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Lee, Sanghyeok,Bae, Yonggyun,Yoon, Kyung Joong,Lee, Jong-Ho,Choi, Wonjoon,Hong, Jongsup Elsevier 2018 Energy conversion and management Vol.174 No.-
<P><B>Abstract</B></P> <P>Key features of hydrocarbon-fueled solid oxide fuel cell stack operation are elucidated by examining its local thermodynamic states with an aid of three-dimensional numerical simulations. A high-fidelity physical model, which resolves the coupling between thermo-chemical reactions and heat and mass transport, is developed and validated. To elucidate important reactions and transport phenomena, local thermodynamic state variables of hydrocarbon-fueled operation are compared with those estimated by assuming pure-hydrogen-supplied operation. Results show that thermochemical reactions proceed at high rates through the thick anode support layer. This induces complete methane conversion as soon as it is introduced to the anode and thermochemical reaction zones concentrated in the vicinity of the fuel inlet. In spite of the fast reaction processes, hydrocarbon-fueled operation has the same electrical current density profile as pure-hydrogen-supplied operation, resulting from changing its local thermodynamic states. Given that the presence of carbon substances and thermochemical reactions, in hydrocarbon-fueled operation, local chemical and electrical conditions are substantially different from those of pure-hydrogen-supplied operation. A lower hydrogen concentration induces a higher concentration overpotential and decreases a reversible electrochemical potential. A lower exchange current density is offset by increasing an activation overpotential at a given applied current. All these reduce the overall cell voltage, as compared to pure-hydrogen-supplied operation. Variation of transport properties such as diffusivities and viscosities influences heat and mass transport such that substantial stresses can be imposed on cell materials. In addition, thermal conditions result in lower incoming-gas heating and a larger heat transfer rate to a neighboring repeating unit. A larger temperature gradient near the fuel inlet may also impose stresses cell materials. A lower power output, attributed to the electrochemical losses in a form of activation and concentration overpotentials, and materials degradation can be accompanied in hydrocarbon-fueled stack operation.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Hydrocarbon-fueled SOFC operation may induce lower power output and reliability. </LI> <LI> Anode thermochemical reaction zones are concentrated in the vicinity of fuel inlet. </LI> <LI> Transport properties alter heat and mass transfer and electrochemical conditions. </LI> <LI> Higher concentration and activation overpotentials reduce an overall cell voltage. </LI> <LI> Larger temperature gradient and heat transfer with a neighboring unit are expected. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Lee, Seunghwan,Shin, Dongwook,Park, Mansoo,Hong, Jongsup,Kim, Hyoungchul,Son, Ji-Won,Lee, Jong-Ho,Kim, Byung-Kook,Lee, Hae-Weon,Yoon, Kyung Joong Elsevier 2017 Journal of the European Ceramic Society Vol.37 No.5
<P><B>Abstract</B></P> <P>Gadolinia-doped ceria (GDC) has emerged as one of the most essential component materials for next-generation solid oxide fuel cells (SOFCs). The refractory nature of GDC has been a major hurdle for its successful implementation, and precise control of the thermal behavior is crucial. Here, we report a particle-dispersed glycine-nitrate process (PD-GNP) that leads to the formation of fast-sintering nanoparticles uniformly conjugated to the surface of slow-sintering inclusion particles. The independent regulation of nanoparticles and sintering aids based on <I>in situ</I> co-assembly process enables precise control over the individual stages of the sintering process and grain growth, resulting in complete densification at desired temperatures. This work highlights a simple and cost-effective way to produce exquisitely tailored GDC nanopowder for specific purposes in the manufacturing of SOFCs; furthermore, it expands opportunities to effectively exploit nanotechnology in the fabrication of a wide range of multilayer ceramic devices.</P>
Lee, Sanghyeok,Park, Mansoo,Kim, Hyoungchul,Yoon, Kyung Joong,Son, Ji-Won,Lee, Jong-Ho,Kim, Byung-Kook,Choi, Wonjoon,Hong, Jongsup Elsevier 2017 ENERGY Vol.120 No.-
<P><B>Abstract</B></P> <P>Elucidating internal thermal conditions of high-temperature solid oxide fuel cell (SOFC) stacks is essential for obtaining a substantial thermal efficiency and reliability for long-term operations prior to their commercialization. To examine simultaneous heat transfer and its generation and their effect on the local thermodynamic state, a high-fidelity physical model that resolves spatially the three-dimensional structure of planar, anode-supported SOFC stacks is used in this study. Results show that thermal conduction through metallic interconnects plays a key role in transferring the heat produced by joule heating and electrochemical reactions and thus determining the internal thermal conditions. The heat generated from the electrolyte and thin reactive electrode layers is transferred to the interconnect predominantly by gaseous convection and conduction through materials in the anode and cathode, respectively. The interconnect subsequently transports this heat conductively towards gas inlets and/or surrounding repeating units, influencing temperature increments, its profile and hot spot formation. Its effect on the internal thermal conditions was further examined by a parametric study with respect to the thermal property and geometry of the interconnect which determine its thermal resistance. They indeed affect significantly heat generation and its transfer within the cell, through its boundaries, between repeating units and to incoming gases.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Internal thermal conditions of solid oxide fuel cell (SOFC) stacks are elucidated. </LI> <LI> Heat transfer in the anode and cathode is attributed to different primary pathways. </LI> <LI> Metallic interconnects play a key role in transferring the heat produced from SOFC. </LI> <LI> Interconnects transport heat towards gas inlets and surrounding repeating units. </LI> <LI> Thermal resistance of interconnects influences substantially thermal conditions. </LI> </UL> </P>
Lee, Sanghyeok,Kim, Hyoungchul,Yoon, Kyung Joong,Son, Ji-Won,Lee, Jong-Ho,Kim, Byung-Kook,Choi, Wonjoon,Hong, Jongsup Elsevier 2016 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.97 No.-
<P><B>Abstract</B></P> <P>The thermo-fluid reacting environment and local thermodynamic state in solid oxide fuel cell (SOFC) stacks were examined by using three-dimensional numerical simulations. Enhancing the performance and durability of the SOFC stacks is essential when a high fuel utilization scheme is implemented to increase the system efficiency and lower system operating costs. In this study, numerical simulations were conducted to elucidate the effect of fuel utilization on heat and mass transfer as the fuel utilization is raised. A high-fidelity three-dimensional physical model was developed incorporating elementary electrochemical reaction kinetics by assuming rate-limiting steps and spatially-resolved conservation equations. The model considers planar anode-supported SOFC stacks and is validated against their electrochemical performance experimentally measured. A parametric study with respect to fuel utilization was conducted by varying a fuel flow rate while maintaining other operating conditions constant. Results show that, when increasing the fuel utilization, a narrow and non-uniform electrochemical reaction zone is observed near the fuel inlet, resulting in substantial depletion of hydrogen in the downstream fuel flow and thus raising the partial pressure of oxygen in the anode. This subsequently lowers the electrochemical potential gradient across the electrolyte and hence induces a large gradient of ionic current density along the cell. Convective flow through porous electrodes also results in pressure gradients in the direction of both cell thickness and length. In addition, the heat balance between conduction through metallic interconnects, convection by gases and the heat generated from charged-species transport and electrochemical reactions determines a temperature gradient along the cell and its maximum location. All of these gradients may induce chemical, mechanical and thermal stresses on SOFC materials and corresponding degradation.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A high-fidelity three-dimensional SOFC physical model was developed and validated. </LI> <LI> Thermo-fluid reacting environment was elucidated as fuel utilization is raised. </LI> <LI> Hydrogen depletion induces a gradient of ionic current density in the electrolyte. </LI> <LI> Conductive flow through porous electrodes results in large pressure gradients. </LI> <LI> The temperature profile, its increments and maximum location were estimated. </LI> </UL> </P>
Highly Dense Mn-Co Spinel Coating for Protection of Metallic Interconnect of Solid Oxide Fuel Cells
Lee, Sung-Il,Hong, Jongsup,Kim, Hyoungchul,Son, Ji-Won,Lee, Jong-Ho,Kim, Byung-Kook,Lee, Hae-Weon,Yoon, Kyung Joong The Electrochemical Society 2014 Journal of the Electrochemical Society Vol.161 No.14
<P>The major degradation issues of solid oxide fuel cells (SOFC) are associated with the oxide scale growth and Cr evaporation of the metallic interconnect. To address these challenges, a highly dense spinel oxide coating was fabricated on a ferritic stainless steel interconnect using a cost-competitive ceramic processing route. The nano-scale Mn<SUB>1.5</SUB>Co<SUB>1.5</SUB>O<SUB>4</SUB> spinel powder was synthesized using a glycine-nitrate method, and the particle agglomerates were effectively disintegrated by a high-energy attrition milling process. The spinel protective coating, which was applied by screen printing, was sintered to a nearly full density, without causing damage to the metallic substrate, by a high-temperature annealing process in a reducing environment, followed by re-oxidation at a moderate temperature. The dense spinel coating remarkably reduced the growth rate of chromia scale and restrained the evaporation of chromium species, as verified by area specific resistance (ASR) measurements and analysis on chromium distribution over the cross-section. Strong adhesion between the coating and substrate was confirmed after 500-hour operation. The sintering mechanism involved in reduction-oxidation heat-treatment was studied based on dilatometry measurements and microstructural features. The implication of the ASR change and the chromium migration for stability of practical SOFC stacks was discussed in detail.</P>