U.S. patent application number 10/897992 was filed with the patent office on 2005-03-17 for oxygen separation through hydroxide-conductive membrane.
Invention is credited to Chen, Muguo, Li, Lin-Feng, Yao, Wenbin.
Application Number | 20050058871 10/897992 |
Document ID | / |
Family ID | 23472550 |
Filed Date | 2005-03-17 |
United States Patent
Application |
20050058871 |
Kind Code |
A1 |
Li, Lin-Feng ; et
al. |
March 17, 2005 |
Oxygen separation through hydroxide-conductive membrane
Abstract
An electrochemical cell for separating a first gas from a
mixture of gas is provided, particularly for separating oxygen from
air. The cell includes a first electrode, a second electrode and a
hydroxide-conducting membrane between the first electrode and the
second electrode.
Inventors: |
Li, Lin-Feng;
(Croton-on-Hudson, NY) ; Yao, Wenbin; (Fort Lee,
NJ) ; Chen, Muguo; (Bedford Hills, NY) |
Correspondence
Address: |
Ralph J. Crispino
Reveo Inc.
85 Executive Blvd.
Elmsford
NY
10523
US
|
Family ID: |
23472550 |
Appl. No.: |
10/897992 |
Filed: |
July 23, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10897992 |
Jul 23, 2004 |
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09836119 |
Apr 17, 2001 |
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6767663 |
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09836119 |
Apr 17, 2001 |
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09373469 |
Aug 12, 1999 |
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6541159 |
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Current U.S.
Class: |
204/242 ;
204/270; 429/482; 429/495 |
Current CPC
Class: |
C01B 2210/0046 20130101;
B01D 67/0069 20130101; B01D 69/10 20130101; B01D 71/02 20130101;
B01D 67/0044 20130101; Y02P 30/30 20151101; B01D 71/022 20130101;
B01D 2325/26 20130101; B01D 67/0046 20130101; Y02P 30/00 20151101;
B01D 53/228 20130101; B01D 53/326 20130101; B01D 69/141 20130101;
C01B 13/0255 20130101; C01B 2210/0051 20130101 |
Class at
Publication: |
429/030 ;
204/270 |
International
Class: |
H01M 008/10; C25B
009/08 |
Claims
1. An electrochemical cell for separating a first gas from a
mixture of gas comprising: a first electrode; a second electrode;
and a hydroxide-conducting membrane between the first electrode and
the second electrode.
2-43. (Canceled)
Description
RELATED CASES
[0001] This is a Continuation-in-Part of copending application Ser.
No. 09/373,469 filed Aug. 12, 1999 entitled "Oxygen Separation
Through Hydroxide Conductive Membrane" by Lin-Feng Li, Wayne Yao
and Muguo Chen, which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to the field of gas separation, and
more particularly to gas separation using a hydroxide ion
conductive membrane.
[0004] 2. Description of the Prior Art
[0005] There is an ever-increasing need for improved systems and
methods for efficient and rapid separation of selective components
from mixtures. In particular, techniques for the separation of
selective components from gaseous mixtures have many significant
technical applications. Many important chemical, environmental,
medical and electronics processing technologies require pure gas,
particularly pure oxygen gas. For example, oxygen is used in
semiconductor fabrication for chemical vapor deposition, reactive
sputtering and reactive ion etching. It finds wide application in
health services, for resuscitation, or, in combination with other
chemicals, for anesthesia. Oxygen can also be used to achieve
environmental benefits by reducing the sulfur emissions of oil
refineries and helping pulp and paper manufacturers meet
regulations relating to bleaching, delignification and lime kiln
enrichment. However, the high cost of pure oxygen typically limits
the wide adoption of such beneficial processes in the chemical,
electronics and medical industries. Further, nitrogen is used, for
example, to protect perishable goods, to protect oxygen-sensitive
materials and to facilitate oxygen sensitive processes.
[0006] High-purity oxygen (e.g., 99.5%+) is largely produced in
cryogenic air separation plants, where the air is cooled down to
the melting point of nitrogen (-210.degree. C.) and its components
separated in large condensation columns. This process requires
expensive, bulky equipment and high-energy consumption, which tends
to militate against the use of oxygen to generate energy. Cryogenic
air separation is typically commercially feasible only on large
scale generation systems.
[0007] Adsorption, both pressure-swing absorption (PSA) and
vacuum-swing absorption (VSA), is widely used for producing
moderate purity oxygen (e.g., 85-90%). The gas separation is
generally based on selective adsorption of nitrogen by synthetic
zeolites. Nitrogen, which is more readily polarizable than oxygen,
interacts more strongly with the electrostatic fields in the
zeolites structure. Nitrogen is thus retained by the zeolites,
while oxygen passes through. When the zeolite is saturated with
nitrogen, it must be stripped, generally by reducing the
pressure.
[0008] Polymer membrane air separation has also been used to
produce air slightly enriched in oxygen (e.g., 28-35%) or for
nitrogen enriched blanketing (e.g., 95-99%). Polymeric air
separation membranes are typically selectively permeable, whereby
the polymeric materials are more permeable to oxygen than nitrogen.
Transport through the membrane is induced by maintaining the vapor
pressure on the permeate side of the membrane lower than the vapor
pressure of the feed mixture. The driving force is typically the
difference in partial vapor pressure of each species across the
membrane.
[0009] One typical configuration includes a system of hollow
fibers, wherein air is passed over the exterior of the fibers, and
enriched oxygen permeates to the interior of the fibers. Another
typical configuration used a spiral wound system, wherein air is
passed through an edge of a configuration of feed channels,
membranes and a permeate channel wound about a porous tube. The
permeate exits the system from the porous tube. Production of
relatively high purity oxygen with air separation based on
selective permeability is not commercially feasible, and is only
suitable for applications where only oxygen-enriched air is
required.
[0010] The permeability of a membrane for a particular gas species
is related to the volumetric flow (cubic centimeters (at standard
pressure and temperature) per second (cc(STP)/s)) of the desired
species through a unit length of membrane thickness (in
centimeters) per area of membrane (in centimeters squared) per
pressure differential (expressed in centimeters of mercury). The
permeability is generally expressed in Barrer units, where: 1 1
Barrer = 1 .times. 10 - 10 [ cc ( STP ) cm cm 2 s cm Hg ] . ( 1
)
[0011] Additionally, selectivity is critical to effective
permeability based gas separation, such that a desired species
permeates at a greater rate than another species. Selectivity
(alpha) is generally expressed as: 2 A , B = Permability A
Permability B . ( 2 )
[0012] A relatively new technology that has emerged for relatively
high purity gas separation involves selective membranes which pass
only the desired components, such as described by H. J. M.
Bouwmeester, A. J. Burggraaf, in "The CRC Handbook of Solid State
Electrochemistry," Ed. P. J. Gellings, H. J. M. Bouwmeester,
chapter 11, CRC Press, Boca Raton, 1997, which is incorporated by
reference herein. Various known membranes include ionic conducting
membranes and mixed ionic-electronic conducting membranes, which
rely on the transportation of oxide (O.sup.2-) ions to separate the
oxygen from air. Although these approaches may offer some
advantages relative to cryogenic oxygen separation, practical
application of the MIEC membrane is hindered by a number of
drawbacks intrinsic to oxide (O.sup.2-) conductive membranes. These
problems include: low oxygen throughput (typically caused by both
low ionic conductivity and low surface oxygen exchange rate);
relatively high operating temperature (>800.degree. C.); costly
materials and costly fabrication; tendency to degrade over time;
and system equipment that is relatively complex and expensive to
build and maintain.
[0013] Turning now to FIG. 1a, a general process that occurs in a
conventional ionic-electronic membrane 10 is shown. Air arrives at
a cathode 12, where oxygen is reduced, but other species do not
react. The oxygen is shuttled across a membrane 10 in the form of
an ion such as O.sup.2- (the process of ionic conduction). At an
anode 14, a complementary chemical reaction evolves pure oxygen,
which is released. Functionally, this type of membrane 10 has three
primary components: a backbone 16, which provides the membrane's
structure; an ionic conductor (not shown), which conducts the ions
across the membrane; and a catalyst (not shown), which aids the
reduction of oxygen at the cathode 12 and the evolution of oxygen
at the anode 14.
[0014] The overall oxygen throughput is determined primarily by two
parameters (the conductivity of the electrons is generally too fast
to be a limiting factor). The first is the ionic conductivity (how
fast the ions can travel across the membrane), which is dependent
on the electrolyte properties. The second is the surface oxygen
exchange rate (how quickly the oxygen is reduced and evolved on
each side), which is dependent on both the ionic conductor and the
catalyst properties of the electrodes.
[0015] Referring now to FIG. 1b, prior art mixed ionic-electronic
conducting membranes 110 typically utilize only a single component,
i.e., a ceramic-noble metal composite, to play all three functional
roles, namely, backbone, ion conduction and catalyst. This material
provides the physical membrane structure 116, reduces and evolves
the oxygen, and relays the O.sup.2- ions and charge-compensating
electrons in opposite directions as shown. To drive the reaction, a
pressure differential is required, with the air pressure at the
cathode 112 exceeding the oxygen pressure at the anode 114, i.e.,
by a factor of about 2 to 10.
[0016] As shown in FIG. 1c, a similar approach known as "active
oxygen pumping" utilizes a membrane 110 in combination with an
external circuit 118, instead of a pressure differential, to drive
the reaction.
[0017] As mentioned hereinabove, there are several disadvantages
associated with the current ceramic-based membranes (ionic
conducting and mixed ionic-electronic conducting) approach, which
are intrinsic to oxide-conducting membranes.
[0018] One such disadvantage is their relatively high operating
temperatures. The chemistry of the ceramic-noble metal membrane
material requires temperatures near 800.degree. C. for the anode
and cathode reactions, as well as the ionic conduction, to
proceed.
[0019] A further disadvantage of ceramic membranes is their
relatively high material and production cost. Since ceramic
membranes generally use the same material to conduct the oxide ions
and as the backbone material, restrictions are placed on the
possible materials that may be used. In particular, noble metals
such as platinum and palladium are generally required to obtain
desirable stability at required operating temperatures and to
promote the oxygen surface reaction, and these metals are
relatively expensive. Also, fabrication of the backbone structure
requires relatively strict and careful control to produce the
correct density and degree of mixing between the ceramic and the
metal. This tends to increase the expense of the process and lower
overall yield.
[0020] Another disadvantage of ceramic based membranes is
relatively low long-term stability under their operating
conditions. At the 800.degree. C. operating temperature of these
membranes, the ceramic and the noble metal tend to react with one
another, generating oxidization of the metal and a concomitant
degradation in performance through lower conductivity. This
instability generally renders such a system impractical for
large-scale applications.
[0021] An engineering problem associated with systems that
incorporate these high-temperature, high-pressure oxide-conducting
membranes involves sealing the membrane so that air cannot leak
past it. Any such leakage tends to disadvantageously lower the
purity of oxygen on the downstream side. This problem has been
addressed by welding the sides of the membrane to input and output
gas lines, but it is a relatively costly and troublesome
solution.
[0022] All of the above problems, which are intrinsic to the
ceramic-noble metal system, illustrate the undesirability of
current oxide-conducting membrane technology.
[0023] Additional ionic conducting membranes have been proposed
based on cation (i.e., proton) exchange membranes, which typically
utilize fluorocarbon-type resins (e.g., the Nafion.RTM. family of
resins which have sulfonic acid group functionality, commercially
available from DuPont Chemicals, Wilmington, Del.), precious metal
(e.g., Pt) anodes and air cathodes. In such systems, separation is
based on proton (i.e., hydrogen cation) conduction through the
membrane. Oxygen reduces at the cathode and forms water with a
provided electrical voltage and hydrogen cations in the membrane,
and evolves at the anode from water. Commercial realization of
cation exchange membrane based air separation systems is limited by
the costs of both the membrane and the catalysts materials.
[0024] Thus, a need exists for an oxygen separation method and
apparatus that addresses the problems associated with conventional
air separation techniques and systems.
SUMMARY OF THE INVENTION
[0025] The above-discussed and other problems and deficiencies of
the prior art are overcome or alleviated by the several methods and
apparatus of the present invention, wherein a hydroxide-conducting
material is utilized to separate gases. An important aspect is that
hydroxide ions (OH.sup.-), rather than oxide ions (O.sup.2-) or
protons, may be utilized to shuttle oxygen molecules through a
membrane at relatively high oxygen throughput. The hydroxide ion
generally has higher conductivity than the oxide ion at any given
temperature. Also, the surface oxygen exchange rate is higher in an
alkaline electrolyte than in oxide electrolytes and acidic
electrolytes (such as in the proton exchange membrane), especially
at low temperature.
[0026] The present invention provides, an electrochemical cell for
separating a first gas from a mixture of gas is provided,
particularly for separating oxygen from air. The cell includes a
first electrode, a second electrode and a hydroxide-conducting
membrane between the first electrode and the second electrode.
[0027] The above-discussed and other features and advantages of the
present invention will be appreciated and understood by those
skilled in the art from the following detailed description and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1a is a schematic representation of the principle of
oxygen separation from air through an inorganic, dense,
ion-conductive membrane of the prior art;
[0029] FIG. 1b is a schematic representation of the principle of
oxygen separation from air through an inorganic, dense, mixed
ionic-electronic conductive membrane of the prior art;
[0030] FIG. 1c is a schematic representation of the principle of
oxygen separation from air through an inorganic, dense, ionic
conductive membrane of the prior art, utilizing active oxygen
pumping;
[0031] FIG. 2 is a schematic diagram of an oxygen separation system
using a hydroxide-conductive membrane embodied herein;
[0032] FIG. 3 is a schematic diagram of another oxygen separation
system using a hydroxide-conductive membrane embodied herein;
[0033] FIG. 4 is a schematic diagram of an oxygen separation system
using the hydroxide-conducting membrane system of FIG. 2;
[0034] FIG. 5 is schematic representation, on an enlarged scale, of
a membrane assembly usable in the oxygen separation system of FIG.
4;
[0035] FIG. 6 is an example of a supporting and conducting
structure for a tested system using a hydroxide-conductive membrane
embodied herein;
[0036] FIG. 7 is a schematic of a system for separating oxygen from
air used in an exemplary embodiment; and
[0037] FIG. 8 is a sampling of sweep in an oxygen generation
process using an exemplary embodiment showing an oxygen peak
only.
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0038] An electrochemical cell for separating a first gas from a
mixture of gases is provided, particularly for separating oxygen
from air. The cell includes a first electrode, a second electrode
and a hydroxide-conducting electrolyte between the first electrode
and the second electrode. In preferred embodiments, the electrolyte
includes a supportive polymeric molecular structure and a
hydroxide-conducting medium.
[0039] Referring to the figures set forth in the accompanying
drawings, illustrative embodiments of the present invention will be
described in detail hereinbelow. For clarity of exposition, like
features shown in the accompanying drawings shall be indicated with
like reference numerals and similar features as shown in alternate
embodiments in the Drawings shall be indicated with similar
reference numerals.
[0040] Briefly described, the present invention includes a
hydroxide (OH.sup.-) conductive membrane that overcomes drawbacks
associated with oxide conductive membranes. The invention is based
in part upon our commonly assigned U.S. patent application Ser. No.
09/259,068, entitled "Solid Gel Membranes", filed on Feb. 26, 1999;
and U.S. Pat. No. 6,183,914, entitled "Polymer-Based Hydroxide
Conducting Membrane," filed on Sep. 17, 1998, both of which are
incorporated by reference herein. Compared to conventional mixed
ionic-electrical conductive membrane technology that operates at
high temperatures (>800.degree. C.), the membrane herein
provides sufficient ionic conductivity and surface oxygen exchange
rate at room temperature. Further, use of room temperature systems
allows for greater flexibility in selecting materials, has robust
construction, and provides reduced overall system cost due to
relatively low material cost and lower temperature operation.
[0041] The membrane used in systems described herein is a
hydroxide-conducting membrane. In general, the membrane is
macroscopically non-aqueous. In certain embodiments, systems
described herein employ membranes having physical characteristics
(e.g., porosity) capable of supporting a hydroxide source, such as
a gelatinous alkaline material. In other embodiments, systems
described herein employ polymer membranes having a composite of a
molecular structure and a hydroxide source, such as an aqueous
electrolyte. In further embodiments, systems described herein
employ anion exchange membranes.
[0042] An electrochemical cell is detailed, having a first
electrode (e.g., oxygen reducing electrode) and a second electrode
(e.g., oxygen evolving electrode) separated by a
hydroxide-conducting membrane. Upon application of a driving force
across the respective sides of the cell (e.g., an electrical
potential across the electrodes or a pressure differential between
the gas reducing side and the gas evolution side), hydroxide ions
are relayed from the oxygen reducing electrode to the oxygen
evolving electrode.
[0043] Turning now to FIG. 2, one embodiment of an electrochemical
cell 200 for gas separation is depicted. The cell 200 includes a
hydroxide-conducting membrane electrolyte 210, and a cathode 212
and an anode 214 disposed on opposite sides of the membrane 210.
The materials utilized for the cathode 212 and the anode 214 may
include oxygen reducing and evolving catalysts, respectively, as
will be discussed in greater detail hereinbelow. The membrane
comprises a hydroxide-conducting membrane such as a physically
supported hydroxide source, a molecularly supported hydroxide
source, or an anion exchange membrane, as will be discussed in
greater detail hereinbelow.
[0044] The cell 200 operates with an external current pumping
mechanism (i.e., circuit 218) such as shown and described
hereinabove with respect to FIG. 1c. In this manner, membrane 210
provides hydroxide conduction. At the cathode 212, O.sub.2
molecules are reduced into hydroxide ions (OH.sup.-) by electrons
transported from the anode 214 through the outside circuit.
Balancing the reaction, OH.sup.- is relayed through the electrolyte
to the anode 214 side, where it is reoxidized into O.sub.2 and
released, so the electrons can return to the cathode side.
Application of DC voltage of a required magnitude causes the
following reactions to occur at the electrodes:
At the cathode: 1/2O.sub.2+H.sub.2O+2e.sup.-.fwdarw.2OH.sup.-
(1)
At the anode: 2OH.sup.-.fwdarw.1/2O.sub.2+H.sub.2O+2e.sup.- (2)
[0045] In an alternative embodiment, the driving force is a
thermodynamic driving force, wherein the pressure of the oxygen on
the inlet side is greater than the pressure on the outlet side.
Electron transfer is accomplished by electrically connecting the
electrodes. Such a configuration is depicted in FIG. 3, wherein a
cell 300 is provided having a cathode 312 and an anode 314 in ionic
communication through a hydroxide-conducting membrane 310. The
anode 314 and the cathode 312 are electrically connected to one
another via a conductor 319. Alternatively, the electrical
connection between the anode 314 and the cathode 312 may be
facilitated by electrically conductive properties within or through
the membrane 310, such as a current collector or dispersion of
electrically conducting material in the case of a physically
supported hydroxide source, or an electrically conducting polymeric
structure or functionality in the case of a molecularly supported
hydroxide source or an anion exchange membrane. Advantageously,
unlike in conventional MIEC membranes, the ionic and electronic
conduction in the present invention are mediated by chemically
separate materials (the electrolyte and the conductor 319,
respectively).
[0046] In various alternatives of providing the electrical
conductive path, a conductor may be in the form of a mesh, porous
plate, metal foam, strip, wire, plate, or other suitable structure
embedded within the membrane between the electrodes. The current
collector is preferably porous to minimize electrolyte obstruction.
The conductor may be formed of various electrically conductive
materials including, but not limited to, copper, ferrous metals
such as stainless steel, nickel, chromium, titanium, and the like
and combinations and alloys comprising at least one of the
foregoing materials.
[0047] At the cathode 312, O.sub.2 molecules are reduced into
hydroxide ions (OH.sup.-) by electrons transported from the anode
314 through the conductor 319. Balancing the reaction, OH.sup.- is
relayed through the electrolyte to the anode 214 side, where it is
reoxidized into O.sub.2 and released, so the electrons can return
to the cathode side.
[0048] The completed cell 200 may be integrated into a system 250
such as shown schematically in FIGS. 4 and 5. A power source is
utilized to drive the oxygen generation process by passing an
electrical current across the electrodes 212, 214. Optionally, the
carbon dioxide content of the air provided to the cell 200 is
minimized. As CO.sub.2 may react with the base, reduced CO.sub.2
content is preferred so as to not adversely affect operation of the
membrane. For example, ambient air may optionally be fed through a
CO.sub.2-scrubber 256. The scrubber 254 is a conventional device,
such as one which utilizes limestone and which may be easily
regenerated as required by heating it off-line. Alternatively, an
air source may be provided having minimal CO.sub.2 content.
[0049] The air is then optionally fed to a water source 258,
generally to humidify the air to facilitate electrochemical
reaction in the system 200. The air is then fed into an oxygen
separation chamber 260.
[0050] As best shown in FIG. 5b, chamber 260 includes an air
chamber 262 and an oxygen chamber 264, with the cell 200 mounted
therebetween in a substantially air-tight (i.e., gas-tight) manner.
As shown, two O-ring-type gaskets 266 made of a sealing material
such as PTFE or natural or synthetic rubber, may be utilized to
provide such gas-tight mounting. Advantageously, the low
temperature operation of the cell 200 obviates the need for
relatively complicated and/or expensive welding or other sealing
techniques generally utilized to seal typical
high-operating-temperature ceramic membranes.
[0051] Oxygen emerging from chamber 264 is coupled to an oxygen
collection device (not shown). A vacuum pump (not shown) also may
optionally be coupled to flow path 257 through a valve 276 to
evacuate the oxygen chamber at the beginning of the operation. An
oxygen gas sensor 272 may be used to monitor the level of oxygen
within the oxygen chamber 264. The system 250 thus provides for
relatively simple and inexpensive oxygen production, which may
occur at ambient temperature and pressure conditions. Various
systems, however, may benefit from the addition of a pressure
differential across the cell system, e.g., by feeding air at
elevated pressures, as described below.
[0052] In an alternative configuration (not shown), the cell 300
may be used in a system similar to system 250, wherein instead of a
voltage applied across the electrodes, a thermodynamic driving
force is used. In such a system, the feed air may be compressed or
otherwise fed at elevated pressures.
[0053] As previously mentioned, the membrane employed in the gas
separation cells described herein are hydroxide conducing
membranes. The membrane may have hydroxide conducing properties by
virtue of: physical characteristics (e.g., porosity) capable of
supporting a hydroxide source, such as a gelatinous alkaline
material; molecular structure that supports a hydroxide source,
such as an aqueous electrolyte; anion exchange properties, such as
anion exchange membranes; or a combination of one or more of these
characteristics capable of providing the hydroxide source.
[0054] In certain embodiments, the membrane comprises a material
having physical characteristics (e.g., porosity) capable of
supporting a hydroxide source, such as a gelatinous alkaline
solution. For example, various separators capable of providing
ionically conducting media are described in: U.S. Pat. No.
5,250,370 entitled "Variable Area Dynamic Battery," Sadeg M. Faris,
Issued Oct. 5, 1993; U.S. application Ser. No. 08/944,507 filed
Oct. 6, 1997 entitled "System and Method for Producing Electrical
Power Using Metal Air Fuel Cell Battery Technology," Sadeg M.
Faris, Yuen-Ming Chang, Tsepin Tsai and Wayne Yao; U.S. application
Ser. No. 09/074,337 filed May 7, 1998 entitled "Metal-Air Fuel Cell
Battery Systems," Sadeg M. Faris and Tsepin Tsai; U.S. application
Ser. No. 09/110,762 filed Jul. 3, 1998 entitled "Metal-Air Fuel
Cell Battery System Employing Metal Fuel Tape and Low-Friction
Cathode Structures," Sadeg M. Faris, Tsepin Tsai, Thomas J.
Legbandt, Muguo Chen and Wayne Yao; U.S. Pat. No. 6,190,792 issued
Feb. 20, 2001 entitled "Ionically-Conductive Belt Structure for Use
in a Metal-Air Fuel Cell Battery System and Method of Fabricating
the Same," Sadeg M. Faris, Tsepin Tsai, Thomas Legbandt, Wenbin Yao
and Muguo Chen; U.S. application Ser. No. 09/116,643 filed Jul. 16,
1998 entitled "Metal-Air Fuel Cell Battery System Employing Means
for Discharging and Recharging Metal-Fuel Cards," Sadeg M. Faris,
Tsepin Tsai, Wenbin Yao and Muguo Chen; U.S. application Ser. No.
09/268,150 filed Mar. 15, 1999 entitled "Movable Anode Fuel Cell
Battery," by Tsepin Tsai and William Morris; U.S. application Ser.
No. 09/526,669 filed Mar. 15, 2000 "Movable Anode Fuel Cell
Battery," Tsepin Tsai, William F. Morris, all of which are herein
incorporated by reference.
[0055] In general, the type of material having physical
characteristics capable of supporting a hydroxide source may
comprise an electrolyte gel. The electrolyte gel may be either
applied directly on the surface of the evolution and/or reduction
electrodes, or applied as a self supported membrane between the
evolution and reduction electrodes. Alternatively, the gel may be
supported by a substrate and incorporated between the evolution and
reduction electrodes.
[0056] The electrolyte (in all variations of the membrane herein)
generally comprises ion conducting material to allow ionic
conduction between the metal anode and the cathode. The electrolyte
generally comprises hydroxide-conducting materials such as KOH,
NaOH, LiOH, RbOH, CsOH or a combination comprising at least one of
the foregoing electrolyte media. In preferred embodiments, the
hydroxide-conducting material comprises KOH. Particularly, the
electrolyte may comprise aqueous electrolytes having a
concentration of about 5% ionic conducting materials to about 55%
ionic conducting materials, preferably about 10% ionic conducting
materials to about 50% ionic conducting materials, and more
preferably about 30% ionic conducting materials to about 40% ionic
conducting materials.
[0057] The gelling agent for the membrane may be any suitable
gelling agent in sufficient quantity to provide the desired
consistency of the material. The gelling agent may be a crosslinked
polyacrylic acid (PAA), such as the Carbopol.RTM. family of
crosslinked polyacrylic acids (e.g., Carbopol.RTM. 675) available
from BF Goodrich Company, Charlotte, N.C., Alcosorb.RTM. G1
commercially available from Allied Colloids Limited (West
Yorkshire, GB) and potassium and sodium salts of polyacrylic acid;
carboxymethyl cellulose (CMC), such as those available from Aldrich
Chemical Co., Inc., Milwaukee, Wis.; hydroxypropylmethyl cellulose;
gelatine; polyvinyl alcohol (PVA); poly(ethylene oxide) (PEO);
polybutylvinyl alcohol (PBVA); combinations comprising at least one
of the foregoing gelling agents; and the like. Generally, the
gelling agent concentration is from about 0.1% to about 50%
preferably about 2% to about 10%.
[0058] The optional substrate may be provided in forms including,
but not limited to, woven, non-woven, porous (such as microporous
or nanoporous), cellular, polymer sheets and the like, which are
capable of allowing sufficient ionic transport between the
reduction and evolution electrodes. In certain embodiments, the
substrate is flexible, to accommodate electrochemical expansion and
contraction of the cell components, and chemically inert to the
cell materials. Materials for the substrate include, but are not
limited to, polyolefin (e.g., Gelgard.RTM. commercially available
from Daramic Inc., Burlington, Mass.), polyvinyl alcohol (PVA),
cellulose (e.g., nitrocellulose, cellulose acetate and the like),
polyamide (e.g., nylon), cellophane, filter paper and combinations
comprising at least one of the foregoing materials. The substrate
may also comprise additives and/or coatings such as acrylic
compounds and the like to make them more wettable and permeable to
the electrolyte.
[0059] In other embodiments of the hydroxide-conducting membrane
herein, a molecular structure is provided that supports a hydroxide
source, such as an aqueous electrolyte. Such membranes are
desirable in that conductivity benefits of aqueous electrolytes may
be achieved in a self supported solid state structure. In certain
embodiments, the membrane may be fabricated from a composite of a
polymeric material and an electrolyte. The molecular structure of
the polymeric material supports the electrolyte. Cross-linking
and/or polymeric strands serve to maintain the electrolyte.
[0060] In one example of a molecular structure that supports a
hydroxide source, a polymeric material such as polyvinyl chloride
(PVC) or poly(ethylene oxide) (PEO) is formed integrally with a
hydroxide source as a thick film. In a first formulation, one mole
of KOH and 0.1 mole of calcium chloride are dissolved in a mixed
solution of 60 milliliters of water and 40 milliliters of
tetrahydrogen furan (THF). Calcium chloride is provided as a
hygroscopic agent. Thereafter, one mole of PEO is added to the
mixture. In a second formulation, the same materials for the first
formula are used, with the substitution of PVC for PEO. The
solution is cast (or coated) as a thick film onto substrate, such
as polyvinyl alcohol (PVA) type plastic material. Other substrate
materials preferably having a surface tension higher than the film
material may be used. As the mixed solvents evaporate from the
applied coating, an ionically-conductive solid state membrane (i.e.
thick film) is formed on the PVA substrate. By peeling the solid
state membrane off the PVA substrate, a solid-state
ionically-conductive membrane or film is formed. Using the above
formulations, it is possible to form ionically-conductive films
having a thickness in the range of about 0.2 to about 0.5
millimeters.
[0061] Another embodiment of a molecular structure is described in
greater detail in U.S. patent application Ser. No. 09/259,068,
entitled "Solid Gel Membrane", to Muguo Chen, Tsepin Tsai, Wayne
Yao, Yuen-Ming Chang, Lin-Feng Li and Tom Karen, filed on Feb. 26,
1999. The polymeric material comprises a polymerization product of
one or more monomers selected from the group of water soluble
ethylenically unsaturated amides and acids, and optionally a water
soluble or water swellable polymer. The polymerized product may be
formed on a support material or substrate. The support material or
substrate may be, but not limited to, a woven or nonwoven fabric,
such as a polyolefin, polyvinyl alcohol, cellulose, or a polyamide,
such as nylon.
[0062] The electrolyte may be added prior to polymerization of the
above monomer(s), or after polymerization. For example, in one
embodiment, electrolyte may be added to a solution containing the
monomer(s), an optional polymerization initiator, and an optional
reinforcing element prior to polymerization, and it remains
embedded in the polymeric material after the polymerization.
Alternatively, the polymerization may be effectuated without the
electrolyte, wherein the electrolyte is subsequently included.
[0063] The water soluble ethylenically unsaturated amide and acid
monomers may include methylenebisacrylamide, acrylamide,
methacrylic acid, acrylic acid, 1-vinyl-2-pyrrolidinone,
N-isopropylacrylamide, fumaramide, fumaric acid,
N,N-dimethylacrylamide, 3,3-dimethylacrylic acid, sodium salt of
vinylsulfonic acid, other water soluble ethylenically unsaturated
amide and acid monomers, or combinations comprising at least one of
the foregoing monomers.
[0064] The water soluble or water swellable polymer, which acts as
a reinforcing element, may include polysulfone (anionic),
poly(sodium 4-styrenesulfonate), carboxymethyl cellulose, sodium
salt of poly(styrenesulfonic acid-co-maleic acid), corn starch, any
other water-soluble or water-swellable polymers, or combinations
comprising at least one of the foregoing water soluble or water
swellable polymers. The addition of the reinforcing element
enhances mechanical strength of the polymer structure.
[0065] Optionally, a crosslinking agent, such as
methylenebisacrylamide, ethylenebisacrylamide, any water-soluble
N,N'-alkylidene-bis(ethylenicall- y unsaturated amide), other
crosslinkers, or combinations comprising at least one of the
foregoing crosslinking agents.
[0066] A polymerization initiator may also be included, such as
ammonium persulfate, alkali metal persulfates and peroxides, other
initiators, or combinations comprising at least one of the
foregoing initiators. Further, an initiator may be used in
combination with radical generating methods such as radiation,
including for example, ultraviolet light, X-ray, .gamma.-ray and
the like. However, the chemical initiators need not be added if the
radiation alone is sufficiently powerful to begin the
polymerization.
[0067] In one method of forming the polymeric material, the
selected fabric may be soaked in the monomer solution (with or
without the ionic species), the solution-coated fabric is cooled,
and a polymerization initiator is optionally added. The monomer
solution may be polymerized by heating, irradiating with
ultraviolet light, gamma-rays, x-rays, electron beam, or a
combination thereof, wherein the polymeric material is produced.
When the ionic species is included in the polymerized solution, the
hydroxide ion (or other ions) remains in solution after the
polymerization. Further, when the polymeric material does not
include the ionic species, it may be added by, for example, soaking
the polymeric material in an ionic solution.
[0068] Polymerization is generally carried out at a temperature
ranging from room temperature to about 130.degree. C., but
preferably at an elevated temperature ranging from about 75.degree.
to about 100.degree. C. Optionally, the polymerization may be
carried out using radiation in conjunction with heating.
Alternatively, the polymerization may be performed using radiation
alone without raising the temperature of the ingredients, depending
on the strength of the radiation. Examples of radiation types
useful in the polymerization reaction include, but are not limited
to, ultraviolet light, gamma-rays, x-rays, electron beam, or a
combination thereof.
[0069] To control the thickness of the membrane, the coated fabric
may be placed in suitable molds prior to polymerization.
Alternatively, the fabric coated with the monomer solution may be
placed between suitable films such as glass and polyethylene
teraphthalate (PET) film. The thickness of the film may be varied
will be obvious to those of skill in the art based on its
effectiveness in a particular application. In certain embodiments,
for example for separating oxygen from air, the membrane or
separator may have a thickness of about 0.1 mm to about 0.6 mm.
Because the actual conducting media remains in aqueous solution
within the polymer backbone, the conductivity of the membrane is
comparable to that of liquid electrolytes, which at room
temperature is significantly high.
[0070] In still further embodiments of the hydroxide-conducting
membrane herein, anion exchange membranes are employed. Some
exemplary anion exchange membranes are based on organic polymers
comprising a quaternary ammonium salt structure functionality;
strong base polystyrene divinylbenzene cross-linked Type I anion
exchangers; weak base polystyrene divinylbenzene cross-linked anion
exchangers; strong base/weak base polystyrene divinylbenzene
cross-linked Type II anion exchangers; strong base/weak base
acrylic anion exchangers; strong base perfluoro aminated anion
exchangers; naturally occurring anion exchangers such as certain
clays; and combinations and blends comprising at least one of the
foregoing materials.
[0071] An example of a suitable anion exchange membrane is
described in greater detail in U.S. Pat. No. 6,183,914 and
incorporated by reference herein. The membrane includes an
ammonium-based polymer comprising (a) an organic polymer having an
alkyl quaternary ammonium salt structure; (b) a
nitrogen-containing, heterocyclic ammonium salt; and (c) a source
of hydroxide anion.
[0072] As a first component, the ammonium-based polymer includes an
organic polymer having an alkyl quaternary ammonium salt structure.
Preferred polymer structures include those having alkyl quaternary
ammonium groups at the ends of the polymer side chains, exemplified
by formula A, below. 1
[0073] wherein
[0074] R is chosen from a direct bond, --C(O)O-- and
--C(O)NH--;
[0075] m is an integer of from 1 to 3;
[0076] n is an integer of from 1 to 4; and
[0077] X.sup.- is a counter anion, preferably chosen from Cl.sup.-,
Br.sup.- and I.sup.-.
[0078] Organic polymers of formula A may be obtained, for example,
as homopolymers from vinyl monomers including the alkyl quaternary
ammonium salt structure, or as copolymers from these vinyl monomers
and other vinyl comonomers. Formula B exemplifies the copolymers
that may be obtained from such a vinyl monomer and a vinyl
comonomers. 2
[0079] U is a polymer constitutive unit from the copolymerized
vinyl comonomer;
[0080] R is chosen from a direct bond, --C(O)O-- and
--C(O)NH--;
[0081] m is an integer from 1 to 3;
[0082] n is an integer from 1 to 4; and
[0083] X.sup.- is a counter anion, preferably chosen from Cl.sup.-,
Br.sup.- and I.sup.-.
[0084] Preferably, the vinyl comonomers that provide polymer
constitutive unit U, will be those having vinylic unsaturated
hydrocarbons. Examples of such vinyl comonomers include, but are
not limited to, acrylic monomers, such as, for example,
CH.sub.2.dbd.CHCOOH and CH.sub.2.dbd.CHCOOR, wherein R is an alkyl
group; methacrylic monomers, such as, for example,
CH.sub.2.dbd.CCH.sub.3COOH and CH.sub.2.dbd.CCH.sub.3COOR, wherein
R is an alkyl group;
CH.sub.2.dbd.[COO(CH.sub.2CH.sub.2O).sub.nCH.sub.3].sub.2, wherein
n is an integer from 1 to 23; CH.sub.2.dbd.CH(C.sub.6Hs);
CH.sub.2.dbd.CHCN; CH.sub.2.dbd.CHCONH.sub.2; vinyl chloride, vinyl
pyrrolidone and the like. The copolymers may be obtained from the
copolymerization of one or more of these vinyl comonomers by any
known process, such as for example, a radical polymerization
process, photopolymerization, or the like.
[0085] In addition to organic polymers of formula A, the
ammonium-based polymer may also comprise monomer units wherein an
alkyl quaternary ammonium salt structure is bonded to the main
chain of the polymer to form a cyclic structure therein,
exemplified by formula C, below. 3
[0086] wherein
[0087] R.sup.2 is chosen from a direct bond and CH.sub.2;
[0088] R.sup.3 and R.sup.4 are each a lower alkyl group;
[0089] n is an integer; and
[0090] X.sup.- is a counter anion, preferably chosen from Cl.sup.-,
Br.sup.- and I.sup.-.
[0091] Organic polymers of formula C may be obtained, for example,
by polymerization of diallyl dialkyl ammonium halide monomers, as
well from other commercial sources. Poly(diallyl-dimethyl-ammonium)
chloride, for example, may be derived from
diallyl-dimethyl-ammonium chloride monomer units. In a preferred
embodiment of the invention, organic polymers of formula C will
have a mean molecular weight of from 20,000 to 500,000.
[0092] As a second constitutive component, the ammonium-based
polymer includes a nitrogen-containing, heterocyclic quaternary
ammonium salt. Preferably, this component is an alkylimidazolium
salt or an alkylpyridinium salt, and more preferably, methyl or
butyl pyridinium salt. In a preferred embodiment, the counter anion
of the salt is chosen from halides such as Cl.sup.-, Br.sup.- and
I.sup.-.
[0093] As a third constitutive component, the ammonium-based
polymer includes a source of hydroxide anion. Preferably, the
source of hydroxide anion is a hydroxide salt, more preferably is a
metal hydroxide salt, and most preferably, is aluminum
hydroxide.
[0094] Without being limited to any particular theory, it is
considered that, in the ammonium-based polymer, the hydroxide
component forms a complex with the counter anion of either the
alkyl quaternary ammonium salt of the organic polymer or the
nitrogen-containing, heterocyclic quaternary ammonium compound. It
is further considered that complexes of both a quasi-tetrahedral
structure and a dimeric quasi-tetrahedral structure with one common
counter ion therein are formed, depending on the ratio of the three
constitutive components. For example, when the hydroxide component
is aluminum hydroxide, both [AlX(OH).sub.3.sup.-] and
Al.sub.2X(OH).sub.6.sup.-] may be formed.
[0095] The preferred ratio of the organic polymer, the
nitrogen-containing, heterocyclic quaternary ammonium salt and the
metal hydroxide salt varies, depending on the type of organic
polymer and ammonium salt utilized. Generally, it is preferred that
for one mole of organic polymer, the amount of the
nitrogen-containing, heterocyclic ammonium salt ranges from 0.2 to
0.6 moles, and the amount of the hydroxide component ranges from
0.3 to 0.5 moles.
[0096] The ammonium-based polymer may be produced by any ordinary
method, such as for example, by uniformly dissolving the components
in an appropriate solvent such as, for example, water, nitromethane
or a lower alcohol, whereby the resulting solution is then spread
over a flat substrate. The solvent is evaporated out and a film
obtained, which is subsequently formed into particulate
material.
[0097] As a means to increase the mechanical strength of an
ammonium-based polymer prepared in accordance with the principles
of the present invention, the composition may further include a
binder, such as for example, an acrylic, polyethylene, or the like.
The binder may be uniformly dissolved along with the other
components during preparation of the initial composition. The
modified membrane exhibits the same order of conductivity as the
three component membrane, along with an increased tensile
strength.
[0098] The oxygen evolving electrode may comprise any suitable
oxygen evolving catalyst, such as nickel or combinations and alloys
comprising nickel. One particularly useful nickel alloy is Raney
nickel. The oxygen evolving electrode may be provided as a discreet
structure, or may be formed directly upon the membrane.
[0099] The oxygen reducing electrode may comprise any suitable air
cathode, such as a carbon supported catalyst structure. The oxygen
reducing electrode may be provided as a discreet structure, or may
be formed directly upon the membrane. An exemplary oxygen reducing
electrode is disclosed in copending, commonly assigned U.S. patent
application Ser. No. 09/415,449, entitled "Electrochemical
Electrode For Fuel Cell", to Wayne Yao and Tsepin Tsai, filed on
Oct. 8, 1999, which is incorporated herein by reference in its
entirety. Other electrodes may instead be used, however, depending
on the performance capabilities thereof, as will be obvious to
those of skill in the art.
[0100] An oxygen reduction/evolution catalyst utilized on the anode
and/or cathode may be fabricated from various materials, including
precious metals, such as platinum and silver,
non-precious-metal-based inorganic catalysts, such as
cobalt-phathanocyanine, cobalt tetramethoxypheny porphyrin,
manganese phathanocyanine, iron phathanocyanine, MnO.sub.x, various
nano-grain perovskites (e.g. LaMnO.sub.3) and spinels (e.g.
LiMnO.sub.2) and combinations comprising at least one of the
foregoing materials. Different types of catalysts may provide
better performance on the anode vs. the cathode, therefore various
embodiments of the present invention may employ two discrete
catalysts.
[0101] The catalyst may be utilized in both ink (dilute) and slurry
(thicker) states applied using various application techniques. One
such technique includes spray coating, in which the catalyst ink is
sprayed onto the membrane. Another technique is the well-known
Doctor Blade method for applying a layer of catalyst onto the
membrane surface. A third approach is silk printing, in which a
silk screen will be used to print the slurry onto the membrane
surface with predetermined thickness. Those skilled in the art will
recognize that a particular application process may be selected
based upon the particular variations in adhesion, uniformity, and
run-to-run thickness consistency provided thereby.
[0102] In certain embodiments, particularly when either or both the
anode and the cathode are provided as discreet structures adjacent
to the membrane, an interfacial material is utilized therebetween.
One suitable interfacial material is described in copending U.S.
Provisional Patent Application No. 60/274,274 filed Mar. 8, 2001
entitled "Interfacial Material For Metal Air Electrochemical
Cells," which is incorporated herein by reference. The interfacial
material may be in the form of a gel comprising an alkaline
solution and a gelling agent, as described above. With the
inclusion of the interfacial layer, it is possible to reducing the
impedance between the electrode and the membrane, and thus improve
the ionic contact between the cathode and the electrolyte. This may
be accomplished while enhancing contact between the electrode and
the membrane. While not wishing to be bound by theory, it is
believed that the interfacial layer serves as a bridge agent to wet
the cathode surface. Further, the internal adhesion of the cathode
itself may be improved (e.g., where cathode particles may be
subject to delaminating from the surface or are loosely packed), as
well as the adhesion between the cathode and the separator (thus)
minimizing or preventing delamination). The gel for the interfacial
layer may further comprise a catalyst material, which may be
similar to the catalyst material used for the electrodes.
[0103] The following illustrative example is intended to
demonstrate certain aspects of the present invention. It is to be
understood that the example should not be construed as limiting.
Further, the example presented was conducted for the purpose of
demonstrating oxygen evolution using a hydroxide conducing membrane
described herein. The system components and parameters have not
been optimized.
EXAMPLE 1
[0104] Cell Construction
[0105] Membrane Construction
[0106] A hydroxide-conducting polymeric membrane is formed
according to following procedure. 0.75 grams
methylenebisacrylamide, 0.56 g acrylamide, 4.70 g methacrylic acid,
and 0.25 g poly(sodium 4-styrenesulfonate) are dissolved in 10
milliliters water and then 20 ml 40% KOH is added to the resulting
solution, which is maintained at room temperature. 0.05 g ammonium
persulfate is then added to the solution. A piece of fabric is
soaked in the resulting monomer solution and then sandwiched
between a piece of glass and a piece of PET transparent film. This
is heated on a 75.degree. C. hotplate for 1 minute and then
irradiated under strong UV light for 5 minutes, whereby a strong
polymer film will be formed. The membrane used in the demonstrative
example herein was 0.55 mm (21.6 mil).
[0107] Electrodes
[0108] An air cathode, commercially available from AluPower, Inc.,
Pawcatuck, Conn. (ACN-175) is disposed on the air side of the
membrane. The cathode has a thickness of 0.5 mm (19.7 mil). A thin
layer of interfacial gel electrolyte is applied to the membrane to
enhance adhesion and conductivity between the cathode and the
membrane. The is was formed from 2.5% polyacrylic acid having a
weight basis average molecular weight of about 3,000,000 and 45%
KOH.
[0109] The anode is cast on the opposite side of the membrane,
wherein the anode comprises Raney nickel (25% solid, commercially
available from Aldrich, Milwaukee, Wis.) in a solution of 5%
Nafion.RTM. (commercially available from E.I. du Pont Nemours and
Company Corp., Wilmington, Del.) in alcohol. The solution is
allowed to dry. The resultant anode thickness in the demonstrative
example herein was 0.2 mm (8 mil)
[0110] Cell Structure
[0111] Referring now to FIGS. 6 and 7, an exemplary cell is
depicted. A conducting/supporting structure 400a is shown in FIG.
6, which is used on the cathode side of the cell. A structure 400b
used on the anode side of the cell (see FIG. 7) is similar. In the
experimental run presented herein, each of the structures 400a and
400b had an overall diameter of 6 cm (2.361 inches) and thickness
of 2.5 cm (0.986 inches), and were formed of stainless steel to
support the membrane electrode assembly (comprising the electrodes
and the membrane therebetween, referred to as the "MEA") and
further to act as current collectors. Faces 402 of the structures
400a and 400b include a recess 404 that supported the active area
of the cell, which in the experimental run presented herein, had a
diameter of 3.6 cm and a depth of 1.01 mm (39.8 mil). An assembly
480 of nickel mesh and foam structures is disposed in the recess
404 of structure 400a, and a cathode 412 is adjacent thereto. In
the experimental run herein, the assembly 480 comprised a first
mesh structure adjacent to structure 400a that was 0.41 mm (16 mil)
thick and 20 mesh, and a second foam structure between the first
mesh structure and the cathode 412 that was 1.3 mm (51 mil) thick
and 110 mesh). A membrane 410, having an anode 414 cast thereon as
described herein, is assembled adjacent to the structure 400b
having an assembly 482. In the experimental run herein, the
assembly 482 comprised a first mesh structure adjacent to structure
400a that was 0.41 mm (16 mil) thick and 20 mesh, and a second foam
structure between the first mesh structure and the cathode 412 that
was 1.3 mm (51 mil) thick and 110 mesh). Matching faces of the
structures 400a and 400b were pressed against each other. Structure
400a used an O-ring (not shown) to seal the joint. The overall unit
was clamped together by two end plates made of non-conducting ABS
plastic, bolts and wing nuts.
[0112] For gas flow to pass over an electrode placed adjacent to
the face 402 of structures 400a and 400b, a passageway 406 (having
a diameter of 0.87 cm (0.343 inches) was formed across the diameter
of structure 400a at the midpoint of the thickness of the
structure, and having suitably threaded ends for insertion of
plastic tubing. Further, a plurality of passageways 408 (having
diameters of 1.98 mm (0.078 inches)) extended from the base of the
recess 404 to passageway 406 at 1.01 cm (0.398 inches) and 1.52 cm
(0.598 inches) from the center point (on both sides) of the recess
404. For passage of current through the cell, electrical leads were
attached to the structures.
[0113] Experimental Procedure
[0114] Humidified air was passed across the cathode, while helium
swiped the anode (for He sweep purity validation). A humidified He
stream was obtained by drawing He from a cylinder and then sending
through a stainless steel bubbler partially filled with water.
Humidified air stream was obtained by drawing air free of carbon
dioxide from a cylinder and passing it through a water column in a
separate stainless steel bubbler. The flow rates of the streams
were monitored using a digital readout and controller module
(Matheson, Model 8251).
[0115] To measure the concentration of O.sub.2 in the exit He, a
slipstream was sent to a Gas Chromatograph (GC) (Hewlett Packard,
Model 5890 Series III). Before being sent to the GC, moisture from
the sample stream was removed by a Nafion.RTM. drier (Perma Pure,
Fort Wayne, Ind.). The GC employed a thermal conductivity detector
(TCD) and a molecular sieve column (6'.times.1/8" SS-13X-60/80,
Alltech, Deerfield, Ill.). The carrier gas was He and its flow rate
was 30 ml/min. The temperatures of the oven, injector and detector
were set at 35.degree. C., 200.degree. C., and 200.degree. C.,
respectively. Retention times for O.sub.2 and N.sub.2 were
approximately 1.5 minutes and 1.7 minutes, respectively.
[0116] After suitable validation procedures (including passing He
across the cathode and anode), the experiment was conducted. A
power source was turned on to apply a DC voltage across the cell.
The air-separation was initiated by sending a humidified air stream
through the cathode chamber at a flow rate of 15 sccm, and the
relative humidity of the exit air stream was measured to be about
74%. The anode chamber had He at a flow rate of 30 sccm. Both the
lines (sweep He and feed air) were open to atmosphere at their
respective ends. The power source was turned on, and for the next
40 minutes, the applied voltage was raised slowly from 0 to 1.2 V.
GC analyses of the sweep He was carried out periodically. At
potential less than 0.6 volts, the current through the cell was of
zero value. The current reading was 0.01 amps at 0.7 V. At 1 V, the
current was of 0.04 A; the corresponding GC sampling yielded a
chromatogram having a single peak with a retention time of 1.59
minutes. Evidently, O.sub.2 was present in the sweep He. As the
potential was raised to 1-1.2 V, the current reached 0.05 A.
Increase of potential to 1.4 V yielded a steady current of 0.05 A.
For the following 45 minutes, the current-voltage condition was
kept steady at 0.05 A and 1.2 V. A number of chromatograms were
obtained from GC analysis of the exit He showing singular peaks of
O.sub.2. The run was ended, after operating for 1.5 hours.
[0117] The result of oxygen generating run-condition is represented
in FIG. 8. The chromatogram has a singular peak with retention time
of 1.59 min, which was oxygen's retention time as obtained from
sampling of air. This confirmed that oxygen was generated at the
anode chamber of the cell. From the GC calibration, it was
determined that the concentration of freshly generated O.sub.2 in
the sweep He was 2350 parts per million based on volume (ppmv). The
effective membrane area was 10.2 cm.sup.2. For the above
concentration of oxygen in a He sweep having a flow rate of 30
scm.sup.3/min, the corresponding flux was found to be
1.15.times.10.sup.-4 scm.sup.3/s.multidot.cm.sup.2. The
thermodynamic driving force was the gradient in the partial
pressure of O.sub.2. which was about 16 cm Hg. It is assumed that
ambient air at the cathode side has 21% O.sub.2, and the O.sub.2 on
the anode side approaches zero (due to the He sweep). Using the
values of thickness of the membrane (0.055 cm), partial pressure
gradient and the flux, the permeability coefficient was calculated
as:
4.0.times.10.sup.-7
(scm.sup.3.multidot.cm)/(cm.sup.2.multidot.s.multidot.- cmHg)=4000
Barrer units.
[0118] The permeability in terms of Barrer units is presented to
provide relative comparison to known art. However, the voltage
applied is not accounted for. Nonetheless, considering the
relatively low current density of the system, it is possible to
attain extremely high oxygen flux upon optimization of the
system.
[0119] By utilizing the hydroxide ion (OH.sup.-) instead of oxide
ion (O.sup.2-) to shuttle the oxygen molecule through a membrane,
oxygen throughput is substantially increased relative to the prior
art.
[0120] Moreover, the operating temperature may be reduced relative
to the prior art, from up to or greater than about 800.degree. C.
to room temperature. This will greatly reduce the system cost and
alleviate many engineering problems, such as difficulties with gas
sealing as discussed hereinabove.
[0121] The hydroxide-conducting membrane of the present invention
advantageously may be utilized in many chemical, electronics and
medical technologies that depend on high-purity oxygen.
Additionally, the present invention may be utilized to assist in
making many industrial processes more environmentally sound.
[0122] While preferred embodiments have been shown and described,
various modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustrations and not limitation.
[0123] Having thus described the invention, what is claimed is:
* * * * *