Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Utilizing fused bath
Reexamination Certificate
1999-11-08
2001-04-10
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Utilizing fused bath
C205S333000, C205S538000, C205S542000, C205S088000, C205S161000, C205S316000
Reexamination Certificate
active
06214194
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the fabrication of layers of solid oxides which have the ability to support the flow of electricity by the diffusion of oxygen ions through the crystal lattice of the oxides. Such oxides are the basis for the technology of solid state electrochemical devices such as solid oxide fuel cells (SOFC), oxygen gas separators, oxygen gas generators, electrolyzers for water vapor and carbon dioxide, and oxygen sensors for combustion control in power generation as well as in automotive internal combustion engine control. SOFCs are devices which produce direct current (DC) electrical energy in the most efficient way known, they are under development by companies and government agencies of most industrialized nations. SOFC generators in the size of hundreds of kilowatts have been built and successfully tested for thousands of hours. The electrochemical cells which make up the SOFC generators are based on the oxygen ion conduction of thin oxide layers of rare earth-element-stabilized zirconium oxide (zirconia) as the electrolyte. Yttria-stabilized zirconia (YSZ) is widely used in many of the electrochemical device applications mentioned above. Oxygen (O
2
) can be separated from air or other gas mixtures and produced in pure form by the application of a DC current between electrodes attached to gas-impervious YSZ layers which separate the O
2
containing gas mixture from pure O
2
. The transport of oxygen from one electrode to the other is supported by the flow of oxygen ions through the gas impervious YSZ electrolyte layer.
Water vapor and carbon dioxide can be decomposed electrochemically in solid oxide electrolysis cells (SOECs) similarly by the application of an external DC power supply in order to produce pure oxygen, hydrogen, and carbon monoxide, respectively. With oxygen sensors one measures the voltage between electrodes of similar, but much smaller, cell configurations only. In this application one electrode is exposed to an oxygen source of known concentration such as air while the other electrode is exposed to the unknown concentration, for instance, of a combustion chamber. The resulting cell voltage is a thermodynamically precise measure for the unknown oxygen concentration.
The described devices use solid oxide electrolyte layers that are nearly pure oxygen ion conductors. YSZ is only one oxide in this group of oxides. Doped cerium oxide (ceria), doped hafnium oxide (hafnia), doped bismuth oxide, and doped lanthanum gallate are further examples for pure oxygen ion conductors that find application as electrolytes in solid oxide electrochemical cells.
Another group of oxides conducts electrical current by oxygen ion diffusion as well as by electrical charges such as electrons or positive holes. Such oxides are called mixed conducting, oxygen-ionic/electronic, conductors. These oxide conductors are also used for electrochemical devices since they allow for oxygen mass transport through the bulk of the solid oxide via ionic as well as electronic charges. A potential application of such oxides is in the form of semipermeable membranes which can be used for oxygen separation from other gases. However, in contrast to the formerly mentioned SOECs, the semipermeable membrane cells require no electrodes and DC power supply. This type of membrane-cell operates by the application of an oxygen gas pressure differential across the membrane which affects oxygen transport through the otherwise structurally dense membrane, resulting in the generation of pure oxygen at the low pressure side of the membrane. These devices are under intense investigation because they offer a new cost effective and efficient method to produce pure oxygen.
Mixed conducting, oxygen-ionic/electronic, oxides are also used in porous layers as electrodes or parts of electrodes in other solid oxide electrochemical cells, such as SOFCs and SOECs, because they allow the achievement of higher current densities and, therefore, reduce cell voltage losses.
Other applications of mixed conducting oxide cell membranes have been proposed for selective oxidation of olefins to achieve a more selective yield of desired chemicals, whereby the mixed conducting oxide membrane controls the oxygen transfer to the organic reactant for selective oxidation.
Both types of electrochemical cells, those with pure ionic electrolytes and electrodes, and those with mixed conducting. oxygen-ionic/electronic membranes without electrodes, are investigated as oxygen generators for life support systems in medical technology, as well as, in aeronautical and space applications.
All devices in the field of this invention operate at elevated temperatures, usually between 300° C. and 1000° C., in order to increase the oxygen ion conductivity of the oxides to useful levels for the desired device operation. The electrical cell resistance is reduced by an increase of the operating temperature of devices because the rate of oxygen ion transfer is increased. An important method, however, for increasing the oxygen transfer rates and current densities in solid oxide devices is the reduction of the thickness of the oxygen ion conducting layers. To achieve this in an economical way is a technological challenge and is the objective of this invention.
2. Description of the Prior Art
The predominant methods and processes to produce the thin and conductive oxide layers for solid oxide electrochemical devices as described in the section: Field of the Invention, can be divided into three categories, namely, powder sinter methods, thermal spay processes, and electrochemical vapor deposition (EVD). The latter process is the most successful one in SOFC technology. EVD is a high temperature process where metal halide vapors react with oxygen in an electrochemical way, whereby the desired reaction product, namely the oxygen ion conductor, is produced as a thin gas-impervious oxide layer of approximately 5 to 50 micrometers, which is deposited onto a suitable support structure for mechanical stability. Thermal spray and sinter methods are best suited for thicker layers.
The process of EVD was first published in Proceedings, ECS—Symposium, Electrode Materials and Processes for Energy Conversion and Storage, 1977, Vol.77-6, pp 572-583 (A. O. Isenberg). The publication describes the basic elements of a vacuum EVD process for making thin layers of oxygen ion conducting oxides such as YSZ and gadolinia doped ceria, and of the mixed conducting complex oxide lanthanum chromite.
U.S. patent application Ser. No. 08/882,579 (A. O. Isenberg) teaches a much improved EVD process that allows the deposition of oxygen ion conducting oxide layers at pressures below or near atmospheric pressure and with the capability to achieve the deposition of more complex compositions for mixed conducting, oxygen-ionic/electronic, oxides through the use of zinc metal vapors as one of the reactants in the deposition zone.
U.S. Pat. No. 4,791,079 (E. A. Hazbun) teaches the formation of mixed conducting oxide layers by EVD for the fabrication of membranes for hydrocarbon oxidation and dehydrogenation processes.
U.S. Pat. No. 3,525,646 (H. Tannenberger et al.) teaches the use of a plasma torch to fabricate solid oxide electrolyte layers for SOFCs.
U.S. Pat. No. 3,402,230 (D. W. White, Jr.) teaches powder coating of stabilized zirconia followed by a sinter process to form the solid oxide electrolyte layers for SOFCs.
The publication “Oxygen Permeability and Phase Transformation of Sr
0.9
Ca
0.1
CoO
2.5+d
” (N. Miura et al.), Journal of The Electrochemical Society, 146 (7) 2581, teaches the fabrication of sintered, mixed conducting, oxygen-ionic/electronic, oxides and the method of measuring oxygen gas permeation through sintered oxide membrane disks.
Mixed conducting, oxygen-ionic/electronic, oxides can be found abundantly in the family of oxides called perovskites of the general formula ABO
3
. The crystal structure of perovskites allows for extensive doping of the host elements in A and B sites. As a consequence, one c
Bach Klaus J.
Gorgos Kathryn
Isenberg Arnold O.
Keehan Christopher M.
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