Patent Grant
RE45100
The invention relates to electrochemical gas sensors, and particularly relates to electrochemical gas sensors having a sensing electrode, a counter reference electrode, and a solid proton conductor for room temperature detection of the concentration of carbon monoxide (CO) in the ambient.
In most prior art solid state commercial gas sensors, it is necessary to heat the sensor element to elevated temperatures in order to acquire both fast response time and high sensitivity to objective gases. For example, N-type semiconductor tin oxide gas sensors and catalytic combustion type Pd/Pt gas sensors must usually be operated in a temperature range of ca. 200° to 500° C. These sensors must be equipped with heaters connected to external power sources. Therefore, room temperature CO gas sensors, which use less power, are desirable.
It is well known that CO reacts with moisture in air at room temperature, and forms protons, electrons, and CO2 in an oxidation reaction of CO.
CO+H2O→CO2+2H++2e− (1)
It is also known that there is a moisture formation reaction by combining protons, electrons, and oxygen in a reduction reaction of oxygen:
2H++2e−+½O2→H2O (2)
These two reactions are the basis of prior art room temperature low power electrochemical gas sensors utilizing a proton conductor.
The current generated by the reactions depicted in
Whether the transport processes shown in
The sensor of
A prior art room temperature proton conductor sensor developed by General Electric using a polymer porous support material saturated by a liquid proton conductor, has been constructed as an electrochemical amperometric CO gas sensor (the G. E. Sensor). In the G. E. Sensor, a liquid reservoir was used to provide the liquid proton conductor to the porous support material. Protons, which are indicative of the ambient CO concentration, were driven across the porous support material through the liquid conductor by a DC voltage. Electrical current response of the sensor to ambient CO concentration was linear. The cost of the sensor with such a complicated design, however, is high and is thus not be suitable for practical consumer applications.
In U.S. Pat. No. 4,587,003, a room temperature CO gas sensor using a liquid proton conductor is taught. Basically, the mechanism and design of the sensor were similar to the G. E. sensor, except that the outside surfaces of the sensing and counter electrodes of the sensor in this patent were coated by porous NAFION™ layers. The CO room temperature gas sensor taught in the patent currently costs about $200.00. The lifetime of such a sensor is about 6-12 months due to the rapid drying of the liquid of the electrolytes. In addition, the sensor requires maintenance due to leakage and corrosion of liquid electrolyte.
The discovery of room temperature solid proton conductors aroused considerable efforts to investigate low cost, all-solid electrochemical room temperature CO gas sensors. One such sensor that was developed was a room temperature CO gas sensor with a tubular design using proton conductors, electronically conductive platinum or the like as the sensing electrode, and electronically conductive silver, gold, graphite or the like as the counter electrode. The sensing electrode decomposed carbon monoxide gas to produce protons and electrons, whereas the counter electrode exhibited no activity to decompose carbon monoxide with the result that a Nemst potential occurred between the two electrodes. Thus, carbon monoxide gas was detected.
In detecting carbon monoxide with the tubular design sensor, protons and electrons are generated at the sensing electrode. For the reaction to be continued, protons and electrons must be removed from the reaction sites, and CO and moisture must be continuously provided from the gaseous phase to the reaction sites. Therefore, the CO reaction only occurs at three-phase contact areas. The three-phase contact areas consist of the proton membrane phase, the platinum electron phase, and the gas phase. Due to the limited three-phase contact areas in the tubular design sensor, the CO reaction was slow. Additionally, the response signal was weak. Further, the Nernst potential was not zero in clean air.
A modified electrochemical CO room temperature gas sensor using a planar or tubular sensor design was a subsequent development to the earlier tubular design CO sensor. In order to overcome the problem that the Nernst potential is not zero in clean air experienced with the earlier tubular design CO sensor, the improved design proposed a four probe measurement method for CO gas detection. The improved design achieved a zero reading in clean air, and the improved sensor was insensitive to variations in relative humility. Theoretical analysis based on electrochemistry, however, indicates that there is no difference between the four probe method and the normal two probe method of the earlier tubular design CO sensor. The improved sensor still used electronic conductors for both the sensing and counter electrodes, and showed slow and weak response signals to CO gas.
A still further improved design of a CO sensor is a room temperature electrochemical gas sensor using a solid polymer proton conductor with a planar sensor design. Response of this further improved sensor to CO was very weak, and was in the nA range even as a DC power source was applied. Apparently, the internal resistance of the sensor was too large. Calculations based on this further improved sensor dimensions indicates that the ionic resistance of the proton conductor membrane is about 400 K-ohm, which is too large to generate a usably strong signal. Further development and improvement of the planar CO gas sensor, which incorporated a sensing mechanism, resulted in performance that was still in nA range of sensor response.
It is an object of this invention to provide a low cost room temperature electrochemical gas sensor, for carbon monoxide and other toxic gases, having a low ionic resistance, a rapid response, and a strong signal to the detection of gaseous CO in the ambient. The toxic gases that can be sensed by the inventive sensor, each of which is referred to herein as an analyte gas, include H2, H2S, H2O vapor, and NOx concentrations.
The inventive electrochemical sensor has both a sensing electrode and a counter electrode. Each of the sensing and counter electrodes can be made of mixed protonic-electronic conductors so as to encourage a high surface area for reactions at the electrodes, which cause fast analyte gas reaction kinetics and a continuity in the transport of electrical charges so as to avoid polarization effects at the electrodes, thus achieving a fast and strong signal response by the sensor in the presence of the analyte gas.
A further aspect of the inventive gas sensor is that only two electrodes are required, whereas prior art gas sensors require three electrodes and a DC power supply.
These objects have been achieved by using a novel button sensor design, which may include mixed proton-electron conductive electrodes, various embodiments of which may also include an electrochemical analyte gas pump to transport analyte gas away from the counter electrode side of the gas sensor. While the inventive sensor is referred to herein as a CO sensor, it is contemplated that the inventive sensor is also capable of sensing other toxic analyte gases disclosed herein.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.
In order that the manner in which the above-recited and other advantages and objects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by counter-reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only a typical embodiment of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
The purpose of a CO electrochemical pump is to prevent an accumulation at the counter electrode of the inventive CO sensor. The CO pump lends increases stability to the sensor response in that the sensor response has less of a propensity to shift with time as in prior art CO sensing devices. When DC power is used as the motivator for the electrochemical CO pump, the passing of electrons from the sensing electrode to the reference electrode is enhanced. By reversing the DC power, the CO is kept away from the reference electrode and does not cause a buildup of CO on the back side of the sensing electrode.
The inventive CO sensor is a solid proton conductor room temperature having a fast and high signal response. To achieve a fast detection time and a high signal response, it is desirable to provide a CO sensor having a low bulk ionic resistance. Bulk ionic resistance Rbulk of the inventive sensor is equal to
Rbulk=Rod/s (3)
where Ro is the ionic resistivity of the protonic conductive membrane, S is the cross section area of the protonic conductive membrane between the two electrodes, and d is the thickness of the protonic conductive membrane.
Resistance of an electrochemical cell includes at least three components: 1) bulk ionic resistance of the membrane, 2) interface resistance between the membrane and electrodes, and 3) electronic resistance of the electrodes. The bulk ionic resistance of the sensor is reduced to about 1 ohm by the inventive button sensor design, such that R bulk is not a performance limit. Electronic resistivity of the electrodes is in order of 10-5 ohm.cm and obviously is not a performance limit. Therefore, the interface resistance, which is relative to the available three-phase contact area, becomes the performance limit.
Assuming that a button NAFION™ CO sensor is exposed to 1,000 ppm CO with air. The Nernst Potential of the sensor is about 200 mV according to our experiment data. If the interface resistance is insignificant, the response shorting current would be about 200 mA (or 250 mA/cm2). For the real case, we only recorded a response current less than 1 mA/cm2
due to existing a large interface resistance. The interface resistance of the sensor according to this invention has been reduced by introducing our mixed proton-electronic conductor.
Two alternative embodiments of the inventive CO sensor are depicted in
In a second embodiment shown in
The amperometric sensor also can be combined with an electrochemical CO pump, as defined hereinafter, and accurate response will be achieved in such combined sensors. In the inventive button sensor design as shown in
Protonic conductors membranes are usually slightly permeable to CO gas. When a membrane is under a carbon monoxide partial pressure difference, a very small amount of carbon monoxide will permeate across the membrane into the counter electrode side.
Influence of the CO permeation to sensor response usually is insignificant because this very small amount of permeated CO is instantly converted into carbon dioxide at the reference electrode. If a precision CO concentration detection is needed, CO concentration in the counter electrode can be minimized by attaching an electrochemical CO pump to the sensor according to this invention. The purpose of an electrochemical pumping circuitry is to prevent the buildup of CO gas at the counter electrode side of the sensor so that a precision CO detection is achieved.
Protonic conductive membrane 12 may be substantially composed of a solid, perfluorinated ion-exchange polymer, or a metal oxide protonic conductor electrolyte material. The following table serves as a further example of solid state protonic conductor which can be used at room temperature in the inventive gas sensor.
Protonic conductive membrane 12 is preferably constructed of materials 6, 7, 8, or 9 which are unreinforced film of perfluorinated copolymers.
As seen in
A further embodiment of the inventive CO sensor is seen in
A DC power source 140 is in electrical contact with pumping electrode 115 and metallic can 130 through electrical contacts 146 and 144. Sensing electrode 116 is in contact with an electrical measurement means 142 through electrical leads 148, 144. DC power supply 140 serves as a CO pump to button sensor 110. Electrical sensing means 142 is used to measure the response of button sensor 110 to concentrations of CO.
Sensing electrode 116 is exposed to the ambient through holes 138A.
CO pumping electrode 115 is exposed through holes 138B to a sealed chamber 115A which serves as a counter environment.
Sensing electrode 116 is exposed to the ambient through holes 138A. First protonic conductive membrane 122 performs the function, in combination with counter and sensing electrodes 114, 116, of sensing CO concentration through the conduction therethrough of protons. Second protonic conductive membrane 112, in combination with pumping electrodes 115, 117, performs the function of pumping CO out of the side of button sensor 110 associated with counter electrode 114 so as to stabilize the sensor response of button sensor 110 upon the detection of a concentration of CO in the ambient.
The ability of the inventive CO sensor to avoid interference with relative humidity is that, with increased relative humidity, bulk ionic resistance of the inventive CO sensor goes down as current flow increases. The resistance decrease and current increase are proportionally the same. Thus, voltage, or sensor response, remains constant as evidenced by the equation V=RI.
In the inventive CO sensor, the sensing electrode is exposed to an environment containing CO, whereas the counter electrode side is sealed air-tight. The sensing mechanism of this sensor is essentially the same as that of the sensor with an opened reference electrode. The protonic conductive membrane can be as thin as 0.2 mm so that the reactant oxygen and the produced water permeate the membrane. A small part of CO gas also permeates through the membrane, but the permeated CO is consumed by the reaction with oxygen electrochemically and catalytically at the counter electrode.
As can be seen from
The mixed conductor material found in the electrode seen in
The mixed conductor material found in the electrode seen in
Button sensor 10 in
Electrical lead 20A electrically contacts sensing electrode 16 through can 30. Electrical lead 20A is connected to an amp meter 24 which is in series with a DC power source 42. DC power source 42 is connected to amplifier 45, which amplifier 45 is connected through to electrical lead 20B, which penetrates cap 32 into counter electrode 14. Amplifier 45 is electrically connected to an electrical lead 20C which penetrates through can 30 into counter electrode 14. The function of the electrical circuitry shown in
The inventive CO gas sensor using the mixed protonic-electronic conductive materials in the electrodes with high surface area of 100 to 1000 M2/g shows a shorting current as high as 150 μA/cm2 to 1,000 ppm CO, which is at least two orders of magnitude higher compared to the sensors with electronic conductive electrodes according to prior art. A preferred composition of such electrodes is as follows:
Other compositions of such electrodes are as follows:
The role of platinum in the sensing electrode is to favor the CO decomposition reaction (1) whereas Ru oxide in the counter electrode is to favor the water formation reaction (2). According to this invention, the Ru oxide, instead of expensive platinum and the like, as reported in prior art, shows excellent CO sensing performance.
It is also contemplated that the electrodes disclosed herein can be composed substantially of carbon, noble metals, or conductive metal oxides. The electrical conducting material in electrodes disclosed here is preferably a proton-electron mixed conductive material having 10-50 wt% of a proton conductor material and 50-90 wt% of a first and a second electrical conductor material. The proton conductor material for the electrodes disclosed herein is preferably a copolymer having a tetrafluorethylene backbone with a side chain of perfluorinated monomers containing at least one of a sulfonic acid group or carboxylic acid group. Preferably, one of the first and second electrical conductor materials for the sensing electrodes disclosed herein is 50-99 wt% of carbon black, and the other of the first and second electrical conductor materials for the sensing electrodes disclosed herein is 1-50 wt% of platinum. Also preferably, one of the first and second electrical conductor materials for the counter electrode is 50-99 wt% of carbon black, and the other of the first and second electrical conductor materials for the counter electrode is 1-50 wt% of Ru oxide.
In a composition of 25 wt% protonic conductor in electrodes, which is a physically continuous phase, there is proton conduction, whereas the rest of the phases in electrodes provide electronic conduction as well as catalytic activity. If without 25 wt% proton conductor in electrodes, the electrodes were only an electronic conductor, and the reactions (1) and (2), above, would only occur at three-phase contact area 86 seen in
While the inventive gas sensor can be used to measure CO concentration, it is also capable of measuring other gases such as H2, H2S, H2O vapor, and NOx concentrations.
Various protonic conductors, including organic protonic conductors and inorganic protonic conductors, can be used in the sensor according to this invention. In what follows, a copolymer protonic conductive membrane based on a tetrafluoroethylene backbone with a side chain of perfluorinated monomers containing sulfonic acid group is used herein as an example of the fabrication of the inventive sensor.
To prevent deterioration of the polymer membrane in the subsequent wetting/drying steps, the membrane must be first converted from the proton form to the sodium form by the following steps A:
The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrated and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
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| Number | Date | Country |
|---|---|---|
| 637 851 | Jun 1994 | EP |
| Entry |
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| Number | Date | Country | |
|---|---|---|---|
| Parent | 08381718 | Jan 1995 | US |
| Child | 10621999 | US |
