Assembly of NiO/Ni(OH)2/PEDOT Nanocomposites on Contra Wires for Fiber-Shaped Flexible Asymmetric Supercapacitors

Dual-Site Biomacromolecule Doped Poly(3, 4-Ethylenedioxythiophene) for Bosting Both Anticoagulant and Electrochemical Performances

Poly(3, 4-ethylenedioxythiophene) (PEDOT) as a new generation of intelligent conductive polymers, is attracting much attention in the field of tissue engineering. However, its water dispersibility, conductivity, and biocompatibility are incompatible, which limit its further development. In this work, biocompatible electrode material of PEDOT doped with sodium sulfonated alginate (SS) which contains two functional groups of sulfonic acid and carboxylic acid per repeat unit of the macromolecule. The as dual-site doping strategy simultaneously boosts anticoagulant and electrochemical performances, for example, good hydrophilicity (water contact angle of 59.40°), well dispersibility (dispersion solution unstratified in 30 days), high conductivity (4.45 S m-1), and enhanced anticoagulant property (extended activated partial thrombin time value of 59.0 s), forming an adjustable PEDOT: biomacromolecule interface; this fills the technical gap of implantable bioelectronics in terms of coagulation and thrombosis risk. At the same time, the assembled all-in-one supercapacitor with anticoagulant properties is prepared by PEDOT: sodium sulfonated alginate as electrode material and sodium alginate hydrogel as electrolyte layer. The dual-site doping strategy provides a new opinion for the design and optimization of functional conductive polymers and its applications in implantable energy storage fields.

An anticoagulant supercapacitor for implantable applications

With the rapid advancement of implantable electronic medical devices, implantable supercapacitors have emerged as popular energy storage devices. However, supercapacitors inevitably come into direct contact with blood when implanted, potentially causing adverse clinical reactions such as coagulation and thrombosis, impairing the performance of implanted energy storage devices, and posing a serious threat to human health. Therefore, this work aims to design an anticoagulant supercapacitor by heparin doped poly(3, 4-ethylenedioxythiophene) (PEDOT) for possible applications in implantable bioelectronics. Heparin (Hep), the as-known anticoagulant macromolecule acts as the counterion for PEDOT doping to enhance its conductivity, and the bioelectrode material PEDOT: Hep with anticoagulant activity is synthesized via chemical oxidation polymerization. Concurrently, the anticoagulant supercapacitor is constructed through in-situ polymerization, where PEDOT: Hep and bacterial cellulose as electrode material and electrolyte layer, respectively. Owing to the incorporation of heparin, the supercapacitor exhibits high hemocompatibility with hemolysis rate <5 %, good anticoagulant performance with coagulation time of 63.4 s, reasonable cycle stability with capacitance retention rate of 76.24 % after 20, 000 cycles, and supplies power for implanted heart rate sensors in female mice. This work provides a platform for implantable electronics to achieve anticoagulant activity in vivo.

Mixed solvent-assisted synthesis of high mass loading amorphous NiCo-MOF as a promising electrode material for supercapacitors

The pursuit of high mass loading metal-organic framework (MOF) materials via a simple method is crucial to achieve high-performance supercapacitors. Herein, an amorphous NiCo-MOF material with a high mass loading of up to 10.3 mg cm-2 was successfully prepared using a mixed solvent system of ethanol and water. In addition, by adjusting the volume ratio of ethanol to water, amorphous NiCo-MOFs with three different morphologies including nanospheres, nanopores, and ultra-thick plates were obtained. It was found that the different solvent systems not only affected the growth rate of MOFs, but also controlled their nucleation rate by changing the coordination environment of the metal ions, and thus achieved morphology and mass loading regulation, thereby influencing their energy storage behavior. Notably, the optimum NiCo-MOF exhibited the superior specific capacitance of up to 9.7 F cm-2 (941.8 F g-1) at a current density of 5 mA cm-2 and high-rate capability of 71.1% even at 20 mA cm-2. Moreover, the corresponding assembled solid-state supercapacitor exhibited an excellent energy density of 0.65 mW h cm-2 at a power density of 2 mW cm-2 and capacity retention of 84.7% after 8000 cycles at 30 mA cm-2. Overall, this work proposes a feasible and effective strategy to achieve high mass loading NiCo-MOFs, impacting their ultimate electrochemical performance, which can possibly be further extended to other MOFs with superior capacitance.

How Practical Are Fiber Supercapacitors for Wearable Energy Storage Applications?

Future wearable electronics and smart textiles face a major challenge in the development of energy storage devices that are high-performing while still being flexible, lightweight, and safe. Fiber supercapacitors are one of the most promising energy storage technologies for such applications due to their excellent electrochemical characteristics and mechanical flexibility. Over the past decade, researchers have put in tremendous effort and made significant progress on fiber supercapacitors. It is now the time to assess the outcomes to ensure that this kind of energy storage device will be practical for future wearable electronics and smart textiles. While the materials, fabrication methods, and energy storage performance of fiber supercapacitors have been summarized and evaluated in many previous publications, this review paper focuses on two practical questions: Are the reported devices providing sufficient energy and power densities to wearable electronics? Are the reported devices flexible and durable enough to be integrated into smart textiles? To answer the first question, we not only review the electrochemical performance of the reported fiber supercapacitors but also compare them to the power needs of a variety of commercial electronics. To answer the second question, we review the general approaches to assess the flexibility of wearable textiles and suggest standard methods to evaluate the mechanical flexibility and stability of fiber supercapacitors for future studies. Lastly, this article summarizes the challenges for the practical application of fiber supercapacitors and proposes possible solutions.

Bridging the Gap between Charge Storage Site and Transportation Pathway in Molecular-Cage-Based Flexible Electrodes

Porous materials have been widely applied for supercapacitors; however, the relationship between the electrochemical behaviors and the spatial structures has rarely been discussed before. Herein, we report a series of porous coordination cage (PCC) flexible supercapacitors with tunable three-dimensional (3D) cavities and redox centers. PCCs exhibit excellent capacitor performances with a superior molecular capacitance of 2510 F mmol^-1, high areal capacitances of 250 mF cm^-2, and unique cycle stability. The electrochemical behavior of PCCs is dictated by the size, type, and open-close state of the cavities. Both the charge binding site and the charge transportation pathway are unambiguously elucidated for PCC supercapacitors. These findings provide central theoretical support for the "structure-property relationship" for designing powerful electrode materials for flexible energy storage devices.

Polyindole Embedded Nickel/Zinc Oxide Nanocomposites for High-Performance Energy Storage Applications

Conducting polymers integrated with metal oxides create opportunities for hybrid capacitive electrodes. In this work, we report a one-pot oxidative polymerization for the synthesis of integrated conductive polyindole/nickel oxide (PIn/NiO), polyindole/zinc oxide (PIn/ZnO), and polyindole/nickel oxide/zinc oxide (PNZ). The polymers were analyzed thoroughly for their composition and physical as well as chemical properties by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), and thermogravimetric analysis (TGA). The PIn and its composites were processed into electrodes, and their use in symmetrical supercapacitors in two- and three-electrode setups was evaluated by cyclic voltammetry (CV), galvanostatic discharge (GCD), and electrochemical impedance spectroscopy (EIS). The best electrochemical charge storage capability was found for the ternary PNZ composite. The high performance directly correlates with its uniformly shaped nanofibrous structure and high crystallinity. For instance, the symmetrical supercapacitor fabricated with PNZ hybrid electrodes shows a high specific capacitance of 310.9 F g^-1 at 0.5 A g^-1 with an energy density of 42.1 Wh kg^-1, a power density of 13.2 kW kg^-1, and a good cycling stability of 78.5% after 5000 cycles. This report presents new electrode materials for advanced supercapacitor technology based on these results.

Nickel(II) Cluster-Based Pillar-Layered Metal-Organic Frameworks for High-Performance Supercapacitors

Most metal-organic frameworks (MOFs) cannot be used as electrode materials for supercapacitors because of their high costs, poor stabilities in aqueous solutions, inferior intrinsic electrocatalytic activities, and poor conductivities. Herein, the application of two nickel(II) cluster-based pillar-layered MOFs, Ni-mba-Na ([Ni₈(mba)₆(Cl)₂Na(OH^-)₃]_n, H₂mba is 2-mercaptobenzoic acid) and Ni-mba-K ([Ni₈(mba)₆(Cl)₂K(OH^-)₃]_n), as electrode materials are reported. They differ from conductive MOFs because they are insulators with small specific surface areas (<10 m² g^-1), and H²mba is an inexpensive raw material. The conductivities of Ni-mba-Na and Ni-mba-K at 30 °C were 4.002 × 10^-10 and >10^-11 S cm^-1, respectively. They showed excellent supercapacitor performance and stabilities and high inherent densities and specific capacitances. The specific powers of their asymmetric supercapacitors could reach up to 16,000 W kg^-1; the specific energies of Ni-mba-Na and Ni-mba-K were 16.9 and 21.8 Wh kg^-1, respectively. Design recommendations for these MOFs are provided based on their structure and performance differences. This paper shows a novel application of nonconductive MOFs in the energy storage field and design of high-performance electrode materials for supercapacitors.

Surface Engineering of Ni wires and Rapid Growth Strategy of Ni-MOF Synergistically Contribute to High-Performance Fiber-Shaped Aqueous Battery

The fiber-shaped aqueous battery (FSAB) has the advantages of flexibility, portability and safety making it promising for energy storage applications. In particular, FSABs based on metal wire current collectors with good electrical conductivity can provide excellent energy storage properties. However, the low adhesion caused by the smooth surface of the metal wire and the unavailability of many electrochemically active materials for use in FSAB is holding back their development. Herein, a substrate is effectively constructed for the strongly applicable growth of the active material via a Ni wire etching strategy. In addition, core-shell structured nanorod arrays consisting of NiCo₂ O₄ and Ni-metal-organic frameworks (MOFs) are constructed, where Ni-MOF can be obtained rapidly via β-Ni(OH)₂ intermediates. The NCO/NM-15 electrode obtained by structural regulation exhibits high capacity and outstanding cycling stability. De calculations further demonstrate that the formation of NiCo₂ O₄ and Ni-MOF heterostructures results in a significant increase in the Fermi level leading to more active internal electrons, which facilitates electron transfer in electrochemical reactions. An assembled FSAB device can provide an energy density of 158.33 µWh cm^-2 and the devices can provide power for a calculator and an electronic watch screen, demonstrating a wide application prospect in the field of energy storage.

Mesoporous vanadium nitride as anion storage electrode for reverse dual-ion hybrid supercapacitor

In traditional dual-ion systems, the cathode usually is employed as anion-storage materials. Herein, we propose a new dual-ion hybrid supercapacitor with reverse anion/cation-storage mechanism, consisting of a mesoporous (MPs) VN anode as a pivotal anion-storage material and K_2-xMn₈O₁₆ nanosheet arrays grown on carbon cloth (NSs/CC) as (K-storage) cathode. During charge/discharge, the anode and cathode reversibly store/release OH⁻ ions and K⁺ ions, respectively. Herein, the MPs VN as anion-storage electrode can operate in an alkaline condition and deliver a high capacitance of 251 mF cm⁻² with desired low-voltage plateau. More importantly, benefiting from unique reverse dual-ion mechanism, the (MPs VN-K_2-xMn₈O₁₆ NSs/CC) hybrid device displays excellent rate performance and satisfying area capacitance along with good durability of 92.2% after 10,000 cycles at a scan rate of 100 mV s⁻¹. It offers new ideas to expand the range of anion-storage materials in dual-ion hybrid supercapacitors.

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The MPs VN anode as anion storage material was firstly proposed

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The dual-ion supercapacitors were firstly constructed by employing the MPs VN anode

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The reverse storage mechanism was firstly applied to hybrid supercapacitors

Synthesis, Characterization and Electrochemical Performance of a Redox-Responsive Polybenzopyrrole@Nickel Oxide Nanocomposite for Robust and Efficient Faraday Energy Storage

A polybenzopyrrole@nickel oxide (Pbp@NiO) nanocomposite was synthesized by an oxidative chemical one-pot method and tested as an active material for hybrid electrodes in an electrochemical supercapattery device. The as-prepared composite material exhibits a desirable 3D cross-linked nanostructured morphology and a synergistic effect between the polymer and metal oxide, which improved both physical properties and electrochemical performance. The unprocessed material was characterized by X-ray diffraction, FTIR and UV-Vis spectroscopy, scanning electron microscopy/energy disperse X-ray analysis, and thermogravimetry. The nanocomposite material was deposited without a binder on gold current collectors and investigated for electrochemical behavior and performance in a symmetrical two- and three-electrode cell setup. A high specific capacity of up to 105 C g-1 was obtained for the Pbp@NiO-based electrodes with a gravimetric energy density of 17.5 Wh kg-1, a power density of 1925 W kg-1, and excellent stability over 10,000 cycles.

Designing flexible, smart and self-sustainable supercapacitors for portable/wearable electronics: from conductive polymers

The rapid development of portable/wearable electronics proposes new demands for energy storage devices, which are flexibility, smart functions and long-time outdoor operation. Supercapacitors (SCs) show great potential in portable/wearable applications, and the recently developed flexible, smart and self-sustainable supercapacitors greatly meet the above demands. In these supercapacitors, conductive polymers (CPs) are widely applied due to their high flexibility, conductivity, pseudo-capacitance, smart characteristics and moderate preparation conditions. Herein, we'd like to introduce the CP-based flexible, smart and self-sustainable supercapacitors for portable/wearable electronics. This review first summarizes the flexible SCs based on CPs and their composites with carbon materials and metal compounds. The smart supercapacitors, i.e., electrochromic, electrochemical actuated, stretchable, self-healing and stimuli-sensitive ones, are then presented. The self-sustainable SCs which integrate SC units with energy-harvesting units in one compact configuration are also introduced. The last section highlights some current challenges and future perspectives of this thriving field.

Phosphate-modified Co-Ni phosphide heterostructure formed by interfacial and electronic tuning for boosted faradaic properties

Rational structural and compositional modulation endows electrode materials with unique physicochemical characteristics due to their adjustable electronic properties. Herein, a phosphate-modified hierarchical nanoarray consisting of a heterojunction with a well-aligned cobalt phosphide nanowire core and nickel phosphide nanosheet shell on flexible carbon cloth (denoted as CoP@Ni₂P-CC) is engineered. The phosphate-modulated heterogeneous phosphide with a tuned electronic structure, additional heterojunction interfaces, and high degree of covalency in the chemical bonds accelerates the reaction kinetics and enhances the energy storage performance. Due to these reasons, the as-obtained phosphide-based heterostructured CoP@Ni₂P-CC electrode delivers a capacity of 475.9 C g^-1 at 0.5 A g^-1 with a satisfying rate capability, which is greatly superior to that of its transition metal counterparts (sulfide, selenide, and oxide). After being assembled into a flexible hybrid supercapacitor (FHSC), a wide operating voltage (1.8 V), high energy/power densities (49.8 W h kg^-1/8.6 kW kg^-1), and long-term stability (85.1% capacity retention after 10 000 cycles) were achieved. This work may provide a general method from multiple strategies for designing reliable pseudocapacitive materials for flexible electronics.

True Meaning of Pseudocapacitors and Their Performance Metrics: Asymmetric versus Hybrid Supercapacitors

The development of pseudocapacitive materials for energy-oriented applications has stimulated considerable interest in recent years due to their high energy-storing capacity with high power outputs. Nevertheless, the utilization of nanosized active materials in batteries leads to fast redox kinetics due to the improved surface area and short diffusion pathways, which shifts their electrochemical signatures from battery-like to the pseudocapacitive-like behavior. As a result, it becomes challenging to distinguish "pseudocapacitive" and "battery" materials. Such misconceptions have further impacted on the final device configurations. This Review is an earnest effort to clarify the confusion between the battery and pseudocapacitive materials by providing their true meanings and correct performance metrics. A method to distinguish battery-type and pseudocapacitive materials using the electrochemical signatures and quantitative kinetics analysis is outlined. Taking solid-state supercapacitors (SSCs, only polymer gel electrolytes) as an example, the distinction between asymmetric and hybrid supercapacitors is discussed. The state-of-the-art progress in the engineering of active materials is summarized, which will guide for the development of real-pseudocapacitive energy storage systems.

A Self-supported Graphene/Carbon Nanotube Hollow Fiber for Integrated Energy Conversion and Storage

Wearable fiber-shaped integrated energy conversion and storage devices have attracted increasing attention, but it remains a big challenge to achieve a common fiber electrode for both energy conversion and storage with high performance. Here, we grow aligned carbon nanotubes (CNTs) array on continuous graphene (G) tube, and their seamlessly connected structure provides the obtained G/CNTs composite fiber with a unique self-supported hollow structure. Taking advantage of the hollow structure, other active materials (e.g., polyaniline, PANI) could be easily functionalized on both inner and outer surfaces of the tube, and the obtained G/CNTs/PANI composite hollow fibers achieve a high mass loading (90%) of PANI. The G/CNTs/PANI composite hollow fibers can not only be used for high-performance fiber-shaped supercapacitor with large specific capacitance of 472 mF cm^-2, but also can replace platinum wire to build fiber-shaped dye-sensitized solar cell (DSSC) with a high power conversion efficiency of 4.20%. As desired, the integrated device of DSSC and supercapacitor with the G/CNTs/PANI composite hollow fiber used as the common electrode exhibits a total power conversion and storage efficiency as high as 2.1%. Furthermore, the self-supported G/CNTs hollow fiber could be further functionalized with other active materials for building other flexible and wearable electronics.

Application Challenges in Fiber and Textile Electronics

Modern electronic devices are moving toward miniaturization and integration with an emerging focus on wearable electronics. Due to their close contact with the human body, wearable electronics have new requirements including low weight, small size, and flexibility. Conventional 3D and 2D electronic devices fail to efficiently meet these requirements due to their rigidity and bulkiness. Hence, a new family of 1D fiber-shaped electronic devices including energy-harvesting devices, energy-storage devices, light-emitting devices, and sensing devices has risen to the challenge due to their small diameter, lightweight, flexibility, and weavability into soft textile electronics. The application challenges faced by fiber and textile electronics from single fiber-shaped devices to continuously scalable fabrication, to encapsulation and testing, and to application mode exploration, are discussed. The evolutionary trends of fiber and textile electronics are then summarized. Finally, future directions required to boost their commercialization are highlighted.

Hierarchical core-shell electrode with NiWO4 nanoparticles wrapped MnCo2O4 nanowire arrays on Ni foam for high-performance asymmetric supercapacitors

Rational construction of MnCo₂O₄-based core-shell nanomaterials with distinctive and desirable architectures possesses great potential in the advanced electrode material of high-performance supercapacitors. Here, a new class of hierarchical core-shell nanowire arrays (NWAs) with a shell of NiWO₄ nanoparticles and a core of MnCo₂O₄ nanowires is reported, which can significantly improve the electrochemical energy storage properties of supercapacitors. The unique core-shell structure endows the MnCo₂O₄@NiWO₄ NWAs electrode with a high areal specific capacitance of 5.09 F cm^-2 at a current density of 1 mA cm^-2 and a superior cyclic retention of 96% after 5000 charge-discharge cycles, which are more preferable than those of MnCo₂O₄ NWAs electrode. More importantly, an aqueous electrochemical energy storage device (core-shell MnCo₂O₄@NiWO₄ NWAs as the positive electrode and active carbon as the negative electrode, MnCo₂O₄@NiWO₄//AC ASC) was assembled and shows a high energy density of 0.23 mWh cm^-2 at a power density of 2.66 mW cm^-2, and 0.09 mWh cm^-2 at 16.00 mW cm^-2, indicating hopeful potential for practical applications. This work highlights the significance of NiWO₄ as a shell for hierarchical core-shell nanostructures, which can further improve the electron transport characteristic of the electrode material, thereby achieving performance breakthroughs in energy storage devices.

Cauliflower-like Nickel with Polar Ni(OH)2/NiO x F y Shell To Decorate Copper Meshes for Efficient Oil/Water Separation

In recent years, superhydrophilic and underwater superoleophobic membranes have shown promising results in advanced oil/water separation. However, these membranes still have some drawbacks, like tedious preparation process and instability, which hinder their application in oil/water separation. Accordingly, the development of a facile approach to prepare superhydrophilic membranes with excellent oil/water separation performance is still coveted. Here, a copper mesh decorated with cauliflower-like nickel (Cu mesh@CF-Ni) is synthesized via a facile one-step electrodeposition method. Due to the surface polar -OH and -O-Ni-F groups of the Ni(OH)₂/NiO _x F _y shell of the cauliflower-like nickel (CF-Ni), this Cu mesh@CF-Ni displays superhydrophilic and underwater superoleophobic wettability. The results show that the Cu mesh@CF-Ni has excellent oil/water separation efficiency (higher than 99.2%) and ultrahigh water flux (around 20 L h^-1 cm^-2). Moreover, it also displays good stability in a 10 wt % NaCl solution and 1 M NaOH solution for oil/water separation. By introducing the CF-Ni with polar Ni(OH)₂/NiO _x F _y components onto the surface of the materials via a simple electrodeposition method, the materials will acquire the capability to not only achieve oil/water separation but also realize many other applications, like self-cleaning, underwater bubble manipulation, and fog harvesting.

Facile Electrodeposition of Poly(3,4-ethylenedioxythiophene) on Poly(vinyl alcohol) Nanofibers as the Positive Electrode for High-Performance Asymmetric Supercapacitor

Poly(vinyl alcohol)/poly(3,4-ethylenedioxythiophene) (PVA/PEDOT) nanofibers were synthesized as a positive electrode for high-performance asymmetric supercapacitor (ASC). PVA/PEDOT nanofibers were prepared through electrospinning and electrodeposition meanwhile reduced graphene oxide (rGO) was obtained by electrochemical reduction. The PVA/PEDOT nanofibers demonstrated cauliflower-like morphology showing that PEDOT was uniformly coated on the smooth cross-linking structure of PVA nanofibers. In addition, the ASC showed a remarkable energy output efficiency by delivering specific energy of 21.45 Wh·kg−1 at a specific power of 335.50 W·kg−1 with good cyclability performance (83% capacitance retained) after 5000 CV cycles. The outstanding supercapacitive performance is contributed from the synergistic effects of both PVA/PEDOT//rGO, which gives promising materials for designing high-performance supercapacitor applications.

Polypyrrole⁻Nickel Hydroxide Hybrid Nanowires as Future Materials for Energy Storage

Hybrid materials play an essential role in the development of the energy storage technologies since a multi-constituent system merges the properties of the individual components. Apart from new features and enhanced performance, such an approach quite often allows the drawbacks of single components to be diminished or reduced entirely. The goal of this paper was to prepare and characterize polymer-metal hydroxide (polypyrrole-nickel hydroxide, PPy-Ni(OH)₂) nanowire arrays demonstrating good electrochemical performance. Nanowires were fabricated by potential pulse electrodeposition of pyrrole and nickel hydroxide into nanoporous anodic alumina oxide (AAO) template. The structural features of as-obtained PPy-Ni(OH)₂ hybrid nanowires were characterized using FE-SEM and TEM analysis. Their chemical composition was confirmed by energy-dispersive x-ray spectroscopy (EDS). The presence of nickel hydroxide in the synthesized PPy-Ni(OH)₂ nanowire array was investigated by X-ray photoelectron spectroscopy (XPS). Both FE-SEM and TEM analyses confirmed that the obtained nanowires were composed of a polymer matrix with nanoparticles dispersed within. EDS and XPS techniques confirmed the presence of PPy-Ni(OH)₂ in the nanowire array obtained. Optimal working potential range (i.e., available potential window), charge propagation, and cyclic stability of the electrodes were determined with cyclic voltammetry (CV) at various scan rates. Interestingly, the electrochemical stability window for the aqueous electrolyte at PPy-Ni(OH)₂ nanowire array electrode was remarkably wider (ca. 2 times) in comparison with the non-modified PPy electrode. The capacitance values, calculated from cyclic voltammetry performed at 20 mV s^-1, were 25 F cm^-2 for PPy and 75 F cm^-2 for PPy-Ni(OH)₂ array electrodes. The cyclic stability of the PPy nanowire array electrode up to 100 cycles showed a capacitance fade of about 13%.

Nickel molybdate nanorods supported on three-dimensional, porous nickel film coated on copper wire as an advanced binder-free electrode for flexible wire-type asymmetric micro-supercapacitors with enhanced electrochemical performances

Wire-shaped micro-supercapacitors attracted extensive attentions in next-generation portable and wearable electronics, due to advantages of miniature size, lightweight and flexibility. Herein, NiMoO₄ nanorods supported on Ni film coated Cu wire are successfully fabricated thorough direct deposition of Ni film onto Cu wire as the conductive substrate, followed by growth of the NiMoO₄ nanorods on Ni film coated Cu wire substrate by means a hydrothermal annealing process. The prepared 3D, porous electrode demonstrates extremely high areal specific capacitance of 12.03F cm^-2 at the current density of 4 mA cm^-2 and retained capacitance of 8.23 F cm^-2 at a much higher current density of 80 mAcm^-2. The electrode, also, shows an excellent cycling stability with capacitance retention of 99.3% after 3000 cycles. The superior electrochemical performance can be attributed to the high area surface, low contact resistance between NiMoO₄ nanorods and Cu wire current collector and presence of a 3D and porous structure provides many electroactive sites and sufficient open space for easy diffusion of the electrolyte ions during redox reactions. Benefiting from their structural features, a fiber shaped asymmetric micro-supercapacitor based on NiMoO₄/Ni film/Cu wire as the positive electrode and carbon fiber coated with reduced graphene oxide as the negative electrode is assembled. The fabricated fiber device presents a wide potential window between 0 and 1.7 V and exhibits high specific capacitance of 0.504F cm^-2 (38.8F cm^-3) at a current density of 4.8 mA cm^-2 with a high energy density of 202 µWh cm^-2 (15.6 mWh cm^-3) at a power density of 4050 µW cm^-2 (313 mWh cm^-3). The energy density retains 124 µWh cm^-2 (9.54 mWh cm^-3) when the power density is increased to 13530 µW cm^-2 (1040.73 mWh cm^-3). In addition, the asymmetric device exhibits an outstanding cycling stability (98.5% capacitance retention after 1000 consecutive cycles) and good mechanical stability. Therefore, this work suggested the promising potential of NiMoO₄ nanorods supported on Ni film coated Cu wire as an advanced electrode material for construction of flexible and portable next-generation energy storage micro-devices with superior electrochemical performances.

Highly Conductive Ti3 C2 Tx MXene Hybrid Fibers for Flexible and Elastic Fiber-Shaped Supercapacitors

Fiber-shaped supercapacitors (FSCs) are promising energy storage solutions for powering miniaturized or wearable electronics. However, the scalable fabrication of fiber electrodes with high electrical conductivity and excellent energy storage performance for use in FSCs remains a challenge. Here, an easily scalable one-step wet-spinning approach is reported to fabricate highly conductive fibers using hybrid formulations of Ti₃ C₂ T_x MXene nanosheets and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate. This approach produces fibers with a record conductivity of ≈1489 S cm^-1 , which is about five times higher than other reported Ti₃ C₂ T_x MXene-based fibers (up to ≈290 S cm^-1 ). The hybrid fiber at ≈70 wt% MXene shows a high volumetric capacitance (≈614.5 F cm^-3 at 5 mV s^-1 ) and an excellent rate performance (≈375.2 F cm^-3 at 1000 mV s^-1 ). When assembled into a free-standing FSC, the energy and power densities of the device reach ≈7.13 Wh cm^-3 and ≈8249 mW cm^-3 , respectively. The excellent strength and flexibility of the hybrid fibers allow them to be wrapped on a silicone elastomer fiber to achieve an elastic FSC with 96% capacitance retention when cyclically stretched to 100% strain. This work demonstrates the potential of MXene-based fiber electrodes and their scalable production for fiber-based energy storage applications.

Research on the High-Performance Electrochemical Energy Storage of a NiO@ZnO (NZO) Hybrid Based on Growth Time

A NiO@ZnO (NZO) hybrid with different reaction times was successfully synthesized by a green hydrothermal method. After comparison, it was found that hydrothermal time had a great impact on specific capacitance. As a supercapacitor electrode of NZO-12h, it exhibited the maximum reversible specific capacitance of 985.0 F/g (3.94 F/cm2) at 5 mA/cm2 and 587.5 F/g (2.35 F/cm2) at 50 mA/cm2, as well as a high retention of 74.9% capacitance after 1500 cycles at 20 mA/cm2. Furthermore, the asymmetric electrode device with ZnO-12h and activated carbon (AC) as the positive and negative electrodes was successfully assembled. In addition, the device exhibited a specific capacitance of 85.7 F/g at 0.4 A/g. Moreover, the highest energy density of 27.13 Wh kg−1 was obtained at a power density of 321.42 W kg−1. These desirable electrochemical properties demonstrate that the NZO hybrid is a promising electrode material for a supercapacitor.

Facile Synthesis of Mesoporous Carbon Spheres Using 3D Cubic Fe-KIT-6 by CVD Technique for the Application of Active Electrode Materials in Supercapacitors

Mesoporous carbon spheres (MCS-750, MCS-800, MCS-850, MCS-900, and MCS-950) have been synthesized by a facile strategy with low temperature and rapid chemical vapor deposition technique. The synthesized MCS possess relatively large surface area (570-670 m² g^-1), good graphitization, remarkable porosity, and redox functionalities on the surface of the synthesized MCS. Combination of these structural and surface properties of the synthesized MCS as an electrode material (MCS-850) showed an excellent charge-storage capacity with a specific capacitance of 338 F/g at 1 mV/s, 217 F/g at 0.5 A/g. MCS-850 shows long-term cycling stability with capacitive retention of more than 96% after 2000 cycles in 6 M KOH electrolyte. In addition, a fabricated two-electrode symmetric cell obtained 86% retention after 2000 cycles. The two-electrode symmetric device exhibited a specific capacitance of 63 F/g at 5 mV/s with an energy density of 7.1 Wh/kg.

Metal-Organic Framework Derived Spindle-like Carbon Incorporated α-Fe2O3 Grown on Carbon Nanotube Fiber as Anodes for High-Performance Wearable Asymmetric Supercapacitors

Iron oxide (Fe₂O₃) has drawn much attention because of its high theoretical capacitance, wide operating potential window, low cost, natural abundance, and environmental friendliness. However, the inferior conductivity and insufficient ionic diffusion rate of a simple Fe₂O₃ electrode leading to the low specific capacitance and poor rate performance of supercapacitors have impeded its applications. In this work, we report a facile and cost-effective method to directly grow MIL-88-Fe metal-organic framework (MOF) derived spindle-like α-Fe₂O₃@C on oxidized carbon nanotube fiber (S-α-Fe₂O₃@C/OCNTF). The S-α-Fe₂O₃@C/OCNTF electrode is demonstrated with a high areal capacitance of 1232.4 mF/cm² at a current density of 2 mA/cm² and considerable rate capability with capacitance retention of 63% at a current density of 20 mA/cm² and is well matched with the cathode of the Na-doped MnO₂ nanosheets on CNTF (Na-MnO₂ NSs/CNTF). The electrochemical test results show that the S-α-Fe₂O₃@C/OCNTF//Na-MnO₂ NSs/CNTF asymmetric supercapacitors possess a high specific capacitance of 201.3 mF/cm² and an exceptional energy density of 135.3 μWh/cm². Thus, MIL-88-Fe MOF derived S-α-Fe₂O₃@C will be a promising anode for applications in next-generation wearable asymmetric supercapacitors.

High-Performance Biscrolled MXene/Carbon Nanotube Yarn Supercapacitors

Yarn-shaped supercapacitors (YSCs) once integrated into fabrics provide promising energy storage solutions to the increasing demand of wearable and portable electronics. In such device format, however, it is a challenge to achieve outstanding electrochemical performance without compromising flexibility. Here, MXene-based YSCs that exhibit both flexibility and superior energy storage performance by employing a biscrolling approach to create flexible yarns from highly delaminated and pseudocapacitive MXene sheets that are trapped within helical yarn corridors are reported. With specific capacitance and energy and power densities values exceeding those reported for any YSCs, this work illustrates that biscrolled MXene yarns can potentially provide the conformal energy solution for powering electronics beyond just the form factor of flexible YSCs.

Graphene-Bridged Multifunctional Flexible Fiber Supercapacitor with High Energy Density

Portable fiber supercapacitors with high-energy storage capacity are in great demand to cater for the rapid development of flexible and deformable electronic devices. Hence, we employed a 3D cellular copper foam (CF) combined with the graphene sheets (GSs) as the support matrix to bridge the active material with nickel fiber (NF) current collector, significantly increasing surface area and decreasing the interface resistance. In comparison to the active material directly growing onto the NF in the absence of CF and GSs, our rationally designed architecture achieved a joint improvement in both capacity (0.217 mAh cm^-2/1729.413 mF cm^-2, 1200% enhancement) and rate capability (87.1% from 1 to 20 mA cm^-2, 286% improvement), which has never been achieved before with other fiber supercapacitors. The in situ scanning electron microscope (SEM) microcompression test demonstrated its superior mechanical recoverability for the first time. Importantly, the assembled flexible and wearable device presented a superior energy density of 109.6 μWh cm^-2 at a power density of 749.5 μW cm^-2, and the device successfully coupled with a flexible strain sensor, solar cell, and nanogenerator. This rational design should shed light on the manufacturing of 3D cellular architectures as microcurrent collectors to realize high energy density for fiber-based energy storage devices.

Design of Novel Wearable, Stretchable, and Waterproof Cable-Type Supercapacitors Based on High-Performance Nickel Cobalt Sulfide-Coated Etching-Annealed Yarn Electrodes

Rapid advances in functional electronics bring tremendous demands on innovation toward effective designs of device structures. Yarn supercapacitors (SCs) show advantages of flexibility, knittability, and small size, and can be integrated into various electronic devices with low cost and high efficiency for energy storage. In this work, functionalized stainless steel yarns are developed to support active materials of positive and negative electrodes, which not only enhance capacitance of both electrodes but can also be designed into stretchable configurations. The as-made asymmetric yarn SCs show a high energy density of 0.0487 mWh cm^-2 (10.19 mWh cm^-3 ) at a power density of 0.553 mW cm^-2 (129.1 mW cm^-3 ) and a specific capacitance of 127.2 mF cm^-2 under an operating voltage window of 1.7 V. The fabricated SC is then made into a stretchable configuration by a prestraining-then-releasing approach using polydimethylsiloxane (PDMS) tube, and its electrochemical performance can be well maintained when stretching up to a high strain of 100%. Moreover, the stretchable cable-type SCs are stably workable under water-immersed condition. The method opens up new ways for fabricating flexible, stretchable, and waterproof devices.

Towards flexible solid-state supercapacitors for smart and wearable electronics

Flexible solid-state supercapacitors (FSSCs) are frontrunners in energy storage device technology and have attracted extensive attention owing to recent significant breakthroughs in modern wearable electronics. In this study, we review the state-of-the-art advancements in FSSCs to provide new insights on mechanisms, emerging electrode materials, flexible gel electrolytes and novel cell designs. The review begins with a brief introduction on the fundamental understanding of charge storage mechanisms based on the structural properties of electrode materials. The next sections briefly summarise the latest progress in flexible electrodes (i.e., freestanding and substrate-supported, including textile, paper, metal foil/wire and polymer-based substrates) and flexible gel electrolytes (i.e., aqueous, organic, ionic liquids and redox-active gels). Subsequently, a comprehensive summary of FSSC cell designs introduces some emerging electrode materials, including MXenes, metal nitrides, metal-organic frameworks (MOFs), polyoxometalates (POMs) and black phosphorus. Some potential practical applications, such as the development of piezoelectric, photo-, shape-memory, self-healing, electrochromic and integrated sensor-supercapacitors are also discussed. The final section highlights current challenges and future perspectives on research in this thriving field.

Dual-Function Metal-Organic Framework-Based Wearable Fibers for Gas Probing and Energy Storage

Metal-organic frameworks (MOFs) coupled with multiwalled carbon nanotubes (MWCNTs) have been developed with an ultrahigh sensitivity for hazardous gas monitoring. Both the MOF/MWCNT and as-derived metal oxides (MOs)/MWCNTs hybrid fibers deliver an ultralow detection limit for NO₂ down to 0.1 ppm without external heating, and they can be further bent into different angles without loss of sensing performance. Also, a high specific capacitance of 110 F cm^-3 can also be obtained for MO/MWCNT hybrid fibers, demonstrating promising application for integrated wearable devices.

High-performance flexible supercapacitors based on electrochemically tailored three-dimensional reduced graphene oxide networks

A simple approach for growing porous electrochemically reduced graphene oxide (pErGO) networks on copper wire, modified with galvanostatically deposited copper foam is demonstrated. The as-prepared pErGO networks on the copper wire are directly used to fabricate solid-state supercapacitor. The pErGO-based supercapacitor can deliver a specific capacitance (C_sp) as high as 81±3 F g⁻¹ at 0.5 A g⁻¹ with polyvinyl alcohol/H₃PO₄ gel electrolyte. The C_sp per unit length and area are calculated as 40.5 mF cm⁻¹ and 283.5 mF cm⁻², respectively. The shape of the voltammogram retained up to high scan rate of 100 V s⁻¹. The pErGO-based supercapacitor device exhibits noticeably high charge-discharge cycling stability, with 94.5% C_sp retained even after 5000 cycles at 5 A g⁻¹. Nominal change in the specific capacitance, as well as the shape of the voltammogram, is observed at different bending angles of the device even after 5000 cycles. The highest energy density of 11.25 W h kg⁻¹ and the highest power density of 5 kW kg⁻¹ are also achieved with this device. The wire-based supercapacitor is scalable and highly flexible, which can be assembled with/without a flexible substrate in different geometries and bending angles for illustrating promising use in smart textile and wearable device.

Nanostructure selenium compounds as pseudocapacitive electrodes for high-performance asymmetric supercapacitor

The electrochemical performance of an energy conversion and storage device like the supercapacitor mainly depends on the microstructure and morphology of the electrodes. In this paper, to improve the capacitance performance of the supercapacitor, the all-pseudocapacitive electrodes of lamella-like Bi₁₈SeO₂₉/BiSe as the negative electrode and flower-like Co_0.85Se nanosheets as the positive electrode are synthesized by using a facile low-temperature one-step hydrothermal method. The microstructures and morphology of the electrode materials are carefully characterized, and the capacitance performances are also tested. The Bi₁₈SeO₂₉/BiSe and Co_0.85Se have high specific capacitance (471.3 F g^-1 and 255 F g^-1 at 0.5 A g^-1), high conductivity, outstanding cycling stability, as well as good rate capability. The assembled asymmetric supercapacitor completely based on the pseudocapacitive electrodes exhibits outstanding cycling stability (about 93% capacitance retention after 5000 cycles). Moreover, the devices exhibit high energy density of 24.2 Wh kg^-1 at a power density of 871.2 W kg^-1 in the voltage window of 0-1.6 V with 2 M KOH solution.

NiCo2S4 nanosheet-decorated 3D, porous Ni film@Ni wire electrode materials for all solid-state asymmetric supercapacitor applications

Wire type supercapacitors with high energy and power densities have generated considerable interest in wearable applications. Herein, we report a novel NiCo₂S₄-decorated 3D, porous Ni film@Ni wire electrode for high performance supercapacitor application. In this work, a facile method is introduced to fabricate a 3D, porous Ni film deposited on a Ni wire as a flexible electrode, followed by decoration with NiCo₂S₄ as an electroactive material. The fabricated NiCo₂S₄-decorated 3D, porous Ni film@Ni wire electrode displays a superior performance with an areal and volumetric capacitance of 1.228 F cm^-2 and 199.74 F cm^-3, respectively, at a current density of 0.2 mA cm^-1 with a maximum volumetric energy and power density (E_V: 6.935 mW h cm^-3; P_V: 1.019 W cm^-3). Finally, the solid state asymmetric wire type supercapacitor is fabricated using the fabricated NiCo₂S₄-decorated 3D, porous Ni film@Ni wire as a positive electrode and N-doped reduced graphene oxide (N-rGO) as a negative electrode and this exhibits good areal and volumetric capacitances of C_A: 0.12 F cm^-2 and C_V: 19.57 F cm^-2 with a higher rate capability (92%). This asymmetric wire type supercapacitor demonstrates a low leakage current and self-discharge with a maximum volumetric energy (E_V: 5.33 mW h cm^-3) and power (P_V: 855.69 mW cm^-3) density.