Jongwoo Han

Generation of B-doped graphene nanoplatelets using a solution process and their supercapacitor applications.

Chemically modified graphene (CMG) nanoplatelets have shown great promise in various applications due to their electrical properties and high surface area. Chemical doping is one of the most effective methods to tune the electronic properties of graphene materials. In this work, novel B-doped nanoplatelets (borane-reduced graphene oxide, B-rG-O) were produced on a large scale via the reduction of graphene oxide by a borane-tetrahydrofuran adduct under reflux, and their use for supercapacitor electrodes was studied. This is the first report on the production of B-doped graphene nanoplatelets from a solution process and on the use of B-doped graphene materials in supercapacitors. The B-rG-O had a high specific surface area of 466 m(2)/g and showed excellent supercapacitor performance including a high specific capacitance of 200 F/g in aqueous electrolyte as well as superior surface area-normalized capacitance to typical carbon-based supercapacitor materials and good stability after 4500 cycles. Two- and three-electrode cell measurements showed that energy storage in the B-rG-O supercapacitors was contributed by ion adsorption on the surface of the nanoplatelets in addition to electrochemical redox reactions.

Coordination chemistry of [Co(acac)2] with N-doped graphene: Implications for oxygen reduction reaction reactivity of organometallic Co-O4-N species

Hybridization of organometallic complexes with graphene-based materials can give rise to enhanced catalytic performance. Understanding the chemical structures within hybrid materials is of primary importance. In this work, archetypical hybrid materials are synthesized by the reaction of an organometallic complex, [Co(II) (acac)2 ] (acac=acetylacetonate), with N-doped graphene-based materials at room temperature. Experimental characterization of the hybrid materials and theoretical calculations reveal that the organometallic cobalt-containing species is coordinated to heterocyclic groups in N-doped graphene as well as to its parental acac ligands. The hybrid material shows high electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline media, and superior durability and methanol tolerance to a Pt/C catalyst. Based on the chemical structures and ORR experiments, the catalytically active species is identified as a Co-O4 -N structure.

Generation of ultra-high-molecular-weight polyethylene from metallocenes immobilized onto N-doped graphene nanoplatelets.

Catalytic natures of organometallic catalysts are modulated by coordinating organic ligands with proper steric and electronic properties to metal centers. Carbon-based nanomaterials such as graphene nanoplatelets are used with and without N-doping and multiwalled carbon nanotube as a ligand for ethylene polymerizations. Zirconocenes or titanocenes are immobilized on such nanomaterials. Polyethylenes (PEs) produced by such hybrids show a great increase in molecular weight relative to those produced by free catalysts. Specially, ultra-high-molecular-weight PEs are produced from the polymerizations at low temperature using the hybrid with N-doped graphene nanoplatelets. This result shows that such nanomaterials act a crucial role to tune the catalytic natures of metallocenes.

Finely tuning oxygen functional groups of graphene materials and optimizing oxygen levels for capacitors

Oxygen-containing, chemically modified graphene (CMG) systems have been intensively investigated for various applications. The development of methods that allow fine control of the oxygen functionality would help better understand the mechanisms associated with CMGs, facilitate optimization of the material properties, and provide standards for chemical characterization purposes. Here, the authors report a new method for finely controlling the levels of oxygen in CMG materials based on the refluxing of aqueous colloidal suspensions of graphene oxide for specific reflux times, which does not require additional reducing agents. Chemical analysis confirmed that the oxygen levels can be finely controlled and they can provide spectroscopic tools to monitor the oxygen levels of CMG-based systems. This system was applied to help provide a fundamental foundation for the correlation between the oxygen groups and capacitive features.