Additive Manufacturing of 3D Aerogels and Porous Scaffolds: A Review

Aerogels are highly porous structures produced by replacing the liquid solvent of a gel with air without causing a collapse in the solid network. Unlike conventional fabrication methods, additive manufacturing (AM) has been applied to fabricate 3D aerogels with customized geometries specific to their applications, designed pore morphologies, multimaterial structures, etc. To date, three major AM technologies (extrusion, inkjet, and stereolithography) followed by a drying process have been proposed to additively manufacture 3D functional aerogels. 3D-printed aerogels and porous scaffolds showed great promise for a variety of applications, including tissue engineering, electrochemical energy storage, controlled drug delivery, sensing, and soft robotics. In this review, the details of steps included in the AM of aerogels and porous scaffolds are discussed, and a general frame is provided for AM of those. Then, the different postprinting processes are addressed to achieve the porosity (after drying); and mechanical strength, functionality, or both (after postdrying thermal or chemical treatments) are provided. Furthermore, the applications of the 3D-printed aerogels/porous scaffolds made from a variety of materials are also highlighted. The review is concluded with the current challenges and an outlook for the next generation of 3D-printed aerogels and porous scaffolds.     

Tunable Assembly of sp3 Cross‐Linked 3D Graphene Monoliths: A First‐Principles Prediction

One of the biggest challenges in graphene applications is how one can fabricate 3D architectures comprising graphene sheets in which the resulting architectures have inherited graphene's excellent intrinsic properties but have overcome its shortcomings. Two series of 3D graphene monoliths (GMs) using zigzag or armchair graphene nanoribbons as building blocks and sp³ carbon chains as junction nodes are constructued, and calculations based on first principles are performed in order to predict their mechanical and electronic properties. The perfect match between sp² nanoribbons and sp³ linkers results in favorable energy and mechanical/dynamic stability. Owing to their tailored motifs, wine‐rack‐like pores, and rigid sp³ linkers, these GMs possess high surface areas, appreciable mechanical strength, and tunable band gaps. Negative linear compressibilities in a wide range are found for the zigzag GMs. By solving the problems of zero gap and dimensionality of graphene sheets simultaneously, these GMs offer a viable strategy towards many applications, e.g., microelectronic devices, energy storage, molecular sieves, sensitive pressure detectors, and telecommunication line systems.     

Recent progress in synthesis of two-dimensional hexagonal boron nitride

Two-dimensional (2D) materials have recently received a great deal of attention due to their unique structures and fascinating properties, as well as their potential applications. 2D hexagonal boron nitride (2D h-BN), an insulator with excellent thermal stability, chemical inertness, and unique electronic and optical properties, and a band gap of 5.97 eV, is considered to be an ideal candidate for integration with other 2D materials. Nevertheless, the controllable growth of high-quality 2D h-BN is still a great challenge. A comprehensive overview of the progress that has been made in the synthesis of 2D h-BN is presented, highlighting the advantages and disadvantages of various synthesis approaches. In addition, the electronic, optical, thermal, and mechanical properties, heterostructures, and related applications of 2D h-BN are discussed.     

Metal–Organic Framework Nanosheets: Programmable 2D Materials for Catalysis,Sensing, Electronics,and Separation Applications

Metal–organic framework nanosheets (MONs) have recently emerged as a distinct class of 2D materials with programmable structures that make them useful in diverse applications. In this review, the breadth of applications that have so far been investigated are surveyed, thanks to the distinct combination of properties afforded by MONs. How: 1) The high surface areas and readily accessible active sites of MONs mean they have been exploited for a variety of heterogeneous, photo-, and electro-catalytic applications; 2) their diverse surface chemistry and wide range of optical and electronic responses have been harnessed for the sensing of small molecules, biological molecules, and ions; 3) MONs tunable optoelectronic properties and nanoscopic dimensions have enabled them to be harnessed in light harvesting and emission, energy storage, and other electronic devices; 4) the anisotropic structure and porous nature of MONs mean they have shown great promise in a variety of gas separation and water purification applications; are discussed. The aim is to draw links between the uses of MONs in these different applications in order to highlight the common opportunities and challenges presented by this promising class of nanomaterials.     

2D Molybdenum Compounds for Electrocatalytic Energy Conversion

The development of advanced nanomaterials is urgent for electrocatalytic energy conversion applications. Recently, 2D nanomaterial-derived heterogeneous electrocatalysts have shown great promise for both fundamental research and practical applications owing to their extremely high surface-to-volume ratio and tunable geometric and electronic properties. Because of their unique electronic structure and physicochemical properties, molybdenum (Mo)-based 2D nanomaterials are emerging as one of the most attractive candidates among the nonprecious materials for electrocatalysts. This review provides a comprehensive overview of the recent advances in the synthesis and modulation of 2D Mo compounds for applications in electrocatalytic energy conversion. The categories based on different compositions and corresponding synthetic approaches of 2D Mo compounds are first introduced. Subsequently, various atomic/plane/synergistic engineering strategies, along with catalytic optimization in the electrochemical process that involves the cycles of water, carbon, and nitrogen, are discussed in detail. Finally, the current challenges and future opportunities for the development of 2D Mo-based electrocatalysts are proposed with the goal of shedding light on these promising 2D nanomaterials for electrocatalytic energy conversion.     

Fabrication,Applications, and Prospects of Aramid Nanofiber

Aramid nanofibers (ANFs) are of great interest in various applications due to its 1D nanoscale, high aspect ratio, high specific surface area, excellent strength, and modulus as well as impressive chemical and thermal stabilities. It is considered as one of the most promising nano‐sized building blocks with excellent properties and has therefore drawn increasing attention since 2011. However, no review has summarized the research progress and the prospective challenges of ANF. Herein, the methods of ANF fabrication and their relative merits are comprehensively discussed together with the challenges and progress in the deprotonation method for preparing ANF. The fabrication methods and development of ANF‐based advanced materials with different macroscopic morphologies, including the 1D ANF aerogel fiber, 2D ANF film/nanopaper/coating, and 3D ANF gel and particle are also described. Furthermore, the applications of ANF in nanocomposite reinforcement, battery separators, electrical insulation nanopaper, flexible electronics, and adsorption and filtration media are presented. Additionally, the possible challenges and outlooks toward the future development of ANF are highlighted. This review indicates that the ANF and ANF‐based materials mentioned herein will boost the development of next‐generation advanced functional materials.     

Cutting Edge Use of Conductive Patterns in Nanocellulose-Based Green Electronics

Green electronics made from degradable materials have recently attracted special attention, because electronic waste (e-waste) represents a serious threat to the environment and to human health worldwide. Among the novel materials used for sustainable technologies, nanocelluloses containing at least 1D in the nanoscale range (1–100 nm) have been widely exploited for various industrial applications owing to their inherent properties, such as biodegradability, mechanical strength, thermal stability, and optical transparency. This review highlights recent advances in research on the development of patterns for conductive material on nanocellulose substrates for use in high-performance green electronics. The advantages of nanocellulose substrates compared to conventional paper substrates for advanced green electronics are discussed. Importantly, this review emphasizes various fabrication strategies for producing conductive patterns on different types of nanocellulose-based substrates, such as cellulose nanofiber (CNF), (2,2,6,6-tetramethylpiperidin-1-yl)oxyl(TEMPO)-oxidized CNF, regenerated cellulose, and bacterial cellulose. In the latter part of this review, emerging engineering applications for green electronics such as circuits, transistors/antennas, sensors, energy storage systems, and electrochromic devices are further discussed.     

Advanced Oxygen Electrocatalysis in Energy Conversion and Storage

Oxygen electrocatalysis is of great significance in electrochemical energy conversion and storage. Many strategies have been adopted for developing advanced oxygen electrocatalysts to promote these technologies. In this invited contribution, recent progress in understanding the oxygen electrochemistry from theoretical and experimental aspects is summarized. The major categories of oxygen electrocatalysts, namely, noble-metal-based compounds, transition-metal-based composites, and nanocarbons, are successively discussed for oxygen reduction and evolution. Design strategies of various oxygen electrocatalysts and their relationship on the structure–activity–performance are comprehensively addressed with the perspectives. Finally, the challenge and outlook for advanced oxygen electrocatalysts are discussed toward energy conversion and storage technologies.     

Recent Developments of Planar Micro‐Supercapacitors: Fabrication,Properties, and Applications

The increasing development of wearable, portable, implantable, and highly integrated electronic devices has led to an increasing demand for miniaturization of energy storage devices. In recent years, supercapacitors, as an energy storage device, have received enormous attention owing to their excellent properties of quick charge and discharge, high power density, and long life cycle with minimal maintenance. Micro‐supercapacitors (MSCs) as a promising candidate for miniaturized energy storage components have undergone considerable theoretical and experimental investigations. Particularly, planar MSCs with a 2D architecture design have more attractive application prospects due to their flexible design and excellent electrochemical performance. However, the major drawbacks of MSCs are their intrinsically low energy density. For this reason, researchers have conducted much investigation to improve their energy density in order to promote their practical application. Herein, the recent development and progress of planar MSCs from the scope of the substrates, electrode materials, fabrication methods, electrochemical properties, and applications are discussed. Finally, the currently existing challenges and developments associated with planar MSCs are also discussed. All in all, planar MSCs have great application potential in various fields of electrochemical energy storage, self‐powered wireless sensors, and stimuli‐responsive and photoresponsive, alternating current line filtering.     

Advanced Flexible Materials from Nanocellulose

With the increase of environmental pollution and depletion of fossil fuel resources, the utilization of renewable biomass resources for developing functional materials or fine chemicals is of great value and has attracted considerable attention. Nanocellulose, as a well-known renewable nanomaterial, is regarded as a promising nano building block for advanced functional materials owing to its unique structure and properties, as well as natural abundance. Typically, its high mechanical strength, structural flexibility, reinforcing capabilities, and tunable self-assembly behavior makes it highly attractive to fabricate flexible materials for various applications. Herein, the recent progress in the design, properties, and applications of advanced flexible materials from nanocellulose is comprehensively summarized. The preparation and properties of nanocellulose are first briefly introduced and discuss its merits in fabricating flexible materials. Then, various advanced flexible materials from nanocellulose are introduced, and the critical role of nanocellulose in constructing flexible materials is highlighted based on its intrinsic properties. After that, their applications in energy storage, electronics, sensor, biomedical, thermally insulating, photonic devices, etc., are presented. At last, the outlook of the current challenges and future perspectives for developing nanocellulose-derived flexible materials are discussed.     

2D Graphitic Carbon Nitride for Energy Conversion and Storage

Graphitic carbon nitride (g-C₃N₄) have attracted great attention in the field of energy conversion and storage due to its unique layered structure, tunable bandgap, metal-free characteristic, high physicochemical stability, and easy accessibility. 2D g-C₃N₄ nanosheets have the features of short charge/mass transfer path, abundant reactive sites and easy functionalization, which are beneficial to optimizing their performance in different fields. However, the reviews of the comprehensive applications of 2D g-C₃N₄ for energy conversion and storage are rare. Herein, this review first introduces the physicochemical properties of bulk g-C₃N₄ and g-C₃N₄ nanosheets, and then summarizes the synthetic strategies of 2D g-C₃N₄ nanosheets in detail, such as thermal oxidation etching, chemical exfoliation, ultrasonication-assisted liquid phase exfoliation, chemical vapor deposition, and others. Emphasis is focused on the rational design and development of 2D g-C₃N₄ nanosheets for the diversified applications in energy conversion and storage, including photocatalytic H₂ evolution, CO₂ reduction, electrocatalytic H₂ evolution, O₂ evolution, O₂ reduction, alkali-metal ion batteries, lithium-metal batteries, lithium–sulfur batteries, metal-air batteries, and supercapacitors. Finally, the current challenges and perspectives of 2D g-C₃N₄ nanosheets for energy conversion and storage applications are discussed.     

Advanced Exfoliation Strategies for Layered Double Hydroxides and Applications in Energy Conversion and Storage

Along with the increasing aggravation of energy and environmental problems, the demand and utilization of renewable energy have increased. The rational design of advanced functional materials serves as a critical point for the improvement of performance and the practical application in renewable energy devices. Layered double hydroxides (LDHs) with 2D layered structures are promising energy materials for their unique physical and chemical properties. Nevertheless, the applications are limited by the structure of stacking with irrational electronic structure, sluggish mass transfer, and low activity. The exfoliation of LDHs into single‐ or few‐layered nanosheets appears to be a promising approach to overcome the above disadvantages. Herein, the recent progress on the development of exfoliation strategies for LDHs including liquid phase exfoliation, plasma‐induced exfoliation, and other advanced exfoliation strategies is highlighted and the applications in energy conversion and storage are systematically introduced.     

Direct Freeform Laser Fabrication of 3D Conformable Electronics

3D conformable electronic devices on freeform surfaces show superior performance to the conventional, planar ones. They represent a trend of future electronics and have witnessed exponential growth in various applications. However, their potential is largely limited by a lack of sophisticated fabrication techniques. To tackle this challenge, a new direct freeform laser (DFL) fabrication method enabled by a 5-axis laser processing platform for directly fabricating 3D conformable electronics on targeted arbitrary surfaces is reported. Accordingly, representative laser-induced graphene (LIG), metals, and metal oxides are successfully fabricated as high-performance sensing and electrode materials from different material precursors on various types of substrates for applications in temperature/light/gas sensing, energy storage, and printed circuit board for circuit. Last but not the least, to demonstrate an application in smart homes, LIG-based conformable strain sensors are fabricated and distributed in designated locations of an artificial tree. The distributed sensors have the capability of monitoring the wind speed and direction with the assistance of well-trained machine-learning models. This novel process will pave a new and general route to fabricating 3D conformable electronic devices, thus creating new opportunities in robotics, biomedical sensing, structural health, environmental monitoring, and Internet of Things applications.     

Engineering the Thermal Conductivity of Functional Phase‐Change Materials for Heat Energy Conversion,Storage, and Utilization

Thermal energy storage technologies based on phase‐change materials (PCMs) have received tremendous attention in recent years. These materials are capable of reversibly storing large amounts of thermal energy during the isothermal phase transition and offer enormous potential in the development of state‐of‐the‐art renewable energy infrastructure. Thermal conductivity plays a vital role in regulating the thermal charging and discharging rate of PCMs and improving the heat‐utilization efficiency. The strategies for tuning the thermal conductivity of PCMs and their potential energy applications, such as thermal energy harvesting and storage, thermal management of batteries, thermal diodes, and other forms of energy utilization, are summarized systematically. Furthermore, a research perspective is given to highlight emerging research directions of engineering advanced functional PCMs for energy applications.     

A Review of Acoustic Devices Based on Suspended 2D Materials and Their Composites

Acoustic devices play an increasingly important role in modern society for information technology and intelligent systems, and recently significant progress has been made in the development of communication, sensing, and energy transduction applications. However, conventional material systems, such as polymers, metals and silicon, show limitations to fulfill the evolving requirements for high-performance acoustic devices of small size, low power consumption, and multifunctional capabilities. 2D materials hold the promise in overcoming the development bottleneck of acoustic devices aforementioned, given their atomic-thin thickness, extensive surface area, superior physical properties, and remarkable layer-stacking tunability. By suspending the 2D materials, mechanical and thermal disruption from substrate will be eliminated, which will enable the development of new classes of acoustic devices with unprecedented sensitivity and accuracy. In this review, the recent progress of acoustic devices based on suspended 2D materials and their composites, especially applications in the audio frequency, static pressure, and ultrasonic frequency range, is briefly summarized, emphasizing the advantageous properties of suspended 2D materials and related outstanding device performance. Together with the development of 2D membrane synthesis, transfer, as well as microelectromechanical fabrication process, suspended 2D materials will shed light on the next-generation high-performance acoustic devices.     

Advanced Applications and Challenges of Electropolymerized Conjugated Microporous Polymer Films

Conjugated microporous polymers (CMPs) are a unique class of porous materials that have an integrated π-conjugated system and permanent intrinsic porosity. They are of great interest because of their outstanding performance in gas sorption, photoredox catalysis, energy storage, organic light-emitting diodes (OLEDs), and sensing applications. However, conventional chemically synthesized CMPs are solid powders and have poor solubility, which makes it difficult to process and integrate devices, and has become a bottleneck preventing their practical applications. Electropolymerization (EP) is a simple and efficient method for the preparation of CMP films, which simultaneously completes material synthesis and film processing. More importantly, the microstructure of the CMP films can be effectively controlled by electrochemical parameters. In this review, first, the basic synthetic principles and strategies of CMP films via EP are introduced, allowing for facile optimization of the structure and properties. Then, the recent progress of the EP CMP films is focused upon in organic electronics, energy storage, sensors, chemical capture and separation, and electrocatalysts. Finally, the challenges and outlook for EP CMP films are addressed.     

Flexible energy storage devices for wearable bioelectronics

With the growing market of wearable devices for smart sensing and personalized healthcare applications,energy stor-age devices that ensure stable power supply and can be constructed in flexible platforms have attracted tremendous research in-terests.A variety of active materials and fabrication strategies of flexible energy storage devices have been intensively studied in recent years,especially for integrated self-powered systems and biosensing.A series of materials and applications for flex-ible energy storage devices have been studied in recent years.In this review,the commonly adopted fabrication methods of flex-ible energy storage devices are introduced.Besides,recent advances in integrating these energy devices into flexible self-powered systems are presented.Furthermore,the applications of flexible energy storage devices for biosensing are summar-ized.Finally,the prospects and challenges of the self-powered sensing system for wearable electronics are discussed.     

Non-van der Waals 2D Materials for Electrochemical Energy Storage

The development of advanced electrode materials for the next generation of electrochemical energy storage (EES) solutions has attracted profound research attention as a key enabling technology toward decarbonization and electrification of transportation. Since the discovery of graphene's remarkable properties, 2D nanomaterials, derivatives, and heterostructures thereof, have emerged as some of the most promising electrode components in batteries and supercapacitors owing to their unique and tunable physical, chemical, and electronic properties, commonly not observed in their 3D counterparts. This review particularly focuses on recent advances in EES technologies related to 2D crystals originating from non-layered 3D solids (non-van der Waals; nvdW) and their hallmark features pertaining to this field of application. Emphasis is given to the methods and challenges in top-down and bottom-up strategies toward nvdW 2D sheets and their influence on the materials’ features, such as charge transport properties, functionalization, or adsorption dynamics. The exciting advances in nvdW 2D-based electrode materials of different compositions and mechanisms of operation in EES are discussed. Finally, the opportunities and challenges of nvdW 2D systems are highlighted not only in electrochemical energy storage but also in other applications, including spintronics, magnetism, and catalysis.     

Wearable Biofuel Cells: Advances from Fabrication to Application

The booming field of wearable devices has nourished progress in developing multifunctional wearable energy sources that can withstand deformations while maintaining their electrochemical functions. Unlike energy storage systems such as rechargeable batteries and supercapacitors, wearable biofuel cells (w-BFCs) generate green electricity from energy-dense carbon-neutral fuels via highly efficient bioelectrochemical reactions, delivering excellent biocompatibility, remarkable environmental sustainability, and exceptional capability of miniaturization. These desirable merits give w-BFCs great potential in the field of wearable applications. Moreover, emerging studies of w-BFCs in self-powered biosensing, controlled drug delivery, and wound dressings have greatly expanded their possible fields of application. Recent progress and strategies to accomplish flexible and stretchable w-BFCs are summarized here. Novel materials and configurations with tailored features that can be employed to fabricate w-BFCs are elaborated and discussed. Current applications and near-future applications of w-BFCs in health-monitoring and medical treatment fields are outlined. Furthermore, challenges and perspectives regarding this emerging field of materials science and engineering are also emphasized.