1 A g-1 in 3 M H2SO4, which represents a ∼2× increase over similarly processed films without BC (309 F g-1) or pure MXene (298 F g-1). The corresponding device has an energy density of 9.63 Wh kg-1 at a power density of 250 W kg-1. BC is inexpensive and enhances the overall performance of MXene-based film electrodes in electronic devices. This method underscores the importance of functional group regulation in enhancing MXene-based materials for energy storage.

Recent Advances of Flexible MXene and its Composites for Supercapacitors

MXenes have unique properties such as high electrical conductivity, excellent mechanical properties, rich surface chemistry, and convenient processability. These characteristics make them ideal for producing flexible materials with tunable microstructures. This paper reviews the laboratory research progress of flexible MXene and its composite materials for supercapacitors. And introduces the general synthesis method of MXene, as well as the preparation and properties of flexible MXene. By analyzing the current research status, the electrochemical reaction mechanism of MXene was explained from the perspectives of electrolyte and surface terminating groups. This review particularly emphasizes the composite methods of freestanding flexible MXene composite materials. The review points out that the biggest problem with flexible MXene electrodes is severe self-stacking, which reduces the number of chemically active sites, weakens ion accessibility, and ultimately lowers electrochemical performance. Therefore, it is necessary to composite MXene with other electrode materials and design a good microstructure. This review affirms the enormous potential of flexible MXene and its composite materials in the field of supercapacitors. In addition, the challenges and possible improvements faced by MXene based materials in practical applications were also discussed.

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.

Sodium Dodecyl Sulfate-Adjusted Phase Composition of Hydrated Tungsten Oxides as Stable Self-Supporting Electrodes for Supercapacitors with High Volumetric Specific Capacitance

High pseudocapacitive activity of hydrated tungsten oxides (WO3·xH2O, x = 1 or 2) makes them promising materials for supercapacitors (SCs). During their synthesis, additives such as complexing agents and surfactants generally can only affect the morphology and/or size of the products. Here, we demonstrate that not only morphology and size of WO3·xH2O were affected, its phase composition could also change from WO3·2H2O to WO3·H2O simply by increasing the amount of sodium dodecyl sulfate (SDS) during its anodization synthesis. To the best of our knowledge, such a phenomenon has not been reported before. In addition, SDS introduced a special structure to the products, i.e., WO3·xH2O nanoplatelets constructed from nanoparticle multilayers with abundant nanogaps between the multilayers, which further arranged into nanoflowers with increased amounts of SDS. Benefiting from such a structure, low internal resistance, enhanced stability, and fast redox kinetics, the as-obtained WO3·xH2O/W-3 self-supporting electrode showed a high volumetric specific capacitance of 1402.92 F cm-3 and good cycling stability (a capacity retention of 106% after 10 000 cycles). In addition, an all-solid-state asymmetric SC device based on WO3·xH2O/W-3 delivered high a volumetric energy density of 44.0 mW h cm-3 at 0.5 W cm-3. Our method demonstrates a potential way to fabricate excellent self-supporting electrodes for SCs.

Spider-Silk-Inspired Tough, Self-Healing, and Melt-Spinnable Ionogels

As stretchable conductive materials, ionogels have gained increasing attention. However, it still remains crucial to integrate multiple functions including mechanically robust, room temperature self-healing capacity, facile processing, and recyclability into an ionogel-based device with high potential for applications such as soft robots, electronic skins, and wearable electronics. Herein, inspired by the structure of spider silk, a multilevel hydrogen bonding strategy to effectively produce multi-functional ionogels is proposed with a combination of the desirable properties. The ionogels are synthesized based on N-isopropylacrylamide (NIPAM), N, N-dimethylacrylamide (DMA), and ionic liquids (ILs) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). The synergistic hydrogen bonding interactions between PNIPAM chains, PDMA chains, and ILs endow the ionogels with improved mechanical strength along with fast self-healing ability at ambient conditions. Furthermore, the synthesized ionogels show great capability for the continuous fabrication of the ionogel-based fibers using the melt-spinning process. The ionogel fibers exhibit spider-silk-like features with hysteresis behavior, indicating their excellent energy dissipation performance. Moreover, an interwoven network of ionogel fibers with strain and thermal sensing performance can accurately sense the location of objects. In addition, the ionogels show great recyclability and processability into different shapes using 3D printing. This work provides a new strategy to design superior ionogels for diverse applications.

High-strength nacre-like composite films based on pre-polymerised polydopamine and polyethyleneimine cross-linked MXene layers via multi-bonding interactions

Demands for high-strength flexible electrodes have significantly increased across various fields, especially in wearable electronics. Inspired by the strong integrated layered structure of the natural nacre via multi-bonding interactions, we report the fabrication of the strong integrated nacre-like composite films based on pre-polymerised polydopamine and polyethyleneimine cross-linked MXene layers (p-DEM), achieving the synergic effect of hydrogen bonding, covalent bonding and electrostatic interactions. As a result, a high-level tensile strength of ∼302 MPa, 10.8 times higher than that of the plain MXene film, is obtained for the prepared p-DE0.5M composite film. Meanwhile, the composite film also delivers superior energy storage (∼1218F cm-3 at 5 mV s-1) and rate performances (capacitance retention of 81.3% at 1000 mV s-1). To demonstrate the practical application of the composite films, a symmetrical supercapacitor based on p-DE0.5M electrodes is assembled, which shows stable energy storage performances under different deformation states such as bending angles at 0, 60, 90 and 180°, or withstand repeated bending times (1000 cycles). This type of multi-bonding interactions induced strong integrated MXene assembly may promote the wide applications of MXene-based films in flexible electronics, artificial intelligence, and tissue engineering, to name a few.

Interfacial Modulation of Ti3 C2 Tx MXene by Cellulose Nanofibrils to Construct Hybrid Fibers with High Volumetric Specific Capacitance

The intrinsic poor rheological properties of MXene inks result in the MXene nanosheets in dried MXene microfibers prone to self-stacking, which is not conducive to ion transport and diffusion, thus affecting the electrochemical performance of fiber-based supercapacitors. Herein, robust cellulose nanofibrils (CNF)/MXene hybrid fibers with high electrical conductivity (916.0 S cm-1 ) and narrowly distributed mesopores are developed by wet spinning. The interfacial interaction between CNF and MXene can be enhanced by hydrogen bonding and electrostatic interaction due to their rich surface functional groups. The interfacial modulation of MXene by CNF can not only regulate the rheology of MXene spinning dispersion, but also enhance the mechanical strength. Furthermore, the interlayer distance and self-stacking effect of MXene nanosheets are also regulated. Thus, the ion transport path within the fiber material is optimized and ion transport is accelerated. In H2 SO4 electrolyte, a volumetric specific capacitance of up to 1457.0 F cm-3 (1.5 A cm-3 ) and reversible charge/discharge stability are demonstrated. Intriguingly, the assembled supercapacitors exhibit a high-volume energy density of 30.1 mWh cm-3 at 40.0 mW cm-3 . Moreover, the device shows excellent flexibility and cycling stability, maintaining 83% of its initial capacitance after 10 000 charge/discharge cycles. Practical energy supply applications (Power for LED and electronic watch) can be realized.

Rheology Engineering for Dry-Spinning Robust N-Doped MXene Sediment Fibers toward Efficient Charge Storage

MXene nanosheets are believed to be an ideal candidate for fabricating fiber supercapacitors (FSCs) due to their metallic conductivity and superior volumetric capacitance, while challenges remain in continuously collecting bare MXene fibers (MFs) via the commonly used wet-spinning technique due to the intercalation of water molecules and a weak interaction between Ti3 C2 TX nanosheets in aqueous coagulation bath that ultimately leads to a loosely packed structure. To address this issue, for the first time, a dry-spinning strategy is proposed by engineering the rheological behavior of Ti3 C2 TX sediment and extruding the highly viscose stock directly through a spinneret followed by a solvent evaperation induced solidification. The dry-spun Ti3 C2 TX fibers show an optimal conductivity of 2295 S cm-1 , a tensile strength of 64 MPa and a specific capacitance of 948 F cm-3 . Nitrogen (N) doping further improves the capacitance of MFs to 1302 F cm-3 without compromising their mechanical and electrical properties. Moreover, the FSC based on N-doped MFs exhibits a high volumetric capacitance of 293 F cm-3 , good stability over 10 000 cycles, excellent flexibility upon bending-unbending, superior energy/power densities and anti-self-discharging property. The excellent electrochemical and mechanical properties endow the dry-spun MFs great potential for future applications in wearable electronics.