• Journal of Energy Engineering
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• v.23 no.3
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• pp.150-156
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• 2014

In this study, we implemented a research for improving performance of redox flow battery (RFB) via enhancing reaction rate of vanadium reaction ($[VO]^{2+}/[VO_2]^+$) that was a rate determining step. For doing that, porous catalyst, CMK3 was employed and its perfoamance was compared with that of Vulcan(XC-72) and commercial Pt/C (Johnson-Matthey Pt 20wt.%). Cyclic voltammetry (CV) was used for inspecting reactivity, while its structural feature was measured by TEM and BET&BJH. Also, Charge-discharge trend was evaluated by single cell tests. As result, CMK3 showed 6 times better catalytic activity and twice better reversibility than Vulcan(XC-72), while it showed larger surface area than Vulcan XR due to its porous structure. Furthermore, CMK3 indicated 85% of reactivity and reversibility of commercial Pt/C despite its Pt-less situation. In single cell tests, when RFB adopted CMK3 as catalyst for positive electrode, its charge-discharge curve result was better than that adopted commercial Pt/C.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.41 no.10
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• pp.788-795
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• 2013

A magnesium bipolar plate whose surface was protected by thinly deposited silver layer was investigated as an alternative to existing graphite bipolar plate of PEM fuel cells. Thin silver layer of $3{\mu}m$ was deposited on a magnesium alloy substrate by physical vapor deposition (PVD) method in an environment of $180^{\circ}C$. A number of tests were conducted on the fabricated magnesium based bipolar plates to determine their suitability for use in PEM fuel cell stacks. The test on corrosion resistance in the same pH condition as in a PEM operation demonstrated the layer protected the magnesium alloy substrate, while unprotected substrate suffered from severe corrosion. The contact resistance of the fabricated bipolar plate was less than $20m{\Omega}-cm^2$ which was superior to the conventional bipolar plates. A single cell was constructed using the fabricated bipolar plates and power output was measured. Due to the enhanced conductivity caused by low contact resistance, slight increase was observed in current density and output voltage. With low density of the magnesium substrate and ease on machining, the weight reduction of the stack of 30~40 % is possible to produce the same power output.

• Journal of the Korean Electrochemical Society
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• v.9 no.3
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• pp.124-131
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• 2006

For commercialization of molten carbonate fuel cell (MCFC), it has some problems to be overcome such as decrease of porosity and thickness of the anode under the operating condition (at $650^{\circ}C$ and working pressure of more than 2 $kg_f/cm^2$). Recently, Ni-Al alloy anode has been proposed to replace the conventional Ni-Cr anode as an alternative material to resist a creep and inhibit the sintering. The objective of this research is to sinter the green sheet of Ni-Al alloy anode during single cell pre-treatment process, which has several advantages like cost down and simplification of manufacturing process. However, the Ni-Al alloy anode prepared with a conventional pre-treatment process showed the phase separation of Ni-Al alloy and formation of micropore(${\leqq}0.4{\mu}m$), resulting in low creep resistance and high electrolyte re-distribution. In order to prevent the Ni-Al alloy anode from phase-separating, nitrogen gas was used in the process of pre-treatment. Introducing the nitrogen, the phase separation from Ni-Al alloy into nickel and alumina was minimized and increased creep resistance. However, there was some micropore formation on the surface of Ni-Al alloy anode during the cell operation due to creation of lithium aluminate. Addition of more amount of electrolyte into a cell, especially at cathode, made the cell performance stable for 2,000 hrs. Consequently, it was possible to make the Ni-Al alloy anode with good creep resistance by the modified in-situ sintering technique.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.152-152
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• 2016

We studied the effect of the silver grid size on graphene transparent conducting films for flexible organic solar cells (OSCs). The silver grid was used an assistant layer of the graphene to reduce the sheet resistance of substrates. Silver grid with various graphene sizes for optimizing transmittance and sheet resistance of substrates were fabricated on polyethylene terephthalate (PET) substrates to form the hybrid films. The optimized grid geometry on the single layer graphene (SLG) was the grid dimension $200{\mu}m{\times}200{\mu}m{\times}50nm{\times}2{\mu}m$ (length ${\times}$ width ${\times}$ height ${\times}$ linewidth), where the sheet resistance was $55.73{\Omega}/square$ with the average transmittance of ~ 92.83 % at 550 nm. The properties of the OSCs fabricated using SLG with optimized silver grids on PET substrates show a short circuit current of $10.9mA/cm^2$, an open circuit voltage of 0.58 V, a fill factor of 60.8 %, and a power conversion efficiency (PCE) of 3.9 %. The PCE was improved about 91% than that of the OSCs using the SLG without the silver grid. These results demonstrate that the optimized grid geometry to the based on the graphene transparent electrodes contribute to improving the performance of OSCs.

• Journal of the Institute of Electronics and Information Engineers
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• v.50 no.1
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• pp.124-130
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• 2013

In this paper, a new motion estimation algorithm with low-computational complexity is proposed to improve the performance of H.264/AVC. The proposed architecture uses the directions of the median motion vector which is computed by the motion vectors of the three neighbor macroblocks in Integer Motion Estimation. By using the directions of the vector, the proposed architecture has a single computational level instead of multi-computational levels in Integer Motion Estimation. The proposed motion estimation is synthesized using the TSMC 0.18um standard cell library. The synthesis result shows that the gate count is about 217.92K at 166MHz and it was improved about 69% compared with previous one.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.5
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• pp.1-9
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• 2003

Detailed flow of a shrouded tail rotor in hover is studied by using a compressible inviscid flow solver on unstructured meshes. The numerical method is based on a cell-centered finite-volume discretization and an implicit Gauss-Seidel time integration. Numerical simulation is made for a single blade attached to the center body and guide by the duct by imposing a periodic boundary condition between adjacent rotor blades. The results show that the performance of an isolated rotor without shroud compares well with experiment. In case of a shrouded rotor, correction of the collective pitch angle is made such that the overall performance matches with experiment to account for the uncertainties of the experimental model configuration. Details of the flow field compare well with the experiment confirming the validity of the present method.

• Transactions of the Korean hydrogen and new energy society
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• v.22 no.1
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• pp.60-68
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• 2011

[ $ZrO_2-TiO_2$ ]binary oxides with various Zr:Ti molar ratios were prepared by sol-gel method and Nafion$^{(R)}$/$ZrO_2-TiO_2$ composite membranes were fabricated for proton exchange membrane fuel cells (PEMFCs) at high temperature and low humidity. Water uptake, Ion exchange capacity (IEC), and proton conductivity of Nafion$^{(R)}$/$ZrO_2-TiO_2$ composite membranes were characterized and these composite membranes were tested in a single cell at $120^{\circ}C$ with various relative humidity (R.H.) conditions. The obtained results were compared with the unmodified membranes (Nafion$^{(R)}$ 112 and Recast Nafion$^{(R)}$). A Nafion$^{(R)}$/$ZrO_2-TiO_2$ composite membrane with 1:3 of Zr:Ti molar ratio showed the highest performance. The performance showed 500 mW/$cm^2$ (0.499V) at $120^{\circ}C$, 50% R. H., and 2 atm.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.23 no.6
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• pp.1514-1525
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• 1998

Two main characteristics of VBR MPEG video traffic are different statistics according to different picture types and the periodic traffic pattern due to GOP structure. Especially, the I-pictures at the beginning of each GOP generate $$\mu$h more traffic than other pictures. When several VBR Mpeg video sources are superposed, the I-picture starting times of these sources may significantly affect the cell loss characteristics of ATM $$\mu$tiplexers. In this paper, we propse a performance model for ATM $$\mu$tiplexers with VBR MPEG video sources whose starting times are arbitrary given. For analysis, both single and superposed source traffic are modeled as NDPPs (non-deterministic periodic processes), and the ATM $$\mu$tiplexer is modeled as a U-state NDPPD/D/1/B queueing system. It is hown that the numerical results are very close to the si$$\mu$ation results. From the relationships between the starting tiem distributions and the corresponding $$\mu$tiplexer performances, some considerations for designing a scheduling policy in order to obtain the maxi$$\mu$ $$\mu$tiplexing gain are presented.

• Transactions of the Korean hydrogen and new energy society
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• v.25 no.1
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• pp.28-38
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• 2014

The organic-inorganic composite membrane in polymer exchange membrane fuel cells (PEMFCs) have several fascinating technological advantages such as a proton conductivity, thermal stability and mechanical properties. As the inorganic filler, silicon carbide (SiC) fiber have been used in various fields due to its unique properties such as thermal stability, conductivity, and tensile strength. In this study, composite membrane was successfully fabricated by modified-silicon carbide fiber. Modified process, as a novel process in SiC, takes reaction by phosphoric acid after oxidation process (generated homogeniusly $SiO_2$ layer on SiC fiber). The mechanical property which was conducted by tensile test of the 5wt% modified-$SiO_2@SiCf$ composite membrane was better than that of Aquivion casting membrane as well as ion cxchange capacity(IEC) and proton conductivity. In addition, the single cell performance was observed that the 5wt% modified-$SiO_2@SiCf$ composite membrane was approximately $0.2A/cm^2$ higher than that of a Aquivion casting electrolyte membrane and electrochemical impedance was improved with the charge transfer resistance and membrane resistance.

• KIEE International Transaction on Electrical Machinery and Energy Conversion Systems
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• v.3B no.3
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• pp.147-154
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• 2003

This paper presents a high-frequency boost type ZVS-PWM chopper-fed DC-DC power converter with a single active auxiliary edge-resonant snubber at the load stage which can be designed for power conditioners such as solar photovoltaic generation, fuel cell generation, battery and super capacitor energy storages. Its principle operation in steady-state is described in addition to a prototype setup. The experimental results of boost type ZVS-PWM chopper proposed here, are evaluated and verified with a practical design model in terms of its switching voltage and current waveforms, the switching v-i trajectory and the temperature performance of IGBT module, the actual power conversion efficiency, and the EMI of radiated and conducted emissions, and then discussed and compared with the hard switching scheme from an experimental point of view. Finally, this paper proposes a practical method to suppress parasitic oscillation due to the active auxiliary resonant switch at ZCS turn-off mode transition with the aid of an additional lossless clamping diode loop, and can be reduced the EMI conducted emission.