Patent Publication Number: US-2004055886-A1

Title: Electrochemical sensor for measuring the concentration of nitrogen oxides

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to an electrochemical sensor for measuring the concentration of nitrogen oxides in a gas to be measured, in particular in the exhaust gas of internal combustion engines in motor vehicles.  
       BACKGROUND INFORMATION  
       [0002] In one sensor, described in e.g., European Patent No. 678 740, the first electrodes of both pump cells are each situated in an inner chamber, the first inner chamber being acted on by the gas to be measured via a first diffusion barrier, and the second inner chamber being connected to the first inner chamber via a second diffusion barrier. Also situated in the first inner chamber is a measurement electrode or Nernst electrode which forms a Nernst cell with the reference electrode situated in the reference gas channel. On the exterior of the solid electrolyte the second electrode in the first pump cell is exposed to the gas to be measured. A pump voltage or direct-current voltage is applied to the first pump cell, and is regulated by the voltage produced in the Nernst cell due to the difference in concentration between the first inner chamber and the reference gas channel. The regulated pump voltage at the first pump cell is used to produce an oxygen partial pressure of constant value in the first inner chamber. The first electrode in the first inner chamber is made of catalytically inert material, and the pump voltage at the first pump cell is set so that the nitrogen oxides entering the first inner chamber do not decompose. The gas volume in the first inner chamber is introduced into the second inner chamber via the second diffusion barrier. The first electrode in the second pump cell is made of catalytically active material, and a constant pump voltage or direct-current voltage is applied to the second pump cell. The nitrogen oxides decompose in the second inner chamber, and the released oxygen ions are pumped from the second inner chamber by the pump voltage. The pump current flowing through the second pump cell, which is measured, is a measure of the concentration of nitrogen oxides in the gas to be measured.  
       SUMMARY  
       [0003] One example embodiment of a sensor according to the present invention may have the advantage that the first pump cell pumps the oxygen into the reference gas instead of into the gas to be measured as the result of positioning the second electrode in the first pump cell in the reference channel. The Nernst cell present in the described conventional sensor may thus be dispensed with for regulating the pump voltage at the first pump cell. The design of the sensor is simplified, resulting in lower manufacturing costs. The pump current flowing through the first pump cell has an essentially linear dependence on the oxygen concentration in the gas to be measured, so that it is possible to use the sensor, using the pump current through the first pump cell as a signal current, to also measure the oxygen concentration in the gas to be measured.  
       [0004] According to one advantageous embodiment of the present invention, the first electrodes of both pump cells are each situated in a first and second inner chamber, the first inner chamber being connected to the gas to be measured and the second inner chamber being connected to the first inner chamber. The diffusion path associated with the first electrode in the first pump cell is formed in the first inner chamber.  
       [0005] According to one advantageous embodiment of the present invention, the diffusion path is formed by the first electrode in the first pump cell itself, which fills the entire first inner chamber. This measure contributes to reduced production costs and a low overall height for the sensor.  
       [0006] In one alternative embodiment of the present invention, a diffusion channel situated in the first inner chamber may be provided as the diffusion path and optionally may be filled with a porous diffusion agent.  
       [0007] In one advantageous embodiment of the present invention, a diffusion path upstream from the first electrode in the second pump cell is omitted, and a cavity is situated between the first inner chamber and the second inner chamber in which the gas volume leaving the first inner chamber is homogenized with respect to its gas component concentration. The cavity, in which a generally constant partial pressure is established, is used as a gas reservoir for the second pump cell from which gas is continuously pumped via the second pump cell.  
       [0008] In one preferred embodiment of the present invention, the cavity is dispensed with altogether, and the second inner chamber directly joins the first inner chamber. The first electrode in the first pump cell is then dimensioned in such a way that a constant, sufficiently low oxygen concentration is achieved at the interface between the first and second inner chamber. In both cases the first electrode in the second pump cell completely fills the second inner chamber. Alternatively, however, a diffusion channel may be provided in the second inner chamber—as in the first inner chamber—which may be filled with diffusion agent.  
       [0009] According to one advantageous embodiment of the present invention, a generally constant direct-current voltage is present at the first pump voltage which is high enough to prevent decomposition of the nitrogen oxides at the first electrode, which is made of catalytically inert material such as platinum and gold, in the first inner chamber. The voltage present at the second pump cell is significantly higher, so that the nitrogen oxides decompose at the first electrode, which is made of catalytically active material such as platinum, in the second inner chamber, and the oxygen thus released is pumped into the reference gas channel. The pump current flowing through the second pump cell is a measure of the residual oxygen and nitrogen oxides concentrations in the gas to be measured. The concentration of nitrogen oxides in the gas to be measured is determined by subtracting the residual oxygen concentration, which is measurable using the pump current in the first pump cell.  
       [0010] It has been shown that, in order to make a highly accurate determination of the concentration of nitrogen oxides in the gas to be measured, it may be necessary to hold the oxygen equilibrium concentration constant within very narrow bounds at the interface between the first and second inner chamber, since the oxygen equilibrium concentration is subject to certain, albeit small, fluctuations, depending on the oxygen concentration in the gas to be measured. In order to achieve this, according to one preferred embodiment of the present invention the pump voltage at the first pump cell is adjusted to the oxygen concentration in the gas to be measured.  
       [0011] According to one advantageous embodiment of the present invention, a characteristics map of the pump current flowing through the first pump cell as a function of the oxygen concentration is stored along with the pump voltage as a parameter. The pump current flowing through the first pump cell is measured and, using the measured value, the instantaneous change in the oxygen concentration is read from the characteristics map. The variable for the change in the voltage is calculated from the ratio of the change in concentration to the oxygen concentration in the gas to be measured. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] The present invention is described in greater detail below, with reference to one exemplary embodiment illustrated in the drawings.  
     [0013]FIG. 1 shows a longitudinal section of a sensor, schematically illustrated.  
     [0014]FIG. 2 shows an illustration of a modified sensor, similar to that of FIG. 1.  
     [0015]FIG. 3 shows a diagram of the progression of the oxygen concentration over the length of the first electrodes in two successively positioned pump cells in the sensor according to FIG. 1.  
     [0016]FIG. 4 shows a diagram of the pump current flowing through the first pump cell in the sensor according to FIG. 1 as a function of the concentration of the oxygen in the gas to be measured.  
     [0017]FIG. 5 shows a diagram of the progression of the oxygen concentration over the length of the first electrode in the first pump cell for two different oxygen concentrations in the gas to be measured and two different pump voltages at the first pump cell.  
     [0018]FIG. 6 shows an enlarged section VI from the diagram shown in FIG. 5.  
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENT  
     [0019] The electrochemical sensor, schematically illustrated in FIG. 1, for measuring the concentration of nitrogen oxides in a gas to be measured, preferably in the exhaust gas of internal combustion engines in motor vehicles, has a gas-sensitive sensor element  10 , the basic structure of which is illustrated in cross section in FIG. 1. Sensor element  10  is usually housed in a sensor housing which is inserted in the exhaust pipe of an internal combustion engine in such a way that sensor element  10  is exposed to the exhaust gas of the internal combustion engine.  
     [0020] Sensor element  10 , which is designed using the planar film technique, has, for example, multiple solid electrolyte layers  11 . Solid electrolyte layers  11  are designed as ceramic films and form a planar ceramic body. The integrated shape of the planar ceramic body is produced by laminating together the ceramic films, which are imprinted with functional layers, and subsequently sintering the laminated structure. Each of the solid electrolyte layers is produced from a solid electrolyte material, such as yttrium-stabilized zirconium oxide (ZrO 2 ), for example, which conducts oxygen ions.  
     [0021] Sensor element  10  includes a first pump cell  12  which is connected to a constant—and in a refinement of the embodiment, adjustable within limits—direct-current voltage, referred to below as pump voltage U 1 , and a second pump cell  13  which is connected to a constant direct-current voltage, referred to below as pump voltage U 2 . Each pump cell  12  or  13  includes a pair of electrodes connected to pump voltage U 1 or U 2  which are situated on a solid electrolyte. Two inner chambers  14 ,  15  are formed in a very thin solid electrolyte layer  11   b  situated between upper solid electrolyte layer  11   a  and subsequent solid electrolyte layer  11   c , first inner chamber  14  having a gas inlet opening  24  to the environment of the gas to be measured, and second inner chamber  15  being connected to first inner chamber  14  via a cavity  16 . The environment of the gas to be measured is symbolized by a flow arrow  27  in FIG. 1. In solid electrolyte layer lid following in the layer compound, a reference gas channel  17  is formed which is acted on by a reference gas  17  and which for example is connected to the ambient air. A resistance heater  18  is situated between the two lower solid electrolyte layers lie and  11   f  which is embedded in electrical insulation  19  made of aluminum oxide (Al 2 O 3 ), for example. Resistance heater  18 , which is connected to a heater voltage, extends over both inner chambers  14 ,  15 , so that these are heated to approximately the same temperature level.  
     [0022] Of the two electrodes of first pump cell  12 , a first electrode  20  is situated in first inner chamber  14  and completely fills it. First electrode  20  is designed in such a way that it forms a diffusion path for the gas to be measured entering inner chamber  14 . First electrode  20  is manufactured from catalytically inert material, for example platinum and gold. Second electrode  21  of first pump cell  12  is situated in reference gas channel  17 . First pump cell  12  is thus connected to pump voltage U 1  in such a way that its higher potential is present at second electrode  21 .  
     [0023] Of the two electrodes of second pump cell  13 , first electrode  22  is situated in second inner chamber  15  and completely fills it. The first electrode is manufactured from catalytically active material, for example platinum. Second electrode  23  of second pump cell  13  is likewise situated in reference gas channel  17 . Pump cell  13  is connected to pump voltage U 2  in such a way that its higher potential is present at second electrode  23 . Both second electrodes  21 ,  23  of both pump cells  12 ,  13  in reference channel  17  are combined into a uniform electrode layer which extends over the entire length of both successively positioned inner chambers  14 ,  15  and of reference gas channel  17 .  
     [0024] If pump voltage U 1 , for example 150 mV, is switched on to first pump cell  12 , a pump current I 1  flows through first pump cell  12 , and a constant oxygen partial pressure, i.e., a constant oxygen concentration, is established at the end of first inner chamber  14  by pumping oxygen ions from first inner chamber  14  into reference channel  17 .  
     [0025]FIG. 3 illustrates the progression of oxygen concentration C in first inner chamber  14  over the length of first electrode  20  of first pump cell  12  for three different oxygen concentrations in the gas to be measured, i.e., the exhaust gas. As an example, curve a shows the progression for an oxygen concentration of 10 −9  mol/mm 3  in the gas to be measured, curve b for an oxygen concentration of 10 −10  mol/mm 3  in the gas to be measured, and curve c for an oxygen concentration of 10 −11  mol/mm 3 . As seen in FIG. 3, the oxygen equilibrium concentration of 1000 ppm, for example, associated with pump voltage U 1  with respect to reference gas (air) is established in the back region of first electrode  20 . The small pump voltage U 1  of just 150 mV, for example, and the catalytically inert material of first electrode  20  at higher pump voltages prevent the nitrogen oxides from decomposing in inner chamber  14 . This end oxygen concentration of 1000 ppm, for example, is established in cavity  16  connected downstream, which in the embodiment has the same cross section in the direction of flow of the gas volume as the two inner chambers  14 ,  15  but may be made several times larger in cross section.  
     [0026] If constant pump voltage U 2 , which is significantly higher than pump voltage U 1  at first pump cell  12 , for example 400 mV, is applied to second pump cell  13 , gas is continuously pumped out of cavity  16  in second pump cell  12  and into second inner chamber  15 . At this high pump voltage U 2  and under the catalytic effect of the catalytically active material of first electrode  22  of second pump cell  13 , the nitrogen oxides decompose in second inner chamber  15 , and the released oxygen ions are pumped into reference gas channel  17  via solid electrolyte layer  11   c.    
     [0027]FIG. 3 illustrates the progression of oxygen concentration C over length s of first electrode  22  of second pump cell  13 , for three different concentrations of nitrogen oxides in the gas to be measured, which in FIG. 3 are given as 0 ppm, 50 ppm, and 100 ppm as examples. Pump current I 2  flowing through second pump cell  13  is a measure of the nitrogen oxides concentration, including a constant residual oxygen concentration R (FIG. 1). The latter is subtracted to determine only the nitrogen oxides concentration in the gas to be measured.  
     [0028]FIG. 4 illustrates pump current I 1  which flows through first pump cell  12  as the result of pumping the oxygen from inner chamber  14  into reference gas channel  17 , as a function of oxygen concentration C in the gas to be measured. It shows that pump current I 1  has a generally linear dependence on oxygen concentration C, so that it is also possible to use the sensor for measuring the oxygen concentration in the gas to be measured.  
     [0029] It has been shown that the equilibrium concentration of oxygen established in cavity  16  is not absolutely constant, but instead fluctuates, albeit within narrow bounds, as a function of the oxygen concentration in the gas to be measured. FIG. 5 illustrates the progression of oxygen concentration C over length s of first electrode  20  in first inner chamber  14  for a concentration C=10 −9  mol/mm 3  and a pump voltage U 1 =0.2 V (curve a), and for a concentration C=10 −11  mol/mm 3  and a pump voltage U 1  which is also 0.2 V (curve b). The curve progression is illustrated in FIG. 6 for enlarged region VI in FIG. 5. It is evident that changing the oxygen concentration in the gas to be measured also results in a change, albeit small, in the oxygen equilibrium concentration at the end of first electrode  20  and in cavity  16 . Consequently, pump current I 2  flowing in second pump cell  13  is no longer precisely constant with respect to portion R which results from the residual oxygen concentration, but instead is dependent on the oxygen concentration in the gas to be measured, so that the measured concentration of nitrogen oxides is somewhat distorted.  
     [0030] To ensure a highly precise measurement, pump voltage U 1  at first pump cell  12  is modified as a function of the oxygen concentration in the gas to be measured, and thus as a function of pump current I 1  flowing through first pump cell  12 . To this end, a characteristic map is used in which pump current I 1  flowing through first pump cell  12  as a function of oxygen concentration C in the gas to be measured, along with pump voltage U 1 , is stored as a parameter. Pump current I 1  flowing through first pump cell  12  is measured, and, using the measured value, instantaneous change ΔC in the oxygen concentration is read from the characteristics map. The variable for the required change in voltage ΔU is calculated from the ratio of change in concentration ΔC to concentration C of oxygen in the gas to be measured, according to  
         Δ                 U     =     K   ·       Δ                 C     C                     
 
     [0031] where constant K is estimated from the Nernst equation. For each decade by which the concentration of oxygen in the gas to be measured changes, the ratio is ΔC/C=0.9.  
     [0032] In the example shown in FIGS. 5 and 6, the change in concentration between curves a and b is 10 −2  mol/mm 3 , and the concentration is thus changed by two decades. If, for example, a voltage reduction ΔU of 2·(−0.014) V=−0.028 V occurs during a drop in concentration from 10 −9  to 10 −11  mol/mm 3 , this results in curve c which generally coincides with the region of curve a. As a result of this voltage reduction ΔU, the oxygen concentration in the end region of first electrode  20  of first pump cell  12  remains absolutely constant, and shows no dependence whatsoever on the concentration of oxygen in the gas to be measured. Thus, current portion R contained in pump current I 2  in second pump cell  13 , which is the result of the oxygen concentration present in cavity  16 , is also constant, and the nitrogen oxides concentration may be determined very precisely by subtracting this constant current portion R.  
     [0033] When pump voltage U 1  present at first pump cell  12  is thus adjusted to the change in oxygen concentration in the gas to be measured, it is possible to dispense with cavity  16  between the two inner chambers  14 ,  15  for homogenizing the gas volume leaving first inner chamber  14  with respect to its gas component concentration, and the layout of the electrodes may be designed so that the two inner chambers  14 ,  15  may merge directly into one another.  
     [0034] In one modification of sensor element  10  illustrated in FIG. 2, a diffusion path is situated upstream from each inner chamber  14 ,  15  of first electrode  20  or  22  in first pump cell  12  or in second pump cell  13 , respectively. This diffusion path includes a diffusion channel  25  or  26  which may be filled with a porous diffusion material such as Al 2 O 3 , as shown in FIG. 2. When the diffusion material is omitted, diffusion channel  25  must be structurally designed so that, in spite of the unrestrained flow of gas to be measured into first inner chamber  14  via gas inlet opening  24 , a sufficiently low, constant oxygen concentration results at the end of first electrode  20  in first pump cell  12 .