• Journal of the Korean Electrochemical Society
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• v.15 no.2
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• pp.95-100
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• 2012

$LiFePO_4$ is an attractive cathode material due to its low cost, good cyclability and safety. But it has low ionic conductivity and working voltage impose a limitation on its application for commercial products. In order to solve these problems, the iron($Fe^{2+}$)site in $LiFePO_4$ can be substituted with other transition metal ions such as $Mn^{2+}$ in pursuance of increase the working voltage. Also, reducing the size of electrode materials to nanometer scale can improve the power density because of a larger electrode-electrolyte contact area and shorter diffusion lengths for Li ions in crystals. Therefore, we chose electrospinning as a general method to prepare $Li[Fe_{0.9}Mn_{0.1}]PO_4$ to increase the surface area. Also, there have been very a few reports on the synthesis of cathode materials by electrospinning method for Lithium ion batteries. The morphology and nanostructure of the obtained $Li[Fe_{0.9}Mn_{0.1}]PO_4$ nanofibers were characterized using scanning electron microscopy(SEM). X-ray diffraction(XRD) measurements were also carried out in order to determine the structure of $Li[Fe_{0.9}Mn_{0.1}]PO_4$ nanofibers. Electrochemical properties of $Li[Fe_{0.9}Mn_{0.1}]PO_4$ were investigated with charge/discharge measurements, electrochemical impedance spectroscopy measurements(EIS).

• Korean Journal of Ecology and Environment
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• v.40 no.1
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• pp.110-120
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• 2007

The objective of this study was to analyze temporal trends of water chemistry and spatial heterogeneity for 13 sampling sites of the Keum River watershed using water quality dataset (obtained from the Ministry of Environment, Korea) during $2001{\sim}2005$. The water quality, based on eight physical and chemical parameters, varied largely depending on the years, seasons, and sampling sites. Seasonal and annual means of conductivity, used as a key indicator for a ionic dilution declined during the monsoon season, and nutrients (TN and TP), based on overall mean of all sites, showed marked declines during the monsoon, compared to those of the premonsoon. In the mean time, BOD and COD had no significant relations with a precipitation, in spite of some differences in the sampling sites. In contrast, major input of SS occurred during the period of summer monsoon. and the variation of TN was similar to that of TP. Spatial trend analyses of all parameters, except for DO and temperature, showed that Site 9 acted as a point source, and thus, water quality at the locations of $S9{\sim}S13$ declined abruptedly over 2 fold, compared to locations of $S1{\sim}S8$. Based on the overall dataset, efficient water quality management in the point source tributary streams is required for better water quality of the main Keum River.

• Journal of Ecology and Environment
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• v.40 no.1
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• pp.24-33
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• 2016

Background: Previous numerous studies on watershed scale demonstrated that the constructions of upper dams may influence the below dams due to modifications of flow regime and nutrient inputs. Little is known about how the dam constructions influence the downstream lakes or reservoirs in the regional scale. This study demonstrates how the construction of upper dam (i.e., Yongdam Dam) influences nutrient regime, trophic relations, and empirical models in Daechung Reservoir (DR). Yongdam Dam was constructed at the upstream region of DR in year 2000. Results: The analysis of hydrological variables showed that inflow and discharge in the DR were largely reduced after the year 2000. The construction of upper dam construction also resulted in increases of water temperature, pH and conductivity (as an indicator of ionic content) in the DR. Empirical models of TP-CHL and N:P ratio-CHL suggested that stronger responses of CHL to the phosphorus were evident after the upper dam construction, indicating that algal production at a unit phosphorus increased after the upper dam construction. Mann-Kendall tests on the relations of N:P ratios to TN showed weak or no relations ($t_{au}=-0.143$, z = -0.371, p = 0.7105) before the dam construction, while the relation of N:P ratios to TP showed strong in the periods of before- ($t_{au}=-0714$, z = -2.351, p = 0.0187) and after the construction ($t_{au}=-0.868$, z = -4.270, p = 0.0000). This outcome indicates that TP is key determinant on N:P ratios in the reservoir. Scatter Plots on Trophic State Index Deviations (TSIDs) of "TSI(SD) - TSI(CHL)" against "TSI(TP) - TSI(CHL)" showed that the dominance of clay turbidity or light limitation was evident before the upper dam construction [TSI(TP) - TSI(CHL) > 0 and TSI(SD) - TSI(CHL) > 0] and phosphorus limitation became stronger after the dam construction [(TSI(TP) - TSI(CHL) < 0 and TSI(SD) - TSI(CHL) > 0]. Conclusions: Overall, our analysis suggests that the upper dam construction modified the response of trophic components (phytoplankton) to the nutrients or nutrient ratios through the alteration of flow regime, resulting in modifications of ecological functions and trophic relations in the low trophic levels.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.25 no.3
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• pp.116-120
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• 2015

$La_2NiO_{4+{\delta}}$ based oxides, a mixed electronic-ionic conductors (MIECs) with $K_2NiF_4$ type structure, have been considerably investigated in recent decades as electrode materials for advanced solid oxide fuel cells (SOFCs) due to their high electrical conductivity, and oxidation reduction reaction (ORR). In this study, structure properties of $La(Ca)_2Ni(Cu)O_{4+{\delta}}$ were studied as a potential cathode for intermediate temperature SOFCs (IT-SOFCs).

• Journal of the Korean Ceramic Society
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• v.56 no.2
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• pp.111-129
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• 2019

Recently, all-solid-state batteries (ASSBs) have attracted increasing interest owing to their higher energy density and safety. As the core material of ASSBs, the characteristics of the solid electrolyte largely determine the performance of the battery. Thus far, a variety of inorganic solid electrolytes have been studied, including the NASICON-type, LISICON-type, perovskite-type, garnet-type, glassy solid electrolyte, and so on. The garnet Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte is one of the most promising candidates because of its excellent comprehensively electrochemical performance. Both, experiments and theoretical calculations, show that cubic LLZO has high room-temperature ionic conductivity and good chemical stability while contacting with the lithium anode and most of the cathode materials. In this paper, the crystal structure, Li-ion transport mechanism, preparation method, and element doping of LLZO are introduced in detail based on the research progress in recent years. Then, the development prospects and challenges of LLZO as applied to ASSBs are discussed.

• Journal of the Korean Electrochemical Society
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• v.9 no.1
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• pp.34-39
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• 2006

Electrochemical properties of Cu foil current collector with a $Li_{0.5}La_{0.5}TiO_3$ Cu a Si thin film deposited by r.f sputtering as an anode for Li free battery were evaluated. The Cu foil current collectors were lied in and out of plasma during sputtering process. The X-ray diffraction results indicated that the as-deposited Si and $Li_{0.5}La_{0.5}TiO_3$ thin films in and out of plasma did not show any crystalline difference. The $Li_{0.5}La_{0.5}TiO_3$ film in plasma and Si film out of plasma showed better cyclability since crystalline $Li_{0.5}La_{0.5}TiO_3$ has much higher ionic conductivity and crystalline Si film is much sensitive far volume change during charge-discharge process. These results suggested that the deposition of amorphous Si on Cu foil current collector is much better for fabrication of Li free battery and it can be useful for the unique battery with a cycling number constraint of below 10.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.11a
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• pp.11-11
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• 2008

ZnO has a very large exciton binding energy (60 meV) as well as thermal and chemical stability, which are expected to allow efficient excitonic emission, even at room temperature. ZnO based electronic devices have attracted increasing interest as the backplanes for applications in the next-generation displays, such as active-matrix liquid crystal displays (AMLCDs) and active-matrix organic light emitting diodes (AMOLEDs), and in solid state lighting systems as a substitution for GaN based light emitting diodes (LEDs). Most of these electronic devices employ the electrical behavior of n-type semiconducting active oxides due to the difficulty in obtaining a p-type film with long-term stability and high performance. p-type ZnO films can be produced by substituting group V elements (N, P, and As) for the O sites or group I elements (Li, Na, and K) for Zn sites. However, the achievement of p-type ZnO is a difficult task due to self-compensation induced from intrinsic donor defects, such as O vacancies (Vo) and Zn interstitials ($Zn_i$), or an unintentional extrinsic donor such as H. Phosphorus (P) doped ZnO thin films were grown on c-sapphire substrates by radio frequency magnetron sputtering with various Ar/ $O_2$ gas ratios. Control of the electrical types in the P-doped ZnO films was achieved by varying the gas ratio with out post-annealing. The P-doped ZnO films grown at a Ar/ $O_2$ ratio of 3/1 showed p-type conductivity with a hole concentration and hole mobility of $10^{-17}cm^{-3}$ and $2.5cm^2/V{\cdot}s$, respectively. X-ray diffraction showed that the ZnO (0002) peak shifted to lower angle due to the positioning of $p^{3-}$ ions with a smaller ionic radius in the $O^{2-}$ sites. This indicates that a p-type mechanism was due to the substitutional Po. The low-temperature photoluminescence of the p-type ZnO films showed p-type related neutral acceptor-bound exciton emission. The p-ZnO/n-Si heterojunction LEO showed typical rectification behavior, which confirmed the p-type characteristics of the ZnO films in the as-deposited status, despite the deep-level related electroluminescence emission.

• Journal of the Korean Electrochemical Society
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• v.19 no.1
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• pp.21-27
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• 2016

Vanadium redox flow batteries (VRFBs) using the electrolytes containing various vanadium ions in sulfuric acid as supporting solution are one of the energy storage devices in alternatively charging and discharging operation modes. The positive electrolyte contains $V^{5+}/V^{4+}$ and the negative electrolyte $V^{2+}/V^{3+}$ depending on the operation mode. To prevent the mixing of two solutions, proton exchange membranes are mainly used in VRFBs. Nafion 117 could be the most promising candidate due to the strong oxidative property of $V^{5+}$ ion, but causes high crossover of electroactive species to result in a decrease in coulombic efficiency. In this study, the composite membranes using Nafion ionomer and porous polyethylene substrate were prepared to keep good chemical stability and to decrease the cost of membranes, and were compared to the properties and performance of the commercially available electrolyte membrane, Nafion 117. As a result, the water uptake and ionic conductivity of the composite membranes increased as the thickness of the composite membranes increased, but those of Nafion 117 slightly decreased. The permeability of vanadium ions for the composite membranes significantly decreased compared to that for Nafion 117. In a single cell test for the composite membranes, the voltage efficiency decreased and the coulombic efficiency increased, finally resulting in the similar energy efficiency. In conclusion, the less cost of the composite membranes by decreasing 6.4 wt.% of the amount of perfluorinated sulfonic acid polymer due to the introduction of porous substrate and lower vanadium ion permeability to decrease self-discharge were achieved than Nafion 117.

• Journal of the Korean Electrochemical Society
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• v.10 no.3
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• pp.207-212
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• 2007

[ $PSCF3737(Pr_{0.3}Sr_{0.7}Co_{0.3}Fe_{0.7}O_3)$ ] is a good candidate cathode material for IT-SOFC(intermediate temperature solid oxide fuel cell) because of high MIEC(mixed ionic electronic conductor) conductivity. In this study, the characteristics of PSCF3737 was investigated and optimizations of sintering temperature and thickness for $PSCF3737(Pr_{0.3}Sr_{0.7}Co_{0.3}Fe_{0.7}O_3)$ was carried out. Impedance responses were divided into two parts by frequency region. Middle frequency part (${\sim}10^2\;Hz$) was concerned with oxygen reduction reaction on surface and low frequency part (${\sim}10^{-1}\;Hz$) was related with oxygen diffusion. The reasonable sintering temperature and thickness of cathode were $1200^{\circ}C$ and about $27\;{\mu}m$ with regard to EIS(electrochemical impedance spectroscopy). ASR(areas specific resistance) of optimized cathode is $0.115\;{\Omega}\;cm^2$ at $700^{\circ}C$.

• Journal of the Korean Electrochemical Society
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• v.15 no.1
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• pp.48-53
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• 2012

Polyethylene (PE) separator is the most popular separator for lithium-ion batteries. However, it suffers from thermal contraction and mechanical rupture. In order to improve the thermal/mechanical dimensional stabilities, this study investigated the effects of $Si_3N_4$ coating. SCS (Silicon-nitride Coated Separator) has been fabricated by applying 10 ${\mu}m$-thick $Si_3N_4$/PVdF coating on one side of PE separator. SCS exhibits enhanced thermal stability over $100{\sim}150^{\circ}C$: its thermal shrinkage is reduced by 10~20% compared with pristine PE separator. In addition, SCS shows higher tensile strength than PE separator. Employing SCS hardly affects the C-rate performance of $LiCoO_2$/Li coin-cell, even though its ionic conductivity is somewhat lower than that of PE separator.