J. R. Selman

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

This paper reports on the stationary polarization of small, differential conversion, molten carbonate fuel cells (3 cm{sup 2}) that was measured between 600 and 700{degrees} 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{degrees} C at low partial pressures of CO{sub 2}. 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.

Electrodeposition of Corrosion‐Resistant Ni‐Zn Alloy I . Cyclic Voltammetric Study

The interaction between different reacting species involved in the initial stage of electrodeposition of nickel-zinc alloy was investigated. A cyclic voltammetric study indicates that codeposition of hydrogen and nickel occurs, with formation of two types of hydrogen-nickel solid solution, i.e., [beta]-Ni and [alpha]-Ni. This nickel hydride formation during Ni-Zn alloy electrodeposition was verified by analyzing the voltammograms of nickel, zinc, and Ni-Zn alloy during initial deposition on various substrates. The dissolution potential of zinc and nickel from electrodeposited nickel-zinc alloy spans a wide range (ca. 400 mV). The influence of the interaction between nickel, hydrogen, and zinc on the nucleation of nickel-zinc electrodeposition is reported in part II of this paper.

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

This paper reports on AC impedance and current interruption measurements which 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{degrees} C. Analysis of data for the cathode indicates that at 650{degrees} C the cathode is under mixed control. Charge-transfer control is approached at lower temperatures (600{degrees} C) and at high CO{sub 2} partial pressures. Control by mass transfer or slow homogeneous reactions (e.g., CO{sub 2} + O{sup =} {yields} CO{sub 3}{sup =}) in the cathode tends to become dominant at higher temperature (700{degrees} C). The kinetic (exchange) reaction orders of the cathode reaction are approximately 0.85 and {minus}0.45 for oxygen and carbon dioxide, respectively. The activation energy of oxygen reduction is approximately 230 kJ/g mol.

Comparison of MCFC Cathode Materials by Porous Electrode Performance Modeling

The polarization characteristics of porous LiCoO[sub 2], LiFeO[sub 2], and NiO 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 cm[sup 2] 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.

Electrodeposition of Ni-Zn alloy. II: Electrocrystallization of Zn, Ni, and Ni-Zn alloy

The initial stages of the electrodeposition of nickel, zinc, and nickel-zinc alloy were investigated by nucleation analysis. A novel deconvolution technique was applied to obtain the individual chronoamperometry of each species and elucidate the nucleation and growth mechanisms of nickel-zinc alloy deposition. In agreement with earlier conclusions (part 1 of this study), codeposition of hydrogen and nickel occurs in the initial stage of deposition. Adsorption and evolution of hydrogen are significant and cause the enhancement of nickel content. From the deconvoluted chronamperograms of nickel-zinc deposition, one sees that the nucleation and growth mechanism of the alloy is similar to that of the pure metals (i.e., nickel or zinc). This similarity implies a weak interaction between nickel and zinc nuclei in the initial stages of deposition.

Meniscus Behavior of Metals and Oxides in Molten Carbonate under Oxidant and Reducing Atmospheres I. Contact Angle and Electrolyte Displacement

The wetting of metals and oxides by molten carbonate is an important factor affecting the performance of a molten carbonate fuel cell (MCFC). The distribution of the electrolyte among electrodes and matrix in the MCFC is dominated by the pore characteristics and wetting properties of these components. However, data on wetting, especially under load (current passage), are limited. In this study, the behavior of the meniscus at a metal is used to obtain information on wetting and electrochemical reactions. Meniscus height and current were measured under varioius atmopsheres. The contact angle was calculated from the meniscus height. The electrolyte distribution in the MCFC was estimated using contact angles thus obtained in oxidant and reducing atmospheres. The results suggest that upon applicatioin of load the electrolyte moves from the anode to the cathode and that capillary effects can worsen the performance of a cell, especially if it is in an unbalanced state of electrolyte filling.

Rotating Disk Studies in Molten Carbonates I . Electrode Design

The objective of the present effort has been to design and fabricate an electrode for the investigation of high temperature molten salts, particularly the alkali carbonates used in the molten carbonate fuel cell (MCFC). A rotating disk electrode has been designed, fabricated, and tested for use in high temperature molten salt systems to determine mass-transport parameters that have been difficult to obtain hitherto due to experimental difficulties. The design incorporates a centrifugally cast and machined solid gold cone soldered to an Inconel shaft, which fits snugly in a precision-machined high purity alumina sheath. A thermal expansion seal obtained when the electrode is heated to the experimental temperature prevents seepage of electrolyte between the gold and insulator. Preliminary tests have shown good performance.

Electrochemical and In Situ Neutron Diffraction Investigations of La‐Ni‐Al‐H Alloys

The performance of selected LaNi{sub 5{minus}y}Al{sub y} hydride electrodes was studied by extensive electrochemical measurements and in situ neutron-diffraction measurements of the deuterated electrode (MD{sub x}) during electrochemical charge-discharge cycles. A small addition of aluminum increased the capacity tenfold under ambient conditions. Increased cell impedance and reduced capacity were noted through the cycle life of LaNi{sub 5{minus}y}Al{sub y}H{sub x}/NiO(OH) cells and were found to be associated with the corrosion and leaching of aluminum from the alloy. A high aluminum content alloy (y = 0.6), however, compensated for the corrosion loss of aluminum by achieving a longer cycle life than that of a low-aluminum-content alloy (y = 0.12). In situ neutron diffraction indicated that only alpha phase was present in the low-aluminum-content alloy, LaNi{sub 4.88}Al{sub 0.12}D{sub 1.1}, while both alpha and beta phases were present in LaNi{sub 4.4}Al{sub 0.6}D{sub 1.8}, the fully charged state. With in situ neutron-diffraction measurements, the absolute values of x in the MD{sub x} formula can be determined for various charge/discharge states, while the coulometry of cell cycling measures only the change in x.

Rotating Disk Studies in Molten Carbonates II . Meniscus and Film Effects

The formation of an electrolyte film between the metal electrode and its insulating sheath produces experimental artifacts and complicates the interpretation of results. Three kinds of fully immersed gold electrodes have been used to study this phenomenon in molten lithium carbonate. Unless adequate precautions are taken such film formation is likely as demonstrated at gold/alumina disk electrodes designed and fabricated for determining the diffusion coefficient and concentration of oxygen species in molten carbonates. At a stationary electrode, film formation produces a spurious voltammetric peak cathodic of the normal peroxide reduction peak in lithium carbonate. This delayed peak, believed to be due to peroxide reduction in the film region, can complicate the interpretation of voltammetric data and lead to erroneous conclusions. At a rotating disk electrode, such film formation can prevent the development of a well-defined limiting current plateau, thereby limiting the usefulness of the electrode and preventing accurate determination of mass-transfer parameters. Such artifacts however, can be used to ones advantage as diagnostic criteria for recognizing film formation which in turn indicates unsatisfactory design or assembly of the disk electrode.

Rotating Disk Studies in Molten Carbonates III . Diffusion Coefficient and Bulk Concentration in Lithium Carbonate

Steady-state and transient measurements have been made at a rotating gold disk electrode in lithium carbonate at 800 C for simultaneously determining the diffusion coefficient and bulk concentration of peroxide ions. At high O[sub 2]/CO[sub 2] ratios where these measurements were made the diffusion coefficient is found to be essentially independent of concentration with a mean value of 3.1[center dot]10[sup [minus]6] cm[sup 2]/s. The bulk concentration of peroxide is two order of magnitude higher than estimates based on thermodynamic calculations, but in agreement with values derived from oxygen solubility measurements in the literature.