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A.r. Balakrishnan - One of the best experts on this subject based on the ideXlab platform.
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Nucleate pool boiling heat transfer of multicomponent mixtures
Chemical Engineering Research and Design, 2004Co-Authors: G. Vinayak Rao, A.r. BalakrishnanAbstract:Steady state pool boiling heat flux data has been obtained for acetone-isopropanol-water and acetone-MEK(methyl ethyl ketone)-water ternary mixtures. The data shows that to obtain a given heat flux, the wall superheat required is greater for mixtures than for the pure components constituting the mixture. The measured heat transfer coefficients were compared with predictions from literature correlations for multicomponent mixtures. In all the cases, overestimation or underestimation of the data was observed. Therefore, a new correlation has been proposed for the heat flux in terms of Archimedes number, Prandtl number, surface–liquid interaction parameter, modified Jakob number, dimensionless surface roughness group, properties-profile parameter and an effective temperature driving force. In general, the effective temperature driving force in Binary mixtures is less than that encountered in pure components and is obtained by incorporating the Binary Diffusivity of the mixture. In multicomponent systems, the multicomponent diffusion coefficients have to be incorporated into the expression for the effective temperature driving force. The heat flux correlation predicts the present experimental data as well as literature data, satisfactorily. The heat flux was found to be a function of the difference between the equilibrium vapour and liquid concentration, (y − x) of the light component(s) and the minimum heat flux occurs at the maximum of (y − x) of the light component(s).
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Heat transfer in nucleate pool boiling of multicomponent mixtures
Experimental Thermal and Fluid Science, 2004Co-Authors: G. Vinayak Rao, A.r. BalakrishnanAbstract:Abstract Steady state pool boiling heat transfer coefficients have been obtained experimentally for acetone–isopropanol–water and acetone–MEK (methyl ethyl ketone)–water ternary systems. The data show that, for a given heat flux, the heat transfer coefficients of mixtures are lower than those obtained for pure components constituting the mixture. The measured heat transfer coefficients were compared with predictions from literature correlations for pool boiling of multicomponent mixtures. In all the cases, overestimation or underestimation of the data was observed. The literature correlations incorporate an `ideal' heat transfer coefficient and a correction term for the presence of other liquids. Part of the uncertainty associated with the literature correlations on comparison with the present data appears to be due to the uncertainty in estimating the ideal heat transfer coefficient. The methods suggested by the earlier authors found to be not applicable for the present heating surface–liquid combinations. Therefore, in the present study, two different correlations were tried for the ideal heat transfer coefficient and it was found that the performance of the literature correlations improved considerably. However, the methods reported in the literature and the correlations tried in the present study to estimate ideal heat transfer coefficient did not consider heating surface–liquid interaction and the effect of heating surface micro-roughness in boiling. Therefore, a correlation to estimate the ideal heat transfer coefficient has been proposed taking into account surface–liquid interaction parameter and surface roughness group. A new correlation to estimate mixture heat transfer coefficients has been proposed in the present study in terms of an ideal heat transfer coefficient and a correction term. In general, the correction term in Binary mixtures is obtained by incorporating the Binary Diffusivity of the mixture. In multicomponent systems, the multicomponent diffusion coefficients have to be incorporated in the expression for the correction term. The mixture heat transfer coefficient correlation along with the correlation for ideal heat transfer coefficient proposed in the present study predicts the present experimental data as well as literature data satisfactorily. The heat transfer coefficient was found to be a function of the difference between the equilibrium vapor and liquid concentration of the light component(s) and the minimum heat transfer coefficient occurs at the maximum of this value.
G. Vinayak Rao - One of the best experts on this subject based on the ideXlab platform.
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Nucleate pool boiling heat transfer of multicomponent mixtures
Chemical Engineering Research and Design, 2004Co-Authors: G. Vinayak Rao, A.r. BalakrishnanAbstract:Steady state pool boiling heat flux data has been obtained for acetone-isopropanol-water and acetone-MEK(methyl ethyl ketone)-water ternary mixtures. The data shows that to obtain a given heat flux, the wall superheat required is greater for mixtures than for the pure components constituting the mixture. The measured heat transfer coefficients were compared with predictions from literature correlations for multicomponent mixtures. In all the cases, overestimation or underestimation of the data was observed. Therefore, a new correlation has been proposed for the heat flux in terms of Archimedes number, Prandtl number, surface–liquid interaction parameter, modified Jakob number, dimensionless surface roughness group, properties-profile parameter and an effective temperature driving force. In general, the effective temperature driving force in Binary mixtures is less than that encountered in pure components and is obtained by incorporating the Binary Diffusivity of the mixture. In multicomponent systems, the multicomponent diffusion coefficients have to be incorporated into the expression for the effective temperature driving force. The heat flux correlation predicts the present experimental data as well as literature data, satisfactorily. The heat flux was found to be a function of the difference between the equilibrium vapour and liquid concentration, (y − x) of the light component(s) and the minimum heat flux occurs at the maximum of (y − x) of the light component(s).
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Heat transfer in nucleate pool boiling of multicomponent mixtures
Experimental Thermal and Fluid Science, 2004Co-Authors: G. Vinayak Rao, A.r. BalakrishnanAbstract:Abstract Steady state pool boiling heat transfer coefficients have been obtained experimentally for acetone–isopropanol–water and acetone–MEK (methyl ethyl ketone)–water ternary systems. The data show that, for a given heat flux, the heat transfer coefficients of mixtures are lower than those obtained for pure components constituting the mixture. The measured heat transfer coefficients were compared with predictions from literature correlations for pool boiling of multicomponent mixtures. In all the cases, overestimation or underestimation of the data was observed. The literature correlations incorporate an `ideal' heat transfer coefficient and a correction term for the presence of other liquids. Part of the uncertainty associated with the literature correlations on comparison with the present data appears to be due to the uncertainty in estimating the ideal heat transfer coefficient. The methods suggested by the earlier authors found to be not applicable for the present heating surface–liquid combinations. Therefore, in the present study, two different correlations were tried for the ideal heat transfer coefficient and it was found that the performance of the literature correlations improved considerably. However, the methods reported in the literature and the correlations tried in the present study to estimate ideal heat transfer coefficient did not consider heating surface–liquid interaction and the effect of heating surface micro-roughness in boiling. Therefore, a correlation to estimate the ideal heat transfer coefficient has been proposed taking into account surface–liquid interaction parameter and surface roughness group. A new correlation to estimate mixture heat transfer coefficients has been proposed in the present study in terms of an ideal heat transfer coefficient and a correction term. In general, the correction term in Binary mixtures is obtained by incorporating the Binary Diffusivity of the mixture. In multicomponent systems, the multicomponent diffusion coefficients have to be incorporated in the expression for the correction term. The mixture heat transfer coefficient correlation along with the correlation for ideal heat transfer coefficient proposed in the present study predicts the present experimental data as well as literature data satisfactorily. The heat transfer coefficient was found to be a function of the difference between the equilibrium vapor and liquid concentration of the light component(s) and the minimum heat transfer coefficient occurs at the maximum of this value.
Anil V Virkar - One of the best experts on this subject based on the ideXlab platform.
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measurement of o 2 n 2 effective Diffusivity in porous media at high temperatures using an electrochemical cell
Journal of The Electrochemical Society, 2003Co-Authors: Feng Zhao, Tad J Armstrong, Anil V VirkarAbstract:The effective Diffusivity of O 2 -N 2 in porous media was measured at high temperatures (650-800°C) using an electrochemical concentration cell. Porous membranes having total porosity between 29 and 48 vol% were fabricated from Sr-doped LaMnO 3 (LSM) with 20 to 30 wt% carbon added as a pore former. The O 2 -N 2 effective Binary Diffusivity, D eff O2-N2 , at 800°C increased from ∼0.016 to ∼0.12 cm 2 /s with increasing open porosity between 15 and 44 vol%. The D eff O 2 -N 2 exhibited a nonlinear dependence on open porosity and increased dramatically for samples with greater than 35 vol% open porosity. The estimated effective Knudsen diffusivities of O 2 and N 2 , D eff O2K and D eff N2K , at 800°C were an order of magnitude higher than the effective Binary Diffusivity, D eff O2-N2 . Thus O 2 -N 2 transport through the porous membranes was governed by the effective Binary Diffusivity, D eff O2-N2 . The effects of O 2 -N 2 effective Binary Diffusivity, D eff O2-N2 , on concentration polarization of cathodes for solid oxide fuel cells were assessed. The nonlinear behavior of the O 2 -N 2 effective Diffusivity as a function of open porosity indicates that a critical amount of porosity in the cathode is necessary to ensure that the overpotential due to concentration polarization is small. The temperature dependence of D eff O2-N2 was investigated between 650 and 800°C, which was found to be in accord with the Chapman-Enskog model.
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Measurement of O 2 N 2 Effective Diffusivity in Porous Media at High Temperatures Using an Electrochemical Cell
Journal of The Electrochemical Society, 2003Co-Authors: Feng Zhao, Tad J Armstrong, Anil V VirkarAbstract:The effective Diffusivity of O 2 -N 2 in porous media was measured at high temperatures (650-800°C) using an electrochemical concentration cell. Porous membranes having total porosity between 29 and 48 vol% were fabricated from Sr-doped LaMnO 3 (LSM) with 20 to 30 wt% carbon added as a pore former. The O 2 -N 2 effective Binary Diffusivity, D eff O2-N2 , at 800°C increased from ∼0.016 to ∼0.12 cm 2 /s with increasing open porosity between 15 and 44 vol%. The D eff O 2 -N 2 exhibited a nonlinear dependence on open porosity and increased dramatically for samples with greater than 35 vol% open porosity. The estimated effective Knudsen diffusivities of O 2 and N 2 , D eff O2K and D eff N2K , at 800°C were an order of magnitude higher than the effective Binary Diffusivity, D eff O2-N2 . Thus O 2 -N 2 transport through the porous membranes was governed by the effective Binary Diffusivity, D eff O2-N2 . The effects of O 2 -N 2 effective Binary Diffusivity, D eff O2-N2 , on concentration polarization of cathodes for solid oxide fuel cells were assessed. The nonlinear behavior of the O 2 -N 2 effective Diffusivity as a function of open porosity indicates that a critical amount of porosity in the cathode is necessary to ensure that the overpotential due to concentration polarization is small. The temperature dependence of D eff O2-N2 was investigated between 650 and 800°C, which was found to be in accord with the Chapman-Enskog model.
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polarization effects in intermediate temperature anode supported solid oxide fuel cells
Journal of The Electrochemical Society, 1999Co-Authors: Anil V Virkar, Kuanzong Fung, Karun Mehta, Subhash C SinghalAbstract:Anode-supported sold oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolyte, Sr-doped LaMnO{sub 3} (LSM) + YSZ cathode, and Ni + YSZ anode were fabricated and their performance was evaluated between 650 and 800 C with humidified hydrogen as the fuel and air as the oxidant. Maximum power densities measured were {approximately} 1.8 W/cm{sup 2} at 800 C and {approximately} 0.82 W/cm{sup 2} at 650 C. Voltage (V) vs. current density (i) traces were nonlinear; V vs. i exhibited a concave-up curvature [d{sup 2}V/di{sup 2} {ge} 0] at low values of i and a convex-up curvature [d{sup 2}V/di{sup 2} {le} 0] at higher values of i, typical of many low temperature fuel cells. Analysis of concentration polarization based on transport of gaseous species through porous electrodes, in part, is used to explain nonlinear V vs. i traces. The effects of activation polarization in the Tafel limit are also included. It is shown that in anode-supported cells, the initial concave-up curvature can be due either to activation or concentration polarization, or both. By contrast, in cathode-supported cells, the initial concave-up curvature is entirely due to activation polarization. From the experimentally observed V vs. i traces for anode-supported cells, effective Binary Diffusivity of more » gaseous species on the anodic side was estimated to be between {approximately} 0.1 cm{sup 2}/s at 650 C and {approximately} 0.2 cm{sup 2}/s at 800 C. The area specific resistance of the cell (ohmic part), varied between {approximately} 0.18 {Omega} cm{sup 2} at 650 C and {approximately} 0.07 {Omega} cm{sup 2} at 800 C with an activation energy of {approximately} 65 kJ/mol. « less
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Polarization Effects in Intermediate Temperature, Anode‐Supported Solid Oxide Fuel Cells
Journal of The Electrochemical Society, 1999Co-Authors: Jai-woh Kim, Anil V Virkar, Kuanzong Fung, Karun Mehta, Subhash C SinghalAbstract:Anode-supported sold oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolyte, Sr-doped LaMnO{sub 3} (LSM) + YSZ cathode, and Ni + YSZ anode were fabricated and their performance was evaluated between 650 and 800 C with humidified hydrogen as the fuel and air as the oxidant. Maximum power densities measured were {approximately} 1.8 W/cm{sup 2} at 800 C and {approximately} 0.82 W/cm{sup 2} at 650 C. Voltage (V) vs. current density (i) traces were nonlinear; V vs. i exhibited a concave-up curvature [d{sup 2}V/di{sup 2} {ge} 0] at low values of i and a convex-up curvature [d{sup 2}V/di{sup 2} {le} 0] at higher values of i, typical of many low temperature fuel cells. Analysis of concentration polarization based on transport of gaseous species through porous electrodes, in part, is used to explain nonlinear V vs. i traces. The effects of activation polarization in the Tafel limit are also included. It is shown that in anode-supported cells, the initial concave-up curvature can be due either to activation or concentration polarization, or both. By contrast, in cathode-supported cells, the initial concave-up curvature is entirely due to activation polarization. From the experimentally observed V vs. i traces for anode-supported cells, effective Binary Diffusivity of more » gaseous species on the anodic side was estimated to be between {approximately} 0.1 cm{sup 2}/s at 650 C and {approximately} 0.2 cm{sup 2}/s at 800 C. The area specific resistance of the cell (ohmic part), varied between {approximately} 0.18 {Omega} cm{sup 2} at 650 C and {approximately} 0.07 {Omega} cm{sup 2} at 800 C with an activation energy of {approximately} 65 kJ/mol. « less
Uday B. Pal - One of the best experts on this subject based on the ideXlab platform.
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H2-H2O Effective Binary Diffusivity Measurement in Solid Oxide Fuel Cell Anode Using an Electrochemical Cell
ECS Transactions, 2019Co-Authors: Kyung Joong Yoon, Ryan S. Eriksen, Srikanth Gopalan, Soumendra N. Basu, Uday B. PalAbstract:The effective Binary Diffusivity of H2 and H2O in a Ni and yittria-stabilized zirconia (YSZ) anode of the solid oxide fuel cells (SOFCs) was measured between 650 and 800oC using an electrochemical cell consisting of an oxygen pump, an oxygen sensor, and a porous SOFC anode pellet. The effective Binary Diffusivity was obtained from the relationship between the current density across the oxygen pump, and the H2 partial pressure gradient across the anode sample measured using the oxygen sensor. The anode limiting current density and concentration polarization were estimated using the experimental results.
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Effect of Anode Active Layer on Performance of Single-Step Cofired Solid Oxide Fuel Cells
Journal of The Electrochemical Society, 2008Co-Authors: Kyung Joong Yoon, Srikanth Gopalan, Uday B. PalAbstract: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.
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Effect of Fuel Composition on Performance of Single-Step Cofired SOFCs
Journal of The Electrochemical Society, 2007Co-Authors: Kyung Joong Yoon, Srikanth Gopalan, Uday B. PalAbstract: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.
Kyung Joong Yoon - One of the best experts on this subject based on the ideXlab platform.
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H2-H2O Effective Binary Diffusivity Measurement in Solid Oxide Fuel Cell Anode Using an Electrochemical Cell
ECS Transactions, 2019Co-Authors: Kyung Joong Yoon, Ryan S. Eriksen, Srikanth Gopalan, Soumendra N. Basu, Uday B. PalAbstract:The effective Binary Diffusivity of H2 and H2O in a Ni and yittria-stabilized zirconia (YSZ) anode of the solid oxide fuel cells (SOFCs) was measured between 650 and 800oC using an electrochemical cell consisting of an oxygen pump, an oxygen sensor, and a porous SOFC anode pellet. The effective Binary Diffusivity was obtained from the relationship between the current density across the oxygen pump, and the H2 partial pressure gradient across the anode sample measured using the oxygen sensor. The anode limiting current density and concentration polarization were estimated using the experimental results.
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out of cell measurements of h2 h2o effective Binary Diffusivity in the porous anode of solid oxide fuel cells sofcs
Journal of Power Sources, 2010Co-Authors: Weidong He, Kyung Joong Yoon, Ryan S. Eriksen, Srikanth Gopalan, Soumendra N. BasuAbstract:Abstract The effective Binary Diffusivity of H 2 and H 2 O in a Ni and yittria-stabilized zirconia (YSZ) anode of the solid oxide fuel cells (SOFCs) was measured between 650 and 800 °C using an electrochemical cell consisting of an oxygen pump, an oxygen sensor, and a porous SOFC anode pellet. The effective Binary Diffusivity was obtained from the relationship between the current density across the oxygen pump, and the H 2 partial pressure gradient across the anode sample measured using the oxygen sensor. The anode limiting current density and concentration polarization were estimated using the experimental results.
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Effect of Anode Active Layer on Performance of Single-Step Cofired Solid Oxide Fuel Cells
Journal of The Electrochemical Society, 2008Co-Authors: Kyung Joong Yoon, Srikanth Gopalan, Uday B. PalAbstract: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.
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Effect of Fuel Composition on Performance of Single-Step Cofired SOFCs
Journal of The Electrochemical Society, 2007Co-Authors: Kyung Joong Yoon, Srikanth Gopalan, Uday B. PalAbstract: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.