Sunhye Yang

High-performance transparent conductive films using rheologically derived reduced graphene oxide.

In this work, we produced large-area graphene oxide (GO) sheets with fewer defects on the basal plane by application of shear stress in solution to obtain high-quality reduced graphene oxide (RGO) sheets without the need for post-annealing processes. This is described as rheologically derived RGO. The large-area GO sheets were generated using a homogenizer in aqueous solution, which induced slippage of the GO in the in-plane direction during the exfoliation process, in contrast with the conventional sonication method. The effects of chemical reduction under mild conditions demonstrated that the formation of structural defects during the exfoliation process affected the RGO properties. In the Raman spectra, the I(D)/I(G) ratio of the homogenized RGO (HRGO) increased more than that of the sonicated RGO (SRGO) due to the large number of ordered six-fold rings on the basal plane. The enhanced sheet resistance of the HRGO thin film was found to be 2.2 kΩ/sq at 80% transmittance. The effective exfoliation method has great potential for application to high-performance RGO-transparent conducting films.

Monolithic Graphene Trees as Anode Material for Lithium Ion Batteries with High C-Rates.

Monolithically structured reduced graphene oxide (rGO), prepared from a highly concentrated and conductive rGO paste, is introduced as an anode material for lithium ion batteries with high rate capacities. This is achieved by a mixture of rGO paste and the water-soluble polymer sodium carboxymethylcellulose (SCMC) with freeze drying. Unlike previous 3D graphene porous structures, the monolithic graphene resembles densely branched pine trees and has high mechanical stability with strong adhesion to the metal electrodes. The structures contain numerous large surface area open pores that facilitate lithium ion diffusion, while the strong hydrogen bonding between the graphene layers and SCMC provides high conductivity and reduces the volume changes that occur during cycling. Ultrafast charge/discharge rates are obtained with outstanding cycling stability and the capacities are higher than those reported for other anode materials. The fabrication process is simple and straightforward to adjust and is therefore suitable for mass production of anode electrodes for commercial applications.

Suppressing spontaneous polarization of p-GaN by graphene oxide passivation: Augmented light output of GaN UV-LED

GaN-based ultraviolet (UV) LEDs are widely used in numerous applications, including white light pump sources and high-density optical data storage. However, one notorious issue is low hole injection rate in p-type transport layer due to poorly activated holes and spontaneous polarization, giving rise to insufficient light emission efficiency. Therefore, improving hole injection rate is a key step towards high performance UV-LEDs. Here, we report a new method of suppressing spontaneous polarization in p-type region to augment light output of UV-LEDs. This was achieved by simply passivating graphene oxide (GO) on top of the fully fabricated LED. The dipole layer formed by the passivated GO enhanced hole injection rate by suppressing spontaneous polarization in p-type region. The homogeneity of electroluminescence intensity in active layers was improved due to band filling effect. As a consequence, the light output was enhanced by 60% in linear current region. Our simple approach of suppressing spontaneous polarization of p-GaN using GO passivation disrupts the current state of the art technology and will be useful for high-efficiency UV-LED technology.

Enhanced response and sensitivity of self-corrugated graphene sensors with anisotropic charge distribution

We introduce a high-performance molecular sensor using self-corrugated chemically modified graphene as a three dimensional (3D) structure that indicates anisotropic charge distribution. This is capable of room-temperature operation, and, in particular, exhibiting high sensitivity and reversible fast response with equilibrium region. The morphology consists of periodic, “cratered” arrays that can be formed by condensation and evaporation of graphene oxide (GO) solution on interdigitated electrodes. Subsequent hydrazine reduction, the corrugated edge area of the graphene layers have a high electric potential compared with flat graphene films. This local accumulation of electrons interacts with a large number of gas molecules. The sensitivity of 3D-graphene sensors significantly increases in the atmosphere of NO2 gas. The intriguing structures have several advantages for straightforward fabrication on patterned substrates, high-performance graphene sensors without post-annealing process.

Characterization of Graphene Nanosheets as Electrode Material and Their Performances for Electric Double-Layer Capacitors

In this work, graphene nanosheets were prepared using the Hummers-Offeman method. We prepared the resultant graphene electrode with kneading type. The prepared graphene nanosheets were characterized by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, transmission electron microscopy, Fourier transform infrared spectroscopy instrument and Raman spectra. Finally, the electrochemical performances of graphene nanosheets in an electrolyte solution of tetraethylammonium tetrafluoroborate ((C2H5)4NBF4, TEABF4) in propylene carbonate (C4H6O3, PC) were examined.

Novel preparation of expanded nano-graphene-based electrodes for EDLC and their improved electrochemical performance

ABSTRACT The electrode raw materials in this work were composed of expanded nano-graphene (ENG)-based active carbon (YP50F) named YEG as an active material; Super-P carbon black (SPB) as an electric conductor; and styrene–butadiene rubber (SBR), sodium salt of carboxymethyl cellulose (CMC), and polytetrafluoroethylene ((C2F4)n, PTFE) as mixed binder materials. We characterized the prepared electrodes by X-ray diffraction, scanning electron microscopy, and Raman spectroscopic techniques. Finally, we examined the electrochemical performances of carbon materials in an electrolyte solution of tetraethylammonium tetrafluoroborate ((C2H5)4NBF4, TEABF4) in propylene carbonate (C4H6O3, PC). The specific capacitance remains the same for smaller values of YEG in the composite electrodes. These results also provide evidence of the optimum loading of ENG in future graphene-based EDLCs.

Enhanced specific capacitance of modified needle cokes by controlling oxidation treatment

The electric double-layer performance of needle cokes can be affected by the morphology of structures. Hence, we introduce modified needle cokes by using simple oxidation treatment. The degree of graphitization with high specific capacitance is controlled by acid and heat treatment. The active sites of cokes are increased with increasing oxidation time. Dilute nitric acid (HNO3) and sodium chlorate (NaClO3) are used for the activation of cokes. In this case, the interlayer distance is dramatically increased from 3.5 to 8.9 A. The specific capacitances are 33 F g−1 and 30 F ml−1, respectively, on a two-electrode system with a potential range of 0–2.5 V. The behaviors of double-layer capacitance are demonstrated by the charge–discharge process and the morphologies of modified needle cokes are analyzed by XRD, FE-SEM, BET and elemental analysis.

Effect of Acid / Heat Treatment on Electric Double Layer Performance of Needle Cokes

In this study, a needle coke was oxidized in a mixture of dilute nitric acid and sodium chlorate () solutions and followed by heat treatment. The samples were analyzed with using XRD, FESEM, elemental analyzer, BET, and Raman spectroscopy. Double layer capacitance was measured with the charge and discharge measurements. The consisting layers of the needle coke were expanded to single phase showing only (001) diffraction peak by the acid treatment for 24 hours. The oxidized coke returned to a graphite structure appearing (002) peak after heat treatment above . The structure returned could be more easily accessible to the ions by the first charge, and improve the double layer capacitance at the second charge. The two-electorde cell from acid treated coke and heat treatment exhibited the maximum capacitances of 32.1 F/g and 29.5 F/ml at the potential of .

New EDLC designed with CNT-AC synthsized via CVD method as additional material for the improved cell resistance

ABSTRACT We report the fabrication of high-performance CNT-AC supercapacitor electrodes. CNTs are also added in the electrode preparation in this work because of high microporosity of activated carbon. The bigger ions can hardly be diffused and adsorbed onto the smaller micropores of activated carbon. The mesoporous nature of CNTs can enhance the ion adsorption through its unique and well-defined hollow core. The as prepared CNT-AC was then used as additive material to study the electrochemical properties of activated carbon based electric double layered supercapacitors (EDLCs). The physiochemical properties of CNT-AC were studied using scanning electron microscopy (SEM), Raman spectroscopic techniques, and transmission electron microscopy (TEM). Coin-type EDLC cells with two symmetrical carbon electrodes were using the synthesized carbon materials for the decrease of resistance. The electrochemical performance of the carbon electrodes was measured by galvanostatic charge/discharge and cyclic voltammetry methods.

Oxidation-treated of Oxidized Carbons and its Electrochemical Performances for Electric Double Layer Capacitor

The oxidation treatment of several carbon materials with a sodium chlorate and 70 wt.% of nitric acid, combined with heat treatment, were attempted to achieve an electrochemical active material with a larger capacitance. Among pitch, needle coke, calcinated needle coke and natural graphite, the structure of needle coke and calacinated needle coke were changed to the graphite oxide structure with the expansion of the inter-layer. On the other hand, the calcinated needle coke after oxidation and heating at has exhibited largest capacitance per weight and volume of 29.5 F/g and 24.5 F/ml at the two-electrode system in the potential range of 0 to 2.5 V. The electrochemical performance of the calcinated needle coke was discussed with the phenomenon of the electric field activation and the formation of new pores between the expanded inter-layer at first charge.