• Journal of Hydrogen and New Energy
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• v.29 no.5
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• pp.450-457
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• 2018

To design the practical core-shell electrocatalysts, combination of core and shell materials is important to meet catalytic activity and durability target. In general, Pd is considered as a good core material due to its best activity caused by strain/ligand effect. Preparing Pd nanoparticles can be a starting point in fabricating core-shell type electrocatalysts, much simplified Pd preparing process is suggested by using carbon monoxide (CO) as a reducing agent and/or capping agent. The solvent composition and reaction temperature can control to nanosheet, tetrahedron, and sphere without using additional stabilizer. Among them, Pd nanosheet which has mainly (111) plane showed about 3 times higher electrocatalytic activity for oxygen reduction reaction (ORR) to the spherical Pd nanoparticles. The enhanced ORR activity of Pd nanosheets can be attributed to the exposure of Pd (111) surface and the high electrochemical surface area. Therefore, we demonstrated that the shape of Pd nanomaterials is easily controlled via a facile reduction method using CO, and (111) plane-oriented Pd nanosheets can be a promising ORR catalysts and core material for polymer electrolyte fuel cells (PEFCs).

• Korean Chemical Engineering Research
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• v.61 no.4
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• pp.604-610
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• 2023

The nitrogen reduction reaction (NRR), which produces NH₃ by reducing N₂ under ambient conditions, is attracting attention as a promising technology that can reduce energy consumption in industrial processes. We investigated the adsorption behaviors at various active sites on the Ni (100) surface, which is widely used among catalytic metal surfaces capable of adsorbing and activating N₂, based on density-functional theory calculations. We also investigated two N₂ adsorption structures, so-called end-on and side-on structures. We find that for the end-on case, N₂ is adsorbed on a top site, and the reaction proceeded in a distal pathway, while for the side-on case, N₂ is adsorbed on a bridge site, and the reaction proceeded with enzymatic pathway. Finally, this study provides insight into the adsorption of metal catalyst surfaces for the NRR reactions based on the calculated Gibbs free energy profiles of the thermodynamically most favorable pathways.

• Journal of Powder Materials
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• v.13 no.5 s.58
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• pp.327-335
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• 2006

In the development of new hydrogen absorbing materials for a next generation of metal hydride electrodes for rechargeable batteries, metastable Mg-Ni-based compounds find currently special attention. Amor phous-nanocrystalline $Mg_{63}Ni_{30}Y_7$ and $Mg_{50}Ni_{30}Y_{20}$ alloys were produced by mechanical alloying and melt-spinning and characterized by means of XRD, TEM and DSC. On basis of mechanically alloyed Mg-Ni-Y powders, complex hydride electrodes were fabricated and their electrochemical behaviour in 6M KOH (pH=14,8) was investigated. The electrodes made from $Mg_{63}Ni_{30}Y_7$ powders, which were prepared under use of a SPEX shaker mill, with a major fraction of nanocrystalline phase reveal a higher electrochemical activity far hydrogen reduction and a higher maximum discharge capacity (247 mAh/g) than the electrodes from alloy powder with predominantly amorphous microstructure (216 mAh/g) obtained when using a Retsch planetary ball mill at low temperatures. Those discharge capacities are higher that those fur nanocrystalline $Mg_2Ni$ electrodes. However, the cyclic stability of those alloy powder electrodes was low. Therefore, fundamental stability studies were performed on $Mg_{63}Ni_{30}Y_7$ and $Mg_{50}Ni_{30}Y_{20}$ ribbon samples in the as-quenched state and after cathodic hydrogen charging by means of anodic and cathodic polarisation measurements. Gradual oxidation and dissolution of nickel governs the anodic behaviour before a passive state is attained. A stabilizing effect of higher fractions of yttrium in the alloy on the passivation was detected. During the cathodic hydrogen charging process the alloys exhibit a change in the surface state chemistry, i.e. an enrichment of nickel-species, causing preferential oxidation and dissolution during subsequent anodization. The effect of chemical pre-treatments in 1% HF and in $10\;mg/l\;YCl_3/1%\;H_2O_2$ solution on the surface degradation processes was investigated. A HF treatment can improve their anodic passivation behavior by inhibiting a preferential nickel oxidation-dissolution at low polarisation, whereas a $YCl_3/H_2O_2$ treatment has the opposite effect. Both pre-treatment methods lead to an enhancement of cathodically induced surface degradation processes.

• Journal of the Korean Electrochemical Society
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• v.12 no.4
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• pp.342-348
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• 2009

Zn/Air secondary batteries are high energy density type and environment-friendly. Also, they have safer properties than batteries of other type by low manufacturing cost and using the electrolyte solution. But, they have a weak concerning large output discharge. Oxygen evolution reaction(OER) and oxgen reduction reaction(ORR) in aqueous solution make a result of a decrease of cell efficiency and life span. Therefore, to minimize the voltage drop from between OCV and charge/discharge voltage is key point. The problem should be solved through developing catalysts of high efficiency. In this study, we synthesized $Pr_{1-x}(Sr,\;Ca)_x\;CoO_3$ powders by citric method and then measured physical characteristics of each powder by XRD, SEM, TGA etc. We examined its electrochemical properties by the cathodic polarization, anodic polarization and cyclic voltammogram. We achieved results that new catalysts showed better performances than existing $La_{1-x}Sr_xCoO_3$, $La_{1-x}Ca_xCoO_3$, ect. catalysts prepared in our lab.

• Journal of the Korean Electrochemical Society
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• v.11 no.4
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• pp.289-296
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• 2008

Electrochemical studies were performed by a half-cell test for the nickel hydroxide (cathode) and hydrogen storage alloy(anode) electrodes for the sealed Ni-MH batteries applicable to industrial use. The electrodes were fabricated and checked a charge efficiency and an internal pressure of the battery during charge-discharge cycling. In order to reduce the internal pressure of the sealed Ni-MH battery, cyclic voltammetry (CV) were performed on the electrodes of nickel hydroxide(cathode) and hydrogen storage alloy(anode), respectively. The results of the test showed clearly the oxidation/reduction and oxygen evolution reaction in a nickel hydroxide electrode and the hydrogenation behavior of a hydrogen storage electrode. The sealed Ni-MH battery of 130Ah was fabricated by using nickel hydroxide of a high over-voltage for an oxygen gas evolution and hydrogen storage alloy of a good performance for activation The battery showed a good characteristics such as a high charge efficiency of 98% at 1 C charge current, a low level internal pressure of 4 atm on a continuous over-charging and a large preservation capacity of 95% at 400 cycle.

• Applied Chemistry for Engineering
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• v.8 no.6
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• pp.1022-1028
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• 1997

This work was performed to study the characteristics of electrochemical intercalation reactions occurring at the interface between the organic electrolyte and tungsten trioxide thin film (thickness of $4000{\AA}$) prepared by e-beam evaporation method as cathodically coloring oxide with regard to the electrochromism by the intercalating reactions of the lithium cation in the 1M $LiClO_4/PC$ organic solution. The characteristics of electrochemical intercalation reactions were investigated by various DC electrochemical methods such as cathodic Tafel polarization test, multiple and the single sweep cyclic voltammetry and the coulomety titrations method. The surfaces of thin films were observed with the patterns of X ray diffraction after the coloring and bleaching reactions. In comparison with the previous results that $WO_3$ thin film intersely detached from the surface of electrode when the hydrogen cation was intercalated into $WO_3$ thin film in the o.1N $H_2SO_4$ aqueous solution, the intercalation reaction of lithium cation into $WO_3$ thin film in the 1M $LiClO_4/PC$ organic solution was shown that the stable bleaching and coloration was appeared within 1.0V of the applied overpotential. When the overpotential of electrochromic reaction for lithium cation in the 1M $LiClO_4/PC$ organic solution had been applied up to 1.5V, the accumulation phenomenon of lithium in amorphous $WO_3$ thin film layer occurred because the inserted lithium into amorphous $WO_3$ thin layer for coloring process was not fully removed from the thin layer to the electrolyte during bleaching process. It was found that there is a limitation of applied overpotential for coloring process by the reduction of the current densities of bleaching and coloration after few number of coloring and bleaching cycles.

• Korean Chemical Engineering Research
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• v.58 no.1
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• pp.142-149
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• 2020

In this study, the electrochemical performance of Graphite/Silicon/Pitch composites as anode material was investigated to improve the low theoretical capacity of artificial graphite. Spherical artificial graphite surface was coated with polyvinylpyrrolidone (PVP) amphiphiles material to synthesize Graphite/Silica material by silica islands growth. The Graphite/Silicon/Pitch composites were prepared by petroleum pitch coating and magnesiothermic reduction. The Graphite/Silicon/Pitch composite electrodes manufactured using poly(vinylidene fluoride) (PVDF), carboxymethyl cellulose (CMC) and polyacrylic acid (PAA) binders. The coin type half cell was assembled using various electrolytes and additives. The Graphite/Silicon/Pitch composites were analysed by X-ray diffraction (XRD), scanning electron microscope (SEM) and a thermogravimetric analyzer (TGA). The electrochemical characteristics of Graphite/Silicon/Pitch composite were investigated by constant current charge/discharge, rate performance, cyclic voltammetry and electrochemical impedance spectroscopy. The Graphite/Silicon/Pitch composites showed high cycle stability at a graphite/silica/pitch ratio (1:4:8 wt%). When the electrode is prepared using PAA binder, the high capacity and stability is obtained. The coin type half cell assembled using EC: DMC: EMC electrolyte showed high initial capacity (719 mAh/g) and excellent cycle stability. The rate performance has an capacity retention (77%) at 2 C/0.1 C and an capacity recovery (88%) at 0.1 C / 0.1 C when the vinylene carbonate (VC) was added.

• Korean Chemical Engineering Research
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• v.57 no.1
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• pp.118-123
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• 2019

In this study, the electrochemical characteristics of Graphite/Silicon/Pitch anode composites were analyzed to improve the low theoretical capacity of graphite as a lithium ion battery. The Graphite/Silica composites were synthesized by bonding silica onto polyvinylpyrrolidone coated graphite. The surface of used silica was treated with (3-Aminopropyl)triethoxysilane(APTES). Graphite/Silicon/Pitch composites were prepared by carbonization of petroleum pitch, the fabrication processes including the magnesiothermic reduction of nano silica to obtain silicon and varying the mass ratio of silica. The Graphite/Silicon/Pitch composites were analysed by XRD, SEM and XRD. Also the electrochemical performances of Graphite/Silicon/Pitch composite as the anode of lithium ion battery were investigated by constant current charge/discharge, rate performance, cyclic voltammetry and electrochemical impedance tests in the electrolyte of $LiPF_6$ dissolved in organic solvents (EC:DMC:EMC=1:1:1 vol%). The Graphite/Silicon/Pitch anode composite (silica 28.5 in weight) has better capacity (537 mAh/g). The cycle performance has an excellent capacity retention to 30th cycle of 95% and the retention rate capability of 98% in 0.1 C/0.2 C.

• Korean Chemical Engineering Research
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• v.62 no.1
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• pp.99-104
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• 2024

In this study, a carbon coated hollow silicon (HSi/C) composite material was prepared for anode material of high-capacity lithiun-ion battery. Hollow silica (HSiO₂) was synthesized by the Stöber method with CTAB (N-Cetyltrimethylammonium bromide). The HSi/C anode composite was manufactured by carbon coating after magnesiothermic reduction of HSiO₂. The physical and electrochemical characteristics of the prepared anode materials were investigated based on CTAB amount. In the FE-SEM analysis, it was found that the HSiO₂ particle size increased as CTAB amount decreased, but shell thickness decreased. The HSi/C composites exhibited high initial discharge capacities of 1866.7, 2164.5 and 2188.6 mAh/g with various CTAB ratios (0.5, 1.0, 1.5), respectively. After 100 cycles of charge-discharge, 0.5-HSi/C demonstrated a high reversible capacity of 1171.3 mAh/g and a capacity retention of 70.9%. Electrochemical impedance spectroscopy (EIS) was employed to analyze the impedance characteristics, and it revealed that 0.5-HSi/C showed more stable resistance characteristics than HSi/C composites with other CTAB amount over 20 cycles.

• Journal of the Korean Electrochemical Society
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• v.27 no.3
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• pp.88-96
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• 2024

Lithium metal has garnered attention as a promising anode active material thanks to its high specific capacity, energy density, and the lowest reduction potential. However, the formation of dendrites, dendritic crystals that arise during the charge and discharge process, has posed safety and lifetime stability challenges. To resolve this, our study has introduced a novel separator design. This separator features a composite coating of vapor-grown carbon fiber, a conductive material in nanofibers, and silver. We have meticulously studied the impact of this innovative separator on the electrochemical properties of the lithium metal anode, unveiling promising results. To confirm the synergistic effect of VGCF and Ag, a separator with no surface treatment and a separator with only VGCF coated on one side were prepared and compared with the Ag-VGCF-separator. In the case of the bare separator, the Li metal surface is covered with dendrites during the initial charge and discharge process. In contrast, both the VGCF-separator and the Ag-VGCF-separator show Li precipitation inside the conductive coating layer coated on the separator surface. Additionally, the Ag-VGCF-separator showed a more uniform precipitate shape than the VGCF-separator. As a result, the Ag-VGCF-separators show improved electrochemical properties compared to the bare separators and the VGCF-separators.