High purity oxygen is required for a number of scientific, medical, and industrial applications. Traditionally, these needs have been met by cryogenic distillation or pressure swing adsorption systems designed to separate oxygen from air. Oxygen separation from air via solid-state zirconia electrolyte technology offers an alternative to these methods. The technology has several advantages over the traditional methods, including reliability, compactness, quiet operation, high purity output, and low power consumption

Clark, Douglas J.

Suitor, Jerry W.

English

NASA Technical Reports Server

N91-24059
OXYGEN PRODUCTION USING SOLID-STATE ZIRCONIA ELECTROLYTE
TECHNOLOGY
Jerry W. Suitor
Douglas J. Clark
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California
ABSTRACT
High purity oxygen is required for a number of scientific, medical, and industrial applications.
Traditionally, these needs have been met by cryogenic distillation or pressure swing adsorption systems
designed to separate oxygen from air. Oxygen separation from air via solid-state zirconia electrolyte
technology offers an alternative to these methods. The new technology has several advantages over the
traditional methods, including reliability, compactness, quiet operation, high purity output, and low power
consumption•
INTRODUCTION
The Process
Solid-state zirconia electrolyte technology is made possible by the fact that zirconia is one of a number of
materials that is electrically insulating but that can conduct ionic oxygen. Because of this attribute,
zirconia can be used as the electrolyte in an electrolytic or galvanic cell. In either case, the zirconia is
sandwiched between two gas permeable electronically conducting electrodes. The electrolytic cell, Figure 1,
is the basis for the oxygen separation cell.
--....
Air (gas)
L:_....:._:_.:_.::-:t_:.)-i)i::!i_!ii iiii Ji_! i_)i"!)i_,, :_:!._._... ... .. ......09 D1;:.............ffusmn_"::::::i...................
Solid-State_
Zirconia Electrolyte ..........
• ......-..-._ :_,_._:.:
[_i'O2 Diffusmn_i i
/
Electron current /
_L
_ Cathode PowerSupply
Figure 1. Electrolytic Cell (Oxygen Separation Cell)
The energy necessary to drive the separation process is supplied by the power supply. Oxygen in the air
diffuses through the cathode to the cathode-electrolyte interface. Under the influence of the applied voltage,
oxygen molecules are dissociated and reduced to oxygen ions, 02- . As an ionic species, oxygen enters the
crystal lattice of the electrolyte and moves toward the anode. At the anode, each ion gives up its two
electrons which then enter the anode and return to the power supply, completing the circuit. The oxygen
atoms recombine into 02 and diffuse out through the anode. Because the zirconia conducts only oxygen
ions, the gas on the output (anode) side of the cell is pure oxygen. Equation 1 describes the reaction.
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https://ntrs.nasa.gov/search.jsp?R=19910014746 2020-03-19T17:44:14+00:00Z
If" _ -
02 (gas 0.2 atm) + 4e- (cathode) _ 20 2" (electrolyte)
202- (electrolyte) _ O2(gas 1.0 atm)+ 4e" (anode) (1)
To impart oxygen ion conductivity to the material, pure zirconia (ZrO2) must be doped with another oxide
such as calcia (CaO) or yttria (Y203). [1] Each dopant cation (Ca 2+ or y3+) replaces a Zr4+ ion; the net
result is that the doped crystal is electrically neutral without having all oxygen ion sites filled. These
oxygen ion vacancies impart in the substance an appreciable oxygen ion conductivity. Because the ionic
conductivity is a strong function of temperature, the typical operating temperature of solid-state zirconia
electrolyte cells is 1000 C.
Thermodynamic Considerations
The laws of thermodynamics play a major role is determining the power consumption of any oxygen
separation system.
Oxygen separation in the solid-state zirconia electrolyte oxygen separator is driven by the current provided
by the power supply. Part of the overall power consumption is due to the thermodynamically reversible
process of separating oxygen from nitrogen and the other gases that make up air. The rest of the power
consumption is caused by irreversible processes including electrical resistance in the electrodes, ionic
resistance in the electrolyte, and parasitic effects that occur at the electrodes.
The reversible part of the power consumption is manifested by a voltage across the oxygen separation cell
called the Nernst voltage. This voltage is a function of the difference in oxygen partial pressure on the two
sides of the cell; the greater the difference, the greater the voltage. For a I000 C cell separating oxygen
from air at atmospheric pressure, the Nernst voltage is 44 mV.
Because the ionization reaction takes place inside the cell (at the cathode-electrolyte interface), the structure
of the electrodes can have a major effect on cell performance. Gaseous oxygen may diffuse through pores in
the electrodes, or it may diffuse through the electrode material itself. Because the presence of electrodes will
reduce the rate of oxygen transport to and from the zirconia electrolyte, regions of oxygen starvation and
oxygen enrichment are formed at the cathode and anode respectively. The presence of these starved and
enriched regions results in an increase in the Nernst voltage. This (irreversible) enhancement of the Nernst
voltage is sometimes called gas-phase polarization. A major portion of the development work performed at
JPL has been directed toward development of electrode materials that minimize gas phase polarization.
ADVANTAGES OF SOLID-STATE ZIRCONIA ELECTROLYTE OXYGEN SEPARATION
Separation of oxygen from air or other oxygen-containing gases by the solid-state zirconia electrolyte
technique has several advantages over traditional methods of separation like cryogenic distillation or
pressure-swing adsorption.
Separation of oxygen by solid-state zirconia electrolyte technology lends itself well to modular
construction, and thus is easily scaled. JPL has developed patented circular oxygen separation cells that can
be stacked, forming a multi-cell stack; multiple stacks can be manifolded together within a common furnace
to form a multi-stack module. The modularity inherent in the technology also simplifies maintenance
procedures.
Equation 1 indicates that transfer of four electrons is required to conduct each oxygen molecule (02) through
the electrolyte. Thus it is a simple matter to control the rate of Oxygen production by controlling the
current applied to the cells. This feature reduces the storage requirements of oxygen separation units and
allows operators to better match production to demand. On a small scale, solid electrolyte cells could be
used to meter precise amounts of oxygen to partial oxidation processes.
The basic solid electrolyte separation cell has no moving parts. This fact contributes to the high inherent
reliability of the technology. The lack of moving parts, combined with the continuous, rather than batch,
nature of the process result in a quiet, vibration-free system.
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Because the separation of oxygen from air by a solid-state zirconia electrolyte cell is an electrochemical
process, the oxygen output stream is 100 percent oxygen. Other separation schemes, such as cryogenic
distillation or pressure swing adsorption, cannot produce oxygen of this purity due the basic processes used.
APPLICATIONS OF SOLD-STATE ZIRCONIA ELECTROLYTE TECHNOLOGY
Medical
Solid-state zirconia electrolyte technology has been proposed for several medical applications. Because of
the modularity of the technology, application in home, hospital and portable settings are feasible. A block
diagram of a home medical unit is shown in Figure 2.
A blower gathers fresh air and forces it through a series of small recuperative heat exchangers. Then, after
passing through the startup heater, the air enters the zirconia oxygen separation module. The module
separates approximately 50 percent of the oxygen from the input air stream. The oxygen and oxygen
depleted air streams pass through heat exchangers before leaving the unit. The heat exchangers thus serve to
preheat the input air, reducing the load on the startup heater, and cool the output streams, reducing the
amount of waste heat leaving the unit.
Product Oxygen Output Oxygen
Air
Input Heat Exchangers
Zirconia
Oxygen
Separation
Module
Blower
StartupHeater
Oxygen-Depleted Air Vent
Oxygen-Depleted Air
DC
Power
Supply
Figure 2. Home Medical Oxygen Supply Unit
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Industrial
Solid-state zirconia electrolyte technology has also been considered in a number of industrial applications.
A similar system to that shown in Figure 2 is used in an industrial application. Of course, the size is
much larger depending on the quantity of oxygen required. The inherent modularity of the zirconia
electrolyte technology permits the construction of units ranging from a few liters/rain of oxygen to tons per
day. Applications considered for this technology include hospital supply, fish farming, weld shop and
metal fabrication, coal gasification, and basic metal refining.
Aerospace
The inherent high reliability of the zirconia oxygen separation process makes the technology attractive for
several aerospace applications.
Feasibility studies are currently underway at JPL to determine the suitability of solid-state zirconia
electrolyte technology for application in spacecraft-borne sensor coolers. In this application, the zirconia
cell would be used as an oxygen pump in a closed-loop Joule-Thomson cooling cycle. The lack of
vibration-inducing moving parts makes this scheme especially attractive for sensitive optical systems.
Another application for the technology in space is In-Situ Resource Utilization (ISRL0. ISRU refers to the
utilization of resources available in space, such as Lunar soil or Martian atmosphere, for propellants, life
support, and fabrication. Solid-state zirconia electrolyte technology has been identified as a key element in
systems to extract oxygen from Lunar soil air and the Martian atmosphere.
JPL ACCOMPLISHMENTS
Project History
JPL has been working in the field of solid-state zireonia electrolyte technology since 1978. In 1984, the
U. S. Department of Energy funded an effort intended to develop oxygen separators for use in coal-fired
power plants. JPL made several major accomplishments in the development of this technology during the
DOE effort [2, 3]. These accomplishments were focused on cell geometry design, component fabrication
techniques, and cell testing.
Cell Geometry
A major factor affecting solid-state zirconia electrolyte oxygen separation cell performance is cell geometry.
The presence of ionic resistivity dictates that the zirconia electrolyte should be made a thin as possible.
Zirconia tubes are in common use but the wall thickness is very large (-lmm). In this case, electrodes are
applied on both the inside and outside surfaces of the tube. Air is passed through the center of the tube, and
oxygen is conducted through the tube wall to the outside. In addition to the wall thickness, another major
drawback to this geometry is the fact that the supply air is depleted of oxygen as it travels the length of the
tube; there is little oxygen left in the air by the time it reaches the far end of the tube. This oxygen
starvation increases the oxygen partial pressure difference across the cell and results in an increased Nernst
voltage andhigher power consumption.
JPL addressed the problem of oxygen starvation by developing a cell geometry based on a circular
electrolyte disk. As shown in Figure 3, the supply air enters one side of the cell around the circumference
of the disk and is directed toward its center by a number of radial ribs. Oxygen is removed as the air travels
toward the center of the disk. Because the cell area available to a given quantity of input air is red_uced as
the center is neared, the increase in Nernst voltage is not as severe as is seen in tubular designs. Computer
models indicating the superior performance of this patented design have been confirmed by experimental
data. The ribs support the zirconia permitting very thin (--0.05 mm) membranes to be used. The ribs also
serve as busbars to carry current to the cell electrodes. [4]
158:
Oxygcn-dcplczd
Air Out
• Air In
Air In Air In
Electrodes _ Electrolyte
Oxygen Out
Figure 3. Radial Air Flow, Axial Current Flow Circular Disk Oxygen Separation Cell Geometry
(Ribs not shown)
Component Fabrication
JPL developed a patented process for fabrication of the zirconia electrolyte membrane. [5] In this process,
conductive ceramic electrodes are applied to the zirconia electrolyte before firing. The method is capable of
producing three layer (electrode-electrolyte-electrode) composite oxygen separation membranes with zirconia
thickness less than 0.05 mm thick. The firing cycle and temperature were substantially reduced in this
patented process. In addition, JPL has developed the ability to fabricate ceramic cell casings.
Testing
As part of the Department of Energy-sponsored effort, JPL built and tested more than sixty single cell
oxygen separation units. In addition, ten multicell stacks were tested; three of these were capable of
producing 1 standard liter of oxygen per minute each.
Current Work
In addition to promotion of earth-bound applications of the technology, current work at JPL is focussed on
studies to determine the suitability of solid-state zirconia electrolyte technology for use in aerospace
applications. Two applications under consideration are In-Situ Resource Utilization, in which oxygen
would be produced from Lunar soil or the Martian atmosphere, and cryogenic refrigeration, which involves
use of a zirconia-based oxygen pump.
TECHNICAL CHALLENGES
The major technological obstacles to commercialization of solid-state zirconia electrolyte technology stem
from the high operating temperature required by the process. To eliminate the need for expensive precious
metal electrodes and interconnects, JPL's oxygen separation cells use strontium-doped lanthanum manganite
(LSM), an electrically conductive ceramic, for both electrodes and cell interconnections. Not only is this
material electrically conductive, it is mechanically and chemically stable at 1000 C.
While the problem of expensive interconnects has been eliminated by the use of LSM, sealing of the cells
continues to be difficult. The majority of JPL-built separation cells to date have relied upon precision-
ground flat seals to contain the the product oxygen. While suitable for use in applications that lack a
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requirementfor pressurized output, the flat seals are incapable of supporting a pressure differential of more
than than 4 kPa (0.6 PSI). Follow-on work at JPL will concentrate on development of higher pressure
seals.
JPL also intends to examine other oxygen ion-conducting materials such as delta-bismuth oxide. Data
reported in the literature indicate that this material has a higher ionic conductivity that doped zirconia, and
can operate at a lower temperature.
CONCLUSIONS
JPL has developed an oxygen separation system that is modular and reliable and without moving parts. The
power consumption is lower than for other oxygen separation systems. The ceramic fabrication technology
has been developed to the point where cells and stacks are easily fabricated. Still required in the
development is the successful use of seal technology to provide high pressure, 100 percent purity oxygen to
a variety of applications including medical, industrial, and aerospace.
How to License The Technology
Patents held on this technology are the property of the California Institute of Technology. Organizations
interested in licensing this technology should contact the California Institute of Technology, Office of
Patents and Licensing, (818) 356-4567.
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REFERENCES
Dell, R. M.; and Hooper, A.: Oxygen Ion Conductors, in Solid Electrolytes: General Principles,
Characterization, Materials, Applications, eds. Paul Hagenmuller and W. van Gool. Academic
(New York): 1978, 291-312.
Suitor, J. W.; Berdahl, C. M.; Ferrall, J. F.; Marner; W. J., Schroeder; J. E.; and Shlichta, P. J.:
Development of an Alternate Oxygen Production Source Using a Zirconia Solid Electrolyte
Membrane: Technical Progress Report for Fiscal Years 1986 and 1987, Jet Propulsion Laboratory
Publication, No. JPL D-4320, May 1987.
Suitor, J. W.; Clark, D. J.; Losey, R. W.: Development of Alternative Oxygen Production Source
Using a Zirconia Solid Electrolyte Membrane: Technical Progress Report for Fiscal Years 1987,
1988, and 1989, Jet Propulsion Laboratory Publication, No. JPL D-7790, August, 1990.
Suitor, J. W.; Berdahl, C. M.; Marner; W. J.: U. S. Patent No. 4,885,142, assigned to Caltech,
Patent No. 4,885,142, December 1989.
Schroeder, J. E.; and Anderson, H. U.: U. S. Patent No. 4,957,673, assigned to the California
Institute of Technology, September 1990.
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Oxygen production using solid-state zirconia electrolyte technology

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