SENSOR ELECTRODE OXYGEN CONTROL IN AN OXYGEN SENSOR

Description

BACKGROUND

[0001] Oxygen sensors can be used to detect the concentration of oxygen in an environment adjacent to the sensor over a range of environmental conditions. When the sensor operates at a certain temperature, a concentration of oxygen can be dissolved in the electrolyte within the sensor. As the temperature increases, the solubility of most gases, including oxygen and nitrogen, can decrease and result in the formation of bubbles of the gas or gases. In an oxygen sensor, these small bubbles of air containing a mixture of nitrogen and oxygen may diffuse to the sensing electrode and react by a process of electrochemical reduction at the catalyst surface, resulting in an artificially high oxygen reading as the finite volume of oxygen gas is consumed at the sensing electrode. Due the transient nature of this response, this type of failure may be referred to as a "spiking" failure.

[0002] US 2006 / 0021873 A1 discloses a planar electrochemical gas sensor is provided with at least one working electrode, at least one counterelectrode, at least one electrolyte-filled planar electrolyte carrier, at least one planar housing upper part and at least one planar housing lower part. The electrodes are arranged such that they are in two-dimensional contact with the electrolyte carrier. The housing upper part and the housing lower part are connected with one another such that the electrodes and the electrolyte carrier are pressed against one another in such a way that they are secured against displacement. The housing upper part and the housing lower part are partially in direct two-dimensional contact with one another, and the connection of the housing upper part and the housing lower part in the area of the direct two-dimensional contact is present at least along a closed figure, which surrounds the electrodes and the electrolyte carrier.SUMMARY

[0003] The present invention is defined by the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0004] For a more complete understanding of the present invention, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. FIG. 1 illustrates a schematic cross section of an electrochemical gas sensor not according to the present invention. FIG. 2 illustrates an isometric view of the oxygen sensor according to the present invention. DETAILED DESCRIPTION

[0005] It should be understood at the outset that although illustrative implementations of one or more embodiments are illustrated below, the disclosed systems and methods may be implemented using any number of techniques, whether currently known or not yet in existence. The invention should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, but may be modified within the scope of the appended claims.

[0006] The following brief definition of terms shall apply throughout the application: The term "comprising" means including but not limited to, and should be interpreted in the manner it is typically used in the patent context; The phrases "in one embodiment," "according to one embodiment," and the like generally mean that the particular feature, structure, or characteristic following the phrase may be included in at least one embodiment of the present invention, and may be included in more than one embodiment of the present invention (importantly, such phrases do not necessarily refer to the same embodiment); If the specification describes something as "exemplary" or an "example," it should be understood that refers to a non-exclusive example; The terms "about" or approximately" or the like, when used with a number, may mean that specific number, or alternatively, a range in proximity to the specific number, as understood by persons of skill in the art field; and If the specification states a component or feature "may," "can," "could," "should," "would," "preferably," "possibly," "typically," "optionally," "for example," "often," or "might" (or other such language) be included or have a characteristic, that particular component or feature is not required to be included or to have the characteristic. Such component or feature may be optionally included in some embodiments, or it may be excluded.

[0007] The oxygen sensor described herein relies upon the principle of an oxygen pump. In this type of sensor, oxygen is reduced at the sensing electrode and water is oxidized at the counter electrode according to the following half reactions:         At the sensing electrode: O 2 + 4H +< + 4e -< → 2H 2 O     (Eq. 1)         At the counter electrode: 2H 2 O→ O 2 + 4H +< + 4e -<      (Eq. 2)

[0008] The overall reaction results in the consumption of oxygen at the sensing electrode with an equivalent production of oxygen at the counter electrode. The overall reaction is maintained by means of a reference electrode and a potentiostat, which lower the potential at the sensing electrode and allow the reaction to proceed. The resulting current between the sensing electrode and the counter electrode is proportional to the oxygen concentration of the ambient gas. In contrast to others sensors, there is no consuming reaction in which the electrodes themselves are consumed.

[0009] The result of the reactions at each electrode is an oxygen concentration gradient in the electrolyte. The concentration of the dissolved oxygen in the electrolyte varies with the composition of the electrolyte, the temperature of the electrolyte, the atmospheric pressure, and the position relative to the sensing electrode and the counter electrode. The oxygen concentration at or near the sensing electrode may be around 0%, while the oxygen concentration in the electrolyte at or near the counter electrode may correspond to a concentration close to or above the ambient gas concentration. Within this gradient, the dissolved oxygen and / or nitrogen concentration may exceed a saturated concentration due to a temperature rise. As the temperature rises above the saturation concentration, a gas phase comprising oxygen can form, and the resulting gas phase bubbles can travel to the sensing electrode where they may react. The resulting spike in the concentration value can result in a false alarm.

[0010] In order to control the oxygen concentration in the electrolyte surrounding the sensing electrode, the air / electrolyte interface can be controlled so that the closest interface is positioned a suitable distance away from the sensing electrode. Specifically, a low oxygen zone can be created around the sensing electrode that is substantially sealed off from the air / electrolyte interface. This zone may limit the oxygen introduction to the sensing electrode to that occurring through the wetted separator. In order to control the inlet oxygen diffusion rate, the relative geometric parameters of the separator can be adjusted along with the relative distances between the sensing electrode, the reference electrode, and the counter electrode so that a flux of the oxygen to the sensing electrode is controlled. This may provide an oxygen sensor having an improved resistance to spiking failures across a broad range of temperatures. A sealed area around the sensing electrode can be used to limit the amount of oxygen reaching the sensing electrode to a diffusional flow. The sealed area may limit or prevent oxygen in a gas from contacting the separator adjacent to the sensing electrode, which may help to avoid any spiking failures.

[0011] FIG. 1 illustrates a cross-section of an oxygen sensor 100, and FIG. 2 illustrates an isometric view of the oxygen sensor of FIG. 1 with the claimed layout of the separator and electrodes illustrated. The oxygen sensor 100 can comprise a multi-part housing including at least a body 102 defining a hollow interior space 110 for receiving and retaining an electrolyte (e.g., forming an electrolyte reservoir), a base 104, and a cap 106. The base 104 and the cap 106 can sealingly engage the body 102 to form an integral unit.

[0012] The body 102 may have a generally rectangular or square shape, though other shapes such as cylindrical, oval, oblong, or the like are also possible. The body 102, the cap 106, and the base 104 can all be formed from materials that are inert to the selected electrolyte. For example, the body 102, the cap 106, and / or the base 104 can be formed from one or more plastic or polymeric materials. The body 102, the cap 106, and / or the base 104 can be formed from a material including, but not limited to, acrylonitrile butadiene styrene (ABS), polyphenylene oxide (PPO), polystyrene (PS), polypropylene (PP), polyethylene (PE) (e.g., high density polyethylene (HDPE)), polyphenylene ether (PPE), or any combination or blend thereof.

[0013] One or more openings can be formed through the body to allow the ambient gas to enter the interior space 110 and / or allow any gases generated within the housing to escape. The oxygen sensor 100 may comprise at least one inlet opening 140 to allow the ambient gas to enter the housing, and at least one exhaust opening 142 to allow the oxygen generated by the counter electrode 1 11 to exhaust from the housing. The inlet opening 140 and / or the exhaust opening 142 can be disposed in the cap 106 when a cap is present and / or in a wall of the body 102. The inlet opening 140 and / or the exhaust opening 142 can comprise a diffusion barrier to restrict the flow of gas (e.g., oxygen) to the sensing electrode 115. The diffusion barrier can be created by forming the inlet opening 140 and / or the exhaust opening 142 as a capillary and / or a film or membrane can be used to control the mass flow rate through one or more of the openings 140, 142.

[0014] The inlet opening 140 and / or the exhaust opening 142 may serve as capillary openings to provide a rate limited exchange of the gases between the interior and exterior of the housing. The inlet opening 140 may have a diameter between about 20 µm and about 200 µm, where the opening can be formed using a convention drill for larger openings and a laser drill for smaller openings. The inlet opening 140 may have a length between about 0.5 mm and about 5 mm, depending on the thickness of the cap 106. The exhaust opening 142 may have a diameter and length in the same ranges as the inlet opening 140. Two or more openings may be present for the inlet gases and / or the exhaust gases. When a membrane is used to control the gas flow into and / or out of the housing, the opening diameter may be larger than the sizes listed above as the film can contribute to and / or may be responsible for controlling the flow rate of the gases into and out of the housing.

[0015] A porous membrane 122 can also be disposed within the sensor 100, a portion of which can serve as a vent membrane to allow any gas forming within the sensor to pass through the membrane and out the exhaust opening 142 in the cap 106 to the atmosphere and / or to control the flow of the ambient gases into the housing through the inlet opening 140. The membrane 122 may be porous to a gas, but can generally form a barrier to the passage of any liquids such as the electrolyte solution in order to form a liquid seal relative to the outside environment. The porous membrane 122 can be formed from polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), polyethylene (PE), polypropylene (PP), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET) polyaryletheretherketone (PEEK), perfluoroalkoxy (PFA), ethylene chlorotrifluoroethylene (E-CTFE), and any combination thereof. The porous membrane 122 can cover the inlet opening 140 and / or the exhaust opening 142. The flow rate of the gas can then be controlled by the relative permeability of the porous membrane 122 to selected gases.

[0016] A higher density, lower porosity bulk flow membrane 124 can be disposed within the cap 106 to serve as a barrier to the bulk flow of gases into the sensor 100, which can impart tolerance to local environmental pressure changes. The opening 140 through the cap 106 and / or the bulk flow membrane may provide a restrictive and / or tortuous diffusional path to allow the gases in the atmosphere to pass into the sensor 100 to react with the electrodes and electrolyte solution at a flowrate that does not cause undesirable sensor response characteristics, which can manifest as significant increased responses over an extended period such as minutes to hours depending upon the magnitude of the bulk flow gas volume that diffused through the capillary.

[0017] The inlet opening 140 may provide an opening into the central space within the housing. The resulting incoming oxygen may contact the electrolyte, for example, within the separator 120. The exhaust opening 142 can be disposed adjacent to the counter electrode 111 and can serve to allow the oxygen generated at the counter electrode 111 to escape from the housing so that the oxygen does not accumulate within the housing and create false readings by flowing to the sensing electrode 115.

[0018] Within the electrochemical gas sensor 100, a separator 120 may be disposed between the body 102 and the cap 106. The separator 120 can comprise a porous member that acts as a wick for the retention and transport of the electrolyte between the reservoir and the electrodes. The separator can comprise a nonwoven porous material (e.g., a porous felt member), a woven porous material, a porous polymer (e.g., an open cell foam, a solid porous plastic, etc.), or the like, and is generally chemically inert with respect to the electrolyte and the materials forming the electrodes. The separator 120 can be formed from various materials that are substantially chemically inert to the electrolyte including, but not limited to, glass (e.g., a glass mat), polymer (plastic discs), ceramics, or the like.

[0019] The electrolyte can comprise any aqueous electrolyte such as a solution of a salt, an acid, or a base depending on the target gas of interest. The electrolyte can comprise a hygroscopic acid such as sulfuric acid for use in an oxygen sensor. Other target gases may use the same or different electrolyte compositions. In addition to aqueous based electrolytes, ion liquid electrolytes can also be used to detect certain gases. The electrolyte can be maintained within the sensor 100 to allow the reactions to occur at the sensing electrode 115 and the counter electrode 111. The electrolyte can be a liquid that is maintained in the separator 120, which acts as an absorbent medium to retain the electrolyte in contact with the electrodes. The electrolyte can be present in the form of a gel.

[0020] The electrodes 111, 113, 115 within the electrochemical gas sensor 100 can be electrically connected to an external circuit through one or more electrical connections. The electrodes may have connector pins 112, 114 extending through the base 104 and / or the body 102 that can be electrically coupled, directly or indirectly, with the electrodes 111, 113, 115. While not shown in FIG. 1, the sensing electrode 115 can have a connector pin disposed through the base 104 to contact the sensing electrode 115. The external surfaces of the connector pins 112, 114 can be electrically coupled to an external circuit. The connector pins 112, 114 may sealingly engage the base 104 and / or the body 102 so that the connector pins 112, 114 are substantially sealed from the interior space 110 of the electrochemical gas sensor 100.

[0021] The connector pins 112, 114 can be formed from an electrically conductive material, which may be plated or coated. The connector pins 112, 114 can be formed from brass, nickel, copper, or the like. The connector pins 112, 114 can be coated to reduce degradation due to the contact with the electrolyte. For example, the connector pins 112, 114 can include a coating of precious metal such as gold, platinum, silver, or the like, or other base metals such as tin.

[0022] The external circuitry can be used to detect a current between the sensing electrode 115 and the counter electrode 111 to determine the oxygen concentration in an ambient gas in contact with the sensor 100. A potentiostatic circuit can be used to maintain the potential of the sensing electrode 115 at a predetermined value relative to the reference electrode 113 independently from the counter electrode, whose potential remains uncontrolled and limited only by the electro catalytic properties of the electrode. The potential of the sensing electrode 115 relative to the reference electrode 113 can be set at a value of between about -300 and -800 mV for the oxygen sensor 100. The sensing electrode 115 and the reference electrode 113 may comprise platinum when the potential of the sensing electrode 115 relative to the reference electrode 113 is set at a value of between about -300 and -800 mV for the oxygen sensor 100.

[0023] The electrodes generally allow for various reactions to take place to allow a current or potential to develop in response to the presence of a target gas such as oxygen. The resulting signal may then allow for the concentration of the target gas to be determined. The electrodes can comprise a reactive material suitable for carrying out a desired reaction. For example, the electrodes can be formed of a mixture of electrically conductive catalyst particles in a binder such as polytetrafluoroethylene (PTFE). For an oxygen sensor, the electrode can comprise carbon (e.g., graphite) and / or one or more metals or metal oxides such as copper, silver, gold, nickel, palladium, platinum, ruthenium, iridium and / or oxides of these metals. The catalyst used can be a pure metal powder, a metal powder combined with carbon, a metal powder supported on electrically conductive medium such as carbon, or a combination of two or more metal powders either as a blend or as an alloy. The materials used for the individual electrodes can be the same or different.

[0024] The electrode can also comprise a backing material or substrate such as a membrane to support the catalyst mixture. The backing material or substrate can comprise a porous material to provide gas access to the electrode through the substrate. The backing material may also be hydrophobic to prevent the electrolyte from escaping from the housing.

[0025] The electrodes can be made by mixing the desired catalyst with a hydrophobic binder such as a PTFE emulsion and depositing the mixture on the backing material. The electrodes might be deposited onto the substrate by for example screen printing, filtering in selected areas from a suspension placed onto the substrate, by spray coating, or any other method suitable for producing a patterned deposition of solid material. Deposition might be of a single material or of more than one material sequentially in layers, so as for example to vary the properties of the electrode material through its thickness or to add a second layer of increased electrical conductivity above or below the layer which is the main site of gas reaction.

[0026] The oxygen concentration at the sensing electrode 115 results from both the oxygen diffusing to the sensing electrode 115 through the separator 120 as well as oxygen entering the separator 120 as a result of a gas / separator 120 interface at any point along the separator 120. When the gas can contact the electrolyte in the separator 120 adjacent to the sensing electrode, a substantial portion of the oxygen contacting the sensing electrode 115 can result from the contact between the gas and the electrolyte in the separator 120 adjacent to the sensing electrode 115. This can result in a localized oxygen concentration in the electrolyte that exceeds the saturation concentration or solubility at certain temperatures. A temperature rise could then result in higher rate of gaseous oxygen diffusion to the active catalyst of the sensing electrode, as a direct result of gas evolution from the electrolyte and subsequent contact with the sensing electrode to create a transient "spike" in the output current.

[0027] In order to avoid the potential for spiking, the oxygen concentration at or near the sensing electrode 115 can be controlled to a level less than the threshold oxygen solubility at the operational temperature. In general, the dissolved oxygen concentration in the electrolyte at or near (e.g., within several millimeters) the sensing electrode 115 should be as close to zero as possible, thereby ensuring that the majority of the measured sensor response results from the controlled diffusion rate of gaseous oxygen through the capillary gas entry hole, rather than the less controlled, internal diffusion rate of dissolved oxygen through the separator. Limiting or preventing the internal diffusion rate of oxygen can improve the correlation between the sensor response and the gaseous oxygen concentration of the external environment. Ideally, the rate at which the oxygen reaches the sensing electrode 115 from the environment in which the oxygen is to be detected should be equal or close to the consumption rate of the oxygen at the sensing electrode 115. Typically, the consumption rate of oxygen at the sensing electrode (e.g., the reduction rate) may be greater than the diffusion rate of gaseous oxygen to the sensing electrode 115 from the environment in which the oxygen is to be detected. When additional internal diffusion of dissolved and / or gaseous oxygen occurs from the electrolyte near the sensing electrode, the current generated in the circuit may correlate poorly with the external oxygen gas concentration.

[0028] The threshold may be a saturation concentration at a design temperature. For example, the threshold may be the oxygen saturation concentration in the electrolyte at the upper operating temperature specified for the sensor 100. The threshold may be a percentage of the saturation concentration at a specified temperature. This may allow for a safety factor to be included in the design of the oxygen sensor. For example, the oxygen concentration may be controlled to less than about 90%, less than about 80%, or less than about 70% of the saturation concentration at a specified temperature.

[0029] The oxygen concentration can be controlled in a number of ways including providing a spacing between the counter electrode 111 and the sensing electrode 115, limiting a gas / electrolyte contact at or near the sensing electrode 115, and / or selecting a geometry for the separator 120 retaining the electrolyte to limit the flux of oxygen to the sensing electrode 1 15 to a rate that is less than a consumption rate of oxygen at the sensing electrode 115 (e.g., an oxygen reduction rate at the sensing electrode 115).

[0030] A dissolved oxygen concentration gradient can be established in response to the operation of the oxygen sensor between the counter electrode 111 and the sensing electrode 115 under normal operational conditions. The dissolved oxygen concentration gradient may generally be expected to represent a concentration either at, or approaching, the solubility limit of dissolved oxygen in the electrolyte surrounding the counter electrode 111 and either low, or approximately zero, concentration of dissolved oxygen in the electrolyte around the sensing electrode 115 when the oxygen sensor 100 is used in typical operational environments. Since the oxygen concentration in the electrolyte decreases towards the sensing electrode 115, the creation of a suitable distance between the counter electrode and the sensing electrode 115 can be used to limit the concentration of dissolved oxygen in the electrolyte near the sensing electrode 1 15 to less than the threshold.

[0031] The separation distance can be provided by forming an ionically conductive pathway between the electrodes having a desired length. The sensing electrode 115, the reference electrode 113, and the counter electrode 111 are disposed on the ionically conductive pathway, with a distance separating each electrode. The ionically conductive pathway can extend in any direction within the housing in order to achieve a desired spacing. The resulting separation may include a labyrinthian configuration so that the ionically conductive pathway is not in a straight line, which would result in an oxygen sensor having relatively large dimensions.

[0032] As shown in FIG. 2, the separator 120 comprises a planar configuration, and each of the electrodes 111, 113, 115 is disposed in a planar configuration in contact with the electrolyte retained in the separator 120. As shown, the separator 120 forms an ionically conductive pathway, hereafter referred to as a "conductive pathway," by virtue of the electrolyte being retained in the separator 120. The separator 120 extends in a plane within the housing between the counter electrode 111, the reference electrode 113, and the sensing electrode 115. Each of the electrodes 111, 113, 115 are disposed in a substantially planar arrangement to contact the electrolyte in the planar separator 120. The conduction pathway does not extend in a straight line within the housing, which allows the conduction pathway to attain the desired length or separation between the electrodes while maintaining a compact sensor design. While the conduction pathway does not extend in a straight line, a single conduction pathway is formed in which the sensors are arranged in series on the pathway. For example, the middle electrode is disposed between the two end electrodes, and a shorter path is not present between the end electrodes than between either of the end electrodes and the middle electrode.

[0033] As shown in FIG. 2, the sensing electrode 115 and the counter electrode 111 are disposed at the ends of the conduction pathway, and the reference electrode 113 is disposed between the sensing electrode 115 and the counter electrode 111. This configuration locates the reference electrode 113 at a position where it is subject to relatively steady concentration gradients of dissolved oxygen and ions diffusing through the electrolyte, which may reduce the current variations generated by the potentiostatic driver circuitry that manifest as measurement "noise". The steady concentration gradients across the reference electrode in this configuration may be considered a more robust design solution for producing a stable reference potential than the highly variable and fluctuating concentrations of protons and dissolved oxygen that might be generated across the reference electrode 113 when it is positioned outside of the potential gradient, which can result in increased noise in the circuit.

[0034] The distance between the electrodes 111, 113, 115 on the conduction pathway may affect the potential for spiking to occur. The distance along the conduction pathway (e.g., along a centerline of the conduction pathway) between the counter electrode 111 and the reference electrode 113 may be between about 1 mm and about 20 mm or between about 2 mm and about 15 mm, where the reference electrode 113 is disposed between the counter electrode 111 and the sensing electrode 115. The distance (e.g., along a centerline of the conduction pathway) between the sensing electrode 115 and the counter electrode 111 may be between about 5 mm and about 30 mm, or between about 7.5 mm and about 25 mm. The relative ratio between the electrodes 111, 113, 115 may also affect the oxygen concentration gradient. A ratio of a distance between the counter electrode 111 and the reference electrode 113 to a distance between the counter electrode 111 and the sensing electrode 115 can be between about 1:1 and about 1:10, or between about 1:1.5 and about 1:5.

[0035] A diffusion barrier may be provided within the sensor to prevent the cross-diffusion of oxygen to the sensing electrode 115. This configuration may allow the conduction pathway to place the sensing electrode 115 relatively close to the counter electrode 111 without any oxygen generated at the counter electrode 111 reaching the sensing electrode 115. The use of a diffusion barrier may also allow the separator 120 to be positioned within a compact sensor while providing the separation needed to control the spiking type errors.

[0036] As shown in FIG. 2, the diffusion barrier can be formed around the sensing electrode 115 while the reference electrode 113 and the counter electrode 111 are disposed outside of the area defined by the diffusion barrier. The diffusion barrier can comprise a seal formed between the cap 106 and the body 102. The seal can comprise a shoulder 202 formed on the body 102 and / or the cap 106, with a corresponding recess 204 or mating structure on the other component. A compliant material may be disposed on the shoulder 202 and / or the recess 204 to form a seal between the body 102 and the cap 106. The shoulder 202 can be formed from the same material as the body 102 and / or the cap 106, and may comprise an integral structure with the body 102 or the cap 106. The compliant seal, when present, may comprise any impermeable materials that is inert with respect to the electrolyte and has a sufficiently low oxygen permeation rate to ensure a low dissolved oxygen concentration is maintained in the electrolyte.

[0037] When the cap 106 and the body 102 are enclosed, the shoulder 202 may contact the recess 204, and any compliant seal may be positioned therebetween so that a seal is formed in the area around the sensing electrode 115. A joining process (e.g., ultrasonic welding, etc.) can be used to fuse the two components to enhance the seal between the body 102 and the cap 106. A similar barrier may be formed around the perimeter of the body 102 and the cap 106 as part of the manufacturing process for the sensor to seal the housing and prevent the electrolyte from leaking.

[0038] A small gap may exist between the shoulder 202 and the recess 204, or at an edge of the joint between the shoulder 202 and the recess 204. When the material of the shoulder 202 and the recess 204 are hydrophilic, a small amount of the electrolyte may be retained in the gap due to capillary action, resulting a layer of the electrolyte being maintained at the seal between the cap 106 and the body 102. The relatively small gap size along with the electrolyte disposed in the gap may form a diffusion barrier that has a greater diffusional resistance than the separator 120, thereby effectively limiting the diffusion of any oxygen through the chamber edge as compared to the diffusional path through the separator 120.

[0039] When the body 102 and the cap 106 are engaged, a chamber 206 can be formed by an inner surface of the body 102, an inner surface of the cap 106, and the inner surface of the shoulder 202. The edge seal may also define a surface of the chamber 206. The chamber 206 has an opening 208 through which the separator 120 retaining the electrolyte extends. Within the chamber 206, the separator 120 is positioned to maintain contact between the electrolyte and the sensing electrode 115. The separator 120 retaining the electrolyte substantially fills the opening 208 so that any gas within the sensor 100 is substantially prevented from entering the chamber 206 through a convective flow.

[0040] When the separator 120 substantially fills the opening 208, the oxygen reaching sensing electrode 115 may originate from oxygen diffusing through the electrolyte retained in the separator 120 from outside of the chamber 206, for example, as resulting from a gas / electrolyte interface outside of the chamber 206. The distance between the opening 208 and the sensing electrode 115 may be small compared to the conduction pathway length between the counter electrode 111 and the sensing electrode 115. The portion of the separator 120 contained within the chamber 206 may be configured to provide an oxygen concentration within the electrolyte within the separator 120 corresponding to less than about a 1 %, less than about a 0.8%, less than about a 0.6%, less than about a 0.4%, less than about a 0.2%, or less than about a 0.1% oxygen concentration at the sensing electrode 115. The distance between the opening 208 and the sensing electrode 115 may be between about 0.1 mm and about 4 mm, or between about 0.5 mm and about 1.5 mm. A similar distance may be provided around the sensing electrode 115 within the chamber in the event that any oxygen is able to enter the chamber 206 through the seal.

[0041] The positioning of the sensing electrode 115 within the chamber 206 may limit the area of the gas / electrolyte interface and reduce or prevent a gas / electrolyte interface within the chamber 206, which can limit the potential for creating a high oxygen concentration at or near the sensing electrode 115. Rather, any oxygen diffusing to the sensing electrode 115 must diffuse through the electrolyte in the separator 120 over a short distance between the exterior of the chamber 206 and the sensing electrode 115 within the chamber 206. The resulting zone of decreased oxygen concentration within the electrolyte may help limit the potential for the oxygen concentration to exceed a saturation concentration in the electrolyte within the chamber 206. Any gas evolving due to a temperature rise may then be prevented from reaching the sensing electrode 115 except through the electrolyte in the separator 120.

[0042] The geometry of the separator 120 may affect the flux of oxygen to the sensing electrode 115 through the electrolyte in the separator 120, and the geometry can be selected so that the rate of oxygen diffusion to the sensing electrode 115 is less than a consumption rate of oxygen at the sensing electrode 115 (e.g., an oxygen reduction rate at the sensing electrode 115). The thickness of the separator 120 (e.g., a distance perpendicular to the plane of the separator 120) near the chamber 206 may be determined by the available distance between the cap 106 and the body 102 when the sensor is assembled. The separator 120 may contact both the cap 106 and the body 102. The thickness of the separator 120 may be between about 0.5 mm and about 5 mm. The width of the separator 120 at the opening 208 may be based on a total area available for the diffusion of oxygen into the chamber 206 through the electrolyte retained in the pores of the separator 120. In general, the area for diffusion (e.g., the product of the width times the thickness along with the porosity of the separator 120) may affect the total amount of oxygen diffusing into the chamber 206 through the electrolyte to contact the sensing electrode 115. The width of the separator 120 at the opening 208 may be between about 0.5 mm and about 20 mm, between about 5 mm and about 17 mm, or between about 6 mm and about 15 mm. The selection of the material for the separator 120, the selection of the electrolyte and electrolyte concentration, and / or the desired oxygen detection range can affect the selection of the available area of the separator 120 at the opening 208.

[0043] In use, the sensor 100 can detect an oxygen concentration of a gas in the environment in which the sensor 100 is disposed. Referring to FIG. 1 and FIG. 2, the gas in the environment around the sensor 100 can enter the housing through an opening 140 so that the oxygen can be received within the housing. As described herein, the housing can comprise the counter electrode 111, the reference electrode 113, and the sensing electrode 115. The oxygen can contact the separator 120 retaining the electrolyte. The separator 120 with the electrolyte retained therein forms the ionically conductive pathway between each of the electrodes 111, 114, 115, which are disposed in a planar alignment. A potentiostatic circuit can be used to maintain a potential of the sensing electrode 115 lower than the reference electrode 1 13, and as a result, the oxygen may begin to be reduced at the sensing electrode 115 while water is oxidized at the counter electrode 111. The oxygen can thus be consumed at the sensing electrode 115 and regenerated at the counter electrode 111. The oxygen generated at the counter electrode 111 can pass through a diffusional barrier and pass out of the sensor through an outlet 142. An oxygen concentration gradient can then be formed in the electrolyte in the separator 120 between the counter electrode 111 and the sensing electrode 115. A current can be developed based on the reaction of the oxygen and water at the sensing electrode 115 and the counter electrode 111, which may allow the oxygen concentration in the gas contacting the separator 120 to be determined.

[0044] During the detection process, the oxygen concentration in the electrolyte at or near the sensing electrode 115 can be limited to less than a threshold amount. In general, the oxygen concentration in the electrolyte in the separator 120 can be limited to less than a saturation concentration at a predetermined temperature, and in some examples, the oxygen concentration in the electrolyte in the separator 120 can be limited to less than a percentage of a saturation concentration at a predetermined temperature. The length of the separator 120 and the distance between the counter electrode 111 and the sensing electrode 115 can be selected so that the oxygen concentration along the oxygen concentration gradient is below the threshold at or near the sensing electrode 115. For example, the oxygen concentration along the oxygen gradient may be below the saturation concentration in the electrolyte at a predetermined temperature, or below a percentage of a saturation concentration at the predetermined temperature, within about 0.5 mm, within about 1 mm, within about 2 mm, within about 4 mm, or within about 5 mm of the sensing electrode.

[0045] The sensing electrode 115 is disposed within the chamber 206 formed within the housing. The separator 120 extends into the chamber 206 to provide contact between the electrolyte in the separator 120 and the sensing electrode 115. The separator 120 is positioned within the chamber 206 and the opening 208 to prevent or limit any gas / electrolyte contact within chamber 206. Some amount of gas / electrolyte contact may occur within the chamber 206, but the gas may not be able to be exchanged with a gas outside of the chamber 206, thereby limiting the potential for the formation of a high-oxygen concentration gas contacting the electrolyte in the separator 120 near to the sensing electrode 115. The use of the chamber 206 may limit the rate at which the oxygen can diffuse to the sensing electrode 115 during the detection process and thereby limit the oxygen concentration along the oxygen gradient near the sensing electrode 115 to less than the threshold amount.

[0046] Limiting the oxygen concentration in the electrolyte can include limiting the diffusional flux of oxygen through the electrolyte in the separator 120 to less than a rate of oxygen reduction at the sensing electrode 115. The choice of the geometry of the separator 120, the geometry of the chamber 206, and / or the relative positioning of the electrodes may all be used to limit the diffusion of oxygen in the electrolyte to the sensing electrode 115.

Claims

1. An oxygen sensor (100) comprising: a housing defining an interior space (110); a sensing electrode (115); a reference electrode (113); a counter electrode (111); a separator (120) extending in a plane within the housing between the sensing electrode (115), the reference electrode (113), and the counter electrode, wherein the sensing electrode (115), the reference electrode (113), and the counter electrode (111) are disposed in a planar arrangement in contact with an electrolyte retained by the separator (120), wherein the electrolyte provides an ionically conductive pathway between each of the sensing electrode (115), the reference electrode (113), and the counter electrode (111), within the housing, wherein the sensing electrode (115), the reference electrode (113), and the counter electrode (111) are arranged in series on the conductive pathway, with the sensing electrode (115) and the counter electrode (111) at the ends of the conductive pathway; and a chamber (206) within the housing, wherein the chamber (206) contains the sensing electrode (115), wherein the chamber (206) comprises an opening (208), and wherein the separator (120) extends into the chamber (206) and substantially fills the opening (208); characterized in that the conductive pathway does not extend in a straight line, and in that a shorter path is not present between the sensing electrode (115) and the counter electrode (111) than between either of the sensing electrode (115) and the counter electrode (111) and the reference electrode (113).

2. The oxygen sensor (100) of claim 1, wherein the housing comprises a cap (106) coupled to a body (102), wherein the chamber (206) is formed by an inner surface of the cap (106), an inner surface of the body (102), and a seal disposed between the cap (106) and the body (102) around the sensing electrode (115).

3. The oxygen sensor (100) of claim 2, wherein a layer of the electrolyte is maintained at the seal (202, 204) between the cap (106) and the body (102).

4. The oxygen sensor (100) of claim 1, wherein the reference electrode (113) and the counter electrode (111) are outside of the chamber (206).

5. The oxygen sensor (100) of claim 1, wherein a portion of the separator (120) is contained within the chamber (206), and wherein the portion of the electrolyte in the separator (120) within the chamber (206) is configured to provide an oxygen concentration at the sensing electrode (115) corresponding to less than 1%.

6. The oxygen sensor (100) of claim 1, wherein the opening (208) and the separator (120) are configured to prevent gas from entering the chamber (206).

7. The oxygen sensor (100) of claim l, wherein a geometry of the separator (120) is configured to limit a diffusional flux of oxygen through the electrolyte to a rate less than a rate of oxygen reduction at the sensing electrode (115).

8. A method of detecting oxygen comprising: receiving oxygen within a housing of an oxygen sensor (100), wherein the oxygen sensor (100) comprises a sensing electrode (115), a counter electrode (111), a reference electrode (113), and a chamber (206) formed within a housing, wherein the chamber (206) contains the sensing electrode (115); contacting the oxygen with a separator (120) extending in a plane within the housing between the sensing electrode (115), the reference electrode (113), and the counter electrode, wherein the sensing electrode (115), the reference electrode (113), and the counter electrode (111) are disposed in a planar arrangement in contact with an electrolyte retained by the separator (120), wherein the retained electrolyte provides an ionically conductive pathway between each of the sensing electrode (115), the counter electrode (111), and the reference electrode (113), wherein the separator (120) and the retained electrolyte forms a planar ionically conductive pathway between the sensing electrode (115), the counter electrode (111), and the reference electrode (113), and wherein the separator (120) extends into the chamber (206) into contact with the sensing electrode (115); forming an oxygen concentration gradient in the electrolyte in the separator (120) between the sensing electrode (115) and the counter electrode (111) by disposing the sensing electrode (115), the reference electrode (113), and the counter electrode (111) in series on the conductive pathway, with the sensing electrode (115) and the counter electrode (111) at the ends of the conductive pathway; limiting an oxygen concentration along the oxygen gradient near the sensing electrode (115) to less than a threshold amount by providing a chamber (206) within the housing, wherein the chamber (206) contains the sensing electrode (115), wherein the chamber (206) comprises an opening (208), and wherein the separator (120) extends into the chamber (206) and substantially fills the opening (208); and detecting a current between the sensing electrode (115) and the counter electrode (111), wherein the current is proportional to a concentration of the oxygen; characterized in that the oxygen concentration along the oxygen gradient near the sensing electrode (115) to less than the threshold amount is limited also by configuring the conductive pathway to not extend in a straight line, wherein a shorter path is not present between the sensing electrode (115) and the counter electrode (111) than between either of the sensing electrode (115) and the counter electrode (111) and the reference electrode (113).

9. The method of claim 8, wherein limiting the oxygen concentration along the oxygen gradient comprises preventing contact between a gas in the oxygen sensor (100) and the electrolyte retained in the separator (120) within the chamber (206).

10. The method of claim 9, wherein limiting the oxygen concentration along the oxygen gradient comprises limiting a diffusional flux of oxygen through the electrolyte retained in the separator (120) to the sensing electrode (115) within the chamber (206) to a rate less than a rate of oxygen reduction at the sensing electrode (115).

11. The method of claim 9, wherein the threshold comprises an oxygen saturation concentration in the electrolyte at an upper operating temperature for the sensor (100).

12. The method of claim 8, wherein the reference electrode (113) is disposed in the planar ionically conductive pathway between the counter electrode (111) and the sensing electrode.