Kyung Joong Yoon

Extremely Thin Bilayer Electrolyte for Solid Oxide Fuel Cells (SOFCs) Fabricated by Chemical Solution Deposition (CSD)

An extremely thin bilayer electrolyte consisting of yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) is successfully fabricated on a sintered NiO-YSZ substrate. Major processing flaws are effectively eliminated by applying local constraints to YSZ nanoparticles, and excellent open circuit voltage and cell performance are demonstrated in a solid oxide fuel cell (SOFC) at intermediate operating temperatures.

Electrochemical Performance of Solid Oxide Fuel Cells Manufactured by Single Step Co-firing Process

Anode-supported planar solid oxide fuel cells (SOFC) were fabricated by a single step co-firing process. The cells were composed of a Ni + yittria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, an industrial Ca-doped LaMnO 3 (LCM) (or lab-made LCM) + YSZ cathode active layer, and an industrial LCM (or lab-made LCM) cathode current collector layer. The fabrication processes involved tape casting of the anode, screen printing of the electrolyte and the cathode, and one step co-firing of the green-state cells at 1300°C for 2 h. The performance of the cells was greatly improved by optimization of these materials and fabrication processes. The electrochemical performance tests of these cells showed that they could provide a stable power density of 0.2-1.0 W/cm 2 with hydrogen as fuel and air as oxidant while operating in the temperature range 700-900°C. The effects of various polarization losses including ohmic polarization, activation polarization, and concentration polarization were studied by impedance spectroscopy measurements and curve-fitting experimentally measured voltage vs current density traces into an appropriate model. Based on these measurements and curve fitting results, the relationships between cell performance and various polarization losses and their dependence on temperature and microstructure, were rationalized.

Analysis of Electrochemical Performance of SOFCs Using Polarization Modeling and Impedance Measurements

Anode-supported planar solid oxide fuel cells (SOFCs) were fabricated by a single-step cofiring process. It comprised of a porous Ni + yttria-stabilized zirconia (YSZ) anode support, a porous and fine-grained Ni + YSZ anode active layer, a dense YSZ electrolyte, a fine-grained porous Ca-doped LaMnO 3 (LCM) + YSZ composite cathode active layer, and a porous LCM cathode current collector layer. The cell was tested between 700 and 800°C with humidified hydrogen (97% H 2 + 3% H 2 O) as the fuel and air as the oxidant. The measured maximum power densities were 1.42 W/cm 2 at 800°C, 1.20 W/cm 2 at 750°C, and 0.87 W/cm 2 at 700°C. The cell was also tested at 800°C with various compositions of fuel and oxidant, and the cell parameters, which included the area specific ohmic resistance, exchange current densities, and anodic and cathodic limiting current densities, were obtained through polarization modeling. The polarization resistances of the cell were calculated using the cell parameters obtained from modeling, and compared with the corresponding values measured using impedance spectroscopy. The effect of cell operating conditions on various polarization losses and performance was analyzed in detail using the polarization modeling results.

Enhanced oxygen diffusion in epitaxial lanthanum–strontium–cobaltite thin film cathodes for micro solid oxide fuel cells

The chemical diffusion coefficient (Dchem) of epitaxial LSC thin films was measured by a newly designed conductivity relaxation method. Dchem of epitaxial thin films was found to be higher than that of bulk LSC by up to two orders of magnitude, and was found to be enhanced further as the film became thinner.

Effect of Fuel Composition on Performance of Single-Step Cofired SOFCs

Anode-supported planar solid oxide fuel cells (SOFCs) were successfully fabricated employing a single-step cofiring process. The cells were comprised of a Ni + yttria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, a Ca-doped LaMnO 3 (LCM) + YSZ cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single-step cofiring of the green-state cell in the temperature range of 1300-1330°C for 2 h. The maximum power densities were 1.50 W/cm 2 at 800°C, 1.20 W/cm 2 at 750°C, and 0.87 W/cm 2 at 700°C, with humidified hydrogen (97% H 2 -3% H 2 O) as fuel and air as oxidant. The experimentally measured voltage-current density (V-i) curves were fitted into a polarization model to obtain the area specific ohmic resistance, exchange current density (anodic and cathodic), anodic limiting current density, cathodic limiting current density, and effective binary diffusivity of hydrogen and water vapor in the anode as well as that of oxygen and nitrogen in the cathode. The cell was also tested at 800°C with various compositions of humidified hydrogen to simulate the effect of practical fuel utilization on the performance of single cells. The V-i curves obtained in various fuel compositions were successfully modeled by fitting only the exchange current density. Anodic and cathodic activation polarizations and the exchange current densities at various fuel compositions were determined. An analytical model describing H 2 -H 2 O reaction at the anode triple-phase boundaries was postulated based on the relationship between the anodic exchange current density and the hydrogen partial pressure in the fuel. The model predicted that the formation of water molecules from adsorbed hydrogen and hydroxyl radical was the rate-determining step in the anodic reaction.

Effect of Anode Active Layer on Performance of Single-Step Cofired Solid Oxide Fuel Cells

Anode-supported planar solid oxide fuel cells (SOFCs) with and without the anode active layer were fabricated by a single-step cofiring process. The cells were comprised of a porous Ni + yittria-stabilized zirconia (YSZ) anode support, a porous fine-grained Ni + YSZ anode active layer for some experiments, a dense YSZ electrolyte, a porous fine-grained Ca-doped LaMnO 3 (LCM) + YSZ cathode active layer, and a porous LCM cathode current collector layer. The fabrication process involved tape casting of the anode support followed by screen printing of the remaining component layers. The cells were then cofired at 1300°C for 2 h. Sintered cells were electrochemically tested between 700 and 800°C with air as oxidant and various compositions of humidified hydrogen as fuel to simulate the effect of fuel utilization on cell performance. Experimentally measured current density-voltage (I-V) characteristics of the cells were fitted into a polarization model, and the cell parameters, including the area-specific ohmic resistance, exchange current density, anodic limiting current density, cathodic limiting current density, effective binary diffusivity of hydrogen and water vapor in the anode, and that of oxygen and nitrogen in the cathode, were obtained. Evaluation of the electrochemical performance and the polarization modeling results indicated that the cell performance is dominated by the cathodic activation polarization at low fuel utilization condition. However, the cell performance loss due to the anodic activation polarization increases as the fuel utilization increases. The anode active layer significantly improves the cell performance at high fuel utilization by lowering the anodic activation polarization under H 2 O-rich fuel because the anode active layer has finer and less porous microstructures and increases the number of the reaction sites near the anode-electrolyte interface. Cell performance at high fuel utilization and the effect of the anode active layer on the cell performance were discussed in detail.

Hydrogen Production Using Solid Oxide Membrane Electrolyzer with Solid Carbon Reductant in Liquid Metal Anode

A laboratory-scale solid oxide membrane (SOM) steam electrolyzer that can potentially utilize the energy value of coal or any hydrocarbon reductant to produce high purity hydrogen has been fabricated and evaluated. The SOM electrolyzer consists of an oxygen-ion-conducting yttria-stabilized zirconia (YSZ) electrolyte with a Ni-YSZ cermet cathode coated on one side and liquid tin anode on the other side. Hydrogen production using the SOM electrolyzer was successfully demonstrated between 900 and 1000{sup o}C by feeding a steam-rich gas to the Ni-YSZ cermet cathode and solid carbon reductant into the liquid tin anode. It was confirmed that the energy required for hydrogen production can be effectively lowered by feeding a solid carbon reductant in the liquid tin anode. A polarization model for the SOM electrolyzer was developed. The experimental data obtained under different operating conditions were curve fitted into the model to identify the various polarization losses. Based on the results of this study, work needed toward increasing the electrochemical performance of the SOM electrolyzer is discussed.

High-performance thin-film protonic ceramic fuel cells fabricated on anode supports with a non-proton-conducting ceramic matrix

A novel strategy to fabricate high-performance thin-film protonic ceramic fuel cells (PCFCs) is introduced by building thin-film PCFC components, including BaCe0.55Zr0.3Y0.15O3−δ (BCZY) electrolytes (1.5 μm) over anode supports consisting of non-proton-conducting ceramic and metal catalytic phases. Ni–yttria-stabilized zirconia (YSZ) was used as supports in this study, which is superior in terms of its well-established facile fabrication process, along with physical and chemical stability, compared to proton-conducting materials. The Ni–YSZ supports provided a flat and smooth deposition surface that facilitates the deposition of the thin film components. A Ni–BCZY anode (∼3 μm), a dense BCZY electrolyte layer (∼1.5 μm), and a porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathode (∼2 μm) were sequentially fabricated over the Ni–YSZ substrates using pulsed laser deposition, followed by post-annealing, and the process was optimized for each component. A fully integrated thin-film PCFC microstructure was confirmed, resulting in high open circuit voltages exceeding 1 V at operating temperatures in the range of 450–650 °C. A promising fuel cell performance was obtained using the proposed fuel cell configuration, reaching a peak power density of 742 mW cm−2 at 650 °C.

Improvement of Sintering, Thermal Behavior, and Electrical Properties of Calcium- and Transition Metal-Doped Yttrium Chromite

The A-site calcium doped yttrium chromite was additionally doped with various transition metals on the B-site to improve the sintering, thermal behavior and electrical properties of these ceramics for future use as an interconnect material in high temperature solid oxide fuel cells (SOFC). With 10 % addition of Co, Cu, Ni, Fe, and Mn, the single phase orthorhombic perovskite structure remained stable over a wide range of oxygen partial pressures, as confirmed by X-ray diffraction. The substitution of Cu for chromium remarkably improved the sinterability and allowed full densification in air by sintering at 1400 degrees C. The substitution of Co and Ni significantly improved the electrical conductivity of yttrium chromites in both oxidizing and reducing environments. This was explained by the increase of charge carrier density with nickel and cobalt doping, as confirmed by Seebeck measurements. With 10% of nickel dopant, the electrical conductivity of Y0.8Ca0.2CrO3±δ increased from 12 to 38 S/cm in air and from 2 to 15 S/cm in reducing atmosphere at 950 degrees C. Mn doping had a negative effect on the sintering and electrical conductivity.

Catalytic behavior of metal catalysts in high-temperature RWGS reaction: In-situ FT-IR experiments and first-principles calculations

High-temperature chemical reactions are ubiquitous in (electro) chemical applications designed to meet the growing demands of environmental and energy protection. However, the fundamental understanding and optimization of such reactions are great challenges because they are hampered by the spontaneous, dynamic, and high-temperature conditions. Here, we investigated the roles of metal catalysts (Pd, Ni, Cu, and Ag) in the high-temperature reverse water-gas shift (RWGS) reaction using in-situ surface analyses and density functional theory (DFT) calculations. Catalysts were prepared by the deposition-precipitation method with urea hydrolysis and freeze-drying. Most metals show a maximum catalytic activity during the RWGS reaction (reaching the thermodynamic conversion limit) with formate groups as an intermediate adsorbed species, while Ag metal has limited activity with the carbonate species on its surface. According to DFT calculations, such carbonate groups result from the suppressed dissociation and adsorption of hydrogen on the Ag surface, which is in good agreement with the experimental RWGS results.