Hai-Soo Chun

Effect of the catalytic ink preparation method on the performance of polymer electrolyte membrane fuel cells

The effect of the preparation method of the catalytic inks on the electrode structure and, thus, on the performance of polymer electrolyte membrane fuel cells (PEMFCs) was investigated. Since the catalytic inks, which are a mixture of Pt/C powders, solvent, and ionomers, change into one of the three states: (i) solution; (ii) colloids and (iii) precipitates, according to the interaction between the ionomers and solvents, two different catalytic inks have been prepared for comparison, the solution inks based on iso-propyl alcohol (IPA) and the colloidal inks based on normal butyl acetate. Performance evaluation, electrochemical analyses, and physical property examination revealed that the electrode prepared by a colloidal method showed better results compared to those of the solution method. The former appeared to secure continuity of the ionomer network and higher porosity in the catalytic layer, resulting in higher proton conductivity and less mass transfer resistance.

Electrochemical characterization of various tin-based oxides as negative electrodes for rechargeable lithium batteries

Abstract Tin oxide and tin-based composite electrodes are examined in both bulk and thin-film form for prospective use as negative electrodes for lithium rechargeable batteries. For bulk electrodes, tin oxides and Sn–Zn–P–O composite materials are compared by charge–discharge testings. Thin films of oxides and composite thin-film electrodes prepared by heat treatment (temperature and time) are characterized by X-ray diffraction analysis, Auger electron spectroscopy, and scanning electron microscopy. The characteristics of thin films are found to depend on the heat-treatment temperature, which influences the structure, the grain size, and adhesion to the substrate. Capacities higher than 350 mA h g −1 are found for bulk electrodes beyond 20 cycles, and beyond 100 cycles for thin-film electrodes.

Mass-transfer and hydraulic characteristics in spray and packed extraction columns for supercritical carbon dioxide-ethanol-water system

Abstract Mass-transfer efficiencies and hydraulic characteristics of a 3.18-cm spray and a packed column for extracting ethanol from aqueous ethanol solution with supercritical carbon dioxide were investigated. Experiments were performed at 308.2, 313.2, and 323.2 K over a pressure range from 9.1 to 12.2 MPa. The influences of fluid properties, phase flow rates, column internals, and phase dispersion on mass-transfer efficiencies and hydraulic characteristics are discussed. An extension of a model for predicting mass-transfer efficiency in conventional liquid-liquid extraction to supercritical-fluid extraction was attempted. The model for mass-transfer efficiencies, developed for conventional spray and packed liquid-liquid extraction columns, was in good agreement with our experimental results.

Reduction of Irreversibility in the First Charge of Tin Oxide Thin Film Negative Electrodes

Sputter deposited 3000 A tin oxide thin films into which metallic lithium (6000 A) was reacted were studied to develop a new negative electrode for thin film rechargeable lithium batteries. The crystal structure and chemical composition of the lithium reacted tin oxide films were characterized by X-ray diffraction (XRD) analysis and Auger electron spectroscopy, respectively. The charge/discharge performances of these films exhibited capacities >400 mAh/g for >75 cycles. There was no irreversible plateau near 0.8 V vs. Li/Li due to reduction of SnO 2 to Sn and Li 2 O during the first charge half-cycle. XRD and scanning transmission electron microscopy results suggest that the lithium-reacted tin oxide thin film consists of metallic tin, lithium oxide, and reduced tin oxide, and the reacted thin films show reduced microcracks created by density fluctuations.

Modeling on lithium insertion of porous carbon electrodes

MCMBs with different crystal structure were tested for an anode of lithium ion batteries (LIB) and the model describing the behavior of porous anodes was simulated numerically by using orthogonal collocation method (OCM). Kinetic parameters such as diffusion coefficients, exchange current densities, and transfer coefficient, describing electrochemical intercalation system of lithium ions, were estimated by fitting the experimental cyclic voltametry (CV) results with the theoretical ones. It was investigated that the theoretical cyclic voltamograms obtained using above parameters fitted well with the experimental curves for the various scan rates from 1 mV s−1 to 5 μV s−1. The parameters were then evaluated on their extended application in various C-rate-charge/discharge cycling tests with showing good agreements between experiments and simulations. As the results show, it was found that numerical simulations based on both potentiometry and galvanometry experimental data resulted in more accurate parameters of electrochemical system. Simulations indicate there exist the optimum design conditions of electrode and separator to obtain the good performance of lithium ion batteries.

Sintering characteristics of a porous Ni/Ni3Al anode for molten carbonate fuel cells

Abstract Studies of sintering behaviour and microstructural pore evolution during sintering of pure Ni particle — and Ni/(4–10 wt.%)Ni 3 Al intermetallics — green sheets for the molten carbonate fuel cell (MCFC) anode are conducted. The pore structure in the Ni/Ni 3 Al anode can be maintained as an open pore network by restraining the initial stage of sintering. Ni 3 Al intermetallics inserted on nickel grain boundaries act as a barrier, which inhibits nickel grain boundary diffusion and controls both densification and nickel grain growth. Pores in the pure nickel anode become smaller than those in the Ni/Ni 3 Al anode by an intermediate stage of sintering which results in densification and nickel grain growth.

Studies of bipolarity in fluidized bed electrodes

Measurements have been made of the anodic reaction products due to the anodic over-potential resulting from the bipolarity of particles in a nominally cathodic fluidized bed electrode reactor. Investigations of the lead particle-basic carbonate solution system show that charge transfer occurs by a bipolar as well as a monopolar mechanism in a nominally monopolar fluidized bed electrode. The bipolar intensity was found to increase with decreasing superficial current density and also with increasing bed voidage.

Electrostatic beneficiation of fly ash using an ejector‐tribocharger

Abstract Tests were conducted to investigate the triboelectrostatic process for separating unburned carbon from coal fly ash. Using a batch type separator composed of two parallel plates and an ejector‐tribocharger, the charge density and the separation efficiency were evaluated under various operating conditions. These included the air flow rate, the solids loading, the relative humidity and the temperature of the air, and the voltage between the two parallel electrodes. It was found that the relative humidity of the surrounding air was the most influencing factor of the charge density as well as the separation efficiency. The maximum charge density was obtained at the air flow rate of 326 l/min, and the air temperature of 74 °C when operated at solids loadings less than 290 g/m3 and at lower than 30% relative humidity. At the optimum conditions, a fly ash of 7.5% LOI could be processed into a 3.0% LOI product with a recovery over 80%.

A simulation of electrochemical kinetics for gas-liquid-solid phase of MCFC anode

A porous Ni-Al alloy anode for the molten carbonate fuel cell has been developed to enhance the creep resistance of the anode as well as to minimize the electrolyte loss A dual-porosity filmed agglomerate model for the Ni-Al alloy anode has been investigated to predict the cell performance The major physicochemical phenomena being modeled include mass transfer ohmic losses and reaction kinetics at the electrode-electrolyte interface The predicted polarization curves are compared with the experimental results obtained from a half cell test The model predicted very well the steady-state cell performance at the given conditions that characterize the state of the electrode

Pack aluminization of nickel anode for molten carbonate fuel cells

The aluminium pack cementation (pack aluminization) process on a porous nickel anode for molten carbonate fuel cells has been studied to improve anode creep resistance. The porous nickel substrates used in this study were fabricated by doctor blade equipment followed by sintering (850 °C). Packs surrounding the Ni anode were made by mixing Al2O3 powder, Al powder, and NaCl as activator. The pack aluminization was performed at 700 to 850 °C for 0.5–5.0 h. After pack aluminization, the principal NiAl intermetallic compounds detected were Ni3Al at 700 °C, NiAl at 750 °C and Ni3Al2 at 800 °C. The aluminum content in the aluminized Ni anode was proportional to the square root of pack aluminizing time. With increasing the Al content in the anode, the creep of the anode decreased. It was nearly constant (2.0%) when the Al content was above 5.0%. Although the exchange current density (24 mA/cm2) for the aluminized (2.5 wt.%) Ni anode was somewhat lower than that of the pure Ni anode (40 mA/cm2), the performance of a single cell using an aluminized Ni anode was similar to that of the one with pure Ni anode.