Eun-Suok Oh

Chemical functionalization of graphene sheets by solvothermal reduction of a graphene oxide suspension in N-methyl-2-pyrrolidone

We report a simple and effective method for reducing and functionalizing graphene oxide into chemically converted graphene by solvothermal reduction of a graphene oxide suspension in N-methyl-2-pyrrolidone (NMP). Graphene oxide sheets were functionalized by free radicals generated during heating of NMP in the presence of air. The degree of functionalization was easily controlled by manipulating the reduction time. High functionalized solvothermally reduced graphene oxide (STRG) shows superior dispersibility in various organic solvents, while slightly functionalized STRG shows excellent electrical conductivity. The superior dispersibility of highly functionalized STRG in organic solvents was attributed to the steric effect of functionalized groups on the surface of STRG sheets. Free-standing STRG paper that was reduced for 1 h exhibited electrical conductivity as high as 21600 S m−1, while the dispersibility of STRG that was reduced for 5 h was as high as 1.4 mg mL−1.

Synthesis of a highly conductive and large surface area graphene oxide hydrogel and its use in a supercapacitor

In this report, we describe the structure of a robust and highly conductive 3D graphene oxide hydrogel. The reduced graphene oxide hydrogel or rGH is fabricated by a crosslinking reaction with ethylene diamine followed by a hydrazine reduction. The material showed a high electrical conductivity of 1351 S m−1 and a specific surface area of 745 m2 g−1 with 10.3 MPa break strength. When used as electrodes for a supercapacitor, it showed a high specific capacitance of 232 F g−1.

Synthesis of B-doped graphene quantum dots as a metal-free electrocatalyst for the oxygen reduction reaction

Boron-doped graphene quantum dots (BGQDs) have been synthesized by a one-step, facile and low temperature method through the hydrothermal treatment of glucose as the precursor in the presence of boric acid. The as-obtained BGQDs possess a high B-doping content up to 4.25% of uniform nm-size. Interestingly, the effect of different types of B–C bond species on the ORR catalytic activity has been investigated to clarify the origin of the electrochemical reduction of O2. Further, the composite of the reduced graphene oxide (rGO) and BGQD (G-BGQDs) was also prepared as a metal-free electrocatalyst for the oxygen reduction reaction (ORR). The G-BGQD composites exhibit a significantly enhanced electrocatalytic activity, including a positive onset potential and a high current density with a one step, four-electron pathway toward the ORR, comparable to the commercial Pt/C catalyst. Among various B–C bond structures in BGQDs, the graphite-like BC3 structure is considered to be an important site for the ORR by improving the electric conductivity and electrocatalytic activity of BGQDs, which is also confirmed by a DFT study. In addition, the G-BGQD composites show an outstanding long-term operational stability and high tolerance to the methanol crossover effect, which are comparable to the commercial Pt/C catalyst. These results demonstrate that the synthesized BGQD, as metal-free catalyst materials, may be inexpensive and efficient electrocatalysts for the replacement of Pt-based catalysts toward the ORR and other electrochemical applications.

Synthesis of polypyrrole-reduced graphene oxide composites by in-situ photopolymerization and its application as a supercapacitor electrode

A highly conductive polypyrrole (PPy)-reduced graphene oxide (RGO) composite with an electrical conductivity of 610 S m−1 was successfully synthesized by the in-situ photopolymerization of pyrrole in a graphene oxide suspension. Graphene oxide (GO) played the role of an electron acceptor and was reduced as it accepted electrons. The reduction of GO was confirmed by the increase in the C/O ratio of RGO with the UV irradiation time as well as the high electrical conductivity of PPy-RGO composite. Through the thermogravimetric analysis, it has been found that the PPy-RGO composite exhibited high thermal stability compared to the GO and PPy. This material was used as an electrode in a supercapacitor cell and showed excellent performance for electrical energy storage. The composite exhibited a specific capacitance of 376 F g−1 at a scan rate of 25 mV s−1.

3D Si/C particulate nanocomposites internally wired with graphene networks for high energy and stable batteries

It is challenging to design silicon anodes exhibiting stable cycling behavior, high volumetric and specific capacity, and low volume expansion for Li-based batteries. Herein, we designed Si/C-IWGN composites (Si/C composites internally wired with graphene networks). For this purpose, we used simple aqueous sol–gel systems consisting of varying amounts of silicon nanoparticles, resorcinol–formaldehyde, and graphene oxide. We found that a small amount of graphene (1–10 wt%) in Si/C-IWGNs efficiently stabilized their cycling behavior. The enhanced cycling stability of Si/C-IWGNs could be ascribed to the following facts: (1) ideally dispersed graphene networks were formed in the composites, (2) these graphene networks also created enough void spaces for silicon to expand and contract with the electrode thickness increase comparable to that of graphite. Furthermore, properly designed Si/C-IWGNs exhibited a high volumetric capacity of ∼141% greater than that of commercial graphite. Finally, a hybrid sample, Si–Gr, consisting of a high capacity Si/C-IWGN and graphite was prepared to demonstrate a hybrid strategy for a reliable and cost-effective anode with a capacity level required for high-energy Li-ion cells. The Si–Gr hybrid exhibited not only high capacity (800–900 mA h g−1 at 100 mA g−1) but also a high electrode volumetric capacity of 161% greater than that of graphite.

Effects of VGCF pretreatment on the characteristics of Fe2O3/VGCF composites as anode materials for Li-ion batteries

Two main topics were handled in this study: loss minimization in the irreversible capacity of vapor-grown carbon fibers (VGCFs) by pretreatment techniques and reversible capacity increase in pretreated VGCFs by the deposition of nano-sized Fe2O3. VGCFs were ball-milled and acid-treated, and nano-sized Fe2O3 particles were supported on the pretreated VGCFs. Their performance as anodes in lithium-ion batteries was characterized using a variety of techniques. Compared to the electrode containing pristine VGCF only, electrodes composed of the pretreated-VGCFs enhanced both charge transfer and discharge capacities due to an increase in lithium-ion mobility. Moreover, the Fe2O3 nanoparticles supported on the pretreated VGCFs could eliminate defects and functional groups on the surface of VGCFs generated by the pretreatment, such that the composite of Fe2O3 and pretreated VGCF showed higher reversible capacity at any rate of charge/discharge than the composite of Fe2O3 and the pristine VGCF. Therefore, the Fe2O3/pretreated VGCF composites could be a good candidate for high capacity anodes.

Compatibility constraint at interfaces with elastic, crystalline solids–I: Theory

Starting with an extended Gibbs–Duhem equation and an expression for stress-deformation behavior derived by Oh and Slattery for elastic crystalline solids, we derive a new compatibility constraint on stress at coherent interfaces. Its use is demonstrated in determining the residual stresses developed during oxidation on the surface of a cylinder.

A double core–shell modification of bulk TiO2 microspheres into porous N-doped-graphene carbon nanoflakes/N-doped TiO2 microspheres for lithium-ion battery anodes

In this study, we modified bulk TiO2 microspheres—using a template-aided, double core–shell modification with N-doped carbon (NC) and N-doped graphene (NG)—for the purpose of forming porous N-doped graphene carbon nanoflakes/N-doped TiO2 (NG–NC/NTiO2) microspheres. The effects of surface modification on the properties of the TiO2 microspheres and the resultant electrochemical performance in a lithium-ion-battery (LIB) anode were thoroughly investigated. The double core–shell modified nanocomposite exhibited a specific capacity of 74 mA h g−1 at a 10 C rate, which was much higher than the capacities of TiO2, carbon/TiO2, and core–shell NC/NTiO2 nanocomposites at rates of 0.2, 1, and 5 C, respectively. The RGO of the double core–shell NC/NTiO2 nanocomposite provided an effective buffering effect for the TiO2 microsphere, resulting in a much lower initial specific-capacity loss of 19.6%, on the 200th cycle, in comparison with the 41.8% loss of the core–shell NC/NTiO2 nanocomposite at the same cyclic stage. Such excellent performances from the TiO2 microspheres with the double core–shell assembly in the LIB anode were attributed to a significant reduction of charge transfer resistance (Rct) and maintenance of electrode stability.

The mechanics and thermodynamics of edge fracture: the critical energy release rate, the compatibility constraint, and the bond potential

Following Gurtin and many others, the critical energy release rate is commonly identified as an ill-defined surface energy. The primary objectives of this paper are to clarify the definition of this surface energy and the role of the entropy inequality in the discussion of critical conditions. In view of an increasing emphasis on ab initio computations, a secondary objective is to show how the critical energy release rate and the compatibility constraint 1 can be used to solve a problem for which we have experimental data, using only ab initio estimates of surface tension and bond potential, both of which are increasingly available.

Compatibility constraint at interfaces with elastic, crystalline solids–II: Applications

Two applications illustrate how the compatibility constraints derived in Part I of this work are to be applied. The analysis of the bending beam experiment of EerNisse and comparison with his experimental data using no adjustable parameters are strong support for their validity.