Flexible, Stretchable, and Rechargeable Fiber‐Shaped Zinc–Air Battery Based on Cross‐Stacked Carbon Nanotube Sheets

Selectively "size-excluding" water molecules to enable a highly reversible zinc metal anode

Significant water-related side reactions hinder the development of highly safe, low-cost aqueous zinc metal batteries (AZMBs) for grid-scale energy storage. Herein, by regulating the length of alkyl chains, we successfully adjust interstitial voids between the polymer chains of a metal soap interface between 1.48 Å (size of a zinc ion) and 4.0 Å (size of a water molecule). Therefore, water molecules are selectively "size-excluded," while smaller zinc ions are permitted to pass through. Consequently, water-related side reactions (including hydrogen evolution and corrosion) could be effectively inhibited. Furthermore, abundant zinc ion tunnels accompanied with zincophilic components facilitate the homogenization of the Zn2+ flux, thus preventing dendrite growth. Therefore, the Zn symmetric cell shows a lifespan of approximately 10 000 cycles at 20 mA cm-2 and 1 mA h cm-2, and the Zn//Na5V12O32 (NVO) full cell delivers much better cycling stability with much higher capacity retention of around 93% after 2000 cycles at 2 A g-1 compared to its bare Zn counterpart (19%). This work provides valuable insights for the utilization of metal soap interfaces and regulation of their channel size between perpendicular alkyl chains to realize precise water shielding, which is not only applicable in ZMBs but also in other aqueous batteries.

A Review of Rechargeable Zinc-Air Batteries: Recent Progress and Future Perspectives

Zinc-air batteries (ZABs) are gaining attention as an ideal option for various applications requiring high-capacity batteries, such as portable electronics, electric vehicles, and renewable energy storage. ZABs offer advantages such as low environmental impact, enhanced safety compared to Li-ion batteries, and cost-effectiveness due to the abundance of zinc. However, early research faced challenges due to parasitic reactions at the zinc anode and slow oxygen redox kinetics. Recent advancements in restructuring the anode, utilizing alternative electrolytes, and developing bifunctional oxygen catalysts have significantly improved ZABs. Scientists have achieved battery reversibility over thousands of cycles, introduced new electrolytes, and achieved energy efficiency records surpassing 70%. Despite these achievements, there are challenges related to lower power density, shorter lifespan, and air electrode corrosion leading to performance degradation. This review paper discusses different battery configurations, and reaction mechanisms for electrically and mechanically rechargeable ZABs, and proposes remedies to enhance overall battery performance. The paper also explores recent advancements, applications, and the future prospects of electrically/mechanically rechargeable ZABs.

Rechargeable Zinc-Air Batteries: Advances, Challenges, and Prospects

Rechargeable zinc-air batteries (Re-ZABs) are one of the most promising next-generation batteries that can hold more energy while being cost-effective and safer than existing devices. Nevertheless, zinc dendrites, non-portability, and limited charge-discharge cycles have long been obstacles to the commercialization of Re-ZABs. Over the past 30 years, milestone breakthroughs have been made in technical indicators (safety, high energy density, and long battery life), battery components (air cathode, zinc anode, and gas diffusion layer), and battery configurations (flexibility and portability), however, a comprehensive review on advanced design strategies for Re-ZABs system from multiple angles is still lacking. This review underscores the progress and strategies proposed so far to pursuit the high-efficiency Re-ZABs system, including the aspects of rechargeability (from primary to rechargeable), air cathode (from unifunctional to bifunctional), zinc anode (from dendritic to stable), electrolytes (from aqueous to non-aqueous), battery configurations (from non-portable to portable), and industrialization progress (from laboratorial to practical). Critical appraisals of the advanced modification approaches (such as surface/interface modulation, nanoconfinement catalysis, defect electrochemistry, synergistic electrocatalysis, etc.) are highlighted for cost-effective flexible Re-ZABs with good sustainability and high energy density. Finally, insights are further rendered properly for the future research directions of advanced zinc-air batteries.

A Minireview of the Solid-State Electrolytes for Zinc Batteries

Aqueous zinc-ion batteries (ZIBs) have gained significant recognition as highly promising rechargeable batteries for the future due to their exceptional safety, low operating costs, and environmental advantages. Nevertheless, the widespread utilization of ZIBs for energy storage has been hindered by inherent challenges associated with aqueous electrolytes, including water decomposition reactions, evaporation, and liquid leakage. Fortunately, recent advances in solid-state electrolyte research have demonstrated great potential in resolving these challenges. Moreover, the flexibility and new chemistry of solid-state electrolytes offer further opportunities for their applications in wearable electronic devices and multifunctional settings. Nonetheless, despite the growing popularity of solid-state electrolyte-based-ZIBs in recent years, the development of solid-state electrolytes is still in its early stages. Bridging the substantial gap that exists is crucial before solid-state ZIBs become a practical reality. This review presents the advancements in various types of solid-state electrolytes for ZIBs, including film separators, inorganic additives, and organic polymers. Furthermore, it discusses the performance and impact of solid-state electrolytes. Finally, it outlines future directions for the development of solid-state ZIBs.

Strategies Toward Stretchable Aqueous Zn-based Batteries for Wearable Electronics from Components to Devices

Recently, aqueous Zn-based batteries (AZBs) are receiving increased attention in wearable and implantable electronics due to the low cost, high safety, high eco-efficiency, and relatively high energy density. However, it is still a big challenge to develop stretchable AZBs (SAZBs) which can be conformally folded, crumpled, and stretched with human body motions. Although a lot of efforts have been dedicated to constructions of SAZBs, a comprehensive review which focuses on summarizing stretchable materials, device configurations and challenges of SAZBs is needed. Herein, this review attempts to critically review the latest developments and progress in stretchable electrodes, electrolytes, packaging materials and device configurations in detail. Furthermore, these challenges and potential future research directions in the field of SAZBs are also discussed.

Rational Design of Flexible Zn-Based Batteries for Wearable Electronic Devices

The advent of 5G and the Internet of Things has spawned a demand for wearable electronic devices. However, the lack of a suitable flexible energy storage system has become the "Achilles' Heel" of wearable electronic devices. Additional problems during the transformation of the battery structure from conventional to flexible also present a severe challenge to the battery design. Flexible Zn-based batteries, including Zn-ion batteries and Zn-air batteries, have long been considered promising candidates due to their high safety, eco-efficiency, substantial reserve, and low cost. In the past decade, researchers have come up with elaborate designs for each portion of flexible Zn-based batteries to improve the ionic conductivities, mechanical properties, environment adaptabilities, and scalable productions. It would be helpful to summarize the reported strategies and compare their pros and cons to facilitate further research toward the commercialization of flexible Zn-based batteries. In this review, the current progress in developing flexible Zn-based batteries is comprehensively reviewed, including their electrolytes, cathodes, and anodes, and discussed in terms of their synthesis, characterization, and performance validation. By clarifying the challenges in flexible Zn-based battery design, we summarize the methodology from previous investigations and propose challenges for future development. In the end, a research paradigm of Zn-based batteries is summarized to fit the burgeoning requirement of wearable electronic devices in an iterative process, which will benefit the future development of Zn-based batteries.

Structural Design and Fabrication of Multifunctional Nanocarbon Materials for Extreme Environmental Applications

Extreme environments represent numerous harsh environmental conditions, such as temperature, pressure, corrosion, and radiation. The tolerance of applications in extreme environments exemplifies significant challenges to both materials and their structures. Given the superior mechanical strength, electrical conductivity, thermal stability, and chemical stability of nanocarbon materials, such as carbon nanotubes (CNTs) and graphene, they are widely investigated as base materials for extreme environmental applications and have shown numerous breakthroughs in the fields of wide-temperature structural-material construction, low-temperature energy storage, underwater sensing, and electronics operated at high temperatures. Here, the critical aspects of structural design and fabrication of nanocarbon materials for extreme environments are reviewed, including a description of the underlying mechanism supporting the performance of nanocarbon materials against extreme environments, the principles of structural design of nanocarbon materials for the optimization of extreme environmental performances, and the fabrication processes developed for the realization of specific extreme environmental applications. Finally, perspectives on how CNTs and graphene can further contribute to the development of extreme environmental applications are presented.

Design of co-continuous structure of cellulose/PAA-based alkaline solid polyelectrolyte for flexible zinc-air battery

In order to prepare high ionic conductivity and robust mechanical properties of alkaline solid polyelectrolyte (ASPE) for applications in flexible wearable devices, a co-continuous structure membrane was designed using in-situ polymerization to introduce cross-linked polyacrylic acid (N-PAA) into the cellulose network constructed by regenerated degreasing cotton (RDC). The resultant ASPE membrane showed high ionic conductivity (430 mS·cm^-1 at 25 °C), strong mechanical properties, and excellent alkaline stabilities, proving the viability of cellulose for use in energy storage systems. Surprisingly, the sandwich-shaped zinc-air battery assembled using RDC/N-PAA/KOH membranes as electrolytes exhibits superior values of cycling stability, discharge time, specific capacity (731.5 mAh·g^-1), peak power density (40.25 mW·cm^-2), and mechanical flexibility. Even under bending conditions, the zinc-air batteries still possess stable energy supply performance, suggesting this novel solid polyelectrolyte has promising application for wearable technology.

S-Doping Promotes Pyridine Nitrogen Conversion and Lattice Defects of Carbon Nitride to Enhance the Performance of Zn-Air Batteries

The efficient operation of Zn-air batteries (ZABs) requires highly active and stable reversible air catalysts. Studies have shown that heteroatom-doped carbonaceous nanomaterials are effective metal-free electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). Herein, we design a facile and scalable catalyst doping scheme to manufacture S-doped carbon nitride (S-C₃N₄). Surprisingly, this metal-free catalyst exhibits excellent OER and ORR electrocatalytic activities in alkaline electrolytes, being comparable to those of commercial Pt/C. For the first time, it is proved by experiments that S doping can not only effectively increase the lattice defects of C₃N₄ but also promote the conversion of pyrrolic nitrogen to pyridine nitrogen, thereby enhancing the bifunctional catalytic activity (OER and ORR). When the catalyst is used as an air electrode for rechargeable ZABs, its performance is obviously better than that provided by commercial Pt/C. Our findings and material design strategies are expected to provide new ideas for the synthesis of various high-performance carbon-based electrocatalysts.

Surface and Interface Engineering of Zn Anodes in Aqueous Rechargeable Zn-Ion Batteries

Rechargeable zinc-ion batteries (ZIBs) have shown great potential as an alternative to lithium-ion batteries. The ZIBs utilize Zn metal as the anode, which possesses many advantages such as low cost, high safety, eco-friendliness, and high capacity. However, on the other hand, the Zn anode also suffers from many issues, including dendritic growth, corrosion, and passivation. These issues are largely related to the surface and interface properties of the Zn anode. Many efforts have therefore been devoted to the modification of the Zn anode, aiming to eliminate the above-mentioned problems. This review gives a comprehensive summary on the mechanism behind these issues as well as the recent progress on Zn anode modification with focus on the strategies of surface and interface engineering, covering the design and application of both the Zn anode supports and surface protective layers, along with abundant examples. In addition, the promising research directions and perspective on these strategies are also presented.

Advanced Multifunctional Aqueous Rechargeable Batteries Design: From Materials and Devices to Systems

Multifunctional aqueous rechargeable batteries (MARBs) are regarded as safe, cost-effective, and scalable electrochemical energy storage devices, which offer additional functionalities that conventional batteries cannot achieve, which ideally leads to unprecedented applications. Although MARBs are among the most exciting and rapidly growing topics in scientific research and industrial development nowadays, a systematic summary of the evolution and advances in the field of MARBs is still not available. Therefore, the review presented comprehensively and systematically summarizes the design principles and the recent advances of MARBs by categories of smart ARBs and integrated systems, together with an analysis of their device design and configuration, electrochemical performance, and diverse smart functions. The two most promising strategies to construct novel MARBs may be A) the introduction of functional materials into ARB components, and B) integration of ARBs with other functional devices. The ongoing challenges and future perspectives in this research and development field are outlined to foster the future development of MARBs. Finally, the most important upcoming research directions in this rapidly developing field are highlighted that may be most promising to lead to the commercialization of MARBs and to a further broadening of their range of applications.

Synergetic Chemistry and Interface Engineering of Hydrogel Electrolyte to Strengthen Durability of Solid-State Zn-Air Batteries

For the challenging pursuit of high energy efficiency and mechanical tolerance in flexible solid-state Zn-air batteries (FSZABs), a hydrogel electrolyte (HE) consisting of dual-network crosslinked polyacrylic acid-Fe³⁺ -chitosan (PAA-Fe³⁺ -CS) polymer host infiltrated with a mixed aqueous electrolyte of NH₄ Cl and ZnCl₂ is developed. The absorbed near-neutral electrolyte renders the HE high ionic conductivity but low corrosiveness to both electrocatalysts and Zn metal anode (ZMA), ensuring more stable Zn-OH-O₂ chemistry compared to that in strong alkaline electrolyte and thus endowing the assembled FSZABs with a landmark cycle life up to 120 h (5 mA cm^-2 ). More intriguingly, the CS molecular beams introduced into the PAA hydrogel backbone will precipitate and fold subjecting to the Hofmeister effect when saturated with the near-neutral electrolyte, which can effectively enhance the interfacial adhesion strength of the HE on both air cathode and ZMA, achieving reliable and robust bonding between them. Thus, the FSZABs simultaneously exhibited a superior tolerance to repeated mechanical deformation during operation, allowing more than 360 continuous bending-recovery cycles without any decline in voltage efficiency. The ingenious chemistry and interface synergetic engineering on the crucial HEs provides a rational methodology to realize boosted electrochemical and mechanical durability of FSZABs forward for future practical implementation.

Supramolecular Self-Assembly of Atomically Precise Silver Nanoclusters with Chiral Peptide for Temperature Sensing and Detection of Arginine

Metal nanoclusters (NCs) as a new type of fluorescent material have attracted great interest due to their good biocompatibilities and outstanding optical properties. However, most of the studies on metal NCs focus on the synthesis, atomic or molecular assembly, whereas metal NCs ability to self-assemble to higher-level hierarchical nanomaterials through supramolecular interactions has rarely been reported. Herein, we investigate atomic precise silver NCs (Ag₉-NCs, [Ag₉(mba)₉], where H₂mba = 2-mercaptobenzoic acid) and peptide DD-5 were used to induce self-assembly, which can trigger an aggregation-induced luminescence (AIE) effect of Ag₉-NCs through non-covalent forces (H-bond, π–π stacking) and argentophilic interactions [Ag(I)–Ag(I)]. The large Stokes shift (~140 nm) and the microsecond fluorescence lifetime (6.1 μs) indicate that Ag₉-NCs/DD-5 hydrogel is phosphor. At the same time, the chirality of the peptide was successfully transferred to the achiral Ag₉-NCs because of the supramolecular self-assembly, and the Ag₉-NCs/DD-5 hydrogel also has good circularly polarized luminescence (CPL) properties. In addition, Ag₉-NCs/DD-5 luminescent hydrogel is selective and sensitive to the detection of small biological molecule arginine. This work shows that DD-5 successfully induces the self-assembly of Ag₉-NCs to obtain high luminescent gel, which maybe become a candidate material in the fields of sensors and biological sciences.

Ion Sieve: Tailoring Zn2+ Desolvation Kinetics and Flux toward Dendrite-Free Metallic Zinc Anodes

Tip-induced dendrites on metallic zinc anodes (MZAs) fundamentally deteriorate the rechargeability of aqueous Zn metal batteries (ZMBs). Herein, an intriguing ion sieve (IS) consisting of 3D intertwined bacterial cellulose, deposited on the surface of MZAs (Zn@IS) through an in situ self-assembly route, is first presented to be effective in inhibiting dendrite-growth on MZAs. Experimental analyses together with theoretical calculations suggested that the IS coating can facilitate the desolvation of [Zn(H₂O)₆]²⁺ clusters via a strong interplay with Zn ions, weaken hydrogen evolution reaction of MZAs, and homogenize the ion flux with the abundant nanopores serving as ion tunnels, synergistically enabling dendrite-free Zn deposition on the Zn@IS anodes. Consequently, a lifespan up to 3000 h at a cutoff capacity of 0.25 mA h cm^-2 was observed in a Zn@IS∥Zn@IS symmetric cell. In terms of application, pairing with a carbon-nanotube@MnO₂ cathode as an example, the full ZMBs acquired enhanced rechargeability with much higher capacity retention over 73.3% after 3000 cycles compared to the counterpart with pristine MZA (21%).

A Smart Lithium Battery with Shape Memory Function

Rapidly growing flexible and wearable electronics highly demand the development of flexible energy storage devices. Yet, these devices are susceptible to extreme, repeated mechanical deformations under working circumstances. Herein, the design and fabrication of a smart, flexible Li-ion battery with shape memory function, which has the ability to restore its shape against severe mechanical deformations, bending, twisting, rolling or elongation, is reported. The shape memory function is induced by the integration of a shape-adjustable solid polymer electrolyte. This Li-ion battery delivers a specific discharge capacity of ≈140 mAh g^-1 at 0.2 C charge/discharge rate with ≈92% capacity retention after 100 cycles and ≈99.85% Coulombic efficiency, at 20 °C. Besides recovery from mechanical deformations, it is visually demonstrated that the shape of this smart battery can be programmed to adjust itself in response to an internal/external heat stimulus for task-specific and advanced applications. Considering the vast range of available shape memory polymers with tunable chemistry, physical, and mechanical characteristics, this study offers a promising approach for engineering smart batteries responsive to unfavorable internal or external stimulus, with potential to have a broad impact on other energy storage technologies in different sizes and shapes.

Stretchable Energy Storage Devices Based on Carbon Materials

Stretchable energy storage devices are essential for developing stretchable electronics and have thus attracted extensive attention in a variety of fields including wearable devices and bioelectronics. Carbon materials, e.g., carbon nanotube and graphene, are widely investigated as electrode materials for energy storage devices due to their large specific surface areas and combined remarkable electrical and electrochemical properties. They can also be effectively composited with many other functional materials or designed into different microstructures for fabricating stretchable energy storage devices. This review summarizes recent advances toward the development of carbon-material-based stretchable energy storage devices. An overview of common carbon materials' fundamental properties and general strategies to enable the stretchability of carbon-material-based electrodes are presented. The performances of the as-fabricated stretchable energy storage devices including supercapacitors, lithium-ion batteries, metal-air batteries, and other batteries are then carefully discussed. Challenges and perspectives in this emerging field are finally highlighted for future studies.

Toward Flexible Zinc-Air Batteries with Self-Supported Air Electrodes

The compelling demand for higher energy performance, flexibility, and miniaturization is the main driving force of the energy storage and conversion industry's quest for flexible devices based on new integration and fabrication process. Herein, the recent advances on the development of flexible zinc-air batteries based on self-supported air electrodes are summarized, focusing on the multiscale and systematic design principles for the design of flexible air electrodes. With the electrocatalytic activity regulation and structural engineering strategies, the rational design of self-supported air electrodes is discussed in integrated devices to underpin the good flexibility for wearable requirement. The perspectives on promising developments of flexible zinc-air batteries and the accumulated knowledge from other flexible devices are also addressed for promoting the advances on flexible zinc-air batteries.

Fabrication of a chiral luminescent hydrogel from gold nanoclusters via molecular recognition

A novel supramolecular chiral hydrogel with enhanced emission was obtained by the co-assembly of achiral thiobarbituric acid-modified gold nanoclusters (TBA-AuNCs) with chiral histidine molecules. Chirality transfer from histidine to the supramolecular hydrogels was achieved through the π-π stacking and intermolecular H-bonding based on molecular recognition. This work gives a creative strategy for the building of chiral nanocluster-based materials.

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.

Self-Healable, High-Strength Hydrogel Electrode for Flexible Sensors and Supercapacitors

Flexible energy storage materials and sensors have become the key equipment of human-machine interface technology. For the preparation of these devices, hydrogel electrodes are relevant because of their unique porous structure, high capacitance, flexibility, small size, and lightweight. In this paper, regular polypyrrole (PPy) is synthesized on a heat-induced phase-separated gel (H-Gel/AS) by the template degradation method, and a gelatin-based PPy hydrogel with high strength, high strain rate, and high conductivity is prepared. Moreover, by adding multiwalled carbon nanotubes (MWCNTs) into a gelatin solution according to the H-Gel/AS method, the electrochemical performance of the resulting H-Gel/AS-MWCNTs-PPy electrode is greatly improved. When the H-Gel/AS-MWCNTs-PPy gel is immersed in an ammonium sulfate solution, wrinkles appear on the surface, resulting in further enhancement of the capacitance. On this basis, a flexible sensor and a solid-state supercapacitor are assembled, and their performance is tested. The sensor can detect tensile, bending, and twisting strains with high sensitivity. Meanwhile, as a flexible solid-state supercapacitor, the specific capacitance is 75 F g^-1, and the capacitance retention rate after 5000 cycles is 98.1% under bending conditions. More importantly, the gelatin-based hydrogel shows great potential for application in wearable devices.

Mapping the Design of Electrolyte Materials for Electrically Rechargeable Zinc-Air Batteries

Electrically rechargeable zinc-air batteries (ERZABs) have attracted substantial research interest as one of the best candidate power sources for electric vehicles, grid-scale energy storage, and portable electronics owing to their high theoretical capacity, low cost, and environmental benignity. However, the realization of ERZABs with long cycle life and high energy and power densities is still a considerable challenge. The electrolyte, which serves as the ionic conductor, is one of the core components of ERZABs, as it plays a significant role during the discharge-charge process and greatly influences the rechargeability, operating voltage, lifespan, power density, and safety of ERZABs. Herein, the fundamental electrochemistry of electrolyte materials for ERZABs and the associated challenges are presented. Furthermore, recent advances in electrolyte materials for ERZABs, including alkaline aqueous electrolytes, nonalkaline electrolytes, ionic liquids, and semisolid-state electrolytes are discussed. This work aims to provide insights into the future exploration of high-performance electrolytes and thus promote the development of ERZABs.

Smart fibers for energy conversion and storage

Fibers have played a critical role in the long history of human development. They are the basic building blocks of textiles. Synthetic fibers not only make clothes stronger and more durable, but are also customizable and cheaper. The growth of miniature and wearable electronics has promoted the development of smart and multifunctional fibers. Particularly, the incorporation of functional semiconductors and electroactive materials in fibers has opened up the field of fiber electronics. The energy supply system is the key branch for fiber electronics. Herein, after a brief introduction on the history of smart and functional fibers, we review the current state of advanced functional fibers for their application in energy conversion and storage, focusing on nanogenerators, solar cells, supercapacitors and batteries. Subsequently, the importance of the integration of fiber-shaped energy conversion and storage devices via smart structure design is discussed. Finally, the challenges and future direction in this field are highlighted. Through this review, we hope to inspire scientists with different research backgrounds to enter this multi-disciplinary field to promote its prosperity and development and usher in a truly new era of smart fibers.

Progress of carbon-based electrocatalysts for flexible zinc-air batteries in the past 5 years: recent strategies for design, synthesis and performance optimization

The increasing popularity of wearable electronic devices has led to the rapid development of flexible energy conversion systems. Flexible rechargeable zinc-air batteries (ZABs) with high theoretical energy densities demonstrate significant potential as next-generation flexible energy devices that can be applied in wearable electronic products. The design of highly efficient and air-stable cathodes that can electrochemically catalyze both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable but challenging. Flexible carbon-based catalysts for ORR/OER catalysis can be broadly categorized into two types: (i) self-supporting catalysts based on the in situ modification of flexible substrates; (ii) non-self-supporting catalysts based on surface coatings of flexible substrates. Methods used to optimize the catalytic performance include doping with atoms and regulation of the electronic structure and coordination environment. This review summarizes the most recently proposed strategies for the synthesis of designer carbon-based electrocatalysts and the optimization of their electrocatalytic performances in air electrodes. And we significantly focus on the analysis of the inherent active sites and their electrocatalytic mechanisms when applied as flexible ZABs catalysts. The findings of this review can assist in the design of more valuable carbon-based air electrodes and their corresponding flexible ZABs for application in wearable electronic devices.

Carbon nanotubes for flexible batteries: recent progress and future perspective

Flexible batteries, which maintain their functions potently under various mechanical deformations, attract increasing interest due to potential applications in emerging portable and wearable electronics. Significant efforts have been devoted to material synthesis and structural designs to realize the mechanical flexibility of various batteries. Carbon nanotubes (CNTs) have a unique one-dimensional (1D) nanostructure and are convenient to further assemble into diverse macroscopic structures, such as 1D fibers, 2D films and 3D sponges/aerogels. Due to their outstanding mechanical and electrical properties, CNTs and CNT-based hybrid materials are superior building blocks for different components in flexible batteries. This review summarizes recent progress on the application of CNTs in developing flexible batteries, from closed-system to open-system batteries, with a focus on different structural designs of CNT-based material systems and their roles in various batteries. We also provide perspectives on the challenges and future research directions for realizing practical applications of CNT-based flexible batteries.

Tendril-Inspired 900% Ultrastretching Fiber-Based Zn-Ion Batteries for Wearable Energy Textiles

Flexible fiber-based Zn-ion batteries represent an ideal power platform for smart wearable energy textiles featuring safety, flexibility, and unique integration. However, the inevitably low elongation limits (<400%) of common fiber-based Zn-ion batteries may restrict applications in highly deformable wearable materials and lead to unstable energy storage performance during practical activities. Herein, an elastic graphene/polyaniline-Zn@silver fiber-based battery (eG/P-Zn@SFB) with a helical structure inspired by the biological structure of luffa tendril is reported. eG/P-Zn@SFB exhibits ultrastretching properties and can be stretched to 900% with a 71% capacity retention ratio. Moreover, the prefabricated battery delivers a high specific capacity of 32.56 mAh/cm³ at 10 mA/cm³ and an energy density of 36.04 mWh/cm³. As a proof of concept, the knitted integrated eG/P-Zn@SFB served as an effective power supply with different bending angles ranging from 0° to 180°, demonstrating potential applications and promising prospects in stretchable flexible electronics and wearable energy textiles.

Recoverable peroxidase-like Fe3O4@MoS2-Ag nanozyme with enhanced antibacterial ability

Antibacterial agents with enzyme-like properties and bacteria-binding ability have provided an alternative method to efficiently disinfect drug-resistance microorganism. Herein, a Fe₃O₄@MoS₂-Ag nanozyme with defect-rich rough surface was constructed by a simple hydrothermal method and in-situ photodeposition of Ag nanoparticles. The nanozyme exhibited good antibacterial performance against E. coli (~69.4%) by the generated ROS and released Ag⁺, while the nanozyme could further achieve an excellent synergistic disinfection (~100%) by combining with the near-infrared photothermal property of Fe₃O₄@MoS₂-Ag. The antibacterial mechanism study showed that the antibacterial process was determined by the collaborative work of peroxidase-like activity, photothermal effect and leakage of Ag⁺. The defect-rich rough surface of MoS₂ layers facilitated the capture of bacteria, which enhanced the accurate and rapid attack of •OH and Ag⁺ to the membrane of E. coli with the assistance of local hyperthermia. This method showed broad-spectrum antibacterial performance against Gram-negative bacteria, Gram-positive bacteria, drug-resistant bacteria and fungal bacteria. Meanwhile, the magnetism of Fe₃O₄ was used to recycle the nanozyme. This work showed great potential of engineered nanozymes for efficient disinfection treatment.

Stretchable Electronics Based on PDMS Substrates

Stretchable electronics, which can retain their functions under stretching, have attracted great interest in recent decades. Elastic substrates, which bear the applied strain and regulate the strain distribution in circuits, are indispensable components in stretchable electronics. Moreover, the self-healing property of the substrate is a premise to endow stretchable electronics with the same characteristics, so the device may recover from failure resulting from large and frequent deformations. Therefore, the properties of the elastic substrate are crucial to the overall performance of stretchable devices. Poly(dimethylsiloxane) (PDMS) is widely used as the substrate material for stretchable electronics, not only because of its advantages, which include stable chemical properties, good thermal stability, transparency, and biological compatibility, but also because of its capability of attaining designer functionalities via surface modification and bulk property tailoring. Herein, the strategies for fabricating stretchable electronics on PDMS substrates are summarized, and the influence of the physical and chemical properties of PDMS, including surface chemical status, physical modulus, geometric structures, and self-healing properties, on the performance of stretchable electronics is discussed. Finally, the challenges and future opportunities of stretchable electronics based on PDMS substrates are considered.

Kirigami-Inspired Flexible and Stretchable Zinc-Air Battery Based on Metal-Coated Sponge Electrodes

The development of efficient and low-cost flexible metal electrodes is significant for flexible rechargeable zinc-air batteries (ZABs). Herein, we reported a new type of flexible metal (zinc and nickel) electrode fabricated via a two-step deposition method on polyurethane sponges (PUS) for flexible ZABs. Compared to conventional electrodes, the metal-coated PUS electrodes exhibited great flexibility, softness, and natural mechanical resilience. In addition, a flexible sandwich-structured ZAB was assembled with the metal-coated PUS electrodes and in situ cross-linked polyacrylic acid (PAA)-KOH hydrogel electrolyte. The flexible ZAB presented stable discharge/charge performance even under complex rolling and twisting deformations. Moreover, inspired by the kirigami-strategy for device-level stretchability, a 100% stretchable fence-shaped ZAB and a 160% stretchable serpentine-shaped ZAB were cut from the above-mentioned flexible ZABs. The kirigami-inspired configuration enabled the battery performance to be stable during stretching, benefiting from the softness of the PUS@metal electrode. These flexible and stretchable ZABs would broaden the promising applications for portable and wearable energy storage devices.

Domiciliary Hospitalization through Wearable Biomonitoring Patches: Recent Advances, Technical Challenges, and the Relation to Covid-19

This article reviews recent advances and existing challenges for the application of wearable bioelectronics for patient monitoring and domiciliary hospitalization. More specifically, we focus on technical challenges and solutions for the implementation of wearable and conformal bioelectronics for long-term patient biomonitoring and discuss their application on the Internet of medical things (IoMT). We first discuss the general architecture of IoMT systems for domiciliary hospitalization and the three layers of the system, including the sensing, communication, and application layers. In regard to the sensing layer, we focus on current trends, recent advances, and challenges in the implementation of stretchable patches. This includes fabrication strategies and solutions for energy storage and energy harvesting, such as printed batteries and supercapacitors. As a case study, we discuss the application of IoMT for domiciliary hospitalization of COVID 19 patients. This can be used as a strategy to reduce the pressure on the healthcare system, as it allows continuous patient monitoring and reduced physical presence in the hospital, and at the same time enables the collection of large data for posterior analysis. Finally, based on the previous works in the field, we recommend a conceptual IoMT design for wearable monitoring of COVID 19 patients.

A Review of the Use of GPEs in Zinc-Based Batteries. A Step Closer to Wearable Electronic Gadgets and Smart Textiles

With the flourish of flexible and wearable electronics gadgets, the need for flexible power sources has become essential. The growth of this increasingly diverse range of devices boosted the necessity to develop materials for such flexible power sources such as secondary batteries, fuel cells, supercapacitors, sensors, dye-sensitized solar cells, etc. In that context, comprehensives studies on flexible conversion and energy storage devices have been released for other technologies such Li-ion standing out the importance of the research done lately in GPEs (gel polymer electrolytes) for energy conversion and storage. However, flexible zinc batteries have not received the attention they deserve within the flexible batteries field, which are destined to be one of the high rank players in the wearable devices future market. This review presents an extensive overview of the most notable or prominent gel polymeric materials, including biobased polymers, and zinc chemistries as well as its practical or functional implementation in flexible wearable devices. The ultimate aim is to highlight zinc-based batteries as power sources to fill a segment of the world flexible batteries future market.

Skin-Like Stretchable Fuel Cell Based on Gold-Nanowire-Impregnated Porous Polymer Scaffolds

Skin-like energy devices can be conformally attached to the human body, which are highly desirable to power soft wearable electronics in the future. Here, a skin-like stretchable fuel cell based on ultrathin gold nanowires (AuNWs) and polymerized high internal phase emulsions (polyHIPEs) scaffolds is demonstrated. The polyHIPEs can offer a high porosity of 80% yet with an overall thickness comparable to human skin. Upon impregnation with electronic inks containing ultrathin (2 nm in diameter) and ultrahigh aspect-ratio (>10 000) gold nanowires, skin-like strain-insensitive stretchable electrodes are successfully fabricated. With such designed strain-insensitive electrodes, a stretchable fuel cell is fabricated by using AuNWs@polyHIPEs, platinum (Pt)-modified AuNWs@polyHIPEs, and ethanol as the anode, cathode, and fuel, respectively. The resulting epidermal fuel cell can be patterned and transferred onto skin as "tattoos" yet can offer a high power density of 280 µW cm^-2 and a high durability (>90% performance retention under stretching, compression, and twisting). The results presented here demonstrate that this skin-thin, porous, yet stretchable electrode is essentially multifunctional, simultaneously serving as a current collector, an electrocatalyst, and a fuel host, indicating potential applications to power future soft wearable 2.0 electronics for remote healthcare and soft robotics.

Directly Grown Vertical Graphene Carpets as Janus Separators toward Stabilized Zn Metal Anodes

Zinc metal anode has garnered a great deal of scientific and technological interest. Nevertheless, major bottlenecks restricting its large-scale utilization lie in the poor electrochemical stability and unsatisfactory cycling life. Herein, a Janus separator is developed via directly growing vertical graphene (VG) carpet on one side of commercial glass fiber separator throughout chemical vapor deposition. A simple air plasma treatment further renders the successful incorporation of oxygen and nitrogen heteroatoms on bare graphene. Thus-derived 3D VG scaffold affording large surface area and porous structure can be viewed as a continuation of planar zinc anode. In turn, the Janus separator harvests homogenous electric field distribution and lowered local current density at the interface of the anode/electrolyte, as well as harnesses favorable zincophilic feature for building-up uniform Zn ionic flux. Such a separator engineering enables an impressive rate and cycle performance (93% over 5000 cycles at 5 A g^-1 ) for Zn-ion hybrid capacitors and outstanding energy density (182 Wh kg^-1 ) for V₂ O₅ //Zn batteries, respectively. This strategy with large scalability and cost-effectiveness represents a universal route to protect prevailing metal anodes (Zn, Na, K) in rechargeable batteries.

Recent Advances on Self-Supported Arrayed Bifunctional Oxygen Electrocatalysts for Flexible Solid-State Zn-Air Batteries

Flexible solid-state Zn-air batteries have been rapidly developed benefiting from the uprising demand for wearable electronic devices, wherein the air electrode integrated with efficient bifunctional oxygen electrocatalysts plays an important role to achieve high performance. Binder-free self-supported bifunctional catalysts can provide large active surface area, fast electron transport path, easy ion diffusion, and excellent structural stability and flexibility, thus acting as promising flexible air cathodes. In this review, recent advances on the application of nanoarrayed electrocatalysts as air cathodes in flexible Zn-air batteries are reviewed. Especially, various types of bifunctional oxygen electrocatalysts, including carbonaceous material arrays, transition metal compound arrays, transition metal/carbon arrays, transition metal compound/carbon arrays, and other hybrid arrays, are discussed. The applications of flexible Zn-air batteries with two configurations (i.e., planar stacks and cable fibers) are also introduced. Finally, perspectives on the optimization of arrayed air cathodes for future development to achieve high-performance flexible Zn-air batteries are shared.

Soft Materials for Wearable/Flexible Electrochemical Energy Conversion, Storage, and Biosensor Devices

Next-generation wearable technology needs portable flexible energy storage, conversion, and biosensor devices that can be worn on soft and curved surfaces. The conformal integration of these devices requires the use of soft, flexible, light materials, and substrates with similar mechanical properties as well as high performances. In this review, we have collected and discussed the remarkable research contributions of recent years, focusing the attention on the development and arrangement of soft and flexible materials (electrodes, electrolytes, substrates) that allowed traditional power sources and sensors to become viable and compatible with wearable electronics, preserving or improving their conventional performances.

A Rechargeable Zn-Air Battery with High Energy Efficiency and Long Life Enabled by a Highly Water-Retentive Gel Electrolyte with Reaction Modifier

Tremendous effort have recently been made in optimizing the air catalysts of flexible zinc-air batteries (ZABs). Unfortunately, the bottleneck factors in electrolytes that largely limit the working life and energy efficiency of ZABs have long been relatively neglected. Herein, an alkaline gel polymer electrolyte (GPE) is fabricated through multiple crosslinking reactions among poly(vinyl alcohol) (PVA), poly(acrylic acid), and graphene oxide followed by intense uptake of an alkali and the KI reaction modifier. The prepared GPE exhibits essentially improved properties compared to traditional PVA gel electrolyte in terms of mechanical strength, ionic conductivity, and water retention capability. In addition, the introduced reaction modifier I^- in the GPE changes the path of the conventional oxygen evolution reaction, leading to a more thermodynamically favorable path. The optimized GPE enables flexible ZABs exhibiting an exceptionally low charge potential of 1.69 V, a long cycling time of 200 h, a high energy efficiency of 73%, and rugged reliability under different extreme working conditions. Moreover, the successful integration of ZABs in a variety of real wearable electronic devices demonstrates their excellent practicability as flexible power sources.

A bimetallic MOF@graphene oxide composite as an efficient bifunctional oxygen electrocatalyst for rechargeable Zn-air batteries

The development of low-cost bifunctional catalysts for replacing precious metal Pt-group electrocatalysts is highly desirable but remains challenging in the energy conversion and storage field. Metal-organic frameworks (MOFs) with a large specific surface area and a controllable pore structure provide a great opportunity to prepare bifunctional catalysts, but their low activity in oxygen electrocatalysis hinders their applications. Herein, we rationally design and fabricate a bimetallic ZnCo-zeolite imidazole framework@graphene oxide (ZnCo-ZIF@GO) hybrid electrocatalyst through a simple in situ growth of a bimetallic ZIF on graphene oxide nanosheets. The obtained ZnCo-ZIF@GO hybrid displays superior electrocatalytic activities toward both the ORR and OER in alkaline solution compared to the pure ZnCo-ZIF. The outstanding bifunctional electrocatalytic performance is attributed to the synergy and strong interaction between the ZnCo-ZIF and GO, enhanced ionic conductivity, and hierarchical porosity. A rechargeable Zn-air battery (ZAB) assembled using ZnCo-ZIF@GO as the air cathode displays excellent charge and discharge performance, high energy density, and cycling stability, demonstrating its great potential as an advanced bifunctional electrocatalyst in the field of energy conversion and storage.

Recent progress, challenges, and prospects of fully integrated mobile and wearable point-of-care testing systems for self-testing

The rapid growth of research in the areas of chemical and biochemical sensors, lab-on-a-chip, mobile technology, and wearable electronics offers an unprecedented opportunity in the development of mobile and wearable point-of-care testing (POCT) systems for self-testing. Successful implementation of such POCT technologies leads to minimal user intervention during operation to reduce user errors; user-friendly, easy-to-use and simple detection platforms; high diagnostic sensitivity and specificity; immediate clinical assessment; and low manufacturing and consumables costs. In this review, we discuss recent developments in the field of highly integrated mobile and wearable POCT systems. In particular, aspects of sample handling platforms, recognition elements and sensing methods, and new materials for signal transducers and powering devices for integration into mobile or wearable POCT systems will be highlighted. We also summarize current challenges and future prospects for providing personal healthcare with sample-in result-out mobile and wearable POCT.

Recent Advances in Fiber-Shaped Supercapacitors and Lithium-Ion Batteries

The rapid development in wearable electronics has spurred a great deal of interest in flexible energy storage devices, particularly fiber-shaped energy storage devices (FSESDs), such as fiber-shaped supercapacitors (FSSCs) and fiber-shaped batteries (FSBs). Depending on their electrode configurations, FSESDs can contain five differently structured electrodes, including parallel fiber electrodes (PFEs), twisted fiber electrodes (TFDs), wrapped fiber electrodes (WFEs), coaxial fiber devices (CFEs), and rolled electrodes (REs). Various rational methods have been devised to incorporate these fiber-shaped electrodes into multifunctional FSESDs, including fiber-shaped supercapacitors, lithium-ion batteries, lithium-sulfur batteries, lithium-air batteries, zinc-air batteries, and aluminum-air batteries. Although significant progress has been made in FSESDs, it remains a major challenge to make high-performance fiber-shaped devices at low cost. A focused and critical review of the recent advancements in fiber-shaped supercapacitors and lithium-ion batteries is provided here. The pros and cons for each of the aforementioned electrode configurations and FSESDs are discussed, along with current challenges and future opportunities for FSESDs.

Flexible 1D Batteries: Recent Progress and Prospects

With the rapid development of wearable and portable electronics, flexible and stretchable energy storage devices to power them are rapidly emerging. Among numerous flexible energy storage technologies, flexible batteries are considered as the most favorable candidate due to their high energy density and long cycle life. In particular, flexible 1D batteries with the unique advantages of miniaturization, adaptability, and weavability are expected to be a part of such applications. The development of 1D batteries, including lithium-ion batteries, zinc-ion batteries, zinc-air batteries, and lithium-air batteries, is comprehensively summarized, with particular emphasis on electrode preparation, battery design, and battery properties. In addition, the remaining challenges to the commercialization of current 1D batteries and prospective opportunities in the field are discussed.

Stretchable electrochemical energy storage devices

Deformable energy storage devices are needed to power the next generation of wearable electronics. This review highlights the most recent advances in stretchable energy storage devices with a focus on batteries and supercapacitors.

Toward Flexible and Wearable Zn-Air Batteries from Cotton Textile Waste

Considering the environmental problems caused by a large amount of cotton textile waste and its possible applications in flexible electrodes, it is very promising to reuse the cotton textile waste as an electrode material, reducing the cost of flexible electrodes and alleviating environmental problems. In this work, we present a rechargeable flexible Zn-air battery based on cotton textile waste, which employs Ni-metallized cotton textile waste (NMCTW) as a flexible substrate for Zn anodes and air cathodes. The transparent NiFe hydroxide thin film horizontally grown on the surface of the NMCTW substrate was synthesized in situ by the electrodeposition method, which exhibits excellent catalytic activity because of the high surface area of the two-dimensional (2D) thin film, large contact area between the thin film and substrate, and fast charge transport of the 2D thin-film structure. In view of the high catalytic performance of the NiFe hydroxide thin film, it was used as the catalytic material of the air cathode for the flexible Zn-air battery. The assembled Zn-air battery based on cotton textile waste demonstrated a good rate performance and outstanding charge and discharge cycling stability. The assembled Zn-air battery was applied to power the light-emitting diode, which exhibits exceptional flexibility and stable output power even under severe mechanical bending deformation, proving the feasibility for its application in flexible electronics.

Investigation of the Environmental Stability of Poly(vinyl alcohol)-KOH Polymer Electrolytes for Flexible Zinc-Air Batteries

Next-generation wearable and portable electronic devices require the development of flexible energy-storage devices with high energy density and low cost. Over the past few decades, flexible zinc–air batteries (FZABs), characterized by their extremely high theoretical energy density from consuming oxygen in air and low cost, have been regarded as one of the most promising power supplies. However, their unique half-open structure poses great challenges for the environmental stability of their components, including the electrolyte and electrodes. As an important ionic conductor, the poly(vinyl alcohol) (PVA)–KOH gel polymer electrolyte (GPE) has been widely utilized in FZABs. To date, most studies have focused on investigations of the electrode, electrocatalyst materials and battery configuration, while very few have paid attention to the influence of the environment on the electrolyte and the corresponding FZAB performance. Herein, for the first time, the environmental stability of PVA–KOH GPE, such as dimensional stability and water and ionic conductivity retention capability, for FZABs in ambient air has been thoroughly studied. Moreover, the properties of the assembled FZABs in terms of cycling stability, discharge performance and power output are investigated. This report aims to play a leading role in examining the environmental stability of electrolytes in FZABs, which is critical for their practical applications.

Recent advances in flexible aqueous zinc-based rechargeable batteries

Flexible aqueous Zn battery has exhibited great potential as a power source for flexible and wearable electronic devices due to its unique features, such as high safety, low cost, and eco-friendliness. Numerous studies on flexible Zn batteries have been reported and exciting achievements have been obtained in the past few years. However, there are still many problems in the electrode design and the assembly process to acquire desirable flexibility without sacrificing the capacity. This review summarizes the up-to-date advances in flexible Zn batteries. We first introduce the recent progresses in anodes, cathodes and solid-state electrolytes. Special emphases are then put on the discussions of differences between various flexible current collectors or substrates. Electrode preparation techniques and flexible battery assembly technologies are also compared and discussed. Finally, challenges toward further developments of flexible Zn batteries with high capacity, excellent flexibility and cycling stability are proposed. Future research trends and highlights are suggested as well.