Highly Flexible,Freestanding Supercapacitor Electrode with Enhanced Performance Obtained by Hybridizing Polypyrrole Chains with MXene

Though polypyrrole (PPy) is widely used in flexible supercapacitors owing to its high electrochemical activity and intrinsic flexibility, limited capacitance and cycling stability of freestanding PPy films greatly reduce their practicality in real‐world applications. Herein, we report a new approach to enhance PPy's capacitance and cycling stability by forming a freestanding and conductive hybrid film through intercalating PPy into layered Ti₃C₂ (l‐Ti₃C₂, a MXene material). The capacitance increases from 150 (300) to 203 mF cm^?2 (406 F cm^?3). Moreover, almost 100% capacitance retention is achieved, even after 20 000 charging/discharging cycles. The analyses reveal that l‐Ti₃C₂ effectively prevents dense PPy stacking, benefiting the electrolyte infiltration. Furthermore, strong bonds, formed between the PPy backbones and surfaces of l‐Ti₃C₂, not only ensure good conductivity and provide precise pathways for charge‐carrier transport but also improve the structural stability of PPy backbones. The freestanding PPy/l‐Ti₃C₂ film is further used to fabricate an ultra‐thin all‐solid‐state supercapacitor, which shows an excellent capacitance (35 mF cm^?2), stable performance at any bending state and during 10 000 charging/discharging cycles. This novel strategy provides a new way to design conductive polymer‐based freestanding flexible electrodes with greatly improved electrochemical performances.     

Antimonene: A Novel 2D Nanomaterial for Supercapacitor Applications

In pursuing higher energy density, without compromising the power density of supercapacitor platforms, the application of an advanced 2D nanomaterial is utilized to maximize performance. Antimonene, for the first time, is characterized as a material for applications in energy storage, being applied as an electrode material as the basis of a supercapacitor. Antimonene is shown to significantly improve the energy storage capabilities of a carbon electrode in both cyclic voltammetry and galvanostatic charging. Antimonene demonstrates remarkable performance with a capacitance of 1578 F g^?1, with a high charging current density of 14 A g^?1. Hence, antimonene is shown to be a highly promising material for energy storage applications. The system also demonstrates a highly competitive energy and power densities of 20 mW h kg^?1 and 4.8 kW kg^?1_, respectively. In addition to the excellent charge storing abilities, antimonene shows good cycling capabilities.     

Metallic Layered Polyester Fabric Enabled Nickel Selenide Nanostructures as Highly Conductive and Binderless Electrode with Superior Energy Storage Performance

Highly flexible and conductive fabric (CF)‐supported cauliflower‐like nickel selenide nanostructures (Ni₃Se₂ NSs) are facilely synthesized by a single‐step chronoamperometry voltage‐assisted electrochemical deposition (ECD) method and used as a positive electrode in supercapacitors (SCs). The CF substrate composed of multi‐layered metallic films on the surface of polyester fibers enables to provide high electrical conductivity as a working electrode in ECD process. Owing to good electrical conductivity, high porosity and intertwined fibrous framework of CF, cauliflower‐like Ni₃Se₂ NSs are densely integrated onto the entire surface of CF (Ni₃Se₂ NSs@CF) substrate with reliable adhesion by applying a chronoamperometry voltage of ?1.0 V for 240 s. The electrochemical performance of the synthesized cauliflower‐like Ni₃Se₂ NSs@CF electrode exhibits a maximum specific capacity (C _SC) of 119.6 mA h g^?1 at a discharge current density of 2 A g^?1 in aqueous 1 m KOH electrolyte solution. Remarkably, the specific capacity of the same electrode is greatly enhanced by introducing a small quantity of redox‐additive electrolyte into the aqueous KOH solution, indicating the C _SC≈251.82 mA h g^?1 at 2 A g^?1 with good capacity retention. Furthermore, the assembled textile‐based asymmetric SCs achieve remarkable electrochemical performance such as higher energy and power densities, which are able to light up different colored light‐emitting diodes.     

Highly Defective Layered Double Perovskite Oxide for Efficient Energy Storage via Reversible Pseudocapacitive Oxygen‐Anion Intercalation

The use of perovskite materials as anion‐based intercalation pseudocapacitor electrodes has received significant attention in recent years. Notably, these materials, characterized by high oxygen vacancy concentrations, do not require high surface areas to achieve a high energy storage capacity as a result of the bulk intercalation mechanism. This study reports that reduced PrBaMn₂O_6–δ (r‐PBM), possessing a layered double perovskite structure, exhibits ultrahigh capacitance and functions as an excellent oxygen anion‐intercalation‐type electrode material for supercapacitors. Formation of the layered double perovskite structure, as facilitated by hydrogen treatment, is shown to significantly enhance the capacitance, with the resulting r‐PBM material demonstrating a very high gravimetric capacitance of 1034.8 F g^?1 and an excellent volumetric capacitance of ≈2535.3 F cm^?3 at a current density of 1 A g^?1. The resultant formation of a double perovskite crystal oxide with a specific layered structure leads to the r‐PBM with a substantially higher oxygen diffusion rate and oxygen vacancy concentration. These superior characteristics show immense promise for their application as oxygen anion‐intercalation‐type electrodes in pseudocapacitors.     

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.     

Highly Stretchable Separator Membrane for Deformable Energy‐Storage Devices

With the emergence of stretchable electronic devices, there is growing interest in the development of deformable power accessories that can power them. To date, various approaches have been reported for replacing rigid components of typical batteries with elastic materials. Little attention, however, has been paid to stretchable separator membranes that can not only prevent internal short circuit but also provide an ionic conducting pathway between electrodes under extreme physical deformation. Herein, a poly(styrene‐b‐butadiene‐b‐styrene) (SBS) block copolymer–based stretchable separator membrane is fabricated by the nonsolvent‐induced phase separation (NIPS). The diversity of mechanical properties and porous structures can be obtained by using different polymer concentrations and tuning the affinity among major components of NIPS. The stretchable separator membrane exhibits a high stretchability of around 270% strain and porous structure having porosity of 61%. Thus, its potential application as a stretchable separator membrane for deformable energy devices is demonstrated by applying to organic/aqueous electrolyte–based rechargeable lithium‐ion batteries. As a result, these batteries manifest good cycle life and stable capacity retention even under a stretching condition of 100%, without compromising the battery's performance.     

3D Printing of Tunable Energy Storage Devices with Both High Areal and Volumetric Energy Densities

Developing advanced supercapacitors with both high areal and volumetric energy densities remains challenging. In this work, self‐supported, compact carbon composite electrodes are designed with tunable thickness using 3D printing technology for high‐energy‐density supercapacitors. The 3D carbon composite electrodes are composed of the closely stacked and aligned active carbon/carbon nanotube/reduced graphene oxide (AC/CNT/rGO) composite filaments. The AC microparticles are uniformly embedded in the wrinkled CNT/rGO conductive networks without using polymer binders, which contributes to the formation of abundant open and hierarchical pores. The 3D‐printed ultrathick AC/CNT/rGO composite electrode (ten layers) features high areal and volumetric mass loadings of 56.9 mg cm^?2 and 256.3 mg cm^?3, respectively. The symmetric cell assembled with the 3D‐printed thin GO separator and ultrathick AC/CNT/rGO electrodes can possess both high areal and volumetric capacitances of 4.56 F cm^?2 and 10.28 F cm^?3, respectively. Correspondingly, the assembled ultrathick and compact symmetric cell achieves high areal and volumetric energy densities of 0.63 mWh cm^?2 and 1.43 mWh cm^?3, respectively. The all‐component extrusion‐based 3D printing offers a promising strategy for the fabrication of multiscale and multidimensional structures of various high‐energy‐density electrochemical energy storage devices.     

MoS2‐Based All‐Purpose Fibrous Electrode and Self‐Powering Energy Fiber for Efficient Energy Harvesting and Storage

Here an all‐purpose fibrous electrode based on MoS₂ is demonstrated, which can be employed for versatile energy harvesting and storage applications. In this coaxial electrode, ultrathin MoS₂ nanofilms are grown on TiO₂ nanoparticles coated carbon fiber. The high electrochemical activity of MoS₂ and good conductivity of carbon fiber synergistically lead to the remarkable performances of this novel composite electrode in fibrous dye‐sensitized solar cells (showing a record‐breaking conversion efficiency of 9.5%) and high‐capacity fibrous supercapacitors. Furthermore, a self‐powering energy fiber is fabricated by combining a fibrous dye‐sensitized solar cell and a fibrous supercapacitor into a single device, showing very fast charging capability (charging in 7 s under AM1.5G solar illumination) and an overall photochemical‐electricity energy conversion efficiency as high as 1.8%. In addition, this wire‐shaped electrode can also be used for fibrous Li‐ion batteries and electrocatalytic hydrogen evolution reactions. These applications indicate that the MoS₂‐based all‐purpose fibrous electrode has great potential for the construction of high‐performance flexible and wearable energy devices.     

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.     

Integration: An Effective Strategy to Develop Multifunctional Energy Storage Devices

Energy storage devices are arousing increasing interest due to their key role in next‐generation electronics. Integration is widely explored as a general and effective strategy aiming at high performances. Recent progress in integrating a variety of functions into electrochemical energy storage devices is carefully described. Through integration at the level of materials: flexible, stretchable, responsive, and self‐healing devices are discussed to highlight the state‐of‐the‐art multi‐functional electronics. Through the integration at the level of devices, the incorporation of photovoltaic and piezoelectric devices is detailed to reflect the advances in self‐powering electronics. Integrated energy storage devices are presented for wearable applications to indicate a new growth direction. The main challenges and important directions are summarized to offer some useful clues for future development.     

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.     

Transition Metal Sulfides Based on Graphene for Electrochemical Energy Storage

Transition metal sulfides, as an important class of inorganics, can be used as excellent electrode materials for various types of electrochemical energy storage, such as lithium‐ion batteries, sodium‐ion batteries, supercapacitors, and others. Recent works have identified that mixing graphene or graphene derivatives with transition metal sulfides can result in novel composites with better electrochemical performance. This review summarizes the latest advances in transition metal sulfide composites with graphene or graphene derivatives. The synthetic strategies and morphologies of these composites are introduced. The authors then discuss their applications in lithium‐ion batteries, sodium‐ion batteries, and supercapacitors. Finally, the authors give their personal viewpoints about the challenges and opportunities for the future development about this direction.     

Fatigue‐Free Aurivillius Phase Ferroelectric Thin Films with Ultrahigh Energy Storage Performance

Dielectric capacitors have become a key enabling technology for electronics and electrical systems. Although great strides have been made in the development of ferroelectric ceramic and thin films for capacitors, much less attention has been given to preventing polarization fatigue, while improving the energy density, of ferroelectrics. Here superior capacitive properties and outstanding stability are reported over 10⁷ charge/discharge cycles and a wide temperature range of ?60 to 200 °C of ferroelectric Aurivillius phase Bi_3.25La_0.75Ti₃O₁₂‐BiFeO₃ (BLT‐BFO), which represents one of the best capacitive performances recorded for the ferroelectric materials. The modification of BLT thin films with BFO overcomes the constraints of ferroelectric Aurivillius compounds and presents an unprecedented combination of the ideal features including improved polarization, reduced ferroelectric hysteresis, and lowered leakage current for high‐energy‐density capacitors. Given the lead‐free and fatigue‐free nature of this Aurivillius phase ferroelectric, this work unveils a new approach towards high‐performance eco‐friendly ferroelectric materials for electrical energy storage applications.     

Metal Sulfide Hollow Nanostructures for Electrochemical Energy Storage

Metal sulfide hollow nanostructures (MSHNs) have received intensive attention as electrode materials for electrical energy storage (EES) systems due to their unique structural features and rich chemistry. Here, we summarize recent research progress in the rational design and synthesis of various metal sulfide hollow micro‐/nanostructures with controlled shape, composition and structural complexity, and their applications to lithium ion batteries (LIBs) and hybrid supercapacitors (HSCs). The current understanding of hollow structure control, including single‐shelled, yolk‐shelled, multi‐shelled MSHNs, and their hybrid micro‐/nanostructures with carbon (amorphous carbon nanocoating, graphene and hollow carbon), is focused on. The importance of proper structural and compositional control on the enhanced electrochemical properties of MSHNs is emphasized. A relationship between structural and compositional engineering with improved electrochemical activity of MSHNs is sought, in order to shed some light on future electrode design trends for next‐generation EES technologies.     

NiS2/FeS Holey Film as Freestanding Electrode for High‐Performance Lithium Battery

In this work, a freestanding NiS₂/FeS holey film (HF) is prepared after electrochemical anodic and chemical vapor deposition treatments. With the combination of good electrical conductivity and holey structure, the NiS₂/FeS HF presents superior electrochemical performance, due to the following reasons: (i) Porous structure of HF provides a large surface area and more active sites/channels/pathways to enhance the ion/mass diffusion. Moreover, the porous structure can reduce the damage from the volumetric expansion. (ii) The as‐prepared electrode combines the current collector (residual NiFe alloy) and active materials (sulfides) together, thus reducing the resistance of the electrode. Additionally, the good conductivity of HF can improve electron transport. (iii) Sulfides are more stable as active materials than sulfur, showing only a small capacity decay while retaining high cyclability performance. This work provides a promising way to develop high energy and stable electrode for Li‐S battery.     

Hydrous RuO2‐Decorated MXene Coordinating with Silver Nanowire Inks Enabling Fully Printed Micro‐Supercapacitors with Extraordinary Volumetric Performance

The fabrication of fully printable, flexible micro‐supercapacitors (MSCs) with high energy and power density remains a significant technological hurdle. To overcome this grand challenge, the 2D material MXene has garnered significant attention for its application, among others, as a printable electrode material for high performing electrochemical energy storage devices. Herein, a facile and in situ process is proposed to homogeneously anchor hydrous ruthenium oxide (RuO₂) nanoparticles on Ti₃C₂T_x MXene nanosheets. The resulting RuO₂@MXene nanosheets can associate with silver nanowires (AgNWs) to serve as a printable electrode with micrometer‐scale resolution for high performing, fully printed MSCs. In this printed nanocomposite electrode, the RuO₂ nanoparticles contribute high pseudocapacitance while preventing the MXene nanosheets from restacking, ensuring an effective ion highway for electrolyte ions. The AgNWs coordinate with the RuO₂@MXene to guarantee the rheological property of the electrode ink, and provide a highly conductive network architecture for rapid charge transport. As a result, MSCs printed from the nanocomposite inks demonstrate volumetric capacitances of 864.2 F cm^?3 at 1 mV s^?1, long‐term cycling performance (90% retention after 10 000 cycles), good rate capability (304.0 F cm^?3 at 2000 mV s^?1), outstanding flexibility, remarkable energy (13.5 mWh cm^?3) and power density (48.5 W cm^?3).