GB2292804A - Electrochemical gas sensor with electrodes deposited on a wick - Google Patents

Abstract

The sensor includes a housing 19 containing electrolyte 35, and at least two electrodes 11, 13, 15 deposited on a wick 17 which supplies electrolyte to the electrodes. Gas reaches the sensing electrode 11 through path 29 which may be open or covered with a porous membrane. The electrodes may comprise PTFE and carbon or silver or a noble metal catalyst of platinum, palladium, iridium, ruthenium, rhodium or gold. The wick may comprise glass fibre filter paper, silica based battery separators, asbestos based battery separators, Nafion, porous philic polyethylene or polypropylene, rubber or non-conducting polymers. Electrical leads to the electrodes may be of carbon fibre or a non-noble metal conductor material. <IMAGE>

Description

ELECTROCHEMICAL GAS SENSOR WITH THE ELECTRODE8 ON A WICK FIELD OF THE INVENTION The invention is related to an electrochemical gas sensor and a method of manufacture therefore and, more particularly, to an electrochemical gas sensor in which the electrodes are fabricated on the wick and the electrical leads are manufactured using low cost carbon fibers or other low cost electrically conducting material composites.

BACKGROUND OF TEE INVENTION A conventional electrochemical gas sensor ("EC sensor") comprises a sensing, or working electrode, a counter electrode and in many cases a third, reference electrode, all of which operate in an electrolyte. The electrolyte may be aqueous or non-aqueous. Conventional EC sensors of the amperometric type and methods of preparation therefor are described in U.S.

Patent Nos. 4,326,927, 4,201,634, 3,992,267, and Re. 31,916, which are herein incorporated by reference. In general, preparation of the sensing electrode involves applying a catalyst/polymer suspension to a porous phobic membrane. The term "phobic" as used herein, defines a material that will not absorb an electrolyte, aqueous or non-aqueous, while the term "philic" defines a material that will absorb the electrolyte.

Thus, where the electrolyte is aqueous, the terms 1,hydrophobic" and "hydrophilic" apply.

After application of the catalyst/polymer suspension, the membrane is cured at an elevated temperature to bond the catalyst/polymer suspension directly to the membrane.

Alternatively a catalyst/polymer composite can be vapor deposited onto the porous membrane as described in U.S. Patent No. 4,326,927. The counter and references electrodes are prepared in the same manner. The catalysts used for the three electrodes are typically metal powders selected from the noble metal group of platinum, palladium, iridium, ruthenium, rhodium or gold, platinum (Pt) being the preferred metal for the counter and reference electrodes. A typical electrode assembly, as shown in U.S. Patent No. 3,992,267, is then assembled by securing the porous phobic membrane and electrodes to a philic electrolyte matrix, or wick, within a sensor housing by means of support members mounted on opposite sides of the wick.

The traditional catalyst/polymer suspension comprises a ti1) mixture of polytetrafluoroethylene ("PTFE" or "Teflon") particles and Pt black powder in a water base vehicle.

Specifically, the mixture comprises a ratio of about 3:1 weight percent Pt Black to Teflon. A dispersion of Teflon in an aqueous matrix is available through E. I. DuPont de Nemours. The water base vehicle is typically prepared using nonionic surfactant, such as Brij-35 and a thickening agent, such as Carbopol 940. Brij-35 is available through Aldrich Chemical Company and Carbopol is available through BF Goodrich. The catalyst/polymer suspension is then sprayed or otherwise deposited onto a porous film which is typically fabricated from Teflon, Zitex or Gortex. Zitex is available through Norton Chemplast and Gortex is available through W. L Gore. Finally, the composite material is dried, pressed and cured at elevated temperatures of approximately 250-300 C.

The porous film is generally made of a material that is not wetted by the electrolyte so as to prevent electrolyte loss through the membrane. The material is porous, however, to allow gas access to the electrode.

A major drawback of the conventional electrode assemblies is that the resultant sensors tend to age fairly rapidly.

"Aging" refers to slowed response time, decreased response to the analyte, or increased electronic noise in the sensor, which ultimately results in failure of the sensor to meet acceptable performance specifications. The typical cause of aging in the case of acid, i.e., aqueous electrolyte, is dryout of the water, which increases acid concentration. The effects of the increased acid concentration include reduced performance of certain detection mechanisms which are sensitive to acid concentration, due to solubility of certain analytes in aqueous acids, and shrinkage of the "wick" material which causes poorer catalyst/lead electrolytic contact or even total separation of electrolyte soaked wick from the catalyst.

In addition, preparation of conventional electrodes has been limited to placement on a porous Teflon film, which is often difficult to handle. In particular, application of an aqueous suspension to a hydrophobic material, such as by spraying, can be tedious. Further, when the electrode is handled and sealed, it is often difficult to keep the wick in contact with the electrode.

Still another drawback of conventional EC sensors is their relatively high cost due to the requirement of significant amounts of noble metals for use in manufacturing the electrodes.

Accordingly, the need exists to provide an improved lowcost sensor which is relatively easy to manufacture and has improved performance in terms of response time, stability of baseline and signal magnitude, and stable lifetime.

SUMMARY OF TEE INVENTION Thus, it is a purpose of the invention to overcome the disadvantages of the prior art and thereby provide a relatively inexpensive EC sensor having an increased operating life.

In accordance with a preferred embodiment of the invention, the EC sensor comprises a sensor housing and a liquid electrolyte contained in the sensor housing. An electrode assembly, comprising at least two electrodes mounted on a wick that is wetted by the liquid electrolyte, is positioned inside the sensor housing. The EC sensor may further include a third electrode. An electrical lead is mounted on each of the electrodes.

The electrochemical gas sensor is preferably manufactured by depositing a catalyst/polymer composite directly on a wick that is wetted by the electrolyte to form at least two electrodes. A third electrode may also be included in the resulting electrode assembly. An electrical lead is then placed onto a surface of each of the electrodes. The electrode assembly is positioned in a s;ensor housing and the sensor housing is sealed.

It is, therefore, an object of the invention to provide an EC sensor and a method of preparation therefor having improved electrode/electrolyte contact.

It is another object of the invention to provide an EC sensor and a method of preparation therefor having a longer operating life than conventional prior art EC sensors.

It is yet another object of the invention to provide an EC sensor and a method of preparation therefor having a reduced cost of materials and assembly and that is amenable to mass production.

These and other objects of the invention will become apparent from the detailed description to follow.

BRIEF DE8CRIPTION OF THE DRAWINGS There follows a detailed description of the preferred embodiments of the invention which are to be taken together with the accompanying drawings, wherein: Figure 1 shows a top view of the electrode assembly of the invention comprising a sensing electrode, a counter electrode and a reference electrode deposited directly on a wick.

Figure 1 shows a cross-sectional view of an assembled EC sensor in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIXENTS Referring now to the drawings, Figure 1 shows a typical three electrode assembly 19 manufactured in accordance with the method of the invention comprising a sensing electrode 11, a counter electrode 13 and a reference electrode 15 fabricated on a wick 17. The electrode assembly may also be fabricated with only two electrodes, i.e., without the reference electrode 15. By depositing the electrodes directly on a wicking material and not on a porous phobic material, i.e., physically or chemically bonding the electrodes to the wick, the electrodes may be directly wetted by the chosen electrolyte, thereby making a positive and good electrolytic contact with the electrodes. This allows the electrolyte to permeate the electrodes 11, 13 and 15 directly, in intimate contact with the catalyst and analyte gas.Further, by placing all three electrodes 11, 13 and 15 on the wick 17, the internal components of the EC sensor are reduced to one, with 3 lead wires. Thus, the complexity of the EC sensor is reduced over conventional, prior art EC sensors using this method.

According to the method of the invention, the electrodes 11, 13 and 15 are isolated individually and can be simultaneously fabricated from a catalyst/polymer suspension.

Each electrode requires only 15 milligrams or less of metals for the catalyst component which results in a tremendous cost savings over the conventional EC sensors. The metals used for the electrodes may be noble metals such as Pt, palladium, iridum, ruthenium, rhodium or gold, or non-noble metals such as carbon, nickel, tin or copper. Preferably, the suspension comprises a platinum powder/phobic Teflon particle dispersion in water. The suspension is typically prepared by suspending 0.5 grams (g) Pt black in 10 cubic centimeters (cc's) of deionized water, using a small magnetic stirrer. Fuel cell grade Pt black is available through Englehard Corporation, Specialty Chemicals Division.Approximately 0.16 g Teflon is then added by pipetting 0.26 g TFE30 into the mixture. TFE30 is a suspension 60% Teflon solids by weight and is available through E.I. Dupont de Nemours.

Once the suspension is uniformly mixed, 100 microliters (A1) of Brij-35 is added while stirring is continued, to improve the stability of the suspension. The resulting suspension contains 1.0 milligram (mg) Pt per 20 p1 suspension, and results in an admixture that is 25% Teflon by weight. A 50 p1 aliquot of the suspension is pipetted directly onto a clean philic substrate, which functions as the wick 17, using a template to control diameter and location of the electrodes. When the suspension is applied to the wick 17, it permeates and intermingles with the upper layers thereof to form an intricate admixture.Suitable philic substrates include glass fiber filter paper, silica based and asbestos based battery separators, Nafion, porous philic polyethylene or polypropylene, rubber polymers such as silicone or any suitable, i.e., stable and not adversely interacting, wicking material for the specific aqueous or nonaqueous electrolyte to be used in the sensor.

The above-mentioned steps are then repeated until all three electrodes 11, 13 and 15 have been deposited on the wic 17. All three electrodes 11, 13 and 15 are preferably placed in the same horizontal plane on a surface of the wick 17 or ir a stacked configuration to eliminate all possibility of electrode leads shorting during assembly due to misalignment of parts. Of course, other geometries are possible and may b found suitable for certain applications.

The resulting electrode assembly 19 is then cured, if necessary. Curing may, for example, be necessary to bake out the Brij-35. Generally, curing is performed under nitrogen a a temperature of 250-350 C for one to seven hours. A preferred curing time and temperature is three hours at 300 C, although any effective equivalent of time, temperature anc pressure can be used to cure in accordance with the size and thickness of the desired electrode and wick materials. Curing allows the Teflon particles to begin to sinter together and immobilize to form a web-like structure which ties the catalyst particles together, as well as bond to the wick.

This web-like structure is somewhat more philic than a structure formed by a curing a catalyst/Teflon mixture onto a Teflon film due to the fact that the philic glass fibers penetrate the Teflon catalyst mixture. The advantage of the more philic structure is that the electrolyte-catalyst interface is more easily established and can be significantly larger. In addition, the curing creates a more rugged catalyst bed which allows the electrodes to withstand more severe sensor assembly processing steps such as ultrasonic welding.

Any chemically inert, low-resistance wire 21, 23 and 25 may be pressed onto the surface of the sensing electrode 11, counter electrode 13, and reference electrode 15, respectively, and used as the electrical leads to the external surface. Alternatively, wires may be incorporated into the mixture during the curing step to form mechanically stable electrical contacts. Pt is the traditional material used for manufacture of electrical leads. Most preferably, however, a less expensive conductor such as carbon is utilized in the EC sensor of the invention, thereby providing a significant cost reduction. Carbon fibers are thousands of times less expensive than Pt wire, yet are conductive enough to allow a good electrical connection from the electrode to the circuit connector outside the sensor housing.Bundles of carbon fibers may be used although a single fiber is preferred because it is easier to seal and make leak-proof inside the sensor housing. The carbon fibers can be platinized, i.e., immersed in hexachloroplatinic acid and cycled in potential until Pt deposits as a fine powder on the surface, or otherwise coated with a conductor to improve contact to the sensing electrode. The electrical leads, 21, 23 and 25, in addition to being electrically conductive, must proceed through the wall of the sensor and form a gas and liquid tight seal so that the electrolyte stays in the sensor and cannot escape. Other materials which may be used for the electrical leads are graphite, carbon filled composites, such as silicone rubber impregnated with carbon or other conductors that are not reactive with carbon or with the electrolyte and that provide a sealed but conductive path through the wall of the sensor housing to the electronic circuit (potentiostatic or galvanic).

Figure 2 shows an assembled EC sensor in accordance with the invention. In the EC sensor, the sensing electrode 11 is pressed close to the gas exposure path 29. The counter and reference electrodes 13, 15 are pressed against the wall 31 of sensor housing 27, and are not exposed to the analyte gas but II are connected electrolytically to the sensing electrode. The gas exposure path 29 can be open to the air or can be covered with a phobic layer in such a manner that there is a close proximity of the catalyst and porous membrane, thereby preventing entrapment of air or electrolyte bubbles and preventing leakage of electrolyte out of the sensor housing.

If an acid electrolyte is used, the phobic layer may be a porous Teflon. Appropriate porous Teflons include Zitex 606223 or Zitex G-100 which are available through Norton Performance Plastics. On the opposite end of the EC sensor, a gas permeable membrane is sealed across a vent port 33, to provide an equilibration path in the event of sudden pressure changes. The electrode assembly 19 is held in place by applying pressure thereto by any conventional means, such as supports or a circular disk mounted on top of the electrode assembly 19. The amount of pressure applied is a function of the material used for the wick 17. If filter paper or silica is used for the wick 17, for example, a sufficient pressure to compress the wick 17 by 25% has been found satisfactory. If the wick 17 is glass, a pressure of less than 30-40 psi is typically required for a properly aligned sensor structure.

The housing 27 is designed with pillars 42 which apply a precise amount of pressure on the electrode assembly 19 so that the spaces 43 between wall 31 and the remaining portion of the housing 27 can be controlled.

The sensor housing 27 of the EC sensor is sealed and then filled with an appropriate electrolyte 35, typically sulfuric acid. The sensor housing 27 may include one or more fill ports (not shown) for adding the electrolyte. Chambers 37 and 39 are circular and may be connected to each other through apertures 41 if only one fill port is utilized. The apertures 41 are generally about 1/4 inch in diameter. The fill port is then sealed. An alternative is to seal the sensor housing 27 ultrasonically, using low power and low amplitude ultrasonic welds in spaces 43. A general guideline is 30 pounds per square inch, 50 micron amplitude, 20 kilohertz. By sealing the sensor housing 27 ultrasonically, the electrode assembly 19 may be saturated with acid prior to welding the housing parts together, thereby eliminating one welding step and potential failure due to a leaky fill port.The electrolyte 35 makes better, more positive contact with the electrodes 11, 13 and 15 and forms a more efficient electrode-electrolyte interface. In addition, the sensor housing 27 may be sealed in a single step.

The EC sensor of the invention provides a more efficient electrode/electrolyte interface as compared with conventional EC sensors in which the electrodes are fabricated on a porous phobic substrate. Good electrolyte contact leads to efficient electrochemical signals so that smaller amounts of catalyst yield larger signals. Moreover, because the electrodes are fabricated directly on the wick, the analyte can dissolve in the pore spaces occupied by the aqueous electrolyte absorbed into the wick.

The positive contact between the electrolyte and electrode is less likely to degrade as the EC sensor ages.

For example, the philic nature of the wick allows the Teflon- Pt dispersion to wet better than just the Pt, which is philic most of the time, along with the phobic Teflon. Thus, as dryout occurs with corresponding shrinkage of the wick material, the positive electrode/electrolyte contact will not be lost. Moreover, the normally observed degradation of signal to noise ratio which is due to loss of physical contact will not occur.

Further advantages observed in the EC sensor are that the sensor stabilizes faster, responds faster, and is extremely well behaved analytically, i.e. very reversible with modest background currents, low temperature coefficients, and pressure coefficients, thereby giving excellent shaped signals when exposed to the gas of interest.

An even further advantage of the EC sensor is that it may be constructed at a reduced cost of materials and assembly.

The amount of catalyst used can be reduced by an order of magnitude and the electrical leads may be manufactured with lower cost materials as compared to high cost Pt wires.

Moreover, the simplified construction reduces the number of housing parts required, reducing the materials cost and the labor required for assembly. The EC sensor is, therefore, amenable to mass production techniques like ultrasonic welding and can be assembled in fewer steps using pick-and-place equipment.

Although the above description has focused on the preparation of a carbon monoxide sensor, the invention can be implemented for any sensor using metal powder suspensions or other metals that can be conveniently bound to the wick and used for preparation of the electrodes. These metals include gold, platinum powder, carbon, silver, iridium, and ruthenium, as well as mixtures which may be used for special applications. Further while the description has focused on preparation of a three electrode sensor, the invention also contemplates preparation of a sensor having only two, or more than three electrodes. Also, even though an electrode comprising a noble-metal Teflon dispersion has been described, any pure metal electrode alone or mixed with other polymers (phobic or philic) or mixed with the wick itself can be envisioned, provided that the electrode is attached directly to the wick. Accordingly, variations and modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention as set forth in the claims.

Claims (26)

CLAIMS We claim:

1. An electrochemical gas sensor comprising: a housing; a liquid electrolyte contained inside said housing; a wick positioned inside said housing for wicking the electrolyte; at least two electrodes deposited on said wick; and an electrical lead mounted on each of said electrodes, to provide means for external electrical connections.

2. The electrochemical gas sensor according to claim 1 wherein said at least two electrodes comprise TeflontànS a noble metal catalyst selected from the group consisting of platinum, palladium, iridium, ruthenium, rhodium, gold, carbon and silver.

3. The electrochemical gas sensor according to claim 2 wherein said noble metal catalyst comprises platinum.

4. The electrochemical gas sensor according to claim 1.

wherein each of said two electrodes comprises no more than 15 milligrams of metal catalyst.

5. The electrochemical gas sensor according to claim 1 wherein said wick is selected from the group consisting of glass fiber filter paper, silica based battery separators, asbestos based battery separators, Nafion, porous philic polyethylene, porous philic polypropylene, rubber and nonconducting polymers.

6. The electrochemical gas sensor according to claim 1 wherein at least one of said electrical leads comprises at least one carbon fiber or a non-noble metal conductor material

7. The electrochemical gas sensor according to claim 1 wherein said housing is sealed by ultrasonic welds.

8. The electrochemical gas sensor according to claim 1 further comprising a gas exposure path covered with a gaspermeable material.

9. The electrochemical gas sensor according to claim 1 further comprising a vent port in said housing, said vent port being sealed with a gas permeable membrane, for providing a pressure equilibration path.

10. The electrochemical gas sensor according to claim 1 wherein said electrodes comprise a sensing electrode, a counter electrode and a reference electrode.

11. A method of manufacturing an electrochemical gas sensor comprising the steps of: depositing at least two electrodes onto a wick to form a electrode assembly; placing an electrical lead onto a surface of each of sai electrodes; positioning said electrode assembly in a sensor housing; and sealing said sensor housing.

12. The method according to claim 11 wherein said electrodes are deposited in the same plane on a surface of said wick.

13. The method according to claim 11 wherein said electrodes are deposited in a stacked configuration on a surface of said wick.

14. The method according to claim 11 further comprising the step of preparing said electrodes by suspending a catalyst and a polymer in a liquid.

15. The method according to claim 14 wherein said catalyst comprises platinum, said polymer comprises Teflon an said liquid comprises water.

16. The method according to claim 11 further comprising the step of curing said electrode assembly after said step of depositing or placing.

17. The method according to claim 11 wherein at least one of said electrical leads comprises a carbon fiber, a bundle of carbon fibers, or a conductive metal composite material.

18. The method according to claim 11 further comprising the step of saturating said electrode assembly with an electrolyte before said step of positioning.

19. The method according to claim 11 further comprising the step of filling said sensor housing with an electrolyte before said step of sealing.

20. The method according to claim 11 wherein said step of sealing comprises ultrasonically welding said sensor housing.

21. The method according to claim 11 wherein said electrode assembly comprises a sensing electrode, a counter electrode and a reference electrode.

22. The method according to claim 21 wherein said step of positioning comprises placing said electrode assembly in said housing such that said sensing, counter and reference electrodes are between a wall of said sensor housing and said wick, said sensing electrode being placed adjacent to a gas exposure path in said wall for allowing exposure of said sensing electrode to an analyte gas and said counter and reference electrodes being placed at a distance from said gas exposure path for preventing exposure of said counter and reference electrodes to an analyte gas.

23. The method according to claim 18 further comprising the step of applying pressure to said saturated electrode assembly to secure said electrode assembly in place inside said housing.

24. The method according to claim 11 wherein said wick is selected from the group consisting of glass fiber filter paper, silica based battery separators, asbestos battery separators, Nafion, porous treated polyethylene, porous hydrophilic polypropylene, polymer composites and rubber polymers.

25. An electrochemical gas sensor, substantially as herein described with reference to, or as shown in, the accompanying drawings.

26. A method of manufacturing an electrochemical gas sensor, substantially as herein described with reference to the accompanying drawings.