Zinc-ion Batteries (ZIBs) are currently considered to be effective energy storage devices for wearable electronics because of their low cost and high safety. Indeed, ZIBs show high power density and safety compared with conventional lithium ion batteries (LIBs) and exhibit high energy density in comparison with supercapacitors (SCs). However, in spite of their advantages, further current collector development is needed to enhance the electrochemical performance of ZIBs. To design the optimized current collector for high performance ZIBs, a high quality graphene film is suggested here, with improved electrical conductivity by controlling the defects in the graphene film. The graphene film showed improved electrical conductivity and good electron transfer between the current collector and active material, which led to a high specific capacity of 346.3 mAh g-1 at a current density of 100 mA g-1, a high-rate performance with 116.3 mAh g-1 at a current density of 2,000 mA g-1, and good cycling stability (68.0 % after 100 cycles at a current density of 1,000 mA g-1). The improved electrochemical performance is firmly because of the defects-controlled graphene film, leading to improved electrical conductivity and thus more efficient electron transfer between the current collector and active material.

Fibrous supercapacitors (FSs), owing to their high power density, good safety characteristic, and high flexibility, have recently been in the spotlight as energy storage devices for wearable electronics. However, despite these advantages, FCs face many challenges related to their active material of carbon fiber (CF). CF has low surface area and poor wettability between electrode and electrolyte, which result in low capacitance and poor long-term stability at high current densities. To overcome these limits, fibrous supercapacitors made using surface-activated CF (FS-SACF) are here suggested; these materials have improved specific surface area and better wettability, obtained by introducing porous structure and oxygen-containing functional groups on the CF surface, respectively, through surface engineering. The FS-SACF shows an improved ion diffusion coefficient and better electrochemical performance, including high specific capacity of 223.6 mF cm2 at current density of 10 μA cm2, high-rate performance of 171.2 mF cm2 at current density of 50.0 μA cm2, and remarkable, ultrafast cycling stability (96.2 % after 1,000 cycles at current density of 250.0 μA cm2). The excellent electrochemical performance is definitely due to the effects of surface functionalization on CF, leading to improved specific surface area and superior ion diffusion capability.

Owing to its low cost, easy fabrication process, and good ionic properties, aqueous supercapacitors are under strong consideration as next-generation energy storage devices. However, the limitation of the current collector is its poor electrochemical stability, leading to low energy storage performance. Therefore, a reasonable design of the current collector and the acidic electrolyte is a necessary, as well as interfacial engineering to enhance the electrochemical performance. In the present study, graphite foil, with excellent electrochemical stability and good electrical properties, is suggested as a current collector of aqueous supercapacitors. This strategy results in excellent electrochemical performance, including a high specific capacitance of 215 F g−1 at a current density of 0.1 A g−1, a superior high-rate performance (104 F g−1 at a current density of 20.0 A g−1), and a remarkable cycling stability of 98 % at a current density of 10.0 A g−1 after 9,000 cycles. The superior energy storage performance is mainly ascribed to the improved ionic diffusion ability during cycling.

Electronic textiles promise to provide an intelligent platform to enlarge the scope of wearable electronic applications. Therefore, the combination of flexible energy storage devies into wearable systems is a key for operating these electronic textiles during bending, knotting, and rolling. Nonetheless, the application of fibrous supercapacitors consisting of a gel-electrolyte and carbon fiber electrode is still obstructed by low capacitance, low rate-performance, and poor cycling stability owing to the inefficient interface between the gel-electrolyte and electrode. Here, a fibrous supercapacitor is obtained using an optimized gelelectrolyte that improves the ionic diffusion capability. The optimized fibrous supercapacitor shows a superior electrochemical performance, including high specific capacitance of 41 mF cm−2 at current density of 2.0 μA cm−2, high-rate performance with 17 mF cm−2 at a current density of 15.0 μA cm−2, and outstanding cycling stability (88% after 3,000 cycles at a current density of 200.0 μA cm−2). The excellent energy storage performance is mainly attributed to the optimzied interface between the gelelectrolyte and electrode material, leading to an improved ionic diffusion capability.

Zinc-ion Batteris (ZIBs) are recently being considered as energy storage devices due to their high specific capacity and high safety, and the abundance of zinc sources. Especially, ZIBs can overcome the drawbacks of conventional lithium ion batteris (LIBs), such as cost and safety issues. However, in spite of their advantages, the cathode materials under development are required to improve performance of ZIBs, because the capacity and cycling stability of ZIBs are mainly influenced by the cathode materials. To design optimized cathode materials for high performance ZIBs, a novel manganese oxide (MnO2) coated graphite sheet is suggested herein with improved zinc-ion diffusion capability thanks to the uniformly decorated MnO2 on the graphite sheet surface. Especially, to optimize MnO2 on the graphite sheet surface, amounts of percursors are regulated. The optimized MnO2 coated graphite sheet shows a superior zinc-ion diffusion ability and good electrochemical performance, including high specific capacity of 330.8 mAh g−1 at current density of 0.1 A g−1, high-rate performance with 109.4 mAh g−1 at a current density of 2.0 A g−1, and remarkable cycling stability (82.2 % after 200 cycles at a current density of 1.0 A g−1). The excellent electrochemical performance is due to the uniformly decorated MnO2 on the graphite sheet surface, which leads to excellent zinc-ion diffusion ability. Thus, our study can provide a promising strategy for high performance next-generation ZIBs in the near future.

Zinc-ion hybrid supercapacitors (ZICs) have recently been spotlighted as energy storage devices due to their high energy and high power densities. However, despite these merits, ZICs face many challenges related to their cathode materials, activated carbon (AC). AC as a cathode material has restrictive electrical conductivity, which leads to low capacity and lifetime at high current densities. To overcome this demerit, a novel boron (B) doped AC is suggested herein with improved electrical conductivity thanks to B-doping effect. Especially, in order to optimize B-doped AC, amounts of precursors are regulated. The optimized B-doped AC electrode shows a good charge-transfer process and superior electrochemical performance, including high specific capacity of 157.4 mAh g−1 at current density of 0.5 A g−1, high-rate performance with 66.6 mAh g−1 at a current density of 10 A g−1, and remarkable, ultrafast cycling stability (90.7 % after 10,000 cycles at a current density of 5 A g−1). The superior energy storage performance is attributed to the B-doping effect, which leads to an excellent charge-transfer process of the AC cathode. Thus, our strategy can provide a rational design for ultrafast cycling stability of next-generation supercapacitors in the near future.

The performance of Li-ion hybrid supercapacitors (asymmetric-type) depends on many factors such as the capacity ratio, material properties, cell designs and operating conditions. Among these, in consideration of balanced electrochemical reactions, the capacity ratio of the negative (anode) to positive (cathode) electrode is one of the most important factors to design the Li-ion hybrid supercapacitors for high energy storing performance. We assemble Li-ion hybrid supercapacitors using activated carbon (AC) as anode material, lithium manganese oxide as cathode material, and organic electrolyte (1 mol L−1 LiPF6 in acetonitrile). At this point, the thickness of the anode electrode is controlled at 160, 200, and 240 μm. Also, thickness of cathode electrode is fixed at 60 μm. Then, the effect of negative and positive electrode ratio on the electrochemical performance of AC/LiMn2O4 Li-ion hybrid supercapacitors is investigated, especially in the terms of capacity and cyclability at high current density. In this study, we demonstrate the relationship of capacity ratio between anode and cathode electrode, and the excellent electrochemical performance of AC/LiMn2O4 Li-ion hybrid supercapacitors. The remarkable capability of these materials proves that manipulation of the capacity ratio is a promising technology for high-performance Li-ion hybrid supercapacitors.

Zn-ion supercapacitors (ZICs) show high energy densities with long cycling life for use in electronic devices. Porous Zn electrodes as anodes for ZICs are fabricated by chemical etching process using optimized conditions. The structures, morphologies, chemical bonding states, porous structure, and electrochemical behavior are examined. The optimized porous Zn electrode shows a root mean square of roughness of 173 nm and high surface area of 153 μm2. As a result, ZIC using the optimized porous Zn electrode presents excellent electrochemical performance with high specific capacitance of 399 F g−1 at current density of 0.5 A g−1, high-rate performance (79 F g−1 at a current density of 10.0 A g−1), and outstanding cycling stability (99 % after 1,500 cycles). The development of energy storage performance using synergistic effects of high roughness and high surface area is due to increased electroactive sites by surface functionalization of Zn electrode. Thus, our strategy will lead to a rational design and contribute to next-generation supercapacitors in the near future.

The design of non-precious electrocatalysts with low-cost, good stability, and an improved oxygen reduction reaction(ORR) to replace the platinium-based electrocatalyst is significant for application of fuel cells and metal-air batteries with high energy density. In this study, we synthesize iron-carbide(Fe3C) embedded nitrogen(N) doped carbon nanofiber(CNF) as electrocatalysts for ORRs using electrospinning, precursor deposition, and carbonization. To optimize electrochemical performance, we study the three stages according to different amounts of iron precursor. Among them, Fe3C-embedded N doped CNF-1 exhibits the most improved electrochemical performance with a high onset potential of −0.18 V, a high E1/2 of −0.29 V, and a nearly four-electron pathway (n = 3.77). In addition, Fe3C-embedded N doped CNF-1 displays exellent long-term stabillity with the lowest ΔE1/2= 8 mV compared to the other electrocatalysts. The improved electrochemical properties are attributed to synergestic effect of N-doping and well-dispersed iron carbide embedded in CNF. Consequently, Fe3C-embedded N doped CNF is a promising candidate for non-precious electrocatalysts for high-performance ORRs.

Mesoporous carbon nanofibers as electrode material for electrical double-layer capacitors(EDLCs) are fabricated using the electrospinning method and carbonization. Their morphologies, structures, chemical bonding states, porous structure, and electrochemical performance are investigated. The optimized mesoporous carbon nanofiber has a high sepecific surface area of 667 m2 g−1, high average pore size of 6.3 nm, and high mesopore volume fraction of 80 %, as well as a unifom network structure consiting of a 1-D nanofiber stucture. The optimized mesoporous carbon nanofiber shows outstanding electrochemical performance with high specific capacitance of 87 F g−1 at a current density of 0.1 A g−1, high-rate performance (72 F g−1 at a current density of 20.0 A g−1), and good cycling stability (92 F g−1 after 100 cycles). The improvement of the electrochemical performance via the combined effects of high specific surface area are due to the high mesopore volume fraction of the carbon nanofibers.

Well-dispersed platinum catalysts on ruthenium oxide nanofiber supports are fabricated using electrospinning, post-calcination, and reduction methods. To obtain the well-dispersed platinum catalysts, the surface of the nanofiber supports is modified using post-calcination. The structures, morphologies, crystal structures, chemical bonding energies, and electrochemical performance of the catalysts are investigated. The optimized catalysts show well-dispersed platinum nanoparticles (1-2 nm) on the nanofiber supports as well as a uniform network structure. In particular, the well-dispersed platinum catalysts on the ruthenium oxide nanofiber supports display excellent catalytic activity for oxygen reduction reactions with a half-wave potential (E1/2) of 0.57 V and outstanding long-term stability after 2000 cycles, resulting in a lower E1/2 potential degradation of 19 mV. The enhanced electrochemical performance for oxygen reduction reactions results from the well-dispersed platinum catalysts and unique nanofiber supports.

Carbon nanofiber (CNF) is used as an electrode material for electrical double layer capacitors (EDLCs), and is being consistently researched to improve its electrochemical performance. However, CNF still faces important challenges due to the low mesopore volume, leading to a poor high-rate performance. In the present study, we prepared the unique architecture of the activated mesoporous CNF with a high specific surface area and high mesopore volume, which were successfully synthesized using PMMA as a pore-forming agent and the KOH activation. The activated mesoporous CNF was found to exhibit the high specific surface area of 703 m2 g−1, total pore volume of 0.51 cm3 g−1, average pore diameter of 2.9 nm, and high mesopore volume of 35.2 %. The activated mesoporous CNF also indicated the high specific capacitance of 143 F g−1, high-rate performance, high energy density of 17.9-13.0Wh kg−1, and excellent cycling stability. Therefore, this unique architecture with a high specific surface area and high mesopore volume provides profitable synergistic effects in terms of the increased electrical double-layer area and favorable ion diffusion at a high current density. Consequently, the activated mesoporous CNF is a promising candidate as an electrode material for high-performance EDLCs.

N-doped carbon nanofibers as catalysts for oxygen-reduction reactions are synthesized using electrospinning and carbonization. Their morphologies, structures, chemical bonding states, and electrochemical performance are characterized. The optimized N-doped carbon nanofibers exhibit graphitization of carbon nanofibers and an increased nitrogen doping as well as a uniform network structure. In particular, the optimized N-doped carbon nanofibers show outstanding catalytic activity for oxygen-reduction reactions, such as a half-wave potential (E1/2) of 0.43 V, kinetic limiting current density of 6.2 mA cm-2, electron reduction pathways (n = 3.1), and excellent long-term stability after 2000 cycles, resulting in a lower E1/2 potential degradation of 13 mV. The improvement in the electrochemical performance results from the synergistic effect of the graphitization of carbon nanofibers and the increased amount of nitrogen doping.

Fluorine-doped tin oxide (FTO) nanoparticles have been successfully synthesized using ultrasonic spray pyrolysis. The morphologies, crystal structures, chemical bonding states, and electrochemical properties of the nanoparticles are investigated. The FTO nanoparticles show uniform morphology and size distribution in the range of 6-10 nm. The FTO nanoparticles exhibit excellent electrochemical performance with high discharge specific capacity and good cycling stability (620mA h g−1 capacity retention up to 50 cycles), as well as excellent high-rate performance (250 mA h g−1 at 700mAg−1) compared to that of commercial SnO2. The improved electrochemical performance can be explained by two main effects. First, the excellent cycling stability with high discharge capacity is attributed to the nano-sized FTO particles, which are related to the increased electrochemical active area between the electrode and electrolyte. Second, the superb high-rate performance and the excellent cycling stability are ascribed to the increased electrical conductivity, which results from the introduction of fluorine doping in SnO2. This noble electrode structure can provide powerful potential anode materials for high-performance lithiumion batteries.

Octahedral Co3O4/carbon nanofiber (CNF) composites are fabricated using electrospinning and hydrothermal methods. Their morphological characteristics, chemical bonding states, and electrochemical properties are used to demonstrate the improved photovoltaic properties of the samples. Octahedral Co3O4 grown on CNFs is based on metallic Co nanoparticles acting as seeds in the CNFs, which seeds are directly related to the high performance of DSSCs. The octahedral Co3O4/CNFs composites exhibit high photocurrent density (12.73 mA/m2), superb fill factor (62.1 %), and excellent power conversion efficiency (5.61 %) compared to those characteristics of commercial Co3O4, conventional CNFs, and metallic Co-seed/CNFs. These results can be described as stemmnig from the synergistic effect of the porous and graphitized matrix formed by catalytic graphitization using the metal cobalt catalyst on CNFs, which leads to an increase in the catalytic activity for the reduction of triiodide ions. Therefore, octahedral Co3O4/CNFs composites can be used as a counter electrode for Pt-free dye-sensitized solar cells.

Perforated polygonal cobalt oxide (Co3O4) is synthesized using electrospinning and a hydrothermal methodfollowed by the removal of a carbon nanofiber (CNF) template. To investigate their formation mechanism, thermogravi-metric analysis, field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy are examined. To obtain the optimum condition of perforated polygonal Co3O4, we pre-pare three different weight ratios of the Co precursor and the CNF template: sample A (Co precursor:CNF template-10:1), sample B (Co precursor:CNF template-3.2:1), and sample C (Co precursor:CNF template-2:1). Among them, sam-ple A exhibits the perforated polygonal Co3O4 with a thin carbon layer (5.7-6.2 nm) owing to the removal of CNF tem-plate. However, sample B and sample C synthesized perforated round Co3O4 and destroyed Co3O4 powders, respectively,due to a decreased amount of Co precursor. The increased amount of the CNF template prevents the formation of polygonalCo3O4. For sample A, the optimized weight ratio of the Co precursor and CNF template may be related to the suc-cessful formation of perforated polygonal Co3O4. Thus, perforated polygonal Co3O4 can be applied to electrode materialsof energy storage devices such as lithium ion batteries, supercapacitors, and fuel cells.

To improve the methanol electro-oxidation in direct methanol fuel cells(DMFCs), Pt electrocatalysts embedded on porous carbon nanofibers(CNFs) were synthesized by electrospinning followed by a reduction method. To fabricate the porous CNFs, we prepared three types of porous CNFs using three different amount of a styrene-co acrylonitrile(SAN) polymer: 0.2 wt%, 0.5 wt%, and 1 wt%, respectively. A SAN polymer, which provides vacant spaces in porous CNFs, was decomposed and burn out during the carbonization. The structure and morphology of the samples were examined using field emission scanning electron microscopy and transmission electron microscopy and their surface area were measured using the Brunauer- Emmett-Teller(BET). The crystallinities and chemical compositions of the samples were examined using X-ray diffraction and X-ray photoelectron spectroscopy. The electrochemical properties on the methanol electro oxidation were characterized using cyclic voltammetry and chronoamperometry. Pt electrocatalysts embedded on porous CNFs containing 0.5 wt% SAN polymer exhibited the improved methanol oxidation and electrocatalytic stability compared to Pt/conventional CNFs and commercial Pt/ C(40 wt% Pt on Vulcan carbon, E-TEK).

Flake-like LiCoO2 nanopowders were fabricated using electrospinning. To investigate their formation mech-anism, field-emssion scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were carriedout. Among various parameters of electrospinning, we controlled the molar concentration of the precursor and the PVPpolymer. When the molar concentration of lithium and cobalt was 0.45 M, the morphology of LiCoO2 nanopowders wasirregular and round. For 1.27 M molar concentration, the LiCoO2 nanopowders formed with flake-like morphology. Forthe PVP polymer, the molar concentration was set to 0.011 mM, 0.026 mM, and 0.043 mM. Irregular LiCoO2 nanop-owders were formed at low concentration (0.011 mM), while flake-like LiCoO2 were formed at high concentration(0.026 mM and 0.043 mM). Thus, optimized molar concentration of the precursor and the PVP polymer may be relatedto the successful formation of flake-like LiCoO2 nanopowders. As a results, the synthesized LiCoO2 nanopowder can beused as the electrode material of Li-ion batteries.