DE102007049713A1 - Sensor element for use in lambda oxygen sensor of motor vehicle, has electrode separated from gas chamber by partition that is selectively permeable for gas component i.e. oxygen, to be detected - Google Patents

Abstract

The sensor element (210) has two electrodes (214, 216) and a solid electrolyte (212) connected with one of the electrodes. The electrode (216) is separated from a gas chamber (114) by a partition (110) that is selectively permeable for a gas component i.e. oxygen, to be detected. The partition has a mixed electronic/ionic conductor that is made of a ceramic/metallic composite material i.e. cermet. The partition has a material that is used as a material of the solid electrolyte. An independent claim is also included for a method for determining a physical characteristic of a gas mixture in a gas chamber .

Description

State of the art

The The invention is based on known sensor elements, which are based on electrolytic Properties of certain solids based, so the ability this solid to conduct certain ions. such Sensor elements are used in particular in motor vehicles, to measure air-fuel gas mixture compositions. Especially are sensor elements of this type under the name "lambda probe" known and play an essential role in the reduction of pollutants in exhaust gases, both in gasoline engines and in diesel technology.

With the so-called air ratio "lambda" (λ) is thereby generally in combustion engineering the relationship between an actually offered air mass and one for the combustion theoretically required (i.e., stoichiometric) Air mass designated. The air ratio is thereby by means of one or more Sensor elements usually at one or more locations in the exhaust system an internal combustion engine measured. Accordingly, "fat" Gas mixtures (ie gas mixtures with a fuel surplus) an air ratio λ <1 whereas "lean" gas mixtures (i.e., gas mixtures with a fuel deficiency) have an air ratio λ> 1. Next Automotive technology will be such and similar Sensor elements in other areas of technology (in particular combustion technology), for example in aviation technology or in the control of burners, z. B. in heating systems or power plants.

Numerous different embodiments of the sensor elements are known from the prior art. One embodiment is, for example, the so-called "jump probe" whose measuring principle is based on the measurement of an electrochemical potential difference between a reference electrode exposed to a reference gas and a measuring electrode exposed to the gas mixture to be measured. In theory, the potential difference between the electrodes, especially at the transition between the rich gas mixture and the lean gas mixture, exhibits a characteristic jump which can be used to actively regulate the gas mixture composition such jump probes, which are also referred to as "Nernst cells" are, for example, in DE 10 2004 035 826 A1 . DE 199 38 416 A1 and DE 10 2005 027 225 A1 described.

alternative or in addition to jump probes also so-called "pump cells" (also called "broadband probes") used in which a electrical "pumping voltage" at two over the solid electrolyte connected electrodes, wherein the "pumping current" is measured by the pumping cell. In contrast to the principle of the jump probes When pumping cells are usually both electrodes with the measured Gas mixture in conjunction. One of the two electrodes (usually over a permeable protective layer) immediately to be measured Exposed to gas mixture. However, the second of the two electrodes is designed such that the gas mixture is not directly to this Electrode can pass, but first penetrate a so-called "diffusion barrier" in order to penetrate into a cavity adjacent to this second electrode to get. As a diffusion barrier is usually a porous ceramic structure with specifically adjustable pore radii used.

kick lean exhaust gas through this diffusion barrier into the cavity a, so by the pumping voltage oxygen molecules at the second, negative electrode electrochemically to oxygen ions reduced by the solid electrolyte to the first, positive Electrode transported and released there as free oxygen again. The Sensor elements are usually in the so-called limit current operation operated, that is in an enterprise, in which the Pumping voltage is chosen such that the through the diffusion barrier incoming oxygen is completely pumped to the counter electrode becomes. In this mode, the pumping current is approximately proportional to the partial pressure of the oxygen in the exhaust gas mixture, see above that such sensor elements often as proportional sensors be designated. In contrast to jump sensors can be such Proportionalsensoren over a comparatively wide Use range for air ratio lambda (broadband sensors).

An example of such a broadband sensor having two electrodes and a solid electrolyte is shown in FIG DE 43 43 748 shown. In this case, an arrangement is proposed in which the electrode reactions are additionally assisted in that in each case a mixed conductor is introduced between the electrodes and the solid electrolyte, which is capable of adsorbing oxygen molecules. The mixed conductor has a conductivity for oxygen ions and for electrons.

In addition to the described principle, in which the first electrode is connected via a protective layer with the gas mixture to be measured, there are still other ways to make broadband sensors. For example, the first Instead electrode should be connected to a separate from the gas mixture reference channel, which is, for example, in connection with the outside air (see, for example DE 38 09 154 C1 ). In this way, this first electrode has a high limiting current. All arrangements can be advantageously produced in planar thick film technology.

In many sensor elements, the sensor principles described above are also combined, so that the sensor elements contain one or more sensors ("cells") operating according to the jump sensor principle and one or more proportional sensors.For example, the principle described above for a pump cell Extending the principle of working "unicellulars" by adding a snap cell (Nernst cell) to a "double cell". Such a structure is for example in EP 0 678 740 B1 described. In this case, by means of a Nernst cell, the oxygen partial pressure is measured in the above-described cavity adjoining the second electrode, and the pumping voltage is adjusted by regulation so that the condition λ = 1 always prevails in the cavity.

Broadband sensor elements according to the above-described limiting current principle in single cell arrangement with two electrodes exposed to the gas mixture, however, have various problems. Thus, a positive pumping current (lean pumping current) with a clear relationship to the oxygen content of the gas mixture is usually measured at a fixed pumping voltage in a lean gas mixture. In the rich gas mixture, however, a positive pumping current is usually measured as well, even if the applied pumping voltage (usually about 600-700 mV) is well below the decomposition voltage of water (about 1.23 V). This positive pumping current is essentially attributable to the molecular hydrogen contained in the gas mixture, which influences the electrochemical potential of the anode, that is to say the first electrode, since the oxygen ions leaving the solid electrolyte instead of molecular oxygen now also form on the first electrode Water can be formed. Similar effects also play a role for other oxygen-supplying redox systems present in the gas mixture, for example CO ₂ / CO. The current is therefore in the range of rich mixtures (fat pump current) by the hydrogen content in the region of the first electrode (eg anode) and the water vapor content (ie in particular the access of the water vapor through the diffusion barrier described above) in the region of the second electrode (eg cathode). The problem now consists, in particular, in that the fat pumping current and the lean pumping current have the same direction electrically, so that a conclusion on the composition of the gas mixture is scarcely possible from the pumping current. In addition to the problems described in the field of rich mixtures, a falsification of the pumping current by the hydrogen is also to be observed in the area of slightly lean exhaust gases, which is already present in this area and provides a positive contribution to the pumping current.

Disclosure of the invention

It Accordingly, a sensor element for determining at least a physical property of a gas mixture in a gas space provided, which in particular in a lambda probe in a Motor vehicle or in other areas of combustion technology can be used and which have the disadvantages described above prevents the known from the prior art devices. In particular, the sensor element can be operated as a broadband sensor with a largely unique characteristic. To this end the current signal in the area of rich gas mixtures is largely suppressed.

A basic idea of the present invention is the reactions taking place in the fatty gas at the anode, such as the reactions CO + O ^2- → CO ₂ + 2e ^- H ₂ + O ^2- > H ₂ O + 2e ^- , to prevent. For this purpose, it is inventively provided to shield a first electrode (which is usually operated as an anode) of oxidizable components from the gas mixture.

Accordingly, the proposed sensor element has at least one first electrode and at least one second electrode and at least one solid electrolyte connecting the at least two electrodes, analogously to the structure of conventional broadband sensors described above. In this case, the at least one first electrode is separated from the gas space by at least one membrane, wherein the at least one membrane is selectively permeable to at least one gas component to be detected. In particular, this at least one gas component may be molecular oxygen, whereas the permeability to the oxidizable components described above is significantly suppressed. For example, the permeability to the oxidizable components may be at least one order of magnitude lower than the permeability to oxygen. For example, the limiting current for oxygen may be about 2 mA, whereas the limiting current The oxidizable components, for example, only about 2-20 microamps. Generally, a ratio of these sizes of in a ratio of 200 to 500 is desirable and preferred. In this way, on the one hand it is ensured that the at least one gas component can be efficiently removed from the at least one first electrode, for example in the form of an oxygen outflow. At the same time, the fuel gas diffusion to the at least one first electrode (anode) is obstructed in the rich gas mode or in the non-equilibrium exhaust gas close to λ = 1. Accordingly, the above-described fuel gas oxidation reactions at the anode are suppressed, and a nearly unique characteristic up to λ = 1.0 is realized.

Of the used above term "membrane" is the furthest Meaning to understand. Thus, on the one hand, the term in the conventional Understood meaning, so also flexible element with a thickness, which is very small compared to the lateral extent of the element. Alternatively, the term "membrane" can also understood in the sense of "layer" without going ahead said dimensional and / or flexibility limitations. Preferred layer thicknesses of the at least one membrane are in the Range between 0.05 mm and 3.0 mm, preferably between 0.1 mm and 1.0 mm.

In particular, mixed electronic / ionic conductors can be used as membranes with selective properties for the various gas components. Such mixed electronic / ionic conductors are known from other fields of materials engineering, such as in the field of fuel cells, and are also referred to as MIEC (mixed ionic electronic conductor). Basically, for example, the materials can be used which in DE 43 43 748 C2 Other embodiments of the production of mixed electronic / ionic conductors, which can be referred to in the present invention with regard to material selection and production as a reference, are disclosed in US Pat US 2004 / 0183005A1 shown. MIECs are mixed conductors which are both internally conductive and electron conducting, preferably the ionic conductivity relates to the at least one gas component to be detected (for example an oxygen ion conductivity for the detection of oxygen ions). For example, it is possible to use ceramic / metallic composite materials, for example so-called "CERMET" materials, Alternatively or additionally, it is also possible to use doped oxide ceramics, in particular perovskite-based and / or fluorite-based oxide ceramics, examples being oxide ceramics which are based on ZrO ₂ and / or or CeO ₂ and / or Y ₂ O _3. In particular, it is possible to use a Tb doping here, as well as mixed ceramics of the oxide ceramics mentioned, such as, for example, a (CeO ₂ ) _X · (Y ₂ O ₃ ) _Y · (ZrO _{2 Z} ceramics, where X, Y and Z are real, advantageously complementary numbers to each other. As a concrete example, a (CeO ₂ ) _0.041 * (Y ₂ O ₃ ) _0.067 * (ZrO ₂ ) _0.892 ceramic is mentioned ,

through the use of the membrane described can be the Measurement signal of broadband sensors in the area of fat gas mixtures strong suppress and in this way a largely unambiguous Realize characteristic. This facilitates the evaluation of the measurement signals especially in the region near λ = 1 strong.

The described sensor element in one of the illustrated embodiments can be advantageously developed in various ways. In particular, these developments relate to the selection of the membrane material. So it has proven to be favorable at first, if materials are used which are compatible with the material of the at least one solid electrolyte. Furthermore it is for usual Sensor elements, in particular for use as lambda probes in the automotive field, advantageous if the at least one membrane has an electronic conductivity in the range of 1/10 1 / Ω to 1/100 1 / Ω, or if neither the electronic nor the ionic conductivity these values are below. In this way can be at usual Probe geometries and operating voltages optimal suppression achieve the measurement signal in the range of rich gas mixtures, the Operation of the probe remains ensured. The exchange coefficient the membrane for oxygen should be sufficient to allow an oxygen flow of preferably 2 mA.

Instead of the at least one membrane directly on the at least one it is preferred to apply one with the at least one first electrode to provide associated cavity, which separated from the at least one gas space by the at least one membrane is. In this way, in the at least one cavity can set a partial pressure of the at least one gas component, which is higher than the partial pressure in the gas space. These Partialdruckdifferenz is the "driving force" for the diffusion of the at least one gas component, in particular the Oxygen, through the at least one membrane. In this way On the one hand, the at least one first electrode on the gas chamber side efficiently screened, but at the same time a large area Diffusion exchange of at least one gas component between Cavity and gas space to be guaranteed.

Furthermore, it is advantageous if the at least one membrane and the at least one first electrode by at least one insulator element are electrically isolated from each other. Also preferably, the at least one solid electrolyte and the at least one membrane are insulated from each other by the at least one insulator element. This isolation prevents a direct electrochemical connection of the at least one first electrode to the fuel gases in the gas space.

Advantageously, furthermore, the at least one second electrode is separated from the gas space by at least one diffusion barrier. In particular, it may, as described above, be a porous ceramic material of high density, whose pore radii are adapted to the application. In particular, a second cavity communicating with the at least one second electrode can also be provided, which is separated from the gas space by the at least one diffusion barrier. Such diffusion barriers are, for example, in Robert Bosch GmbH: "Sensors in the motor vehicle", 2001, page 116 , described in various embodiments. Advantageously, materials with a static pressure dependence k are used, which is at least 1 bar, but is preferably higher. The static pressure dependence refers to the ratio between Knudsendiffusion and gas phase diffusion, which is just the same at k = 1 bar.

The described sensor element in one of the illustrated embodiments can be realized for example in a layer technology, wherein different layer structures can be used. For example, a layer structure with opposite Electrodes are used, wherein the first electrode in a higher layer plane is arranged than the at least a second electrode. Also a layer construction with opposite Electrodes in reverse order is conceivable. Alternatively, you can also be used a layer structure in which the electrodes arranged on the same side of the at least one solid electrolyte are.

advantageously, the sensor element described is operated such that the at least a first electrode is operated as an anode, wherein the at least a second electrode is used as the cathode. It is preferred a pump voltage between 100 mV and 1.0 V, preferably between 300 mV and 800 mV, and more preferably between 600 mV and 700 mV applied between the at least two electrodes, wherein the pumping current is measured by the sensor element. This chosen Pumping voltage is usually sufficient to the sensor element operate in limit current operation, but the pump voltage is simultaneously below the decomposition voltage of water, so that no decomposition of water occur at the second electrode should.

Brief description of the drawings

embodiments The invention is illustrated in the drawings and in the following Description explained in more detail.

It demonstrate:

1 a schematic representation of a transport of oxygen ions through a MIEC membrane;

2 a first embodiment of a sensor element according to the invention with external anode;

3 A second embodiment of a sensor element according to the invention with internal anode; and

4 a third embodiment of a sensor element with adjacent electrodes.

In 1 is a schematic example of a membrane 110 represented, which consists of a MIEC material. The MIEC material shown here is selectively permeable to oxygen ions and is otherwise gas-tight. It should be in the schematic representation in 1 assumed that the membrane 110 an overhead anode compartment 112 (For example, a cavity in front of an anode, see below) separates from a gas space 114 , It is assumed that in the anode compartment 112 a higher partial pressure p _O2 * of the oxygen in the anode compartment 112 is greater than the partial pressure p _O2 ** in the gas space 114 , The essential, at the interfaces and in the membrane 110 ongoing processes or transport reactions are shown as reaction equations. However, these reaction equations are also shown only symbolically, so that no value was placed on stoichiometric completeness.

As a first reaction step takes place at the interface between anode space 112 and membrane 110 (Interfacial 116 in 1 ) an adsorption reaction of molecular oxygen instead. In this case, the membrane material (with the oxygen vacancies V •• / O contained therein) releases two electrons to the oxygen, so that it passes into an ionized state, which symbolically here with OX / O (in the grid built-in oxygen ions) is shown. This ionized oxygen then migrates in the form of oxygen ions which are in the membrane 110 are shown as O ^2- , in the direction of the gas space 114 (Oxygen ion diffusion 118 in 1 ). In this Diffusion step thus becomes the ion-conducting property of the MIEC material of the membrane 110 exploited.

At the opposite interface 120 between membrane 110 and gas room 114 then finds, in reverse, the reaction at the interface 116 Again, an oxidation of the oxygen ions takes place, which thereby their excess electrons to the material of the membrane 110 submit. Subsequently, the oxygen molecules are desorbed in the form of O ₂ , so that again forms molecular, gaseous oxygen, which from the membrane 110 to the gas room 114 is delivered. The excess electrons migrate again towards the interface 116 between anode space 112 and membrane 110 , ie in the opposite direction to the oxygen ion diffusion 118 , This electron movement, which in 1 symbolically with the reference number 122 Thus, the second essential property of the MIEC material, ie the electronic conductivity, is exploited. While the oxygen ion diffusion 118 is driven by the partial pressure difference of the oxygen between anode space 112 and gas room 114 , the electron movement 122 driven by the potential difference between the interfaces 116 and 120 , which is due to the oxygen ion diffusion 118 builds. The system thus aims at an equilibrium state in which the partial pressure difference is reduced as in a short-circuited concentration cell (Nernst cell).

In 2 is an embodiment of a sensor element 210 shown, which can be used for example in a lambda probe to determine the composition of combustion chamber gases. In this case, a layer structure is used, in which a solid electrolyte 212 with an internal cathode 214 and an external anode 216 is contacted. In this case, a radially symmetrical structure is used, in which of the gas space 114 Gas mixture through a central bore 218 and a diffusion barrier 220 in a cathode compartment 222 can get. As a diffusion barrier 220 For example, as described above, porous materials of high density, such as ceramic materials, are commonly used.

According to the oxygen partial pressure in the cathode compartment 222 and according to one between cathode 214 and anode 216 applied pump voltage migrate oxygen ions through the solid electrolyte 212 to the anode 216 , wherein the pumping current is measured and (in limiting current operation, see above) a measure of the oxygen concentration in the gas mixture of the gas space 114 represents. At the anode 216 molecular oxygen is formed again. Above the anode 216 is a cavity 224 arranged, which the anode space 112 in 1 equivalent. In sensor operation is usually the oxygen partial pressure in this cavity 224 over the anode 216 greater than the oxygen partial pressure in the gas mixture in the gas space 114 , The cavity 224 is through a membrane 110 which the cavity 224 and the anode 216 covering a large area, from the gas space 114 separated, so that in 1 proceed described reactions. Due to the concentration gradient between the oxygen concentration in the cavity 224 and over the anode 216 (Partial pressure usually about 1 bar to 100 bar) and the oxygen concentration in the gas space 114 (Partial pressure usually a maximum of about 0.2 bar) transports the MIEC membrane 110 at the anode 216 arising oxygen electrochemically selective in the gas space 114 , Penetration of fuel gas components (eg H ₂ , CO, etc.) from the gas space 114 in the cavity 224 However, due to the blocking properties of the membrane 110 avoided. Accordingly, fatty gas reactions at the anode 216 suppressed. This results in a unique characteristic in the range air> lambda ≥ 1. Accordingly, the sensor element 210 a cost-effective, designed as protozoa without air reference sensor element 110 which can be used, for example, in diesel vehicles for measuring mixed gas compositions.

As described above, it is about the anode 216 electrochemically from the gas space 114 to separate, usually required, the membrane 110 electrically from the anode 216 to decouple. For this purpose, in the layer plane of the anode 216 an insulator element 226 provided in the form of an insulator layer, which is the membrane 110 electrically against the anode 216 isolated. For example, as the material of the insulator element 226 an electrically insulating polymer (if this has sufficient temperature stability) and / or an inorganic insulator material (eg., Alumina, Al ₂ O ₃ ) are used.

The sensor element 210 in the embodiment according to 2 is constructed in a radially symmetrical arrangement. Alternatively, however, could also be another arrangement, such as a linear design, are used, analogous to, for example, the below in 4 illustrated design. It is essential that the dimensioning and the structure of the sensor element 210 be tuned so that the at the anode 216 resulting amount of oxygen by diffusion through the membrane 110 into the exhaust gas in the gas space 114 can be transported. Accordingly, for example, the parameters influencing the oxygen pumping current (such as, for example, the geometry and / or composition of the diffusion barrier 220 , the temperature of the sensor element 210 and the like) the diffusivity of the MIEC membrane 110 be adjusted. For example, NEN can for this purpose Limit currents of preferably 0.1-3 mA, more preferably from 1.0 to 1.8 mA, can be set. Furthermore, it should be noted that usually with increasing air ratio λ of the oxygen partial pressure in the cavity 224 must rise. The mechanical stability of the membrane must not be exceeded.

Below the layer structure cathode 214 - solid electrolyte 212 - anode 216 is separated by additional substrate layers 228 , a heating element 230 arranged in the form of a heating foil. This heating element 230 includes insulator layers 232 and heating resistors 234 , for example in the form of resistance loops. insulator layers 232 and heating resistors 234 are on a substrate 236 arranged. Also other types of construction of heating elements 230 are usable. The heating element 230 serves to set the optimum operating temperature, for example, the diffusion properties through the diffusion barrier 220 to selectively influence, which in turn, for example, the limiting current can be adjusted.

In 2 is a second embodiment of a sensor element 210 represented, which also again a layer structure with each other on a solid electrolyte 212 opposite cathode 214 and anode 216 includes. However, the selected layer structure differs essentially from the exemplary embodiment in two points 2 , Thus, on the one hand, a structure is chosen in which the anode 216 inside (ie on the one from the gas room 114 opposite side of the solid electrolyte 212 ), and the cathode 214 on the top, so on the gas space 114 facing side of the solid electrolyte 212 , Again, there is a cathode compartment 222 as a cavity above the cathode 214 provided, which by a gas-impermeable cover layer 310 from the surrounding gas space 114 is disconnected. In addition, again, between solid electrolyte 212 and cover layer 310 , a diffusion barrier 220 intended.

A second difference is that in the embodiment according to 3 no radially symmetric design was chosen, but a linear design. This means in particular that the construction is not symmetrical about the hole 218 is constructed by which gas mixture from the gas space 114 to the membrane 110 , and from there into the anode room 112 can get.

Otherwise, the structure of the sensor element corresponds 210 according to the embodiment in 3 largely according to the structure 2 , Again, a heating element 230 with insulator layers 232 , Heating resistors 234 and substrate 236 provided, which is constructed for example as a film element and which the temperature of the sensor element 210 can adjust or regulate. In this way, for example, as a result of the immediate proximity between the heating element 230 and membrane 110 which specifically influence the line properties (ion conduction and / or electron conduction) of the membrane material.

In 4 is finally a third embodiment of a sensor element according to the invention 210 shown in perspective. Again, a layer structure is used, which realizes a linear arrangement. In this embodiment, however, are cathode 214 and anode 216 on the same side of the solid electrolyte 212 arranged. cathode 214 and anode 216 are thereby of electrode contacts (cathode contact 410 and anode contact 412 ) in the form of on the solid electrolyte 212 contacted printed conductors. Furthermore are about cathode 214 and anode 216 again the cathode compartment 222 or the anode space 112 provided, this against the surrounding gas space 114 again through a cover layer 310 are separated. Furthermore, there is a diffusion of gas mixture into the cathode compartment 220 from the gas space 114 to enable, in turn, a diffusion barrier 220 in front of the cathode room 222 , intended. Likewise, again, a membrane 110 provided a selective gas diffusion from the anode compartment 112 in the gas space 114 to enable. Under the membrane 110 is to isolate the membrane 110 opposite the solid electrolyte 212 and the anode 216 , an insulator element 226 provided in the form of an insulator layer. In that regard, the operation of the operation of the structures described above in the corresponds 2 and 3 , In this case, the polarity of anode 216 (with anode compartment 112 and membrane 110 ) and cathode 214 (with cathode compartment 220 ) in a further preferred embodiment also be interchanged with respect to the above description.

Again, according to the embodiment as well 3 a heating element 230 provided, with heating resistors 234 which is between two insulator layers 232 embedded and on a substrate 236 are applied. The contacting of the heating resistors 234 takes place on the gas space 114 remote side of the sensor element 210 via two heating contacts 414 which via vias 416 in the substrate 236 with the heating resistors 234 are connected.

Finally, it should be noted that the sensor element 210 in the embodiments in the 2 . 3 and 4 specifically described for the measurement of oxygen. However, the inventive idea can also be analogously applied to the measurement of other types of gases, which are based on the measurement of a pumping current through a solid electrolyte. It is essential that forms on one of the two electrodes (but mutatis mutandis this can also be done on two electrodes) during "pumping" a gas which is selectively abtranspor benefits from the range of the respective electrode, without certain other gases from the surrounding gas space 114 can get to the respective electrode. Accordingly, then, for example, the materials of the solid electrolyte 212 and / or the membrane 110 to choose.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• DE 102004035826 A1 [0003]
• - DE 19938416 A1 [0003]
• DE 102005027225 A1 [0003]
• - DE 4343748 [0006]
• - DE 3809154 C1 [0007]
• EP 0678740 B1 [0008]
• - DE 4343748 C2 [0014]
• US 2004/0183005 A1 [0014]

Cited non-patent literature

• - Robert Bosch GmbH: "Sensors in motor vehicles", 2001, page 116 [0019]

Claims (12)

Sensor element ( 210 ) for determining at least one physical property of a gas mixture in a gas space ( 114 ), in particular for determining an oxygen concentration in the exhaust gas of an internal combustion engine, having at least one first electrode ( 216 ) and at least one second electrode ( 214 ) and at least one the at least one first electrode ( 216 ) and the at least one second electrode ( 214 ) connecting solid electrolyte ( 212 ), characterized in that the at least one first electrode ( 216 ) from the gas space ( 114 ) is separated by at least one membrane ( 110 ), wherein the at least one membrane ( 110 ) is selectively permeable to at least one gas component to be detected. Sensor element ( 210 ) according to the preceding claim, characterized in that the at least one gas component is oxygen. Sensor element ( 210 ) according to one of the two preceding claims, characterized in that the at least one membrane ( 110 ) has at least one mixed electronic / ionic conductor. Sensor element ( 210 ) according to the preceding claim, characterized in that the at least one mixed electronic / ionic conductor comprises at least one of the following materials: a ceramic / metallic composite material, in particular a CERMET; a doped oxide ceramic, in particular a perovskite-based and / or fluorite-based oxide ceramic; an oxide ceramic based on ZrO ₂ and / or CeO ₂ and / or Y ₂ O ₃ , in particular with a Tb doping; a (CeO ₂ ) _X • (Y ₂ O ₃ ) _Y • (ZrO ₂ ) _Z ceramic, wherein X, Y and Z are real numbers complementary to one, preferably one (CeO ₂ ) _0.041 * (Y ₂ O ₃ ) _0.067 × (ZrO ₂ ) _0.892 ceramic, in particular doped with Tb. Sensor element ( 210 ) according to one of the preceding claims, characterized in that the at least one membrane ( 110 ) has at least one material which is also used as the material of the at least one solid electrolyte ( 212 ) is used. Sensor element ( 210 ) according to one of the preceding claims, characterized in that the at least one membrane ( 110 ) has at least one of the following properties: - the at least one membrane ( 110 ) has an electronic conductivity, which does not fall below a value of 1/100 1 / Ω, preferably does not fall below a value of 1/10 1 / Ω; and - the at least one membrane ( 110 ) has an ionic conductivity which does not fall below a value of 1/100 1 / Ω, preferably does not fall below a value of 1/10 1 / Ω. Sensor element ( 210 ) according to one of the preceding claims, characterized in that the at least one membrane ( 110 ) in the direction of the passage of the at least one gas component to be detected has an expansion in the range between 0.05 mm and 3.0 mm, preferably between 0.1 mm and 1.0 mm. Sensor element ( 210 ) according to one of the preceding claims, characterized in that at least one with the at least one first electrode ( 216 ) communicating cavity ( 224 ), wherein the at least one cavity ( 224 ) from the gas space ( 114 ) is separated by the at least one membrane ( 110 ). Sensor element ( 210 ) according to one of the preceding claims, characterized in that the at least one membrane ( 110 ) and the at least one first electrode ( 216 ) by at least one isolator element ( 226 ) are electrically isolated from each other, wherein preferably also the at least one solid electrolyte ( 212 ) and the at least one membrane ( 110 ) by the at least one insulator element ( 226 ) are isolated from each other. Sensor element ( 210 ) according to one of the preceding claims, characterized in that the at least one second electrode ( 214 ) from the gas space ( 114 ) is separated by at least one diffusion barrier ( 220 ), in particular a porous diffusion barrier ( 220 ), wherein preferably at least one with the at least one second electrode ( 214 ) associated second cavity ( 222 ) is provided, which of the gas space ( 114 ) through the at least one diffusion barrier ( 220 ) is disconnected. Sensor element ( 210 ) according to one of the preceding claims, characterized by at least one of the following layer constructions: a layer structure in which the at least one first electrode ( 216 ) is arranged in a higher layer plane than the at least one second electrode ( 214 ), wherein the at least one first electrode ( 216 ) and the at least one second electrode ( 214 ) on opposite sides of the at least one solid electrolyte ( 212 ) are arranged; A layer structure in which the at least one first electrode ( 216 ) is arranged in a deeper layer plane than the at least one second electrode ( 214 ), wherein the at least one first electrode ( 216 ) and the at least one second electrode ( 214 ) on opposite sides of the at least one solid electrolyte ( 212 ) are arranged; A layer structure in which the at least one first electrode ( 216 ) and the at least one second electrode ( 214 ) on the same side of the at least one solid electrolyte ( 212 ) are arranged. Method for determining at least one physical property of a gas mixture, in particular for determining an oxygen concentration in the exhaust gas of an internal combustion engine, using a sensor element ( 210 ) according to one of the preceding claims, characterized in that the at least one first electrode ( 216 ) is operated as an anode that the at least one second electrode ( 214 ) is operated as a cathode and that a pumping voltage between 100 mV and 1.0 V, preferably between 300 mV and 800 mV and particularly preferably between 600 mV and 700 mV, between the at least one first electrode ( 216 ) and the at least one second electrode ( 214 ) is applied, wherein a pumping current through the sensor element ( 210 ) is measured.