GB1591898A

GB1591898A - Electrochemical cell having enhanced-surface non-conduction solid electrolyte and method of making same

Info

Publication number: GB1591898A
Application number: GB51960/77A
Authority: GB
Original assignee: UOP LLC
Current assignee: Honeywell UOP LLC
Priority date: 1976-12-15
Filing date: 1977-12-14
Publication date: 1981-07-01
Also published as: FR2374640A1; ES465653A1; ES487745A1; IT1089129B; FR2374640B1; CA1096441A; DE7738098U1; DE2755650A1; JPS53101485A

Description

(54) ELECTROCHEMICAL CELL HAVING ENHANCED-SURFACE NON CONDUCTION SOLID ELECTROLYTE AND METHOD OF MAKING SAME (71) We, UOP Inc, a corporation organized under the laws of the State of Delaware United States of America, of Ten UOP Plaza, Algonquin & Mt. Prospect Roads, Des Plaines, Illinois, 60016, United States of America, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: This invention relates to electrochemical cells such as oxygen sensors and fuel cells but particularly to oxygen-sensing cells which incorporate an oxygen-ion-conducting solid electrolyte such as yttria stabilized zirconia. Such oxygen-sensing cells are good ionic conductors and are used to generate a voltage signal in accordance with the familiar Nernst equation in response to differences in the partial pressures of oxygen on a reference side (usually air) and a sensing side.

As discussed at length in U.S. Patent 3,935,089, a conventional oxygen sensing cell having Pt electrodes deposited on either side of a stabilized oxygen-ion conducting solid electrolyte will experience a rather mild change in EMF as the engine air-fuel ratio (A/F) is varied about the stoichiometric ratio (S) rather than the sharp step change predicted by the Nernst equation. The mild transition is presumed to be caused by the fact that oxidation reactions in an engine do not attain equilibrium, so that oxygen concentration in the exhaust gas is always higher than the theoretical value (the value predicted by the Nernst equation) when an A/F lower than S (rich mixture) is employed. The Pt electrode in the exhaust gas acts as a catalyst for reactions of oxygen in the exhaust gas with unburned hydrocarbons and carbon monoxide but the catalytic effect is insufficient to allow the reactions to obtain equilibrium. The aforementioned patent proposes that a much sharper EMF transition can be made as the A/F passes through S. This is accomplished by extending the electrode surface by applying a coating of Al203 to it which is impregnated with a catalyst.

Although the aforementioned patent teaches a way to increase the surface area of the electrode to better achieve equilibrium of the gases, the cell of the patent would necessarily still have a relatively high internal impedance due to the relatively small surface area of the solid electrolyte, even after sandblasting. This is so since the oxygen ions are only conducted through the zirconia electrolyte material. Where solid electrolyte cells are used to sense the oxygen content of an automotive engine's exhaust gas they must necessarily be quite compact so they can be inserted in the fashion of a spark plug into the side of an exhaust pipe.

Typically, the solid electrolyte is in the form of a stabilized zirconia wafer or a thimble which must have a sufficient thickness to provide the strength necessary to resist damage in the rugged exhaust environment. Unfortunately, the relatively small size of the cell and its relatively great thickness combine to provide the cell with a substantial internal impedance.

To maximize output, such cells are usually operated at relatively high temperatures, about 540"C, even though such high temperatures enhance the degradation of the catalyst electrode. Obviously, it would be desirable to be able to decrease the internal impedance of such cells to increase their voltage output and/or to permit operation at lower temperatures to provide increased life.

The present invention seeks to provide an improved electro-chemical cell having a significantly lower internal impedance than prior art cells of the same thickness.

According to the present invention there is provided an electrochemical cell comprising a dense, non-porous self-supporting solid oxygen-ion-conducting electrolyte member, a porous layer of less dense solid oxygen-ion-conducting electrolyte in intimate contact with at least one surface of the dense electrolyte member, and a porous catalytic electrode on at least portions of the porous layer, the porous layer having a surface area exposed to the electrode which exceeds the surface area of the dense electrolyte member which it overlies by a factor of at least 50.

The invention further provides a method of producing an electrochemical cell having a dense substrate of an ion-conducting solid electrolyte material of a predetermined thickness, including the steps of coating at least a portion of one surface of the substrate with a coating of very fine particles of a porous ion-conducting solid electrolyte of the same chemical composition as the dense substrate; applying current collectors to each side of the substrate; and applying a catalyst-containing solution to at least portions of the coating to catalytically activate said coating, the current collectors and the catalytically activated coating being in electrically conducting relationship and the surface area of the coating being at least 50 times greater than the surface area of the substrate to which it is applied, whereby the cell has a reduced output impedance.

In one embodiment the invention provides an oxygen sensor for sensing the difference in oxygen content between an exhaust gas from a combustion process and a reference gas, said sensor including a body member and an oxygen ion-conductive member of dense non-porous stabilized zirconia sealed in the body member so that one side is exposed to the exhaust gas and the other to the reference gas, one end of the zirconia member being coated on at least a portion of the surface thereof which is designed to be exposed to the exhaust gas with a porous thick film coating of stabilized zirconia, a porous catalytic electrode in contact with the exposed surface of the coating, and current collectors in contact with the opposite sides of said zirconia member, the porous thick film coating having a surface area exposed to the electrode which exceeds the surface area of the dense zirconia member which it overlies by a factor of at least 50.

In a preferred embodiment of the present invention, a porous stabilized zirconia network is fired onto the surface of a dense stabilized zirconia substrate such as a wafer or disc or a thimble and a porous electrode coating is then applied. Unlike the non-ion-conductive alumina coating proposed by U.S. Patent 3,935,089, the porous yttria-zirconia coating is an integral part of the solid electrolyte matrix and as such provides a very large surface area for ionic conduction at the gas-solid interfaces as compared to the area of the dense yttriazirconia substrate. The large surface area, which is at least 50 times and often at least 100 times up to as much as 1000 times the area of the underlying substrate, serves to reduce or eliminate polarizations that can develop at the sensing surface since it provides many more 3-phase sites (platinum, zirconia, and gas phase) where oxygen ions can pass through the surface and react with CO to form CO2. Since diffusional processes limit the removal of reaction products, CO2 tends to form an immobilized film over the surface which restricts the access of the CO to the oxygen ions at the 3-phase sites. It is obvious that the more sites there are for the oxygen ions to react with the CO, the greater will be the extent of the reaction.

Obviously, the larger surface area for ion "transduction" (ionic transfer to and from the gas phase just across the gas-solid interface) denotes a lowered impedance and the possibility for an increased flow of current as compared to a cell not having an extended ion conductive surface. The lowered impedance permits lower temperature operation as compared to prior art cells for a given voltage signal. Thus, the improved cell can begin producing signals sooner after engine startup, or, alternatively, could be positioned further downstream from the exhaust manifold than present devices where it could be expected to provide a longer life due to the less rugged nature of the environment.

Preferably, a porous yttria-zirconia extension of the dense solid electrolyte matrix is provided on the reference-gas side of the oxygen sensor in addition to that described above.

This further minimizes the development of a polarizing potential within the solid electrolyte by providing more sites for oxygen ingress to the solid electrolyte matrix to accommodate the higher total flux of 0 - ions as demanded by the external circuit on the new and improved low impedance device of this invention.

The preferred embodiment of the invention takes advantage of the fact that a thinner section of the dense electrolyte will be design have a lower intrinsic impedance. In this embodiment, recesses are formed in the central area of both sides of a disc- or wafer-shaped electrolyte. The hoop strength of the material surrounding the recesses minimizes the weakening of the structure produced by the recesses. By utilizing the extended surface zirconia coating, the effective surface area for transduction can still be much greater than not only the projected surface areas of the recesses, but greater than the entire surface area of a disc not having recesses or a zirconia coating.

The invention is further illustrated with reference to the accompanying drawings in which Figure 1 is a fragmentary cross-section on line 1-1 of Figure 2 showing one embodiment of an oxygen sensor inforporating the invention; Figure 2 is an end view of the sensor of Figure 1; Figure 3 is a fragmentary cross-section of an oxygen sensor incorporating a preferred embodiment of the invention; Figure 4 is a cross-section of a recessed surface solid electrolyte having an artificial macropore matrix; Figure 5 is a photomicrograph of 125cm thick layer of porous zirconia on a zirconia substrate; Figure 6 is a graph plotting the output voltage of a cell of this invention as a function of increasing A/F ratio and operating temperatures; Figure 7 is a graph plotting internal cell resistance versus A/F for cells which do and do not have porous zirconia coatings under different temperature and flow conditions; and Figure 8 is a cross-section of a fuel cell having very large artificial macropores for pumping fuel or gas to the cell.

Referring to Figure 1 of the drawings, a fragmentary portion of an oxygen sensing cell 10 is shown with the eimensions exaggerated for clarity. A wafer or disc 12 of dense, yttria stabilized zirconia is hermetically sealed by a glass frit 13 in a recess 14 in the end of a non-ion conducting ceramic insulating tube 16 made of a material such as forsterite. The tube 16 could be of a short length as disclosed in our copending GB patent application No. 4612/77 (Serial No. 1563940), or it could be of the more conventional longer length designed to extend well into the exhaust gas stream.

A layer or coating 18 or porous stabilized zirconia of the same chemical composition as the dense zirconia substrate 12 overlies the substrate and defines an extended ion-conducting surface for the substrate of at least 50-1000 times the area of the substrate. A platinum collector ring 20 surrounds the extended layer 18 and stripes 22 of platinum are deposited on top of layer 18 and joined to the ring 20. A lead 24 of platinum also joins the ring 20 with a jumper portion 24' and runs down the exterior surface of the tube 16 where it is available for connection into an electrical circuit (not shown).

Although not as much improvement is derived from providing an extended ion-conductive porous stabilized zirconia layer 28 on the reference side of the sensor or cell 10 as on the sensing side, a significant improvement is provided which makes such layer 28 desirable. A platinum lead 32 is connected to the layer 28 by a current collector ring 34 in similar fashion to the lead 24. A solution of chloroplatinic acid deposited over the layers 18, 28 and the current collectors 20, 22 and 34 provides a lattice of porous platinum particles 38 (Fig. 2) which extends throughout the sponge-like pores in the porous layers and provides a myriad of 3-phase catalyst reaction sites. The particles 38 provide improved conductivity across the solid electrolyte surface to the current collector members.

The cell 10' shown in Figure 3 represents a preferred embodiment of the invention which is generally identical to the cell 10 of Figure 1 but differs in that the substrate 12' is recessed in its center at 42, 44 so that the porous layers 18', 28' can be flush with the outer rim portion 46. As previously discussed, the recessing of the substrate lowers its impedance.

Figure 5 is microphotograph taken at 2000X magnification with a scanning electron microscope of a 125,u thick porous zirconia layer (such as the layer 18 in Figure 1) on a zirconia substrate. It is clearly evident that two ranges of porosity are distributed in the sintered porous zirconia (white areas) coating: a large macropore structure (black areas) of approximately 1-10,u in diameter and a microporous network of approximately 0.01 to 0.1,u in pore diameter which is continuous between the fine particles of the sintering network.

These macro- and microporous networks are naturally developed in the sintering process of loosely compacted fine powders.

Artificial macropore and super-macropore distributions, the latter ranging from 10,a to 1000/1. in diameter, (Fig. 4) can be designed into fine-particle zirconia matrices 48 by incorporating small rod-like elements 50 of organic fibers such as are made with acrylic or nylon resins, loading them into the ceramic paste-body to a degree to assure sufficient tangential contact between the fibers, drying to set the matrix, and then burning out the fibers leaving the artificial macropore structure intact. This permits a graduation in pore sizes of diffusion regions to be incorporated into the extended solid electrolyte matrix so that deeper layers within the matrix are in nearer diffusion-equilibrium with the reactant equilibrium gas.

A final (Figure 8) extension of artificial macropore construction applies to fuel cells, and would permit construction of an array of continuous linear pipes 54 in the pore bed 56, lying parallel to and on both sides of the dense electrolyte surface 58. The pipes or tubes may have a diameter of from .040-0.500 inches with a length to diameter ratio of ten or more. With one end of the pipe-array plugged as shown at 60, entrant fuel or oxidant gas may be pumped into either side of the solid reactor surface-bed 56, allowing high temperature fuel cell reactors to be constructed with higher current capacity than could be obtained under the limiting restriction of a gas diffusional process. The solid oxygen ion conducting electrolyte 58 in the fuel cell embodiment shown can be stabilized zirconia or one of several other suitable materials such as cerium oxide.

In order that the present invention may be more completely understood, the following examples are given to describe the method of making a low impedance cell and to demonstrate the dual function of the extended surface of this construction: the cell functions firstly as a high surface area catalyst support, which when activated, serves in the mode disclosed in Patent 3,935,089 to bring the incompletely reacted sampled gases to thermodynamic equilibrium by offering energetic surface sites which promote equilibration reactions; secondly, as a "fanned-out" extension of the 0= conductmg solid electrolyte matrix, it minimizes cell polarizations caused by accretion of gaseous reaction products generated by the cell operation. In this latter polarization-limiting mode, it will be shown that the operationallygenerated occluding gas films are diffusion-limited in their removal, and that improved cell performance in the form of lower impedance and higher 0= transport capability, can be obtained with the extended surface solid electrolyte system at higher rates of sampled gas transport across the face of the sensor.

EXAMPLE I A planar-surfaced dense 8-1/2 mole % Y203-ZrO2 disc 12 shown incorporated in the sensor housing of Figure 1 was coated on both sides with circular patterns of fine 8-1/2 mole No Y2O3-ZrO2 powder suspended as a pigment in a thick film ink. The dried ink pattern 18 was approximately 1/4" dia x .005" thick. After air drying, firing at 1500"C. for 1 hour sintered the powder into ionic continuity with the dense disc, still retaining the porosity shown on the SEM photomicrograph of Figure 5. A current-collector ring 20 of platinum ink (Figure 2) was drawn around the periphery of the 0.25" dia film coating 18, and fine lines 22 crossed the fired pad in a spoke-and-wheel configuration. Firing of the current collector was achieved at 9500C. in an air oven.

The disc was then glass-sealed by a glass frit 13 into the recessed end of a forsterite ceramic tube 16 to provide isolation of the sensing and reference electrode faces. Platinum-paste jumper interconnections 32', 24' were fired over the sealing-ring 20 to connect the current collectors 20, 22 on the disc surfaces to prepare platinum conductor stripes 32, 24 axiallylocated on the inner and outer surfaces of the forsterite tube. The sensor faces were then catalytically activated by applying 5 mg Pt as chloroplatinic acid solution to each porous yttria-zirconia sintered film, and hydrogen firing for 30 minutes at 2250C. The tube was then mounted in a metal housing for insertion into a bench test apparatus which passed preheated mixtures of CO and air to generate equivalent A/F ratios.

Results Figure 6 shows the output of the sensor fabricated as per Example I as a function of A/F ratio. Not only is the Nerstian transition abrupt at stoichiometry (S) as reported by U.S.

Patent 3,935,089, where: E = t14F in Gef and where R is the gas constant, T is the absolute temperature, F is the Faraday constant, O2 is the partial pressure of oxygen in the reference and gas atmospheres respectively, but the cell output at low A/F ratios shows the inversion of temperature dependence not predicted by the Nerstian relationship above. Eddy (IEE Transactions on Vehicular Technology, Vol. VT-23, No. 4, Nov., 1974) has shown that this inverted temperature relationship is due to the increasing dissociation of CO2 at higher temperatures, giving CO2 f CO + 1/2 02.

The excess 2 liberated produces a leaner (lower output) signal than might otherwise be obtained. Eddy describes this inverted temperature relationship as due to ideal catalytic activity in bringing the exhaust gas to thermodynamic equilibrium at the sensing electrode.

This result has been achieved without the use of alumina in the catalyst-support system as prescribed by Patent 3,935,089 and shows the dual function of the extended surface coating, in that the measured gas sample may be brought to catalyzed equilibrium, giving not only the sharp Nerstian step function at stoichiometry, but also providing the appropriate inverted thermal response characteristic at low A/F ratios.

EXAMPLE II A second cell was prepared as in Example I except that 6.25 mg of Pt were deposited on each electrode in the catalytic activation step. The cell was mounted in the test apparatus described above and cell output was measured at various temperatures and sample gas transport rates as a function of the resistive load across the cell. The cell impedance was indirectly measured as equal to the adjusted resistive load which lowered the cell terminal voltage to one-half the open circuit potential.

Results The data of Figure 7 detail the test results of two cells. The cells comprise an extendedsurface Y2O3-ZrO2 cell as shown in Fig. 1 and a planar Y203-ZrO2 prior art type cell fabricated as shown in Figure 1 but without the sintered Y203-ZrO2 extended surface. From the data the following conclusions can be drawn: 1) In comparing curves A and C, the extended surface zirconia cell has a lower impedance than the plane-surfaced cell (approximately 10,000 Qversus~60,000Q) when oper ated at 5400C.

2) At 650"C. (compare curves E and F) the extended surface cell has an impedance in the range of 750 + 150Q while the planar surface zirconia cell has an impedance in the 7 + 2 kfl range.

3) Increasing gas flow rate (compare A versus B, C versus D) over the same sensor showed a decrease in Rl of h, 50% for the planar ZrO2 sensor and an approximate 2/3 decrease in Ri for the extended surface ZrO2 sensor. This indicates that at these flow rates the nature of the polarization developed on discharge during cell measurement in an accumulation of oxidized by-product at the sensing interface and possibly reduced by-product (nitrogen enriched air) at the reference interface, and that higher gas velocities aid in the removal of these collected impurities. It is also apparent that when comparing the two types of sensors, the cell with extended surface ZrO2 interfaces will allow more of this by-product to collect on its larger surfaces before developing surface polarizations which add Ri, its internal resistance.

In overall conclusion, it has been shown that an enhanced-surface stabilized zirconia solid electrolyte can perform the dual function of: (1) a catalyst support, which when platinum activated, promotes the attainment of thermodynamic equilibrium in partially reacted gas mixtures at the outermost regions of the particulate matrix from the dense electrolyte body, and (2) an increased surface providing a greater number of sites for 0= ion transduction across the solid gas interfaces. On the sensing electrode surface, considering the resistance of the longer ionic conduction paths within the extended particulate matrix, it would appear that this latter function is served in the innermost regions of the particulate matrix adjacent to the dense electrolyte body, interacting the transduced oxygen with the downwardly-diffusing thermodynamically equilibrated sensed gas.

WHAT WE CLAIM IS: 1. An electrochemical cell comprising a dense, non-porous self-supporting solid oxygenion-conducting electrolyte member, a porous layer of less dense solid oxygen-ion-conducting electrolyte in intimate contact with at least one surface of the dense electrolyte member, and a porous catalytic electrode on at least portions of the porous layer, the porous layer having a surface area exposed to the electrode which exceeds the surface area of the dense electrolyte member which it overlies by a factor of at least 50.

2. An electrochemical cell as claimed in claim 1 wherein the porous layer has a surface area exposed to the electrode which exceeds the surface area of the dense electrolyte member which it overlies by a factor of at least 100.

3. An electrochemical cell as claimed in claim 1 or 2 wherein the dense electrolyte member comprises stabilized zirconia.

4. An electrochemical cell as claimed in claim 1 or 2 wherein the dense electrolyte member comprises cerium oxide.

5. An electrochemical cell as claimed in any of claims 1 to 4 wherein the dense electrolyte member includes a central portion of reduced thickness, and the porous layer of less dense electrolyte overlies this central portion.

6. An electrochemical cell as claimed in any of claims 1 to 5 wherein a metal current collector overlies spaced apart portions of the porous layer and the porous catalytic electrode overlies and is in electrical contact with the current collector.

7. An electrochemical cell as claimed in Claim 6 wherein the dense electrolyte member is in the form of a wafer.

8. An electrochemical cell as claimed in Claim 7 wherein the current collector overlies the porous layer in a pattern similar to a spoked wheel.

9. An electrochemical cell as claimed in claim 8 wherein the porous catalytic electrode comprises platinum.

10. An electrochemical cell as claimed in any of claims 3 to 9 wherein the dense electrolyte member comprises zirconia stabilized with yttria.

11. A method of producing an electrochemical cell having a dense substrate of an ion-conducting solid electrolyte material of a predetermined thickness, including the steps of coating at least a portion of one surface of the substrate with a coating of very fine particles of a porous ion-conducting solid electrolyte of the same chemical composition as the dense substrate; applying current collectors to each side of the substrate; and applying a catalystcontaining solution to at least portions of the coating to catalytically activate said coating, the current collectors and the catalytically activated coating being in electrically conducting relationship and the surface area of the coating being at least 50 times greater than the surface area of the substrate to which it is applied, whereby the cell has a reduced output impedance.

12. A method as claimed in claim 11 wherein the ion-conducting solid electrolyte is designed to be exposed on one surface to a gas to be sensed and on another surface to a reference gas, and the method includes the step of applying the coating to both the sensing gas surface and the reference gas surface.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

ated at 5400C.

2) At 650"C. (compare curves E and F) the extended surface cell has an impedance in the range of 750 + 150Q while the planar surface zirconia cell has an impedance in the 7 + 2 kfl range.

3) Increasing gas flow rate (compare A versus B, C versus D) over the same sensor showed a decrease in Rl of h, 50% for the planar ZrO2 sensor and an approximate 2/3 decrease in Ri for the extended surface ZrO2 sensor. This indicates that at these flow rates the nature of the polarization developed on discharge during cell measurement in an accumulation of oxidized by-product at the sensing interface and possibly reduced by-product (nitrogen enriched air) at the reference interface, and that higher gas velocities aid in the removal of these collected impurities. It is also apparent that when comparing the two types of sensors, the cell with extended surface ZrO2 interfaces will allow more of this by-product to collect on its larger surfaces before developing surface polarizations which add Ri, its internal resistance.

In overall conclusion, it has been shown that an enhanced-surface stabilized zirconia solid electrolyte can perform the dual function of: (1) a catalyst support, which when platinum activated, promotes the attainment of thermodynamic equilibrium in partially reacted gas mixtures at the outermost regions of the particulate matrix from the dense electrolyte body, and (2) an increased surface providing a greater number of sites for 0= ion transduction across the solid gas interfaces. On the sensing electrode surface, considering the resistance of the longer ionic conduction paths within the extended particulate matrix, it would appear that this latter function is served in the innermost regions of the particulate matrix adjacent to the dense electrolyte body, interacting the transduced oxygen with the downwardly-diffusing thermodynamically equilibrated sensed gas.

WHAT WE CLAIM IS: 1. An electrochemical cell comprising a dense, non-porous self-supporting solid oxygenion-conducting electrolyte member, a porous layer of less dense solid oxygen-ion-conducting electrolyte in intimate contact with at least one surface of the dense electrolyte member, and a porous catalytic electrode on at least portions of the porous layer, the porous layer having a surface area exposed to the electrode which exceeds the surface area of the dense electrolyte member which it overlies by a factor of at least 50.
2. An electrochemical cell as claimed in claim 1 wherein the porous layer has a surface area exposed to the electrode which exceeds the surface area of the dense electrolyte member which it overlies by a factor of at least 100.
3. An electrochemical cell as claimed in claim 1 or 2 wherein the dense electrolyte member comprises stabilized zirconia.
4. An electrochemical cell as claimed in claim 1 or 2 wherein the dense electrolyte member comprises cerium oxide.
5. An electrochemical cell as claimed in any of claims 1 to 4 wherein the dense electrolyte member includes a central portion of reduced thickness, and the porous layer of less dense electrolyte overlies this central portion.
6. An electrochemical cell as claimed in any of claims 1 to 5 wherein a metal current collector overlies spaced apart portions of the porous layer and the porous catalytic electrode overlies and is in electrical contact with the current collector.
7. An electrochemical cell as claimed in Claim 6 wherein the dense electrolyte member is in the form of a wafer.
8. An electrochemical cell as claimed in Claim 7 wherein the current collector overlies the porous layer in a pattern similar to a spoked wheel.
9. An electrochemical cell as claimed in claim 8 wherein the porous catalytic electrode comprises platinum.
10. An electrochemical cell as claimed in any of claims 3 to 9 wherein the dense electrolyte member comprises zirconia stabilized with yttria.
11. A method of producing an electrochemical cell having a dense substrate of an ion-conducting solid electrolyte material of a predetermined thickness, including the steps of coating at least a portion of one surface of the substrate with a coating of very fine particles of a porous ion-conducting solid electrolyte of the same chemical composition as the dense substrate; applying current collectors to each side of the substrate; and applying a catalystcontaining solution to at least portions of the coating to catalytically activate said coating, the current collectors and the catalytically activated coating being in electrically conducting relationship and the surface area of the coating being at least 50 times greater than the surface area of the substrate to which it is applied, whereby the cell has a reduced output impedance.
12. A method as claimed in claim 11 wherein the ion-conducting solid electrolyte is designed to be exposed on one surface to a gas to be sensed and on another surface to a reference gas, and the method includes the step of applying the coating to both the sensing gas surface and the reference gas surface.
13. A method as claimed in Claim 11 wherein the ion-conducting solid electrolyte is

designed to be exposed on one surface to a gas to be sensed and on another surface to a reference gas, and the coating of fine particles is applied to at least a portion of the electrolyte surface which is adapted to be exposed to a gas to be sensed.
14. A method as claimed in claim 13 wherein the substrate has a central portion of its surface area removed to reduce its center thickness before the coating is applied.
15. A method as claimed in any of claims 11 to 14 wherein the coating is fired and sintered before the current collectors and catalyst-containing solution are applied.
16. A method as claimed in claim 15 wherein the catalyst-containing solution is chloroplatinic acid.
17. An oxygen sensor for sensing the difference in oxygen content between an exhaust gas from a combustion process and a reference gas, said sensor including a body member and an oxygen ion-conductive member of dense non-porous stabilized zirconia sealed in the body member so that one side is exposed to the exhaust gas and the other to the reference gas, one end of the zirconia member being coated on at least a portion of the surface thereof which is designed to be exposed to the exhaust gas with a porous thick film coating of stabilized zirconia, a porous catalytic electrode in contact with the exposed surface of the coating, and current collectors in contact with the opposite sides of said zirconia member, the porous thick film coating having a surface area exposed to the electrode which exceeds the surface area of the dense zirconia member which it overlies by a factor of at least 50.
18. An oxygen sensor as claimed in claim 17 wherein the body member is of tubular, non-ion-conducting ceramic and the oxygen ion-conductive member is in the shape of a disc, which is hermetically sealed in one end of the tubular ceramic body member.
19. An oxygen sensor as claimed in 17 or 18 wherein the current collector which is in contact with the exhaust gas side of the zirconia member overlies the porous catalytic electrode and is arranged in a spoke-like pattern.
20. An oxygen sensor as claimed in any of Claims 17 to 19 wherein the zirconia member is stabilized with yttria and the porous electrode and current collectors are platinum.
21. An electrochemical cell as claimed in any of claims 1 to 10 wherein the porous layer of electrolyte has a pore structure in which at least some of the pores have a diameter in the range of 1 - 10,a .
22. An electrochemical cell as claimed in any of Claims 1 to 10 wherein the porous layer of electrolyte has a pore structure in which at least some of the pores have a diameter in the range of 0.01 - 0.lit.
23. An electrochemical cell as claimed in any of Claims 1 to 10 wherein the porous layer of electrolyte has a pore structure in which at least some of the pores have a diameter in the range of 0.01 - 10,(1.
24. An electrochemical cell as claimed in any of Claims 1 to 10 wherein the porous layer of electrolyte has a pore structure in which at least some of the pores have a diameter in the range of l 0 - 1000,u
25. An electrochemical cell as claimed in any of Claims 1 to 10 or 10 to 24 wherein the porous layer of electrolyte has tubular passages formed therein which have at least one end open externally of the cell for permitting gas to be pumped into said porous layer, said tubular passages having a diameter in the range of 0.040 - 0.500 inches.
26. An electrochemical cell as claimed in claim 1 and substantially as hereinbefore described with reference to the oxygen sensor shown in Figures 1 and 2 or in Figure 3 of the accompanying drawings or with reference to the solid electrolyte shown in Figure 4 of the accompanying drawings.
27. An electrochemical cell as claimed in claim 1 and substantially as hereinbefore described in Example I or Example II.
28. A method as claimed in claim 11 and substantially as hereinbefore described with reference to the oxygen sensor shown in Figures 1 and 2 or in Figure 3 of the accompanying drawings or with reference to the solid electrolyte shown in Figure 4 of the accompanying drawings.
29. A method as claimed in claim 11 and substantially as hereinbefore described in Example I or Example II.
30. An oxygen sensor as claimed in claim 17 and substantially as hereinbefore described with reference to, and as shown in, Figures 1 and 2 or Figure 3 of the accompanying drawings or incorporating in a structure so shown and described a solid electrolyte substantially as hereinbefore described with reference to, and as shown in, Figure 4 of the accompanying drawings.
31. An oxygen sensor as claimed in claim 17 and substantially as hereinbefore described in Example I or Example II.
32. An oxygen sensor comprising a body member and an electrochemical cell member as claimed in claim 1 sealed in said body member so as to enable simultaneous exposure of opposed sides of said cell member to an exhaust gas on the one hand and a reference gas on the other hand so as to detect a difference between said exhaust gas and said reference gas with respect to their oxygen concentrations.