Tough-Hydrogel Reinforced Low-Tortuosity Conductive Networks for Stretchable and High-Performance Supercapacitors

High-performance supercapacitors based on coarse nanofiber bundle and ordered network hydrogels

Most of the developed flexible hydrogel supercapacitors struggle to maintain their electrochemical stability and structural integrity under tensile strain. Therefore, developing a flexible supercapacitor with excellent mechanical properties and stable electrochemical performance under different strains remains a challenge. Based on the previous cartilage-like structure, we designed a new coarse nanofiber bundle and ordered network. A coarse nanofiber bundle and ordered network skeleton was constructed by directional freezing and filled with polyvinyl alcohol (PVA) to serve as a soft matrix to prepare PVA-SNF-CNTs-PPy-3 hydrogel electrode, which has high tensile strength (6.22 MPa) and fatigue threshold (8759.8 J/m2). In addition, the loading of carbon nanotubes and polypyrrole onto the SNF-ordered network enabled the conductive material to form an ordered conductive energy storage network along the skeleton, providing an area-specific capacitance of up to 23.96 F/cm2. The coarse nanofiber bundle and ordered network provided supercapacitors with the least capacitance consumption under 150 % deformation, and the capacitance retention was >98.2 %. After repeated stretching (3000 times), the capacitance remained >91.45 %. This study provides new ideas for the development of flexible supercapacitors with high capacitance and high mechanical reliability.

Novel Thermoplastic Polyurethanes Enable Biaxially Stretchable Conductor for Supercapacitors with High Areal Capacitance

Stretchable supercapacitors are essential components in wearable electronics due to their low heat generation and seamless integration capabilities. Thermoplastic polyurethane elastomers, recognized for their dynamic hydrogen-bonding structure, exhibit excellent stretchability, making them well-suited for these applications. This study introduces fluorine-based interactions in the hard segments of thermoplastic polyurethanes, resulting in polyurethanes with a low elastic modulus, high fracture strength, exceptional fatigue resistance, and self-healing properties. By utilizing these polyurethanes as binders and meshed fabric as scaffolds, we developed highly stretchable conductors. These conductors maintain low resistance (∼26 ohms) under biaxial stretching and exhibit a stable bidirectional conductivity after 1600 stretching cycles. The fabricated supercapacitor electrode, incorporating fabric current collectors, polyurethane, and MXene, achieves an ultrahigh areal specific capacitance of 7200 mF cm-2 and retains 100% capacity after 2300 cycles. This material design strategy offers significant potential in elastic materials, stretchable conductors, and high-performance energy storage for wearable electronics.

Highly deformable bi-continuous conducting polymer hydrogels for electrochemical energy storage

Conducting polymer hydrogels with inherent flexibility, ionic conductivity and environment friendliness are promising materials in the fields of energy storage. However, a trade-off between mechanical and electrochemical properties has limited the development of flexible/stretchable conducting polymer hydrogel electrodes, owing to the intrinsic conflict among mechanical and electrical phases. Here, we report a reliable design to enable conducting polymer with both exceptional mechanical and electrical/electrochemical performance through the construction of bi-continuous conducting polymer crosslinked network. The resultant bi-continuous conducting polymer hydrogels (BCPH) demonstrate significantly improved mechanical and electrochemical properties compared to the conventional conducting polymer hydrogel (CPH) electrode. BCPH presents a high specific capacitance of 715 F g-1 at 0.5 A/g, a high mechanical strength (∼1 MPa) and a large stretchability (∼300%). Enabled by such intrinsically deformability and electrochemical properties, we further demonstrate its utility in flexible solid-state supercapacitor (FSSC), which exhibits an outstanding specific capacitance of 760 mF cm-2 at 2 mA cm-2, excellent electrochemical stability with 81% capacitance retention after 5000 charge/discharge cycles, and superior bending cycle stability. This simple and scalable strategy provides a platform for the fabrication of high-performance conducting hydrogel electrodes for various wearable electronic equipment.

In Situ Polymerization of Hydrogel Electrolyte on Electrodes Enabling the Flexible All-Hydrogel Supercapacitors with Low-Temperature Adaptability

All-hydrogel supercapacitors are emerging as promising power sources for next-generation wearable electronics due to their intrinsic mechanical flexibility, eco-friendliness, and enhanced safety. However, the insufficient interfacial adhesion between the electrode and electrolyte and the frozen hydrogel matrices at subzero temperatures largely limit the practical applications of all-hydrogel supercapacitors. Here, an all-hydrogel supercapacitor is reported with robust interfacial contact and anti-freezing property, fabricated by in situ polymerizing hydrogel electrolyte onto hydrogel electrodes. The robust interfacial adhesion is developed by the synergistic effect of a tough hydrogel matrix and topological entanglements. Meanwhile, the incorporation of zinc chloride (ZnCl2) in the hydrogel electrolyte prevents the freezing of water solvents and endows the all-hydrogel supercapacitor with mechanical flexibility and fatigue resistance across a wide temperature range of 20 °C to -60 °C. Such all-hydrogel supercapacitor demonstrates satisfactory low-temperature electrochemical performance, delivering a high energy density of 11 mWh cm-2 and excellent cycling stability with a capacitance retention of 90% over 10000 cycles at -40 °C. Notably, the fabricated all-hydrogel supercapacitor can endure dynamic deformations and operate well under 2000 tension cycles even at -40 °C, without experiencing delamination and electrochemical failure. This work offers a promising strategy for flexible energy storage devices with low-temperature adaptability.

Surfactant-assisted preparation of all-gel-state flexible supercapacitor with remarkable electrochemical performance based on polyaniline-polyacrylamide/sodium alginate hydrogels

The electrochemical performance of polyaniline-based all-gel-state supercapacitor (AGSSC) is significantly depended on the dispersity and mass loaded of polyaniline (PANI). In this manuscript, inspired by the properties of surfactant, sodium dodecylbenzene sulfonate (SDBS) was introduced to prepare various PANI-polyacrylamide/sodium alginate/SDBS (PANIy-PSSx) AGSSCs. With presence of SDBS, the electrochemical performance of PANIy-PSSx AGSSCs was greatly improved, displaying a trend of initial rise and then decrease with increasing concentration of SDBS from 0 to 0.75 wt%. As the content of SDBS was 0.5 wt%, the resulting PANI1.0-PSS0.5 AGSSC displayed the optimum electrochemical properties with area capacitance and energy density of 913.79 mF/cm2 and 81.23 μWh/cm2, respectively. The capacitance rate of PANI1.0-PSS0.5 AGSSC was still more than 93 % after 2000 cycles of sequential CV scans at the scan rate of 200 mV/s. These data were greatly higher than many reported PANI-based AGSSCs. Moreover, the resultant PANI1.0-PSS0.5 AGSSC could maintain high electrochemical performance even after various operations, such as compression, puncture, fluctuating temperature, bending situations and various voltage windows and series-parallel connections. The resultant PANI1.0-PSS0.5 AGSSC had the wide potentials to satisfy the real application requirements. This study offered a facile strategy for design and preparation of flexible supercapacitor with excellent electrochemical performance.

Low-impedance tissue-device interface using homogeneously conductive hydrogels chemically bonded to stretchable bioelectronics

Stretchable bioelectronics has notably contributed to the advancement of continuous health monitoring and point-of-care type health care. However, microscale nonconformal contact and locally dehydrated interface limit performance, especially in dynamic environments. Therefore, hydrogels can be a promising interfacial material for the stretchable bioelectronics due to their unique advantages including tissue-like softness, water-rich property, and biocompatibility. However, there are still practical challenges in terms of their electrical performance, material homogeneity, and monolithic integration with stretchable devices. Here, we report the synthesis of a homogeneously conductive polyacrylamide hydrogel with an exceptionally low impedance (~21 ohms) and a reasonably high conductivity (~24 S/cm) by incorporating polyaniline-decorated poly(3,4-ethylenedioxythiophene:polystyrene). We also establish robust adhesion (interfacial toughness: ~296.7 J/m2) and reliable integration between the conductive hydrogel and the stretchable device through on-device polymerization as well as covalent and hydrogen bonding. These strategies enable the fabrication of a stretchable multichannel sensor array for the high-quality on-skin impedance and pH measurements under in vitro and in vivo circumstances.

Robust and sensitive conductive nanocomposite hydrogel with bridge cross-linking-dominated hierarchical structural design

Conductive hydrogels have a remarkable potential for applications in soft electronics and robotics, owing to their noteworthy attributes, including electrical conductivity, stretchability, biocompatibility, etc. However, the limited strength and toughness of these hydrogels have traditionally impeded their practical implementation. Inspired by the hierarchical architecture of high-performance biological composites found in nature, we successfully fabricate a robust and sensitive conductive nanocomposite hydrogel through self-assembly-induced bridge cross-linking of MgB2 nanosheets and polyvinyl alcohol hydrogels. By combining the hierarchical lamellar microstructure with robust molecular B─O─C covalent bonds, the resulting conductive hydrogel exhibits an exceptional strength and toughness. Moreover, the hydrogel demonstrates exceptional sensitivity (response/relaxation time, 20 milliseconds; detection lower limit, ~1 Pascal) under external deformation. Such characteristics enable the conductive hydrogel to exhibit superior performance in soft sensing applications. This study introduces a high-performance conductive hydrogel and opens up exciting possibilities for the development of soft electronics.

Ultrahigh-Ionic-Conductivity, Antifreezing Poly(amidoxime)-Grafted Polyzwitterion Hydrogel for Facile Integrated into High-Performance Stretchable Flexible Supercapacitor

Developing wearable supercapacitors (SCs) with high stretchability, arbitrary deformability, and antifreezing ability is still a challenge. In the present work, an ultrahigh-ionic-conductivity, antifreezing poly(amidoxime)-graft-polyzwitterion (PAO-g-PSBMA) hydrogel electrolyte is fabricated by grafting PSBMA in PAO. Owing to the abundant hydrophilic and high ionic adsorption capacity of amidoxime groups in PAO and zwitterion groups in PSBMA, the as-prepared PAO-g-PSBMA hydrogel can facilitate the dissociation of lithium salt and exhibit an ultrahigh ionic conductivity of 29.8 S m-1 at 25 °C and 3.4 S m-1 even at -30 °C. Employing mATi3C2Tx and mSTi3C2Tx, which contain small amounts of PAO-AGE and PAO-g-PSBMA dispersions, respectively, coated onto both sides of the PAO-g-PSBMA hydrogel, we followed a thermal treatment to facilely form integrated stretchable flexible SCs. The as-prepared SCs show an outstanding recoverable tensile stain of 80% and an excellent electrochemical stability under many types and times of arbitrary deformation. More importantly, as-prepared mATi3C2Tx- and mSTi3C2Tx-based SCs present fantastic antifreezing ability and excellent stability with 74.6 and 78.3% retention of the initial capacitance, respectively, even after 1000 times of stretching to 60% at -30 °C. This work offers a new strategy of using PAO-grafted polyzwitterion for obtaining an antifreezing stretchable SC, which shows a high potential for application in next-generation integrated stretchable devices in various fields.

Ultrastrong Ionotronic Films Showing Electrochemical Osmotic Actuation

A multifunctional soft material with high ionic and electrical conductivity, combined with high mechanical properties and the ability to change shape can enable bioinspired responsive devices and systems. The incorporation of all these characteristics in a single material is very challenging, as the improvement of one property tends to reduce other properties. Here, a nanocomposite film based on charged, high-aspect-ratio 1D flexible nanocellulose fibrils, and 2D Ti3 C2 Tx MXene is presented. The self-assembly process results in a stratified structure with the nanoparticles aligned in-plane, providing high ionotronic conductivity and mechanical strength, as well as large water uptake. In hydrogel form with 20 wt% liquid, the electrical conductivity is over 200 S cm-1 and the in-plane tensile strength is close to 100 MPa. This multifunctional performance results from the uniquely layered composite structure at nano- and mesoscales. A new type of electrical soft actuator is assembled where voltage as low as ±1 V resulted in osmotic effects and giant reversible out-of-plane swelling, reaching 85% strain.

Mesoporous Iron Family Element (Fe, Co, Ni) Molybdenum Disulfide/Carbon Nanohybrids for High-Performance Supercapacitors

As the demand for fuel continues to increase, the development of energy devices with excellent performance is crucial. Supercapacitors (SCs) are attracting attention for their advantages of high specific energy and a long cycle life. At present, the development of high-performance electrode materials is the main point for research and development of SCs. Transition metal sulfides have the advantages of a large interlayer space and high theoretical capacity, making them promising electrode materials. Herein, we reported a series of ultrathin mesoporous iron family element (Fe, Co, Ni) molybdenum disulfide (MxMo1-xS2/C, M = Fe, Co, and Ni) by a template method. The original monolayer mesoporous structure of MoS2/C was maintained, and accumulation and agglomeration of MoS2/C were avoided. Based on our investigations, the best performance was that of CoxMo1-xS2/C nanohybrids. Furthermore, the concentrations of Co and Mo ions were modulated to obtain the best performance, in which Mo and Co ions were released at 1:1, 1:2, and 1:3 ratios and they were named CoxMo1-xS2/C-1, CoxMo1-xS2/C-2, and CoxMo1-xS2/C-3, respectively. Overall, these materials represent a significant improvement and show promise as high-performance SC electrode materials due to their enhanced capacitance and stability. At a current density of 0.5 A g-1, CoxMo1-xS2/C-2 has the optimal specific capacitance of 184 F g-1. CoxMo1-xS2/C-2 as an SC electrode exhibited better reversible capacity and cycling stability than MoS2/C, which is an improvement over MoS2/C regarding reversible capacity and cycling stability.

A novel catalyst derived from Co-ZIFs to grow N-doped carbon nanotubes for all-solid-state supercapacitors with high performance

Carbon nanotubes (CNTs) have been widely used as electrode materials for electrochemical energy storage devices (e.g., supercapacitors) due to their excellent chemical and physical properties. However, conventional approaches (e.g., electron-beam vapor deposition and atomic layer deposition) to fabricate catalysts for the growth of CNTs are complex and demand high energy consumption. Herein, we report a facile method to synthesize catalysts derived from cobalt-containing zeolitic imidazolate frameworks (Co-ZIFs), which is exploited to in situ construct the three-dimensional (3D) CNT hybrid materials for all-solid-state supercapacitors. In brief, Co-ZIFs with a controllable structure is first grown on the inner porous surface of carbon foams pyrolyzed from commercial melamine foams, followed by thermal annealing and chemical vapor deposition to grow CNTs, achieving 3D free-standing CNT-based hybrids. The well-distributed Co-ZIFs in the carbon foam enable the grown CNTs with uniform structures and morphologies. Using the fabricated CNT-based hybrid as electrodes, the assembled all-solid-state supercapacitors show a high specific capacitance of 19.4 mF cm-2 at a current density of 0.5 mA cm-2, which could be further optimized to as high as 871.8 mF cm-2 by incorporating the pseudocapacitive material of manganese dioxide in CNT-based hybrids. This study provides a facile solution approach to fabricate the catalyst for the growth of a CNT inner porous substrate; the resultant 3D free-standing hybrids could be used as efficient electrodes for high-performance energy storage devices beyond supercapacitors.

Solution-based self-assembly synthesis of two-dimensional-ordered mesoporous conducting polymer nanosheets with versatile properties

Conducting polymers with conjugated backbones have been widely used in electrochemical energy storage, catalysts, gas sensors and biomedical devices. In particular, two-dimensional (2D) mesoporous conducting polymers combine the advantages of mesoporous structure and 2D nanosheet morphology with the inherent properties of conducting polymers, thus exhibiting improved electrochemical performance. Despite the use of bottom-up self-assembly approaches for the fabrication of a variety of mesoporous materials over the past decades, the synchronous control of the dimensionalities and mesoporous architectures for conducting polymer nanomaterials remains a challenge. Here, we detail a simple, general and robust route for the preparation of a series of 2D mesoporous conducting polymer nanosheets with adjustable pore size (5-20 nm) and thickness (13-45 nm) and controllable morphology and composition via solution-based self-assembly. The synthesis conditions and preparation procedures are detailed to ensure the reproducibility of the experiments. We describe the fabrication of over ten high-quality 2D-ordered mesoporous conducting polymers and sandwich-structured hybrids, with tunable thickness, porosity and large specific surface area, which can serve as potential candidates for high-performance electrode materials used in supercapacitors and alkali metal ion batteries, and so on. The preparation time of the 2D-ordered mesoporous conducting polymer is usually no more than 12 h. The subsequent supercapacitor testing takes ~24 h and the Na ion battery testing takes ~72 h. The procedure is suitable for users with expertise in physics, chemistry, materials and other related disciplines.

Scalable Quasi-Solid-State Supercapacitor for Wide-Temperature Wearable Devices

Quasi-solid-state supercapacitors have wide application prospects in flexible and scalable electronics, which require high capacity, simple form factor, and excellent mechanical robustness. However, it is a challenge to have all these benefits in one material. Addressing this, we report a composite hydrogel with excellent mechanical durability and freezing resistance. The designed composite hydrogel acts both as a load-bearing layer to maintain its structure during deformation and as a permeable binder to stimulate the interfacing between the conductive electrode and the electrolyte to reduce the interface resistance. Flexible supercapacitors are assembled with composite hydrogels and high-performance MnO₂/carbon cloth, which has excellent performance and can store energy at different temperatures or bending states. These results show that the tough hydrogel facilitates the improvement of electrical and mechanical stability, showing great potential in wide-temperature wearable devices.

Low-Temperature Resistant Stretchable Micro-Supercapacitor Based on 3D Printed Octet-Truss Design

Recently, stretchable micro-supercapacitors (MSCs) that can be easily integrated into electronic devices have attracted research and industrial attentions. In this work, three-dimensional (3D) stretchable MSCs with an octet-truss electrode (OTE) design have been demonstrated by a rapid digital light processing (DLP) process. The 3D-printed electrode structure is beneficial for electrode-electrolyte interface formation and consequently increases the number of ions adsorbed on the electrode surface. The designed MSCs can achieve a high capacitance as ≈74.76 mF cm^-3 under 1 mA cm^-3 at room temperature even under a high mechanical deformation, and can achieve 19.53 mF cm^-3 under 0.1 mA cm^-3 at a low temperature (-30 °C). Moreover, finite element analysis (FEA) reveals the OTE structure provides 8 times more contact area per unit volume at the electrode-electrolyte interface compared to the traditional interdigital electrode (IDE). This work combines structural design and 3D printing techniques, which provides new insights into highly stretchable MSCs for next-generation electronic devices.

Cartilage structure-inspired elastic silk nanofiber network hydrogel for stretchable and high-performance supercapacitors

Flexible supercapacitors are an important portable energy storage but suffer from low capacitance, inability to stretch, etc. Therefore, flexible supercapacitors must achieve higher capacitance, energy density, and mechanical robustness to expand the applications. Herein, a hydrogel electrode with excellent mechanical strength was created by simulating the collagen fiber network and proteoglycan in cartilage using silk nanofiber (SNF) network and polyvinyl alcohol (PVA). The Young's modulus and breaking strength of the hydrogel electrode increased by 205 % and 91 % compared with PVA hydrogel owing to the enhanced effect of the bionic structure, respectively, which are 1.22 MPa and 1.3 MPa. The fracture energy and fatigue threshold reached 1813.5 J/m² and 1585.2 J/m², respectively. The SNF network effectively connected carbon nanotubes (CNTs) and polypyrrole (PPy) in series, affording a capacitance of 13.62 F/cm² and energy density of 1.2098 mWh/cm². This capacitance is the highest among currently reported PVA hydrogel capacitors, which can maintain >95.2 % after 3000 charge-discharge cycles. This capacitance Notably, the cartilage-like structure endowed the supercapacitor with high resilience; thus, the capacitance remained >92.1 % under 150 % deformation and >93.35 % after repeated stretching (3000 times), which was far superior to that of other PVA-based supercapacitors. Overall, this effective bionic strategy can endow supercapacitors with ultrahigh capacitance and effectively ensure the mechanical reliability of flexible supercapacitors, which will help expand the applications of supercapacitors.

Simultaneous Polymerization Acceleration and Mechanical Enhancement for Printing a Biomimetic PEDOT Adhesive by Coordinative and Orthogonal Ruthenium Photochemistry

Conductive hydrogels are promising material candidates in fields ranging from flexible sensors and electronic skin applications to personalized medical monitoring. However, developing intrinsically conductive polymer hydrogels (ICPHs) with high mechanical properties and excellent printability is still challenging. Here, we introduce a simultaneous polymerization acceleration and mechanical enhancement (SPAME) strategy to construct PEDOT-based ICPHs via the rational design of coordinative and orthogonal ruthenium photochemistry (CORP). This orthogonal photochemistry triggers the oxidative polymerization of EDOT and the coupling of phenols within seconds under blue light irradiation. Benefiting from the bifunctional EDTA-Fe design, the photoreleased Fe(III) accelerated the EDOT polymerization and shortened the preparation time of ICPHs to a few seconds. At the same time, the addition of EDTA-Fe enhanced their mechanical properties, and both the critical strains and maximum stresses of the hydrogel doubled. Furthermore, the introduction of phenol residues in PAA-Ph significantly shortened the gelation time from several minutes to about 7 s. Thus, this fast and controllable CORP chemistry is compatible with standard printing techniques for engineering hydrogels for complex multifunctional structures for multifunctional bioelectronics and devices.

Nanosheet-hydrogel composites: from preparation and fundamental properties to their promising applications

Hydrogels are an important class of soft materials with elastic and intelligent properties. Nevertheless, these traditional hydrogels usually possess poor mechanical properties and limited functions, which greatly restrict their further applications. With the rapid development of nanotechnology, there have been significant advances in the design and fabrication of functional nanocomposite hydrogels with unique properties and functions. Among various materials, nanosheets with planar topography, large specific surface areas, and versatile physicochemical properties have attracted intense research interest. Herein, this review summarises the synthesis mechanisms, fundamental properties, and promising applications of nanosheet-incorporated hydrogels. In particular, how the nanosheet structure is applied to improve the overall performance of the hydrogel in each application is emphasized. Additionally, the current challenges and prospects are briefly discussed in this area. We expect that the combination of nanosheets and hydrogels can attract more researchers' interest and bring new opportunities in the future.

Tissue-Mimetic Supramolecular Polymer Networks for Bioelectronics

Addressing the mechanical mismatch between biological tissue and traditional electronic materials remains a major challenge in bioelectronics. While rigidity of such materials limits biocompatibility, supramolecular polymer networks can harmoniously interface with biological tissues as they are soft, wet, and stretchable. Here, an electrically conductive supramolecular polymer network that simultaneously exhibits both electronic and ionic conductivity while maintaining tissue-mimetic mechanical properties, providing an ideal electronic interface with the human body, is introduced. Rational design of an ultrahigh affinity host-guest ternary complex led to binding affinities (>10¹³  M^-2 ) of over an order of magnitude greater than previous reports. Embedding these complexes as dynamic cross-links, coupled with in situ synthesis of a conducting polymer, resulted in electrically conductive supramolecular polymer networks with tissue-mimetic Young's moduli (<5 kPa), high stretchability (>500%), rapid self-recovery and high water content (>84%). Achieving such properties enabled fabrication of intrinsically-stretchable stand-alone bioelectrodes, capable of accurately monitoring electromyography signals, free from any rigid materials.

Structural Color Ionic Hydrogel Patches for Wound Management

Ionic hydrogels have attracted extensive attention because of their wide applicability in electronic skins, biosensors, and other biomedical areas. Tremendous effort is dedicated to developing ionic hydrogels with improved detection accuracy and multifunctionality. Herein, we present an inverse opal scaffold-based structural color ionic hydrogel with the desired features as intelligent patches for wound management. The patches were composed of a polyacrylamide-poly(vinyl alcohol)-polyethylenimine-lithium chloride (PAM-PVA-PEI-LiCl) inverse opal scaffold and a vascular endothelial growth factor (VEGF) mixed methacrylated gelatin (GelMA) hydrogel filler surface. The scaffold imparted the composite patches with brilliant structural color, conductive property, and freezing resistance, while the VEGF-GelMA surface could not only prevent the ionic hydrogel from the interference of complex wound conditions but also contribute to the cell proliferation and tissue repair in the wounds. Thus, the hydrogel patches could serve as electronic skins for in vivo wound healing and monitoring with high accuracy and reliability. These features indicate that the proposed structural color ionic hydrogel patches have great potential for clinical applications.

Mechanically Ultra-Robust, Elastic, Conductive, and Multifunctional Hybrid Hydrogel for a Triboelectric Nanogenerator and Flexible/Wearable Sensor

Flexibility/wearable electronics such as strain/pressure sensors in human-machine interactions (HMI) are highly developed nowadays. However, challenges remain because of the lack of flexibility, fatigue resistance, and versatility, leading to mechanical damage to device materials during practical applications. In this work, a triple-network conductive hydrogel is fabricated by combining 2D Ti₃ C₂ T_x nanosheets with two kinds of 1D polymer chains, polyacrylamide, and polyvinyl alcohol. The Ti₃ C₂ T_x nanosheets act as the crosslinkers, which combine the two polymer chains of PAM and PVA via hydrogen bonds. Such a unique structure endows the hydrogel (MPP-hydrogel) with merits such as mechanical ultra-robust, super-elasticity, and excellent fatigue resistance. More importantly, the introduced Ti₃ C₂ T_x nanosheets not only enhance the hydrogel's conductivity but help form double electric layers (DELs) between the MXene nanosheets and the free water molecules inside the MPP-hydrogel. When the MPP-hydrogel is used as the electrode of the triboelectric nanogenerator (MPP-TENG), due to the dynamic balance of the DELs under the initial potential difference generated from the contact electrification as the driving force, an enhanced electrical output of the TENG is generated. Moreover, flexible strain/pressure sensors for tiny and low-frequency human motion detection are achieved. This work demonstrates a promising flexible electronic material for e-skin and HMI.

Self-Healing and Shape-Editable Wearable Supercapacitors Based on Highly Stretchable Hydrogel Electrolytes

Shape editability combined with a self-healing capability and long-term cycling durability are highly desirable properties for wearable supercapacitors. Most wearable supercapacitors have rigid architecture and lack the capacity for editability into desirable shapes. Through sandwiching hydrogel electrolytes between two electrodes, a suite of wearable supercapacitors that integrate desirable properties namely: repeated shape editability, excellent self-healing capability, and long-term cycling durability is demonstrated. A strategy is proposed to enhance the long-term cycling durability by utilizing hydrogel electrolytes with unique cross-linking structures. The dynamic crosslinking sites are formed by quadruple H bonds and hydrophobic association, stabilizing the supercapacitors from inorganic ion disruption during charge-discharge processes. The fabricated supercapacitors result in the capacitance retention rates of 99.6% and 95.8% after 5000 and 10 000 charge-discharge cycles, respectively, which are much higher than others reported in the literature. Furthermore, the supercapacitor sheets can be repeatedly processed into various shapes without any capacitance loss. The supercapacitors exhibit a 95% capacitance retention rate after five cutting/self-healing cycles, indicative of their excellent self-healing performance. To demonstrate real-life applicability, the wearable supercapacitors are successfully used to power a light-emitting diode and an electronic watch.

Scalable Manufacturing of Solid Polymer Electrolytes with Superior Room-Temperature Ionic Conductivity

A scalable manufacturing protocol is developed to prepare polymer-based solvent-free all-solid flexible energy storage devices based on a two-roll mill and adapted rubber mixing technology. The as-prepared solid polymer electrolytes (SPEs) consisting of commercial poly(methyl methacrylate)-grafted natural rubber (MG30) and lithium bis(trifluoromethanesulfonyl)imide achieve a superior ionic conductivity of 2.7 × 10-3 S cm-1 at 30 °C. The superior ionic conductivity is attributed to the formation of an ionic cluster network in the composite as proved by small-angle X-ray scattering and infrared spectroscopy measurements. Moreover, the as-prepared SPEs show good mechanical stability over a broad temperature range, that is , a storage modulus above 1 × 104 Pa from 30 to 120 °C as indicated by the rheology data. Furthermore, the SPEs were assembled with the carbon black-filled MG30 (i.e., MG30C) electrode into a flexible supercapacitor cell, which had a wide voltage window of 3.5 V, good energy density of 28.4 μW h·cm-2 at 160 °C, and good temperature tolerance up to 160 °C. This scaling-up manufacture strategy shows tremendous potential to the advancing of SPEs in applications of flexible energy storage device.

Plant-inspired conductive adhesive organohydrogel with extreme environmental tolerance as a wearable dressing for multifunctional sensors

Conductive hydrogels have attracted significant attention as a promising material in electrical and biomedical fields. However, the simultaneous realization of good conductivity, toughness, high tissue adhesiveness, excellent biocompatibility, and extreme environmental tolerance remains a challenge. Inspired by the antifreezing/antiheating behavior of natural plants, a calcium chloride/TEMPO-oxidized cellulose nanofiber-dopamine/ polyacrylamide (CaCl₂/TOCNF-DOPA/PAM) glycerol/water organohydrogel with antifreezing and antiheating properties, good transparency, conductivity, stability, excellent biocompatibility, mechanical properties, and tissue adhesiveness was fabricated. The organohydrogel has about 700% stretchability, with about 90% transparency. The organohydrogel exhibits good conductivity of 4.9 × 10^-4 S/cm and high tissue adhesiveness of 50 kPa, which can monitor various human activities. The organohydrogel displays excellent extreme environmental tolerance to maintain the conductivity and mechanical properties under an extremely wide temperature range (-24 to 50 °C) for a long period due to its water-locking effect between glycerol and water molecules. The biocompatible organohydrogel is able to protect the skin from frostbite or burns in harsh environments. The plant-inspired stable and durable organohydrogel is used as a wearable dressing for multifunctional sensors.

Supramolecular Gel-Derived Highly Efficient Bifunctional Catalysts for Omnidirectionally Stretchable Zn-Air Batteries with Extreme Environmental Adaptability

Most existing stretchable batteries can generally only be stretched uniaxially and suffer from poor mechanical and electrochemical robustness to withstand extreme mechanical and environmental challenges. A highly efficient bifunctional electrocatalyst is herein developed via the unique self-templated conversion of a guanosine-based supramolecular hydrogel and presents a fully integrated design strategy to successfully fabricate an omnidirectionally stretchable and extremely environment-adaptable Zn-air battery (ZAB) through the synergistic engineering of active materials and device architecture. The electrocatalyst demonstrates a very low reversible overpotential of only 0.68 V for oxygen reduction/evolution reactions (ORR/OER). This ZAB exhibits superior omnidirectional stretchability with a full-cell areal strain of >1000% and excellent durability, withstanding more than 10 000 stretching cycles. Promisingly, without any additional pre-treatment, the ZAB exhibits outstanding ultra-low temperature tolerance (down to -60 °C) and superior waterproofness, withstanding continuous water rinsing (>5 h) and immersion (>3 h). The present work offers a promising strategy for the design of omnidirectionally stretchable and high-performance energy storage devices for future on-skin wearable applications.

Metal-Functionalized Hydrogels as Efficient Oxygen Evolution Electrocatalysts

Conductive polymer hydrogels have large surface areas and electrical conductivities. Their properties can be further tailored by functionalizing them with metals and nonmetals. However, the potential applications of metal-functionalized hydrogels for electrocatalysis have rarely been investigated. In this work, we report the synthesis of transition-metal-functionalized polyaniline-phytic acid (PANI-PA) hydrogels that show efficient electrocatalytic activities for the oxygen evolution reaction (OER). Among the many transition metals studied, Fe is accommodated by the hydrogel the most due to the favorable affinity of the PA groups in the hydrogel for Fe. Meanwhile, those containing both Fe and Co are found to be the most effective electrocatalysts for OER. The most optimized such hydrogel, NF@Hgel-Fe_0.3Co_0.1, which is made using a solution that has a 3:1 ratio of Fe and Co, needs an overpotential of only 280 mV to catalyze OER in 1 M KOH solution with a current density of 10 mV cm^-2. Furthermore, these metal-functionalized PANI-PA hydrogels can easily be loaded on the nickel foam or carbon cloth via a simple soak-and-dry method to generate free-standing electrodes. Overall, this work demonstrates a facile synthesis and fabrication of sustainable and efficient OER electrocatalysts and electrodes that are composed of easily processable hydrogels functionalized with earth-abundant transition metals.

Highly Flexible and Broad-Range Mechanically Tunable All-Wood Hydrogels with Nanoscale Channels via the Hofmeister Effect for Human Motion Monitoring

Wood-based hydrogel with a unique anisotropic structure is an attractive soft material, but the presence of rigid crystalline cellulose in natural wood makes the hydrogel less flexible. In this study, an all-wood hydrogel was constructed by cross-linking cellulose fibers, polyvinyl alcohol (PVA) chains, and lignin molecules through the Hofmeister effect. The all-wood hydrogel shows a high tensile strength of 36.5 MPa and a strain up to ~ 438% in the longitudinal direction, which is much higher than its tensile strength (~ 2.6 MPa) and strain (~ 198%) in the radial direction, respectively. The high mechanical strength of all-wood hydrogels is mainly attributed to the strong hydrogen bonding, physical entanglement, and van der Waals forces between lignin molecules, cellulose nanofibers, and PVA chains. Thanks to its excellent flexibility, good conductivity, and sensitivity, the all-wood hydrogel can accurately distinguish diverse macroscale or subtle human movements, including finger flexion, pulse, and swallowing behavior. In particular, when "An Qi" was called four times within 15 s, two variations of the pronunciation could be identified. With recyclable, biodegradable, and adjustable mechanical properties, the all-wood hydrogel is a multifunctional soft material with promising applications, such as human motion monitoring, tissue engineering, and robotics materials.

Interface Engineering on Cellulose-Based Flexible Electrode Enables High Mass Loading Wearable Supercapacitor with Ultrahigh Capacitance and Energy Density

For practical energy storage devices, a bottleneck is to retain decent integrated performances while increasing the mass loading of active materials to the commercial level, which highlights an urgent need for novel electrode structure design strategies. Here, an active nitrogen-doped carbon interface with "high conductivity, high porosity, and high electrolyte affinity" on a flexible cellulose electrode surface is engineered to accommodate 1D active materials. The high conductivity of interface favors fast electron transport, while its high porosity and high electrolyte affinity properties benefit ion migration. As a result, the flexible anode accommodated by carbon nanotubes achieves an ultrahigh capacitance of 9501 mF cm^-2 (315.6 F g^-1 ) at a high mass loading of 30.1 mg cm^-2 , and the flexible cathode accommodated by polypyrrole nanotubes realizes a remarkably high capacitance of 6212 mF cm^-2 (248 F g^-1 , 25 mg cm^-2 ). The assembled flexible quasi-solid-state asymmetric supercapacitor delivers a maximum energy density of 1.42 mWh cm^-2 (2.2 V, 2105 mF cm^-2 ), representing the highest value among all reported flexible supercapacitors. This versatile design concept provides a new way to prepare high performance flexible energy storage devices.

"Porous and Yet Dense" Electrodes for High-Volumetric-Performance Electrochemical Capacitors: Principles, Advances, and Challenges

With the ever-rapid miniaturization of portable, wearable electronics and Internet of Things, the volumetric performance is becoming a much more pertinent figure-of-merit than the conventionally used gravimetric parameters to evaluate the charge-storage capacity of electrochemical capacitors (ECs). Thus, it is essential to design the ECs that can store as much energy as possible within a limited space. As the most critical component in ECs, "porous and yet dense" electrodes with large ion-accessible surface area and optimal packing density are crucial to realize desired high volumetric performance, which have demonstrated to be rather challenging. In this review, the principles and fundamentals of ECs are first observed, focusing on the key understandings of the different charge storage mechanisms in porous electrodes. The recent and latest advances in high-volumetric-performance ECs, developed by the rational design and fabrication of "porous and yet dense" electrodes are then examined. Particular emphasis of discussions then concentrates on the key factors impacting the volumetric performance of porous carbon-based electrodes. Finally, the currently faced challenges, further perspectives and opportunities on those purposely engineered porous electrodes for high-volumetric-performance EC are presented, aiming at providing a set of guidelines for further design of the next-generation energy storage devices.

Correlating Ionic Conductivity and Microstructure in Polyelectrolyte Hydrogels for Bioelectronic Devices

Hydrogels have become the material of choice in bioelectronic devices because their high-water content leads to efficient ion transport and a conformal interface with biological tissue. While the morphology of hydrogels has been thoroughly studied, systematical studies on their ionic conductivity is less common. Here, we present an easy-to-implement strategy to characterize the ionic conductivity of a series of polyelectrolyte hydrogels with different amounts of monomer and crosslinker and correlate their ionic conductivity with microstructure. Higher monomer increases the ionic conductivity of the polyelectrolyte hydrogel due to the increased charge carrier density, but also leads to excessive swelling that may cause device failure upon integration with bioelectronic devices. Increasing the amount of crosslinker can reduce the swelling ratio by increasing the crosslinking density and reducing the mesh size of the hydrogel, which cuts down the ionic conductivity. Further investigation on the porosity and tortuosity of the swollen hydrogels correlates the microstructure with the ionic conductivity. These results are generalizable for various polyelectrolyte hydrogel systems with other ions as the charge carrier and provide a facile guidance to design polyelectrolyte hydrogel with desired ionic conductivity and microstructure for applications in bioelectronic devices. This article is protected by copyright. All rights reserved.

Growing Co–Ni–Se nanosheets on 3D carbon frameworks as advanced dual functional electrodes for supercapacitors and sodium ion batteries

The reasonable design of electrode materials is crucial for tuning the electrochemical performances of advanced energy storage systems.