Joongmyeon Bae

Advanced Electrochemical Properties of LnBa0.5Sr0.5Co2O5 + δ (Ln = Pr , Sm, and Gd) as Cathode Materials for IT-SOFC

Excellent area-specific-resistance (ASR) values have been exhibited by cathode materials with a Sr-doped layer perovskite type structure and therefore show themselves to be possible candidates for intermediate-temperature-operating solid oxide fuel cell (IT-SOFC, 600-800°C) applications. SmBa 0.5 Sr 0.5 Co 2 O 5+δ (SBSCO) electrode was sintered onto 10 mol % gadolinia-doped ceria (Ce 0.9 Gd 0.1 O 2 , CG091) at 1000°C to form symmetrical cells and exhibited an ASR value of 0.092 Ω cm 2 at 700°C. The lowest ASR value was observed when the composite cathode of 50 wt % of SBSCO and 50 wt % of CG091 (SBSC050) was used in conjunction with an interlayer of CGO91 applied between the electrode and 8 mol % Y 2 O 3 stabilized ZrO 2 electrolyte. These were 0.12 Ω cm 2 at 600°C and 0.019 Ω cm 2 at 700°C, respectively. The coefficient of thermal expansion (CTE) of SBSCO was 21.9 × 10 -6 K -1 at 700°C. However, the CTE of the composite cathode of SBSC050 was shown to be 13.6 × 10 -6 K -1 at 700°C, this being more compatable with the other components within the cell.

Chromium deposition and poisoning at Ba0.5Sr0.5Co0.8Fe0.2O3 cathode of solid oxide fuel cells

Chromium deposition and poisoning at Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) cathodes of solid oxide fuel cells are investigated. BSCF cathode shows an excellent and stable performance in the absence of metallic interconnects. However, in the presence of a chromia-forming metallic interconnect performance of the BSCF cathodes degrades quickly. Chromium deposition was observed on the BSCF cathode surface after 1200 min polarization at 900 and 800C. The Cr2O3/SrO and Cr2O3/BaCO3 oxide couple study indicates that SrCrO4, BaCrO4, and BaCr2O4 start to form at temperatures above 500C. The results indicate that BSCF cathode is not chromium-tolerant despite its superior activity for the O2 reduction reaction.

Numerical Study on Operating Parameters and Shapes of a Steam Reformer for Hydrogen Production from Methane

Abstract The steam reformer for hydrogen production from methane is studied by a numerical method. Langmuir-Hinshelwood model is incorporated for catalytic surface reactions, and the pseudo-homogeneous model is used to take into account local equilibrium phenomena between a catalyst and bulk gas. Dominant chemical reactions are Steam Reforming (SR) reaction, Water-Gas Shift (WGS) reaction, and Direct Steam Reforming (DSR) reaction. The numerical results are validated with experimental results at the same operating conditions. Using the validated code, parametric study has been numerically performed in view of the steam reformer performance. As increasing a wall temperature, the fuel conversion increases due to the high heat transfer rate. When Steam to Carbon Ratio (SCR) increases, the concentration of carbon monoxide decreases since WGS reaction becomes more active. When increasing Gas Hourly Space Velocity (GHSV), the fuel conversion decreases due to the heat transfer limitation and the low residence time. The reactor shape effects are also investigated. The length and radius of cylindrical reactors are changed at the same catalyst volume. The longer steam reformer is, the better steam reformer performs. However, system energy efficiency decreases due to the large pressure drop. 기호설명 c

Numerical Analysis of the Heat and Mass Transfer Characteristics in an Autothermal Methane Reformer

This paper discusses a numerical analysis of the heat and mass transfer characteristics in an autothermal methane reformer. Assuming local thermal equilibrium between the bulk gas and the surface of the catalyst, a one-medium approach for the porous medium analysis was incorporated. Also, the mass transfer between the bulk gas and the catalysts surface was neglected due to the relatively low gas velocity. For the catalytic surface reaction, the Langmuir-Hinshelwood model was incorporated in which methane (CH 4 ) is reformed to hydrogen-rich gases by the autothermal reforming (ATR) reaction. Full combustion, steam reforming, water-gas shift, and direct steam reforming reactions were included in the chemical reaction model. Mass, momentum, energy, and species balance equations were simultaneously calculated with the chemical reactions for the multiphysics analysis. By varying the four operating conditions (inlet temperature, oxygen to carbon ratio (OCR), steam to carbon ratio, and gas hourly space velocity (GHSV)), the performance of the ATR reactor was estimated by the numerical calculations. The SR reaction rate was improved by an increased inlet temperature. The reforming efficiency and the fuel conversion reached their maximum values at an OCR of 0.7. When the GHSV was increased, the reforming efficiency increased but the large pressure drop may decrease the system efficiency. From these results, we can estimate the optimal operating conditions for the production of large amounts of hydrogen from methane.

A Simple Descriptor to Rapidly Screen CO Oxidation Activity on Rare-Earth Metal-Doped CeO2: From Experiment to First-Principles

Ceria (CeO2) is an attractive catalyst because of its unique properties, such as facile redoxability and high stability. Thus, many researchers have examined a wide range of catalytic reactions on ceria nanoparticles (NPs). Among those contributions are the reports of the dopant-dependent catalytic activity of ceria. On the other hand, there have been few mechanistic studies of the effects of a range of dopants on the chemical reactivity of ceria NPs. In this study, we examined the catalytic activities of pure and Pr, Nd, and Sm-doped CeO2 (PDC, NDC, and SDC, respectively) NPs on carbon monoxide (CO) oxidation. Density functional theory (DFT) calculations were also performed to elucidate the reaction mechanism on rare-earth (RE)-doped CeO2(111). The experimental results showed that the catalytic activities of CO oxidation were in the order of CeO2 > PDC > NDC > SDC. This is consistent with the DFT results, where the reaction is explained by the Mars-van Krevelen mechanism. On the basis of the theoretical interpretation of the experimental results, the ionic radius of the RE dopant can be used as a simple descriptor to predict the energy barrier at the rate-determining step, thereby predicting the entire reaction activity. Using the descriptor, a wide range of RE dopants on CeO2(111) were screened for CO oxidation. These results provide useful insights to unravel the CO oxidation activity on various oxide catalysts.

Characterization and Electrochemical Performance of Composite BSCF Cathode for Intermediate-temperature Solid Oxide Fuel Cell

The composite barium strontium cobalt ferrite (BSCF) cathodes were investigated in the intermediate temperature range of solid oxide fuel cells (SOFCs). The characteristics and electrochemical performances of composited BSCF/samarium doped ceria (SDC); BSCF/gadolinium doped ceria (GDC); and BSCF/SDC/GDC were compared to single BSCF cathode. The BSCF used in this study were synthesized using glycine nitrate process and mechanically mixing was used to fabricate a composite cathode. Using a composite form, the thermal expansion coefficient (TEC) could be reduced and BSCF/SDC/GDC exhibited the lowest TEC value at . The electrochemical performance from half cells and single cells exhibited nearly the same trend. All the composite cathodes gave higher electrochemical performance than the single BSCF cathode (0.22 ); however, when two kinds of electrolyte were used (BSCF/SDC/GDC, 0.36), the electrochemical performance was lower than when the BSCF/SDC (0.45 ) or BSCF/GDC (0.45 ) was applied as cathode (, 97%/3% to the anode and ambient air to the cathode).

Numerical Analysis of a Steam Reformer Coupled With a Combustion Burner

This study focuses on a numerical simulation of a steam reforming system. The steam reforming system consisted of a cylindrical steam reformer and a combustion burner. The heat was supplied to an endothermic steam reformer from combustion gases. The correlation between the performance and the shape of the system was studied using two different configurations. The first configuration utilized a flame guide between the combustion burner and the steam reformer, whereas the other did not. The flame guide changed the flow of the combustion gas, which affected the heat transfer rate from the burner to the reformer. Reactor temperature profiles, heat transfer rates, fuel conversions, and hydrogen yields were calculated. In addition, the fuel feed ratio between the burner and the steam reformer was manipulated as an operating parameter.

Numerical Study on Correlation between Operating Parameters and Reforming Efficiency for a Methane Autothermal Reformer

The objective of this paper is to investigate characteristics of an autothermal reformer at various operating conditions. Numerical method has been used, and simulation model has been developed for the analysis. Pseudo-homogeneous model is incorporated because the reactor is filled with catalysts of a packed-bed type. Dominant chemical reactions are Full Combustion reaction, Steam Reforming(SR) reaction, Water-Gas Shift(WGS) reaction, and Direct Steam Reforming(DSR) reaction. Simulation results are compared with experimental results for code validation. Operating parameters of the autothermal reformer are inlet temperature, Oxygen to Carbon Ratio(OCR), Steam to Carbon Ratio(SCR), and Gas Hourly Space Velocity(GHSV). Temperature at the reactor center, fuel conversion, species at the reformer outlet, and reforming efficiency are shown as simulation results. SR reaction rate is improved by increased inlet temperature. Reforming efficiency and fuel conversion reached the maximum at 0.7 of OCR. SR reaction and WGS reaction are activated as SCR increases. When GHSV is increased, reforming efficiency increases but pressure drop from the increased GHSV may decrease the system efficiency.

고체산화물 연료전지의 전극과 스택운영의 기능적 분석

This study aims to investigate the functional analysis of anode and cathode materials in Anode supported Solid Oxide Fuel Cell. The concentration polarization of single cell was investigated with CFD (Computational Fluid Dynamics) method for the case of the different morphology by using four types of unit cell and discussed to reduce the concentration polarization. The concentration polarization at anode side effected the voltage loss in Anode supported Solid Oxide Fuel Cell and increased contact areas between fuel gas and anode side could reduce the concentration polarization. For intermediate temperature operation, Anode-supported single cells with thin electrolyte layer of YSZ (Yttria-Stabilized Zirconia) were fabricated and short stacks were built and evaluated. We also developed diesel and methane autothermal reforming (ATR) reactors in order to provide fuels to SOFC stacks. Influences of the H₂O/C (steam to carbon ratio), O₂/C (oxygen to carbon ratio) and GHSV (Gas Hourly Space Velocity) on performances of stacks have been investigated. Performance of the stack operated with a diesel reformer was lower than with using hydrogen as a fuel due to lower Nernst voltage and carbon formation at anode side. The stack operated with a natural gas reformer showed similar performances as with using hydrogen. Effects of various reformer parameters such as H₂O/C and O₂/C were carefully investigated. It is found that O₂/ C is a sensitive parameter to control stack performance.

The Tests of 1 kWe Diesel Reformer and Solid Oxide Fuel Cell System

The high temperatures required to operate solid oxide fuel cells (SOFCs) allow for internal reforming of hydrocarbon fuels over a Ni-based anode. With their capability of being fuel flexible, SOFCs have operated under a wide range of fuels including diesel as examined in this study. But in order to reduce high possibilities of deposit formation in diesel internal reforming, additional external reforming technology was used for our system. The final goal of this research is to develop 1 kWe diesel-powered SOFC systems for residential power generation. Before constructing a complete 1 kWe SOFC system, a series of durability experiments were conducted on individual components of the system including the fuel reformer and stack. After testing the full-scale 1 kWe diesel reformer, deposit formation was visible within the catalyst and on the surface of the reactor head, which seriously degraded the performance. With several individual components tested, the construction of one-box type 1 kWe SOFC system is in progress. In a preliminary six-cell stack test using sulfur-free synthetic diesel, the system initially showed an output power of ∼110 kWe at a 0.8 V average cell potential. However, there was a significant drop off in output power after a few hours of operation, which was likely caused by severe deposit formation on the SOFC stack. Light hydrocarbons such as ethylene and/or “less reformed” heavier hydrocarbons caused by gas reactions under the incomplete fuel mixing upstream of the catalyst were likely responsible for the deposit formation.