GB2303710A - Electrochemical toxic gas sensor with gas permeable membrane - Google Patents
Electrochemical toxic gas sensor with gas permeable membrane Download PDFInfo
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- GB2303710A GB2303710A GB9618782A GB9618782A GB2303710A GB 2303710 A GB2303710 A GB 2303710A GB 9618782 A GB9618782 A GB 9618782A GB 9618782 A GB9618782 A GB 9618782A GB 2303710 A GB2303710 A GB 2303710A
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- gas
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- permeable membrane
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- electrochemical sensor
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- 239000012528 membrane Substances 0.000 title claims abstract description 71
- 239000007789 gas Substances 0.000 title claims description 93
- 239000002341 toxic gas Substances 0.000 title claims description 23
- 239000003792 electrolyte Substances 0.000 claims abstract description 20
- -1 polyethylene Polymers 0.000 claims abstract description 14
- 229920001343 polytetrafluoroethylene Polymers 0.000 claims abstract description 14
- 239000004810 polytetrafluoroethylene Substances 0.000 claims abstract description 14
- 239000004698 Polyethylene Substances 0.000 claims abstract description 9
- 229920000573 polyethylene Polymers 0.000 claims abstract description 9
- 239000011260 aqueous acid Substances 0.000 claims abstract description 6
- 229920001577 copolymer Polymers 0.000 claims abstract description 6
- 239000010409 thin film Substances 0.000 claims abstract description 6
- 230000007935 neutral effect Effects 0.000 claims abstract description 4
- 239000004417 polycarbonate Substances 0.000 claims abstract description 4
- 229920000515 polycarbonate Polymers 0.000 claims abstract description 4
- 229920005573 silicon-containing polymer Polymers 0.000 claims abstract description 4
- YSYRISKCBOPJRG-UHFFFAOYSA-N 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole Chemical compound FC1=C(F)OC(C(F)(F)F)(C(F)(F)F)O1 YSYRISKCBOPJRG-UHFFFAOYSA-N 0.000 claims abstract description 3
- 229920009441 perflouroethylene propylene Polymers 0.000 claims abstract description 3
- 229920001296 polysiloxane Polymers 0.000 claims abstract description 3
- 150000003839 salts Chemical class 0.000 claims abstract description 3
- BFKJFAAPBSQJPD-UHFFFAOYSA-N tetrafluoroethene Chemical group FC(F)=C(F)F BFKJFAAPBSQJPD-UHFFFAOYSA-N 0.000 claims abstract description 3
- 239000004812 Fluorinated ethylene propylene Substances 0.000 claims abstract 2
- HQQADJVZYDDRJT-UHFFFAOYSA-N ethene;prop-1-ene Chemical group C=C.CC=C HQQADJVZYDDRJT-UHFFFAOYSA-N 0.000 claims abstract 2
- 150000007522 mineralic acids Chemical class 0.000 claims abstract 2
- 239000011356 non-aqueous organic solvent Substances 0.000 claims abstract 2
- 239000011244 liquid electrolyte Substances 0.000 claims description 19
- 239000000463 material Substances 0.000 description 21
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 16
- 230000035699 permeability Effects 0.000 description 13
- 239000010408 film Substances 0.000 description 12
- 229910002091 carbon monoxide Inorganic materials 0.000 description 11
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 description 10
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 9
- 229910052719 titanium Inorganic materials 0.000 description 9
- 239000010936 titanium Substances 0.000 description 9
- 229920006362 Teflon® Polymers 0.000 description 8
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 6
- 229920003023 plastic Polymers 0.000 description 6
- 239000004033 plastic Substances 0.000 description 6
- 229910052697 platinum Inorganic materials 0.000 description 6
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 6
- 239000004809 Teflon Substances 0.000 description 5
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 5
- 229910052737 gold Inorganic materials 0.000 description 5
- 239000010931 gold Substances 0.000 description 5
- 239000000565 sealant Substances 0.000 description 5
- 239000000126 substance Substances 0.000 description 5
- 238000001514 detection method Methods 0.000 description 4
- 230000002209 hydrophobic effect Effects 0.000 description 4
- 229910052751 metal Inorganic materials 0.000 description 4
- 239000002184 metal Substances 0.000 description 4
- 125000006850 spacer group Chemical group 0.000 description 4
- QTBSBXVTEAMEQO-UHFFFAOYSA-N Acetic acid Chemical compound CC(O)=O QTBSBXVTEAMEQO-UHFFFAOYSA-N 0.000 description 3
- KWYUFKZDYYNOTN-UHFFFAOYSA-M Potassium hydroxide Chemical compound [OH-].[K+] KWYUFKZDYYNOTN-UHFFFAOYSA-M 0.000 description 3
- GWEVSGVZZGPLCZ-UHFFFAOYSA-N Titan oxide Chemical compound O=[Ti]=O GWEVSGVZZGPLCZ-UHFFFAOYSA-N 0.000 description 3
- 230000008859 change Effects 0.000 description 3
- 238000006243 chemical reaction Methods 0.000 description 3
- 229920002313 fluoropolymer Polymers 0.000 description 3
- 229910000510 noble metal Inorganic materials 0.000 description 3
- 231100000572 poisoning Toxicity 0.000 description 3
- 230000000607 poisoning effect Effects 0.000 description 3
- 230000009467 reduction Effects 0.000 description 3
- 230000004044 response Effects 0.000 description 3
- OGIDPMRJRNCKJF-UHFFFAOYSA-N titanium oxide Inorganic materials [Ti]=O OGIDPMRJRNCKJF-UHFFFAOYSA-N 0.000 description 3
- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 description 2
- KDLHZDBZIXYQEI-UHFFFAOYSA-N Palladium Chemical compound [Pd] KDLHZDBZIXYQEI-UHFFFAOYSA-N 0.000 description 2
- 239000004743 Polypropylene Substances 0.000 description 2
- WCUXLLCKKVVCTQ-UHFFFAOYSA-M Potassium chloride Chemical compound [Cl-].[K+] WCUXLLCKKVVCTQ-UHFFFAOYSA-M 0.000 description 2
- RAHZWNYVWXNFOC-UHFFFAOYSA-N Sulphur dioxide Chemical compound O=S=O RAHZWNYVWXNFOC-UHFFFAOYSA-N 0.000 description 2
- XECAHXYUAAWDEL-UHFFFAOYSA-N acrylonitrile butadiene styrene Chemical compound C=CC=C.C=CC#N.C=CC1=CC=CC=C1 XECAHXYUAAWDEL-UHFFFAOYSA-N 0.000 description 2
- 239000004676 acrylonitrile butadiene styrene Substances 0.000 description 2
- 229920000122 acrylonitrile butadiene styrene Polymers 0.000 description 2
- 239000000443 aerosol Substances 0.000 description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 2
- 239000011248 coating agent Substances 0.000 description 2
- 238000000576 coating method Methods 0.000 description 2
- 239000004811 fluoropolymer Substances 0.000 description 2
- 239000011888 foil Substances 0.000 description 2
- LELOWRISYMNNSU-UHFFFAOYSA-N hydrogen cyanide Chemical compound N#C LELOWRISYMNNSU-UHFFFAOYSA-N 0.000 description 2
- 229910000037 hydrogen sulfide Inorganic materials 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 150000002739 metals Chemical class 0.000 description 2
- 239000011255 nonaqueous electrolyte Substances 0.000 description 2
- 239000001301 oxygen Substances 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 229920001155 polypropylene Polymers 0.000 description 2
- 239000010970 precious metal Substances 0.000 description 2
- 230000035945 sensitivity Effects 0.000 description 2
- MGWGWNFMUOTEHG-UHFFFAOYSA-N 4-(3,5-dimethylphenyl)-1,3-thiazol-2-amine Chemical compound CC1=CC(C)=CC(C=2N=C(N)SC=2)=C1 MGWGWNFMUOTEHG-UHFFFAOYSA-N 0.000 description 1
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 description 1
- 229920001780 ECTFE Polymers 0.000 description 1
- 239000004677 Nylon Substances 0.000 description 1
- QCWXUUIWCKQGHC-UHFFFAOYSA-N Zirconium Chemical compound [Zr] QCWXUUIWCKQGHC-UHFFFAOYSA-N 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 230000002378 acidificating effect Effects 0.000 description 1
- 239000000460 chlorine Substances 0.000 description 1
- 229910052801 chlorine Inorganic materials 0.000 description 1
- 230000000295 complement effect Effects 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 230000003292 diminished effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000010411 electrocatalyst Substances 0.000 description 1
- 239000007772 electrode material Substances 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 230000006353 environmental stress Effects 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 229920002457 flexible plastic Polymers 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 229910001867 inorganic solvent Inorganic materials 0.000 description 1
- 239000003049 inorganic solvent Substances 0.000 description 1
- 229910052741 iridium Inorganic materials 0.000 description 1
- GKOZUEZYRPOHIO-UHFFFAOYSA-N iridium atom Chemical compound [Ir] GKOZUEZYRPOHIO-UHFFFAOYSA-N 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 229910001092 metal group alloy Inorganic materials 0.000 description 1
- 238000000034 method Methods 0.000 description 1
- 230000005012 migration Effects 0.000 description 1
- 238000013508 migration Methods 0.000 description 1
- JCXJVPUVTGWSNB-UHFFFAOYSA-N nitrogen dioxide Inorganic materials O=[N]=O JCXJVPUVTGWSNB-UHFFFAOYSA-N 0.000 description 1
- 229920001778 nylon Polymers 0.000 description 1
- 239000005486 organic electrolyte Substances 0.000 description 1
- 239000003960 organic solvent Substances 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 230000001590 oxidative effect Effects 0.000 description 1
- 229910052763 palladium Inorganic materials 0.000 description 1
- 239000011148 porous material Substances 0.000 description 1
- 239000001103 potassium chloride Substances 0.000 description 1
- 235000011164 potassium chloride Nutrition 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 230000035939 shock Effects 0.000 description 1
- 239000007921 spray Substances 0.000 description 1
- 229910052715 tantalum Inorganic materials 0.000 description 1
- GUVRBAGPIYLISA-UHFFFAOYSA-N tantalum atom Chemical compound [Ta] GUVRBAGPIYLISA-UHFFFAOYSA-N 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
- 230000005068 transpiration Effects 0.000 description 1
- 229910052726 zirconium Inorganic materials 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/403—Cells and electrode assemblies
- G01N27/404—Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid, e.g. Clark-type oxygen sensors
- G01N27/4045—Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid, e.g. Clark-type oxygen sensors for gases other than oxygen
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- Chemical & Material Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Health & Medical Sciences (AREA)
- Physics & Mathematics (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Molecular Biology (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Measuring Oxygen Concentration In Cells (AREA)
Abstract
The sensor is separated from the gas to be measured by a gas permeable membrane 70. The sensor comprises a housing containing a working electrode 68, counter electrode 60 and electrolyte. Pins 54 with circumferential grooves pass through the housing and are connected to the electrodes. The working electrode may be adjacent to or formed on the gas permeable membrane or may be separated from it by a gas porous membrane. The membrane is a thin film and may be formed from a copolymer of perfluoro(2,2-dimethyl-1,3-dioxole) and tetrafluoroethylene, polyethylene, polytetrafluoroethylene, fluorinated ethylene propylene copolymer, fluorinated silicone polymer or a copolymer of silicone and polycarbonate. The electrolyte may be aqueous acid, aqueous neutral, aqueous alkaline, non-aqueous organic solvent with a salt or a non-aqueous inorganic acid.
Description
TITLE ELECTROCREMICAL TOXIC GAS SENSOR FIELD OF THE INVflTION The present invention relates to electrochemical gas sensors that are used for the detection of a variety of toxic gases such as carbon monoxide, chlorine, hydrogen cyanide, hydrogen sulfide, nitrogen dioxide and sulfur dioxide.
BACKGROUND OF THE INVENTION
In a typical electrochemical gas sensor, the gas to be measured diffuses from the atmosphere through a gas porous membrane to a working electrode where a chemical reaction occurs. The type, rate, and efficiency of the chemical reaction is controlled by the material used to make the working electrode, the diffusion rate of the gas to the working electrode, and the potential at which the working electrode is set in relation to a reference, another electrode. The working electrode potential is commonly set with the aid of a potentiostat circuit, but this is not necessarily a required operating mode. At the counter electrode, a chemical reaction complementary to the one occurring at the working electrode takes place. The current flow between the working electrode and the counter electrode is proportional to the concentration of the gas being measured.An ionically conductive liquid electrolyte contacts all the electrodes and allows charge balance to be maintained within the sensor. Such electrochemical gas sensors are generally disclosed and described in U.S. Patent
Nos. 4,132,616; 4,324,632; 4,474,648; and in European Patent
Application No. 0 496 527 Al.
An exploded view of a presently-known electrochemical gas sensor used for detecting carbon monoxide is shown in FIG. 1. During assembly of such a sensor, a platinum counter electrode 2 is placed on the inside bottom of a sensor housing 4. The counter electrode 2 typically includes a gas porous membrane 3 such as Vortex' or Zitex.
Next, a gold-coated current collector 6 is placed in the sensor housing 4 in a manner allowing the gold-plated ring to contact the perimeter of the counter electrode 2 and the tab to extend through a lower hole (not shown) in the sensor housing 4 on top of the current collector 6. An O-ring 8 is then placed in the sensor housing 4 with the main spacer 10 being placed on top of the O-ring 8.
With these pieces in place, an electrically insulating but porous separator 12 is placed within the main spacer 10 and then a wick 14 is placed over the separator 12. Preferably, the wick is dumbbell-shaped and made from porous polyethylene or polypropylene which has been treated to make it hydrophilic. A second O-ring 16 is then placed over the main spacer 10 and a second gold-coated current collector 18 is placed on top of this O-ring 16 with the tab extending through a middle hole (not shown) in the sensor housing 4. Next, a platinum reference electrode 20 which has a center hole 22 is placed over the current collector 18 in a manner allowing them to make electrical contact. A third O-ring 24 is then placed over the reference electrode 20 followed by a spacer 26 and one or more separators 26.
Separators 28 are similar to separator 12.
A third gold-coated current collector 30 is then placed over this assembly with its tab extending through an upper hole (not shown) in the sensor housing 4. A platinun working electrode 32 is placed over the current collector 30. The working electrode 32 is similar in structure to the counter electrode 2 and also includes a gas porous membrane such as Gortexe or Zitex . The working electrode 32 is inserted face down whereas the counter electrode is face up.
The sensor inlet assembly 34 which includes a baffle 36 to reduce convection is then pushed down over the stack and forms the top of sensor housing 4. The entire structure is maintained under some pressure in sensor housing 4 by fitting a retaining ring 38 into a groove 40 at the top of the sensor housing 4. Later, the tabs of the current collectors which extend through the lower, middle and upper holes are bent parallel to, and heat sealed to, the outer wall of the sensor housing 4. The area around where the tabs protrude through the holes in the sensor housing 4 is then coated with a hydrophobic sealant. liter this sealant has dried, the sensor is filled with an ionically conductive aqueous sulfuric acid electrolyte through a fill hole 42 near the bottom of the sensor housing 4 which is then sealed with plug 44.
Toxic gas sensors utilizing this configuration have several disadvantages. Among them are high manufacturing costs which are due to the numerous parts used in the sensor as well as the labor involved with assembling the sensor and with applying the heat seal and hydrophobic seal ants. Additionally, the high cost of precious metals such as the gold in the current collectors requires the use of fragile, laminated leads which are not very sturdy and which must be protected from mechanical abuse while still allowing for reliable external electrical connections. Even with the use of gaskets or 0ring, and hydrophobic sealants, sensors of this type still tend to leak electrolyte after long periods of use or after exposure to elevated temperatures.The leakage of the liquid electrolyte, typically aqueous sulfuric acid, not only reduces the performance characteristics of the sensor but can also corrode and destroy the instrument in which the sensor is located. Still another drawback to this type of sensor is its size which is over one inch in height.
Toxic gas sensors such as the one shown in FIG. 1 and described above do not use gas permeable membranes because the permeability of well known materials to gases and vapors is either too low or the materials are not sufficiently inert to withstand typical toxic gas sensing environments. As a result, toxic gas sensors utilize gas porous membranes such as Gore-raw or Zitexe. These gas porous membranes are usually made out of PTFE (polytetrafluoroethylene) which contain a large number of microscopically visible holes which are on the order of several microns in diameter. These holes typically cover about 60-70% of the geometric area of the gas porous membrane.
Although electrochemical toxic gas sensors made with gas porous membranes work acceptably in many applications, they are generally acknowledged to have several drawbacks. For example, the porosity of a gas porous membrane, for the most part, limits the choice of acceptable liquid electrolytes which can be used in the sensor primarily to aqueous acids. Even when using such aqueous acids, application of a pressure differential to the sensor can cause the electrolyte to weep through the porous membrane.
Additionally, water vapor rapidly transpires through a gas porous membrane as temperature and humidity change. This makes it necessary to leave space in the body of the sensor for a relatively large electrolyte reservoir.
This increases the complexity of the sensor and causes the sensor to be larger than desirable. It also causes the pX of the electrolyte to change which changes the potential of the reference electrode in the sensor. The drift in the potential of the reference electrode results in zero drift, span drift and temperature compensation problems when using the sensor in an instrument. Also, aerosols, particles, and high molecular weight gases can easily pass through the microscopic holes in a porous membrane, causing poisoning of the sensing electrode. This poisoning phenomena results in a slow decline in sensor output with time until the sensor is chemically destroyed and no longer useable.
It would be desirable, therefore, to have an electrochemical toxic gas sensor which did not have these drawbacks.
The present invention relates to a toxic gas sensor which uses a gas permeable membrane instead of a gas porous membrane as the primary boundary layer between the toxic gas to be sensed and the contents of the electrochemical gas sensor. A gas permeable membrane is one wherein the gas first dissolves in the material of the membrane before it can diffuse through the material since there are no holes in the membrane.
This is different from the gas porous membrane described above wherein the gas diffuses directly through the microscopic holes in the membrane. The gas permeable membrane may be placed directly over a conventional working electrode which typically includes a gas porous membrane. In this configuration, the gas permeable membrane may be heat-sealed to the top of the sensor housing along with the gas porous membrane of the working electrode or it may be sealed to the housing along its perimeter by using other conventional means such as 0-ring seals. In a preferred embodiment of the present invention, a second gas permeable membrane is used to seal the bottom of the sensor housing.
The electrocatalytic material such as platinum used to form the working electrode may be applied directly on or adjacent to the gas permeable membrane.
If this method of fabrication is chosen, the gas porous membrane can be completely eliminated. Alternately, the gas permeable membrane can be heat-laminated directly to a gas porous membrane that has been coated with an electrode material on one face.
In the following description, references to 'fluted' or 'fluting' should be construed as meaning that the electrically conducting feedthrough has a circumferential groove.
Other details, objects and advantages of the present invention will become apparent as the following description of the presently preferred embodiments of practicing the invention proceeds.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, preferred embodiments of the invention are illustrated in which:
FIG. 1 is an exploded view of a presently known electrochemical toxic gas sensor for carbon monoxide;
FIG. 2 is an exploded view of an electrochemical toxic gas sensor having a circumferentially grooved electrically conducting feedthrough.
FIG. 3 is a top view of the sensor shown in FIG. 2 taken along line 3-3.
FIG. 4 shows a circumferentially grooved electrically-conducting feedthrough used in a electrochemical toxic gas sensor;
FIG. 5 is a graph of the data shown in TABLE 1.
FIG. 6 is a graph of the data shown in TABLE 2.
FIG. 7 is a graph of the data shown in TABLE 4.
DETAILED DESCRIPTION OF TEE INVENTION
An electrochemical toxic gas sensor is shown in FIG.
2 as it would be assembled to detect for carbon monoxide.
The sensor has a housing 50 preferably made of a plastic material such as polyethylene with a small hole 52 in the bottom. The small hole 52 allows for the addition of the liquid electrolyte during assembly. The sensor also has a plurality of fluted titanium pins 54, which are proferably insert-molded into the plastic housing 50. These pins 54 are the electrically-conducting feedthroughs which allow for electrical contact between the electrodes inside the housing 50 and the outside of the sensor. Within the housing 50 is placed an electrode table or shelf 56. The table 56 is preferably made of a flexible plastic material similar to the housing 50 and is held in place by four tabs or posts 58 as shown in FIG. 3. The electrochemical components of the sensor are then stacked on the table 56.The space under the table 56 serves as a reservoir for the liquid electrolyte.
During assembly of the sensor, the electrode table 56 is inserted into the plastic housing 50. An electrically conducting and inert metal lead, such as short strips of platinum foil (not shown) having a thickness of about 3 mil, are spot-welded to the top of the titanium pins 54 and then bent upright. A counter electrode 60 is placed face up on the electrode table 56 and a first appropriate platinum foil lead is bent back over the counter electrode 60 allowing for good electrical contact. A separator 62 is then placed over this assembly. Next, a platinum reference electrode 64 is placed over the separator 62 and a second appropriate platinum foil lead is bent back contacting this electrode.
A second separator 66 is then placed over this assembly.
This separator 66 preferably has tabs which extend below the level of the electrode table 56 to allow liquid electrolyte from the reservoir to wick into the electrochemical components and ionically connect all the electrodes with the sensor placed in any orientation.
Next, the remaining platinum foil lead is bent back over the second separator 66. The platinum working electrode 68 is typically located on the under side of a gas porous membrane 70 which is then placed over this assembly so that the gas porous membrane 70 is exposed to the atmosphere and the working electrode 68 contacts the foil lead. The gas porous membrane 70 is then heat-sealed to the top of the plastic housing 50. The housing 50 is then turned upside down and partially filled through small hole 52 with a liquid electrolyte such as aqueous sulfuric acid.
Preferably a piece of gas porous material is placed over the small hole 52 and is heat-sealed to the housing 50. The electrodes and the separators described above are similar to those described in connection with FIG. 1.
The sensor of the present invention can then be placed within an outer housing (not shown) which is used to improve the electrical contact of the foil leads to the electrodes by compressing the electrochemical components, minimize air flow sensitivity, control temperature compensation requfrements, improve signal iinea6ity and uniformity, and adjust output sensitivity. The outer housing is not unlike the housing 4 and inlet assembly 34 'shown in FIG. 1, but is smaller. Preferably the hole or holes in the outer housing above the working electrode are covered with a second gas porous membrane to prevent liquid transport into the sensor. The sensor remains open to the transpiration of water vapor and gases due to the gas porous membranes used therein.
It was expected that over time the titanium pins 54 in the sensor would become covered with oxide and the liquid electrolyte would then wet the oxide layer and leak through it to the outside of the housing 50 because there is no chemical bond between the titanium pins 54 and the polyethylene housing SO. This is precisely what happens if non-fluted titanium pins are used. In practice, however, it has been found that there is no liquid electrolyte leakage in the gas sensor of the present invention. This is believed to be caused by the oxidation of the titanium surface in a flute of the electrically-conductive feedthrough. It appears that the creepage rate of the liquid electrolyte slows down or stops after an initial period of time if a fluted feedthrough such as a fluted titanium pin is used.A titanium oxide layer 72 appears to form on the upper end of the pin and stops increasing in length at the bottom of the neck of the first flute 74 as shown in FIG. 4. This phenomena can be observed under a microscope if the titanium pin 54 is removed from the polyethylene housing SO. What is observed is a titanium oxide layer starting at the top of the pin 54 which is exposed to the liquid electrolyte inside the housing. The titanium oxide layer continues to form within the polyethylene housing 50 as the pin narrows down into the first flute 74. It is believed that the growth of the oxide layer increases the effective diameter of the pin 54.The forces exerted by the increased pin diameter at the bottom of the neck of the first flute apparently push the pin 54 against the polyethylene housing 50 sufficiently hard to retard any further migration of liquid electrolyte past the bottom of the neck of the first flute 74 under normal sensor use conditions. No oxide layer is observed in the second flute 76 or on the surface of the pin 54 in the area between the flutes 74 and 76.
Sensors of the present invention are easy to assemble and have a much lower cost compared to presently known sensors. This is due in part to the reduction in the number of parts required for the sensor, i.e., no O-ring seals, gaskets, or hydrophobic seal ants, as well as the reduced labor in assembling the sensor.It is also because the sensor requires less precious metal parts and simpler current collectors. - tt; The present invention also eliminates the leakage path which inevitably occurs through the O-ring seals in the sensor shown in FIG. 1. Sensors such as shown in FIG. 2 have been maintained at elevated temperatures of up to 60 0C for extended periods of time and have been temperature shocked repeatedly between 0 0C and 500C with no leakage of liquid electrolyte having been observed.
The sensor of the present invention is not limited to configurations having three electrodes but may be used in electrochemical sensors having two, four or more than four electrodes. Also, the sensor housing need not be polyethylene, although it should be insert-moldable and compatible with the liquid electrolyte chosen. Plastics which meet these requirements when aqueous acid electrolytes are used include fluoropolymers such as Teflon, Halars, and Tefzele. Other plastics which could be used include polypropylene, nylon, ABS (acrylonitrile-butadiene styrene), and polycarbonate.
Other materials which can be used for the electrically-conductive feedthroughs include those nonprecious metals or metal alloys which form tenacious oxide films and therefore do not corrode at an appreciable rate in aqueous acid electrolytes. Such semi-nöble metals include tantalum and zirconium in addition to titanium. Of course, the pins could be made from precious or noble metals such as gold, palladium, platinum and iridium since these materials are inert and do not form an oxide coating which will wick the liquid electrolyte out of the sensor housing at low potentials. However, at high enough oxidizing potentials, even noble metals will form a tenacious oxide coating in the same manner as semi-noble metals. Noble metal-plated seminoble metal pins could also be used.
It is preferable to have more than one flute in the electrically-conducting feedthrough. Similarly, flutes of various shapes and sizes can be used in the sensor of the present invention. It is believed that a V-shaped or U shaped flute, or any shape in between, will also work. The shape of the flutes shown in FIG. 4 was chosen for their ability to mechanically hold the pin in the housing after the pin was insert-molded into the housing.
According to the invention, the sensor shown in FIG. 2 is also provided with a gas permeable membrane. Thin PTFE membranes, such as a 1/4 mil PTFE film, have been found to provide sufficient gas permeability to allow the sensing of toxic gases. In one test, a 1/4 mil PTE film was placed'ove'rthe porous membrane of the sensor shown in FIG. 2. An O-ring was placed over the film. An outer housing was then placed around the assembly compressing the O-ring and seating the film to gas transport around the perimeter via the O-ring. The working electrode was then set to 0.00 volts using a low cost potentiostat.
After ten minutes in air, the base current stabilized.
Using flow meters to adjust the gas concentration to +10% accuracy the sensor was then tested for response to carbon monoxide yielding the data shown in TABLE 1.
TABLE 1 ppm CO (in air) Sensor Output ( A) 0 -0.02
114 0.44
206 0.84
281 1.08
468 1.54
515 1.65
562 1.77
634 1.90
687 2.10
736 2.17
792 2.25
858 2.45 883 2.50
951 2.70
1030 2.90
When plotted such as is shown in FIG. 5, this data indicates that, within experimental error, the sensor was linear over the entire carbon monoxide concentration range studied. The electrical output of the sensor was approximately 2.8 nanoamps/ppm carbon monoxide.
In a second, more controlled set of experiments with the same sensor, a PAR Model 363 potentiostat was used to set the working electrode potential to 0.000 volts vs.
the reference electrode. A Sierra mass flow gas proportioner with a +2S accuracy was used to mix air and a tank of 5.14% carbon monoxide in air to various concentrations while maintaining a flow rate of 300 cc/min.
The data shown in TABLE 2 was obtained:
TABLE 2
Percent Co (in air) Sensor Output (uA) O -O .1 0.17 5.0
0.34 9.2
0.69 18.3
1.37 36.4
2.06 54.6
2.74 73.3
3.43 90.8 4.11 109
4.45 122
4.80 132
4.97 136
5.14 142
When plotted such as shown in FIG. 6, it is found that the sensor was linear over the entire concentration range studied. The electrical output of the sensor was approximately 27 microamps/percent CO and agreed well with the results in TABLE 1.
The output of the sensor as a function of gas flow rate at 5.14% Co in air was then studied and the data in
TABLE 3 was obtained:
Flow Rate (cc/min) Sensor Output ( A)
0
10 132
20 135
40 137
80 138
160 140
300 143
The electrical output of the sensor at a gas flow rate of zero declined slowly as the carbon monoxide was consumed.
However, only an 8% change in electrical output was noted from 10 to 300 cc/min gas flow.
The response time of the sensor to 5.14% CO at 300 cc/min was then studied yielding the data shown in TABLE 4:
TABLE 4
Time (sec) Sensor Output ruA) O -O .1 5 52.0
10 86.2
15 107.8
25 124.5
30 130.0
40 134.5
45 135.7
50 136.6
60 137.6
75 138.7
90 139.4
120 139.4
150 140.2
180 141.6
240 140.2
360 139.2
540 138.8
660 138.8
720 138.8
780 138.9
840 138.8
When plotted such as shown in FIG. 7, a well defined curve is obtained with 90% of final output achieved in 25 seconds and 97% of final output achieved in 45 seconds.
Another sensor was fabricated which was similar to the above sensor except that the 1/4 nil PTFE film (i.e., the gas permeable membrane) was heat-laminated to the membrane which was part of the working electrode. This process changed the crystallinity of the PTFE thereby reducing its permeability. Therefore, output of this sensor to carbon monoxide was only 0.75 nanoampstppm. This sensor was also exposed to 1,893 ppm H2S in N2. A rapid response of 4.35 microamps or 2.3 nanoamps/ppm B2S was observed.
The 1/4 mil PTFE film was supplied by CZEMFAB.
This is not the only gas permeable film currently available with sufficient permeability and chemical stability to meet the requirements of electrochemical toxic gas sensors. For example, CREMPLAST supplies a 1/2 mil PTFE material and
Dupont supplies a 1/2 mil FEP Teflon material which should also work.
Materials having higher gas permeabilities would provide increased outputs. This would allow the potentiostat to be less complex and/or allow for lower gas detection limits. Materials with the chemical stability associated with fluoropolymers but with a much higher gas permeability than PTFE are currently available. Examples of such materials are Teflons AF-1600 and Teflons AF-2400 manufactured by Dupont. These materials are copolymers of perfluoro (2, 2-dimethyl -1,3 -dioxole) and tetrafluoroethylene Teflon AF-1600 is available in 1, 2 and 10 mil films and has gas permeabilities approximately two orders of magnitude higher than PTFE. Silicone polymers can also be fluorinated resulting in a material which combines the high gas permeability of silicone polymers with the chemical inertness expected from fluorinated polymers.
Another material with gas permeability several orders of magnitude higher than that of PTFE while having adequate chemical inertness for use in some electrochemical gas sensing applications is a copolymer of silicone and polycarbonate. It is manufactured by Membrane Products
Company and sold as MEM-213 in films between 1 and 10 mile in thickness. MEM-213 is also available in ultrathin films on micro porous supports.
Comparable gas permeability data to toxic gases does not exist for the above mentioned gas permeable membranes. However, the relative gas permeability of these materials may be grasped by considering the data for oxygen which is provided below:
Material Oxygen Permeability centi-barrer1 PTFE 420
MEM-213 16,000
Teflon AF-1600 34,000
Teflon AF-2400 99,000
Note that the permeability of Teflons AF-1600 and AF-2400 are high enough to allow outputs with thin films of this material to be comparable to those obtained with gas porous membranes.
Toxic gas sensors which have a gas permeable membrane between the electrochemically active portion of the device and the gas to be sensed are generally more robust and rugged than presently known sensors which use gas porous membranes. As a result of using a gas permeable membrane, the sensors will be more able to withstand environmental stresses such as, shock, bump, and vibration without leaking electrolyte. They will also be able to withstand and operate over a wider range of temperatures and pressures than sensors using only gas porous membranes.
The advantages of using a gas permeable membrane instead of a gas porous membrane will be fully realizable when using acid, neutral, or alkaline aqueous electrolytes such as aqueous acetic acid, aqueous potassium chloride, or aqueous potassium hydroxide, respectively. Non-aqueous electrolytes, with organic as well as inorganic solvents, can also be used with gas permeable membranes. In contrast, only aqueous acidic electrolytes can be readily used with sensors having only gas porous membranes. When neutral aqueous or organic electrolytes are used with sensors having only gas porous membranes, the sensors experience a significant reduction in their ability to withstand environmental abuse. Successful use of aqueous alkaline or inorganic electrolytes has never been achieved with toxic gas sensors using only gas porous membranes.
When using aqueous electrolytes with sensors having only a gas permeable membrane, water vapor exchange rates with the atmosphere will be minimized. When using low vapor pressure non-aqueous electrolytes with minimal water solubility, water exchange will be virtually eliminated.
This will stabilize the potential of the reference electrode allowing calibration frequencies to be significantly reduced and detection limits to be lowered.
Lower detection limits will also be achievable because the gas permeable membrane will control the temperature compensation requirements of the sensor. This is a significant improvement because the properties of the gas permeable film will remain constant with time. Accurate temperature compensation has been difficult to achieve using sensors having only gas porous membranes in part, because pH changes shift the reference electrode potential.
With the use of gas permeable membranes, poisoning of the sensing electrode will be minimized because aerosols, particles, and high molecular weight gases will not have direct access to the electrochemically active portion of the sensor. For example, the effects of salt spray in marine environments will be greatly diminished. This will reduce the calibration frequency as well as increase the useful life of the sensor. It will also allow for use of less electrocatalyst when making the working electrode and the counter electrode. This will further reduce costs and could lead to a reduction in noise levels.
Sensors having a gas permeable membrane are expected to be less expensive to construct because the complex separator configurations required for wicking the liquid electrolyte to ensure omni-positional use may no longer be necessary. Also, since water exchange will be minimized, the size of the cavity or reservoir that must be set aside for the liquid electrolyte can be reduced and perhaps eliminated. This will allow for the design of significantly smaller electrochemical toxic gas sensors than was previously possible.
While presently preferred embodiments of the invention have been shown and described with particularity in connection with the accompanying drawings, the invention may be otherwise embodied within the scope of the following claims.
Subject matter described herein is described and claimed in co-pending United Kingdom Patent Application
No. 9404128.2 (Publication No. 2277378)
Claims (12)
1. An electrochemical toxic gas sensor comprising a housing; a working electrode, a counter electrode, and a liquid electrolyte within the housing; the electrodes being electrically separate from one another but ionically connected via the electrolyte; an electrical contact for each electrode which passes through the housing; and a gas permeable membrane.
2. The electrochemical sensor of claim 1 wherein the gas permeable membrane comprises a thin film of a copolymer of perfluoro(2,2-dimethyl-1,3-dioxole) and tetrafluoroethylene.
3. The electrochemical sensor of claim l wherein the gas permeable membrane comprises a thin film of polyethylene, or polytetrafluoroethylene or fluorinated ethylene propylene copolymer.
4. The electrochemical sensor of claim I wherein the gas permeable membrane comprises a thin film of a fluorinated silicone polymer.
5 The electrochemical sensor of claim 1 wherein the gas permeable membrane comprises a thin film of a copolymer of silicone and polycarbonate.
6. The electrochemical sensor of claim 1 wherein the working electrode includes a gas porous membrane, the gas permeable membrane being positioned such that the toxic gas must first pass through the gas permeable membrane before reaching the working electrode.
7. The electrochemical sensor of claim 6 wherein the working electrode is placed adjacent to the gas permeable membrane.
8. The electrochemical sensor of claim 1 wherein the working electrode is fabricated on the inner surface of the gas permeable membrane.
9. The electrochemical sensor of claim 1 wherein the liquid electrolyte is selected from the group consisting of an aqueous acid electrolyte, an aqueous neutral electrolyte and an aqueous alkaline electrolyte.
10. The electrochemical sensor of claim 1 wherein the electrolyte comprises a non-aqueous organic solvent and a salt.
11. The electrochemical sensor of claim 1 wherein the electrolyte comprises a non-aqueous inorganic acid.
12. The electrochemical sensor as claimed in any preceding claim, in which the electrical contact for each electrode comprises an electrically conducting feed through having a circumferential groove.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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US08/027,044 US5338429A (en) | 1993-03-05 | 1993-03-05 | Electrochemical toxic gas sensor |
GB9404128A GB2277378B (en) | 1993-03-05 | 1994-03-03 | Electrochemical toxic gas sensor |
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Publication Number | Publication Date |
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GB9618782D0 GB9618782D0 (en) | 1996-10-23 |
GB2303710A true GB2303710A (en) | 1997-02-26 |
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GB2367136A (en) * | 2000-09-21 | 2002-03-27 | Draegerwerk Ag | Electrochemical gas sensor with membrane of specified copolymer |
WO2006081377A1 (en) * | 2005-01-26 | 2006-08-03 | Rosemount Analytical Inc. | Amperometric sensor comprising counter electrode isolated from liquid electrolyte |
EP2731129A1 (en) * | 2012-11-07 | 2014-05-14 | ams AG | Molded semiconductor sensor device and method of producing the same at a wafer-level |
EP2871472A1 (en) * | 2013-11-06 | 2015-05-13 | Life Safety Distribution AG | Support for electrode stack & provision for venting of a gas sensor using an internally mounted table |
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WO1985004482A1 (en) * | 1984-03-27 | 1985-10-10 | University Of Strathclyde | Electrochemical assembly |
GB2225859A (en) * | 1988-12-10 | 1990-06-13 | Draegerwerk Ag | Electrochemical measuring cell for determining ammonia and its derivatives. |
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GB2053482A (en) * | 1979-06-22 | 1981-02-04 | Hydro Quebec | Device for detecting and measuring the concentration of gaseous hydrogen dissolved in a fluid |
EP0096417A1 (en) * | 1982-06-09 | 1983-12-21 | Hitachi, Ltd. | Apparatus for measuring dissolved hydrogen concentration |
WO1985002465A1 (en) * | 1983-12-01 | 1985-06-06 | Honeywell Inc. | Electrochemical sensing of carbon monoxide |
WO1985004482A1 (en) * | 1984-03-27 | 1985-10-10 | University Of Strathclyde | Electrochemical assembly |
GB2225859A (en) * | 1988-12-10 | 1990-06-13 | Draegerwerk Ag | Electrochemical measuring cell for determining ammonia and its derivatives. |
US5076904A (en) * | 1989-04-29 | 1991-12-31 | Dragerwerk Aktiengesellschaft | Electrochemical measuring cell for determining ammonia or hydrazine in a measuring sample |
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GB2367136A (en) * | 2000-09-21 | 2002-03-27 | Draegerwerk Ag | Electrochemical gas sensor with membrane of specified copolymer |
WO2006081377A1 (en) * | 2005-01-26 | 2006-08-03 | Rosemount Analytical Inc. | Amperometric sensor comprising counter electrode isolated from liquid electrolyte |
EP2731129A1 (en) * | 2012-11-07 | 2014-05-14 | ams AG | Molded semiconductor sensor device and method of producing the same at a wafer-level |
WO2014072114A1 (en) * | 2012-11-07 | 2014-05-15 | Ams Ag | Semiconductor sensor device and method of producing a semiconductor sensor device |
US9543245B2 (en) | 2012-11-07 | 2017-01-10 | Ams Ag | Semiconductor sensor device and method of producing a semiconductor sensor device |
EP2871472A1 (en) * | 2013-11-06 | 2015-05-13 | Life Safety Distribution AG | Support for electrode stack & provision for venting of a gas sensor using an internally mounted table |
US9874540B2 (en) | 2013-11-06 | 2018-01-23 | Life Safety Distribution Ag | Support for electrode stack and provision for venting of a gas sensor using an internally mounted table |
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