Cheol-Min Yang

Edge-enriched, porous carbon-based, high energy density supercapacitors for hybrid electric vehicles.

Supercapacitors can store and deliver energy by a simple charge separation, and thus they could be an attractive option to meet transient high energy density in operating fuel cells and in electric and hybrid electric vehicles. To achieve such requirements, intensive studies have been carried out to improve the volumetric capacitance in supercapacitors using various types and forms of carbons including carbon nanotubes and graphenes. However, conventional porous carbons are not suitable for use as electrode material in supercapacitors for such high energy density applications. Here, we show that edge-enriched porous carbons are the best electrode material for high energy density supercapacitors to be used in vehicles as an auxiliary powertrain. Molten potassium hydroxide penetrates well-aligned graphene layers vertically and consequently generates both suitable pores that are easily accessible to the electrolyte and a large fraction of electrochemically active edge sites. We expect that our findings will motivate further research related to energy storage devices and also environmentally friendly electric vehicles.

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.

Adsorption properties of nitrogen-alloyed activated carbon fiber

Abstract N-alloyed activated carbon fibers (ACFs) were prepared by chemical vapor deposition (CVD) of pyridine on pitch-based ACF at 1023 K and 1273 K for 1 h. The N-alloyed ACFs were characterized by using the N2 adsorption at 77 K, CO2 adsorption at 273 K, elemental analysis, and X-ray photoelectron spectroscopy (XPS). The nitrogen content increases as a result of the pyridine-CVD. XPS examination showed that the percent of quaternary nitrogens in nitrogen structures increases remarkably from 60 to 91% for CVD temperatures of 1023 and 1273 K, respectively. The effects of N-alloying on adsorption properties of ACFs for C2H5OH and H2O were examined at 303 K. Both N-alloyed ACFs have a larger fractional filling of C2H5OH molecules. The uptake pressure of the H2O adsorption branches of both N-alloyed ACFs shifts to a lower relative pressure, compared with that of pristine ACF. Furthermore, the ratios of the VH2O (saturated amount of H2O at P/P0=1) to W0N2 (pore volume determined by N2 adsorption at 77 K) of N-alloyed ACFs are much larger than that of pristine ACF.

Defect healing of reduced graphene oxide via intramolecular cross-dehydrogenative coupling

A chemical defect healing of reduced graphene oxide (RGO) was carried out via intramolecular cross-dehydrogenative coupling (ICDC) with FeCl3 at room temperature. The Raman intensity ratio of the G-band to the D-band, the IG/ID ratio, of the RGO was increased from 0.77 to 1.64 after the ICDC reaction. From XPS measurements, the AC=C/AC-C ratio, where the peak intensities from the C=C and C-C bonds are abbreviated as AC=C and AC-C, of the RGO was increased from 2.88 to 3.79. These results demonstrate that the relative amount of sp(2)-hybridized carbon atoms is increased by the ICDC reaction. It is of great interest that after the ICDC reaction the electrical conductivity of the RGO was improved to 71 S cm(-1), which is 14 times higher than that of as-prepared RGO (5 S cm(-1)).

Effect of cooling condition on chemical vapor deposition synthesis of graphene on copper catalyst.

Here, we show that chemical vapor deposition growth of graphene on copper foil is strongly affected by the cooling conditions. Variation of cooling conditions such as cooling rate and hydrocarbon concentration in the cooling step has yielded graphene islands with different sizes, density of nuclei, and growth rates. The nucleation site density on Cu substrate is greatly reduced when the fast cooling condition was applied, while continuing methane flow during the cooling step also influences the nucleation and growth rate. Raman spectra indicate that the graphene synthesized under fast cooling condition and methane flow on cool-down exhibit superior quality of graphene. Further studies suggest that careful control of the cooling rate and CH4 gas flow on the cooling step yield a high quality of graphene.

Flexible electrochromic films based on CVD-graphene electrodes

Graphene synthesized via chemical vapor deposition is a notable candidate for flexible large-area transparent electrodes due to its great physical properties and its 2D activated surface area. Electrochromic devices in optical displays, smart windows, etc are suitable applications for graphene when used as a transparent conductive electrode. In this study, various-layer graphene was synthesized via chemical vapor deposition, and inorganic WO(x) was deposited on the layers, which have advantageous columnar structures and W(6+) and W(4+) oxidation states. The characteristics of graphene and WO(x) were verified using optical transmittance, Raman spectroscopy, x-ray photoelectron spectroscopy and scanning electron microscopy. The optimum transparent conductive electrode condition for controlling graphene layers was investigated based on the optical density and cyclic voltammetry. Electrochromic devices were fabricated using a three-layer graphene electrode, which had the best optical density. The graphene in the flexible electrochromic device demonstrated a potential for replacing ITO in flexible electronics.

High-temperature treatment effect of microporous carbon on ordered structure of confined SO2

Abstract The X-ray diffraction (XRD) of SO 2 molecules adsorbed in micropores of activated carbon fiber (ACF) was measured at 303 K. Effect of pore width and high-temperature treatment (HTT) of ACF on the XRD of SO 2 in micropores were examined. The HTT gave rise to a more dipole-oriented structure of SO 2 molecules in micropores of 1.1 nm width, whereas the opposite effect was observed in micropores of 0.7 nm. The intensified image potential of an SO 2 molecule for the heat-treated graphitic pore wall should stabilize the dipole-oriented structure in the case of 1.1 nm micropores, while the heat-treatment provides defective structures in the narrow micropore system. The presence of O 2 stabilized the dipole-oriented structure of SO 2 molecules in the micropore.

Atomic layer coating of hafnium oxide on carbon nanotubes for high-performance field emitters

Carbon nanotubes coated with hafnium oxide exhibit excellent electron emission characteristics, including a low turn-on voltage, a high field enhancement factor, and exceptional current stability. Their enhanced emission performance was attributed to a decrease in the work function and an increase in the electron density of states at the carbon nanotube Fermi level closest to the conduction band minimum of hafnium oxide. In addition, the enhanced current stability was attributed to the ability of hafnium oxide to protect the carbon nanotubes against ions and free radicals created in the electron field emission process.

Electrochemical role of oxygen containing functional groups on activated carbon electrode

We have experimentally and theoretically clarified the effect of oxygen functional groups on the capacitive performance of a photochemically treated activated carbon electrode. A high density of CO groups at the mouth of the micropores, where the chemically active edge sites are predominantly available, increases the energy barrier for ions to enter the pores, thereby resulting in a large decrease in the specific capacitance.

An environmentally friendly approach to functionalizing carbon nanotubes for fabricating a strong biocomposite Film

Water-soluble, highly functionalized multiwalled carbon nanotubes (MWNTs) were prepared via a facile, environmentally benign method. The effectiveness of the highly functionalized MWNTs as a reinforcing filler in a water-soluble mechanically weak chitosan biopolymer was evaluated. We observed a substantial improvement in the mechanical properties of the film; the stretchability of the film was maintained when 5 wt% MWNTs was added. In addition, the biocomposite film exhibited long-term antibacterial activity. The reinforcing effect can be explained by the homogeneous dispersion of the nanotubes in the polymer matrix through the strong hydrogen bonding between the sulfonic acid groups (–SO3H) on the sidewalls of the MWNTs and the amine (–NH2) and hydroxyl (–OH) groups on the chitosan backbone. The improvement in the mechanical properties of the MWNT–chitosan biocomposite may make it suitable for many environmental and clinical applications.