ONE PIECE SHAPED PLANAR SEPARATOR
Embodiments of the disclosure include an electrochemical sensor comprising a housing defining an interior space; a sensing electrode; a counter electrode; and a separator retaining an electrolyte, wherein the electrolyte provides an ionically conductive pathway between each of the sensing electrode and the counter electrode within the housing, wherein the electrodes are orientated in a common plane, the separator contacts each of the electrodes, the separator comprises a variety of geometrical features, including tabs, thinned portions, cut-outs, and rings, and the separator comprises variations in compression, density, and surface area.
This application is a continuation of and claims priority to PCT Application Serial No. PCT/US2016/036637 entitled “One Piece Shaped Planar Separator”, file Jun. 9, 2016, which is a continuation-in-part of both: 1) PCT Patent Application Serial No. PCT/US15/41449 entitled “Inert Corrosion Barriers for Current Collector Protection”, filed 22 Jul. 2015, and 2) PCT Patent Application Serial No. PCT/US15/46461 entitled “Sensing Electrode Oxygen Control in an Oxygen Sensor”, filed 24 Aug. 2015, all of which are incorporated herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTNot applicable.
REFERENCE TO A MICROFICHE APPENDIXNot applicable.
BACKGROUNDElectrochemical gas sensors generally comprise electrodes in contact with an electrolyte for detecting a gas concentration. The electrodes are electrically coupled to an external circuit though lead wires that are coupled to connector pins. When a gas contacts the electrolyte and the electrodes, a reaction can occur that can create a potential difference between the electrodes or cause a current to flow between the electrodes. The resulting signal can be correlated with a gas concentration in the environment.
In some instances, the sensors can be used to detect a concentration of oxygen in an environment adjacent to the sensor over a range of environmental conditions. Electrochemical sensors such as oxygen sensors can experience a number of issues during operation. For example, when the sensor operates at a certain temperature, a concentration of a target gas, or a reaction product formed by the oxidation or reduction of a target gas, 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 cause instability in the signal at the sensing electrodes, where the oxygen may react by a process of electrochemical reduction at the catalyst surface, resulting in a temporary or transient artificially high oxygen reading as the finite volume of oxygen gas is consumed at the sensing electrode. Other issues, including the availability of electrolyte in contact with the electrodes and issues with corrosion, can also be present. The presence of bubbles of even an inert gas such as nitrogen within the sensor can also be a problem as they may create an easy gas phase path through which oxygen can rapidly diffuse and get to the sensing electrode.
SUMMARYEmbodiments of the disclosure include an electrochemical sensor comprising a housing defining an interior space; a sensing electrode; a counter electrode; and a separator retaining an electrolyte, wherein the electrolyte provides an ionically conductive pathway between each of the sensing electrode and the counter electrode within the housing, wherein the electrodes are orientated in a common plane, the separator contacts each of the electrodes, the separator comprises a variety of geometrical features, including tabs, thinned portions, cut-outs, and rings, and the separator comprises variations in compression, density, and surface area.
Embodiments of the disclosure also include a method for manufacturing a planar separator for use in an electrochemical sensor comprising forming a planar separator, wherein the ratio of the length of the separator to the overall thickness of the separator is at least 20/1; forming geometric features within the separator comprising tabs, thinned portions, cut-outs, and rings; and assembling the planar separator within a housing, wherein the separator contacts a plurality of electrodes.
These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims.
For a more complete understanding of the present disclosure, 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.
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 disclosure 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 along with their full scope of equivalents.
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.
Described herein are various designs and configurations for electrochemical sensors. Electrochemical gas sensors generally detect the presence of a gas in the atmosphere adjacent the sensor by allowing a controlled flow (or diffusion) rate of the ambient gases to enter and react within the sensor. The composition of the electrodes and electrolyte within the sensor can be selected to react with different gases, thereby enabling a degree of selectivity in determining the ambient concentration of a targeted gas. The electrochemical sensors can comprise components and electronic instrumentation (e.g., electronic circuitry such as potential meters, current meters, potentiostats, and the like) that collectively are capable of detecting gases or vapors that are susceptible to electrochemical oxidation or reduction at the sensing electrode, such as carbon monoxide, hydrogen sulphide, sulphur dioxide, nitric oxide, nitrogen dioxide, chlorine, hydrogen, hydrogen cyanide, hydrogen chloride, ozone, ethylene oxide, hydrides, and/or oxygen. While some embodiments described herein refer to an oxygen sensor, the same configurations and methods can be used with the appropriate materials to perform electrochemical oxidation and/or reduction to enable detection of any suitable target gas.
In some aspects, the electrochemical sensors described herein may comprise an oxygen sensor which 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: O2+4H++4e−→2H2O (Eq. 1)
At the counter electrode: 2H2O→O2+4H++4e− (Eq. 2)
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 drive the sensing electrode to a potential which allows 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. As embodied by Fick's Law, this proportionality applies for sensors operating under diffusion limited gas access of the capillary and full consumption of the target gas at the sensing electrode. In contrast to other sensors, there is no consuming reaction in which the electrodes or the electrolyte themselves are consumed.
Described herein are a number of features for use with an oxygen sensor that can contribute to the operation of the sensor. In an aspect of the present sensor, contact between an electrode material and an electrical lead can be maintained over a limited area of the sensor. This may allow the separator in contact with the electrode to be separately compressed, generally at a lower compression level than that of the electrical lead. This may allow the separator that acts to retain the electrolyte in contact with the electrode to avoid being over-compressed, which can limit the amount of available electrolyte in contact with the electrode. Also, limiting the compression to the contact between the electrode and electrical lead could even remove the need for a separator at all, if the electrolyte is operable to wet the materials in the sensor sufficiently to remain in contact with them (e.g. ionic liquids). At the same time, adequate compression between the electrode and the electrical lead may be maintained so that the electrical connection between the electrical lead and the electrode is maintained.
In an aspect of the present sensor, 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 and/or nitrogen 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.
In some aspects, the air/electrolyte interface can be controlled so that the closest interface is positioned a suitable distance away from the sensing electrode in order to control the oxygen concentration in the electrolyte surrounding the sensing electrode within the sensor itself. 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. In some embodiments, a sealed area around the sensing electrode can be used to limit the amount of oxygen reaching the sensing electrode to a liquid-phase diffusional flow, which may be orders of magnitude slower than gas phase 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.
In other aspects, the interface for the sensing electrode can occur based on gas diffusion through a gas diffusion membrane that is part of the sensing electrode. In this embodiment, the gas can contact the membrane within the sensor and diffuse to the electrode interface to form a three-phase gas/electrolyte/electrode interface for carrying out a reaction, which can produce the signal used to indicate an amount of the gas present.
In some aspects, the separator used with the sensor can be planar, and the use of a specific geometry may allow for various oxygen gradients to be controlled. Various features such as the cross-sectional area along the length of the separator can be controlled through the shape of the separator. In some aspects, various ablation patterns can be used to control a diffusion rate through a portion of the separator. The overall design of the separator may then allow a specific gradient of reactants (e.g., dissolved oxygen, ions, etc.) to be maintained during operation of the sensors.
In an aspect of the sensor, a breather tab can be used to allow gases that collect within the sensor body to be vented. Problems can arise when a liquid electrolyte level within the sensor body covers a breather vent. By limiting the contact between any accumulated gas phase and the breather material, the gas cannot pass through the vent to an exterior of the sensor body. As disclosed herein, a breather tab can be used that passes through the sensor body along an elongated path. The path allows the breather tab to be in contact with the gas phase, and thereby vent gas from the sensor body at a wide variety of electrolyte liquid levels and orientations of the sensor.
In still other aspects, various wicking designs can be used with the sensor to aid in electrolyte transport between an electrolyte reservoir and the separator. The sensor can generally be designed so that the electrolyte is in contact with the separator, which serves as a wick to retain the electrolyte in contact with the electrodes. In some instances (e.g., low electrolyte levels, differing orientations, etc.), the electrolyte may not properly wick into the separator. In order to transport the electrolyte from a reservoir to the separator, wicking channels can be included within the sensor body or housing to further transport the electrolyte to the separator. Various structures such as capillary channels can be formed in one or more portions of the inner surface of the housing. This may serve to wick the electrolyte to the separator at a wide variety of angles and electrolyte levels.
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. In an embodiment, 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.
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. In an embodiment, the electrochemical 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 any gases generated by the counter electrode 111 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.
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. In an embodiment, the inlet opening 140 may have a diameter between about 20 μm and about 200 μm, where the opening can be formed using a conventional 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. In some embodiments, 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 (or diffusion) 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. In general, the gas access between the ambient environment and the interior of the electrochemical sensor 100 is intended to occur through the inlet opening 140. When exhaust opening 142 is present, the exhaust opening 142 can be configured so that the rate of diffusion through the exhaust opening 142 may be less than that through the inlet opening 140, thereby reducing any access through the exhaust opening 142 relative to the inlet opening 140.
A porous membrane (e.g., vent membrane 314 in
A porous membrane 122 can also be disposed within the sensor 100, a portion of which can serve as a breather tab to allow any gas forming within the sensor to pass through the membrane to the vent membrane. In some embodiments, the porous membrane 122 can serve as a vent membrane over the exhaust opening 142. The porous 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 allow any gas to pass through the porous membrane 122 to the vent membrane and/or the exhaust opening 142. In an embodiment, 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 exhaust opening 142. The flow rate of the gas between the reservoir 110 and the porous membrane 122 can then be controlled by the relative permeability of the porous membrane 122 to selected gases. When the vent membrane is present, the porous membrane 122 may have a higher gas permeability to allow the gases within the sensor 100 to pass to the vent membrane.
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 while allowing for relatively free diffusion of gas through the bulk flow membrane 124, which can impart tolerance to local environmental pressure changes that can disrupt the diffusion controlled operation of the inlet opening 140. The inlet opening 140 through the cap 106 and/or the bulk flow membrane 124 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.
The inlet opening 140 may provide an opening into the central space within the housing. The resulting incoming gases (e.g., including the target gas) may contact the electrolyte, for example, within the separator 120. In an embodiment, the exhaust opening 142 can be disposed adjacent to the counter electrode 111 and can serve to allow gases generated at the counter electrode 111 to escape from the housing so that the gases do not accumulate within the housing and create false readings by flowing to the sensing electrode 115.
It can be useful to minimize the parts count in the sensor design due to the implications for cost and complexity of manufacture and reproducibility of assembly. It can also be advantageous during the assembly of the sensors to use full or partial automation when there are fewer parts to handle. There is a constraint within most instrument designs that the gas access must be on the upper x-y plane due to the need to allow clear gas access while the instrument is being held in the hand with the display clearly visible. This constrains many of the geometrical options for the designer. One aspect of the design which helps to meet this requirement is the cap section, and in particular, the portion of the cap 106 containing the inlet opening 140. This is designed to be located in the outer shell of the instrument. In this way, the internal sensor volume can be created in a region which would normally be left as a void in traditional sensors. In some aspects, the cap can contain various elements such as the filter, bulk flow membrane, and inlet opening.
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. In general, the separator 120 is electrically insulating to prevent a direct electrical connection between the electrodes through the separator material itself (e.g., as opposed to through the electrolyte). In an embodiment of the separator, the separator may comprise high volume, non-woven separator materials such as a porous felt and/or a non-woven, unbound, glass fiber separator material described as Whatman GFA separator. This typical separator material is manufactured in roll form where the glass density (or “grammage”) is nominally uniform along and across the roll. The separator 120 can comprise 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. In an embodiment, 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.
In some aspects, the electrolyte can comprise an aqueous electrolyte, an ionic liquid, a solid electrolyte, and/or a liquid non-aqueous, non-ionic additive. In an aspect, the electrolyte can comprise any aqueous electrolyte such as an aqueous solution of a salt, an acid, or a base depending on the target gas of interest. In an embodiment, 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 electrolytes, ionic 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. In some embodiments the electrolyte can comprise a non-ionic, non-aqueous fluid, gel, or solid, which can be appropriately doped to provide the ionic characteristics to the electrolyte. In an embodiment, 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. In some embodiments, the electrolyte can be present in the form of a gel. In an embodiment, the electrolyte may comprise a carrier or other additive, such as poly(ethylene glycol), poly(ethylene oxide), poly(propylene carbonate).
In an embodiment, the electrolyte can comprise a solid electrolyte. Solid electrolytes can include electrolytes adsorbed or absorbed into a solid structure such as a solid porous material and/or materials that allow protonic and or electronic conduction as formed. In an embodiment, the solid electrolyte can be a protonic conductive electrolyte membrane. The solid electrolyte can be a perfluorinated ion-exchange polymer such as Nafion. Nafion is a hydrated copolymer of polytretafluoroethylene and polysulfonyl fluoride vinyl ether containing pendant sulfuric acid groups. When used, a Nafion membrane can optionally be treated with an acid such as H3PO4, sulfuric acid, or the like, which improves the moisture retention characteristics of Nafion and the conductivity of hydrogen ions through the Nafion membrane. The sensing, counter and reference electrodes can be hot-pressed onto the Nafion membrane to provide a high conductivity between the electrodes and the solid electrolyte.
The counter electrode 111, reference electrode 113, and sensing electrode 115 within the electrochemical gas sensor 100 can be electrically connected to an external circuit through one or more electrical connections. In an embodiment, the electrodes 111, 113, 115 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
The connector pins 112, 114 can be formed from an electrically conductive material, which may be plated or coated. In an embodiment, 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, other base metals such as tin, or other metals such as niobium and tantalum. In an embodiment of an electrochemical sensor, the pins may comprise gold flash plated, nickel coated brass pins.
In some aspects, the connector pins 112, 114 can contact the electrode directly, through a conductive layer, and/or one or more current collectors such as a metal wire or conductive ribbon can be used to conduct electrical charges generated at active electrode surfaces to the connector pins 112, 114, which tend to be more robust than the metal wire or ribbon. When current collectors are used, the connector pins 112, 114 can facilitate connection between the current collectors and an external electronic circuit.
The external circuitry can be used to detect a current between the sensing electrode 115 and the counter electrode 111 to determine the target gas 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 111, whose potential remains uncontrolled and limited only by the electrocatalytic properties of the electrode. In an embodiment, 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 electrochemical sensor 100 when the target gas is oxygen. In some embodiments, 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 electrochemical sensor 100 when the sensing gas is oxygen.
The electrodes 111, 113, 115 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 111, 113, 115 can comprise a reactive material suitable for carrying out a desired reaction. For example, the electrodes 111, 113, 115 can be formed of a mixture of electrically conductive catalyst particles in a binder such as polytetrafluoroethylene (PTFE). For an oxygen sensor, an exemplary 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.
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.
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.
When the target gas is oxygen in a sensor using a diffusion limited opening (e.g., a capillary, etc.), the oxygen concentration at the sensing electrode 115 within the sensor 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 within the sensor 100. When the gas can contact the electrolyte in the separator 120 adjacent to the sensing electrode 115, 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 a higher rate of gaseous oxygen diffusion to the active catalyst of the sensing electrode 115, as a direct result of gas evolution from the electrolyte and subsequent contact with the sensing electrode 115 to create a transient “spike” in the output current.
In some aspects, the design of the separator 120 can be used to help prevent spiking failures by designing the separator 120 to provide a pressure resistance to a bubble gas containing oxygen from passing through the separator 120 and reaching the sensing electrode 115 during environmental pressure changes. The separator 120 can be designed so that the pressure resistance is higher than the pressure resistance through the vent, thereby routing the gas through the vent to exit the sensor housing. As an example, pressure changes of up to about 400 millibar can be associated with temperature changes from −40° C. to +60° C. and back again. The separator 120 can be designed to provide a pressure resistance to a gas bubble of at least about 400 millibar, at least about 500 millibar, or at least about 600 millibar.
In order to avoid the potential for spiking in some aspects, 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 120. 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 less than the consumption rate of the oxygen at the sensing electrode 115 (such that the electrode may be diffusion limited over all conditions over the life of the sensor). Typically, for most embodiments of this sensor type, the consumption rate of oxygen at the sensing electrode 115 (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 target gas is to be detected through all openings in the sensor 100. When additional internal diffusion of dissolved and/or gaseous target gas occurs from the electrolyte near the sensing electrode 115, the current generated in the circuit may correlate poorly with the external target gas concentration.
In an embodiment, 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. In some embodiments, 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 electrochemical sensor 100. For example, the target gas (e.g., 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.
The target gas concentration at or near the sensing electrode 115 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 the target gas to the sensing electrode 115 to a rate that is less than a consumption rate of the target gas at the sensing electrode 115 (e.g., a target gas reduction rate at the sensing electrode 115).
In an oxygen sensor, a dissolved target gas concentration gradient can be established in response to the operation of the electrochemical sensor between the counter electrode 111 and the sensing electrode 115 under normal operational conditions. Alternatively, in another sensor such as a CO sensor or even in oxygen sensors having a different configuration, there may be no gradient of target gas within the separator, since CO is not evolved at the counter electrode and the incoming CO from the capillary should be consumed by the sensing electrode 115. In some oxygen sensors, the dissolved target gas concentration gradient may generally be expected to represent a concentration either at, or approaching, the solubility limit of dissolved target gas in the electrolyte surrounding the counter electrode 111 and either low, or approximately zero, concentration of dissolved target gas in the electrolyte around the sensing electrode 115 when the electrochemical sensor 100 is used in typical operational environments. Since the target gas concentration in the electrolyte decreases towards the sensing electrode 115, the creation of a suitable distance between the counter electrode 111 and the sensing electrode 115 can be used to limit the concentration of dissolved target gas in the electrolyte near the sensing electrode 115 to less than the threshold.
In an embodiment, the separation distance can be provided by forming an ionically conductive pathway between the electrodes 111, 113, 115 having a desired length. The sensing electrode 115, the reference electrode 113, and the counter electrode 111 can be disposed on the ionically conductive pathway, with a distance separating each electrode. In an embodiment, 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 labyrinth configuration so that the ionically conductive pathway is not in a straight line, which would result in an oxygen sensor having relatively large dimensions.
In an embodiment as shown in
In an embodiment as shown in
The distance between the electrodes 111, 113, 115 on the conduction pathway may affect the potential for spiking to occur. In an embodiment, 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. In some embodiments, 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 target concentration gradient. In an embodiment, 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.
In some embodiments, a diffusion barrier may be provided within the sensor 100 to prevent the cross-diffusion of target gas 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 target gas 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. In some embodiments of the sensor 100, the body 102 may comprise one or more breather slots 220 (described in more detail below).
As shown in
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 there between so that a seal is formed in the area around the sensing electrode 115. In some embodiments, 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 100 to seal the housing and prevent the electrolyte from leaking.
In some embodiments, 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 in 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 gas through the chamber edge as compared to the diffusional path through the separator 120.
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 may have an opening 208 through which the separator 120 retaining the electrolyte can extend. Within the chamber 206, the separator 120 may be positioned to maintain contact between the electrolyte and the sensing electrode 115. The separator 120 retaining the electrolyte may substantially fill the opening 208 so that any gas within the sensor 100 is substantially prevented from entering the chamber 206 through a convective flow.
When the separator 120 substantially fills the opening 208, the target gas reaching the sensing electrode 115 may originate from target gas 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. In an embodiment, the portion of the separator 120 contained within the chamber 206 may be configured to provide a target gas 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% target gas concentration at the sensing electrode 115. In an embodiment, 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 206 in the event that any target gas is able to enter the chamber 206 through the seal.
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 target gas concentration at or near the sensing electrode 115. Rather, any target gas 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 target gas concentration within the electrolyte may help limit the potential for the target gas 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.
In any of the embodiments described herein, the geometry of the separator 120 may affect the flux of target gas to the sensing electrode 115 through the electrolyte in the separator 120, and the geometry can be selected so that the rate of target gas diffusion to the sensing electrode 115 is less than a consumption rate of target gas 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. In an embodiment, the separator 120 may contact both the cap 106 and the body 102. In some embodiments, 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 target gas 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 target gas diffusing into the chamber 206 through the electrolyte to contact the sensing electrode 115. In some embodiments, 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 target gas detection range can affect the selection of the available area of the separator 120 at the opening 208.
In use, the sensor 100 can detect a target gas concentration of a gas in the environment in which the sensor 100 is disposed. Referring to
During the detection process, the target gas concentration in the electrolyte at or near the sensing electrode 115 can be limited to less than a threshold amount. In general, the target gas concentration in the electrolyte in the separator 120 can be limited to less than a saturation concentration at a predetermined temperature, and in some embodiments, the target gas concentration in the electrolyte in the separator 120 can be limited to less than a percentage of a saturation concentration at a predetermined temperature. In an embodiment 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 target gas concentration along the target gas concentration gradient is below the threshold at or near the sensing electrode 115. For example, the target gas concentration along the target gas 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 115.
In some embodiments, the sensing electrode 115 can be disposed within the chamber 206 formed within the housing. The separator 120 can extend into the chamber 206 to provide contact between the electrolyte in the separator 120 and the sensing electrode 115. The separator 120 can be positioned within the chamber 206 and the opening 208 to prevent or limit any gas/electrolyte contact within chamber 206. Controlling how gas moves within the sensor 100 may rely on the fact that gas diffusion through the gas phase is orders of magnitude faster than gas diffusion through liquid. Therefore, while a wetted separator is a very effective barrier to gas diffusion, even a small void or aperture can immediately create a relatively facile gas path. Thus, the separator must completely fill the opening 208 in the barrier to provide the required control. In some embodiments, 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-target gas 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 target gas can diffuse to the sensing electrode 115 during the detection process and thereby limit the target gas concentration along the target gas gradient near the sensing electrode 115 to less than the threshold amount.
In some embodiments, limiting the target gas concentration in the electrolyte can include limiting the diffusional flux of target gas through the electrolyte in the separator 120 to less than a rate of target gas oxidation and/or reduction at the sensing electrode 115. In some embodiments, the diffusional flux may be controlled to be lower than the lowest detection limit required of the sensor. The choice of the geometry of the separator 120, the geometry of the chamber 206, the use of one or more breather slots, and/or the relative positioning of the electrodes may all be used to limit the diffusion of target gas in the electrolyte to and/or from the sensing electrode 115.
In the sensor 300 shown in
The sensor 300 may be assembled by over-molding the plastic body component using a suitable material, in this case TPE. One or more applications or components could be used, although for cost reasons, a single over-molded application to create the pin seals, oxygen cross over barrier, and/or reference cross over barrier can be used. The compression of the seals may be in the same direction as the component assembly, in this case the Z direction, to facilitate fault finding, although it is possible to assemble in the Z direction and seal in the X and Y directions. The area of the electrode backing tape that is not covered by catalyst may be compressed with the TPE to remove any voids in this area.
In order to reduce any current loss resulting from internal electrical resistance between the electrodes and the current collectors and/or contact pins, a low contact resistance between the current collector and the electrode surface is useful. Typical contact solutions within gas sensors rely on the compression of porous separator components to physically push the current collector into contact with the electrode material. However, this type of compression can result in over-compression of materials such as the separator that is electrically non-conducting and has a low mechanical strength and poor elasticity. Geometric tolerances of the internal sensor components surrounding the compressed separator, variations in the mechanical properties of the separator under compressive load forces, and the susceptibility of all supporting components to compressive creep affect the compression force used to obtain a low contact resistance. Additionally, throughout the operational lifetime of the sensor, the compressive force applied by the separator material on the current collector can be affected by the mechanical forces resulting from external stresses such as vibration, impact, and thermal cycling. For many sensor designs, the resultant increase in electrical contact resistance can potentially result in various failure modes such as partial/complete loss of sensor output and/or slow speed of response to the target gas.
In order to address potential over-compression issues, the elements of the sensor may be designed to separate the compression requirements for ensuring contact between the electrolyte-retaining materials with the electrodes (e.g., the separator, etc.) and between the electrical contacts by which the electrodes are connected to the external circuit. The separation of the compression regions may allow a solid pin (e.g., a contact pin surface) to contact the electrode, rather than requiring an intermediate connection to a flexible ribbon current connector, which is compatible with compression within a stack.
The sensor 400 may comprise a housing 401 defining an interior space (or reservoir) 430. The sensor 400 may comprise one or more separators 404 (also shown in
In some aspects, a lead frame can be used to improve contact between a current collector or contact pin and an electrode in an electrochemical gas sensor. This configuration may help to avoid the use of current collectors and improve sealing around the edges of electrodes to prevent gas access via diffusion, bulk flow etc. As shown in
In
As shown in
The material of the contact pins 802 in any of the configurations described in this entire disclosure can be chosen to protect the pins from the electrolyte. For example, the connector pins can be formed from an electrically conductive and corrosive material (brass, nickel, copper, or the like) which may be plated or coated to reduce degradation due to the contact with the electrolyte. For example, the coating may comprise one of gold, tungsten, niobium, tantalum, platinum, or any alloy or combination thereof. Alternatively, the material of the connector pins may be non-corrosive. Exemplary materials include gold, tungsten, niobium, tantalum, platinum, or any alloy or combination thereof.
While the above sensor is described in the context of an oxygen sensor, similar concepts and practices could be used in a variety of electrochemical sensors.
In some aspects, wicking structures can be used with the sensor to improve the sensor performance. Electrochemical systems employ a variety of electrolytes having a range of physical properties. Typically, successful practical designs for small, low cost, low power sensors may rely on liquid electrolytes such as sulfuric acid. Despite the challenges involved in retaining such aggressive materials within housings and dealing with material compatibility issues, liquid electrolytes still may offer improved environmental performance, which is a key driver for devices which must operate in a wide range of environmental conditions, including a large range in temperatures and humidity.
Proper transport of the electrolyte within the sensor allows the separator, and more particularly the desired portions of the separator, to remain wetted to maintain proper operation of the sensors. In general, the electrochemical reactions occur at the three-phase interface between the catalysts, gas, and electrolyte. For systems utilizing liquid electrolytes, the electrolyte must be transported from a reservoir to the areas adjacent to the electrodes to form the three-phase interface. If the electrolyte is not adequately transported to the appropriate locations within the separator, then a sensor placed under environmental stress may fail due to an inadequate formation of the three-phase interface. For example, the separator may dry out at or near an electrode, resulting in an inadequate performance of the sensor.
In order to improve the electrolyte transport within the sensor, channels can be used within the sensor that preferentially move the liquid to the perimeter of the reservoir where the electrolyte can be transported into contact with the separator and/or a specially formed and treated separator, which is differentially compressed by mating features on the sensor casing parts.
The channels can be used in conjunction with wicking features to transport the electrolyte to the internal components in order for them to function correctly. For example, gas diffusion electrodes need to be partially wetted and provided with ionic connection between other electrodes in order to produce a functional electrochemical cell and separators require the electrolyte to form barriers to internal gas flow. Additionally, the ratio between the total available internal free space and the electrolyte volume contributes to (and may define) the environmental “window” (or time to failure) under specified operational environmental conditions of temperature and humidity, and so it is desirable to utilize a component that fills as small a volume as possible in order to improve the environmental window for the product.
In an embodiment of a sensor, the walls, floor, and ceiling of the reservoir may be utilized to create surface “buttresses” provided with channels, where the dimensions of the channels are chosen to promote electrolyte transport. In some aspects, larger channels can be used to direct the fluid flow to certain areas within the reservoir where the electrolyte can contact wicking features. The dimensions of the wicking features may be small enough to create a capillary effect, allowing the liquid electrolyte to be transported by the channels to the separator. The one or more molded or machined channels located within the reservoir may be used to transport the free electrolyte towards a separator material, which has a pore size volume distribution under the local conditions of compression to produce a differential capillary attraction which draws the electrolyte from the mechanical wicking channel into the separator to thereby enable both the separator and contacting gas diffusion electrodes to function correctly. By using the appropriate combination of larger channels to direct the electrolyte and wicking features to transport the electrolyte from the reservoir to the separator, the location of the wetting of the separator can be controlled to some degree. For example, the wicking features may be aligned to provide the electrolyte at or near one or more of the electrodes.
As shown in
In some embodiments, one or more surfaces of the channels 808 and/or the capillaries can be surface treated to provide a suitably attractive finish to aid in directing the fluid flow. For example, when the electrolyte comprises an aqueous fluid, the surfaces can be treated to be hydrophilic to attract the electrolyte and aid in transporting the electrolyte to the desired area in the base and into the separator. Various surface treatments such as a plasma treatment can be used to modify the surface. In some embodiments, portions of the base can be treated to be hydrophobic to direct the electrolyte to the desired wicking areas, which can then be suitably hydrophilic. The selection of material for the formation of the base may also be based on the type of electrolyte to take into account the desire to direct the flow of the fluid into the desired area. For non-aqueous electrolytes, the surface treatment can be selected to produce the desired attractive or repulsive forces to help direct the electrolyte to the wicking features.
In some sensors, the channels 808 may comprise a width of approximately 250 micrometers (μm). In some sensors, the channels 808 may comprise a width of approximately 300 μm. In some sensors, the channels 808 may comprise a width between approximately 200 μm and 400 μm. In some sensors, the channels 808 may comprise a width between approximately 100 μm and 500 μm. In some sensors, the channels 808 may comprise a width less than approximately 500 μm. In some sensors, the channels 808 may comprise a depth of approximately 500 μm. In some sensors, the channels 808 may comprise a depth less than approximately 600 μm. In some embodiments, the channels 808 can be significantly larger (e.g., to server as bulk flow channels) and only the wicking features 806 at or around the bosses 805 may comprise small dimensions.
The use of the channels, as described above, may ensure that even when there is relatively little electrolyte within the sensor, the electrolyte is localized at or near wicking features and then moved (via the channels) to the points where the buttresses intersect the separator. The separator may then absorb the liquid, thereby ensuring that the electrodes are preferentially wetted. This functionality is assisted by the use of differential compression and/or selective control of the degree of hydrophobicity of different regions of the separator. The buttresses may access the separator at a number of cut-outs around the perimeter of the support table which align with the capillary buttresses in the reservoir walls below.
When the electrolyte volume is high (for example after extended periods spent in high humidity environments), there is likely to be free electrolyte (i.e. not localized within the separator or the buttress slots) within the reservoir. In this case, depending upon the orientation of the sensor, the electrolyte may additionally contact the separator through the free space where the counter electrode breather tab (as described in more detail herein) can pass through the support table and/or through the breather slots in the support table under the counter electrode.
The use of channels within the reservoir may reduce the part count necessary to produce the internal capillary forces required for directional flow of an aqueous electrolyte. Introducing sharp geometric edges, either by insert molding or machining, may create high energy surfaces that preferentially wet and produce desirable differential surface energies that effectively retain the electrolyte within the mechanical wicking “channel”. The effectiveness and wettability of a wicking channel (generated by higher surface energy) seems from practical experimentation to be superior for channels previously wetted by acid and/or plasma conditioning.
Referring now to
As the pin bosses 903 no longer reach the table, additional upstanding buttresses 905 have been added to transport electrolyte up the side walls Cup′ refers to the ‘normal’ sensor orientation but is understood to have little meaning in practical applications where the device can rest in any orientation). Referring now to
Referring now to
The sensor table 910 may comprise a rectangular cutout 916 to allow a protrusion feature 925 of the top cap 924, as well as a breather tab 931 to pass through the table 910 into the housing 904 (as described in more detail below).
The sensors as described herein can use a suitable electrolyte to provide the ionic conduction of charge necessary for correct electrochemical operation. Typically, sensor designs have electrode components nominally perpendicular to a common axis, referred to hereafter as a “stacked” design. Such stacked designs can include a plurality of separators between the stacked electrodes to electrically isolate the electrodes while maintaining the electrolyte in contact with the electrodes.
Typical separators may be formed from fiber sheet material that is cut or punched into a simple geometric shape for ease of manufacture. When filled with an electrolyte, the separator may bias the effective cross sectional areas of the interfacial phases towards solid/liquid/solid (e.g., catalyst/electrolyte/catalyst) interfaces rather than the liquid/gas (e.g., electrolyte/internal “air” reservoir) interfaces at the edges of these stacked components.
In an embodiment of a sensor, the sensor may comprise a shaped separator containing electrolyte that is in intimate contact with two or three electrodes orientated nominally in a common plane, referred to hereafter as a “planar” design, to provide the ionic connectivity required for operation of an electrochemical sensor. A planar arrangement of electrodes offers a practical solution to the primary issue of separately improving the compression required for fluid transport and electrical connectivity, as described in more detail herein.
Lateral separation of electrodes allows separation and simplified, more accurate control of the compression applied to fluid transport and connection aspects. For example, one separator overlaying an electrode (which can be connected electrically via a completely different part of the structure) has a more controllably defined compressibility than a conventional stacked design having 3 electrodes, 3 or more separators, insulators, and current collectors, all of which can have competing compression needs. In addition to better fundamental control, the planar arrangement also allows lateral variation of compression across these well-defined structures which can be beneficial (e.g., in promoting and/or selectively controlling electrolyte transport). This can be achieved by hard features on the sensor casing and/or table to increase/decrease pressure on chosen areas of absorbents. For example, a distance or gap between the sensor casing and table can vary laterally to thereby variably compress the separator across the sensor. Another benefit of planar designs is that the overall volume of separator material required to ensure the full electrode areas remain in contact with electrolyte is reduced, even without shrinking the size of the electrodes themselves.
A planar electrode arrangement can use a planar separator arrangement. A planar separator allows for the use of a single shaped component rather than separate components for each separator, thereby allowing for simplifications in assembly and improvements in reliability. Planar separators can suffer from electrolyte transport issues, which may result in portions of the separator having an insufficient supply of electrolyte, even while other portions may be saturated with the electrolyte. This may be true, for example, when a gas is evolved from an electrode, which can result in the evaporation of a portion of the electrolyte. As a result, electrolyte transport within the planar separator may need to be controlled during operation.
The variance in geometry of the shaped planar separator may also allow the ionic resistance between the electrode pairs to be selectively modified during the design and construction of the sensor. For example, a thin strip of wetted separator may connect an “isolated” (e.g., not located directly on a flow path between the sensing and counter electrode) reference electrode to the potentiostatic controlled sensing electrode, providing similar functionality to that of a “Luggin” capillary.
Forming the shaped separator may comprise laser cutting of a sheet material, which may allow for complex and accurate parts to be manufactured, thereby reducing geometric tolerances in the X-Y plane and reducing variations in the stack compression due to the localized glass density, pore volume, and tortuosity. By reducing the number of separator layers used for a typical stacked sensor design and the large cross sectional areas of the separator that are used to provide a sufficiently large area of contact between opposing separators, the “environmental window” (as described above) of operation can be increased as a result of needing less electrolyte to wet the smaller volume of separator material.
Further modification of the glass density, and thereby the porosity of the separator, can be made in the Z axis by local compression of the separator between opposing faces of the sensor enclosure, care being required not to damage the fibers (e.g., glass fibers, polymer fibers, etc.) of the separator. The separator might be configured to improve the electrochemical performance of the electrochemical sensor, for example, by reducing any “polarization” effects associated with the local proton concentration in the electrolyte which can lead to a dramatic disturbance in the electrochemistry occurring at the three electrodes.
Additional benefits to a planar electrode and separator configuration can include the effective formation of a feature known in the industry as a “partition” between the reference and sensing electrodes. According to standard fluid dynamics for a non-compressible liquid in a tube (e.g. the Hagen-Poiseuille equation states that the volumetric flow rate is inversely proportional the length of the tube, analogous to the length of the porous wetted separator in this case) the increased resistance to electrolyte flow will effectively make the sensor more tolerant to internal pressure differences resulting from environmental transients of pressure and/or temperature that might generate a current “spike” or “glitch,” as described in more detail herein. However, any increase in the resistance to electrolyte flow may also reduce the ability for the electrolyte to flow and wet various areas of the separator.
The shaped separator 1012 can be used to create an ionic path with the width of the separator 1012 controlling the ionic resistance between each electrode. Also, the separator 1012 can create electrical isolation between the electrodes when required. The separator 1012 also provides water management by compression of specific areas of the separator 1012, or by thinning the separator 1012 with a laser, both of which can be used to control the local density and porosity of the separator 1012. The separator 1012 may create a wetted barrier to prevent gases (e.g., gases comprising oxygen) from directly contacting the sensing electrode 1009. By using the shaped separator 1012, the force (or pressure) required to create an electrical contact with the electrodes may be separately controlled from the compression force required for the desired level of capillary action through the separator 1012.
Controlling the density and/or compression of the separator 1012 along its length may allow for one or more gradients to be formed within the separator 1012 material, where the flow of the electrolyte through the separator 1012 may be controlled by the gradients. The separator 1012 may be formed of a fiber material (such as glass fibers) where the fibers may have a consistent size or diameter. Therefore, when portions of the fiber material are removed, and the separator is compressed within the assembled sensor, different sized voids may be created between the glass fibers. This may create differences in capillary attraction within the separator 1012 and therefore directionality for the electrolyte to flow within the separator 1012. The voids may also create localized reservoirs for retention of the electrolyte. The retained electrolyte can then flow into an area as needed in the event of electrolyte loss, for example as a result of drying out at one or more locations (e.g., at or near an electrode).
An application of this technique may allow for localization of electrolyte around one or more of the electrodes. For example, water exchange between the interior and exterior of the sensor may occur more rapidly at certain locations within the sensor (such as the counter electrode and/or the sensing electrode). By altering the density and compression of the separator, the flow of the electrolyte within the separator may be directed toward these locations where water is lost, increasing the performance of the sensor. Additionally, the flow of the electrolyte within the separator may be controlled for sensors used in extreme dry or wet conditions, depending on the problems created within the sensor by these conditions.
In an embodiment of a planar separator 1012, the ratio of the length 1104 and/or width 1102 to the overall thickness 1106 may be at least 20/1. In an embodiment of a planar separator 1012, the ratio of the length 1104 and/or width 1102 to the overall thickness 1106 may be at least 40/1. In an embodiment of a planar separator 1012, the ratio of the length 1104 and/or width 1102 to the overall thickness 1106 may be approximately 70/1.
In an embodiment of a planar separator 1012, the thickness 1106 may be less than approximately 10% of the width 1102 and/or length 1104. In an embodiment of a planar separator 1012, the thickness 1106 may be less than approximately 2% of the width 1102 and/or length 1104. In an embodiment of a planar separator 1012, the thickness 1106 may be approximately 1.5% of the width 1102 and/or length 1104.
In some cases, the separator 1012 may have a thickness 1106 of less than approximately 0.5 millimeters (mm). In some cases, the separator 1012 may have a thickness 1106 of approximately 0.25 mm. In some cases, the separator 1012 may have a width 1102 of at least approximately 5 mm. In some cases, the separator 1012 may have a width 1102 of at least approximately 10 mm. In some cases, the separator 1012 may have a width 1102 of approximately 17.5 mm. In some cases, the separator 1012 may have a length 1104 of at least approximately 5 mm. In some cases, the separator 1012 may have a length 1104 of at least approximately 10 mm. In some cases, the separator 1012 may have a length 1104 of approximately 17 mm.
The separator 1012 may comprise openings (or contacts) 1110, 1112, and 1114 for providing contact points between the electrodes and the pins. The distance along the separator 1012 between the contacts 1110 and 1112 may be at least approximately 10 mm (between counter electrode 1010 and reference electrode 1008). In some cases, the distance along the separator 1012 between the contacts 1110 and 1112 may be at least approximately 15 mm.
The distance along the shaped separator 1012 between the contacts 1112 and 1114 may be at least approximately 10 mm (between reference electrode 1008 and sensing electrode 1009). In some cases, the distance along the separator 1012 between the contacts 1112 and 1114 may be at least approximately 10 mm.
The distance along the separator 1012 between contacts 1110 and 1112 may be greater than the length along the separator 1012 between contacts 1112 and 1114. The width of the separator 1012 between the contacts 1110 and 1112 may be greater than the width of the separator 1012 between the contacts 1112 and 1114. In other words, the overall area of the separator 1012 between the contacts 1110 and 1112 may be greater than the overall area of the separator 1012 between the contacts 1112 and 1114.
As shown in
A typical “reservoir” design would involve an internal volume that is free of glass fiber material (maximum volume with minimum electrolyte retention) that contains “free” electrolyte that might contact the compressed separator material (i.e. a step change to uniform porosity) and thereby be wicked away from the reservoir by capillary attraction. A separator may also comprise additional separator geometrical features (e.g. tabs, rings etc.) to improve the effectiveness and likelihood of electrolyte contact under the target application conditions.
The separator materials illustrated in
The holes in the material may be created using a laser cutter. For example, a CO2 laser cutter may be used with a wave length of approximately 10600 nanometers. The laser cutter may comprise a 370 mm lens which gives a spot size of approximately 540 microns. The laser cutter may comprise an 80 watt air cooled laser.
Reducing the volume of material in defined sections of separator material (which may be GFA) could be achieved using the laser in spot marking mode to add perforations to areas of the GFA. Reducing the volume of material in defined sections of separator material may also be achieved using the laser on different power or speed settings to remove layers of GFA in certain sections.
While the above sensor is described in the context of an oxygen sensor, similar concepts and practices could be used in a variety of electrochemical sensors.
Some sensors use three electrodes orientated nominally in a common plane. This configuration allows for a shift of the bias of the effective cross sectional areas of the interfacial phases further towards the liquid/gas (electrolyte/internal “air” reservoir) interfaces rather than the solid/liquid/solid (catalyst/electrolyte/catalyst) interfaces. By designing the geometry of internal components to modify the effective contact areas between the electrolyte and gas phase within the sensor, the local rates of interfacial diffusion of oxygen between the electrolyte and the internal gas volumes can be controlled to improve the electrochemical performance of the electrochemical sensor. This may minimize any “polarization” effects associated with the limiting diffusion rates of dissolved oxygen away from the site of generation at the counter electrode and may prevent the local formation of bubbles or micro-bubbles of gases (typically oxygen and nitrogen) in the electrolyte which can lead to a disturbance in the electrochemistry occurring at the three electrodes, reducing counter electrode (anode) “activity” as a result of the reduction in the effective contact area between the catalyst and electrolyte or producing transients currents at the sensing electrode (cathode).
The inclusion of openings or cavities in internal components of the sensors, referred to hereafter as “breather slots,” in the body component of a planar sensor may provide control of the gas exchange between the electrolyte in the separator and the gas phase in the reservoir of the sensor. In other words, the breather slots may provide sufficient contact area between the electrolyte contained in the separator and the gas phase in the reservoir of the sensor to allow for control of the equilibrium between the gases dissolved in the electrolyte and the gas phase. The breather slots may provide gas access to the free space in the reservoir, and may also allow excess free electrolyte (that is not localized elsewhere in the reservoir) to contact the separator directly if the sensor is in the right orientation. Additionally, the breather slots may allow for the electrolyte concentration in portions of the separator material to be controlled, further affecting and controlling the movement of the electrolyte within the separator.
For example, breather slots located near the counter electrode may be operable to vent bubbles (or gases) generated at the electrode into the reservoir, so that the counter electrode may remain wetted by the electrolyte and functioning. The breather slots near the counter electrode may prevent the produced gases from drying out the electrode.
In another example, breather slots located near the reference and/or sensing electrodes may allow for faster exchange of water vapor from the reservoir to the electrodes, where the electrodes may function in dry conditions. The breather slots near the reference and/or sensing electrodes may allow gas from the reservoir to more quickly contact the electrodes. Water transport through the gas phase is orders of magnitude faster than diffusion through a liquid, so localised increases in electrolyte concentration due to drying out are better managed by allowing gas phase access of water to the dried regions, rather than requiring it to diffuse through the electrolyte itself from wetter parts.
Electrochemical oxygen pump sensors can employ three electrodes that operate in an electrolyte having different concentrations of dissolved oxygen in intimate contact with the electrocatalyst. To improve the electrochemical reactions occurring at all electrodes, any constraining concentrations of reactants and products (Le Chatelier's principle) can be reduced within their local environments.
The effective cross sectional areas of the breather slots may be optimized for the specific requirements of each of the electrodes. Around the counter electrode in an oxygen sensor, the cross sectional area of the breather slots may be increased, as it is beneficial to increase the rate of diffusion of a dissolved target gas (such as oxygen) away from the site of generation into the internal sensor reservoir. Around the reference electrode, the cross sectional area of the breather slots may be balanced to control the dissolved target gas concentration in the vicinity of the reference electrode, imparting greater sensor tolerance to external target gas concentrations. Around the sensing electrode, the cross sectional area of the breather slots may be controlled to create an anaerobic zone around the sensing electrode, thereby reducing baseline currents generated in oxygen free environments and also improving the response times to reach baseline conditions.
Referring back to
While the above sensor is described in the context of an oxygen sensor, similar concepts and practices could be used in a variety of electrochemical sensors.
In order to avoid spiking and glitch issues as described herein, one or more features to control the pressure within the sensor can be used. Such a feature can allow the gas to move from bulk free space of the body as the gas expands or contracts due to pressure and temperature effects. When the electrolyte level in the sensor is at a high level, e.g. when the sensor is new, in a filled state, and/or operated in high relative humidity conditions, it is not likely that there will be spaces connecting into a gas path. Therefore, it is important to ensure that good gas communication is maintained at all times in all orientations.
In an electrochemical sensor, a highly porous breather tab may be used to form a pathway for air contained within the sensor to vent out through the breather tab (in the case of an increase in internal pressure) and/or vent in through the breather tab (in the case of a decrease in internal pressure) via a suitable aperture (capillary or other) in the sensor enclosure to provide gaseous communication between the sensor interior and environment. A sensor may normally contain sources of air or other gases, where gas can be produced by the electrochemical reaction at an electrode (normally the counter electrode), or air can be found in voids between mechanical parts or in the free volume in the reservoir. The breather tab can comprise a hydrophobic yet porous material such as PTFE. The hydrophobic nature of the membrane may reduce the likelihood of a liquid entering the membrane and blocking gas flow. Further, the porous nature of the breather tab can allow gases to flow through the membrane between a surface of the membrane in contact with a gaseous space in the sensor and a surface of the membrane in contact with a vent hole. The hydrophobic nature of the membrane may also aid in preventing any liquid (e.g., the electrolyte) from reaching the vent hole and leaking from the sensor.
If the breather tab is not secured in position by design, or if the sensor takes on water when in a high humidity environment, a vent blockage or partial blockage could be caused by the electrolyte, thereby preventing gas from accessing the breather tab. Also, the sensor may be positioned, either by design or during use, in an orientation which allows the electrolyte to prevent the gas from accessing the membrane. In some instances, an attachment (such as a heat stake or other connection between the breather tab and the housing) can block or partially block the vent by compressing and thus restricting the air flow through the breather tab. This may prevent a gas from flowing between an access surface on the membrane to a vent located on an opposite side of the attachment point.
If a vent is blocked or expanding air within the sensor cannot access the vent, oxygen or other gas can find its way to one of the other electrodes and create a spike in the sensor output. This spiking can cause issues for the user of the sensor by creating false alarms. It is possible to arrange the breather tab so that it can be positioned to access the gas produced at the counter electrode, in voids and the gas in the reservoir, in a repeatable way which will remain so during the sensor's life.
Referring now to
In
The breather tab 1811 may comprise one continuous piece of highly porous PTFE to enable continuous lateral movement of gas though its full length. The breather tab 1811 may extend outwards in one dimension from the vent 1802 and extend down away from the vent 1802 through the full length of the reservoir 1830. The breather tab 1811 may then extend back up to the base/table 1808 under the electrode compartment 1806. The breather tab 1811 may be secured in one or more locations by heat sealing 1821 to ensure it does not move during operation.
While the above sensor is described in the context of an oxygen sensor, similar concepts and practices could be used in a variety of electrochemical sensors.
Embodiments of the disclosure include an electrochemical sensor comprising a housing defining an interior space; a sensing electrode; a counter electrode; and a separator retaining an electrolyte, wherein the electrolyte provides an ionically conductive pathway between each of the sensing electrode and the counter electrode within the housing, wherein the electrodes are orientated in a common plane, the separator contacts each of the electrodes, the separator comprises a variety of geometrical features, including tabs, thinned portions, cut-outs, and rings, and the separator comprises variations in compression, density, and surface area.
In an embodiment of the electrochemical sensor, the ratio of the length of the separator to the overall thickness of the separator is at least 20/1. In an embodiment of the electrochemical sensor, the ratio of the length of the separator to the overall thickness of the separator is approximately 70/1. In an embodiment of the electrochemical sensor, the thickness of the separator is less than approximately 10% of the width of the separator. In an embodiment of the electrochemical sensor, the thickness of the separator is approximately 1.5% of the width of the separator. In an embodiment of the electrochemical sensor, the separator has a thickness of less than approximately 0.5 millimeters (mm). In an embodiment of the electrochemical sensor, the separator has a thickness of approximately 0.25 mm. In an embodiment of the electrochemical sensor, the separator has a width of at least approximately 5 mm. In an embodiment of the electrochemical sensor, the separator has a width of approximately 17.5 mm. In an embodiment of the electrochemical sensor, the separator has a length of at least approximately 5 mm. In an embodiment of the electrochemical sensor, the separator has a length of approximately 17.25 mm. In an embodiment of the electrochemical sensor, the sensor may further comprise a plurality of electrical contacts, wherein the separator comprises openings that surround contact points between the electrodes and the electrical contacts. In an embodiment of the electrochemical sensor, the sensor may further comprise a reference electrode, wherein the distance along the separator between the openings for the counter electrode and the reference electrode is different from the length along the separator between openings for the reference electrode and the sensing electrode. In an embodiment of the electrochemical sensor, the sensor may further comprise a reference electrode, wherein the overall surface area of the separator between the openings for the counter electrode and the reference electrode is different from the overall surface area of the separator between openings for the reference electrode and the sensing electrode. In an embodiment of the electrochemical sensor, the separator comprises a variety of patterns of holes throughout the material of the separator, wherein the holes retain and wick (by capillary attraction) aqueous electrolyte. In an embodiment of the electrochemical sensor, the holes create a graduated glass density within the separator. In an embodiment of the electrochemical sensor, the holes are created using a laser, and wherein the laser ablates material from the target surface to modify the glass density locally.
Embodiments of the disclosure include a method for manufacturing a planar separator for use in an electrochemical sensor comprising forming a planar separator, wherein the ratio of the length of the separator to the overall thickness of the separator is at least 20/1; forming geometric features within the separator comprising tabs, thinned portions, cut-outs, and rings; and assembling the planar separator within a housing, wherein the separator contacts a plurality of electrodes.
In an embodiment of the method, the method may further comprise applying variant compression forces in different areas of the separator. In an embodiment of the method, the method may further comprise creating a variety of patterns of holes through the separator to create a graduated glass density within the separator.
While various embodiments in accordance with the principles disclosed herein have been shown and described above, modifications thereof may be made by one skilled in the art without departing from the spirit and the teachings of the disclosure. The embodiments described herein are representative only and are not intended to be limiting. Many variations, combinations, and modifications are possible and are within the scope of the disclosure. Alternative embodiments that result from combining, integrating, and/or omitting features of the embodiment(s) are also within the scope of the disclosure. Accordingly, the scope of protection is not limited by the description set out above, but is defined by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated as further disclosure into the specification and the claims are embodiment(s) of the present invention(s). Furthermore, any advantages and features described above may relate to specific embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages or having any or all of the above features.
Additionally, the section headings used herein are provided for consistency with the suggestions under 37 C.F.R. 1.77 or to otherwise provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings might refer to a “Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered as a limiting characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.
Use of broader terms such as comprises, includes, and having should be understood to provide support for narrower terms such as consisting of, consisting essentially of, and comprised substantially of. Use of the term “optionally,” “may,” “might,” “possibly,” and the like with respect to any element of an embodiment means that the element is not required, or alternatively, the element is required, both alternatives being within the scope of the embodiment(s). Also, references to examples are merely provided for illustrative purposes, and are not intended to be exclusive.
While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted or not implemented.
Also, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.
Claims
1. An electrochemical sensor comprising:
- a housing defining an interior space;
- a sensing electrode;
- a counter electrode; and
- a shaped separator retaining an electrolyte, wherein the electrolyte provides an ionically conductive pathway between the sensing electrode and the counter electrode within the housing,
- wherein the electrodes are orientated in a common plane,
- wherein the shaped separator contacts each of the electrodes,
- wherein the shaped separator comprises at least one choke point between the contact between the shaped separator and the sensing electrode and the contact between the shaped separator and the counter electrode, and
- wherein the shaped separator comprises variations in compression, density, and surface area.
2. The electrochemical sensor of claim 1, wherein the at least one choke point of the shaped separator comprises a narrowed width of the separator material.
3. The electrochemical sensor of claim 1, wherein the at least one choke point of the shaped separator defines the ion concentration gradient between the counter and sensing electrode.
4. The electrochemical sensor of claim 1, wherein the at least one choke point is located and sized to create localization of electrolyte around one or more of the electrodes.
5. The electrochemical sensor of claim 1, wherein the ratio of the length of the separator to the overall thickness of the separator is at least 20:1.
6. The electrochemical sensor of claim 1, wherein the ratio of the length of the separator to the overall thickness of the separator is approximately 70:1.
7. The electrochemical sensor of claim 1, wherein the thickness of the separator is less than approximately 10% of the width of the separator.
8. The electrochemical sensor of claim 1, wherein the thickness of the separator is approximately 1.5% of the width of the separator.
9. The electrochemical sensor of claim 1, wherein the separator has a width of at least approximately 5 mm.
10. The electrochemical sensor of claim 1, wherein the separator has a length of at least approximately 5 mm.
11. The electrochemical sensor of claim 1, further comprising: a plurality of electrical contacts, wherein the shaped separator comprises openings that surround contact points between the electrodes and the electrical contacts.
12. The electrochemical sensor of claim 11, further comprising a reference electrode, wherein the distance along the shaped separator between the openings for the counter electrode and the reference electrode is different from the length along the shaped separator between openings for the reference electrode and the sensing electrode.
13. The electrochemical sensor of claim 11, further comprising a reference electrode, wherein the overall surface area of the shaped separator between the openings for the counter electrode and the reference electrode is different from the overall surface area of the shaped separator between openings for the reference electrode and the sensing electrode.
14. The electrochemical sensor of claim 1, wherein the shaped separator is formed of a glass fiber material.
15. The electrochemical sensor of claim 14, wherein the at least one choke point of the shaped separator comprises a variety of patterns of holes throughout the material of the shaped separator, wherein the holes retain and wick (by capillary attraction) aqueous electrolyte.
16. The electrochemical sensor of claim 15, wherein the holes create a graduated glass density within the shaped separator.
17. The electrochemical sensor of claim 14, wherein the holes are created using a laser, and wherein the laser ablates material from the target surface to modify the glass density locally.
18. A method for providing electrical contact between two electrodes using a planar separator in an electrochemical sensor, the method comprising:
- providing a housing defining a reservoir comprising an electrolyte;
- forming a planar shaped separator, wherein the ratio of the length of the shaped separator to the overall thickness of the shaped separator is at least 20/1, and wherein the shaped;
- forming at least one choke point within the shaped separator; and
- stacking the planar separator with a plurality of electrodes within the housing, wherein the separator contacts the plurality of electrodes
- wherein the choke point of the separator is located between the contact between the separator and a first electrode and the contact between the separator and a second electrode.
19. The method of claim 18, further comprising: applying variant compression forces in different areas of the separator.
20. The method of claim 18, further comprising: creating a variety of patterns of holes through the separator to create a graduated density within the separator.
Type: Application
Filed: Jan 22, 2018
Publication Date: May 31, 2018
Inventors: Neils Hansen (Poole), Stuart Harris (Bournemouth), Richard Brown (Southampton)
Application Number: 15/877,208
