U.S. patent number RE28,792 [Application Number 05/454,021] was granted by the patent office on 1976-04-27 for electrochemical method for separating o.sub.2 from a gas; generating electricity; measuring o.sub.2 partial pressure; and fuel cell.
This patent grant is currently assigned to Westinghouse Electric Corporation. Invention is credited to Roswell J. Ruka, Joseph Weissbart.
United States Patent |
RE28,792 |
Ruka , et al. |
April 27, 1976 |
Electrochemical method for separating O.sub.2 from a gas;
generating electricity; measuring O.sub.2 partial pressure; and
fuel cell
Abstract
.Iadd.A fuel cell and method of operation of same is detailed
for measuring oxygen pressure, separating oxygen from a gas, or
generating electrical energy. The fuel cell has first and second
electrodes of selected materials with a solid partition of a solid
electrolyte between the electrodes. The solid electrolyte consists
of a solid solution of selected oxides having a high degree of
oxygen ion conductivity. .Iaddend.
Inventors: |
Ruka; Roswell J. (Pittsburgh,
PA), Weissbart; Joseph (Palo Alto, CA) |
Assignee: |
Westinghouse Electric
Corporation (Pittsburgh, PA)
|
Family
ID: |
27037310 |
Appl.
No.: |
05/454,021 |
Filed: |
March 22, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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126098 |
Jul 24, 1961 |
|
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Reissue of: |
534322 |
Mar 15, 1966 |
03400054 |
Sep 3, 1968 |
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Current U.S.
Class: |
429/410; 205/784;
204/426; 204/425; 204/427; 205/785; 429/430; 429/442; 429/496 |
Current CPC
Class: |
H01M
8/12 (20130101); G01N 27/4073 (20130101); H01M
14/00 (20130101); B01D 53/326 (20130101); Y02E
60/50 (20130101) |
Current International
Class: |
B01D
53/32 (20060101); H01M 14/00 (20060101); H01M
8/12 (20060101); H01M 008/12 (); C25B 001/02 () |
Field of
Search: |
;204/195S,129 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1,386,878 |
|
Dec 1964 |
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FR |
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1,039,412 |
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Aug 1966 |
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UK |
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Other References
H Steinmetz, "Development of a Continuous Meter for Oxygen in
Sodium," United Nuclear Corp., White Plains, N.Y., 8/1962, p. 11.
.
Kiukkola et al., "Measurements on Galvanic Cells Involving Solid
Electrolytes," in J. Elect. Chem. Soc., 6/1957, pp. 379-386. .
J. Weissbart et al., "Oxygen Gauge," in the Review of Scientific
Instruments, Vol. 32, No. 5, May 1961, pp. 593-595. .
Baur et al.: "Uber Brennstoff-Ketten Mit Festleitern," in
Zeitschrift Electrochemie Bd. 43, NR. 9, 1937, pp.
728-732..
|
Primary Examiner: Curtis; Allen B.
Attorney, Agent or Firm: Lynch; M. P.
Parent Case Text
This application is a continuation-in-part of application Ser. No.
126,098, filed July 24, 1961, now abandoned.
Claims
What is claimed is:
1. A fuel cell comprising: a solid partition of a solid electrolyte
consisting essentially of a solid solution of oxides having a high
degree of oxygen ion conductivity compared with either electronic
conductivity or cation conductivity, said solid electrolyte having
the formula
where M represents at least one tetravalent element forming an
oxide highly stable at temperatures of about 500.degree.C to about
1200.degree.C selected from the group consisting of zirconium,
thorium and hafnium, R represents at least one element from the
group consisting of elements of Groups II-A and III-B of the
Periodic Table which form cations with stable +2 and +3 valences in
the oxide, x represents a number having a value from about 0.05 to
about 0.3 and y and z represent numbers having values necessary to
make (R.sub.y O.sub.z) electrically neutral; a first porous
electrode consisting of lanthanum nickel oxide applied to a first
face of said partition; a second porous electrode consisting of a
mixture of nickel and platinum applied to the opposite face of said
partition; said electrodes being stable at temperatures of about
500.degree. C to about 1200.degree. C; .[.mean.]. .Iadd.means
.Iaddend.for supplying gas containing oxygen to said first
electrode; means for supplying a fuel capable of reacting with
oxygen on the opposite side of said partition near to said second
electrode; means for heating said electrodes to an elevated
temperature of about 500.degree. C to 1200.degree. C whereby oxygen
forms ions at said first electrode which migrate through said
partition upon placing it in a closed circuit and react with said
fuel at the second electrode, said electrodes thereby developing a
potential difference; and means for withdrawing the reaction
products from near said second electrode.
2. A fuel cell as in claim 1 wherein: R represents at least one
element from the group consisting of calcium, barium, strontium,
yttrium and lanthanum.
3. A fuel cell as in claim 1 wherein: said solid electrolyte has
the formula (ZrO.sub.2).sub.1-x (CaO).sub.x where x represents a
number having a value from about 0.1 to about 0.3.
4. A method for separating oxygen from a gas containing oxygen or
for producing substantially pure oxygen comprising: supplying a gas
containing oxygen to a first electrically conductive porous
electrode of an electrochemical cell comprising a solid electrolyte
consisting essentially of a solid solution of oxides having a high
degree of oxygen ion conductivity compared with either electronic
conductivity or cation conductivity, said solid electrolyte having
the formula (MO.sub.2).sub.1-x (R.sub.y O.sub.z).sub.x where M
represents at least one tetravalent element from the group
consisting of zirconium, thorium, valent element from the group
consisting of zirconium, thorium, and hafnium, R represents at
least one element from the group consisting of elements of Groups
II-A and III-B of the Periodic Table which form cations with stable
+2 and +3 valences in the oxide, x represents a number having a
value of from about .[.0.05.]. .Iadd.0.1 .Iaddend.to about 0.3 and
y and z represent numbers having values sufficient to make R.sub.y
O.sub.z electrically neutral; said first electrically conductive
porous electrode and a second electrically conductive porous
electrode being disposed on opposite surfaces of said solid
electrolyte; heating said electrolyte and electrodes, while
supplying said gas, to a temperature within the range of from about
500.degree.C to 1200.degree.C; applying, while supplying said gas
and heating said electrolyte and electrodes, a DC potential
difference across said electrodes of a polarity such that said
first electrode is negative with respect to said second electrode
so that substantially pure oxygen is produced at said second
electrode.
5. A method as in claim 4 wherein: said gas containing oxygen
contains said oxygen in molecular form. .[.6. A method as in claim
4 wherein: x
represents a number having a value from about 0.1 to about 0.3..].
7. A method as in claim .[.6.]. .Iadd.4 .Iaddend.wherein R
represents at least one element from the group consisting of
calcium, barium, strontium, yttrium and lanthanum; and said gas
containing oxygen contains said oxygen
in molecular form. 8. A method for generating electrical energy
comprising: establishing unequal oxygen partial pressures at first
and second electrically conductive porous electrodes of an
electrochemical cell comprising said electrodes on opposite
surfaces of a solid electrolyte consisting essentially of a solid
solution of oxides having a high degree of oxygen ion conductivity
compared with either electronic conductivity or cation
conductivity; said solid electrolyte having the formula
(MO.sub.2).sub.1-x (R.sub.y O.sub.z).sub.x where M represents at
least one tetravalent element from the group consisting of
zirconium, thorium and hafnium, R represents at least one element
from the group consisting of elements of Groups II-A and III-B of
the Periodic Table which form cations with stable +2 and +3
valences in the oxide, x represents a number having a value of from
about .[.0.05.]. .Iadd. 0.1 .Iaddend.to about 0.3 and y and z
represent numbers having values sufficient to make R.sub.y O.sub.z
electrically neutral; said electrodes being electrically connected
across a load in an external circuit; excluding oxidizable fuel
from said electrodes to develop electrical power across said load
by reason of said unequal oxygen partial pressures at said
electrodes. .[.9. A method as in claim 8 wherein: x represents
a
number having a value of from about 0.1 to about 0.3..]. 10. A
method as in claim .[.9.]. .Iadd.8 .Iaddend.wherein: R represents
at least one element from the group consisting of calcium, barium,
strontium, yttrium
and lanthanum. 11. A method as in claim .[.9.]. .Iadd.8
.Iaddend.wherein: the establishing of unequal oxygen partial
pressures and the excluding of oxidizable fuel are effected by
providing an enclosure completely enclosing said electrolyte and
said electrodes with said enclosure containing oxygen gas and a
compressor means disposed therein to maintain said partial
pressures unequal. .[.12. A method for the measurement of oxygen
pressure comprising: establishing and maintaining an oxygen partial
pressure of known value at a first electrically conductive porous
electrode of an electrochemical cell including a solid electrolyte
consisting essentially of a solid solution of oxides having a high
degree of oxygen ion conductivity compared with either electronic
conductivity or cation conductivity, said solid electrolyte has the
formula (MO.sub.2).sub.1-x (R.sub.y O.sub.z).sub.x where M
represents at least one tetravalent element from the group
consisting of zirconium, thorium, and hafnium, R represents at
least one element from the group consisting of elements of Groups
II-A and III-B of the Periodic Table which form cations with stable
+2 and +3 valences in the oxide, x represents a number having a
value of from about 0.05 to about 0.3 and y and z represent numbers
having values sufficient to make R.sub.y O.sub.z electricallly
neutral, said first electrically conductive porous electrode and a
second electrically conductive porous electrode being disposed on
opposite surfaces of said solid electrolyte; supplying a gas
containing an unknown oxygen partial pressure to said second
electrode; measuring an electrical output across said electrodes as
a measure of said unknown oxygen partial
pressure..]. 13. A method .[.as in claim 12 wherein:.]. .Iadd.for
the measurement of oxygen pressure comprising: establishing and
maintaining an oxygen partial pressure of known value at a first
electrically conductive porous electrode of an electrochemical cell
including a solid electrolyte consisting essentially of a solid
solution of oxides having a high degree of oxygen ion conductivity
compared with either electronic conductivity or cation
conductivity, said solid electrolyte has the formula
(MO.sub.2).sub.1-x (R.sub.y O.sub.z).sub.x where M represents at
least one tetravalent element from the group consisting of
zirconium, thorium and hafnium, R represents at least one element
from the group consisting of elements of Groups II-A and III-B of
the Periodic Table which form cations with +2 and +3 valences in
the oxide, .Iaddend.x represents a number having a value of from
about 0.1 to about 0.3 .Iadd.and y and z represent numbers having
values sufficient to make R.sub.y O.sub. z electrically neutral,
said first electrically conductive porous electrode and a second
electrically porous electrode being disposed on opposite surfaces
of said solid electrolyte; supplying a gas containing an unknown
oxygen partial pressure to said second electrode; measuring an
electrical output across said electrodes as a measure of said
unknown oxygen partial pressure.
.Iaddend. 14. A method as in claim 13 wherein: R represents at
least one element of the group consisting of calcium, barium,
strontium, yttrium and lanthanum; and further comprising measuring
the cell temperature, while measuring said electrical output, so
that electrical output variations due to temperature varitions can
be accounted for.
Description
This invention relates generally to electrochemical devices, and
particularly fuel cells, for conversion between chemical and
electrical forms of energy and, more particularly, to those having
a solid electrolyte through which oxygen ions are conducted.
The present invention will be discussed principally in connection
with fuel cells because it is a typical application of the
invention which is of considerable interest. It should be
particularly noted, however, that the invention in its broader
aspects is applicable to electrochemical devices for other purposes
including electrical energy generation as part of a heat engine,
the separation of oxygen from a gas mixture such as air and the
measurement of oxygen pressure.
In general, a fuel cell is an electrochemical cell to which a fuel
and an oxidant are supplied and permitted to react in a manner
resulting in the direct production of useful electrical energy. The
reactants and the reaction products are supplied and removed
continuously. For practical levels of power generation it is, of
course, desirable that low cost materials be used such as air for
the oxidant and carbonaceous gases or coal for the fuel. Therefore,
a fuel cell is limited, not by necessity but as a matter of
practice, to those capable of operating on conventional fuels.
Conventional fuels are today the fossil fuels, naturally occurring
gaseous hydrocarbons and fuels, easily derived from them, such as
hydrogen and carbon monoxide.
Fuel cells and batteries are each a class of devices for the
conversion of chemical energy to electrical energy. They are
distinct, however, as the terms are currently used in the art. A
battery is a device which itself stores chemical energy and
converts it to electrical energy on demand. A primary battery is
one that is used for a single energy discharge only while a
secondary battery is one in which the products of the chemical
reaction are converted back to the original chemical reactants by
electrical charging. On the other hand, a fuel cell is merely an
energy converter; it is not a source of stored energy. A fuel cell
is operated by having reactants supplied continually to it and
ideally its life is not affected by the chemical reaction per
se.
Fuel cells are not limited in potential application merely to small
power sources but are also of interest for large scale power
generation because they are not subject to Carnot cycle limitations
and hence offer the possibility of a higher conversion efficiency
than such devices as steam turbine generators.
Many investigators believe that cells having molten salt
electrolytes and operating at about 500.degree. C. to 800.degree.
C. are the most promising for low cost power generation. However,
problems have been encountered with devices of this type, such as
difficulty in adequately containing the electrolyte, corrosion of
the electrodes, gas leakage and the maintenance of the activity of
the catalysts at the electrodes to facilitate or speed up the
electrochemical reactions taking place there. In addition, the gas
pressure at the electrodes must be regulated so that "cell
drowning" and "cell gassing" do not occur. In cell drowning, the
liquid electrolyte is forced into the electrode pores by capillary
action and in cell gassing the gas forces the electrolyte out of
the pores. In both cases, the three phase electrode-electrolyte-gas
reaction zone necessary for efficient cell operation is destroyed.
Such problems have been so serious that only small cells have been
built which require an external heat source to maintain them at
operating temperature.
Another type of cell is that which utilizes a solid electrolyte and
operates at generally higher temperatures. Heretofore, such cells
have been characterized by their very low current density resulting
from a high internal resistance, corrosion problems and
irreversible changes in the electrolyte during operation. This has
led to the general feeling that such devices will not reach a
practical level of operation.
As a result of our investigations into solid electrolyte fuel
cells, it was found that the problems reported by prior
investigators are not applicable to all solid electrolytes under
all conditions and that certain high temperature devices show
promise of practical power generation.
It is, therefore, an object of the present invention to provide an
improved electrochemical device having a solid electrolyte and
which is suitable for fuel cells and other applications.
Another object is to provide a solid electrolyte fuel cell whose
electrolyte and electrodes are quite stable and do not deteriorate
in operation.
Another object is to provide a practical fuel cell which avoids
problems of electrolyte containment, gas leakage and cell drowning
and gassing, and reduces catalysts problems.
Another object is to provide an electrochemical device for power
generation which can utilize any heat source.
Another object is to provide an electrochemical device for the
separation of oxygen from a gas mixture.
Another object is to provide an electrochemical device for the
measurement of oxygen pressure.
In accordance with the most general aspects of the present
invention, there is provided an electrochemical cell which has a
solid electrolyte consisting essentially of a solid solution of
ionically conductive oxides. The electrolyte can be represented by
the formula:
where M represents at leasat one tetravalent element from the group
consisting of zirconium, thorium, and hafnium, R represents at
least one element from the group consisting of elements which form
cations with stable +2 and +3 valences in the oxide, x represents a
number having a value of from about 0.05 to about 0.3 and y and z
represent numbers having values sufficient to make R.sub.y O.sub.z
electrically neutral. Examples of elements which are suitable for
the element R are calcium, barium, strontium, yttrium, lanthanum,
scandium, ytterbium and samarium which form the oxides CaO, BaO,
SrO, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Sc.sub.2 O.sub.3, Yb.sub.2
O.sub.3 and Sm.sub.2 O.sub.3, respectively. Porous conductive
layers serving as electrodes are disposed on opposite sides of the
electrolyte and are selected of material which is stable under the
operating conditions of the device.
According to other features of the invention, an electrochemical
cell having the general structure just described is provided for
operation under various particular operating conditions for various
purposes. The devices are operated at temperatures of about
500.degree. C. to about 1200.degree. C.
In one form of the invention, a fuel is provided at one electrode
and a gas containing molecular oxygen such as air or pure oxygen at
the other. The electrodes are electrically connected across a load
for the development of the electrical energy. In another form, no
fuel is used at the electrodes but rather the existence of oxygen
at different pressures at the two electrodes permits conversion to
electrical energy to by operating the cell in a heat engine.
Embodiments are also disclosed for purposes of separating oxygen
from a gas mixture and for measuring oxygen pressure.
The invention, both as to its physical arrangement and its manner
of operation, together with the above-mentioned and further objects
and advantages thereof may best be understood by reference to the
following description taken in connection with the accompanying
drawings, in which:
FIG. 1 is a partial cross-sectional view of a fuel cell in
accordance with this invention;
FIG. 2 is a cross-sectional view of a fuel cell made in accordance
with this invention;
FIG. 3 is a curve of voltage against current for typical fuel cell
such as that of FIG. 2;
FIG. 4 is a cross-sectional view in schematic form of an electrical
energy producing system utilizing an electrochemical cell with
oxygen at different pressures on opposite sides of the solid
electrolyte;
FIG. 5 is a curve of voltage against current for a typical cell in
a system like that of FIG. 4;
FIG. 6 is a cross-sectional view of an electro-chemical cell
utilized as a device for separating oxygen from a gas mixture;
FIG. 7 is a cross-sectional view of an electro-chemical cell
utilized as a device for measuring oxygen pressure; and
FIG. 8 is a cross-sectional view of a plurality of electrochemical
cells joined in a unitary structure from which a single output may
be obtained.
Referring now to FIG. 1 there is shown for purposes of explanation
a schematic view of a portion of a solid electrolyte fuel cell. The
cell consists essentially of a partition comprising a solid
electrolyte 10 on one side of which there is a first electrode 12
(cathode) and on the other side is a second electrode 14 (anode).
The electrodes 12 and 14 are porous to an extent that gas may
readily diffuse through them. To the outer side of the first
electrode 12 there is supplied oxygen or an oxygen containing gas
such as air and to the outer side of second electrode 14 there is
supplied an oxidizable fuel which may be either a solid, such as
carbon, or a gas, such as carbon monoxide or hydrogen.
On the first side oxygen is electrochemically reduced at the
interface between the electrode 12 and the electrolyte 10 according
to the reaction O.sub.2 +4 electrons.fwdarw.20=, the electrons
being supplied by the conductive electrode 12 and the oxygen ions
become a part of the electrolyte crystal structure. The ions
migrate through the solid electrolyte 10 and at the second
electrode 14 they are electrochemically oxidized in the presence of
a fuel by a reaction such as 20=+2H.sub.2 .fwdarw.2H.sub.2 O+4
electrons, where hydrogen is the fuel. Hence, an oxidized fuel is
produced which is carried away and electrons are released in the
second electrode 14. The electrons flow through an external circuit
comprising a lead 16 from the second electrode 14 to a load 18 and
then through another lead 20 to the first electrode 12.
The electrochemical reaction of oxygen with a fuel produce a
potential difference across external load 18 which maintains a
continuous electron and oxygen ion flow in the closed circuit and
useful power can be derived.
Among the requirements for the solid electrolyte 10 is that it have
a high ratio of ionic conductivity to electronic conductivity. This
is necessary so that the potential difference between the
electrodes is not reduced by simultaneous diffusion of ions and
electrons. Cation conductivity must be very low to prevent
destruction of the electrode-electrolyte interface. Furthermore,
the electrolyte 10 must act as a barrier to the fuel and to oxygen
gas so that direct reaction of the fuel with the oxygen does not
occur.
The material making up the solid electrolyte 10 consists
essentially of a solid solution of oxides having a high degree of
oxygen ion conductivity compared with either electronic
conductivity or cation conductivity. The electrolyte can be
represented by the general formula:
where M represents at least one tetravalent element from the group
consisting of zirconium, thorium and hafnium, R represents at least
one element from the group consisting of the elements which form
cations with stable +2 and +3 valences in the oxide, x represents a
number having a value of from about 0.05 to about 0.3 and y and z
represent numbers having values sufficient to make R.sub.y O.sub.z
electrically neutral. Examples of elements which are suitable for
the element R are calcium, barium, strontium, yttrium, lanthanum,
scandium, ytterbium and samarium which form the oxides CaO, BaO,
SrO, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Sc.sub.2 O.sub.3, Yb.sub.2
O.sub.3 and Sm.sub.2 O.sub.3, respectively. The first three oxides
being of elements of Group II-A and the remaining five oxides being
of elements of Group III-B of the Periodic Table as shown on pages
400 and 401 of the Handbook of Chemistry and Physics, 39th edition,
and published by Chemical Rubber Publishing Company of Cleveland,
Ohio. It should be particularly noted that mixtures of the
designated oxides, both those making up MO.sub.2 and R.sub.y
O.sub.z, are suitable such as a mixture of zirconium and thorium
oxides for (MO.sub.2) and a mixture of calcium and yttrium oxides
for (R.sub.y O.sub.z). The quantity x retains the same range of
values.
By a stable valence, it is meant that the element R remains in a
single valence state in the oxide over an appreciable range of
conditions, particularly in the oxidizing and reducing atmospheres
existing at the cathode and anode, respectively, at the temperature
of operation.
The optimum value of x, the mole fraction of oxide R.sub.y O.sub.z,
is that for which cell resistivity is lowest and conduction is by
oxygen ions. Present information indicates this optimum value is
about 0.13 to about 0.15 for (ZrO.sub.2).sub.1-x (CaO).sub.x.
Conductivity studies to date indicate the optimum value of x varies
for different elements R. For example, the work of Strickler and
Carlson, reported in part in J. Am. Ceram. Soc., 48, 286 to 289,
June 1965, indicates that the maximum conductivity of some binary
mixed oxide compositions (wherein element M in the above formula is
Zr) occurs near the cubic-monocolinic phase boundary. This
indicates the following optimum values of x for some elements R: R
R.sub.y O.sub.z X ______________________________________ Sc
Sc.sub.2 O.sub.3 0.06 Yb Yb.sub.2 O.sub.3 0.07 Y Y.sub.2 O.sub.3
0.09 Sm Sm.sub.2 O.sub.3 0.10
______________________________________
It also appears from present information that conductivity is
greater for compositions where the optimum x value is small
compared with compositions where the optimum x value is large. For
example,
is about twice as conductive as (ZrO.sub.2).sub..91 (Y.sub.2
O.sub.3).sub..09.
The second oxide (R.sub.y O.sub.z) increases the ionic conductivity
of the structure and also tends, incidentally, at least where the
first oxide is ZrO.sub.2, to place the structure in cubic or
distorted cubic phase. Thus, such structures are sometimes referred
to as stabilized zirconia. The same is believed true of
compositions containing hafnia. Thoria is in the cubic form without
an additional oxide.
The solid electrolyte 10 preferably has a thickness of about 1.5
mm. or less. It is not desirable for the electrolyte to be thicker
because the internal resistance is then increased. The electrolyte
should be as thin as it can be fabricated consistent with necessary
mechanical strength and ability to permit only oxygen ions to pass
through it. To prevent diffusion of molecular gases through the
electrolyte, is also necessary that the material be relatively
dense or at least have no interconnecting pores.
The electrolyte material is selected for its high ratio of ionic
conductivity to electronic conductivity, a high ratio of oxygen ion
to cation conductivity and its chemical stability. By cation
conductivity is meant the conductivity of the positive ions of the
elements M and R in the above formula. It has been found that at
relatively high temperatures the above materials have considerably
greater ionic than electronic conductivity and the ionic
conductivity is relatively high. For example, experiments with
(ZrO.sub.2).sub..85 (CaO).sub..15 at 1000.degree. C. have shown
that ionic conductivity comprises more than 98% of the total
conductivity. The high oxygen ion conduction in structures of this
type is believed to result in the following manner, which is
presented merely by way of explanation since an understanding of
the conduction process is not essential to the practice of the
invention. Considering (ZrO.sub.2).sub.1-x (CaO).sub.x as an
example, all of the Ca.sup..sup.+2 ions entering the lattice
replace Zr.sup..sup.+4 ions in the lattice. An equivalent number of
oxygen vacancies are thereby formed in the oxygen lattice in order
to preserve electrical neutrality of the crystal. The oxygen
vacancies are distributed throughout the lattice and hence provide
a path for oxygen ion migration. It has been found suitable to
employ an electrolyte having a fluorite-like crystal structure.
However, other crystalline structures such as pervoskite and
pyrochlorite may be used. Material suitable for use as the solid
electrolyte is commercially available and methods of making it are
well known. For example, (ZrO.sub.2).sub.1-x (CaO).sub.x, commonly
known as calcium stabilized zirconia is widely available in dense
forms for the general uses of a ceramic material. The electrolyte
should be substantially free of impurities which may increase the
electronic and cationic conductivity of it.
The ionic conductivity is found to be temperature dependent and is
greater at higher temperatures. Accordingly, it is desirable to
operate at high temperatures, particularly about 800.degree. C. and
above in the case of fuel cells. At extremely high temperatures
certain factors become significant which offset the increase in
conductivity. One of these factors is the chemical stability of the
electrodes which imposes a practical high temperature limit of
about 1200.degree. C.
The selection of suitable electrodes must be done carefully to
ensure good low resistance contact to the electrolyte, high
temperature chemical stability, sufficient conductivity, porosity
and a coefficient of thermal expansion compatible with the solid
electrolyte.
Among the electrode materials which have been found suitable are,
for the second electrode 14, a mixture of nickel and platinum
formed by placing nickel pellets of 50 mesh size wetted with
chloroplatinic acid solution in contact with the electrolyte and
heating to about 1100 C. For the first electrode 12, an electrode
can be used of a mixed meal oxide such as lanthanum nickel oxide
(LaNiO.sub.3) or calcium lanthanum manganese oxide (Ca.sub.x
La.sub.y MnO.sub.3). Such an electrode may be made by spraying with
a plasma jet. One or more metals of the platinum group comprising
platinum, palladium, rhodium and iridium and their alloys are also
suitable for use as either of the electrodes 12 and 14. Methods of
forming such a layer of such material suitable for use as an
electrode are well known. For example, a paste comprising the
powdered metal, such as platinum, in an organic binder can be
applied to the electrolyte and heated to remove the binder and fuse
the platinum to the electrolyte. It of course simplifies the
fabrication procedure if the electrodes 12 and 14 are each of the
same material, such as those of the platinum group. Furthermore,
such a symmetrical arrangement makes it possible to reverse the
fuel and oxygen supply to the electrodes and thus minimize the
effect of small cation contributions to the oxygen ion
conduction.
The high temperature necessary in order to have good oxygen ion
conductivity is ideally maintained as a result of the theoretical
heat losses associated with the electrochemical reaction as well as
i.sup.2 R losses within the cell. Initially of course some other
means must be provided to heat the cell to temperature, for
example, direct mixing of the oxidant and fuel. In a device of
sufficiently large electrode are compactly arranged, heat generated
within the system will be sufficiently large to maintain the cell
at the operating temperature. The fact that a high operating
temperature is required is not necessarily a disadvantage, for
example the catalytic problems are minimized. In applications such
as torpedoes, for example, batteries may overheat and be damaged
due to high current drain. A fuel cell as described here would take
advantage of such operating conditions.
Referring now to FIG. 2, there is shown a fuel cell in accordance
with the present invention. The fuel cell comprises a partition of
a solid electrolyte 10 as was above described having first and
second electrodes 12 and 14 on opposite sides thereof. This basic
structure is contained within a thermally insulated enclosure 22
which is initially heated to the necessary operating temperature by
any suitable heat source.
On one side of the solid electrolyte 10 there are provided an inlet
means 24 for oxygen or an oxygen containing gas and outlet means 26
for that portion of the oxidant which is not used by the cell. On
the other side of the electrolyte 10 there are similarly provided
input means 28 for a gaseous fuel and an outlet means 30 for unused
fuel and oxidized fuel. Electrical leads 20 and 16 are provided in
conductive contact with the first and second electrodes as in FIG.
1.
The device of FIG. 2 is designed for use with a gaseous fuel which
may be, for example, a mixture of hydrogen with a small amount of
water vapor as well as others previously mentioned.
In FIG. 3 there are shown typical performance curves for a device
as shown in FIG. 2. Each curve was obtained with a device using an
electrolyte 10 of
(ZrO.sub.2).sub..9 (CaO).sub..1
The oxidant was pure oxygen.
Curve 40 is derived from a device using an electrode 12 of
LaNiO.sub.3 on the oxygen side. The electrode 14 on the fuel side
was a platinum-nickel mixture. The fuel was H.sub.2 and H.sub.2 O
at a pressure ratio of about 28, the operating temperature was
1074.degree. C., the electrode areas were each 3 square centimeters
and the electrolyte thickness was 1.5 mm.
Curve 42 is derived from a device also using an electrode of
LaNiO.sub.3 on the oxygen side with an effective cell area of 3
square centimeters and an electrolyte 1.5 mm. thick but with an
electrode on the fuel side of Pt, a fuel of CO and CO.sub.2 at a
pressure ratio of the about 1 and an operating temperature of
1197.degree. C.
Curve 44 was derived from a device having electrodes of Pt on both
sides, an effective cell area of 2.27 square centimeters, an
electrolyte thickness of 0.9 mm., a fuel of H.sub.2 and H.sub.2 O
at a pressure ratio of about 30, i.e. 30 millimeters of hydrogen
per millimeter of water vapor, and operating temperature of
1050.degree. C.
The curves 40, 42 and 44 indicate that an electrochemical fuel cell
in accordance with the present invention can achieve relatively
high current densities.
Cells as described have been operated continuously for periods as
long as 625 hours at which time little deterioration was evident.
Much longer life is apparently possible. The curves 40, 42 and 44
were taken with experimental devices which due to their small size
had to be heated by an independent source but which can be scaled
up to adequate size for self-sustained operation.
Other useful electrochemical devices besides fuel cells have been
developed using as the solid electrolyte a material as above
described. Referring now to FIG. 4 there is shown a schematic of an
electrochemical device for the conversion of heat to electrical
energy. As contrasted with the device of FIG. 2, the device of FIG.
4 uses no conventional fuel at the electrodes. Instead of operating
by reason of oxygen at one side and such a fuel at the other, the
device of FIG. 4 operates by reason of there being oxygen on both
sides of the solid electrolyte 10 but at different partial pressure
so that there will be a potential different at the electrodes. The
device of FIG. 4 is shown as a closed system in which oxygen
continually circulates.
The solid electrolyte 10 is disposed with first and second
electrodes 12 and 14 on either side, each of which has a conductive
lead 16 and 20 extending therefrom so that power may be generated
across a load 18. The electrolyte 10 and electrodes 12 and 14 are
enclosed with a furnace 46 or are in some manner provided so that
they are heated to the necessary operating temperature for the
device. Some suitable heat source is necessary throughout operation
since the oxygen transfer process is endothermic. Besides ordinary
heat sources such sources as jet engine exhausts and nuclear
reactors can be employed. External to the furnace region and hence
at a lower temperature is means 48 for compressing the oxygen gas.
Such an oxygen compressor 48 may be for example any of the well
known mechanical devices for compressing gases. Oxygen passing
through the compressor 48 is compressed to the higher pressure
existing on the side of the solid electrolyte 10 having the first
electrode 12. As a result of the higher pressure there existing a
potential difference is developed between the electrodes which
causes electrons released at the anode 14 to pass through the
external electrical circuit back to the cathode 12. At the same
time a current of oxygen ions passes through the electrolyte to the
second electrode 14, or from right to left in the drawing, with
electrons being released at the anode resulting in the oxidation of
oxygen ions and the release of oxygen gas which recirculates
through the compressor.
The pertinent reaction at the cathode 12 is therefore, O.sub.2 +4
electrons.fwdarw.20=and at the anode 14 it is 20=.fwdarw.O.sub.2 +4
electrons.
It is of course desirable that the device of FIG. 4 be operable
with a net output power, that is, the oxygen compressor 48 must be
such as to consume less power than that produced by the
electrochemical cell.
Another means for maintaining an oxygen pressure difference at the
two electrodes 12 and 14 of the cell is by the use of a metal or
metal oxide or possibly a molecular sieve type material capable of
reacting with oxygen formed at the low pressure chamber and then
being decomposed at a higher temperature to build up the oxygen
pressure at the cathode 12.
A performance curve 55 obtained for the electrochemical cell
portion of a heat engine device similar to that shown in FIG. 4 is
shown in FIG. 5. Curve 55 was obtained by maintaining an oxygen
partial pressure of 726.2 torr at the cathode and 152.5 torr
pressure at the anode and operating at a temperature of about
1024.degree. C. The cell electrolyte had the composition (Z.sub.F
O.sub.2).sub..85 (CaO).sub..15 and the electrodes on each side were
of platinum. The experimental open circuit voltage agrees closely
with the theoretical EMF of an oxygen concentration cell as
calculated from the theoretical relation
EMF=4.96.times.10.sup..sup.-5 ##EQU1## where T is the temperature
in .degree.K, cP.sub.02 is the pressure at the cathode and
aP.sub.02 is the pressure at the anode.
In FIG. 6 is shown another device using the solid electrolyte and
porous electrode structure as previously described but in a
cylindrical configuration which may also be used in the previously
described devices. Here, however, this structure is utilized as an
electrolytic oxygen concentration cell for the purpose of
separating oxygen from a mixture of gases such as air rather than
using the cell as a galvanic device as in the previous embodiments.
For the operation of the device, it is necessary that a DC power
source 54 be provided when the oxygen partial pressure on opposite
sides of the electrolyte is the same or if the oxygen is at a
higher partial pressure at the pure oxygen electrode 14. Even with
a higher oxygen partial pressure at the impure oxygen electrode 12
a power supply 54 is desirable in order to transfer oxygen through
the cell more rapidly. The nature of the electrolyte 10 is such
that gases other than oxygen cannot be transferred electrolytically
through it. As a result, the gas obtained at the second electrode
14 is substantially pure oxygen while that discharge from the first
electrode 12 is a nitrogen rich fraction of air.
This device is useful in any application where it is desirable to
separate oxygen and provides an alternative for low temperature
fractionation processes. It may also be used for pressurizing
oxygen. It may be operated with a device as shown in FIG. 4 used as
the power source 54. Configurations are also obvious wherein a
device like that of FIG. 6 is used to pump oxygen from a lower to a
higher pressure for the operation of the device of FIG. 4, so long
as the compressor cell operates at a lower temperature.
Another useful device utilizing the structure of the solid
electrolyte 10 and porous electrodes 12 and 14 is that of FIG. 7
which is a device for the measurement of oxygen pressure. Here the
oxygen on the cathode side 12 is maintained at a known pressure
within the sealed portion 90. The pressure of the oxygen on the
other side is to be measured and is provided with a tubulation 92
to communicate with an oxygen gas containing system. The pressure
is measured by measuring the potential difference between the
electrodes 12 and 14 by leads 93 and 94. That is, the device of
FIG. 7 operates substantially as that of the electrochemical cell
portion of FIG. 4, however, rather than drawing current for the
purpose of power generation in FIG. 7 the EMF is measured as a
measure of oxygen pressure since there is a direct relation between
the EMF established between the electrodes and the oxygen pressure
at the electrodes 12 and 14. Since the EMF also varies with
temperature, it is desirable to use a temperature indicator such as
a thermocouple 95 at the cell. It should be noted that the oxygen
at unknown pressure existing on the anode side is assumed lower
than that on the cathode, otherwise the polarity of the EMF is
reversed. Also, the oxygen on either side may exist in a gas
mxiture without operation being substantially affected since
mixture the oxygen partial pressure ratio determines the EMF
established between the electrodes.
In order to generate useful amounts of power with devices as
described herein the useful electrode area must be fairly large. It
is then desirable to form efficient and compact structures for
combining a plurality of individual cells in a unit providing a
single electrical output. Such a device is shown in FIG. 8 where
each of the individual cells 60 and 70 comprises a solid
electrolyte partition 62 and 72 with electrodes 64 and 65 and 74
and 75 on their opposite sides. Between the cells 60 and 70 there
is provided a conductive member 80 comprising a solid plate 82
non-porous to gases to prevent mixing of fuel and oxidant with
projections 83 and 84 on opposite sides in contact with the
electrodes 65 and 74, respectively. The member 80 is electronically
conductive. The projections 83 and 84 extend through gas channels
for supplying the fuel gas and the oxidant to the respective
electrodes and are not so large as to impair gas flow. It is of
course obvious that such a configuration is useful with any solid
electrolyte fuel cell design and is not necessarily limited to that
disclosed herein. Also, additional fuel cells may be incorporated
in the structure, each separated by an electronically conductive
member 80.
In this way the thermal losses due to leads extending from the
heated device is reduced since only one pair of external leads is
necessary. Also, electrical loss (i.sup.2 R) in the leads are
reduced since a plurality of contacts is provided in parallel
between adjacent cells.
While the present invention has been shown and described in certain
forms only, it will be obvious to those skilled in the art that it
is not so limited but is susceptible of various changes and
modifications without departing from the spirit and scope
thereof.
* * * * *