Abstract:
An electrochemical sensor having at least two electrodes, and a reservoir chamber containing electrolyte. The reservoir chamber is internally coated with a wicking material to spread the electrolyte evenly over the walls of the reservoir.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Electrochemical sensors are an importance devices in industrial safety, environmental and emissions monitoring, quality and process control and clinical diagnostic applications. They are sensitive, accurate, and low cost, and are used in many portable instruments as well as fixed systems. 
         [0002]    Each sensor includes at least two electrodes and an electrolyte. The electrolyte can be an aqueous solution of an acid, an alkali, or a mineral salt; examples are sulfuric acid, phosphoric acid, potassium hydroxide, lithium chloride, and lithium perchorate. The electrolyte can also be of an organic type such as tetraethylammonium perchlorate (TEAP) in a low vapor pressure organic solvent. Because the volume of the electrolyte can change with time and with environmental conditions, a reservoir chamber is usually incorporated into the sensor to provide additional amounts of electrolyte and/or to allow for expansion of the electrolyte in certain environments. 
         [0003]    For example, an aqueous electrolyte can lose water to the atmosphere when the relative humidity is low, and absorb water from the atmosphere when the relative humidity is high. The volume of the electrolyte therefore fluctuates with ambient conditions. If the relative humidity is held constant, the electrolyte will adjust its volume and concentration until equilibrium is reached with the surrounding atmosphere. 
         [0004]    In order to use the electrolyte in the reservoir, a wick is typically employed inside the sensor cell, disposed in contact with both the electrolyte in the reservoir and the electrolyte between the electrodes. The wick draws the liquid electrolyte and transports it by capillary action. Because electrolyte is free to flow inside the reservoir chamber, some sensors have an adsorbent pad installed in the reservoir to immobilize the electrolyte. 
         [0005]    Sensors including wicks are disclosed, for example, in U.S. Pat. No. 4,587,003 and U.S. Pat. No. 7,534,333, both assigned to City Technology Limited, and incorporated herein by reference. 
         [0006]    Commercially available gas sensors typically have a wick. Examples of such gas sensors include the 4 Series toxic gas sensors from City Technology Ltd. in the UK, and the 6A-CO and 6A-H 2 S sensors from Industrial Scientific Corporation in the US. Most available sensors have a wick that reaches an area in which the electrolyte is most likely to reside. 
         [0007]    Current portable instruments are required to be small and convenient to carry. They are often limited in size, however, by the size of the sensors used therein. When sensors are small, the electrolyte reservoir is proportionally reduced in size. In such miniature sensors, it is not practical to place an adsorbent pad in the reservoir as it not only takes up the very limited free space, but also competes with the adsorbent material in the electrode stack for the limited volume of electrolyte. Without an adsorbent pad in the reservoir, however, the electrolyte tends to stay in corners due to surface tension, and loses contact with the wick that is required to transport the electrolyte. As a consequence, the electrode stack can dry out, even though there is a sufficient amount of electrolyte left in the reservoir, and when this occurs, the sensor has either no or low sensitivity, and/or long response times to the analyte of interest. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore the object of the invention to provide a miniature sensor that delivers reliable performance under varying environmental conditions. 
         [0009]    It is a further object of the invention to provide a miniature sensor that is robust and easy to manufacture. 
         [0010]    To accomplish these and other objects, the invention is directed to a miniature sensor in which is incorporated a small reservoir chamber that is internally coated with a wicking material which has an affinity to the electrolyte and is chemically stable. The wicking material provides a connection between electrolyte in the reservoir and electrolyte in the electrode stack. 
         [0011]    In particular, the invention is directed to an electrochemical sensor comprising a housing including an opening for admitting gas for analysis, an electrode stack disposed within the housing, and comprising at least two electrodes separated by an electrolyte-absorbent separator (typically fiberglass) placed therebetween, a reservoir for electrolyte disposed adjacent the electrolyte stack, the reservoir being defined by walls, and a wicking material coating on the walls of the reservoir, the wicking material coating being constructed and arranged for spreading electrolyte evenly over the walls of the reservoir. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an exploded view of an electrochemical gas sensor according to the invention, which includes an electrode stack, an electrode reservoir chamber and a wick between therebetween; 
           [0013]      FIG. 2  is lateral cross-sectional view of the sensor of  FIG. 1 ; and 
           [0014]      FIG. 3  is a schematic diagram of the electrochemical gas sensor according to the invention, connected to a potentiostat circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    With reference to  FIGS. 1 and 2 , an electrochemical sensor  1  contains three electrodes, a working electrode  2 , a reference electrode  4  and a counter electrode  6 . These electrodes are separated by separators  8  and  10 , with separator  8  between working electrode  2  and reference electrode  4 , and separator  10  between reference electrode  4  and counter electrode  6 . The electrodes  2 ,  4 ,  6  and separators  8  and  10 , which is referred to as an electrode stack  11 , is supported on a slotted plastic disk  12 , an insulating support, which has a wick  14  extending downwardly. The electrode stack  11 , with support disk  12  on one surface thereof and O-ring  16  on the other surface thereof, is housed in a plastic enclosure formed by attaching a cap  18  to a sensor housing  20 , typically by ultrasonic welding. 
         [0016]    As can be seen more clearly in  FIG. 2 , the plastic housing  20  has three external electrical connector pins  21 ,  22 ,  23  molded in. Within the housing, there are three metal wires or strips  25 ,  26 , and  27  connected to the pins  21 ,  22 , and  23 , respectively, at one end thereof, and the three wires or strips  25 ,  26  and  27  are connected to the three electrodes  2 ,  4  and  6 , respectively, at the other end thereof. 
         [0017]    Under the support disk  12  is an electrolyte reservoir  32 , with a vent  34 . After electrolyte is filled into the reservoir through the  34 , vent is plugged with a plastic insert  36 . 
         [0018]    The wick  14  maintains physical contact with both the electrode stack  11  through the slotted disk  14 , and the inner surface of the reservoir chamber  32 . The wick  14  wicks up electrolyte from the reservoir and wets the separators  8  and  10  in the electrode stack  11 . 
         [0019]    As shown in  FIG. 3 , when the three electrodes  2 ,  4  and  6  are connected to a potentiostat circuit  50  with an appropriate bias voltage, the sensor is ready to work. The target gas enters the sensor through capillary  28  in cap  18 , and electrochemical reactions take place within the stack  11  generating a current output at the working electrode, which is amplified by the potentiostat circuit, and detected at output  52 . The potentiostat circuit is well known in the art, and includes ground  54  and a connection  56  to a power source. 
         [0020]    The capillary  28  is protected from dust and moisture by a porous membrane  30 . 
         [0021]    When the overall size of the sensor is small, for example a cylinder 5-15 mm in diameter×5-15 mm in height, the electrolyte reservoir  32  is also small, for example in the range of 0.3-1 ml in volume. With projections in the areas of pins  21 ,  22  and  23 , and the vent  34  in the bottom of the reservoir, the reservoir has many small corners that easily trap the electrolyte, and due to surface tension, the small amount of liquid electrolyte is difficult to spread over the whole surface of the bottom of the reservoir. The amount of electrolyte in the reservoir is also quite small, for example in the range of 0.10-0.15 ml, typically 0.12 ml. 
         [0022]    The sensor may also be in the shape of a cuboid, of a size, for example, 5-15 mm in each of length, width and height. 
         [0023]    While the invention is especially advantageous with respect to such miniature sensors, it is not limited to miniature sensors, and may be used with sensors of all sizes and shapes. 
         [0024]    The electrode stack  11  is the heart of the sensor in which electrochemical reactions take place. The separators  8  and  10  must be charged with electrolyte at all times in order to keep the sensor in operating condition. If there is not enough electrolyte, the ionic conductivity between electrodes will be poor and the sensor will not function properly, for example exhibiting a long response time and low output. 
         [0025]    According to the invention, the surface of the reservoir is coated with a wicking material  40 ; when there is an excess of electrolyte in the reservoir, it is spread over the whole surface of the reservoir by the wicking material  40 . The wicking material is in physical contact with the surface of the reservoir, and will therefore wick up electrolyte to the electrode stack  11 , and keep the sensor in operating condition. The wicking material  40  also assists in the filling process, as it wicks the electrolyte away from the vent, where the electrolyte might otherwise block the vent during the filling process, and/or remain in the vent area and prevent further filling. 
         [0026]    A large variety of materials may be used for the wicking material coating. For example, fiberglass paper may be crushed into particles, and dispersed in water to form a colloid and dried. Other suitable materials include fumed silica (SiO 2 ), powdered alumina (Al 2 O 3 ), titania (TiO 2 ), zirconia (ZrO 2 ), and other ceramic materials and mixtures thereof. Carbon powder and absorbent polymers, such as polypropylene and polyethylene in fiber form may also be used. The wicking material must be compatible with the electrolyte, and be chemically stable. 
         [0027]    When the wicking material coating in the reservoir is in physical contact with the electrolyte in the electrode stack, the wick may be omitted. 
         [0028]    While the disclosed embodiment shows the reservoir arranged below the stack, other arrangements, for example by the side of the stack, above the stack, or even within the stack (between the electrodes) may be used. 
       EXAMPLES 
     Example 1 
       [0029]    0.5 g of a Whatman GF/A binder-free, glass microfiber filter disk is crushed at 10,000+ lbs, dispersed in 100 mL water, and then mixed to form a colloid. About 0.5 ml of the colloid is injected into a 0.5 ml electrolyte reservoir and allowed to dry at 60° C. for 1 hr. The colloid is evenly distributed and uniformly coated the internal walls of the reservoir. The coating adheres strongly to the walls. When the reservoir is filled with 0.15 ml diluted sulfuric acid electrolyte, it absorbs a minimal amount of the electrolyte and spreads it quickly over the entire surface of the coated area. 
         [0030]    The reservoir is deployed in a sensor to detect carbon monoxide gas. 
       Example 2 
       [0031]    0.3 g of fumed silicon dioxide (SiO 2 ), 99.5%, 400 mesh from Alfa Aesar, US, is added to 100 ml of de-ionized water and then mixed at room temperature to form a colloid. About 0.3 ml of the mixture is injected into a 0.3 ml electrolyte reservoir and then allowed to dry at 50° C. A uniform coating is formed on the internal walls of the reservoir. The reservoir is filled with 0.1 ml of diluted phosphoric acid (H 3 PO 4 ) electrolyte, and deployed in a sensor to detect hydrogen sulfide gas.