Patent Publication Number: US-2006000723-A1

Title: Electrochemical sensor having improved response time

Description:
PRIORITY APPLICATION  
      This application is a Continuation application of U.S. patent application Ser. No. 10/345,772 for an “Electrochemical Sensor Having Improved Response Time,” filed Jan. 16, 2003 which is a Continuation-in-Part application of U.S. patent application Ser. No. 09/443,875 for a “Film Type Solid Polymer Ionomer Sensor and Sensor Cell,” filed Nov. 19, 1999, now U.S. Pat. No. 6,682,638 issued on Jan. 27, 2004. 
    
    
     FIELD OF THE INVENTION  
      The invention relates to an electrochemical gas sensor having an improved response time.  
     BACKGROUND OF THE INVENTION  
      Detecting gases is useful for a variety of reasons. With respect to environmental concerns, an apparatus for detecting pollution or industrial emission is beneficial to help limit such contaminants entering water systems or the atmosphere. A gas detection unit may also be used for detecting the presence of dangerous chemical compounds, such as carbon monoxide, in a mixture of gases. In the medical field, a gas detection unit may be used for detecting a particular gas in equipment, such as an oxygen inhalation machine, for alerting staff as to the amount of oxygen remaining in the reservoir or given to the patient.  
      Known methods and apparatuses have been developed to detect the presence of gases. Typical systems include gas chromatography, ion chromatography, electrolytic conductivity detection, and conductometric measurement. However, these manners for detecting gases have generally been expensive, cumbersome, or shown to have low sensitivities and slower response times. In situations where a generally quick response time may be desired, such as detecting toxic gases or a lack of oxygen in an oxygen inhalation machine, gas detection systems having enhanced abilities to quickly detect particular gases are usually favorable.  
      Electrochemical sensors were provided to overcome these limitations. Electrochemical sensors typically provide signals which tend to exhibit acceptable sensitivity and usually have quick response times relative to gas chromatography, ion chromatograph, and electrolytic conductivity detection systems.  
      Other electrochemical gas sensors typically include metal layers or electrodes in contact with and beneath an electrolytic film of, for example, Nafion or Teflon. However, because the gas usually needs to diffuse through the ionic medium before reaching the sensing electrode, the response time may be negatively affected.  
      Recently, planar thin film sensors have been developed by constructing three planar electrodes on an insulating substrate and covering them with a thin polymer electrolyte, such as Nafion. J. A. Cox and K. S. Alber,  Amperometric Gas Phase Sensor for the Determination of Ammonia in a Solid State Cell Prepared by a Sol - Gel Process,  143, No. 7 J. Electrochem. Soc. L126-L128 (1996) developed a solid state cell in which microelectrode arrays were coated with a film of vanadium oxide xerogel for detection of ammonia. However, this film needs to be soaked in an electrolyte solution in order to provide ionic conductivity. These methodologies, in which a planar substrate with metal electrodes is covered with a thin film of solid state electrolytic material, are suitable for automated mass production, but they have longer response times since gas needs to diffuse through a relatively thick film of electrolyte.  
      As shown electrochemical gas sensor  10  includes substrate  11 , electrode  3 , and ionomer membrane  5 . Gas enters and exits sensor  10  through the inlet and outlet as shown. A portion of the gas entering sensor  10  diffuses through diffusion hole  20  and contacts electrode  3 , which detects the type of gas present in sensor  10 .  
      To enhance sensitivity to sensor  10 , a reservoir  9  is provided containing electrolyte solution to wet ionomer membrane  5 . As shown, reservoir  9  and, therefore, the electrolyte solution is in contact with inomer membrane  5 . Because reservoir  9  is located on a same side of ionomer membrane  5  as diffusion hole  20 , a length of diffusion hole is typically at least as long as a height of reservoir  9 .  
      What is desired, therefore, is an electrochemical sensor that overcomes the limitations of the prior art to provide a further improved response time. What is also desired is an electrochemical sensor having a wetted electrolytic medium to maintain sensitivity. A further desire is to provide an electrochemical sensor having a diffusion control passage for controlling the flow of gas leading to the sensing electrode.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an object of the invention to provide an electrochemical gas sensor having an improved response time.  
      It is an object of the invention to provide an electrochemical gas sensor having improved sensitivity.  
      A further object of the invention to provide an electrochemical gas sensor having a control passage for controlling the flow of gas leading to the sensing electrode and/or for controlling the flow of electrolyte solution to wet the ionomer membrane.  
      These and other objects of the invention are achieved by provision an electrochemical gas sensor for detecting a gas having a substrate having a first surface and a second surface and an electrode deposited on the first surface. The sensor also includes an ionomer membrane in contact with the first surface and the electrode. The ionomer membrane has an opening in a location proximate to the electrode for permitting gas introduced into the sensor to diffuse through the opening to simultaneously contact the electrode and the ionomer membrane within the opening. The substrate further includes at least one hole extending from the first surface to the second surface for permitting moisture to diffuse through the at least one hole to contact the ionomer membrane for enhancing sensitivity.  
      The invention further includes a reservoir for containing moisture, or electrolyte solution, to moisten the ionomer membrane. In some embodiments, the reservoir may be located adjacent to the substrate on a side of the substrate opposite the ionomer membrane. The electrolyte solution diffuses from the reservoir through the at least one hole in the substrate to contact the ionomer membrane on the opposite side of the substrate.  
      The sensor may optionally include a wicking material in contact with the second surface to facilitate drawing moisture from the reservoir toward the substrate. In some embodiments, the wicking material may be located in the at least one hole of the substrate.  
      In other embodiments, the reservoir may be spaced apart from the second surface and the wicking material may be between the second surface and the reservoir. Optionally, the substrate may also be a thin foil. The sensor may further include a film of electrolytic material on the electrode to increase a three phase contact between the gas, electrode, and ionomer membrane.  
      In another aspect, the invention includes a method for detecting a gas, including the steps of providing a substrate having a surface, providing at least one hole in the substrate that extends from a first surface of the substrate to a second surface of the substrate, and depositing an electrode on the first surface. The method further includes the steps of contacting an ionomer membrane with the electrode, providing an opening in the ionomer membrane in an approximate area of the electrode, introducing a gas into the opening toward the electrode, and simultaneously contacting the gas with both the electrode and ionomer membrane.  
      The method may further include the step of providing a reservoir containing moisture to moisten the ionomer membrane, wherein the the reservoir is positioned adjacent to the substrate on a side of the substrate opposite the ionomer membrane.  
      The method includes the step of diffusing moisture from the reservoir to the at least one hole to contact the ionomer membrane.  
      Optionally, the method includes the step of placing a wicking material in contact with the second surface and a solution in the reservoir for drawing moisture from the reservoir toward the substrate. In some embodiments, the method may include placing a wicking material in the at least one hole.  
      The method may also include the step of directing gas through the opening toward the electrode. Additionally, the method may further include the step of controlling the gas as it passes through the opening toward the electrode. Similarly, the method may include the step of controlling the solution as it passes through the at least one hole in the substrate.  
      To enhance detection of the gas being introduced into the sensor, the method may include the steps of oxidizing the gas as the gas contacts the surface of the electrode and/or reducing the gas as the gas contacts the surface of the electrode.  
      The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  depicts an electrochemical gas sensor in accordance with the invention.  
       FIG. 2  depicts a method for providing the electrochemical gas sensor shown in  FIG. 1 .  
       FIG. 3  depicts an exploded view of the electrode shown in  FIG. 1 . 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
       FIG. 1  depicts the electrochemical gas sensor  30  in accordance with the invention. Sensor  30  includes substrate  32 , ionomer membrane  34 , and electrode  38  placed within housing  48 . Gas enters sensor  30  through inlet  42  and is detected after diffusing through diffusion hole  44  to contact electrode  38 , which is in contact with ionomer membrane  34 . Gas exits sensor  30  through outlet  46 . It is understood that the gas may flow in a reversed direction where outlet  46  is the inlet and inlet  42  is the outlet.  
      Sensor  30  of  FIG. 1  overcomes this disadvantage by wetting ionomer membrane  34 , via hole  36  in substrate  32 , with solution  52  located on a side of substrate  32  opposite from electrode  38 . Because of the position of reservoir  56 , length L′ can be shortened, thereby reducing gas diffusion time and improving the sensitivity of sensor  30 . The more length L′ is reduced, the faster the response time of sensor  30 . In some embodiments, length L′ is less than 1.4 mm. In other embodiments, length L′ is less than 1 mm. In further, embodiments, length L′ is less than 0.5 mm. In still further embodiments, length L′ is less than 0.1 mm. In fact, length L′ or a thickness of ionomer membrane  34  may be reduced until it is flush with or below a surface of electrode  38 . In some embodiments, diffusion hole  44  is eliminated because length L′ is flush with or below a surface of electrode  38 . All that is required is for ionomer membrane  34 , of any length L′, to be in contact with electrode  38  so that gas entering through inlet  42  provide a desired gas/ionomer membrane/electrode interface.  
      As a result of the reduced length L′ of sensor  30 , the response time of sensor  30  is less than approximately 2 seconds, more preferably less than approximately 1 second, and most preferably less than approximately 0.5 seconds. In some embodiments, the response time is less than approximately 0.1 seconds.  
      To further enhance sensitivity, a thickness of substrate  32  is reduced to improve wetting by solution  52 . Substrate  32  is of an electrically non-conductive material for providing a surface upon which electrode  38  is placed. Optionally, substrate  32  is a thin foil having insulative, or electrically non-conductive, properties, such as Kapton or any other material. The foil is not metallic or conductive. The foil may also be flexible as compared to ceramic or glass. The thickness of the foil, or substrate  32 , is generally less than approximately 4 mils and preferably less than approximately 1 mil. The thinner substrate  32 , the faster ionomer membrane  34  is wetted and this positively affects sensor response time. Therefore, as the thickness of substrate  32  approaches 0 mils, the response time is further reduced.  
      Optionally, in some embodiments, sensor  30  may include wicking material  54  to facilitate or enhance wetting of ionomer membrane  34  by solution  52 . Wicking material  54  is typically of a material that absorbs liquid, such as a sponge. Hence, as shown in  FIG. 1 , wicking material  54  will draw solution  52  upwardly from reservoir  56  toward ionomer membrane  34 .  
      As shown, reservoir  56  and substrate  32  are separable from one another where wicking material  54  is placed between reservoir  56  and substrate  32 . In other embodiments, wicking material is placed within reservoir  56  and reservoir comes in contact with substrate  32 . In further embodiments, substrate  32  and reservoir  56  are made not separable from one another but are formed as one unit. Wicking material  54  may optionally be used with any of these embodiments of reservoir  56  and substrate  32 .  
      As shown in  FIG. 1 , substrate  32  further includes at least one hole  36  extending from a first surface  62  of substrate  32  to a second surface  64  of substrate  32 , thereby forming a thru-hole, for permitting solution  52  to pass, or diffuse, through at least one hole  36  to contact ionomer membrane  34 . In the embodiments where substrate  32  is a foil, or a thin non-conductive material, wicking material  54  would be positioned in a closer relationship to ionomer membrane  34  than where substrate  32  is of a thick material. Where substrate  32  is a foil, solution  52  absorbed by wicking material  54  would more easily wet ionomer membrane  34 . Optionally, wicking material  54  would be in contact, through at least one hole  36 , with ionomer membrane  34 . In some embodiments, wicking material  54 , in addition to or instead of being between substrate  32  and solution  52 , is placed within at least one hole  36 .  
      To further facilitate wetting of ionomer membrane  34  by solution  52 , or optional wicking material  54 , a plurality of holes  36  are placed in substrate  32 . It is understood that hole  36  is of any diameter, length, shape, or dimension. Also, the more holes  36  in substrate  32 , in any location, the better ionomer membrane  34  is wetted. Hence, the hole  36  or plurality of holes  36  may act as a form of wetting control to ionomer membrane  34 , as too much wetting or too little wetting negatively affects sensitivity. Moreover, hole  36  may be, in addition or instead of being round, a square shaped or polygonal shaped hole. Hole  36  may further be a slit or aperture of any kind. All that is required of hole  36  is that it provides a passage from first surface  62  to second surface  64  so that solution  52  diffuses through hole  36  to contact ionomer membrane  34 .  
      As shown in  FIG. 3 , to enhance sensitivity of sensor  30  in some embodiments, a thin film  134  of ionomer membrane  34  may be placed on electrode  38  to increase the area of contact between ionomer membrane  34 , electrode  38 , and gas to include the surface of electrode  38 . Gas diffuses throughout film  134 , which is in contact with the surface of electrode  38 . As a result of the increased contact area, the sensing area is increased and response time is minimized. Gas diffuses faster through film  134  when film  134  has a minimal thickness. Hence, the thinner film  134  is, the faster the response time is for sensor  30 .  
      Without film  134 , the interface in the approximate area of electrode  38  would be substantially smaller, limited to an area where ionomer membrane  34  comes in contact with electrode  38 . This contact area would generally be a linear contact point defining an approximate circumference of electrode  38 .  
      In some embodiments, film  134  has a thickness less than 2 micrometers. Ideally, film  134  should be as thin as possible to maximize sensor response time and sensitivity. Hence, sensor  30  may further comprise film  134  having a thickness of less than 1 micrometer. A film having such reduced thickness permits faster gas diffusion and, thus, faster response times. Film  134  is an electrolytic medium, which includes all the limitations of ionomer membrane  34  and may be, but need not be, the same material as ionomer membrane  34 .  
      Film  134  is in a solid state or dry electrolyte for it has more structural integrity than liquid state electrolyte, thereby permitting a consistently uniform thickness over electrode  38 . This enhances sensor repeatability and facilitates functionality for liquid state electrolyte would be difficult to maintain in a fixed position on the surface of sensing electrode  38 .  
      Optionally, the response time of sensor  30  may further be improved by reducing the size of inlet  42  and outlet  46 . In this effort, the gas is more concentrated while inside sensor  30  due to there being less internal volume for the gas to disperse. Less dispersion and a more concentrated gas generally results in a more easily detected gas and, therefore, reduced response time of sensor  30 . As shown in  FIG. 1 , the dispersion in the horizontal direction is reduced, which is generally referred to as axial dispersion because the dispersion is approximately along the axis containing a center point of sensor  30 , is reduced due to a reduction in size of inlet  42  and outlet  46 . In some embodiments, inlet  42  and outlet  46  have a diameter of approximately 1 mm. Inlet  42  and outlet  46  need not be round but may be of any shape so long as gas may be injected into and extracted from sensor  30 . Such shapes include 3 sided, 4 sided, or polygonal geometries.  
      Optionally, as shown in  FIGS. 1 and 3 , sensor  30  may also include cover  50  on ionomer membrane  34  for minimizing the vaporization or evaporation of solution  52  as solution  52  is absorbed and passed upwardly through ionomer membrane  34 . Cover  50  is in contact with the surface of ionomer membrane  34  opposite from substrate  32 . Cover  50  does not block any portion of either diffusion hole  44  or electrode  38  because doing so would hinder gas detection and negatively affect sensor sensitivity. Cover  50  is not needed for sensor  30  to operate properly and may be eliminated entirely from sensor  30 . For embodiments where sensor  30  includes cover  50 , it is understood that the length L′ of the diffusion path is the height of both ionomer membrane  34  and cover  50 . For embodiments where sensor  30  does not include cover  50 , length L′ is the height of membrane  34 .  
      In another aspect of the invention, a method  80  is shown in  FIG. 2  for detecting a gas entering sensor  30  of  FIG. 1 . Method  80  includes the steps of providing  82  a substrate and providing  84  at least one hole in the substrate that extends from a first surface of the substrate to a second surface of the substrate. Method  80  also includes the steps of depositing  86  an electrode on the first surface, contacting  88  an ionomer membrane with the electrode, and providing  98  an opening in the ionomer membrane in an approximate area of the electrode. Method  80  further includes the steps of introducing  90  a gas into the opening toward the electrode and simultaneously contacting  92  the gas with both the electrode and the ionomer membrane. Detection includes oxidizing and/or reducing the gas.  
      Method  80  detects gas entering the sensor by directing some of the gas through the opening in the ionomer membrane and toward the electrode. By varying the diameter, length, or shape of the opening, method  80  controls the gas passing through the opening toward the electrode.  
      In some embodiments, method  80  may include providing  96  a reservoir containing moisture to wet the ionomer membrane so that the sensor&#39;s sensitivity is enhanced. In these embodiments, method  80  includes positioning the reservoir adjacent to the substrate and on a side of the substrate opposite the ionomer membrane. Solution for wetting the ionomer membrane is contained in the reservoir and comes in contact with the ionomer membrane by diffusing  94  through the at least one hole in the substrate.  
      Optionally, method  80  includes placing  100  wicking material, such as a sponge or other liquid absorbing material, in contact with the second surface of the substrate and the solution in the reservoir. In this position, the wicking material will draw the solution from the reservoir upwards toward the ionomer membrane. In some embodiments, method  80  includes placing wicking material in the at least one hole of the substrate to further facilitate wetting of the ionomer membrane.  
      Method may also control wetting, or the amount of solution passing through the at least one hole, the ionomer membrane by increasing or decreasing the amount of holes in the substrate and/or varying the diameter, length, or shape of the at least one hole in the substrate.  
      Although the invention has been described with reference to a particular arrangement of parts, features and the like, these are not intended to exhaust all possible arrangements or features, and indeed many other modifications and variations will be ascertainable to those of skill in the art