Haejune Kim

Crumpled Nitrogen‐Doped Graphene Nanosheets with Ultrahigh Pore Volume for High‐Performance Supercapacitor

Continuous scientifi c endeavors have been directed toward the optimization of graphene by manipulating its electronic, mechanical, chemical, and structural properties, such as surface area, pore geometry, and functional sites, in order to advance various potential applications, including nanoelectronics, energy storage/conversion, and catalysis. [ 1 ] The structural reformation of graphene, from pore generation to morphology transformation, is receiving growing attention, because the reconstruction of graphene could potentially result in localized highly reactive regions and thus unexpected properties for specifi c applications. [ 2 ] For instance, it was reported that crumpled graphene allows for the fabrication of polymer-graphene nanocomposite fi lms with low O 2 permeability and effective reduction of transparency. [ 3 ] Chemical functionalization of graphene (e.g., graphene oxide or GO) is another effective method for manipulating physical and chemical properties of graphene, because enriched reactive oxygen functional groups in GO can provide ample covalent bonding sites for the chemical functionalization. The functionalized GO can be easily converted to graphene-like materials through chemical or thermal reduction of GO. [ 4 ] For instance, nitrogen-doped graphene (NG) can be synthesized through thermal annealing of GO in ammonia, and the resulting NG showed some unique properties including improved conductivity and excellent catalytic activity. Actually, NG has been intensively investigated as electrode materials for lithium-ion batteries, catalysts for oxygen reduction reac-

A General Approach to One-Pot Fabrication of Crumpled Graphene-Based Nanohybrids for Energy Applications

Crumpled graphene oxide (GO)/graphene is a new type of carbon nanostructure that has drawn growing attention due to its three-dimensional open structure and excellent stability in an aqueous solution. Here we report a general and one-step approach to produce crumpled graphene (CG)-nanocrystal hybrids, which are produced by direct aerosolization of a GO suspension mixed with precursor ions. Nanocrystals spontaneously grow from precursor ions and assemble on both external and internal surfaces of CG balls during the solvent evaporation and GO crumpling process. More importantly, CG-nanocrystal hybrids can be directly deposited onto various current-collecting substrates, enabling their tremendous potential for energy applications. As a proof of concept, we demonstrate the use of hybrid electrodes of CG-Mn(3)O(4) and CG-SnO(2) in an electrochemical supercapacitor and a lithium-ion battery, respectively. The performance of the resulting capacitor/battery is attractive and outperforms conventional flat graphene-based hybrid devices. This study provides a new and facile route to fabricating high-performance hybrid CG-nanocrystal electrodes for various energy systems.

Binding Sn-based nanoparticles on graphene as the anode of rechargeable lithium-ion batteries

A facile method has been developed to synthesize Sn-based nanoparticle-decorated graphene through simultaneous growth of SnO2 nanoparticles and a carbonaceous polymer film on graphene oxide sheets followed by heat treatment at various temperatures (250, 550, 750, and 900 °C). Detailed characterization of the resulting composite material using transmission electron microscopy and field emission scanning electron microscopy suggests that Sn-based nanoparticles were reliably bound to the graphene surface through a carbon film. Cyclic voltammograms and galvanostatic technique were used to investigate electrochemical properties of the Sn-based composite material as the anode of lithium-ion batteries (LIBs). Samples obtained with 550 °C heat treatment, which contained mixed Sn-based components (Sn, SnO, SnO2), exhibit the best electrochemical performance among the series of nanocomposites in terms of specific capacity and cycling stability.

Hierarchical vertically oriented graphene as a catalytic counter electrode in dye-sensitized solar cells

Vertically oriented graphene (VG) nanosheets are synthesized for counter electrodes (CEs) of dye-sensitized solar cells (DSSCs). The VG electrode exhibits charge transfer resistance about 1% of the Pt electrode and improves power conversion efficiency of DSSCs from 4.68% (for Pt CEs) to 5.36%.

Straightforward fabrication of a highly branched graphene nanosheet array for a Li-ion battery anode

A new graphene-based hybrid nanostructure is designed for anode materials in lithium-ion batteries. The highly branched graphene nanosheets (HBGNs) directly grown on a copper current collector exhibited promising electrochemical performance due to their unique morphology, high electrical conductivity, large interfacial surface area, and high porosity.

Rational design of mesoporous NiFe-alloy-based hybrids for oxygen conversion electrocatalysis

A mesoporous NiFe-based alloy was synthesized through a hard-template technique and investigated as the electrocatalyst for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Precious metal-free elements, i.e., nitrogen (N), nickel (Ni), and iron (Fe), were merged together through either doping or alloying to form a mesoporous structure supporting NiFe-alloy electrocatalyst (m-NiFe/CNx). The synergetic effects from multiple active sites, in combination with structural merits, endowed the m-NiFe/CNx electrocatalyst with an excellent electrocatalytic activity for both OER and ORR, including high activity, fast kinetics, modest overpotential, and excellent stability. The m-NiFe/CNx also featured a simple and scalable synthesis process with a low cost, which makes it an attractive alternative catalyst to precious metal-based electrocatalysts for OER and ORR.

Selective Deposition of CdSe Nanoparticles on Reduced Graphene Oxide to Understand Photoinduced Charge Transfer in Hybrid Nanostructures

A linker-free reduced graphene oxide (R-GO)-CdSe nanoparticle (CdSe NP) hybrid nanostructure was synthesized using a chemical vapor deposition method. CdSe NPs were selectively deposited on the surface of R-GO with controlled NP size and coverage. The distribution and morphology of CdSe NPs on R-GO were characterized by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The resulting hybrid nanostructure exhibited photoresponse to both laser and simulated sunlight AM 1.5G excitation. The hybrid structure with low CdSe NP coverage showed distinct photoresponse times in air, N(2), NH(3), and NO(2), while high CdSe NP coverage led to nearly constant but three orders of magnitude smaller response time in all gases. Such a difference in photoresponse as a function of NP coverage is attributed to the energy band bending at the interface between the R-GO and the CdSe NP. The selective deposition of CdSe NPs on R-GO and the understanding of the subsequent photoinduced charge transfer can potentially lead to high-performance optoelectronic devices.

Novel Hybrid Carbon Nanofiber/Highly Branched Graphene Nanosheet for Anode Materials in Lithium-Ion Batteries

The novel hybrid carbon nanofiber (CNF)/highly branched graphene nanosheet (HBGN) is synthesized via a simple two-step CVD method and its application as the anode material in a lithium-ion battery (LIB) is demonstrated. The CNFs offer a good electrical conductivity and a robust supporting structure, while the HBGNs provide increased Li storage sites including nanoporous cavities, large surface area, and edges of exposed graphene platelets. The hybrid material showed a reversible capacity of 300 mAh g(-1) with excellent cycling stability. Our study provides a new avenue for design and synthesis of carbon-carbon hybrid materials for versatile applications.

Novel hybrid Si film/carbon nanofibers as anode materials in lithium-ion batteries

The hybrid Si film/carbon nanofiber (CNF) as an anode in lithium-ion batteries (LIBs) was synthesized using a two-step chemical vapour deposition (CVD) method. This binder- and conductive additive-free electrode delivered a discharge capacity of 1000 mA h g−1 over 200 cycles. CNFs as a support material were directly grown on a stainless steel foil, while the stress-resilient Si films coated on the CNFs offered high Li storage capacity.