Note: Descriptions are shown in the official language in which they were submitted.
~ 1114~;;20
52-EE~0-248 ~
,:
The instant invention related to an electrochemical
gas or vapor sensor and more particularly, to an electrochemical
sensor which does not utilize a liquid electrolyte and de-
tects gases or vapor such as carbon monoxide, NO2 alcohol,
etc.
Electrochemical cells to sense various gaseous
constituents such as hydrogen, oxygen, carbon monoxide, etc.,
are ~nown and have been described in various publications
and patents. An electrochmical gas sensor may be defined as
a sensor in which the gaseous contituents to be sensed are
brought in contact with a catalytic electrode so that the
constituent is either oxidized or reduced with the exchange
of electrons. The flow of current due to the oxidation and
reduction of the gaseous constituent is then a measure of the
concentration of the constituent to be detected. One form of
electrochmical gas sensor for gases such as hydrogen, carbon - -
monoxide, hydrocarbons, etc., is described in an article
entitled~
Electrochemical Detection of H2, CO and
Hydrocarbons in -Inert of Oxygen Atmospheres -
by A.B. LaConti, and H.~J.R. Maget printed in the Journal of
Electrochemical Society, ~olume 118 No. 3, March of 1971.
The eIectrochemical gas sensor~ described in the aforesaid
article is one in which an eIectrical biasing potential is
applied between the sensing and counter electrodes so that the
sensing electrode is maintainea at a potential which is
exactly to the rest potential of a Pt~O2 eIectrode, thus
making the device insensitive to oxygen and air; consequently,
the desired constituent may be sensed as it is oxidized
at the electrode even though contained in an oxygen containing
gas stream. In the absence of the constituent to be detected
no current flows in the external circuit because of the electrical
- 11140Z0 52-EE-0-248
biasing and the power consumption of the device is reduced
substantially in that the cell consumes virtually no power
during those intervals when the gas to be detected is not
present. The electrochemical sensors described in the
LaConti/Maget article were of the solid polymer electrolyte
and did not use liquid electrolytes. However, as pointed
out in the article, in order to maintain the potential at the
sensing electrode constant so that the cell current is an
accurate measure of the gas concentration, it was necessary
either to maintain a hydrogen supply at the counter-reference
electrode (as referred to in Col. 1 of page 507 of the article)
or alternatively, the system had to be operated as a three-
electrode device with the remaining electrodes operated in a
potentiostatic mode. However, when the system was operated ~ -
in the potentiostatic mode, either a salt bridge or sulfuric
acid electrolyte bridge was required between the reference
electrode and the sensing electrode in order to produce a
usable and invariant output from the sensor and to alleviate
potential fluctuations at the sensor electrode. The use of
salt bridges or acid electrolyte bridges in the gas sensor
while accomplishing the desired result of preventing potential
fluctuations at the sensing electrode, introduce many dif-
ficulties, if small portable sensors, particularly ones that
may be small enough that may be easily carried or worn by the
individual, are required. Ob~iously, the other approach,
described in the article, namely these provisions of a separate
source of hydrogen gas so as to maintain a stable hydrogen
counter-reference electrode on the other side of the membrane,
presents even greater difficulties from the standpoint of
obtaining a small, portable device.
Another existing electrochmical gas sensor which
operates in a potentiostatic mode is described in U.S. Patent
1114~20
-~ 52-EE-0-248
No. 3,776,832, issued December 4, 1973 entitled:
Electrochemical Detection Cell -
and in an article entitled:
Controlled Potential Electrochemical
Analysis of Carbon Monoxide
found in a 1974 issue of American Laboratory on pages 50
et seq. The device described in these two publications is a
three-electrode cell, i.e., the cell contains a sensing
electrode, a counter electrode and a reference electrode.
A potentiostat is coupled between the reference and the
sensing electrodes to maintain the potential at the sensing
electrode constant with variations at ambient or environment. . -
This electrochemical gas sensor maintains a constant voltage
at the reference eIectrode without the need for a source of
hydrogen. This latter arrangement does, however, utilize a
liquid acid electrolyte and is subject to all of the shortcomings
of any electrochemical system which utilizes a liquid electro-
lyte, acid or alkaline. In the first instance, there is the
problem of changing electrolyte concentration with time and
the impact that it has on the output, sensitiYity and accuracy
of the device. In fact, the above cited patent points out
that a liquid electrolyte reserYoir must ~e supplied for a
wick or matrix in order to maintain the concentration relatiYely
constant~ T,-is obYiously results in a large, fairly cumbersome
arrangement for the sensor and one which is not very attractive -
from the standpoint of a portable device which may be carried
or worn on a person.
In addition to changes in eIectrolyte concentration
with time, there's always the risk of electrolyte spillage,
materials corrosion, the probIem of gasketing and sealing in
order to prevent the eIectrolyte from leaking. It is also
apparent that the electrode utilized in the gas sensor in the
--3--
020
52-EE-0-248
above identified patent not only functions as an oxidizing
electrode, but must also act as a sealant for the liquid
electrolyte. Hence, it has to be large enough, sturdy enough,
etc., configuration, dimension and strength to perform this
function. Utilization of a liquid electrolyte can also produce
masking of active catalytic sides on the sensing electrode
by electrolyte migration thereby reducing the sensitivity and
output of the device.
It will be obvious from the aforesaid discussion, that
electrochemical sensors of the type utilizing liquid electro-
lytes are inherently of the sort that make a small portable
device difficult, if not impossible, to achieve because of all
of the problems and structural requirements that are associated
with the use of liquid electrolytes. A need therefore exists
for an electrochemical gas sensor which utilizes a hydrated
solid polymer electrolyte which eliminates all liquid electro-
lytes and thus makes possible a much smaller device which is
invariant with time, etc., and which does not require salt
bridges or acid bridyes between the reference and sensing
electrode to enhance response and sensitivity. By the elimin-
ation of the external bridges, further reductions in size and
bulk of the device become possible. Applicants have found
that a sensor which uses a hydrated solid polymer electrolyte
of the ion-exchange membrance type with an ionically conductive
hydrated membrane bridge between the sensing and reference
electrodes can be integrated into a form instrinsic part of
the solid polymer electrolyte membrane itself to stable, high
level output.
In addition, applicant has found that the use of a
solid polymer electrolyte cation exchange membrane which
exhibits rapid water vapor phase transport, the electrochemical
gas becomes self-humidifying. As a consequence, separate
52-EE-0-248
humidifying devices such as bubblers, etc., may be
eliminated from the sensor again leading to a smaller, more
compact and less expensive device. In addition, by elimin-
ating independent humidifiers such as bubblers, there is
essentially no chance that liquid H2O in the form of mist
or droplets carried into the sensor. Where separate bubbler
type saturators are used and the gas is first passed through
the bubbler to bring it to a 100~ relative humidity obviously
carryover of liquid H2O as a possibility. Furthermore, the
gas to be sensed does not contact liquid water thus minimizing
the possibility of "scrubbing out" a gas such as N02, for
example, which is to be sensed. By using an integrated humidi-
fier/sensor cell, the sensor cell is maintained at essentially
100% relative humidity during operation, thus minimizing
performance drop-off due to drying sensor sell SPE.
Applicant has found that this self-humidifying
arrangement is possible in SPE-type gas sensor by maintaining
the side of the SPE membrane away from the gas side, i.e. away
from the side containing the sensing electrode flooded with
distilled water. The distilled water is rapidly transported
across the SPE membrane in the vapor phase rapidly bringing the
incoming gases on the other side of the SPE to 100% relative
humidity~
It is therefore a principal objective of this invention
to provide an electrode, potentiostated electrochemical gas
sensor containing an integrated ionically conductive bridge
between the sensing and reference electrode.
A further objective of this invention is to provide
a three-electrode, electrically biased, electrochemical gas
sensor having an ionically conductive bridge between the
sensing and reference electrode integrated into the solid
polymer electrolyte.
52-EE-0-248
Another objective of the invention is to provide a
self-humidifying, potentiostated, three-electrode, electro-
chemical gas sensor utilizing a solid polymer electrolyte.
Still another objective of the invention is to
provide a self-humidifying, electrically biased, three-electrode
electrochemical gas sensor utilizing a solid polymer electrolyte.
Yet another objective of the invention is to provide
a self-humidifying, electricaly biased, three-electrode electro-
chemical gas sensor utilizing a solid polymer electrolyte with
an integrated ionically conductive bridge between two of the
electrodes.
Still other objectives and advantages of the invention -
will become apparent as the description thereof proceeds.
The various ob~ectives and advantages of the invention
are realized in an electrochemical gas sensor of the solid
polymer electrolyte type in which a catalytic sensing
electrode and catalytic reference electrode are positioned on
one side of the membrane and a catalytic counter electrode
is positioned on the other side of the membrance opposite
the sensing electrode. An ionically conductive, solid polymer
electrolyte bridge is provided by swelling the mem~rance
between the counter electrode and a point which is opposite
the reference electrode. This reduces the resistance between
the reference and sensing electrodes. thereby maximizing
the output of the device. In order to provide self-humidification
and bring the gases at the sensing electrode to 100% relative
humidity without the utilization of separate humidifiers of
the bubbling type, the counter electrodes side of the solid
polymer electrolyte is flooded with distilled water in the
region of the counter electrode. By flooding one side, water
in the vapor phase is transported rapidly across the membrane
to the sensing electrode thereby rapidly bringing any gases
~ ZO 52-EE-0-248
in the vicinity of the sensing electrode to 100~ relative
humidity.
The novel features which are believed to be character-
istic of this invention are set forth in the appended claims.
The invention itself, however, both as to organization and
mode of operation, toge~er with further objectives and
advantages thereof, are best understood by reference to the
following description taken in connection with accompanying
drawings in which:
Figure 1 is a perspective view of the cell in its
assembled condition;
Figure 2 is an exploded, sectional view taken along
the line 2-2 of Figure l;
Figure 3 is a partially broken away view of the
solid polymer of Figure 2 electrolyte showing the
ionically conducted bridge on one side of the
polymer etc.;
Figure 4 is a schematic diagram showing the gas
sensing cell and a potentiostatic circuit for
controlling operation of the cell and maintaining
the reference and sensing electrodes at a desired
potential.
Operation of the electrically biased, potentiostated,
three-electrode eIectrochemical gas sensor is based on the
oxidation or reduction of the constituent to be detected at
the catalytic sensing electrode. The sensing electrode is
maintained at a potential to produce rapid oxidation in the
case of carbon monoxide. It is also biased at or above the
rest potential of an electrode for oxygen or air so that
oxidation or reduction of air has no effect on the output from
the cell. The sensing electrode potenital must, however, be
below that at which water is dissociated to produce hydrogen
~ 20 52-EE-0-248
and oxygen. Thus,the sensing electrode must be maintained
at a potential which is much higher or more anodic than the
oxidation potential of the particular gaseous constituent.
For example, in the case of carbon monoxide, the electrode
potential for the CO2/CO redox couple is -0.12 volts with
reference to a Pt/H2 electrode. By maintaining the reference
potential in the range from 1~0 to 1.3 volts, there is rapid
and immediate oxidation of the carbon monoxide reaching the
sensing electrode in accordance with the following reactions
at the sensing electrode and at the counter electrode:
Sensing Electrode
CO + H2O = 2H + CO2 + 2e- (1)
Counter Electrode
-
+ _ (2)
2H + 1/2 2 (air) + 2e = H2) (3)
It can be seen that as the carbon monoxide is oxidized to
carbon dioxide, electrons are released which flow in the
external circuit and hydrogen ions are transported through
the electrolyte through the counter electrode and are
reduced there to form either molecular hydrogen or water.
~he current flowing in the external circuit as a result of
this rapid oxidation of carbon monixide to carbon dioxide is
thus directly proportional to the concentration of the ~ ~-
carbon monoxide in the gaseous stream.
Since the potential on the sensing electrode is
maintained at the level which is substantially greater in
the oxidizing direction, the potential required to oxidize
carbon monoxide to carbon dioxide, the incoming carbon monoxide
is rapidly oxidized at the sensing electrode. That is, as
pointed out previously, the o~idation/reduction potential for
the CO/CO2 couple is -0.12 volts. The voltage maintained at
the sensing electrode is in the range from 1.0 to 1.3 volts.
--8--
- 1~14~0 52-EE-0-248
It produces almost immediate and complete oxidation of the
carbon noxide. By liminting the voltage with the particular
catalyst used to 1.3 volts, the potential at the sensing
electrode is not sufficient to produce oxidation of water to
produce hydrogen and oxygen thereby minimizing background
current due to current flow produced by the oxidation of the
2' H /H2O couple. That is, the theoretical oxygen/water
redox couple is at +1.23 volts. However, due to normal over-
voltages at any sensing electrodes, the oxidation of the water
will take place at some voltage greater than 1.23. With the
catalytic electrode used in the instant invention which is a
platinum -5% iridium catalyst of an alloy of the reduced oxides
of platinum and iridium, there is no oxidation of water at
1.3 volts thereby ensuring that the current flow in the sensing
cell is due exclusively to the oxidation of the incoming gaseous
constituent such as carbon monoxide.
A potentiostatic circuit coupled between the re~erence ;~
and sensing electrodes is utilized to control the electrode
potentials with the sensing electrode potential preferably
maintained at 1.1 volts and the reference electrode potential
at +50 m~ below that, namely 1.05 volts. At 1.05 volts, which
roughLy represents the rest potential for an oxygen electrode
in a hydrated SPE acid system, the oxygen in the air which
contains the constituent to be sensed has no effect on the
sensing device. That is, by maintaining the reference at 1.05
which is essentially the rest potential of the sensing electrode,
no current flow due to the oxidation or reduction of the oxygen -
in the air, thus eliminating errors. Any shift of the potential
at the sensing electrode affects the rate of oxidation at the
electrode. That is, a drop in potential at the sensing electrode
reduces the oxidation rate while a rise increases the oxidation
rate. If such changes are per~itted to occur, the current
~ 20 52-EE-0-248
sensed will not be an accurate reflection of the gas con-
centration. Consequently, a potentiostatic circuit is
provided to hold the voltage at the sensing electrode
constant with reference to the hydrogen reference potential
while at the same time, maintaining a potential differential
between the reference and the sensing electrode to maintain
good zero background current characteristics as is explained
in Canadian Application Serial Number 308,237, filed
July 27, 1978, entitled:
Zero Background Current Operation Temperature
Compensated SPE Gas Detecting Cell
by A.B. La Conti, et al assigned to the General Electric
Company, the assignee of the present invention.
Inasmuch as the voltage at the sensing electrode and
the voltage difference between the reference and sensing
electrodes play such an important part in the accuracy and
of the output current from the device, it is highly desirable
that a good ionically conductive path is maintained between
these two electrodes. That is, although ideally no current
is supposed to flow between the sensing and reference elec-
trodes in a potentiostatic mode, there are small currents that
do flow between these electrodes. Since such small currents
do flow, there can be a significant IR drop between these two
electrodes if the resistance of the path is very high. Any
such IR drop changes not only the differential voltage between
these two electrodes, but may change the actual potential on
the sensing electrode that produces errors in the concentration
indication while differences in the voltage differential between
these two electrodes may have undesirable effects on the back-
ground current at zero-air flow, also affects the accuracy
of the overall indication since background current must be
subtracted from the actual indication to obtain an accurate
-- 10 --
--- 11141~Z()
52-EE-0-24
reading of the gas concentration.
In a gas sensing cell using a liquid electrolyte,
both the sensing electrode and the reference electrode
contact the same reservoir of electrolyte which are usually
acids such as sulfuric acid or phosphoric acid. Such acid
electrolyte have very low resistance, and consequently the
IR drop between the reference and the sensing electrode is
very low. However, in cells using a solid polymer electro-
lyte, even though it is hydrated, the surface of the SPE
membrane after assembly always has a tendency to dry out
somewhat. As the surface of the membrane dries slightly,
the resistance of the membrane across the dried surface
can increase substantially. Hence, the resistance of the
lateral path between the sensing and reference electrodes
across the partially dehydrated solid-polymer electrolyte
ion exchange membrane increases resulting in IR drops which
introduce the errors referred to above. Applicant has found,
as will be described in detail later, that the effect of
membrance drying and the resulting IR drop may be substantially
eliminated by forming an ionically conductive hydrated SPE
bridge across the surface and through the membrane. The
conductivity of the hydrated ionically conductive bridge, if
positioned on the counter electrode side, plus the two paths
through the me~brane, is sufficiently high to reduce the IR
drop between the sensing and reference electrode sufficiently
to reduce variations in the sensing electrode potential and
Yariations in the differential Yoltage therebetween to a
minimum, thereby producing a time invariant output from the
sensor, good background response.
Figure 1 shows a perspective view of the assembled
sensor, constructed in accordance with the instant invention.
Thus, the gas sensor includes a bottom plate fabricated of
~ ZO 52-EE-0-248
a suitable plastic material which does not react with the
gaseous constituent. Plate 1 has a pair of gas flow ports,
2 and 3, to allow a gas stream containing the constituent to
be sensed, to be brought into a sensing chamber, not shown in
Figure 1, which communicates with a catalytic sensing electrode, -
also not shown, positioned on the near surface of a hydrated,
solid-polymer electrolyte tSPE~ membrane 4 which preferably
is a sulfonated perfluorocarbon cation exchange membrane.
Plate 1 also has an opening 5 covered by a barrier film 6
which communicates, in the preferred embodiment, with a cat-
alytic reference electrode, not shown, positioned on the same
surface of the SPE membrane tube as the catalytic sensing
electrode. In order that reversibility of the air/O2 electrode
is optimized, the reference electrode and its active surface
area should be as large as possible. The membrane area in
contact with the reference area should also be fully hydrated
so all of the catalyst is in contact with a highly dissociated
sulfonic acid group. This permits full use of the full
membrane/electrolyte area. The effective electrodes are -
decreased if there is:
1~ Deficiency in catalyst (as by small electrode
area, low surface area, etc.~
2) Deficiency of sulfonic acid groups tnot
dissociated, low IEC~.
3) Air/O2 deficiency at electrode surface.
Barrier film 6 is selectively permeable to permit passage of
oxygen while sufficiently blocking the gas to be sensed so
that the potential of the reference electrode accurately
represents the rest potential of an oxygen electrode and is
not affected by the gas to be sensed. Thus, in a carbon
monoxide sensor, the barrier film may be a 0.001 inch to
0.01 thick silicone film which is selectively permeable to
-12-
--'` 1114020 55-EE-0-248
to oxygen while substantially blocking CO. Membrane 4
and bottom plate 1 are secured by means of a double-sided
adhesive tape, not shown, which is positioned at all areas,
but the electrodes and the openings in bottom plate 1 and,
as a result, the membrane and plate, adhere firmly while at
the same time blocking passage of gas between the reference
and sensing electrodes. Positioned on the other side of
membrane 4 is a catalytic counter electrode which is spatially
oriented so to be opposite to the sensing electrode. A
hydrated, ionically conductive bridge is formed between the
counter electrode and a point on the other side of the mem-
brane which is spatially oriented with the reference electrode
on the near side of the membrane.
Positioned on the far side of membrane 4 is a -
gasket 7 which is rigidly secured to it by double-sided ad-
hesive tape. Gasket 7, as will be pointed out in detail later,
includes a pair of circular hydration ports which communicate
with a reservoir 8 to maintain selected portions of the far
side of the SPE membrane flooded to permit transport of
water in the vapor phase across the membrane to the sensing
electrode to permit humidification of the incoming gases to
bring them to 100% relative humidity. The hydration ports
are connected by means of a water channel which is in alignment
with a ionically conductive hydrated SPE bridge on the far
side of the membrane and thus maintains a continuous water
contact with the surface of the swollen, hydrated membrane
bridge to prevent drying out of the ionically conductive
bridge.
Reservoir 8 contains a fluid filler cap shown gen-
erally at 9 which allows the introduction of distilled water
which is used for flooding the far surface of the solid
polymer electrolyte for self-humidification of the gases as
, ~ ,: ` ~ ' '
-13-
~2-E~-0-248
well as for maintaining the ionically conducted hydrated
SPE bridge flooded at all times.
As may be seen more clearly in Figure 2, reservoir
8 is filled with distilled water shown generally at 10 and
is thus in continuous contact with the upper surface of
gasket 7. Gasket 7, as pointed out below, contains a pair
of hydration ports, 11 and 12, connected by means of a water
channeI 13 which is aligned generally to be positioned over
the upper surface of a swollen, ionically conductive hydrated
membrane bridge 4 on the hydrated SPE cation membrane 4. Hyd-
ration port 11 is generally aligned spatially with one end
of the SPE bridge and hydration port 12 is spatially aligned
with a counter electrode 15 which is bonded to and embedded
in the upper surface of membrane 4. Thus, membrane bridge 14
generally extends between the counter electrode 15 to a point
on the upper surface which is a spatial alignment with the
catalytic reference electrode 16 bonded and embedded in the
lower surface of the SPE membrane 4. The openings 11 and 12
are generally larger in area than the counter electrode and
the reference electrode so that the surface of the counter
. ...... .
electrode and in the membrane area around the electrode is
flooded. As a result, water in the vapor phase diffuses
rapidly through the membrane to the other side of membrane
4 particularly to sensing electrode 17 which is bonded to and
embedded in the lower surface of membrane 4 and spatially
in alignment with the counter electrode. Gases are brought
into contact with the sensing electrode through a circular
sensing port 18 in the upper surface of bottom plate 1 with
the sensing port communicating through suitable channels
not shown, with flow ports 2 and 3 positioned in bottom plate -
1. Reference electrode 16 is in direct communication with
opening 5, which is covered by the silicone barrier film 6
-14-
~1~4~ZO
- 52-EE-0-248
to pass oxygen to the reference electrode while bloaking
the gaseous constituents, such as CO. Each of the sensing
reference and counter electrodes have suitable conductive
tabs 19, 20, and 21, which may, for example, be a small
tantalum wire approximately 0.012 inches in diameter spot
welded to the individual electrodes. The tabs are connected
to a potentiostatic circuit, presently to be described, to
maintain the potentials on these electrodes constant to permit
invariant, accurate operation.
A tape 22, which has adhesive on both sides, is
positioned between gasket 7 and membrane 4. The tape is located
on the surface of the membrane at a location away from the
electrodes and the conductiYe, hydrated SPE bridge to fasten
gasket 7 and the membrane 4 securely together. A similar tape,
not shown, having ahdesi~e on both sides is positioned between
the lower surface of membrane 4 and the top side of bottom
plate 1. The tape is located between electrodes 16 and 17
to secure the membranes to the ~ottom plate and between elec-
trodes 16 and 17 to secure the membranes to the bottom plate
and to block flow of the gases between the reference and
sensing electrodes. ReserYoir chamber 8 is securely fastened
to gasket 7 by means of a suitable adhesi~e tape located
between flange portion of the reserYOir housing and the edges
of gasket 7, thereby securing the housing firmly against
gasket 7 and sealing it against leakage.
Figure 3 is a partially broken away perspective
view of SPE membrane 4 and shows the hydrated bridge as well
as the reference eIectrode on the bottom surface of the membrane.
Thus, it may be seen that counter electrode 15 is bonded to
and embedded into the upper surface of the membrane and a
conducting tabe 20 is attached to the counter electrode by
spot weIding or the like. The catalytic electrode, as will
-15-
~14~21)
52-EE-0-248
be described in detail later, is a bonded mass of particles
of a platinum -5% iridium alloy catalyst and hydrophobic
particles such as polyetrafluoroethylene (a material sold
by duPont Company under its trade designation, Teflon). The
bonded mass of catalytic material and hydrophobic particles
is supported in a metallic, current conducting screen which
is then bonded to and embedded in the surface of the membrane. - ,
m e membrane is swollen to produce a hydrated ionically
conductive SPE bridge 14 which extends from counter electrode
15 along the lateral surface of the membrane and through the '
membrane to a position which is spatially aligned with reference
electrode 16 which is bonded to and embedded in the lower
surface of membrane 4. Thus, there is a good ionically con-
ducting path from the sensing electrode 17, not shown in Figure
3, (which is spatially aligned with counter eIectrode 15)
through the membrane to reference electrode 16. This low
resistance, and highIy ionically conductive path between the
reference and the sensing electrodes thereby substantially -
eliminates or minimizes IR drops between the sensing and -
reference electrodes which are likely to introduce changes
in the potential at the sensing eIectrode as well as changes
in the differential voltage to be maintained between the
sensing and referencing electrodes; changes which introduce
undesirable errors into the operation of the device. '-
Hydrated SPE Exchange Membrane
The solid polymer eIectrolyte ion-exchange membrane
4 which separates the sensing eIectrode from the counter elec-
trode, is characterized by ion transport selectivity. Being
a cation exchange membrane, it permits passage of positively
charged ions, i.e. cations, and rejects and blocks pasage of
negatiyely charged ions,i~e., anions. Thus, the hydrogen ions
produced through the oxidation of the carbon monoxide at the
-16-
14~ZO
~ 52-EE-0-248
sensing electrodes are transported through the ion-exchange-
membrane to the counter electrodes where it is reduced by
the addition of electrons to produce molecular hydrogen
or reacts with O2(air) to form water. There are various
classes of ion exchange resins which may be fabricated into
membranes to provide selective transport of cations. Two
broad classes are the so-called sulfonic acid cation exchange
resins and carboxylic cation exchange resins. In the sul-
fonic acid membranes, the ion exchange groups are hydrated
sulfonic acid radicals (i.e., SO3H+. XH2O) which are attached
to a polymer backbone by sulfonation. In the carboxylic
resins, the ion exchanging group is - CO OH. The ion
exchanging acid radicals in both classes are fixedly attached
to the backbone of the polymer by sulfonation and otherwise
and thus provide ion exchange capacity. me concentration
of the electrolyte, however, remains fixes since the electro-
lyte, i.e., the acid radical, is attached to the polymer
backbone and is not ~obile within the membrane. While
cation exchange membranes of either kind may be utilized in
the invention, the sulfonatea polymer type is preferred.
There are many types of sulfonated polymer exchange resins.
However, the perfluorocarbon sulfonic acid membranes are
preferred because they not only provide excellent cation
transport, but they are also highly stable, are not affected
by acids and strong oxidants, and have excellent thermal
stability. In addition to these highly desirable chemical
and physical properties, they are further characterized
by the fact that they are essentially invariant with time
and thus do not degrade. A preferred cation polymer membrane
is one in which the polymer is a hydrated copolymer of
polytetrafluoreothylene (PTFE) and polysulfonyl fluoride
vinyl ether containing pendant sulfonic (SO3) acid groups.
-17-
-- 1114~20 52-EE-0-248
The sulfonic groups are chemically bound to the perfluorocarbon
backbone. The membrane is hydrated by soaking it in 100C
water for 30 minutes. This yields a membrane having 30% to 40~
water based on dry weight of membrane. The water content remains
invariant providing the membrane is not allowed to dry out. The
structure of the sulfonated perfluorocarbon is as follows:
[ (CF2 CF2 X (CF2 CFy]n
[OCF2 CF )3]2----OCF2 CF2 SO3 ~ x H2
The ionic conductivity is provided by the mobility
of the hydrated hydrogen ions (H XH20). Electrodes 15,
16 and 17 are in the form of a decal mounted in a titanium
screen and are integrally bonded to and embedded in the
surface of the polymeric cation exchange membrane. One process
for doing so is described in detail in United States Patent
No. 3,134,697, entitled "Fuel Cell", issued May 26, 1964 in the
name of L.W. Niedrach and assigned to General Electric Company,
the assignee of the instant application. Briefly speaking,
the electrode structure is forced into the surface of a per-
fluorocarbon ion-exchange-membrane, thereby integrally
bonding the gas absorbing hydrophobic particle catalyst
mixture to and embedding it in the surface of the ion-exchange
resin membrane.
The membrane is equilibrated by immersing it in
boiling water, i.e., 100C to produce a hydrated SPE ion-exchange-
membrane.~ Thereafter, the membrane is further processed
to form theionically conductive hydrated SPE bridge
14 over a selected portion of the membrane. To this end,
the membrane is further hydrated by an "in situ" addition
of boiling water (three additions, ten minutes apart). That
is, boiling water is poured in and allowed to pass through
the hydration ports into the water channel 13. The water
- 18 -
~ Q20 52-EE-0-248
swells the area underneath the hydration channel which
extends between the counter electrode and the spatial
projection of the reference electrode on the lower surface
of the membrane. Exposure to the boiling water produces a
swelling of the surface underneath the water channel to
produce the bridge which is thus hydrated to produce a good
ionically conductive path. This procedure is repeated three
times, ten minutes apart, thereby producing the hydrated
bridge 14.
As was pointed out previously, the gas sensor des-
cribed in the instant application is a self-humidifying
arrangement in which water transport across the membrane is
achieved by transporting water across the membrane to the
sensing electrode to bring the incoming gases to 100~ relative
humidity.
In order to obtain the maximum transport of water
in the vapor phase across the membrane, the SPE membrane
should have the highest possible water content, i.e., the
highest possible ion exchange capacity (IEC). The ion ex-
change capacity and hence the water content of the membrane
is controlled ~y making the meg/gram of dry membrane as high
as possible. Thus, fcr a sulfonated perfluorocarbon membrane
of the type sold by duPont by their trade designation Nafion,
exceIlent water vapor transport will be achieved whenever
the membrane has an meg/gram of dry membrane is in the range
.83 to .95.
Catalytic Electrodes
The cell electrodes are gas permeable, noble metal
alloy catalytic electrodes comprising noble metal alloy
particles bonded to pakticles of a hydrophobic polymer
such as polytetrafluoroethylene. The catalytic electrodes
are preferably a bonded mixture of reduced oxides of a
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52-EE-0-248
platinum -5% iridium to platinum 15% iridium alloy and of - -
PTFE hydrophobic particles. The manner of fabricating the
reduced oxides of platinum-iridium are described in detail
in U.S. Patent No. 3,992,271, Ivan F. Danzig, et al, issued
November 16, 1976 and assigned to the General Electric Company,
the assignee of the present application. The nature and
character of an electrode comprising a mixture of particles
of a gas-absorbing noble metal bonded with particles of hydro-
phobic material such as polytetrafluoroethylene and process
for fabricating the same is described in detail in U.S. Patent
No. 3,432,355, entitled "Polytetrafluroethylene Coated and
Bonded Cell Structurel' issued March 11, 1969 in the name of
L.W. Niedrach, et al and assigned to the Canadian General
Electric Company, the assignee of the present application
and in U.S. Patent No, 3,297,484, entitled IlElectrode Structure
and Fuel Incorporating the Samell issued January 10, 1967 -
in the name of LW. Niedrach which is also assigned to the
General Electric Company, the assigneee of the present
invention. --
As pointed out in U.S. Patent No. 3,992,271, the
Platinum -5~6 Iridium catalyst which consists of reduced
oxides of the Platinum Iridium alloy is prepared by thermally
decomposing mixed metal salts of the elements of the alloy.
The actual method of preparation is a modification of the
Adams method of platinum preparation by the inclusion of
a thermally decomposable iridium halide such as iridium
chloride. In one example of the method finely divided halide
salts of platinum and iridum are mixed in the same eight
ratio of platinum iridium as is desired in the final alloy.
An excess of sodium is incorporated in the mixture fused in
a silica dish at 500C for three hours. The residue is
then thoroughly washed to remove the nitrates and halides
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` ~114~20 52-EE-0-248
present. The resulting suspension of mixed oxides is
reduced at room temperature by using an electro-chemical
reduction technique. The product is dried thoroughly and
ground and sieved through a mesh nylon screen. The reduced
oxide of platinum -5% iridium alloy thus produced is then
bonded with the hydrophobic polytetrafluorethylene particles
in accordance with the procedure described in Niedrach,
Patent No. 3,432,355 referred to above and bonded and em-
bedded into the surface of the membrane by the process
described in Patent No. 3,134,697 above.
Figure 4 illustrates schematically an arrangement in
which the electrodes of an SPE-type gas sensor are coupled
to a potentiostatic circuit which maintains the potential
at the sensing electrode constant at the desired level and
maintains the proper potential difference between the sensing
and the reference electrodes. Potentiostatic devices are
well known in the art and only a brief description thereof
will be provided in connection with Figure 4 for the sake of
completeness. Thus, the SPE sensing cell is shown at 20 and
includes a solid polymer electrolyte ion exchange membrane
having sensing electrode 21, a reference electrode 22 on one
side of the membrane, and a counter electrode 23 at the other
side. The reference electrode 22 is coupled to the inverting
input terminal of an operational amplifier 25 and is compared
to a reference voltage from a DC supply source 24 which is
applied to the non-inverting input of amplifier 25. DC
source 24 includes a battery or other power source 26, the
positive terminal of which is grounded. Potentiometer
resistor 27 is connected across battery 26 and has a slider
which is connected to the non-inverting terminal. The pos-
ition of this slider is so adjusted to represent the potential
at which the sensing electrode is to be maintained. Thus,
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~il4~0 52-EE-0-248
in the case of a carbon monoxide sensor, reference electrode
22 is maintained at 50 mv below the sensing electrode poten-
tial so that the reference voltage at potentiometer resistor
27 is maintained at one 1-1 volts. With a voltage differential
of +50 mv between the sensing and reference electrode, the
sensing electrode is maintained at 1.1 volts with respect to
a platinum/H2 electrode. The potentiostatic circuit thus
shown in Figure 4 senses any change in the voltage between
the reference and sensing electrode and compares it to the
preset value at the potentiometer slider. Any changes are
used to generate a current at the output of operational amp-
lifier 25 between counter electrode 23 and sensing electrode
21 to eliminate the difference voltage producing it. The
output current is sensed across resistor 28 by a suitable
meter 29.
The current required to drive the system back to null ~ -
balance is representative of the concentration of the gas
being sensed. That is, as a gas such as carbon monoxide is
oxidized to CO2 and the electrons are liberated by the
potential at the sensing electrode tends to shift as does the
differential Yoltage. A current is generated to drive the
; system through a negative feedback mode back to its null point,
nameIy to maintain the sensing electrode at 1.1 volt and the
differential voltage between the reference and sensing voltage
at +50 mv. The current required to do so is then a direct
indication of the gas concentration since it is the oxidation
of the gas which has produced the change in the differential
voltage which requires a shift in the current to drive the
system back to null balance.
As has been pointed out previously, the voltage range
at the sensing electrode with respect to a platinum/hydrogen
reference electrode is between 1.0 volts and 1.3 volts with
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.
li~4~3Z0 52-EE-0-248
the preferred voltage being l.l volts. The upper voltage is
limited to a maxi~um of 1.3 to avoid oxidation of the water
since this water reaction competes with the oxidation of the
carbon monoxide and introduces current flow which produces
errors in the sensing of the gas concentration. The voltage
should not be allowed to go below l.0 volts in order to
maintain an oxide coating on the surface of the catalytic
sensing electrode. That is, at l.0 volts or above the platinum
-5% iridium reduced oxide alloy has a thin oxide film on the
surface which enhances operation and also prevents carbon
monoxide poisoning of the catalyst. Below l.0 volts, the
oxide at the surface of the catalyst is removed and there is
a risk of the carbon monoxide poisoning the catalyst. In
addition, there is a possibility of reducing oxygen, thus
introducing a competing reaction which produces a current flow
which introduces an error factor into the measurement of the
gas concentration. That is, any current flow which is due to
anything but the oxidation of carbon monoxide, as for example,
by reduction of oxygen or oxidation of water, which are com-
peting reactions, introduce errors.
In order to illustrate the manner in which an ionically
conductive bridge integral with the SPE membrane results in
impro~ed performance, eIectrode sensor cells were fabricated
and tested in the followin~ fashion. A cell was built in
which Teflon bonaed platinum -5% iridium electrodes were
bonded to a sulfonated fluorocarbon membrane. Sensing and
counter eIectrodes were approximately ll/16ths inch in diameter
and the reference eIectrode approximately 5/16ths inch in
diameter. The sensing and reference electrodes were on the
same side of the solid polymer electrode. Tantalum wires of
approximately .012 of an inch were spot welded to the electrode
for current collection. The counter electrode side was
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,
'
~ 14~20 52-EE-0-248
flooded with 10 cc distilled water at ambient temperature
through the use of a gasket having hydratlon ports. The cell,
however, did not have a hydrated membrane bridge. The cell
was activated using a potentiostatic circuit. The sensing
electrode was maintained at 1.1 volts +50 mv above the reference
electrode. Testing was conducted by exposing the reference
electrodes to ambient air and the sensing electrode to an
18 parts per million carbon monoxide in air feed. The
results were as follows:
Background Sensor
Total Signal (uA) Signal (uA) Output (uA) Reservoir
With 17 ppm CO Due to Air Due to CO Alone Solution
+4 to 6 +2 4 Distilled H O
+5 to 6 +2 4 ~
Add boiling distilled H2O and then allow to cool to 25C
-4 to -5 2 4
The cell assembly was then modified by the addition
of liquid electrolytes such as sulfuric acid at various con-
centrations to rhe reservoir side and the output measured
again with an 18 part per million concentration of carbon
monoxide in air stream. The output date for the cell was as
follows:
Background Sensor
Total Signal (uA) Signal (uA) Output (uA) Reservoir
With 18 ppm CO Due to Air Due to CO Alone Solution
+10 to 12 +2 +10 2 4
14 +1 +13 H2 4
+18 to 20 NIL +20 1.0 NH2SO4
+22 to 24 +2 +22 2 4
me data indicates that a liquid electrolyte bridge between
the sensing and the reference electrode increases magnitude of
response. However, by adding liquid electrolyte, it becomes
difficult to maintain the cell output invariant as the electro-
lyte concentration change with time. The danger of acid
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`: 1114~Z0 52-EE-0-248
spillage and migration of electrolyte to cover catalytic
sites is also still present. The sensor cell was disassembled
and a small water channel of the type shown in Figur~ 2 was
inserted in the gasket with the water channel extending between
the counter electrode and a spot on the surface of the mem-
brane which was in spatial alignment with the reference electrode
on the other side of the membrane. The entire SPE membrane
was boiled in distilled water prior to assembly to hydrate
it. The cell was then exposed to an air stream containing 18
parts per million of carbon monoxide. A liquid electrolyte in
the form of 1.5 normal sulfuric acid solution was added to
the reservoir side and the sensor output measured. The
results were as follows:
Background Sensor
Total Signal (uA) Signal (uA) Output (uA) Reservoir
With 18 ppm CO Due to Air Due to CO Alone Solution
+12 +2 +10 Distilled H2)
+24 +2 +20 1.5 NH2SO4
It can be seen that the partially hydrated bridge formed due
to the addition of boiling distilled water does increase
the sensitivity of the gas sensor ~rom 4 to 10 microamps for
an 18 ppm concentration, but it is still substantially below
the cell output for the same gas concentration, but utilizing
a 1.5N sulfuric acid liquid electrolyte~bridge.
Thereafter, the SPE membrane was further hydrated
along a selected portion of the membrane, i.e., in the bridge
area, by "in situ" addition of boiling water (three additions - -
10 minutes apart). The sensor was allowed to cool to 25C
and then retested with an 18 ppm CO concentration in air.
The output of the sensor with the counter side flooded by
distilled was as follows:
-25-
~ ZO 52-EE-0-248
Background Sensor
Total Signal (UA) Signal (uA) Output (uA) Reservoir
Time W th 18 PPM CO Due to Air Due to CO Alone Solution
0800 23--24 2 22 DiSt. H2O
1200 24 2 22 "
1745 23 2 21 "
It is apparent from this data that the hydrated SPE
bridge formed by "in sitUI~ addition of boiling water to the
reservoir increases the output signal and also maintains it
invariant with time. The output of the cell with an integrated
SPE bridge is equal to the produced by a sensor in which a
liquid electrolyte bridge while eliminating all of the problems
normally associated with the use of liquid electrolytes.
The same cell was used to detect NO in an air feed
at 30 cc/min at various concentrations with the following
results:
NO Conc Signal
(ppm) (uA)
38
53-5 4
41 - 74- 75
It is apparent .from this data that NO may be detected
at low concentrations in the same fashion as CO and that
good sensor output in range 1. 5 - 2 uA/ppm is obtained with
this ceIl construction~ .
It is therefore apparent that an improved, compact
gas sensor has been provided which utilizes a solid polymer
electrolyte having an integral ionicaly conductive bridge
between a r~ference and sensing electrodes which is not ~ ::
subject to all of the shortcomings normally associated with
potentiostated, three electrode, gas sensors utilizing liquid
electrolytes.
While embodiments of this invention have been
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lil4~20 52-EE-0-248
shown and described, it will, of course, be understood
that the invention is not limited thereto, since many other
arrangements both in the devices and structures and in the
process steps may be employed. It is contemplated by the
appended claims to cover any such modifications that fall
wtihin the true scope and spirit of this invention.
-27~
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