Myeong Seong Kim

Scalable fabrication of micron-scale graphene nanomeshes for high-performance supercapacitor applications

Graphene nanomeshes (GNMs) with nanoscale periodic or quasi-periodic nanoholes have attracted considerable interest because of unique features such as their open energy band gap, enlarged specific surface area, and high optical transmittance. These features are useful for applications in semiconducting devices, photocatalysis, sensors, and energy-related systems. Here, we report on the facile and scalable preparation of multifunctional micron-scale GNMs with high-density of nanoperforations by catalytic carbon gasification. The catalytic carbon gasification process induces selective decomposition on the graphene adjacent to the metal catalyst, thus forming nanoperforations. The pore size, pore density distribution, and neck size of the GNMs can be controlled by adjusting the size and fraction of the metal oxide on graphene. The fabricated GNM electrodes exhibit superior electrochemical properties for supercapacitor (ultracapacitor) applications, including exceptionally high capacitance (253 F g−1 at 1 A g−1) and high rate capability (212 F g−1 at 100 A g−1) with excellent cycle stability (91% of the initial capacitance after 50 000 charge/discharge cycles). Further, the edge-enriched structure of GNMs plays an important role in achieving edge-selected and high-level nitrogen doping.

Silica-assisted bottom-up synthesis of graphene-like high surface area carbon for highly efficient ultracapacitor and Li-ion hybrid capacitor applications

We report a facile bottom-up approach for the synthesis of pure and macro-sized (>500 nm) graphene-like carbon by precisely employing sp2 carbon rich 1,2,4,5-benzene tetracarboxylic acid (BTCA) as a precursor. We also addressed the features, such as high specific surface area (SSA) and sp2 hybridized carbon content, of the BTCA-derived carbon (BTCADC) over conventional top-down processed reduced graphene oxide (RGO). For instance, a two fold enhancement in SSA (960 m2 g−1) and C : O atomic ratio (∼19) was noted for BTCADC when compared to RGO (SSA: 402 m2 g−1 and C : O ratio ∼ 10). The SSA of BTCADC was further extended to 2673 m2 g−1via a chemical activation process (A-BTCADC) along with a high pore volume (2.15 cm3 g−1). Furthermore, we attempted to explain the unsolved issue of carbon layer stacking (π–π stacking) in RGO by precisely adopting a bottom-up approach. From an application point of view, we explored the possibility of using such carbonaceous materials as promising electrodes for both symmetric and Li-ion hybrid supercapacitor configurations in an organic medium. The A-BTCADC based symmetric cell in a 1 M tetraethylammonium tetrafluoroborate (TEA·BF4) in acetonitrile (ACN) electrolyte displayed a specific capacitance (Csp) of 225 F g−1 (at 0.5 A g−1) with a stable cycling profile of up to 10 000 cycles (at 10 A g−1) between 0 and 3 V. This bottom-up approach opens new avenues to extend graphene-based science and technology to the next level.

A lithium iron phosphate/nitrogen-doped reduced graphene oxide nanocomposite as a cathode material for high-power lithium ion batteries

A LiFePO4/nitrogen-doped reduced graphene oxide nanocomposite was synthesized using a solution-based method followed by heat treatment. The nitrogen-doped reduced graphene oxide surrounding the LiFePO4 nanoparticles facilitates the transfer of electrons throughout the electrodes, which significantly reduces the internal resistance of the electrodes, resulting in high utilization of LiFePO4. Electrodes fabricated with the LiFePO4/nitrogen-doped reduced graphene nanocomposite show high discharge capacities and voltages at high rates including sub-zero temperature conditions, even at commercially acceptable loading levels.

Synthesis of Reduced Graphene Oxide-Modified LiMn0.75Fe0.25PO4 Microspheres by Salt-Assisted Spray Drying for High-Performance Lithium-Ion Batteries.

Microsized, spherical, three-dimensional (3D) graphene-based composites as electrode materials exhibit improved tap density and electrochemical properties. In this study, we report 3D LiMn0.75Fe0.25PO4/reduced graphene oxide microspheres synthesized by one-step salt-assisted spray drying using a mixed solution containing a precursor salt and graphene oxide and a subsequent heat treatment. During this process, it was found that the type of metal salt used has significant effects on the morphology, phase purity, and electrochemical properties of the synthesized samples. Furthermore, the amount of the chelating agent used also affects the phase purity and electrochemical properties of the samples. The composite exhibited a high tap density (1.1 g cm−3) as well as a gravimetric capacity of 161 mA h g−1 and volumetric capacity of 281 mA h cm−3 at 0.05 C-rate. It also exhibited excellent rate capability, delivering a discharge capacity of 90 mA h g−1 at 60 C-rate. Furthermore, the microspheres exhibited high energy efficiency and good cyclability, showing a capacity retention rate of 93% after 1000 cycles at 10 C-rate.

A chemically bonded NaTi2(PO4)3/rGO microsphere composite as a high-rate insertion anode for sodium-ion capacitors

We report on the synthesis of a high rate NaTi2(PO4)3/graphene composite for use as an anode material for high power Na-ion hybrid capacitors with the following characteristics; (1) reduction of the particle size of NaTi2(PO4)3 to the nanometer scale in order to reduce the Na+ ion diffusion length, (2) chemical bonding between NaTi2(PO4)3 nanoparticles and graphene in order to improve electrical conductivity, and (3) interconnected nanoporous structures in order to allow easy access of Na+ ions to NaTi2(PO4)3. For this, the NaTi2(PO4)3/rGO microsphere composite was prepared via a facile spray drying method using a solution mixture of graphene oxide, NaH2PO4·2H2O, Ti(OC2H5)4 and NH4H2(PO4)3, in which all the components of the titanium were present as ionic species in order to facilitate the chemical bonding between NaTi2(PO4)3 and rGO in the composite. The NaTi2(PO4)3/rGO microsphere composite had a Ti–O–C bond between NaTi2(PO4)3 nanoparticles (<80 nm) and rGO and interconnected nanoporous structures. The NaTi2(PO4)3/rGO microsphere composite exhibited a near theoretical specific capacity of 133 mA h g−1 at a 0.1 C-rate and excellent rate capability (70% capacity retention at a 50 C-rate) with very stable cycling performance (only 2% capacity loss after 200 cycles at a high rate of 10C). Furthermore, the energy density and power density of the NHC assembled with a NaTi2(PO4)3/rGO anode and an AC-based cathode are far better than those of other NHCs assembled using other metal oxide-based anodes and AC cathodes.

A study of the effects of synthesis conditions on Li 5 FeO 4 /carbon nanotube composites

Li5FeO4/carbon nanotube (LFO/CNT) composites composed of sub-micron sized LFO and a nanocarbon with high electrical conductivity were successfully synthesized for the use as lithium ion predoping source in lithium ion cells. The phase of LFO in the composite was found to be very sensitive to the synthesis conditions, such as the heat treatment temperature, type of lithium salt, and physical state of the precursors (powder or pellet), due to the carbothermic reduction of Fe3O4 by CNTs during high temperature solid state reaction. Under optimized synthesis conditions, LFO/CNT composites could be synthesized without the formation of impurities. To the best of our knowledge, this is the first report on the synthesis and characterization of a sub-micron sized LFO/CNT composites.

Orderly meso-perforated spherical and apple-shaped 3D carbon microstructures for high-energy supercapacitors and high-capacity Li-ion battery anodes

The Stober synthesis, which is composed of two steps of the formation of RF resin spheres in presence of an NH3 catalyst and the carbonization of RF resin spheres under an inert atmosphere, is a well-known approach to the preparation of carbon spheres (CSs). We herein modified the first step of the Stober procedure to introduce morphological and physicochemical changes to CSs. Two different fully perforated 3D carbon-based micromaterials were prepared, namely spherical meso-perforated carbon (SSMPC) and apple-shaped meso-perforated carbon (ASMPC). In the preparation of these materials, we adopted colloidal silica-mediated spray drying method followed by carbonization and silica removal. High specific surface areas and pore volumes were achieved for both ASMPC (1141 m2 g−1 and 3.2 cm3 g−1) and SSMPC (1050 m2 g−1 and 2.1 cm3 g−1). We then evaluated the charge storage properties in organic media from supercapacitor (SC) as well as Li-ion battery (LIB) perspectives. An ASMPC-based symmetric SC was capable of delivering a specific capacitance and energy density of 260 F g−1 and 75.56 W h kg−1, respectively, in addition to an excellent cyclability of 30 000 cycles. In the LIB, ASMPC exhibited a maximum capacity of 1698 mA h g−1 after 175 cycles at 200 mA g−1. We systematically elaborated that inaccessible interior sites of the 3D CSs could become accessible through the introduction of meso-perforations on the periphery and in the interior. We expected that the 3D shape and meso-perforations were responsible for the exceptional performance of CSs in SCs and LIBs.