Scalable 3D Self-Assembly of MXene Films for Flexible Sandwich and Microsized Supercapacitors

The self-assembly of large-area MXene films is the main step to realize their applications in various energy storage devices. However, the scalable self-assembly of flexible thin MXene films with high conductivity as well as excellent mechanical and electrochemical properties is still a challenge. Herein, a synchronous reduction and self-assembly strategy to fabricate flexible MXene films is developed, where MXene films are synchronously reduced and self-assembled on the Zn foil surface. Furthermore, the self-assembly of MXene films can be scaled up by controlling the area of Zn substrates. By adjusting the patterns of Zn substrates, the interdigital MXene patterns can also be obtained via a selectively reducing/assembling process. The resultant MXene films demonstrate high electrical conductivity, large specific surface area, and excellent mechanical properties. Thus they can serve as the electrodes of flexible supercapacitor devices directly. As a proof of concept, flexible sandwich and microsized supercapacitors are designed based on the above MXene film electrodes. Both sandwich and microsized supercapacitors display stable electrochemical performance under various bending states. This study provides a route to achieve large-area MXene-based films or microsized structures for applications in the field of energy storage.     

Oxygen‐Deficient Bismuth Oxide/Graphene of Ultrahigh Capacitance as Advanced Flexible Anode for Asymmetric Supercapacitors

Oxygen‐deficient bismuth oxide (r‐Bi₂O₃)/graphene (GN) is designed, fabricated, and demonstrated via a facile solvothermal and subsequent solution reduction method. The ultrafine network bacterial cellulose (BC) as substrate for r‐Bi₂O₃/GN exhibits high flexibility, remarkable tensile strength (55.1 MPa), and large mass loading of 9.8 mg cm^?2. The flexible r‐Bi₂O₃/GN/BC anode delivers appreciable areal capacitance (6675 mF cm^?2 at 1 mA cm^?2) coupled with good rate capability (3750 mF cm^?2 at 50 mA cm^?2). In addition, oxygen vacancies have great influence on the capacitive performance of Bi₂O₃, delivering significantly improved capacitive values than the untreated Bi₂O₃ flexible electrode, and ultrahigh gravimetric capacitance of 1137 F g^?1 (based on the mass of r‐Bi₂O₃) can be obtained, achieving 83% of the theoretical value (1370 F g^?1). Flexible asymmetric supercapacitor is fabricated with r‐Bi₂O₃/GN/BC and Co₃O₄/GN/BC paper as the negative and positive electrodes, respectively. The operation voltage is expanded to 1.6 V, revealing a maximum areal energy density of 0.449 mWh cm^?2 (7.74 mWh cm^?3) and an areal power density of 40 mW cm^?2 (690 mW cm^?3). Therefore, this flexible anode with excellent electrochemical performance and high mechanical properties shows great potential in the field of flexible energy storage devices.     

Large‐Area Flexible Core–Shell Graphene/Porous Carbon Woven Fabric Films for Fiber Supercapacitor Electrodes

New porous materials are of great importance in many technological applications. Here, the direct synthesis of multi‐layer graphene and porous carbon woven composite films by chemical vapor deposition on Ni gauze templates is reported. The composite films integrate the dual advantages of graphene and porous carbon, having not only the excellent electrical properties and flexibility of graphene but also the porous characteristics of amorphous carbon. The multi‐layer graphene/porous carbon woven fabric film creates a new platform for a variety of applications, such as fiber supercapacitors. The designed composite film has a capacitance of 20 μF/cm², which is close to the theoretical value and a device areal capacitance of 44 mF/cm².     

Superior Micro‐Supercapacitors Based on Graphene Quantum Dots

Graphene quantum dots (GQDs) have attracted tremendous research interest due to the unique properties associated with both graphene and quantum dots. Here, a new application of GQDs as ideal electrode materials for supercapacitors is reported. To this end, a GQDs//GQDs symmetric micro‐supercapacitor is prepared using a simple electro‐deposition approach, and its electrochemical properties in aqueous electrolyte and ionic liquid electrolyte are systematically investigated. The results show that the as‐made GQDs micro‐supercapacitor has superior rate capability up to 1000 V s^?1, excellent power response with very short relaxation time constant (τ₀ = 103.6 μs in aqueous electrolyte and τ₀ = 53.8 μs in ionic liquid electrolyte), and excellent cycle stability. Additionally, another GQDs//MnO₂ asymmetric supercapacitor is also built using MnO₂ nanoneedles as the positive electrode and GQDs as the negative electrode in aqueous electrolyte. Its specific capacitance and energy density are both two times higher than those of GQDs//GQDs symmetric micro‐supercapacitor in the same electrolyte. The results presented here may pave the way for a new promising application of GQDs in micropower suppliers and microenergy storage devices.     

Flexible MXene/Graphene Films for Ultrafast Supercapacitors with Outstanding Volumetric Capacitance

A strategy to prepare flexible and conductive MXene/graphene (reduced graphene oxide, rGO) supercapacitor electrodes by using electrostatic self‐assembly between positively charged rGO modified with poly(diallyldimethylammonium chloride) and negatively charged titanium carbide MXene nanosheets is presented. After electrostatic assembly, rGO nanosheets are inserted in‐between MXene layers. As a result, the self‐restacking of MXene nanosheets is effectively prevented, leading to a considerably increased interlayer spacing. Accelerated diffusion of electrolyte ions enables more electroactive sites to become accessible. The freestanding MXene/rGO‐5 wt% electrode displays a volumetric capacitance of 1040 F cm^?3 at a scan rate of 2 mV s^?1 , an impressive rate capability with 61% capacitance retention at 1 V s^?1 and long cycle life. Moreover, the fabricated binder‐free symmetric supercapacitor shows an ultrahigh volumetric energy density of 32.6 Wh L^?1, which is among the highest values reported for carbon and MXene based materials in aqueous electrolytes. This work provides fundamental insight into the effect of interlayer spacing on the electrochemical performance of 2D hybrid materials and sheds light on the design of next‐generation flexible, portable and highly integrated supercapacitors with high volumetric and rate performances.     

High Electroactive Material Loading on a Carbon Nanotube@3D Graphene Aerogel for High‐Performance Flexible All‐Solid‐State Asymmetric Supercapacitors

Freestanding carbon‐based hybrids, specifically carbon nanotube@3D graphene (CNTs@3DG) hybrid, are of great interest in electrochemical energy storage. However, the large holes (about 400 µm) in the commonly used 3D graphene foams (3DGF) constitute as high as 90% of the electrode volume, resulting in a very low loading of electroactive materials that is electrically connected to the carbon, which makes it difficult for flexible supercapacitors to achieve high gravimetric and volumetric energy density. Here, a hierarchically porous carbon hybrid is fabricated by growing 1D CNTs on 3D graphene aerogel (CNTs@3DGA) using a facile one‐step chemical vapor deposition process. In this architecture, the 3DGA with ample interconnected micrometer‐sized pores (about 5 µm) dramatically enhances mass loading of electroactive materials comparing with 3DGF. An optimized all‐solid‐state asymmetric supercapacitor (AASC) based on MnO₂@CNTs@3DGA and Ppy@CNTs@3DGA electrodes exhibits high volumetric energy density of 3.85 mW h cm^?3 and superior long‐term cycle stability with 84.6% retention after 20 000 cycles, which are among the best reported for AASCs with both electrodes made of pseudocapacitive electroactive materials.     

Fabrication of Highly Flexible,Scalable, and High‐Performance Supercapacitors Using Polyaniline/Reduced Graphene Oxide Film with Enhanced Electrical Conductivity and Crystallinity

With developments in technology, tremendous effort has been devoted to produce flexible, scalable, and high‐performance supercapacitor electrode materials. This report presents a novel fabrication method of highly flexible and scalable electrode material for high‐performance supercapacitors using solution‐processed polyaniline (PANI)/reduced graphene oxide (RGO) hybrid film. SEM, TEM, Raman, and XPS analyses show that the PANI/RGO film is successfully synthesized. The percentages of the PANI component in the film are controlled (88, 76, and 60%), and the maximum electrical conductivity (906 S cm^?1) is observed at the PANI percentage of 76%. Notably, electrical conductivity of the PANI/RGO film (906 S cm^?1) is larger than both PANI (580 S cm^?1) and RGO (46.5 S cm^?1) components. XRD analysis demonstrates that the strong π–π interaction between the RGO and the PANI cause more compact packing of the PANI chains by inducing more fully expanded conformation of the PANI chains in the solution, leading to increase in the electrical conductivity and crystallinity of the film. The PANI/RGO film also displays diverse advantages as a scalable and flexible electrode material (e.g., controllable size and great flexibility). During the electrochemical tests, the film exhibits high capacitance of 431 F g^?1 with enhanced cycling stability.     

Asymmetric Supercapacitors Based on Graphene/MnO2 Nanospheres and Graphene/MoO3 Nanosheets with High Energy Density

Asymmetric supercapacitors with high energy density are fabricated using a self‐assembled reduced graphene oxide (RGO)/MnO₂ (GrMnO₂) composite as a positive electrode and a RGO/MoO₃ (GrMoO₃) composite as a negative electrode in safe aqueous Na₂SO₄ electrolyte. The operation voltage is maximized by choosing two metal oxides with the largest work function difference. Because of the synergistic effects of highly conductive graphene and highly pseudocapacitive metal oxides, the hybrid nanostructure electrodes exhibit better charge transport and cycling stability. The operation voltage is expanded to 2.0 V in spite of the use of aqueous electrolyte, revealing a high energy density of 42.6 Wh kg^?1 at a power density of 276 W kg^?1 and a maximum specific capacitance of 307 F g^?1, consequently giving rise to an excellent Ragone plot. In addition, the GrMnO₂//GrMoO₃ supercapacitor exhibits improved capacitance with cycling up to 1000 cycles, which is explained by the development of micropore structures during the repetition of ion transfer. This strategy for the choice of metal oxides provides a promising route for next‐generation supercapacitors with high energy and high power densities.     

Hierarchical Ni–Co Hydroxide Petals on Mechanically Robust Graphene Petal Foam for High‐Energy Asymmetric Supercapacitors

A hierarchical structure consisting of Ni–Co hydroxide nanopetals (NCHPs) grown on a thin free‐standing graphene petal foam (GPF) has been designed and fabricated by a two‐step process for pseudocapacitive electrode applications. The mechanical behavior of GPFs has been, for the first time to our knowledge, quantitatively measured from in situ scanning electron microscope characterization of the petal foams during in‐plane compression and bending processes. The Young's modulus of a typical GPF is 3.42 GPa, indicating its outstanding mechanical robustness as a nanotemplate. The GPF/NCHP electrodes exhibit volumetric capacitances as high as 765 F cm^?3, equivalent to an areal capacitance of 15.3 F cm^?2 and high rate capability. To assess practical functionality, two‐terminal asymmetric solid‐state supercapacitors with 3D GPF/NCHPs as positive electrodes are fabricated and shown to exhibit outstanding energy and power densities, with maximum average energy density of ≈10 mWh cm^?3 and maximum power density of ≈3 W cm^?3, high rate capability (a capacitance retention of ≈60% at 100 mA cm^?2), and excellent long‐term cyclic stability (full capacitance retention over 15 000 cycles).     

Microporous Polypyrrole‐Coated Graphene Foam for High‐Performance Multifunctional Sensors and Flexible Supercapacitors

This study reports on the fabrication of pressure/temperature/strain sensors and all‐solid‐state flexible supercapacitors using only polydimethylsiloxane coated microporous polypyrrole/graphene foam composite (PDMS/PPy/GF) as a common material. A dual‐mode sensor is designed with PDMS/PPy/GF, which measures pressure and temperature with the changes of current and voltage, respectively, without interference to each other. The fabricated dual‐mode sensor shows high sensitivity, fast response/recovery, and high durability during 10 000 cycles of pressure loading. The pressure is estimated using the thermoelectric voltage induced by simultaneous increase in temperature caused by a finger touch on the sensor. Additionally, a resistor‐type strain sensor fabricated using the same PDMS/PPy/GF could detect the strain up to 50%. Flexible, high performance supercapacitor used as a power supply is fabricated with electrodes of PPy/GF for its high surface area and pseudocapacitance. Furthermore, an integrated system of such fabricated multifunctional sensors and a supercapacitor on a skin‐attachable flexible substrate using liquid–metal interconnections operates well, whereas sensors are driven by the power of the supercapacitor. This study clearly demonstrates that the appropriate choice of a single functional material enables fabrication of active multifunctional sensors for pressure, temperature, and strain, as well as the supercapacitor, that could be used in wirelessly powered wearable devices.     

Inkjet Printed Large‐Area Flexible Few‐Layer Graphene Thermoelectrics

Graphene‐based organic nanocomposites have ascended as promising candidates for thermoelectric energy conversion. In order to adopt existing scalable printing methods for developing thermostable graphene‐based thermoelectric devices, optimization of both the material ink and the thermoelectric properties of the resulting films are required. Here, inkjet‐printed large‐area flexible graphene thin films with outstanding thermoelectric properties are reported. The thermal and electronic transport properties of the films reveal the so‐called phonon‐glass electron‐crystal character (i.e., electrical transport behavior akin to that of few‐layer graphene flakes with quenched thermal transport arising from the disordered nanoporous structure). As a result, the all‐graphene films show a room‐temperature thermoelectric power factor of 18.7 µW m^?1 K^?2, representing over a threefold improvement to previous solution‐processed all‐graphene structures. The demonstration of inkjet‐printed thermoelectric devices underscores the potential for future flexible, scalable, and low‐cost thermoelectric applications, such as harvesting energy from body heat in wearable applications.     

A Large‐Area,Flexible, and Flame‐Retardant Graphene Paper

Similar to the paper‐making process, the efficient flame retardant graphene paper is conveniently obtained by using graphene oxide (GO) and hexachlorocyclotriphosphazene (HCCP) aqueous pulp. The “paper pulp” can also conceivably be used as ink to make other hydrophilic films become flame retardant paper. Further, the as‐prepared reduced GO‐HCCP paper (RGO‐HCCP paper), compared with GO‐HCCP paper, can maintain its intact structure for a longer time in an ethanol flame. As a consequence of these preparation methods, the bearing temperature of the as‐prepared graphene papers shows a significant increase.     

Facile Synthesis of Hematite Quantum‐Dot/Functionalized Graphene‐Sheet Composites as Advanced Anode Materials for Asymmetric Supercapacitors

For building high‐energy density asymmetric supercapacitors, developing anode materials with large specific capacitance remains a great challenge. Although Fe₂O₃ has been considered as a promising anode material for asymmetric supercapacitors, the specific capacitance of the Fe₂O₃‐based anodes is still low and cannot match that of cathodes in the full cells. In this work, a composite material with well dispersed Fe₂O₃ quantum dots (QDs, ≈2 nm) decorated on functionalized graphene‐sheets (FGS) is prepared by a facile and scalable method. The Fe₂O₃ QDs/FGS composites exhibit a large specific capacitance up to 347 F g^?1 in 1 m Na₂SO₄ between –1 and 0 V versus Ag/AgCl. An asymmetric supercapacitor operating at 2 V is fabricated using Fe₂O₃/FGS as anode and MnO₂/FGS as cathode in 1 m Na₂SO₄ aqueous electrolyte. The Fe₂O₃/FGS//MnO₂/FGS asymmetric supercapacitor shows a high energy density of 50.7 Wh kg^?1 at a power density of 100 W kg^?1 as well as excellent cycling stability and power capability. The facile synthesis method and superior supercapacitive performance of the Fe₂O₃ QDs/FGS composites make them promising as anode materials for high‐performance asymmetric supercapacitors.     

Multilayer‐Folded Graphene Ribbon Film with Ultrahigh Areal Capacitance and High Rate Performance for Compressible Supercapacitors

Limited by 2D geometric morphology and low bulk packing density, developing graphene‐based flexible/compressible supercapacitors with high specific capacitances (gravimetric/volumetric/areal), especially at high rates, is an outstanding challenge. Here, a strategy for the synthesis of free‐standing graphene ribbon films (GRFs) for high‐performance flexible and compressible supercapacitors through blade‐coating of interconnected graphene oxide ribbons and a subsequent thermal treatment process is reported. With an ultrahigh mass loading of 21 mg cm^?2, large ion‐accessible surface area, efficient electron and ion transport pathways as well as high packing density, the compressed multilayer‐folded GRF films (F‐GRF) exhibit ultrahigh areal capacitance of 6.7 F cm^?2 at 5 mA cm^?2, high gravimetric/volumetric capacitances (318 F g^?1, 293 F cm^?3), and high rate performance (3.9 F cm^?2 at 105 mA cm^?2), as well as excellent cycling stability (109% of capacitance retention after 40 000 cycles). Furthermore, the assembled F‐GRF symmetric supercapacitor with compressible and flexible characteristics, can deliver an ultrahigh areal energy density of 0.52 mWh cm^?2 in aqueous electrolyte, almost two times higher than the values obtained from symmetric supercapacitors with comparable dimensions.     

All‐Solid‐State Asymmetric Supercapacitors with Metal Selenides Electrodes and Ionic Conductive Composites Electrolytes

All‐solid‐state flexible asymmetric supercapacitors (ASCs) are developed by utilization of graphene nanoribbon (GNR)/Co_0.85Se composites as the positive electrode, GNR/Bi₂Se₃ composites as the negative electrode, and polymer‐grafted‐graphene oxide membranes as solid‐state electrolytes. Both GNR/Co_0.85Se and GNR/Bi₂Se₃ composite electrodes are developed by a facile one‐step hydrothermal growth method from graphene oxide nanoribbons as the nucleation framework. The GNR/Co_0.85Se composite electrode exhibits a specific capacity of 76.4 mAh g^?1 at a current density of 1 A g^?1 and the GNR/Bi₂Se₃ composite electrode exhibits a specific capacity of 100.2 mAh g^?1 at a current density of 0.5 A g^?1. Moreover, the stretchable membrane solid‐state electrolytes exhibit superior ionic conductivity of 108.7 mS cm^?1. As a result, the flexible ASCs demonstrate an operating voltage of 1.6 V, an energy density of 30.9 Wh kg^?1 at the power density of 559 W kg^?1, and excellent cycling stability with 89% capacitance retention after 5000 cycles. All these results demonstrate that this study provides a simple, scalable, and efficient approach to fabricate high performance flexible all‐solid‐state ASCs for energy storage.     

Micro‐Supercapacitors Based on Interdigital Electrodes of Reduced Graphene Oxide and Carbon Nanotube Composites with Ultrahigh Power Handling Performance

A novel method for fabricating micro‐patterned interdigitated electrodes based on reduced graphene oxide (rGO) and carbon nanotube (CNT) composites for ultra‐high power handling micro‐supercapacitor application is reported. The binder‐free microelectrodes were developed by combining electrostatic spray deposition (ESD) and photolithography lift‐off methods. Without typically used thermal or chemical reduction, GO sheets are readily reduced to rGO during the ESD deposition. Electrochemical measurements show that the in‐plane interdigital design of the microelectrodes is effective in increasing accessibility of electrolyte ions in‐between stacked rGO sheets through an electro‐activation process. Addition of CNTs results in reduced restacking of rGO sheets and improved energy and power density. Cyclic voltammetry (CV) measurements show that the specific capacitance of the micro‐supercapacitor based on rGO–CNT composites is 6.1 mF cm^?2 at 0.01 V s^?1. At a very high scan rate of 50 V s^?1, a specific capacitance of 2.8 mF cm^?2 (stack capacitance of 3.1 F cm^?3) is recorded, which is an unprecedented performance for supercapacitors. The addition of CNT, electrolyte‐accessible and binder‐free microelectrodes, as well as an interdigitated in‐plane design result in a high‐frequency response of the micro‐supercapacitors with resistive‐capacitive time constants as low as 4.8 ms. These characteristics suggest that interdigitated rGO–CNT composite electrodes are promising for on‐chip energy storage application with high power demands.     

Polyoxometalate‐coupled Graphene via Polymeric Ionic Liquid Linker for Supercapacitors

The integration of electrical double‐layer capacitive and pseudocapacitive materials into novel hybrid materials is crucial to realize supercapacitors with high energy and power densities. Here, high levels of energy and power densities are demonstrated in supercapacitors based on a new type of nanohybrid electrode consisting of polyoxometalate (POM)‐coupled graphene in which a polymeric ionic liquid (henceforth simply PIL) serves as an interfacial linker. The adoption of PIL in the construction of nanohybrids enables a uniform distribution of discrete POM molecules along with a large surface area of graphene sheets. When testing electrochemical characteristics under a two‐electrode system, as‐prepared supercapacitors exhibit a high specific capacitance (408 F g^?1 at 0.5 A g^?1), rapid rate capability (92% retention at 10 A g^?1), a long cycling life (98% retention during 2000 cycles), and high energy (56 Wh kg^?1) and power (52 kW kg^?1) densities. First‐principles calculations and impedance spectroscopy analysis reveal that the PILs enhance the redox reactions of POMs by providing efficient ion transfer channels and facilitating the charge transfer in the nanohybrids.     

An Ultrafast,High-Loading,and Durable Poly(p-aminoazobenzene)/Reduced Graphene Oxide Composite Electrode for Supercapacitors

Although challenging, the fabricated supercapacitor electrodes with excellent rate capability, long cycling stability, and high mass-loading are crucial for practical applications. Herein, a novel 3D porous poly(p-aminoazobenzene)/reduced graphene oxide hydrogel is designed and prepared as an ultrafast, high-loading, and durable pseudocapacitive electrode through a facile two-step self-assembly approach. Owing to abundant stable redox-active sites, fast electrolyte diffusion, and efficient charge conduction, the PRH electrode (5 mg cm⁻²) shows a high specific capacitance (701 F g⁻¹ at 2 A g⁻¹) and ultrafast rate (97% capacitance retention at 100 A g⁻¹). Furthermore, even with a mass-loading of 10 mg cm⁻², the electrode still exhibits comparable high performance and excellent long-term cycling life (only 6.7% capacitance loss after 10 000 cycles). This work demonstrates novel polyaniline analog composites for constructing novel electrodes, promising to open an avenue toward practical applications.     

Self‐Healing Reduced Graphene Oxide Films by Supersonic Kinetic Spraying

The industrial scale application of graphene and other functional materials in the field of electronics has been limited by inherent defects, and the lack of simple deposition methods. A simple spray deposition method is developed that uses a supersonic air jet for a commercially available reduced graphene oxide (r‐GO) suspension. The r‐GO flakes are used as received, which are pre‐annealed and pre‐hydrazine‐treated, and do not undergo any post‐treatment. A part of the considerable kinetic energy of the r‐GO flakes entrained by the supersonic jet is used in stretching the flakes upon impact with the substrate. The resulting “frozen elastic strains” heal the defects (topological defects, namely Stone‐Wales defect and C₂ vacancies) in the r‐GO flakes, which is reflected in the reduced ratio of the intensities of the D and G bands in the deposited film. The defects can also be regenerated by annealing.