Method for Determining Relative Amount of Oxygen Containing Gas in a Gas Mixture

FIG. 1 is a schematic of an electrochemical device for measuring the relative amount of a measurement gas in a gas mixture according to one embodiment of the present invention.

FIG. 2 is a graph relating the relative amount of H.sub.2 O in a combined intake air and exhaust mixture to the relative amount of EGR.

FIG. 3 is a graph showing the output of the measurement gas sensor according to the embodiment of the present invention shown in FIG. 1 as a function of EGR.

FIG. 4A is a schematic of a single-cell oxygen pumping device according to the prior art.

FIG. 4B is a graph showing the relation between the pumping current and the pumping voltage of the oxygen pumping device of FIG. 4A in O.sub.2 /N.sub.2 gas mixtures.

FIG. 4C is a graph showing the relation between the pump current and the pumping voltage of the oxygen pumping device of FIG. 4A in CO.sub.2 /N.sub.2 gas mixtures.

FIG. 4D is a graph showing the relation between the pumping current and the pumping voltage of the oxygen pumping device of FIG. 4A in O.sub.2 /CO.sub.2 /N.sub.2 gas mixtures.

FIG. 5 is a graph showing the relation between the pumping current and the pumping voltage of the prior art oxygen pumping device of FIG. 4A in O.sub.2 /H.sub.2 O gas mixtures.

FIG. 6 is a graph showing the relation between the pumping current and the pumping voltage of the present invention oxygen pumping device of FIG. 1 in O.sub.2 /H.sub.2 O gas mixtures.

FIG. 7 is a schematic of an electrochemical device for measuring the relative amount of a measurement gas in a gas mixture according to a second embodiment of the present invention.

FIGS. 8A and 8B are schematics of electrochemical devices for measuring the relative amount of a measurement gas in a gas mixture according to other embodiments of the present invention.

FIG. 9 is a schematic of an electrochemical device having a planar configuration for measuring the relative amount of a measurement gas in a gas mixture according to another embodiment of the present invention.

This invention is directed to an electrochemical oxygen pumping device and method for determining the relative amount of a measurement gas consisting essentially of at least one oxygen containing gas in a gas mixture comprising in addition at least a second oxygen containing gas which is capable of being pumped out of the device or disassociated at a voltage less than that which is capable of disassociating the measurement gas. One embodiment of the device is shown in FIG. 1. This device 20 has two electrochemical cells 21 and 31 arranged with a supporting structure so that a restricted volume 40 is defined. Volume 40 communicates with a gas mixture (e.g., O.sub.2, H.sub.2, and H.sub.2 O) through at least one aperture 41. Each of the two cells consists of a platelet 22, 32 made from an oxygen ion conducting solid electrolyte such as ZrO.sub.2, and two electrodes 23, 24, 33, 34 applied on the two sides of the platelets. These electrodes are made from platinum or some other material according to procedures well established in the area of oxygen sensors. Electrochemical cells 21 and 31 are operated as oxygen pumps having currents I.sub.1 and I.sub.2 passing through them when appropriate voltages are applied across each cell. Advantageously, a heater 29 is positioned adjacent sensor 20 to provide an elevated temperature of about at least 500.degree. C. suitable for operation of sensor 20.

In order to more fully understand the operation of device 20, first consider a single ZrO.sub.2 device 60 of FIG. 4A well known in the art for use in sensing oxygen gas (O.sub.2) percentage in the intake air/exhaust mixture of internal combustion engines. Operation of this prior art type of device is based on oxygen pumping. It has a single ZrO.sub.2 cell 61 made from a ZrO.sub.2 platelet with two platinum electrodes 63 and 64 arranged in a structure so that a volume 65 is defined. Volume 65 communicates with a gas mixture comprising O.sub.2, e.g., an intake air/exhaust gas mixture of an internal combustion engine, through an aperture 66. When a voltage is applied across cell 61 so that electrode 63 is negative, a current I.sub.3 passes through the ZrO.sub.2 material as a result of a motion of oxygen ions from electrode 63 to electrode 64.

As the oxygen ions formed at electrode 63 travel through the ZrO.sub.2 platelet to electrode 64, more O.sub.2 molecules from the gas phase disassociate and react with electrons at electrode 63 to form oxygen ions (O.sup.=). By means of this electrochemical reaction, as oxygen ions are depleted at electrode 63 (in traveling to electrode 64) more oxygen ions are formed from O.sub.2 gas molecules in volume 65. By means of an inverse electrochemical reaction, oxygen ions at electrode 64 are released as O.sub.2 molecules into the ambient gas. The net effect of the current through the cell is to pump O.sub.2 out of volume 65. Because of the lower concentration of O.sub.2 inside volume 65, there will be a diffusional flux of O.sub.2 from the gas mixture (e.g., intake air/exhaust gas mixture) into volume 65 through aperture 66. Under steady state conditions, the diffusional flux of O.sub.2 into volume 65 will be equal to the flux of O.sub.2 pumped out of volume 65 by the pumping current.

In an O.sub.2 /N.sub.2 mixture, the current voltage (I-V) characteristic of prior art device 60 is shown in FIG. 4B. For small voltages, the current increases with voltage as more oxygen is pumped out of volume 65. For sufficiently large voltages, the pumping current saturates. This corresponds to the condition that all oxygen inside volume 65 is pumped out by the current. The saturation current I.sub.s is proportional to the relative amount of O.sub.2 in the ambient. FIG. 4C shows the I-V characteristic of sensor 60 in a CO.sub.2 and N.sub.2 mixture. Because of the lack of oxygen, the pumping current is zero for low voltages. Above a threshold value of about 0.7 volts, the pumping current increases with voltage due to electrodisassociation of CO.sub.2. For still higher voltages, saturation currents are again observed corresponding to the disassociation of all CO.sub.2 inside volume 65. The saturation current is proportional to the relative amount of CO.sub.2 in the gas. A similar I-V characteristic is obtained with H.sub.2 O/N.sub.2 mixtures except that the threshold voltage for the disassociation of H.sub.2 O is somewhat larger (about 1.1 volt). It is apparent that the prior art device of FIG. 4A can be used to measure the total concentration of oxygen containing gases, e.g., O.sub.2, H.sub.2 O, and CO.sub.2, in a gas mixture additionally containing inert gases (i.e., non-oxygen containing gases), e.g. N.sub.2. This can be done by applying a sufficiently large voltage to achieve complete disassociation of all of the oxygen containing gases. FIG. 4D shows the I-V characteristic of prior art device 60 in O.sub.2, CO.sub.2, and N.sub.2 gas mixtures, in particular for a mixture containing a fixed concentration of O.sub.2 (2%) and variable concentrations of CO.sub.2. The saturation current observed at lower voltages (V<1 volt) corresponds to the complete removal of O.sub.2 from inside volume 65; the saturation current obtained at the higher voltage (V>1.1 volts) is proportional to the concentration of O.sub.2 plus CO.sub.2 in the mixture. FIG. 5 shows the I-V characteristics of prior art device 60 in gas mixtures comprising a fixed concentration of O.sub.2 (20%) and variable amounts of H.sub.2 O as may be present in air. The saturation current I.sub.s is proportional to the concentration of O.sub.2 plus H.sub.2 O in the mixture. However, since the concentration of the O.sub.2 remains fixed, the relative amount of the H.sub.2 O in the gas mixture could be determined from the value of the saturation current, whose variation would be due then to the varying concentration of the H.sub.2 O as seen in FIG. 5. Toshio Usui and Yoichi Kurumiya in "Humidity Sensing Properties Of The Limiting Current Type Oxygen Sensor", Transducers '87, pp. 701-704, disclose an oxygen sensor of the single-cell ZrO.sub.2 type similar to prior art device 60 shown in FIG. 4A. This reference discloses I-V characteristics for gas mixtures of O.sub.2, N.sub.2 and O.sub.2, and H.sub.2 O.

Device 60 of FIG. 4A cannot be used, however, to determine the relative amount of one oxygen containing gas, such as H.sub.2 O, in a gas mixture containing at least one other oxygen containing gas, such as O.sub.2, when a variable amount of O.sub.2 exists in the gas mixture. In such a situation, the saturation current would be proportional to the total concentration of O.sub.2 +H.sub.2 O. Since both concentrations are variable, using such a prior art device, one could not determine what proportion of the saturation current was due to each gas individually.

The measurement of the relative amount of a gas, e.g., H.sub.2 O, in a mixture comprising at least one other oxygen containing gas, e.g., O.sub.2 can be accomplished, however, with device 20 of FIG. 1, an embodiment of the present invention. For a gas mixture consisting essentially of O.sub.2, H.sub.2 O, a constant voltage of less than 0.8 volts, and advantageously in the range of 0.2 to 0.8 volts, is applied across cell 21 so that electrode 24 is negative to pump all O.sub.2 out of volume 40. A second constant voltage larger than about 1.0 volts, and advantageously in the range of 1.2 to 2.0 volts is applied across cell 31 so that electrode 33 is negative to disassociate all H.sub.2 O inside volume 40. The current I.sub.2 through cell 31 is proportional to the H.sub.2 O concentration in the gas mixture. If the gas mixture were to comprise O.sub.2, CO.sub.2, and H.sub.2 O and it were desired to again measure the H.sub.2 O concentration, a constant voltage greater than 0.7 volts but less than about 1.0 volts (i.e. the voltage necessary to pump out all O.sub.2 and disassociate all CO.sub.2 without disassociating H.sub.2 O) would be applied across cell 21 with electrode 24 being made the negative electrode. A second constant voltage larger than about 1.0 volts, and advantageously in the range of 1.3 to 2.0 volts, is applied across cell 31 so that electrode 33 is negative to disassociate all the H.sub.2 O in the volume 40. The current I.sub.2 through cell 31 is proportional to the H.sub.2 O concentration in the gas mixture.

As described above, the gas mixture comprises at least two oxygen containing gases. The oxygen containing gases can be selected from the group consisting essentially of oxygen, O.sub.2, and oxygen containing gas compounds such as NO.sub.2, CO.sub.2, H.sub.2 O, SO.sub.2, CO, etc. Exemplary of gas mixtures which may be employed according to the invention are O.sub.2 and H.sub.2 O; O.sub.2, H.sub.2 and H.sub.2 O; H.sub.2 O and CO.sub.2 ; H.sub.2 O, CO.sub.2, N.sub.2 and CO; O.sub.2, CO.sub.2 and H.sub.2 O; O.sub.2, N.sub.2 and H.sub.2 O; O.sub.2, N.sub.2 and CO.sub.2 ; O.sub.2 and CO.sub.2 ; air; and the like. Still other gas mixtures which may be employed in the present invention device and method will be apparent to those skilled in the art in view of the present disclosure.

If the gas mixture employed in the invention is selected from a gas mixture comprising H.sub.2 O in addition to at least one other oxygen containing gas, the device can function as a humidity sensor. That is, the device can be used to determine the relative amount of water vapor (H.sub.2 O) in the gas mixture. During practice of the invention, oxygen containing gases which can be pumped out of the device or disassociated at a voltage lower than that required to disassociate any measurement gas, e.g., H.sub.2 O, will be pumped out or disassociated by first pump cell 21. Accordingly, in the first six gas mixtures disclosed in the previous paragraph, if the device is used to measure humidity, the following gases will be respectively pumped out or disassociated at first pump cell 21: O.sub.2 ; O.sub.2 ; CO.sub.2 ; CO and CO.sub.2 ; O.sub.2 and CO.sub.2 ; and O.sub.2. Since, according to the invention the measurement gas is disassociated by the second cell, in these mixtures, the voltage applied across second pump cell 31 would be that sufficient to disassociate substantially all of the H.sub.2 O molecules. As mentioned above, the humidity (H.sub.2 O content) of air as measured by the present device is the absolute humidity. Relative humidity can be determined by further employing a thermistor to measure the temperature.

Should it be desired to measure, e.g., CO.sub.2 in a O.sub.2, CO.sub.2 and H.sub.2 O gas mixture, a voltage sufficient to pump out O.sub.2 at first pump cell 21 would be applied and a voltage sufficient to disassociate CO.sub.2 (but less than that which will disassociate H.sub.2 O) would be applied across the second pump cell 31. The device may be employed, e.g., to maintain a desired relative amount of CO.sub.2 in a controlled environment. Should it be desired to determine the amount of CO.sub.2 +H.sub.2 O in this gas mixture, a voltage would be applied across first pump cell 21 sufficient to pump out substantially all O.sub.2 and a higher voltage, sufficient to disassociate both the CO.sub.2 and H.sub.2 O, would be applied across second pump cell 31. This embodiment is the subject of the Logothetis et al U.S. application Ser. No. 055,821 described above.

As disclosed above, the present invention device is capable of measuring the relative amount of a measurement gas in a gas mixture. The measurement gas whose concentration is measured according to the present invention can be one or more gases selected from the gas mixture which can be disassociated at second pump cell 31 at a voltage greater than that at which the second oxygen containing gas can be pumped out or disassociated at the first pump cell 21. The second oxygen containing gas pumped out or disassociated at pump cell 21 can be one or more gases. If only one second oxygen containing gas is removed at pump cell 21 and should it be desired to additionally measure the concentration of this gas, e.g., the O.sub.2 concentration in an O.sub.2, CO.sub.2, H.sub.2 O gas mixture, currently available oxygen sensors could be used. Alternately, according to another embodiment of this invention, the sensor of this invention could be used as described above with the addition that the pumping current I.sub.1 through cell 21 is also measured by suitable circuitry connected across cell 21. In this case I.sub.1 is proportional to the O.sub.2 concentration. According to yet another embodiment of-this invention, the sensor of this invention could be used to measure the concentration of the gas removed at cell 21 by applying voltage, V=0, across first pump cell 21 and then applying across second pump cell 31 a voltage sufficient to pump out the O.sub.2, but less than that which will disassociate any other oxygen containing gases (CO.sub.2, H.sub.2 O). The current generated across cell 31 being proportional to the concentration of O.sub.2. In effect, using the present invention device in this second way converts it, for a time, to a single cell oxygen sensor.

Similarly, in a CO.sub.2, H.sub.2 O gas mixture, according to the present invention, CO.sub.2 would be disassociated at first pump cell 21 and the H.sub.2 O would be disassociated and its concentration measured at second pump cell 31. Should it be desired to additionally measure the concentration of CO.sub.2, known oxygen sensors could be employed as described above. Alternately, according to another embodiment of this invention, the sensor of this invention could be used with the addition that the pumping current I.sub.1 through cell 21 is also measured. In this case, I.sub.1 is proportional to the CO.sub.2 concentration. According to still another embodiment of this invention, the sensor of this invention could be used to measure the concentration of the CO.sub.2 gas removed at cell 21 by applying voltage, V=0, across first pump cell 21 and applying a voltage across second pump cell 31 sufficient to disassociate CO.sub.2 but less than that which will disassociate H.sub.2 O. While the discussions of this and the prior paragraph for measuring the concentration of the gas removed at pump cell 21 have been directed to device 20, it will be apparent to those skilled in the art in view of the present disclosure that this techniques can be similarly applied to the planar device of this invention.

If the O.sub.2, CO.sub.2 and H.sub.2 O mixture is an exhaust gas/intake air mixture of an internal combustion engine, this invention can be used to determine the EGR based on the relative amount of H.sub.2 O in the exhaust gas/intake air mixture. As is known in the art, for an engine controlled at the stoichiometric air-to-fuel ratio, the relative amount of H.sub.2 O in the exhaust gas is constant for constant humidity in the air. For example, for a fuel with hydrogen-to-carbon ratio of about 2.3 (stoichiometric A/F value equal to 14.7), the relative amount of H.sub.2 O in the exhaust gas at A/F=14.7 is about 13% by volume. FIG. 2 shows the relative amount of H.sub.2 O in the combined exhaust gas/intake air mixture as a function of the relative amount of EGR. FIG. 3 shows the output (pumping current of cell 31) of sensor 20 as a function of EGR. The effect of humidity in the air is indicated by showing plots for 0% and 100% humidity at a temperature of 70.degree. F. It is apparent that the relative amount of H.sub.2 O in the combined exhaust gas/intake air depends appreciably on humidity. The effect of humidity normally present in air can be eliminated by measuring the H.sub.2 O for zero EGR (completely closed EGR valve), i.e., by using the device to measure the relative amount of water vapor in the air& Alternately, a second (separate) device of this type could be used to measure the relative amount of water vapor in the air in order to correct for the ambient water vapor and thus accurately determine EGR based on the relative amount of H.sub.2 O in the exhaust gas/intake air mixture.

As disclosed above, this invention is capable of measuring the relative amount of a measurement gas consisting essentially of at least one oxygen containing gas in a gas mixture comprising in addition at least a second oxygen containing gas which is capable of being pumped out of the device or disassociated at a voltage less than that which is capable of disassociating the measurement gas. Accordingly, in a gas mixture containing O.sub.2, A, B, and C (A, B and C being oxygen containing gases) and if O.sub.2 can be pumped out at a voltage lower than that capable of disassociating A, B, and C, the relative amount of each of A, B and C in the gas mixture can individually be measured. In addition, also as disclosed above, this invention is capable of measuring the total relative amount of two or more oxygen containing gases (also termed "measurement gas") in a gas mixture containing at least a second (i.e., one additional) oxygen containing gas. For the sake of explanation, let us assume that gas A disassociates above 1.3 volts, that gas B disassociates above 1.9 volts and that gas C disassociates above 2.3 volts. According to an embodiment of this invention, in order to measure the total concentration of gases A+B, a voltage of about 0.8 volts is applied across cell 21 (electrode 24 being negative) to pump out all O.sub.2 from volume 40 and a voltage of 2.2 volts is applied across cell 31 (electrode 33 being negative) to disassociate all A and B (but not C) gas molecules in volume 40. The saturation current of cell 31 would be proportional to the concentration of A plus B in the mixture. In this case, O.sub. 2 would be considered the second oxygen containing gas and A plus B would be considered the measurement gas. Similarly, the total concentration of A+B+C would be determined by applying a voltage of about 0.8 volts across cell 21 and a voltage of, for example, 2.6 volts across cell 31. To measure the total relative amount of B+C in the gas mixture a voltage in the range of 1.3-1.8 volts is applied across cell 21 to remove O.sub.2 and disassociate all A gas molecules in volume 40 and a voltage of, for example, 2.6 volts is applied across cell 31 to disassociate all B and C molecules in volume 40. The saturation current across cell 31 would be proportional to the concentration of B plus C in the mixture. In this latter case, B plus C would be the measurement gas and O.sub.2 plus A would be the second oxygen containing gas. The voltage at which each of the oxygen containing gases in a mixture disassociate can be obtained by scanning the voltages as would be apparent to one skilled in the art in view of the present disclosure. Effective use of the invention to measure a measurement gas, individually or in total, would depend in part on the difference between the voltages applied across the two cells. If the difference is very small, measurement of the gas would be less effective, i.e., it is more difficult to separate the gases at each of the electrodes. Other factors which influence the effective use of the invention include factors such as type of electrode material (fast electrode processes), physical state (microstructure) of the electrodes, and the geometry of the device. The techniques described herein for measuring the relative amount of, e.g., B plus C in the mixture are similarly applicable to the planar device disclosed herein as would be apparent to one skilled in the art in view of the present disclosure.

The device and method of this invention (including the aspects disclosed herein directed to the planar device and method) can be used in a continuous way (i.e., to continuously monitor the relative amount of an individual gas or a combination of gases in a gas mixture) or in an intermittent (sampling) way. That is, the values of the appropriate individual voltages across the first and second pump cell may be maintained for an extended period of time during which it is desired to measure the relative amount of the measurement gas in the restricted volume, e.g., the humidity in air during the entire operation of an automobile. On the other hand, the determination of one gas need not be done continuously. Measurement of e.g., the humidity of air can be done on a periodic (intermittent) basis. If it is desired to determine the individual concentration of two or more gases in a gas mixture, e.g., the concentration of CO.sub.2 in air and also the humidity of the same air sample, the voltages across the first and second pump cells would be alternated, first to that combination which will allow measurement of the CO.sub.2 in the air sample (as has been described above) and then to that combination which will allow measurement of the humidity (H.sub.2 O) in the air (as also has been described above). In the same way, the present invention is capable of measuring the concentration of one oxygen containing gas and also the total of two or more of the oxygen containing gases in the gas mixture. Numerous ways of applying the invention to measure the relative amount of oxygen containing gas in a gas mixture will be apparent to those skilled in the art in view of the present disclosure.

The device of FIG. 1 operates under the assumption that cell 21 can pump all oxygen entering volume 40 through aperture 41 so that only a measurement gas reaches cell 31, (since aperture 41 is in close proximity to electrode 24 and remote with respect to electrode 33). If this is not the case with the configuration of FIG. 1, the desired condition can be accomplished by modifying the device structure 50 as shown in FIG. 7. In this structure, a porous layer 38 is deposited on top of the inner electrode of cell 31. This porous layer is made from ZrO.sub.2 or an inert material (e.g. spinel or aluminum oxide) and acts as a barrier to O.sub.2 diffusion so that all O.sub.2 is pumped out by cell 21.

Several other device configurations are possible. According to other embodiments of this invention, the two pumping cells are more strongly decoupled by placing between them barriers to oxygen diffusion. For example, the porous layer in the device of FIG. 7 may be relilaced with a "wall" 42 having an aperture 43 as shown in device 50A of FIG. 8A. Another type of configuration is shown in device 50B of FIG. 8B. Device 50B uses a pump cell to remove the O.sub.2 or decompose oxygen containing gases other than the measurement gas and a pump cell/sensor cell structure 70 (similar to the sensor structure described in U.S. Pat. No. 4,272,329 by Hetrick et al) to measure the measurement gas.

As disclosed above, the electrochemical device for determining relative amount of the measurement gas can also be made in a planar configuration. FIG. 9 shows one embodiment of a planar device 80 according to this invention. One starts with a dense platelet 82 and deposits porous electrodes 83 and 84 on both sides of platelet 82 to form a first oxygen pump cell 81. Platlet 82 is made from an oxygen ion conducting solid electrolyte such as ZrO.sub.2. The electrodes are made of platinum or some other material according to procedures well established in the area of oxygen sensors. A first porous layer 89 made of ZrO.sub.2 or an inert material such as spinel or alumina is deposited on top of electrode 83 to form a barrier to diffusion of O.sub.2 molecules. A first porous electrode 85 is deposited on first porous layer 89 followed by another porous layer 86 made from an oxygen ion conducting electrolyte such as ZrO.sub.2. Finally, a second porous electrode 87 is deposited on top of second porous layer 86. Porous layer 86 and electrodes 85 and 87 form second pump cell 88. The gas mixture communicates with the first porous layer 89, by means of, for example diffusion of the gas mixture into porous layer 89 through its sides. The sides may be exposed or covered with porous electrode material through which the gas mixture may diffuse into the first porous layer 89. The gas mixture may also communicate with the first porous layer 89 by diffusion of the gas mixture into second porous layer 86 and subsequently diffusion of the gas mixture through first porous electrode 85 into first porous layer 89.

An appropriate voltage is applied across second pump cell 88, the second electrode 87 being biased positively, to pump out or decompose oxygen containing gases which can be pumped out or disassociated at voltages lower than that which decompose the measurement gas. This voltage is less than that which is capable of disassociating the measurement gas in the porous parts of the structure. An appropriate voltage is applied across first pump cell 81 to completely disassociate only all of the measurement gas inside the porous layer 89. The saturation current of pump cell 81 is proportional to the relative amount of measurement gas in the gas mixture.

Various modifications and variations will no doubt occur to those skilled in the art to which this invention pertains. For example, the particular construction of the two cell oxygen pumping device may be varied from that disclosed herein. These and all other variations which basically rely on the teachings through which this invention has advanced the art are properly considered within the scope of this invention.