WO2008080730A1

WO2008080730A1 - Sensor element with additional diagnosis function

Info

Publication number: WO2008080730A1
Application number: PCT/EP2007/063175
Authority: WO
Inventors: Detlef Heimann; Torsten Handler; Henrico Runge; Harald Guenschel
Original assignee: Robert Bosch Gmbh
Priority date: 2006-12-29
Filing date: 2007-12-03
Publication date: 2008-07-10
Also published as: DE102006062051A1

Abstract

Proposed is a method for measuring at least one gas component in a gas space (112) using a sensor element (110), wherein the at least one sensor element (110) has at least one first electrode (116), at least one second electrode (118) and at least one solid electrolyte (114, 514) which connects the at least two electrodes (116, 118). Here, the at least one first electrode (116) has at least one mixed-potential electrode (116; 710). The at least one second electrode is screened off relative to the gas space (112). The method comprises at least two operating states: at least one amperometric operating state, in which a voltage is applied between the two electrodes (116, 118) and a pump current is measured, and at least one potentiometric measurement state, in which a voltage between the at least one mixed-potential electrode (116; 710) and the at least one second electrode (118) is detected.

Description

description

title

Sensor element with additional diagnostic function

State of the art

The invention is based on known sensor elements which are based on electrolytic properties of certain solids, ie the ability of these solids to conduct certain ions. Such sensor elements are used in particular in motor vehicles to measure air-fuel-gas mixture compositions. Such sensor elements are also known as "lambda probe" 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), the relationship between an air mass actually offered and a theoretically required (that is to say stoichiometric) air mass is generally referred to in combustion technology

Air ratio is measured by one or more sensor elements usually at one or more locations in the exhaust system of an internal combustion engine. Correspondingly, "rich" gas mixtures (i.e., gas mixtures with a fuel surplus) have an air ratio λ <1, whereas "lean" gas mixtures (i.e., gas mixtures with a fuel deficiency) have an air ratio λ> 1. In addition to automotive technology, such and similar

Sensor elements in other areas of technology (in particular combustion technology) used, for example in aeronautical engineering or in the control of burners, z. B. in heating systems or power plants.

Such sensor elements are now known in numerous different embodiments. One embodiment is the so-called "jump probe" whose measurement principle is based on the measurement of an electrochemical potential difference between a reference gas exposed reference electrode and a measuring gas exposed to the measured measuring electrode is based. Reference electrode and measuring electrode are connected to one another via the solid electrolyte, wherein doped zirconium dioxide (eg yttrium-stabilized ZrO 2) or similar ceramics are generally used as the solid electrolyte due to its oxygen-ion-conducting properties. Theoretically, the potential difference between the electrodes, especially at the transition between the rich gas mixture and the lean gas mixture, has a characteristic jump, which can be used to actively regulate the gas mixture composition by the jump point λ = 1. Various exemplary embodiments of such jump probes, which are also referred to as "Nernst cells", are described, for example, in DE 10 2004 035 826 A1, DE 199 38 416 A1 and DE 10 2005 027 225 A1.

Alternatively or in addition to jump probes, so-called "pump cells" are used in which an electrical "pump voltage" is applied to two electrodes connected via the solid electrolyte, the "pump current" being measured by the pump cell In the case of pump cells, generally both electrodes are connected to the gas mixture to be measured, whereby one of the two electrodes is exposed directly to the gas mixture to be measured (usually via a permeable protective layer) can not get directly to this electrode, but must first penetrate a so-called "diffusion barrier" to get into a cavity adjacent to this second electrode. In this case, the diffusion barrier used is usually a porous ceramic structure with specifically adjustable pore radii. If lean exhaust gas passes through this diffusion barrier into the cavity, oxygen molecules at the second, negative electrode are electrochemically reduced to oxygen ions by means of the pumping voltage, are transported through the solid electrolyte to the first, positive electrode and released there again as free oxygen , The sensor elements are usually operated in the so-called limiting current operation, that is, in an operation in which the pumping voltage is selected such that the oxygen entering through the diffusion barrier is completely pumped to the counterelectrode. In this operation, the pumping current is approximately proportional to the partial pressure of the oxygen in the exhaust gas mixture, so that such sensor elements are often referred to as proportional sensors. in the Contrary to jump sensors, such proportional sensors can be used as so-called broadband sensors over a comparatively wide range for the air ratio lambda.

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 by adding a snap cell (Nernst cell) to a "double cell." Such a construction is described, for example, in EP 0 678 740 B1. Measured to the second electrode adjacent cavity and tracked the pumping voltage by a control so that in the cavity always the condition λ = 1 prevails.

Broadband sensor elements in a 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 also measured, 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 due essentially to the molecular hydrogen contained in the gas mixture, which influences the electrochemical potential of the anode, ie the first electrode, since water now forms on the first electrode from the oxygen ions leaving the solid electrolyte instead of molecular oxygen can be. 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 thus 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 by the above-described diffusion barrier) in the region of the second electrode ( eg cathode). The problem now lies in particular in that the fat pumping current and the lean pumping current have the same direction in electrically, so that a conclusion about the composition of the pumping current can be drawn from the pumping current. tion of the gas mixture is hardly possible. In addition to the problem described in the field of rich mixtures, a falsification of the pumping current by the hydrogen is also to be observed in the region of slightly lean non-equilibrium exhaust gases, which already exists in this area and provides a positive contribution to the pumping current.

A further problem of known broadband sensor elements in single cell arrangement is that the sensor elements should advantageously also be usable for diagnostic purposes for the monitoring of exhaust aftertreatment components. Stricter legislation, especially in the USA, in the field of diagnosis of mission-relevant components, require in a so-called "on-board diagnosis" (OBD) the monitoring of all exhaust aftertreatment components to predetermined OBD limits, which are usually specified as a factor of the emission limit. Coated catalysts (eg oxidation catalyst and NOx storage catalyst) also have to be tested for the effectiveness of their coating or their functionality, and the oxidation catalyst is responsible for the conversion of unburned hydrocarbons (HC) and CO. II Legislation that the oxidation catalyst be tested for catalytic coating effectiveness and recognized as defective if the emission exceeds 1.75 times the limit.

Previous approaches use the temperatures in front of and behind the catalyst to estimate the conversion (exothermic reaction) in the catalyst. This turnover in turn serves as a measure of the functionality of the coating. However, these methods can only give a qualitative statement regarding the conversion capability of the catalyst and thus ensure no limit monitoring. As a rule, legal requirements are difficult to achieve.

Conventional broadband sensor elements in a single cell arrangement, however, are only suitable for carrying out the above-mentioned difficulties of unambiguity of the characteristic curve in order to deduce an oxygen concentration from ion streams, but not, for example, for measuring fuel gas components. Although in principle a (at least temporary) use of such broadband sensor elements for a potentiometric measurement would be possible. However, conventional sensor elements are only sensitive to oxygen and have as jump cells a steep potential jump at λ = 1. Outside this range, the potential characteristics are almost flat, so that virtually no information about the gas composition can be obtained from this.

Disclosure of the invention

Accordingly, a method is proposed for measuring at least one gas component of a gas mixture in a gas space, which avoids the disadvantages described above. Furthermore, an electronic control device for carrying out the method and a system for carrying out the method are proposed. The method uses at least one sensor element with at least one first electrode, at least one second electrode and at least one electrolyte connecting the at least one first electrode and the at least one second electrode. One idea of the invention consists in achieving, on the one hand, a unique pumping current characteristic in that the at least one second electrode is shielded from the at least one gas chamber. In this way, fuel gas reactions are avoided at the at least one second electrode. Another idea of the invention is to design the at least one first electrode as a fuel gas-sensitive electrode. For this purpose, it is proposed that the at least one first electrode be designed in such a way that it comprises at least one mixed potential electrode which is sensitive to the gas composition, that is, in particular sensitive to hydrocarbons and carbon monoxide.

Accordingly, the proposed method comprises at least two operating states: at least one amperometric measuring state in which a pumping voltage is applied between the at least two electrodes and a pumping current is detected, wherein preferably at least one first gas component in the gas mixture is deduced from the at least one pumping current. The term "concentration" is to be construed broadly and includes both a concentration in the classical sense as a particle number per unit volume, but also other variables that can be correlated with the concentration, such as, for example, partial pressure, mass percent or volume percent, molar fraction or the like or simply a Vohandensein or nonexistence For example, this may be an oxygen concentration. D-

usually voltages between 300 mV and 800 mV, in particular between 600 mV and 700 mV used. This amperometric measurement state thus corresponds to the usual operating state of single-cell broadband probes. Furthermore, however, at least one potentiometric measurement state is provided in the proposed method, in which at least one voltage between the at least one mixed potential electrode and the at least one second electrode is detected. From the at least one detected voltage is preferably closed to a concentration (which term is again interpreted broad, see above) at least one second gas component in the gas mixture, for example, as described above, to a fuel gas concentration, in particular hydrocarbons and / or carbon monoxide ,

In contrast to the commonly used platinum electrodes, the electrode function (electrode potential) of mixed potential electrodes (fuel gas-sensitive electrodes, electrodes with non-Nernstian behavior) is no longer thermodynamic, but kinetically determined. The electrode potentials deviate from the Nernst equation and, depending on the gas mixture composition, mixing potentials are produced. These mixed potentials lead to sensor signals which are in direct correlation with the non-equilibrium exhaust gases such as CO or HC. The reference in potentiometric operation forms the shielded, at least one second electrode.

In particular, the at least one mixed potential electrode may comprise at least one electrode material which has a lower electrocatalytic activity than platinum. Furthermore, it is also possible to use a platinum electrode which has an admixture of a catalytically inactive or less catalytically active metal than platinum, in particular gold and / or silver and / or copper and / or lead. Such metal mixtures have proven to be particularly suitable for reasons of compatibility with the production process (for example sintering conditions of the ceramic) and the operating conditions (temperature, atmosphere, etc.). For example, since gold accumulates predominantly on a platinum surface, even small amounts of gold (0.1 to 1%) massively influence the electrode activity and lead to a measurable fuel gas sensitivity of the electrode. However, since gold has a high vapor pressure at typical operating temperatures of the sensor elements (for example, 780 ⁰ C) and partially evaporates, the gold concentration should be selected so that the fuel gas sensitivity is maintained over the lifetime of the sensor element. Accordingly, admixtures of gold, silver, copper or lead in the range between 0.05 wt .-% to 5 wt .-%, in particular between 0.1 wt .-% and 1.0 wt .-%, proposed.

Alternatively, it is also possible to use an oxide electrode as the mixed potential electrode, in particular a metal oxide electrode, for example based on perovskite and / or chromite and / or gallate. Also ceramic-metal composites can be used, as well as mixtures of an oxide ceramic with gold, silver, copper and / or lead.

In order to realize a monitoring of at least one function of at least one exhaust aftertreatment device (for example at least one filter and / or at least one catalytic converter), one of the described probes can be used upstream of the exhaust aftertreatment device and another downstream of the at least one exhaust aftertreatment device. For example, a sensor element after an exhaust gas turbocharger and a further sensor element after an oxidation catalyst and / or diesel particulate filter and / or storage catalyst can be placed. In amperometric normal operation, the oxygen concentration or lambda is then measured and is available in the usual way to the corresponding software functions. If appropriate monitoring conditions are present, the operating mode of the lambda probe can be changed to potentiometric operation, so that, for example, hydrocarbon and carbon monoxide emissions before and after the catalysts can be measured. From this information, the conversion behavior of the catalysts can be determined qualitatively and / or quantitatively. The monitoring may be carried out either passively, for example during a regeneration operation (for example of a diesel particulate filter and / or storage catalytic converter) or actively (for example by temperature regulation in the catalytic converter, for example by a corresponding post-injection).

To achieve the unique pump current characteristic in amperometric operation by means of the abovementioned shielding of the at least one second electrode, the at least one first electrode can be connected to the at least one surrounding gas space via at least one flow resistance element, and the at least one second electrode can be connected by means of at least one diffusion resistance element. This can be at least -O-

a flow resistance element and the at least one diffusion resistance element be designed such that the limiting current of the at least one second electrode (usually the pump anode) is smaller than the limiting current of the at least one first electrode. In this case, it is particularly preferable to set limit currents in which a ratio of less than 1/100, in particular less than 1/1000, is present. Preferably, the limiting current of the at least one second electrode is 1 to 20 μA, more preferably 10 μA, and the limiting current of the at least one first electrode is 500 μA to 6 mA, preferably 1.5 mA. The limiting current of an electrode is defined as the saturation pumping current, d. H. the maximum pumping current which can be achieved when the pumping voltage between the at least two electrodes is increased. This limiting current can be defined, for example, for oxygen and oxygen ion transport through the solid electrolyte as the current which is achieved when all the oxygen molecules which reach the electrode operated as a cathode are completely transported through the solid electrolyte to the anode. Usually, the sensor element is operated with this limiting current, i. H. In this mode, the pump current is approximately proportional to the gas molecule concentration, and the reverse current of the opposite electrode, which was previously operated as an anode, becomes experimentally polarized by reverse polarity determined, so that now the former anode is operated as a cathode.

The setting of the condition for the limiting current ratio can be fulfilled in particular in that the at least one diffusion resistance element has a greater diffusion resistance than the at least one flow resistance element. The diffusion resistance is the resistance which an element opposes to a concentration difference between the two sides of the element and thus hinders diffusion. For this embodiment of the diffusion resistors, for example, the same diffusion medium (for example, a porous material) for the at least one diffusion resistance element and the at least one flow resistance element can be used, but in different layer thicknesses, so that, for example, before the at least one second electrode, a higher layer thickness is used than before the at least one first electrode. Alternatively or in addition, an adjustment of the surface of the resistance elements can take place. The limiting current increases at least approximately proportionally the available for the diffusion Querschnittsfiäche, and inversely proportional to the length or layer thickness of the resistive elements.

In addition, however, the at least one flow resistance element preferably has a greater flow resistance than the at least one diffusion resistance element. In this case, the flow resistance is defined as that resistance which an element opposes to a pressure difference between both sides of the element and thus prevents a flow between both sides of the element. The flow resistance can be adjusted, for example, by increasing or decreasing a pore size of a porous medium, and / or by varying a channel cross-section, a channel geometry or a channel length. The above-described advantageous relationship between the limiting currents causes the above-described shielding effect of the at least one second electrode against the reducing gases, such as hydrogen. It is particularly favorable if this shielding is effected by the at least one diffusion resistance element having a diffusion channel which connects the at least one first electrode to the at least one gas space and / or at least one reference space. This diffusion channel should preferably have a large length, ie a length which is large in relation to the mean free path of the gas molecules at the corresponding operating temperature of the sensor element (for example 700 to 800 ^° C.). In this way, the difference between gas phase diffusion and flow resistance can be maximally utilized in order to bring about a shielding of the at least one first electrode. If gas molecules in the diffusion channel (although of course also several diffusion channels can be used) have no collision partners other than the walls of the diffusion channel, transport would only occur via Knudent diffusion with the same behavior for flow and diffusion. By virtue of the design as a diffusion channel, on the other hand, there is only a slight diffusion transport of fatty gas to the at least one second electrode, and thus only a small fat pumping current. Advantageously, the at least one diffusion channel is provided with a height in the range between 2 L to 25 L and a width in the range of 2 L to 25 L and a length in the range between 0.5 mm and 20 mm. Here L is the mean free path of the molecules of the gas mixture at an operating pressure of the sensor element, which is usually in the range of the normal pressure. This dimensioning of the at least one diffusion channel has proved to be particularly favorable to prevent the diffusion of grease gas to at least a first electrode.

As an alternative or in addition to the written shielding method, the at least one second electrode can also be shielded in that it is completely separated from the gas space. In this case, this at least one second electrode can be connected, for example, to a reference gas space, for example via an exhaust air duct. This reference gas space is completely separated from the at least one gas space, which may be, for example, an engine room, which is separated from the exhaust line. In the at least one exhaust duct, for example, a porous element may then again be provided, for example once again an Al ₂ O ₃ ceramic.

The at least one first electrode may, for example, comprise a single electrode, wherein in each case switching should be made between amperometric and potentiometric measurement states. Thus, for example, in this case, a clocking occur, preferably not overlap amperometric and potentiometric clock cycles. The clock cycles may, for example, be selected such that during the amperometric measurement state, a predetermined amount of the at least one first gas component to be detected is pumped from the at least one first electrode to the at least one second electrode to be there as reference during the subsequent potentiometric clock cycle to stand.

As an alternative or in addition to the described configuration of the at least one first electrode, it is also possible to use a construction in which the at least one first electrode further comprises at least one pump electrode in addition to the at least one mixed potential electrode. The pumping voltage is then applied between the pumping electrode and the at least one second electrode and the pumping current is measured between these electrodes. In the potentiometric measurement state, on the other hand, the voltage between the at least one mixed potential electrode and the at least one second electrode is measured. In this way, the potentiometric measurements, for example, at least temporarily make the same time for amperometric measurement. Short description of the drawing

Exemplary embodiments of the invention are illustrated in the drawings and explained in more detail in the following description. Show it:

FIG. 1 shows a first exemplary embodiment of a sensor element which can be used according to the invention with two possibilities of shielding the pump anode;

FIG. 2 shows a pumping current characteristic in the case of a shielded anode;

3 shows a second exemplary embodiment of an inventively usable sensor element with internal pump anode;

FIG. 4 shows a further exemplary embodiment of a sensor element with external electrodes;

Figure 5 shows another exemplary embodiment of a sensor element with internal

electrodes;

FIG. 6 shows an exemplary embodiment of a sensor element with an outer cathode, an inner anode and an exhaust air duct;

FIG. 7 shows an exemplary embodiment of a sensor element in which the mixed potential electrode and the pump cathode are separated;

FIG. 8A shows a first exemplary embodiment of a system for measuring a gas component; and

FIG. 8B shows a second exemplary embodiment of a system for measuring a gas component. 1 shows a first embodiment of a sensor element 110 is shown, which is used for the erfϊndungsgemäße method. This is a sensor element 110, which can be used, for example, in a lambda probe or as a lambda probe in order to determine the gas composition (air ratio) in a gas space 112. In this example, the sensor element 110 is designed as a radial design (whereby linear designs are also possible) with a solid electrolyte 114, on which on opposite sides an inner pumping cathode 116 is arranged as the first electrode and an outer side, on the side facing the gas chamber 112 Pump anode 118 are arranged as a second electrode. In operation, voltages are applied between the two pumping electrodes 116, 118 in the range described above, and a current is measured between the two electrodes 116, 118 (pumping current I _p ). In the following, it is generally assumed that the at least one first electrode is connected as pump cathode 116 and the at least one second electrode of sensor element 110 is pump anode 118. However, another circuit is also conceivable, or even a circuit in which, depending on the operating state , the cathode and anode function change. In particular, the term "pump cathode" or "pump anode" is used, without prejudice to the possibility of using these electrodes in the potentiometric (ie, generally currentless) measuring state as potential electrodes, between which the voltage is measured.

The pump cathode 116 is in this case designed as a fuel gas-sensitive electrode and has, for example, a platinum electrode with a gold admixture. Other configurations than mixed potential electrode according to the above description are possible.

In front of the pumping cathode 116, a cathode cavity 120 in the form of a rectangular cavity is formed. Through a gas inlet hole 122 in the sensor element 110, gas mixture from the gas space 122 enters the sensor element 110 and can pass from there into the cathode cavity 120. Between gas inlet hole 122 and cathode cavity 120, a flow resistance element 124 in the form of a porous, dense material is arranged, which limits the limiting current of the pump cathode 116 and thus substantially determines the slope of the pump current characteristic. In the exemplary embodiment of the sensor element 110 shown in FIG. 1, two different possibilities are illustrated for suppressing gaseous gas reactions at the pump anode 118 and thus achieving a unique pumping current characteristic. While the pump anode 118 is shielded from the gas space 112 by a diffusion resistance element in the form of a porous protective layer 126 in the right-hand part of the diagram (FIG. 1), the pump anode 118 in the left part of this schematic illustration (in FIG denotes) a geometrically configured diffusion resistance element 128. In this case, the pump anode 118 is surrounded by a gas-impermeable cover layer 130, in which, above the pump anode 118, a rectangular cavity 132 is formed. This cavity 132 is connected to the gas inlet hole 122 via a long diffusion channel 134, which opens into the gas inlet hole 122. For the advantageous dimensions of the diffusion channel 134 reference is made to the above description. At the mouth of the diffusion channel 134 in the gas inlet hole, a widening 136 is provided to prevent the diffusion channel 134 is added by entering from the gas space 112 dirt. On the one hand, it is possible through the diffusion channel 134 for oxygen, which forms at the pump anode 118, to be able to flow out into the gas space 112. On the other hand, combustion chamber gases are made difficult to penetrate into the cavity 132 above the pump anode 118 by the long diffusion path. The cavity 132 additionally causes a spatial possibility for the reaction of penetrating fuel gases, such as hydrogen, before they reach the pump anode 118 and there can trigger unwanted anode reactions.

The sensor element 110 according to the exemplary embodiment in FIG. 1 can be modified in many ways. Thus, unlike the radial design shown here, a linear design can also be selected. The two options A and B shown can be implemented individually or in combination. Furthermore, it can be seen in FIG. 1 that below the pump anode 118, pump cathode 116 and solid electrolyte 114, which together form a pump cell 138, a heating element 140 is provided, which is composed of insulator layers 142 and heating resistors 144 arranged between them. By means of this heating element 140, which acts as a tempering element 146 can be, for example, an operating temperature of the sensor element 110 to typically 700-800 ⁰ C set, the temperature is adjusted, for example, to optimize the electrolytic properties of the solid electrolyte 114.

FIG. 2 schematically shows the effect of the measures described above on the characteristic curve (pumping current I _p as a function of the air ratio λ) of the sensor element 110 shown in FIG. The pumping current I _p is plotted against the air ratio λ. Theoretically, the pumping current I _p in the rich region 210 should be at zero, ie on the λ axis. At λ = 1 and larger λ values (lean region 212), the pump current I _{p should} then increase approximately linearly with the air ratio λ, which is shown in dashed lines in FIG. 2 by the theoretical characteristic curve 214. In fact, however, characteristic curve 216 is observed in single cell sensor elements 110 exposed to the gas mixture pump anode 118, which is approximated to the theoretical curve 214 only at high λ values. However, in the slightly lean region, approximately at λ = 1, the characteristic curve 216 deviates from the theoretical curve 214 and even increases again to smaller λ values. The characteristic curve 220 shows the pump anode 118 shown in B in FIG. 1, in which the diffusion resistance element 128 is realized. It can clearly be seen that this characteristic curve 220 closely approximates the theoretical course 214. Thus, a measurement down to small λ values, ie λ-numbers just above 1, is possible.

FIG. 3 shows a further exemplary embodiment of a sensor element 110, which in turn has a pump cell 138 with a pump cathode 116 and a pump anode 118 and a solid electrolyte 114 located therebetween. The pump cathode 116 is in turn, as described above, wholly or partially made of the combustible gas-sensitive material. In contrast to the exemplary embodiment according to FIG. 1, however, in the exemplary embodiment according to FIG. 3, the pump cathode 116 is arranged on top of the solid electrolyte 114, and the pump anode 118 is located on the inside. A gas-impermeable cover layer 130 is arranged above the pump cathode 116 for shielding from the gas space 112, so that an approximately rectangular cathode cavity 120 again forms above the pump cathode 116. This cathode cavity 120 is shielded from the gas space 112 by the flow resistance element 124, which is designed, for example, as in the exemplary embodiment according to FIG. Again, a gas inlet hole 122 is provided, which in this case, however, does not serve the purpose of gas supply to the pump anode 118 (as in the exemplary embodiment according to FIG. 1 for gas supply to the pump cathode 116), but rather an escape of oxygen from a cavity 132 in the interior of the sensor element 110, FIG. in which the pump anode 118 is arranged. Accordingly, the gas inlet hole 122, which in this case is no longer an "access hole", may be configured, for example, with a smaller cross-section than the gas inlet hole 122 in the embodiment of FIGURE 1. Thus, the diffusion resistance is further increased 3 shows a part of a diffusion resistance element 128, which prevents or reduces a diffusion of fuel gases from the gas space 112 into the cavity 132 above the pump anode 118 and at the same time allows an outflow of oxygen from the cavity 132. In addition, in FIG 132 is shielded from the gas space 112 by a porous element 310, which is advantageously a coarse-pored, porous element.

Finally, FIG. 4 shows a third exemplary embodiment of a sensor element 110 which realizes a layer structure with pump cathode 116 and pump anode 118 arranged on the same side of the solid electrolyte 114. Again pump anode 118, pump cathode 116 and solid electrolyte 114 form a pumping cell 138, but now the pumping current flows in a substantially horizontal direction, parallel to the layer planes, between the electrodes 116, 118. Above the pump cathode 116, which in turn is designed as a mixed potential electrode, again a cathode cavity 120 is formed, which is shielded from the gas space 122 by a gas-tight cover layer 130. The cathode cavity 120 is separated with the gas space 122 via a flow resistance element 124 in the form of a dense, small-pored ceramic layer, analogous to the preceding embodiments.

Above the pump anode 118, in turn, a cavity 132 is formed, which is likewise separated off from the gas space 122 by the gas-tight cover layer 130. The hollow space 132 is separated from the gas space 122 by the one diffusion channel 134, wherein a porous element 310, analogous to the exemplary embodiment in FIG. 3, is introduced into the diffusion channel 134. Diffusion channel 134 and porous element 310 act together as a diffusion - -

On resistance element 128, with respect to the dimensioning of the diffusion channel 134 can be made to the above description.

Furthermore, as in the previous exemplary embodiments, a heating element 140 is again provided in the exemplary embodiment according to FIG. In contrast to the preceding exemplary embodiments, in the case of this planar arrangement with adjacent electrodes 116, 118, however, an asymmetric heating is realized in the example according to FIG. 4, in which pump anode 118 and diffusion resistance element 128 are heated in the spatial average at a temperature which is approximately 20% below the average temperature of pumping cathode 116 and flow resistance element 124 is located. For this purpose, the heating element 140 is arranged such that laterally the pump anode 118 and the diffusion resistance element 128 are not completely covered since the heating element 140 does not extend to the same extent to the right edge of the sensor element 110 as to the left edge. As a result of this increased operating temperature on the pump cathode 116 side, a gas inlet, which is indicated symbolically in FIG. 4 by 410, from the gas space 122 into the cathode cavity 120 through the porous flow resistance element 124 (diffusion process) is favored. At the same time, due to the lower operating temperature on the pump anode 118 side, an outflow of oxygen (gas outflow 412) from the cavity 132 into the gas space 122 is allowed, but diffusion of fuel gases from the gas space 122 into the cavity 132 through the diffusion channel 134 and the porous element 310 are suppressed due to the lower temperature.

In the exemplary embodiments in FIGS. 1, 3 and 4, although the pump anode 118 is shielded from the gas space 112, it is nevertheless connected to the latter (via the diffusion resistance element 128). In contrast, FIG. 5 shows an exemplary embodiment of a sensor element 110 which can also be used according to the invention, in which the pump anode 118 is completely separated from the gas space 112 and instead is connected via an exhaust air duct 510 to a reference gas space 512, for example an engine compartment of a motor vehicle.

The sensor element 110 has on the side facing the gas space 112 a first solid electrolyte 114, for example, again an yttrium-stabilized zirconium dioxide Electrolytes, on. In addition, a second solid electrolyte 514 arranged inside the sensor element 110 is provided. The solid electrolytes 114, 514 are contacted by a pump cathode 116 and a pump anode 118, which are each formed in two parts, with an upper, the upper solid electrolyte 114 contacting partial cathode 516, a lower part cathode 518, which contacts the lower solid electrolyte 514 a upper part anode 520, which contacts the upper solid electrolyte 114 and a lower partial anode 522, which in turn contacts the lower solid electrolyte 514. The two partial cathodes 516 and 518 and the two partial anodes 520, 522 are each connected to one another and thus together form the pumping cathode 116 and the pumping anode 118. The splitting into a plurality of partial electrodes causes an enlarged electrode surface and thus a reduced internal resistance of the sensor element 110. A cathode cavity 120 is provided between the partial cathodes 516, 518, and an anode cavity 132 is provided between the two partial anodes 520, 522.

Analogous to the first exemplary embodiment according to FIG. 1, the cathode cavity 120 communicates with the gas space 120 via a gas inlet hole and a flow resistance element 124. The pump cathode 116 is in turn, as described above, made of fuel gas-sensitive material. The pumping cathode 218 is electrically contacted by a cathode lead 524 disposed on the lower solid electrolyte 514. Via a cathode connection 526 on the upper side of the solid electrolyte 114 and an electrical through-connection 528, the pumping cathode 116 can be connected to a corresponding electronic control device (compare FIGS. 8A and 8B below) and, for example, supplied with a voltage.

The anode cavity 132 is connected to the reference gas space 512 via the exhaust air duct 510. Anodenhohlraum 132 and exhaust duct 510 are either unfilled or filled with an oxygen-permeable porous filling element 530 on Al ₂ θ ₃ basis. In the above-described at least one amperometric measurement state, in which oxygen is pumped from the pump cathode 116 to the pump anode 118, the exhaust air channel 510 serves to discharge oxygen with comparatively low flow resistance to the reference gas chamber 512, so that no or only a small amount in the anode cavity 132 Overpressure is created. In the potentiometric measuring state, on the other hand, here as well as in the previous - o¬

exemplary embodiments, the anode cavity 132 and the exhaust air duct 510, in each case with the porous filling element 530, as an air reference for the potentiometric determination of the potential difference between the electrodes 116, 118.

The pump anode 118 is electrically contacted via an anode feed line 532 and connected via a further electrical feedthrough 534 in the solid electrolyte 114 to an anode connection 536 arranged on the upper side of the solid electrolyte 114. Via this anode connection 536, the pump anode 118 can also be connected, for example, to the electronic control device (see FIGS. 8A and 8B below) so that, for example (in the amperometric measurement state), a voltage (for example a constant voltage) is applied between pump anode 118 and pump cathode 116 can be and / or a pumping current can be measured. Anode supply line 532 and cathode supply line 524 are arranged side by side lying in the embodiment of Figure 5 and separated by the exhaust duct 510 against each other. Alternatively, a superimposed arrangement of the supply line 524, 532 can be realized.

Below the lower solid electrolyte 514, in turn, a heating element 140 is arranged, in which a heating resistor 144 between two insulator layers 142 is embedded. The heating resistor 144 can be electrically contacted via through-holes 538 in a carrier substrate 540 and heating contacts 542 and acted upon by a heating current. For example, this heating current can be regulated with a regulation which, for example, sets a constant internal resistance of the sensor element 110.

Also, by means of the arrangement of the sensor element 110 shown in FIG. 5 with the pump anode 118 completely shielded from the gas space 112, the unique characteristic 220 shown in FIG. 2 can be realized. When used as a lambda probe, a pumping current corresponding to the oxygen partial pressure is measured in the lean region 212, but no current in the rich region 210, since there is no free oxygen and since the selected pumping voltage is advantageously below the decomposition voltage of the water. Thus, fuel gas oxidation may occur at the inner, shielded, fuel gas-blind pump anode 118. This can be a cost-effective, built as a single cell Realize sensor element 210, which is suitable for example for use in diesel vehicles.

FIG. 6 shows a further exemplary embodiment of a sensor element 110 which can be used according to the invention. This sensor element 110 combines essential features of the exemplary embodiments according to FIGS. 3 and 5. The sensor element 110 in turn has a solid electrolyte 114, wherein, analogously to the exemplary embodiment in FIG. 3, a pump cathode 116 is arranged on the side of the solid electrolyte 114 facing the gas space 112. and on the gas chamber 112 facing away from the inner side of the solid electrolyte 114 a pump anode 118. The pump cathode 116 is again formed as a mixed potential electrode, in this embodiment, a cermet electrode 610 and a contact frame 612. Analogous to the embodiment in Figure 3 is the Cathode shielded by a gas-impermeable cover layer 130 relative to the gas space 112, wherein in turn a flow resistance element 124 is provided which allows penetration of gas mixture from the gas space 112 into the cathode cavity 120. The pump cathode 116 is in turn electrically contacted by a cathode feed line 524, which is arranged on the upper side of the solid electrolyte 114, and a cathode connection 526.

In contrast to the exemplary embodiment according to FIG. 3, however, the inner pump anode 118 does not communicate with the gas space 112 via a gas inlet hole 122, but is completely separated from the gas space 112, analogously to the exemplary embodiment in FIG. 5, and is located via an exhaust air duct 510 with a reference gas space 512 in connection. Below the pump cathode 118, an anode cavity 132 is again provided. Anode cavity 132 and exhaust duct 510 are in turn filled with a porous filling element 530. Analogous to the above description of FIG. 5, depending on the measurement state, the exhaust air duct 510 or the anode cavity 132 serves to remove oxygen from the anode region or as a reference gas chamber for a potentiometric measurement.

The pump anode 118 is in turn electrically contacted via an anode feed line 532, an electrical through-connection 534 and an anode connection 536 on the surface of the solid electrolyte 114. In this embodiment, the anode lead 532 and the exhaust duct 510 partially formed overlapping. The construction of the sensor element 110 according to the exemplary embodiment in FIG. 6 offers substantially the same advantages as the structure according to FIG. 5, but one of the technically complicated plated-through holes in the upper solid electrolyte 114 can be dispensed with.

The exemplary embodiments of the sensor element 110 according to FIGS. 1, 3, 4, 5 and 6 have as the first electrode only the pump cathode 116. This pumping cathode 116 is thus used both in the amperometric measuring state (in this case only being a true "pumping electrode") and in the potentiometric measuring state Since amperometric measurement and potentiometric measurement are difficult to carry out simultaneously, the used above described clocked or cyclic operation in which is switched between the measurement states.

In contrast, FIG. 7 shows an exemplary embodiment in which pump cathode 116 and mixed potential electrode 710 are formed as separate electrodes. In this case, the structure of the sensor element 110 essentially corresponds to the structure of the sensor element 110 according to FIG. 5, so that with respect to the individual elements and the construction, reference can be made largely to the above description of this figure. Again, two-piece, juxtaposed, internal pumping electrodes 116, 118 are provided, wherein the pumping cathode 116 can be acted upon via a gas inlet hole 122 and a flow resistance element 124 with gas mixture from the gas space. In this case, however, the pump cathode 116 is preferably of a conventional, i. H. non-fuel gas sensitive material, such as platinum (or a platinum cermet). As in the exemplary embodiment according to FIG. 5, the pump cathode 118 is completely shielded from the gas space 112 and connected to the reference gas space 512 via an exhaust air duct 510.

In addition to the pump cathode 116, the sensor element 110 according to FIG. 7 has the described mixed potential electrode 710 on the upper side of the upper solid electrolyte 114 facing the gas space 112, which is arranged such that it overlaps in its perpendicular projection at least partially with the pump anode. The mixed potential electrode 710 has one of the fuel gas-sensitive electrode materials described above. The mixed potential electrode 710 is connected via a mixed potential electrode lead 712, which is also arranged on the surface of the solid electrolyte 114, connected to a potential contact 714, so that the sensor element 110 now has 3 electrode contacts 526, 536 and 714 on its surface.

While in the preceding examples, the pump cell and the potential cell each component-identical composed of the electrodes 116, 118 and the solid electrolyte 114, 514, pump cell and potential cell are now separated. While the pump cell is still formed from the electrodes 116, 118 and the solid electrolytes 114, 514, the mixed potential electrode 710, the upper solid electrolyte 114 and the pump anode 118 now additionally form the potential cell.

FIGS. 8A and 8B show two exemplary embodiments of systems 810 by means of which at least one gas component of a gas mixture in the gas space 112 can be measured with the aid of the sensor elements 110 described above. The system 810 in each case has at least one sensor element 110, for example one of the sensor elements described above. Preferably, a plurality of sensor elements 110 are used, for example, sensor elements in the exhaust system before and after corresponding exhaust aftertreatment devices. For simplicity, however, only one sensor element 110 is shown here. The exemplary embodiment according to FIG. 8A shows an example in which a sensor element 110 is used, in which the at least one first electrode 116 is formed in one piece. In contrast, in the exemplary embodiment according to FIG. 8B, the first electrode is formed in two pieces, with a mixed potential electrode 710 which is arranged separately from the pump cathode 116, analogously to the exemplary embodiment according to FIG.

The erfmdungs contemporary system 810 includes, in addition to the at least one sensor element 110, an electronic control device 812, which is shown only schematically and symbolically in these figures. The electronic control device 812 can be designed, for example, as an integral part of the sensor element 110, as a separate component, or it can be decentralized and partially integrated into the sensor element 110. The electronic control device comprises an amperometric measuring device 814 and a potentiometric measuring device 816. The amperometric measuring device has a voltage source 818, for example a controllable voltage source, by means of which the pumping electrodes 116, 118 can be subjected to a pumping voltage in the at least one amperometric measuring state. Furthermore, a current measuring device 820 is provided, by means of which the pumping current I _p can be measured in the at least one amperometric measuring state. Pump voltage source 818 and current measuring device 820 are driven or read by a central control unit 822, which may contain, for example, electronic components and / or a microcomputer.

The potentiometric measuring device has a voltage measuring device 824. While in the exemplary embodiment according to FIG. 8A the voltage measuring device 824 measures the voltage between the two pumping electrodes 116, 118, in the exemplary embodiment according to FIG. 8B a measurement takes place between the mixed potential electrode 710 and the pump anode 118, the latter in this case not acting as a pumping electrode but as a potential electrode. The potentiometric measuring device 816 is also actuated and read out via the central control unit 822. The central control unit 822 is connected to other components via an interface 826, so that, for example, control signals for controlling an internal combustion engine and / or exhaust aftertreatment devices can be issued by the electronic control device 812, or the electronic control device 812 can be correspondingly activated from the outside For example, in order to set the potentiometric measurement state in a regeneration operation of a catalytic converter, and in a normal operation of the motor vehicle, the amperometric measurement state. Furthermore, the electronic control device 812 may also contain a device for clocking the measurement states, for example to set a regular change between an amperometric and a potentiometric measurement state. The information obtained by the electronic control device 812 can be used, for example, in the context of an on-board diagnosis (OBD), be used by a central engine control for controlling the internal combustion engine and / or transmitted via an on-board computer to a driver. In this way, for example, a malfunction of a catalyst, ie exceeding a predetermined Limit values with respect to, for example, a fuel gas emission, immediately detected and issued a warning.

Claims

claims

1. A method for measuring at least one gas component of a gas mixture in a gas space (112), wherein at least one sensor element (110) having at least one first electrode (116), at least one second electrode (118) and at least one at least one first electrode (116 ) and the at least one second electrode (118) connecting solid electrolyte (114, 514) is used, wherein the at least one first electrode (1 16) at least one mixed potential electrode (116, 710) and can be acted upon with the gas mixture, wherein the at least one second electrode (118) is shielded from the gas space (112), characterized in that the method comprises at least two operating states: at least one amperometric measurement state, wherein between the at least one first electrode (116) and the at least one second electrode (118) a pumping voltage is applied, wherein at least one between the at least one first electrode (116) and the minde at least one second electrode (118) is detected flowing pumping current; and at least one potentiometric measurement state, wherein at least one voltage between the at least one mixed potential electrode (116; 710) and the at least one second electrode (118) is detected.

2. Method according to the preceding claim, characterized in that the concentration of at least one first gas component in the gas mixture is deduced from the at least one pumping current and that from the at least one detected

Voltage to a concentration of at least one second gas component in the gas mixture is closed.

3. Method according to one of the two preceding claims, characterized in that the at least one mixed potential electrode (116; 710) has at least one of the following properties: the at least one mixed potential electrode (116; 710) comprises at least one electrode material having a lower electrocatalytic activity than platinum; the at least one mixed potential electrode (116; 710) has a platinum electrode with an admixture of a catalytically inactive metal, in particular of gold and / or silver and / or copper and / or lead, in particular in the range between

0.05 wt% to 5 wt%, more preferably between 0.1 wt% and 1.0 wt%; the at least one mixed potential electrode (116; 710) has a platinum electrode with at least partial coverage by a catalytically inactive metal, in particular gold and / or silver and / or copper and / or lead, wherein the at least partial covering is preferably incomplete ; the at least one mixed potential electrode (116; 710) comprises an oxide, in particular a metal oxide, in particular a perovskite-based and / or chromium-based and / or gallate-based metal oxide; - The at least one mixed potential electrode (116; 710) has a ceramic-metal

Composite on; the at least one mixed potential electrode (116; 710) comprises a mixture of at least one oxide ceramic and at least one of the following metals: gold, silver, copper, lead.

4. The method according to any one of the preceding claims, characterized in that the at least one first electrode (1 16) via at least one flow resistance element (124) with the at least one gas space (112) is connected, wherein the at least one second electrode (118) via at least one diffusion resistance element (128) is connected to the at least one gas space (112) and / or at least one reference gas space (512), characterized in that the at least one flow resistance element (124) and the at least one diffusion resistance element (128) are configured such that the limiting current of the at least one second electrode (118) is smaller than the limiting current of the at least one first electrode (116).

5. Method according to the preceding claim, characterized in that the at least one flow resistance element (124) and the at least one diffusive element - -

On resistance element (128) are configured such that the at least one flow resistance element (124) has a greater flow resistance than the at least one diffusion resistance element (128).

6. The method according to one of the two preceding claims, characterized in that the at least one diffusion resistance element (128) has a greater diffusion resistance than the at least one flow resistance element (124).

7. The method according to any one of the preceding claims, characterized in that the limiting current of the at least one second electrode (118) is smaller than 1/100 of

Limit current of the at least one first electrode (116) and preferably less than 1/1000 of the limiting current of the at least one first electrode (116).

8. The method according to any one of the preceding claims, characterized in that the limiting current of the at least one second electrode (118) is 1 to 20 microamps, preferably 10 microamps, and that the limiting current of the at least one first electrode (116) 500 microamps to 6 milliamperes is preferably 1.5 milliamperes.

9. Method according to claim 1, characterized in that the at least one first electrode (116) comprises, in addition to the at least one mixed potential electrode (116; 710), at least one pumping electrode (116), wherein the pumping voltage in the at least one amperometric measuring state between the at least one pumping electrode (116) and the at least one second electrode (118) is applied, wherein the at least one pumping current between the at least one pumping electrode (116) and the at least one second electrode (118) is measured and wherein in the at least one potentiometric measuring state the at least one voltage between the at least one mixed potential electrode (116; 710) and the at least one second electrode (118) is measured.

10. The method according to the preceding claim and one of claims 3-7, characterized in that the at least one pumping electrode (116) of the at least one NEN gas space (112) is separated by the at least one flow resistance element (124).

11. The method according to any one of claims 9 and 10, characterized in that the at least one amperometric measurement state and the at least one potentiometric

Messzustand be used at least temporarily simultaneously.

12. The method according to any one of the preceding claims, characterized in that the at least one amperometric measurement state and the at least one potentiomet- rische measurement state are used clocked at least temporarily, the clock cycles are selected such that during the amperometric measurement state, a predetermined amount of at least one The first gas component to be detected is pumped from the at least one first electrode (116) to the at least one second electrode (118) in order to act as a reference there during the at least one potentiometric measurement state.

13. The method according to claim 1, wherein the potentiometric measurement state is used to generate at least one information about at least one function of at least one exhaust aftertreatment device, wherein the at least one exhaust aftertreatment device comprises at least one catalyst and / or at least one has a filter.

14. The method according to the preceding claim, characterized in that the potentiometric measurement state is used during a regeneration operation of the at least one exhaust aftertreatment device.

15. The method according to one of the two preceding claims, characterized in that the at least one information is used to regulate the at least one function.

16. The method according to one of the three preceding claims, characterized in that at least two sensor elements (110) are used, wherein at least one first - -

Sensor element (110) is used in the exhaust line before the at least one exhaust aftertreatment device and at least one second sensor element (110) in the exhaust line after the at least one exhaust aftertreatment device.

17. Method according to claim 1, characterized in that the at least one sensor element (110) has at least one of the following layer structures: the at least one first electrode (116) and the at least one second electrode (118) are on one of the at least one Gas side (112) facing side of the at least one solid electrolyte (114, 514), wherein the at least one first electrode (1 16) via at least one flow resistance element (124) is connected to the at least one gas space (112), wherein the at least one second Electrode (118) via at least one diffusion resistance element (128) is connected to the at least one gas space (112); - The at least one first electrode (116) is arranged in the interior of the at least one sensor element (110) and connected to the at least one gas space (112) via at least one gas inlet hole (122) and at least one flow resistance element (124), wherein the at least one second electrode (118) is arranged on the at least one gas chamber (112) facing side of the at least one solid electrolyte (114, 514) and is connected to the at least one gas chamber (112) via at least one diffusion resistance element (128); the at least one first electrode (116) and the at least one second electrode (118) are arranged inside the at least one sensor element (110), wherein the at least one first electrode (116) with the at least one gas space (112) via at least one gas inlet hole (122) and at least one flow resistance element (124) is connected and wherein the at least one second electrode (118) from the at least one gas space (112) shielded and connected to at least one reference gas space (512); the at least one second electrode (118) is arranged in the interior of the at least one sensor element (110) and connected to the at least one gas space (112) via at least one gas inlet hole (122) and at least one diffusion resistance element (128), the at least one first electrode (116) on the at least one gas space (112) facing side of the at least one solid electrolyte (114, 514) is arranged and with the at least one gas chamber (112) via at least one flow resistance element (124) is connected; the at least one second electrode (118) is arranged in the interior of the at least one sensor element (110) and connected to at least one reference gas space (512) via at least one gas inlet hole (122), wherein the at least one first electrode (116) the at least one gas space (112) facing side of the at least one solid electrolyte (114, 514) is arranged and with the at least one gas space (112) via at least one flow resistance element (124) is connected; the at least one first electrode (116) comprises at least one mixed potential electrode (116; 710) arranged on a side of the at least one solid electrolyte (114, 514) facing the at least one gas space (112) and at least one inside the at least one sensor element (110) arranged pump electrode (116), wherein the at least one pump electrode (116) with the at least one

Gas space (112) via at least one gas inlet hole (122) and at least one flow resistance element (124) is connected and wherein the at least one second electrode (118) from the at least one gas space (112) shielded and connected to at least one reference gas space (512).

18. An electronic control device (812) for controlling at least one sensor element (110), comprising means (814, 816, 818, 820, 822, 824, 826) for carrying out a method according to one of claims 1-17.

19. System for measuring at least one gas component of a gas mixture in a gas space (112), comprising at least one electronic control device (812) according to the preceding claim and at least one sensor element (110).