A high-performance, all-textile and spirally wound asymmetric supercapacitors based on core–sheath structured MnO2 nanoribbons and cotton-derived carbon cloth

Nickel cobaltite nanowire arrays grown on nitrogen-doped carbon nanotube fiber fabric for high-performance flexible supercapacitors

Flexible supercapacitors have received considerable attention for their potential application in flexible electronics, but usually suffer from relatively low energy density. Developing flexible electrodes with high capacitance and constructing asymmetric supercapacitors with large potential window has been considered as the most effective approach to achieve high energy density. Here, a flexible electrode with nickel cobaltite (NiCo₂O₄) nanowire arrays on nitrogen (N)-doped carbon nanotube fiber fabric (denoted as CNTFF and NCNTFF, respectively) was designed and fabricated through a facile hydrothermal growth and heat treatment process. The obtained NCNTFF-NiCo₂O₄ delivered a high capacitance of 2430.5 mF cm^-2 at 2 mA cm^-2, a good rate capability of 62.1 % capacitance retention even at 100 mA cm^-2 and a stable cycling performance of 85.2 % capacitance retention after 10,000 cycles. Moreover, the asymmetric supercapacitor constructed with NCNTFF-NiCo₂O₄ as positive electrode and activated CNTFF as negative electrode exhibited a combination of high capacitance (883.6 mF cm^-2 at 2 mA cm^-2), high energy density (241 μW h cm^-2) and high power density (80175.1 μW cm^-2). This device also had a long cycle life after 10,000 cycles and good mechanical flexibility under bending conditions. Our work provides a new perspective on constructing high-performance flexible supercapacitors for flexible electronics.

Modern Developments for Textile-Based Supercapacitors

Smart textiles are transforming the future of wearable technology, and due to that, there has been a great deal of new research looking for alternative energy storage. Supercapacitors offer high discharge rates, flexibility, and long life cycles and can be integrated fully into a textile. Optimization of these new systems includes utilizing electrically conductive materials, employing successful electrostatic charge and/or faradaic responses, and fabricating a textile-based energy storage system without disrupting comfort, washability, and life cycle. This paper examines recent developments in fabrication methods and materials used to create textile supercapacitors and what challenges still remain.

Constructing N-doped and 3D Hierarchical Porous graphene nanofoam by plasma activation for supercapacitor and Zn ion capacitor

Traditional electrode materials still face vital challenges of few active sites, low porosity, complex synthesis process, and low specific capacitance. Herein, N-doped and 3D hierarchical porous graphene nanofoam (N-GNF) is created on carbon fibers (CFs) by employing a facile, fast, and environmentally friendly strategy of N₂ plasma activation. After an appropriated N₂ plasma activation, the graphene nanosheets (GNSs) synthesized by Ar/CH₄ plasma deposition transform into N-GNF successfully. N doping donates rich active sites and increases the hydrophilia, while hierarchical nanoarchitecture exposes an enlarged effective contact area at the interface between electrode and electrolyte and affords sufficient space for accommodating more electrolytes. The as-assembled flexible N-GNF@CFs//Zn NSs@CFs Zn ion capacitor delivered a high energy density of 105.2 Wh kg^-1 at 378.6 W kg^-1 and initial capacity retention of 87.9% at the current of 2 A g^-1 after a long cycle of 10,000.

Smart Electronic Textile-Based Wearable Supercapacitors

Electronic textiles (e-textiles) have drawn significant attention from the scientific and engineering community as lightweight and comfortable next-generation wearable devices due to their ability to interface with the human body, and continuously monitor, collect, and communicate various physiological parameters. However, one of the major challenges for the commercialization and further growth of e-textiles is the lack of compatible power supply units. Thin and flexible supercapacitors (SCs), among various energy storage systems, are gaining consideration due to their salient features including excellent lifetime, lightweight, and high-power density. Textile-based SCs are thus an exciting energy storage solution to power smart gadgets integrated into clothing. Here, materials, fabrications, and characterization strategies for textile-based SCs are reviewed. The recent progress of textile-based SCs is then summarized in terms of their electrochemical performances, followed by the discussion on key parameters for their wearable electronics applications, including washability, flexibility, and scalability. Finally, the perspectives on their research and technological prospects to facilitate an essential step towards moving from laboratory-based flexible and wearable SCs to industrial-scale mass production are presented.

A new potassium dual-ion hybrid supercapacitor based on battery-type Ni(OH)2 nanotube arrays and pseudocapacitor-type V2O5-anchored carbon nanotubes electrodes

Hybrid supercapacitors (HSCs) with the characteristics of high energy density, long cycle life and without altering their power density need to be developed urgently. Herein, a novel dual-ion hybrid supercapacitors (DHSCs) with Ni(OH)₂ nanotube arrays (NTAs) as positive electrode and V₂O₅ directly grown on freestanding carbon nanotubes (CNTs) as negative electrode is assembled. In charging mechanism of DHSCs, K⁺ are inserted into the V₂O₅ negative while OH^- react with Ni(OH)₂ positive; during discharge, the K⁺ and OH^- are released from V₂O₅ negative and Ni(OH)₂ positive, respectively, and return back to the electrolyte, which is quite different from traditional metal ion or alkaline supercapacitors. Because of the merits combining dual-ion mechanism and HSCs, the DHSC displays excellent capacity retention of ∼ 81.4% after 10,000 cycles, high energy density of ∼ 25.4 μWh cm^-2 and high power density of ∼ 4.66 mW cm^-2, indicating the potential applications in the further on flexible wearable electronics.

The Electrochemical Stability of Starch Carbon as an Important Property in the Construction of a Lithium-Ion Cell

This paper shows use of starch-based carbon (CSC) and graphene as the anode electrode for lithium-ion cell. To describe electrochemical stability of the half-cell system and kinetic parameters of charging process in different temperatures, electrochemical impedance spectroscopy (EIS) measurement was adopted. It has been shown that smaller resistances are observed for CSC. Additionally, Bode plots show high electrochemical stability at higher temperatures. The activation energy for the SEI (solid–electrolyte interface) layer, charge transfer, and electrolyte were in the ranges of 24.06–25.33, 68.18–118.55, and 13.84–15.22 kJ mol⁻¹, respectively. Moreover, the activation energy of most processes is smaller for CSC, which means that this electrode could serve as an eco-friendly biodegradable lithium-ion cell element.

Comparative study on supercapacitive and oxygen evolution reaction applications of hollow nanostructured cobalt sulfides

Due to the diversity of sulfur valence in cobalt-based sulfides, it is difficult to control the crystal phase and composition of the products during synthesis. Herein, a one-pot hydrothermal method is reported to self-assemble the cobalt sulfides (CoS₂, Co₉S₈and Co₃S₄) with hollow nanostructures. The whole preparation process is simple and mild, avoiding high temperature calcination. The performances of the three kinds of cobalt sulfide in superior supercapacitors and electrocatalytic oxygen evolution performance applications follow the order of CoS₂ > Co₉S₈ > Co₃S₄. Further analysis demonstrates that the performance difference in these cobalt sulfides may be attributed to three factors: the presence ofS22-,the coordination environment of Co and the presence of continuous network of Co-Co bonds. The distinctive electrochemical performance of CoS₂and Co₉S₈may help us to better understand the excellent electrochemical activity of metal polysulfides and metal sulfides after doping or alloying. Therefore, this work may provide a reference in understanding and designing the electrode materials for highly efficient applications in the fields of energy storage and conversion.

Boosting supercapacitive performance of flexible carbon via surface engineering

The relatively low specific capacitance of flexible carbons hinders their practical application for fabricating high-performance flexible supercapacitors. In this work, a surface engineering method is proposed to boost the supercapacitive performance of the flexible carbon. In this method, a flexible carbon was fabricated from carbon felt via co-activation with potassium argininate and potassium hydroxide (KOH) as activators, and the resulting material is abbreviated as AKCF. Unlike traditional KOH activation processes, the addition of potassium argininate can produce a micro-graphitized carbon layer to be the outer layer of AKCF fibers for achieving better electronic transfer. Due to the improved conductivity and lower charge transfer resistance endowed by a thin micro-graphitized carbon layer, the capacitance of the AKCF-0.1 (0.1 M arginine was used) electrode obtained by the co-activation process is elevated to a 1.8-fold higher value of 403 C·g^-1 (2583 mC·cm^-2) relative to the AKCF-0 (0 M arginine was used) electrode prepared by KOH activation alone (222 C·g^-1 or 1369 mC·cm^-2). Moreover, this AKCF-0.1 electrode also displays satisfactory rate capability (66% capacitance retention after a 20-fold current increase) and highly stable cycling performance (no capacitance decline after 20,000 cycles). In addition, the asymmetric supercapacitors constructed with this AKCF-0.1 electrode as the flexible negative electrode expresses high energy densities of 68.4 Wh·kg^-1 and 0.139 mWh·cm^-2 in aqueous and gel electrolytes, respectively.

Rationally designed hierarchical C/TiO2/Ti multilayer core-sheath wires for high-performance energy storage devices

Fiber-shaped supercapacitors (FSCs) are promising power sources for wearable electronic devices due to their small size, excellent flexibility and deformability. The performance of FSCs has been severely affected by the framework of the fibrous electrodes and the interface between the electrode materials and current collector. Herein, we propose an ingenious strategy that combines anodizing etching and CVD methods to transform the less-active titanium wires into unique hierarchical carbon/TiO₂ nanotube/Ti (CTNT) core-sheath wires, which have high conductivity, good mechanical strength and porous structure on the surface. CTNT wires can be used not only as a high-performance electrode, but also as an ideal substrate for depositing active materials. We have demonstrated the deposition of MnO₂ and MoS₂ on the surface of CTNT to prepare MnO₂@CTNT and MoS₂@CTNT core-sheath composite wires through electrochemical deposition and hydrothermal reaction, respectively. The specific areal capacitance of a single wire (MoS₂@CTNT) can reach up to 557.83 mF cm^-2 in a three-electrode system. Two such wires were further used as electrodes for making an all-solid-state asymmetric fiber-shaped supercapacitor (AFSC). The prepared AFSC has a wide voltage window of 2.7 V, a large areal capacitance of 121.42 mF cm^-2 and an excellent energy density of 74.37 μW h cm^-2. It also shows good rate performance and stability, and even after 10 000 cycles of charging and discharging, a capacitance retention rate of 76.5% can be achieved.

Ultra-high rate capability of nanoporous carbon network@V2O5 sub-micron brick composite as a novel cathode material for asymmetric supercapacitors

A green biomass-derived nanoporous carbon network (NCN) has been prepared and integrated with V2O5 sub-micron bricks (SMBs). The large surface area and high pore volume of the NCN can not only provide abundant sites for electrochemical reactions but also stabilize the structure of the V2O5 SMBs. The NCN@V2O5 SMB composite, acting as a novel cathode material, delivers a high areal capacitance of 786 mF cm-2 at 0.2 mA cm-2 and superior cycling stability with 89.5% capacitance retention after 5000 cycles. Besides, the electrode achieves an ultra-high rate capability (82% capacitance retention as the current density increases from 0.2 to 5 mA cm-2) since the contribution from the non-diffusion-controlled process is estimated to be as high as 95.5%-98.5% according to the kinetic analysis. Furthermore, the micropores are more favorable than the mesopores at lower current densities (0.2-2 mA cm-2), while the contribution of the external surface area becomes more significant for current densities higher than 2 mA cm-2. Moreover, an asymmetric supercapacitor assembled using this cathode and the NCN anode shows superior electrochemical properties, such as wide operating voltage, long cycle life and large energy density (72.2 μW h cm-2). Their excellent electrochemical features and good eco-friendliness confirm the potential of the NCN@V2O5 SMBs for use as supercapacitors.

Advances in Manufacturing Composite Carbon Nanofiber-Based Aerogels

This article provides an overview on manufacturing composite carbon nanofiber-based aerogels through freeze casting technology. As known, freeze casting is a relatively new manufacturing technique for generating highly porous structures. During the process, deep cooling is used first to rapidly solidify a well-dispersed slurry. Then, vacuum drying is conducted to sublimate the solvent. This allows the creation of highly porous materials. Although the freeze casting technique was initially developed for porous ceramics processing, it has found various applications, especially for making aerogels. Aerogels are highly porous materials with extremely high volume of free spaces, which contributes to the characteristics of high porosity, ultralight, large specific surface area, huge interface area, and in addition, super low thermal conductivity. Recently, carbon nanofiber aerogels have been studied to achieve exceptional properties of high stiffness, flame-retardant and thermal-insulating. The freeze casting technology has been reported for preparing carbon nanofiber composite aerogels for energy storage, energy conversion, water purification, catalysis, fire prevention etc. This review deals with freeze casting carbon nanofiber composite materials consisting of functional nanoparticles with exceptional properties. The content of this review article is organized as follows. The first part will introduce the general freeze casting manufacturing technology of aerogels with the emphasis on how to use the technology to make nanoparticle-containing composite carbon nanofiber aerogels. Then, modeling and characterization of the freeze cast particle-containing carbon nanofibers will be presented with an emphasis on modeling the thermal conductivity and electrical conductivity of the carbon nanofiber network aerogels. After that, the applications of the carbon nanofiber aerogels will be described. Examples of energy converters, supercapacitors, secondary battery electrodes, dye absorbents, sensors, and catalysts made from composite carbon nanofiber aerogels will be shown. Finally, the perspectives to future work will be presented.

3D nanoflower-like MoSe2 encapsulated with hierarchically anisotropic carbon architecture: a new and free-standing anode with ultra-high areal capacitance for asymmetric supercapacitors

An anisotropic carbon-supported MoSe₂ nanoflowers is designed and acts as an ultra-high areal capacitance of free-standing anode. The energy density of assembled asymmetric supercapacitor is higher than or comparable to that of some Li-ion batteries.

Facile Activation of Commercial Carbon Felt as a Low-Cost Free-Standing Electrode for Flexible Supercapacitors

High cost, low capacitance, and complicated synthesis process are still the key limitations for carbon-negative materials to meet their industrial production and application in high-energy-density asymmetric supercapacitors (ASCs). In this work, we demonstrate the facile preparation of ultrahigh-surface-area free-standing carbon material from low-cost industrial carbon felt (CF) and its application for flexible supercapacitor electrode with outstanding performance. Through a simple freeze-drying-assisted activation method, the as-prepared activated CF (ACF) was endowed with satisfactory flexibility, ultrahigh specific surface area of 2109 m² g^-1, good electric conductivity (311 S m^-1), and excellent wettability to aqueous electrolyte. Owing to these merits, the ACF expressed an ultrahigh areal capacitance of 1441 mF cm^-2, a high specific capacitance ( C_s) of 280 F g^-1 based on the mass of the whole electrode, and an impressive cycling stability (87% retention after 5000 cycles). When applied as a flexible freestanding electrode for MnO₂//ACF ASCs, the ACF-based device provided satisfactory areal energy densities of 0.283 and 0.104 mWh cm^-2 in aqueous and quasi-solid electrolytes, respectively. The values outperform many previously reported carbon-based electrochemical devices. The low cost of raw material and the facile fabrication process, together with the high electrochemical performance, make our ACF electrode highly applicable for the mass production of flexible energy-storage devices.