Kap Seung Yang

Tailoring the pore structure of carbon nanofibers for achieving ultrahigh-energy-density supercapacitors using ionic liquids as electrolytes

The low energy density of commercially available activated carbon-based supercapacitors has limited their widespread applications. In the current work, we demonstrated fabrication of carbon nanofiber-based supercapacitors that exhibited ultra-high energy density by rationally tailoring their pore structure in an ionic liquid system. To gain control on the pore structure, three different methods were employed for the synthesis of an electrospinning-derived freestanding carbon nanofiber web. They are incorporation of a pore generator (i.e., tetraethyl orthosilicate) in the electrospinning step, physical activation (e.g., H2O or CO2), and hydrogen treatment. We observed finely tuned pore sizes ranging from 0.734 to 0.831 nm and accompanying changes in BET surface areas ranging from 1160 to 1624 m2 g−1. The entrapped TEOS within the electrospun organic nanofiber web provided high tuning ability of the pore structure in the following carbonization step, and decreased the activation energy of the pore formation. Both high specific capacitance (161 F g−1) and ultra-high energy density (246 W h kg−1) were achieved when the pore size on the surface of carbon nanofibers matched with the ionic size of the electrolyte. Our results demonstrate the importance of a finely tuned pore structure to secure high-temperature operable carbon nanofiber-based supercapacitors with ultrahigh energy density using ionic liquids as electrolytes.

Further carbonization of anisotropic and isotropic pitch-based carbons by microwave irradiation

Abstract Isotropic (PFO) and anisotropic (NMP) pitch-based carbon powders thermally carbonized above 600°C were carbonized further by microwave irradiation under inert gases, Ar and N2. The temperature of the carbon powder during microwave irradiation was measured by an Inconel sheathed thermocouple. The sample under Ar gas showed higher temperature than that under N2 gas before exhibition of the Ar arc discharge. The temperature increased in the early stage and decreased when the irradiation was continued. The irradiation increased the Lc(002) value and electric conductivity with no significant differences in values between isotropic and anisotropic pitch-based carbons. The results were explained on the basis of the mechanism of inductive heating from the material’s resistance to electron movement leading to similar morphological structure with lapse of irradiation time. However, the morphological structural differences between the isotropic and anisotropic carbons reflect on the variations of Lc(002) and d(002) values with irradiation time. d(002) of anisotropic carbon showed minimum and maximum behaviors with the increase in Lc(002) value indicating stacking in advance and subsequently reduction in d(002) values of the uniformly ordered crystallites. On the other hand, d(002) values of isotropic carbons decreased monotonically with increasing Lc(002) values, reflecting the wide distributions of the crystallite sizes in the isotropic carbon. Microwave irradiation for 30 min effectively increased the Lc(002) value comparable with the value of NMP-based carbon thermally heat treated at 1500°C for 1 h.

Facile Synthesis of Carbon-Coated Silicon/Graphite Spherical Composites for High-Performance Lithium-Ion Batteries.

A high-performance Si/carbon/graphite composite in which Si nanoparticles are attached onto the surface of natural graphite by carbonization of coal-tar pitch is proposed for use in lithium-ion batteries. This multicomponent structure is favorable for improving Li(+) storage capability because the amorphous carbon layer encapsulating Si nanoparticles offers sufficient electric conductivity and strong elasticity to facilitate relaxation of strain caused by electrochemical reaction of Si during cycles. The Si/carbon/graphite composite exhibits a specific capacity of 712 mAh g(-1) at a constant current density of 130 mA g(-1), and maintains more than 80% of its initial capacity after 100 cycles. Moreover, it shows a high capacity retention of approximately 88% even at a high current density of 5 C (3250 mA g(-1)). On the basis of electrochemical and structural analyses, we suggest that a rational design of the Si/carbon/graphite composite is mainly responsible for delivering a high reversible capacity and stable cycle performance. Furthermore, the proposed synthetic route for the Si/carbon/graphite composite is simple and cost-effective for mass production.

Preparation and ionic conductivity of sulfonated-SEBS/SiO2/plasticizer composite polymer electrolyte for polymer battery

Abstract Sulfonated poly(styrene–ethylene–buthylene–styrene) (SSEBS) for a polymer electrolyte membrane was prepared according to the capacity of ion exchange. SSEBS was sufficient to utilize with the polymer electrolyte membrane from the viewpoint of appearance, tensile strength, and ionic conductivity. The SSEBS/SiO2/plasticizer composite polymer electrolyte for the polymer battery was fabricated with various silica contents. LiClO4 or LiCF3SO3 was used as a salt, EC/PC (1:1 vol.%) as solvents, SiO2 as a filler, and DBP as a plasticizer, respectively. The ionic conductivity was enhanced with increasing the degree of sulfonation. The ionic conductivity reached 2.6×10−3 S/cm when the content of silica was 12 wt.% at 1 M LiClO4 in EC/PC (1:1 vol.%) and 40 wt.% for plasticizers. However, if the content of silica was too excessive, the ionic conductivity was decreased due to restriction of ionic movement.

Synthesis, antimycobacterial screening and ligand-based molecular docking studies on novel pyrrole derivatives bearing pyrazoline, isoxazole and phenyl thiourea moieties

We report here the synthesis, antibacterial and antitubercular evaluation of 61 novel pyrrolyl derivatives bearing pyrazoline, isoxazole and phenyl thiourea moieties. Molecular docking was carried out on enoyl ACP reductase from Mycobacterium tuberculsosis using Surflex-Dock, which is one of the key enzymes involved in type II fatty acid biosynthetic pathway of Mycobacterium tuberculosis, an attractive target for designing novel antitubercular agents. Docking analysis of the crystal structure of ENR performed using Surflex-Dock in Sybyl-X 2.0 software indicates the occupation of substituted pyrrolyl derivatives into hydrophobic pocket of InhA enzyme. Compounds 9b and 9d exhibited the highest antitubercular activity almost close to isoniazid (0.4 μg/mL) with a MIC value of 0.8 μg/mL. All other compounds showed the good activity with a MIC value of 6.25-100 μg/mL. The compounds were further tested for mammalian cell toxicity using human lung cancer cell-line (A549) and were nontoxic. Some compounds exhibited inhibition activities against InhA.

Electrospun Nanocomposite Fiber Mats of Zinc-Oxide Loaded Polyacrylonitrile

We have demonstrated the feasibility of using electrospinning method to fabricate long and continuous composite nanofiber sheets of polyacrylonitrile (PAN) incorporated with zinc oxide (ZnO). Such PAN/ZnO composite nanofiber sheets represent an important step toward utilizing carbon nanofibers (CNFs) as materials to achieve remarkably enhanced physico-chemical properties. In an attempt to derive these advantages, we have used a variety of techniques such as field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high resolution X-ray diffraction (HR-XRD) to obtain quantitative data on the materials. The CNFs produced are in the diameter range of 100 to 350 nm after carbonization at . Electrical conductivity of the random CNFs was increased by increasing the concentration of ZnO. A dramatic improvement in porosity and specific surface area of the CNFs was a clear evidence of the novelty of the method used. This study indicated that the optimal ZnO concentration of 3 wt% is enough to produce CNFs having enhanced electrical and physico-chemical properties.

Heat treatment temperature effects on structural and electrochemical properties of PVDC-based disordered carbons

X-ray diffraction (XRD), BET surface area, Raman spectroscopy and electrochemical measurement were used to investigate the morphological behaviors of poly(vinylidene chloride) (PVDC)-based carbons in the heat treatment temperature range 700 to 3000°C. X-ray diffraction and Raman scattering results proposed that PVDC-based carbons heat treated up to 3000°C behaved as typically non-graphitizable carbons. The structural parameters were correlated with discharge/charge characteristics of Li ions into PVDC-based carbons. It was realized that the Li ion storage capacity of PVDC-based carbons during the first cycle tended to decrease with increasing the heat-treatment temperature due to the reduction of hydrogen content and the reduction of the specific surface area from pores.

Electron Beam Irradiation-Enhanced Wettability of Carbon Fibers

A simple but controllable way of altering the surface nature of carbon fibers, without sacrificing their intrinsic mechanical properties, is demonstrated using electron beam irradiation. Such treatment leads to physically improved roughness as well as chemically introduced hydrophilic oxygen-containing functional groups on the surface of carbon fibers that are essential for assuring an efficient stress transfer from carbon fibers to a polymer matrix.

Isotropic Carbon and Graphite Fibers from Chemically Modified Coal-Tar Pitch

A precursor for a general purpose carbon fiber was prepared from coal tar pitch (CP) modified with 10 % p-benzoquinone (BQ) at 380 ‡C for 3 hours. Such a modification raised the softening of the pitch from 85 ‡C to 271 ‡C at a yield of 43 %. The modified pitch was spun smoothly at a rate of 480 m/min into a fiber of 20 Μm diameter. The fiber was stepwise stabilized at 236 ‡C (5 ‡C/min) and 312 ‡C (1 ‡C/min) for 3 hours at each temperature. Successively,carbonization and graphitization were performed at 1,000 ‡C and 2,400 ‡C, respectively, for one hour. Both the carbonized and graphitized fibers exhibited tensile strength of 570 MPa. The structural parameters of carbon and graphite fibers were their orientation values of 56.2 and 58.1 %, relatively low Lc(002) of 11.24 and 25 å, and large interlayer spacing (d002) of 3.86 and 3.49 å, respectively.

Rationally engineered surface properties of carbon nanofibers for the enhanced supercapacitive performance of binary metal oxide nanosheets

The hybridization of an electrochemically active metal oxide with electrically conductive carbon nanofibers (CNFs) has been utilized as a solution to overcome the energy density limitation of carbon-based supercapacitors as well as the poor cyclic stability of metal oxides. Herein, we have demonstrated the growth of binary metal oxide nanosheets on the engineered surface of CNFs to fully exploit their electrochemical activity. Metal oxide nanosheets were observed to grow vertically from the surface of CNFs. The high structural toughness of the CNF–metal oxide composite under strong sonication indicated strong interfacial binding strength between the metal oxide and the CNFs. The rationally designed porous CNFs presented a high specific surface area and showed high capacity for adsorbing metal ions, where the active edge sites acted as anchoring sites for the nucleation of metal oxides, thereby leading to the formation of a well dispersed and thin layer structure of binary metal oxide nanosheets. Excellent electrochemical performance (e.g., specific capacitance of 2894.70 F g−1 and energy density of 403.28 W h kg−1) was observed for these binary metal oxide nanosheets, which can be attributed to the large increase in the accessible surface area of the electrochemically active metal oxide nanosheets due to their homogeneous distribution on porous CNFs, as well as the efficient charge transfer from the metal oxide to the CNFs facilitated the improvement in the performance.