Optimization of Pore Characteristics of Graphite-Based Anode for Li-Ion Batteries by Control of the Particle Size Distribution.

We investigate the reassembly techniques for utilizing fine graphite particles, smaller than 5 µm, as high-efficiency, high-rate anode materials for lithium-ion batteries. Fine graphite particles of two sizes (0.4-1.2 µm and 5 µm) are utilized, and the mixing ratio of the two particles is varied to control the porosity of the assembled graphite. The packing characteristics of the assembled graphite change based on the mixing ratio of the two types of fine graphite particles, forming assembled graphite with varying porosities. The open porosity of the manufactured assembled graphite samples ranges from 0.94% to 3.55%, while the closed porosity ranges from 21.41% to 26.51%. All the assembled graphite shows improved electrochemical characteristics properties compared with anodes composed solely of fine graphite particles without granulation. The sample assembled by mixing 1.2 µm and 5 µm graphite at a 60:40 ratio exhibits the lowest total porosity (27.45%). Moreover, it exhibits a 92.3% initial Coulombic efficiency (a 4.7% improvement over fine graphite particles) and a capacity of 163.4 mAh/g at a 5C-rate (a 1.9-fold improvement over fine graphite particles).

Optimization of the Filler-and-Binder Mixing Ratio for Enhanced Mechanical Strength of Carbon-Carbon Composites.

In this paper, a method for optimizing the mixing ratio of filler coke and binder for high-strength carbon-carbon composites is proposed. Particle size distribution, specific surface area, and true density were analyzed to characterize the filler properties. The optimum binder mixing ratio was experimentally determined based on the filler properties. As the filler particle size was decreased, a higher binder mixing ratio was required to enhance the mechanical strength of the composite. When the d50 particle size of the filler was 62.13 and 27.10 µm, the required binder mixing ratios were 25 and 30 vol.%, respectively. From this result, the interaction index, which quantifies the interaction between the coke and binder during carbonization, was deduced. The interaction index had a higher correlation coefficient with the compressive strength than that of the porosity. Therefore, the interaction index can be used in predicting the mechanical strength of carbon blocks and optimizing their binder mixing ratios. Furthermore, as it is calculated from the carbonization of blocks without additional analysis, the interaction index can be easily used in industrial applications.

Effect of Porosity in Activated Carbon Supports for Silicon-Based Lithium-Ion Batteries (LIBs).

Activated carbon supports for Si deposition with different porosities were prepared, and the effect of porosity on the electrochemical characteristics was investigated. The porosity of the support is a key parameter affecting the Si deposition mechanism and the stability of the electrode. In the Si deposition mechanism, as the porosity of activated carbon increases, the effect of particle size reduction due to the uniform dispersion of Si was confirmed. This implies that the porosity of activated carbon can affect the rate performance. However, excessively high porosity reduced the contact area between Si and activated carbon, resulting in poor electrode stability. Therefore, controlling the porosity of activated carbon is essential to improving the electrochemical characteristics.

Effect and Mechanism of Pitch Coating on the Rate Performance Improvement of Lithium-Ion Batteries.

This study evaluated the effect of pitch coating on graphite anode materials used in lithium-ion batteries and investigated the mechanism whereby pitch coating improves the electrochemical properties. The FG (flake graphite) and pitch were mixed in weight ratios of 95:5-80:20. The mixture was pressed and prepared into a block form. Additionally, heat treatment was performed at 900 °C for 1 h and pulverized in the size range of 10-25 µm. The results showed that the particles of uniform pitch-coated graphite became more spherical. However, when the pitch is added excessively, pitch aggregation occurs rather than a thicker coating, indicating a nonuniform particle shape. Pitch has a randomly oriented structure and a small crystal size. Therefore, pitch serves as a lithium-ion diffusion pathway, resulting in an improved rate of performance. Notably, the uniform pitch-coated graphite exhibited an outstanding rate of performance owing to the relieving of particle orientation in the electrode rolling process. During the rolling process, the particles are oriented perpendicular to the lithium-ion diffusion pathway, making it difficult for the lithium ions to diffuse. Adding an excessive amount of pitch was found to deteriorate the rate of performance. Pitch aggregation increased the interfacial resistance by forming a heterogeneous surface.

Control of cyclic stability and volume expansion on graphite-SiO x -C hierarchical structure for Li-ion battery anodes.

To increase the energy density of today's batteries, studies on adding Si-based materials to graphite have been widely conducted. However, adding a Si-based material in the slurry mixing step suffers from low distribution due to the self-aggregation property of the Si-based material. Herein, a hierarchical structure is proposed to increase the integrity by using APS to provide a bonding effect between graphite and SiO x . Additionally, to endow a protection layer, carbon is coated on the surface using the CVD method. The designed structure demonstrates enhanced integrity based on electrochemical performance. The MSG (methane decomposed SiO x @G) electrode demonstrates a high ICE of 85.6% with 429.8 mA h g-1 initial discharge capacity. In addition, the MSG anode has superior capacity retention (89.3%) after 100 cycles, with enhanced volumetric expansion (12.7%) after 50 cycles. We believe that the excellent electrochemical performance of MSG is attributed to increased integrity by using APS (3-aminopropyltrimethoxysilane) with a CVD carbon coating.

Fabrication of Binder Pitches Allowing for Low-Temperature Formation and High Coking Values and Examination of Mechanical Properties of Artificial Graphite Blocks Made of Binder Pitches.

The present study focused on the development of a binder pitch to allow for low-temperature forming processes when fabricating coke-based artificial graphite blocks while increasing the density of the resultant blocks. To this end, high-softening-point (200 °C) pitches were fabricated. The pitch and byproducts obtained from the pitch synthesis were then used as binders to fabricate blocks with high mechanical strength and low porosity. Pitches were fabricated using pyrolyzed fuel oil (PFO), a petroleum residue. A high-softening-point (200 °C) pitch synthesized at 420 °C for 3 h was used as a binder pitch, and conventional pitch (124 °C) was synthesized at 400 °C for 1 h and then used. Pitch byproducts were extracted according to the boiling point of naphthalene (two rings) and anthracene (three rings) with varying numbers of aromatic rings by distillation. The largest amount of pitch byproduct was obtained in the temperature range from 220 to 340 °C, and the content of naphthalene in the byproduct was the highest over the entire temperature range. The fabricated pitches at 420 °C and byproducts were mixed to form modified pitches. It was found that their softening point and coking value (CV) decreased with the increasing content of the pitch byproduct. Low-boiling point components of the byproducts were removed from the modified pitches at the kneading process temperature (200 °C), and the mass-loss rate observed in the carbonization process temperature range (200-900 °C) was comparable to that of the high-softening-point pitch. The kneading rate of the pitch and byproduct was determined and selected based on the mass-loss rate described above, and blocks were then fabricated using a hot press. Subsequently, the fabricated blocks were subjected to heat treatment for carbonization (900 °C) and graphitization (2700 °C). After the heat treatment, the true density and apparent density of the blocks were measured, and the porosity of the blocks was calculated based on these values. The porosity of the graphite block fabricated using the pitch with a softening point of 120 °C was 21.84%, while the porosity of the graphite block fabricated using the modified pitch was 14.9%. For mechanical strength analysis, their compressive strength was measured. The compressive strength of the graphite block made of the conventional pitch (CP) was measured to be 47.59 MPa, while the compressive strength of the graphite block made of pitch mixed with a byproduct distilled at 220-340 °C was 58.79 MPa. This result suggested that a decrease in the porosity resulted in increased mechanical strength. The application of the modified pitches developed in the present study temporarily decreased the softening point of the high-softening-point pitch due to the effect of the added byproducts, allowing for a low-temperature forming process. It was also possible to fabricate artificial graphite blocks with low porosity due to the high CV of the high-softening-point pitch. As a result, blocks with high mechanical strength could be obtained.

Study of the Molecular-Weight Distribution of Binder Pitches for Carbon Blocks.

The present study aimed to identify the required characteristics of binder pitches in the filler-binder mixing process to effectively manufacture graphite blocks. To this end, a binder pitch was separated into pitch fractions of varying molecular-weight segments. The role and effectiveness of each pitch fraction were then analyzed with respect to their molecular-weight distribution. As a result, the optimal molecular-weight distribution was determined. More specifically, a coal-tar pitch was separated into solvent-soluble and solvent-insoluble fractions. The molecular-weight distribution was determined according to this classification, and the characteristics of each pitch fraction were examined. The pitch separation process was conducted using three solvents: hexane, toluene, and quinoline. The resulting pitch was separated into the following pitch fractions: hexane-soluble (HS), hexane-insoluble-toluene-soluble (HI-TS), toluene-insoluble-quinoline-soluble (TI-QS), and quinoline-insoluble (QI). Fourier transform infrared (FT-IR) spectrum, matrix-assisted laser desorption ionization-time of flight (MALDI-TOF), and softening point of each pitch fraction were measured. Also, pitch samples were refabricated while varying the mixing ratio of these pitch fractions, and carbon blocks were then prepared using them. The compressive strength and porosity of these blocks were measured and compared. The P154_B pitch with a high content of TI-QS was used to fabricate a green block. Due to the high viscosity of the binder used, the fluidity was not sufficiently high, and thus, the green block made of this pitch had relatively low strength. The other blocks had similar levels of strength. After the carbonization process, the carbon block with a high content of HS (P352_B-C) and the carbon block with the HS content removed (P073_B-C) showed lower compressive strength than their respective green-block counterparts (P352_B and P073_B). However, their strength was higher compared to those of the other carbon blocks. In the case of carbon block P073_B-C, the HS content was completely removed, and thus, the content of TI-QS (ß-resin) was relatively high. Accordingly, this carbon block ended up with large amounts of components that had high coking values (CVs), and this contributed to limiting the formation of pores. Therefore, the compressive strength of this carbon block was high. In the case of the carbon block with a high content of HS (P352_B-C), a suitable level of viscosity was achieved because the HS components ensured high fluidity. As a result, blocks with higher density and compressive strength could be fabricated. The major findings of the present study confirm that producing carbon blocks with high mechanical properties requires binder pitches with a balanced combination of suitable viscosity to ensure sufficiently high fluidity and a proper level of CV to effectively suppress the formation of pores in the mixing and molding process.

Ultrathin rGO-wrapped free-standing bimetallic CoNi2S4-carbon nanofibers: an efficient and robust bifunctional electrocatalyst for water splitting.

Electrochemical water splitting represents an ideal strategy for producing clean hydrogen as an energy carrier that serves as an alternative to fossil fuels. As an effective method for hydrogen production, an efficient inexpensive multifunctional electrocatalyst with high durability is designed. Herein, we describe the heterostructural design of a three-dimensional catalytic network with self-embedded CoNi2S4 nanograins grown on electrospun carbon nanofibers (CoNi2S4-CNFs) with anchored thin-layer reduced graphene oxide. This is achieved via facile electrospinning followed by carbonization, low-temperature sulfidation, and surface functionalization. As a bifunctional catalyst, CoNi2S4-CNFs exhibited robust high activity toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline medium. The anchored ultrathin graphene oxide layer promoted the stability and durability of the catalytic network with an efficient path for the transportation of electrons. The rGO-anchored CoNi2S4-CNFs yielded overpotential values of 228 mV and 205 mV for the HER and OER, respectively, that drives a current density of 20 mA cm-2 in an alkaline medium. Notably, the excellent electrochemical properties are attributed to the functional effect of the CoNi2S4 on the CNF network. The ultrathin feature of rGO improved the durability of the catalytic network. Moreover, using the rGO-anchored CoNi2S4-CNFs as a cathode and anode in a two-electrode water splitting system required a cell voltage of only 1.55 V to reach a current density of 10 mA cm-2. These CNFs exhibited outstanding durability for 48 h. The present work offers new insight for the design of a catalytic network with a non-noble metal catalyst that exhibits excellent electrocatalytic activity and durability on the metal sulfides in overall water splitting.

Activated carbon fibers for toxic gas removal based on electrical investigation: Mechanistic study of p-type/n-type junction structures.

In this study, we evaluated the potential use of CuO-ZnO combination structures with activated carbon fibers (ACFs) for the adsorption (by ACFs) and electrochemical detection (by CuO-ZnO) by of SO2 gas. The gas adsorptivity was concluded to improve as a result of the synergetic effects of physical adsorption by the micropores and mesopores, the specific surface area developed by chemical activation and the chemical adsorption reaction between SO2 and the transition metals introduced in the CuO-ZnO combination structures. From comparison of the SO2 sensing properties, the CuO-ZnO combination structures with ACFs exhibited the fastest sensing capability. This result can be attributed to the larger specific surface area of the semiconductor, which extended its depletion layer by forming p-type CuO/n-type ZnO junctions. This phenomenon led to good SO2 detection through a decrease in the resistance; thus, the contributions of the sensing responses of p-type CuO and n-type ZnO represent a predominant characteristic of the sensor. These types of mechanisms were proven through various physicochemical and electrical characterization methods, especially through evaluation of the SO2 sensing capability of the CuO-ZnO combination structures with ACFs. The reversible sensing capability indicates that the p-n junction structure changed the electrical properties of the ACFs, leading to an intriguing sensing mechanism.

Effects of pore structure on the high-performance capacitive deionization using chemically activated carbon nanofibers.

Capacitive deionization (CDI) electrodes were constructed from activated carbon fibers prepared using electrospinning and chemical activation. The CDI efficiencies of these electrodes were studied as a function of their specific surface areas, pore volumes and pore sizes via salt ion adsorption. The specific surface areas increased approximately 90 fold and the pore volume also increased approximately 26 fold with the use of greater amounts of the chemical activation agent. There was a relative increase in the mesopore fraction with higher porosity. A NaCI solution was passed through a prepared CDI system, and the salt removal efficiency of the CDI system was determined by the separation of the Na+ and Cl- ions toward the anode and cathode. The CDI efficiency increased with greater specific surface areas and pore volumes. In addition, the efficiency per unit pore volume increased with a reduction in the micropore fraction, resulting in the suppressed overlapping effect. In conclusion, the obtained improvements in CDI efficiency were mainly attributed to mesopores, but the micropores also played an important role in the high-performance CDI under conditions of high applied potential and high ion concentrations.

Electrical resistance behavior of oxyfluorinated graphene under oxidizing and reducing gas exposure.

The electrical resistance behavior of graphene was studied under oxidizing and reducing gas exposure. The graphene surface was modified via oxyfluorination to obtain a specific surface area and oxygen functional groups. Fluorine radicals provided improved pore structure and introduction of an oxygen functional group. A high-performance gas sensor was obtained based on enlarged target gas adsorption sites and an enhanced electron charge transfer between the target gas and carbon surface via improved pore structure and the introduction of oxygen functional groups, respectively.

Improvement in transdermal drug delivery performance by graphite oxide/temperature-responsive hydrogel composites with micro heater.

Transdermal drug delivery system (TDDS) was prepared with temperature-responsive hydrogel. The graphite was oxidized and incorporated into hydrogel matrix to improve the thermal response of hydrogel. The micro heater was fabricated to control the temperature precisely by adopting a joule heating method. The drug in hydrogel was delivered through a hairless mouse skin by controlling temperature. The efficiency of drug delivery was improved obviously by incorporation of graphite oxide due to the excellent thermal conductivity and the increased interfacial affinity between graphite oxide and hydrogel matrix. The fabricated micro heater was effective in controlling the temperature over lower critical solution temperature of hydrogel precisely with a small voltage less than 1 V. The cell viability test on graphite oxide composite hydrogel showed enough safety for using as a transdermal drug delivery patch. The performance of TDDS could be improved noticeably based on temperature-responsive hydrogel, thermally conductive graphite oxide, and efficient micro heater.

Photocatalytic degradation of procion blue dye in aqueous solution by a TiO2-carbon nano-composite.

A nano-composite consisting of TiO2-CNT was prepared via the sol-gel technique by using titanium n-butoxide along with carbon nano-tubes (CNTs) followed by calcination at 450 degrees C. Spectral analysis reveals that the TiO2 formed was present on the carbon in anatase form. The effect of adsorption was investigated in an aqueous solution of procion blue dye in a darkroom and the photochemical reaction in aqueous suspensions of titania composite under UV illumination. The reaction was studied by monitoring the discoloration of dye via employing a UV-Visible spectrophotometeric technique as a function of irradiation time. The composite catalyst was found to be efficient in the photodegradation of the dye.

Improved photodegradation properties and kinetic models of a solar-light-responsive photocatalyst when incorporated into electrospun hydrogel fibers.

The capacity of a photocatalyst system to degrade water pollutants was optimized using solar-light-sensitive TiO(2) and the swelling behavior of a hydrogel. TiO(2) synthesized via a sol-gel process was modified by multielement doping to change its solar-light-responsive properties. A hydrogel was used for the rapid absorption of both anionic and cationic water pollutants. TiO(2) particles were immobilized in/on hydrogel fibers by an electrospinning method for the easy recovery of TiO(2), and the ability of the hydrogel/TiO(2) composite to degrade dye molecules was studied. The TiO(2) particles were observed to have maintained their original anatase-type crystallinity in/on the electrospun hydrogel fibers. The dye degradation capacity of the hydrogel/TiO(2) composite was investigated using both anionic and cationic dyes under sunlight. Two mechanisms were suggested by which the hydrogel/TiO(2) composite can remove dye particles from the water: (1) the absorption of dyes by the hydrogel and (2) the degradation of the dye by the TiO(2) in the hydrogel. Both of these mechanisms were investigated in this study. We found that the dye was effectively absorbed by the hydrogel fibers as demonstrated by the swelling behavior of the hydrogel and the nano-size effects. The dye was then introduced to the TiO(2) particles for degradation.

Fluorination of electrospun hydrogel fibers for a controlled release drug delivery system.

Electrospinning and fluorination were carried out in order to obtain a controlled release drug delivery system to solve the problem of both an initial burst of the drug and a limited release time. Poly(vinyl alcohol) was electrospun with Procion Blue as a model drug and heat treated in order to obtain cross-linked hydrogel fibers. Two different kinds of electrospun fibers of thin and thick diameters were obtained by controlling the electrospinning conditions. Thin fibers offer more available sites than thick fibers for surface modification during fluorination. Fluorination was conducted to control the release period by introducing hydrophobic functional groups on the surface of fibers. With an increase in the reaction pressure of the fluorine gas hydrophobic C-F and C-F(2) bonds were more effectively introduced. Over-fluorination of the fibers at higher reaction pressures of fluorine gas led to the introduction of C-F(2) bonds, which made the surface of the fibers hydrophobic and resulted in a decrease in their swelling potential. When C-F bonds were generated the initial drug burst decreased dramatically and total release time increased significantly, by a factor of approximately 6.7 times.

Effects of fluorination modification on pore size controlled electrospun activated carbon fibers for high capacity methane storage.

Electrospun carbon fibers were prepared as a methane storage medium. Chemical activation was carried out using potassium carbonate to develop the pore structure, which can provide sites for the uptake of methane, and then fluorination surface modification was conducted to enhance the capacity of storage. Chemical activation provided a highly microporous structure, which is beneficial for methane storage, with a high specific surface area greater than 2500m(2)/g. The pore size distribution showed that the prepared samples have pore sizes in the range of 0.7-1.6nm. The effect of fluorination surface modification was also investigated. The functional groups, which were confirmed by XPS analysis, played an important role in guiding methane gas into the carbon silt pores via the attractive force felt by the electrons in the methane molecules due to the high electronegativity of fluorine. Eventually, the methane uptake increased up to 18.1wt.% by the synergetic effects of the highly developed micropore structure and the guiding of methane to carbon pores by fluorine.

Investigation of multielemental catalysts based on decreasing the band gap of titania for enhanced visible light photocatalysis.

Novel photocatalysts based on carbon-, nitrogen-, boron-, and fluorine-codoped TiO(2) have been successfully prepared from a single precursor in order to obtain titania with a decreased band gap. Three kinds of catalytic mechanisms are suggested. Initially, boron acts as an initiator to lead the movement of electrons in the valence band, and then nitrogen and fluorine provide electrons in the valence band. Eventually, electrons in the valence band can travel to the conduction band through a carbon bridge. The effect of the calcination temperature was also evaluated for the photodegradation of dyes. Excellent photoactivity results were obtained in the case of samples treated at 400 degrees C and the phase transformation from anatase to rutile did not occur up to calcination temperatures of 800 degrees C. The photodegradation followed the pseudo-first-order kinetic expression. The exceptional visible photoactivities of the prepared catalysts can be predominantly attributed to the effects of doping on titania, reducing its band gap.

Improved capacitance characteristics of electrospun ACFs by pore size control and vanadium catalyst.

Nano-sized carbon fibers were prepared by using electrospinning, and their electrochemical properties were investigated as a possible electrode material for use as an electric double-layer capacitor (EDLC). To improve the electrode capacitance of EDLC, we implemented a three-step optimization. First, metal catalyst was introduced into the carbon fibers due to the excellent conductivity of metal. Vanadium pentoxide was used because it could be converted to vanadium for improved conductivity as the pore structure develops during the carbonization step. Vanadium catalyst was well dispersed in the carbon fibers, improving the capacitance of the electrode. Second, pore-size development was manipulated to obtain small mesopore sizes ranging from 2 to 5 nm. Through chemical activation, carbon fibers with controlled pore sizes were prepared with a high specific surface and pore volume, and their pore structure was investigated by using a BET apparatus. Finally, polyacrylonitrile was used as a carbon precursor to enrich for nitrogen content in the final product because nitrogen is known to improve electrode capacitance. Ultimately, the electrospun activated carbon fibers containing vanadium show improved functionality in charge/discharge, cyclic voltammetry, and specific capacitance compared with other samples because of an optimal combination of vanadium, nitrogen, and fixed pore structures.

The study of controlling pore size on electrospun carbon nanofibers for hydrogen adsorption.

Polyacrylonitrile (PAN)-based carbon nanofibers (CNFs) were prepared by using electrospinning method and heat treatment to get the media for hydrogen adsorption storage. Potassium hydroxide and zinc chloride activations were conducted to increase specific surface area and pore volume of CNFs. To investigate the relation between pore structure and the capacity of hydrogen adsorption, textural properties of activated CNFs were studied with micropore size distribution, specific surface area, and total pore volume by using BET (Brunauer-Emmett-Teller) surface analyzer apparatus and the capacity of hydrogen adsorption was evaluated by PCT (pressure-composition-temperature) hydrogen adsorption analyzer apparatus with volumetric method. The surface morphology of activated CNFs was observed by SEM (scanning electron microscope) images to investigate the surface change through activation. Even though specific surface area and total pore volume were important factors for increasing the capacity of hydrogen adsorption, the pore volume which has pore width (0.6-0.7 nm) was a much more effective factor than specific surface area and pore volume in PAN-based electrospun activated CNFs.

Preparation and characteristics of electrospun activated carbon materials having meso- and macropores.

Mesoporous activated carbon samples were prepared from electrospun PAN-based carbon fibers using physical activation with silica. Textural characterization was performed using nitrogen adsorption at 77 K. The BET specific surface area and pore size distribution of silica activated carbon materials were investigated. According to the increment of silica, BET specific surface area was increased about thirty times and it was found that silica activated carbon materials were highly mesoporous by studying pore surface distribution and pore volume distribution. Surface morphology of silica activated carbon materials were observed by SEM images. The spherical typed carbon materials were investigated. The diameter of spherical typed carbon materials was increased in proportional of the increment of silica.