Molten Carbonate Fuel Cell Cathode Research Articles

Influence of gas phase mass transfer limitations on molten carbonate fuel cell cathodes

The purpose of this paper is to elucidate to what extent mass transfer limitations in the gas phase affect the performance of porous molten carbonate fuel cell cathodes. Experimental data from porous nickel oxide cathodes and calculated data are presented. One and two-dimensional models for the current collector and electrode region have been used. Shielding effects of the current collector are taken into account. The mass balance in the gas phase is taken into account by using the Stefan-Maxwell equation. For standard gas composition and normal operating current density, the effect of gas-phase diffusion is small. The diffusion in the gaseous phase must be considered at operation at higher current densities. For low oxygen partial pressures, the influence of mass transfer limitations is large, even at low current densities. To eliminate the influence of conversion on polarization curves recorded on laboratory cell units, measurements should always be performed with a five to tenfold stoichiometric excess of oxygen. Two-dimensional calculations show rather large concentration gradients in directions parallel to the current collector. However, the influence on electrode performance is still small, which is explained by the fact that most of the current is produced close to the electrolyte matrix.

Investigation of Porous Electrodes by Current Interruption: Application to Molten Carbonate Fuel Cell Cathodes

A transient agglomerate model for simulation and analysis of experimental data, obtained by current interruption on porous molten carbonate fuel cell cathodes, is presented. The initial fast change of the potential after current interruption on a polarized electrode is due to the closed‐circuit potential distribution in the electrode. Conventional estimation of the iR corrected overvoltage by current interruption on porous electrodes, with finite electronic conductivity in the solid phase and a finite ionic conductivity of the pore electrolyte, leads to an overcompensation of the external potential drop and an underestimation of the total steady‐state overvoltage due to the internal currents passing in the electrode after interruption. The overcompensation of the external potential drop is directly proportional to the geometric current density and to the thickness of the electrode and inversely proportional to the sum of the effective conductivities in the electrode matrix and the pore electrolyte.

Comparison of MCFC Cathode Materials by Porous Electrode Performance Modeling

The polarization characteristics of porous , , and structures when used as molten carbonate fuel cell cathodes are compared. A film agglomerate model, which accounts for kinetic activation, mass transfer, and both electronic and ionic ohmic resistance, is developed and applied to polarization data obtained for various cathode structures tested in 3 cm2 lab cells. The model is used to evaluate the intrinsic catalytic properties of the materials and accounts for the influence of porous microstructure and electrolyte filling.

Polarization Characteristics of Novel Molten Carbonate Fuel Cell Cathodes

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A high performance molten carbonate fuel cell cathode

A highly-conducting ceramic material, identified as stable in molten carbonate electrolyte, has been fabricated by partial sintering of the powder into porous gas-diffusion electrodes. These have been tested with commercial molten carbonate electrolyte ‘tiles’, to compare performance with that of state-of-the-art NiO cathodes. Although the fabrication technique was quite simple, the resultant La0.8Sr0.2CoO3 electrodes showed kinetic performance equivalent to the NiO. This, or other perovskite or spinel-structure ceramic shown to be totally system compatible in long-term tests, can be formed into effective electrodes to replace NiO.

The Effect of Oxidant Composition on the Performance of Molten Carbonate Fuel Cells

Theoretical and experimental studies were performed to determine the effect of oxidant composition on the overpotential of molten carbonate fuel cell cathodes. Pressure level, carbon dioxide and oxygen mole fractions, the diluent species (nitrogen vs. helium), and electrolyte content were varied. The overpotential was found to increase with decreasing carbon dioxide and oxygen mole fraction in most cases, due to an increase in gas‐phase diffusional resistance within the cathode and its current collector. The effective carbon dioxide and oxygen reaction orders for current at constant overpotential depend on oxidant composition, cathode and current collector geometry, and electrolyte content. The data were rationalized by a theoretical model which assumes that the rate controlling step is the reaction of oxide ion with carbon dioxide to form carbonate ion. However, further clarification of the reaction path is desirable to facilitate performance optimization.

A Homogeneous/Agglomerate Model for Molten Carbonate Fuel Cell Cathodes

A molten carbonate fuel cell cathode model was developed which enables the estimation of performance as a function of cathode electrolyte content. This model is neither a thin film nor a conventional agglomerate model, but involves the calculation of the effective agglomerate diameter, porosity, tortuosity, and number based on knowledge of the electrode's pore spectrum and electrolyte content. Calculated electrode performance and experimental results are compared for cathodes of different pore spectra over a range of electrolyte contents.

Molten Carbonate Fuel Cell Cathode Materials Study

Data are presented on the electronic conductivity and molten alkali carbonate stability (Li:K binary of 62:38 mole ratio) of various materials at 923 K under a 30% atmosphere. Of the examined materials, only and satisfied both criteria sufficiently for acceptable use as molten carbonate fuel cell cathode materials.

Polarization of the Molten Carbonate Fuel Cell Anode and Cathode

A dual‐porosity, agglomerate‐type model for the porous anode and cathode of the molten carbonate fuel cell is developed and used to predict electrode performance in a small, differential‐conversion, cell. The model is based on a phenomenological treatment of mass transport, electrode kinetics, and ionic conduction, combined with structural assumptions. The model predicts the steady‐state performance, given a minimum number of structural parameters. Comparison with experimental data for a 3 cm2 anode and cathode shows good agreement for plausible values of these parameters.

NiO Solubility in Molten Li/K Carbonate under Molten Carbonate Fuel Cell Cathode Environments

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Film Thickness and Distribution of Electrolyte in Porous Fuel Cell Components

The DLVO theory of liquid film stability is used to estimate the thickness of electrolyte films that may be present in molten carbonate fuel cell cathodes. Our result is ≈ 30Å, which is 2 orders of magnitude thinner than the values postulated in thin film models of cathode performance. We also examine carefully the thermodynamic basis of an equation governing the equilibrium of electrolyte among the different porous components of the fuel cell. We show that the validity of this equation is more general than previously recognized in that its derivation does not depend on the assumption of cylindrical or spherical pore geometry.

Metal Oxide Solubility in Eutectic Li/K Carbonate Melts

The equilibrium solubility of NiO, CuO, ZnO, LiFeO/sub 2/, and LaNiO/sub 3/ in binary Li/K carbonate was quantitatively measured between 823 and 1223 K under a simulated molten carbonate fuel cell cathode environment. The solubility of all materials, except LaNiO/sub 3/, was found to increase with increasing temperature. Equilibrium solubilities at 923 K, the operating temperature of a molten carbonate fuel cell, were determined to be 13 X 10/sup -6/, 32 X 10/sup -6/, 63 X 10/sup -6/, and 16 X 10/sup -6/ mole fraction for NiO, CuO, ZnO, and LiFeO/sub 2/, respectively. The LaNiO/sub 3/ solubility remained below the quantitative detection limit until the melt's temperature exceeded 1123 K, at which time material decomposition was detected.

The Effect of Thickness on the Performance of Molten Carbonate Fuel Cell Cathodes

Polarization data were obtained for the reduction of oxygen on porous lithiated nickel oxide fuel cell cathodes in lithium‐potassium carbonate electrolyte at 650°C. Cathode thickness was varied from 0.013 to 0.112 cm with the maximum performance occurring for cathodes of approximately 0.08 cm thickness. The polarization data were optimized for each electrode by varying the electrode electrolyte content in a half‐cell apparatus. The optimum content occurred when the combination of internal ionic resistive, diffusional, and activation losses resulted in minimum overvoltage.