Molten Carbonate Fuel Cell Electrodes Research Articles

Determination of Optimum Production Conditions of Casting Slurry in the Manufacture of Molten Carbonate Fuel Cell Electrodes with the Tape Casting Technique

In this study, Ni anode and LiAlO2 electrolyte green sheets were manufactured for MCFC system. Experimental scale production of green sheets was carried out by using tape casting technique. The method of casting slurry, which is the first stage of preparation, has a direct impact on the final product. Therefore, cast slurries with different ratios of a solvent, a binder, a dispersant, plasticizer loading to the organic compounds were prepared to produce green sheets with the technique of tape casting in order to investigate the viscosity, shear stress, thixotropy and flow characteristics. After sheets were dried, physical properties (mechanical tests) were determined. Mixing method (balling mill and mechanical stirrer) and mixing duration of slurry, mixing weight ratio of binder, solvent, plasticizer, dispersant, Li/K carbonate, Ni, and LiAlO2 powder in the slurry was examined.

Dynamic behaviors of a molten carbonate fuel cell under a sudden shut-down scenario: The effects on temperature gradients

A three-dimensional (3-D), dynamic model of a molten carbonate fuel cell (MCFC) is developed in the current work. The model takes into account the heat and mass transfers of various reacting gas species. Yuh and Selman's model is implemented to solve the voltage–current relationship. In addition, water-gas shift (WGS) reaction is also included in the model. The simulation result is validated with published experimental data. The model is used to study the effects of steady and pulsating flows on the temperature gradients of MCFC electrodes in a shut-down event. Co-flow and counter-flow of MCFC are included in this study. The results show that higher thermal conductivity and smaller gas temperature difference before and after shut down event ensure better structural integrity of the fuel cell. In addition, under the simulated condition, the counter-flow operating condition is better for MCFC due to its overall lower temperature and also temperature gradients. In the case of the pulsating flow, the change of electrode temperature gradient is greater in co-flow MCFC compared to that of counter-flow MCFC. The sinusoidal profile of an electrode temperature gradient is more visible when both anode and cathode gas channels are subject to pulsation. A steady gas flow ensures a lower maximum temperature gradient in an electrode. Nevertheless, the simulation result also shows that under certain pulsating flow conditions, the cathode electrode will have a lower average temperature magnitude compared to that of steady-flow.

Analysis of a molten carbonate fuel cell: Numerical modeling and experimental validation

A detailed dynamic model incorporating geometric resolution of a molten carbonate fuel cell (MCFC) with dynamic simulation of physical and electrochemical processes in the stream-wise direction is presented. The model was developed using mass and momentum conservation, electrochemical and chemical reaction mechanisms, and heat-transfer. Results from the model are compared with data from an experimental MCFC unit. Furthermore, the model was applied to predict dynamic variations of voltage, current and temperature in an MCFC as it responds to varying load demands. The voltage was evaluated using two different approaches: one applying a model developed by Yuh and Selman [C.Y. Yuh, J.R. Selman, The polarization of molten carbonate fuel cell electrodes: I. Analysis of steady-state polarization data, J. Electrochem. Soc. 138 (1991) 3642–3648; C.Y. Yuh, J.R. Selman, The polarization of molten carbonate fuel cell electrodes: II. Characterization by AC impedance and response to current interruption, J. Electrochem. Soc. 138 (1991) 3649–3655] and another applying simplified equations using average local temperatures and pressures. The results show that both models can be used to predict voltage and dynamic response characteristics of an MCFC and the model that uses the more detailed Yuh and Selman approach can predict those accurately and consistently for a variety of operating conditions.

Experimental determination of effective conductivities in porous molten carbonate fuel cell electrodes

In this work an electrochemical impedance spectroscopy method for the determination of the effective conductivities of the pore electrolyte and electrode matrix in porous electrodes has been used. The technique has been employed on porous nickel oxide and lithium cobaltite cathodes partly flooded with lithium–potassium carbonate melt in cathode gas environment. The experimental results show that the effective conductivity of the pore electrolyte of a porous nickel oxide cathode is 0.9–2.2 Ω −1 m −1 at the most. If data are approximately corrected for the faradaic reaction the effective conductivity becomes 0.1–0.7 Ω −1 m −1. For the lithium cobaltite cathode the measured conductivity of the solid phase is similar to the data measured ex-situ. The effective conductivity of the pore electrolyte is 0.8 Ω −1 m −1, i.e. close to the results found for nickel oxide cathodes. The effective conductivity of the pore electrolyte calculated by means of a theoretical model is 0.5–3.5 Ω −1 m −1.

A Comparative Study of NiO ( Li ) , LiFeO2, and LiCoO2 Porous Cathodes for Molten Carbonate Fuel Cells

Porous cathodes of , Co‐doped , and for the molten carbonate fuel cell (MCFC) were examined in a comparative study using electrochemical impedance spectroscopy at temperatures of 923, 973, and 1023 K. Using this technique the contributions of charge transfer and diffusion to the impedance could be separated. The impedance results as a function of gas composition were compared with theoretical predictions using the thin‐film model leading to the conclusion that the most predominant diffusing species in porous MCFC electrodes are molecular oxygen and carbon dioxide. The reaction mechanism is probably the same for all three cathodes involving either the reduction of peroxy‐carbonate or the reduction of dissociated oxygen. The remaining difference in gas dependencies can then be explained by assuming a low coverage of oxide ions on while and have intermediate coverage by oxide ions. From the temperature dependence of the impedance an estimate may be given of the activation energies of the polarization processes.

Porous‐Electrode Modeling of the Molten‐Carbonate Fuel‐Cell Electrodes

Porous‐electrode modeling of molten‐carbonate fuel‐cell (MCFC) electrodes has been critically reviewed. Comparisons of the existing MCFC electrode models indicate that the modeling is still not sufficient to predict electrode performance over the entire range of gas compositions. A complete and accurate set of polarization data has been used to provide a solid basis for critically reconsidering the agglomerate model as applied to MCFC electrodes. The steady‐state polarization of the anode can be fitted satisfactorily over the entire range of fuel‐gas compositions by the dry‐agglomerate model. The steady‐state polarization of the cathode cannot be fitted equally well by the film‐agglomerate model, which appears to indicate that the reaction mechanisms used are not representative for the entire range of oxidant compositions. Incorporation of the slow homogeneous recombination reaction or of various chemical equilibria involving peroxide and superoxide does not improve the fit.

The Polarization of Molten Carbonate Fuel Cell Electrodes: II . Characterization by AC Impedance and Response to Current Interruption

AC impedance and current interruption measurements have been investigated as an in situ method for characterizing rate‐limiting processes in molten carbonate fuel cell porous electrodes. Analysis of the data for the fuel cell anode indicates that the anode is always under strong mass‐transfer control, in the temperature range of 600–700°C. Analysis of data for the cathode indicates that at 650°C the cathode is under mixed control. Charge‐transfer control is approached at lower temperature (600°C) and at high partial pressures. Control by mass transfer or slow homogeneous reactions (e.g.,) in the cathode tends to become dominant at higher temperature (700°C). The kinetic (exchange) reaction orders of the cathode reaction are approximately 0.85 and −0.45 for oxygen and carbon dioxide, respectively. The activation energy of oxygen reduction is approximately 230 kJ/g mol.

The Polarization of Molten Carbonate Fuel Cell Electrodes: I . Analysis of Steady‐State Polarization Data

The stationary polarization of small, differential conversion, molten carbonate fuel cells (3 cm2) was measured between 600 and 700°C under various gas compositions. Multiple linear regression was used to correlate the experimental data and to infer the rate‐limiting processes in fuel cell electrodes. The analysis indicates that both the anode and the cathode are primarily under mixed control at 700°C at low partial pressures of . The anode does not exhibit charge‐transfer control under normal operating conditions, due to its very fast kinetics. The Superoxide mechanism appears to be the dominant reaction in a fuel cell cathode.

Characterization of fuel cell electrode processes by AC impedance

AbstractAlternating current impedance measurements can be used as an in situ method for characterizing rate‐limiting processes in fuel cell electrodes. Theoretical models have been derived and solved analytically. Theoretical analysis indicates that kinetic information can be extracted from the effect of temperature and gas composition on the size of the kinetic loop. The relative importance of kinetics and mass transfer can be inferred from AC impedance analysis. Several examples are given for the case of molten carbonate fuel cell (MCFC) electrodes. Such electrodes are not accessible to classical kinetic investigation techniques due to the high operating temperature and corrosive environment. Relevant kinetic information about MCFC electrodes has been obtained successfully using this in situ technique.

ChemInform Abstract: Conducting Ceramic Oxides for Use as Molten Carbonate Fuell Cell Electrodes.

AbstractThe dc resistivities of doped (gel‐resin process) mixed cation oxides have been measured under conditions similar to those found for molten carbonate fuel cell electrodes.

Conducting Ceramic Oxides for Use as Molten Carbonate Fuel Cell Electrodes

Mixed cation oxides were fabricated and doped using gel‐resin processing techniques in order to increase the compound's electronic conductivities. The dc resistivities of the various compounds were measured under gas compositions and temperatures similar to those found for molten carbonate fuel cell electrodes. Low resistivities were measured at high temperatures for all the titanate‐based systems (, , , , and ) when: (i) doping with M+5impurities and (ii) the atmosphere was necessarily reducing (low ). These materials would be especially good candidates for anodes but, due to increased resistivities under oxidizing conditions, probably are not well suited for cathodes. Several possible explanations for the increased resistivities under oxidizing conditions are discussed.

A study of electrolyte-phobic molten carbonate fuel cell electrodes

The incorporation of electrolyte-phobic material into gas diffusion electrodes enhances their performance. The principles governing the choice of electrolyte-phobic materials in molten carbonate electrolyte are discussed. Contact angle measurements of molten NaLiCO3 on materials which have hexagonal layer-like structures confirmed that providedγ LV γ SV, the contact angle will be greater than 90°. Of the materials tested (C, BN, BaO.6.Fe2O3, PbO.6Fe2O3), only C possessed adequate corrosion resistance. Electrolyte-phobic anodes, prepared by incorporation of graphite fibres into the electrocatalyst mixture, significantly reduced concentration polarization and paste electrolyte cells using such electrodes gave 160 mA/cm2 at 0.6 V on 80% H2/20% CO2 fuel at 600°C, as opposed to 80 mA/cm2 for conventional cells.