Strong and Robust Polyaniline‐Based Supramolecular Hydrogels for Flexible Supercapacitors

A Highly Robust and Conducting Ultramicroporous 3D Fe(II)-Based Metal-Organic Framework for Efficient Energy Storage

Exploitation of metal-organic framework (MOF) materials as active electrodes for energy storage or conversion is reasonably challenging owing to their poor robustness against various acidic/basic conditions and conventionally low electric conductivity. Keeping this in perspective, herein, a 3D ultramicroporous triazolate Fe-MOF (abbreviated as Fe-MET) is judiciously employed using cheap and commercially available starting materials. Fe-MET possesses ultra-stability against various chemical environments (pH-1 to pH-14 with varied organic solvents) and is highly electrically conductive (σ = 0.19 S m-1) in one fell swoop. By taking advantage of the properties mentioned above, Fe-MET electrodes give prominence to electrochemical capacitor (EC) performance by delivering an astounding gravimetric (304 F g-1) and areal (181 mF cm-2) capacitance at 0.5 A g-1 current density with exceptionally high cycling stability. Implementation of Fe-MET as an exclusive (by not using any conductive additives) EC electrode in solid-state energy storage devices outperforms most of the reported MOF-based EC materials and even surpasses certain porous carbon and graphene materials, showcasing superior capabilities and great promise compared to various other alternatives as energy storage materials.

Synergistic Coassembly of Folic Acid-Based Supramolecular Polymer with a Covalent Polymer Toward Fabricating Functional Antibacterial Biomaterials

Supramolecular biomaterials can recapitulate the structural and functional facets of the native extracellular matrix and react to biochemical cues, leveraging the unique attributes of noncovalent interactions, including reversibility and tunability. However, the low mechanical properties of supramolecular biomaterials can restrict their utilization in specific applications. Combining the advantages of supramolecular polymers with covalent polymers can lead to the fabrication of tailor-made biomaterials with enhanced mechanical properties/degradability. Herein, we demonstrate a synergistic coassembled self-healing gel as a multifunctional supramolecular material. As the supramolecular polymer component, we chose folic acid (vitamin B9), an important biomolecule that forms a gel comprising one-dimensional (1D) supramolecular polymers. Integrating polyvinyl alcohol (PVA) into this supramolecular gel alters its ultrastructure and augments its mechanical properties. A drastic improvement of complex modulus (G*) (∼3674 times) was observed in the folic acid-PVA gel with 15% w/v PVA (33215 Pa) compared with the folic acid gel (9.04 Pa). The coassembled hydrogels possessed self-healing and injectable/thixotropic attributes and could be printed into specific three-dimensional (3D) shapes. Synergistically, the supramolecular polymers of folic acid also improve the toughness, durability, and ductility of the PVA films. A nanocomposite of the gels with silver nanoparticles exhibited excellent catalytic efficiency and antibacterial activity. The folic acid-PVA coassembled gels and films also possessed high cytocompatibility, substantiated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and live-dead assays. Taken together, the antibacterial and cell-adhesive attributes suggest potential applications of these coassembled biomaterials for tissue engineering and wound healing.

Ultra-Stable High-Capacity Polythiophene Derivative for Wide-Potential-Window Supercapacitors

Conducting polymer (CP)-based supercapacitors show great promise for applications in the field of wearable and portable electronics. However, these supercapacitors face persistent challenges, notably low energy density and inadequate stability. In this study, we introduce a polythiophene derivative, designated as poly(EPE), synthesized via the electrochemical polymerization of 8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-3,3-dimethyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine (EPE). The resulting poly(EPE) polymer exhibits an exemplary 3D porous network-like structure, significantly enhancing its capacitance performance. When employed as the electrode material, the symmetric supercapacitor demonstrates an exceptionally high specific capacitance of 1342 F g-1 at a current density of 4.0 A g-1, along with impressive energy and power densities of 119.3 W h kg-1 and 38.83 kW kg-1, respectively. These capacitance values surpass those of previously reported pristine CP-based supercapacitors. Notably, the supercapacitor showcases outstanding stability, maintaining a retention rate of 92.5% even after 50,000 charge-discharge cycles. These findings underscore the substantial potential of poly(EPE) as an electrode material for the advancement of the supercapacitor technology.

Nature's coatings: Sodium alginate as a novel coating in safeguarding plants from frost damages

Frost damage remains a significant challenge for agricultural practices worldwide, leading to substantial economic losses and food insecurity. Practically, traditional methods for frost management have proven ineffective and come with several drawbacks, such as energy consumption and limited efficacy. Hence, proposing an anti-freezing coating can be an innovative idea. The potential of sodium alginate (SA) to construct anti-freezing hydrogels has been explored in several sciences. SA hydrogels can form protective films around plants as a barrier against freezing temperatures and ice crystals on the plant's surface. Sodium alginate exhibits excellent water retention, enhancing plant hydration during freezing conditions. This coating can provide insulation, effectively shielding the plant from frost damage. The advantages of SA as a coating material, such as its biocompatibility, biodegradability, and non-toxic nature, are highlighted. Therefore, the proposed use of SA as an innovative coating material holds promise for safeguarding plants from frost damage. Following SA potential and frost's huge damage, the present review provides a comprehensive overview of the recent developments in SA-based anti-freezing hydrogels, their applications, and their potential in agriculture as anti-freezing coatings. However, further research and field trials are necessary to optimize the application methods and understand the long-term effects on productivity.

Liquid Metal-Polymer Conductor-Based Conformal Cyborg Devices

Gallium-based liquid metal (LM) exhibits exceptional properties such as high conductivity and biocompatibility, rendering it highly valuable for the development of conformal bioelectronics. When combined with polymers, liquid metal-polymer conductors (MPC) offer a versatile platform for fabricating conformal cyborg devices, enabling functions such as sensing, restoration, and augmentation within the human body. This review focuses on the synthesis, fabrication, and application of MPC-based cyborg devices. The synthesis of functional materials based on LM and the fabrication techniques for MPC-based devices are elucidated. The review provides a comprehensive overview of MPC-based cyborg devices, encompassing their applications in sensing diverse signals, therapeutic interventions, and augmentation. The objective of this review is to serve as a valuable resource that bridges the gap between the fabrication of MPC-based conformal devices and their potential biomedical applications.

Co-containing metal-organic framework for high-performance asymmetric supercapacitors with functionalized reduced graphene oxide

Nowadays, supercapacitors are the most coveted eco-friendly and sustainable next-generation energy storage devices. In this regard, developing supercapacitors with high energy density and power density has always been a challenge for researchers. Herein, we have exploited an electroactive Co-containing metal-organic framework (Co-MOF) using cheap and commercially available starting materials under refluxing conditions and explored its energy storage properties in three- and two-electrode methods. The Co-MOF exhibited a specific capacitance of 425 F g-1 at 2 A g-1, maintaining a capacitance of ∼78% over 2200 successive charge-discharge cycles in a three-electrode system. The two-electrode asymmetric supercapacitor (ASC) using Co-MOF as the working electrode and as-synthesized p-phenylenediamine (PPD)-functionalized reduced graphene oxide (PPD-rGO) as the counter electrode divulged a specific capacitance of 72.5 F g-1 at 2 A g-1 current density with ∼70% capacitive retention after 2200 successive charge-discharge cycles over a broad potential window of 0-1.6 V. Moreover, the ASC demonstrated a maximum power density of 11.9 kW kg-1 at 10 A g-1 and a maximum energy density of 25.8 W h kg-1 at 2 A g-1 current density. Owing to the stable electrochemical redox (Co2+/Co3+)-mediated pseudocapacitive behavior of the Co-MOF and the high surface area and electrical conductivity of in situ generated PPD-intercalated rGO, the fabricated ASC unveiled high-performance supercapacitive behaviors. To investigate the practical applicability of this material, solid-state (ASC) devices were fabricated by employing the Co-MOF as the positive electrode and PPD-rGO as the negative electrode in a KOH-based gel electrolyte, which could power a commercially available light-emitting diode bulb (∼1.8 V) for several seconds. Therefore, the elucidated high electrochemical energy storage performance of the prepared Co-MOF makes it a very promising electrode material for supercapacitors.

Engineering Smart Composite Hydrogels for Wearable Disease Monitoring

Growing health awareness triggers the public's concern about health problems. People want a timely and comprehensive picture of their condition without frequent trips to the hospital for costly and cumbersome general check-ups. The wearable technique provides a continuous measurement method for health monitoring by tracking a person's physiological data and analyzing it locally or remotely. During the health monitoring process, different kinds of sensors convert physiological signals into electrical or optical signals that can be recorded and transmitted, consequently playing a crucial role in wearable techniques. Wearable application scenarios usually require sensors to possess excellent flexibility and stretchability. Thus, designing flexible and stretchable sensors with reliable performance is the key to wearable technology. Smart composite hydrogels, which have tunable electrical properties, mechanical properties, biocompatibility, and multi-stimulus sensitivity, are one of the best sensitive materials for wearable health monitoring. This review summarizes the common synthetic and performance optimization strategies of smart composite hydrogels and focuses on the current application of smart composite hydrogels in the field of wearable health monitoring.

Flexible Supercapacitors Based on Stretchable Conducting Polymer Electrodes

Supercapacitors are widely used in various fields due to their high power density, fast charging and discharging speeds, and long service life. However, with the increasing demand for flexible electronics, integrated supercapacitors in devices are also facing more challenges, such as extensibility, bending stability, and operability. Despite many reports on stretchable supercapacitors, challenges still exist in their preparation process, which involves multiple steps. Therefore, we prepared stretchable conducting polymer electrodes by depositing thiophene and 3-methylthiophene on patterned 304 stainless steel (SS 304) through electropolymerization. The cycling stability of the prepared stretchable electrodes could be further improved by protecting them with poly(vinyl alcohol)/sulfuric acid (PVA/H₂SO₄) gel electrolyte. Specifically, the mechanical stability of the polythiophene (PTh) electrode was improved by 2.5%, and the stability of the poly(3-methylthiophene (P3MeT) electrode was improved by 7.0%. As a result, the assembled flexible supercapacitors maintained 93% of their stability even after 10,000 cycles of strain at 100%, which indicates potential applications in flexible electronics.

Recent progress in polyaniline-based composites as electrode materials for pliable supercapacitors

Significant contributions have been made towards the development of flexible energy storage devices to meet the ever-growing energy demand. Flexibility, mechanical stability, and electrical conductivity are three critical qualities that distinguish conducting polymers from other materials. Polyaniline (PANI) has drawn considerable attention among the various conducting polymers for use in flexible supercapacitors. PANI offers several desirable properties including high porosity, a large surface area, and high conductivity. Despite its merits, it also suffers from poor cyclic stability, low mechanical strength, and notable discrepancy between theoretical and actual capacitance. These shortcomings have been addressed by creating composites of PANI with structurally sturdy elements such as graphene, carbon nanotubes (CNTs), metal-organic framework (MOFs), MXenes, etc., thus enhancing the performance of supercapacitors. This review outlines the several schemes adopted to prepare diverse binary and ternary composites of PANI as the electrode material for flexible supercapacitors and the significant impact of composite formation on the flexibility and electrochemical performance of the fabricated pliable supercapacitors.

Environment tolerant, adaptable and stretchable organohydrogels: preparation, optimization, and applications

Multiple stretchable materials have been successively developed and applied to wearable devices, soft robotics, and tissue engineering. Organohydrogels are currently being widely studied and formed by dispersing immiscible hydrophilic/hydrophobic polymer networks or only hydrophilic polymer networks in an organic/water solvent system. In particular, they can not only inherit and carry forward the merits of hydrogels, but also have some unique advantageous features, such as anti-freezing and water retention abilities, solvent resistance, adjustable surface wettability, and shape memory effect, which are conducive to the wide environmental adaptability and intelligent applications. This review first summarizes the structure, preparation strategy, and unique advantages of the reported organohydrogels. Furthermore, organohydrogels can be optimized for electro-mechanical properties or endowed with various functionalities by adding or modifying various functional components owing to their modifiability. Correspondingly, different optimization strategies, mechanisms, and advanced developments are described in detail, mainly involving the mechanical properties, conductivity, adhesion, self-healing properties, and antibacterial properties of organohydrogels. Moreover, the applications of organohydrogels in flexible sensors, energy storage devices, nanogenerators, and biomedicine have been summarized, confirming their unlimited potential in future development. Finally, the existing challenges and future prospects of organohydrogels are provided.

Vertically Aligned Micropillar Arrays Coated with a Conductive Polymer for Advanced Pseudocapacitance Energy Storage

With the development of high-performance supercapacitors, a conductive polymer (CP)-based pseudocapacitance electrode with good electrical conductivity and low processing costs holds a promising application prospect. As the core component of supercapacitors, the CP electrode with adjustable spacing, a high specific surface area, and a faster ion diffusion path has been extensively investigated. Herein, based on accurate top-down photolithography and electropolymerization approaches, we fabricate a CP-coated vertically aligned micropillar array (MPA) electrode. The electrode presents an overwhelming enhanced areal specific capacitance compared with that of a flat configuration, which is partially ascribed to an increased electroactive surface area, two rapid channels for ion diffusion and electron transfer, and enhanced electric field intensity that is provided by the MPA structure. Based on the fabricated CP-based MPA electrode, an asymmetric supercapacitor is assembled with two thiophene derivatives, presenting a high energy density and excellent cycling stability. A supercapacitor system cascading with three asymmetric supercapacitor devices further demonstrates the practical applications by driving the light-emitting diodes. This work provides a good reference for the further development of CP-based energy storage devices with high energy density and superior cycling stability.

Preparation of chitosan/sodium alginate conductive hydrogels with high salt contents and their application in flexible supercapacitors

Conductive hydrogels (CHs) are a potential material for flexible electronics. However, most of CHs display disadvantages of low ionic conductivities and intolerance to low temperatures. Herein, a novel physical CHs with salt contents as high as 30 wt% was prepared with chitosan (CTS) and sodium alginate (SA) by combining the anti-polyelectrolyte effect and semi-dissolution acidification sol-gel transition (SD-A-SGT) method. The obtained hydrogels show extremely high ionic conductivities up to 2.96 × 10^-1 S·cm^-1 at room temperature and 4.9 × 10^-2 S·cm^-1 at -20 °C. The effects of different salts on the ion mobility and electrochemical properties of CTS/SA CHs were predicted and analyzed. The flexible supercapacitor assembled using CTS/SA CHs as the electrolyte exhibits the specific capacitance as high as 405 F·g^-1 at the current density of 0.25 A·g^-1 and satisfying electrochemical stability with 74.91% capacitance retention in 1000 cycles. Our work has provided a new strategy for constructing green CHs with high ionic conductivities.

Facile fabrication of a polyvinyl alcohol-based hydrophobic fluorescent film via the Hantzsch reaction for broadband UV protection

Excessive exposure to ultraviolet (UV) light is harmful to human health. However, the traditional preparation of anti-UV films through the doping of UV absorbers leads to unstable products. Chemical modification of polyvinyl alcohol (PVA) to fabricate functional derivatives expand the application of these materials. Herein, a 1,4-dihydropyridine (DHP) fluorescent ring with a conjugated structure as a strong UV-absorber group was introduced onto a polyvinyl alcohol acetoacetate (PVAA) film to improve its UV-blocking performance. Firstly, PVAA was prepared via transesterification using tert-butyl acetoacetate (t-BAA). Then, the Hantzsch reaction was carried out on the surface of the PVAA film at room temperature. The resulting film showed high transparency, bright fluorescence emission, good mechanical properties, and outstanding stability. The introduction of the hydrophobic carbon chain reduced the hydrophilicity and swelling capacity of the PVAA film. In addition, the conjugated structure endowed the fluorescent film with excellent UV-blocking performance, where almost 100% UVA and UVB spectra could be shielded. The UV-blocking properties of the prepared films were persistent when they were exposed to UV irradiation, solvents, and subjected to thermal treatment. This work presents a facile and environmentally-friendly strategy by which to fabricate a multifunctional PVA-based film, which holds great potential for application in the anti-counterfeiting and UV-blocking fields.

Design of Fe3+-Rich, High-Conductivity Lignin Hydrogels for Supercapacitor and Sensor Applications

Preparation of natural polymer-based highly conductive hydrogels with tunable mechanical properties for applications in flexible electronics is still challenging. Herein, we report a facile method to prepare lignin-based Fe³⁺-rich, high-conductivity hydrogels via the following two-step process: (1) lignin hydrogels are prepared by cross-linking sulfonated lignin with poly(ethylene glycol) diglycidyl ether (PEGDGE) and (2) Fe³⁺ ions are impregnated into the lignin hydrogel by simply soaking in FeCl₃. Benefiting from Fe³⁺ ion complexation with catechol groups and other functional groups in lignin, the resultant hydrogels exhibit unique properties, such as high conductivity (as high as 6.69 S·m^-1) and excellent mechanical and hydrophobic properties. As a strain sensor, the as-prepared lignin hydrogel shows high sensitivity when detecting various human motions. With the flow of moist air, the Fe³⁺-rich lignin hydrogel generates an output voltage of 162.8 mV. The assembled supercapacitor of the hydrogel electrolyte demonstrates a high specific capacitance of 301.8 F·g^-1, with a maximum energy density of 26.73 Wh·kg^-1, a power density of 2.38 kW·kg^-1, and a capacitance retention of 94.1% after 10 000 consecutive charge-discharge cycles. These results support the conclusion that lignin-based Fe³⁺-rich, high-conductivity hydrogels have promising applications in different fields, including sensors and supercapacitors, rendering a new platform for the value-added utilization of lignin.

Wearable Self-Powered Smart Sensors for Portable Nutrition Monitoring

Self-powered sensors have attracted great attention in the field of analysis owing to the necessity of power resources for the routine use of sensor devices. However, it is still challenging to construct wearable self-powered sensors in a simple and efficient way. Herein, wearable self-powered textile smart sensors based on advanced bifunctional polyaniline/reduced graphene oxide (PANI/RGO) films have been successfully developed for remote real-time detection of vitamin C. Specifically, a pH-assisted oil/water (O/W) self-assembly strategy was proposed to boost the O/W self-assembled PANI/RGO films via proton regulation. The as-obtained PANI/RGO films could be directly loaded on the textile substrate, with good capacitive and biosensing performance due to the multifunctionality of PANI and RGO, respectively. Moreover, both wearable power supply devices and wearable biosensors based on PANI/RGO films possess good electrochemical performance, which paves the way for the actual application of self-powered nutrition monitoring. Significantly, obvious signals have been obtained in the detection of vitamin C beverages, exhibiting promising application values in daily nutrition track necessities. Prospectively, this study would provide an effective and simple strategy for integrating wearable self-powered sensors, and the developed smart sensing system is an ideal choice for the portable detection of nutrition.

Skin-like hydrogel devices for wearable sensing, soft robotics and beyond

Skin-like electronics are developing rapidly to realize a variety of applications such as wearable sensing and soft robotics. Hydrogels, as soft biomaterials, have been studied intensively for skin-like electronic utilities due to their unique features such as softness, wetness, biocompatibility and ionic sensing capability. These features could potentially blur the gap between soft biological systems and hard artificial machines. However, the development of skin-like hydrogel devices is still in its infancy and faces challenges including limited functionality, low ambient stability, poor surface adhesion, and relatively high power consumption (as ionic sensors). This review aims to summarize current development of skin-inspired hydrogel devices to address these challenges. We first conduct an overview of hydrogels and existing strategies to increase their toughness and conductivity. Next, we describe current approaches to leverage hydrogel devices with advanced merits including anti-dehydration, anti-freezing, and adhesion. Thereafter, we highlight state-of-the-art skin-like hydrogel devices for applications including wearable electronics, soft robotics, and energy harvesting. Finally, we conclude and outline the future trends.

Near-Infrared-Light-Modulated Lubricating Coating Enabled by Photothermal Microgels

As a micro-/nano-sized hydrogel, polymeric microgel not only has three-dimensional (3D) molecular networks but also displays the small-size effect, which has been widely used in various fields, such as drug nanocarrier, photonic crystal, functional coating, and aqueous lubrication. In this work, a photothermal lubricating coating was prepared using polymeric/inorganic hybrid microgels and its surficial friction was deliberately modulated by the remote irradiation of near-infrared (NIR) light. Specifically, a photothermal hybrid microgel, Fe₃O₄@poly(N-isopropylacrylamide-co-polyacrylic acid) (Fe₃O₄@PNA), was first fabricated and then sprayed onto poly(ethyleneimine) (PEI)-modified substrate to form a lubricating microgel coating. At room temperature, this microgel coating was hydrophilic and achieved good hydration lubrication with relatively low friction. After the introduction of NIR light, the photothermal microgel coating converted light energy into heat energy for increasing its own temperature rapidly. Due to the thermosensitive PNA shell, the wettability of the coating was transformed to hydrophobicity above the lower critical solution temperature (LCST), resulting in a remarkable increase in friction. In other words, the surficial friction of this microgel coating could be reversibly modulated using NIR light. This work expands the application scope of microgels in the field of aqueous lubrication and introduces the functional microgels into making the smart lubricating coating for the first time. This basic research in the field of friction control may provide an efficient strategy for the design of interfacial sensing, controlled transmission, and intelligent manipulator.

Switchable PNIPAm/PPyNT Hydrogel for Smart Supercapacitors: External Control of Capacitance for Pulsed Energy Generation or Prolongation of Discharge Time

Supercapacitors based on nonresponsive polymer hydrogels are gaining significant attention due to their fabrication simplicity and high potential for wearable electronics. However, the use of smart hydrogels in supercapacitor design remains unexplored. In this work, a smart externally controlled supercapacitor based on a temperature-responsive hydrogel doped with polypyrrole nanotubes (PPyNTs) is proposed. The redistribution of PPyNTs in the poly(N-isopropylacrylamide) (PNIPAm) hydrogel can be reversibly controlled by light illumination or temperature increase, leading to on-demand formation/disruption of the nanotube conductive network, due to release/entrapping of the nanotubes from PNIPAm globule volume on surface. The switchable material was introduced in a supercapacitor design as an active and smart electrode, responsible for external control of charge transport and storage. The created device showed a switchable supercapacitor performance with an ability to significantly and rapidly change capacity under heating/cooling or light illumination. The external trigger was applied for static or dynamic control of supercapacitor behavior: prolongation of discharge time (with constant electric loading) or vice-versa pronounced acceleration of supercapacitor discharge. The proposed smart material-based supercapacitor can find a range of attractive applications in backup energy storage or high power pulse generation.

Preparation of lignosulfonate ionic hydrogels for supercapacitors, sensors and dye adsorbent applications

Lignin, an abundant natural polymer but presently under-utilized, has received much attention for its green/sustainable advantages. Herein, we report a facile method to fabricate lignosulfonate (LS) ionic hydrogels by simple crosslinking with poly (ethylene glycol) diglycidyl ether (PEGDGE). The as-obtained LS-PEGDGE hydrogels were comprehensively characterized by mechanical measurements, FT-IR, and SEM. The rich sulfonic and phenolic hydroxyl groups in LS hydrogels play key roles in imparting multifunctional smart properties, such as adhesiveness, conducting, sensing and dye adsorption, as well as superconductive behavior when increasing the moisture content. The hydrogels have a high adsorption capacity for cationic dyes, using methylene blue as a model, reaching 211 mg·g^-1. As a moist-induced power generator, the maximum output voltage is 181 mV. The LS-PEGDGE hydrogel-based flexible strain sensors exhibit high sensitivity when detecting human movements. As the hydrogel electrolyte, the assembled supercapacitor shows high specific capacitance of 236.9 F·g^-1, with the maximum energy density of 20.61 Wh·kg^-1, power density of 2306.4 W·kg^-1, and capacitance retention of 92.9% after 10,000 consecutive charge-discharge cycles. Therefore, this multifunctional LS hydrogels may have promising applications in various fields, providing a new platform for the value-added utilization of lignin from industrial waste.

Tissue adhesive hydrogel bioelectronics

Flexible bioelectronics have promising applications in electronic skin, wearable devices, biomedical electronics, etc. Hydrogels have unique advantages for bioelectronics due to their tissue-like mechanical properties and excellent biocompatibility. Particularly, conductive and tissue adhesive hydrogels can self-adhere to bio-tissues and have great potential in implantable wearable bioelectronics. This review focuses on the recent progress in tissue adhesive hydrogel bioelectronics, including the mechanism and preparation of tissue adhesive hydrogels, the fabrication strategies of conductive hydrogels, and tissue adhesive hydrogel bioelectronics and applications. Some perspectives on tissue adhesive hydrogel bioelectronics are provided at the end of the review.

Heat-Resistant and High-Performance Solid-State Supercapacitors Based on Poly(para-phenylene terephthalamide) Fibers via Polymer-Assisted Metal Deposition

Heat-resistant supercapacitors with excellent mechanical strength are highly required for flexible and wearable electronics in our daily life. Here, high-performance and heat-resistant solid-state supercapacitors based on poly(para-phenylene terephthalamide) fibers/Ag/poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS)&carbon nanotubes (PPTA/Ag/PCNTs) have been fabricated for the first time. Due to the toughness of the PPTA fibers and the excellent electroconductivity of the composite fiber, the PPTA/Ag/PCNTs-based supercapacitor exhibits good electrochemical performance together with brilliant mechanical strength and bending durability. Moreover, the heat resistance of tough PPTA/Ag/PCNTs fiber and ionic liquid gel electrolyte endow the supercapacitor with stability at temperatures as high as 80 °C and the capacitance can return to more than 80% after the heat treatment. The power density or energy density of the supercapacitor assembled on PPTA/Ag/PCNTs fiber electrodes is higher than that of any other polymer fiber-related supercapacitor. This work adds to the study of high-performance supercapacitors based on polymer fibers and provides the possibility to apply PPTA fiber for energy storage devices with flexibility and heat resistance.

Self-Standing Hydrogels Composed of Conducting Polymers for All-Hydrogel-State Supercapacitors

Conducting polymer hydrogels that are capable of contacting with electrolytes at the molecular level, represent an important electrode material. However, the fabrication of self-standing hydrogels merely composed of conducting polymers is still challenging owing to the absence of reliable methods. Herein, a novel and facile macromolecular interaction assisted route is reported to fabricate self-standing hydrogels consisting of polyaniline (PANi: providing high electrochemical activity) and poly(3,4-ethylenedioxythiophene) (PEDOT: enabling high electronic conductivity). Owing to the synergistic effect between them, the self-standing hydrogels possess good mechanical properties and electronic/electrochemical performances, making them an excellent potential electrode for solid-state energy storage devices. A proof-of-concept all-hydrogel-state supercapacitor is fabricated, which exhibits a high areal capacitance of 808.2 mF cm^-2 , and a high energy density of 0.63 mWh cm^-3 at high power density of 28.42 mW cm^-3 , superior to many recently reported conducting polymer hydrogels based supercapacitors. This study demonstrates a novel promising strategy to fabricate self-standing conducting polymer hydrogels.

Recent Progress on Graphene/Polyaniline Composites for High-performance Supercapacitors

Electrode materials are crucial for the electrochemical performance of supercapacitors. In view of the high specific surface area, high conductivity of graphene nanosheets and the high pseudocapacitance of polyaniline (PANI), the combination of graphene with PANI has become a research hotspot. In this work, we summarize the recent advance on the synthesis of PANI and graphene/PANI composites, and their application in supercapacitors. The synthesis of PANI is the basis of preparing graphene/PANI composites, so we first introduce the synthesis methods of PANI. Then, the advances of two dimensional (2D) and three dimensional (3D) graphene/PANI composites are summarized according to the inherent feature of graphene. The 2D composites of pristine graphene and functionalized graphene with PANI are introduced separately; furthermore, the 3D composites are classified into three sections, including flexible graphene/PANI composites, graphene framework based composites, and printable graphene/PANI composites. At last, aiming at solving the current challenges of graphene/PANI composites, we put forward some strategies for preparing high performance graphene/PANI composite electrodes.

Conductive Polymer Hydrogel Microfibers from Multiflow Microfluidics

Conductive hydrogels are receiving increasing attention for their utility in electronic area applications requiring flexible conductors. Here, it is presented novel conductive hydrogel microfibers with alginate shells and poly (3, 4-ethylenedioxythiophene): poly (4-styrenesulfonate) (PEDOT: PSS) cores fabricated using a multiflow capillary microfluidic spinning approach. Based on multiflow microfluidics, alginate shells are formed immediately from the fast gelation reaction between sodium alginate (Na-Alg) and sheath laminar calcium chloride flows, while PEDOT: PSS cores are solidified slowly in the hollow alginate hydrogel shell microreactors after their precursor solutions are injected in situ as the center fluids. The resultant PEDOT: PSS-containing microfibers are with features of designed morphology and highly controllable package, because material compositions or the sizes of their shell hydrogels can be tailored by using different concentrations or flow rates of pregel solutions. Moreover, the practical values of these microfibers in stretch sensitivity and bending stability are explored based on various electrical characterizations of the compound materials. Thus, it is believed that these microfluidic spinning PEDOT: PSS conductive microfibers will find important utility in electronic applications requiring flexible electronic systems.

Improving Ion Rejection of Conductive Nanofiltration Membrane through Electrically Enhanced Surface Charge Density

Nanofiltration (NF) is considered a promising candidate for brackish and seawater desalination. NF exhibits high multivalent ion rejection, but the rejection rate for monovalent ions is relatively low. Besides, great challenges remain for conventional NF membranes to achieve high ion rejection without sacrificing water flux. This work presents an effective strategy for improving the ion rejection of conductive NF membrane without decreasing the permeability through electrically assisted enhancement of surface charge density. When external voltage is increased from 0 to 2.5 V, the surface charge density of the membrane increases from 11.9 to 73.0 mC m^-2, which is 6.1× higher than that without external voltage. Correspondingly, the rejection rate for Na₂SO₄ increases from 81.6 to 93.0% and that for NaCl improves from 53.9 to 82.4%; meanwhile, the membrane retains high permeabilities of 14.0 L m^-2 h^-1 bar^-1 for Na₂SO₄ filtration and 14.5 L m^-2 h^-1 bar^-1 for NaCl filtration. The Donnan steric pore model analysis suggests that the Donnan potential difference between the membrane and bulk solution is increased under electrical assistance, leading to increased ion transfer resistance for improved ion rejection. This work provides new insight into the development of advanced NF technologies for desalination and water treatment.

Hydrogel bioelectronics

Hydrogels have emerged as a promising bioelectronic interfacing material. This review discusses the fundamentals and recent advances in hydrogel bioelectronics.

Confining Redox Electrolytes in Functionalized Porous Carbon with Improved Energy Density for Supercapacitors

It is a big challenge to improve the energy density of the carbon-based supercapacitors for wide applications. In this work, considering the properties of redox electrolytes, functionalized porous carbon has been synthesized with interconnected pores and oxygen functional groups, which is employed to well hold the redox electrolyte ions. As a result, the functionalized porous carbon shows a high capacitance of 454 F g^-1 at a current density of 1 A g^-1 and can maintain 88% of the initial capacitance after 10 000 charge-discharge cycles at 10 A g^-1. Especially, the as-prepared asymmetric supercapacitor obtains high energy density of 36.9 W h kg^-1 at the power density of 225 W kg^-1. This new design strategy by coordinating carbon materials with the redox electrolytes will guide the development of high-energy density supercapacitors.

Wearable Wire-Shaped Symmetric Supercapacitors Based on Activated Carbon-Coated Graphite Fibers

With the advantages of being lightweight, flexible, and wearable, wire-shaped supercapacitors have received tremendous attention in wearable and portable power sources in recent years. Considering the demands for large-scale applications, it is necessary to explore a facile and convenient preparation approach for wire-shaped supercapacitors. Herein, we reported a simple approach to fabricate wire-shaped electrodes by a dipping method, which possessed a nitric acid-activated graphite fiber core and an activated carbon-coating layer structure. Parallel and symmetric all-solid-state wire-shaped supercapacitors (PWSCs) based on the electrodes were fabricated. The as-fabricated PWSC showed high energy density (6.60 W h/kg, 8.08 mW h/cm, and 1 mV/s) and power density (253 mW/kg, 0.31 mW/cm, and 100 mV/s) and excellent flexibility. Furthermore, this wire-shaped supercapacitor may bring broader application prospects for energy storage devices in future wearable electronic areas.

Distinctive Construction of Chitin-Derived Hierarchically Porous Carbon Microspheres/Polyaniline for High-Rate Supercapacitors

Recently, nanostructured porous carbons are attracting significant interest in various important applications. However, a green and innovative method to fabricate hierarchically porous-structured carbon is still a challenge. In the present work, hierarchically porous carbon microspheres (HCMs) were prepared by pyrolyzing the chitin microspheres fabricated from a chitin/chitosan blend solution, in which chitosan was used as a forming agent of nanopores/nanochannels to construct the microspheres. The HCM displayed hierarchical porous structure and improved specific surface area of 1450 m²/g. For the application of HCM in hybrid electrode materials as supercapacitors, polyaniline (PANI) nanoclusters were further deposited on the surface of HCM. A symmetric supercapacitor based on HCM-PANI exhibited high rate capability with retaining over 64% of the capacitance as the scan rate increased from 2 to 500 mV/s. This work introduced a distinctive and green method to fabricate hierarchically porous carbon materials, having considerable application prospect for energy storage.

A Wearable Supercapacitor Engaged with Gold Leaf Gilding Cloth Toward Enhanced Practicability

Flexible energy storage devices have attracted wide attention because of the increasing requirement of wearable electronics. However, comfortability, productivity, and feasibility, to name a few, are still far from satisfactory in the current wearable supercapacitors (SCs). This is largely due to the missing of an ideal low-cost flexible substrate/current collector that should not only exhibit high conductivity, but also be compatible with modern textile technologies. Herein, we apply the traditional gilding technique to cloth and successfully convert the cloth to be an excellent current collector which is available at a reasonable cost and compatible with textile technologies. Thanks to the strong electrostatic interaction, we found that a positively charged gold leaf could be laminated on a negatively charged polyester cloth intimately. This substrate could perfectly act as an integrated compact electrode after the electrodeposition of polypyrrole nanorods. The resulting electrode is mechanically strong enough to withstand the tortures of repeated bending, cutting, or puncturing, and is readily assembled into wearable SCs and energy cloth with outstanding practicability, for example, safety and breathability. It is foreseeable that our work will inspire a series design of wearable electronics based on the fascinating gilding art.

Dual-Functional Graphene Carbon as Polysulfide Trapper for High-Performance Lithium Sulfur Batteries

The lithium sulfur (Li-S) battery has attracted much attention due to its high theoretical capacity and energy density. However, its cycling stability and rate performance urgently need to improve because of its shuttle effect. Herein, oxygen-doped carbon on the surface of reduced graphene oxide (labeled as ODC/rGO) was fabricated to modify the separators of Li-S batteries to limit the dissolution of the lithium polysulfides. The mesoporous structure in ODC/rGO can not only serve as the physical trapper, but also provide abundant channels for fast ion transfer, which is beneficial for effective confinement of the dissoluble intermediates and superior rate performance. Moreover, the oxygen-containing groups in ODC/rGO are able to act as chemical adsorption sites to immobilize the lithium polysulfides, suppressing their dissolution in electrolyte to enhance the utilization of sulfur cathode in Li-S batteries. As a result, because of the synergetic effects of physical adsorption and chemical interaction to immobilize the soluble polysulfides, the Li-S batteries with the ODC/rGO-coated separator exhibit excellent rate performance and good long-term cycling stability with 0.057% capacity decay per cycle at 1.0 C after 600 cycles.