WO1997041428A1

WO1997041428A1 - Solid-state gas sensor

Info

Publication number: WO1997041428A1
Application number: PCT/US1997/006981
Authority: WO
Inventors: Fernando H. Garzon; Eric L. Brosha
Original assignee: The Regents Of The University Of California
Priority date: 1996-04-30
Filing date: 1997-04-24
Publication date: 1997-11-06
Also published as: EP0896668A1; BR9709755A; EP0896668A4; CA2262755A1; AU2926197A

Abstract

A solid-state sensor (50) provides for sensitive detection of a selected gas in a gaseous mixture to output a signal useful for monitoring and control. The sensor has a solid-state oxide electrolyte (54), a first electrode (52) contacting the electrolyte for oxidizing or reducing the selected gas in the gaseous mixture at a first reaction rate, and a second electrode (56) contacting the electrolyte for reducing oxygen in the gaseous mixture at a second reaction rate. The difference in the first and second rates produces a measurable electrochemical potential across the electrolyte. The improvement in the sensor comprises the first electrode formed from a dense thin film of a mixed conductor suitable for catalyzing the oxidation or reduction of the selected gas. Suitable mixed conductor films are formed from perovskite-type and fluorite-type oxides with a thickness less than about 1 um.

Description

SOLID-STATE GAS SENSOR

BACKGROUND OF THE INVENTION This invention relates to gas sensors, and, more particularly, to gas sensors that use a potential difference between exposed electrodes as an output signal related to gas concentration. This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the

U.S. Department of Energy. The government has certain rights in the invention.

It is known to detect a specific analyte gas in a gas mixture by a potential difference generated between electrodes disposed on a solid electrolyte from electrooxidation and electroreduction reactions that occur at different rates at the electrode/electrolyte interface. For example, the electrode reactions occurring on electrodes exposed to a gas stream containing CO and O₂ may be written:

CO_(g) + O² _(βl) o CO_{2 (g)} + 2e-_w O² _(βl) o y₂O_2(g) + 2e-_(d) where (el) denotes the electrolyte phase, (g) the gas analyte phase, and (ct) the catalytic electrode phase. The reduction and oxidation reactions occurring at the electrodes may proceed at different rates, leading to a potential difference between electrodes that is functionally related to the concentration of CO in a stream of mixed gases.

D. E. Williams et al., "Solid Electrolyte Mixed Potential Phenomena," Solid State Chemistry, pp. 275-278 (1982), teach CO sensor 10 that uses two different noble metal electrodes, e.g., one gold electrode 16 and one platinum electrode 14. separated by yttrium-zirconium oxide (YSZ) electrolyte 12, as shown in Figure 1 A. The reaction rate on the surface of gold electrode 16 is not the same as the rate on platinum electrode 14. Since the local exchange current density depends on surface coverage of several species and on reaction rate constants, these generally will be different for different electrode materials. The result is a measured electric potential 18 across the yttrium-zirconium oxide electrolyte 12 due to a difference in oxygen activity (oxygen chemical potential or oxygen partial pressure) across electrolyte 12.

N. Li et al., "High-Temperature Carbon Monoxide Potentiometric Sensor," 140 J. Electrochem. Soc, No. 4, pp. 1068-1072 (1993), teach a CO sensor 20,_shown in Figure 1 B, that functions on the same principles as the Williams device, above. But different oxidation and reduction (redox) rates across oxygen ion conductor 22 are obtained by coating Pt electrode 24 with an oxide coating 26, which may be a mixture of CuO and ZnO. This two- phase coating 26 of the oxides of copper and zinc is a known CO oxidation catalyst. The differing rates of CO oxidation at coated electrode 24 and uncoated Pt electrode 28 lead to a difference in oxygen chemical potential across solid electrolyte 22, with a concomitant measurable electric potential 32 across sensor 20.

N. Miura et al., "Mixed Potential Type NO_x Sensor Based on Stabilized Zirconia and Oxide Electrode." 143 J. Electrochem. Soc, pp. L33-L35 (1996), teach the reduction of NO_x using Pt electrodes, where one Pt electrode covers a metal oxide layer, similar to Li et al., above. It was found that CdMn₂O₄ operates to catalyze the oxidation of NO and the reduction of NO₂

The reported sensors do not provide adequate response times for control applications and the operating temperature range is not adequate for many applications. At least in part, the sensitivity of the prior art devices are limited by the area available for the electrochemical sensing reactions to occur. The active area is the area of gas phase contact with both the solid electrolyte surface and the electrically conductive metal electrode surface. This three phase contact occurs over only a small area on conventional devices, such as taught by Williams et al. and Li et al., supra, since the metal electrodes do not have a high coefficient of diffusion for the oxygen or oxygen ions needed at the interface. Accordingly, it is an object of the present invention to increase the interface area over which the gas electrochemical sensing reactions take place.

Another object of the present invention is to provide a gas sensor having a response time suitable for real-time monitoring and control applications.

One other object of the present invention is improve the operating temperature range of solid-state gas sensors.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the apparatus of this invention may comprise a sensor for detecting a selected gas in a gaseous mixture. The sensor has a solid-state oxide electrolyte, a first electrode contacting the electrolyte for oxidizing or reducing the selected gas in the gaseous mixture at a first reaction rate, and a second electrode contacting the electrolyte for reducing oxygen in the gaseous mixture at a second reaction rate. The difference in the first and second rates produces a measurable electrochemical potential across the electrolyte. The improvement in the sensor comprises the first electrode formed from a dense thin film of a mixed conductor suitable for catalyzing the oxidation or reduction of the selected gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incoφorated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIGURE 1 A is a cross-sectional representation of a prior art solid-state carbon monoxide sensor. FIGURE 1 B is a cross-sectional representation of a prior art solid-state gas sensor.

FIGURES 2A, 2B and 2C are representations of solid-state gas sensors according to the present invention.

FIGURE 3 graphically compares the response of a sensor in accordance with the present invention with a prior art sensor.

FIGURE 4 graphically compares the effect of temperature on the response of a sensor in accordance with the present invention with a prior art sensor.

FIGURE 5 graphically depicts the effect of current biasing on the response of a sensor according to the present invention.

FIGURES 6A and 6B graphically depict the response of a sensor according to the present invention with mixed conductor films having thicknesses of 1 μm and 0.15 μm, respectively.

FIGURE 7 graphically compares the response of a sensor having a thin film mixed conductor according to the present invention with a sensor having a thick film mixed conductor.

DETAILED DESCRIPTION OF THE INVENTION Referring first to Figure 2A, gas sensor 40 is provided with one electrode 42 formed of a dense thin film of mixed electronic and ionic conducting oxide. In one embodiment, the oxide is a perovskite-type oxide, i.e., an oxide of the general formula

and related structures such as A_2.A'MNO₄ (M=Cu, Ni), where A and A' are cations of larger size than B, B'. In another embodiment, the oxide forming electrode 42 is a fluorite-type oxide. Both the perovskite-type and fluorite-type oxides are referred to herein as mixed conductors, which may be used for electrode 42. Electrode 42 is deposited on one surface area of solid electrolyte 44 and acts both as a catalyst for electrooxidation or electroreduction of the analyte gas and as a source of oxygen for the reaction. Exemplary analyte gases include carbon monoxide (CO), hydrogen (H₂), hydrocarbons (such as alcohols), and nitric oxide (NO_x). Counter electrode 46 is deposited on a second surface of solid electrolyte 44. A suitable counter electrode is a noble metal, such as gold or platinum, or another mixed conductor that catalyzes the reduction of oxygen. Since a mixed conductor is an electronic conductor, no auxiliary metal electrode is required, although a small contact pad (not shown) may be applied to facilitate connecting an external circuit, e.g., voltmeter 48. In a particular embodiment, mixed conductor electrode 42 is deposited by a thin film deposition process, such as RF magnetron sputtering vapor deposition or an electron beam deposition, that yields a dense film. The term "dense" as used herein means a film that is near theoretical crystallographic density, i.e., porosity less than about 20%. Further, as used herein, the term "thin film" means a film having a thickness less than about 1 μm to prevent signal degradation from the large oxygen storage capacity of the mixed conductor material and preferably in the range of about 1000-2000 A (0.1 μm to 0.2 μm) to provide a fast response to the analyte gas. A metal electrode pad (not shown) of a conventional material may be sputter deposited on a portion of the mixed oxide film wherein an external circuit lead is soldered to the electrode pad.

An exemplary solid electrolyte 44 is a yttrium-zirconium oxide (500 μm thick with an area of 1 cm² available from CeraFlex®). Indeed, the film of mixed conductor 44 having a thickness as noted above does not inhibit the flow of oxygen to the interface area between mixed conductor 44 and electrolyte 44 but distributes oxygen ions over the entire interface area for rapid response time and increased sensitivity to analyte gas concentration changes. The thickness of solid electrolyte 44 is not critical, but a minimum thickness should be selected to provide structural support for the mixed conductor film. Other suitable solid electrolyte materials include rare earth doped cerium oxide and lanthanum-strontium-gallium-magnesium oxide. In one embodiment, mixed conductor electrode 42 is substantially contiguous with solid electrolyte 44 and forms a large reaction area therewith for interfacing all three material phases - oxygen phase, gas phase, and catalyst phase - needed for the sensing reaction to occur. Counter electrode 46 is a metal electrode, e.g., a sputtered film of gold (Au) platinum (Pt) or the like, approximately 2500 A, or another mixed conductor, where counter electrode 46 is a relatively poor oxygen reduction catalyst to provide a differential reaction rate with the mixed conductor film electrode and a concomitant voltage difference detected by a voltage/current sensor, control circuit, or the like 48. A bias current source 49 may be included in the external circuit to optimize response to the sensed gas, as discussed below. Thus, in gas sensor 40 operating in the oxidation mode, the reduction of oxygen in the gas stream being analyzed occurs at counter electrode 46 as electrons are returned from an external circuit, e.g., voltage/current sensor 48. the oxygen ions are conducted through electrolyte 44 to mixed conductor electrode 42, where the oxygen ions diffuse throughout mixed conductor 42 to a reaction surface for oxidizing the analyte gas. The intrinsic catalytic activity of mixed conductor 42 effectively promotes the oxidation of the analyte gas to release electrons in the external circuit. As noted above, the reaction rates at mixed conductor electrode 42 and counter electrode 46 are different so that a voltage difference is produced between the electrodes and sensed by voltage/current sensor 48.

When gas sensor 40 operates in the reduction mode, the analyte gas is reduced at the surface of mixed conductor electrode 42 to produce oxygen ions that move through mixed conductor electrode 42 and electrolyte 44 to counter electrode 42. The catalytic nature of counter electrode 42 then oxidizes the oxygen ions to remove the excess electrons from the oxygen ions and release oxygen to the gas stream. The excess electrons act to complete the circuit with the reduction reaction at mixed conductor electrode 42.

Figure 2B illustrates the processes that occur in the film layers of a gas sensor 50 according to the present invention in the oxidation mode with reference to an exemplary gas, carbon monoxide CO. Thus, oxygen is reduced at oxygen reduction electrode 56, 1/2 O₂+2e →O⁼. The oxygen ions move through oxygen ion conductor 54 to mixed conductor electrode 52. Electrons are removed from the oxygen ions in mixed conductor electrode and return through an external circuit (not shown) to electrode 56. This reaction yield elemental oxygen at the surface of mixed conductor electrode 52. Mixed conductor electrode 52 electrocatalytically promotes the oxidation of the gas to be sensed, here CO, with the oxygen generated in the lattice of mixed conductor electrode 52, CO+O|_attice→CO₂. Thus, it will be appreciated that the gas contacting the oxygen reduction electrode must contain oxygen. The analyte gas may be used if it contains oxygen or a separate stream of oxygen-containing gas, e.g., air, may be used. Where the gas stream containing the analyte gas also contains oxygen, both electrodes may be located on the same side of the sensor as the analyte gas stream. An exemplary embodiment of this configuration is shown in Figure 2C, where sensor 60 has a reducing catalytic electrode 62 of a perovskite-type or fluorite-type oxide and an oxidation electrode 64 disposed on the same side of solid electrolyte 66, which separates electrodes 62 and 64 by a distance approximating the electrolyte thickness as described above. In a preferred embodiment, electrodes 62 and 64 are interdigitated to maximize the facing edges for sensitive performance. Again, a bias current source 68 may be included in the external circuit connecting electrodes 62 and 64 and an output sensor, such as voltmeter 72 or the like, may detect an output from sensor 60 for measurement and control purposes. The above configurations offer several performance advantages over the designs presented by Li et al. and Williams et al.: (1) increased sensitivity, (2) high operating temperatures with a concomitant wider operating range, and (3) faster response time. Comparative data shown in Figure 3 between the present device and data reported for the Li et al. device indicates that sensor 40 (Figure 2A), described above, has larger response output values and a greater response slope at 500°C, even at relatively low CO concentrations (500 ppm) in an air background. Figure 4 compares the response of our device having a catalytic metal electrode 46 (Figure 2A) with the Li et al. device as a function of increasing temperature. Although the sensor output decreases with increasing temperature for both sensor designs, the sensor according to our invention exhibits appreciable sensitivity even at 700°C. Thus, the output of our sensor at 700° C is equal to or greater than the output of Li et al. at 450°C, as shown in Figure 4 In addition, our device, unlike noble metal-based CO sensors, can be configured to show no response to hydrogen, i.e., is highly selective for the analyte gas CO. Further, Li et al. requires a time of about 4 min. to reach an equilibrium voltage compared to about 10 sec. for our device.

In one aspect of the invention, a current bias is applied between the electrode surfaces (see, e.g., current source 49 in Figure 2A and source 68 in Figure 2C). The effect of a current bias for a gas stream containing CO is shown in Figure 5. By biasing the sensor to promote the flow of oxygen ions through the electrolyte toward a mixed conductor film, here a Lao₈Sr₀₂CoO₃ film ( this current direction is designated by a "+" current notation), the rate of CO oxidation is enhanced with respect to the rate of oxygen reduction on the counter electrode surface. The application of only +0.15 μA increased sensor output by almost 20 mV over the unbiased case because of the increased oxygen differential across the electrolyte. Reversing the current reverses the process so that La₀₈Sr_{Q 2}CoO₃ becomes a poor CO oxidation catalyst, with a concomitant decrease in sensor response.

In another aspect of the present invention, the mixed conductor is formed as a thin film, typically much less than a micron in thickness, to provide adequate sensitivity. Thin films enable the surface electroactive reactions, e.g., the oxidation of CO, hydrogen, and hydrocarbons, or reduction of NO_x, to have greater influence on the oxygen chemical potential at the interface between the mixed conductor and the solid electrolyte by minimizing the distance between the film surface and the interface. The flux of oxygen to the surface of the material, the reaction that changes the interfacial oxygen chemical potential, is controlled by the diffusion coefficient of oxygen in the mixed conductor electrode and the diffusion distance, which is controlled by the film thickness. A film thickness is selected so that oxygen diffusion is not a limiting factor in the sensor response.

The effect of mixed conductor film thickness is shown by Figures 6A and 6B for a device having lanthanum strontium cobalt oxide on a yttria stabilized zirconia (YSZ) electrolyte with a Pt counter electrode. Figure 6A illustrates the performance of a device with a film of about 1 μm. While the device is responsive to changes in CO concentration, steady state conditions are not achieved. Figure 6B illustrates the response of a device with a film of about 0.15 μm. It should be noted that a step-like response, i.e., equilibrium is reached, occurs for each incremental change in CO concentration, a condition that is needed for a control device.

Figure 7 graphically compares the performance of a CO sensor where one mixed conductor film was formed to a thickness of about 0.15 μm using a thin film technique and the other conductor film was formed with a thick film technique, e.g., ink printing, with a thickness of more than about 100 μm. The thick film showed a small output response (about 8 mV) with a long response time to a step increase of 500 ppm CO compared to the thin film response (greater than 40 mV) with a short response time. The response characteristics were obtained under identical conditions of background atmosphere, temperature, and bias conditions.

Suitable electrocatalytic mixed conductors are well known and can be selected from a number of reference sources. An overview discussion with reference to other supporting literature is found in L.G. Tejuca et al., "Structure and Reactivity of Perovskite-Type Oxides," 36 Advances in

Catalysis, pp. 237-328 (Academic Press, Inc., 1989) and H.L. Tuller, "Mixed ionic-electronic conduction in a number of fluorite and pyrochlore compounds," 52 Solid State Ionics, pp. 135-146 (1992), both incorporated herein by reference. Tejuca et al. generally discuss the catalytic effect of perovskite-type oxides on the oxidation and reduction of numerous gases.

Exemplary gases include CO, NO, H₂, and vapor phase alcohols. Tuller discusses fluorite mixed conductors as catalytic oxidation electrodes for solid oxide fuel cells, oxygen sensors, and water electrolyzers. Thus, the selection of a perovskite-type or fluorite-type oxide to catalyze the oxidation or reduction of a selected gas can be routinely done by a person of ordinary skill in catalysis.

Our invention is not directed to the general application of perovskite- type or fluorite-type oxides to catalyze the oxidation and reduction of gases, but is directed to the application of perovskite-type oxides and fluorite-type oxides to a gas sensor as a thin, dense film. We have recognized that the mixed conductor characteristic of perovskite-type and fluorite-type oxides advantageously interacts with the catalytic property to enable a sensitive gas sensor to be realized.

The sensor described herein has been shown to be useful in detecting many different gases in a mixed gas stream, where the sensed gas undergoes an electrocatalytic reaction on the surface of a selected mixed conductor. Each sensor is characterized by its mixed electrode/oxygen ion conductor/counter electrode. Example 1.

La₀₈₄Sr_01βMnO₃/zirconia/Pt: Two sensors were prepared, a thick film mixed conductor (100 μm) and a thin film sensor (1000 A). The response of the sensor was determined at a temperature of 600 °C to a step increase of 500 ppm CO in a 1% oxygen gas - balance nitrogen gas. The thin film sensor exhibited about a 42 mV square wave response to the CO input, whereas the thick film exhibited a response of less than 10 mV with a response occurring over several hundred seconds.

Example 2.

La₀₈₄Sr_01βMnO₃/YSZ/La₀₈Sr₀₂CoO₃: The sensor surfaces were formed from two different mixed conductor films, each having a thickness of about 1500 A. The response of the sensor was determined at a temperature of 600 °C to a step increase of 330 ppm CO in an air background. The sensor exhibited a step response of 15 mV (0.0 μA bias current). Example 3.

La₀₈Sr₀₂CoO₃/YSZ/Pt: The sensor was formed from a mixed conductor with a thickness of 1500 A effective to catalyze the oxidation of hydrogen. The response of the sensor was determined at a temperature of 500 °C in an air background with step increases in H₂ of 236/702/1274/1508 ppm with concomitant signal response increases of about 55/15/15/5 mV for a single step, with the signal increasing from about 55mV to about 90 mV (0.0 μA bias current) over the entire range.

Example 4. LaCoO₃/YSZ/Au: The sensor was formed with a perovskite mixed conductor at a thickness of 1500 A effective to catalyze the oxidation of ethanol. The response of the sensor was determined at a temperature of 600 °C to a step increase of 330 ppm ethanol in an air background. The sensor exhibited a response of about 182 mV (+0.15 μA bias current), 190 mV (0.0 μA bias current), 210 mV (-0.15 μA bias current).

Example 5.

La_{! 8}Sr₀₂CuO₄/YSZ/Au: The sensor was formed from a perovskite mixed conductor with a thickness of 1500 A effective to catalyze the reduction of nitric oxide to nitrogen gas. The response of the sensor was determined at a temperature of 500 °C in a background of 1% O₂-balance nitrogen to a step increase of 36 ppm nitric oxide. The increase produced a step signal output change from about 2mV to about 9mV.

Example 6.

Tb-Y CeO₂/YSZ/Pt: The sensor was formed from a fluorite mixed conductor with a thickness of 1000-1200 A effective to catalyze the oxidation of CO. The response of the sensor was determined at a temperature of 600 °C in a background of 1 %O₂-balance nitrogen to an increase of 400 ppm CO.

The signal output change was about 8 mV to 46 mV (+0.15 μA bias), 0 mV to 30 mV (0.0 μA bias), and -6 mV to 24 mV (-0.15 μA bias).

The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A sensor for determining the concentration of a selected gas in a gaseous mixture, said sensor having a solid-state oxide electrolyte, a first electrode contacting said electrolyte for oxidizing or reducing said selected gas in said gaseous mixture at a first reaction rate, and a second electrode contacting said electrolyte for reducing or oxidizing oxygen, respectively, in said gaseous mixture at a second reaction rate, wherein the difference in said first and second reaction rates produces a measurable electrochemical potential across said electrolyte, the improvement comprising said first electrode formed from a dense thin film of a first mixed conductor suitable for catalyzing said oxidizing or reducing of said selected gas.

2. A sensor according to Claim 1 , wherein said film of mixed conductor has a thickness substantially less than 1 μm.

3. A sensor according to Claim 1 , wherein said film of mixed conductor has a thickness less than about 0.2 μm.

4. A sensor according to any one of Claims 1-3, wherein said mixed conductor is a perovskite-type oxide.

5. A sensor according to any one of Claims 1-3, wherein said mixed conductor is a fluorite-type oxide.

6. A sensor according to any one of Claims 1-3, wherein said selected gas is selecting from the group consisting of carbon dioxide, nitric oxide, hydrogen, and ethanol.

7. A sensor according to Claim 1 , further including means for producing a bias current between said first and second electrodes in a

direction to enhance the movement of oxygen ions through said electrolyte toward an oxidizing or reducing one of said first and second electrodes.

8. A sensor according to Claim 1 , wherein said second electrode is formed from a dense thin film of a second mixed conductor.

9. A sensor according to Claim 8, wherein said film of said first and second mixed conductors each have a thickness substantially less than 1 μm.

10. A sensor according to Claim 8, wherein said films of said first and second mixed conductors each have a thickness less than 0.2 μm.

11. A sensor according to any one of Claims 8-10, wherein said first and second mixed conductors are different perovskite-type oxides.

12. A sensor according to any one of Claims 8-10, wherein one of said first and second mixed conductors is a perovskite-type oxide and the other is a fluorite-type oxide.