Abstract:
A mediator-based electrochemical gas sensor reacts selectively with hydrogen sulfide. The gas sensor has an electrolyte solution (9), which contains a mediator compound in the form of metallates of transition metals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119 of German Patent Application DE 10 2008 033 828.1 filed Jul. 19, 2008, 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 mediator compound. 
       BACKGROUND OF THE INVENTION 
       [0003]    An electrochemical gas sensor with a mediator dissolved in the electrolyte is known from DE 10 2004 062 051 A1. The presence of a mediator offers the possibility of providing sensors that are highly selective for the analyte gas. The mode of operation of a gas sensor with a mediator is based on the fact that analyte gas diffuses through the measuring electrode into the electrolyte solution and is oxidized or reduced by the mediator. The analyte is converted in the process into a decomposition product and the mediator into an intermediate product, which is reoxidized or re-reduced at the measuring electrode. The electron transfer needed for this, which is proportional to the percentage of analyte gas in the gas sample, can be detected as the measured current. 
         [0004]    Electrochemical gas sensors with mediators are characterized by a low residual current, high long-term stability and low cross sensitivity to interfering gases. Suitable mediators are available so far for special detection reactions only. However, the sensitivity of detection of the electrochemical gas sensor is also affected by the electrode material of the gas sensor. 
         [0005]    An electrochemical gas sensor with a plurality of electrodes and a measuring electrode made of diamond-like carbon (DLC) is known from DE 199 39 011 B1. The measuring electrode is produced by means of a coating process, in which diamond-like carbon is applied to a gas-permeable membrane by means of a sputtering process. The measuring electrodes made of DLC have very high long-term stability. 
         [0006]    An electrochemical gas sensor with a measuring electrode consisting of boron- or nitrogen-doped diamond (BDD) is known from DE 101 44 862 A1. The measuring electrode material is applied as a thin layer to a porous, gas-permeable substrate. Such measuring electrodes have a very high long-term stability and an extremely large potential window, so that very difficult-to-oxidize substances can be reacted as well. 
         [0007]    An electrochemical measuring device, in which the measuring electrode has carbon nanotubes, is known from DE 10 2006 014 713 B3. This sensor contains a mediator based on a transition metal, it selectively detects SO 2  and avoids the formation of elemental sulfur, but has only a low sensitivity for H 2 S. 
       SUMMARY OF THE INVENTION 
       [0008]    A basic object of the present invention is to propose a mediator-based electrochemical gas sensor, which selectively detects hydrogen sulfide. 
         [0009]    According to the invention, an electrochemical gas sensor is provided for detecting H 2 S in a gas sample. The electrochemical gas sensor comprises a measuring electrode, another electrode and an electrolyte solution containing a mediator compound in the form of metallates of transition metals. 
         [0010]    The mediator system specified according to the present invention is based on the fact that the hydrogen sulfide is oxidized into sulfuric acid and the formation of elemental sulfur is thus avoided and high sensitivity is reached at the same time. 
         [0011]    Mediators are preferably not fully soluble in an electrolyte solution. The use of suspensions or solutions of the mediator with solid undissolved solute offers a number of advantages, such as:
       constant mediator concentration at variable atmospheric humidity,   identical equilibrium potentials at the measuring electrode and the auxiliary electrode if the measuring electrode and the auxiliary electrode consist of carbon,   filtering action of the solid undissolved solute, and   the sensor can be operated under anaerobic conditions if the measuring electrode and the auxiliary electrode consist of carbon and the mediator determines the potential of these electrodes.       
 
         [0016]    Metallates of transition metals are used in the case of the mediator compound according to the present invention. The suitable metallates are vanadates, chromates, molybdates, tungstates, and permanganates. Molybdates of a transition metal salt are especially advantageous. 
         [0017]    A 2 molar to 10 molar and preferably 3 molar aqueous lithium chloride solution, which covers a wide range of temperatures and humidities, is preferably used as the conductive electrolyte. It is also possible to use ammonium halides if organic solvents, for example, ethylene carbonate or propylene carbonate, are used. It is also possible to use ionic liquids, for example, substituted ammonium, phosphonium or imidazolium compounds. 
         [0018]    The preparation of an aqueous electrolyte suspension will be described below. 
         [0019]    As much copper chloride is added to an aqueous lithium chloride solution as is needed to obtain a concentration between 0.5 molar and 5.0 molar and preferably 3.0 molar. The following reagents are also added to the resulting chloro complexes:
       Metallates: Chromates, vanadates, tungstates, but preferably molybdates. The concentration of the metallates is between 0.2 molar and 2 molar and preferably 1.0 molar,   inorganic acids or salts such as NaHSO 4 . Both the residual current and the t90 time can be markedly reduced with these additives.       
 
         [0022]    Polybasic carboxylic acids and their salts, preferably citric acid, phthalic acid as well as citrates and phthalates, are suitable for stabilizing the pH value. Boric acid or its salts may also be used as a polybasic acid. 
         [0023]    The resulting concentration of the reagents should be 0.5 mol to 5.0 mol and preferably 1.0 mol per L. Besides the catalytic activity, these compounds also have pH-buffering properties, so that the gas can be admitted to the sensors over many hours without loss of sensitivity. 
         [0024]    When the individual components are combined, a green solution is formed at first, from which a precipitate precipitates after some time. Hygroscopic alkali or alkaline earth metal halides, preferably chlorides, may also be used as conductive electrolytes in an aqueous solution. An especially preferred formula is 3.0 molar LiCi, 3.0 molar CuCl 2  and 1.0 molar Li 2 MoO 4 . 
         [0025]    The measuring electrode preferably consists of diamond-like carbon. However, it is also possible to use other carbon materials, for example, carbon nanotubes or measuring electrodes made of boron- or nitrogen-doped diamond (BDD) or precious metal thin-layer electrodes. 
         [0026]    Measuring electrodes made of carbon nanotubes (CNT) have long-term stability, can be integrated in existing sensor constructions in a simple manner, are suitable for many mediators, and can be purchased at a low cost. There are only a few cross sensitivities caused by the electrode material. This applies especially to multiwall carbon nanotubes (MW CNT). The carbon nanotubes are preferably used without binder. Such measuring electrodes are wetted by the electrolyte solution over their entire surface, as a result of which a large surface is obtained for the electrochemical reaction. The measuring electrode according to the present invention is preferably also permeable to gases. A measuring electrode made of CNT has better conductivity than a comparable measuring electrode made of DLC. 
         [0027]    The layer thickness of the carbon nanotubes at a measuring electrode depends on the structure of the measuring electrode. If the carbon nanotubes are in the form of multiwall carbon nanotubes, the layer thickness is between one μm and a thousand μm, and preferably between 50 μm and 150 μm. The layer thickness is between 0.5 μm and 100 μm and preferably between 10 μm and 50 μm in case of single-wall carbon nanotubes. 
         [0028]    The layer thickness also depends on the purity of the material. The layer thickness is closer to the lower end of the range in case of materials of an especially high purity. 
         [0029]    A large-area contact is obtained between the material of the measuring electrode and the analyte or with the converted mediator due to the use of carbon nanotubes, so that complete oxidation or reduction takes place. Part of the analyte or of the reacted mediator is thus prevented from diffusing into the electrolyte space. 
         [0030]    If the measuring electrode is designed as a precious metal thin-layer electrode, the layer thickness is less than 600 μm. Thick-layer electrodes have not proved to be successful because they have high residual currents and low selectivities. 
         [0031]    The auxiliary electrode advantageously consists of a precious metal, for example, gold, platinum or iridium, and alternatively of carbon nanotubes. 
         [0032]    A reference electrode or a protective electrode, which consists of a precious metal or carbon nanotubes, may also be present in addition to the auxiliary electrode. 
         [0033]    An exemplary embodiment of the present invention is shown in the figures and will be explained below. 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 
         [0034]    In the drawings: 
           [0035]      FIG. 1  is a sectional schematic view of an electrochemical gas sensor according to the invention; 
           [0036]      FIG. 2  is a gas admission curve; and 
           [0037]      FIG. 3  is a graph showing a comparison of cross sensitivities. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    Referring to the drawings in particular, a measuring electrode  3  made of diamond-like carbon (DLC) on a diffusion membrane  4 , a protective electrode  5 , a reference electrode  6  in a wick  7  and an auxiliary electrode  8  are arranged in a sensor housing  2  in the embodiment of the electrochemical sensor  1  illustrated in  FIG. 1 . The interior space of the sensor housing is filled with an electrolyte-mediator mixture  9 , wherein the mediator is additionally also present as a solid undissolved solute  10 . The electrodes  3 ,  5 ,  6 ,  8  are held at fixed distances from one another by means of liquid-permeable nonwoven mats  11 ,  12 ,  13 ,  14 . The gas enters through an opening  15  in the sensor housing  2 . The electrochemical sensor  1  is connected in the known manner to a potentiostat, not shown in more detail. 
         [0039]      FIG. 2  shows a typical gas admission curve  16  with the sensor  1  according to the present invention. Sensor  1  was exposed to a concentration of 1.96 ppm of H 2 S for 6 minutes at a temperature of 20° C. and 50% relative humidity. The gas admission time in seconds is plotted on the abscissa in  FIG. 2  and the sensor current in microAmperes is plotted on the ordinate. 
         [0040]    The following values are obtained as mean values from five sensors and five measurements: 
         [0000]        I   0 =7±2 nA (residual current) 
         [0000]        S= 3.0±0.1 μA ppm −1  (sensor signal in μA per ppm of H 2 S) 
         [0000]        D =3.4±1.6% (drift) 
         [0000]        t   0-90 =41.8±18.6 sec (jump response up to 90% of the maximum sensor signal). 
         [0000]    Sensor  1  according to the present invention is characterized by a very low residual current I 0  and by the broad measuring range, because both concentrations of a few ppm and gas concentrations in the percent range can be measured. 
         [0041]      FIG. 3  illustrates the cross sensitivities of a conventional electrochemical gas sensor with precious metal thick-layer electrode compared to the sensor  1  according to the present invention with the mediator compound. The darkly shaded bars  17  relate to the conventional gas sensor and the lightly shaded bars  18  to the sensor  1  according to the present invention. The tested gases are plotted on the abscissa in  FIG. 3  and the sensor signal S in μA per ppm of H 2 S is plotted on the ordinate. As can be determined from  FIG. 3 , both sensors yield an approximately equal measured signal when H 2 S gas is admitted. However, the conventional gas sensor has marked cross sensitivities under the effect of moisture and in case of the gases NO, PH 3 , AsH 3 , SO 2  and B 2 H 6 . Sensor  1  according to the present invention has, by contrast, a cross sensitivity in case of SO 2  only. Since this is only one gas, this cross sensitivity can be compensated in a simple manner, for example, by a second sensor, which measures the SO 2  component only. 
         [0042]    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.