High performance lithium-ion batteries (LIBs) have attracted considerable attention as essential energy sources for high-technology electrical devices such as electrical vehicles, unmanned drones, uninterruptible power supply, and artificial intelligence robots because of their high energy density (150-250 Wh/kg), long lifetime (> 500 cycles), low toxicity, and low memory effects. Of the high-performance LIB components, cathode materials have a significant effect on the capacity, lifetime, energy density, power density, and operating conditions of high-performance LIBs. This is because cathode materials have limitations with respect to a lower specific capacity and cycling stability as compared to anode materials. In addition, cathode materials present difficulties when used with LIBs in electric vehicles because of their poor rate performance. Therefore, this study summarizes the structural and electrochemical properties of cathode materials for LIBs used in electric vehicles. In addition, we consider unique strategies to improve their structural and electrochemical properties.

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• Estimation of Representative Mechanical Property of Porous Electrode for Secondary Batteries with Homogenization Method
  Changmin Pyo, Jaewoong Kim
  Journal of the Korean Society of Manufacturing Process Engineers.2022; 21(9): 85.     CrossRef

Sb-doped SnO₂ (ATO) transparent conducting films are fabricated using horizontal ultrasonic spray pyrolysis deposition (HUSPD) to form uniform and compact film structures with homogeneously supplied precursor solution. To optimize the molar concentration and transparent conducting performance of the ATO films using HUSPD, we use precursor solutions of 0.15, 0.20, 0.25, and 0.30 M. As the molar concentration increases, the resultant ATO films exhibit more compact surface structures because of the larger crystallite sizes and higher ATO crystallinity because of the greater thickness from the accelerated growth of ATO. Thus, the ATO films prepared at 0.25 M have the best transparent conducting performance (12.60±0.21 Ω/□ sheet resistance and 80.83% optical transmittance) and the highest figure-of-merit value (9.44±0.17 × 10^-3 Ω^-1). The improvement in transparent conducting performance is attributed to the enhanced carrier concentration by the improved ATO crystallinity and Hall mobility with the compact surface structure and preferred (211) orientation, ascribed to the accelerated growth of ATO at the optimized molar concentration. Therefore, ATO films fabricated using HUSPD are transparent conducting film candidates for optoelectronic devices.

Uniform TiO₂ blocking layers (BLs) are fabricated using ultrasonic spray pyrolysis deposition (USPD) method. To improve the photovoltaic performance of dye-sensitized solar cells (DSSCs), the BL thickness is controlled by using USPD times of 0, 20, 60, and 100 min, creating TiO₂ BLs of 0, 40, 70, and 100 nm, respectively, in average thickness on fluorine-doped tin oxide (FTO) glass. Compared to the other samples, the DSSC containing the uniform TiO₂ BL of 70 nm in thickness shows a superior power conversion efficiency of 7.58±0.20% because of the suppression of electron recombination by the effect of the optimized thickness. The performance improvement is mainly attributed to the increased open-circuit voltage (0.77±0.02 V) achieved by the increased Fermi energy levels of the working electrodes and the improved short-circuit current density (15.67±0.43 mA/cm2) by efficient electron transfer pathways. Therefore, optimized TiO₂ BLs fabricated by USPD may allow performance improvements in DSSCs.

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• Flexible Dye-sensitized Solar Cell Using Titanium Gel at Low Temperature
  Seung Hwan Ji, Hyunsu Park, Doyeon Kim, Do Hyung Han, Hye Won Yun, Woo-Byoung Kim
  Korean Journal of Materials Research.2019; 29(3): 183.     CrossRef
• Surface tailoring of zinc electrodes for energy storage devices with high-energy densities and long cycle life
  Geon-Hyoung An, SeungNam Cha, Jung Inn Sohn
  Applied Surface Science.2019; 467-468: 1157.     CrossRef
• Crystallinity Control Effects on Vanadium Oxide Films for Enhanced Electrochromic Performances
  Kue-Ho Kim, Ju-Won Bae, Tae-Kuen Lee, Hyo-Jin Ahn
  Korean Journal of Materials Research.2019; 29(6): 385.     CrossRef

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 (E_1/2) of 0.57 V and outstanding long-term stability after 2000 cycles, resulting in a lower E_1/2 potential degradation of 19 mV. The enhanced electrochemical performance for oxygen reduction reactions results from the well-dispersed platinum catalysts and unique nanofiber supports.

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• Fabrication of Porous Electrodes for Zinc-Ion Supercapacitors with Improved Energy Storage Performance
  Geon-Hyoung An
  Korean Journal of Materials Research.2019; 29(8): 505.     CrossRef
• Surface tailoring of zinc electrodes for energy storage devices with high-energy densities and long cycle life
  Geon-Hyoung An, SeungNam Cha, Jung Inn Sohn
  Applied Surface Science.2019; 467-468: 1157.     CrossRef
• Synthesis of Nitrogen Doped Protein Based Carbon as Pt Catalysts Supports for Oxygen Reduction Reaction
  Young-geun Lee, Geon-hyeong An, Hyo-Jin Ahn
  Korean Journal of Materials Research.2018; 28(3): 182.     CrossRef

Transparent conducting electrodes (TCEs) are attracting considerable attention as an important component for emerging optoelectronic applications such as liquid crystal displays, touch panels, and solar cells owing to their attractive combination of low resistivity (< 10^-3 Ω cm) and high transparency (>80%) in the visible region. The solutionbased process has unique properties of an easy fabrication procedure, scalability, and low cost compared to the conventional vacuum-based process and may prove to be a useful process for fabricating TCEs for future optoelectronic applications demanding large scale and flexibility. In this paper, we focus on the introduction of a solution-based process for TCEs. In addition, we consider the powder materials used to fabricate solution-based TCEs and strategies to improve their transparent conducting properties.

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• Electrically conductive and anti-corrosive coating on copper foil assisted by polymer-nanocomposites embedded with graphene
  Han Kim, Hyemin Lee, Hyo-Ryoung Lim, Hong-Baek Cho, Yong-Ho Choa
  Applied Surface Science.2019; 476: 123.     CrossRef

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 E_1/2 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.

Carbon nanofiber (CNF) composites coated with spindle-shaped Fe₂O₃ nanoparticles (NPs) are fabricated by a combination of an electrospinning method and a hydrothermal method, and their morphological, structural, and chemical properties are measured by field-emission scanning electron microscopy, transmission electron microscopy, Xray diffraction, and X-ray photoelectron spectroscopy. For comparison, CNFs and spindle-shaped Fe₂O₃ NPs are prepared by either an electrospinning method or a hydrothermal method, respectively. Dye-sensitized solar cells (DSSCs) fabricated with the composites exhibit enhanced open circuit voltage (0.70 V), short-circuit current density (12.82 mA/cm2), fill factor (61.30%), and power conversion efficiency (5.52%) compared to those of the CNFs (0.66 V, 11.61 mA/cm2, 51.96%, and 3.97%) and spindle-shaped Fe₂O₃ NPs (0.67 V, 11.45 mA/cm2, 50.17%, and 3.86%). This performance improvement can be attributed to a synergistic effect of a superb catalytic reaction of spindle-shaped Fe₂O₃ NPs and efficient charge transfer relative to the one-dimensional nanostructure of the CNFs. Therefore, spindle-shaped Fe₂O₃-NPcoated CNF composites may be proposed as a potential alternative material for low-cost counter electrodes in DSSCs.

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• Ni Nanoparticles-Graphitic Carbon Nanofiber Composites for Pt-Free Counter Electrode in Dye-Sensitized Solar Cells
  Dong-Hyeun Oh, Bon-Ryul Koo, Yu-Jin Lee, HyeLan An, Hyo-Jin Ahn
  Korean Journal of Materials Research.2016; 26(11): 649.     CrossRef

Perforated polygonal cobalt oxide (CO₃O₄) is synthesized using electrospinning and a hydrothermal method followed by the removal of a carbon nanofiber (CNF) template. To investigate their formation mechanism, thermogravimetric analysis, field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Xray photoelectron spectroscopy are examined. To obtain the optimum condition of perforated polygonal CO₃O₄, we prepare 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, sample A exhibits the perforated polygonal CO₃O₄ with a thin carbon layer (5.7-6.2 nm) owing to the removal of CNF template. 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 polygonal CO₃O₄. For sample A, the optimized weight ratio of the Co precursor and CNF template may be related to the successful formation of perforated polygonal CO₃O₄. Thus, perforated polygonal CO₃O₄ can be applied to electrode materials of energy storage devices such as lithium ion batteries, supercapacitors, and fuel cells.

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• Synthesis of Nitrogen Doped Protein Based Carbon as Pt Catalysts Supports for Oxygen Reduction Reaction
  Young-geun Lee, Geon-hyeong An, Hyo-Jin Ahn
  Korean Journal of Materials Research.2018; 28(3): 182.     CrossRef
• Electrochemical Behavior of Well-dispersed Catalysts on Ruthenium Oxide Nanofiber Supports
  Geon-Hyoung An, Hyo-Jin Ahn
  Journal of Korean Powder Metallurgy Institute.2017; 24(2): 96.     CrossRef

The triboelectric property of a material is important to improve an efficiency of triboelectric generator (TEG) in energy harvesting from an ambient energy. In this study, we have studied the TEG property of a semiconducting SnO₂ which has yet to be explored so far. As a counter triboelectric material, PET and glass are used. Vertical contact mode is utilized to evaluate the TEG efficiency. SnO₂ thin film is deposited by atomic layer deposition on bare Si wafer for various thicknesses from 5.2 nm to 34.6 nm, where the TEG output is increased from 13.9V to 73.5V. Triboelectric series are determined by comparing the polarity of output voltage of 2 samples among SnO₂, PET, and glass. In conclusion, SnO₂, as an intrinsic n-type material, has the most strong tendency to be positive side to lose the electron and PET has the most strong tendency to be negative side to get the electron, and glass to be between them. Therefore, the SnO₂-PET combination shows the highest TEG efficiency.

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• Triboelectric charge generation by semiconducting SnO2 film grown by atomic layer deposition
  No Ho Lee, Seong Yu Yoon, Dong Ha Kim, Seong Keun Kim, Byung Joon Choi
  Electronic Materials Letters.2017; 13(4): 318.     CrossRef

WS₂-W-WC embedded carbon nanofiber composites were fabricated by using electrospinning method for use in high-performance supercapacitors. In order to obtain optimum electrochemical properties for supercapacitors, WS₂ nanoparticles were used as precursors and the amounts of WS₂ precursors were controlled to 4 wt% (sample A) and 8 wt% (sample B). The morphological, structural, and chemical properties of all samples were investigated by means of field emission photoelectron spectroscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. These results demonstrated that the embedded phases of samples A and B were changed from WS₂ to WS₂-W-WC through carbothermal reaction during carbonization process. In particular, sample B presented high specific capacitance (~119.7 F/g at 5 mV/s), good high-rate capacitance (~60.5%), and superb cycleability. The enhanced electrochemical properties of sample B were explained by the synergistic effect of the using 1-D structure supports, increase of specific surface area, and improved conductivity from formation of W and WC phases.

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• WS2 Nanoparticles Embedded in Carbon Nanofibers for a Pseudocapacitor
  Ki-Wook Sung, Jung Soo Lee, Tae-Kum Lee, Hyo-Jin Ahn
  Korean Journal of Materials Research.2021; 31(8): 458.     CrossRef

SnO₂-CoO/carbon-coated CoO core/shell nanowire composites were synthesized by using electrospinning and hydrothermal methods. In order to obtain SnO2-CoO/carbon-coated CoO core/shell nanowire composites, SnO₂-Co₃O₄ nanowire composites and SnO₂-Co₃O₄/polygonal Co₃O₄ core/shell nanowire composites are also synthesized. To demonstrate their structural, chemical bonding, and morphological properties, field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were carried out. These results indicated that the morphologies and structures of the samples were changed from SnO₂-Co₃O₄ nanowires having cylindrical structures to SnO₂-Co₃O₄/Co₃O₄ core/shell nanowires having polygonal structures after a hydrothermal process. At last, SnO₂-CoO/carbon-coated CoO core/shell nanowire composites having irregular and high surface area are formed after carbon coating using a polypyrrole (PPy). Also, there occur phases transformation of cobalt phases from Co₃O₄ to CoO during carbon coating using a PPy under a argon atmosphere.

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• Co-Embedded Graphitic Porous Carbon Nanofibers for Pt-Free Counter Electrode in Dye-Sensitized Solar Cells
  혜란 안, 혜린 강, 효정 선, 지호 한, 효진 안
  Korean Journal of Materials Research.2015; 25(12): 672~677.     CrossRef
• Synthesis of Perforated Polygonal Cobalt Oxides usinga Carbon Nanofiber Template
  Dong-Yo Sin, Geon-Hyoung An, Hyo-Jin Ahn
  Journal of Korean Powder Metallurgy Institute.2015; 22(5): 350.     CrossRef

Flake-like LiCoO₂ nanopowders were fabricated using electrospinning. To investigate their formation mechanism, field-emssion scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were carried out. Among various parameters of electrospinning, we controlled the molar concentration of the precursor and the PVP polymer. When the molar concentration of lithium and cobalt was 0.45 M, the morphology of LiCoO₂ nanopowders was irregular and round. For 1.27 M molar concentration, the LiCoO₂ nanopowders formed with flake-like morphology. For the PVP polymer, the molar concentration was set to 0.011 mM, 0.026 mM, and 0.043 mM. Irregular LiCoO₂ nanopowders 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 related to the successful formation of flake-like LiCoO₂ nanopowders. As a results, the synthesized LiCoO₂ nanopowder can be used as the electrode material of Li-ion batteries.

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• Electrochemical Behavior of Well-dispersed Catalysts on Ruthenium Oxide Nanofiber Supports
  Geon-Hyoung An, Hyo-Jin Ahn
  Journal of Korean Powder Metallurgy Institute.2017; 24(2): 96.     CrossRef
• Synthesis and Characterization of SnO2-CoO/carbon-coated CoO Core/shell Nanowire Composites
  Yu-Jin Lee, Bon-Ryul Koo, Hyo-Jin Ahn
  Journal of Korean Powder Metallurgy Institute.2014; 21(5): 360.     CrossRef