Abstract:
A gas detector includes an electrochemical gas sensor. The sensor includes a plurality of electrodes. At least one of the electrodes is formed of a catalyst/binder slurry which is halftone printed onto a substrate. The composite printed element and substrate are sintered to form the electrode.

Description:
FIELD 
       [0001]    The application pertains to electrochemical gas sensors and gas detectors which incorporate such sensors. More particularly, the application pertains to such sensors which can be formed in part by printing. 
       BACKGROUND 
       [0002]    Electrochemical gas sensors are well known for detecting and quantifying toxic gases such as carbon monoxide, oxygen and the like. Such sensors can be implemented using electrochemical cells. Such cells operate in an amperometric mode providing a current output which is related to the concentration of the particular analyte gas. 
         [0003]    Such sensors usually include a sensing electrode. Known electrodes are made by a solution based method. 
         [0004]    In such solution-based methods, a catalyst is initially ultrasonically dispersed in an aqueous solution to form a suspension. Polytetrafluoroethylene (PTFE) is added to the suspension for form a flocculate mixture. The flocculate mixture is then transferred onto a substrate, which is sintered at an elevated temperature. The sintered mixture is then transferred onto a microporous PTFE membrane, then pressed. The ratio of PTFE in the electrode not only affects gas diffusion parameters in the sensor, it also supports the electrocatalyst and maximizes the interfaces between catalyst, gas and electrolyte at which the key electrochemical processes occur. 
         [0005]    As is apparent, many steps are needed in this solution-based method to manufacture an electrode. The consequences include high manufacturing costs, material costs and labor costs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  illustrates a gas detector in accordance herewith; 
           [0007]      FIG. 2  is a flow chart illustrating aspects of a method in accordance herewith; 
           [0008]      FIG. 3  is a graph illustrating response time to O 2  in air for an electrode including a mixture of GEFC-IES and platinum; 
           [0009]      FIG. 4  is a graph illustrating response time to CO in air for an electrode including a mixture of GEFC-IES and platinum; and 
           [0010]      FIG. 5  is a graph illustrating response time to O 2  in air for an electrode including a mixture of GEFC-IES, platinum and graphite. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    While disclosed embodiments can take many different forms, specific embodiments hereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles hereof, as well as the best mode of practicing same, and is not intended to limit the claims hereof to the specific embodiment illustrated. 
         [0012]    In one aspect, an electrochemical gas sensors having improved productivity can advantageously be implemented by using screen printing technology. A catalyst slurry, or recipe, can be screen printed or halftone printed on an electrode membrane by a printer, then sintered. 
         [0013]    The printed element can then be used as an electrode of an electrochemical sensor. Exemplary types of sensors include O 2  sensors or CO sensors. In another aspect, alternative types of sensors in accordance herewith include, without limitation, oxygen pumps and toxic gas sensors. 
         [0014]    The slurry can be made simply and quickly without any need for complicated equipment. The slurry can include a catalyst, binder, and diluents. Unlike known processes, the screen printing method, in accordance herewith, has fewer steps. 
         [0015]    The catalyst can be platinum, platinum black, a mixture of graphite and platinum, a mixture of carbon and platinum black, a noble metal, mixtures thereof. 
         [0016]    A solution of perfluorinated ion electrolyte solution (GEFC-IES the copolymer of perfluorosulfonic acid and PTFE) commercially available from Golden Energy Fuel Cell Co., Ltd. or Nafion® (copolymer of tetrafluoroethylene (Teflon®) and perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid) commercially available from Dupont™, can be used as a binder. Glycol or other similar chemicals can be used as a diluent to form a catalyst slurry, recipe or catalyst system, which can be printed on a PTFE membrane by a printer. The printed element is sintered at an elevated temperature to form an electrode which can be used in an electrochemical sensor such as O 2  sensor or CO sensor. 
         [0017]    GEFC-IES&#39;s or Nafion®&#39;s function is that of a binder. Its ratio in the electrode not only affects gas diffusion parameters in the sensor whilst supporting the electrocatalyst and maximizing the interfaces between catalyst, gas and electrolyte at which the key electrochemical processes occur. The slurry made from GEFC-IES or Nafion® is suitable for use in halftone screen printing. 
         [0018]    As illustrated in  FIG. 1 , an exemplary oxygen sensor  10  can be carried in a housing  12  and include, a gas diffusion sensing or working electrode  14 , a reference electrode  16  and a counter electrode  18 . One or more of the electrodes can be formed by a printing process as described below in detail. The electrodes need not be identical. 
         [0019]    As would be understood by those of skill in the art, electrodes formed by the present printing based process can be incorporated into gas detectors, such as detector  30 . Detector  30  can include a housing  34  which carries the sensor  10 , as well as electrodes 14-18 manufactured as described herein. Control circuits  36  can be coupled to the electrodes to make gas concentration determinations. An audio and/visual output device  38  can be provided to alert users to a current, sensed gas concentration. 
         [0020]      FIG. 2  illustrates aspects of a method  100  in accordance herewith. A slurry, including a platinum catalyst along with glycol and a solution of GEFC is mixed together to get a uniform mixture as at  102 . The slurry is then heated, to a certain volume, as at  104 . 
         [0021]    The screen printable catalyst is then halftone printed on a PTFE sheet using a printer, as at  106 . The printed element or shape is then sintered at a predetermined temperature, as at  108 , to obtain an electrode which can be used as a sensing, reference, or counter electrode, as at  110 . 
         [0022]    In accordance herewith, the electrode catalysts can be made from 80% weight Platinum black and 20% weight of GEFC-IES binder. The binder in the slurry not only affects gas diffusion parameters in the sensor it also supports the platinum electrocatalyst and maximizes the interfaces between catalyst, gas and electrolyte at which the key electrochemical processes occur. 
         [0023]    Relative to  FIG. 1 , sensor  10  can be implemented as a O 2  sensor or CO sensor using the electrode created by the above described process  100 . Operationally, at the sensing electrode for an O 2  sensor the O 2  is reduced: 
         [0000]      O 2 +4H + +4 e   − →2H 2 O  (1)
 
         [0024]    At the counter electrode there is a counter balancing oxidation: 
         [0000]      2H 2 O→4H + +O 2 +4 e   −   (2)
 
         [0025]      FIG. 3  illustrates a graph of the response of an O 2  sensor with time to 20.9% O 2  in air and N 2  with respect to the above described catalyst material. At a sensing electrode for a CO sensor the CO is oxidized: 
         [0000]      CO+H 2 O→CO 2  2H + +4 e   −   (3)
 
         [0026]    At the counter electrode there is a counter balancing reduction: 
         [0000]      O 2 +4H + +4 e   − →2H 2 O  (4)
 
         [0027]      FIG. 4  illustrates a graph of the response with time to air and 50 ppm carbon monoxide using a mixture of GEFC-IES and Platinum as a sensing electrode formed by screen printing. 
         [0028]    In another example a predetermined ratio of platinum and graphite is mixed together with glycol and a solution of GEFC-IES to get a uniform mixture. Then the slurry is heated to a predetermined volume. The catalyst is then halftone printed on a PTFE sheet using a printer. After printing, the printed element is then sintered at a predetermined temperature to obtain the electrode which can be used as a sensing, reference, or counter electrode for an O 2  sensor. 
         [0029]    The electrode catalyst in this second example is made from 75% weight Platinum black, 10% weight graphite and 15% weight GEFC-IES binder.  FIG. 5  illustrates a graph of the response to O 2  sensor with time in air and N 2  with respect to the second catalyst material. 
         [0030]    In summary, the above disclosed electrode manufacturing process using screen printing method has fewer steps than known processes. First, a catalyst (e.g. Platinum Black or mixture of Carbon and Platinum Black or other noble metal catalyst) is mixed with GEFC-IES or Nafion® or a mixture of GEFC-IES and Nafion®. Glycol is then added to form a slurry by stirring. 
         [0031]    An electrode form can then be screen printed on a PTFE membrane and sintered at an elevated temperature. Platinum electrodes usable in both sensors and CO sensors can be formed using this screen printing process. 
         [0032]    Those of skill will also understand that the graphs of  FIG. 3-5  are illustrative only and not limitations hereof. Variations in electrode structures may lead to differing response times without departing from the spirit and scope hereof. 
         [0033]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. 
         [0034]    Further, logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be add to, or removed from the described embodiments.