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Choi, Mingi,Yong, Kijung The Royal Society of Chemistry 2014 Nanoscale Vol.6 No.22
<P>Vertically aligned high-quality single crystalline brookite TiO2 nanoarrays were synthesized for the first time using an environmentally benign one-step hydrothermal reaction. They have a unique bullet-shaped structure which has a length of 700-1000 nm and a width of 150-250 nm with a sharpened tip structure. By adjusting the concentration of NaOH in hydrothermal reaction, we could also synthesize other types of TiO2 nanostructures including anatase TiO2 nanotubes/nanowires. The morphologies and crystal structures of the products were confirmed by scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis. Their vertically aligned structures facilitate their application as photoanodes in photoelectrochemical cells, and the photoelectrochemical properties such as photocurrent density and open circuit voltage were measured in a three-electrode electrochemical cell with TiO2 nanoarrays, Ag/AgCl and a Pt flag as the working, reference and counter electrodes, respectively, incorporating a 0.1 M NaOH electrolyte solution. The fabricated brookite TiO2 nanoarrays exhibited a highly enhanced photocurrent density and a longer electron lifetime compared with anatase TiO2 nanoarrays with similar lengths.</P>
Enhanced charge transfer with Ag grids at electrolyte/electrode interfaces in solid oxide fuel cells
Choi, Mingi,Hwang, Sangyeon,Byun, Doyoung,Lee, Wonyoung The Royal Society of Chemistry 2016 Journal of Materials Chemistry A Vol.4 No.12
<▼1><P>This paper demonstrated the effect of Ag grids at the electrolyte/electrode interfaces on the electrochemical performance of solid oxide fuel cells (SOFCs).</P></▼1><▼2><P>We systematically investigated the effect of Ag grids at the electrolyte/electrode interfaces on the electrochemical performance of solid oxide fuel cells (SOFCs). Electrohydrodynamic jet printing was employed to fabricate Ag grids with a precise control of the geometry. A substantial reduction in the polarization resistance was observed when the pitch of the Ag grids decreased from 400 to 50 μm, indicating enhanced charge transfer through efficient supply and distribution of electrons along the Ag grids at the electrolyte/electrode interfaces. Our results demonstrate the possibility of engineering interfaces with metallic grids to achieve enhanced electrochemical performances for SOFCs operating at intermediate temperatures.</P></▼2>
Nano-film coated cathode functional layers towards high performance solid oxide fuel cells
Choi, Mingi,Lee, Jongseo,Lee, Wonyoung The Royal Society of Chemistry 2018 Journal of Materials Chemistry A Vol.6 No.25
<P>Nano-structured composite electrodes, from a carefully conducted infiltration process, are one of the most promising electrode structures for intermediate temperature solid oxide fuel cells (IT-SOFCs), due to their ability to promote the oxygen reduction reaction (ORR) and enlarge triple phase boundaries (TPBs). Here, we demonstrate rational design of infiltrated nano-structures for IT-SOFCs with enhanced ORR kinetics and excellent stability. We systematically designed a nano-structured cathode functional layer (CFL) from the infiltration of Sm0.5Sr0.5CoO3−δ (SSC) into porous Gd0.2Ce0.8O2−δ (GDC). By controlling the drying process during the infiltration process, two distinct nano-structures of SSC, discrete coating and film-like coating, were fabricated on the GDC scaffolds. The CFL with the film-like coating showed ∼30% reduction in polarization resistance (<I>R</I>p) and ∼15% increase in peak power density at 650 °C compared to the CFL with the discrete coating in spite of ∼12-fold lower loading of infiltration materials. Furthermore, the CFL with the film-like coating showed an excellent stability, maintaining an <I>R</I>p of 0.029 Ω cm<SUP>2</SUP> for 100 h at 650 °C. Our results demonstrated that the high performance and stability of composite electrodes for IT-SOFCs can be achieved through a thin-film coated CFL.</P>
Choi, Mingi,Hwang, Sangyeon,Byun, Doyoung,Lee, Wonyoung The Korean Ceramic Society 2015 한국세라믹학회지 Vol.52 No.5
The specific role of current collectors was investigated at the electrolyte/electrode interface of solid oxide fuel cells (SOFCs). Ag grids were fabricated as current collectors using electrohydrodynamic (EHD) jet printing for precise control of the grid geometry. The Ag grids reduced both the ohmic and polarization resistances as the pitch of the Ag grids decreased from $400{\mu}m$ to $100{\mu}m$. The effective electron distribution along the Ag grids improved the charge transport and transfer at the interface, extending the active reaction sites. Our results demonstrate the applicability of EHD jet printing to the fabrication of efficient current collectors for performance enhancement of SOFCs.
Choi, Mingi,Lim, Jonghun,Baek, Minki,Choi, Wonyong,Kim, Wooyul,Yong, Kijung American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.19
<P>Among three polymorphs of TiO2, the brookite is the least known phase in many aspects of its properties and photoactivities (especially comparable to anatase and rutile) because it is the rarest phase to be synthesized in the standard environment among the TiO2 polymorphs. In this study, we address the unrevealed photocatalytic properties of pure brookite TiO2 film as an environmental photocatalyst. Highly crystalline brookite nanostructures were synthesized on titanium foil using a well-designed hydrothermal reaction, without harmful precursors and selective etching of anatase, to afford pure brookite. The photocatalytic degradation of rhodamine B, tetramethylammonium chloride, and 4-chlorophenol on UV-illuminated pure brookite were investigated and compared with those on anatase and ruffle TiO2. The present research explores the generation of OH radicals as main oxidants on brookite. In addition, tetramethylammonium, as a mobile OH radical indicator, was degraded over both pure anatase and brookite phases, but not rutile. The brookite phase showed much higher photoactivity among TiO2 polymorphs, despite its smaller surface area compared with anatase. This result can be ascribed to the following properties of the brookite TiO2 film: (i) the higher driving force with more negative flat-band potential, (ii) the efficient charge transfer kinetics with low resistance, and (iii) the generation of more hydroxyl radicals, including mobile OH radicals. The brookite-nanostructured TiO2 electrode facilitates photocatalyst collection and recycling with excellent stability, and readily controls photocatalytic degradation rates with facile input of additional potential.</P>
분무응용 기술 : Gaseous sphere 분사모델을 이용한 CNG 직분사 엔진 모델링
최민기 ( Mingi Choi ),송진근 ( Jingeun Song ),박성욱 ( Sungwook Park ) 한국액체미립화학회 2015 한국액체미립화학회 학술강연회 논문집 Vol.2015 No.-
This paper describes the modeling of CNG (compressed natural gas) direct injection engine using gaseous sphere injection model. Numerical modeling was conducted using KIVA-3V Release 2 code with some modifications. Three dimensional mesh included 4 valves was used for computational grid. Gaseous sphere injection model could be integrated in KIVA-3V Release 2 code with some modification of liquid injection model and RNG (re-normalization group) k-ε turbulence model. This model could simulate gaseous sphere injection using coarse mesh which saves calculation time. The fine mesh is not required to resolve the inflow boundary for gaseous sphere injection. Likewise with liquid injection model, gaseous spheres are injected as parcels which represent a group of gaseous spheres and these parcels evaporate at a time. The evaporation of gaseous spheres occurs without energy change. Particularly, poppet type injector was used for CNG direct injection therefore hollow cone type injection model was modified. The RNG k-ε turbulence model need some modifications. Since this model is known to over-predict gas jet diffusion, turbulence kinetic energy and turbulence length scale values were adjusted depend on grid location. The modified RNG k-ε turbulence model is applied only for the injection period. After the fuel injection was finished, the conventional RNG k-ε turbulence model is applied. Chemkin chemistry solver 2 was coupled with KIVA-3V Release 2 code to simulate combustion process of CNG fuel. CNG is represented by methane and GRI 3.0 mechanism which optimized for combustion process of natural gas was used. In order to calculate the turbulent flame speed, G-equation model was used in which flame front is specified by zero level set of G. In addition, experiments of gaseous fuel injection was performed for gas-jet visualizations using PLIF (planar laser induced fluorescence) method. For safety reasons, compressed nitrogen was used instead of compressed natural gas in the experiments. The tracer which plays an important role in PLIF experiments was acetone that has a very low boiling point, a high saturation pressure, a good fluorescence and low toxicity. Furthermore, experiments of CNG combustion was performed using single cylinder SI (spark ignition) engine. For CNG direct injection, poppet type gaseous fuel injector was used and this injector was centrally mounted. In this study, the simulation results of CNG direct injection engine were compared to experimental results. The gaseous sphere injection model can reliably predict CNG direct injection. Furthermore, the results of ignition and combustion process agreed well with experiment results.