Electrochemical gas sensor and method of using same

An electrochemical gas sensor having a solid ionic conductive electrolyte therein and adapted to sense trace levels of reactive gases in the parts per million range. The electrochemical sensing cell has a pair of catalytic electrodes attached to the opposite sides of the solid electrolyte. The sensing cell is supported and contained to permit the distribution of reactant gases over the electrode surfaces for reacting thereat and thereby providing an external electrical current flow representative of the quantity of the reactant gas undergoing sensing.

FIELD OF INVENTION 
This invention relates to electrochemical gas sensors for electrically 
signalling the concentration of an electrochemically active gas, such as 
hydrogen and oxygen, in a gas mixture and, more particularly, to a sensing 
cell utilizing a solid electrolyte capable of sensing electrochemically 
active gases in ranges down to parts per million. 
CROSS-REFERENCE 
This invention is an electrochemical gas sensor of the type disclosed in 
the earlier filed copending application bearing Ser. No. 404,680 and 
assigned to the same assignee as the present invention, in that the 
invention disclosed herein is alsc an electrochemical gas sensor capable 
of sensing reactive gases in the parts per million (p.p.m.) range. The 
aforementioned application was abandoned and refiled as a continuation 
application bearing Ser. No. 563,811 on Aug. 6, 1990 and issued as U.S. 
Pat. No. 5,085,760 on Feb. 4, 1991. The present invention, however, 
utilizes a solid electrolyte element, rather than an aqueous electrolyte 
solution, as is typical of known, prior art electrochemical gas sensors 
and the gas sensor disclosed in the aforementioned copending patent 
application utilizes an aqueous electrolyte solution. 
BACKGROUND OF INVENTION 
One of the major problems associated with the present day, known 
electrochemical gas sensors that utilize an aqueous electrolyte solution 
is the loss of water from the sensor during the sensing operations. When 
these prior art sensors are utilized and if the gases to be sensed are not 
humidified prior to applying them to the electrochemical gas sensor, the 
loss of water from the sensor may even be severe. The loss of water from 
these prior art electrochemical gas sensors can significantly affect the 
output signal as well as the life of the sensor. Certain types of 
electrochemical gas sensors can be used continuously by adding water to 
the sensor's body periodically, typically once a month. Some gas 
analyzers, such as the Teledyne Analytical Instruments Model 306WA Trace 
Oxygen Analyzer, have the capability to humidify the gas prior to exposure 
of gas to the electrochemical sensor. To utilize such equipment, however, 
adds substantial complications to an analysis procedure and particularly 
in the control of the extent to which the gases are humidified. As should 
be apparent, the improper control of the gas humidification of the gases 
undergoing sensing could either increase or decrease the water level in 
the sensor and, accordingly, adversely affect the output signals derived 
from the sensor. At the present time, therefore, there is no other known 
solution to the loss of water problem from an aqueous electrolyte 
solution. 
Solid, ionic conductive elements are known and have been used in 
hydrogen-oxygen fuel cells, as is well-known to those skilled in the fuel 
cell art. The use of such solid, ionic conductive electrolyte elements in 
an electrochemical gas sensor, however, has not been heretofore proposed 
or used in such electrochemical gas sensors to solve the problems of the 
loss of water in the typical prior art gas sensor, as we presently 
understand the prior art. 
SUMMARY OF INVENTION 
The present invention provides an improved, inexpensive and significantly 
simpler construction for an electrochemical gas sensor as well as a 
simplified operation without the prior problems of the drying of the 
electrolyte used in the sensor. The electrochemical gas sensing cell of 
the present invention is capable of sensing concentrations of 
electrochemically active gases in gas mixtures in the parts per million 
range, as well. The use of a solid electrolyte element in the gas sensor 
eliminates the problem of the loss of water in sensors utilizing the 
aqueous electrolyte solution. The present invention utilizes a solid ionic 
polymer membrane that when equilibrated with water, causes the membrane to 
achieve significant ionic conductivity at room tempratures and, when 
properly treated, up to temperatures of 180 degrees Centigrade. The 
polymer membrane is utilized with a high surface area, metal catalyzed gas 
diffusion electrodes for both the anode and cathode electrodes. The anode 
may have a platinum catalyzed metal, while the oxygen sensing cathode 
electrode may have a platinum, or silver, or gold catalyzed element. The 
cathode electrode, then, will sense the quantity of oxygen in the gas 
mixture applied thereto while hydrogen will be fed to the anode electrode 
in proportion to the general requirements of the quantity of the oxygen 
that is contained in the gas mixture applied to the cathode. The same cell 
may be utilized for sensing hydrogen in the gas mixture with the amount of 
oxygen applied to the cell being of a known concentration. 
The method comprehended by the present invention is one for sensing the 
concentration of electrochemically reactive gas in a gas mixture, 
including concentrations in the parts per million range, as well as the 
steps of providing a solid electrolyte membrane capable of being rendered 
ionic conductive when equilibrated with water or phosphoric acid. The 
method includes a pair of catalytic electrode means at opposite sides of 
the solid membrane with each electrode means being attached to an 
individual side of the solid membrane, each of the electrodes being 
constructed of a high surface area metal catalyzed gas diffusing 
electrode. The method then comprehends distributing a gas mixture having a 
reactive gas therein to one of the electrodes for causing an 
electrochemical reduction of the active gas thereat and producing water, 
while distributing another gas of known concentration to the other 
electrode means to cause an electrochemcial oxidation thereof for 
producing an electronic charge that flows between the electrodes during 
the electrochemical reactions. 
From a specific structural standpoint, the electrochemical sensing cell 
comprises a solid ionic conductive electrolyte element with anode 
catalytic electrode means and cathode catalytic electrode means arranged 
at opposite sides of the solid element and attached to the solid element 
whereby in combination they constitute the sensing cell. The anode and 
cathode electrode means are characterized as a composite structure with 
electrically conducting gas diffusion hydrophobic surfaces and having the 
electrically conductive hydrophobic gas diffusing layer adapted for 
conveying gas therethrough and a catalyst layer of a high surface area 
metal dispersed on the high surface area carbon support. Container means 
enclose the sensing cell, including means for supporting the thus defined 
electrochemical sensing cell whereby the anode and cathode electrode 
surfaces are exposed within the container. The sensing cell includes means 
for continuously circulating a gas mixture having an unknown quantity of 
an electrochemcially active gas therein to be sensed for exposure to the 
gas diffusion surface of one of the electrode means to be diffused 
therethrough to the catalyst layer to be reacted thereat and means for 
circulating a gas of a known constant concentration for exposure to the 
gas diffusing surface of the other electrode means to be diffused 
therethrough to the catalyst layer. The gases applied to the electrodes 
are selected for producing the reactions at the electrodes to cause the 
thus generated electronic charge to flow between the electrodes through an 
external circuit for signalling the sensed concentration of the reactive 
gas in the applied gas mixture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention contemplates solving the problem of electrochemical 
gas sensors utilizing an aqueous electrolyte solution associated with the 
loss of water from the sensor during its use, by the use of a solid ionic 
conductive electrolyte, preferably in the form of a solid, ionic polymer 
as the electrolyte element. In addition, the electrochemical gas sensor 
having a solid electrolyte is contemplated for use in sensing gas traces 
in the parts per million (ppm) range. An example of a solid ionic polymer 
element that may be used in an electrochemical gas sensor of the type 
under consideration is commercially available from the E. I. duPont de 
Nemours & Company, Inc., of Wilmington, Del. Similar ionic conductive 
polymers are available from the Dow Chemical Company of Midland, Mich. 
Prior to embarking on the detailed disclosure of the electrochemical gas 
sensor utilizing an ionic conductive, solid element as the electrolyte, it 
would facilitate the understanding of the present invention if the 
functioning of the solid electrolyte material is in mind. A solid ionic 
conductive polymer that is manufactured and sold by the duPont Company 
under the trademark "NAFION" will be described. The polymer strand of the 
duPont ionic polymer consists of perfluorinated carbon chains with ether 
linkage and a sulfonic acid group attached at the end of the polymer 
strand as is represented by the following general structure: 
##STR1## 
The equivalent weight of the aforementioned duPont solid ionic polymer is 
1100 and the molecular weight is more than a million. Reference may be 
made to the duPont Technical Information Bulletin, Number AL-163, on 
"Nafion Resins" for a more detailed description of this product and which 
disclosure is incorporated herein by reference. 
It is known that when the duPont solid ionic polymer is equilibrated with 
water, the sulfonic groups protonate water molecules, and, as a result, 
the solid polymer membrane achieves significant ionic conductivity at room 
temperature. The loss of the protonated water from said duPont solid ionic 
polymer is insignificant at room temperature and is therefore in itself a 
solution to the loss of water in the prior art type of electrochemical 
sensors. However, it has been found that at temperatures greater than 50 
degrees Centigrade, the water from the selected solid ionic polymer will 
be slowly lost, and, as a result, the ionic conductivity of the element 
will accordingly decrease. This will render the polymer resin unsuitable 
for use as a solid electrolyte in an electrochemical cell at these higher 
temperatures. 
It has been found that the ionic conductivity of the polymer element can be 
maintained even at temperatures above room temperature, up to 180 degrees 
Centigrade, provided the polymer membrane is pre-equilibrated with 
phosphoric acid by soaking the membrane in phosphoric acid at 100 degrees 
Centigrade for several hours. At 100 degrees Centigrade water from the 
Nafion element is lost and to the most extent is replaced by phosphoric 
acid (98 percent) since the sulfonic groups of the Nafion element can 
easily protonate the phosphoric acid molecules. At the 180 degree 
Centigrade temperature, phosphoric acid contains less than 4 percent by 
weight water. This will then permit the ionic conductivity of the polymer 
element to be maintained at temperatures up to 180 degrees Centigrade. 
Since the vapor pressure of phosphoric acid at 180 degrees Centigrade is 
relatively low, the loss of phosphoric acid from the membrane will be 
insignificant at the 180 degrees temperature and ionic conductivity of the 
polymer membrane will not be seriously affected at this higher 
temperature. At temperatures greater than 180 degrees Centigrade the 
Nafion element slowly degrades and adversely affects the kinetics of the 
electrochemical reactions at the electrode surfaces, and therefore the 
sensor shows unstable output over extended periods of time, i.e., the 
sensor becomes useless. The protonated form of the solid polymer carries 
the ionic charge from one electrode to another of a sensing cell during 
the electrochemical reactions at the anode and cathode electrodes provided 
for the cell. The use of a solid ionic polymer as a solid electrolyte 
element in hydrogen-oxygen fuel cells is well-known to those working in 
the fuel cell art. As is indicated hereinabove, the use of a solid polymer 
electrolyte pre-equilibrated with phosphoric acid in an electrochemical 
gas sensor, however, has not been reported to date, and no such disclosure 
is known to the applicants herein. It has been found that the use of a 
solid polymer electrolyte in an electrochemical gas sensor of the type 
under consideration would significantly simplify the sensor design and the 
operation thereof and avoid the problems of the prior art type of gas 
sensor utilizing the aqueous electrolyte solutions and the loss of water 
therefrom. With the use of a such a pre-equilibrated solid electrolyte 
element there would be no free electrolyte in the resulting sensor and, 
therefore, there would be no danger from the sensor becoming dry during 
its operation and thereby inoperative, or impaired in operation. 
Now referring to the drawings and specifically to FIG. 3, the configuration 
of the cell 10 utilized in the sensor of the present invention will be 
first examined in detail based on a schematic representation of the cell 
10 therein. The solid electrolyte selected for use in the cell 10 and 
illustrated in FIG. 3 as the element 12 is constructed of the resin 
identified as a "Nafion" resin of E. I. duPont de Nemours Company, Inc., 
as discussed hereinabove. The important characteristic of the solid 
electrolyte element 12 is that the "Nafion" resin is a perfluorinated 
ion-exchange polymer that becomes ionic conductive when equilibrated with 
water or other molecules which are protonated by the sulfonic group of the 
"Nafion" element. The solid electrolyte membrane 12 selected for use in 
the cell 10 of the present invention has a circular configuration of 1.25 
inches in diameter and a thickness of 7/1000 of an inch (7 mils). The cell 
10 is completed by sandwiching the membrane 12 between two high surface 
area metal catalyzed gas diffusion electrodes 13 and 14, respectively, 
identifying the cathode and anode electrodes of the cell 10. The cathode 
electrode 13 and anode electrode 14 are constructed identically, and each 
has a diameter of 0.92 inches and a thickness of 17/1000 of an inch. The 
cathode electrode 13 has a silver, Ag, catalyst thereon while the anode 
electrode 14 has a platinum, Pt, catalyst thereon. The electrodes that may 
be utilized in accordance with the present invention are commercially 
available from Prototech Company of Newton, Mass., and other suppliers. 
Specifically, the structural organization of such electrodes is well-known 
for use in fuel cells and is disclosed in detail in U.S. Pat. No. 
4,647,359 and assigned to said Prototech Company. The commercial 
embodiments of such electrodes as disclosed in the Prototech patent are 
available from the Prototech Company and are useful in the sensor S. The 
presently preferred embodiment of the electrode that is commercially 
available is identified as a gas diffusion electrode on Toray paper and 
has been utilized in the sensor S. The disclosure of U.S. Pat. No. 
4,647,359 is incorporated herein by reference. Although such electrodes 
are utilized in fuel cells for generating electric power, there is no 
known utilization of such electrodes in electrochemical gas analyzers of 
the type which is the subject of the present invention and, particularly, 
in sensing gas traces, such as oxygen and hydrogen in the parts per 
million range. The use of high surface area metal catalyzed gas diffusion 
electrodes affords the possibility of achieving much higher effective 
surface areas for the electrodes of an electrochemical gas analyzing cell 
without increasing its geometric area.The typical effective surface area 
of a gas diffusion electrode may be up to 600 times greater than its 
geometric area when compared to a smooth, metal screen electrode where the 
effective area is usually twice the geometric area. The aforementioned 
commercially available electrodes useful in the present invention 
typically may have a high surface area for the catalyst surface of at 
least 150 m.sup.2 /gram, (150 square meters per gram). In terms of 
particle sizes of metal catalyst, the sizes fall within the 15-25 
Angstroms range. The relationship of particle sizes and surface area is 
well-known in the prior art, and this particle size relates to a surface 
area of 178-112 m.sup.2 /gram. 
The electrodes 13 and 14 are a composite structure having a gas diffusing 
Teflon-carbon backing layer on one side thereof bonded to a relatively 
thin layer of high surface area catalyst metal dispersed on a high surface 
area carbon support. The gas diffusing portions of the electrodes consist 
of a microporous structure of Teflon and carbon mixture. The microporous 
structure functions as gas wicks to convey the gas subjected thereto 
through the microporous, electrically conductive, hydrophobic structure of 
the Teflon-carbon mixture to the catalyst surface. This gas receiving and 
diffusing layer permits the conveyance of the gas therethrough by means of 
the gas wicks formed by Teflon-carbon structures. The catalyst layer of 
the cathode is a relatively thin layer having approximately 1/10th of the 
thickness of the backing layer. This layer can be termed the catalyst 
layer as it is exposed to the solid electrolyte of the cell S. The 
catalyst layer consists of a high surface area metal catalyst, such as 
silver, platinum, gold and the like metals, dispersed on a high surface 
area carbon support. A relatively small amount of Teflon or a plymeric, 
fluorinated hydrocarbon material is used as a binder of the metallic 
catalyst to the carbon support. The electrodes 13 and 14 may e utilized 
without a backing layer and still be operative and those types of 
electrodes are also commercially available from the above identified 
Prototech Company. The thus defined gas diffusing layers of the electrodes 
13 and 14 allow the gases to be analyzed to diffuse through its 
micorporous structure and reach the metallic catlyst surfaces where an 
electrochemically active gas, such as oxygen, reacts. 
As illustrated in FIG. 3, a metal ring 15 is secured to the cathode 
electrode 13 in electrical contact with the electrode and provided with a 
lead wire 15L connected to the ring 15 for use as an output lead terminal. 
Similarly, the anode electrode 14 has a metal ring 16 secured adjacent the 
ends thereof with its individual output lead 16L connected to the ring 16. 
The cell 10 functions, for example, when a gas mixture having an unknown 
concentration of oxygen is delivered to the cathode electrode 13 and a 
reactive gas such as hydrogen is coupled to the anode electrode 14. 
Assuming these operating conditions, the cell 10 will have the overall 
reactions at the anode electrode 14 and the cathode electrode 13 as 
follows: 
##STR2## 
During the reaction at the cathode electrode 13, the anode electrode 14 
will give off hydrogen ions and electrons. The electrons thus released at 
the anode electrode flow to the cathode electrode through an external 
circuit with a magnitude proportional to the oxygen concentration of the 
gas applied to the cathode. As the aforementioned equations symbolize 
during the operation of the cell 10, water is produced at the cathode 
electrode 13. The amount of water produced at the cathode electrode 13 
will depend on the concentration of oxygen in the gas mixture applied to 
the electrode and the length of time for which the sensor S is used. Since 
the gases to be sensed are usually dry, water concentration will not build 
up in the sensors at least when the sensor S is used for sensing 
electrochemical, reactive gases with low oxygen concentrations for 
relatively short periods of time. If the sensor is to be used for sensing 
gases with high concentrations of oxygen for longer periods of time, it 
would be advantageous to use the sensor S at temperatures above 50 degrees 
Centigrade to minimize the water buildup in the sensor S, as will be 
detailed hereinafter. 
Now considering the treatment required of the "Nafion" resin membrane 12 
for use at temperatures above room temperatures up to 180 degrees 
Centigrade, it will be noted that the resin membrane as received from the 
supplier is in the form of a sheet having a thickness of 7/1000 (7 mils) 
and is then cut into a circular configuration of a desired diameter (1.25 
inches). Then it is normally cleaned using a dilute hydrogen peroxide and 
deionized water in accordance with the procedures well-known to those 
skilled in the art of handling such solid electrolyte materials. After the 
cleaning process, the membrane 12 is soaked in a concentrated phosphoric 
acid at a temperature of 100 degrees Centigrade for several hours, 
preferably for at least twelve hours. During this soaking period, most of 
the water in the membrane is replaced with the phosphoric acid. The 
membrane 12 is then removed from the phosphoric acid, wiped quickly and 
sandwiched between the electrodes 13 and 14 using a hydraulic press at 
approximately 1000 pounds per square inch. This assembly constitutes the 
electrochemical cell 10 and is then in condition to be housed for defining 
the sensor S. 
With the construction of the cell 10 in mind and now referring to FIG. 1, 
the incorporation of the cell 10 in the sensor S as illustrated in FIGS. 1 
and 2 will now be examined. The sensor 10 is first placed between two 
nonconductive blocks, such as a pair of acrylic plastic blocks 20 and 21 
of a circular configuration. The blocks 20 and 21 each have an opening 
through their centers of an inside diameter of approximately 0.7 inches. 
The two blocks 20 and 21 with cell 10 placed centrally thereof, as 
illustrated, are secured together for holding the cell 10 in position by 
two stainless steel plates 22 and 23 arranged on opposite sides of the 
blocks 20 and 21, respectively. Each of the stainless steel plates 22 and 
23 have stainless steel tubes welded thereto for circulating the gases to 
be sensed by the cell 10. The stainless steel plates 22 and 23 are secured 
together for securing the blocks 20 and 21 holding the cell 10 
therebetween by means of the fasteners 30 and 31 secured in suitable 
apertures for each of the plates 22 and 23 adjacent the ends thereof with 
the heads of the fasteners 30 and 31 illustrated in FIG. 1 on the left 
hand side and threaded into suitable threaded apertures provided for the 
right hand plate 22. 
Assuming that the cathode electrode 13 is illustrated on the right hand 
side of FIG. 1 and the anode electrode 14 on the left side, as 
illustrated, the cathode electrode 13 will have a silver catalyst secured 
thereto while the anode electrode 14 will have a platinum catalyst secured 
to the anode electrode. The system for delivering the gases to the anode 
and cathode electrodes in accordance with the above arrangement will 
include a tubular element 33 welded to the plate 23 for delivering the gas 
to the anode electrode 14 of the cell 10. The plate 23 is also provided 
with an outlet tubular element 34 spaced from the inlet tubular element 33 
for causing the gas entering the tubular element 33 to be circulated past 
the high surface area platinum catalyzed anode electrode 14 and then exit 
through the tubular element 34. It will be recognized that when the 
hydrogen gas is coupled to the tubular element 33, the hydrogen will be 
oxidized at the anode electrode in accordance with the above described 
reaction equation giving off hydrogen ions and electrons. Similarly 
arranged with the cathode electrode 13 on the opposite side of the cell 
10, there are provided two tubular elements 35 and 36 arranged in a spaced 
apart relationship and welded to the stainless steel plate 22. The tubular 
element 35 may function as an entry element for the gas containing the 
reactive gas to be sensed, such as oxygen, and coupled past the cathode 
electrode 13 and then exits the space between the electrode and the inside 
wall of the plate 22 by means of the exit tubular element 36, as indicated 
hereinabove. With the oxygen in the gas mixture applied to the cathode 
electrode, the oxygen would be reduced in accordance with the above 
identified equation. With the reactions taking place simultaneously at the 
anode and cathode electrodes, the current will flow between the electrodes 
and through the exterior circuit for sensing the oxygen concentration and 
providing an electrical output signal representative of the concentration 
of the sensed reactive gas or oxygen in the gas mixture coupled to the 
entry tubular element 35. FIG. 1 eliminates the conductive rings secured 
to the anode and cathode electrodes, illustrated in FIG. 3 merely for 
simplification purposes in view of the small sizes of the elements under 
consideration. The arrangement illustrated also includes sealing the 
blocks 20 and 21 by means of the O-rings 37 and 38 secured between the 
inner wall of the plate 22 and the block 20 and the inner wall of the 
plate 23 and the block 21, preventing the escape of the gases undergoing 
sensing coupled into the spaces on the opposite sides of the cell 10. The 
conductive lead wires 15L and 16L illustrated in FIG. 3 that are secured 
to their respective rings 15 and 16 are illustrated in FIG. 1 whereby the 
lead wire 15L functions as the cathode contact and is threaded through the 
blocks 20 and 21 at the bottom portions thereof, as is evident from FIG. 1 
and the cathode contact lead is a wire constructed of metallic wire, while 
the lead wire 16L extends through the upper portions of the blocks 20 and 
21 in the sealed relationship for providing the external output signal 
between the anode and cathode electrodes. The lead wire 16L is constructed 
of the same metallic wire as the lead wire 15L. 
The sensor S utilizing the "Nafion" resin membrane 12 as the solid polymer 
electrolyte and a high surface area platinum catalyzed gas diffusion anode 
electrode 14 attached to the membrane 12 and a high surface area silver 
catalyzed gas diffusion cathode electrode 13 constructed as described 
hereinabove has been successfully operated. The sensor S response that 
resulted produced a linear output with varying concentrations. The output 
signal can be increased by wetting of the cathode or anode electrodes or 
by coating the electrode with a thin film of the "Nafion" resin by soaking 
the electrode in a "Nafion" resin solution. This, however, would require 
the gas diffusion electrode to be modified so as not to have the 
hydrophobic backing layer. Such gas diffusion electrodes are also 
available from the Prototech Company referenced hereinabove. The 
electrode, then, after being coated with a thin film of the selected 
resin, can be attached to the solid membrane 12 using pressure in a 
similar fashion as described hereinabove. 
Now referring to FIG. 4, the electrical circuit for processing the 
electrical signals derived from the sensing cell S at suitable external 
cathode and the external anode terminals will be considered. Any 
conventional sensing circuit may be employed by coupling it to the 
external anode and cathode terminals and in FIG. 4, a conventional 
operational amplifier Amp is illustrated connected to the external anode 
and cathode terminals with the anode electrode illustrated connected to 
the negative input terminal of the amplifier Amp. The output circuit for 
the amplifier Amp is connected in series circuit relationship with an 
output resistor AO connected to a common voltage level or ground. A 
feedback resistor AF is connected between the output terminal common to 
the resistors AF and AO, to the negative input terminal of the amplifier 
Amp. A meter M may be connected between the output terminal O and ground 
that is calibrated to read the concentration of the sensed oxygen of the 
gas undergoing analysis for a direct read-out of the sensor S. 
In the use of the sensor S, the concentrations of the electrochemically 
reactive gases such as hydrogen admitted to the anode electrode will be 
dependent on whether the gas to be analyzed has a high or low oxygen 
concentration. If the sensor S is to be used to analyze a gas which 
contains a low concentration of oxygen, in the parts per million (ppm) 
range or lower, the content of hydrogen gas in an inert gas such as 
nitrogen could be approximately 1 percent. If the quantity of oxygen to be 
sensed is at an approximately 1 percent level, pure hydrogen could be used 
at the anode electrode. 
It should be noted that the same sensor S could be used as a hydrogen 
sensor with the same identical construction. This can be achieved by 
keeping the oxygen concentration that is coupled to the cathode electrode 
13 constant while analyzing the gas mixture containing hydrogen at the 
anode electrode 14.