Engineering 2D Architectures toward High‐Performance Micro‐Supercapacitors

The rise of micro‐supercapacitors is satisfying the demand for power storage in portable devices and wireless gadgets. But the miniaturization of the energy‐storage components is significantly limited by their energy density. Electrode materials with adequate electrochemical active surfaces are therefore required for improving performance. 2D materials with ultralarge specific surface areas offer a broad portfolio of the development of high‐performance micro‐supercapacitors in spite of their several critical drawbacks. An architecture engineering strategy is therefore developed to break these natural limits and maximize the significant advantages of these materials. Based on the approaches of phase transformation, intercalation, surface modification, material hybridization, and hierarchical structuration, 2D architectures with improved conductivity, enlarged specific surface, enhanced redox activity, as well as the unique synergetic effect exhibit great promise in the application of miniaturized supercapacitors with highly enhanced performance. Herein, the architecture engineering of emerging 2D materials beyond graphene toward optimizing the performance of micro‐supercapacitors is discussed, in order to promote the application of 2D architectures in miniaturized energy‐storage devices.     

Enhancing the supercapacitor performance of graphene/MnO2 nanostructured electrodes by conductive wrapping

MnO2 is considered one of the most promising pseudocapactive materials for high-performance supercapacitors given its high theoretical specific capacitance, low-cost, environmental benignity, and natural abundance. However, MnO2 electrodes often suffer from poor electronic and ionic conductivities, resulting in their limited performance in power density and cycling. Here we developed a "conductive wrapping" method to greatly improve the supercapacitor performance of graphene/MnO2-based nanostructured electrodes. By three-dimensional (3D) conductive wrapping of graphene/MnO2 nanostructures with carbon nanotubes or conducting polymer, specific capacitance of the electrodes (considering total mass of active materials) has substantially increased by ～20% and ～45%, respectively, with values as high as ～380 F/g achieved. Moreover, these ternary composite electrodes have also exhibited excellent cycling performance with >95% capacitance retention over 3000 cycles. This 3D conductive wrapping approach represents an exciting direction for enhancing the device performance of metal oxide-based electrochemical supercapacitors and can be generalized for designing next-generation high-performance energy storage devices.     

Chemically Integrated Inorganic‐Graphene Two‐Dimensional Hybrid Materials for Flexible Energy Storage Devices

State‐of‐the‐art energy storage devices are capable of delivering reasonably high energy density (lithium ion batteries) or high power density (supercapacitors). There is an increasing need for these power sources with not only superior electrochemical performance, but also exceptional flexibility. Graphene has come on to the scene and advancements are being made in integration of various electrochemically active compounds onto graphene or its derivatives so as to utilize their flexibility. Many innovative synthesis techniques have led to novel graphene‐based hybrid two‐dimensional nanostructures. Here, the chemically integrated inorganic‐graphene hybrid two‐dimensional materials and their applications for energy storage devices are examined. First, the synthesis and characterization of different kinds of inorganic‐graphene hybrid nanostructures are summarized, and then the most relevant applications of inorganic‐graphene hybrid materials in flexible energy storage devices are reviewed. The general design rules of using graphene‐based hybrid 2D materials for energy storage devices and their current limitations and future potential to advance energy storage technologies are also discussed.     

3D打印石墨烯制备技术及其在储能领域的应用研究进展

石墨烯优异的力学和物理性能使其成为理想的储能材料。因结构精确可控,易实现规模化制备,3D打印石墨烯材料有望在储能领域得到广泛应用。本文全面综述了3D打印石墨烯制备技术及其在储能领域的应用研究进展。石墨烯墨水的黏度和可打印性是实现石墨烯3D打印的制约因素。实现工艺简单、浓度可控、无黏结剂石墨烯墨水的规模化打印将成为3D打印石墨烯制备技术未来的研究热点。石墨烯超级电容器、锂硫电池、锂离子电池等储能元件一体化打印成型是3D打印石墨烯在储能领域应用的发展方向。     

Extrusion‐Based 3D Printing of Hierarchically Porous Advanced Battery Electrodes

A highly porous 2D nanomaterial, holey graphene oxide (hGO), is synthesized directly from holey graphene powder and employed to create an aqueous 3D printable ink without the use of additives or binders. Stable dispersions of hydrophilic hGO sheets in water (≈100 mg mL^?1) can be readily achieved. The shear‐thinning behavior of the aqueous hGO ink enables extrusion‐based printing of fine filaments into complex 3D architectures, such as stacked mesh structures, on arbitrary substrates. The freestanding 3D printed hGO meshes exhibit trimodal porosity: nanoscale (4–25 nm through‐holes on hGO sheets), microscale (tens of micrometer‐sized pores introduced by lyophilization), and macroscale (<500 µm square pores of the mesh design), which are advantageous for high‐performance energy storage devices that rely on interfacial reactions to promote full active‐site utilization. To elucidate the benefit of (nano)porosity and structurally conscious designs, the additive‐free architectures are demonstrated as the first 3D printed lithium–oxygen (Li–O₂) cathodes and characterized alongside 3D printed GO‐based materials without nanoporosity as well as nanoporous 2D vacuum filtrated films. The results indicate the synergistic effect between 2D nanomaterials, hierarchical porosity, and overall structural design, as well as the promise of a freeform generation of high‐energy‐density battery systems.     

Ultrathin planar graphene supercapacitors

With the advent of atomically thin and flat layers of conducting materials such as graphene, new designs for thin film energy storage devices with good performance have become possible. Here, we report an "in-plane" fabrication approach for ultrathin supercapacitors based on electrodes comprised of pristine graphene and multilayer reduced graphene oxide. The in-plane design is straightforward to implement and exploits efficiently the surface of each graphene layer for energy storage. The open architecture and the effect of graphene edges enable even the thinnest of devices, made from as grown 1-2 graphene layers, to reach specific capacities up to 80 μFcm(-2), while much higher (394 μFcm(-2)) specific capacities are observed multilayer reduced graphene oxide electrodes. The performances of devices with pristine as well as thicker graphene-based structures are examined using a combination of experiments and model calculations. The demonstrated all solid-state supercapacitors provide a prototype for a broad range of thin-film based energy storage devices.     

Active electrode materials of graphene balls and their composites for supercapacitors: A perspective view

In recent years, supercapacitors have received considerable research attention for energy storage systems due to their high-power density, fast charge-discharge processes, and long cycle life. The superior performance of supercapacitors is considerably dependent on the electrode materials. Among electrode materials, graphene balls (GBs) and their composites have recently attracted strong interest. They are considered ideal for the fabrication of electrode materials because of their unique characteristics of large specific surface area and superior electric conductivity, which should make them very effective for use in supercapacitors. In particular, GBs and their microstructured composites have recently been proven promising candidates for supercapacitor electrodes. Their unique 3D morphology provides highly porous graphene structures for decoration with active materials. In this perspective, recent studies were highlighted and discussed that focus on GBs and their composites for the potential energy storage devices called supercapacitors, (i.e., electric double layer capacitors and pseudocapacitors).     

A Flexible Film with SnS2 Nanoparticles Chemically Anchored on 3D‐Graphene Framework for High Areal Density and High Rate Sodium Storage

The design and construction of flexible electrodes that can function at high rates and high areal capacities are essential regarding the practical application of flexible sodium‐ion batteries (SIBs) and other energy storage devices, which remains significantly challenging by far. Herein, a flexible and 3D porous graphene nanosheet/SnS₂ (3D‐GNS/SnS₂) film is reported as a high‐performance SIB electrode. In this hybrid film, the GNS/SnS₂ microblocks serve as pillars to assemble into a 3D porous and interconnected framework, enabling fast electron/ion transport; while the GNS bridges the GNS/SnS₂ microblocks into a flexible framework to provide satisfactorily mechanical strength and long‐range conductivity. Moreover, the SnS₂ nanocrystals, which chemically bond with GNS, provide sufficient active sites for Na storage and ensure the cycling stability. Consequently, this flexible 3D‐GNS/SnS₂ film exhibits excellent Na‐storage performances, especially in terms of high areal capacity (2.45 mAh cm^?2) and high rates with superior stability (385 mAh g^?1 at 1.0 A g^?1 over 1000 cycles with ≈100% retention). A flexible SIB full cell using this anode exhibits high and stable performance under various bending situations. Thus, this work provide a feasible route to prepare flexible electrodes with high practical viability for not only SIBs but also other energy storage devices.     

Synergistic effects from graphene and carbon nanotubes enable flexible and robust electrodes for high-performance supercapacitors

Flexible and lightweight energy storage systems have received tremendous interest recently due to their potential applications in wearable electronics, roll-up displays, and other devices. To manufacture such systems, flexible electrodes with desired mechanical and electrochemical properties are critical. Herein we present a novel method to fabricate conductive, highly flexible, and robust film supercapacitor electrodes based on graphene/MnO(2)/CNTs nanocomposites. The synergistic effects from graphene, CNTs, and MnO(2) deliver outstanding mechanical properties (tensile strength of 48 MPa) and superior electrochemical activity that were not achieved by any of these components alone. These flexible electrodes allow highly active material loading (71 wt % MnO(2)), areal density (8.80 mg/cm(2)), and high specific capacitance (372 F/g) with excellent rate capability for supercapacitors without the need of current collectors and binders. The film can also be wound around 0.5 mm diameter rods for fabricating full cells with high performance, showing significant potential in flexible energy storage devices.     

Hollow MXene Spheres and 3D Macroporous MXene Frameworks for Na‐Ion Storage

2D transition metal carbides and nitrides, named MXenes, are attracting increasing attentions and showing competitive performance in energy storage devices including electrochemical capacitors, lithium‐ and sodium‐ion batteries, and lithium–sulfur batteries. However, similar to other 2D materials, MXene nanosheets are inclined to stack together, limiting the device performance. In order to fully utilize MXenes' electrochemical energy storage capability, here, processing of 2D MXene flakes into hollow spheres and 3D architectures via a template method is reported. The MXene hollow spheres are stable and can be easily dispersed in solvents such as water and ethanol, demonstrating their potential applications in environmental and biomedical fields as well. The 3D macroporous MXene films are free‐standing, flexible, and highly conductive due to good contacts between spheres and metallic conductivity of MXenes. When used as anodes for sodium‐ion storage, these 3D MXene films exhibit much improved performances compared to multilayer MXenes and MXene/carbon nanotube hybrid architectures in terms of capacity, rate capability, and cycling stability. This work demonstrates the importance of MXene electrode architecture on the electrochemical performance and can guide future work on designing high‐performance MXene‐based materials for energy storage, catalysis, environmental, and biomedical applications.     

Compact energy storage enabled by graphenes: Challenges,strategies and progress

Storing as much energy as possible in as compact a space as possible is an ever-increasing concern to deal with the emerging “space anxiety” in electrochemical energy storage (EES) devices like batteries, which is known as “compact energy storage”. Carbons built from graphene units can be used as active electrodes or inactive key materials acting as porous micro- or even nano-reactors that facilitate battery reactions and play a vital role in optimizing the volumetric performance of the electrode and the battery. In this review, we discuss and clarify the key issues and specific strategies for compact energy storage, especially in batteries. The use of shrinkable carbon networks to produce small yet sufficient reaction space together with smooth charge delivery is highlighted as the simplest structure–function-performance relationship when used in supercapacitors and is then extended to overcome problems in compact rechargeable lithium/sodium/potassium batteries. Special concerns about cycling stability, fast charging and safety in compact batteries are discussed in detail. Strategies for compact energy storage ranging from materials to electrodes to batteries are reviewed here to provide guidance for how to produce a compact high energy battery by densifying the electrodes using customized carbon structures.     

3D Printing of Freestanding MXene Architectures for Current‐Collector‐Free Supercapacitors

Additive manufacturing (AM) technologies appear as a paradigm for scalable manufacture of electrochemical energy storage (EES) devices, where complex 3D architectures are typically required but are hard to achieve using conventional techniques. The combination of these technologies and innovative material formulations that maximize surface area accessibility and ion transport within electrodes while minimizing space are of growing interest. Herein, aqueous inks composed of atomically thin (1–3 nm) 2D Ti₃C₂T_x with large lateral size of about 8 µm possessing ideal viscoelastic properties are formulated for extrusion‐based 3D printing of freestanding, high specific surface area architectures to determine the viability of manufacturing energy storage devices. The 3D‐printed device achieves a high areal capacitance of 2.1 F cm^?2 at 1.7 mA cm^?2 and a gravimetric capacitance of 242.5 F g^?1 at 0.2 A g^?1 with a retention of above 90% capacitance for 10 000 cycles. It also exhibits a high energy density of 0.0244 mWh cm^?2 and a power density of 0.64 mW cm^?2 at 4.3 mA cm^?2. It is anticipated that the sustainable printing and design approach developed in this work can be applied to fabricate high‐performance bespoke multiscale and multidimensional architectures of functional and structural materials for integrated devices in various applications.     

Doped graphene electrodes for organic solar cells

In this work graphene sheets grown by chemical vapor deposition (CVD) with controlled numbers of layers were used as transparent electrodes in organic photovoltaic (OPV) devices. It was found that for devices with pristine graphene electrodes, the power conversion efficiency (PCE) is comparable to their counterparts with indium tin oxide (ITO) electrodes. Nevertheless, the chances for failure in OPVs with pristine graphene electrodes are higher than for those with ITO electrodes, due to the surface wetting challenge between the hole-transporting layer and the graphene electrodes. Various alternative routes were investigated and it was found that AuCl(3) doping on graphene can alter the graphene surface wetting properties such that a uniform coating of the hole-transporting layer can be achieved and device success rate can be increased. Furthermore, the doping both improves the conductivity and shifts the work function of the graphene electrode, resulting in improved overall PCE performance of the OPV devices. This work brings us one step further toward the future use of graphene transparent electrodes as a replacement for ITO.     

An All‐Stretchable‐Component Sodium‐Ion Full Battery

Stretchable energy‐storage devices receive considerable attention due to their promising applications in future wearable technologies. However, they currently suffer from many problems, including low utility of active materials, limited multidirectional stretchability, and poor stability under stretched conditions. In addition, most proposed designs use one or more rigid components that fail to meet the stretchability requirement for the entire device. Here, an all‐stretchable‐component sodium‐ion full battery based on graphene‐modified poly(dimethylsiloxane) sponge electrodes and an elastic gel membrane is developed for the first time. The battery exhibits reasonable electrochemical performance and robust mechanical deformability; its electrochemical characteristics can be well‐maintained under many different stretched conditions and after hundreds of stretching–release cycles. This novel design integrating all stretchable components provides a pathway toward the next generation of wearable energy devices in modern electronics.     

Three-dimensional porous graphene-based ultra-lightweight aerofoam exhibiting good thermal insulation

Three-dimensional (3D) architectures of graphene and carbon nanotubes (CNTs) were prepared via the in situ self-assembly of graphene oxide and carboxyl-functionalized CNTs by mild chemical reduction and freeze-drying. The prepared 3D architectures exhibited densities of 7.35–9.68 mg/mL and thermal conductivities of 0.0192–0.0414 W/mK. These features were created by a highly porous structure, including millimeter-, micrometer-, and nanometer-scale pores. A number of analytical techniques were used to characterize the 3D architectures of graphene. The highly porous graphene-based aerofoams exhibited low densities, good thermal stabilities, and low thermal conductivities, making them excellent candidates for application in thermal protection materials, as well as in super capacitors and energy storage systems.     

3D knitted energy storage textiles using MXene-coated yarns

Textile-based energy storage devices offer an exciting replacement for bulky and uncomfortable batteries in commercial smart garments. Fiber and yarn-based supercapacitors, currently dominating research in this field, have demonstrated excellent performance below ∼4 cm in length, but suffer at longer lengths due to increased resistance. Herein, a new architecture of wearable energy storage devices, 3D knitted supercapacitors, is designed and prototyped with the intention of exploiting the architecture of a knit textile to improve the performance of long yarn electrodes. While Computer-Aided Design (CAD) knitting is a ubiquitous technology for producing textiles, knitted energy storage devices have been largely unexplored due to the need for meters of highly conductive yarn electrodes that meet the strenuous strength and flexibility requirements for CAD knitting. MXenes, a family of solution processable and conductive two-dimensional (2D) materials, have been realized as inks, slurries, pastes, and now dyes for the development of on-paper, on-plastic, and on-textile microsupercapacitor electrodes. In this work, Ti₃C₂T_x MXene was adopted as an active material for coating meters of commercial natural and synthetic yarns, enabling the production of knitted planar microsupercapacitors. The impact on electrochemical performance of knit structure and geometry was systematically studied in an attempt to produce energy storing textiles with power and energy densities that can be used for practical applications. The resulting energy storing textiles demonstrate high capacitance, up to 707 mF cm⁻² and 519 mF cm⁻² at 2 mV s⁻¹ in 1 M H₃PO₄ and PVA-H₃PO₄ gel electrolyte, respectively, and excellent cycling stability over 10,000 cycles. This work represents an important step towards the mass production of MXene-based conductive yarns and 3D knitted energy storage devices and demonstrates how knit structure plays a significant role on device performance.     

Cathode Architectures for Rechargeable Ion Batteries: Progress and Perspectives

To satisfy the rising demand for energy, battery electrodes with higher loading, to simultaneously increase areal energy and power, are necessary. Nevertheless, in conventional thin-film electrodes, there is mutual exclusion between energy (capacity) and power. Increasing the thickness of electrodes alone is not feasible since this will lead to reductions in ion-diffusion efficiency, as well as electrode flexibility. To address this difficulty, 3D electrode architectures, especially cathode architectures, are proposed to pave a new path for the design and optimization of battery devices. Recent research suggests that 3D cathode architectures may optimize the configuration and engineering processes of battery technologies. Herein, the state-of-the-art progress of cathode architectures in various rechargeable-ion-battery technologies is summarized. Emphasis is placed on the different architecture strategies, areal loading, and mechanical understanding of 3D electrodes. Upcoming research directions are further outlined for future development in this field.     

Intertwined Nanocarbon and Manganese Oxide Hybrid Foam for High‐Energy Supercapacitors

Rapid charging and discharging supercapacitors are promising alternative energy storage systems for applications such as portable electronics and electric vehicles. Integration of pseudocapacitive metal oxides with single‐structured materials has received a lot of attention recently due to their superior electrochemical performance. In order to realize high energy‐density supercapacitors, a simple and scalable method is developed to fabricate a graphene/MWNT/MnO₂ nanowire (GMM) hybrid nanostructured foam, via a two‐step process. The 3D few‐layer graphene/MWNT (GM) architecture is grown on foamed metal foils (nickel foam) via ambient pressure chemical vapor deposition. Hydrothermally synthesized α‐MnO₂ nanowires are conformally coated onto the GM foam by a simple bath deposition. The as‐prepared hierarchical GMM foam yields a monographical graphene foam conformally covered with an intertwined, densely packed CNT/MnO₂ nanowire nanocomposite network. Symmetrical electrochemical capacitors (ECs) based on GMM foam electrodes show an extended operational voltage window of 1.6 V in aqueous electrolyte. A superior energy density of 391.7 Wh kg^?1 is obtained for the supercapacitor based on the GMM foam, which is much higher than ECs based on GM foam only (39.72 Wh kg^?1). A high specific capacitance (1108.79 F g^?1) and power density (799.84 kW kg^?1) are also achieved. Moreover, the great capacitance retention (97.94%) after 13 000 charge–discharge cycles and high current handability demonstrate the high stability of the electrodes of the supercapacitor. These excellent performances enable the innovative 3D hierarchical GMM foam to serve as EC electrodes, resulting in energy‐storage devices with high stability and power density in neutral aqueous electrolyte.     

The application of graphene as electrodes in electrical and optical devices

Graphene is a promising next-generation conducting material with the potential to replace traditional electrode materials such as indium tin oxide in electrical and optical devices. It combines several advantageous characteristics including low sheet resistance, high optical transparency and excellent mechanical properties. Recent research has coincided with increased interest in the application of graphene as an electrode material in transistors, light-emitting diodes, solar cells and flexible devices. However, for more practical applications, the performance of devices should be further improved by the engineering of graphene films, such as through their synthesis, transfer and doping. This article reviews several applications of graphene films as electrodes in electrical and optical devices and discusses the essential requirements for applications of graphene films as electrodes.     

From Flatland to Spaceland: Higher Dimensional Patterning with Two‐Dimensional Materials

The creation of three‐dimensional (3D) structures from two‐dimensional (2D) nanomaterial building blocks enables novel chemical, mechanical or physical functionalities that cannot be realized with planar thin films or in bulk materials. Here, we review the use of emerging 2D materials to create complex out‐of‐plane surface topographies and 3D material architectures. We focus on recent approaches that yield periodic textures or patterns, and present four techniques as case studies: (i) wrinkling and crumpling of planar sheets, (ii) encapsulation by crumpled nanosheet shells, (iii) origami folding and kirigami cutting to create programmed curvature, and (iv) 3D printing of 2D material suspensions. Work to date in this field has primarily used graphene and graphene oxide as the 2D building blocks, and we consider how these unconventional approaches may be extended to alternative 2D materials and their heterostructures. Taken together, these emerging patterning and texturing techniques represent an intriguing alternative to conventional materials synthesis and processing methods, and are expected to contribute to the development of new composites, stretchable electronics, energy storage devices, chemical barriers, and biomaterials.