Textile‐Based Electrochemical Energy Storage Devices

In the past few years, insensitive attentions have been drawn to wearable and flexible energy storage devices/systems along with the emergence of wearable electronics. Much progress has been achieved in developing flexible electrochemical energy storage devices with high end‐use performance. However, challenges still remain in well balancing the electrochemical properties, mechanical properties, and the processing technologies. In this review, a specific perspective on the development of textile‐based electrochemical energy storage devices (TEESDs), in which textile components and technologies are utilized to enhance the energy storage ability and mechanical properties of wearable electronic devices, is provided. The discussion focuses on the material preparation and characteristics, electrode and device fabrication strategies, electrochemical performance and metrics, wearable compatibility, and fabrication scalability of TEESDs including textile‐based supercapacitors and lithium‐ion batteries.     

Graphene‐Based Nanocomposites for Energy Storage

Since the first report of using micromechanical cleavage method to produce graphene sheets in 2004, graphene/graphene‐based nanocomposites have attracted wide attention both for fundamental aspects as well as applications in advanced energy storage and conversion systems. In comparison to other materials, graphene‐based nanostructured materials have unique 2D structure, high electronic mobility, exceptional electronic and thermal conductivities, excellent optical transmittance, good mechanical strength, and ultrahigh surface area. Therefore, they are considered as attractive materials for hydrogen (H₂) storage and high‐performance electrochemical energy storage devices, such as supercapacitors, rechargeable lithium (Li)‐ion batteries, Li–sulfur batteries, Li–air batteries, sodium (Na)‐ion batteries, Na–air batteries, zinc (Zn)–air batteries, and vanadium redox flow batteries (VRFB), etc., as they can improve the efficiency, capacity, gravimetric energy/power densities, and cycle life of these energy storage devices. In this article, recent progress reported on the synthesis and fabrication of graphene nanocomposite materials for applications in these aforementioned various energy storage systems is reviewed. Importantly, the prospects and future challenges in both scalable manufacturing and more energy storage‐related applications are discussed.     

Novel Metal@Carbon Spheres Core–Shell Arrays by Controlled Self‐Assembly of Carbon Nanospheres: A Stable and Flexible Supercapacitor Electrode

The high performance of electrochemical energy‐storage devices relies largely on scrupulous design of nanoarchitectures and smart hybridization of bespoke active materials. Carbon nanopsheres (CNSs) are widely used for energy storage and conversion devices. Here, the directional assembly of CNSs on a vertical‐standing metal scaffold into a core/shell array structure is reported. The method uses a three‐step all‐solution synthesis strategy (chemical bath deposition, electrodeposition, and hydrothermal) and begins from ZnO microrod arrays as a sacrificial template. The self‐assembly of CNSs can be correlated to a simultaneous etching effect to the ZnO accompanying the polymerization of glucose precursor. The Ni microtube/CNSs arrays are selected as an example for structural and electrochemical characterizations. The novel type of metal/CNSs arrays is demonstrated to be a highly stable electrode for supercapacitors. The electrodes of metal/CNSs arrays are assembled into symmetric supercapacitors and exhibit high capacitances of 227 F g^?1 (at 2.5 A g^?1) and an outstanding cycling stability with capacitance retention of 97% after 40 000 cycles.     

Hierarchical Structures Based on Two‐Dimensional Nanomaterials for Rechargeable Lithium Batteries

Two‐dimensional (2D) nanomaterials (i.e., graphene and its derivatives, transition metal oxides and transition metal dichalcogenides) are receiving a lot attention in energy storage application because of their unprecedented properties and great diversities. However, their re‐stacking or aggregation during the electrode fabrication process has greatly hindered their further developments and applications in rechargeable lithium batteries. Recently, rationally designed hierarchical structures based on 2D nanomaterials have emerged as promising candidates in rechargeable lithium battery applications. Numerous synthetic strategies have been developed to obtain hierarchical structures and high‐performance energy storage devices based on these hierarchical structure have been realized. This review summarizes the synthesis and characteristics of three styles of hierarchical architecture, namely three‐dimensional (3D) porous network nanostructures, hollow nanostructures and self‐supported nanoarrays, presents the representative applications of hierarchical structured nanomaterials as functional materials for lithium ion batteries, lithium‐sulfur batteries and lithium‐oxygen batteries, meanwhile sheds light particularly on the relationship between structure engineering and improved electrochemical performance; and provides the existing challenges and the perspectives for this fast emerging field.     

Recent Progress in Biomass‐Derived Electrode Materials for High Volumetric Performance Supercapacitors

Tremendous efforts have been spent on the development of electrical energy storage (EES) systems with high volumetric performance in the past few years due to the evergrowing demand of miniaturized, portable electronic devices, and electric vehicles. Among all the EES devices, supercapacitors with electrode materials derived from biosources have attracted special attention due to their eco‐friendliness, natural abundance, their intrinsic porous structures as well as their renewable and sustainable features. However, the relatively low packing densities make their specific volumetric capacitance intrinsically low, which has largely limited their further application in the supercapacitors. To address these issues, various promising approaches ranging from structural manufacture to compositional design are applied and significant breakthroughs are witnessed in recent years. In this progress report, key factors influencing the volumetric performance of biomass‐derived electrode materials are systematically discussed with a particular focus spanning from fundamental to operational aspects. This work provides insights into the development of high‐volumetric‐performance biomass‐derived supercapacitors by comprehensively summarizing recent advances in the rational structural design and fabrication. Perspectives regarding the future challenges and promising research directions on the design of next‐generation EES devices are also provided.     

Transition‐Metal (Fe,Co, Ni) Based Metal‐Organic Frameworks for Electrochemical Energy Storage

Transition‐metal (Fe, Co, Ni) based metal‐organic framework materials with controllable structures, large surface areas and adjustable pore sizes have attracted wide research interest for use in next‐generation electrochemical energy‐storage devices. This review introduces the synthesis of transition‐metal (Fe, Co, Ni) based metal‐organic frameworks and their derivatives with the focus on their application in supercapacitors and batteries.     

Highly Efficient Materials Assembly Via Electrophoretic Deposition for Electrochemical Energy Conversion and Storage Devices

Featuring pronounced controllability, versatility, and scalability, electrophoretic deposition (EPD) has been proposed as an efficient method for film assembly and electrode/solid electrolyte fabrication in various energy storage/conversion devices including rechargeable batteries, supercapacitors, and fuel cells. High‐quality electrodes and solid electrolytes have been prepared through EPD and exhibit advantageous performances in comparison with those realized with traditional methods. Recent advances in the application of EPD materials in electrochemical energy storage and conversion devices are summarized. In particular, the parameters that influence the efficiency of an EPD process from colloidal preparation to deposition are evaluated with the aim to provide insightful guidance for realizing high‐performance electrochemical energy conversion materials and devices.     

Heteroatom‐Doped and Oxygen‐Functionalized Nanocarbons for High‐Performance Supercapacitors

Electrochemical capacitors (best known as supercapacitors) are high‐performance energy storage devices featuring higher capacity than conventional capacitors and higher power densities than batteries, and are among the key enabling technologies of the clean energy future. This review focuses on performance enhancement of carbon‐based supercapacitors by doping other elements (heteroatoms) into the nanostructured carbon electrodes. The nanocarbon materials currently exist in all dimensionalities (from 0D quantum dots to 3D bulk materials) and show good stability and other properties in diverse electrode architectures. However, relatively low energy density and high manufacturing cost impede widespread commercial applications of nanocarbon‐based supercapacitors. Heteroatom doping into the carbon matrix is one of the most promising and versatile ways to enhance the device performance, yet the mechanisms of the doping effects still remain poorly understood. Here the effects of heteroatom doping by boron, nitrogen, sulfur, phosphorus, fluorine, chlorine, silicon, and functionalizing with oxygen on the elemental composition, structure, property, and performance relationships of nanocarbon electrodes are critically examined. The limitations of doping approaches are further discussed and guidelines for reporting the performance of heteroatom doped nanocarbon electrode‐based electrochemical capacitors are proposed. The current challenges and promising future directions for clean energy applications are discussed as well.     

Nature Driven Bio‐Piezoelectric/Triboelectric Nanogenerator as Next‐Generation Green Energy Harvester for Smart and Pollution Free Society

Electronics wastes (e‐wastes) are the major concern in the rapid expansion of smart/wearable/portable electronics in modern high‐tech society. Informal processing and enormous gathering of e‐wastes can lead to adverse human/animal health effects and environmental pollution worldwide. Currently, these issues are a big headache and require the scientific community to develop effective green energy harvesting technologies using biodegradable/biocompatible materials. Piezoelectric/triboelectric nanogenerators (PNGs/TNGs) are considered one of the most promising renewable green energy sources for the conversion of mechanical/biomechanical energies into electricity. However, organic/inorganic material based PNGs/TNGs are very much incompatible, and considered e‐wastes for their non‐biodegradability. This review covers potential uses of biodegradable/biocompatible materials which are wasted every day as nature driven material based bio‐nanogenerators with a particular focus on their applications in flexible PNGs/TNGs fabrication. Structural investigation and possible working principles are described first in order to outline the basic mechanism of bio‐inspired materials behind energy harvesting. Then, energy harvesting abilities and the mechanical sensing of bio‐inspired integrated flexible devices are discussed under various mechanical/biomechanical activities. Finally, their potential applications in various flexible, wearable, and portable electronic fields are demonstrated. These bio‐inspired energy harvesting devices can make huge changes in fields as diverse as portable electronics, in vitro/in vivo biomedical applications, and many more.     

Recent Progress in Flexible Electrochemical Capacitors: Electrode Materials,Device Configuration,and Functions

With increasing demand for portable, flexible, and even wearable electronic devices, flexible energy storage systems have received increasing attention as a key component in this emerging field. Among the options, supercapacitors, commonly referred to as ultracapacitors or electrochemical capacitors, are widely recognized as a potential energy storage system due to their high power, fast charge/discharge rate, long cycling life‐time, and low cost. To date, considerable effort has been dedicated to developing high‐performance flexible supercapacitors based on various electrode materials; including carbon nanomaterials (e.g., carbon nanotubes, graphene, porous carbon materials, carbon paper, and textile), conducting polymers (e.g., polyaniline, polypyrrole, polythiophene), and hybrid materials. A brief introduction to the field is provided and the state‐of‐the‐art is reviewed with special emphasis on electrode materials and device configurations.     

Liquid–Solid Interfacial Assemblies of Soft Materials for Functional Freestanding Layered Membrane–Based Devices toward Electrochemical Energy Systems

Freestanding layered membrane–based devices have broad applications in highly efficient energy‐storage/conversion systems. The liquid–solid interface is considered as a unique yet versatile interface for constructing such layered membrane–based devices. In this review, the authors outline recent developments in the fabrication of soft materials to functionalize layered devices from the aspect of liquid–solid interfacial assembly and engineering arts. Seven liquid–solid interfacial assembly strategies, including flow‐directed, superlattice, solvent‐casting, evaporation‐induced, dip‐coating, spinning, and electrospinning assemblies, are comprehensively highlighted with a focus on their synthetic pathways, formation mechanisms, and interface engineering strategies. Meanwhile, recent representative works on layered membrane–based devices for electrochemical energy applications are presented. Finally, challenges and opportunities of this research area are highlighted in order to stimulate future developments. This review not only offers comprehensive and practical approaches to assemble liquid–solid interfaces with soft materials for various important layered electrochemical energy devices but also sheds lights on fundamental insights by thoughtful discussions on performance enhancement mechanisms of these electrochemical energy systems.     

Alveoli‐Inspired Facile Transport Structure of N‐Doped Porous Carbon for Electrochemical Energy Applications

Heteroatom‐doped porous carbon materials have attracted much attention because of their extensive application in energy conversion and storage devices. Because the performance of fuel cells and the rate capability of supercapacitors depend significantly on multiple factors, such as electrical conductivity and transport rate of ions and reactants, designing these carbon‐based materials to optimize performance factors is vital. In order to address these issues, alveoli that possess a hollow cavity where oxygen exchange can occur are synthesized, inspired by N‐doped carbon materials with a high surface area and low transport resistance. By incorporating a dopamine coating on zeolitic imidazolate framework (ZIF), pore size is modified and electrical conducting pathways are constructed, resulting in changes to the reaction kinetics. These highly interconnected electron connection channels and proper pore sizes facilitate the diffusion of reactants and the conduction of electrons, leading to high activity of the oxygen reduction reaction (ORR), which is comparable to Pt, and high rate performance in supercapacitors.     

Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion

Mixed metal sulfides (MMSs) have attracted increased attention as promising electrode materials for electrochemical energy storage and conversion systems including lithium‐ion batteries (LIBs), sodium‐ion batteries (SIBs), hybrid supercapacitors (HSCs), metal–air batteries (MABs), and water splitting. Compared with monometal sulfides, MMSs exhibit greatly enhanced electrochemical performance, which is largely originated from their higher electronic conductivity and richer redox reactions. In this review, recent progresses in the rational design and synthesis of diverse MMS‐based micro/nanostructures with controlled morphologies, sizes, and compositions for LIBs, SIBs, HSCs, MABs, and water splitting are summarized. In particular, nanostructuring, synthesis of nanocomposites with carbonaceous materials and fabrication of 3D MMS‐based electrodes are demonstrated to be three effective approaches for improving the electrochemical performance of MMS‐based electrode materials. Furthermore, some potential challenges as well as prospects are discussed to further advance the development of MMS‐based electrode materials for next‐generation electrochemical energy storage and conversion systems.     

An Efficient Ultra‐Flexible Photo‐Charging System Integrating Organic Photovoltaics and Supercapacitors

Flexible and biocompatible integrated photo‐charging devices consisting of photovoltaic cells and energy storage units can provide an independent power supply for next‐generation wearable electronics or biomedical devices. However, current flexible integrated devices exhibit low total energy conversion and storage efficiency and large device thickness, hindering their applicability towards efficient and stable self‐powered systems. Here, a highly efficient and ultra‐thin photo‐charging device with a total efficiency approaching 6% and a thickness below 50 µm is reported, prepared by integrating 3‐µm‐thick organic photovoltaics on 40 µm‐thick carbon nanotube/polymer‐based supercapacitors. This flexible photo‐charging capacitor delivers much higher performance compared with previous reports by tuning the electrochemical properties of the composite electrodes, which reduce the device thickness to 1/8 while improving the total efficiency by 15%. The devices also exhibit a superior operational stability (over 96% efficiency retention after 100 charge/discharge cycles for one week) and mechanical robustness (94.66% efficiency retention after 5000 times bending at a radius of around 2 mm), providing a high‐power and long‐term operational energy source for flexible and wearable electronics.     

3D Nanostructures for the Next Generation of High‐Performance Nanodevices for Electrochemical Energy Conversion and Storage

Among the different nanostructures that have been demonstrated as promising materials for various applications, 3D nanostructures have attracted significant attention as building blocks for constructing high‐performance nanodevices. Particularly over the last decade, considerable research efforts have been devoted to designing, fabricating, and evaluating 3D nanostructures as electrodes for electrochemical energy conversion and storage devices. Although remarkable progress has been achieved, the performance of electrochemical energy devices based on 3D nanostructures in terms of energy conversion efficiency, energy storage capability, and device reliability still needs to be significantly improved to meet the requirements for practical applications. Rather than simply outlining and comparing different 3D nanostructures, this article systematically summarizes the general advantages as well as the existing and future challenges of 3D nanostructures for electrochemical energy conversion and storage, focusing on photoelectrochemical water splitting, photoelectrocatalytic solar‐to‐fuels conversion from nitrogen and carbon dioxide, rechargeable metal‐ion batteries, and supercapacitors. A comprehensive understanding of these advantages and challenges shall provide valuable guidelines and enlightenments to facilitate the further development of 3D nanostructured materials, and contribute to the achieving more efficient energy conversion and storage technologies toward a sustainable energy future.     

Bio‐Nanotechnology in High‐Performance Supercapacitors

The use of bio‐nanotechnology for the fabrication of diverse functional nanomaterials with precisely controlled morphologies and microstructures is attracting considerable attention due to its sustainability and renewability. As one of the key energy storage devices, supercapacitor (SC) requires the active electrode material to have high specific surface area, interconnected porous structure, excellent electronic conductivity, and appropriate heteroatom doping for promoting the transfer of electrons and electrolyte ions. The combination of bio‐technology and SC will open up a new avenue for the large‐scale fabrication of high performance functional energy storage devices. In this review, the most state‐of‐the‐art research progress in bio‐nanotechnological fabrication of different nanomaterials, including carbon materials, metal oxides, conducting polymers, and their corresponding composites are reviewed with the following three bio‐nanotechnical approaches covered: (1) biomass carbonization technologies; (2) bio‐template methods; and (3) bio‐complex technologies, while also highlighting their applications as functional SC electrodes.     

Advanced Electrode Materials Comprising of Structure‐Engineered Quantum Dots for High‐Performance Asymmetric Micro‐Supercapacitors

Micro‐supercapacitors (MSCs) as a new class of energy storage devices have attracted great attention due to their unique merits. However, the narrow operating voltage, slow frequency response, and relatively low energy density of MSCs are still insufficient. Therefore, an effective strategy to improve their electrochemical performance by innovating upon the design from various aspects remains a huge challenge. Here, surface and structural engineering by downsizing to quantum dot scale, doping heteroatoms, creating more structural defects, and introducing rich functional groups to two dimensional (2D) materials is employed to tailor their physicochemical properties. The resulting nitrogen‐doped graphene quantum dots (N‐GQDs) and molybdenum disulfide quantum dots (MoS₂‐QDs) show outstanding electrochemical performance as negative and positive electrode materials, respectively. Importantly, the obtained N‐GQDs//MoS₂‐QDs asymmetric MSCs device exhibits a large operating voltage up to 1.5 V (far exceeding that of most reported MSCs), an ultrafast frequency response (with a short time constant of 0.087 ms), a high energy density of 0.55 mWh cm^?3, and long‐term cycling stability. This work not only provides a novel concept for the design of MSCs with enhanced performance but also may have broad application in other energy storage and conversion devices based on QDs materials.     

Smart Windows: Electro‐, Thermo‐, Mechano‐, Photochromics,and Beyond

A smart window that dynamically modulates light transmittance is crucial for building energy efficiently, and promising for on‐demand optical devices. The rapid development of technology brings out different categories that have fundamentally different transmittance modulation mechanisms, including the electro‐, thermo‐, mechano‐, and photochromic smart windows. In this review, recent progress in smart windows of each category is overviewed. The strategies for each smart window are outlined with particular focus on functional materials, device design, and performance enhancement. The advantages and disadvantages of each category are summarized, followed by a discussion of emerging technologies such as dual stimuli triggered smart window and integrated devices toward multifunctionality. These multifunctional devices combine smart window technology with, for example, solar cells, triboelectric nanogenerators, actuators, energy storage devices, and electrothermal devices. Lastly, a perspective is provided on the future development of smart windows.     

Flexible Pseudocapacitive Electrochromics via Inkjet Printing of Additive‐Free Tungsten Oxide Nanocrystal Ink

Direct inkjet printing of functional inks is an emerging and promising technique for the fabrication of electrochemical energy storage devices. Electrochromic energy devices combine electrochromic and energy storage functions, providing a rising and burgeoning technology for next‐generation intelligent power sources. However, printing such devices has, in the past, required additives or other second phase materials in order to create inks with suitable rheological properties, which can lower printed device performance. Here, tungsten oxide nanocrystal inks are formulated without any additives for the printing of high‐quality tungsten oxide thin films. This allows the assembly of novel electrochromic pseudocapacitive zinc‐ion devices, which exhibit a relatively high capacity (≈260 C g^?1 at 1 A g^?1) with good cycling stability, a high coloration efficiency, and fast switching response. These results validate the promising features of inkjet‐printed electrochromic zinc‐ion energy storage devices in a wide range of applications in flexible electronic devices, energy‐saving buildings, and intelligent systems.     

Emerging 3D‐Printed Electrochemical Energy Storage Devices: A Critical Review

Three‐dimensional (3D) printing, a layer‐by‐layer deposition technology, has a revolutionary role in a broad range of applications. As an emerging advanced fabrication technology, it has drawn growing interest in the field of electrochemical energy storage because of its inherent advantages including the freeform construction and controllable 3D structural prototyping. This article focuses on the topic of 3D‐printed electrochemical energy storage devices (EESDs), which bridge advanced electrochemical energy storage and future additive manufacturing. Basic 3D printing systems and material considerations are described to provide a fundamental understanding of printing technologies for the fabrication of EESDs. The performance metrics of 3D‐printed EESDs are then given and the related performance optimization strategies are discussed. Next, the recent advances of 3D‐printed EESDs, including sandwich‐type and in‐plane architectures, are summarized. Conclusions and future perspectives with some unique challenges and important directions are then discussed. It can be expected that, with the help of 3D printing technology, the development of advanced electrochemical energy storage systems will be greatly promoted.