Abstract:
An electrochemical gas sensor ( 10 ) includes a housing ( 11 ) which has a number of electrodes ( 31, 32 ), i.e. at least one working electrode ( 31 ) and at least one counter electrode ( 32 ), in addition to a liquid electrolyte ( 60 ). At least one of said electrodes ( 31, 32 ) and/or the housing ( 11 ) are at least partially formed of an absorption agent composition. A method of detecting acid gases employs the electrochemical gas sensor ( 10 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a United States National Phase Application of International Application PCT/EP2015/000292 filed Feb. 11, 2015 and claims the benefit of priority under 35 U.S.C. §119 of German Application 10 2014 002 500.4 filed Feb. 21, 2014, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains to an electrochemical gas sensor with a housing, with a plurality of electrodes including, at least one working electrode and one counterelectrode as well as with a liquid electrolyte, especially to an electrochemical gas sensor that can be used to detect sour gases and/or gas mixtures containing sour gases. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electrochemical gas sensors are generally known. They usually have a plurality of electrodes, which are in conductive contact with an electrolyte liquid and form in this way a galvanic cell, hereinafter also called electrochemical measuring cell. There are both sensors that are used stationarily and sensors that are used in portable devices. 
         [0004]    The industrial field of application of such sensors ranges, for example, from the chemical industry to the monitoring of refrigerating plants up to agricultural plants. The sensors are used especially to detect critical concentrations and flammable and/or toxic gases in time and to warn against a corresponding hazard. In many areas of the chemical industry, just as in the semiconductor industry, especially the detection of so-called sour gases is of great interest in this connection in order to guarantee safety at the workplace. Sour gases are generally defined in the sense of the present invention as gases that form (weak) acids, for example, halogen halides, such as HF and HCl, hydrogen sulfide or even acetic acid, when dissolved in water. 
         [0005]    GB 2129562 B discloses in this connection the electrochemical detection of hydrogen fluoride (HF). Both the cathode and the anode of the sensor proposed there are platinum wires. A mixture of calcium bromide and calcium bromate is used as the electrolyte. However, this detection method is especially flow- and temperature-dependent. 
         [0006]    An alternative solution is known from WO 2002/031485 A1. A measuring electrode consisting of an electrochemically active metal oxide powder is proposed here. However, such metal oxide powders may have high cross-sensitivities to other gases, as a result of which low concentrations of the target gas cannot always be detected reliably. 
         [0007]    WO 1999/001758 A1 also discloses an electrochemical sensor, which is said to be used especially to detect hydrogen chloride. The working electrode in this sensor has an electrochemically active surface consisting of gold. However, this is dissolved over time, which may lead to the failure of the sensor in an emergency. 
       SUMMARY OF THE INVENTION 
       [0008]    Based on this, an object of the present invention is to overcome these and other drawbacks of the state of the art and to provide an improved electrochemical gas sensor. In particular, a gas sensor shall be provided that has the highest possible measuring sensitivity and the best possible signal stability under permanent load. Furthermore, the gas sensor shall be able to be manufactured as cost-effectively and simply as possible. 
         [0009]    As a solution, the present invention provides an electrochemical gas sensor comprising a housing, a plurality of electrodes, namely, at least one working electrode and one counterelectrode, as well as a liquid electrolyte, wherein at least one of the electrodes and/or the housing is comprised of an absorbent composition. 
         [0010]    The part of the gas sensor that defines the gas sensor towards the outside is typically considered to be the housing in this connection. The housing is usually in contact with the ambient air towards the outside and forms a receptacle for the electrolyte and the electrodes inside. 
         [0011]    An absorbent composition is defined as a composition that has at least one absorbent, the absorbent being designed such that it can react with at least one reaction product formed at the counterelectrode. An absorbent composition in the sense of the present invention may therefore consist exclusively of an absorbent in the simplest case. However, the absorbent composition preferably also contains further constituents, as they will be described in more detail below, in addition to the absorbent. 
         [0012]    The gas to be analyzed diffuses, in principle, at first to the working electrode in such an electrochemical sensor. It is reduced there. The reaction products formed in the process migrate as ions to the counterelectrode, where they are reoxidized (reverse reaction). 
         [0013]    Therefore, the gas to be analyzed follows in the housing a diffusion path over the course of which it first enters the electrolyte from the ambient air and at the end of which the reaction products formed at the counterelectrode flow again out of the housing. In the simplest case, the housing has at least one opening for the gas exchange with the surrounding area and an electrolyte-filled reaction chamber, in which the electrodes are located. The diffusion path of the gas to be analyzed then passes, for example, through the opening into the reaction chamber. The gas to be analyzed, dissolved in the electrolyte, flows in the reaction chamber to the working electrode, at which the direct reaction takes place. At the same time, gas formed at the counterelectrode during the reverse reaction, which is likewise dissolved in the electrolyte, flows back to the opening and from there out of the sensor. It is also conceivable that the housing has a gas inlet and a gas outlet, in which case the diffusion path leads from the gas inlet to the working electrode and from the counterelectrode to the gas outlet. 
         [0014]    If an absorbent is provided, gas that is formed during the reverse reaction can be absorbed. It is possible in this manner to prevent this gas both from collecting at the counterelectrode and from being released to the surrounding area. 
         [0015]    The absorbent composition may absorb especially reaction products that are formed during the reaction taking place at the counterelectrode. It is possible in this way to prevent, for example, signal breakdowns, which could otherwise develop due to an increase in the concentration of these reaction products at the counterelectrode. The absorbent composition is arranged in the gas sensor such that reaction products formed at the counterelectrode come into contact with the absorbent composition when they are moving along the above-described diffusion path in the gas sensor. 
         [0016]    It is conceivable, for example, that one of the electrodes consists of the absorbent composition. It is also conceivable that one of the electrodes has one or more sections, which consists/consist of the absorbent composition. In other words, it is conceivable that at least one of the electrodes has an absorbent composition. 
         [0017]    It is especially favorable if the absorbent composition is arranged at or in the counterelectrode. It is conceivable in this connection, for example, that the counterelectrode consists of the absorbent composition. The reaction products formed at the counterelectrode can be directly trapped at the counterelectrode in this manner. 
         [0018]    It is conceivable, for example, that fluoride is formed at first during the direct reaction at the working electrode, and this fluoride will react at the counterelectrode to form HF. However, the release of HF from the gas sensor is not desirable, as a rule, because HF is highly toxic and corrosive. In addition, the released HF may lead to the above-described sensor drift when it collects in the sensor. It is therefore advantageous if the HF formed at the counterelectrode can react with the absorbent contained in the absorbent composition of the counterelectrode. It is conceivable, for example, that the HF is converted into a precipitated solid in this reaction. 
         [0019]    It is also conceivable that at least one section of the housing consists of the absorbent composition. A section of the housing, which section is in contact with the electrolyte, is preferably formed from the absorbent composition. For example, HF formed at the counterelectrode or even another reaction product can be trapped in this manner from the electrolyte before it is released into the area surrounding the gas sensor. It is thus favorable, for example, if the housing has a recess, which forms a gas outlet, and the absorbent composition is arranged fully or partially in the recess. 
         [0020]    For example, the absorbent composition may be arranged in the gas outlet in a plug-like manner. The absorbent composition may form a filter, through which the gas flowing out of the gas sensor must flow. The absorbent contained in the absorbent composition can thus react with the reaction product, so that release into the environment is effectively preventable. At the same time, the absorbent composition may form an outer limitation of the electrolyte-filled reaction chamber. 
         [0021]    It is conceivable that either one or more of the electrodes consists/consist of the absorbent composition or that a part of the housing, for example, the gas outlet, consists of the absorbent composition. It is also conceivable that both at least one of the electrodes and a part of the housing consist of the absorbent composition. Furthermore, it is conceivable that one or more of the electrodes consists/consist of a first absorbent composition, while a part of the housing consists of a second absorbent composition. 
         [0022]    It is favorable if the absorbent composition contains at least one absorbent, a carrier material and an additive. The absorbent is preferably a substance or substance mixture that can react with at least one reaction product formed at the counterelectrode. The selection of the particular concrete composition may depend, on the one hand, on which reaction products are to be expected at the counterelectrode, and, on the other hand, on whether the absorbent composition shall be used as an electrode material and/or as a component of the housing, for example, as a filter in the gas outlet. In any case, the absorbent composition may be a composite consisting of absorbent, carrier material and additive. 
         [0023]    If the absorbent composition is used as an electrode material, it preferably contains both material that will hereinafter be called active electrode material and material that will hereinafter be called passive electrode material. An active electrode material is defined here as a material that is electrochemically active and participates in the electrochemical reaction of the gas sensor, which takes place at the corresponding electrode. A passive electrode material is defined as a material that does not participate in the electrochemical reaction in the strict sense of the word. The electrochemical reaction in the strict sense of the word is defined as the reaction taking place between the gas to be analyzed and the working electrode or the reverse reaction taking place at the counterelectrode. For example, the additive of the absorbent composition may be an active electrode material. It is conceivable, for example, that the active electrode material consists of carbon. However, it is also possible, of course, to use other materials, for example, metal, preferably noble metals, for example, platinum, iridium or gold, as the active electrode material. Provisions are made in a preferred embodiment for the absorbent composition to have carbon nanotubes as the additive. The carbon nanotubes are used as active electrode material and ensure the electrical conductivity as well the reactivity of the electrode. 
         [0024]    The carrier material and/or the absorbent may be passive electrode materials. 
         [0025]    It is, in addition, favorable, especially if the absorbent composition shall be used as the electrode material, if the absorbent composition contains an absorbent that is poorly soluble or insoluble in the electrolyte. It is preferred in this connection if the absorbent is poorly soluble or especially preferably insoluble in the electrolyte. The absorbent composition and therefore the electrode material can be prevented in this manner from gradually dissolving. By contrast, there should be a minimum solubility for the reaction product formed, so that the reaction product is not entirely insoluble but is preferably soluble only poorly. The electrode surface can renew in this way continuously to a low extent due to dissolution of the reaction product. The slow but steady dissolution of the absorbent taking place in this process takes place so slowly and over such a long period of time that the consumption of the electrode material is ideally irrelevant in relation to the overall service life of the gas sensor. At the same time, the slow but steady surface renewal makes it possible to prevent clogging of the electrode surface with reaction products. 
         [0026]    It is favorable, furthermore, if the absorbent composition contains a carbonate compound, preferably an alkali carbonate compound or an alkaline earth carbonate compound, especially preferably BaCO 3 , as the absorbent. This is advantageous above all if the gases to be detected are sour gases, as defined above. For example, HF formed at the counterelectrode can react with BaCO 3  into BaF 2  and H 2 CO 3  corresponding to the following reaction equation. The BaF 2  formed precipitates in this case as a solid: 
         [0000]      2HF+BaCO 3 →BaF 2 ↓+H 2 CO 3  
 
         [0000]    Even though the precipitated BaF 2  is deposited on or in the counterelectrode, it does so without poisoning the counterelectrode (i.e., without clogging the surface of the electrode and thus without sealing it for further reactions). 
         [0027]    It is also favorable in this connection if the absorbent composition contains as the carrier material a fibrous material, preferably a microfibrous material, especially preferably glass fibers, microfibers and/or nanofibers, preferably polymer microfibers and/or polymer nanofibers. The absorbent contained in the absorbent composition can be applied on this carrier material. It is conceivable, for example, that the carrier material is coated with the absorbent. It is, however, especially favorable if the absorbent composition is a mixture of comminuted carrier material and powdered absorbent as well as the additive. This mixture may be prepared first, for example, as a pulpy mass similar to a pulp and then brought to the desired form, for example, as an electrode, filter or plug for the gas outlet. The carrier material ensures that the mixture of the absorbent and the particular additive can form an effective composite, on the one hand, and is, on the other hand, sufficiently moldable to be brought to the desired shape. In a first embodiment, the carrier material may be glass fiber material. It is also conceivable that they are nanofibers or microfibers or a composite of nanofibers and microfibers made of a polymer or a polymer mixture, for example, electrospun or melt-spun nanofibers, nanofiber nonwovens, microfibers or microfiber nonwovens or even comminuted composite nonwovens from nanofibers and microfibers. 
         [0028]    If, however, the absorbent composition shall be used exclusively as a material in the area of the housing, it may be favorable if the absorbent composition contains polytetrafluoroethylene (PTFE) or a PTFE derivative as an additive. This additive may facilitate and thus improve the diffusion of gas through the absorbent composition. It is conceivable, for example, that the PTFE or PTFE derivative is PTFE fibers. 
         [0029]    To detect sour gases, especially halogen halide gases, it is advantageous, furthermore, if the electrolyte is a composition that contains an organic solvent, preferably a composition that contains a quinoid system, and a conducting salt, which has an organic cation. The organic solvent may be selected, for example, from the group containing alkylene carbonate, alkylene carbonate mixture and/or butyrolactone, preferably from the group containing propylene carbonate, ethylene carbonate or a mixture of propylene carbonate and ethylene carbonate. It is especially preferred if the organic solvent is sulfolane. 
         [0030]    The conducting salt may be, for example, an ionic liquid. The anion of the conducting salt is preferably selected from the group containing halides, carbonate, sulfonate, phosphate and/or phosphonate, preferably an anion selected from the group containing alkyl sulfonate, alkenyl sulfonate, aryl sulfonate, alkyl phosphate, alkenyl phosphate, aryl phosphate, substituted alkyl sulfonate, substituted alkenyl sulfonate, substituted aryl sulfonate, substituted alkyl phosphate, substituted alkenyl phosphate, substituted aryl phosphate, halogenated phosphate, halogenated sulfonate[,] halogenated alkyl sulfonate, halogenated alkenyl sulfonate, halogenated aryl sulfonate, halogenated alkyl phosphate, halogenated alkenyl phosphate, halogenated aryl phosphate, especially preferably an anion selected from the group containing fluorophosphate, alkyl fluorophosphate, aryl sulfonate, especially preferably from the group containing perfluoroalkyl fluorophosphate, and toluene sulfonate. 
         [0031]    It is advantageous in this connection if the conducting salt contains as cations metal ions, onium ions or a mixture of metal ions and onium ions. For example, the metal ions may be selected from among alkali metal ions or alkaline earth metal ions, preferably from among Li, K and/or Na. It is favorable if the onium ions are selected from among ammonium, phosphonium, guanidium cations and heterocyclic cations, preferably selected from among alkyl ammonium and heterocyclic cations, especially preferably selected from among alkyl ammonium, imidazolium and/or substituted imidazolium ions, wherein the substituted imidazolium ions preferably have a structure corresponding to 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    in which R1, R2, R3, R4 and R5 may be selected, independently from one another, from among —H, straight-chain or branched alkyl containing 1 to 20 C atoms, straight-chain or branched alkenyl containing 2 to 20 C atoms and one or more double bonds, straight-chained or branched alkinyl containing 2 to 20 C atoms and one or more triple bonds, saturated, partially or fully unsaturated cycloalkyl containing 3-7 C atoms, which may be substituted with alkyl groups containing 1 to 6 C atoms, saturated or fully unsaturated heteroaryl, heteroaryl-C1-C6 alkyl or aryl-C1-C6 alkyl, wherein R2, R4 and R5 are especially preferably H and R1 and R3 denote, each independently from one another, a straight-chain or branched alkyl containing 1 to 20 C atoms. 
         [0032]    It is especially conceivable, for example, that tetrabutylammonium toluene sulfonate or 1-hexyl-3-methylimidazolium-tris(pentafluoroethyl)-trifluorophosphate is used as the conducting salt. It is thus especially advantageous if the electrolyte is a mixture of a solvent, a conducting salt and/or an organic mediator, alkylammonium toluene sulfonate and ionic liquids, with a perfluoroalkyl fluorophosphate anion. 
         [0033]    It is favorable, furthermore, if the organic mediator is a polyhydroxy compound, which forms a quinoid system or a naphthalene system during oxidation. The organic mediator may be selected, for example, from the group containing ortho-dihydroxybenzene, para-dihydroxybenzene, substituted ortho-dihydroxybenzenes and substituted para-dihydroxybenzenes, dihydroxynaphthalene, substituted dihydroxynaphthalene, anthrahydroquinone, substituted anthrahydroquinone, preferably 1,2-dihydroxybenzene, 1,4-dihydroxybenzene, naphthohydroquinone, substituted 1,2- or 1,4-dihydroxybenzene, substituted hydroquinone, substituted naphthohydroquinone, substituted anthrahydroquinone, especially preferably substituted hydroquinone, and substituted 1,2-dihydroxybenzene. It is especially favorable in this connection if the substituents of the substituted anthraquinone, substituted 1,2-dihydroxybenzene and/or substituted 1,4-hydroquinone are selected from the group containing sulfonyl, tert.-butyl, hydroxyl, alkyl, aryl, preferably and/or tert.-butyl. 
         [0034]    It is conceivable in another embodiment that the counterelectrode is rod-shaped. The counterelectrode may consist now at least partially of the absorbent composition. It is conceivable, furthermore, that the working electrode surrounds the rod-shaped counterelectrode in a tube-like manner. For example, the working electrode may be wound simply around the counterelectrode. It is also conceivable that the working electrode is configured as a tube and pushed over the rod-shaped counterelectrode. It is favorable in this connection if at least one separating layer, preferably a hydrophilic and/or electrolyte-impregnated separating layer, is arranged between the counterelectrode and the working electrode. 
         [0035]    It is also conceivable in all other conceivable embodiments that one or more separating layers are arranged between the counterelectrode and the working electrode. The separating layer is preferably likewise hydrophilic and/or electrolyte-impregnated. 
         [0036]    In any case, the separating layer may be in a fluidic connection with an electrolyte reservoir. For example, the gas sensor may be configured such that it has a reaction chamber, in which the electrodes are arranged, and an electrolyte reservoir, in which the liquid electrolyte is contained. The electrolyte can be sent in this manner to the electrodes from the electrolyte reservoir by means of the separating layer. The separating layer preferably extends over and beyond the reaction chamber to the electrolyte reservoir. It can thus have a wick effect, by which the electrolyte is transported to the electrodes. The separating layer can thus have a wick effect. In other words, it is seen that the separating layer can form an electrolyte line between the electrolyte reservoir and the electrodes. To keep the distance over which the electrolyte is transported by means of the wick effect of the separating layer as short as possible, it is conceivable that the electrolyte channels are formed laterally parallel to the electrodes covered by the separating layer. 
         [0037]    It is also conceivable in all these exemplary embodiments that the gas sensor has, furthermore, a protective electrode. It is especially favorable in this connection if the protective electrode is arranged between the working electrode and the counterelectrode. The protective electrode can prevent, for example, gas generated at the counterelectrode from flowing back to the working electrode and from leading to incorrect signals there. It is advantageous in this connection if a diffusion-limiting separating layer is arranged between the counterelectrode and the protective electrode, e.g., in the form of a glass fiber nonwoven. 
         [0038]    Furthermore, the gas sensor may have a reference electrode. For example, a reference electrode having a flat configuration may be arranged laterally from the working electrode and/or laterally from the counterelectrode. 
         [0039]    All conceivable embodiments of the gas sensor according to the present invention are especially suitable for use for detecting sour gases and/or gas mixtures that contain sour gases, preferably for detecting HF, HCl and/or acetic acid. The HF and/or HCl formed at the counterelectrode can effectively be prevented from being able to be released by means of the absorbent composition. The use of a corresponding gas sensor has especially the advantage that the sensor is resistant to continuous gas exposure. Such a sensor has a good measured signal decay characteristic, i.e., the sensor will again show the zero value in a short time at the end of the measured signal. The development of drift of the measured signal during continuous gas exposure can be prevented to the greatest extent possible. Furthermore, the use of such a gas sensor according to the present invention has the advantage that no toxic and/or corrosive gases are released from the sensor. 
         [0040]    Further features, details and specifics appear from the figures and exemplary embodiments described below. It is obvious that these exemplary embodiments are only exemplary and that further variants and exemplary embodiments will readily appear to the person skilled in the art on the basis of this specification. 
         [0041]    The present invention is described in detail below with reference to the attached figures. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]    In the drawings: 
           [0043]      FIG. 1 a    is a schematic top view showing a design of a gas sensor according to the present invention, in which the counterelectrode consists of the absorbent composition; 
           [0044]      FIG. 1 b    is a schematic cross sectional view through the reaction chamber of the gas sensor along line A-A in  FIG. 1   a;    
           [0045]      FIG. 1 c    is a schematic side view of the gas sensor of  FIG. 1   a;    
           [0046]      FIG. 2 a    is a schematic top view showing a design of a gas sensor according to the present invention with an auxiliary electrode, which consists of the absorbent composition; 
           [0047]      FIG. 2 b    is a schematic cross sectional view through the reaction chamber of the gas sensor along line A-A in  FIG. 2   a;    
           [0048]      FIG. 2 c    is a schematic side view of the gas sensor of  FIG. 2   a;    
           [0049]      FIG. 3 a    is a schematic top view showing a design of a gas sensor according to the present invention, in which the absorbent composition forms a plug in the gas outlet of the gas sensor; 
           [0050]      FIG. 3 b    is a schematic cross sectional view through the reaction chamber of the gas sensor along line A-A in  FIG. 3 a   ; and 
           [0051]      FIG. 3 c    is a schematic side view of the gas sensor. 
           [0052]      FIG. 4 a    is a schematic top view showing a design of a gas sensor according to the present invention; 
           [0053]      FIG. 4 b    is a cross sectional view through the reaction chamber of the gas sensor along line A-A in  FIG. 4 a   ; and 
           [0054]      FIG. 4 c    is a schematic side view of the gas sensor; 
           [0055]      FIG. 5  is a schematic perspective view showing a design of an electrode arrangement according to the present invention, in which a separating layer and a working electrode are wound around a rod-shaped counterelectrode consisting of the absorbent composition; 
           [0056]      FIG. 6 a    is a schematic sectional view showing an alternative schematic design of a gas sensor according to the present invention; and 
           [0057]      FIG. 6 b    is a schematic sectional view showing the sensor shown of  FIG. 6 a    rotated by 90°. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0058]    Referring to the drawings, The gas sensor  10  shown in  FIGS. 1 a  through 1 c , 2 a  through 2 c , 3 a  through 3 c , 4 a  through 4 c , 6 a  and 6 b    has a housing  11 , which encloses a reaction chamber  21 . A first recess  23  and a second recess  24  are formed in the housing  11 . The first recess  23  is a gas inlet, through which gas to be analyzed can flow into the reaction chamber  21 . The second recess  24  is a gas outlet, through which gas formed at the counterelectrode  32  can flow out of the reaction chamber  21 . The electrodes  31 ,  32  of the gas sensor  10 , namely, the working electrode  31  and the counterelectrode  32 , are arranged in the reaction chamber  21 . There is a separating layer  50  between the working electrode  31  and the counterelectrode  32 . The separating layer  50  is hydrophilic. 
         [0059]    Furthermore, there is a hydrophobic membrane  41  between the working electrode  31  and the first recess  23 . Another hydrophobic membrane  43  is located between the counterelectrode  32  and the second recess  24 . The two hydrophobic membranes  41 ,  43  prevent electrolyte  60  from escaping from the reaction chamber  21  and at the same time protect the electrodes  31 ,  32  located opposite the recesses  23 ,  24  from dust and contamination, which could otherwise possibly be introduced into the reaction chamber  21  through the recesses  23 ,  24 . 
         [0060]    The gas sensor  10  has, furthermore, an electrolyte reservoir  12 , in which a liquid electrolyte  60  is contained. It is seen that the separating layer  50  connects the electrolyte reservoir  12  with the reaction chamber  21 . The separating layer  50  is impregnated with the liquid electrolyte  60 . The reaction chamber  21  is in fluidic connection in this way with the electrolyte reservoir  12 . Electrolyte channels  51  are formed on the side of the separating layer  50 . These are used to support the fluidic connection between the electrolyte reservoir  12  and the reaction chamber  21 . 
         [0061]    The gas sensor  10  shown in  FIGS. 2 a  through 2 c  and 4 a  through 4 c    has, furthermore, a reference electrode  33 . It is seen that the reference electrode  33  is also connected with the electrolyte reservoir  12  via the separating layer  50 . As can also be seen, the reference electrode  33  is covered by the separating layer  50  in the examples shown. It is, however, also conceivable in a variant, not shown, that the reference electrode  33  is embedded in the separating layer  50 . In any case, the reference electrode  33  is also in fluidic connection with the electrolyte reservoir  12 , so that the electrolyte  60  is sent to the reference electrode  33  from the electrolyte reservoir  12  by means of the separating layer  50 . It is seen in the top view shown in  FIG. 2 a    that the reference electrode  33  is arranged at a laterally spaced location from the electrodes  31 ,  32 ,  34  arranged in the reaction chamber  21 . However, all electrodes of the gas sensor  10  are at the same time in a conductive contact with one another through the electrolyte  60 . 
         [0062]    The gas sensor  10  shown as an example in  FIGS. 2 a  through 2 c  and 4 a  through 4 c    has, in addition to the electrodes  31 ,  32 ,  33  already described, a protective electrode  34  arranged between the working electrode  31  and the counterelectrode  32 . It is seen that the electrodes  31 ,  32 ,  34  are arranged in a sandwich-like manner. The separating layer  50  is arranged above the working electrode  31 . It is seen that the separating layer  50  fully covers the working electrode  31 . The protective electrode  34  is arranged above the separating layer  50 . The protective electrode  34  is covered by a membrane  42 . The membrane  42  is a diffusion-limiting membrane in a special variant. Another layer of the separating layer  50  is formed above the membrane  42 . The counterelectrode  32  is arranged on this separating layer  50 . The protective electrode  34  is designed in this arrangement of the electrodes  31 ,  32 ,  34  such that the surface area of the protective electrode  34  is larger than the surface area of the working electrode  31  and larger than the surface area of the counterelectrode  32 . 
         [0063]    In the embodiment shown in  FIGS. 4 a  through 4 c    as well as  6   a  and  6   b , the housing  11  is formed by an electrolyte reservoir  12  and an electrode carrier  20 . The part of the housing  11  facing the viewer is shown as being transparent in the view shown in  FIG. 4 a    especially in the area of the electrode carrier  20  to illustrate how the arrangement of the components described below may be in the interior of the electrode carrier  20 . 
         [0064]    A liquid electrolyte  60  is present in the electrolyte reservoir  12  here as well. The electrode carrier  20  has a reaction chamber  21 . Gas can enter the reaction chamber  21  through a first recess  23  (can be seen in  FIGS. 4 b  and 6 b   ) and escape through a second recess  24 . The first recess  23  is formed on the underside of the electrode carrier  20  in the example being shown, while the second recess  24  is formed in the opposite top side of the electrode carrier  20 . It is obvious that it is also conceivable, as an alternative, that the first recess  23  is formed in the top side and the second recess  24  in the underside of the electrode carrier  20 . It is thus seen that the electrode carrier  20  has at least one first recess  23 . 
         [0065]    The reaction chamber  21  is connected with the electrolyte reservoir  12  via a separating layer  50  here as well, so that the reaction chamber  21  is in a fluidic connection with the electrolyte reservoir  12  via the separating layer  50 . To improve the feed of the electrolyte  60  even more, electrolyte channels  51  are formed on the side of the separating layer  50 . It is, of course, also conceivable that only one electrolyte channel  51  is formed or that more than two electrolyte channels  51  are present. 
         [0066]    It is seen in both  FIG. 4 b    and  FIG. 4 c    as well as in  FIG. 6 b    that the reaction chamber  21  is defined by a first wall section  25  and a second wall section  26  of the electrode carrier  20 . Therefore, the electrode carrier  20  forms the reaction chamber  21 . The electrodes  31 ,  32 ,  34  are arranged here between the first wall section  25  and the second wall section  26 . Furthermore, it is seen that the reaction chamber  21  is connected with the ambient air via the first and second recesses  23 ,  24 . It is seen, furthermore, in  FIG. 4 c    that the electrode carrier  20  forms a section of the housing  11  of the gas sensor  10 . The housing  11  comprises here the wall of the electrolyte reservoir  12  and the wall of the electrode carrier  20 . The separating layer  50  is arranged in the electrode carrier  20  such that it is in direct contact with the electrolyte  60  present in the electrolyte reservoir  12 . 
         [0067]    The separating layer  50 , especially a section  52  of the separating layer  50  protruding from the electrode carrier  20 , is in direct contact with the electrolyte  60  present in the electrolyte reservoir  12  in the alternative embodiment of the gas sensor  10  shown in  FIGS. 6 a  and 6 b    as well. It is seen here as well that the electrode carrier  20  forms a section of the housing  11 . The electrode carrier  20  has a fastening section  22 . The electrode carrier  20  is fixed with this fastening section  22  in a receptacle  13  of the electrolyte reservoir  12 . The electrode carrier  20  is arranged here such that the part of the electrode carrier  20  in which the reaction chamber  21  is formed forms a section of the housing  11 . The electrode carrier  20  has an inner surface and an outer surface  28  in this area. The inner surface defines the reaction chamber  21 . The outer surface  28  forms a section of the outer housing wall of the gas sensor  10 . 
         [0068]    In the exemplary embodiments shown in  FIGS. 1 a  through 1 c  and 2 a  through 2 c   , the counterelectrode  32  consists of the absorbent composition  70 . In a first variant, the absorbent composition  70  consists of a mixture of BaCO 3  as an absorbent, comminuted glass fibers as the carrier material and carbon nanotubes as the additive. 
         [0069]    The absorbent composition  70  is arranged in the form of a plug in the recess  24 , which forms the gas outlet, in the gas sensor  10  shown in  FIGS. 3 a  through 3 b   . Gas flowing out of the gas sensor  10  must now flow through the recess  24 , i.e., through the gas outlet, and therefore through the absorbent composition  70 . In a first variant, the absorbent composition  70  consists of a composition that contains BaCO 3  as the absorbent, glass fibers as the carrier material and Teflon fibers as an additive. In a second variant, the absorbent composition  70  consists of CaCO 3  as an absorbent, glass fibers as a carrier material and Teflon fibers as an additive. It is also conceivable, as an alternative, that another alkali carbonate or alkaline earth carbonate is contained as the absorbent in the absorbent composition  70 . 
         [0070]    In one embodiment, not shown, the gas sensor  10  is configured as shown in  FIGS. 1 a  through 1 c   . In addition to the counterelectrode  32  consisting of a first absorbent composition, absorbent composition  70  may also be arranged in this embodiment in the form of a plug in the recess  24  that forms the gas outlet. In a first variant, the absorbent composition  70  has the same composition as the first absorbent composition, of which the counterelectrode  32  consists. The absorbent composition contains here BaCO 3  as an absorbent, glass fibers as a carrier material and carbon nanotubes as an additive. In a second variant, the absorbent composition  70  has a different composition than the absorbent composition of which the counterelectrode  32  consists. In this variant, the absorbent composition, of which the counterelectrode  32  consists, contains BaCO 3  as an absorbent, glass fibers as a carrier material and carbon nanotubes as an additive. The absorbent composition  70 , which is arranged in the recess  24 , i.e., in the gas outlet, contains BaCO 3  or, in other variants, another alkali or alkaline earth carbonate as the absorbent, glass fibers as the carrier material and Teflon fibers as the additive. 
         [0071]    In another embodiment, not shown, the gas sensor  10  is configured corresponding to  FIGS. 2 a  through 2 c   , and absorbent composition  70  is additionally arranged in the recess  24 , which forms the gas outlet. The absorbent composition  70  has, in a first variant, the same composition as the absorbent composition of which the counterelectrode  32  consists. As in the variants of the embodiment shown in  FIGS. 1 a  through 1 c   , the absorbent composition contains BaCO 3  as the absorbent, glass fibers as the carrier material and carbon nanotubes as the additive. In a second variant, the absorbent composition  70  has a different composition than the absorbent composition of which the counterelectrode  32  consists. The absorbent composition, of which the counterelectrode  32  consists, contains BaCO 3  as the absorbent, glass fibers as the carrier material and carbon nanotubes as the additive in this variant as well. The absorbent composition  70 , which is arranged in the recess  24 , i.e., in the gas outlet, contains BaCO 3  or, in other variants, another alkali carbonate or alkaline earth carbonate, glass fibers as the carrier material and Teflon fibers as the additive. 
         [0072]    In yet another embodiment, not shown, the gas sensor  10  is configured corresponding to the  FIGS. 3 a  through 3 c   , and absorbent composition  70  is arranged not only in the recess  24 , which forms the gas outlet, but the counterelectrode  32  also consists of absorbent composition. In a first variant, the absorbent composition  70  has the same composition as the absorbent composition of which the counterelectrode  32  consists. In a second variant, the absorbent composition  70  has a different composition than the absorbent composition of which the counterelectrode  32  consists. The respective absorbent compositions may have the compositions as described above in both variants. 
         [0073]    The counterelectrode  32  consists of the absorbent composition in the embodiments of the gas sensor  10  shown in  FIGS. 4 a  through 4 c  and 6 a    as well as  6   b  as well. In a first variant, not shown, absorbent composition  70  is additionally arranged in the form of a plug in the recess  24  in this case as well. The absorbent composition  70 , which is arranged in the gas outlet, i.e., in the recess  24 , as the same as the absorbent composition of which the counterelectrode  32  consists. Absorbent composition  70  is also additionally arranged in the form of a plug in the recess  24 , which forms the gas outlet, in a second variant, not shown. However, the composition of the absorbent composition  70  arranged in the gas outlet, i.e., in the recess  24 , differs from the composition of the absorbent composition of which the counterelectrode  32  consists. 
         [0074]    A special embodiment variant of the arrangement of the working and counterelectrodes  31 ,  32  is seen in  FIG. 5 . The counterelectrode  32  is formed here from the absorbent composition and is rod-shaped. The working electrode  31  is configured as a tube (is tubular) and is plugged onto the counterelectrode  32 . The separating layer  50  is formed between the working electrode  31  and the counterelectrode  32 . In an alternative embodiment, the counterelectrode  32  may also be in the form of a tube (tubular). 
         [0075]    An example of a composition of an electrolyte, as it can be used together with the above-described exemplary embodiments, is a mixture of about 60 wt. % of propylene carbonate, about 40 wt. % of ethylene carbonate, about 0.1 mole of HMIM FAP [1-hexyl-3-methyl-imidazolium-tris(pentafluoroethyl)-trifluorophosphate] and about 0.5 mole of tert.-butyl hydroquinone. This composition is, of course, variable, and the ratio of propylene carbonate to ethylene carbonate is not limited to a ratio of 60:40 wt. % by any means. The molar quantity of HMIM FAP and tert.-butyl hydroquinone contained is also variable. 
         [0076]    All the features and advantages, including configuration details, arrangements in space and method steps appearing from the claims, the specification and the drawings may be essential both in themselves and in the many different combinations. 
         [0077]    While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.