Patent Publication Number: US-6656335-B2

Title: Micro-fuel cell sensor apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/185,497, filed Feb. 28, 2000, the intire content of which is hereby incorporated by reference in this application. 
    
    
     This invention relates to a sensor for the measurement of hydrogen content in a gas stream. More particularly, it relates to a sensor for determining the hydrogen partial pressure by measuring the electric current generated by the reaction of hydrogen in a gas stream with oxygen from ambient air. 
     BACKGROUND OF THE INVENTION 
     Industrial uses of hydrogen require a simple and sensitive device for detecting hydrogen leaks and for measuring hydrogen concentrations. Prior art detectors have a long response time to hydrogen. For example, one such detector sold under the trade name Hydran is devoted primarily for the continuous monitoring of slowly variable hydrogen concentrations and has a response time on the order of minutes. Several attempts have been made in the past to improve the response time of hydrogen detectors without much success. 
     Moreover, known hydrogen detectors failed to consider characteristics influencing the sensor response time. Thus, there is a need for an efficient sensor with a fast response time for analyzing hydrogen content and determining hydrogen partial pressure in gas streams. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a micro-fuel cell sensor apparatus and method for the measurement of hydrogen content and hydrogen partial pressure in a gas stream. The sensor is disposed in a fuel-cell housing. The sensor includes a sensing element having first and second gas diffusing electrodes spaced from one another. A fuel-cell spacer having an acidic electrolyte is disposed between the two electrodes. The first electrode is spaced from a first gas permeable membrane by a first cavity, the first membrane being disposed proximate to the housing base. 
     A second gas permeable membrane is disposed opposite to the first membrane and away from the housing base. Oxygen from atmospheric air is continuously supplied to the second gas diffusing electrode by way of natural diffusion through the second gas permeable membrane. The second electrode is spaced from the second membrane by a second cavity. The amount of oxygen supplied to the second electrode exceeds the amount required for stochiometric reaction with hydrogen diffused through the first membrane. 
     The above described sensor is disposed in a sensor body having a chamber defined therein for accommodating the sensor. An external gas stream is received in the sensor body via an opening therein. A sensor cover having a recess sealingly mates with the sensor body, the recess in the cover opening into the chamber in the sensor body. 
     The sensor cover further includes a connector for providing electrical connection to the sensor and also for facilitating measurement of the sensor output. The sensor cover also includes a third gas permeable membrane for supplying oxygen by way of natural diffusion from atmospheric air. Oxygen diffused into the sensor body through the third membrane enters the sensor by way of further diffusion through the second membrane. Excess oxygen may be furnished at the second electrode by an appropriate selection of second and third membranes. The first membrane is chosen to have a high permeability to hydrogen and lower permeability to gases having molecular dimensions that are higher than hydrogen. 
     In its assembled state, when hydrogen from a gas stream diffuses selectively through the first membrane into the first cavity facing the first gas diffusing electrode, electrochemical charging of the first electrode occurs at a potential corresponding to hydrogen concentration in the first cavity, while the potential of the second electrode remains unchanged. The potential difference created between the first and second electrodes produces a current flow measured by connecting the first and second electrodes through a load resistance. The current measured as a voltage drop across the load resistance represents the micro-fuel cell sensor output. 
     In one aspect, the present invention thus provides a sensor for measuring partial hydrogen pressure in a gas stream, the sensor including a housing, a sensing element comprising first and second gas diffusing electrodes spaced from one another, a fuel-cell spacer having an acidic electrolyte disposed between the first and second electrodes, a first gas permeable membrane separating the first electrode from the gas stream; and a second gas permeable membrane separating the second electrode from atmospheric air. Preferably, the first membrane has higher permeability to hydrogen and lower permeability to gases with molecular dimensions greater than that of hydrogen. The oxygen rate of permeation through the second membrane is higher than hydrogen rate of permeation through the first membrane, whereby oxygen furnished at the second electrode exceeds stochiometric oxygen necessary for the reaction with hydrogen. The first and second electrodes are preferably connected through a load resistance to measure the sensor output. 
     Oxygen furnished at the second electrode is controlled by an appropriate choice of the second membrane. The first and second membranes are preferably made of a polymeric material. A hydrogen partial pressure gradient is maintained between the first electrode and an external gas stream. The first and second electrodes are preferably identical. 
     In another embodiment, the first membrane of the micro-fuel cell sensor is disposed adjacent to the first electrode, thus varying the volume of the first cavity. It may further be possible to dispose the first membrane in close contact with the first electrode. 
     In another aspect, the present invention provides an apparatus for measuring partial hydrogen pressure in a gas stream. The apparatus includes a housing, a micro-fuel cell sensor disposed in the housing, a cover member, the sensor including a sensing element having first and second gas diffusing electrodes spaced from one another, a fuel-cell spacer with an acidic electrolyte interposed between the first and second electrodes, a first gas permeable membrane spaced from the first electrode, a second gas permeable membrane spaced from the second electrode to supply oxygen to the second electrode by natural diffusion of atmospheric air. The cover member includes a connector for providing an electrical connection to the sensor, a third gas permeable membrane disposed in one of the cover member and the housing for receiving atmospheric air. The apparatus further includes means for sealingly attaching the housing to an assembly carrying a gas stream. 
     In yet another aspect, the present invention provides a method of measuring hydrogen content in a gas stream by mounting the housing to an assembly carrying the external gas stream, permitting the gas stream to diffuse through the first membrane, thereby effecting an electrochemical charging of the first electrode, permitting oxygen to diffuse through the second membrane to furnish oxygen at the second electrode, the amount of oxygen exceeding an amount necessary for a stochiometric reaction with hydrogen present in the external gas stream, electrically connecting the first and second electrodes by a resistance, and measuring the current flow created by the potential difference between the first and second electrodes to produce a sensor output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded cross-sectional view of a micro-fuel cell sensor assembly; 
     FIG. 2 is a cross-sectional view a micro-fuel cell sensor body, with cover assembly as shown in FIG. 3, for accommodating the micro-fuel cell sensor of FIG. 1; 
     FIG. 3 is a cross-sectional view of a cover assembly of the micro-fuel cell sensor body of FIG. 2; 
     FIG. 4 is a cross-sectional view of another embodiment of the invention wherein the first gas permeable membrane is located adjacent to the first gas diffusing electrode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 there is illustrated a detailed view of a micro-fuel cell sensor assembly  10  for measuring partial hydrogen pressure in gas streams. The sensor  10  includes a fuel-cell housing  2  having a base portion  16  and a fuel-cell cover  9 . An aperture  15  is defined in the base portion  16  for facilitating diffusion of hydrogen from an external gas stream into a first cavity  17 . The sensing element of the sensor  10  includes a first electrode  4 - 1  disposed in housing  10  towards the base portion  16 . A second electrode  4 - 2  is disposed opposite to the first electrode  4 - 1  with a fuel-cell spacer  5  comprising an acidic electrolyte disposed therebetween. A first membrane  14  is disposed on base portion  16  to separate the first electrode  4 - 1  from an external gas stream. The first membrane  14  is spaced from the first electrode  4 - 1  by a first cavity  17 . A second membrane  12  is disposed adjacent to the fuel cell cover  9  and separates the second electrode  4 - 2  from atmospheric air diffusing into the sensor body through a third gas permeable membrane  34  as illustrated in FIG.  3 . The second membrane  12  is spaced from the second electrode  4 - 2  by a second cavity  18 . The second cavity  18  is continuously supplied with oxygen by natural diffusion from the atmospheric air through the second membrane  12 . Excess oxygen may be furnished at the second electrode  4 - 2  by an appropriate choice of the second membrane  12 . The second membrane  12  is chosen to supply the second electrode  4 - 2  with an excess amount of oxygen than otherwise required for a stochiometric reaction with diffused hydrogen. The concentration polarization of the second electrode  4 - 2  may thus be avoided, realizing a sensor with anodic control. Sensor leads  6 - 1  and  6 - 2  are disposed in housing  2  to contact first and second electrodes  4 - 1  and  4 - 2 , respectively. Output of the sensor  10  is measured between the sensor leads  6 - 1  and  6 - 2  through a resistor  37  as illustrated in FIG.  3 . 
     The sensor  10  as described above is adapted to be placed in a sensor body  20  as illustrated in FIG.  2 . The sensor body  20  includes an upper portion  21  and a lower base portion  23  with an aperture  24  defined therein. An external gas stream is received in the sensor body  20  through orifice  25  defined between apertures  24 ,  26 . An opening  22  in sensor body  20  accommodates sensor  10 . Aperture  15  communicates with aperture  26  defined in opening  22  of sensor body  20 . 
     FIG. 3 illustrates a cover member  30  for covering the sensor body  20  in an airtight manner. Cover member  30  includes a slot  31  having an upper end  36  and a lower end  33 . The cover member  30  sealingly covers the sensor body  20  as illustrated in FIG.  2 . Cover member  30  further includes a vent  35  for permitting oxygen from atmospheric air to enter the second cavity  18  of sensor  10  through slot  31 . At least one fastener may be used to secure the cover member  30  to the sensor body  20  as illustrated in FIG.  2 . The third gas permeable membrane  34  separates vent  35  from the atmospheric air. A perforated vent cover plate  40  overlies and protects the third membrane. A connector member  38  having a end portion  41  is disposed in an airtight manner in the upper portion  36  of slot  31 . The connector  38  includes a resistor  37  which projects out into the upper portion  36  of slot  31 . Sensor leads  6 - 1  and  6 - 2  connected on one side to the first electrode  4 - 1  and  4 - 2 , respectively, terminate in connector  38 . The output of the sensor  10  is represented by the potential difference between sensor leads  6 - 1  and  6 - 2  through resistor  37 . 
     In its assembled state, the base portion  23  of the sensor body  20  is adapted to be tightly attached on assemblies carrying a gas stream to measure hydrogen content in the gas stream. In this state, the upper portion  21  of the sensor body faces atmospheric air. Thus, the second cavity  18  facing the second electrode  4 - 2  is continuously supplied with oxygen by natural diffusion from the atmospheric air. Hydrogen gas present in the gas stream enters the sensor through aperture  24 , diffuses through the first membrane  14  to enter the first cavity  17  in order to contact the first electrode  4 - 1 . The first and second electrodes may have noble metal electro-catalyst and graphite paper or carbon cloth backing. Since the first membrane  14  is chosen to have high permeability to hydrogen, but is less permeable to gases with higher molecular dimensions than hydrogen, the sensor is primed to be highly selective for hydrogen. 
     Selective diffusion of hydrogen gas from a gas stream through the first membrane  14  into the first cavity  17  causes electrochemical charging of the first electrode  4 - 1  at a potential corresponding to the hydrogen concentration in the first cavity  17  facing the first electrode  4 - 1 , while the potential at the second electrode  4 - 2  remains unchanged. The potential difference created between the first and second electrodes produces a current flow by connecting the electrodes through a resistor  37 . This current measured as a voltage drop across the resistor  37  represents the sensor output. In the illustrated configuration of the sensor, the first membrane  14  is a diffusion barrier for the linearity of the sensor output toward hydrogen concentration. Since the hydrogen concentration at the first electrode  4 - 1  is always zero, and since the sensor  10  consumes the hydrogen at a faster rate than the rate of permeation through the first membrane, as long as hydrogen is present in the gas stream, a partial pressure gradient between the outside and the inside of the sensor exists, thus permitting diffusion of hydrogen into the sensor. 
     Referring now to FIG. 4, a second embodiment is illustrated where elements in common with the sensor of FIG. 1 are indicated by similar reference numerals, but with a prefix “1” added. Here, the first membrane  114  is located on or directly adjacent the surface of the first electrode  14 - 1 , thus varying the volume of the first cavity  117 . 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.