Continuous Energy Harvesting and Motion Sensing from Flexible Electrochemical Nanogenerators: Toward Smart and Multifunctional Textiles

NaxSb Alloy-Based Low-Frequency Mechanical Energy Harvesters for Virtual Taste Sensations

Batteries can be activated by external mechanical force and generate current, enabling a smart class of electrochemical-mechanical strain energy harvesters therefrom. Here, we have developed a NaxSb alloy-based harvester that is able to electrochemically convert low-frequency bending or pressing mechanical energy into electrical energy. The device is designed as a flexible symmetric cell incorporating two sodiated antimony nanoflake electrodes, whose peak power and energy output are more than twice those of other sodium-alloyed electrochemical-mechanical strain energy harvesters reported. We demonstrate that the open-circuit voltage of the device is an asymptotic function of the curvature radius in the bending mode and a linear function of pressure in the pressing mode. Taking advantage of the tunability of voltage, we present a new technology that simulates various tastes by releasing low-voltage electrical signals from the harvester. This technology can not only help people with impaired taste but also be integrated into a virtual reality system to create immersive taste experiences.

Redesigning Natural Materials for Energy, Water, Environment, and Devices

The United Nations Framework Convention on Climate Change (UNFCCC) acknowledges that global cooperation is paramount to mitigate climate change and further warming. The global community is committed to renewable energy and natural materials to tackle this challenge for all humankind. The widespread use of natural materials is embraced as one such action to reach net-zero carbon emissions. Given the hierarchical framework and earth abundance, cellulose-based materials extend their negative carbon benefits to our daily products and accelerate our pace toward carbon neutrality. Here, we present an overview of recent developments of cellulose-based materials in upsurging applications in radiative cooling, thermal insulation, nanofluidics, and wearable devices. We also highlight various modifications and functionalized processes that transform massive amounts of cellulose into green products. The prosperous development of functionalized cellulose materials aligns with a circular economy. Expedited interdisciplinary fundamental investigations are expected to make fibrillated cellulose penetrate more into carbon downdraw at speed and scale.

Muscle-inspired anisotropic carboxymethyl cellulose-based double-network conductive hydrogels for flexible strain sensors

Conductive hydrogels are considered one of the most promising materials for preparing flexible sensors due to their flexible and extensible properties. However, conventional hydrogels' weak mechanical and isotropic properties are greatly limited in practical applications. Here, the internal structure of the hydrogel was regulated by pre-stretching synergistic ion crosslinking to construct a carboxymethyl cellulose-based double network-oriented hydrogel similar to muscle. The introduction of pre-stretching increased the tensile strength of the double-network hydrogel from 1.45 MPa to 4.32 MPa, and its light transmittance increased from 67.3 % to 84.5 %. In addition, the hydrogel's thermal stability and electrical conductivity were improved to a certain extent. Its good mechanical properties and conductive properties can be converted into stable electrical signal output during deformation. The carboxymethyl cellulose-based double network oriented hydrogels were further assembled as flexible substrates into flexible sensor devices. The hydrogel sensors can monitor simple joint movements as well as complex spatial movements, which makes them have potential application value in the research field of intelligent response electronic devices such as flexible wearables, intelligent strain sensing, and soft robots.

Stretchable and Self-Healable Fiber-Shaped Conductors Suitable for Harsh Environments

Fiber-shaped conductors with high electrical conductivity, stretchability, and durability have attracted increasing attention due to their potential for integration into arbitrary wearable forms. However, these fiber conductors still suffer from low reliability and short life span, particularly in harsh environments. Herein, a conductive, environment-tolerant, stretchable, and healable fiber conductor (CESH), which consists of a self-healable and stretchable organohydrogel fiber core, a conductive and buckled silver nanowire coating, and a self-healable and waterproof protective sheath, is reported. Such a multilayer core-sheath design not only offers high stretchability (≈2400%), high electrical conductivity (1.0 × 106 S m-1 ), outstanding self-healing ability and durability, but also possesses unprecedented tolerance in harsh environments including wide working temperature (-60-20 °C), arid (≈10 % RH (RH: room humidity)), and underwater conditions. As proof-of-concept demonstrations, CESHs are integrated into various wearable formats as interconnectors to steadily perform the electric function under different mechanical deformations and harsh conditions. Such a new type of multifunctional fiber conductors can bridge the gap in stretchable and self-healing fiber technologies by providing ultrastable electrical conductance and excellent environmental tolerance, which can greatly expand the range of applications for fiber conductors.

Hierarchically Plied Mechano-Electrochemical Energy Harvesting Using a Scalable Kinematic Sensing Textile Woven from a Graphene-Coated Commercial Cotton Yarn

Wearable sensing systems are suitable for monitoring human motion. To realize a cost-effective and self-powered strain-sensing fiber, we developed a mechano-electrochemical harvesting yarn and textile using hierarchically arranged plied yarns composed of meter-long graphene-coated cotton yarns. Such a fiber relies on the principle of electrochemical capacity change to convert mechanical energy to electric energy. Further, this harvester can be used as a self-powered strain sensor because its output depends on mechanical stimuli. Additionally, the yarn can be woven into a kinematic sensing textile that measures the strength and direction of the applied force. The textile-type harvester can successfully detect various human movements such as pressing, bending, and stretching. The proposed sensing fiber will pave the way for the development of advanced wearable systems for ubiquitous healthcare in the future.

Thermoelectric properties of X3N2O2 (X = Hf, Zr) MXene monolayers: a first-principles study

MXene monolayers have received increasing attention due to their unique properties, particularly their high conductivity, which shows great potential in thermoelectric materials. In this paper, we present a theoretical study of the thermoelectric properties of X₃N₂O₂ (X = Hf, Zr) MXene monolayers, taking electron-phonon coupling into consideration. Owing to their similar geometrical structures, electronic band structures, and phonon dispersions, X₃N₂O₂ MXene monolayers exhibit homogeneous electron and phonon transport properties. The conduction band shows multi-valley characteristics which leads to better n-type electron transport properties than p-type ones. The maximum values of the n-type power factor can reach 32 μW cm^-1 K^-2 for the Hf₃N₂O₂ monolayer and 23 μW cm^-1 K^-2 for the Zr₃N₂O₂ monolayer. In terms of phonon transport, the lattice thermal conductivity for the Zr₃N₂O₂ monolayer is higher than that for the Hf₃N₂O₂ monolayer, due to larger phonon group velocity. Our results show that the Hf₃N₂O₂ monolayer is more suitable for thermoelectric materials than the Zr₃N₂O₂ monolayer, with optimal n-type thermoelectric figure of merit (ZT) values of 0.36 and 0.15 at 700 K, respectively. These findings may be useful for the development of wearable thermoelectric devices and sensor applications based on X₃N₂O₂ MXene monolayers.

Recent Advances in Carbon Nanotube-Based Energy Harvesting Technologies

There has been enormous interest in technologies that generate electricity from ambient energy such as solar, thermal, and mechanical energy, due to their potential for providing sustainable solutions to the energy crisis. One driving force behind the search for new energy-harvesting technologies is the desire to power sensor networks and portable devices without batteries, such as self-powered wearable electronics, human health monitoring systems, and implantable wireless sensors. Various energy harvesting technologies have been demonstrated in recent years. Among them, electrochemical, hydroelectric, triboelectric, piezoelectric, and thermoelectric nanogenerators have been extensively studied because of their special physical properties, ease of application, and sometimes high obtainable efficiency. Multifunctional carbon nanotubes (CNTs) have attracted much interest in energy harvesting because of their exceptionally high gravimetric power outputs and recently obtained high energy conversion efficiencies. Further development of this field, however, still requires an in-depth understanding of harvesting mechanisms and boosting of the electrical outputs for wider applications. Here, various CNT-based energy harvesting technologies are comprehensively reviewed, focusing on working principles, typical examples, and future improvements. The last section discusses the existing challenges and future directions of CNT-based energy harvesters.

Evolution of nanostructured skin patches towards multifunctional wearable platforms for biomedical applications

Recent advances in the field of skin patches have promoted the development of wearable and implantable bioelectronics for long-term, continuous healthcare management and targeted therapy. However, the design of electronic skin (e-skin) patches with stretchable components is still challenging and requires an in-depth understanding of the skin-attachable substrate layer, functional biomaterials and advanced self-powered electronics. In this comprehensive review, we present the evolution of skin patches from functional nanostructured materials to multi-functional and stimuli-responsive patches towards flexible substrates and emerging biomaterials for e-skin patches, including the material selection, structure design and promising applications. Stretchable sensors and self-powered e-skin patches are also discussed, ranging from electrical stimulation for clinical procedures to continuous health monitoring and integrated systems for comprehensive healthcare management. Moreover, an integrated energy harvester with bioelectronics enables the fabrication of self-powered electronic skin patches, which can effectively solve the energy supply and overcome the drawbacks induced by bulky battery-driven devices. However, to realize the full potential offered by these advancements, several challenges must be addressed for next-generation e-skin patches. Finally, future opportunities and positive outlooks are presented on the future directions of bioelectronics. It is believed that innovative material design, structure engineering, and in-depth study of fundamental principles can foster the rapid evolution of electronic skin patches, and eventually enable self-powered close-looped bioelectronic systems to benefit mankind.

Integrated Mechano-Electrochemical Harvesting Fiber and Thermally Responsive Artificial Muscle for Self-Powered Temperature-Strain Dual-Parameter Sensor

Significant progress in healthcare fields around the world has inspired us to develop a wearable strain−temperature sensor that can monitor biomedical signals in daily life. This novel self-powered temperature−strain dual-parameter sensor comprises a mechano-electrochemical harvester (MEH) and a thermally responsive artificial muscle (TAM). The MEHTAM system generates electricity from strain and thermal fluctuations. In addition, the sensor is comfortable to wear, owing to its stretchability (>100%), softness (<3 MPa), and one-dimensional fibers (diameter 230 μm). The MEH induces a change in the electrochemical capacitance, resulting in an electrical signal under applied strain (34 μA/m) and stress (20 μA/(m·MPa)). The TAM can be used as a mechanical temperature sensor, because the tensile stroke responds linearly to changes in temperature. As the harvester and artificial muscle are combined, the MEHTAM system generates electricity, owing to external and internal mechanical stimuli caused by muscle contractions as a response to temperature changes. The MEHTAM system that we have developed—a self-powered, strain−temperature dual-parameter sensor that is soft, stretchable, and fiber-shaped—is an interesting candidate for the production of comfortable, wearable, dual-parameter sensors.

Microbuckled Mechano-electrochemical Harvesting Fiber for Self-Powered Organ Motion Sensors

Mechanical harvesters have attracted tremendous attention as self-powered strain sensors; previous harvesters required high stress to stretch the fiber because of their high Young's modulus and low elasticity. We report on a mechano-electrochemical harvesting (MECH) fiber based on the new buckle structure, which has a low Young's modulus (2 MPa) with high elasticity (up to 100%) in a similar physiological fluid. MECH converts mechanical energy into electrical energy by changing the capacitance due to changing the surface area caused by the microbuckle on the surface. The damage to the cells can be minimized by their softness; the fiber was stitched on the tissue of the pig stomach while maintaining the performance like a suture fiber. Additionally, the fiber successfully operated in an organ-similar system, which is composed of the stomach or bladder of a pig. The fiber has a high potential to be applied in wearable energy sources and self-powered strain sensors.

Chemo-Mechanical Energy Harvesters with Enhanced Intrinsic Electrochemical Capacitance in Carbon Nanotube Yarns

Predicting and preventing disasters in difficult-to-access environments, such as oceans, requires self-powered monitoring devices. Since the need to periodically charge and replace batteries is an economic and environmental concern, energy harvesting from external stimuli to supply electricity to batteries is increasingly being considered. Especially, in aqueous environments including electrolytes, coiled carbon nanotube (CNT) yarn harvesters have been reported as an emerging approach for converting mechanical energy into electrical energy driven by large and reversible capacitance changes under stretching and releasing. To realize enhanced harvesting performance, experimental and computational approaches to optimize structural homogeneity and electrochemical accessible area in CNT yarns to maximize intrinsic electrochemical capacitance (IEC) and stretch-induced changes are presented here. Enhanced IEC further enables to decrease matching impedance for more energy efficient circuits with harvesters. In an ocean-like environment with a frequency from 0.1 to 1 Hz, the proposed harvester demonstrates the highest volumetric power (1.6-10.45 mW cm^-3 ) of all mechanical harvesters reported in the literature to the knowledge of the authors. Additionally, a high electrical peak power of 540 W kg^-1 and energy conversion efficiency of 2.15% are obtained from torsional and tensile mechanical energy.

Multifunctional Carbon Fiber Composites: A Structural, Energy Harvesting, Strain-Sensing Material

Multifunctional structural materials are capable of reducing system level mass and increasing efficiency in load-carrying structures. Materials that are capable of harvesting energy from the surrounding environment are advantageous for autonomous electrically powered systems. However, most energy harvesting materials are non-structural and add parasitic mass, reducing structural efficiency. Here, we show a structural energy harvesting composite material consisting of two carbon fiber (CF) layers embedded in a structural battery electrolyte (SBE) with a longitudinal modulus of 100 GPa─almost on par with commercial CF pre-pregs. Energy is harvested through mechanical deformations using the piezo-electrochemical transducer (PECT) effect in lithiated CFs. The PECT effect creates a voltage difference between the two CF layers, driving a current when deformed. A specific power output of 18 nW/g is achieved. The PECT effect in the lithiated CFs is observed in tension and compression and can be used for strain sensing, enabling structural health monitoring with low added mass. The same material has previously been shown capable of shape morphing. The two additional functionalities presented here result in a material capable of four functions, further demonstrating the diverse possibilities for CF/SBE composites in multifunctional applications in the future.

Human Skin-Inspired Electrospun Patterned Robust Strain-Insensitive Pressure Sensors and Wearable Flexible Light-Emitting Diodes

Wearable skin-inspired electronic skins present remarkable outgrowth in recent years because their promising comfort device integration, lightweight, and mechanically robust durable characteristics led to significant progresses in wearable sensors and optoelectronics. Wearable electronic devices demand real-time applicability and factors such as complex fabrication steps, manufacturing cost, and reliable and durable performances, severely limiting the utilization. Herein, we nominate a scalable solution-processable electrospun patterned candidate capable of forming ultralong mechanically robust nano-microdimensional fibers with higher uniformity. Nanofibrous patterned substrates present surface energy and silver nanoparticle crystallization shifts, contributing to strain-sensitive and -insensitive conductive electrodes (10 000 cycles of 50% strain). Synergistic robust stress releasing and durable electromechanical behavior engenders stretchable durable health sensors, strain-insensitive pressure sensors (sensitivity of ∼83 kPa^-1 and 5000 durable cycles), robust alternating current electroluminescent displays, and flexible organic light-emitting diodes (20% improved luminescence and 300 flex endurance of 2 mm bend radius).

Stretchy Electrochemical Harvesters for Binarized Self-Powered Strain Gauge-Based Static Motion Sensors

The human monitoring system has motivated the search for new technology, leading to the development of a self-powered strain sensor. We report on the stretchable and soft stretchy electrochemical harvester (SECH) bilayer for a binarized self-powered strain gauge in dynamic and static motion. The active surface area participating in the electrochemical reaction was enhanced after stretching the SECH in the electrolyte, leading to an increase in the electrochemical double-layer capacitance. A change in the capacitance induced a change in the electrical potential of the bilayer, generating electrical energy. The SECH overcomes several challenges of the previous mechano-electrochemical harvester: The harvester had high elasticity (50%), which satisfied the required strain during human motion. The harvester was highly soft (modulus of 5.8 MPa), 103 times lower than that of the previous harvester. The SECH can be applied to a self-powered strain gauge, capable of measuring stationary deformation and low-speed motion. The SECH created a system to examine the configuration of the human body, as demonstrated by the human monitoring sensor from five independent SECH assembled on the hand. Furthermore, the sensing information was simplified through the binarized signal. It can be used to assess the hand configuration for hand signals and sign language.

Hydrogels as Soft Ionic Conductors in Flexible and Wearable Triboelectric Nanogenerators

Flexible triboelectric nanogenerators (TENGs) have attracted increasing interest since their advent in 2012. In comparison with other flexible electrodes, hydrogels possess transparency, stretchability, biocompatibility, and tunable ionic conductivity, which together provide great potential as current collectors in TENGs for wearable applications. The development of hydrogel-based TENGs (H-TENGs) is currently a burgeoning field but research efforts have lagged behind those of other common flexible TENGs. In order to spur research and development of this important area, a comprehensive review that summarizes recent advances and challenges of H-TENGs will be very useful to researchers and engineers in this emerging field. Herein, the advantages and types of hydrogels as soft ionic conductors in TENGs are presented, followed by detailed descriptions of the advanced functions, enhanced output performance, as well as flexible and wearable applications of H-TENGs. Finally, the challenges and prospects of H-TENGs are discussed.

The Effect of Mechanical Frequency on Piezoelectrochemical Energy Harvesters

Microenergy harvesters such as piezoelectro-chemical (PEC) devices allow the extraction of low-frequency mechanical energy, which might otherwise be lost. Recent literature on PEC harvesters has noted that the input mechanical frequency affects the device current outputs, but this effect is not well understood. Mechanical energy sources often have variable frequencies, so understanding PEC harvester performance as a function of frequency is vital for the optimization of these devices. Using a commercially available lithium ion pouch cell as a test system, this work finds that applying a square-wave frequency with the fastest strain rate and longest hold time maximizes PEC current output. There is a monotonic increase in peak power, maximum half-cycle energy, and energy conversion efficiency as the input mechanical frequency approaches zero. This indicates that PEC harvesters have more flexibility in operating frequency than piezoelectric harvesters, as PEC harvesters do not have resonance or antiresonance frequencies.

Celery-derived porous carbon materials for superior performance supercapacitors

Supercapacitors are of paramount importance for next-generation applications, demonstrating high energy output and an ultra-long cycle life, and utilizing green and sustainable materials. Herein, we utilize celery, a common biomass from vegetables, by a facile low-cost pyrolysis and activation method for use in high-voltage, high-energy, and high-power supercapacitors. The as-synthesized hierarchically porous carbon materials with a high surface area of 1612 m2 g-1 and a large quantity of nitrogen and phosphorus heteroatoms exhibit a high specific capacitance of 1002.80 F g-1 at 1 A g-1 and excellent cycling stability of 95.6% even after 10 000 cycles (10 A g-1) in aqueous electrolytes. Moreover, the assembled symmetric cell delivers a high energy density of 32.7 W h kg-1 at 1200 W kg-1 and an ultra-high stability (loss of 4.8% after 10 000 cycles). Therefore, the outstanding electrochemical performance of the materials will be of use in the development of high-performance, green supercapacitors for advanced energy storage systems.

Hydrogel Ionic Diodes toward Harvesting Ultralow-Frequency Mechanical Energy

Energy harvesting from human motion is regarded as a promising protocol for powering portable electronics, biomedical devices, and smart objects of the Internet of things. However, state-of-the-art mechanical-energy-harvesting devices generally operate at frequencies (>10 Hz) well beyond human activity frequencies. Here, a hydrogel ionic diode formed by the layered structures of anionic and cationic ionomers in hydrogels is presented. As confirmed by finite element analysis, the underlying mechanism of the hydrogel ionic diode involves the formation of the depletion region by mobile cations and anions and the subsequent increase of the built-in potential across the depletion region in response to mechanical pressure. Owing to the enhanced ionic rectification ratio by the embedded carbon nanotube and silver nanowire electrodes, the hydrogel ionic diode exhibits a power density of ≈5 mW cm^-2 and a charge density of ≈4 mC cm^-2 at 0.01 Hz, outperforming the current energy-harvesting devices by several orders of magnitude. The applications of the self-powered hydrogel ionic diode to tactile sensing, pressure imaging, and touchpads are demonstrated, with sensing limitation is as low as 0.01 kPa. This work is expected to open up new opportunities for ionic-current-based ionotronics in electronics and energy devices.

Energy Autonomous Sweat-Based Wearable Systems

The continuous operation of wearable electronics demands reliable sources of energy, currently met through Li-ion batteries and various energy harvesters. These solutions are being used out of necessity despite potential safety issues and unsustainable environmental impact. Safe and sustainable energy sources can boost the use of wearables systems in diverse applications such as health monitoring, prosthetics, and sports. In this regard, sweat- and sweat-equivalent-based studies have attracted tremendous attention through the demonstration of energy-generating biofuel cells, promising power densities as high as 3.5 mW cm-2 , storage using sweat-electrolyte-based supercapacitors with energy and power densities of 1.36 Wh kg-1 and 329.70 W kg-1 , respectively, and sweat-activated batteries with an impressive energy density of 67 Ah kg-1 . A combination of these energy generating, and storage devices can lead to fully energy-autonomous wearables capable of providing sustainable power in the µW to mW range, which is sufficient to operate both sensing and communication devices. Here, a comprehensive review covering these advances, addressing future challenges and potential solutions related to fully energy-autonomous wearables is presented, with emphasis on sweat-based energy storage and energy generation elements along with sweat-based sensors as applications.

Fiber‐Shaped Electronic Devices

Textile electronics embedded in clothing represent an exciting new frontier for modern healthcare and communication systems. Fundamental to the development of these textile electronics is the development of the fibers forming the cloths into electronic devices. An electronic fiber must undergo diverse scrutiny for its selection for a multifunctional textile, viz., from the material selection to the device architecture, from the wearability to mechanical stresses, and from the environmental compatibility to the end‐use management. Herein, the performance requirements of fiber‐shaped electronics are reviewed considering the characteristics of single electronic fibers and their assemblies in smart clothing. Broadly, this article includes i) processing strategies of electronic fibers with required properties from precursor to material, ii) the state‐of‐art of current fiber‐shaped electronics emphasizing light‐emitting devices, solar cells, sensors, nanogenerators, supercapacitors storage, and chromatic devices, iii) mechanisms involved in the operation of the above devices, iv) limitations of the current materials and device manufacturing techniques to achieve the target performance, and v) the knowledge gap that must be minimized prior to their deployment. Lessons learned from this review with regard to the challenges and prospects for developing fiber‐shaped electronic components are presented as directions for future research on wearable electronics.

Biomechanical Energy Harvesters Based on Ionic Conductive Organohydrogels via the Hofmeister Effect and Electrostatic Interaction

The recent use of cryoprotectant replacement method for solving the easy drying problem of hydrogels has attracted increasing research interest. However, the conductivity decrease of organohydrogels due to the induced insulating solvent limited their electronic applications. Herein, we introduce the Hofmeister effect and electrostatic interaction to generate hydrogen and sodium bonds in the hydrogel. Combined with its double network, an effective charge channel that will not be affected by the solvent replacement, is therefore built. The developed organohydrogel-based single-electrode triboelectric nanogenerator (OHS-TENG) shows low conductivity decrease (one order) and high output (1.02-1.81 W/m²), which is much better than reported OHS-TENGs (2-3 orders, 41.2-710 mW/m²). Moreover, replacing water with glycerol in the hydrogel enables the device to exhibit excellent long-term stability (four months) and temperature tolerance (-50-100 °C). The presented strategy and mechanism can be extended to common organohydrogel systems aiming at high performance in electronic applications.

Application of textile technology in tissue engineering: A review

One of the key elements in tissue engineering is to design and fabricate scaffolds with tissue-like properties. Among various scaffold fabrication methods, textile technology has shown its unique advantages in mimicking human tissues' properties such as hierarchical, anisotropic, and strain-stiffening properties. As essential components in textile technology, textile patterns affect the porosity, architecture, and mechanical properties of textile-based scaffolds. However, the potential of various textile patterns has not been fully explored when fabricating textile-based scaffolds, and the effect of different textile patterns on scaffold properties has not been thoroughly investigated. This review summarizes textile technology development and highlights its application in tissue engineering to facilitate the broader application of textile technology, especially various textile patterns in tissue engineering. The potential of using different textile methods such as weaving, knitting, and braiding to mimic properties of human tissues is discussed, and the effect of process parameters in these methods on fabric properties is summarized. Finally, perspectives on future directions for explorations are presented. STATEMENT OF SIGNIFICANCE: Recently, biomedical engineers have applied textile technology to fabricate scaffolds for tissue engineering applications. Various textile methods, especially weaving, knitting, and braiding, enables engineers to customize the physical, mechanical, and biological properties of scaffolds. However, most textile-based scaffolds only use simple textile patterns, and the effect of different textile patterns on scaffold properties has not been thoroughly investigated. In this review, we cover for the first time the effect of process parameters in different textile methods on fabric properties, exploring the potential of using different textile methods to mimic properties of human tissues. Previous advances in textile technology are presented, and future directions for explorations are presented, hoping to facilitate new breakthroughs of textile-based tissue engineering.

Effect of pomelo seed-derived carbon on the performance of supercapacitors

Electrochemical ultracapacitors derived from green and sustainable materials could demonstrate superior energy output and an ultra-long cycle life, which could contribute to next-generation applications. Herein, we utilize pomelo seeds, a bio-waste from pomelo, in high-energy and high-power supercapacitors by a facile low-cost pyrolysis and activation method. The as-synthesized hierarchically porous carbon is surface-engineered with a large quantity of nitrogen and sulfur heteroatoms to give a high specific capacitance of ∼845 F g-1 at 1 A g-1. An ultra-high stability of ∼93.8% even after 10 000 cycles (10 A g-1) is achieved at room temperature. Moreover, a maximum energy density of ∼85 W h kg-1 at a power density of 1.2 kW kg-1 could be achieved in 1.2 V aqueous symmetrical supercapacitors. The results provide new insights that will be of use in the development of high-performance, green supercapacitors for advanced energy storage systems.

Stamping Flexible Li Alloy Anodes

Li metal holds great promise to be the ultimate anode choice owing to its high specific capacity and low redox potential. However, processing Li metal into thin-film anode with high electrochemical performance and good safety to match commercial cathodes remains challenging. Herein, a new method is reported to prepare ultrathin, flexible, and high-performance Li-Sn alloy anodes with various shapes on a number of substrates by directly stamping a molten metal solution. The printed anode is as thin as 15 µm, corresponding to an areal capacity of ≈3 mAh cm^-2 that matches most commercial cathode materials. The incorporation of Sn provides the nucleation center for Li, thereby mitigating Li dendrites as well as decreasing the overpotential during Li stripping/plating (e.g., <10 mV at 0.25 mA cm^-2 ). As a proof-of-concept, a flexible Li-ion battery using the ultrathin Li-Sn alloy anode and a commercial NMC cathode demonstrates good electrochemical performance and reliable cell operation even after repetitive deformation. The approach can be extended to other metal/alloy anodes such as Na, K, and Mg. This study opens a new door toward the future development of high-performance ultrathin alloy-based anodes for next-generation batteries.

Aligned carbon nanotube fibers for fiber-shaped solar cells, supercapacitors and batteries

Aligned carbon nanotube (CNT) fibers have been considered as one of the ideal candidate electrodes for fiber-shaped energy harvesting and storage devices, due to their merits of flexibility, lightweight, desirable mechanical property, outstanding electrical conductivity as well as high specific surface area. Herein, the recent advancements on the aligned CNT fibers for energy harvesting and storage devices are reviewed. The synthesis, structure, and properties of aligned carbon nanotube fibers are briefly summarized. Then, their applications in fiber-shaped energy harvesting and storage devices (i.e., solar cells, supercapacitors, and batteries) are demonstrated. The remaining challenges are finally discussed to highlight the future research direction in the development of aligned CNT fibers for fiber-shaped energy devices.

A hierarchical porous P-doped carbon electrode through hydrothermal carbonization of pomelo valves for high-performance supercapacitors

Porous carbon materials are synthesized from pomelo valves by the hydrothermal activation of H3PO4 followed by simple carbonization. The as-synthesized hierarchically porous carbon electrode exhibits a high specific capacitance of 966.4 F g-1 at 1 A g-1 and an ultra-high stability of 95.6% even after 10 000 cycles. Moreover, the supercapacitor also demonstrates a maximum energy of 36.39 W h kg-1 and a maximum power of 33.33 kW kg-1 with an energy retention of 25.56 W h kg-1, which paves the way for the development of high-performance, green supercapacitors for advanced energy storage systems.

Bath Electrospinning of Continuous and Scalable Multifunctional MXene-Infiltrated Nanoyarns

Electroactive yarns that are stretchable are desired for many electronic textile applications, including energy storage, soft robotics, and sensing. However, using current methods to produce these yarns, achieving high loadings of electroactive materials and simultaneously demonstrating stretchability is a critical challenge. Here, a one-step bath electrospinning technique is developed to effectively capture Ti₃ C₂ T_x MXene flakes throughout continuous nylon and polyurethane (PU) nanofiber yarns (nanoyarns). With up to ≈90 wt% MXene loading, the resulting MXene/nylon nanoyarns demonstrate high electrical conductivity (up to 1195 S cm^-1 ). By varying the flake size and MXene concentration, nanoyarns achieve stretchability of up to 43% (MXene/nylon) and 263% (MXene/PU). MXene/nylon nanoyarn electrodes offer high specific capacitance in saturated LiClO₄ electrolyte (440 F cm^-3 at 5 mV s^-1 ), with a wide voltage window of 1.25 V and high rate capability (72% between 5 and 500 mV s^-1 ). As strain sensors, MXene/PU yarns demonstrate a wide sensing range (60% under cyclic stretching), high sensitivity (gauge factor of ≈17 in the range of 20-50% strain), and low drift. Utilizing the stretchability of polymer nanofibers and the electrical and electrochemical properties of MXene, MXene-based nanoyarns demonstrate potential in a wide range of applications, including stretchable electronics and body movement monitoring.

Electrode Effects on Flexible and Robust Polypropylene Ferroelectret Devices for Fully Integrated Energy Harvesters

This work presents a characterization study of the electrode interface in polypropylene ferroelectret nanogenerators. An emphasis is made on the comparison of carbon nanotube fiber electrodes with traditional metallic thin film electrodes. Multiple experiments were performed on samples with the same electrode dimensions for a range of applied pressures. Results showed higher open-circuit voltage peak values for the thin film metal electrodes, regardless of the applied pressure. Interestingly, the difference in short-circuit current values between metal and carbon nanotube-based fiber electrodes was not as significant. The carbon nanotube fiber electrode was further investigated by post-treating the fiber with acetone and comparing the results with untreated carbon nanotube film electrodes and thin film metal electrodes. In an effort to enable a monolithic integration of ferroelectret energy harvesters with flexible energy storage elements, this work also presents studies on generation and leakage of induced free charge in the electrodes of flexible ferroelectret energy harvesters. It was found the current leakage through parasitic elements is a faster process than dipole relaxation in the polypropylene film. Finally, an electrode reliability study shows no significant difference in the electrical output of the devices with metallic thin film electrodes after single folding but shows a significant deterioration after crumpling; meanwhile, these processes had no effect on the performance of similar devices with carbon nanotube fiber-based electrodes.