Gradiently Foaming Ultrasoft Hydrogel with Stop Holes for Highly Deformable, Crack-Resistant and Sensitive Conformal Human-Machine Interfaces.

Hydrogels are considered as promising materials for human-machine interfaces (HMIs) owing to their merits of tailorable mechanical and electrical properties; nevertheless, it remains challenging to simultaneously achieve ultrasoftness, good mechanical robustness and high sensitivity, which are the pre-requisite requirements for wearable sensing applications. Herein, for the first time, this work proposes a universal phase-transition-induced bubbling strategy to fabricate ultrasoft gradient foam-shaped hydrogels (FSHs) with stop holes for high deformability, crack-resistance and sensitive conformal HMIs. As a typical system, the FSH based on polyacrylamide/sodium alginate system shows an ultralow Young's modulus (1.68 kPa), increased sustainable strain (1411%), enhanced fracture toughness (915.6 J m-2), improved tensile sensitivity (21.77), and compressive sensitivity (65.23 kPa-1). The FSHs are used for precisely acquiring and identifying gesture commands of the operator to remotely control a surgical robot for endoscopy and an electric ship in a first-person perspective for cruising, feeding crabs and monitoring the environmental change in real-time.

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.

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

MXene fibers are promising candidates for weaveable and wearable energy storage devices because of their good electrical conductivity and high theoretical capacitance. Herein, we propose a nacre-inspired strategy for simultaneously improving the mechanical strength, volumetric capacitance, and rate performance of MXene-based fibers through synergizing the interfacial interaction and interlayer spacing between Ti3C2TX nanosheets. The optimized hybrid fibers (M-CMC-1.0%) with 99 wt % MXene loading exhibit an improved tensile strength of ∼81 MPa and a high specific capacitance of 885.0 F cm-3 at 1 A cm-3 together with an outstanding rate performance of 83.6% retention at 10 A cm-3 (740.0 F cm-3). As a consequence, the fiber supercapacitor (FSC) based on the M-CMC-1.0% hybrid delivers an output capacitance of 199.5 F cm-3, a power density of 1186.9 mW cm-3, and an energy density of 17.7 mWh cm-3, respectively, implying its promising applications as portable energy storage devices for future wearable electronics.

Ti3 C2 TX MXene Ink Direct Writing Flexible Sensors for Disposable Paper Toys.

Flexible electronics have gained great attention in recent years owing to their promising applications in biomedicine, sustainable energy, human-machine interaction, and toys for children. Paper mainly produced from cellulose fibers is attractive substrate for flexible electronics because it is biodegradable, foldable, tailorable, and light-weight. Inspired by daily handwriting, the rapid prototyping of sensing devices with arbitrary patterns can be achieved by directly drawing conductive inks on flat or curved paper surfaces; this provides huge freedom for children to design and integrate "do-it-yourself (DIY)" electronic toys. Herein, viscous and additive-free ink made from Ti3 C2 TX MXene sediment is employed to prepare disposable paper electronics through a simple ball pen drawing. The as-drawn paper sensors possess hierarchical microstructures with interweaving nanosheets, nanoflakes, and nanoparticles, therefore exhibiting superior mechanosensing performances to those based on single/fewer-layer MXene nanosheets. As proof-of-concept applications, several popular children's games are implemented by the MXene-based paper sensors, including "You say, I guess," "Emotional expression," "Rock-Paper-Scissors," "Arm wrestling," "Throwing game," "Carrot squat," and "Grab the cup," as well as a DIY smart whisker for a cartoon mouse. Moreover, MXene-based paper sensors are safe and disposable, free from producing any e-waste and hazard to the environment.

Green Flexible Electronics: Natural Materials, Fabrication, and Applications.

The emergence of plastic electronics satisfies the increasing demand for flexible electronics. However, it has caused severe ecological problems. Flexible electronics based on natural materials are increasing to hopefully realize the "green" and eco-friendly concept. Herein, recent advances in the design and fabrication of green flexible electronics are reviewed. First, this review comprehensively introduces various natural materials and derivatives, focusing particularly on fibroin and silk, wood and paper, plants, and biomass. Second, fabrication techniques for modifying natural materials, including physical and chemical methods, are presented, after which their merits and demerits are thoroughly discussed. Green flexible electronics based on natural materials, comprising electrical wires/electrodes, antennas, thermal management devices, transistors, memristors, sensors, energy-harvesting devices, energy-storage devices, displays, actuators, electromagnetic shielding, and integration systems, are described in detail. Finally, perspectives on the existing challenges and opportunities to employ natural materials in green flexible electronics are presented.

Universal Strategy for Preparing Highly Stable PBA/Ti3C2Tx MXene toward Lithium-Ion Batteries via Chemical Transformation.

Prussian blue analogues (PBAs) are believed to be intriguing anode materials for Li+ storage because of their tunable composition, designable topologies, and tailorable porous structures, yet they suffer from severe capacity decay and inferior cycling stability due to the volume variation upon lithiation and high electrical resistance. Herein, we develop a universal strategy for synthesizing small PBA nanoparticles hosted on two-dimensional (2D) MXene or rGO (PBA/MX or PBA/rGO) via an in situ transformation from ultrathin layered double hydroxides (LDH) nanosheets. 2D conductive nanosheets allow for fast electron transport and guarantee the full utilization of PBA even at high rates; at the meantime, PBA nanoparticles effectively prevent 2D materials from restacking and facilitate rapid ion diffusion. The optimized Ni0.8Mn0.2-PBA/MX as an anode for lithium-ion batteries (LIBs) delivers a capacity of 442 mAh g-1 at 0.1 A g-1 and an excellent cycling robustness in comparison with bare PBA bulk crystals. We believe that this study offers an alternative choice for rationally designing PBA-based electrode materials for energy storage.

Mechanisms for self-templating design of micro/nanostructures toward efficient energy storage.

The ever-growing demand in modern power systems calls for the innovation in electrochemical energy storage devices so as to achieve both supercapacitor-like high power density and battery-like high energy density. Rational design of the micro/nanostructures of energy storage materials offers a pathway to finely tailor their electrochemical properties thereby enabling significant improvements in device performances and enormous strategies have been developed for synthesizing hierarchically structured active materials. Among all strategies, the direct conversion of precursor templates into target micro/nanostructures through physical and/or chemical processes is facile, controllable, and scalable. Yet the mechanistic understanding of the self-templating method is lacking and the synthetic versatility for constructing complex architectures is inadequately demonstrated. This review starts with the introduction of five main self-templating synthetic mechanisms and the corresponding constructed hierarchical micro/nanostructures. Subsequently, the structural merits provided by the well-defined architectures for energy storage are elaborately discussed. At last, a summary of current challenges and future development of the self-templating method for synthesizing high-performance electrode materials is also presented.

The Jahn-Teller Effect for Amorphization of Molybdenum Trioxide towards High-Performance Fiber Supercapacitor.

Amorphous pseudocapacitive nanomaterials are highly desired in energy storage applications for their disordered crystal structures, fast electrochemical dynamics, and outstanding cyclic stability, yet hardly achievable using the state-of-the-art synthetic strategies. Herein, for the first time, high capacitive fiber electrodes embedded with nanosized amorphous molybdenum trioxide (A-MoO3-x) featuring an average particle diameter of ~20 nm and rich oxygen vacancies are obtained via a top-down method using α-MoO3 bulk belts as the precursors. The Jahn-Teller distortion in MoO6 octahedra due to the doubly degenerate ground state of Mo5+, which can be continuously strengthened by oxygen vacancies, triggers the phase transformation of α-MoO3 bulk belts (up to 30 µm long and 500 nm wide). The optimized fibrous electrode exhibits among the highest volumetric performance with a specific capacitance (C V ) of 921.5 F cm-3 under 0.3 A cm-3, endowing the fiber-based weaveable supercapacitor superior C V and E V (energy density) of 107.0 F cm-3 and 9.5 mWh cm-3, respectively, together with excellent cyclic stability, mechanical robustness, and rate capability. This work demonstrates a promising strategy for synthesizing nanosized amorphous materials in a scalable, cost-effective, and controllable manner.

Ti3C2TX MXene for Sensing Applications: Recent Progress, Design Principles, and Future Perspectives.

Sensors are becoming increasingly significant in our daily life because of the rapid development in electronic and information technologies, including Internet of Things, wearable electronics, home automation, intelligent industry, etc. There is no doubt that their performances are primarily determined by the sensing materials. Among all potential candidates, layered nanomaterials with two-dimensional (2D) planar structure have numerous superior properties to their bulk counterparts which are suitable for building various high-performance sensors. As an emerging 2D material, MXenes possess several advantageous features of adjustable surface properties, tunable bandgap, and excellent mechanical strength, making them attractive in various applications. Herein, we particularly focus on the recent research progress in MXene-based sensors, discuss the merits of MXenes and their derivatives as sensing materials for collecting various signals, and try to elucidate the design principles and working mechanisms of the corresponding MXene-based sensors, including strain/stress sensors, gas sensors, electrochemical sensors, optical sensors, and humidity sensors. In the end, we analyze the main challenges and future outlook of MXene-based materials in sensor applications.

Mechanistic insight in site-selective and anisotropic etching of prussian blue analogues toward designable complex architectures for efficient energy storage.

Engineering coordination compounds, e.g., prussian blue (PB) and its analogues (PBAs), with designable complex nanostructures via chemical etching holds great opportunities for improving energy storage performances by adjusting topological geometry, selectively exposing active sites, tuning electronic properties and enhancing accessible surface area. Unfortunately, it remains ambiguous particularly on site-selective and anisotropic etching behaviors. Herein, for the first time, we propose that two distinct regions are formed inside NiCo PBA (NCP) cubes due to the competition between classical ion-by-ion crystallization and non-classical crystallization based on aggregation. Such a unique structure ultimately determines not only the etching position but also the anisotropic pathway by selectively exposing unprotected Ni sites. According to this principle, complex PBA architectures, including nanocages, open nanocubes (constructed by six cones sharing the same apex), nanocones, and chamfer nanocubes can be intentionally obtained. After thermal annealing, NCP nanocones are converted to morning glory-like porous architectures composed of NiO/NiCo2O4 heterostructures with a mean particle size of 5 nm, which show improved rate performance and cycling stability.

Solution-Processed Sensing Textiles with Adjustable Sensitivity and Linear Detection Range Enabled by Twisting Structure.

Wearable strain sensors are emerging rapidly for their promising applications in human motion detection for diagnosis, healthcare, training instruction, and rehabilitation exercise assessment. However, it remains a bottleneck in gaining comfortable and breathable devices with the features of high sensitivity, linear response, and tunable detection range. Textiles possess fascinating advantages of good breathability, aesthetic property, tailorability, and excellent mechanical compliance to conformably attach to human body. As the meandering loops in a textile can be extended in different directions, it provides plenty of room for exploring ideal sensors by tuning a twisting structure with rationally selected yarn materials. Herein, textile sensors with twisting architecture are designed via a solution-based process by using a stable water-based conductive ink that is composed of polypyrrole/polyvinyl alcohol nanoparticles with a mean diameter of 50 nm. Depending on the predesigned twisting models, the thus-fabricated textile sensors show adjustable performances, exhibiting a high sensitivity of 38.9 with good linearity and a broad detection range of 200%. Such sensors can be integrated into fabrics and conformably attached to skin for monitoring subtle (facial expressions, breathing, and speaking) and large (stretching, jumping, running and jogging, and sign language) human motions. As a proof-of-concept application, by integrating with a wireless transmitter, the signals detected by our sensors during exercise (e.g., running) can be remotely received and displayed on a smartphone. It is believed that the integration of our textile sensors with selected twisting models into a cloth promises full-range motion detection for wearable electronics and human-machine interfaces.

Electrostatically Assembling 2D Nanosheets of MXene and MOF-Derivatives into 3D Hollow Frameworks for Enhanced Lithium Storage.

As an essential member of 2D materials, MXene (e.g., Ti3 C2 Tx ) is highly preferred for energy storage owing to a high surface-to-volume ratio, shortened ion diffusion pathway, superior electronic conductivity, and neglectable volume change, which are beneficial for electrochemical kinetics. However, the low theoretical capacitance and restacking issues of MXene severely limit its practical application in lithium-ion batteries (LIBs). Herein, a facile and controllable method is developed to engineer 2D nanosheets of negatively charged MXene and positively charged layered double hydroxides derived from ZIF-67 polyhedrons into 3D hollow frameworks via electrostatic self-assembling. After thermal annealing, transition metal oxides (TMOs)@MXene (CoO/Co2 Mo3 O8 @MXene) hollow frameworks are obtained and used as anode materials for LIBs. CoO/Co2 Mo3 O8 nanosheets prevent MXene from aggregation and contribute remarkable lithium storage capacity, while MXene nanosheets provide a 3D conductive network and mechanical robustness to facilitate rapid charge transfer at the interface, and accommodate the volume expansion of the internal CoO/Co2 Mo3 O8 . Such hollow frameworks present a high reversible capacity of 947.4 mAh g-1 at 0.1 A g-1 , an impressive rate behavior with 435.8 mAh g-1 retained at 5 A g-1 , and good stability over 1200 cycles (545 mAh g-1 at 2 A g-1 ).

Identifying the active site of ultrathin NiCo LDH as an efficient peroxidase mimic with superior substrate affinity for sensitive detection of hydrogen peroxide.

Nanozymes have been extensively investigated to imitate protein enzymes in biomimetic chemistry and the identification of the active site is believed to be the pre-requisite before one can effectively regulate their activity. Herein, ultrathin NiCo LDH nanosheets are synthesized via a fast co-precipitation at room temperature and can be stably dispersed in water without any additives of surfactants or organic solvents. By tuning the ratio between Ni and Co in LDH nanosheets, the activity is tuned and their peroxidase-like activity is determined by Co sites that show higher affinity to both 3,3',5,5'-tetramethylbenzidine (TMB) and hydrogen peroxide (H2O2) due to the strong Lewis acidity of Co3+ and the low redox potential of Co3+/Co2+. Together with their small crystallite size, ultra-thin thickness and tunable composition, NiCo LDH is used as a nanozyme for highly sensitive colorimetric detection of H2O2 and the limit of detection (LOD) reaches 0.48 µM.

Polarity-assisted formation of hollow-frame sheathed nitrogen-doped nanofibrous carbon for supercapacitors.

Heteroatom-doped carbon nanostructures with uniform size and morphology, well-designed architectures, and minimized interfacial resistance have been recognized as promising electrode materials for energy storage, but remain a crucial challenge. Herein, we develop a general approach of polarity-induced decoration of a monolayer sheath of metal-organic framework (MOF) particles with excellent uniformity in size and morphology on electrospun polymer nanofibers. These hybrid nanofibers are facilely converted into nitrogen-doped nanofibrous carbon (denoted as N-NFC) during pyrolysis. The thus-obtained N-NFC features (1) a one-dimensional nanofibrous structure with a highly conductive core, (2) a monolayer sheath of hollow carbon-frames with uniform size and morphology, (3) plenty of micro/mesopores with a highly accessible surface area, and (4) a high N-doping level, all of which guarantee its good electrochemical performance with a high capacitance of 387.3 F g-1 at 1 A g-1. In a solid-state supercapacitor, it delivers excellent rate capability (78.0 F g-1 at 0.2 A g-1 and 64.0 F g-1 at 1 A g-1), an enhanced energy density of 7.9 W h kg-1 at a power density of 219 W kg-1, and outstanding cycling stability with 90% capacity retained over 10 000 cycles at 1 A g-1.

Oxygen vacancy enriched hollow cobaltosic oxide frames with ultrathin walls for efficient energy storage and biosensing.

Transition metal oxides (TMOs) with desired morphologies and atomic structures have promising applications in energy storage, catalysis, and biosensing because of their large specific surface area, high theoretical capacitance, abundant active sites, etc. In this study, hierarchical Co3O4 with enriched oxygen vacancies and finely tuned nanostructures, e.g. high porosity, thin wall thickness, hollow or yolk-shell structure, is prepared by dynamically balancing the decomposition and oxidation of the zeolitic imidazolate framework-67 (ZIF-67) precursor directly calcined in air at 300 °C. The optimized Co3O4 hollow frame inherits the original shape of the parent ZIF-67 with a high volume retention of 83% and features an ultrathin wall thickness of 10 nm, a large accessible surface area of 63.7 m2 g-1 and a high content of surface oxygen vacancies. It thus delivers an excellent specific capacitance of 770 F g-1 at 1 A g-1, a rate capability of 570.9 F g-1 at 20 A g-1 and excellent cycling stability for energy storage, and a high sensitivity of 0.7 mA mM-1 cm-2, a low detection limit of 0.2 µM (S/N = 3), and a wide linear detectable range of 0.005-1.175 mM for electrochemical non-enzymatic detection of glucose.