WO2009030547A1

WO2009030547A1 - Method and sensor element for determining the water and/or carbon dioxide concentration in a gas

Info

Publication number: WO2009030547A1
Application number: PCT/EP2008/058815
Authority: WO
Inventors: Thomas Wahl; Bernd Schumann; Thomas Classen
Original assignee: Robert Bosch Gmbh
Priority date: 2007-09-05
Filing date: 2008-07-08
Publication date: 2009-03-12
Also published as: EP2191262A1; DE102007042219A1

Abstract

The present invention relates to a method for determining the water or carbon dioxide concentration in a gas with a sensor element comprising a solid electrolyte body (2), a first electrochemical pump cell comprising a first inner pump electrode (12a, 12b), which is arranged in a first inner gas chamber (11) of the solid electrolyte body (2), and a first outer pump electrode (13) which is arranged on that side of the solid electrolyte body (2) which faces the measurement gas, wherein the first inner pump electrode (12a, 12b) and the first outer pump electrode (13) are connected to one another by means of a current source, and a second electrochemical pump cell comprising a second inner pump electrode (22a, 22b), which is arranged in a second inner gas chamber (21) of the solid electrolyte body (2), and a second outer pump electrode (23) which is arranged on that side of the solid electrolyte body (2) which faces the measurement gas, wherein the second inner pump electrode (22a, 22b) and the second outer pump electrode (23) are connected to one another by means of a current source. In said method: the first pump cell is operated with a pump voltage which only reduces molecular oxygen to O2- ions, and the second pump cell is operated with a higher pump voltage which both reduces molecular oxygen to O2- ions and cleaves water to form O2- ions, and the water concentration in the measurement gas is determined from the difference between the pump currents of the two pump cells and/or from the voltage difference resulting from the pump currents; or the first pump cell is operated with a pump voltage which both reduces molecular oxygen to O2- ions and cleaves water to form O2- ions, and the second pump cell is operated with a higher pump voltage which both reduces molecular oxygen to O2- ions and cleaves water to form O2- ions and cleaves carbon dioxide to form O2- ions, and the carbon dioxide concentration in the measurement gas is determined from the difference between the pump currents of the two pump cells and/or from the voltage difference resulting from the pump currents. The invention also relates to a sensor element for determining the water and/or carbon dioxide concentration in a gas and to the use thereof.

Description

ROBERT BOSCH, 70432 Stuttgart

title

Method and sensor element for determining the water and / or carbon dioxide concentration in a gas

description

The present invention relates to a method and a sensor element for determining the water and / or carbon dioxide concentration in a gas and their use.

State of the art

Modern vehicles with internal combustion engines have an exhaust aftertreatment for exhaust gas purification. While in gasoline engines, the exhaust gas cleaning with the so-called three-way catalyst is considered dominated, the exhaust gas purification in a diesel engine is relatively complex. An alternative to current direct injection diesel engines may in the future be diesel engines that use homogeneous combustion, at least in the partial load range, to significantly reduce emissions or alter the composition of the exhaust gas compared to conventional combustion processes such that the primary emissions of the engine become easier to be eliminated. Such combustion processes are summarized by the term "homogeneously charched combustion ignition" (HCCI) These combustion processes operate with a high exhaust gas recirculation rate, but also react very sensitively to the last mentioned method, for which it is particularly advantageous if the exhaust gas recirculation rate can be measured directly. This can be done using the guide components water or carbon dioxide, which represent a measure of the exhaust gas recirculation rate A water sensor (H ₂ θ sensor) or a carbon dioxide sensor (CO ₂ sensor, carbon dioxide sensor) can be located in the exhaust gas recirculation or in the air intake tract The combination of air supply and exhaust gas recirculation must be accommodated in such sensors towards condensed water in the parked vehicle and have low response times in the range «1 s.

It is known that a carbon dioxide or water concentration measurement can be carried out by light absorption in the infrared range. However, with this principle

Dirt on the optical components (e.g., the light window) which leads to a decrease in sensitivity or signal drift over the operating time, which is why such sensors are not applicable in the field of an internal combustion engine.

The state of the art is also the determination of water in gaseous media by a capacitive measurement. Typically, a polymer membrane, whose reversible water absorption correlates with the water content of the surrounding atmosphere, is brought into the electric field between two electrodes. Water absorption increases the dielectric and thus increases the capacitance of the capacitor. However, such sensors are not in the range of. Due to the very limited operating temperature of the polymer material

Internal combustion engine applicable.

In addition, carbon dioxide sensors are known that use high-temperature ionic conductors such as complex phosphates (NASICON) or Na-β "-aluminate for carbon dioxide determination. The basic principle of such sensors is based on a powerless

Voltage measurement on a galvanic cell with alkaline ion-conducting solid electrolytes. However, such carbon dioxide sensors can be irreversibly damaged both during operation and in the non-operated state by the action of liquid water and leaching of alkali or alkaline earth metal ions.

Disclosure of the invention

Advantages of the invention

The inventive method for determining the concentration of water and / or carbon dioxide in a gas with the characterizing features of the independent claim has the advantage that based on the inventively determined water or carbon dioxide concentration, the exhaust gas recirculation rate of a on a "homogeneous charched combustion ignition" Method-based internal combustion engine can be determined directly, which has an advantageous effect on its operation Sensor elements good resistance in the exhaust gas and condensed water in the parked vehicle and low response times of «1 s and in particular <100 ms on. Furthermore, sensor elements according to the invention can advantageously be produced in large numbers.

Brief description of the drawings

Ausffihrungsbeispiele the inventions are illustrated in the drawings and explained in more detail in the following description.

Fig.l shows a longitudinal section of a conventional oxygen broadband probe in

planar;

FIG. 2 a shows a first embodiment of a sensor element according to the invention with two pump cells, wherein the electrodes of the two pump cells apart from the

Electrically separated solid electrolyte Connection contacts are in longitudinal section perpendicular to the main surfaces; FIG. 2b shows the sensor element from FIG. 2a in a longitudinal section parallel to the main faces along the line Al-Al '; FIG. 2c shows the sensor element from FIG. 2a in a longitudinal section parallel to the main faces along the line A2-A2 '; FIG. Fig. 2d shows a schematic circuit diagram of a possible embodiment of a

Operating electronics for the sensor element of FIGS. 2a and 2b; 3a shows a second embodiment of a sensor element according to the invention with two pump cells, wherein the inner pumping electrodes of the two pump cells are electrically conductively connected to one another, in longitudinal section perpendicular to the main surfaces;

Fig. 3b shows the sensor element of Fig. 3a in longitudinal section parallel to the Hauptfiächen along the line B-B '; 3c shows a schematic circuit diagram of a possible embodiment of a

Operating electronics for the sensor element from FIGS. 3a and 3b;

4a shows a third embodiment of a sensor element according to the invention with two pump cells and a Nernst cell, wherein the inner pumping electrodes of the two pump cells are electrically conductively connected to one another, in longitudinal section perpendicular to the main surfaces; - A -

Fig. 4b shows the sensor element of Fig. 4a in longitudinal section parallel to the major surfaces along the line C-C; 4c shows a schematic circuit diagram of a possible embodiment of a

Operating electronics for the sensor element from FIGS. 4a and 4b; 5a shows a fourth embodiment of a sensor according to the invention with two pump cells and two Nernst cells, wherein the inner pumping electrodes of the two pump cells are electrically conductively connected to one another, in longitudinal section perpendicular to the main surfaces;

Fig. 5b shows the sensor element of Fig. 5a in longitudinal section parallel to the major surfaces along the line D-D ';

Fig. 5c shows a schematic circuit diagram of a possible embodiment of a

Operating electronics for the sensor element from FIGS. 5a and 5b; Fig. 6 shows I / U characteristics for the cathodic oxygen reduction of 20.9% by volume O ₂ in

N ₂ against air at various electrodes comprising platinum and / or gold; Fig. 7 shows the dependence of the decomposition voltage of 20.9% by volume of O ₂ in N ₂ against

Air from the gold content of the cathode; Fig. 8 shows the dependence of the decomposition voltage of 3 vol .-% H ₂ O in N ₂ against

Air from the gold content of the cathode;

9 shows the dependence of the decomposition voltage of 7% by volume, 28% by volume and 50% by volume of CO ₂ in N ₂ against air on the gold content of the cathode.

FIG. 1 shows a longitudinal section of a conventional oxygen broadband probe in planar technology, for example a universal lambda probe (LSU). A conventional oxygen broadband probe is based on a ceramic base body 2 with a

Gas inlet opening 3 and a separated by a diffusion barrier 4 inner gas space. In the inner gas space an inner pump electrode pair IIa, IIb is arranged, which is connected together with the arranged on the outside of the ceramic body 2 outer pumping electrode 8 via terminal contacts 7 to a power source and forms a pumping cell. In addition, the inner pumping electrode IIa, IIb forms over

Terminal contacts 7 with a reference electrode 10 a Nernst cell. As shown in FIG. 1, the inner pump electrode pair IIa, IIb, the outer pumping electrode 8 and the reference electrode 10 are connected to surfaces for electrical connection contact 7 via separate lines. In addition, the outer pumping electrode 8 is covered by a protective layer 9 in order to protect it from abrasion and / or deposits.

Further, a conventional oxygen broadband probe has an electrical insulation 6 insulated heating device 5 with two connection contacts 7a, 7b for connection to a power source.

2a, 2b and 2c show a first embodiment of a sensor element according to the invention with two pump cells, wherein the electrodes of the two pump cells are electrically isolated apart from the solid electrolyte body 2, in longitudinal section perpendicular to the main surfaces, in longitudinal section parallel to the main surfaces along the line Al -Al 'and in longitudinal section parallel to the major surfaces along the line A2-A2'. 2 a and 2 b show that the two electrochemical pumping cells of a sensor element according to the invention are arranged in such a way that the measurement gas is conveyed via a gas inlet opening 10 in a solid electrolyte body 2 and via two, in particular identical, diffusion-limiting porous structures 16, 26, a so-called diffusion barrier a first 11 and a second gas space 21 of the electrochemical pumping cells can penetrate. The gas inlet opening 10 is in the context of this inventive embodiment shape designed in the form of a main surface to the main shaft, which branches in the interior of the solid electrolyte 2 T-shaped and connects to two parallel to the main surface arranged channels, which the diffusion barriers 16, 26 and the inner Gas spaces 11, 21 include. A first 12 a, 12 b and a second inner 22 a, 22 b pumping electrode (IPE), which adjoins an oxygen ion-conducting solid electrolyte body 2, are arranged in each of these inner gas spaces 11, 21. As the figures 2a and 2b show the inner set

Pumping electrodes 12a, 12b; 22a, 22b in the context of this embodiment according to the invention each consist of a pair of electrodes 12a, 12b placed on the two main surfaces of the respective inner gas space 11, 21; 22a, 22b together, which are connected to each other electrically conductive and low impedance. On the gas to be measured facing the outer side of the solid electrolyte body 2, in particular the outer side of the solid electrolyte body 2 in the

Gas inlet opening 10 is recessed, a first 13 and a second 23 outer pumping electrode (APE) are arranged, which via a power source with the inner pumping electrodes (IPE) 12a, 12b; 22a, 22b are connected and together with the associated first and second inner pumping electrode (IPE) 12a, 12b; 22a, 22b form a first and second electrochemical pumping cells. Within the scope of this embodiment, both the outer 13, 23 and the inner 12 a, 12 b; 22a, 22b pump electrodes of the two pumping cells each have separate lines and connection contacts 32a, 32b, 32c, 32d. Thus, at least a relatively high-impedance decoupling of the two measuring cells can be accomplished. In addition, the outer pumping electrodes 13, 23 in the context of this embodiment are covered by a protective layer 14, 24 in order to protect them from corrosion. As shown in Fig. 2c, this embodiment of the invention further comprises an electrical Isolation 31 insulated heater 30 with two connection contacts 33a, 33b for connection to a power source, which is arranged parallel to the main surface in the solid electrolyte body 2 below the plane of the electrochemical pumping cells. Preferably, the measurement gas which reaches the pump cells has an excess of oxygen.

To determine the water concentration in a measurement gas, the first pump cell is operated such that only molecular oxygen is cathodically reduced to O ions on the first inner pump electrode 12a, 12b. The O ions are then transported through the heated solid electrolyte body 2, oxidized back to molecular oxygen at the first outer pumping electrode 13 and released to the gas. The one for the first

Pump cell to be applied voltage must fall below the decomposition voltage of water in the limiting current case. This voltage is approximately 1 V in the case of planar pump cells (Ch pumping voltage in the limiting current case plus the decomposition voltage of water).

Furthermore, the second pumping cell for determining the water concentration in a measurement gas is operated in such a way that molecular oxygen is cathodically reduced at the second inner pumping electrode 22a, 22b and water is split. The O ^{2 "} ions are then transported through the heated solid electrolyte body 2, oxidized back to molecular oxygen at the second outer pumping electrode 23 and delivered to the measurement gas, the pumping voltage of the second pumping cell being> 500 mV to <800 mV above that of the first pumping cell.

This ensures that not only molecular oxygen, but also water is split and contributes to the ion current (pumping current). Since the difference between the pumping currents of the first and second pumping cells and the voltages generated therefrom is proportional to the water concentration in the measuring gas, a measurement of these pumping currents or the voltages generated can be used to determine the water concentration in the measuring gas.

In analogy to the determination of the water concentration in a measurement gas, the first pump cell for determining the carbon dioxide concentration in a measurement gas is operated in such a way that molecular oxygen is cathodically reduced and water is split at the first inner pump electrode 22a, 22b. The voltage to be applied to the first pump cell must be below the decomposition voltage of carbon dioxide in the limiting current case. In the case of planar pump cells, this voltage to be undershot is approx.> 1200 mV to <1800 mV (Ch / FfcO pump voltage in the limiting current case plus the decomposition voltage of carbon dioxide). Furthermore, the second pumping cell for determining the carbon dioxide concentration in a measuring gas is operated in such a way that molecular oxygen is cathodically reduced at the second inner pumping electrode 22a, 22b and water and carbon dioxide are split. The O ^{2 ~} ions are then transported through the heated solid electrolyte body 2 analogously to the already explained method for determining the water concentration in a measurement gas, oxidized again to molecular oxygen at the second outer pump electrode 23 and delivered to the measurement gas. The pumping voltage of the second pumping cell is> 100 to <300 mV above that of the first pumping cell. This ensures that not only molecular oxygen is reduced and water is split, but also the cleavage of carbon dioxide contributes to the ion current (pumping current). Since the difference between the pumping currents of the first and second pumping cells and the voltages generated therefrom is proportional to the carbon dioxide concentration in the measuring gas, a measurement of these pumping currents or of the voltages generated can be used to determine the carbon dioxide concentration in the measuring gas.

Both in the context of this first embodiment and the second, third and fourth embodiment explained in connection with FIGS. 3 a to 5 c, the outer 13, 23 and inner 12 a, 12 b; 22a, 22b pump electrodes, a platinum group metal or a platinum group metal alloy, in particular selected from the group comprising platinum, palladium, rhodium, iridium, and / or ruthenium. Preferably, that is inner

Pump electrode at the both reduced oxygen and water to be split, from a platinum group metal-gold alloy, in particular with a gold content of> 0.1 wt .-% to <10 wt .-%, formed.

FIG. 2 d shows a schematic circuit diagram of a possible embodiment of a

Operating electronics for the sensor element of Fig. 2a, 2b and 2c. Within the scope of this embodiment, two current sources provide the electrical pumping currents with the pumping voltages already explained. The operational amplifiers OVl and OV2 amplify the voltage dropped across the measuring resistors and the voltage difference between the two is determined. This voltage difference serves as a measuring signal and is the

Water or carbon dioxide concentration in the sample gas proportional.

3a and 3b show a second embodiment of a sensor element according to the invention with two pumping cells, wherein the inner pumping electrodes of the two pumping cells are electrically conductively connected to one another, in longitudinal section perpendicular to the main surfaces and in

Longitudinal section parallel to the main surfaces along the line B-B '. The shown in Fig. 3a and 3b second embodiment of a sensor element according to the invention has the same elements and a similar element arrangement as the first, already explained in connection with FIGS. 2a to 2d, embodiment of a sensor element according to the invention. As in the first embodiment of a sensor element according to the invention, the outer pumping electrodes 13, 23 of the two pumping cells each have separate lines and

Connection contacts 32a, 32b. However, the second embodiment differs from the first embodiment in that the first 12a, 12b and the second 22a, 22b inner pumping electrodes of the first and second pumping cells are connected to each other via an electric line and via a single further electrical line with a single electrical Terminal contact 32e are connected.

The mode of operation of the second embodiment of a sensor element according to the invention of the pumping cells corresponds to the mode of operation explained in connection with the first embodiment.

3c shows a schematic circuit diagram of a possible embodiment of an operating electronics for the sensor element from FIGS. 3a and 3b. As a comparison of FIG. 3c with FIG. 2d shows, the operating circuit of the second embodiment of a sensor element according to the invention is similar except that the operational amplifiers OV1, OV2, measuring resistors RM _ΘSS , I, Rλfess, 2 and internal pumping electrodes IPE1, IPE2 to ground are laid, the operation circuit of the first embodiment.

4a and 4b show a third embodiment of a sensor element according to the invention two pumping cells and a Nernst cell, wherein the inner pumping electrodes of the two pumping cells are electrically conductively connected to each other, in a longitudinal section perpendicular to the

Principal surfaces and in longitudinal section parallel to the major surfaces along the line C-C. The third embodiment of a sensor element according to the invention differs from the second embodiment shown in FIGS. 3a and 3b in that a first reference electrode (RE) 15 is arranged co-planar with the first inner pumping electrode 12a, 12b via an electrical line connected to a terminal contact 32f. The

Reference electrode 15 forms a Nernst cell with the first inner pumping electrode 12a, 12b. The reference electrode 15 is exposed to a gas atmosphere of high, constant oxygen content. This can be achieved by a direct supply of air through a corresponding channel structure or via a so-called pumped reference, in which from the first inner pumping electrode 12a, 12b to the reference electrode 15 a smaller

Oxygen pump current is driven done. If the first pump cell operated such that the molecular oxygen in the first inner pumping electrode 12a, 12b is cathodically reduced and the resulting O ^{2 "ions} by the heated solid electrolytic body 2 is transported, is oxidized at the first external pump electrode 13 back to molecular oxygen and Due to the oxygen partial pressure difference between the first inner pumping electrode 12a, 12b and the reference electrode 15, a Nernst voltage can be measured in accordance with Nernst's law For example, the pumping voltage of the first pumping cell should be controlled in such a way that the Nernst cell reaches a Nernst voltage of 450 mV.

Furthermore, if the second pump cell is operated in such a way that the molecular oxygen is cathodically reduced and water is split at the second inner pumping electrode 22a, 22b and the resulting O ^{2 "} ions are then transported through the heated solid electrolyte body 2, then again at the second outer pumping electrode 23 oxidized molecular oxygen and delivered to the sample gas, so the oxygen partial pressure in the inner gas chamber 21 of the second pump cell decreases.The pumping voltage of the second outer pumping electrode 23 should be> 500 mV to <800 mV above that of the first pumping cell The difference of the pumping currents of the first and second pumping cell and the voltages generated therefrom is proportional to the water concentration in the measuring gas, so that a measurement of this Pump currents or the voltages generated n can be used to determine the water concentration in the sample gas.

In order to determine the carbon dioxide concentration in a measuring gas, the first pumping cell of the third embodiment is operated in such a way that molecular oxygen is cathodically reduced at the first inner pumping electrode 22a, 22b and water is split. For example, the pumping voltage of the first pumping cell should be controlled in such a way that the Nernst cell reaches a Nernst voltage of> 950 to <1250 mV. Furthermore, the second pumping cell for determining the carbon dioxide concentration in a measurement gas is operated in such a way that molecular oxygen is cathodically reduced at the second inner pumping electrode 22a, 22b and water and carbon dioxide are split. The O ^{2 "} - ions are then analogous to the already described method for determining the water concentration in a measuring gas transported through the heated solid electrolyte body 2, oxidized at the second outer pumping electrode 23 back to molecular oxygen and delivered to the sample gas. The pumping voltage of the second pumping cell is> 300 to <600 mV above that of the first pumping cell. This ensures that not only molecular oxygen is reduced and water is split, but also the cleavage of carbon dioxide to the ionic current

(Pumping current) contributes. Since the difference between the pumping currents of the first and second pumping cells and the voltages generated therefrom is proportional to the carbon dioxide concentration in the measuring gas, a measurement of these pumping currents or of the voltages generated can be used to determine the carbon dioxide concentration in the measuring gas.

4c shows a schematic circuit diagram of a possible embodiment of an operating electronics for the sensor element from FIGS. 4a and 4b. The operational amplifier OV1 shown in the operating circuit in Fig. 4c is connected as a comparator and sets the voltage at the output so that at the first outer pumping electrode 13, a voltage is applied which leads to such a pumping current that the inverting input the same

Has voltage, as the non-inverting. For this purpose, the amount of voltage in the Nernst cell must be the same as the reference voltage. The Nernstzellenspannung and thus the partial pressure of the gas molecules are adjusted via the driven by the pumping voltage pumping current through the first pump cell. OV2.1 decouples the high-impedance pumping voltage from the subsequent circuit as an impedance converter. OV2.2 is connected together with Rl, R2 and R3 as an inverting adder and adds to the voltage of the first outer pumping electrode 13, the voltage L ¹ _ReE , wherein the over the ratio of the resistors Rl, R2 and R3 to be set gain factor is one. The pump current is again inverted in the same direction as the pump current from OVl .1 via the inverter OV2.3. U _Reß has a value between 500 and 800 mV. As a result, the second outer pumping electrode 13 receives a pumping voltage higher by this amount than the first outer pumping electrode 23, whereby a pumping current is driven through the second pumping cell, which is due to the decomposition of oxygen and water. The voltage drops at the measuring resistors R _Mess are converted into voltage signals by the operational amplifiers OVl.2 and OV 2.4 and the voltage difference between the two is determined. This voltage difference serves as a measurement signal and is proportional to the water concentration in the sample gas.

5a and 5b show a fourth embodiment of a sensor element according to the invention with two pump cells and two Nernst cells, wherein the inner pumping electrodes of the two pump cells are electrically conductively connected to one another, in longitudinal section perpendicular to the

Principal surfaces and in longitudinal section parallel to the major surfaces along the line D-D '. The Fourth embodiment of a sensor element according to the invention differs from the third, shown in Fig. 4a and 4b embodiment, that co-planar to the second inner pumping electrode 22a, 22b, an additional second reference electrode (RE) 25 is arranged, which via an electrical line connected to a terminal contact 32g. Co-planar with the inner pumping electrodes 12a, 12b; 22a, 22b is therefore one each

Reference electrode (RE) 15; 25 arranged. Thus, the reference electrode 15 with the first inner pumping electrode 12a, 12b forms a first Nernst cell and the reference electrode 25 with the second inner pumping electrode 22a, 22b forms a second Nernst cell. The first and second reference electrodes 15; 25 are exposed to a gas atmosphere with high, constant oxygen content. This can be achieved by a direct supply of air through a corresponding channel structure or via a so-called pumped reference, in which from the first inner pumping electrode 12a, 12b to the reference electrode 15 and / or from the second inner pumping electrode 22a, 22b to the reference electrode 25, a small oxygen pumping current is done.

The reference electrodes 15 and 25 of the third embodiment shown in FIGS. 4a and 4b and the fourth embodiment shown in FIGS. 5a and 5b preferably comprise a platinum group metal or a platinum group metal alloy, in particular selected from the group comprising platinum, palladium, rhodium, iridium, and / or ruthenium.

The first pumping cell is operated as in the third embodiment explained in FIGS. 4a and 4b.

Furthermore, if the second pump cell is operated in such a way that the molecular oxygen and water molecules at the second inner pumping electrode 22a, 22b are cathodically reduced, as O -

Ions transported through the heated solid electrolyte, oxidized to molecular oxygen at the second outer pumping electrode 23 and delivered to the sample gas, the oxygen partial pressure in the second inner gas chamber 21 of the second pumping cell is lowered. Due to the oxygen partial pressure difference between the second inner pumping electrode 22a, 22b and the reference electrode 25, a Nernst voltage can be measured according to Nernst's law. This serves as a controlled variable for the regulation of the pumping voltage of the second pumping cell. The second Nernst cell should reach a Nernst voltage, which leads to a second pumping voltage, which is> 500 mV to <800 mV above the pumping voltage of the first pump cell. For example, the Nernst voltage of the second Nernst cell is in a range of> 950 mV to <1250 mV. This ensures that not only molecular oxygen is reduced, but also water is split and the Ion current (pumping current) contributes. The difference between the pumping currents of the first and second pumping cells and the voltages generated therefrom is proportional to the water concentration in the measuring gas, so that a measurement of these pumping currents or of the voltages generated can be used to determine the water concentration in the measuring gas.

To determine the carbon dioxide concentration in a measurement gas, the first Nernst cell and the first pump cell are operated as in the context of the carbon dioxide concentration measurement explained in FIGS. 4a and 4b with the third embodiment of a sensor element according to the invention. In the fourth embodiment, the second Nernst cell is operated in such a way that the second Nernst cell reaches a Nernst voltage which leads to a second pumping voltage which is> 300 to <600 mV above the pumping voltage of the first pump cell. This ensures that not only molecular oxygen is reduced and water is split, but also the cleavage of carbon dioxide contributes to the ion current (pumping current). Since the difference between the pumping currents of the first and second pumping cells and the voltages generated therefrom is proportional to

Carbon dioxide concentration in the sample gas, a measurement of these pumping currents or the voltages generated can be used to determine the carbon dioxide concentration in the sample gas.

Fig. 5c shows a principle circuit sketch of a possible embodiment of a

Operating electronics for the sensor element from FIGS. 5a and 5b;

The operating circuit in Fig. 5c, shows two operational amplifiers OVl .1 and OV2.1, which are connected as comparators and at the output adjust the voltage such that at the outer pumping electrode a voltage is applied which leads to such a pumping current that the inverting input has the same voltage as the non-inverting one. For this purpose, the amount of voltage in the Nernst cell must be the same as the reference voltage. The Nernstzellenspannung and thus the partial pressure of the gas molecules are adjusted via the driven by the pumping voltage pumping current through the pumping cell. The reference _voltage U _ref i and U _ref2 differ in such a way that the pumping voltage of the second outer pumping electrode 23,> 500 mV to <800 mV, is higher than the pumping voltage of the first outer pumping electrode 13. The voltage drops across the measuring resistors Rjvi _ess are detected by the operational amplifiers OVl .2 and OV 2.2 are converted into voltage signals and the voltage difference between them is determined. This voltage difference serves as a measurement signal and is proportional to the water concentration in the sample gas. FIG. 6 shows a family of curves of current / voltage characteristics for the cathodic oxygen reduction of 20.9% by volume O ₂ in N ₂ at various electrodes comprising platinum and / or gold. To record the current / voltage characteristics, a broadband probe was operated in the above-mentioned oxygen / nitrogen gas mixture. In this case, the outer pumping electrode was cathodically (negatively) and the inner pumping electrode connected anodically in order to avoid limiting current effects as far as possible. Since the two electrodes were exposed to an identical atmosphere, no significant Nernst voltages had to be considered. Fig. 6 shows that electrodes made of platinum-gold alloys with a gold content of> 1 wt .-%, an overvoltage effect occurs, while electrodes made of a platinum-gold alloy with a gold content of 0.01 wt .-% similar like pure

Behave platinum electrodes. The overvoltages can be read from the traces at the point where the flat region of the current / voltage curve transitions to a steeply sloping area. The minimum of the first derivative of the current / voltage characteristic corresponds to the overvoltage in the context of the present invention.

Fig. 7, 8 and 9 show the dependence of the decomposition voltage of 20.9 vol .-% O ₂ , 3 VoL% H ₂ O and 7 vol .-%, 28 vol .-% and 50 vol .-% CO ₂ in N ₂ of the gold content of the cathode, each measured against an anode of platinum. The decomposition stresses were determined as explained in connection with FIG. 6. Figures 7 to 9 show that as the gold content of the cathode increases, the decomposition voltage for oxygen and carbon dioxide increases due to overvoltage effects, while the decomposition voltage for water remains almost unaffected. For an electrode with a gold content of> 0.1 wt .-% to <10 wt .-%, in particular from> 2 wt .-% to <4 wt .-%, the decomposition voltage of carbon dioxide is significantly increased, so that a larger

There is a distance between the decomposition voltage of carbon dioxide and water, as would be the case with a pure platinum electrode. Although the decomposition voltage for oxygen is increased, but in particular in the case of a gold content of> 2 wt .-% to <4 wt .-% results comparable to the decomposition voltage of water decomposition voltage at a great distance from the decomposition voltage of carbon dioxide. In the already explained embodiments of a sensor element according to the invention, therefore, preferably those electrodes at which only oxygen is reduced and water split (that is, at which no carbon dioxide cleavage is to take place) are formed from a platinum-gold alloy having a gold content according to the invention. The present invention is a method for determining the concentration of water or carbon dioxide in a gas with a sensor element comprising a solid electrolyte body, a first electrochemical pumping cell comprising a first inner pumping electrode arranged in a first inner gas space of the solid electrolyte body and one on the

The first inner pumping electrode and the first outer pumping electrode are connected to each other via a current source, and a second electrochemical pumping cell comprising a second inner pumping electrode arranged in a second inner gas space of the solid electrolyte body and one on the Measuring gas facing side of the solid electrolyte body second outer pumping electrode, wherein the second inner pumping electrode and the second outer pumping electrode are connected to each other via a power source,

by doing

- The first pump cell is operated with a pump voltage, which leads only to the reduction of molecular oxygen to O-ions, and

- The second pumping cell is operated at a higher pumping voltage, both for the reduction of molecular oxygen to O-ions and for the splitting of water to

O ^{2 "} ions leads, and

- From the difference of the pumping currents of the two pumping cells and / or from the resulting from the pumping currents voltage difference, in particular using the 1st Fick 'see law, the water concentration is determined in the sample gas; or

- The first pump cell is operated with a pumping voltage, which leads both to the reduction of molecular oxygen to O ^{2 ~} ions and to the cleavage of water to O ^{2 "} ions, and

- The second pumping cell is operated at a higher pumping voltage, both for the reduction of molecular oxygen to O ^{2 ~} ions and for the splitting of water to

O ions and carbon dioxide leads to O ions, and

from the difference of the pumping currents of the two pumping cells and / or from the voltage difference resulting from the pumping currents, in particular by using the

1. Fick 'see law, the carbon dioxide concentration is determined in the sample gas. The method according to the invention is based on the principle that the electrochemical decomposition voltages of molecular oxygen, water and carbon dioxide on electrodes of the electrode materials according to the invention differ by several 100 mV. For example, in the present invention, for an electrochemical pumping cell having a zirconia solid electrolyte body and an outer and inner platinum pumping electrode, the electrochemical decomposition voltages for molecular oxygen, water and carbon dioxide shown in the following table were determined. The values given in the following table were corrected by the build-up-related ohmic loss.

It is clear from the table that when a voltage of a few hundredths of a volt is applied to a pumping cell, molecular oxygen is reduced to 0-ions at the cathode, with no decomposition / decomposition of water and carbon dioxide. If the voltage is increased to about 0.5 V, in addition to the decomposition of molecular oxygen, a decomposition of water into O ^{2 "} ions occurs, in which case no decomposition of carbon dioxide occurs. 8 volts is also the decomposition of carbon dioxide to O ions.

In principle, the inventive method can be carried out with sensor elements whose first and / or second, inner pumping electrode, and / or their first and / or second outer pumping electrode, and / or their first and / or second reference electrode, a platinum metal or a platinum metal alloy, in particular selected from the group comprising platinum, palladium, rhodium, iridium, and / or ruthenium. That is, the method according to the invention can be carried out, for example, with sensor elements which have two inner pump electrodes made of platinum. In the context of the present invention, however, it has turned out to be advantageous if those inner pumping electrodes, at which molecular oxygen and water or molecular oxygen, water and carbon dioxide are decomposed, are formed from a platinum-gold alloy according to the invention. That is, the erfmdungsgemäße method can also be performed with sensor elements that have an inner

Pumping electrodes of platinum and an inner pumping electrode made of a platinum-gold alloy according to the invention, or via two inner pumping electrodes of a platinum-gold alloy according to the invention have.

Within the scope of one embodiment of the method according to the invention, if the sensor element according to the invention used in the determination of the water or carbon dioxide concentration has no reference electrodes,

- For determining the water concentration to the first pumping cell, a pumping voltage of> 200 mV to <700 mV, for example, from> 300 mV to <650 mV, in particular of

> 400 mV to <650 mV applied, and to the second pumping cell, a pump voltage, the

> 500 mV to <800 mV, for example> 550 mV to <750 mV, in particular> 600 mV to <700 mV, is higher than the pumping voltage of the first pumping cell, applied or

- For determining the carbon dioxide concentration to the first pumping cell applied a pumping voltage of> 700 mV to <1200 mV, for example, from> 750 mV to <1150 mV, in particular from> 800 mV to <1000 mV, and to the second pumping cell, a pumping voltage , which is> 300 mV to <600 mV, for example> 35OmV to <550 mV, in particular> 375 mV to <525 mV higher than the pumping voltage of the first pumping cell

Within the scope of a further embodiment of the method according to the invention, if the sensor element according to the invention used in the determination of the water or carbon dioxide concentration has a reference electrode which forms a Nernst cell with a pump cell, the pumping voltage of the pump cell, which determines the water concentration with the reference electrode forms a Nernst cell, regulated such that the Nernst cell has a Nernst voltage of> 350 mV to <550 mV, for example from> 400 mV to <500 mV, in particular from> 430 mV to <470 mV, and to the pumping cell which does not form a Nernst cell with the reference electrode, a pump voltage is applied which is> 500 mV to <800 mV, for example> 550 mV to <750 mV, in particular> 600 mV to <700 mV higher than the pumping voltage of the pumping cell, which forms a Nernst cell with the reference electrode; or

- For determining the carbon dioxide concentration, the pumping voltage of the pumping cell, which forms a Nernst cell with the reference electrode, controlled such that the Nernstzelle a Nernst voltage of> 950 mV to <1250 mV, for example, from

> 1000 mV to <1200 mV, in particular from> 1050 mV to <1150 mV, and to the pump cell, which does not form a Nernst cell with the reference electrode, a pump voltage is applied which is> 300 mV to <600 mV, for example> 350 mV to

<550 mV, in particular> 400 mV to <500 mV, higher than the pumping voltage of the pumping cell, which forms a Nernst cell with the reference electrode.

Within the scope of a further embodiment of the method according to the invention, if the sensor element according to the invention used in the determination of the water or carbon dioxide concentration has two reference electrodes, of which in each case a reference electrode with a pump cell forms a Nernst cell,

for determining the water concentration, the pumping voltage of the first pumping cell is controlled such that the first Nernst cell has a Nernst voltage of> 350 mV to <550 mV, for example from> 400 mV to <500 mV, in particular from> 430 mV to <470 mV, wherein the second Nernst cell has a Nernst voltage that is> 500 V to <800 mV, for example> 550 mV to <750 mV, in particular> 600 mV to <700 mV, higher than the Nernst voltage of the first Nernst cell; or

- For determining the carbon dioxide concentration, the pumping voltage of the first pumping cell controlled such that the first Nernstzelle a Nernst voltage of> 950 mV to

<1250 mV, for example, from> 1000 mV to <1200 mV, in particular from> 1050 mV to <1150 mV, wherein the second Nernst cell has a Nernst voltage, the

> 300 V to <600 mV, for example> 350 mV to <550 mV, in particular> 400 mV to

<500 mV, higher than the Nernst voltage of the first Nernst cell.

Another object of the present invention is a sensor element for determining the

Water and / or carbon dioxide concentration in a gas, for example, while carrying out the inventive method, comprising a solid electrolyte body, a first electrochemical pumping cell comprising a disposed in a first inner gas space of the solid electrolyte body first inner pumping electrode and one on the

Measuring gas facing side of the solid electrolyte body arranged first outer Pump electrode, wherein the first inner pumping electrode and the first outer pumping electrode are connected to each other via a power source, and a second electrochemical pumping cell comprising a second inner pumping chamber disposed in a second inner gas space of the solid electrolyte body second inner pumping electrode and arranged on the side facing the measuring gas side of the solid electrolyte body second outer

Pump electrode, wherein the second inner pumping electrode and the second outer pumping electrode are connected to each other via a power source,

characterized in that

the first inner pumping electrode and / or the second inner pumping electrode comprises a platinum metal-gold alloy.

The term "gas" in the context of the present invention also gas mixtures, for example, exhaust gases from an internal combustion engine, such as a gasoline or

Diesel engine, and / or from an incinerator, such as an oil heater.

In the context of the present invention, the first inner pumping electrode and / or the second inner pumping electrode can be formed, for example, from a platinum-gold alloy. Preferably, the first inner pumping electrode and / or the second inner pumping electrode is / are made of a platinum metal-gold alloy, in particular platinum-gold alloy, with a gold content of> 0.1% by weight to <10% by weight, for example, from> 1 wt .-% to <7% by weight, in particular from> 2 wt .-% to <4 wt .-%, based on the total weight of the alloy formed.

Advantageously, in the context of the present invention, those internal pumping electrodes, on which molecular oxygen and water or molecular oxygen, water and carbon dioxide are decomposed, are formed from a platinum-metal-gold alloy according to the invention, in particular platinum-gold alloy. A platinum metal-gold alloy according to the invention has the advantage that, in contrast to conventional

Electrode materials in which the difference in the decomposition voltage of water and carbon dioxide is in a range of 0.3 V and in which the decomposition voltages are also dependent on the concentration of the components of the gas mixture, a clear separation between the decomposition of water and carbon dioxide possible is. The platinum metal-gold alloy according to the invention advantageously also allows a Decomposition of water at very low water concentrations and high carbon dioxide concentrations without decomposing carbon dioxide.

Insofar as at least one inner pumping electrode comprises a platinum-metal-gold alloy according to the invention, the first and / or second inner pumping electrode, and / or the first and / or second outer pumping electrode, and / or the first and / or second reference electrode, can be a platinum metal or a platinum metal alloy, in particular selected from the group comprising platinum, palladium, rhodium, iridium, and / or ruthenium. Preferably, the first and / or second, inner pumping electrode and / or the first and / or second outer pumping electrode, and / or the first and / or second reference electrode, platinum.

Preferably, the first and second pumping cells have a common gas inlet opening. The gas inlet opening is configured, for example, in the form of a shaft which is perpendicular to the main surfaces of the sensor element and which branches in a T-shape in the interior of the solid electrolyte and adjoins two channels arranged parallel to the main surfaces, each of which comprises a diffusion barrier and an inner gas space. In each of these inner gas spaces, the first and second inner pumping electrodes (IPE) of the first and second pumping cells are arranged. Conveniently, the first and second inner pumping electrodes (IPE) adjoin the solid state electrolyte body.

In one embodiment, the first and second pumping cells are configured substantially mirror-symmetrically with respect to the axis of the gas inlet opening. In this case, "substantially" means that deviations from a perfect symmetry of <30%, for example of <20%, in particular of <10%, are possible in the context of this embodiment.

Preferably, the first pumping cell has a first diffusion barrier defining the gas inlet opening and the second pumping cell has a second diffusion barrier defining the gas inlet opening. In this case, the diffusion barriers can be configured identically, for example. Via the common gas inlet opening and the diffusion barriers, the inner gas spaces of the first and second pumping cells are in contact with the gas to be measured. Preferably, the first and / or second diffusion barriers are based on a porous material, for example porous, partially stabilized or fully stabilized zirconium oxide and / or aluminum oxide. In the context of the present invention, "partially stabilized or fully stabilized zirconium oxide and / or aluminum oxide", for example doped zirconium oxide and / or aluminum oxide, in particular yttrium, Calcium-, magnesium- and / or scandium-doped zirconium oxide and / or alumina, understood.

In the context of the present invention, the first inner pumping electrode and the second inner pumping electrode can be connected via respective separate electrical lines, each with an electrical connection contact; or the inner pumping electrodes of the first and second pumping cells can be connected to one another via an electrical line and be connected via one, in particular single, further electrical line to one, in particular single, electrical connection contact; or the inner pumping electrodes of the first and second pumping cells can be connected to one another via an electrical line and to an electrical connection contact.

For example, the outer pumping electrodes of the two pumping cells each have separate lines and connection contacts. Advantageously, the outer pumping electrodes are covered by a protective layer to prevent them from abrasion and / or

To protect deposits. Such a protective layer may be formed, for example, of porous, partially stabilized or fully stabilized zirconium oxide and / or aluminum oxide.

The sensor element according to the invention may further comprise one or two reference electrodes. Within the scope of an embodiment of the present invention, a first reference electrode is arranged co-planar with the first inner pumping electrode and forms a first Nernst cell with the first pumping electrode. In the context of a further embodiment of the present invention, a second reference electrode is additionally arranged co-planar with the second inner pumping electrode and has a second reference electrode connected to the second pumping electrode

Nernst cell forms.

The reference electrode / s is / are exposed to a gas atmosphere with a high, constant oxygen content. For example, the reference electrode (s) may be provided by a direct supply of air through a corresponding channel structure or via a pumped one

Reference, in which a small oxygen pump current is driven from the inner pumping electrode to the reference electrode, to be exposed to a gas atmosphere with a high, constant oxygen content.

In the context of the present invention, it is provided that the sample gas reaching the pump cells has an excess of oxygen. The solid electrolyte material of the solid electrolyte body is expediently oxygen-conducting. For example, the solid electrolyte body of partially stabilized or fully stabilized zirconia, in particular yttrium, calcium, magnesium and / or scandium-doped zirconia formed. The use of a ZrC ^ technology has proved to be particularly advantageous in the context of the present invention due to the robustness and exhaust gas resistance.

In the context of a further embodiment, the first inner pumping electrode and / or the second pumping electrode are a pair of electrodes made of two electrically conductive electrodes, in particular low-resistance electrically conductive electrodes. For example, the electrodes of an inner pump electrode pair are arranged on opposite major surfaces of the inner gas space.

Advantageously, a sensor element according to the invention also has a heating device. Conveniently, the heating device is surrounded by an electrically insulating layer. For example, the heating device is arranged parallel to the main surfaces of the sensor element in the solid electrolyte body, in particular below the plane of the electrochemical pumping cells.

Another object of the present invention is the use of a sensor element according to the invention and / or a method according to the invention for monitoring the operation of an internal combustion engine, such as a gasoline or diesel engine, in particular one on a "homogeneous charched combustion ignition" -

Method based on the internal combustion engine, and / or for determining the exhaust gas recirculation rate of an internal combustion engine, in particular a based on a "homogenious charched combustion ignition" method engine, or for monitoring the operation of an incinerator, such as an oil heater, or for monitoring of chemical manufacturing processes, exhaust air systems and /or

Exhaust after-treatment systems.

Further advantages and advantageous embodiments of the subject invention are described in the description, the drawings and the claims.

Claims

claims

1. A method for determining the concentration of water or carbon dioxide in a gas with a sensor element comprising a solid electrolyte body (2), - a first electrochemical pumping cell comprising a in a first inner

Gas chamber (11) of the solid electrolyte body (2) arranged first inner pumping electrode (12a, 12b) and arranged on the side facing the measuring gas of the solid electrolyte body (2) arranged first outer pumping electrode (13), wherein the first inner pumping electrode (12a, 12b) and the first outer pumping electrode (13) are connected to one another via a current source, and a second electrochemical pumping cell comprising a second inner pumping electrode (22a, 22b) arranged in a second inner gas space (21) of the solid electrolyte body (2) and one on the measuring gas Side of the solid electrolyte body (2) arranged second outer pumping electrode (23), wherein the second inner pumping electrode (22a, 22b) and the second outer pumping electrode (23) via a

Power source are connected to each other,

by doing

- The first pump cell is operated with a pumping voltage, which leads only to the reduction of molecular oxygen to O-ions, and the second pumping cell is operated at a higher pumping voltage, both for the reduction of molecular oxygen to O-ions and for the cleavage of Water leads to O ^{2 "} ions, and - from the difference of the pumping currents of the two pumping cells and / or out of the

Pumping currents resulting voltage difference, the water concentration is determined in the sample gas; or

the first pumping cell is operated with a pumping voltage, which can be used both for the reduction of molecular oxygen to O ^{2 "} ions and for the cleavage of

Water leads to O ^{2 "} ions, and the second pumping cell is operated at a higher pumping voltage, which leads both to the reduction of molecular oxygen to O-ions and to the splitting of water to O ^{2 "} ions and carbon dioxide to O ^2" ions, and from the difference of the pumping currents of the two pumping cells and / or from the resulting from the pumping currents voltage difference, the carbon dioxide

Concentration in the measured gas is determined.

2. The method according to claim 1, characterized in that for the determination of the water concentration to the first pumping cell, a pumping voltage of> 200 mV to <700 mV is applied, and to the second

Pumping a pump voltage that is> 500 mV to <800 mV higher than the pumping voltage of the first pump cell is applied, or applied to determine the carbon dioxide concentration to the first pumping cell, a pumping voltage of> 700 mV to <1200 mV, and to the second pumping cell a pumping voltage, the> 300 V to <600 mV higher than the

Pumping voltage of the first pump cell is applied.

3. The method of claim 1, wherein the sensor element additionally comprises a reference electrode (15) which forms a Nernst cell with a pump cell, characterized in that for determining the water concentration, the pumping voltage of the pump cell, which with the reference electrode (15) a Nernst cell is controlled so that the Nernst cell has a Nernst voltage of> 350 mV to <550 mV, and to the pump cell, which is not Nernstzelle with the reference electrode, a pumping voltage is applied, the> 500 V to <800 mV higher than that Pump voltage of the pump cell, which forms a Nernst cell with the reference electrode is; or for determining the carbon dioxide concentration, the pumping voltage of the pumping cell, which forms a Nernst cell with the reference electrode (15), is controlled such that the Nernst cell has a Nernst voltage of> 950 mV to <1250 mV, and to the pumping cell, which is not a Nernst cell with the reference electrode

(15), a pump voltage is applied which is> 300 mV to <600 mV higher than the pumping voltage of the pumping cell, which forms a Nernst cell with the reference electrode (15).

4. The method according to claim 1, wherein the sensor element additionally has two reference electrodes (15, 25), of which in each case a reference electrode forms a Nernst cell with a pump cell, characterized in that the pump voltage of the first pump cell is regulated in such a way to determine the water concentration, that the first Nernst cell has a Nernst voltage of> 350 mV up to

<550 mV, wherein the second Nernst cell has a Nernst voltage that is> 500 V to <800 mV higher than the Nernst voltage of the first Nernst cell; or for determining the carbon dioxide concentration, the pumping voltage of the first pumping cell is controlled such that the first Nernst cell has a Nernst voltage of> 950 mV to <1250 mV, wherein the second Nernst cell has a Nernst voltage, which is> 300 V to <600 mV higher than that Nernst voltage of the first Nernst cell is.

5. Sensor element for determining the concentration of water and / or carbon dioxide in a gas comprising a solid electrolyte body (2), a first electrochemical pumping cell comprising a first inner pumping electrode (12a) arranged in a first inner gas space (11) of the solid electrolyte body (2) 12b) and a first outer pumping electrode (13) arranged on the side of the solid electrolyte body (2) facing the measurement gas, the first inner pumping electrode (13)

A second electrochemical pumping cell comprising a second inner pumping electrode (22a, 22b) arranged in a second inner gas space (21) of the solid electrolyte body (2) and a side of the solid electrolyte body facing the measurement gas

(2) arranged second outer pumping electrode (23), wherein the second inner Pumpelekτrode (22a, 22b) and the second outer pumping electrode (23) are connected to each other via a power source,

characterized in that

the first inner pumping electrode (12a, 12b) and / or the second inner pumping electrode (22a, 22b) comprises a platinum-metal-gold alloy.

6. Sensor element according to claim 5, characterized in that those inner

Pump electrodes (12a, 12b, 22a, 22b), involving molecular oxygen and water or molecular oxygen, water and carbon dioxide is decomposed, is formed from a platinum metal-gold alloy / are.

7. Sensor element according to claim 5 or 6, characterized in that the first inner pumping electrode and / or the second inner pumping electrode (12a, 12b, 22a, 22b) of a

Platinum metal-gold alloy is formed with a gold content of> 0.1 wt .-% to <10 wt .-% / are.

8. Sensor element according to one of the preceding claims, characterized in that the first inner pumping electrode (12a, 12b) and / or the second inner pumping electrode (22a,

22b) is formed of a platinum-gold alloy / are.

9. Sensor element according to one of the preceding claims, characterized in that the first and second pumping cell have a common gas inlet opening (10).

10. Sensor element according to one of the preceding claims, characterized in that the first pumping cell has a first diffusion barrier (16) delimiting the gas inlet opening (10) and the second pumping cell has a second diffusion barrier (26) delimiting the gas inlet opening (10).

11. Sensor element according to one of the preceding claims, characterized in that the first inner pumping electrode (12a, 12b) and the second inner pumping electrode (22a, 22b) via respective separate electrical lines, each having an electrical connection contact (32c, 32d) are connected ; or - the inner pumping electrodes (12a, 12b, 22a, 22b) of the first and second pumping cells are connected to one another via an electrical line and to an electrical connection contact (32e).

12. Sensor element according to one of the preceding claims, characterized in that - co-planar with the first inner pumping electrode (12a, 12b), a first reference electrode

(15) is arranged, which with the first pumping electrode (12a, 12b) forms a first Nernst cell, and / or co-planar to the second inner pumping electrode (22a, 22b), a second reference electrode (25) is arranged, which with the second Pumping electrode (22a, 22b) forms a second Nernst cell.

13. Sensor element according to one of the preceding claims, characterized in that the first and / or second, inner pumping electrode (12a, 12b, 22a, 22b), and / or the first and / or second outer pumping electrode (13, 23) and / or the first and / or second reference electrode (15; 25) a platinum metal or a platinum metal alloy, in particular selected from the group comprising platinum, palladium, rhodium, iridium, and / or

Ruthenium, includes.

14. Sensor element according to one of the preceding claims, characterized in that the first inner pumping electrode (12a, 12b) and / or the second pumping electrode (22a, 22b) comprises a pair of electrodes comprising two electrically conductively connected electrodes (12a, 12b;

22a, 22b).

15. Use of a method according to any one of claims 1 to 4 and / or a sensor element according to one of claims 5 to 14 for monitoring the operation of an internal combustion engine and / or for determining the exhaust gas recirculation rate of a

Internal combustion engine or for monitoring the operation of an incinerator or for monitoring of chemical manufacturing processes, exhaust air systems and / or exhaust aftertreatment systems.