Yong Jung Kim

High performance of cup-stacked-type carbon nanotubes as a Pt–Ru catalyst support for fuel cell applications

The potential of cup-stacked-type carbon nanotubes (CSCNTs) as a catalyst support for the direct methanol fuel cells has been investigated by the electrochemical oxidation of methanol at various temperatures. The CSCNT-supported platinum–ruthenium (Pt–Ru) bimetallic catalyst exhibited twice as high a power density as the Pt–Ru catalyst supported on Vulcan XC-72 carbon, which is widely used as a catalyst support for the DMFC electrodes. The microscopic analysis of the CSCNT-supported Pt–Ru catalysts revealed that the bimetallic electrocatalysts were well dispersed on the CSCNT supports, and the particle size of the electrocatalysts was ca.5nm . The results of this work indicate that the performance of the carbon support materials is largely influenced by their electrical properties, morphology and crystallographic structures.

The Reinforcing Effect of Combined Carbon Nanotubes and Acetylene Blacks on the Positive Electrode of Lithium-Ion Batteries

Here, we demonstrate the preparation of high-performance positive electrodes for lithium-ion batteries by adding small amounts of both carbon nanotubes and acetylene blacks to LiCoO2-based active materials. The merits of using carbon nanotubes together with acetylene blacks as cathode fillers include not only the enhancement of the electrical and the thermal properties of the electrode but also the enhancement of the density of the electrode and the shortening of the electrolyte absorption time. We envisage that the use of carbon nanotubes as multifunctional fillers will increase in both cathode and anode materials for lithium-ion secondary batteries. Since the development of lithium-ion batteries in 1990, they have had an enormous influence on our lives. 2] At present, portable electronic devices and hybrid vehicles have evergrowing requirements for safe and high-performance lithiumion batteries. Therefore, new types of the nanostructure electrode materials or fillers including carbon nanotubes have been examined to improve the electrochemical performance of lithium-ion batteries (e.g. , large capacity, high rate capability and long life cycle), as well as for developing new end-use products (e.g. , cosmetics). In commercial lithium-ion batteries, up to 100 tons per year of highly pure crystalline carbon nanotubes are incorporated as effective fillers in anode materials, in which the resilience and the electrical properties of carbon nanotubes are believed to play an important role in extending the life cycle of the batteries. Similarly, several studies have examined the capability of carbon nanotubes to enhance the electrical conductivity of cathode materials in relation to that of conventionally used carbon blacks as lithium metal oxides, which have low electrical conductivity, experience structural deterioration or capacity degradation during charging and discharging cycles. However, there appears to be a critical question regarding the complete replacement of acetylene blacks by carbon nanotubes in cathodes owing to the capability of acetylene blacks to store a significant amount of electrolyte in their primary structure in addition to enhancing the conductivity. Also, previous studies have emphasized the electrical conductivity of the cathode as the only advantage of the incorporated carbon nanotubes, even though homogeneously distributed carbon nanotubes appear to give rise to additional functions. In this study, we examine the advantages of adding a hybridtype filler, consisting of acetylene blacks and high-purity crystalline thick multiwalled carbon nanotubes, to a LiCoO2-based cathode as compared to a cathode with added acetylene blacks or carbon nanotubes, from the viewpoint of their electrical and thermal properties and electrolyte adsorption capabilities as well as their electrochemical performance. Consequently, we demonstrate that optimally combined carbon nanotubes within a cathode act as electrical, thermal and structure-linking segments and provide suitably created pores, thereby decreasing the electrolyte absorption time. The prepared electrode consisted of three different morphological components: micrometer-sized LiCoO2 particles, long carbon nanotubes and nanometer-sized acetylene blacks. The technical reason for selecting LiCoO2 (Figure 1 c) as an active

High energy-density capacitor based on ammonium salt type ionic liquids and their mixing effect by propylene carbonate

Novel ionic liquids comprised of a quaternary ammonium salt type cation have been applied to an electrolyte for high-performance electric double-layer capacitors (EDLCs). The novel ionic liquids [IL-B; N, N-diethyl-N-methyl(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF 4 ) and IL-T; N, N-diethyl-N-methyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI)] are promising candidates for EDLC electrolytes in terms of high decomposition voltage (wide voltage window), nonflammability, easy handling, nonvolatility, and low production costs. Notably, the wide voltage window indicates that IL-B and IL-T are more advantageous in energy density than typical propylene carbonate-based electrolytes (i.e., TEA-BF 4 /PC) and a conventional imidazolium type ionic liquid (i.e., 1-ethyl-3-methylimidazolium tetrafluoroborate, EMI-BF 4 ). The effectiveness of IL-B and IL-T on the application to EDLC electrolytes has been confirmed by using KOH-activated mesophase pitch-based carbon fibers (MPCFs) as an electrode material. The combination of IL-T (IL-B) and KOH-activated MPCFs has provided 56 F/g (51 F/g) of high specific capacitance at maximum (1 mA/cm 2 discharge current density, 3.5 V charging voltage), which is equivalent to 224 F/g (204 F/g) in a conventional three-compartment measuring system. In addition, the specific capacitance of both ionic liquids has increased proportional to the increase in the applied voltage from 2.5 to 3.5 V, in contrast to the decline observed for TEA-BF 4 /PC at 3.5 V. Furthermore, the mixture of the IL-B exhibiting high viscosity with propylene carbonate (1 M of IL-B in PC) has been found to provide an excellent capacitance behavior comparable to that observed for the pure IL-B. This indicates that the mixture has great potential for application to EDLC electrolytes, similar to pure IL-B and IL-T.

PVDC-Based Carbon Material by Chemical Activation and Its Application to Nonaqueous EDLC

An organic solvent-type electrolytic solution was employed for application on an electric double-layer capacitor (EDLC) made of a polyvinylidene chloride (PVDC) based carbon material. Although the PVDC-based carbon material showed an excellent capacitance over a 100 F/g in an aqueous solvent-type electrolytic solution, it showed that a very small capacitance value was obtained in an organic solvent-type electrolyte solution, which did not even exceed the value of 5 F/g. It also confirmed the most effective temperature to get a capacitance shifted was from 700 to 900°C. At this time, chemical activation with KOH was tried for improving of the performance of the capacitance on PVDC-based carbon. The double-layer capacitance affected by variations of the pore size distribution was explained on the basis of the conventional pore analysis by means of gas adsorption. It was confirmed that the chemical activation with KOH was quite effective for improving capacitance. After the activation with KOH, especially by the 400 wt % addition, the capacitance obtained a value as high as 55 F/g. which is equivalent to 220 F/g for a conventional three-compartment system with a reference electrode. It was confirmed that the impregnation of KOH is effective for widening the pore diameter and affect the improvement of the capacitance. The effect of the functional groups on the surface of PVDC-carbons is also mentioned.

Carbon-supported Pt–Ru nanoparticles prepared in glyoxylate-reduction system promoting precursor–support interaction

A high dispersion of carbon-supported Pt–Ru alloy nanoparticles have been prepared in an alkaline aqueous solution by using glyoxylate as a reducing agent. The glyoxylate monoanion is converted to oxalate dianion with the reduction of metal precursor ions. The surface-potential analysis of unsupported Pt–Ru black in the presence of all the anion species coexistent in the preparation system suggests a possibility of oxalate dianion as the most effective particle stabilizer. In the glyoxylate-reduction system, a precursor–support interaction is significantly promoted to affect the formation and stabilization of nanoparticles. The glyoxylate reduction at 20-wt% metal loading has provided a higher Pt(0)/Ru concentration ratio in the near-surface region than that of 60-wt% catalyst. The structural and morphological features and advantageous surface composition of the 20-wt% catalyst contribute to the high mass activity for methanol oxidation in the anode overpotential range of typical direct methanol fuel cells.

Edge-enriched, porous carbon-based, high energy density supercapacitors for hybrid electric vehicles.

Supercapacitors can store and deliver energy by a simple charge separation, and thus they could be an attractive option to meet transient high energy density in operating fuel cells and in electric and hybrid electric vehicles. To achieve such requirements, intensive studies have been carried out to improve the volumetric capacitance in supercapacitors using various types and forms of carbons including carbon nanotubes and graphenes. However, conventional porous carbons are not suitable for use as electrode material in supercapacitors for such high energy density applications. Here, we show that edge-enriched porous carbons are the best electrode material for high energy density supercapacitors to be used in vehicles as an auxiliary powertrain. Molten potassium hydroxide penetrates well-aligned graphene layers vertically and consequently generates both suitable pores that are easily accessible to the electrolyte and a large fraction of electrochemically active edge sites. We expect that our findings will motivate further research related to energy storage devices and also environmentally friendly electric vehicles.

Freestanding, bendable thin film for supercapacitors using DNA-dispersed double walled carbon nanotubes

Freestanding, thin, and bendable electrodes for supercapacitors are fabricated by filtering DNA-dispersed double walled carbon nanotubes (DWNTs) into a thin film and thermally treating the film in argon. We found that DNA has the ability to disperse the strongly bundled DWNTs and is converted to phosphorus-enriched carbons, which give rise to strong redox peaks at around 0.4 V. The combination of the large capacitance from the DNA-derived carbons and the high electrical conductivity of carbon nanotubes allow DWNT/DNA films to be used as a potential electrode material for supercapacitors.

An environmentally friendly dispersion method for cup-stacked carbon nanotubes in a water system

A dry process using VUV light was confirmed as a novel technique to attach functional groups onto cup-stacked carbon nanotubes and to develop their isolation in a water system without the use of dispersing agents.

Anomalous cyclic voltammetric response from pores smaller than ion size by voltage-induced force

Nanoporous carbons, with different micropore size distributions, were prepared based on waste coffee grounds by a chemical activation process in order to elucidate the correlation between desolvated ions and pores smaller than the sizes of ions using an organic electrolyte. The pore structure of the coffee-based nanoporous carbon was strongly dependent on the heat-treatment temperature prior to the activation process. Cyclic voltammograms of the nanoporous carbons mainly dominated by the smaller pore relative to that of the bare ion size clearly showed deviation from an ideal feature of the current response. It was clearly envisaged that even a bare ion of a size larger than the pore size can penetrate into the pore by voltage-induced force.

Carbonaceous Anode Materials

The use of the engineered carbon materials as an anode allowed the lithium ion batteries (LIBs) to achieve a large advance in the last decade. The performance of LIBs strongly depends on the microtexture of the carbon materials. Due to the contribution of the carbon materials, the electrochemical performance of the LIBs has been improved almost double for the last 10 years. Thus, intensive work has focused on the identification of key factors of carbon materials that can improve anode performance. Recently, there is the active work on the development of nanosized carbon materials (e.g., carbon nanotube and graphene). In this chapter, we describe the correlation between the microstructural parameters and anode performance of conventional and novel types of carbon materials for Li ion batteries by connecting with the market demand and the trends in Li ion secondary batteries.