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Highly flexible freestanding carbon nanofibers were electroplated with silver for use in supercapacitor applications. The brittle carbon nanofibers were encased within bendable silver shells to provide superior flexibility and resilience of the supercapacitors. The enhanced electrical conductivity derived from the silver shell structure dramatically increased the capacitance of the supercapacitor. The silver shell also conferred structural stability to the carbon core, thus furnishing stable, long-term electrode performance. Nearly 100% of the specific capacitance was retained after N  = 10,000 galvanostatic charge-discharge cycles. The mechanical endurance or stability of the fabricated electrode was evaluated using 1,000 bending cycles, demonstrating that the electrode performance remained unchanged. Cyclic voltammetry and galvanostatic discharge curves were measured at various scan rates and current densities. The fabricated electrodes were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy, which clearly illustrated the carbon-core and silver-shell structure. [ABSTRACT FROM AUTHOR]

Currently, lightweight wearable energy storage devices are in great demand owing to their use in wearable electronics and energy-efficient electric vehicles. Freestanding carbon nanofibers replace the need for metal substrates while providing a rapid electrical network owing to their excellent electrical properties. Bimetallic oxides with multivalent oxidation states facilitate the rapid transfer of electrolytic ions owing to efficient Faradaic reactions, thereby enhancing the overall energy storage capability. In this study, CoOx@CNF was derived from ZIF-67 (zeolitic-imidazolate framework) @PAN-2MI fibers that were stabilized in air at 280°C and then annealed in argon at 900°C. Subsequently, Co was seeded on the annealed CoOx@CNF and subjected to a hydrothermal process in sodium molybdate dihydrate solution to grow CoMoO4 nanosheets, eventually forming bimetallic CoMoO4@CNF. The concentration of sodium molybdate solution was varied to determine the optimal growth conditions for CoMoO4 nanosheets. The energy density of the optimal bimetallic CoMoO4@CNF sample was 166.5 μWh cm-2 at a power density of 200 μW cm-2; this represented a nearly twofold increase compared to that of the single metallic CoOx@CNF. Powering humidity sensors using only one CoMoO4@CNF supercapacitor was demonstrated. The optimal sample remained stable during long-term galvanostatic charge and discharge cycles ( N cyc = 30,000) and retained 100% of its specific capacitance. [ABSTRACT FROM AUTHOR]

We demonstrate electrospun ternary Fe 2 O 3 -SnO x -carbon nanofiber (CNF) composites as high-performance, flexible, and freestanding anodes for Li ion batteries (LIBs). In the ternary composites, the CNF matrix accommodates volume changes of Fe and Sn during lithiation and delithiation, while also providing short Li ion diffusion pathways, which ultimately enhances the capacity of the LIB. The concentrations of Fe and Sn were varied to find the optimal composition. Higher Sn content increased capacity at slow discharge rates, but decreased capacity at higher discharge rates. Increasing Fe to Sn ratio improved performance at high discharge rates. The Fe 2 O 3 -SnO x -CNF composite fabricated at a Fe:Sn weight ratio of 3:1 exhibited a reversible capacity value of 756 mAh·g −1 after 55 cycles. The materials and methods demonstrated here are unique in producing a flexible, free-standing mat with high gravimetric capacity using a simple and scalable production process. [ABSTRACT FROM AUTHOR]

• The review focuses on potential of flexible, freestanding binder-free nanofiber electrodes for real time applications. • The advancement of the hollow/porous composite nanofibers, and process parameters are presented. • Morphology engineering over composite nanofibers for enhancing electrochemical performance is reviewed. • Current challenges and future prospect of binder free composite nanofibers for flexible SCs are suggested. The versatile electrospinning technique is scalable and suitable to fabricate highly conducting freestanding carbon nanofiber composite electrodes for energy storage devices. Freestanding/flexible electrodes hold enormous potential for use in wearable electronic devices. Carbon-yielding polymers and the optimal use of sacrificial polymers, metal oxides, and sulfides retain the flexibility and enhance the surface area and pseudocapacitance of electrodes. Both as-prepared electrospun fibers and carbonized nanofibers are compatible with surface decoration via various chemical and electrochemical routes. Metal oxides/sulfides with various morphologies, such as nanocones and nanosheets, can be grown on the carbon nanofibers or on the as-prepared electrospun fibers using chemical synthesis methods such as electrodeposition, hydrothermal processes, and chemical impregnation to enhance the pseudocapacitance of the electrodes. Similarly, the deposition of metal organic frameworks on as-prepared electrospun fibers embellishes these fibers with nanostructures of specific morphologies such as dodecahedral and spindle-shaped structures. Under optimal conditions, these morphologies do not hamper the flexibility of the fibers, and binders are not required to retain them or maintain the electrode integrity. The engineering of electrodes with various morphologies and process parameters is presented systematically. Electrospinning-derived electrodes that have demonstrated significant electrochemical performance are highlighted and critically analyzed, and the energy storage mechanisms of these supercapacitors are described in detail. [ABSTRACT FROM AUTHOR]

• Flexible, freestanding, binder-free nanofiber electrodes for lithium-ion batteries are introduced. • Significance and limitations of electrospinning for fabrication of freestanding electrodes are discussed. • Parameters and polymers for the design of porous and core–shell nanofibers are analyzed. • Feasibility of decorating fibers with nanosheets, nanocones, and polyhedra of active materials is discussed. Carbon nanofibers derived from electrospun precursors show great promise for electronic applications owing to their flexibility, conductivity, high surface area, and open structure. The integration of metal oxides and sulfides in carbon nanofibers, rather than using them with other binders, eliminates many problems caused by poor adhesion, nanomaterial agglomeration, excess mass contributed by inactive binders, and low conductivity of embedded active materials. The engineering of electrospun fibers with novel morphologies, such as core–shell, hollow, or porous structures, and the use of decorated carbon nanofibers (e.g. , by electrodeposition or co-precipitation) are discussed in this review. Representative schematic illustrations of the lithium-storage mechanism for these binder-free electrodes are presented. We describe how the electrospinning technique can offer a cost-effective strategy for fabrication of lightweight lithium-ion batteries with high capacity and excellent bendability. This review presents the fascinating morphologies of these specially designed carbon nanofiber electrodes, which enhance the electrochemical performance of metal oxides and sulfides, illustrating their enormous potential for use in wearable electronic devices and hybrid electric vehicles. [ABSTRACT FROM AUTHOR]

• Electrospun nanofiber mats are surface-decorated with Mn-doped ZIF-8 nanoparticles. • Freestanding flexible Mn@ZnO/CNF is demonstrated as a promising supercapacitor electrode. • Enhanced capacitance of 501 F·g−1 and 92% capacitance retention after 10,000 cycles are obtained. We demonstrate freestanding, flexible, and cost-effective supercapacitor electrodes comprising carbon nanofibers (CNFs) decorated with metal oxide framework (MOF)-derived manganese-doped zinc oxide (Mn@ZnO). Nanoparticles of manganese-doped zeolitic imidazolate framework (ZIF-8) were grown directly on electrospun polyacrylonitrile nanofibers by a simple solution-phase synthesis. Carbonization of these composite fibers produced high surface area dodecahedral Mn@ZnO on core CNFs that provide fast electron-transfer pathways. The synergy between Mn@ZnO (active sites for Faradaic reactions) and the highly electrically conductive carbon nanofiber improves the performance of the supercapacitor electrode. The Mn@ZnO/CNF electrodes exhibit a high specific capacitance of 501 F·g−1 and retain >92% of their initial capacitance after 10,000 cycles. The optimized Mn@ZnO/CNF electrodes deliver impressive energy densities of 72.1 W·h·kg−1 and 33.3 W·h·kg−1 at power densities of 500 W·kg−1 and 5000 W·kg−1, respectively. This electrochemical performance demonstrates that the Mn@ZnO/CNF nanostructured composite is a robust electrode material for long-lifetime high-rate energy storage/delivery devices. [ABSTRACT FROM AUTHOR]

Zeolitic imidazolate framework-derived carbon and metal oxide composites are promising for applications in lithium-ion batteries due to their morphology and porous structure. Herein, we report freestanding carbon nanofibers grafted with amorphous ZnO where the rhombic dodecahedral morphology of the parent (ZIF8) is maintained. The impacts of ZIF8 loading temperature on particle size after pyrolysis and on the electrochemical properties of these hybrid structures for LIBs are investigated. Polyacrylonitrile nanofiber loaded with ZIF8 at 45 °C exhibits a perfect sodalite topology and optimized particle size. The carbonized freestanding composite fibers have a high specific capacity of 818 mAh·g −1 at a current density of 100 mA·g −1 after 100 cycles. This remarkable performance of ZIF8-derived ZnO is due to its loading and coordination chemistry, which facilitates flexible volume changes and continuity in electrical conductivity through a continuous carbon nanofiber. [ABSTRACT FROM AUTHOR]

We demonstrate the fabrication of core-shell SnO x /carbon nanofiber (CNF) composite mats via single-nozzle one-step electrospinning for use as flexible freestanding electrodes in supercapacitors. The freestanding and flexible nature of the composites is essential for their use in lightweight, portable, and foldable electronic devices and eliminates the need for a separate current collector. We fully characterized the structural and morphological properties of the SnO x /CNF mats and optimized the SnO x to CNF precursor ratio. The optimized SnO x /CNF-based symmetric supercapacitor exhibited a capacitance of 289 F·g −1 at a scan rate of 10 mV·s −1 . Moreover, it retained more than 88% of its initial capacitance after 5000 cycles, highlighting the long-term stability of supercapacitors based on these SnO x /CNF mats. [ABSTRACT FROM AUTHOR]

We demonstrate the fabrication of a MnO x /carbamide carbon nanofiber (CCNF) composite consisting of MnO particles embedded in CCNFs as a highly flexible and freestanding electrode material for supercapacitors. A sacrificial polymer component, polymethylmethacrylate, included in the precursor solution, pyrolyzes during heating, resulting in pores in the fibers, some of which are filled by the MnO nanocrystals. Carbamide is added to control the size of the MnO x particles as well as to increase the carbon content of the composite and hence its conductivity. The X-ray diffraction and Raman spectra of the composite show that the MnO particles formed have low crystallinity. Transmission electron microscopy confirms that the MnO particles are distributed very uniformly over the CCNFs. Symmetric supercapacitors constructed using electrodes of this composite exhibit specific capacitances of 498 F∙g −1 at a scan rate of 10 mV∙s −1 and 271 F∙g −1 at a current density of 1 A∙g −1 . They also exhibit excellent long-term cycling performance, retaining 93% of their initial capacity after 5000 cycles of galvanostatic charging/discharging. [ABSTRACT FROM AUTHOR]

We have produced flexible, freestanding, and light weight mats of FeO x -decorated carbon nanofibers (CNFs) and demonstrated their use in supercapacitors with high energy and power density and excellent long term capacitance retention. Highly flexible carbon-iron oxide nanofibers were synthesized by electrospinning a solution of polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), and iron acetylacetonate (FeAcAc), followed by annealing to carbonize the PAN, pyrolyze the PMMA to produce pores, and convert FeAcAc to FeO nanoparticles. The morphology of the FeO x /CNF composite was determined by scanning and transmission electron microscopies, which showed that the embedded FeO x nanoparticles were well distributed in the CNF electrode. We employed cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy to evaluate the electrochemical performance of symmetric supercapacitors prepared from the FeO x /CNF composite. The supercapacitors exhibited high specific capacitance (427 F·g −1 at 10 mV·s −1 and 436 F·g −1 at 1 A·g −1 in the optimal case) and good stability, retaining 89% of their initial capacitance after 5000 cycles at a current density of 1 A·g −1 . The optimal device achieved an energy density of 167 Wh·kg −1 at a power density of 0.75 kW·kg −1 , and an energy density of 66 Wh·kg −1 at a power density of 7.5 kW·kg −1 . These combinations of energy and power densities can meet the needs of many emerging supercapacitor applications. [ABSTRACT FROM AUTHOR]

We demonstrate the fabrication of flexible electrospun carbon nanofibers (CNFs) containing uniformly distributed carbon nanotubes (CNTs) and iron oxide (FeO x ) nanoparticles (NPs) as a composite material for lithium-ion battery (LIB) anodes. The uniform incorporation of CNTs and FeO x NPs within the CNFs was confirmed by transmission electron microscopy. Embedding even a small amount of CNTs (0.1 wt%) in the CNFs increased the LIB performance by a factor of two compared with pure CNFs. The specific capacity was also increased, by 25%, when an iron source (iron (III) acetylacetonate) was added at a concentration of 2 wt%. The FeO x -CNT/CNF electrode maintained a capacity of 1008 mAh∙g −1 at a current density of 100 mA∙g −1 after 100 cycles. Further, the high-performance anode material was freestanding, flexible, and lightweight. This makes it suitable for next-generation LIBs, which must deliver high power over long periods while also being light and flexible. [ABSTRACT FROM AUTHOR]