EP1362237A1

EP1362237A1 - Gas sensor and method for measuring a gas component in a gas mixture

Info

Publication number: EP1362237A1
Application number: EP02708205A
Authority: EP
Inventors: Ulrich Alkemade; Bernd Schumann; Berndt Cramer; Marget Schuele; Thorsten Ochs; Sabine Thiemann-Handler
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2001-02-10
Filing date: 2002-02-05
Publication date: 2003-11-19
Also published as: JP4051287B2; WO2002065113A1; US20040112765A1; DE10106171A1; US7309415B2; JP2004518150A

Abstract

A gas sensor, based on a solid electrolyte, for measuring a gas component in a gas mixture, with at least one sensitive region is disclosed, comprising a first means for generating a reaction gas from a further gas component in the gas mixture. A second means is arranged in the sensitive region of the gas sensor, by means of which the residual content of the reaction gas can be determined after a reaction between the reactive gas and the gas component to be measured has occurred. A corresponding method is also disclosed.

Description

gas sensor

The invention relates to a gas sensor and a method for measuring a gas component in a gas mixture according to the preamble of the independent claims.

State of the art

In the course of progressive environmental legislation, the need for sensors is increasing, with the help of which even the smallest quantities of pollutants can be reliably determined. Gas sensors play a major role here, making it possible to determine gaseous pollutants in the ppm range regardless of the temperature of the sample gas. The measurement signals of the gas sensor which are proportional to the amount of pollutant are often so small that a high measurement inaccuracy cannot be avoided. Indirect determination of pollutants is one possible way out of this dilemma.

EP 241 751 A2 shows a gas sensor with which the content of ammonia, carbon onoxide, hydrocarbons, nitrogen oxides or sulfur dioxide in gas mixtures can be monitored, but not the oxygen content. To be able to determine nitrogen oxides, a measuring method is used, among other things proposed, in which a known amount of ammonia is added as a reaction gas to a gas mixture, which reacts with the nitrogen oxides on a catalyst of the gas sensor. If the amount of ammonia originally added is known, the NOx concentration in the gas mixture can be determined by determining the residual ammonia content. A disadvantage of this method is that an addition device for ammonia must be provided.

The object of the present invention is to provide a gas sensor which enables the measurement of various gas components of a gas mixture in a reliable and timely manner.

Advantages of the invention

The gas sensor according to the invention, or the method according to the invention, with the characterizing features of the independent claims advantageously achieves the object on which the invention is based. The gas component to be measured is determined indirectly by determining the residual content of a reaction gas after it has completely reacted with the gas component to be measured. It is particularly advantageous that the reaction gas does not first have to be added to the gas mixture, which would require one or more appropriate devices, but is instead generated in the gas sensor itself.

For this purpose, the gas sensor has a first means with which, in a first step, a reaction gas is generated from a further gas component of the gas mixture that is not the gas component to be measured. In a second step, this reacts with the gas component to be measured. The gas sensor also contains a sensitive area in which a second means is arranged which, in a third step, permits the determination of the residual content of reaction gas after its reaction with the gas component to be measured. If the reaction gas is generated in excess based on the amount of gas component to be measured and the amount of reaction gas generated is known, the residual amount of reaction gas can be used to infer the amount of gas component to be measured originally present in the gas mixture.

It is advantageous if, as the first means, an electrochemical pump cell is provided, at the electrode facing the gas mixture, the further gas component can be reduced or oxidized as required to form a reaction gas. Depending on the application, an electrochemical pump cell, an electrochemical concentration cell or a resistive measuring element can be considered as a second means.

Furthermore, it is advantageous if a catalyst is provided in the gas sensor, which catalyzes the reaction between the gas component to be measured and the reaction gas. The catalyst can at least largely cover one of the electrodes of the first or second agent.

In a further advantageous embodiment of the present invention, the first or second means is preceded by an electrochemical pump cell which regulates the oxygen content in the gas mixture before it reaches the sensitive area of the gas sensor. If the electrochemical pump cell is combined with an electrochemical concentration cell, this increases the accuracy with which the oxygen content of the gas mixture can be regulated and at the same time enables the additional determination of the oxygen content in the gas mixture. The Regulation of the oxygen content takes place, for example, in a first area of a measurement gas space of the gas sensor, the reaction of the gas component to be measured with the reaction gas and the determination of the residual content of reaction gas in a second area of the measurement gas space.

The gas component to be measured is advantageously determined by first generating a reaction gas from another gas component of the gas mixture and reacting the gas component to be measured within the gas sensor with the reaction gas. It is important to ensure that the reaction gas is always present in excess based on the amount of gas component to be measured. After the reaction, the residual content of reaction gas is determined and the original concentration of gas component to be measured is inferred from the residual content, knowing the amount of reaction gas initially generated.

drawing

Nine embodiments of the invention are shown in the drawing and explained in more detail in the following description. Show it:

FIG. 1 shows a cross section through the large area of a sensor element according to a first exemplary embodiment,

2 shows a cross section through the large area of a sensor element according to a first variant of the first exemplary embodiment, in which the positions of the first and second means or catalyst are interchanged,

Figure 3 shows a cross section through the large area of a Sensor element according to a second variant of the first exemplary embodiment, the second means and the catalyst being arranged in a separate layer plane of the gas sensor,

FIG. 4: a cross section through the large area of a sensor element according to a third variant of the first exemplary embodiment, the first means being arranged in a separate layer plane of the gas sensor,

FIG. 5 shows a cross section through the large area of a sensor element according to a second exemplary embodiment, in which the catalytic converter is integrated in the second means,

Figure 6: a cross section through the large area of a sensor element according to a third embodiment, the measuring gas space is divided by a diffusion barrier.

FIG. 7: a cross section through the large area of a sensor element according to a fourth exemplary embodiment, the measuring gas space of which is divided by a diffusion barrier and in which the catalyst is integrated in the second means,

FIG. 8: a cross section through the large area of a sensor element according to a fifth exemplary embodiment, in which the diffusion barrier for dividing the measuring gas space is arranged between the first and second means,

FIG. 9: a cross section through the large area of a sensor element according to a first variant of the fifth exemplary embodiment, the second means and the catalyst being arranged in a separate layer plane of the gas sensor, FIG. 10: a cross section through the large area of a sensor element according to a sixth exemplary embodiment, in which the gas component to be measured is determined by potentiometric means,

FIG. 11 shows a cross section through the large area of a sensor element according to a seventh exemplary embodiment, in which the gas component to be measured is determined in a resistive way,

FIG. 12: a cross section through the large area of a sensor element according to an eighth exemplary embodiment, in which the catalyst is combined with the first means,

Figure 13: a cross section through the large area of a sensor element according to a ninth embodiment in which the catalyst is combined with the first and the second means.

Unless stated otherwise, the reference symbols used in FIGS. 1 to 13 always denote functionally identical structural components of a sensor element.

embodiments

Figure 1 shows a basic structure of a first embodiment of the present invention. 10 designates a planar sensor element of an electrochemical gas sensor which has, for example, a plurality of oxygen-ion-conducting solid electrolyte layers 11a, 11b, 11c, lld, lle, llf and 11g. The solid electrolyte layers 11a-11g are designed as ceramic films and form a planar ceramic Body. The integrated form of the planar ceramic body of the sensor element 10 is X manufactured by laminating together the printed with Funktioήsschichten ceramic sheets and then ^'sintering of the laminated structure in a conventional manner Each of the solid electrolyte layers lla-llg is made of oxygen ion conducting solid electrolyte material such as Y ₂ 0 ₃ partially or fully stabilized Zr'0 ₂ executed. The solid electrolyte layers 11a-11g can alternatively be replaced at least partially by aluminum oxide foils at locations where ion conduction in the solid electrolyte is not important or even undesirable.

The sensor element 10 contains a measuring gas space 13 which is in contact with a gas mixture surrounding the gas sensor via a gas inlet opening 15. The gas inlet opening 15 is designed, for example, as a bore penetrating the solid electrolyte layer 11a, but it can also be arranged in the same layer plane 11b as the measuring gas space 13. A buffer space 17 and a diffusion barrier 19, for example made of porous ceramic material, are provided between the gas inlet opening 15 and the measurement gas space 13 in the direction of diffusion of the measurement gas. The buffer space 17 serves to avoid signal peaks in the case of rapidly changing gas concentrations in the gas mixture.

A reference gas channel 30, which contains a reference gas atmosphere, is formed in a further layer plane 11d of the sensor element. The reference gas atmosphere can be air, for example. For this purpose, the reference gas channel 30 has an opening (not shown) on a side of the sensor element facing away from the measurement gas, which ensures the gas exchange with the ambient air.

In the ceramic base body of the sensor element 10 there is also a resistor between two insulation layers 32, 33. Floor heater 35 embedded. The resistance heater is used to heat the sensor element 10 to the necessary operating temperature.

One or two first inner electrodes 20 are arranged in the first measuring gas space 13. An outer electrode 22 is located on the outer side of the solid electrolyte layer 11a directly facing the gas mixture, which can be covered with a porous protective layer (not shown). The electrodes 20, 22 form a first electrochemical pump cell. The mode of operation as a pump cell comprises the application of a voltage between the electrodes 20, 22 of the pump cell, which results in ion transport between the electrodes 20, 22 through the solid electrolyte 11a. The number of "pumped" ions is directly proportional to a pump current flowing between the electrodes 20, 22 of the pump cell.

In the diffusion direction of the measuring gas, a second and a third inner electrode 24, 26 are provided downstream of the first inner electrode 20 in the measuring gas space 13. The associated common outer electrode, which serves as the reference electrode 28, is located in the reference gas channel 30. The second inner electrode 24 forms a second electrochemical pump cell with the reference electrode 28 and the third inner electrode 26 forms a third electrochemical pump cell with the reference electrode 28. In addition, the inner electrode 20 can be interconnected with the reference electrode 28 to form an electrochemical Nernst or concentration cell. A Nernst or concentration cell is generally understood to mean a two-electrode arrangement in which both electrodes 20, 28 are exposed to different gas concentrations and a difference in the potentials applied to the electrodes 20, 28 is measured. According to the Nernst equation, this potential difference allows a conclusion to be drawn about the gas concentrations present at the electrodes 20, 28.

Another possibility is to interconnect the second inner electrode 24 with the outer electrode 22 to form a second electrochemical pump cell or the third inner electrode 26 with the outer electrode 22 to form a third electrochemical pump cell.

The electrode material for all electrodes is used in a manner known per se as a cermet in order to sinter with the ceramic films.

For the operation of the sensor element 10 as a gas sensor, the first pump cell together with the concentration cell is used to regulate the oxygen content of the gas mixture diffusing into the measuring gas space 13. By pumping in or pumping out oxygen, a constant oxygen partial pressure of, for example, 0.1 to 1000 pprα is set in the measuring gas chamber 13. The oxygen partial pressure in the measuring gas space 13 is checked by means of the concentration cell. In this case, the pumping voltage is varied at the pumping cell so that 20, 28 of the cell concentration is adjusted between the ^electrodes' a constant potential difference. The pump current flowing within the pump cell is a measure of the oxygen concentration present in the diffusing gas mixture and enables the additional function of the gas sensor as an oxygen probe. Since premature decomposition of the gas component to be measured on the first inner electrode 20 is undesirable, the first inner electrode 20 is preferably made of a catalytically inactive material such as gold or a gold-platinum alloy. If the application of the gas sensor to the determination stabx- ler gas components, the inner electrode 20 can also have platinum, a rhodium-platinum alloy or some other suitable material.

If it is to be assumed that the present gas mixture has only a small proportion of oxygen, the first inner electrode and thus the first electrochemical pump cell can also be dispensed with. This is the case, for example, with exhaust gases from motor vehicles that are operated constantly with a lambda value = 1. This simplifies the sensor structure.

The gas mixture in the measuring gas space 13 which is set to a constant oxygen partial pressure now reaches the sensitive area 40 of the gas sensor. The second inner electrode 24 of the second pump cell is arranged in this. At the second inner electrode 24, which preferably, but not necessarily, also has a catalytically inactive material such as gold or a gold-platinum alloy, by applying a corresponding potential, a further gas component of the gas mixture, which is not the gas component to be measured, generates a reaction gas that is reacted with the gas component measuring z: u. If the gas sensor is used, for example, to determine nitrogen oxides, a potential of, for example, -500 to -750 mV with respect to the reference electrode 28 is set on the second inner electrode 24 and water or carbon dioxide is reduced to hydrogen: or carbon monoxide. The oxygen released is electrochemically reduced and pumped out.

H ₂ 0 + 2e ^~ <=> H ₂ + CT (pumped out) (e ^" = electron) (1) C0 ₂ + 2e ^" . CO + 0 ^{2 "} (pumped out) (2) The second inner electrode 24 is dimensioned such that the reaction gas generated (hydrogen or carbon monoxide) is present in excess based on the amount of gas component (nitrogen oxides) to be measured contained in the gas mixture. In order to prevent the gas component to be measured (nitrogen oxides) from being decomposed due to the strongly negative potential of the second inner electrode 24 and thus no longer being available for measurement, the second inner electrode 24 is preferably provided with a protective device 36. The protective device 36 can, for example, as shown in FIG. 1, be made of solid electrolyte material or another suitable ceramic material. The geometric design of the protective device 36 in the form of a slotted or perforated cover layer means that only a small part of the diffusing gas mixture comes into contact with the second inner electrode 24. Since this small part of the gas mixture also has a sufficiently high proportion of the further gas component (water, carbon dioxide), an excess of reaction gas can still be made available. Gas mixtures that contain air, for example, or exhaust gases from internal combustion engines meet this requirement.

The gas mixture enriched with the reaction gas (hydrogen or carbon monoxide) now reaches a part of the sensitive area 40 facing away from the gas inlet opening 15. There, a catalyst 38 in the form of a catalytically active layer is applied in the measuring gas space 13, which catalyzes the conversion of the reaction gas (hydrogen or Carbon monoxide) catalyzed with the gas component to be measured (nitrogen oxides) according to equations (3), (4).

x H ₂ + NO _x ox H ₂ 0 +% N ₂ (3) x CO + NO _x x C0 ₂ + N ₂ (4)

Since the reaction gas is in excess, a complete conversion of the gas component to be measured is ensured. On the side of the sensitive region 40 facing away from the gas inlet opening 15, a third inner electrode 26 is also arranged, which together with the reference electrode 28 forms the third pump cell. Optionally, the third inner electrode 26 can be applied to an additional solid electrolyte layer 37 in order to shorten the diffusion distance between the catalyst 38 and the third inner electrode 26.

The potential of the third inner electrode 26 is selected so that oxygen is pumped from the reference gas channel 30 to the third inner electrode 26 and reacts there with the remaining reaction gas. Since this reaction is the reverse reaction of reaction (1), (2), the further gas component (water or carbon dioxide) is formed (equation (5), _ (6)). For this purpose, a potential of -300 to -500 mV is set on the third inner electrode 26.

H ₂ + 0 ^{2 ~} <=> H ₂ 0 + 2e ^~ (5) CO + 0 ^{2 "} O C0 ₂ + 2e ^" (6)

The third inner electrode 26 is made of a catalytically active material such as platinum or an alloy of platinum, rhodium and / or palladium. The pump current flowing in the third pump cell is determined and is directly proportional to the residual concentration of the reaction gas. Since the originally on the second inner elec- trode 24 generated initial concentration of reaction gas in the

Gas mixture is approximately constant and can easily be determined by a calibration measurement, from the difference l of the initial concentration and the residual concentration of the reaction gas after its reaction with the gas component to be measured, the original content of gas component present in the gas mixture to be measured. The smaller the measured residual concentration of the reaction gas, the greater was originally the concentration of gas component to be measured in the gas mixture.

The use of a gas sensor having the sensor element 10 is not limited to the determination of nitrogen oxides. In principle, the second pump cell can be used to generate reaction gases either by electrochemical reduction or oxidation. In the first case, reducible gas components can be determined, in the second case oxidizable ones.

If a reducing potential is set on the second inner electrode 24 of the second pumping cell, not only hydrogen and carbon monoxide can be generated as reaction gases, but in principle also nitrogen monoxide from nitrogen dioxide or sulfur monoxide from sulfur dioxide or trioxide.

NO _x + 2X e ^" NO + XO ^2" (pumped out) (7)

SO _x + 2X e ^" SO + XO ² - (pumped out) (8)

The reaction gases generated according to equations (7), (8) can be reacted with reducible gas components and thus used to determine them. The selection of the reaction gas suitable for the individual case is based on the standard electrochemical potentials of the tion or implementation of the reaction gas occurring redox reactions, as well as reaction kinetics.

The measurement of oxidizable gas components is also possible without a change in the embodiment of the gas sensor being necessary. Only the potential of the second inner electrode 24 is now selected so that one or more gas components of the gas mixture are oxidized in a targeted manner at suitable temperatures. These can be, for example, water, nitrogen monoxide, sulfur monoxide or sulfur dioxide.

N ₂ + 20 ^{2 "} = 2 NO + 4e ^~ (9)

NO + O ^{2 "} N0 ₂ + 2e ^" (10)

SO + x O ^{2 "} <-> S0 _X + 2x e ^" x = 1, 2 (11)

2 0 ^{2 "} o 0 ₂ + 2e ^" . (12)

The catalyst 38 then reacts with the oxidizing reaction gas with the reducing gas components to be determined, such as ammonia, hydrogen, methane or hydrocarbons.

3 0 ₂ + CH ₄ C0 ₂ + 2 H ₂ 0 (13) 3 N0 ₂ + 4 NH ₃ O 6 H ₂ 0 + 3.5 N ₂ (14) 2 N0 ₂ + CH ₄ <= C0 ₂ + 2 H ₂ 0 + N ₂ (15)

In order to be able to determine the residual content of the oxidizing reaction gas at the third inner pump electrode 26, the potential of this electrode compared to the reference electrode 28 is selected such that the residual content of reaction gas at the third pump electrode 26 is reduced and the oxygen released in the process Pumped reference gas channel 30 becomes. The pump current now occurring with the opposite sign is used as the measurement signal. From the difference between the initial concentration initially present and the residual concentration of the reaction gas remaining after the reaction, the concentration of gas component to be measured originally present in the measuring gas is concluded. The present gas sensor is therefore suitable, depending on the choice of the potentials on the inner electrodes 24, 26, for determining both reducing and oxidizing gas components of a gas mixture.

If a reducing potential is set on the second inner electrode 24 and an oxidizing potential on the third inner electrode 26, then oxidizing gas components can be determined by means of the gas sensor. If an oxidizing potential is set on the second inner electrode 24 and a reducing potential is set on the third inner electrode 26, then reducing gas components can be determined. The second inner electrode has a selective effect with regard to the generation of oxygen and prevents oxidation of the gas component to be detected.

By fine-tuning the oxidizing or reducing potential at the second inner electrode 24, specific oxidizing or reducing reaction gases or mixtures of different oxidizing or reducing reaction gases can be generated in a targeted manner. The potentials to be set for this result, taking into account possible overvoltages, from the standard potentials of the reactions, in which the required reaction gases are formed from further gas components. Since the potentials applied to the electrodes 24, 26 can be varied at short notice, there is also the possibility, periodically or in short time intervals, of alternately determining one or more reducing or oxidizing gas components in succession using a sensor.

In a further embodiment it is provided that the second inner electrode 24 is contacted by means of a constant current source, for example a large electrical resistance between the current source and the electrode 24 is provided in relation to the electrical resistance of the second inner electrode 24, such that the major part of the applied to electrical resistance and electrode 24 -electrical voltage drops at the electrical resistance.

Since the amount of reaction gas generated at the second inner electrode 24 is a function of the electrode current of the electrode 24, the application of a constant current enables a constant volume of reaction gas to be provided per unit of time. In order to enable the most effective possible operation of the sensor element, it makes sense to select the potential applied to the second inner electrode 24 so that the electrochemical generation of the desired reaction gas from a further gas component is ensured and on the other hand the electrode current at the electrode 24 limit that the reaction gas generated is in a slight excess with respect to the gas component to be measured. In this way, the sensor signal of the sensor element becomes independent of the concentration of the further gas component from which the reaction gas is generated on the second inner electrode 24. FIGS. 2, 3 and 4 show variants of the sensor element shown in FIG. 1. In the variant shown in FIG. 2, the second inner pump electrode 24 and the protective device 36 are displaced into that part of the sensitive region 40 of the sensor element which faces away from the gas inlet opening 15. The catalyst 38 or the third inner pump electrode 26, on the other hand, are arranged on the side of the sensitive region 40 facing the gas inlet opening 15. Since in this variant the gas component to be measured hits the catalyst 38 in the sensitive area 40 without first passing through the second inner pump electrode 24, the probability is very low that the gas component to be measured undesirably reaches the second inner pump electrode 24 without m Catalyst 38 to be implemented. A sufficient provision of the reaction gas is also ensured in this arrangement, since the further gas component required for this can penetrate unhindered to the second inner electrode 24.

A second variant of the sensor element according to the first exemplary embodiment is shown in FIG. 3. The sensor element comprises two additional solid electrolyte layers llcl, llc2. A further measuring gas space 14 is located in the layer 11cl, which is in contact with the first measuring gas space 13 via an opening 16 through the solid electrolyte layer 11c. The catalyst 38 and the third pump electrode 26 are located in the second measuring gas chamber 14. This construction of the sensor element causes an increase in the diffusion distance - within the sensitive area 40 of the sensor element without the sensor element having to be extended at the same time. The larger diffusion distance decouples the generation of the reaction gas from its reaction with the gas component to be measured or from the detection of the residual content of the reaction gas. FIG. 4 shows a third variant of the sensor element according to the first embodiment. Its structure has the layer sequence of the sensor element shown in FIG. 3. In this variant, the second inner pump electrode 24 and the protective device 36 are located in the second measuring gas chamber 14 and the catalyst 38 or the third pump electrode 26 are arranged in the first measuring gas chamber 13. This variant combines the advantages of the first variant according to FIG. 2 with the advantages of the second variant according to FIG. 3. The fact that the gas component to be measured reaches the catalyst 38 in the sensitive area 40 without first passing through the second inner pump electrode 24, and the extended diffusion distance between the catalyst 38 and the second inner electrode 24 minimize the probability that the gas component to be measured can reach the second inner electrode 24.

FIG. 5 shows a sensor element according to a second exemplary embodiment of the present invention. Instead of a catalyst 38 and a separate third inner electrode 26, the sensor element has a catalytically active, preferably porous combination electrode 27. This is arranged in that part of the sensitive region 40 of the sensor element which faces away from the gas inlet opening 15. The combination electrode 27 can be designed in the form of an electrode partially or completely covered with a catalytically active layer or consist entirely of a catalytically active, preferably porous material. The combination electrode 27 has the advantage that the spatial separation of reaction of the reaction gas with the gas component to be measured and detection of the residual content of reaction gas is eliminated. In addition, with; With the help of the combination electrode 27 also the implementation of Catalyze reaction gases or gas components to be measured, which cannot be catalyzed by means of a catalytically active material alone, but additionally requires the application of a corresponding potential to the catalyst. It is furthermore advantageous that, in the production of a sensor element according to the second exemplary embodiment, a work step which involves the application of a separate catalytic converter 38 is avoided compared to the previously described variants.

A variant of the sensor element shown in FIG. 5 consists, in analogy to the variant shown in FIG. 2, of exchanging reaction gas-generating inner electrode 24 or protective device 36 and combination electrode 27. The second inner pump electrode 24 or the protective device 36 is thus arranged in that part of the sensitive region 40 of the sensor element which faces away from the gas inlet opening 15. At the same time, the combination electrode 27 is provided in that part of the sensitive region 40 which faces the gas inlet opening 15. In this variant, the probability that the gas component to be measured comes into contact with the second inner pump electrode 24 and is lost for the measurement is minimized.

In a further variant of the sensor element shown in FIG. 5, which largely corresponds to that already shown in FIG. 3, the combination electrode 27 is shifted into a separate layer 11cl. Here, too, there is an extension of the diffusion distance between the inner electrode 24 generating reaction gas and the combination electrode 27 given while maintaining the linear expansion of the sensor element.

A third variant of the sensor element described in FIG. 5 includes, analogously to the third variant of the first embodiment already shown in FIG. 4, the arrangement of the second inner pump electrode 24 or the protective device 38 in the separate solid electrolyte layer 11cl. This variant combines the advantages of the two variants of the sensor element shown in FIG. 5 already described.

FIG. 6 shows a sensor element according to a third exemplary embodiment of the present invention, in which the measuring gas space 13 additionally contains a diffusion barrier 42 which divides the measuring gas space 13 into an area 44 regulating the oxygen content of the measuring gas and the sensitive area 40. In the embodiments of the sensor element described so far, there is basically the possibility that reaction gas, despite the protective device 36, penetrates into the part of the measuring gas space 13 facing the gas inlet opening 15. If this is a reducing reaction gas, it is converted due to the higher oxygen content there; if it is an oxidizing reaction gas, it is decomposed at the first inner electrode 20. The diffusion barrier 42 prevents undesired diffusion of the reaction gas generated at the second inner electrode 24 into the part of the measuring gas space 13 facing the gas inlet opening 15. This increases the measuring accuracy of the sensor element, since the concentration of the generated reaction gas in the sensitive area 40 of the sensor element only diffuses from the diffusing one Amount of gas component to be measured is dependent. A variant of the sensor element shown in FIG. 6 consists in interchanging the positions of the inner electrodes 24, 26 or the protective device 36 and the catalyst 38 in accordance with the sensor element already shown in FIG. Another variant, similar to that in FIG. 3, is based on the arrangement of the third inner electrode 26 or the catalyst 38 in a separate layer plane 11cl. A third variant, comparable to the variant shown in FIG. 4, results from the arrangement of the second inner electrode 24 or the protective device 36 in the separate test electrolyte layer 11cl.

FIG. 7 shows a sensor element according to a fourth embodiment of the present invention. The sensor element shown in FIG. 7 combines the advantages of the features of the second with those of the third embodiment. It includes a diffusion barrier 42 between the oxygen-regulating and the sensitive area 40, 44 of the measuring gas space 13 as well as the combination of the catalyst 38 and the third inner electrode 26 to form the combination electrode 27. Here too, the positions of the second inner electrode 24 or Protection device 36 and combination electrode 27 are possible, and the displacement of combination electrode 27 or second inner electrode 24 into a separate layer plane 11cl.

FIG. 8 shows a sensor element according to a fifth exemplary embodiment of the present invention. The sensor element shown in FIG. 8 is based on the one shown in FIG. 6 and has a diffusion barrier 46 within the measuring gas space 13, which is connected downstream of the second inner pump electrode 24 in the direction of flow of the gas mixture and spatially faces the sensitive area 40 into one of the gas inlet opening 15 and one of the gas dividing opening 15 facing away part. Such an embodiment is primarily intended for applications in which the reaction gas generated is inert to oxygen or is not subject to decomposition at the first inner electrode 20. Diffusion barrier 46 makes diffusion to catalyst 38 or third inner electrode 26 more difficult, so that the effect of an extended diffusion path between second and third inner electrodes 24, 26 is further enhanced.

If, in addition, as shown in FIG. 9, the catalyst 38 or the third inner pump electrode 26 is laid in a separate layer plane 11cl, then according to this first variant of the fifth exemplary embodiment, by applying the diffusion barrier 46 in the opening 16 of the solid electrolyte layer 11c, the effect of a Difficult diffusion further increased. Difficult diffusion eliminates inhomogeneities in the gas mixture.

Another variant consists in combining the catalytic converter 38 and the third inner electrode 26 to form a combination electrode 27 which, for example, like the second inner electrode 24, can be laid in the separate layer plane 11cl.

Variations of the sensor element on which the invention is based while maintaining the measurement principle are also the subject of the invention. For example, the measuring gas space 13, 14 can optionally be filled with porous material as diffusion resistance or contain several diffusion barriers. In addition, more than one electrochemical cell can be provided for regulating the oxygen content of the measuring gas or more than one electrochemical cell for generating a reducing or oxidizing reaction gas. That too Means serving to determine the residual content of the reaction gas can be provided in multiple versions.

The detection of the residual content of reaction gas in the gas mixture is carried out in the described exemplary embodiments by means of the third pump cell using amperometric methods. However, it is also possible to carry out the detection potentiologically using a concentration cell. For this purpose, the third inner electrode 26 is connected together, for example, with the reference electrode 28 to form a Nernst or concentration cell.

The potentiometric detection of reaction gases such as hydrogen or carbon monoxide is carried out particularly advantageously by using a so-called imbalance sensor. Such a sensor element is shown in FIG. 10. In the measuring gas chamber 13 there is also a fourth inner electrode 29, which is catalytically inactive and is connected to the catalytically active third inner electrode 26 to form a Nernst or concentration cell. Since a different potential is formed on the catalytically active third inner electrode 26 than on the catalytically inactive fourth inner electrode 29, a voltage can be determined as a measurement signal. This effect is particularly pronounced if a combination electrode 27 is used as the third inner electrode, without the catalyst 38.

Another way of detecting the reaction gas is to use a resistive measuring element. A corresponding exemplary embodiment is shown in FIG. 11. A voltage is applied to the third and fourth inner electrodes 26, 29 and the resistance a gas-sensitive layer 50 between the two inner electrodes 26, 29 is determined.

In a further advantageous embodiment, the catalytic converter 38 is combined with the second inner electrode 24 according to FIG. The catalyst can partially or completely cover the second inner electrode 24, an additional, preferably porous, solid electrolyte layer 48 being arranged between the catalyst 38 and the surface of the second inner electrode 24, in order to prevent the gas component to be measured from being electrochemical on the catalyst 38 is implemented. The combination of catalyst 38 and second inner electrode 24 particularly effectively prevents access of the gas component to be measured to the second inner electrode 24, since the gas component to be measured must first pass through the porous catalyst 38 before it reaches the second inner electrode 24. In the catalyst 38 it encounters an excess of the diffusing in the opposite direction _. Reaction gas and is fully implemented.

FIG. 13 shows a sensor element in accordance with a ninth exemplary embodiment. The catalyst 38 is combined both with the second inner electrode 24 and with the third inner electrode 26. This arrangement leads to a particularly effective avoidance of the diffusion of the gas component to be measured to the second inner electrode 24.

In order to ensure a particularly favorable gas flow within the gas sensor, one or more devices for guiding the gas flow within the measuring gas space 13, 14 can be provided in addition to the protective device 36. The possible uses of the gas sensor on which the invention is based can be seen, for example, in the detection of pollutants in exhaust gases from internal combustion engines. In particular, the detection of nitrogen oxides enables control, for example, of the functionality or the loading condition of a NOx storage catalytic converter. For this purpose, the gas sensor is arranged downstream of the NOx storage catalytic converter in the exhaust gas line in the flow direction of the exhaust gas. In addition, the determination of ammonia enables the control of SCR systems operated with ammonia or urea. The gas sensor is arranged in the exhaust line between the exhaust aftertreatment unit and the exhaust and the ammonia content of the exiting exhaust gas is checked.

The gas sensor can also be used for pollutant analysis in combustion plants for heating purposes. It is also possible to check the completeness of the combustion by detecting methane, for example.

In principle, the gas sensor enables both the purely qualitative detection of the existence of a gas component to be measured and the determination of its concentration in a gas mixture.

Claims

Gas Sensor claims

1. Gas sensor based on solid electrolyte for measuring a gas component in a gas mixture, with at least one sensitive area, characterized in that the gas sensor has a first means for generating a reaction gas from a further gas component of the gas mixture, and that in the sensitive area (40) a second means is arranged with which the residual content of the reaction gas can be determined after a reaction taking place between the reaction gas and the gas component to be measured.

2. Gas sensor according to claim 1, characterized in that the first means is an electrochemical pump cell which has an electrode (24) facing the gas mixture.

3. Gas sensor according to claim 2, characterized in that a constant current is applied to the electrode facing the gas mixture (24).

4. Gas sensor according to claim 2, characterized in that on the electrode facing the gas mixture (24) of the elektroαhemi- see pump cell the further gas component water to the reaction gas hydrogen and / or the further gas component carbon dioxide to the reaction gas carbon monoxide is reducible.

5. Gas sensor according to claim 2, characterized in that on the gas mixture facing electrode (24) of the electrochemical pump cell as further gas components nitrogen monoxide and / or a sulfur oxide are each oxidizable to a reaction gas.

6. Gas sensor according to one of claims I to 5, characterized in that the second means is an electrochemical pump cell.

7. Gas sensor according to one of claims 1 to 5, characterized in that the second means is an electrochemical concentration cell.

8. Gas sensor according to one of claims 1 to 5, characterized in that the second means is a resistive measuring element.

9. Gas sensor according to one of the preceding claims, characterized in that a catalyst (38) is provided for the reaction of the reaction gas with the gas component to be measured.

10. Gas sensor according to claim 9, characterized in that the catalyst (38) is arranged in the immediate vicinity of the second means.

11. Gas sensor according to claim 9, characterized in that the catalyst (38) at least largely covers an electrode (24, 26, 27, 29) of the first and / or second means.

12. Gas sensor according to one of the preceding claims, characterized in that the first and / or second means is preceded by an electrochemical pump cell for regulating the oxygen content in the gas mixture.

13. Gas sensor according to claim 12, characterized in that electrodes (20, 22) of the electrochemical pump cell for regulating the oxygen content are arranged in a different layer plane (11a, 11b) of the gas sensor than the catalyst (38) and / or the electrodes (24 , 26, 27, 28, 29) of the first and / or second means.

14. Gas sensor according to one of the preceding claims, characterized in that a measuring gas chamber (13, 14) surrounded by solid electrolyte layers (11a, 11b, 11c, llcl, llc2) is provided, to which the gas mixture can be supplied via a diffusion resistor (19, 42) and in which the electrodes (20, 24, 26, 27, 29) of the first means, the second means or the electrochemical pump cell for regulating the oxygen content and / or the catalyst (38) are located.

15. Gas sensor according to claim 14, characterized in that the measuring gas space (13) is divided into two areas (40, 44) and contains a diffusion barrier (42) as a subdivision.

16. Gas sensor according to one of the preceding claims, characterized in that a means (36) for steering the diffusing gas mixture is provided.

17. A method for measuring a component of a gas mixture, in particular by means of a gas sensor according to one of claims 1 to 15, characterized in that an excess of a reaction gas is generated from a further component of the gas mixture, which is reacted with the component to be measured that the residual content of the reaction gas after the reaction is determined and that the original concentration of the component to be measured in the gas mixture is inferred from the residual content of the reaction gas.

18. The method according to claim 17, characterized in that the reaction gas is generated from the further component of the gas mixture by reduction or oxidation.

19. The method according to claim 17 or 18, characterized in that the reaction gas is generated from the further component of the gas mixture in an inner region (13, 14) of the gas sensor, which is largely separated from a gas space surrounding the gas sensor.

20. The method according to any one of claims 17 to 19, characterized in that for the determination of a first gas component to be measured, a first reaction gas is generated by reduction and alternately to determine a second gas component to be measured, a second reaction gas is generated by oxidation.

21. The method according to any one of claims 17 to 20, characterized in that the reaction gas is generated in a stoichiometric or volumetric excess with respect to the amount of gas component to be measured.

22. The method according to any one of claims 17 to 21, dadurcii characterized in that the oxygen content of the gas mixture is additionally determined.

23. Use of a gas sensor according to one of claims 1 to 16 and / or a method according to one. of claims 17 to 22 for determining the concentration of a gas component in a gas mixture.

24. Use of a gas sensor according to one of claims 1 to 16 and / or a method according to one of claims 17 to 22 for determining a gas component in the exhaust gas of an internal combustion engine.

25. Use of a gas sensor according to one of claims 1 to 16 and / or a method according to one of claims 17 to 22 for determining nitrogen oxides and / or ammonia.

26. Use of a gas sensor according to one of claims 1 to 16 and / or a method according to one of claims 17 to 22 for monitoring the functionality and / or the loading state of a NO _x storage catalytic converter.